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Warm saline intermediate waters in theCretaceous tropical Atlantic Ocean

OLIVER FRIEDRICH1,2*, JOCHEN ERBACHER1, KAZUYOSHI MORIYA2, PAUL A. WILSON2

AND HENNING KUHNERT3

1Bundesanstalt fur Geowissenschaften, Stilleweg 2, 30655 Hannover, Germany2National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK3Center for Marine Environmental Sciences, Universitat Bremen, Klagenfurter Strasse, 28359 Bremen, Germany*e-mail: of2w07@noc.soton.ac.uk

Published online: 30 May 2008; doi:10.1038/ngeo217

During the mid-Cretaceous period, the global subsurface oceanswere relatively warm, but the origins of the high temperaturesare debated. One hypothesis suggests that high sea levels and thecontinental configuration allowed high-salinity waters in low-latitude epicontinental shelf seas to sink and form deep-watermasses1–3. In another scenario, surface waters in high-latituderegions, the modern area of deep-water formation, were warmedthrough greenhouse forcing4, which then propagated throughdeep-water circulation. Here, we use oxygen isotopes and Mg/Caratios from benthic foraminifera to reconstruct intermediate-water conditions in the tropical proto-Atlantic Ocean from 97 to92 Myr ago. According to our reconstruction, intermediate-watertemperatures ranged between 20 and 25 ◦C, the warmest everdocumented for depths of 500–1,000 m. Our record also revealsintervals of high-salinity conditions, which we suggest reflect aninflux of saline water derived from epicontinental seas aroundthe tropical proto-North Atlantic Ocean. Although derived fromonly one site, our data indicate the existence of warm, salineintermediate waters in this silled basin. This combination ofwarm saline intermediate waters and restricted palaeogeographyprobably acted as preconditioning factors for the prolongedperiod of anoxia and black-shale formation in the equatorialproto-North Atlantic Ocean during the Cretaceous period.

The middle of the Cretaceous period (∼120–80 Myr ago) iswidely interpreted to represent the best example of a long-livedgreenhouse interval from the entire geological record with highglobal temperatures deep-ocean temperatures at least 10 ◦C warmerthan today5, high atmospheric carbon dioxide levels6 and minimal7,possibly transient8, continental ice budgets. Characteristic of thisgreenhouse interval is the deposition of abundant organic-richsediments in the geological record, many of them finely laminatedand well known as some of the world’s most important petroleumsource rocks. Some of these black shales are widely, if notglobally, distributed. Among the global Oceanic Anoxic Events(OAEs), OAE 2 is the most prominent, lasting approximately500 kyr and falling during the interval of peak sustained globalwarmth of the past 150 Myr (refs 5,9). It is associated withsignificant turnover within numerous biotic groups, and aprominent carbon isotope increase in marine records10,11. Otherblack shales are mainly present in the Atlantic–Tethyan realm andrecent results show that the equatorial proto-North Atlantic was,for some reason, particularly prone to anoxia with black-shale

deposition at Demerara Rise spanning over 20 Myr of Albian toSantonian time11.

Numerous studies have focused on the origin of OAEs andthe cause of mid-Cretaceous warmth. However, with the exceptionof some modelling experiments, comparatively few studies haveconcentrated on the issue of water-mass formation and oceancirculation during this peak greenhouse interval. Furthermore,most palaeoceanographic studies of the mid-Cretaceous arelimited9,12 by problems of diagenetic alteration of carbonatesediments and calcareous microfossils and/or extremely lowsampling resolution. Thus, controversy has developed regardingthe importance of ocean circulation mode to the question ofthe warmth of the Cretaceous world. The hypothesis of a modeof water-mass formation and oceanic circulation, to some extentdriven by or at least influenced by the sinking of warm, saline watersin low-latitudinal regions with extreme net evaporation is deeplyrooted in the palaeoceanographic literature about the Cretaceousperiod and the early Cenozoic era1–3. Numerical circulation modelexperiments for a restricted Atlantic basin during the Albian havesimulated formation of warm, saline intermediate- to deep-watermasses along the northern coastlines of South America and Africa13.Similarly, δ18O data from Albian and Cenomanian molluscs andradiaxial fibrous calcite cements have been interpreted to recordwarm, saline water masses originating from the ancestral Gulfof Mexico14. Yet the nonlinearity of the equation of state forsea water means that the sensitivity of density to changes insalinity is small at high temperatures. Furthermore, other modelexperiments conclude4 that the global picture of water-massformation must be dominated by sinking at high latitudes,and further petro-geochemical analysis15 of molluscs and marinecements calls into question the δ18O-based interpretations of theGulf of Mexico data because of diagenetic alteration.

To investigate the hydrography of the low-latitude proto-NorthAtlantic, we generated stable isotope records in glassy9

foraminiferal calcite (see the Methods section) from over 100 m ofblack shales recovered from Ocean Drilling Program sites 1258 and1260 at Demerara rise, which have present water depths of 3,192 mand 2,549 m respectively (Supplementary Information, Fig. S1).On the basis of seismic stratigraphy and sediment composition,these sites record the history of intermediate waters during theCenomanian age (palaeodepths of ∼1,000 m at site 1258 and∼500 m at site 1260 (refs 16,17)). We generated a chronology

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97.0

96.5

96.0

95.5

95.0

94.5

94.0

93.5

93.0

92.5

92.0

Age (Myr)

Site 1258

MCE

Site 1260

OAE 2

Turo

nian

Stage

–28–30 –26 –24 –22

Ceno

man

ian

Decreasing benthic foraminiferal abundances

and diversities

–30 –28 –26 –24 –22

δ13Corg (‰ VPDB)

δ18Ocarb (‰ VPDB)

36 32 28 24

Palaeotemperature (°C)

20 16 12

1

2

3

4

5

0–1–2–3–4–5

δ13Corg (‰ VPDB)

Figure 1 δ18O for planktic (black) and benthic foraminifera of sites 1258 (orange) and 1260 (red). See the Methods section for palaeotemperature calculation. δ13Corgpartly from ref. 11. Planktic foraminifera: Hedbergella delrioensis (open black circles, site 1258); benthic foraminifera (site 1258, this study and ref. 7; site 1260, this study):Bolivina anambra (horizontal crosses), B. cf. incrassata (open downward triangles), Gavelinella dakotensis (open squares), G. intermedia (diagonal crosses), Gavelinella spp.(asterisks), Lenticulina spp. (open upward triangles), Neobulimina albertensis (circles), Osangularia schloenbachi (open diamonds) Praebulimina prolixa (filled diamonds),Tappanina sp. 1 (filled squares), mixed benthics (filled downward triangles). Grey fields: 90% of data around interval mean. MCE: Middle Cenomanian event.

for these sequences using new and published11 organic carbonisotope (δ13Corg) data together with a recently published calcareousnannofossil biostratigraphy18 (Fig. 1). Our benthic oxygen isotoperecord is of much higher resolution than any previously publisheddata set for the mid-Cretaceous and indicates that intermediatewaters at Demerara Rise were extremely warm throughout the Earlyand Middle Cenomanian (Fig. 1, Supplementary Information,Fig. S2). Assuming reasonable, conservative estimates for seawaterδ18O (see the Methods section), we estimate intermediate-watertemperatures for Demerara Rise between 20 and 25 ◦C—thewarmest yet reported from well-preserved benthic foraminiferal

calcite of the mid-Cretaceous and far exceeding today’s watertemperatures at comparable depths (4–7 ◦C (ref. 19)).

Our benthic oxygen isotope record exhibits noticeablestratigraphic structure and these data fall into five time intervalson this basis (Fig. 1). During the first of these intervals(∼97.3–96.5 Myr), benthic δ18O shows a relatively narrow spreadabout a mean of about −2.25h Vienna PeeDee Belemnite (VPDB).The mean benthic δ18O in interval 2 (∼96.5–95.5 Myr) is onlyslightly higher but the spread of data is noticeably wider, especiallyat site 1258 (Fig. 1). For interval 3 (∼95.5–94 Myr), mean benthicδ18O is noticeably higher than for any other part of our record with

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Site 1260

Age (Myr)

Benthics (n × 103/g)

δ18Obenthic (‰ VPDB)

N. albertensis

G. dakotensis

Mixed benthics

P. prolixa

B. anambra

Tappanina sp.1

0 1 2 –3 –2 –1 –2 –1–3

Site 1258

δ18Obenthic (‰ VPDB)

Benthics (n × 103/g)

2

3

G. intermedia

B. cf. incrassata

1 2094.4

94.6

94.8

95.0

95.2

95.4

95.6

95.8

96.0

96.2

96.4

Figure 2 Benthic foraminiferal abundances and δ18O values of sites 1258 and 1260 for the time interval between 94.4 and 96.4 Myr. Abundances as individuals pergram dried sediment. Intervals 2 and 3 and isotopic markers are the same as in Fig. 1.

the lowest values recorded in interval 4 on the run up to OAE 2.Benthic δ18O values in the remainder of our record (interval 5)show a narrow spread about a mean of about 2.0h VPDB. The datacomprising this record do not come from a single species, but thesample-to-sample consistency and the similar values recorded bydifferent taxa in the same samples indicate that the variability seencannot be attributed to taxon-dependent isotopic offsets (Fig. 2).Instead, we interpret the changes in our record (Fig. 1) to indicatereal environmental signals. These signals are large and requiresubstantial shifts in the δ18O of sea water and/or temperature ofCretaceous sea water. For example, the shift in mean benthic δ18Oacross the interval 2/3 boundary requires an increase in seawaterδ18O of about 1h or a cooling of about 4 ◦C. Similarly, withininterval 2 at site 1258, particularly well-defined, high-amplitudevariations in benthic δ18O are seen on a timescale of about100 kyr that require changes in intermediate-water δ18O of about2–2.5h or of temperature by about 8 ◦C. Despite these pronouncedchanges in our benthic record, very little variation is seen inour accompanying planktic record (Fig. 1). This observationindicates7 that the benthic δ18O record cannot be explained interms of whole-ocean changes in seawater δ18O (for example, dueto glaciation), but leaves the origin of the large changes in benthicδ18O unexplained.

To help decipher the origin of the large changes in benthicδ18O seen in our records, we have generated faunal records forthe key time intervals (top of 2, base of 3, Fig. 2). This dataset reveals large shifts in benthic foraminiferal abundance withinthe late stages of interval 2 and a rapid decline to near-zeroabundance on the run up to the interval 2/3 boundary (Fig. 2). Thistransition is slightly earlier and sharper in the deeper site (1258).We interpret the high-amplitude changes in abundance withininterval 2 to reflect a combination of changes in food availabilityand oxygenation at the sea floor. Yet, changes in food availabilitycannot have contributed to the marked decline in foraminiferalabundance near the interval 2/3 boundary because this wouldrequire a total loss of productivity—something that is inconsistentwith the high total organic carbon content of these sediments.

Thus, this signal must predominantly reflect decreased sea-flooroxygenation. These changes in faunal abundance occur in tandemwith pronounced shifts in benthic δ18O, where low abundancesare coincident with high benthic δ18O and vice versa, and thisrelationship is particularly clear at the deeper site (site 1258, Fig. 2).The sign of this relationship points to pronounced increases in localseawater δ18O and salinity (not cooling) as the cause of the highbenthic δ18O intervals because benthic foraminiferal assemblagesare relatively insensitive to temperature.

To test our interpretation that much of the structure in ourbenthic δ18O record within intervals 2 and 3 reflects pronouncedchanges in local seawater δ18O, which reflects salinity rather thantemperature, we measured Mg/Ca ratios in benthic foraminiferalcalcite, which are a proxy for calcification temperature. Althoughthe foraminiferal calcite in our samples is extremely well preserved,the tests are small in size and rare in many samples (Figs 1,2).Therefore, it was not possible to generate a high-resolution recordusing conventional techniques. Instead, we measured Mg/Caratios in a small subset of samples using a laser-ablation method(Table 1). This limited data set cannot be used to evaluate short-term environmental change, and absolute temperatures cannot becalculated with accuracy because of the uncertainty associated withthe Mg/Ca ratio of Cretaceous sea water20 and species-specific Mgpartitioning between sea water and foraminiferal calcite. However,the data available show only modest variation with the highestMg/Ca ratios indicated for interval 3—a pattern consistent withthe interpretation that the comparatively high benthic δ18O valuesduring interval 3 are attributable to the local incursion of warmhigh-δ18O, high-salinity waters (rather than colder waters).

To assess the probable mechanism of formation of thepostulated high-δ18O, high-salinity waters, it is important toconsider the tectonic configuration of the Atlantic Ocean during themid-Cretaceous. During the Late Cenomanian, the central NorthAtlantic was an isolated basin (see Supplementary Information,Fig. S1) with limited exchange of intermediate and deep watersoccurring with adjacent basins. Connection to the Tethys Oceanwas restricted by continental terranes21, the Equatorial Atlantic

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Table 1 Comparison of Mg/Ca with δ18O for three distinct intervals. (Depth in metre composite depth (mcd), age in million years (Myr), Mg/Ca ratios and δ18O dataversus VPDB. Interval numbers refer to those shown in Fig. 1; s.d., standard deviation; n of ind., number of measured individuals; holes, number of individual holesablated; B. anambra, Bolivina anambra ; N. albertensis, Neobulimina albertensis.)

Depth Age Mg/Ca Mg/Ca Mg/Ca Mg/Ca Mg/Ca δ18O Interval Mg/Ca δ18O δ18O(mcd) (Myr) species n of ind. holes sample mean sample s.d. sample (h) number interval mean interval mean (h) interval s.d.

417.25 92.58 B. anambra 3 6 5.84 2.19 −2.69 5 7.33 −1.97 0.35417.6 92.65 B. anambra 3 10 8.82 1.98 −1.88439.375 94.83 B. anambra 5 18 11.13 3.36 −1.13 3 10.13 −1.56 0.52440.11 94.89 B. anambra 5 16 9.12 1.99 −1.06467.67 96.96 N. albertensis 5 21 8.20 2.46 −2.15 1 9.21 −2.56 0.35468.32 97.00 N. albertensis 5 23 10.22 3.19 −2.09

Gateway and the Caribbean Gateway allowed exchange of onlysurface waters22,23, whereas connection to the Arctic Ocean wasblocked until the opening of the Greenland–Iceland–NorwegianSea during the Oligocene epoch24. Hence, the intermediate- todeep-water masses of the central Atlantic Ocean at this timewould have been formed locally within the basin. One mode ofwater-mass formation within the basin must presumably haveinvolved winter cooling at the highest latitudes available. Tectonicreconstructions, however, show that the basin extended onlyto the mid-latitudes (∼40–45◦ N, Supplementary Information,Fig. S1). On the basis of brachiopod δ18O data and observedlatitudinal gradients in the preservation of organic material inthe Atlantic Ocean, Cenomanian southwestern European shelf seasare proposed as a potential source region contributing to proto-North Atlantic intermediate and deep waters25. Our data suggestthe operation of an extra mode of water-mass formation—low-latitude evaporation-led modification of waters in epicontinentalbasins and/or on shelf areas. Our sites can be excluded as a sourcefor this type of water formation owing to the increased gradientbetween planktic and benthic foraminiferal calcite during interval 3relative to 2 and 4. More likely source regions for the proposed high-saline waters would have been the contemporaneous epicontinentalseas of northern South America and northern Africa. Manywere the sites of contemporaneous evaporite formation (seeSupplementary Information, Fig. S1). Some of the Cretaceousshelf areas closest to Demerara Rise are well known to havebecome marginal anoxic basins accumulating vast deposits oforganic-carbon-rich sediments during intervals of sea-level highstand. Sedimentological and palynofloral data from, for example,eastern Venezuela, point to semi-arid climate conditions, which areproposed to result in high salinities and low oxygen contents inshallow shelf areas26.

It seems probable that the main reason for the extremewarmth of the Cretaceous ocean interior was the warmth ofsurface waters at mid- to high-latitude sites of deep convection4,25.But our data show that high-δ18O, high-salinity water massesexisted at least sporadically over Demerara Rise during the LateCenomanian. Data of this type from one locality cannot beused to determine the regional extent or global significance ofsuch water masses. However, the mid-Cretaceous was a time ofextremely high eustatic sea levels27 resulting in vast epicontinentalseas during the Cenomanian and Turonian—potentially favourablelocales for the sporadic formation of low-latitude warm and salinewater masses akin to those that form today during the summeron the Bahama Bank complex28, with potential implications forour understanding of ocean circulation in a greenhouse world.We interpret the existence of warm saline waters at depth onDemerara Rise to the unique plate tectonic configuration of themid-Cretaceous proto-North Atlantic, the extremely high sea levelsand the greenhouse climate of the Cenomanian. We postulatethat delivery of these dense water masses acted, together with

the restricted and silled nature of the basin, to precondition theregion in favour of anoxia, helping to explain the exceptionally longhistory of laminated black shale and ultimately hydrocarbon sourcerock formation.

METHODS

AGE MODELWe use the numerical ages of ref. 29. Tie points are the beginning of thecarbon isotope excursion paralleling the Middle Cenomanian event (lower partAcanthoceras rhotomagense and upper part Cunningtoniceras inerme ammonitezone25; 95.7 Myr), the last occurrence of the nannofossil marker Corollithionkennedyi18 (94.1 Myr) and the initiation of the OAE 2 positive carbon isotopeexcursion, estimated as ∼300 kyr before the Cenomanian/Turonian boundary(93.8 Myr (ref. 25)). At site 1258, a sharp decline of carbon isotope values inthe uppermost part of OAE 2 points to the existence of a short-term hiatus11.Accordingly, our Turonian data from site 1258 are plotted against age bycorrelating the carbon isotope values to site 1260.

ISOTOPE AND Mg/Ca MEASUREMENTS AND PALAEOTEMPERATURE RECONSTRUCTIONForaminiferal tests from Demerara Rise are generally extremely well preserved(‘glassy’), lacking internal cements and recrystallization9 (see SupplementaryInformation, Fig. S3). A few intervals including most parts of the OAE 2,however, show significant calcite infilling of the shells. Tests from these intervalsare not used in this study. The small numbers of individuals and shifts inassemblages made it necessary to generate species-specific records usingmultiple taxa (see Supplementary Information, Tables). Different habitat effectsare believed to explain a part of the strong scattering in our data. Furthermore,it is possible that the mechanism of changes in the bottom-water mass, asproposed in this study, also operated on short timescales influencing the benthicoxygen isotope values. Hence, we focus mainly on the long-term changes of thestable isotope record.

Stable isotope measurements were obtained from single taxon separatesusing between 5 and 50 individuals mainly from the fractions 63–125 µm and125–250 µm. Measurements were carried out using a Finnigan MAT 251 massspectrometer at the Leibniz-Labor (Kiel, Germany), coupled online to theCarbo-Kiel device I for automated CO2 preparation. External precision is betterthan 0.07h. The data are given in the usual δ-notation and refer to the VPDBstandard as established using the NBS20 carbonate isotope standard.

Palaeotemperature calculations are based on equations (1) of ref. 30,assuming a δ18Ow of Late Cretaceous sea water of −1.0hSMOW for an ice-freeworld31 and that the foraminiferal calcite was formed in isotopic equilibriumwith Cretaceous sea water. This universal application of the mean value for δwis appropriate for deep waters (and thus for most of the presented data), butwill lead to an underestimation of SSTs calculated from planktic foraminiferalδ18O values due to latitudinal gradients for δw of the surface ocean9.

Mg/Ca measurements were carried out using a New Wave UP 193solid-state laser ablation system with 193 nm wavelength, coupled to a FinniganElement 2 sector field inductively coupled plasma mass spectrometer at theUniversity of Bremen. The calibrations are based on the NIST 612 glass standardreference material. For each sample, the final Mg/Ca ratio was calculated byaveraging the measurements of all ablated holes from all specimens (maximumof five specimens per sample and five holes per specimen, see Table 1 fornumber of ablated holes). The relative standard deviation for Mg/Ca basedon 10 measurements on the NIST 612 was 4.5%; the standard error of the

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average was 1.4%. The latter value gives an estimate of the precision of theforaminiferal Mg/Ca data. On the basis of the uncertain Mg/Ca of Cenomaniansea water, possible temporal changes in seawater Mg/Ca composition and theunknown relation between ambient water temperature and the Mg/Ca ofextinct foraminifera, we refrain from calculating palaeotemperatures based onthis proxy and discuss inferred changes in only relative temperature.

Received 7 December 2007; accepted 29 April 2008; published 30 May 2008.

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Supplementary Information accompanies this paper on www.nature.com/naturegeoscience.

AcknowledgementsWe are grateful to the Leg 207 Shipboard Scientific Party, W. Hale, N. Westphal, K. Noeske and S. Feller.D. Panten, N. Andersen and H. Erlenkeuser are thanked for measuring stable isotopes. Our paperbenefited from discussions with H. Brumsack, A. Forster, P. Hardas, A. Hetzel, J. Mutterlose, R. Norrisand P. Sexton. This research used samples provided by the Ocean Drilling Program. ODP is sponsoredby the US National Science Foundation and participating countries under the management of JointOceanographic Institutions. Financial support for this study was provided by the German ResearchFoundation (to O.F., J.E. and H.K.), KAKENHI (to K.M.) and UK ODP NERC (to P.A.W.).

Author informationReprints and permission information is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests for materials should be addressed to O.F.

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