Paleogene Glyptodontidae Propalaehoplophorinae (Mammalia ...
Circulation in the Southern Ocean during the Paleogene inferred ...
-
Upload
duongkhuong -
Category
Documents
-
view
213 -
download
0
Transcript of Circulation in the Southern Ocean during the Paleogene inferred ...
www.elsevier.com/locate/epsl
Earth and Planetary Science Le
Circulation in the Southern Ocean during the Paleogene inferred
from neodymium isotopes
Howie D. Scher*, Ellen E. Martin
Department of Geological Sciences, University of Florida, Gainesville, FL, USA
Received 14 April 2004; received in revised form 10 October 2004; accepted 10 October 2004
Editor: E. Boyle
Abstract
Long-term records of neodymium (Nd) isotopes from sedimentary archives can be influenced by both changes in water mass
mixing and continental weathering. Results of Nd isotopic analyses of fossil fish teeth from ODP Site 689 (Maud Rise,
Southern Ocean) provide a long, continuous, high-resolution marine sediment Nd isotope record (expressed in eNd units).
Correlation of down core secular variations between the eNd record, d13C values from benthic foraminifera, and clay mineral
assemblages demonstrates that long-term variability of Nd isotope ratios reflect changes in ocean circulation, and that only
minor fluctuations in eNd values are associated with changes in continental weathering on Antarctica.
Nonradiogenic eNd values at Site 689 during the middle Eocene require the contribution of an end member with eNdb�9.5.
Southern Ocean deep water may have been too radiogenic in the middle Eocene (eNd=�8.5), though this end member may not
be fully characterized. A possible source of deep water outside of the Southern Ocean in the middle Eocene is the Tethys Sea
(eNd=�9.3 to �9.8). The presence of Warm Saline Deep Water (WSDW) on Maud Rise is consistent with the Nd isotope
results. The onset of more radiogenic eNd values at ~40.8 Ma coincides with other changes at Site 689 which are consistent with
a switch from a warm bottom water mass in the middle Eocene to a colder bottom water mass in the late middle Eocene.
A rapid shift to radiogenic eNd values beginning at 37 Ma is best explained by the opening of Drake Passage. The shift
coincides with increases in phytoplankton production throughout the Atlantic sector of the Southern Ocean that document the
development of upwelling cells presumably related to more effective latitudinal circulation.
After the Eocene/Oligocene boundary when large-scale ice sheets developed on Antarctica, Southern Ocean sourced water
masses, such as Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW), had a greater influence on the
hydrography of the study area. An early Oligocene trend to nonradiogenic compositions resulted in similar values to the modern
eNd values of these water masses. The modern eNd values of AAIW and AABW reflect a significant contribution of North
Atlantic Deep Water (NADW), thus decreasing eNd values in the early Oligocene may have resulted from the export of Northern
Component Water (NCW, similar to modern NADW).
During the late Oligocene and early Miocene, the long-term trends of the record follow benthic d13C values. Variability in
the Nd isotope record most likely reflects fluctuations in ocean circulation arising from changes in the relative contributions of
0012-821X/$ - s
doi:10.1016/j.ep
* Correspon
E-mail addr
tters 228 (2004) 391–405
ee front matter D 2004 Elsevier B.V. All rights reserved.
sl.2004.10.016
ding author. Tel.: +1 352 392 2231; fax: +1 352 392 9294.
ess: [email protected] (H.D. Scher).
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405392
different end member water masses to the Southern Ocean. An interval where eNd values and d13C values are not correlated may
reflect the influence of a short-lived weathering event on the eNd record. Early Miocene eNd values resemble those of modern
Southern Ocean water masses, indicating a shift toward present-day patterns of ocean circulation.
D 2004 Elsevier B.V. All rights reserved.
Keywords: fossil fish teeth; neodymium isotope ratios; ocean circulation; Warm Saline Deep Water; Drake Passage
1. Introduction
The Nd isotope ratio of seawater (143Nd/144Nd)
demonstrates a strong correlation with water mass.
Dissolved Nd has a short seawater residence time
(600–2000 years; [1–3]) and is sourced predomi-
nantly from the continents, thus the Nd isotope ratio
of a water mass tends to reflect the geology of its
source area. Sedimentary archives that contain
measurable levels of Nd are gaining recognition in
paleoceanography because recent studies suggest that
water mass mixing, i.e., ocean circulation, influences
the Nd isotopic ratio of seawater. Therefore, Nd
isotopes have the potential to yield patterns of past
ocean circulation, for which there are few reliable
proxies.
Fig. 1. Paleogeographic reconstruction of the late Eocene showing the loc
discussed in the text, and relevant tectonic gateways. Arrows illustrate poss
the Ocean Drilling Stratigraphic Network (OSDN).
Proxies for ocean circulation are critical for
examining links between ocean circulation and global
climate change. However, applying Nd isotopes as a
proxy for ocean circulation has been complicated by
the competing influence of another paleoclimate
signal, changes in continental weathering. The com-
position and/or provenance of material entering the
source area of a water mass, can also bear upon the
Nd isotopic ratio of seawater. Distinguishing between
ocean circulation and continental weathering as
sources of variability in long-term Nd isotope records
is crucial for the application of Nd isotopes to
Cenozoic paleoceanography.
This paper reports the findings of a multi-proxy
approach designed to deconvolve the contributing
signals to the Nd isotope ratio of seawater in the
ation of ODP Site 689, the locations of other DSDP and ODP sites
ible pathways for deep water exchange between ocean basins. From
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 393
Atlantic sector of the Southern Ocean. Nd isotope
variability is compared to proxy records from the
same location that record a strong response to
changes in either continental weathering, as docu-
mented by clay mineralogy, or ocean circulation, as
defined by d13C values from benthic foraminifera. In
this study, fossil fish teeth from ODP Site 689
(64.318S, 3.068E, 2080 m) (Fig. 1) were used to
generate a high-resolution (average 270 ky) Nd
isotope record over a 20-My interval from the
middle Eocene to early Miocene. The high yield of
fossil fish teeth permitted the construction of this
record at a resolution that is unparalleled during this
interval in the Cenozoic.
ODP Site 689 has been extensively studied, and
many proxy records have been generated that span
the relevant interval. d18O and d13C values of
benthic foraminifera were measured by Kennett and
Stott [4], Mackensen and Ehrmann [5], and Diester-
Haass and Zahn [6]. The relative abundance of clay
minerals, which reflect continental weathering con-
ditions on Antarctica, was determined by Ehrmann
and Mackensen [7] and Robert et al. [8]. Paleopro-
ductivity in the surface waters overlying Maud Rise
have been derived from benthic foraminiferal accu-
mulation rates [6]. Nd isotope variability is suffi-
ciently resolved and can be directly compared to
variability in other proxy records. The length of the
record coupled with close spacing of samples
provides an excellent opportunity to better under-
stand the nature and causes of long-term secular
variability of Nd isotopes in seawater.
2. Background
2.1. Seawater eNd values
Nd isotope investigations of seawater samples
demonstrate that modern North Atlantic Deep Water
(NADW) has an eNd value of �13.5 [9], reflecting
the weathering input of Archean age rocks into the
Labrador Sea. eNd units represent the difference of
the 143Nd/144Nd ratio in parts per 104 from the
chondritic uniform reservoir (CHUR) [10]. The eNdvalue of modern Pacific seawater is very distinct
(eNd=�4) [11], which is due to the contribution of
young, circum-Pacific volcanogenic sources. Water
masses sourced in the Southern Ocean such as
Antarctic Bottom Water (AABW) and Antarctic
Intermediate Water (AAIW) have intermediate eNdvalues ~�8 [1,12,13], which reflect mixing between
Pacific and Atlantic seawater. The input of weathered
material in the dissolved and suspended load of
rivers can modify eNd values of seawater proximal to
such sources. For example, the Orange River in
southern Africa drains terrains of Proterozoic age
such as the Orange River Group (eNd=�13.5 to �24)
[14]. Seawater around South Africa has a non-
radiogenic signature [15] presumably resulting from
input from the Orange River.
2.2. Constraining sources of variability in Southern
Ocean Nd records
Long-term Nd isotope records from ferromanga-
nese (Fe–Mn) crusts have demonstrated that present-
day provinciality between the Pacific and Atlantic
oceans has been maintained for much of the
Cenozoic [16–20]. It follows that variability of Nd
isotope ratios in Southern Ocean sedimentary records
during the Cenozoic should reflect changes in the
proportion of end member water masses mixing in
the Southern Ocean, as well as changes in con-
tinental weathering.
Although the eNd values of water masses that are
likely to influence the interpretation of Paleogene eNdrecords from the Southern Ocean have been loosely
constrained, the Nd isotopic contribution from con-
tinental weathering during the relevant interval must
also be examined. The portion of the Nd isotope signal
in Southern Ocean records that is attributable to
continental weathering is likely to reflect the growth
of ice sheets on Antarctica. However, it is difficult to
estimate the eNd value from weathering of Antarctic
rocks because much of the continent is ice covered.
From exposures above the ice, the spatial limit of
Precambrian basement rocks have been traced and
amount to a significant portion of the continent [21].
Geochemical investigations of basement exposures
yield nonradiogenic Nd isotope signatures. Gabbros
from northern Victoria Land demonstrate eNd values
that range from �14 to �19 [22] and granulites from
the Wilson Terrain have eNd values of �16 [23]. eNdvalues from Grenville-age gneisses in the Maud
Province are �10.5 [24]. Enderby Land outcrop
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405394
samples have very low Nd isotope ratios (typically eNd~�30 to�50), consistent with its Archean age [25–27].
Borg et al. [28–31] measured a large range of Nd
isotope ratios (eNd=0 to�35) on outcrop samples from
the Transantarctic Mountains near the western Ross
Sea. Thus, it is likely that material entering the
Southern Ocean from weathering on Antarctica has
an eNd value that is less radiogenic than the modern
seawater value of �8.
2.3. Archives of seawater Nd
The application of Nd isotopes from Fe–Mn sedi-
ment archives to questions in paleoceanography have
provided insight into gateway events [16,32], Northern
Hemisphere glaciation [33], and the history of NADW
export to the Southern Ocean [20,34]; however, these
studies have been limited to the Neogene. Robust Nd
isotope records from the Paleogene have been more
elusive. Fe–Mn crusts do not adequately resolve the
natural variability of Nd isotopes on Paleogene time
scales due to very slow growth rates (1–15 mm/My)
and poor age control beyond 10 Ma (see Frank [35] for
a recent review). The resolution and age control
limitations imposed on Fe–Mn Nd isotope records
have not permitted precise correlation to other paleo-
ceanographic proxy records, and have limited the
ability to deconvolve contributing signals from ocean
circulation and continental weathering, although it is
clear that some long-term records do reflect weathering
inputs [36].
In recent years, fossil fish teeth have been used to
construct relatively high resolution Nd isotope
records [37–40]. In a post-mortem mineralogical
transformation of hydroxyfluorapatite to fluorapatite
that occurs at the sediment–water interface, fish teeth
acquire Nd concentrations that average 300 ppm.
The post-mortem addition of Nd to fish teeth
overwhelm very low levels of Nd that were
incorporated in vivo, thus passing the eNd signal of
bottom water into fish teeth [41]. The eNd signal
carried by fossil fish teeth is resistant to alteration
during burial and diagenesis. When exposed to
similar bottom waters, Nd isotope data from fossil
fish teeth corroborate those from Fe–Mn crusts [37]
and authigenic Fe–Mn coatings [39]. The occurrence
of high Nd concentrations in fossil fish teeth within
precisely dated ODP sections has provided a means
to more accurately examine seawater Nd isotope
variability during the Paleogene.
3. Methods
3.1. Analytical methods
Fossil fish teeth were hand picked out of the
N125-Am fraction of samples that had were washed
and sieved with deionized water. Most samples
consisted of three to seven teeth and were cleaned
using the oxidative/reductive procedure after Boyle
[42], Boyle and Keigwin [43], and Boyle (personal
communication, 1993) to chemically remove Fe–Mn
coatings. Samples were dissolved in aqua regia and
the solution was transferred to clean Teflonkbeakers. All samples were spiked for Nd concen-
tration measurements. Selected samples were also
spiked for Sm concentration measurements. Samples
were then evaporated to dryness in preparation for
cation exchange chemistry.
Samples were redissolved in 0.75 N HCl and the
solution was passed through a quartz glass column
packed with Mitsubishik cation exchange resin. The
column was washed with 1.7 N HCl to remove co-
existing elements such as Ca and Mg. Sr was then
eluted with 1.7 N HCl. The Sr isotope data for these
samples is discussed in Martin and Scher [39]. Ba,
which negatively impacts the analysis of Nd isotopes,
was removed by washing the column with 2 N HNO3.
The rare earth elements (REE) were then eluted with
4.5 N HCl. The solution containing the REE was
evaporated to dryness then redissolved in 0.75 N HCl
and passed through a separate though identical column
packed with Mitsubishik cation exchange resin
treated with NH4OH. The column was washed with
distilled 0.2 M Alpha hydroxyisobuteric acid (Alpha-
HIBA) buffered to pH ~4.6 with NH4OH, to isolate Sm
and subsequently to isolate Nd. The solutions contain-
ing purified Sm and Nd were evaporated to dryness,
redissolved in ~20 Al of aqua regia to remove Alpha-
HIBA, and evaporated to dryness. The total Nd blank
for this procedure is 6 pg.
Isotopic ratios were analyzed on a Micromass
Sector 54 thermal ionization mass spectrometer
(TIMS) in dynamic mode at the University of Florida.
Samples for Nd analysis were redissolved in 8 N
Fig. 2. Nd isotope ratios from fossil fish teeth and polarity reversal stratigraphy vs. depth for ODP Site 689. Nd isotope ratios are plotted as
eNd(T) values, which are calculated from the average 147Sm/144Nd values of selected (samples Table 1 in the Appendix). Error bars are the 2rexternal reproducibility of the JNdi-1 and Ames Nd standards. The polarity reversal stratigraphy is from Spieh [47] and is shown with reference
to geologic epochs. The diagonal lines in the polarity reversal stratigraphy at 66.86 and 65.5 mbsf represent the unconformities discussed in
Section 4.2.
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 395
nitric acid and loaded onto zone-refined Re filaments
with silica oxide gel, and analyzed as NdO+. Using142Nd16O as the monitor peak, beams of 0.5 V were
measured for 200 ratios. Mass fractionation was
corrected to 146Nd16O/144Nd16O=0.722254. Samples
for Sm analysis were redissolved in 8 N nitric acid
and loaded onto Tantalum filaments.
Replicate analyses of an internal standard (AMES
Nd) during the 6 months in which samples were
analyzed yielded 0.512138 (F0.000012, 2r external
reproducibility, n=40). Replicate analyses of the
international Nd standard JNdi-1 from September,
2003 to February, 2004 yielded 0.512102
(F0.000012, 2j external reproducibility, n=65). No
correction has been applied to the Nd isotope data.
Internal measurement errors of samples are listed in
Table 1 in the Appendix.
Ten samples from this study were spiked and
analyzed for Sm in order to determine the 147Sm/144Nd
ratios preserved by the teeth at various levels in the
core. The range of 147Sm/144Nd values from teeth at
ODP Site 689 is 0.1212–0.1303, in agreement with147Sm/144Nd values from fossil fish teeth in other
marine cores [37,38,40]. An average 147Sm/144Nd
value of 0.1248 was applied to all samples to calculate
eNd(T) values. This correction ranges from 0.4 to 0.2
eNd units between the oldest and youngest samples
(see Table 1).
3.2. Age model
The age model used in this study is from Mead
and Hodell [44] and was modified to the time scale
of Cande and Kent [45]. Two unconformities are
present in the upper part of the section. At 66.86
mbsf, early Miocene sediments lie unconformably
over late Oligocene sediments (~5 My hiatus). The
second, at 65.5 mbsf, lies within a normally
magnetized interval and was recognized by biostra-
tigraphy [46] and Sr isotopes [44]. The sediments
overlying this unconformity (b1 My hiatus) have
been assigned to Chron C5En by means of Sr
isotope chemostratigraphy [44].
4. Results
Sm andNd isotope data are reported in Table 1 in the
Appendix. Nd concentrations of these samples are
discussed in Martin and Scher [39]. Nd isotope results
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405396
are presented as eNd(T) values and are plotted as a
function of depth (Fig. 2). To effectively compare
other proxy records from Site 689, the Nd isotope data
has been plotted against depth, and the magneto-
stratigraphy [47] is shown so the results can be
discussed with reference to relevant geologic epochs.
In addition the Nd isotope data are plotted against age
to show the ages of paleoceanographic events inferred
from the data.
Clearly, there is variability in the Nd isotope data
that exceeds analytical precision (Fig. 2). The eNdrecord is dominated by a pattern of secular variability
that begins with very nonradiogenic values in the
middle Eocene, then shifts stepwise to the most
radiogenic values observed in the record by the latest
Eocene. Through the Oligocene and Miocene there are
variations of ~1 eNd unit. First-order fluctuations are
generally smooth, well-resolved shifts in eNd values
with amplitudes exceeding reported external reprodu-
cibility. The Nd isotopic results are discussed with
Fig. 3. eNd record (top panel), clay mineral assemblages (middle panels), an
for the clay mineral assemblages are from Ehrmann and Mackensen [7]. Be
originating at the bottom of the diagram call attention to dramatic chang
emanating from the top of the diagram highlight the dramatic shifts in Nd
reference to four time slices that display unique Nd
isotopic patterns.
4.1. Middle Eocene (183–140 mbsf, 46–37 Ma)
This interval is distinguished by a prominent step
in eNd values that occurs at 162 mbsf. Below the step,
eNd values average �9.25 and display little variability.
At 162 mbsf, eNd values step up 0.75 eNd units to an
average value of �8.5 for the remainder of the middle
Eocene.
4.2. Late Eocene (140–121 mbsf, 37–33.7 Ma)
The late Eocene interval is dominated by a very
pronounced increase of 1.15 eNd units. In a well-
defined shift beginning at 139.50 mbsf, values
increase from �8.5 in the late Eocene to �7.35 at
125 mbsf, in the latest Eocene. The values that
culminate this increasing trend are the most radiogenic
d benthic d18O (bottom panel) vs. depth for ODP Site 689. The data
nthic d18O data are from Diester-Haass and Zahn [6]. The gray bars
es in the clay mineral assemblage and d18O record. The gray bars
isotope ratios.
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 397
values measured and are more radiogenic than
present-day eNd values at this location [1]. eNd valuesrapidly decrease to �8.35 in the latest Eocene, but
radiogenic values are restored by the lowermost
Oligocene.
4.3. Early Oligocene (121–93 mbsf, 33.7–28.5 Ma)
In the early Oligocene interval, eNd values decreasetowards nonradiogenic compositions averaging �8.5
by the late early Oligocene.
4.4. Late Oligocene to early Miocene (93–60 mbsf,
28.5–16 Ma)
During the late Oligocene, eNd values again
increase, reaching radiogenic compositions around
�7.9; however, this trend is interrupted by a rapid
excursion to nonradiogenic values of �9.1 begin-
ning at 88 mbsf. The departure to nonradiogenic
compositions is brief and by 80 mbsf eNd values
have recovered to radiogenic compositions of
�7.65. The late Oligocene interval ends abruptly
at the hiatus at 66.86 mbsf and is overlain by
middle early Miocene sediments with eNd values
averaging �8.7.
5. Sources of long-term Nd isotope variability
5.1. Comparison to records of continental weathering
The group of primary clay minerals consisting of
smectite, illite, kaolinite and chlorite present in deep
sea sediment are initially formed on nearby continents.
The relative abundances of these clay minerals are
indicative of various weathering processes, which are
ultimately controlled by climate [48]. Illite and chlorite
are chemically immature and dominate clay mineral
assemblages in regions characterized by physical
weathering [49–51]. The occurrence of smectite in
marine sedimentary sequences can be indicative of
chemical weathering under warm, humid conditions
[48]. Kaolinite is generally indicative of intense
chemical weathering conditions, though it can occur
in polar sedimentary sequences from the mechanical
weathering of kaolinite deposits [48,52,53]. Changes in
the style of continental weathering on Antarctica that
resulted from the growth of ice sheets are recorded in
the clay mineral assemblage from Site 689 [7,54,55].
Dramatic changes in continental weathering style are
indicated by two intervals of pronounced change in the
middle Eocene and early Oligocene (Fig. 3).
The pre-Oligocene history of Antarctic glaciation
has been inferred from direct evidence in the form of
glaciomarine sediments in cores from the Antarctic
margin [53,56–58]. Deposits of waterlain glacial tills,
sands and diamictites indicate brief and localized
episodes of glaciation on Antarctica in the middle
and late Eocene. At Site 689, the appearance of chlorite
in detectable quantities at 154 mbsf (38.6 Ma) reflects
an increase in physical weathering associated with
these early glaciations (Fig. 3). The simultaneous
appearance of kaolinite in this assemblage is likely
due to the physical weathering of older kaolinite
bearing sediments on Antarctica, such as those found
in the Beacon Supergroup [7]. There is no shift in Nd
isotopes at Site 689 that corresponds with the change in
clay minerals at 154 mbsf; instead eNd values between154.44 and 153.33mbsf are virtually unchanged. There
is a 0.5 eNd unit increase in slightly younger material
between 153.33 and 151.40mbsf (Fig. 3), however, it is
unlikely that the proximity of the increase in eNd unitsto the shift in clay minerals is significant. This is in part
because an increase in weathered material derived from
Antarctica should result in decreasing eNd values.
Moreover, shifts in eNd values of this magnitude are
observed in other parts of the record, including earlier
intervals when there is no documented evidence of
glaciation on Antarctica.
At approximately 122 mbsf, coinciding with the
oxygen isotope shift at the Eocene/Oligocene boun-
dary, a major change occurs in the character of the clay
mineral assemblage (Fig. 3). During the Eocene, illite
amounted to less than 20% of the total clay accumulat-
ing at this site. At ~122 mbsf, illite increased to about
60%, and remained high throughout the remainder of
the record. This shift reflects the switch from predom-
inant chemical weathering conditions, which prevailed
on Antarctica before the early Oligocene glaciation, to
a physical weathering regime following the build up of
ice sheets. A rapid excursion to nonradiogenic eNdvalues occurs from 124.24 to 120.95 mbsf that may be
related to the early Oligocene glaciation as recorded by
d18O values (Fig. 3). The shift may reflect a brief
interval when a very large amount of nonradiogenic Nd
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405398
was delivered to the Southern Ocean, released by
mechanical weathering of Precambrian basement
provinces on Antarctica during rapid ice sheet growth.
Radiogenic values are restored by 118.7 mbsf, thus if
eNd values did respond to such a weathering event, the
effects were transitory.
The changes observed in the clay mineral assem-
blage described above are associated with short term
fluctuations in eNd values. However, long-term vari-
ability of eNd values is independent from changes in
the style of continental weathering on Antarctica (Fig.
3). There is no change in the clay mineral assemblage
surrounding the large step in eNd values during the
middle Eocene. Likewise, the sharp increase in eNdvalues observed during the late Eocene is not
associated with large changes in the relative abun-
dance of clay minerals. The difference in the timing of
the shifts between Nd isotope ratios and clay minerals
suggests that changes in continental weathering are
not responsible for the first order secular variability
observed in the Nd isotope record from Site 689. It is
likely, then, that the Nd isotope record reflects
changes in ocean circulation.
5.2. Comparison to benthic d13C
Similarities between the Nd isotope record and the
record of d13C values of benthic foraminifera Cibici-
doides [6] also support the idea the Nd isotope record
Fig. 4. Nd isotope ratios from fossil fish teeth and y13C from benthic fora
record (dashed line) is from Diester-Haass and Zahn [6].
reflects changes in ocean circulation. After the late
Eocene, variations in eNd values display a close
coherence to the long-term trend of the benthic d13Crecord (Fig. 4). While benthic foraminiferal d13C is
often used as a nutrient proxy to reconstruct ocean
circulation, the signal can be overprinted by produc-
tivity changes in surface waters and changes in the
size of the oceanic carbon reservoir. Mackensen and
Ehrmann [5] demonstrated that benthic d13C trends at
Site 689 do not reflect ocean circulation during the
Eocene on the basis that similar trends are observed in
sites at a range of depths on the Kerguelan Plateau.
However, it was concluded that benthic d13C varia-
bility in the Oligocene do reflect ocean circulation
because the trend to lighter d13C values at Site 689 is
not observed at Sites 738 and 744 (Fig. 1). Moreover,
an increase in local productivity over Maud Rise that
could have led to lighter benthic d13C values during
the Oligocene is not supported by paleoproductivity
proxies (Fig. 5).
Despite differences in the geochemical cycling of
Nd and carbon in seawater, and different host phases
for these elements in marine sediments, the similarity
between long-term trends of the eNd and d13C records
during the Oligocene suggests that both tracers
respond to the same paleoceanographic signal. The
advantage of the eNd record is that it provides more
information regarding the source area of water mass
end members.
minifer Cibicidoides vs. depth for ODP Site 689. The benthic d13C
Fig. 5. Nd isotope ratios from fossil fish teeth and paleoproductivity vs. age for ODP Site 689. The age model used for Site 689 is from Mead
and Hodell [44] and has been recalibrated to the ages of Cande and Kent [45]. The paleoproductivity record is from Diester-Haass and Zahn [6]
and is derived from the abundance of benthic foraminifera.
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 399
6. Southern Ocean paleoceanography from Nd
isotopes
A rudimentary understanding of global seawater
Nd isotope patterns during the Paleogene is provided
by previous investigations of marine glauconite
deposits [59], Fe–Mn crusts [16–20] and fossil fish
teeth [38–40]. With this limited Nd isotope database
for the Paleogene ocean, the Nd isotope record from
Site 689 can be interpreted in terms of water mass
Fig. 6. Nd isotope ratios vs. age for the early Miocene through middle Eoc
records from the Atlantic, Pacific and Indian were measured from ferrom
denotes the range of qNd values estimated of the Tethys Sea from middle E
Alps [59].
mixing; though some water mass end member
compositions are poorly constrained.
6.1. Middle Eocene deep water sources
Reconstructions of deep water circulation during
the Paleogene indicate that the Southern Ocean was
the predominant source of deep water [60–63].
However, eNd values at Site 689 during the middle
Eocene (eNd=�9.1 to �9.5) are slightly less radio-
ene. The data for ODP Site 689 are from this study. The Nd isotope
anganese crusts from abyssal depths [16–18]. The shaded rectangle
ocene age authigenic glauconite deposits in the Helvetic belt of the
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405400
genic than the only published estimate of Southern
Ocean deep water (eNd=�9.1 in the early Eocene
[38]). Thomas et al. [38] did observe eNd values as
low as �10.6 on Maud Rise briefly in the early
Eocene, however eNd values from ODP Site 1090
(Fig. 1) are much less radiogenic (eNd=�8.5 [64])
during the middle Eocene.
There are two possible explanations for the Nd
isotope data at Site 689. First, if Southern Ocean
deep water during the middle Eocene is not fully
constrained by available data, then Site 689 eNdvalues may simply reflect a middle Eocene analogue
to the deep water source described in Thomas et al.
[38]. The other possibility is that middle Eocene eNdvalues at Site 1090 are characteristic of Southern
Ocean deep water [64] and the observation that Site
689 is less radiogenic than this end member requires
that the nonradiogenic signal was propagated to Site
689 from a deep water source outside of the
Southern Ocean. The only known sources of sea-
water outside of the Southern Ocean with such a
nonradiogenic signal (eNdb�9.5) in the middle
Eocene were the North Atlantic (eNd=�11)
[16,18,19] and the Tethys Sea (eNd=�9.3 to �9.8)
[59] (Fig. 6). The North Atlantic end member eNdvalue is based on Fe–Mn crusts, while the Tethys
seawater estimate is based on eNd values of Rb–Sr
and K–Ar dated glauconite deposits from the
Helvetic belt of the Alps, interpreted as the northern
continental shelf of the Tethys Sea.
Downwelling in the North Atlantic may have been
possible during the middle Eocene on the basis of sea-
surface temperature (SST) constraints from d18O
values of planktonic foraminifera, which indicate a
temperature of ~13 8C in the northeastern Atlantic
[65]. However, export of deep water from the North
Atlantic to the Southern Ocean in the middle Eocene
is unlikely based on reconstructions of deep ocean
circulation using d13C values of benthic foraminifera
[61,63]. In these reconstructions, values in the South-
ern Ocean remain high relative to the North Atlantic
and Pacific indicating the dominance of Southern
Ocean deep water.
The hypothesis that a subtropical source of deep
water, known as Warm Saline Deep Water (WSDW),
formed in regions of net evaporation during warm
climate intervals has been contentious since it was
first proposed by Chamberlin [66]. Many geochem-
ical, faunal, and sedimentological records have been
interpreted as reflecting a shift from high latitude deep
water production to an inferred source of low latitude
deep water production [4,60,61,62,67–72]. However,
much of the evidence for a low latitude deep water
source is equivocal, no direct evidence exists for
WSDW, and general circulation models (GCMs) with
early Paleogene boundary conditions fail to produce a
stable mode of salinity-induced downwelling in the
Tethys Sea, which was a large, low latitude seaway at
the time [73]. Yet the presence of Tethys-derived
seawater in the Southern Ocean appears to be the most
likely explanation for the Nd isotope data from Site
689, and thus supports the production of WSDW in
the low latitude Tethys Sea and subsequent export to
the Southern Ocean.
Further, Nd isotope investigations may strengthen
or weaken the WSDW interpretation, however it
does corroborate previously published geological
evidence for a warm water mass on Maud Rise.
Kennett and Stott [4] attributed reversed depth
gradients of oxygen isotope data from Sites 689
and 690 to differences in bottom water temperatures,
concluding that incursions of a warm deep water
mass beneath colder Southern Ocean seawater led to
the 0.5x difference in d18O values. Other changes at
Site 689 that coincide with the onset of radiogenic
values at 40.8 Ma (~162 mbsf), such as increasing
d18O values (Fig. 3) and decreased carbonate
preservation [74], are also consistent with a change
from a warm bottom water mass during the middle
Eocene to colder bottom water in the late middle
Eocene.
6.2. Constraints on Drake Passage
The timing of the opening of Drake Passage has
been a long-standing debate driven by the hypothesis
of Kennett [75] linking the opening of Drake Passage
to initiation of the Antarctic Circumpolar Current
(ACC) and development of ice sheets on Antarctica.
Estimates for the opening of Drake Passage to deep
water flow range from around the Oligocene/Miocene
boundary [76–78] to the early Oligocene [79, 80].
Despite numerous attempts to constrain the opening
by dating the onset of the ACC with other proxies (see
recent review by Barker and Thomas [81]) the debate
has endured.
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 401
Nd isotopes offer an interesting approach to this
problem in that the Nd isotopic ratio of Pacific and
Atlantic deep waters comprise, respectively, the most
and least radiogenic seawater in the ocean, a
distinction that has persisted for much of the Cenozoic
[16–19] (Fig. 6). Assuming that the radiogenic
signature of Pacific seawater was effectively absent
from the Atlantic sector of the Southern Ocean when
Drake Passage was closed, the opening of Drake
Passage should introduce the radiogenic fingerprint of
Pacific seawater to the study area.
In the late Eocene, a dramatic shift in eNd values
leads to the most radiogenic values observed in this
study. The resulting value (eNd=�7.3) is more
radiogenic than any intermediate or deep water mass
in the present-day Southern Ocean [1,12,13,82]. The
only obvious explanation for such radiogenic eNdvalues is the influx of Pacific deep water into the
study area. The calculated paleodepth curve for Site
689, using the thermal subsidence model of Parsons
and Sclater [83] indicates a depth of 1600 m in the
late Eocene [72]. Thus, the data supports that Drake
Passage was open to shallow and possibly inter-
mediate depths by the late Eocene (~37 Ma). This
estimate is in excellent agreement with Diester-Haass
and Zahn [6], who reached a similar conclusion
based on an increase in proxy measurements of local
surface productivity during the late Eocene (Fig. 5).
Increased phytoplankton production also occurred in
other parts of the Atlantic sector of the Southern
Ocean during the late Eocene [84], indicating a
change in the nutrient profile of the surface layer,
presumably resulting from the development of
upwelling cells. Based on the Nd isotope data,
widespread changes in productivity in the Atlantic
sector of the Southern Ocean can be linked to the
opening of Drake Passage to intermediate depths and
the association of accelerated ocean currents with
more effective latitudinal circulation. Variability of
benthic d13C values which accompany the shift in
eNd values likely reflect more pronounced changes in
ventilation of the water column [6].
An alternative pathway for Pacific seawater to the
vicinity of Site 689 in the late Eocene was through the
Central American Seaway and subsequent export into
the southern high latitudes (Fig. 1). It is intriguing that
the Nd isotope record from DSDP Site 357 on Rio
Grande Rise shows a shift to radiogenic eNd values in
the middle Eocene [85], which supports a pathway for
Pacific water into the tropical South Atlantic. How-
ever, the preferred interpretation for the Nd isotope
data from Site 689 is the pathway through Drake
Passage, as it is a more direct route to the study area
and is consistent with other observations.
6.3. Oligocene to early Miocene ocean circulation
During the Oligocene and Miocene, long-term
variability of eNd values follows the benthic d13C
record (Fig. 4), and most likely reflects fluctuations in
ocean circulation arising from changes in the relative
contributions of different end member water masses to
the Southern Ocean.
Following the rapid shift to radiogenic values in
the late Eocene, eNd values decrease slightly in the
early Oligocene to a lower mean value of ~�8.5,
close to values observed in present-day Southern
Ocean sourced water masses, AAIW and AABW
[1,12,13,82]. The increasing influence of these water
masses following the major glaciation of Antarctica
likely played a more significant role in the hydrog-
raphy of the study area [65]. The modern eNd values
of Southern Ocean sourced water masses reflect a
significant contribution of NADW, so decreasing eNdvalues at this time may have resulted from the export
of Northern Component Water (NCW, similar to
modern NADW) [86]. Southwesterly dipping down-
lap reflections of early Oligocene sediments within
the Southeast Faeroes drift provide evidence of a
southerly flow regime in the North Atlantic [87].
During the late Oligocene, there is a long-term
increase in eNd values, followed by a decrease in eNdvalues through 24.8 Ma (66.90 mbsf), just below the
hiatus at 66.86 mbsf where the Oligocene section
ends. Strengthening of the ACC during the late
Oligocene as suggested by Barker [78] may account
for some of the positive fluctuations in eNd values. Ashort-lived excursion is superimposed upon the long-
term trend, from 28.2 to 27.13 Ma (87.96 to 80.23
mbsf ) when eNd values fall to very nonradiogenic
values (eNd=�9.1), approaching values observed
during the middle Eocene. It is not immediately clear
what this excursion represents. Based on the similarity
of the values during the excursion to middle Eocene
values, it is possible that WSDW was present briefly
at the study area in the late Oligocene, though there is
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405402
no other evidence to support this supposition. An
alternative explanation is that the excursion represents
a pulse of NCW. There is stable isotopic evidence for
a similar pulse of NCW during the early Oligocene
[86]. A final possibility is that a climatically induced
weathering event introduced nonradiogenic Nd into
the Southern Ocean.
The long-term decrease in eNd values at the top of
the Oligocene section follows the trend of the
benthic d13C record (Fig. 4). Above the hiatus at
66.86 mbsf, early Miocene eNd values average �8.5.
This value is similar to modern eNd values of deep
and intermediate waters in the Southern Ocean and
perhaps reflects the beginning of modern deep water
circulation patterns as suggested by Woodruff and
Savin [88].
7. Conclusions
Nd isotope ratios have been measured from middle
Eocene to early Miocene age fossil fish teeth from
ODP Site 689. The record represents the longest high-
resolution record of Paleogene Nd isotopes. Using
multiple paleoceanographic proxy records, the con-
tributing signals from ocean circulation and continen-
tal weathering were deconvolved from the Nd isotope
record enabling an examination of the nature and
causes of secular variability of Nd isotopes at Site 689
through the Paleogene.
Long-term secular variations of Nd isotopes are
not associated with major changes in continental
weathering on Antarctica as revealed by changes in
clay mineral abundances. Instead, a close corre-
spondence with d13C values is observed, suggesting
the Nd isotope variability reflects changes in deep
water circulation. Though the eNd and d13C records
demonstrate similar trends, the eNd record provides
more information about the circulation of water
mass end members. Therefore, the Nd isotope data
can be used to examine the evolution of ocean
circulation in the Southern Ocean during the
Paleogene.
The eNd values of the water mass overlying Site
689 during the middle Eocene are less radiogenic than
estimates for Southern Ocean deep water at the time
and require the contribution of a water mass with eNdb�9.5. The contribution from a Tethys Sea end
member, known as WSDW, provides an intriguing
explanation for the data. This interpretation is
consistent with oxygen isotope data and sedimento-
logical evidence for WSDW on Maud Rise during the
Paleogene.
A dramatic shift toward radiogenic eNd values in
the late Eocene is best explained by an influx of
Pacific seawater into the Atlantic Ocean, signifying
the opening of Drake Passage by 37 Ma. Increases in
phytoplankton production throughout the Atlantic
sector of the Southern Ocean also occur in the late
Eocene, and indicate the development of upwelling
cells associated with more effective latitudinal circu-
lation. The Nd isotope data places important con-
straints on the timing of the opening of Drake Passage
and indicates that flow through Drake Passage was
established to shallow, and possibly intermediate
depths, prior to large-scale development of ice sheets
on Antarctica.
Long-term variability of eNd values during the
Oligocene and early Miocene closely follows the
trend of benthic d13C values. The only exception
occurs in the late Oligocene during a short-lived
excursion to nonradiogenic eNd values similar to
those observed in the middle Eocene. It is unclear
whether the excursion reflects the fingerprint of a
pulse of nonradiogenic seawater to the study area,
such as NCW or WSDW, or a climatically induced
weathering event that introduced nonradiogenic
material from Antarctica into the Southern Ocean.
During the early Miocene, eNd values average �8.5
which is similar to modern values for deep and
intermediate water in the Southern Ocean and
perhaps reflects the emergence of circulation patterns
similar to today.
Acknowledgments
We appreciate reviews by Tim Bralower and
Martin Frank whose comments have led to improve-
ments in the manuscript. HDS is grateful to J. Lyons
for providing valuable assistance in the lab. A debt of
gratitude is extended to R. Thomas for his technical
expertise with the TIMS at UF. Samples were
provided by the Ocean Drilling Program.
This research was supported by an NSF Career
award to EEM (OCE-962970).
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 403
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.epsl.2004.10.016.
References
[1] C. Jeandel, Concentration and isotopic composition of Nd in
the South Atlantic Ocean, Earth Planet. Sci. Lett. 117 (1993)
581–591.
[2] C. Jeandel, J.K. Bishop, A. Zindler, Exchange of neodymium
and its isotopes between seawater and small and large particles
in the Sargasso Sea, Geochim. Cosmochim. Acta 59 (1995)
535–547.
[3] K. Tachikawa, C. Jeandel, M. Roy-Barman, A new approach
to the Nd residence time in the ocean; the role of atmospheric
inputs, Earth Planet. Sci. Lett. 170 (1999) 433–446.
[4] J.P. Kennett, L.D. Stott, Proteus and Proto-Oceanus; ancestral
Paleogene oceans as revealed from Antarctic stable isotopic
results; ODP Leg 113, Proc. Ocean Drill. Program Sci. Results
113 (1990) 865–880.
[5] A. Mackensen, W.U. Ehrmann, Middle Eocene through early
Oligocene climate history and paleoceanography in the
Southern Ocean; stable oxygen and carbon isotopes from
ODP Sites on Maud Rise and Kerguelen Plateau, Mar. Geol.
108 (1992) 1–27.
[6] L. Diester-Haass, R. Zahn, Eocene–Oligocene transition in the
Southern Ocean; history of water mass circulation and
biological productivity, Geology 24 (1996) 163–166.
[7] W.U. Ehrmann, A. Mackensen, Sedimentological evidence for
the formation of an East Antarctic ice sheet in Eocene/
Oligocene time, Palaeogeogr. Palaeoclimatol. Palaeoecol. 93
(1992) 85–112.
[8] C. Robert, L. Diester-Haass, H. Chamley, Late Eocene–
Oligocene oceanographic development at southern high
latitudes, from terrigenous and biogenic particles; a compar-
ison of Kerguelen Plateau and Maud Rise, ODP Sites 744 and
689, Mar. Geol. 191 (2002) 37–54.
[9] D.J. Piepgras, G.J. Wasserburg, Rare earth element trans-
port in the western North Atlantic inferred from Nd
isotopic observations, Geochim. Cosmochim. Acta 51
(1987) 1257–1271.
[10] D.J. DePaolo, G.J. Wasserburg, Nd isotopic variations and
petrogenetic models, Geophys. Res. Lett. 3 (1976) 249–252.
[11] D.J. Piepgras, S.B. Jacobsen, The isotopic composition of
neodymium in the North Pacific, Geochim. Cosmochim. Acta
52 (1988) 1373–1381.
[12] D.J. Piepgras, G.J. Wasserburg, Isotopic composition of
neodymium in waters from the Drake Passage, Science 217
(1982) 207–214.
[13] C.J. Bertram, H. Elderfield, The geochemical balance of the
rare earth elements and neodymium isotopes in the oceans,
Geochim. Cosmochim. Acta 57 (1993) 1957–1986.
[14] D.L. Reid, H.J. Welke, A.J. Erlank, A. Moyes, The Orange
River Group; a major Proterozoic calcalkaline volcanic belt in
the western Namaqua Province, Southern Africa; geochemis-
try and mineralization of Proterozoic volcanic suites, Geol.
Soc. London Spec. Publ. 33 (1987) 327–346.
[15] F. Albarede, S.L. Goldstein, World map of Nd isotopes in sea-
floor ferromanganese deposits, Geology 20 (1992) 761–763.
[16] K.W. Burton, H. Ling, R.K. O’Nions, Closure of the Central
American Isthmus and its effect on deep-water formation in
the North Atlantic, Nature 386 (1997) 382–385.
[17] H.F. Ling, K.W. Burton, R.K. O’Nions, B.S. Kamber, F. von
Blanckenburg, A.J. Gibb, J.R. Hein, Evolution of Nd and Pb
isotopes in Central Pacific seawater from ferromanganese
crusts, Earth Planet. Sci. Lett. 146 (1997) 1–12.
[18] R.K. O’Nions, M. Frank, F. von Blanckenburg, H.F. Ling,
Secular variation of Nd and Pb isotopes in ferromanganese
crusts from the Atlantic, Indian and Pacific oceans, Earth
Planet. Sci. Lett. 155 (1998) 15–28.
[19] K.W. Burton, D. Lee, J.N. Christensen, A.N. Halliday, J.R.
Hein, Actual timing of neodymium isotopic variations
recorded by Fe–Mn crusts in the western North Atlantic,
Earth Planet. Sci. Lett. 171 (1999) 149–156.
[20] M. Frank, N.Whiteley, S. Kasten, J.R. Hein, K. O’Nions, North
Atlantic DeepWater export to the Southern Ocean over the past
14 Myr; evidence from Nd and Pb isotopes in ferromanganese
crusts, Paleoceanography 17 (2002) 12.1–12.10.
[21] S.L. Harley, Archaean–Cambrian crustal development of East
Antarctica; metamorphic characteristics and tectonic implica-
tions; Proterozoic East Gondwana; supercontinent assembly
and breakup, Geol. Soc. London Spec. Publ. 206 (2003)
203–230.
[22] L. Dallai, C. Ghezzo, Z.D. Sharp, Oxygen isotope evidence for
crustal assimilation and magma mixing in the Granite Harbour
Intrusives, northern Victoria Land, Antarctica, Lithos 67
(2003) 135–151.
[23] F. Talarico, L. Borsi, B. Lombardo, Relict granulites in the
Ross Orogen of northern Victoria Land (Antarctica): II.
Geochemistry and palaeo-tectonic implications; tectonics of
East Antarctica, Precambrian Res. 75 (1995) 157–174.
[24] C.D. Wareham, R.J. Pankhurst, R.J. Thomas, B.C. Storey,
G.H. Grantham, J. Jacobs, B.M. Eglington, Pb, Nd, and Sr
isotope mapping of Grenville-age crustal provinces in Rodinia,
J. Geol. 106 (1998) 647–659.
[25] D.J. DePaolo, W.I. Manton, E.S. Grew, M. Halpern, Sm–Nd,
Rb–Sr and U–Th–Pb systematics of granulite facies rocks
from Fyfe Hills, Enderby Land, Antarctica, Nature 298 (1982)
614–618.
[26] L.P. Black, J.W. Sheraton, P.R. James, Late Archaean granites
of the Napier Complex, Enderby Land, Antarctica; a compar-
ison of Rb–Sr, Sm–Nd and U–Pb isotopic systematics in a
complex terrain, Precambrian Res. 32 (1986) 343–368.
[27] L.P. Black, M.T. McCulloch, Evidence for isotopic equilibra-
tion of Sm–Nd wholerock systems in early Archaean crust of
Enderby Land, Antarctica, Earth Planet. Sci. Lett. 82 (1987)
15–24.
[28] S.G. Borg, E. Stump, B.W. Chappell, M.T. McCulloch, D.
Wyborn, R.L. Armstrong, J.R. Holloway, Granitoids of
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405404
northern Victoria Land, Antarctica; implications of chemical
and isotopic variations to regional crustal structure and
tectonics, Am. J. Sci. 287 (1987) 127–169.
[29] S.G. Borg, D.J. DePaolo, B.M. Smith, Isotopic structure and
tectonics of the central Transantarctic Mountains, J. Geophys.
Res., B, Solid Earth Planets 95 (1990) 6647–6667.
[30] S.G. Borg, D.J. DePaolo, A tectonic model of the Antarctic
Gondwana margin with implications for southeastern Aus-
tralia; isotopic and geochemical evidence; accretionary tec-
tonics and composite continents, Tectonophysics 196 (1991)
339–358.
[31] S.G. Borg, D.J. DePaolo, Laurentia, Australia, and Antarctica
as a late Proterozoic supercontinent; constraints from isotopic
mapping, Geology 22 (1994) 307–310.
[32] M. Frank, B.C. Reynolds, R.K. O’Nions, Nd and Pb isotopes
in Atlantic and Pacific water masses before and after closure of
the Panama Gateway, Geology 27 (1999) 1147–1150.
[33] F. von Blanckenburg, T.F. Naegler, Weathering versus
circulation-controlled changes in radiogenic isotope tracer
composition of the Labrador Sea and North Atlantic deep
water, Paleoceanography 16 (2001) 424–434.
[34] R.L. Rutberg, S.R. Hemming, S.L. Goldstein, Reduced North
Atlantic deep water flux to the glacial Southern Ocean inferred
from neodymium isotope ratios, Nature 405 (2000) 935–938.
[35] M. Frank, Radiogenic isotopes; tracers of past ocean circu-
lation and erosional input, Rev. Geophys. 40 (2002)
DOI:10.1029/2000RG000094.
[36] M. Frank, van de Flierdt, Tina, A.N. Halliday, P.W. Kubik, B.
Hattendorf, D. Gunther, Evolution of deep water and weath-
ering inputs in the central Atlantic Ocean over the past 33 Myr,
Paleoceanography 18 (2003) DOI:10.1029/2003PA000919.
[37] E.E. Martin, B.A. Haley, Fossil fish teeth as proxies for
seawater Sr and Nd isotopes, Geochim. Cosmochim. Acta 64
(2000) 835–847.
[38] D.J. Thomas, T.J. Bralower, C.E. Jones, Neodymium isotopic
reconstruction of late Paleocene–early Eocene thermohaline
circulation, Earth Planet. Sci. Lett. 209 (2003) 309–322.
[39] E. Martin, H. Scher, D., Preservation of seawater Sr and Nd
isotopes in fossil fish teeth: bad news and good news, Earth
Planet. Sci. Lett. 220 (2004) 25–39.
[40] D.J. Thomas, Evidence for deep-water production in the North
Pacific Ocean during the early Cenozoic warm interval, Nature
430 (2004) 65–68.
[41] H. Elderfield, R. Pagett, Rare earth elements in ichthyoliths:
variations with redox conditions and depositional environ-
ments, Sci. Total Environ. 49 (1986) 175–197.
[42] E.A. Boyle, Cadmium, zinc, copper, and barium in foramin-
ifera tests, Earth Planet. Sci. Lett. 53 (1981) 11–35.
[43] E.A. Boyle, L.D. Keigwin, Comparison of Atlantic and Pacific
paleochemical records for the last 215,000 years; changes in
deep ocean circulation and chemical inventories, Earth Planet.
Sci. Lett. 76 (1985) 135–150.
[44] G.A. Mead, D.A. Hodell, Controls on the 87Sr/86Sr composi-
tion of seawater from the middle Eocene to Oligocene; Hole
689B, Maud Rise, Antarctica, Paleoceanography 10 (1995)
327–346.
[45] S.C. Cande, D.V. Kent, Revised calibration of the geomagnetic
polarity timescale for the Late Cretaceous and Cenozoic, J.
Geophys. Res., B, Solid Earth Planets 100 (1995) 6093–6095.
[46] R.E. Gersonde, L.H. Burckle, Neogene diatom biostratigraphy
of ODP Leg, 113, Weddell Sea (Antarctic Ocean), Proc. Ocean
Drill. Program Sci. Results 113 (1990) 761–789.
[47] V. Spiess, Cenozoic magnetostratigraphy of Leg 113 drill sites,
Maud Rise, Weddell Sea Antarctica, Proc. Ocean Drill.
Program Sci. Results 113 (1990) 261–315.
[48] H. Chamley, Clay Sedimentology, Springer-Verlag, Berlin,
1989, 623 pp.
[49] P.E. Biscaye, Mineralogy and sedimentation of Recent deep-
sea clay in the Atlantic Ocean and adjacent seas and oceans,
Geol. Soc. Am. Bull. 76 (1965) 803–831.
[50] J.J. Griffin, H. Windom, E.D. Goldberg, The distribution of
clay minerals in the world ocean, Deep-Sea Res. Oceanogr.
Abstr. 15 (1968) 433–459.
[51] K.C. Moriarty, Clay minerals in Southeast Indian Ocean
sediments, transport mechanisms and depositional environ-
ments, Mar. Geol. 25 (1977) 149–174.
[52] D.A. Darby, Kaolinite and other clay minerals in Arctic Ocean
sediments, J. Sediment. Petrol. 45 (1975) 272–279.
[53] M.J. Hambrey, W.U. Ehrmann, B. Larsen, Cenozoic glacial
record of the Prydz Bay continental shelf, East Antarctica,
Proc. Ocean Drill. Program Sci. Results 119 (1991) 77–132.
[54] J.P. Kennett, P.F. Barker, Latest Cretaceous to Cenozoic
climate and oceanographic developments in the Weddell Sea,
Antarctica; an ocean-drilling perspective, Proc. Ocean Drill.
Program Sci. Results 113 (1990) 937–960.
[55] L. Diester-Haass, C. Robert, H. Chamley, Paleoceanographic
and paleoclimatic evolution in the Weddell Sea (Antarctica)
during the middle Eocene–late Oligocene, from a coarse
sediment fraction and clay mineral data (ODP Site 689), Mar.
Geol. 114 (1993) 233–250.
[56] P.J. Barrett, M.J. Hambrey, D.M. Harwood, A.R. Pyne, P.N.
Webb, Synthesis, in: P.J. Barrett (Ed.), Antarctic Cenozoic
history from the CIROS-1 drillhole, McMurdo Sound, Depart-
ment of Scientific and Industrial Research (DSIR), Wellington,
New Zealand, 1989, pp. 241–251.
[57] S.W. Wise Jr, J.R. Breza, D.M. Harwood, W. Wei, Paleogene
glacial history of Antarctica, in: D.W. Mueller, J.A. McKenzie,
H. Weissert (Eds.), Controversies in modern geology, Acad.
Press, London, United Kingdom, 1991, pp. 133–171.
[58] J.R. Breza, S.W. Wise Jr., Lower Oligocene ice-rafted debris
on the Kerguelen Plateau; evidence for East Antarctic
continental glaciation, Proc. Ocean Drill. Program Sci. Results
120 (1992) 161–178.
[59] P. Stille, H. Fischer, Secular variation in the isotopic
composition of Nd in Tethys seawater, Geochim. Cosmochim.
Acta 54 (1990) 3139–3145.
[60] G.S. Mountain, K.G. Miller, Seismic and geologic evidence
for early Paleogene deepwater circulation in the western North
Atlantic, Paleoceanography 7 (1992) 423–439.
[61] D.K. Pak, K.G. Miller, Paleocene to Eocene benthic foramini-
feral isotopes and assemblages; implications for deepwater
circulation, Paleoceanography 7 (1992) 405–422.
H.D. Scher, E.E. Martin / Earth and Planetary Science Letters 228 (2004) 391–405 405
[62] J.C. Zachos, D.K. Rea, K. Seto, R. Nomura, N. Niitsuma,
Paleogene and Early Neogene deep water paleoceanography of
the Indian ocean as determined from benthic foraminifer stable
carbon and oxygen isotope records, in: R.A. Duncan, D.K.
Rea, R.B. Kidd, U. von Rad, J.K. Weissel (Eds.), The Indian
Ocean: A Synthesis of Results from the Ocean Drilling
Program, AGU Monograph, vol. 70, 1992, pp. 351–386.
[63] J.D. Wright, K.G. Miller, Southern Ocean influences on late
Eocene toMiocene deepwater circulation, in: J.P. Kennett, D.A.
Warnke (Eds.), The Antarctic paleoenvironment; a perspective
on global change: Part two, American Geophysical Union,
Washington, DC, United States (USA), 1993, pp. 1–25.
[64] H.D. Scher, E.E. Martin, Eocene To Miocene Southern Ocean
deep water circulation revealed from fossil fish teeth Nd
isotopes, Eos Trans. AGU 82 (2001) Fall Meet. Suppl.,
Abstract OS31C-0468.
[65] J.C. Zachos, L.D. Stott, K.C. Lohmann, Evolution of early
Cenozoic marine temperatures, Paleoceanography 9 (1994)
353–387.
[66] T.C. Chamberlin, On a possible reversal of deep-sea circu-
lation and its influence on geologic climates, Proc. Am. Philos.
Soc. (1906) 33–43.
[67] N.J. Shackleton, E.B. Kraus, H.P. Hanson, The deep-sea
sediment record of climate variability; climatic variability in
the oceans; papers from an international symposium held at the
Cooperative Institute for Marine and Atmospheric Studies,
Prog. Oceanogr. 11 (1982) 199–218.
[68] M.L. Prentice, R.K. Matthews, Cenozoic ice-volume history;
development of a composite oxygen isotope record; with
Suppl. Data 88-26, Geology 16 (1988) 963–966.
[69] J.P. Kennett, L.D. Stott, Abrupt deep-sea warming, palae-
oceanographic changes and benthic extinctions at the end of
the Palaeocene, Nature 353 (1991) 225–229.
[70] D.K. Rea, J.C. Zachos, R.M. Owen, P.D. Gingerich, Global
change at the Paleocene–Eocene boundary; climatic and
evolutionary consequences of tectonic events, Palaeogeogr.
Palaeoclimatol. Palaeoecol. 79 (1990) 117–128.
[71] J.C. Zachos, K.C. Lohmann, J.C.G. Walker, S.W. Wise, Abrupt
climate changes and transient climates during the Paleogene; a
marine perspective; Centennial special issue, J. Geol. 101
(1993) 191–213.
[72] G.A. Mead, D.A. Hodell, P.F. Ciesielski, Late Eocene to
Oligocene vertical oxygen isotopic gradients in the South
Atlantic; implications for warm saline deep water, in: J.P.
Kennett, D.A. Warnke (Eds.), The Antarctic paleoenviron-
ment; a perspective on global change: Part two,
American Geophysical Union, Washington, DC, 1993,
pp. 27–48.
[73] K.L. Bice, J. Marotzke, Numerical evidence against reversed
thermohaline circulation in the warm Paleocene/Eocene ocean,
J. Geophys. Res., C, Oceans 106 (2001) 11–11542.
[74] L. Diester-Haass, Middle Eocene to early Oligocene paleo-
ceanography of the Antarctic Ocean (Maud Rise ODP Leg
113, Site 689): change from a low to a high productivity
ocean., Palaeogeogr. Palaeoclimatol. Palaeoecol. 113 (1995)
311–334.
[75] J.P. Kennett, Cenozoic evolution of Antarctic glaciation, the
Circum-Antarctic Ocean, and their impact on global paleo-
ceanography, J. Geophys. Res. 82 (1977) 3843–3860.
[76] P.F. Barker, J. Burrell, The opening of Drake Passage, Mar.
Geol. 25 (1977) 15–34.
[77] P.F. Barker, J. Burrell, The influence upon Southern Ocean
circulation, sedimentation, and climate of the opening of
Drake Passage; Antarctic geoscience, in: C. Craddock (Ed.),
Antarctic Geoscience, University of Wisconsin Press, Madi-
son, WI, 1982, pp. 377–385.
[78] P.F. Barker, Scotia Sea regional tectonic evolution; implica-
tions for mantle flow and palaeocirculation, Earth-Sci. Rev. 55
(2001) 1–39.
[79] L.A. Lawver, L.M. Gahagan, Opening of Drake Passage and
its impact on Cenozoic ocean circulation, in: T.J. Crowley,
K.C. Burke (Eds.), Tectonic Boundary Conditions for
Climate Reconstructions, Oxford University Press, Oxford,
1998, pp. 212–223.
[80] L.A. Lawver, L.M. Gahagan, Evolution of Cenozoic seaways
in the Circum-Antarctic region; Antarctic Cenozoic palae-
oenvironments; geologic record and models, Palaeogeogr.
Palaeoclimatol. Palaeoecol. 198 (2003) 11–37.
[81] P.F. Barker, E. Thomas, Origin, signature and palaeoclimatic
influence of the Antarctic Circumpolar Current, Earth-Sci.
Rev. 66 (2004) 143–162.
[82] F. Albarede, S.L. Goldstein, D. Dautel, The neodymium
isotopic composition of manganese nodules from the Southern
and Indian oceans, the global oceanic neodymium budget, and
their bearing on deep ocean circulation, Geochim. Cosmo-
chim. Acta 61 (1997) 1277–1291.
[83] B. Parsons, J.G. Sclater, An analysis of the variation of ocean
floor bathymetry and heat flow with age, J. Geophys. Res. 82
(1977) 803–827.
[84] B. Diekmann, G. Kuhn, R. Gersonde, A. Mackensen, Middle
Eocene to early Miocene environmental changes in the sub-
Antarctic Southern Ocean; evidence from biogenic and
terrigenous depositional patterns at ODP Site 1090, Glob.
Planet. Change 40 (2004) 295–313.
[85] M.R. Palmer, H. Elderfield, Rare earth elements and neo-
dymium isotopes in ferromanganese oxide coatings of
Cenozoic foraminifera from the Atlantic Ocean, Geochim.
Cosmochim. Acta 50 (1986) 409–417.
[86] K. Miller, G., Middle Eocene to Oligocene stable isotopes,
climate, and deep water history: the terminal Eocene event? in:
D. Prothero, W.A. Berggren (Eds.), Eocene–Oligocene Cli-
matic and Biotic Evolution, Princeton University, Princeton,
NJ, 1992, pp. 160–177.
[87] R. Davies, J. Cartwright, J. Pike, C. Line, Early Oligocene
initiation of North Atlantic Deep Water formation, Nature 410
(2001) 917–920.
[88] F. Woodruff, S.M. Savin, Miocene deepwater oceanography,
Paleoceanography 4 (1989) 87–140.