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

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doi:10.1016/j.ep

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tters 228 (2004) 391–405

ee front matter D 2004 Elsevier B.V. All rights reserved.

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ding author. Tel.: +1 352 392 2231; fax: +1 352 392 9294.

ess: [email protected] (H.D. Scher).

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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


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.


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


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.


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


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.


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


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.


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


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).


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.

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