Article

Reconstruction of intermediate water circulation in the tropical North Atlantic during the past 22,000 years

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Abstract

Decades of paleoceanographic studies have reconstructed a well-resolved water mass structure for the deep Atlantic Ocean during the Last Glacial Maximum (LGM). However, the variability of intermediate water circulation in the tropics over the LGM and deglacial abrupt climate events is still largely debated. This study aims to reconstruct intermediate northern- and southern-sourced water circulation in the tropical North Atlantic during the past 22 kyr and attempts to confine the boundary between Antarctic Intermediate Water (AAIW) and northern-sourced intermediate water (i.e., upper North Atlantic Deep Water (NADW) or Glacial North Atlantic Intermediate Water) in the past. High-resolution Nd isotopic compositions of fish debris and acid-reductive leachate of bulk sediment in core VM12-107 (1079 m depth) from the Southern Caribbean are not in agreement. We suggest that the leachate method does not reliably extract the Nd isotopic compositions of seawater at this location, and that it needs to be tested in more detail in various oceanic settings. The fish debris εNd values display a general decrease from the early deglaciation to the end of the Younger Dryas, followed by a greater drop toward less radiogenic values into the early Holocene. We propose a potentially more radiogenic glacial northern endmember water mass and interpret this pattern as recording a recovery of the upper NADW during the last deglaciation. Comparing our new fish debris Nd isotope data to authigenic Nd isotope studies in the Florida Straits (546 m and 751 m depth), we propose that both glacial and deglacial AAIW do not penetrate beyond the lower depth limit of modern AAIW in the tropical Atlantic.

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... The prevalence of GNAIW or CPDW sourced AAIW is also consistent with nearby foraminifera/fish debris Nd isotope data from intermediate water depths ( typical for modern AAIW in the tropical W-Atlantic (Huang et al., 2014;Osborne et al., 2014). Therefore, although the decrease in εNd from a GNAIW like signature of −9.7 (Gutjahr et al., 2008) in the LGM to −11.4 in the early Holocene can be explained by a change from GNAIW to NADW (Xie et al., 2014), these changes could also be explained by variability of εNd within AAIW. A change in the εNd signature of AAIW is consistent with a change of source waters feeding AAIW. ...
... With GNAIW as the main source for AAIW during the LGM and a combination of NADW and CPDW today as suggested from proxy data and modelling by Talley (2013) and Ferrari et al. (2014), this change in source waters would have altered the AAIW εNd signature accordingly. The offsets between the two εNd records (Huang et al., 2014;Xie et al., 2014) from the Cd w and δ 13 C reconstructions (following Talley, 2013 andFerrari et al., 2014). Water masses and flow are indicated by arrows. ...
... Relatively positive benthic δ 13 C values imply a considerable contribution from northern sources. further confirmed suggestions of Xie et al. (2014) that the reliability of different techniques to extract the seawater εNd near ocean margins needs to be tested and confirmed for each location (Howe et al., 2016). Sedimentary exchange process may thus be an important cause of the divergence of εNd time series from the W-Atlantic (Pahnke et al., 2008;Xie et al., 2012Xie et al., , 2014Huang et al., 2014). ...
Article
As part of the return flow of the Atlantic overturning circulation, Antarctic Intermediate Water (AAIW) redistributes heat, salt, CO2 and nutrients from the Southern Ocean to the tropical Atlantic and thus plays a key role in ocean–atmosphere exchange. It feeds (sub)tropical upwelling linking high and low latitude ocean biogeochemistry but the dynamics of AAIW during the last deglaciation remain poorly constrained. We present new multi-decadal benthic foraminiferal Cd/Ca and stable carbon isotope ( ) records from tropical W-Atlantic sediment cores indicating abrupt deglacial nutrient enrichment of AAIW as a consequence of enhanced deglacial Southern Ocean upwelling intensity. This is the first clear evidence from the intermediate depth tropical W-Atlantic that the deglacial reconnection of shallow and deep Atlantic overturning cells effectively altered the AAIW nutrient budget and its geochemical signature. The rapid nutrient injection via AAIW likely fed temporary low latitude productivity, thereby dampening the deglacial rise of atmospheric CO2.
... Some records indicate a greater presence of AAIW in the Atlantic during abrupt Northern Hemisphere cold events [Pahnke et al., 2008], while others suggest the exact opposite [Xie et al., 2012;Huang et al., 2014]. These interpretations are complicated by the fact that some of the records consist of leachate measurements [Pahnke et al., 2008;Xie et al., 2012], which are susceptible to detrital contamination [Elmore et al., 2011;Wilson et al., 2013], or come from enclosed basins [Xie et al., 2012[Xie et al., , 2014 where the seawater has been shown to be subject to modification by boundary exchange [Osborne et al., 2014]. At least one study utilized cores from below the main flow of AAIW [Pahnke et al., 2008]. ...
... Furthermore, this contamination must have come from a subcomponent of the detrital material being preferentially leached as the bulk detrital ε Nd values are less radiogenic than the foraminiferal values ( Figure 3). This pattern of leachates being more radiogenic than foraminiferal or fish debris ε Nd values but bulk detrital values being less radiogenic has also been reported for intermediate-depth cores on the Demerara Rise further north in the Atlantic [Huang et al., 2014] and in the southern Caribbean Sea [Xie et al., 2014]. It is also clear that the offset between leachates and foraminifera is not constant through time (Figure 3), and therefore, a simple correction cannot be applied to the leachate results to convert them to seawater ε Nd values. ...
... The stability of the foraminiferal ε Nd records reveals that the deglacial peaks in the leachate record from KNR 159-5-36GGC (Figure 3) are not due to changes in AAIW penetration into the South Atlantic [Pahnke et al., 2008]. These observations support the assertion that decarbonated leachate ε Nd records should be verified by other authigenic phases such as foraminifera or fish debris before being interpreted as a hydrographic signal Xie et al., 2014] and indicate that unverified leachate records should no longer be included in studies that model changes in ε Nd of Atlantic seawater during millennial-scale climate events [Friedrich et al., 2014]. ...
Article
Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.
... However, the low temporal resolution of Nd isotope data (tens of thousands of years per sample) and high chronological uncertainty of ferromanganese crust samples 13 and the poor temporal coverage of deep-sea coral samples, notably their absence during the LGM in the western North Atlantic 15 , raise questions about the inferred endmember stability, especially for the LGM interval. Previous studies have hypothesized that the North Atlantic ε Nd endmember was more radiogenic during the LGM 16,17 , but so far there is no reliable record that demonstrates the glacial-interglacial evolution of this endmember. ...
... The compilation of ε Nd suggests that the deep Atlantic sector of the Southern Ocean was strongly stratified, implying a very dense AABW that was isolated from the overlying Circumpolar Deep Water during the LGM (Fig. 5a), similar to the Pacific sector 52 . Our results showing similar values in the shallow tropical Atlantic and at our North Atlantic core site also suggest a significantly weaker AAIW intrusion during the LGM ( Supplementary Fig. 7), consistent with previous studies 16,53 . ...
Article
Full-text available
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. The Nd isotope composition of seawater has been used to reconstruct past changes in the various contributions of different water masses to the deep ocean, with the isotope signatures of endmember water masses generally assumed to have been stable during the Quaternary. Here, the authors show that deep water produced in the North Atlantic had a significantly more radiogenic Nd signature during the Last Glacial Maximum compared to today.
... A recent study directly modeling changes in the εNd of AAIW in the study region during the deglaciation ( Gu et al., 2017) reproduces an existing εNd reconstruction from the Bonaire Basin (Core VM12-107; Xie et al., 2014) and our new εNd record from Tobago Basin Core 235 (Figures 1 and 3). The variability of these data was previously ascribed to a gradual change from GNAIW-dominated admixture during the LGM to NADW-dominated admixture during the Holocene, but Gu et al. (2017) suggested a significant influence from the enhanced upwelling of radiogenic Caribbean deep waters during the LGM, which then declined during the deglaciation. ...
... Nutrient and ventilation reconstructions from the tropical W-Atlantic may suggest a change in the composi- tion of water masses forming AAIW due to Southern Ocean upwelling changes ( Poggemann et al., 2017). However, the similarity of the εNd signatures of GNAIW and CPDW during the last glacial ( Gutjahr et al., 2008) and the alteration of the AAIW εNd signature via mixing on its pathway into the N-Atlantic mean that large changes in the εNd of AAIW are not to be expected ( Xie et al., 2014). Assuming a more radiogenic εNd signature of GNAIW near À10 ( Gutjahr et al., 2008), the glacial εNd values of ~À9.7 in both records are con- sistent with the notion of GNAIW being the main source water mass for AAIW during full glacial times (e.g., Paleoceanography and Paleoclimatology Osborne et al., 2014) means that the Nd isotope records cannot provide unambiguous or quan- titative information on changes in the mixture of AAIW at its source region or its volume flow. ...
Article
Antarctic Intermediate Water (AAIW) is an important conduit for nutrients to reach the nutrient-poor low-latitude ocean areas. In the Atlantic, it forms part of the return path of the Atlantic Meridional Overturning Circulation (AMOC). Despite the importance of AAIW, little is known about variations in its composition and signature during the prominent AMOC and climate changes of the last deglaciation. Here we reconstruct benthic foraminiferal Mg/Ca-based intermediate water temperatures (IWTMg/Ca) and intermediate water neodymium (Nd) isotope compositions at submillennial resolution from unique sediment cores located at the northern tip of modern AAIW extent in the tropical W-Atlantic (850- and 1018-m water depth). Our data indicate a pronounced warming of AAIW in the tropical W-Atlantic during Heinrich Stadial 1 and the Younger Dryas. We argue that these warming events were induced by major AMOC perturbations resulting in the pronounced accumulation of heat in the surface Southern Ocean. Combined with published results, our data suggest the subsequent uptake of Southern Ocean heat by AAIW and its rapid northward transfer to the tropical W-Atlantic. Hence, the rapid deglacial northern climate perturbations directly controlled the AAIW heat budget in the tropical W-Atlantic after a detour via the Southern Ocean. We speculate that the ocean heat redistribution via AAIW effectively dampened Southern Hemisphere warming during the deglaciation and may therefore have been a crucial player in the climate seesaw mechanisms between the two hemispheres.
... The magnitude and spatial extent of tropical Atlantic middepth warming and its temporal and causal relationship to the middepth warming in the North Atlantic is, however, not well established. Most of the middepth oceanographic reconstructions in the tropical Atlantic are limited to the western part of the basin and quantitative reconstruction of middepth temperature is very limited [Came et al., 2007[Came et al., , 2008Freeman et al., 2015;Huang et al., 2014;Lund et al., 2015;Lynch-Stieglitz et al., 2014;Mangini et al., 2010;Oppo et al., 2015;Rühlemann et al., 2004;Schmidt et al., 2012;Sortor and Lund, 2011;Xie et al., 2012Xie et al., , 2014. Here we focus on the middepth of the eastern equatorial Atlantic (EEA) and examine last deglacial oceanographic changes using sensitive Mg/Ca paleothermometry and benthic foraminiferal Ba/Ca, δ 13 C, and Δ 14 C reconstructions. ...
... Linking the very old middepth North Atlantic water to advection of Southern Ocean-sourced middepth water is at odds with our results from the middepth of tropical Atlantic that serves as a conduit for the AAIW. The upper part of northern tropical Atlantic middepth (≤1000 m water depth) appears to be marked by a strong influence of relatively young North Atlantic water [Came et al., 2008;Huang et al., 2014;Xie et al., 2012Xie et al., , 2014. Consistent with previous studies Cléroux et al., 2011;Freeman et al., 2015;Lund et al., 2015;Mangini et al., 2010;Sortor and Lund, 2011] our results reveal the absence of a radiocarbon-depleted middepth water in the tropical Atlantic during H1 (Figure 7). ...
Article
Full-text available
We present a benthic foraminiferal multi-proxy record of eastern equatorial Atlantic (EEA) mid depth water (1295 m) covering the last deglacial. We show that EEA mid depth water temperatures were elevated by 3.9 ± 0.5 °C and 5.2 ± 1.2 °C during Heinrich event 1 (H1) and Younger Dryas (YD), respectively. The radiocarbon content of the EEA mid depth during H1 and YD is relatively low and comparable to the values of the pre-H1 episode and Bølling-Allerød, respectively. A transient earth system model simulation, which mimics the observed deglacial AMOC history, qualitatively reproduces the major features of the EEA proxy records. The simulation results suggest that fresh water-induced weakening of the AMOC leads to a vertical shift of the horizon of Southern Ocean-sourced water and a stronger influence of EEA sea surface temperatures via mixing. Our findings reaffirm the lack of a distinctive signature of radiocarbon depletion and therefore do not support the notion of interhemispheric exchanges of strongly radiocarbon-depleted mid-depth water across the tropical Atlantic during H1 and YD. Our temperature reconstruction presents a critical zonal and water depth extension of existing tropical Atlantic data and documents a large-scale and basin-wide warming across the thermocline and mid-depth of the tropical Atlantic during H1 and YD. Significant difference in the timing and pace of H1 mid-depth warming between tropical Atlantic and North Atlantic likely points to a limited role of the tropical Atlantic mid-depth warming in the rapid heat build up in the North Atlantic mid depth.
... This is supported by studies based on the ε Nd signatures of the authigenic fraction of marine sediments and of unclean foraminifera (Böhm et al., 2015;Gutjahr et al., 2008;Howe, Piotrowski, & Rennie, 2016;Lippold et al., 2016;Pahnke et al., 2008;Roberts et al., 2010). However, a growing number of studies on ε Nd signatures of the authigenic fraction of marine sediments and foraminifera suggest reduced presence of SSW in the North Atlantic and a much larger fraction of Northern Source Water (NSW) in the intermediate and deep tropical North Atlantic Ocean during the LGM and deglaciation (Casme et al., 2003;Howe, Piotrowski, Noble, et al., 2016;Howe, Piotrowski, Oppo, et al., 2016, Howe et al., 2018Huang et al., 2014;Pöppelmeier et al., 2020;Xie et al., 2012Xie et al., , 2014Zhao et al., 2019). Recently published records of distinctly unradiogenic ɛ Nd signatures prevailing during the early Holocene in areas within and outside of the Labrador Sea Howe, Piotrowski, & Rennie, 2016;Jaume-Segui et al., 2020;Pöppelmeier et al., 2018) were interpreted to result from either enhanced production of LSW (Roberts et al., 2010), enhanced partial dissolution of poorly chemically weathered detrital material deposited in the Labrador Sea following Laurentide Ice Sheet retreat Howe, Piotrowski, & Rennie, 2016;Pöppelmeier et al., 2018), or the admixture of a particularly dense water mass with highly unradiogenic Nd isotope composition to the abyssal Yellow dots indicate locations of surface sediment samples from this study. ...
Article
Full-text available
Limited constraints on the variability of the deep‐water production in the Labrador Sea complicate reconstructions of the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Quaternary. Large volumes of detrital carbonates were repeatedly deposited in the Labrador Sea during the last 32 kyr, potentially affecting radiogenic Nd isotope signatures. To investigate this the Nd isotope compositions of deep and intermediate waters were extracted from the authigenic Fe‐Mn oxyhydroxide fraction, foraminiferal coatings, the residual silicates and leachates of dolostone grains. We provide a first order estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea. During the Last Glacial Maximum the Nd isotope signatures in the Labrador Sea would allow active water mass mixing with more radiogenic ɛNd values (−12.6 and −14) prevailing in its eastern part whereas less radiogenic values (ɛNd ∼ −18.4) were found on the western Labrador slope. The deposition of detrital carbonates during Heinrich stadials (2,1) was accompanied by negative detrital and authigenic Nd isotope excursions (ɛNd ∼ −31) that were likely controlled by dissolution of dolostone or dolostone associated mineral inclusions. This highly unradiogenic signal dominated the authigenic phases and individual water masses in the Labrador Sea, serving as potential source of highly unradiogenic Nd to the North Atlantic region, while exported southward. The Holocene authigenic ɛNd signatures of the coatings and leachates significantly differed from those of the detrital silicates, approaching modern bottom water mass signatures during the Late Holocene.
... This has resulted in the exploitation of Nd isotopes as a geochemical tracer of ocean circulation in the past and present (e.g. Lambelet et al., 2016;Lacan and Jeandel, 2005a;Lippold et al., 2016;Piotrowski et al., 2012;van de Flierdt and Frank, 2010;Frank, 2002;Gutjahr et al., 2008;Piotrowski et 35 al., 2004;Roberts et al., 2010;Howe et al., 2017;Basak et al., 2015;Dausmann et al., 2017;Hu and Piotrowski, 2018;Jonkers et al., 2015;Wilson et al., 2015;Xie et al., 2014;Pöppelmeier et al., 2020bPöppelmeier et al., , 2022Blaser et al., 2019b;Goldstein and Hemming, 2003). With specific regard to the radiogenic properties of this rare earth element, its isotope composition is typically expressed in epsilon (εNd) units, where εNd=[( 143 Nd/ 144 Nd)sample/( 143 Nd/ 144 Nd)CHUR-1] × 10 4 , thus describing the parts per 10,000 deviation of a sample from the chondritic uniform reservoir (CHUR) (Jacobsen and Wasserburg, 1980). ...
Preprint
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The neodymium (Nd) isotope composition (εNd) of seawater can be used to trace large-scale ocean circulation features. Yet, due to the elusive nature of marine Nd cycling, particularly in discerning non-conservative particle-seawater interactions, there remains considerable uncertainty surrounding a complete description of marine Nd budgets. Here, we present an optimisation of the Nd isotope scheme within the fast coupled atmosphere-ocean general circulation model (FAMOUS), using a statistical emulator to explore the parametric uncertainty and optimal combinations of three key model inputs relating to: (1) the efficiency of reversible scavenging, (2) the magnitude of the seafloor benthic flux, and (3) a riverine source scaling, accounting for release of Nd from river sourced particulate material. Furthermore, a suite of sensitivity tests provide insight on the regional mobilisation and spatial extent (i.e., testing a margin-constrained versus a seafloor-wide benthic flux) of certain reactive sediment components. In the calibrated scheme, the global marine Nd inventory totals 4.27 × 1012 g and has a mean residence time of 727 years. Atlantic Nd isotope distributions are represented well, and the weak sensitivity of North Atlantic Deep Water to highly unradiogenic sedimentary sources implies an abyssal benthic flux is of secondary importance in determining the water mass εNd properties under the modern vigorous circulation condition. On the other hand, Nd isotope distributions in the North Pacific are 3 to 4 εNd-units too unradiogenic compared to water measurements, and our simulations indicate that a spatially uniform flux of bulk sediment εNd does not sufficiently capture the mobile sediment components interacting with seawater. Our results of sensitivity tests suggest that there are distinct regional differences in how modern seawater acquires its εNd signal, in part relating to the complex interplay of Nd addition and water advection.
... Literature data are plotted against their originally published age models. 111,112 ]. Grey and light blue shadings and blue circles are defined in Extended Data Fig. 7. εNd at 26JPC was well within the range of NSW values (grey shading) during HS1, but shifted towards SSW values (light blue shading) at the Bølling onset. ...
Article
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For its greenhouse effects, atmospheric CO2 can critically influence the global climate on millennial and centennial timescales. Pleistocene atmospheric CO2 variations must involve changes in ocean storage of carbon, but the mechanisms and pathways of carbon transfer between the oceanic and atmospheric reservoirs are poorly understood due, in part, to complications associated with interpretation of carbonate system proxy data. Here we employ a recently developed approach to reconstruct upper Atlantic air–sea CO2 exchange signatures through the last deglaciation. Using this approach, proxy and model data each suggest that there was a net release of CO2 via the Atlantic sector of the Southern Ocean during the early deglaciation, which probably contributed to the millennial-scale atmospheric CO2 rise during Heinrich Stadial 1 at ~18.0–14.7 kyr ago. Moreover, our data reveal a previously unrecognized mechanism for the centennial-scale atmospheric CO2 rise at the onset of the Bølling warming event around 14.7 kyr ago, namely, the expansion of Antarctic Intermediate Water, a water mass that is especially inefficient at sequestering atmospheric CO2. Our findings highlight the role of the Southern Ocean outgassing and intermediate water-mass production and volume variations in governing millennial- and centennial-timescale atmospheric CO2 rises during the last deglaciation. Expansions of Antarctic Intermediate Water can help explain centennial-scale atmospheric CO2 highs during the last deglaciation, according to a reconstruction of the marine carbonate system in the Southern Ocean.
... Thus, water column ε Nd measured in-situ for the present day or in sedimentary/seafloor archives for the past has been observed to track basin-scale ocean transports such as meridional overturning circulation (e.g. Basak et al., 2015;Dausmann et al., 2017;Howe et al., 2016;Hu and Piotrowski, 2018;Jonkers et al., 2015;Pöppelmeier et al., 2020aPöppelmeier et al., , 2020bRoberts and Piotrowski, 2015;Wilson et al., 2015;Xie et al., 2014) and to reconstruct marine gateway events (Horikawa et al., 2010;Khélifi et al., 2014;Martin and Scher, 2006;Newkirk and Martin, 2009;Scher and Martin, 2006;Sepulchre et al., 2014;Stumpf et al., 2015;Stumpf et al., 2010). Such interpretations of ε Nd rely on the assumption that in the absence of local lithogenic input, ε Nd behaves conservatively. ...
Article
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Understanding the role of sediment-water interactions in the oceanic cycling of neodymium (Nd) isotopes is essential for its reliable use as a modern and palaeoceanographic tracer of ocean circulation. However, the exact processes that control Nd cycling in the ocean are poorly defined and require an up-to-date knowledge of the sources, sinks and transformation of this tracer to and within the ocean (e.g. as per the GEOTRACES core mission). We propose a considerable improvement of Nd-source identification by providing an extensive and up-to-date compilation of published terrestrial and marine sedimentary Nd isotopic measurements. From this database, we construct high resolution, gridded, global maps that characterise the Nd-isotopic signature of the continental margins and seafloor sediment. Here, we present the database, interpolation methods, and final data products. Consistent with the previous studies that inform our compilation, our global results show unradiogenic detrital Nd isotopic values (εNd ≈ -20) in the North Atlantic, εNd values of ≈ -12 to -7 in the Indian and Southern Ocean, and radiogenic values (εNd ≈ -3 to +4) in the Pacific. The new, high-resolution interpolation is useful for improving conceptual knowledge of Nd sources and sinks and enables the application of isotope-enabled ocean models to understand targeted Nd behaviour in the oceans. Such applications may include: examining the strength and distribution of a possible benthic flux required to reconcile global Nd budgets, establishing the potential use of Nd isotopes as a kinematic tracer of ocean circulation, and a general quantification of the non-conservative sedimentary processes that may contribute to marine Nd cycling.
... In the Equatorial and South Atlantic at~1000 m, we find negative deglacial ε Nd anomalies (Fig. 9B) in the same region as in the LGM (Fig. 9A). This pattern again may be explained by reduced AAIW contribution (Howe et al., 2016b;Huang et al., 2014;Poggemann et al., 2018;Xie et al., 2014). We can perform a mixing calculation to quantify this factor as done for the LGM (section 5.1.2). ...
Article
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The Global Overturning Circulation is linked to climate change on glacial-interglacial and multi-millennial timescales. The understanding of past climate-circulation links remains hindered by apparent conflicts among proxy measures of circulation. Here we reconstruct circulation changes since the Last Glacial Maximum (LGM) based on a global synthesis of authigenic neodymium isotope records (εNd). We propose the bottom-up framework of interpreting seawater and authigenic εNd considering not only conservative watermass mixing, but also the preformed properties and the non-conservative behavior of εNd , both subject to sedimentary influences. We extract the major spatial-temporal modes of authigenic εNd using Principal Component Analysis, and make a first-order circulation reconstruction with budget-constrained box model simulations. We show that during the LGM, the source region of North Atlantic overturning shifted southward, which led to more radiogenic preformed εNd of glacial Northern Source Water (NSW). Considering this preformed effect, we infer that glacial deep Atlantic had a similar proportion of NSW as today, although the overall strength of glacial circulation appears reduced from both North Atlantic and Southern Ocean sources, which increased the relative importance of non-conservative behavior of εNd and may have facilitated accumulation of respired carbon in the deep ocean. During the deglaciation, we find that Southern Ocean overturning increased, which offset suppressed North Atlantic overturning and resulted in a net stronger global abyssal circulation. Faster global scale deglacial circulation reduced the relative importance of non-conservative effects, resulting in Atlantic-Pacific convergence of abyssal εNd signatures. Variations of Southern Ocean overturning likely drove a significant fraction of deglacial changes in atmospheric CO2 and oceanic heat budget.
... Several studies used Nd isotopes to reconstruct the history of AAIW over the past 30 ka (e.g. Howe et al., 2016b;Huang et al., 2014;Poggemann et al., 2018;Xie et al., 2014) with conflicting conclusions regarding the extent of GAAIW during the last ice age. If the scenario we proposed here, namely reduced GAAIW end member Nd concentration and a more radiogenic signature, is correct, it would have substantial impact on the interpretation of AAIW-depth Nd isotope records. ...
Article
Antarctic Intermediate Water (AAIW) plays a central role in the Atlantic Meridional Overturning Circulation (AMOC) as the return flow of Northern Sourced Water (NSW) and is therefore of significant importance for the global climate. Past variations of the boundary between AAIW and NSW have been extensively investigated, yet available results documenting the prevailing depth of this boundary and the southern extent of NSW during the last ice age remain ambiguous. Here, we present five new timeseries focusing on the authigenic neodymium isotope signal in sediment cores retrieved from the Southwest Atlantic covering the past 25,000 years. The sites are situated along the southern Brazil Margin and form a bathymetric transect ranging between 1000 and 3000 m water depth, encompassing the modern water mass boundaries of AAIW and NSW and therefore allow their reconstruction since the Last Glacial Maximum (LGM). The new Nd isotope records show little change between the LGM and early deglaciation as well as relatively homogeneous values over the full depth range of the cores during these times. These results strongly contrast with epibenthic foraminiferal stable carbon isotope records (d13C) from the same sites which exhibit highest glacial values at mid-depths, presumably related to NSW mixing into southern sourced water. We propose that the discrepancy between these two independent water mass proxies is partly related to changes in Nd end member properties of glacial AAIW. The combination of elevated glacial dust fluxes and, as a result, sustained export productivity caused high sinking particle flux in the western South Atlantic, where AAIW is forming. Higher particle flux would have increased the removal (scavenging) of Nd from shallow waters thus reducing the Nd concentration and overprinting the isotopic signature of the glacial AAIW end member. Only under consideration of changes in Nd end member properties along with non-conservative processes such as remineralization of organic matter influencing past seawater d13C can we reconcile the water mass reconstructions from both proxies.
... The results suggest a similar decrease in the fraction of AAIW at the Demerara Rise (~8°N), contrasting with neodymium isotope data, which at face value suggest a higher fraction of AAIW in the shallow tropics during the LGM (Huang et al., 2014). However, interpretation of neodymium isotopes at this location may be complicated by possible end-member changes (Huang et al., 2014;Xie et al., 2014), boundary exchange (e.g., Howe et al., 2018), and/or by changes in the contribution of the radiogenic signature from the Caribbean Sea (Gu et al., 2017). ...
Article
The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end-member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air-sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high-nutrient AAIW with nutrient-depleted, carbonate-rich waters sourced from the region of the modern-day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.
... We propose that whilst there may be an increase in AABW formation coincident with the increased ventilation of the deep Southern Ocean and South Atlantic (Burke and Robinson, 2012;Skinner et al., 2010) that the same may not be true for AAIW. Although still debated (e.g. the export of AAIW to the intermediate depths of the Caribbean Sea was strong during late HS1, Poggemann et al., 2017;Valley et al., 2017), there is increasing evidence that AAIW did not penetrate significantly further north in the Atlantic during HS1 (Howe et al., 2016a;Huang et al., 2014;Xie et al., 2014). This evidence is consistent with a recent deglacial transient simulation performed with a neodymium-enabled ocean model (Gu et al., 2017). ...
Article
The delivery of freshwater to the North Atlantic during Heinrich Stadial 1 (HS1) is thought to have fundamentally altered the operation of Atlantic meridional overturning circulation (AMOC). Although benthic foraminiferal carbon isotope records from the mid-depth Atlantic show a pronounced excursion to lower values during HS1, whether these shifts correspond to changes in water mass proportions, advection, or shifts in the carbon cycle remains unclear. Here we present new deglacial records of authigenic neodymium isotopes – a water mass tracer that is independent of the carbon cycle – from two cores in the mid-depth South Atlantic. We find no change in neodymium isotopic composition, and thus water mass proportions, between the Last Glacial Maximum (LGM) and HS1, despite large decreases in carbon isotope values at the onset of HS1 in the same cores. We suggest that the excursions of carbon isotopes to lower values were likely caused by the accumulation of respired organic matter due to slow overturning circulation, rather than to increased southern-sourced water, as typically assumed. The finding that there was little change in water mass provenance in the mid-depth South Atlantic between the LGM and HS1, despite decreased overturning, suggests that the rate of production of mid-depth southern-sourced water mass decreased in concert with decreased production of northern-sourced intermediate water at the onset of HS1. Consequently, we propose that even drastic changes in the strength of AMOC need not cause a significant change in South Atlantic mid-depth water mass proportions.
... Dense SOW may spread isopycnally into the northern Hemisphere from the deep Southern Ocean with δ 13 C value of <−0.2% (Curry and Oppo 2005). There is also evidences that SOW may penetrate into the deep North Atlantic as far as 60°N effecting water shallower than 2000 m (Curry and Oppo 2005;Rickaby and Elderfield 2005;Praetorius et al. 2008;Xie et al. 2014). It has generally been observed that low benthic δ 13 C resulting from mixing with southern sourced water mass occurred only during glacial and deglacial intervals Keigwin 1982, Oppo andFairbanks 1987;Curry et al. 1988;Raymo et al. 1990;McManus et al. 1999;Flower et al. 2000;Oppo et al. 2015). ...
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Foraminifera abundance and stable isotope records from ODP Site 984 (61.25°N, 24.04°W, 1648 m) in the North Atlantic are used to reconstruct surface circulation variations and the relative strength of the North Atlantic Deep Water (NADW) formation over the period spanning the peak warmth of Marine Interglacial Stage (MIS) 9e (~324–336 ka). This interval includes the preceding deglaciation, Termination 4 (T4), and the subsequent glacial inception of MIS 9d. The records indicate a greatly reduced contribution of NADW during T4, as observed in more recent deglaciations. In contrast with the most recent deglaciation, the lack of a significant NADW signal extended from T4 well into the peak interglacial MIS 9e and persisted nearly until the transition to the subsequent glacial stage MIS 9d. Although NADW formation resumed during MIS 9e, only depths greater than 2000 m appear to have been ventilated. The poorly ventilated intermediate depth of Site 984 (<2000 m) may have resulted on one hand from a general reduction of deep water ventilation by NADW during the study interval or, on the other hand, from different pathways of the spread of newly formed NADW that bypassed the study location. The intermediate depths may have also been invaded by southern-sourced waters as the formation of intermediate depth NADW weakened. The absence of any significant NADW signal at the water depth of Site 984 during the climatic optimum contrasts sharply with subsequent interglacial peaks (MIS 5e and the Holocene). Despite the perturbed intermediate depth circulation, oceanic heat transport northeastward was not interrupted and may have contributed to the relatively mild interglacial conditions of MIS 9e.
... Reconstructions of authigenic (seawater-derived) Nd isotope records have been performed for a wide range of time scales and resolutions, from the Archaean to the present day. In particular, a growing number of Late Pleistocene records lend strong support to the notion that Nd isotopes may serve as a tracer for past water mass provenance and ocean circulation, particularly in the Atlantic Ocean [11][12][13][14][15][16][17][18][19]. However, quantitative use of the tracer is difficult at best, and simplifications have been necessary in order to derive (qualitative) interpretations of past changes. ...
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The neodymium (Nd) isotopic composition of seawater has been used extensively to reconstruct ocean circulation on a variety of time scales. However, dissolved neodymium concentrations and isotopes do not always behave conservatively, and quantitative deconvolution of this non-conservative component can be used to detect trace metal inputs and isotopic exchange at ocean–sediment interfaces. In order to facilitate such comparisons for historical datasets, we here provide an extended global database for Nd isotopes and concentrations in the context of hydrography and nutrients. Since 2010, combined datasets for a large range of trace elements and isotopes are collected on international GEOTRACES section cruises, alongside classical nutrient and hydrography measurements. Here, we take a first step towards exploiting these datasets by comparing high-resolution Nd sections for the western and eastern North Atlantic in the context of hydrography, nutrients and aluminium (Al) concentrations. Evaluating those data in tracer– tracer space reveals that North Atlantic seawater Nd isotopes and concentrations generally follow the patterns of advection, as do Al concentrations. Deviations from water mass mixing are observed locally, associated with the addition or removal of trace metals in benthic nepheloid layers, exchange with ocean margins (i.e. boundary exchange) and/or exchange with particulate phases (i.e. reversible scavenging). We emphasize that the complexity of some of the new datasets cautions against a quantitative interpretation of individual palaeo Nd isotope records, and indicates the importance of spatial reconstructions for a more balanced approach to deciphering past ocean changes. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
... Eleven cores were selected from the North Atlantic [Colin et al., 2010;Crocket et 3 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 Table 1 for details regarding the cores and Figure 1 for core locations. Gutjahr et al., 2008;Gutjahr and Lippold, 2011;Pahnke et al., 2008;Roberts et al., 2010;Xie et al., 2012;Piotrowski et al., 2012;Xie et al., 2014;Huang et al., 2014], four from the South Atlantic [Rutberg et al., 2000;Piotrowski et al., 2004Piotrowski et al., , 2005Piotrowski et al., , 2008Pahnke et al., 2008;Skinner et al., 2013] . Two cores are located in the North Pacific Basak et al., 2010]. ...
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Over the past decade, records of the seawater neodymium isotopic composition (εND) have become a widely-used proxy to reconstruct changes in ocean circulation. Our study investigates the transient response of εND to large-scale ocean circulation changes using an Earth system model of intermediate complexity. It is shown that a weakening of the North Atlantic Deep Water (NADW) formation results in positive εND anomalies in the Atlantic and the Pacific below 1000 m water depth whereas variations in Antarctic Bottom Water (AABW) production generate a Pacific-Atlantic dipole pattern of deep ocean εND changes. Further experiments explore which ocean regions are suitable to record the temporal evolution of the overturning in the North Atlantic and the Southern Ocean by means of εND data. High local correlations occur between simulated Southern Ocean overturning changes and simulated εND anomalies in the deep North Pacific and almost globally for simulated North Atlantic overturning changes respectively; clearly indicating the strong potential of εND to work as a proxy of past ocean circulation changes. Finally, the compromising effects of simultaneously occurring anomalies in the North Atlantic and the Southern Ocean overturning cells on reconstructions of past ocean circulation changes are identified. Combining our model simulations with currently available core data, our study demonstrates that changes in εND documented in numerous Atlantic paleo-records clearly support the notion of a strengthening in the Atlantic Meridional Overturning Circulation over the course of Termination 1.
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Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal and detrital neodymium isotope data from three intermediate depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25 kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southwards shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.
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Boundary Exchange (BE – exchange of elements between continental margins and the open ocean) has recently been emphasized as a key process in the oceanic cycle of neodymium (Nd). We here use a regional eddy-permitting resolution Ocean General Circulation Model (1/4°) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental marings. These exchange times are significantly lower than the previous evaluations using a low resolution model (6 months to 10 years), but however in agreement with the available seawater Nd isotope data, highlighting the importance of the model dynamics in simulating the BE process.
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This atlas consists of a description of data analysis procedures and horizontal maps of annual, seasonal, and monthly climatological distribution fields of salinity at selected standard depth levels of the world ocean on a one-degree latitude-longitude grid. The aim of the maps is to illustrate large-scale characteristics of the distribution of ocean salinity. The fields used to generate these climatological maps were computed by objective analysis of all scientifically quality-controlled historical salinity data in the World Ocean Database 2009. Maps are presented for climatological composite periods (annual, seasonal, monthly, seasonal and monthly difference fields from the annual mean field, and the number of observations) at selected standard depths.
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Two new records of paired benthic foraminiferal Mg/Ca and δ18O from two low latitude western Atlantic sediment cores—one taken from within the Florida Current and the other from the Little Bahama Bank—provide insights into the spatial distribution of intermediate depth temperature and salinity variability during the deglaciation. During the Younger Dryas cold event, both temperature and salinity increased at the Florida Current site and decreased at the Little Bahama Bank site. The temperature increase within the Florida Current is consistent with a reduction in the strength of the northward-moving surface return flow of the Atlantic meridional overturning circulation; the temperature decrease at the Little Bahama Bank is consistent with a cooling of high latitude North Atlantic surface waters. To test the possibility that a freshening of the surface North Atlantic caused the paleoceanographic changes during the Younger Dryas, the Geophysical Fluid Dynamics Laboratory (GFDL) R30 coupled ocean-atmosphere general circulation model was forced using a North Atlantic freshwater perturbation of 0.1 Sv for a period of 100 years. The freshwater flux causes an overall reduction in the Atlantic overturning from 25 Sv to 13 Sv. However, at ˜1,100 m water depth, ventilation increases, causing decreases in both temperature and salinity throughout much of the intermediate depth, open-ocean North Atlantic. At the western boundary, intermediate depth temperatures and salinities increase due to weakened overturning, and also due to an increase in runoff from the Amazon River, which causes a surface stability and a decrease in the upwelling of colder, deeper waters.
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The radiogenic neodymium (Nd) isotope composition of foraminiferal shells provides a powerful archive to investigate past changes in sources and mixing of water masses. However, seawater Nd isotope ratios extracted from foraminiferal shells can be biased by contaminant phases such as organic matter, silicates, or ferromanganese coatings, the removal of which requires rigorous multiple step cleaning of the samples. Here we investigate the efficiency of Flow Through and batch cleaning methods to extract seawater Nd isotope compositions from planktonic foraminifera in a shelf setting in the Gulf of Guinea that is strongly influenced by riverine sediment inputs. Nd isotope analyses of reductively and oxidatively cleaned mono-specific planktonic foraminiferal samples and reductively cleaned mixed benthic foraminifera were complemented by analyses of non-reductively cleaned mono-specific planktonic foraminiferal samples, Fe–Mn coatings of de-carbonated bulk sediment leachates, and the residual detrital fraction of the same sediment.
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Antarctic Intermediate Water (AAIW) is a key player in the global ocean circulation, contributing to the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and influencing interhemispheric heat exchange and the distribution of salinity, nutrients and carbon. However, the deglacial history of AAIW flow into the North Atlantic is controversial. Here we present a multicore-top neodymium isotope calibration, which confirms the ability of unclean foraminifera to faithfully record bottom water neodymium isotopic composition (εNdεNd) values in their authigenic coatings. We then present the first foraminifera-based reconstruction of εNdεNd from three sediment cores retrieved from within modern AAIW, in the western tropical North Atlantic. Our records reveal similar glacial and interglacial contributions of AAIW, and a pronounced decrease in the AAIW fraction during North Atlantic deglacial cold episodes, Heinrich Stadial 1 (HS1) and Younger Dryas (YD). Our results suggest two separate phases of reduced fraction of AAIW in the tropical Atlantic during HS1, with a greater reduction during early HS1. If a reduction in AAIW fraction also reflects reduced AMOC strength, this finding may explain why, in many regions, there are two phases of hydrologic change within HS1, and why atmospheric CO2 rose more rapidly during early than late HS1. Our result suggesting less flow of AAIW into the Atlantic during North Atlantic cold events contrasts with evidence from the Pacific, where intermediate-depth εNdεNd records may indicate increased flow of AAIW into the Pacific during the these same events. Antiphased εNdεNd behavior between intermediate depths of the North Atlantic and Pacific implies that the flow of AAIW into Atlantic and Pacific seesawed during the last deglaciation.
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During the Last Glacial Maximum, the northwestern North Atlantic constituted a major conduit for Labrador and Greenland ice sheet meltwaters. Vertical density gradients in its upper water masses have been reconstructed by combining information from transfer functions based on dinocysts and from oxygen isotope measurements (δ18O) in planktonic foraminifera. Transfer functions yield temperature and salinity and thus potential density (σθ) for the warmest (August) and coldest (February) months in the photic zone. The δ18O values in different size fractions of epipelagic (Globigerina bulloides) and mesopelagic (Neogloboquadrina pachyderma left-coiled (Npl)) foraminifera allow us to assess σθ gradients through the pycnocline between surface and intermediate waters, based on the calibration of a σθ versus δ18O relationship from transfer function reconstructions. The size and density of Npl shells provide further constraints on these σθ gradients. The results show the development of a very strong pycnocline during the LGM with a difference of about 3 (summer) to 1.5 (winter) σθ units between surface and underlying waters. They indicate conditions unfavorable for vertical convection and support the hypothesis of the spreading of a shallow, low-salinity buoyant layer over the northern North Atlantic. This layer depicted a strong E-W gradient, with maximum seasonal contrast and minimum absolute σθ values westward.
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The North Atlantic Deep Water (NADW) neodymium isotopic composition (Nd IC) is increasingly used in oceanography and paleoceanography to trace large-scale circulation and weathering processes, notably to investigate past variations of the global thermohaline circulation. Although the present-day NADW Nd IC is well characterized at ε\varepsilonNd = -13.5, the acquisition of this isotopic signature (in other words, the causes of this value) has so far been very sparsely documented. Such an understanding is, however, fundamental to the interpretation of paleo records. Nd IC and rare earth element concentrations were measured at 9 stations within the North Atlantic Subpolar Gyre (SIGNATURE cruise, summer 1999). The comparison of this data set with our understanding of water mass circulation provides a description of how the three layers constituting the NADW, the Labrador Sea Water (LSW, ε\varepsilonNd = -13.9 +/- 0.4), North East Atlantic Deep Water (NEADW, ε\varepsilonNd -13.2 +/- 0.4), and North West Atlantic Bottom Water (NWABW, ε\varepsilonNd -14.5 +/- 0.4), acquire their Nd IC through distinct water mass mixings and lithogenic inputs. These different mechanisms, acting upon water masses from very diverse sources, seem to bring the Nd IC of the three NADW layers to values close together and similar to that of the NADW. It is suggested that sediment/seawater interactions significantly lower the NEADW and NWABW Nd IC along the South East Greenland margin. Since these interactions do not significantly modify the Nd content of these water masses, sediment remobilizations leading to the Nd IC variations are probably associated with Nd removal fluxes from the water mass toward the sediment, a process called boundary exchange. On the other hand, LSW seems to acquire its Nd IC from the Subpolar Mode Waters from which it is formed by deep convection, and no other mechanism needs to be invoked. Its unradiogenic signature could ultimately be linked to fresh water runoff from the Canadian Shield. These conclusions should allow more precise interpretations of paleoceanographic Nd IC records, taking into account the distinct histories of the three NADW layers, including distinct water mass mixings and distinct lithogenic inputs.
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Neodymium isotopes provide a paleoceanographic proxy for past deep water circulation and local weathering changes and have been measured on various authigenic marine sediment components, including fish teeth, ferromanganese oxides extracted by acid-reductive leaching, cleaned foraminifera, and foraminifera with Fe-Mn oxide coatings. Here we compare Nd isotopic measurements obtained from ferromanganese oxides leached from bulk sediments and planktonic foraminifera, as well as from oxidatively-reductively cleaned foraminiferal shells from sediment cores in the North Atlantic. Sedimentary volcanic ash contributes a significant fraction of the Nd when the ferro-manganese (Fe-Mn) oxide coatings are leached from bulk sediments. Reductive leachates of marine sediments from North Atlantic core tops near Iceland, or directly downstream from Iceland-Scotland Overflow Waters, record ε\varepsilonNd values that are significantly higher than seawater, indicating that volcanic material is easily leached by acid-reductive methods. The ε\varepsilonNd values from sites more distal to Iceland are similar to modern seawater values, showing little contamination from Iceland-derived volcanogenic material. In all comparisons, core top planktonic foraminifera ε\varepsilonNd values more closely approximate modern deep seawater than the bulk sediment reductive leached value suggesting that the foraminifera provide a route toward quantifying the Nd isotopic signature of deep North Atlantic water masses.
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A hypothesis is presented for the origin of Pleistocene climate instability, based on expansion of Antarctic sea ice and associated changes in the oceans' salinity structure. The hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of Toggweiler and Samuels [1993b], in which deepwater upwelling is restricted to high southern latitudes. The reconfigured conveyor is shown to be potentially stabilized in an ``on'' mode by precipitation at high southern latitudes and potentially destabilized into ``on'' and ``off'' modes by the counteracting influence of Antarctic sea ice. The mechanism is clarified by the use of a hydraulic analogue. We hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between Pleistocene warm and cold periods. The hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods, including aspects of interhemispheric timing.
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Sm-Nd concentrations and Nd isotopes were investigated in the fine fraction of two Labrador Sea cores to reconstruct the deep circulation patterns through changes in sedimentary supply since the last glacial stage. Three sources are involved: the North American Shield, Palaeozoic rocks from northeastern Greenland, and mid-Atlantic volcanism. The variable input of these sources provides constraints on the relative sedimentary supply, in conjunction with inception of deep currents. During the last glacial stage a persistent but sluggish current occurred inside the Labrador Basin. An increasing discharge of volcanic material driven by the North East Atlantic Deep Water is documented since 14.3 kyr, signaling the setup of a modern-like deep circulation pattern throughout the Labrador, Irminger, and Iceland basins. During the last deglacial stage the isotopic record was punctually influenced by erosion processes related mainly to ice-sheet instabilities, especially 11.4, 10.2, and 9.2 kyr ago.
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Abrupt changes in the African monsoon can have pronounced socioeconomic impacts on many West African countries. Evidence for both prolonged humid periods and monsoon failures have been identified throughout the late Pleistocene and early Holocene epochs 1,2. In particular, drought conditions in West Africa have occurred during periods of reduced North Atlantic thermohaline circulation, such as the Younger Dryas cold event 1. Here, we use an ocean–atmosphere general circulation model to examine the link between oceanographic changes in the North Atlantic Ocean and changes in the strength of the African monsoon. Our simulations show that when North Atlantic thermohaline circulation is substantially weakened, the flow of the subsurface North Brazil Current reverses. This leads to decreased upper tropical ocean stratification and warmer sea surface temperatures in the equatorial South Atlantic Ocean
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Formation of North Atlantic Deep Water (NADW) represents a transfer of upper layer water to abyssal depths at a rate of 15 to 20 x 10 6 m3/s. NADW spreads throughout the Atlantic Ocean and is exported to the Indian and Pacific Oceans by the Antarctic Circumpolar Current and deep western boundary currents. Naturally, there must be a compensating flow of upper layer water toward the northern North Atlantic to feed NADW production. It is proposed that this return flow is accomplished primarily within the ocean's warm water thermocline layer. In this way the main thermoclines of the ocean are linked as they participate in a thermohaline-driven global scale circulation cell associated with NADW formation. The path of the return flow of warm water is as follows: Pacific to Indian flow within the Indonesian Seas, advection across the Indian Ocean in the 10ø-15øS latitude belt, southward transfer in the Mozambique Channel, entry into the South Atlantic by a branch of the Agulhas Current that does not complete the retroflection pattern, northward advection within the subtropical gyre of the South Atlantic (which on balance with the southward flux of colder North Atlantic Deep Water supports the northward oceanic heat flux characteristic of the South Atlantic), and cross-equatorial flow into the western North Atlantic. The magnitude of the return flow increases along its path as more NADW is incorporated into the upper layer of the ocean. Additionally, the water mass characteristics of the return flow are gradually altered by regional ocean-atmosphere interaction and mixing processes. Within the Indonesian seas there is evidence of strong vertical mixing across the thermocline. The cold water route, Pacific to Atlantic transport of Subantarctic water within the Drake Passage, is of secondary importance, amounting to perhaps 25% of the warm water route transport. The continuity or vigor of the warm water route is vulnerable to change not only as the thermohaline forcing in the northern North Atlantic varies but also as the larger-scale wind-driven criculation factors vary. The interocean links within the Indonesian seas and at the Agulhas retroflection may be particularly responsive to such variability. Changes in the warn: water route continuity may in turn influence formation characteristics of NADW.
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The degree of similarity of the ∂13C records of the planktonic foraminiferal species N. pachyderma and of the benthic foraminiferal genus Cibicides in the high-latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle. Whereas continuous formation of deep water is recognized in the southern ocean, the Norwegian Sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation. However, deep water formed in the north Atlantic south of the Norwegian Sea during the last climatic cycle as early as isotope substage 5d, and this area was also the only active northern source during stages 4-2. A detailed reconstruction of the geographic distribution of ∂13C in benthic foraminifera in the Atlantic Ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean, whereas the Glacial North Atlantic Deep Water cannot be traced south of 40°N. At shallower depth an oxygenated 13C rich Intermediate Water mass extended from 45°N to 15°S. In the Pacific Ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700-2600 m and is best explained by stronger formation of Intermediate Water in high northern latitudes.
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Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution of east Atlantic deepwater and intermediate water masses over the last 30,000 years. Our results show that there have been three distinct modes of deepwater circulation: Near the stage 3-2 boundary, the origin of North Atlantic Deep Water (NADW) was similar to today (mode 1). However, after late stage 3 the source region of the NADW end-member shifted from the Norwegian-Greenland Sea to areas south of Iceland (mode 2). A reduced NADW flow persisted during the last glacial maximum, with constant preformed δ13C values. The nutrient content of NADW increased markedly near the Azores fracture zone from north to south, probably because of the mixing of upwelled Antarctic Bottom Water (AABW) from below, which then advected with much higher flux rates into the northeast Atlantic. Later, the spread of glacial meltwater over the North Atlantic led to a marked short-term ventilation minimum below 1800 m about 13,500 14C years ago (mode 3). The formation of NADW recommenced abruptly north of Iceland 12,800–12,500 years ago and reached a volume approaching that of the Holocene, in the Younger Dryas (10,800–10,350 years B.P.). Another short-term shutdown of deepwater formation followed between 10,200 and 9,600 years B.P., linked to a further major meltwater pulse into the Atlantic. Each renewal of deepwater formation led to a marked release of fossil CO2 from the ocean, the likely cause of the contemporaneous 14C plateaus. Over the last 9000 years, deepwater circulation varied little from today, apart from a slight increase in AABW about 7000 14C years ago. It is also shown that the oxygenated Mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30,000 years.
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Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition.
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1] The ocean's role in regulating atmospheric carbon dioxide on glacial‐interglacial timescales remains an unresolved issue in paleoclimatology. Reduced mixing between deep water masses may have aided oceanic storage of atmospheric CO 2 during the Last Glacial Maximum (LGM), but data supporting this idea have remained elusive. The d 13 C of benthic foraminifera indicate the Atlantic Ocean was more chemically stratified during the LGM, but the nonconservative nature of d 13 C complicates interpretation of the LGM signal. Here we use benthic foraminiferal d 18 O as a conservative tracer to constrain the ratio of meridional transport to vertical diffusivity in the deep Atlantic. Our calculations suggest that the ratio was at least twice as large at the LGM. We speculate that the primary cause was reduced mixing between northern and southern component waters, associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor. The shallower water mass boundary yields an order of magnitude increase in the volume of southern component water, suggesting its residence time may have increased substantially. Our analysis supports the idea that an expanded volume of Antarctic Bottom Water and limited vertical mixing enhanced the abyssal ocean's ability to trap carbon during glacial times.
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a b s t r a c t Mediterranean Outflow Water (MOW) is characterised by higher temperatures and salinities than other ambient water masses. MOW spreads at water depths between 500 and 1500 m in the eastern North Atlantic and has been a source of salinity for the Atlantic Meridional Overturning Circulation in the North Atlantic. We used high-resolution Nd and Pb isotope records of past ambient seawater obtained from authigenic ferromanganese coatings of sediments in three gravity cores at 577, 1745 and 1974 m water depth in the Gulf of Cadiz and along the Portuguese margin complemented by a selection of surface sediments to reconstruct the extent and pathways of MOW over the past 23 000 years. The surface and downcore Nd isotope data from all water depths exhibit only a very small variability close to the present day composition of MOW but do not reflect the present day Nd isotopic stratification of the water column as determined from a nearby open ocean hydrographic station. In contrast, the Pb isotope records show significant and systematic variations, which provide evidence for a significantly different pattern of the MOW pathways between 20 000 and 12 000 years ago compared with the subsequent period of time.
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A neodymium oxide with relative Nd-143/Nd-144 ratio 1.000503 +/- 1(1 sigma(m)) to LaJolla Nd was prepared as a new isotopic reference. The neodymium reagent was selected from two points of view as follows. The first is low abundance of neighboring elements Ce and Sm, which affects isobaric interference. The second is high Nd-143/Nd-144 ratio, which is closer to those of chondritic and mantle-derived materials. The Nd-143/Nd-144 ratio of the reagent was measured alternately with LaJolla Nd to get a coherency with LaJolla Nd using 12 mass spectrometers in ii laboratories in Japan. Aliquots of this neodymium oxide reagent named JNdi-1 are available upon request from the Geological Survey of Japan and may be useful for precise interlaboratory calibration of Nd isotopes.
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The history of deep water formation and abyssal flow is poorly known but important to establish in order to develop a better understanding of changes in oceanic mass, heat, salt, and nutrient transport. North Atlantic high-latitude regions currently are the dominant deep water producers, but paleogeographic constraints, proxy interpretations, and physical models have suggested other modes for the past, such as those characterized by high-latitude Pacific sources, subtropical sources, or widespread, nonlocalized sources. Here we present new North Pacific Late Cretaceous–Paleogene Nd isotope data from fossil fish debris and detrital silicates, combined with results of coupled climate model simulations to test these hypothesized circulation modes. The data and model simulations support a circulation mode characterized by high-latitude, bipolar Pacific convection. Deep convection in the North Pacific, and likely the South Pacific, was most intense during the relatively “cool” portion of the Late Cretaceous–Paleocene and waned prior to the peak global warmth of the Early Eocene (ca. 52 Ma).
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Ocean circulation is closely linked to climate change on glacial-interglacial and shorter timescales. Extensive reorganizations in the circulation of deep and intermediate-depth waters in the Atlantic Ocean have been hypothesized for both the last glaciation and the subsequent Younger Dryas cold interval,, but there has been little palaeoceanographic study of the subtropical gyres. These gyres are the dominant oceanic features of wind-driven circulation, and as such they reflect changes in climate and are a significant control on nutrient cycling and, possibly, atmospheric CO2 concentrations. Here we present Cd/Ca ratios in the shells of benthic foraminifera from the Bahama banks that confirm previous suggestions, that nutrient concentrations in the North Atlantic subtropical gyre were much lower during the Last Glacial Maximum than they are today (up to 50% lower according to our data). These contrasting nutrient burdens imply much shorter residence times for waters within the thermocline of the Last Glacial Maximum. Below the glacial thermocline, nutrient concentrations were reduced owing to the presence of Glacial North Atlantic Intermediate Water. A high-resolution Cd/Ca record from an intermediate depth indicates decreased nutrient concentrations during the Younger Dryas interval as well, mirroring opposite changes at a nearby deep site,. Together, these observations suggest that the formation of deep and intermediate waters - North Atlantic Deep Water and Glacial North Atlantic Intermediate Water, respectively - wax and wane alternately on both orbital and millennial timescales.
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Past glacial-interglacial climate transitions were accompanied by millennial-scale pulses in atmospheric CO2 that are widely thought to have resulted from the release of CO2 via the Southern Ocean. However, direct proxy evidence for a Southern Ocean role in regulating past ocean-atmosphere CO2 exchange is scarce. Here we use combined radiocarbon and neodymium isotope measurements from the last deglaciation to confirm greatly enhanced overturning and/or air-sea exchange rates relative to today, in particular during the Bolling-Allerod warm interval. We show that this deglacial pulse in ocean ventilation was not driven by the North Atlantic overturning alone, and must have involved an increase in the ventilation of southern-sourced deep waters. Our results thus confirm the removal of a physical and/or dynamical barrier to effective air-sea (CO2) exchange in the Southern Ocean during deglaciation, and highlight the Antarctic region as a key locus for global climate/carbon-cycle feedbacks.
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Neodymium (Nd) isotopes, tracers of deep water mass source and mixing, were measured on sedimentary planktic foraminifera with authigenic coatings from a depth-transect of cores (1400–4800 m) from Chatham Rise in the southwest Pacific, over the past 30 ka. We observe deglacial variations in the Nd isotopic composition, which showed an average glacial composition of εNd=−5.0εNd=−5.0 (1σ; ±0.3n=4) for cores sites below 3200 mbsl. No significant deglacial variation was observed in the Nd isotopic composition of intermediate depth waters (1400 mbsl), in contrast with benthic foraminifera δC13 data. The deglacial εNdεNd shift of CDW in the southwest Pacific is consistent with changes observed in the deep South Atlantic and Equatorial Indian Ocean, but εNdεNd values are offset by ∼1εNd∼1εNd-unit to more radiogenic values throughout the deglacial records, likely due to admixture of a Nd isotope signal which was modified in the Southern Ocean or Pacific, perhaps by boundary exchange. However, this modification did not overprint the deglacial Nd isotope change. The consistent deglacial evolution of εNdεNd in the South Atlantic, Equatorial Indian and southwest Pacific CDW, is evidence for the connection of CDW during the glacial, and propagation of diminished North Atlantic Deep Water export to the glacial Southern Ocean. In contrast, spatial heterogeneities in the benthic foraminifera δC13 of CDW have been observed in the Atlantic, Indian and Pacific basins of the deep glacial Southern Ocean. The Nd isotope data implies a well-connected deep Southern Ocean, which transported waters from the Atlantic to the Indian and Pacific oceans, during the glacial. This suggests that basin-scale variability in the glacial δC13 composition of CDW was unrelated to circulation changes.
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Benthic foraminiferal stable isotope records from four high-resolution sediment cores, forming a depth transect between 1237 m and 2303 m on the South Iceland Rise, have been used to reconstruct intermediate and deep water paleoceanographic changes in the northern North Atlantic during the last 21 ka (spanning Termination I and the Holocene). Typically, a sampling resolution of ˜100 years is attained. Deglacial core chronologies are accurately tied to North Greenland Ice Core Project (NGRIP) ice core records through the correlation of tephra layers and changes in the percent abundance of Neogloboquadrina pachyderma (sinistral) with transitions in NGRIP. The evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic δ13C and δ18O values, which do not lie on glacial or Holocene water mass mixing lines. These periods correlate with the late Younger Dryas/Early Holocene (11.5-12.2 ka) and Heinrich Stadial 1 (14.7-16.8 ka) during which time freshwater input and sea-ice formation led to brine rejection both locally and as an overflow exported from the Nordic seas into the northern North Atlantic, as earlier reported by Meland et al. (2008). The export of brine with low δ13C values from the Nordic seas complicates traditional interpretations of low δ13C values during the deglaciation as incursions of southern sourced water, although the spatial extent of this brine is uncertain. The records also reveal that the onset of the Younger Dryas was accompanied by an abrupt and transient (˜200-300 year duration) decrease in the ventilation of the northern North Atlantic. During the Holocene, Iceland-Scotland Overflow Water only reached its modern flow strength and/or depth over the South Iceland Rise by 7-8 ka, in parallel with surface ocean reorganizations and a cessation in deglacial meltwater input to the North Atlantic.
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The dissolved neodymium (Nd) isotopic distribution in the deep oceans is determined by continental weathering inputs, water mass advection, and boundary exchange between particulate and dissolved fractions. Reconstructions of past Nd isotopic variability may therefore provide evidence on temporal changes in continental weathering inputs and/or ocean circulation patterns over a range of timescales. However, such an approach is limited by uncertainty in the mechanisms and importance of the boundary exchange process, and the challenge in reliably recovering past seawater Nd isotopic composition (εNd) from deep sea sediments. This study addresses these questions by investigating the processes involved in particulate–solution interactions and their impact on Nd isotopes. A better understanding of boundary exchange also has wider implications for the oceanic cycling and budgets of other particle-reactive elements.
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During the last few decades, neodymium isotopes have been increasingly used as a paleoceanographic and paleoclimate proxy on million-year through millennial time-scales. The widespread use of the Nd isotope proxy depends on whether the Nd isotopic composition of past seawater can be reliably extracted from authigenic sediment phases. Here we show evidence that planktonic foraminifera with authigenic coatings preserve a deep water Nd isotopic signature and thus can be used to reconstruct past deep water Nd isotopes. Applying this method downcore on South Atlantic Deep Cape Basin core TNO57-21 (41.1°S, 7.9°E, 4981 m), the Nd isotopic composition of the planktonic foraminiferal Fe–Mn oxide coatings is identical with and confirms that of the reductively leached bulk sediment fraction (36, 37 and 38) as reflecting changes in bottom water sourcing. In the case of northeastern North Atlantic core BOFS 8K (52.5°N 22.1°W, 4045 m), the planktonic foraminiferal Fe–Mn oxide coatings have different Nd isotopes from reductive sediment leaches. Clearly acid-reductive leaching must be tested in more detail. We observe that the Nd isotopic composition of the deep northeastern Atlantic core shifted during the deglaciation, from glacial values similar to the South Atlantic Cape Basin towards Holocene values similar to the Bermuda Rise (Roberts et al., 2010). Taken as a whole, the Nd isotope record from foraminiferal authigenic coatings suggests large-scale changes in the proportion of northern and southern deep water sources occurred throughout the Atlantic Ocean, with substantial chemical gradients at times, during the last deglaciation.
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The use of neodymium (Nd) isotopes to reconstruct past water mass mixing relies upon the quasi-conservative behaviour of this tracer, whereas recent studies in the modern oceans have suggested that boundary exchange, involving the addition of Nd from ocean margin sediments, may be an important process in the Nd cycle. Here we suggest that the relative importance of water mass advection versus boundary exchange can be assessed where the deep western boundary current in the Indian Ocean flows past the Madagascan continental margin; a potential source of highly unradiogenic Nd. Foraminiferal coatings and bulk sediment reductive leachates are used to reconstruct bottom water Nd isotopic composition (εNd) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of ∼4 εNd units along the flow path of Circumpolar Deep Water; εNd≈−8.8 in the deep southern inflow upstream of Madagascar, which evolves towards εNd≈−11.5 offshore northern Madagascar, whereas εNd≈−7.3 where deep water re-circulates in the eastern Mascarene Basin. This variability is attributed to boundary exchange and, together with measurements of detrital sediment εNd, an isotope mass balance suggests a deep water residence time for Nd of ≤400 yr along the Madagascan margin. Considering deglacial changes, a core in the deep inflow upstream of Madagascar records εNd changes that agree with previous reconstructions of the Circumpolar Deep Water composition in the Southern Ocean, consistent with a control by water mass advection and perhaps indicating a longer residence time for Nd in the open ocean away from local sediment inputs. In contrast, sites along the Madagascan margin record offset εNd values and reduced glacial–interglacial variability, underlining the importance of detecting boundary exchange before inferring water mass source changes from Nd isotope records. The extent of Madagascan boundary exchange appears to be unchanged between the Holocene and Late Glacial periods, while a consistent shift towards more radiogenic εNd values at all sites in the Late Glacial compared to the Holocene may represent a muted signal of a change in water mass source or composition.
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Understanding intermediate water circulation across the last deglacial is critical in assessing the role of oceanic heat transport associated with Atlantic Meridional Overturning Circulation variability across abrupt climate events. However, the links between intermediate water circulation and abrupt climate events such as the Younger Dryas (YD) and Heinrich Event 1 (H1) are still poorly constrained. Here, we reconstruct changes in Antarctic Intermediate Water (AAIW) circulation in the subtropical North Atlantic over the past 25 kyr by measuring authigenic neodymium isotope ratios in sediments from two sites in the Florida Straits. Our authigenic Nd isotope records suggest that there was little to no penetration of AAIW into the subtropical North Atlantic during the YD and H1. Variations in the northward penetration of AAIW into the Florida Straits documented in our authigenic Nd isotope record are synchronous with multiple climatic archives, including the Greenland ice core δ18O record, the Cariaco Basin atmosphere Δ14C reconstruction, the Bermuda Rise sedimentary Pa/Th record, and nutrient and stable isotope data from the tropical North Atlantic. The synchroneity of our Nd records with multiple climatic archives suggests a tight connection between AAIW variability and high-latitude North Atlantic climate change.
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The neodymium concentration and isotopic composition of the Mediterranean outflow in the Strait of Gibraltar has been directly determined. The outflow has a dissolved Nd concentration of ~23 pmoles/kg with ϵ_Nd(0) = −9.4. These results are in clear disagreement with previous values inferred for the Mediterranean. This concentration and ϵ_Nd(0) do not sharply contrast with North Atlantic Deep Water (NADW) values and can produce isotopic anomalies no larger than 1 ϵ unit in the North Atlantic. A new profile in the eastern North Atlantic does not exhibit an ϵ_Nd(0) anomaly at the depth of the Mediterranean Water core, in accordance with the above inferences, while concentrations and ϵNd(0) at this site indicate that Nd is conservative in the deep eastern North Atlantic. A previously reported ϵNd(0) anomaly in the eastern North Atlantic, originally associated with the Mediterranean outflow, has a different source. The direct contribution of the Mediterranean outflow to the NADW Nd budget is small, ~10% of the high latitude sources. However, the outflow plus entrained water supply ~30% of the total Nd associated with northern latitude deep water sources. Concentrations of dissolved Nd in the outflow are enriched compared to the Atlantic inflow, which has 16 pmoles/ kg with ϵ_Nd(0) = −11.8. The principal source of the excess dissolved Nd is inferred to be partial dissolution of detrital particles. Simple mass balance calculations suggest that ~0.6% of the Nd associated with detrital river-transported sediments and/or ~3.5% of the Nd associated with eolian particles delivered to the Mediterranean is mobilized during passage into the marine environment. If general, these mobilization efficiencies imply that the dissolution of detrital particles is a significant source of dissolved Nd (and other rare earth elements) in seawater.
Article
The circulation of Antarctic Intermediate Water is thought to make an important contribution to the global ocean-climate system, but the details of this interaction are not fully understood. Furthermore, the behaviour of Antarctic Intermediate Water under glacial and interglacial conditions is not well constrained. Here we present a 25,000-year-long record of neodymium isotopic variations-a tracer of water-mass mixing-from the middle depths of the tropical Atlantic Ocean. Our data reveal abruptly enhanced northward advection of Antarctic Intermediate Water during periods of reduced North Atlantic overturning circulation during the last deglaciation. These events coincide with an increase in the formation of Antarctic Intermediate Water and warming in the southwest Pacific Ocean, which suggests a tight link with Southern Hemisphere climate. In contrast, the initial incursion of southern source water into the North Atlantic ~19,000years ago coincided with weak Antarctic Intermediate Water formation in the Pacific and reduced overturning in the North Atlantic. We conclude that reduced competition at intermediate water depth at this time allowed expansion of Antarctic Intermediate Water into the North Atlantic. This early incursion of Antarctic Intermediate Water may have contributed to freshening of the North Atlantic, perhaps spurring the subsequent collapse of North Atlantic deep convection.
Article
Ocean circulation controls climate in two key ways: it transports heat and it determines the rate at which the carbon-laden deep ocean equilibrates with the atmosphere. In the Atlantic the meridional overturning circulation is a particularly important mechanism for the inter-hemispheric transport of heat. Recent studies1 have used neodymium (Nd) isotopes in the Southern Ocean to confirm a weakening of southward deep water export from the North Atlantic, and by implication a diminution in northward heat transport at the surface, during glacial periods. The timing of the Nd shift also suggests that such weakening post-dates both ice-sheet growth and major re-organisation of carbon reservoirs at glaical-interglacial boundaries. The use of Nd isotopes in the Southern Ocean to track changes in deep water export from the North Atlantic requires a knowledge of the secular evolution in the North Atlantic source itself. Here we report results of un-precedently high resolution (20 kyr) Nd isotope analyses of Fe-Mn crusts, obtained by laser ablation MC-ICPMS. The crusts we have analysed come from 1800-2000m in western North Atlantic and are well-placed to sample North Atlantic Deep Water (NADW). The data show that NADW has maintained an epsilon Nd of around -12.5 to -13.5 for the past 500 kyr, through 5 glacial-interglacial (G-IG) cycles. There is a hint of a slight shift towards more radiogenic values during glacials, but these variations are within the uncertainties on the analyses (about 0.5 epsilon units). The first conclusion from these data is that attempts to use the Nd isotope composition of the deep Southern Ocean as a record of the strength of NADW1 are not complicated by changes in the isotopic composition of the northern source itself. However, the constancy of the Nd isotopic composition of NADW through G-IG climate change is in itself a surprising result. The unradiogenic character of Nd in NADW at the present day is determined by the contribution from Labrador Sea Water. Our data require that the NADW mix during glacial periods resulted in a co-incidentally similar Nd isotope composition despite the changes in the sites of convection that models of the circulation seem to demand2. 1 Piotrowski, A. et al. 2005, Science 307, 1933 2 Rahmstorf, S. 2002, Nature 419, 207
Article
The provenance of eolian dust supplied to deep-sea sediments has the potential to offer insights into changes in past atmospheric circulation. Specifically, measuring temporal changes in dust provenance can shed light on changes in the mean position of the Intertropical Convergence Zone (ITCZ), a region acting as a barrier separating wind-blown material derived from northern versus southern hemisphere sources. Here we have analyzed Nd, Sr, and Pb isotope ratios in the operationally-defined detrital component extracted from deep-sea sediments in the eastern equatorial Pacific (EEP) along a meridional transect at 110°W from 3°S to 7°N (ODP Leg 138, sites 848–853).
Article
Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from twenty five piston cores, gravity cores and multi-cores from the Brazil margin. The cores span water depths from about 400 m to 3000 m and intersect the major water masses in this region. The Holocene and glacial bathymetric profiles of benthic foraminifera delta 13C show significant differences. The Holocene bathymetric profile along the Brazil margin shows the presence of North Atlantic Deep Water as a local maximum in delta 13C centered at about 2500 m, intersecting the northward flowing water masses Antarctic Intermediate Water/Circum-polar Deep Water and Antarctic Bottom Water. The glacial bathymetric profile of delta 13C of Sigma CO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow ( ˜1000 m), southern-source water mass with an end-member delta 13C value of about 0.3-0.5 0/00 VPDB, a mid-depth ( ˜1500 m), northern-source water mass with an end-member value of about 1.5 0/00 , and a deep (>2000 m), southern-source water with an end-member value of
Article
We present evidence that the characteristic chemical signature (based on coupled benthic foraminiferal Cd/Ca and δ13C) of Antarctic Intermediate waters (AAIW) penetrated throughout the intermediate depths of the Atlantic basin to the high-latitude North Atlantic during the abrupt cooling events of the last deglaciation: Heinrich 1 and the Younger Dryas. AAIW may play the dynamic counterpart to the "bipolar seesaw" when near-freezing salty bottom waters from the Antarctic (AABW) sluggishly ventilate the deep ocean. Our data reinforce the concept that interglacial circulation is stabilized by salinity feedbacks between salty northern sourced deep waters (NADW) and fresh southern sourced waters (AABW and AAIW). Further, the glacial ocean may be susceptible to the more finely balanced relative densities of NADW and AAIW, due to either freshwater input or a reversal of the salinity gradient, such that the ocean is poised for NADW collapse via a negative salinity feedback. The unstable climate of the glacial period and its termination may arise from the closer competition for ubiquity at intermediate depths between northern and southern sourced intermediate waters.
Article
Observations with bottom-moored current meters in the Jungfern passage show that, contrary to previous conclusions, deep water in the Venezuela basin is being renewed. During a 5-day record, the flow was mainly oscillatory at tidal frequency, with a net inflow by the tidal mechanism of only 6 X 103 m 8 sec -. During a single 11-hour surge the inflow rate was 50 to 200 X 103 m 8 sec -. Steady-state heat-budget and oxygen-budget calculations require a renewal rate of about 100 X 103 m 3 sec -. The sill depth is 1860 meters. STD observations near the sill reveal a sharp interface between Caribbean water above and oceanic water below sill depth. The north-south hydrographic sections of IGY data show an abrupt change in vertical temperature gradient near 2600 to 2800 meters. This feature, also found in other basins, is interpreted to result from differences in renewal between tidal inflow and stronger sporadic surges, with the earth's rotation being an important controlling parameter. In recent studies of the renewal of deep water in the Caribbean Sea, Wiist [1964] inferred renewal whereas Worthington [1966] and Sturges [1965] concluded that there was no evidence for present renewal because the water types were discontinuous between the available oceanic water and the deep wa.ter found inside the basins. In this paper we. present data obtained aboard Trident in February 1969 that show clear evidence for renewal of the deep water, discuss the mixing of this renewal water, and explain the lack of continuity of water types. The deepest passage into the Caribbean Sea is thought to lead through the Anegada. passage near Puerto Rico and then through the Jungfern passage into the Venezuela basin. The other deep sill is through the Windward passage, which leads into the Cayman basin. The still depth in the Windward passage has been ?
Article
Nine depth-profiles of dissolved Nd concentrations and isotopic ratios (εNd) were obtained in the Levantine Basin, the Ionian, the Aegean, the Alboran Seas and the Strait of Gibraltar. Thirteen core-top sediments and Nile River particle samples were also analyzed (leached with 1 N HCl, acetic acid or hydroxylamine hydrochloride). The seawater εNd values become more radiogenic during the eastward circulation in the Mediterranean Sea. The relationship between salinity and the seawater εNd shows that the Nd isotopic signature is more conservative than salinity in the Mediterranean Sea. The water mass with the highest εNd (−4.8) is found at about 200 m in the easternmost Levantine basin. The average εNd value for deep waters is −7.0 in the eastern basin, 2.5 ε-units higher than in the western basin. By examining the sensitivity of seawater εNd to Nd inputs from the Nile, we conclude that the most significant radiogenic Nd source is partially dissolved Nile River particles. The Nd flux from the Nile River water has a minor influence on the Mediterranean seawater εNd. Except for the easternmost Levantine Basin, the leachate εNd values are consistent with the seawater values. In the easternmost Levantine Basin, the leachate εNd values obtained with HCl leaching are systematically higher than the seawater values. The relationship between leachate and residual εNd values indicates that the HCl leaching partially dissolves lithogenic Nd, so the dissolution of Nile River particles is the cause of the observed shift. Some εNd values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with HCl leaching. Although the reason for this shift is not clear, 87Sr/86Sr successfully detects the presence of a nonmarine component in the leachate. Our results suggest that leaching performance may vary with the mineralogy of marine sediments, at least in the case of the Mediterranean Sea.
Article
Quantifying past circulation is a vital part of testing our understanding of the modern and future climate system. The isotopic composition of neodymium (Nd) in marine precipitates has considerable promise as a recorder of past circulation patterns, but its robust application requires knowledge of the end-member compositions in order to correctly deconvolute a downstream signal. We show here, using in situ, high temporal resolution analyses of ferro-manganese crusts from the North Atlantic, that the Nd isotopic composition of deep water during times of much more extensive Northern Hemisphere ice cover was no different than the modern-day interglacial value. This result is surprising, but greatly simplifies the use of Nd isotopes as tracers of the strength and patterns of circulation in the Atlantic in the past.
Article
Atlantic d13C transect of Duplessy et al. (1988). The distribution of d13 Co fSCO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (1000 m), southern source water mass with an end-member d13C value of about 0.3-0.5% VPDB, a middepth (1500 m), northern source water mass with an end-member value of about 1.5%, and a deep (>2000 m), southern source water with an end-member value of less than 0.2%, and perhaps as low as the 0.9% values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal d18O. A revised glacial section of deep water d13C documents the positions and gradients among these end-member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.
Article
We present the first combined dissolved hafnium (Hf) and neodymium (Nd) concentrations and isotope compositions of deep water masses from the Atlantic sector of the Southern Ocean. Eight full depth profiles were analyzed for Hf and twelve for Nd. Hafnium concentrations are generally depleted in the upper few hundred meters ranging between 0.2 pmol/kg and 0.4 pmol/kg and increase to relatively constant values of around 0.6 pmol/kg in the deeper water column. At the stations north of the Polar Front (PF), Nd concentrations increase linearly from about 10 pmol/kg at depths of ~ 200 m to up to 31 pmol/kg close to the bottom indicating particle scavenging and release. Within the Weddell Gyre (WG), however, Nd concentrations are essentially constant at 25 pmol/kg at depths greater than ~ 1000 m. The distributions of both elements show a positive correlation with dissolved silicon implying a close linkage to diatom biogeochemistry. Hafnium essentially shows invariant isotope compositions with values averaging at εHf = + 4.6, whereas Nd isotopes mark distinct differences between water masses, such as modified North Atlantic Deep Water (NADW, εNd = − 11 to − 10) and Antarctic Bottom Water (AABW, εNd = − 8.6 to − 9.6), but also waters locally advected via the Agulhas Current can be identified by their unradiogenic Nd isotope compositions. Mixing calculations suggest that a small fraction of Nd is removed by particle scavenging during mixing of water masses north of the PF. Nevertheless, the Nd isotope composition has apparently not been significantly affected by uptake and release of Nd from particles, as indicated by mixing calculations. A mixing envelope of an approximated North Pacific and a North Atlantic end-member shows that Nd isotope and concentration patterns in the Lower Circumpolar Deep Water (LCDW) can be fully explained by ~ 30:70 percentage contributions of these respective end-members.
Article
The oceanic Nd budget is calculated using a steady state 10-box model and a compilation of field data. This is the first attempt to propose consistent estimates of the Nd fluxes entering the ocean, as well as indicating possible Nd sources and the proportion of Nd fluxes exchanged between dissolved and particulate fractions. With presently available Nd data the best estimates give a total Nd influx of 9 × 109 g/yr, which leads to an oceanic Nd residence time of 500 years. From modeling tests we suggest that the authigenic Nd scavenged by particulates is 100% remineralized in the deep ocean. The total exchanged Nd flux may be as high as 2 × 1010 g/yr. The εNd(0) values of the influxes are -22, -11, +1, and -4 for the North Atlantic, surface Atlantic, North Pacific, and surface Indo-Pacific regions, respectively. Atmospheric and riverine Nd fluxes are insufficient to explain the magnitude and regional variability of calculated Nd influxes and εNd(0). We propose continental margins as an additional source supplying Nd to the ocean. Using the model calibrated for Nd, we examine the sensitivity of deep water εNd(0) to variations of Nd inputs to the ocean. Deep water Nd concentrations and εNd(0) vary with the changes in Nd influxes and their εNd(0). As Nd sources to the ocean may change during glacial/interglacial periods, the εNd(0) shifts recorded in ferromanganese nodules and crusts do not necessarily reflect changes in paleoceanic circulation. The effects of continental erosion should be considered in reconstructing patterns of ocean circulation using Nd isotopes.
Article
Holocene and glacial carbon isotope data of benthic foraminifera from shallow to mid-depth cores from the northeastern subpolar Atlantic show that this region was strongly stratified, with carbon-13-enriched glacial North Atlantic intermediate water (GNAIW) overlying carbon-13-depleted Southern Ocean water (SOW). The data suggest that GNAIW originated north of the polar front and define GNAIW end-member carbon isotope values for studies of water-mass mixing in the open Atlantic. Identical carbon isotope values in the core of GNAIW and below the subtropical thermocline are consistent with rapid cycling of GNAIW through the northern Atlantic. The high carbon isotope values below the thermocline indicate that enhanced nutrient leakage in response to increased ventilation may have extended into intermediate waters. Geochemical box models show that the atmospheric carbon dioxide response to nutrient leakage that results from an increase in ventilation rate may be greater than the response to nutrient redistribution by conversion of North Atlantic deep water into GNAIW. These results underscore the potential rule of Atlantic Ocean circulation changes in influencing past atmospheric carbon dioxide values.
Article
146 Sm– 142 Nd and 147 Sm– 143 Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207 Pb/ 206 Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ɛ 142 Nd = − 6 ± 12 ppm, ɛ 143 Nd = −32.5, and 147 Sm/ 144 Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142 Nd excess in South African cratonic mantle. The lack of a 142 Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ɛ 142 Nd = 8 ± 10 ppm, ɛ 143 Nd = 45 ± 1, and 147 Sm/ 144 Nd = 0.5891. Back-calculation of this present-day ɛ 143 Nd yields an unrealistic estimate for the initial ɛ 143 Nd of − 160 ɛ-units, clearly indicating post-crystallization disturbance of the 147 Sm– 143 Nd system. Examination of 146,147 Sm– 142,143 Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146 Sm– 142 Nd systematics in Jack Hills zircons.
Article
The extraction of a deepwater radiogenic isotope signal from marine sediments is a powerful, though under-exploited, tool for the characterisation of past climates and modes of ocean circulation. The radiogenic and radioactive isotope compositions (Nd, Pb, Th) of ambient deepwater are stored in authigenic Fe–Mn oxyhydroxide coatings in marine sediments, but the unambiguous separation of the isotopic signal in this phase from other sedimentary components is difficult and measures are needed to ensure its seawater origin. Here the extracted Fe–Mn oxyhydroxide phase is investigated geochemically and isotopically in order to constrain the potential and the limitations of the reconstruction of deepwater radiogenic isotope compositions from marine sediments.
Article
Records of the past neodymium (Nd) isotope composition of the deep ocean can resolve ambiguities in the interpretation of other tracers. We present the first Nd isotope data for sedimentary benthic foraminifera. Comparison of the ɛNd of core-top foraminifera from a depth transect on the Cape Basin side of the Walvis Ridge to published seawater data, and to the modern dissolved SiO2–ɛNd trend of the deep Atlantic, suggests that benthic foraminifera represent a reliable archive of the deep water Nd isotope composition. Neodymium isotope values of benthic foraminifera from ODP Site 1264A (Angola Basin side of the Walvis Ridge) from the last 8 Ma agree with Fe–Mn oxide coatings from the same samples and are also broadly consistent with existing fish teeth data for the deep South Atlantic, yielding confidence in the preservation of the marine Nd isotope signal in all these archives. The marine origin of the Nd in the coatings is confirmed by their marine Sr isotope values. These important results allow application of the technique to down-core samples.The new ɛNd datasets, along with ancillary Cd/Ca and Nd/Ca ratios from the same foraminiferal samples, are interpreted in the context of debates on the Neogene history of North Atlantic Deep Water (NADW) export to the South Atlantic. In general, the ɛNd and δ13C records are closely correlated over the past 4.5 Ma. The Nd isotope data suggest strong NADW export from 8 to 5 Ma, consistent with one interpretation of published δ13C gradients. Where the ɛNd record differs from the nutrient-based records, changes in the pre-formed δ13C or Cd/Ca of southern-derived deep water might account for the difference. Maximum NADW-export for the entire record is suggested by all proxies at 3.5–4 Ma. Chemical conditions from 3 to 1 Ma are totally different, showing, on average, the lowest NADW export of the record. Modern-day values again imply NADW export that is about as strong as at any stage over the past 8 Ma.
Article
Paired benthic Cd/Ca and δ13C records have been generated along core M35003 in the western tropical Atlantic. Decreased glacial water column dissolved cadmium (Cdw) and increased benthic δ13C indicate enhanced ventilation with nutrient-deplete intermediate waters, in line with similar inferences from other North Atlantic mid-depth records. An abrupt early deglacial δ13C collapse that is associated with a marked positive Cdw anomaly indicates a transient collapse of mid-depth ventilation from North Atlantic sources, conceivably in conjunction with the H1 meltwater anomaly. The Cdw record displays fine-scale fluctuations that mimic the Greenland Dansgaard/Oeschger (D/O) cycles and show decreased Cdw during stadials. This pattern is opposite to Cdw variations in a deep water record from Bermuda Rise that display increased Cdw concentrations during stadials. The divergent pattern between mid-depth and deep water Cdw records indicates millennial-scale switches between deep and shallow convection in the glacial North Atlantic, at the pace of the D/O climatic cycles. Several high-amplitude anomalies occur in the Cdw record that reach levels similar to those observed today in the North Pacific. While a substantial nutrient increase in the mid-depth North Atlantic cannot be ruled out during these events, changes of pore water chemistry and Cd/P fractionation during biological uptake offer alternative scenarios to explain the peak Cdw maxima.
Article
Oceanic and atmospheric circulation patterns varied considerably during the Tertiary and Quaternary and influenced the geochemical cycles of elements in seawater. We report the first resolution lead and neodymium isotopic record of such changes at a high time resolution in two depths profiles from a hydrogenous FeMn crust. The crust, Va13-2, is located in the central Pacific (146°W, 9°25′N, 4830 m) and has previously been dated by ²³⁰Th and ¹⁰Be. The first profile was drilled with a sample time resolution of ∼3 kyr and allows evaluation of short-term changes to lead and neodymium sources to central Pacific seawater over the last 400 kyr (marine δ¹⁸O stages 2 to 11). Longer-term changes were monitored at lower time resolution in a second profile to an age of 10 Ma.
Article
The variation of North Atlantic Deep Water (NADW) formation over the Last Glacial cycle, from Oxygen Isotopic Substage 5e (OIS-5e; the Eemian) to future global warming projections, is investigated using the UVic Earth System Climate Model. The results are compared with available micropaleontological and stable isotope proxy paleo-reconstructions. Equilibrium simulations for the Eemian ( BP) and the Last Glacial Maximum (LGM— BP) both reveal the absence of Labrador Sea Water (LSW) formation although NADW formation still occurs, albeit at a reduced rate relative to the modern times. For the Eemian, the location of convection in the eastern North Atlantic is similar to the present, although it is generally shallower and less extensive. In the case of the LGM, deep convection has moved southward to the western coast of Europe and is much more localised. The inferred inception of a modern-like circulation slightly before BP revealed by proxy reconstructions is not captured by the model unless the meltwater forcing from the Laurentide ice sheet is applied in a long transient simulation. This raises questions concerning the applicability of equilibrium simulations in capturing the early Holocene climate. In all global warming projections, the LSW formation initially ceases as atmospheric CO2 rises, but recovers once the level is held fixed in the atmosphere. Convection in the north extends further into the Nordic Seas as the sea ice edge retreats. In all simulations convection remains active in the eastern North Atlantic, with its latitude depending on the position of the sea ice edge, suggesting that the formation of lower NADW is a robust feature of Late Quaternary climate. As the Labrador Sea is found to be very sensitive to the freshwater forcing, it suggests that this region represents an ideal location for the concentration of observational studies to monitor a possible oceanic response to anthropogenic climate change.
Article
Continental margins are, via river sediment discharges, the major source of a number of elements to the ocean. They are also, for several reactive elements, sites of preferential removal from the water column, due to enhanced scavenging [1] [M.P. Bacon, Tracers of chemical scavenging in the ocean: Boundary effects and large-scale chemical fractionation, Philos. Trans. R. Soc. Lond., A 325 (1988) 147–160.]. They can therefore be understood as sources of elements for the ocean, sinks or both. Although exchanges of matter are suspected to occur at the continent/ocean interface [2] [P.H. Santschi, L. Guo, I.D. Walsh, M.S. Quigley, M. Baskaran, Boundary exchange and scavenging of radionuclides in continental margin waters of the Middle Atlantic Bight: implications for organic carbon fluxes, Cont. Shelf Res. 19 (1999) 609–636.] and despite their probable importance for the ocean chemistry, closed budgets have still yet to be determined. Here, based on neodymium isotopic composition data obtained during the past 6 yr, we document and quantify significant neodymium exchange at ocean boundaries, in areas covering a large spectra of hydrographical, biological and geochemical characteristics : Eastern Indian Ocean, Western Equatorial Pacific, Western Tropical Pacific and Northwestern Atlantic, with neodymium removal fluxes accounting for 74±23%, 100±38%, 62±54% and 84±45% of the neodymium input fluxes, respectively. Recognition of boundary exchange and its potential globalization have important implications for (1) our understanding of margin/ocean interactions and their influence on the oceanic isotopic chemistry, and (2) geochemical cycling of reactive elements (including pollutants) at ocean margins.