Nd Isotopic Composition Of Seawater Research Articles

Glacial\u2013interglacial Nd isotope variability of North Atlantic Deep Water modulated by North American ice sheet

The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atlantic Deep Water (NADW), a major component of the global overturning ocean circulation, was significantly more radiogenic than modern during the Last Glacial Maximum (LGM), and shifted towards modern values during the deglaciation. We propose that weathering contributions of unradiogenic Nd modulated by the North American Ice Sheet dominated the evolution of the NADW Nd isotope endmember. If water mass mixing dominated the distribution of deep glacial Atlantic Nd isotopes, our results would imply a larger fraction of NADW in the deep Atlantic during the LGM and deglaciation than reconstructed with a constant northern endmember.

New insights into hydrological exchange between the South China Sea and the Western Pacific Ocean based on the Nd isotopic composition of seawater

Rare Earth Element (REE) concentrations and Nd isotopic compositions (εNd) of 16 seawater profiles collected in the northern South China Sea (SCS) and the Philippine Sea were investigated (1) to establish the distribution of the Nd isotopic composition of water masses along the tropical Western Pacific and the SCS until now underexplored, (2) to constrain hydrological exchanges between the SCS and the Philippine Sea through the Luzon Strait, and (3) to test, in the context of the semi-closed marginal sea (SCS), the possible impact of the process of exchange of Nd between SCS water masses and unradiogenic sediments from its north-western margin.εNd values for mid- and deep-water masses of the Philippine Sea and the SCS range from −2.3 to −4.4 and generally increase slightly as water depth increases. In the Philippine Sea, εNd values for the North Pacific Intermediate Water (NPIW) reach −2.7±0.4 at mid-depths (500 to 1400m). Below ~1800m, the Pacific Deep Water (PDW) is characterised by less radiogenic Nd (−4.1±0.5) indicating the intrusion of southern-sourced water. For most of the stations in the Northern SCS, water masses below 1500m (PDW) display homogenous εNd values (~−4.1) identical to those of the PDW in the Philippine Sea. εNd values for the South China Sea Intermediate Water (SCSIW, 500–1500m) vary from −3.0 to −3.9 as a result of the vertical mixing of the NPIW with the PDW in the SCS. Seawater εNd values for the SCS display local modification (~−5.3 to −7.0) in areas where water passes above sediment drift deposit systems. This implies locally confined “boundary exchange’’ with unradiogenic sediments (around −11) insufficient to alter the Nd isotopic composition of the PDW in the Northern SCS. In addition, εNd values analysed for the first time in nepheloid layers collected along the north-western margin of the SCS do not exhibit any significant modification of the seawater εNd which remains into a narrow range between −3.7 and −4.3, similar to that of the PDW. A compilation of εNd from the sea surface water of the SCS indicates lower εNd values in the western and the central SCS (−3.3 to −9.5), induced by sediments riverine input which are characterised by unradiogenic εNd (around −11). Finally, as significant variations are observed in seawater εNd at mid-depth between the SCS and the Philippine Sea, εNd values could be used in the future to track intermediate water mass exchange in the Luzon Strait, a process which, at present, remains poorly understood.

Reconstruction of the Nd isotope composition of seawater on epicontinental seas: Testing the potential of Fe–Mn oxyhydroxide coatings on foraminifera tests for deep-time investigations

The Fe–Mn oxide fraction leached from deep-sea sediments has been increasingly used to reconstruct the Nd isotope composition of deep water masses, that can be used to track changes in oceanic circulation with a high temporal resolution. Application of this archive to reconstruct the Nd isotope composition of bottom seawater in shallow shelf environments remained however to be tested. Yet as the Nd isotope composition of seawater on continental margins is particularly sensitive to changes in erosional inputs, establishment of neritic seawater Nd isotope evolution around areas of deep water formation would be useful to discriminate the influence of changes in oceanic circulation and in isotopic composition of erosional inputs on the Nd isotope record of deep waters. The purpose of this study is to test the potential of Fe–Mn coatings leached from foraminifera tests to reconstruct the Nd isotope composition of seawater in shelf environments for deep-time intervals. Albian to Turonian samples from two different outcrops have been recovered, from the Paris Basin (Wissant section, northern France) and from the Western Interior Seaway (Hot Spring, South Dakota, USA), that were deposited in epicontinental seas. Rare Earth Element (REE) spectra enriched in middle REEs in the foraminifera leach at Wissant highlight the presence of Fe–Mn oxides. The similarity of the Nd isotopic signal of the Fe–Mn oxide fraction leached from foraminifera tests with that of fish teeth suggests that Fe–Mn oxides coating foraminifera can be good archives of shelf bottom seawater Nd isotopic composition. Inferred bottom shelf water Nd isotope compositions at Wissant range from −8.5 to −9.7 ε-units, about 1.5–2 ε-units higher than that of the contemporaneous local detrital fraction. At Hot Spring, linear REE spectra characterizing foraminifera leach may point to an absence of authigenic marine Fe–Mn oxide formation in this area during the Late Cenomanian–Early Turonian, consistent with dysoxic to anoxic conditions at Hot Spring, contemporaneous to an Oceanic Anoxic Event. The similarity of the Nd isotopic signal of the carbonate matrix of foraminifera with that of fish teeth suggests that it records the Nd isotope composition of bottom shelf seawater as well. Inferred bottom shelf water Nd isotope compositions at Hot Spring are quite radiogenic, between −7 and −6 ε-units, about 2.5–4 ε-units higher than that of the contemporaneous local detrital fraction. In contrast, in both sections Fe–Mn oxides leached directly from the decarbonated sediment tend to yield a less radiogenic Nd isotopic composition, typically between 0.2 and 0.8 ε-units lower, that is intermediate between that of fish teeth and of the detrital fraction. This suggests the contribution of pre-formed continental Fe–Mn oxides to the Nd isotopic signal, along with authigenic marine oxides, or a detrital contamination during leaching.

Oceanwide imprint of large tectonic and oceanic events on seawater Nd isotope composition in the Indian Ocean from 90 to 40 Ma

We have analyzed four sediment cores from the Southern Indian Ocean (ODP sites 757, 758, 1135 and 762) with high carbonate content, in order to reconstruct the neodymium isotopic composition (εNd) of ancient intermediate South Indian seawater from Late Cretaceous (90 Ma) to Early Eocene (40 Ma). The εNdvariations are highly consistent and exhibit reproducible patterns over a very large geographic area, confirming the seawater origin of the signal. Combining geochemical constraints with paleogeographic reconstructions, we highlight the respective roles of (1) large‐scale tectonic events, (2) continental weathering from surrounding Precambrian terrains (90–65 Ma), (3) oceanic circulation changes (50–40 Ma) and, possibly, (4) local volcanism of the ultra‐fast spreading South East Indian Ridge (SEIR) (60–50 Ma) on the Nd isotopic composition of South Indian seawater. Between 60 Ma and 50 Ma, the regional Nd isotopic variations closely mimic changes in SEIR spreading rate. We suggest that the Nd isotopic composition of seawater could be influenced by Nd of volcanic origin in the vicinity of ultra‐fast spreading ridges (>13 cm/yr). The India‐Asia collision closed the Equatorial Seaway between Asia and India and drastically changed oceanic circulation patterns in the Indian Ocean: warm and more radiogenic Pacific equatorial seawater was diverted to the South by the East Indian coast. A stronger mixing of this Pacific seawater with South Indian seawater would explain the rapid shift ofεNd from 50 Ma (−11) to 40 Ma (−8).

Modeling the distribution of Nd isotopes in the oceans using an ocean general circulation model

Nd isotopic compositions of authigenic phases in marine archives are increasingly used as a tracer of past changes in ocean circulation. Such applications assume that water masses are “tagged” with distinct Nd isotope compositions in source regions and that away from these regions the Nd isotope ratios observed in deep waters mainly reflect water mass mixing. However, the degree to which the simple assumption of “quasi-conservative mixing” reflects the complicated reality of general circulation in the oceans is not well understood. Here, we present numerical simulations of the Nd isotope distribution in seawater using an offline ocean general circulation model (OGCM), treating the Nd isotopic composition of seawater as a conservative tracer. Nd isotope data from modern surface waters are used to generate an interpolated map of Nd isotope compositions for the entire surface ocean. This map is treated as a fixed surface boundary condition that is propagated into the interior ocean by the advective–diffusive transport of the ocean model until a steady state is reached. This treatment of Nd isotopes was chosen because it mimics the behavior of well-known and well-understood conservative tracers, such as potential temperature and salinity. The model results for all non-surface grid points are then compared to published data. This simple approach produces Nd isotope estimates for North Atlantic Deep Water that are consistent with observations, but produces values lower than observed in the deep Pacific and Southern Oceans, indicating the need for a source of radiogenic Nd directly to the deep Pacific Ocean. Sensitivity tests show this to be a robust result. By fixing the return flow of the Pacific Ocean to the Southern Ocean with Nd isotope ratios that are consistent with modern observations, the model results are brought into close agreement with a majority of the available data. With the addition of a deep Pacific source of radiogenic Nd, more than half of the simulated values from the interior ocean fall within one ε-unit of the corresponding observations. Our results thus indicate that the deep Pacific Ocean requires a source of radiogenic Nd in addition to inputs to the surface ocean, and that when the deep waters of the North Atlantic and North Pacific are set with realistic values, a majority of intermediate areas are consistent with quasi-conservative mixing of these two end-members.

Records of the Nd isotope composition of seawater from the Bay of Bengal: Implications for the impact of Northern Hemisphere cooling on ITCZ movement

This study presents a record of planktonic foraminiferal neodymium isotopic gradients along a north–south transect in the Bay of Bengal during time slices of late Holocene and last glacial maximum (LGM) age, together with a record of planktonic foraminiferal variation in the northern Bay of Bengal (15°N) over the last 195 ky. In late Holocene core top planktonic foraminifera, the north–south ε Nd gradient rises from nonradiogenic values of − 12 at 20°N to − 10 at 5°N, in parallel with the modern surface salinity gradient controlled by discharge of Himalayan rivers in the northern Bay. During the LGM, ε Nd increased throughout the Bay, the contrast between northernmost and southernmost sites decreased, and maximum ε Nd values of − 6.5 occurred between 12 and 15°N. A small part of the shift to higher mean ε Nd throughout the Bay during the glacial may arise from a uniform increase in deposition of far-field dust from Arabian and Persian Gulf regions. However, the spatial pattern of ε Nd variation between LGM and late Holocene also suggests a shift from modern dominance of nonradiogenic Nd sources from the Ganges–Brahmaputra basin to LGM dominance of more radiogenic Nd sources from Arakan coastal rivers. Over the last 195 ky at 15°N, the most radiogenic ε Nd values of − 7 occur at glacial maxima and the most nonradiogenic values of − 11 occur during interglacials. ε Nd values are highly correlated with glacial interglacial variations in planktonic foraminiferal δ 18O. In this record, shifts in river sources from the more northerly Ganges–Brahmaputra watershed to the more southerly Arakan coastal river systems respond dominantly to Inter-Tropical Convergence Zone (ITCZ) movement driven by Northern Hemisphere cooling during 100 ky glacial/interglacial cycles, with a small component of variation on precessional timescales. The nonlinear correlation of ε Nd with ice volume suggests that ITCZ movement responds to aerial coverage of ice sheets and snow rather than to ice thickness and volume. These data add support to recent general circulation models of ITCZ response to Northern Hemisphere ice sheets, which simulate decreased glacial precipitation in the Ganges–Brahmaputra basin and increased glacial precipitation in the southern Arakan coastal basin.

The estuarine chemistry of rare earth elements: comparison of the Amazon, Fly, Sepik and the Gulf of Papua systems

A comparison of the Amazon, Fly/Gulf of Papua and Sepik River systems has shown that there are two distinct processes operating on dissolved rare earth elements (REE) in estuaries, large scale salt-induced coagulation in the low salinity region and small to extensive release in the mid to high salinity region. Fractionation of the REE occurs during both removal and release. The order of removal follows LREE≥MREE>HREE and the order of release follows HREE>MREE≥LREE where L, M and H refer to light, middle and heavy REE, respectively. Hence, estuarine reactions can modify the relative abundance of dissolved REE reaching the oceans. Operating in tandem, the removal and release lead to the preferential removal of the LREE and the preferential release of HREE during estuarine mixing. This combination, in turn, results in an ‘effective’ river water flux (river flux after modifications by estuarine chemistry) which is evolved toward the REE composition of seawater, which has a heavy REE-enriched composition when normalized against the REE composition of rivers or the Earth’s continental crust. The increase of dissolved REE in the mid to high salinity waters of the Amazon and Fly estuaries suggests a sediment and/or suspended sediment source. The lack of REE increases in the Sepik River estuary strengthens this explanation as the mixing zone of this estuary is located over a deep shelf where there is little physical contact between bottom sediment and estuarine waters. In marked contrast, the Amazon and Fly estuaries are dominated by the active resuspension and deposition of bottom sediments. The resupply of REE, accompanied by fractionation, may result from REE–carbonate complexation during the reaction of seawater with suspended particles and/or bottom sediment. Estuarine and shelf sediments may be important sources of dissolved Nd to the oceans, thereby helping to maintain the inter-ocean differences in the Nd isotopic composition of seawater by reducing the residence time of Nd in the oceans.

Evidence for Decreased NADW Flux During Glacial Stages 2 and 4 from Nd Isotopes in Southern Ocean Sediments

It has been suggested that North Atlantic Deep Water (NADW) production was reduced during the Last Glacial Maximum thereby altering the distribution of heat around the globe, and the partitioning of CO2 between the ocean and atmosphere (Broecker and Denton, 1989). Although the thermohaline circulation concept has great appeal in explaining much of the evidence for climate changes that have taken place in the past, currently employed proxies have produced conflicting results regarding the changes in NADW production in the late Quaternary. In the Southern Oceart ~5C measurements on benthic foraminifera have been interpreted to indicate a switching off of NADW during glacial times. However, this interpretation is in conflict with interpretations based on Cd/Ca measurements of benthic foraminifera (Boyle, 1992). In support of the Cd/Ca results, 23 ipaj230Th data from core top and late glacial marine sediments have been interpreted to indicate that export of deep water formed in the North Atlantic was similar to modem conditions (Yu et al., 1996). The conflicting interpretations based on commonly employed proxies highlights the need for new tracers. We report here on work in progress to validate and apply the Nd isotope composition of marine authigenic precipitates as a proxy for variation in deep water mass composition. The Nd isotope composition of deep seawater varies systematically with water mass properties. The unradiogenic character of Nd in the North Atlantic reflects the ancient shield rocks surrounding the area of NADW formation. The very young rocks surrounding the Pacific Ocean impart a more radiogenic signature to Pacific waters. In the Atlantic Ocean the Nd isotope composition of today's deep water appears to be the product of mixing of Southern Component Waters and Northern Component waters. The observation that the Nd isotope composition of present-day water conforms to water mass properties is an important line of evidence in evaluating the potential for Nd isotopes as a tracer of water mass mixing in the past. However, it is also necessary that there be some extractable phase that sequesters seawater Nd in marine sediment in order to make use of this observation. Published work has demonstrated that ferromanganese precipitates, specifically crusts and nodules, generally record the approximate eNd of the bottom water mass that bathes them, with only a few exceptions (e.g. Albarbde and Goldstein, 1992). Although ferromanganese crusts have been demonstrated to provide a reliable measure of bottom water Nd composition, and recent studies have used these substrates to examine changes in seawater composition over time, crusts are limited in their availability and growth rate, generally integrating over l0 thousand to 1 million years per subsample. Accordingly, we are in the process of testing the application of leaching methods to extract the ferromanganese phase from marine sediments in order to examine secular variation in Nd isotope composition of seawater at glacial to interglacial timescales. A preliminary test of leaches from a suite of core tops has yielded positive results that are entirely overlapping with seawater measurements. Core RCll-83 (41.36~ 9.48~ 4718 m) is located in the Cape Basin of the southeastern Atlantic Ocean. Extensive stable isotope data have been reported for this core (Charles et al., 1996). Charles et al. (1996) have interpreted the large variations in the ~5C to reflect a reduced contribution of NADW to this region during Stage 2 and Stage 4. RC11-83 also records a significant glacial to interglacial contrast in the Nd isotopic composition of the ferromanganese component (Fig. 1). A core top sample and a sample from isotope Stage 3 provide Nd isotope ratios similar to nearby water measurements. In contrast, values much closer to those of the modem Pacific are found in two samples measured from Stage 2. Two measurements come from samples that were taken from transitional intervals based on stable isotope measurements, and these samples also provide intermediate Nd isotope values.

Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents

The isotopic composition of Nd is uniform in metalliferous sediments formed at distances varying from >1000 km to within 10 km of the East Pacific Rise (EPR) palaeoridge. These data indicate that hydrothermal vent fluids, despite having concentrations more than 500 times greater, have no effect on the Nd isotopic composition of seawater. This implicates efficient scavenging of hydrothermal Nd by particulates, resulting in extremely short residence times (< 1 year) close to the hydrothermal vents. Therefore Nd isotopic studies of ancient seawater precipitates, particularly metalliferous sediments, cannot be used to delimit the magnitude of past hydrothermal circulation without independent constraints on local scavenging rates (or residence times) relative to those of modern oceans.

The isotopic composition of Nd in different ocean masses

Sm-Nd data for authigenic ferromanganese sediments from the oceans indicate that the Atlantic, Pacific, and Indian Oceans each have a distinct range in Nd isotopic composition which are characteristic of each ocean basin and reflect the dissolved load of Nd in the water mass. Within each basin, Nd isotopic compositions show some variability but are relatively well defined. Isotopic compositions of Nd measured in these samples are all far less than the 143Nd/ 144Nd ratios of source rocks with oceanic affinities. Direct measurements of the Nd isotopic composition of seawater presented here support the view that REE in ferromanganese sediments are derived by the direct precipitation of these elements out of seawater. Nd isotopic variations in ferromanganese sediments cannot be explained by contributions from continental detritus. It is therefore believed that the Nd isotopic variations found for ferromanganese sediments represent true variations in the isotopic composition of Nd dissolved in seawater in various ocean masses. These variations reflect primarily the age and 147Sm/ 144Nd of the continental masses being sampled, which is believed to be the major source of REE in seawater. These variations indicate that the residence time of Nd in seawater must be very short relative to the mixing rates between ocean masses. Nd isotopic studies, both in seawater and sediments should, therefore, be useful as a monitor of ocean currents and interocean mixing over the past several million years.