Cretaceous Oceanography and Foraminifera

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Geological Society of America Special Papers

doi: 10.1130/0-8137-2332-9.3011999;332;301-328Geological Society of America Special Papers

Isabella Premoli Silva and William V. SliterCretaceous paleoceanography: Evidence from planktonic foraminiferal evolution

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INTRODUCTION

Modern planktonic foraminiferal distributions define five

latitudinally constrained bioprovinces from the equator to the

poles. Altogether, the bioprovinces are characterized by a marked

decrease in species richness (or diversity) from the equator

toward the high latitudes where the latter assemblages are almost

monospecific and are dominated by the most opportunistic

species with simple morphologies (B, 1982). Similar patterns ofdistribution can be reconstructed as far back in time as the Pale-

ocene, although several of the Paleogene Epoch morphologic

groups are extinct and apparently did not behave like their mod-

ern counterparts (Boersma and Premoli Silva, 1983, 1991). How

do the modern, or at least the Paleogene, patterns apply to Creta-

Geological Society of America

Special Paper 332

1999

Cretaceous paleoceanography: Evidence from planktonic

foraminiferal evolution

Isabella Premoli Silva

Dipartimento di Scienze della Terra, Universita di Milan, 20133 Milano, Italy

William V. Sliter (deceased October 31, 1997)

U.S. Geological Survey, 345 Middlefield Road, MS 975, Menlo Park, California 94025-3591

ABSTRACT

The evolution of planktonic foraminifers in the Cretaceous shows pulses of diver-

sification and stasis interrupted by brief extinction events and faunal turnover. The

overall record shows a threefold pattern; from the early Valanginian to the latest Apt-

ian/Albian boundary, then to the latest Albian, and finally, to the end of the Creta-

ceous. The pattern of evolution in the first and second intervals is similar and is

characterized by increasing diversity, size, and morphologic complexity. The third

interval differs in pattern and shows short periods of rapid diversification and

turnover separated by longer periods of stasis. We equate these evolutionary changes

to parallel changes in the physical and chemical structure of the Cretaceous ocean.

The first and second intervals represent the progression from a mixed, eutrophic,

upper-water column to the development of a thermocline, stratification, and niche-

partitioning in an oligotrophic column. The third interval is more complex and is

characterized by periods of mixing and stability above the thermocline probably con-

trolled by climatic conditions and nutrient runoff, and by the development of well-

defined latitudinal bioprovinces in the Campanian and Maastrichtian.This overall record is interrupted by five events of paleoceanographic signifi-

cance. Three of the events, the Selli Event in the early Aptian, the Aptian/Albian

boundary event, and the Bonarelli Event in the latest Cenomanian, are defined by the

deposition of organic carbon-rich sediment. The fourth, or Santonian event, precipi-

tated the largest foraminiferal turnover during the Cretaceous, affecting all plank-

tonic foraminiferal trophic groups. This event also ushered in a Cretaceous ocean with

modern attributes. The fifth event was the catastrophic extinction at the end of the

Cretaceous that terminated the Mesozoic Era. Each of these events shows evidence of

upper-water column disruption likely related to increased upwelling.

Premoli Silva, I., and Sliter, W. V., 1999, Cretaceous paleoceanography: Evidence from planktonic foraminiferal evolution, in Barrera, E., and Johnson, C.C., eds., Evolution of the Cretaceous Ocean-Climate System: Boulder, Colorado, Geological Society of America Special Paper 332.

301

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ceous planktonic foraminifers that are extinct except for the

opportunistic Guembelitria? Can the evolutionary changes dis-

played by Cretaceous planktonic foraminifers be used to interpret

the paleoceanographic evolution of the Cretaceous ocean?

Here, we summarize the state of the art and interpret the

sequence of major ecological changes in the pelagic realm during

the Cretaceous based primarily on the evolutionary development(morphologic complexity, diversity, life strategy) and spatial dis-

tribution of planktonic foraminifers, from their first diversification

in the early Valanginian to the crisis at the Cretaceous/Paleogene

Series boundary. Our intent is to extend our paleoceanographic

interpretation to the fullest to foster future discussion and compar-

ative analysis. While nowhere rigorous, we also incorporate

selected significant changes shown by other fossil groups such as

calcareous nannofossils, radiolarians, dinoflagellates, benthic

foraminifers, and megafossils to augment our conclusions.

Earlier preliminary paleoceanographic reconstructions were

based on Albian to Maastrichtian planktonic foraminiferal faunas

from the remarkably continuous pelagic record from central Italy

(Wonders, 1980; Premoli Silva and Sliter, 1994). Here, we extend

the record through the Early Cretaceous by incorporating the

results of paleontologic investigations from the Rio Argos section

in southern Spain (Coccioni and Premoli Silva, 1994) and the

Gorgo a Cerbara section in Italy (Coccioni et al., 1992; Cecca et

al., 1994). The Bottaccione and Rio Argos sections together span

the entire evolutionary succession of Cretaceous planktonic

foraminifers. Both sections are well calibrated to integrated bio-

and magnetostratigraphic schemes and to most stages within the

latest chronologic time scale (Erba et al., 1995b, mainly after

Gradstein et al., 1994; Channell et al., 1995).

CRETACEOUS PALEOCEANOGRAPHIC CHANGES

The record of planktonic foraminiferal evolution through the

Cretaceous is characterized by periods of diversification that

alternate with periods of stasis during which the assemblages

apparently underwent little or no change (Fig. 1). These alternat-

ing periods of slow to rapid diversification and stasis display dif-

ferent duration through time. We equate these evolutionary

patterns to parallel changes in the physical and chemical struc-

ture of the worlds oceanic water masses.

The overall Cretaceous record shows a general threefold pat-

tern. The first interval extends from the first diversification in the

early Valanginian until the latest Aptian. The pattern shows a con-

tinuously increasing diversification that is interrupted by a singleepisode of moderate turnover near the Selli Event, an episode of

organic carbon-rich (Corg-rich) sediment deposition in the late

early Aptian (Coccioni et al., 1992; Erba, 1994). The second

interval extends from the Aptian/Albian boundary, another Corg-

rich episode (Brhret et al., 1986; Tornaghi et al., 1989; Arthur

et al., 1990), and extends until the latest Albian. Again the pat-

tern is characterized by continuously increasing diversification

similar to that of the first interval but in half of the time (12 m.y.

versus 22 m.y.). The difference in duration is mainly related to

the prolonged stasis (about 10 m.y.) during most of the Valangin-

ian and Hauterivian after the first diversification, whereas the fol-

lowing periods of diversification occurred over a comparable

length of time in both intervals.

The third interval, from near the close of the Albian until the

end of the Cretaceous, differs in pattern from the earlier intervals

(Fig. 1). This interval is characterized by short periods of rapiddiversification and turnover separated by longer periods of stasis

except around the Corg-rich Bonarelli Event near the Cenoman-

ian/Turonian boundary (Arthur et al., 1990) when the cycles of

alternation were strongly accelerated (two periods of diversifica-

tion separated by a stasis within


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Figure 1. Major planktonic foraminiferal evolutionary patterns and paleoceanographic changes through the Cretaceous plotted against planktonicforaminiferal zonal scheme and major stratigraphic events, magnetostratigraphy, and absolute age (from Erba et al., 1995b; and Channell et al.,1995). Arrows for evolutionary changes not in scale and exaggerated for changes in diversification. P, precursor events; T, turnover; E, extinctions.



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the Bonarelli Event in the latest Cenomanian, was preceded by

increased oceanic mixing and closely spaced temperature fluctu-

ations possibly controlled by Milankovitch orbital cycles (see

Herbert et al., 1995; Streel et al., 1995).

Below, we examine the basis of our paleoceanographic con-

clusions derived from the evolution and distribution patterns of

Cretaceous planktonic foraminifers and other selected fossilgroups as well as generalized information from sedimentary and

isotopic records.

PATTERNS OF EVOLUTIONARY CHANGES IN

CRETACEOUS PLANKTONIC FORAMINIFERS

The evolutionary patterns exhibited by Cretaceous plank-

tonic foraminifers are based on the records of species and generic

richness determined from published biostratigraphic ranges (e.g.,

Sigal, 1977; Wonders, 1979, 1980; Robazynski et al., 1984;

Caron, 1985; Huber, 1990, 1991; Sliter, 1992; Coccioni and Pre-

moli Silva, 1994; Premoli Silva and Sliter, 1994; Robazynski and

Caron, 1995). The derived values are plotted graphically in Fig-

ure 2 against the planktonic foraminiferal zonal scheme and time

scale of Erba et al. (1995b, mainly after Gradstein et al., 1994),

and updated in the Early Cretaceous according to new ages pro-

vided by Channell et al. (1995). The genera and species used in

our computations are listed in Appendix A. Our analyses do not

include the near-shore favusellids or globuligerinids in the Early

Cretaceous, species recently described as endemic to the Antarc-

tic region, or those recorded only at one location (e.g., Haig,

1992; Huber, 1988; Nederbragt, 1991). Consequently, species

richness is generally underestimated.

Figure 2 clearly shows the diversity of planktonic foraminifers

increasing overall at both the generic and specific levels from the

first diversification in the early Valanginian to the end of the Creta-

ceous. The relative number of species generally parallels the num-

ber of genera (see also Premoli Silva and Sliter, 1994), however,

the plot of species diversity (Fig. 3) shows that species richness

increased discontinuously throughout the Cretaceous. For exam-

ple, the Early Cretaceous Period is characterized by a slow gradual

diversification from the early Valanginian to the base of the Glo-

bigerinelloides ferreolensis Zone in the early late Aptian.

This diversification is followed by a progressive but rela-

tively rapid decrease in species richness during the remainder of

the Aptian until very low numbers were reached in the interval

around the Aptian/Albian boundary. Species diversification then

slowly rebounded during most of the Albian, with an accelerationin the late Albian Rotalipora appenninica Zone. After a slight

decrease in the latest Albian and early Cenomanian, a gradual

increase in diversity characterized the remainder of the Ceno-

manian and Turonian. The exception in this pattern occurs at the

Bonarelli Event; a short episode characterized by low diversity

within the planktonic foraminifers balanced against a high abun-

dance of heterohelicids andHedbergella planispira.

Diversity decreased slightly in the Coniacian and then

peaked in the SantonianDicarinella asymetrica Zone. Following

a decrease in the early Campanian, species diversity then

increased continuously from the late early Campanian until the

Gansserina gansseri Zone near the base of the Maastrichtian,

when there were more than 60 species, representing the highest

diversification noted in the Cretaceous. After this peak, species

diversity decreased progressively to 30 species just prior to the

end of the Cretaceous. Within this overall trend, slow diversifica-tion and/or nondiversification characterize the upper part of the

Dicarinella concavata Zone in the Coniacian and the middle part

of the Globotruncanita elevata Zone in the early Campanian.

The species diversity curve in general reflects the number of

originations (Fig. 3). Major peaks in origination are recorded in

the early BarremianHedbergella similisHedbergella kuznetso-

vae Zone, at the base of the latest AlbianRotalipora appenninica

Zone, in the upper part of the late CenomanianDicarinella alge-

riana Subzone of theR. cushmani Zone, in the lower part of the

late Turonian D. primitivaMarginotruncana sigali Zone,

throughout much of the lower and middle parts of the Santonian

Dicarinella asymetrica Zone, at the base of the middle Campan-

ian Globotruncana ventricosa Zone, and from the Globotrun-

cana aegyptiaca to the base of Gansserina gansseri Zones in the

latest Campanian.

In contrast, extinctions contribute more significantly to the

evolutionary overturn in the late Aptian, in the upper part of the

latest AlbianRotalipora appenninica Zone, in the lower part of

theD. asymetrica Zone, in the lower part of the early Campanian

Globotruncanita elevata Zone, and in the middle to latest Maas-

trichtian. Overall throughout the Cretaceous, however, origina-

tions predominate over extinctions.

The curve for the degree of turnover shown in Figure 3,

which was derived by adding the number of originations and

extinctions, is more significant than the separate curves of the lat-

ter two criteria. In particular, two peaks stand out, one in the San-

tonianDicarinella asymetrica Zone and a second, larger one just

prior to the end of the Cretaceous. Smaller but discrete peaks are

also recorded in the early Barremian, early Aptian, latest Albian,

late Cenomanian, late middle Turonian, and latest Campanian.

All significant peaks in turnover are expressed schematically but

not to scale in Figure 1 by the larger inverted triangles in the col-

umn for planktonic foraminiferal changes.

MODERN PLANKTONIC FORAMINIFERAL LIFE

STRATEGIES

Here we examine the life strategies of modern planktonicforaminifers as a model for our interpretation of the evolution of

Cretaceous species. Most modern planktonic foraminifers live

discretely stratified within the upper part of the water column

above the thermocline, in the so-called mixed layer (Hemleben et

al., 1989). Because the thermocline is typically deeper in low-lat-

itude tropical regions and shallows toward high latitudes where it

reaches the surface at the circumpolar front, the volume of the

mixed layer decreases with increased latitude, resulting in the

progressive elimination of niches inhabited by planktonic

304 I. Premoli Silva and W. V. Sliter



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foraminifers. As a consequence, planktonic foraminifers inhabit-

ing the mixed layer characteristically decrease in diversity from

the tropics toward the high latitudes and in general are absent in

polar waters (B, 1977).

Accompanying the latitudinal decrease in species richness

from tropics to high latitude is the progressive loss of the less-tol-

erant planktonic foraminiferal species, which are characterized

by complex morphologies and/or high-energy requirements. As

a result, high-latitude assemblages become dominated by the

most tolerant cosmopolitan forms that are characterized by sim-

ple morphologies (Margalef, 1965; B, 1980, 1982; Hemleben et

al., 1989). Similar decreases in morphologic complexity are

observed in planktonic foraminiferal assemblages associated with

the inception and intensification of seasonal upwelling regimes

(Kroon and Ganssen, 1988).

Further, laboratory experiments and the results of plankton

Cretaceous paleoceanography: Evidence from planktonic foraminiferal evolution 305

Figure 2. Stratigraphic distribution of planktonic foraminiferal genera and number of species per genus plotted against planktonic foraminiferal zonalscheme, magnetostratigraphy,and absolute age (as in Fig. 1). Compilation based mainly on Pessagno (1967), Longoria (1974),Robaszynski et al. (1979,1984,1990,1993),Caron (1985), Coccioni and Premoli Silva (1994), Premoli Silva and Sliter (1994), and Robaszynski and Caron (1995). Columns onthe right show the timing of the five events that interrupt the Cretaceous planktonic foraminiferal record and the broad threefold evolutionary pattern.



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Figure 3. Number of species, originations, extinctions, and degree of turnover in Cretaceous planktonic foraminifers (foraminiferal sources as inFig. 2, chronology as in Fig. 1). Degree of turnover is the sum of the number of originations and extinctions.



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tows from vertical transects in key areas of the oceans (see Hem-

leben et al., 1989, and references therein) show that the vertical and

horizontal distribution and abundance of planktonic foraminifers

are controlled by numerous biotic and abiotic factors, which can

enhance or limit population growth. Among these are reproductive

capacity, presence or absence of symbionts, diversity of nutritional

sources, and varied physical and chemical parameters. Althoughwe are far from fully understanding these interacting relationships,

we can state the following:

1. The different depths occupied by planktonic foraminifers

in the upper water column likely reflect the avoidance of compe-

tition and the efficient exploitation of unique conditions prevail-

ing in each water horizon.

2. Maximum vertical separation of species occurs in warm

waters of tropical and subtropical regions that are characterized

by greater diversity of physical and biotic variables with depth.

Vertical separation is much less evident, if present, in cooler

waters, which become more uniform in structure with increased

latitude.

3. The wide range of niches inhabited by planktonic

foraminifers in tropical waters is due to large variations in salin-

ity and temperature in the upper few hundred meters; this corre-

lates to variations in abundance and diversity of potential prey,

diversified habitats, and multiple adaptive strategies. The result is

a rich species diversity.

4. The various preferred depth habitats are characterized by

distinct changes in morphology, surface ornamentation, posses-

sion/absence of symbionts, and reproductive cycle. In general,

globigerinids, especially spinose species, mainly abound in sur-

face waters, whereas a number of nonspinose species, including

globorotaliid morphologies, are found at depth, although some

may be abundant in temperate surface waters during the winter

months.

5. Shell calcification occurs mainly in the upper hundred

meters, although there is evidence for continuing calcification,

thus growth, while sinking to greater depths (deeper than 100 m)

and even below the thermocline (i.e., Globorotalia menardii,

Truncorotalia truncatulinoides,Globorotalia hirsuta,Neoglobo-

quadrina dutertrei, among others).

6. Reproduction occurs with a lunar or semilunar periodicity

in several species.

7. Gamete release occurs at the thermocline at the base of the

mixed layer. The thermocline is a stable layer that coincides with

the deep chlorophyll maximum (DCM) layer and is rich in phy-

toplankton. Most juveniles of spinose and nonspinose speciesthat require phytoplankton prey are found in the thermocline

region even if, as adults, some of these switch to a more carnivo-

rous diet (Fairbanks and Wiebe, 1980; Hemleben et al., 1989).

8. Surface-dwelling spinose species show a distinct prefer-

ence for zooplankton prey. In contrast, nonspinose species, which

include most of the deep dwellers, show a tendency to consume

phytoplankton prey. Consequently, phytoplankton distribution

and composition influence the spatial and temporal distribution

of each species.

9. Some species with complex morphologies (i.e.,T. trun-

catulinoides and G. hirsuta) apparently reproduce in annual

cycles at locations where spring blooms provide ample phyto-

plankton prey and water temperatures are suitable for survival.

During the rest of the year they live in cooler, deeper water in

tropical areas, probably feeding on decaying organic matter and

marine snow, but move progressively closer to the surfacetoward higher latitudes. These patterns emphasize the importance

of temperature as a controlling factor for reproduction.

10. Spinose species are more abundant in oligotrophic waters,

whereas nonspinose species are more abundant in eutrophic

waters that are richer in phytoplankton (e.g., upwelling areas,

diatom-rich Antarctic waters).

11. The latitudinal and vertical distribution of planktonic for-

aminiferal species is reflected in the oxygen and carbon isotopic

ratios of their tests.

Relationship to nutrients

As reported by Hallock (1987), habitat diversity is inversely

related to nutrients because (a) the length and complexity of food

chains tend to be an inverse function of food supply, (b) eutrophic

(nutrient-rich) environments are inherently unstable, and (c) the

greater depths of low-nutrient (oligotrophic) euphotic zones,

characterized by a range of light intensities, provide greater

potential for specialization than do the shallower and more varia-

ble euphotic zones found in areas influenced by upwelling,

runoff, or seasonal blooms.

Some taxonomic groups, however, cannot take advantage of

the potentially numerous niches of the low-nutrient environments

because they metabolically require more energy than is available

under such conditions. For instance, phytoplankton respond to

nutrient supply as follows (Rhyther, 1969): diatoms bloom and

flourish where upwelling and other sources of turbulence supply

abundant nutrients, dinoflagellates succeed the diatoms as nutri-

ent supplies decline and/or physical stability is established, and

when nutrient supplies become scarce, nano- and picoplankton,

including coccolithophorids, dominate the phytoplankton. This

sequence of plankton communities is similar along a nutrient gra-

dient in tropical-subtropical regions as well as through latitudes

(Hallock, 1987). In oligotrophic subtropical gyres, complex food

webs are nano- and picoplankton based (Rhyther, 1969). Dinofla-

gellates dominate areas or seasons having intermediate nutrient

supplies, as in temperate waters, whereas low- and middle-lati-

tude upwelling zones and high-latitude regions are dominated bydiatoms (Hallock et al., 1991). Similarly,Globigerina bulloides,

although the index species of the subpolar bioprovince (B,

1977; Kennett, 1982), is typically abundant in meridional

upwelling zones in the tropics, even where water temperatures

are depressed only a few degrees (De Mir, 1971).

The similarity between low-latitude upwelling assemblages

and those of high-latitude regions is easily explained in terms of

energy requirements (Hallock et al., 1991). In organisms whose

body temperatures conform closely with ambient temperatures,




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the ectotherms, metabolic rates decline with temperature. Phys-

iologically, the rate of reactions doubles for every 10 C increase

in temperature. That means that four times as much food is

required to support an organism in tropical waters as compared to

subpolar waters. Thus, the rate of nutrient input that supports

oligotrophic communities in warm waters might support

mesotrophic or even eutrophic communities in cold waters. Aminimal four-fold increase in food supply, associated with a drop

in temperature of only few degrees during seasonal low-latitude

upwelling, can shift a community from oligotrophic warm-water

taxa to eutrophic cool-water taxa.

Life-history strategy

Organisms have been categorized by their life-history strate-

gies, or their reproductive potential as related to competition and

resource utilization (MacArthur and Wilson, 1967; Valentine,

1973). At one extreme are r-selected opportunists, able to rapidly

increase their population densities usually by early maturation

and faster reproduction (Hallock, 1985). Opportunists proliferate

in resource-rich, low-stability regimes where their numbers fluc-

tuate widely. At the other extreme are K-selected specialists, char-

acterized by long individual life and low reproductive potential.

The K-selected strategy is most advantageous in highly stable,

typically oligotrophic environments where organisms compete by

specialization and habitat partitioning. In between are a range of

organisms adapted to mesotrophic environments, where tenden-

cies towards r- or K-selected strategies vary relative to the two

end members.

In planktonic foraminifers, size is often directly related to

reproductive potential; the faster a foraminifer matures and

reproduces, the higher its reproductive potential (Hallock, 1985).

Generally, small-sized foraminifers tend more towards the r-

selected end of the reproductive spectrum, whereas large-sized

foraminifers tend towards the K-selected end (Caron and Home-

wood, 1983; Hallock et al., 1991). Opportunistic planktonic

species are typically small, morphologically variable, widely dis-

tributed, and rapidly increase in abundance when nutrients

become available. K-selected planktonic species may be larger,

morphologically more complex and specialized, host algal sym-

bionts, and are typically most diverse in oligotrophic waters

where they compete by specialization and habitat partitioning.

In terms of the sedimentological record and fossilization

potential for the past, plankton diversity is high below olig-

otrophic and eutrophic environments because high proportions ofsuch biotas produce calcareous and siliceous shells, respectively.

While pelagic carbonate production is active in oligotrophic to

mildly eutrophic waters (Hallock et al., 1991), under truly

eutrophic conditions, short diatom-based food chains dominate at

the expense of most planktonic foraminiferal and coccol-

ithophorid taxa. In addition, high CO2 concentrations in subsur-

face waters in eutrophic regions severely limit carbonate

preservation. Planktonic foraminiferal diversity, similar to that of

other organisms, should be maximal in boundary regions domi-

nated by mesotrophic conditions. In these cases, taxa represent-

ing different water masses or ecologic conditions, ranging from

relatively eutrophic during seasonal blooms to oligotrophic when

upwelling or runoff decline seasonally or yearly, accumulate into

the sediments. Diversity in these areas can also increase because

of the presence of species unique to the ecotone.

In summary, planktonic foraminifers inhabiting the mixedlayer today characteristically decrease in number from the tropics

toward the high latitudes and in general are absent in polar waters

(B, 1980, 1982). The decrease in species richness from tropics

to high latitude is reflected in the planktonic foraminiferal assem-

blages that progressively lose the less tolerant species (K-selected

strategists), characterized by complex morphologies and/or high-

energy requirements, and become dominated by the most toler-

ant, cosmopolitan, and opportunistic forms, characterized by

small-sized simple morphologies (Margalef, 1965; B, 1980,

1982; Hemleben et al., 1989).

Similar patterns characterize intensifying upwelling regimes

(Kroon and Ganssen, 1988). Diversity decreases with increasingly

less stable conditions in the upper water column. As a conse-

quence, the decrease or disappearance of stratification in these

areas results in a decrease in the number of ecologic niches. When

conditions are extreme, the assemblages are dominated by a single

species; Globigerina bulloides in the meridional upwelling areas

(Kroon and Ganssen, 1988) andNeogloboquadrina pachyderma

in the Polar bioprovince (B, 1977).

LATITUDINAL AND VERTICAL DISTRIBUTION OF

CRETACEOUS PLANKTONIC FORAMINIFERS

We believe that Cretaceous planktonic foraminifers, although

all extinct except for Guembelitria, behaved for the most part like

their modern counterparts (Davids, 1966; Sliter, 1972; Wonders,

1980; Caron and Homewood, 1983; Caron, 1983; Leckie, 1989;

Coccioni et al., 1992; Keller et al., 1993). This interpretation is

supported by the few studies dealing with biogeography and iso-

topic analyses of Cretaceous planktonic foraminifers (e.g.,

Laughton et al., 1972; Hart and Tarling, 1974; Douglas and Savin,

1975; Hart and Bailey, 1979; Boersma and Shackleton, 1981;

Huber, 1988, 1991; Haig, 1992; Huber et al., 1995) and is consis-

tent with Paleogene reconstructions (Boersma and Premoli Silva,

1988, 1991; Premoli Silva and Boersma, 1989).

At the same time, we acknowledge that differences existed

between Cretaceous planktonic foraminifers and the modern

stock, such as the fact that even the most simple Cretaceous mor-photypes with globular chambers like the hedbergellids are finely

perforated, do not bear spines, and probably lacked symbionts.

Despite these differences, we believe that Cretaceous planktonic

species with complex morphotypes preferentially inhabited trop-

ical and subtropical oligotrophic waters, while high-latitude

assemblages were dominated by small-sized opportunistic

species with simple morphologies. Further, these patterns

resulted in a progressive decrease in species richness from the

tropics to the poles.




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In terms of vertical stratification, another useful modern

parameter, Sliter (1972), Hart (1980), Caron and Homewood

(1983), Caron (1983), Hart and Ball (1986), and Leckie (1987,

1989) suggested that the hedbergellids and heterohelicids lived in

surface waters and were the most ubiquitous forms on the

shelves. The same authors proposed that the ornamented globular

chamber-bearing forms lived slightly deeper in the water column,the double-keeled globotruncanids lived deeper yet, and the sin-

gle-keeled forms inhabited the deepest parts of the spectrum. This

hypothesis can only be partially tested through isotopic analysis

and/or through depth transects from near-shore to open-marine

environments.

The hypothesis is poorly supported by stable isotopes whose

values show little interspecies variation even during the Maas-

trichtian (DHondt and Arthur, 1995, 1996) when Cretaceous bio-

provinciality was at its strongest (Davids, 1966). Still, the degree

of vertical stratification in Cretaceous planktonic foraminifers is

unresolved because isotopic values of Cretaceous planktonic

foraminifers are too scarce to allow a reliable reconstruction of

their habitats and latitudinal and vertical temperature gradients in

the Cretaceous ocean were weak (Douglas and Savin, 1975;

Boersma and Shackleton, 1981; Barrera and Huber, 1990).

Further, most of the ad hoc depth transects completed to date

are located in middle latitudes (Hart, 1980) where the

presence/absence of ornamented forms cannot be related unam-

biguously to depth habitat but may be due to climatic conditions.

The only data from shallow-water tropical regions is derived from

the shelf edge of two Pacific guyots, Wodejebato and MIT (Erba

et al., 1995a), and is limited to very narrow time spans. At Wode-

jebato,Heterohelix andArchaeoglobigerina were found consis-

tently with rare specimens of Gansserina,Rugoglobigerina, and

Globotruncana in very low-diversity Maastrichtian faunas

deposited during times of more open-marine conditions. Very rare

specimens of Globotruncanita,Pseudoguembelina,Pseudotextu-

laria, and Globigerinelloides also were found occasionally. The

more limited planktonic foraminiferal assemblages of late Aptian

and late Albian age from MIT Guyot yielded rare specimens of

hedbergellids and globigerinelloidids among which, however,H.

trocoidea,G. ferreolensis, a doubtful G. algerianus, and one pos-

sible rotaliporid were identified (Erba et al., 1995a). These latter

records, however, are too fragmentary to be fully useful.

As a consequence, we derive information about the life strat-

egy of Cretaceous planktonic foraminifers from their reported lat-

itudinal distribution (Table 1). Although the record is far from

complete, the recent campaigns of the Ocean Drilling Program(ODP) provided a discrete latitudinal coverage from the Antarctic

margins to the northern high middle-latitudes (Hart, 1980;

Ciesielski et al., 1988; Huber, 1990, 1991; Leckie, 1990; Haig,

1992; Premoli Silva, 1992; Wonders, 1992). Next we examine the

high-latitude record to augment the wealth of information from

low latitudes (Coccioni and Premoli Silva, 1994; Premoli Silva

and Sliter, 1994, and references therein) as the basis for our

worldwide Cretaceous paleoceanographic interpretations.

Northern high latitudes

The oldest record outside the tropical Tethys, north or south,

is from the North Sea area where Barremian, probably late Bar-

remian, planktonic foraminiferal assemblages are composed of

several hedbergellids and a few globigerinelloidids similar to

assemblages from low latitudes (Ascoli, 1976; Banner and Desai,1988; Coccioni and Premoli Silva, 1994; Banner et al., 1993).

This similarity can be extended to early Aptian assemblages,

whereas the late Aptian and most of the Albian assemblages were

still composed of hedbergellids and globigerinelloidids and

lacked the ticinellids north of 35 paleolatitude (Hart, 1976). In

the latest Albian, assemblages from southern England, although

still dominated by hedbergellids, yielded few or occasionally

common, ornamented taxa such as Rotalipora appenninica,

Planomalina buxtorfi, Praeglobotruncana delrioensis, and

Costellagerina (Magniez-Jannin, 1981).

During the Cenomanian, the northern limit for rotaliporids

moved further north reaching the Danish coasts in theRotalipora

reicheli Zone (Hart, 1979). Thereafter, although ornamented taxa

continued to be recorded from mid-high northern latitudes, the

surviving species displayed short stratigraphic ranges and repre-

sented a minor component of the assemblages that was increas-

ingly overwhelmed by hedbergellids, whiteinellids, and later the

archaeoglobigerinids (Robaszynski et al., 1979; Caron, 1985).

Among the marginotruncanids,M. marginata was the most abun-

dant, had the broadest latitudinal range, and may have appeared

earlier in higher latitudes than in the Tethys according to

Hilbrecht et al. (1992). Other species recorded from high lati-

tudes are M. renzi and M. pseudolinneiana associated with

Dicarinella canaliculata in the Coniacian and Globotruncana

linneiana in the Santonian, whereas theDicarinella concavata

group is recorded discontinuously (Bailey and Hart, 1979). Sim-

ilar assemblages are recorded from the Great Valley sequence of

northern California perhaps influenced by the southward flowing

eastern boundary current (see Douglas, 1969). The most

northerly record of planktonic foraminifers is from the Arctic

slope of Alaska where rare specimens of Hedbergella andHet-

erohelix are present in the middle Turonian (Tappan, 1962).

During the Campanian, ornamented planktonic foraminifers

were greatly reduced in high-latitude assemblages (Berggren,

1962) and faunas consist of limited Heterohelix (Stenestad,

1969), Archaeoglobigerina, and Globigerinelloides (Caron,

1985). By the late Campanian through Maastrichtian, the assem-

blages were dominated by Rugoglobigerina along with a fewGlobigerinelloides and Heterohelix (Berggren, 1962). Orna-

mented species migrated as far north as southern Scandinavia

twice during this interval (Berggren, 1962); Globotruncana

linneiana andRugotruncana subcircumnodiferin the late Cam-

panian, and a much more diversified fauna during the late Maas-

trichtian Abathomphalus mayaroensis Zone. This latter fauna

included Globotruncana arca, G. rosetta, G. mariei, Contuso-

truncana contusa, Globotruncanita stuarti, Rugoglobigerina

macrocephala, Kuglerina rotundata, Racemiguembelina fructi-




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13/29

cosa, and Pseudotextularia elegans along with globotruncanel-

lids andAbathomphalus. A similar assemblage is recorded on the

Nova Scotian shelf and Orphan Knoll in the northwestern

Atlantic. The assemblage includes the above species plus

Gansserina gansseri, less common rugoglobigerinids, very com-

mon Heterohelix, and, among the hedbergellids, H. mon-

mouthensis is well represented (Davids, 1966; our personal

observation). Davids (1966) also noted the absence of Globotrun-

canella citae (=G. havanensis) north of 50 latitude.

Southern high latitudes

In the austral realm, the oldest planktonic foraminifers

recorded are Caucasella hauterivica and Guembelitria sp. from

Site 765 in the Argo Abyssal Plain off northwestern Australia that

are probably early Aptian in age (Gradstein, Ludden et al., 1990).

Upper Aptian sediments and planktonic faunas were recovered

higher in the sequence at the same site, as well as nearby at Site

766 (Haig, 1992) and on Maud Rise (Leckie, 1990). Recurrent

species areHedbergella planispira andH. delrioensis whereasH.

trocoidea, H. gorbachikae, Globigerinelloides ferreolensis, G.

barri, H. infracretacea, and possibleH. sigali are less common.

Clavate forms are missing from this interval at all sites. A very

depauperate fauna is recorded through most of the Albian, where

the most consistent species areH. planispira andH. delrioensis,

except from Exmouth Plateau where assemblages are limited to

H. planispira (Wonders, 1992).

As noted by Haig (1992), a uniform planktonic fauna con-

sisting of common Planomalina, Rotalipora, Praeglobotrun-

cana, Clavihedbergella, andHedbergella occupied the latitudinal

range between the Papuan Basin and Site 766 in the latest Albian.At the Exmouth Plateau, assemblages are still dominated by hed-

bergellids and contain Planomalina buxtorfi and Praeglobotrun-

cana delrioensis but no rotaliporids (Wonders, 1992). In contrast,

the coeval fauna on Naturaliste Plateau (Site 258) is much less

diversified and consists of rare specimens of Hedbergella

planispira, H. delrioensis, Clavihedbergella simplex, and Prae-

globotruncana delrioensis (Herb, 1974) in the absence ofRotal-

ipora and Planomalina. Of interest is the absence of true

Ticinella at all sites.

In the Cauvery Basin at the tip of the Indian subcontinent,

planktonic foraminiferal assemblages from questionable latest

Aptian through most of the Albian contain diverse hedbergellids,

Globigerinelloides bentonensis, G. caseyi, and rare Ticinella

roberti (Venkatachalapathy and Ragothaman, 1995). In the latest

Albian, however, theRotalipora appenninica Zone assemblage

also yieldedR. balernaensis and Praeglobotruncana stephani

(Venkatachalapathy and Ragothaman, 1995). Complete Tethyan

faunas are recorded from the Ladack and Nepal areas, both of

which are located at the northern edge of the Indian subcontinent.

There, the biostratigraphic succession from theRotalipora sub-

ticinensis Subzone through theR. appenninica Zone was identi-

fied (Premoli Silva et al., 1991).

According to the reconstructions of Golonka et al. (1994)

and Ricou (in Baumgartner et al., 1992) and Dercourt et al.

(1993), the western Antarctic and northwestern Australian mar-

gins were located between 55 S and 6062 S latitude in the

early Aptian whereas the Naturaliste Plateau lay some 10 south-

ward of this belt and the Cauvery Basin was almost at the same

latitude as Maud Rise at 64 S. In the late Albian, plate motion

moved the Australian margin and India about 10 northward

whereas Antarctica probably did not change latitudinally.

Middle to late Cenomanian assemblages from Exmouth

Plateau are more similar to those of the Tethys with rotaliporids

and praeglobotruncanids (Wonders, 1992).Rotalipora appen-

ninica is very rare butR. brotzeni, R. deeckei, andR. reicheli in

the presence ofR. cushmani are apparently more common than in

the Tethys. Rotaliporids became extinct just prior to the Bonarelli

equivalent and whiteinellids are common across the Ceno-

manian/Turonian boundary. Helvetoglobotruncana helvetica,

Dicarinella hagni, andD. imbricata are present as well as themarginotruncanids, which evolved biostratigraphically as in the

Tethys. Important components of the faunas are Falsotruncana

maslakovae andMarginotruncana marianosi along withHed-

bergella flandrini. All species of marginotruncanids are appar-

ently present, including the large marginotruncanids, whereas the

Dicarinella concavata group is poorly and discontinuously rep-

resented. Globotruncanita elevata seems to appear at the same

level as in the tropics. Odd is the highest record ofMarginotrun-

cana sinuosa, M. undulata, and H. flandrini within the lower




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Campanian and well above the first occurrence of the nannofossil

Aspidiscus parcus (=basal Campanian) and the last occurrence of

D. asymetrica. Typically these species do not extend into the

Campanian (see Caron, 1985; Premoli Silva and Sliter, 1994) and

their occurrence, if not reworked, may represent an extension of

their ranges in southern high latitudes.

Depauperate faunas occur in most of the Campanian andthrough the lower half of the Maastrichtian on Exmouth Plateau.

Globotruncana arca, Globigerinelloides, andHeterohelix (espe-

ciallyH. globulosa) are dominant and G. bulloides, C. fornicata,

and G. linneiana are present in discontinuous pulses (Wonders,

1992).Rugoglobigerina rugosa, R. milamensis, Globotruncanella

havanensis, Gublerina sp., and Planoglobulina carseyae first

occur at the appropriate biostratigraphic levels. Late Maastrichtian

faunas again exhibit strong Tethys affinities with C. contusa and

A. mayaroensis among others, whereas Globotruncanita, Gansse-

rina, and G. rosetta are poorly represented.

Antarctic assemblages in the Campanian and Maastrichtian

are dominated by Heterohelix and Globigerinelloides (Huber,

1990). Hedbergellids are common but never reach the abundance

of the previous two groups. The hedbergellid maximum abun-

dance is related toH. sliteri, which is endemic to the Antarctic

region, whereasH. holmdelensis andH. monmouthensis often are

poorly represented or even absent in the late Maastrichtian. The

archaeoglobigerinids, frequently represented by endemic species,

are common at Maud Rise but less common in subantarctic

regions.Abathomphalus mayaroensis is present or even abundant

along the Antarctic margin and apparently appears earlier there

than in the tropics (Huber, 1992). Globotruncanellids are repre-

sented by G. petaloidea and, in minor amounts, by G. havanenis

and G. pschadae. Among the large heterohelicids only Pseudo-

textularia elegans and Gublerina are recorded. Keeled forms are

rare and mainly are represented by the genus Rugotruncana

(Huber, 1992).

CRETACEOUS PLANKTONIC FORAMINIFERAL

LIFE STRATEGIES

Comparing the known latitudinal and vertical distributions

of Cretaceous planktonic foraminifers to their morphologic char-

acteristics provides a means to interpret their life strategies. As

with modern planktonic foraminifers, we believe that the size of

the Cretaceous taxa likewise was related to resource utilization

and reproductive potential. Similarly, we employ the terms r-

selected and K-selected to describe the reproductive strategyof the Cretaceous species while recognizing there is a continuum

between these end members. The life history strategy for a given

Cretaceous species in this continuum was related to a particular

environment at a given time. To aid in this comparison we recog-

nize the following 14 recurrent taxonomic clusters ranked

according to progressively more complex gross test morphology,

ornamentation, and wall-surface porosity:

1. Simple low-trochospiral morphotypes with globular cham-

bers, which include the thin- and smooth-walled hedbergellids.

2. Low to medium-high trochospiral morphotypes mainly

with subglobular chambers and possessing marked, more or less

organized rugosities on the surface; these include Costellagerina,

Whiteinella, Archaeoglobigerina, Rugoglobigerina, Kuglerina,

and Trinitella.

3. Low trochospiral morphotypes with subglobular cham-

bers, secondary apertures, and a tendency to thicken the walls ofthe inner whorls; the ticinellids.

4. Low trochospiral morphotypes with subacute to acute

peripheral margins lacking true keels; includes Praeglobotrun-

cana and Globotruncanella.

5. Trochospiral morphotypes, mainly planoconvex, possess-

ing juvenile subglobular chambers and hemispherical chambers

in the adult with or without marginal keel(s); these include

Dicarinella, Helvetoglobotruncana, Gansserina, and possibly

Rugotruncana.

6. Single- to double-keeled, mostly trochospiral morphotypes

with acute and/or truncated margins and raised sutures on both

spiral and umbilical sides; these include Rotalipora, Margino-

truncana, Globotruncana, Contusotruncana, and Globotrun-

canita. We include the ornamented planispiral genus Planomalina

in this group.

7. Double-keeled, trochospiral morphotypes with truncated

margins and raised sutures only on the spiral side; includes Fal-

sotruncana andAbathomphalus.

8. Planispiral morphotypes, mainly small-sized, with com-

pressed to inflated chambers; Globigerinelloides.

9. Thin, microperforate biserial heterohelicids with a smooth

to finely striate surface; includesHeterohelix andLaeviheterohelix.

10. Biserial heterohelicids with supplementary apertures;

Pseudoguembelina.

11. Flaring heterohelicids with more than two chambers per

row; Ventilabrella, Planoglobulina, and Gublerina.

12. Medium-sized heterohelicids with chambers arranged

from biserial to annular and a surface from smooth to costate;

includes Pseudotextularia andRacemiguembelina.

13. Large, thick-walled plurigeneric cluster includingHed-

bergella trocoidea, Ticinella roberti, andBiticinella breggiensis,

which display some features in common with groups 2 and 3 but

lack the marked surface ornamentation, and Globigerinelloides

algerianus and G. barri, which are planispiral like group 8.

14. Clavate to tubulospinous-chambered cluster; includes

Clavihedbergella, Hedbergella rhinoceros, Leupoldina, Schack-

oina, Plummerita, andRadotruncana.

These 14 clusters are further lumped into 10 discrete groups(A to J) with apparently similar environmental behavior. These

groups are based on latitudinal distribution, from cosmopolitan

to progressively more endemic. (A) The simplest morphologies

in both trochospiral and biserial clusters (1 and 9) are the most

cosmopolitan and opportunistic. (B) Small Globigerinelloides

(cluster 8) behave similarly to the simplest group but are slightly

less tolerant of variable conditions. (C) Praeglobotruncana and

Globotruncanella (cluster 4) are the most cosmopolitan taxa

among the more complex clusters. (D) The round chambered




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pustulose forms (equated to cluster 2); among them Whiteinella

andArchaeoglobigerina tend to display high dominance in their

respective assemblages, suggesting higher tolerance to eutrophic

conditions and a broader latitudinal distribution with respect to

the other genera of the cluster. (E) Ticinellids (cluster 3),

planoconvex forms (cluster 5), and the various species of cluster

13 display similar behavior in their respective time intervals; theyare mainly missing at high latitudes. (F) Rare complex heterohe-

licids of clusters 10 and 12 are occasionally present in high-lati-

tude assemblages. (G) Among the flaring heterohelicids (cluster

11),Gublerina is the only form recorded, although rarely, at high

latitudes. (H) Falsotruncana and especially Abathomphalus

(cluster 7) may be important components of the middle- to high-

latitude assemblages in their respective time intervals and are

much rarer at low latitudes except for the higher abundance of

Falsotruncana in marginal seas such as in Tunisia (Caron, 1981).

(I) The keeled cluster (6) includes less tolerant forms that domi-

nate low- to middle-latitude assemblages with high diversity.

Among them,Marginotruncana marginata, M. coronata, M.

pseudolinneiana,Globotruncana bulloides, and G. linneiana dis-

play a slightly broader latitudinal range than the other forms in

this cluster. (J) Representatives of the clavate cluster (14) show

a very discontinuous stratigraphic range with a distinct prefer-

ence for low to middle latitudes.

In terms of life strategy (Fig. 4, Table 2), the simplest mor-

phologic groups (A and B above) are opportunists, or r-strate-

gists, with high reproductive potential and inhabiting more

nutrient-rich waters close to the eutrophic part of the resource

spectrum. The number of species in this group is small through-

out the entire Cretaceous. In contrast, the complex, large-sized

keeled morphotypes (I above), are the most specialized and least

tolerant to environmental variation, or K-strategists, that inhabit

low-nutrient waters close to the oligotrophic part of the spectrum

and compete by habitat partitioning. This group is highly diversi-

fied. In between these reproductive end members are the r/K

intermediate morphotypes that display a range of trophic strate-

gies. In Table 2, we have attempted to place members along the

continuum as more r- or K-selected. The number of species in

each genus in the intermediate group is small (see Fig. 4) but the

group is highly diversified overall, although less diversified than

the K-strategists. In particular, the high dominance behavior of

Whiteinella andArchaeoglobigerina suggests that these forms

had a high reproductive potential that seems to characterize

upwelling regions. Finally, the clavate group in analogy with

Eocene hantkeninids may have inhabited oxygen-depleted sur-face to near-surface waters (Boersma et al., 1987).

Life strategies with time

As shown in Figure 4 , the abundance of the r-selected

opportunists,r/Kintermediate group, and K-selected strategists

changed through time. In the Early Cretaceous, planktonic fora-

miniferal assemblages are composed only of r-selected oppor-

tunistic hedbergellids and globigerinelloidids: at the beginning

they are rare and very small in size, then gradually increase both

in size and abundance, reaching their maximum diversification in

the AptianLeupoldina cabri and Globigerinelloides ferreolensis

Zones. During the late Aptian, the assemblages became enriched

by the first r/Kintermediate forms such as the large-sized, thick-

walled Globigerinelloides algerianus,G. barri,Hedbergella tro-

coidea, Ticinella roberti, and T. bejaouaensis. As theseintermediate forms disappear in the late Aptian, the early to mid-

dle Albian planktonic foraminiferal assemblages were once again

dominated initially by small-sized opportunists, but they rapidly

acquired new r/Kintermediate forms represented by the ticinel-

lids, first of small size then progressively of increasing size and

complexity (see Brhret et al., 1986). Morphologic diversity

similar to that of the late Aptian resumed close to the base of the

Biticinella breggiensis Zone. This is followed by the continued

diversification and size increase of the r/Kintermediate group.

The late Albian marks the appearance of the first K-selected

strategists, the rotaliporids, which rapidly diversified. Diversifi-

cation, however, also continued among the r-selected opportunist

and r/Kintermediate groups with the appearance of thin, biserial

Heterohelix and praeglobotruncanids, respectively. The maxi-

mum diversity in this period was reached in the lower part of the

Rotalipora appenninica Zone where the stratigraphic distribution

of new taxa overlaps with older r/Kintermediate forms, such as

the ticinellids.

After a period during which the planktonic foraminiferal

community remained rather stable (early to early late Cenoman-

ian), a new diversification within the r/Kintermediate group in the

late Cenomanian announced the organic carbon-rich (Corg-rich)

Bonarelli Event. The Bonarelli Event (latest Cenomanian) is char-

acterized by the dominance of the r/Kintermediate whiteinellids

associated with abundant r-selected opportunists and much rarer

r/Kdicarinellids in the absence of the K-selected strategists.

K-strategists reappear in the middle Turonian, represented by

the genus Marginotruncana, which rapidly diversifies. These

species are joined in the Coniacian to Santonian by the first

occurrence of the globotruncanids, contusotruncanids, and

globotruncanitids, contributing to the high diversity in the Late

Cretaceous. The highest diversification within the K-strategists is

reached in the late Campanian to early Maastrichtian, followed

by a slow decline in diversity toward the end of the Maastrichtian.

The decreasing diversity, however, is not paralleled by a decrease

in abundance of the K-strategists, except just prior to the K/T

boundary.

An increasing diversification within the r/K intermediategroup accompanies that of the K-strategists with the appearance

of new genera and species. However, their abundance in the

assemblages commonly is modest except at very high latitudes

where they may dominate the faunas, such as the archaeoglo-

bigerinids in the Maastrichtian from the Antarctic margin (Huber,

1990, 1992). Moreover, representatives of the r/Kintermediate

group became progressively more important components of the

assemblages when the K-selected strategists decreased in diver-

sity just prior to the K/T boundary. It is also worth mentioning




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that the r-selected opportunists, the most consistent component

of the Cretaceous planktonic faunas, are hedbergellid-dominated

in the Early and early Late Cretaceous, but later in the Late Cre-taceous they become heterohelicid-dominated in both diversity

and abundance.

Representatives of the clavate group display a random

stratigraphic distribution (Fig. 4) with schackoinids the most con-

sistent forms in low latitudes (Table 2). Most of the time they are

represented by rare specimens of a single species except in two

intervals; in the Barremian to the early late Aptian, where they

display the highest diversity, and in the late Campanian. Pecu-

liarly, the Barremian to Aptian interval is characterized by the

widespread occurrence of several Corg-rich layers that culminated

in the Selli horizon with a Corg content greater than 8% total

weight (Weissert, 1989), but no Corg-rich layers are recorded inthe late Campanian. As a consequence, the paleoceanographic

significance of the clavate group is still ambiguous.

Additionally, we note the following faunal characteristics:

a. Hedbergellids are confirmed as shallow-water dwelling as

well as the most tolerant of the trochospiral planktonics with sev-

eral species described from high latitudes (Douglas and Rankin,

1969; Herb, 1974; Leckie, 1990; Venkatachalapathy and

Ragothaman, 1995).

b. Narrow heterohelicids are more tolerant than flaring types.


Figure 4. Stratigraphic distribution of Cretaceous planktonic foraminifers grouped according to their inferred life strategy (data from Fig. 2) plottedagainst the main ecological changes (chronology as in Fig. 1).

http://specialpapers.gsapubs.org/http://specialpapers.gsapubs.org/


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The former are ubiquitous and often abundant, suggesting a life

strategy close to that of the r-selected hedbergellids.

c. Among more complex heterohelicids,Pseudoguembelina,

Pseudotextularia, and Gublerina migrated into higher latitudes

toward Antarctica. The data, however, are far from complete as

heterohelicids often were overlooked in previous studies.

d. Large and small forms with globular chambers with vari-

ous ornamentation show a life strategy more tolerant to environ-

mental changes. Included here are species of Hedbergella,

Ticinella, Costellagerina, Whiteinella, Helvetoglobotruncana,

Archaeoglobigerina, and Gansserina.

e. Among the ticinellids,T. primula is more tolerant than T.

praeticinensis or T. roberti.

f. The presence of Gansserina andArchaeoglobigerina on



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Pacific guyots (Erba et al., 1995a) and in the subantarctic region

(Huber, 1991), and the latter genus from the Antarctic margin,

suggests that both groups preferred a shallower-water habitat and

are environmentally tolerant. This interpretation is supported by

stable isotope values from Archaeoglobigerina (Barrera and

Huber, 1990).

g. The abundance and earlier appearance ofAbathomphalusmayaroensis in middle to high southern latitudes suggests a pre-

ferred extratropical habitat for this r/Kintermediate taxon (Huber,

1992; Huber et al., 1995).

h. Marginotruncana marginata and Globotruncana bul-

loides are the most tolerant of the K-strategists to changes in the

environment.

CRETACEOUS PALEOCEANOGRAPHIC CHANGES

WITH TIME

The evolutionary changes that we observe in Cretaceous

planktonic foraminifers provide a means to interpret the history

of paleoceanographic changes. In the Late Jurassic to Early Cre-

taceous, the surface of the world ocean can be described as a

rather eutrophic paleoenvironment inhabited by r-selected oppor-

tunists even though the record is limited mainly to rare low-lati-

tude sites. Globuligerinids dominated until the first diversification

of the Cretaceous planktonic foraminifers in the early Valangin-

ian (Fig 5). This diversification coincides in general with several

events in other pelagic fossil groups as well as with geochemical

and sedimentological events. For example, hedbergellids first

occured as the calpionellids drastically decreased in abundance

shortly before their extinction. At almost the same level, nanno-

conids, the main constituent of the Early Cretaceous low-latitude

pelagic sediments (Erba and Quadrio, 1987), decrease in abun-

dance whereas coccolithophorids diversify, boreal dinoflagellates

migrate to low latitudes (Leereveld, 1995), and diatoms become

somewhat better preserved (Erba and Quadrio, 1987). These

events portend changes in the water column derived from the

increased flow of horizontal and vertical oceanic currents.

The biologic events slightly precede a positive shift in 13C

that Weissert (1989) correlated to an increase in river discharge

during a warm, humid, climatic regime. These conditions con-

tributed to the increased preservation of organic matter. Corre-

spondingly, the isotopic shift coincides with the first deposition of

Corg-rich black shales intercalated with Cretaceous pelagic lime-

stone. The increased seasonality from dry to more humid climatic

conditions with increased fresh-water runoff likely increased thenutrient supply in surface waters, creating more eutrophic condi-

tions suitable for diversification of the hedbergellids.

The decrease in nannoconids at this time supports this inter-

pretation. Erba (1994) suggested the nannoconids are analogous

to extant Florisphaera, a nannofossil that proliferates when the

deep chlorophyll maximum (DCM) lies in the lower part of the

photic zone. On the other hand, shoaling of the DCM favors the

proliferation of coccolithophorids in the upper part of the photic

zone. If Erbas interpretation is correct, the hedbergellids may

have appeared when nutrients were concentrated in near-surface

waters concomitant with the increased river discharge hypothe-

sized by Weissert (1989; see also Lini et al., 1992) and upwelling

in the upper water column.

After their first diversification, planktonic foraminifers

underwent a prolonged period of evolutionary stasis in the late

Valanginian to earliest Barremian. At the same time, their strati-graphic distribution and abundance fluctuated rhythmically (Coc-

cioni et al., 1992) in accordance with alternating limestone/marl

bedding. These lithologic rhythms have been related to climatic

changes induced by orbitally produced Milankovitch cycles (Her-

bert and Fischer, 1986). The existence at that time of discrete sea-

sonality is supported by provincialism that affected calcareous

nannofossils, ammonites, and belemnites in northern Europe

(Mutterlose, 1991).

Fluctuations in seasonality apparently accelerated and

strengthened in the early Barremian inducing more contrasting

nutrient levels in surface waters. New niches were developed for

the gradual diversification of opportunists and the first occurrence

of clavate forms represented by ClavihedbergellaandLeupold-

ina. Planktonic foraminifers became abundant for the first time

in pelagic carbonates and increased in size. The first occurrence

of benthic genera such as Gavelinella, that characterize middle-

slope water depths later in the Cretaceous, suggests the devel-

opment of an intermediate water layer (Sliter, 1980). Still,

environmental conditions maintained the deep position of the

DCM for nannoconid proliferation (Erba, 1994). The new niches

for opportunists apparently extended into the northern high lati-

tudes (Banner et al., 1993) and, although we lack control from

the southern high latitudes, the planktonic fauna apparently was

cosmopolitan.

In the late Barremian, planktonic foraminifera continued to

diversify but rhythmic fluctuations in abundance increased in

intensity (Coccioni et al., 1992). Paleoenvironmental conditions

were alternately more to less suitable for the globigerinelloidids

as well, ranging from less eutrophic during their presence to more

eutrophic during their absence. Such environmental fluctuations

doubtless were a widespread phenomena and not confined to low

latitudes, however; the spotty record from northern high latitudes

does not provide the answer. The fact that benthic foraminifers

increased in diversity and black-shale layers became more com-

mon in the same interval may indicate that the changing paleoen-

vironmental conditions affected the entire water column.

The first turnover in planktonic foraminifers is associated

with the Corg-rich deposition at the Selli Level (Figs. 3, 5). Theevent was preceded by a diversification of hedbergellids associ-

ated with the temporary disappearance of nannoconids (Erba,

1994) and a major turnover in radiolarians (Erbacher, 1994).

Closely following or perhaps still associated with the waning

phase of the increased burial of organic matter of marine origin

and the second major Cretaceous positive shift of 13C, there is a

temporary decrease of hedbergellids and globigerinelloidids

coeval with a diversification within the clavate group. Paleoen-

vironmental conditions surrounding the Selli Event were proba-




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bly quite peculiar, with increased upwelling and the probable

expansion of oxygen-depleted waters (Arthur et al., 1990). Still,

these conditions did not prevent diversification within the hed-

bergellids and globigerinelloidids. Several interpretations regard-

ing the significance of the Selli Event have been put forward.

Larson (1991) related it to the acme of volcanic activity, or super-

plume, which disrupted the previous steady state leading to thetemporary nannoconid crisis (see Erba, 1994). Erbacher (1994)

spoke of an increase in productivity due to enhanced nutrient

input into surface water from flooded coastal areas during a rise

in sea level. This increased productivity caused an expansion of

the oxygen minimum zone. That these events reached to oceanic

surface waters is attested to by the turnover in planktonic forami-

niferal opportunists.

Following these events, planktonic foraminifers rapidly

increased in size and diversity. Niche-partitioning of the mixed

layer is evident by the middle of the late Aptian, as indicated by

the first occurrence of large,r/Kintermediate hedbergellids and

thick-walled globigerinelloidids. Nannoconids registered a new

acme (N. truittii Acme, see Erba, 1994), 13C values became

more positive (Weissert, 1989), and radiolarians underwent grad-

ual extinction (Erbacher, 1994). The appearance of the interme-

diate planktonic foraminifers signals the onset or strengthening

of a weak thermocline. For the first time, surface waters were

characterized by less eutrophic, probably oligotrophic to

mesotrophic, paleoenvironmental conditions. At the same time,

the occurrence of localized black shales such as those of the

Calera Limestone, originally located in the ancestral Pacific

Ocean basin (Sliter, 1989), attest to continued, if brief, fluctua-

tions in the strength of the thermocline.

The post-Selli paleoenvironmental conditions expanded

toward high southern latitudes but apparently not into northern

high latitudes. Northern assemblages consist only of oppor-

tunists, perhaps reflecting the shallow-water depths of the

reported sites as the nannoconid acme is also recorded in north-

ern Europe (Mutterlose, 1989; Erba, 1994).

This episode of diversification was interrupted before the

close of the Aptian when the weak thermocline was completely

disrupted, provoking the extinction and temporary disappearance

of r/Kintermediate forms. The early Albian planktonic forami-

niferal assemblages once again were composed of only r-selected

opportunists, suggesting the return of eutrophic conditions in a

likely well-developed upwelling regime. These more eutrophic

conditions were accompanied by a new Corg-rich accumulation

event (OAE 1b; Arthur et al., 1990; Erba, 1994), strong carbonatedissolution (Brhret et al., 1986; Premoli Silva et al., 1989a;

Erba, 1992), and a turnover among radiolarians (Erbacher, 1994).

Planktonic foraminifers did not diversify during this period.

As the upwelling regime decreased, niche-partitioning

within the mixed layer gradually resumed, first accommodating

the r/Kintermediate group and then returning to a late Aptian

state by the Ticinella praeticinensis Subzone. Contrary to the late

Aptian, diversification continued into the late Albian and the

occurrence of the first K-selected strategists in theRotalipora tici-

nensis Zone heralded for the first time during the Cretaceous the

development of a pronounced stratification of the mixed layer

above a well-defined thermocline. Paleoenvironmental conditions

shifted to more oligotrophic even if still within the mesotrophic

range as indicated by the as yet abundant r-opportunists. Climate-

driven rhythmic fluctuations within Milankovitch frequencies

dominated during this period of subtle environmental changes(Herbert and Fischer, 1986; Premoli Silva et al., 1989b; Tornaghi

et al., 1989; Erba, 1992; Herbert et al., 1995).

The onset of a stronger thermocline requires an increase in

equator-to-pole temperature gradients. The expected increase in

temperature gradient is supported by the occurrence of less diver-

sified planktonic foraminiferal assemblages on the Antarctic mar-

gin (Herb, 1974), indicating that a weak bioprovince perhaps

was differentiated at the highest southern latitudes (>60 S) by

the late Albian (see Baumgartner et al., 1992). Diversification of

planktonic foraminifers is paralleled by diversification within

radiolarians during most of the Albian; moreover, the foraminif-

eral turnover and extinction event in the upper part of theRotali-

pora appenninica Zone also is registered within the radiolarians

(Erbacher, 1994). The widespread hiatus at this level (Schlanger,

1986) and the apparent coeval demise of several carbonate plat-

forms in the Atlantic and Pacific Oceans (e.g., Hallock and

Schlager, 1986; Winterer et al., 1995; Haggerty et al., 1995)

appear to be associated with a change in paleotemperature

(Erbacher, 1994; Sliter, 1995).

This late Albian event was followed by a period of evolu-

tionary stasis that spanned the early Cenomanian (Fig. 5). This

stasis was associated with the expansion of warm niches toward

higher latitudes. In the Northern Hemisphere, low-latitude plank-

tonic foraminiferal assemblages with K-selected strategists first

occurred on the English margin during the Cenomanian trans-

gression. These faunas show a strong similarity to austral tem-

perate faunas from Exmouth Plateau (Wonders, 1992) that by the

Cenomanian had been transported northward through oceanic

plate motion. During most of the Cenomanian, however, the

southern high-latitude bioprovince had become more discrete and

was characterized only by rare K-selected strategists at the lati-

tude of the Falkland Islands and Tierra del Fuego (Sliter, 1976;

Krasheninnikov and Basov, 1983), much farther north than dur-

ing the late Albian.

In the late Cenomanian, diversification among planktonic

foraminifers occurred primarily within r/Kintermediate forms

and to a minor extent within r-selected opportunists, whereas K-

selected strategists decreased in diversity. These trends indicate afurther change toward less stable and more eutrophic paleoenvi-

ronmental conditions that culminated in the Corg-rich Bonarelli

Event. At that time the thermocline was once again disrupted and

surface to subsurface waters became very unstable and controlled

by an upwelling regime.

The Bonarelli Event, the worldwide OAE 2 (see Arthur et

al., 1990), coincides with the highest accumulation of organic

matter of marine origin in pelagic sediments during the Creta-

ceous (>23% total weight; Arthur and Premoli Silva, 1982), a




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major turnover within radiolarians (e.g., Marcucci Passerini et al.,

1991; Thurow et al., 1992; Erbacher, 1994), mollusks (Elder,

1989), and calcareous nannofossils (Erba et al., 1995b), and a

major positive shift of 13C. Several authors have associated this

event with a pulse of very high primary productivity induced by

an exceptional increase in nutrients in surface and subsurface

waters (Herbin et al., 1986; Arthur et al., 1987; Coccioni et al.,1991; Bellanca et al., 1996). The Bonarelli Event is preceded by

precursor events such as numerous black-shale layers and the

increasing abundance ofHeterohelix and r/Kintermediate forms

(whiteinellids and dicarinellids) paralleled by the gradual extinc-

tion of the K-selected rotaliporids.

In terms of diversity, a short period of stasis followed the

Bonarelli Event (Fig. 5). This interval is unusual, however, as the

gradually decreased dominance of the whiteinellids is balanced

by the increased dominance of the praeglobotruncanids,

dicarinellids, heterohelicids, and hedbergellids. The appearance

ofHelvetoglobotruncana helvetica, a single-keeled descendant of

the rounded-shaped whiteinellids, indicates that the upper water

column slowly restratified. The thermocline again was well

defined by the middle Turonian, when the K-selected margin-

otruncanids first occurred, and remained a permanent feature of

the Late Cretaceous ocean until the end of the era.

Shortly after the increased stratification of the upper water

column,K-selected strategists became highly diversified and

dominated the large size-fraction of the assemblages, while the

r-selected opportunists were still abundant in fine fraction. The

r/Kintermediate group underwent a marked turnover in the late

Turonian with the extinction of the praeglobotruncanids and

some of the dicarinellids inherited from the latest Cenomanian.

As a result, despite the appearance of the archaeoglobigerinids,

the intermediate group overall became a minor component of the

assemblages (Fig. 4). This situation continued with little change

for more than 4 m.y.

From the Bonarelli Event through the Coniacian, tropical

paleoenvironmental conditions expanded over a large latitudinal

band extending to the subantarctic region and provinciality

decreased. Only a few K-strategists seemed better adapted to high

latitudes than to low latitudes such asMarginotruncana mari-

anosi, Falsotruncana, andHedbergella flandrini, all of which

were more abundant outside the tropical belt to at least the south-

ern mid-high latitudes. However, the spotty record of true high-

latitude oceanic sites for this time period prevents understanding

the complete latitudinal provincial gradient. For example, the

presence of two opportunistic species from Turonian strata on theArctic margin (Tappan, 1962) likely reflects a depauperate fauna

associated with coastal shallow-water environments.

The largest turnover of planktonic foraminifers in the Creta-

ceous occurred in the SantonianDicarinella asymetrica Zone and

affected all trophic groups, (Fig. 5). Both originations and

extinctions exemplify the interval (Fig. 3), however, the origina-

tion of new genera and species outnumbered extinctions (Fig. 2).

Diversity is high and new forms include new morphologies such

as those of the compressed heterohelicid Laeviheterohelix, the

flaring heterohelicids Sigalia and Ventilabrella, inflated globiger-

inelloidids, and the clavateEohastigerinella. The interval repre-

sents the transition between K-selected faunas dominated by

marginotruncanids to one dominated by globotruncanids and

globotruncanitids (Wonders, 1980). The interval further repre-

sents the ecotone between two major ocean types; the earlier

Greenhouse ocean with variable sediments consisting of multi-colored carbonate typical of redox cycles, chert, and black shale

and with weak bioprovinces, and the later modern ocean domi-

nated by more uniform carbonate and with well-defined bio-

provinces.

Accompanying these changes in theD. asymetrica Zone is a

major turnover in calcareous nannofossils, the last occurrence of

widespread black-shale deposition (Zimmerman et al., 1987), a

strong episode of carbonate dissolution close to the top of the

zone, a pulse of volcanic activity, and the final opening of a deep-

water connection between the North and South Atlantic Oceans

(Kennett, 1982). The latter event may have been key to under-

standing the scope of paleoceanographic changes. Opening of

full oceanic communication through a wider South Atlantic

altered global deep-water circulation as noted by Wonders (1980)

and likely precipitated changes in ocean chemistry, marine cli-

mate, and water structure. The unusual combination of both

major originations and extinctions in all trophic groups indi-

cates that changes occurred throughout the water column and

supports the interpretation of an altered deep-water circulation.

In the Campanian and Maastrichtian, tropical planktonic

foraminiferal assemblages are dominated by highly diversified

K-selected strategists that now include abundant globotrun-

canids, fewer globotruncanitids and Contusotruncana, and,

among the opportunists, thin heterohelicids. In contrast, the for-

merly important hedbergellids became very rare. The r/Kinter-

mediate group increased considerably in diversity with the

appearance of new morphotypes in both the complex heteroheli-

cids and trochospiral forms. This signifies that the number of

niches within the mixed layer increased measurably with respect

to previous times to accommodate the more than 60 species rec-

ognized in low latitudes.

This extreme partitioning implies that tropical surface waters

were characterized by oligotrophic conditions as they are today.

Moreover, as in the modern ocean, niches for K-selected strate-

gists were progressively eliminated toward high latitudes. As

expected, species diversity also decreased with increased latitude,

allowing the identification of discrete bioprovinces that extended

to the Antarctic margin (see also Davids, 1966). The onset of thebioprovinces occurred in the early Campanian during a period of

taxonomic stasis and became more pronounced as the diversity

of K-selected strategists increased in the late Campanian.

Near the base of the Maastrichtian, the decrease in diversity

within K-selected strategists and the opposite within the r/Kinter-

mediate group indicate a shift to less oligotrophic conditions

accompanied by a progressive weakening of the thermocline.

This trend strongly accelerated near the end of the Maastrichtian,

except for a brief episode in the earlyAbathomphalus mayaroen-




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sis Zone (at 66.7 Ma according to Huber and Watkins, 1992),

when a few specimens of rugotruncanids and Globotruncana bul-

loides were recorded on the Antarctic margin (Huber, 1992), and

Contusotruncana contusa migrated into the North Sea (Caron,

1985). At the close of the Cretaceous, the thermocline was totally

disrupted and all niches disappeared. Planktonic foraminifers that

survived this drastic event include the opportunistic Guembeli-tria, which flooded the ocean surface at the K/T boundary (i.e.,

Delacotte et al., 1985; Kroon and Nederbragt, 1990), very rare

small hedbergellids (Keller, 1993; Olsson et al., 1992), and an

ancestral form of the Paleocene chiloguembelinid

Cretaceous Oceanography and Foraminifera

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