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Neodymium (Nd) isotopes have been utilized as a tracer of water mass source in the modern ocean and in palaeoceanographic studies, though the oceanic cycling of Nd is not yet fully constrained. Recent studies have highlighted the importance of processes that occur near the seawater – sediment interface in altering the Nd isotopic composition of bottom waters. The two major observed processes “boundary exchange” and “benthic flux” have been suggested as playing an important role in setting water mass compositions, however, more studies are needed to constrain their chemical mechanism and the extent to which these processes set the composition of deep waters. The Antarctic continental margin is an important place to study these processes because Antarctic-sourced waters dominate the Southern Ocean and ventilate the global deep ocean. This study is the first to measure and compare seawater, porewater and sediment data from along the margin of Antarctica to examine the nature of potential boundary processes. We show that a process similar to boundary exchange seems to be occurring within porewaters, modifying porewater chemistry by shifting its Nd isotopic ratios to more radiogenic values without significantly increasing the concentration of dissolved Nd. We hypothesize that this shift results from partial dissolution of radiogenic detrital particles, such as smectite, amphibole and/or volcanic glass, while re-scavenging maintains low Nd concentrations. We infer the existence of benthic flux of porewaters to deep waters by examining chemical gradients in porewaters and show that it is much lower on the Antarctic Margin compared to other studies. Benthic flux appears to be slightly enhanced along the Antarctic Peninsula than in the Bellingshausen Sea due to partial degradation of organic matter and associated dissolution of Fe-Mn oxyhydroxides. Taken together, boundary processes do not significantly change the Nd isotopic composition of Antarctic margin seawater because while the porewaters have an altered Nd isotopic composition the Nd concentration of these porewaters is low compared to other settings.

Significant gaps in our understanding of the oceanic cycling of neodymium (Nd) and the other rare earth elements (REEs) remain despite decades of research. One important observation which has not been adequately explained is that the concentration of dissolved Nd typically increases with depth, similar to nutrient profiles, while Nd isotopes appear to reflect conservative water mass mixing in the intermediate and deep ocean; this has been termed the “Nd paradox”. Here we present a detailed study of the dissolved Nd isotopic composition across a section at 40°S in the South Atlantic, collected by UK GEOTRACES cruise (section GA10). The South Atlantic represents a natural laboratory for our understanding of spatial controls on ocean geochemistry, because of the large variability of inputs, spatial differences in particulate cycling, and horizontal advection and mixing at depth between major northern- and southern-sourced water masses. This variability has also made the South Atlantic a critical region subject to intense investigations that aim at reconstructing past changes in ocean processes, such as changes in biological productivity and deep ocean circulation. Our Nd isotope results from the GA10 section provide observational data show the signal of water mass mixing and reversible scavenging. In the surface ocean (0–600 m), Nd isotopic compositions are distinct between different surface ocean currents and spatially can be tied to various continental sources. In the intermediate ocean (600–2500 m), the vertical Nd isotope distribution exhibits distinct signals of different water masses by horizontal advection, including upper North Atlantic Deep Water and Antarctic Intermediate Water formed in the Atlantic Ocean or the Indian Ocean. The Nd isotope distribution also reflects influence of reversible scavenging that smears the signals downwards in the water column (i.e., offset to more radiogenic values). In the deep ocean below 2500 m, Nd isotope distribution largely follows conservative water mass mixing model. Nd concentration in the deep ocean, however, deviates from conservative mixing and increases constantly with depth. We also observe that Nd isotopes appear to be shifted towards the composition of overlying water masses. These observations suggest that reversible scavenging of Nd onto organic and other types of particles is a major vertical process throughout the water column. We also suggest that this process can resolve the “Nd paradox” of decoupling of Nd concentration and isotopic composition due to mixing dynamics. Because abyssal water masses already have a high Nd concentration, a given amount of Nd added from the vertical process has less of an effect on Nd isotopic compositions in deep water masses than it does for intermediate water masses which have comparatively low Nd concentration.

International audience; Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty arises from the difficulty in separating the respectivecontribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. Here, we present a novel method for tracing the origin of weathered rocks on continents, based on the measurement of REE and Nd isotopes (eNd) in leached iron oxide fractions of river sediments [1,2]. We show that the degree of mid-REE enrichment in leached sediment phases provides information on the source of Fe oxides, indicating the presence of ancient marine Fe oxides derived from the erosion of sedimentary rocks or more recent secondary oxides formed in soils via silicate weathering. We also demonstrate that the e Nd difference between paired Fe-oxide and detrital fractions in river sediments (DeNd Feox-Det) reflects the relative contribution of sedimentary vs crystalline silicate rocks during weathering. Rivers draining old cratons and volcanic provinces display near-zero DeNd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1), while multi-lithological catchments hosting sedimentary formationsyield systematically higher values (2.7 ± 1.2). Taken together, these findings show that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. Finally, the influence of climate and geomorphic parameters on the Nd isotopic composition of sedimentary Fe oxides will be discussed, together with future perspectives.

Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present.

Significant gaps in our understanding of the oceanic cycling of neodymium (Nd) and the other rare earth elements (REEs) remain despite decades of research. One important observation which has not to date been adequately explained is that the concentration of dissolved Nd typically increases with depth, similar to nutrient profiles, while Nd isotopes appear to reflect conservative water mass mixing in the intermediate and deep ocean; this has been termed the “Nd paradox”. Here we present a detailed study of the dissolved Nd isotopic composition across a section at 40°S in the South Atlantic, collected by UK GEOTRACES cruise (section GA10). The South Atlantic represents a natural laboratory for our understanding of spatial controls on ocean geochemistry, because of the large variability of inputs, spatial differences in particulate cycling, and horizontal advection and mixing at depth between major northern- and southern- sourced water masses. This variability has also made the South Atlantic a critical region for reconstructing past changes in ocean processes such as changes in biological productivity and deep ocean circulation, using paleoceanographic proxies such as foraminiferal carbon and Nd isotopes.This GA10 section of Nd isotopes provides observational data showing chemical changes across water mass boundaries as a result of the horizontal advection of water mass chemistry by ocean circulation but also containing the imprint of reversible scavenging as the major vertical process. In the surface ocean (0 - 600 m), Nd isotopic compositions are distinct between different surface ocean currents and spatially can be tied to various continental sources. In the intermediate ocean (600 - 2500 m), the vertical Nd isotope distribution exhibits distinct signals of different water masses (i.e. a-AAIW, i-AAIW, and u-NADW) by horizontal advection, as well as influence of reversible scavenging that smears the signals downwards (i.e. offset to more radiogenic values). In the deep ocean below 2500 m, Nd isotope distribution follows conservative water mass mixing model. Nd concentration in the deep ocean, however, deviates from conservative mixing and increases constantly with depth. We also observe that Nd isotopes appear to be shifted towards the composition of overlying water masses. These observations suggests that reversible scavenging onto organic and other types of particles is a major vertical process throughout the water column. We also suggest that this process can resolve the “Nd paradox” of decoupling of Nd concentration and isotopic composition because abyssal water masses already have a high [Nd], so that a given amount of added Nd has less effect on ƐNd in deep water masses than it does for low [Nd] intermediate water masses.

Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. In this study, we investigate the use of rare earth element (REE) and neodymium isotopes (εNd) in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents. A new index, called ‘concavity index’ (CI), is defined for measuring the degree of mid-REE enrichment in geological samples, which enables the determination of the source of iron oxides in sediments, such as seawater-derived Fe-oxyhydroxide phases, ancient marine Fe oxides derived from the erosion of sedimentary rocks, and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation. Using this index, we demonstrate that the εNd difference between paired Fe-oxide and detrital fractions in river sediments (defined here as ∆εNd Feox-Det) directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering. While rivers draining old cratons and volcanic provinces display near-zero ∆εNd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1; n = 30), multi-lithological catchments hosting sedimentary formations yield systematically higher values (2.7 ± 1.2; n = 44), showing that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. This assumption is reinforced by the evidence that calculated ∆εNd Feox-Det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins. Examining the influence of climate and tectonics on measured Nd isotopic compositions, we find that ∆εNd Feox-Det is strongly dependent on temperature in lowlands, following an Arrhenius-like relationship that reflects enhanced alteration of silicate rocks and formation of secondary Fe oxides in warmer climates. In contrast, in high-elevation catchments, ∆εNd Feox-Det defines striking correlation with maximum basin elevation, which we also interpret as reflecting the intensification of silicate weathering and associated Fe oxide formation as elevation decreases, due to the combined effects of thicker soils and warmer temperature. Overall, our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high-elevation environments, while silicate weathering dominates in floodplains. This novel approach combining REE and Nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record, which could be used in future studies to reassess the causal relationships between mountain uplift, erosion and climate throughout Earth's history.

Knowledge of the geometry and strength of the deep overturning circulation is central to the understanding of past climate variability on glacial–interglacial (G–I) timescales. In this study, neodymium (Nd) isotopic ratios on planktonic foraminifera are used to reconstruct the water mass source and mixing history of intermediate and deep water in the Southeast (SE) Atlantic and Southwest (SW) Pacific over the past 200 kyr from five sediment cores. Nd isotopes from a depth transect of cores in the SE Atlantic displayed a stronger geochemical gradient around 3.5 km at the LGM, with higher εNd values of ∼−6.0 below that boundary than those of ∼−7.8 above. In contrast, a similar εNd value (∼−9.5) is observed at both the intermediate and abyssal depths in the Holocene. The glacial upper ocean is 1.8 εNd units less radiogenic than the lower ocean, reflecting an increase in the amount of North Atlantic Component Water (NACW) in the upper SE Atlantic. A coherent Nd isotope change was observed in the SE Atlantic and SW Pacific intermediate and deep water with major excursion to more radiogenic εNd values during Marine Isotope Stage (MIS) 2, 4 and 6. This suggests the flux of NACW to the Southern Ocean was reduced during cold marine stages, possibly accompanied by changes in the water mass geometry. The constant 2 εNd-unit offset between intermediate water in the SE Atlantic and SW Pacific suggests the persistent propagation of less radiogenic NACW to the upper Pacific Ocean over the last glacial cycle. Moreover, the larger glacial vertical εNd gradients between the intermediate and deep waters in both the SE Atlantic and SW Pacific indicate a decreased proportion of NACW entrainment into the deep oceans, consistent with studies proposing reduced mixing between the upper and lower meridional overturning cells.

Most conceptual models of ocean circulation during past glacial periods invoke a shallowed North Atlantic‐sourced water mass overlying an expanded, poorly ventilated Southern Ocean (SO)‐sourced deep water mass (Southern Component Water, or SCW), rich in remineralised carbon, within the Atlantic basin. However, the ventilation state, carbon inventory and circulation pathway of SCW sourced in the Pacific sector of the SO (Pacific SO) during glacial periods are less well understood. Here, we present multi‐proxy data – including δ18O and δ13C measured on the benthic and planktic foraminifera Cibicidoides wuellerstorfi, and Neogloboquadrina pachyderma, and productivity proxies including percent CaCO3, total organic carbon (TOC) and Ba/Ti – from a sediment core located in the high latitude (71°S) Pacific SO spanning the last 800 kyr. Typical glacial δ13C values of SCW at this core site are ~0 ‰. We find no evidence for SCW with extremely low δ13C values during glacials in the high latitude Pacific SO. This leads to a spatial gradient in the stable carbon isotope composition of SCW from the high latitude SO, suggesting there are different processes of deep‐ and bottom‐water formation around Antarctica. A reduced imprint of air‐sea gas exchange is evident in the SCW formed in the Atlantic SO compared with the Pacific SO. A spatial δ13C gradient in SCW is apparent throughout much of the last 800,000 years, including interglacials. A SO‐wide depletion in benthic δ13C is observed is early MIS 16, coinciding with the lowest atmospheric pCO2 recorded in Antarctic ice cores.

The deep Pacific Ocean holds the largest oceanic reservoir of carbon which may interchange with the atmosphere on climatologically important timescales. The circulation of the deep Pacific during the Last Glacial Maximum (LGM), however, is not well understood. Neodymium (Nd) isotopes of ferromanganese oxide coatings precipitated on planktonic foraminifera are a valuable proxy for deep ocean water mass reconstruction in paleoceanography. In this study, we present Nd isotope compositions ( e Nd ) of planktonic foraminifera for the Holocene and the LGM obtained from 55 new sites widely distributed in the Pacific Ocean. The Holocene planktonic foraminiferal e Nd results agree with the proximal seawater data, indicating that they provide a reliable record of modern bottom water Nd isotopes in the deep Pacific. There is a good correlation between foraminiferal e Nd and seawater phosphate concentrations ( R 2 = 0.80 ), but poorer correlation with silicate ( R 2 = 0.37 ). Our interpretation is that the radiogenic Nd isotope is added to the deep open Pacific through particle release from the upper ocean during deep water mass advection and aging. The data thus also imply the Nd isotopes in the Pacific are not likely to be controlled by silicate cycling. In the North Pacific, the glacial Nd isotopic compositions are similar to the Holocene values, indicating that the Nd isotope composition of North Pacific Deep Water (NPDW) remained constant (−3.5 to −4). During the LGM, the southwest Pacific cores throughout the water column show higher e Nd corroborating previous studies which suggested a reduced inflow of North Atlantic Deep Water to the Pacific. However, the western equatorial Pacific deep water does not record a corresponding radiogenic excursion, implying reduced radiogenic boundary inputs during the LGM probably due to a shorter duration of seawater–particle interaction in a stronger glacial deep boundary current. A significant negative glacial e Nd excursion is evident in mid-depth (1–2 km) cores of the eastern equatorial Pacific (EEP) which may suggest a stronger influence of NPDW return flow to the core sites and decreased local input in the EEP. Taken together, our Nd records do not support a dynamically slower glacial Pacific overturning circulation, and imply that the increased carbon inventory of Pacific deep water might be due to poor high latitude air–sea exchange and increased biological pump efficiency in glacial times.

An ocean of climate impacts Large decreases in Atlantic meridional overturning circulation accompanied every one of the cold Northern Hemispheric stadial events that occurred during the heart of the last glacial period. These events, lasting on average around 1000 years each, have long been thought to result from changes in deep ocean circulation. Henry et al. used a suite of geochemical proxies from marine sediments to show that reductions in the export of northern deep waters occurred before and during stadial periods (see the Perspective by Schmittner). This observation firmly establishes the role of ocean circulation as a cause of abrupt glacial climate change during that interval. Science , this issue p. 470 ; see also p. 445

The identification of sediment sources to the ocean is a prerequisite to using marine sediment cores to extract information on past climate and ocean circulation. Sr and Nd isotopes are classical tools with which to trace source provenance. Despite considerable interest in the Arctic Ocean, the circum-Arctic source regions are poorly characterised in terms of their Sr and Nd isotopic compositions. In this study we present Sr and Nd isotope data from the Paleogene Central Basin sediments of Svalbard, including the first published data of stream suspended sediments from Svalbard. The stream suspended sediments exhibit considerable isotopic variation (εNd = −20.6 to −13.4; 87Sr ∕ 86Sr = 0.73421 to 0.74704) which can be related to the depositional history of the sedimentary formations from which they are derived. In combination with analysis of the clay mineralogy of catchment rocks and sediments, we suggest that the Central Basin sedimentary rocks were derived from two sources. One source is Proterozoic sediments derived from Greenlandic basement rocks which are rich in illite and have high 87Sr ∕ 86Sr and low εNd values. The second source is Carboniferous to Jurassic sediments derived from Siberian basalts which are rich in smectite and have low 87Sr ∕ 86Sr and high εNd values. Due to a change in depositional conditions throughout the Paleogene (from deep sea to continental) the relative proportions of these two sources vary in the Central Basin formations. The modern stream suspended sediment isotopic composition is then controlled by modern processes, in particular glaciation, which determines the present-day exposure of the formations and therefore the relative contribution of each formation to the stream suspended sediment load. This study demonstrates that the Nd isotopic composition of stream suspended sediments exhibits seasonal variation, which likely mirrors longer-term hydrological changes, with implications for source provenance studies based on fixed end-members through time.

We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first-order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single-domain (SD), pseudo-single-domain (PSD), and multi-domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end-member (EM) FORC diagrams, located at user-defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt-sized MD component, a fine silt-sized PSD component, and a mixed clay-sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA-based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four-EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi-component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.

International audience; Neodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years).At water depths ≥ 1500 m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥ 200 m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths.

Over the last 2 decades, significant advances have been made in reconstructing past rates of ocean circulation using sedimentary proxies for the dynamics of abyssal waters. In this study we combine the use of two rate proxies, sortable silt grain size, and sedimentary 231Pa/230Th, measured on a depth transect of deep-sea sediment cores from the northern NE Atlantic, to investigate ocean circulation changes during the last deglacial. We find that at two deep sites, the core-top 231Pa/230Th ratios reflect Holocene circulation rates, while during Heinrich Stadial 1, the deglacial ratios peaked as the sortable silt grain size decreased, reflecting a general circulation slowdown. However, the peak 231Pa/230Th significantly exceeded the production ratio in both cores, indicating that 231Pa/230Th was only partially controlled by ocean circulation at these sites. This is supported by a record of 231Pa/230Th from an intermediate water depth site, where values also peaked during Heinrich Stadial 1, but were consistently above the production ratio over the last 24 ka, reflecting high scavenging below productive surface waters. At our study sites, we find that preserved sediment component fluxes cannot be used to distinguish between a scavenging or circulation control, although they are consistent with a circulation influence, since the core at intermediate depth with the highest 231Pa/230Th recorded the lowest particle fluxes. Reconstruction of advection rate using 231Pa/230Th in this region is complicated by high productivity, but the data nevertheless contain important information on past deep ocean circulation.

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 e Nd = − 5.0 (1σ; ± 0.3 n = 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 δ C 13 data. The deglacial e Nd shift of CDW in the southwest Pacific is consistent with changes observed in the deep South Atlantic and Equatorial Indian Ocean, but e Nd values are offset by ∼ 1 e Nd -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 e Nd 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 δ C 13 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 δ C 13 composition of CDW was unrelated to circulation changes.

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 (Piotrowski et al., 2004, 2005, 2008) 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.

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 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.

Cycling Down The Mid-Pleistocene Transition, which lasted from approximately 1.25 million to 700 thousand years ago, was a period during which the dominant periodicity of Earth's climate cycles inexplicably changed from 41 thousand to 100 thousand years. This change is clearly apparent in the oxygen isotopic composition of many calcifying marine organisms, but changes in both ice volume and temperature affect the signal, and so exactly what the signal means has remained unclear. Elderfield et al. (p. 704 ; see the Perspective by Clark ) separated these two effects by measuring both the oxygen isotopic makeup and the Mg/Ca (a proxy that reflects changes in temperature only) of certain benthic foraminifera. The findings reveal the contributions of ice volume and temperature to glacial cycles, suggest when and why the Mid-Pleistocene Climate Transition occurred, and clarify how carbon is lost from the ocean-atmosphere during deglaciations but also changes because of ocean circulation.

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 (eNd) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of ∼4 eNd units along the flow path of Circumpolar Deep Water; eNd≈−8.8 in the deep southern inflow upstream of Madagascar, which evolves towards eNd≈−11.5 offshore northern Madagascar, whereas eNd≈−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 eNd, 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 eNd 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 eNd 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 eNd 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.

One of the key activities during the initial phase of the international GEOTRACES program was an extensive international intercalibration effort, to ensure that results for a range of trace elements and isotopes (TEIs) from different cruises and from different laboratories can be compared in a meaningful way. Here we present the results from the intercalibration efforts on neodymium isotopes and rare earth elements in seawater and marine particles. Fifteen different laboratories reported results for dissolved 143Nd/144Nd ratios in seawater at three different locations (BATS 15 m, BATS 2000 m, SAFe 3000 m), with an overall agreement within 47 to 57 ppm (2σ standard deviation of the mean). A similar agreement was found for analyses of an unknown pure Nd standard solution carried out by 13 laboratories (56 ppm), indicating that mass spectrometry is the main variable in achieving accurate and precise Nd isotope ratios. Overall, this result is very satisfactory, as the achieved precision is a factor of 40 better than the range of Nd isotopic compositions observed in the global ocean. Intercalibration for dissolved rare earth element concentrations (REEs) by six laboratories for two water depths at BATS yielded a reproducibility of 15% or better for all REE except Ce, which seems to be the most blank-sensitive REE. Neodymium concentrations from 12 laboratories show an agreement within 9%, reflecting the best currently possible reproducibility. Results for Nd isotopic compositions and REE concentrations on marine particles are inconclusive, and should be revisited in the future.

Reconstructing past detrital flux and provenance in the SouthernOcean provides information about changes in source regions associated with climate variations and transport pathways. We present a LastGlacial Maximum (LGM) to Holocene comparison of 230Th normalised fluxes combined with sediment provenance data (Pb, Nd and Sr isotopes) from a latitudinal core transect in the eastern Atlanticsector of the SouthernOcean (ODP Leg 177 cores). We compare the radiogenic isotopic composition (IC) of detritus in these cores to that of cores proximal to potential source areas. We observe a well-defined latitudinal Holocene gradient in both detrital flux and provenance of sediment. High detrital fluxes in the north are associated with terrigenous material derived from southern Africa, while low detrital fluxes in the south are associated with supply from southern South America, West Antarctica and the South Sandwich Islands. The data suggest that this well-defined Holocene gradient in detrital flux and sediment provenance is controlled by the flow of the Antarctic Circumpolar Current (ACC) and the position of its frontal zones. The LGM is characterised by 2 to 6 times higher than modern detrital fluxes at most ODP Leg 177 sites. The LGM detrital fluxes do not show a latitudinal trend and suggest a greatersupply of glaciogenic detritussourced from southern South America. GlacialPatagonian outwash sediments (< 5 μm fraction) were analysed and compared to the bulk compositions of the marine sediments. The Pb IC of the Patagonian sediments is very similar to the glacial IC of sediments in the Scotia Sea and at ~ 49° S latitude in the eastern Atlanticsector. We propose that the glacial IC of sediments is controlled by increased delivery of Patagoniandetritus initially supplied by glaciers and then transported at depth via the ACC.

The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. We present neodymium (Nd) and carbon (C) isotope records from an equatorial Indian Ocean core (SK129–CR2) spanning the last 150 kyr. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (Marine Isotope Stages; MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11–83/TNO57–21 South Atlantic deep Cape Basin Nd isotope record ( Piotrowski et al., 2005 ). Since the deep Cape Basin is oceanographically upstream of the Indian Ocean from a deep water circulation perspective, the Nd isotopes suggest that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial–interglacial timescales, in particular suggesting higher productivity during MIS 5. Increased equatorial productivity during MIS 5 is supported by benthic infaunal–epifaunal foraminiferal δ 13 C gradients, as well as benthic foraminiferal δ 13 C gradients along deep-water flow. Concurrent warming, indicated by planktonic foraminiferal Mg/Ca during MIS 5 at the site is consistent with changing thermocline temperature and may indicate a link to surface-ocean hydrographic changes.

Seawater carbonate ion and δ13C are affected by many processes including biology, air–sea exchange, alkalinity change, and mixing between different water masses. Study of modern ocean data shows that deep ocean carbonate ion and δ13C of dissolved inorganic carbon can be used together as useful tracers for deep water mass reconstructions in the past. We present records of deep water carbonate ion concentration ([CO32−]) changes of the North Atlantic Ocean water column since the last glacial, quantitatively reconstructed using benthic foraminiferal boron/calcium (B/Ca) ratios. Records from six cores over 1 to 4 km reveal that the carbonate chemistry of the glacial North Atlantic was more stratified than the modern ocean, with higher [CO32−] by ~ 20–30 μmol kg− 1 at 1–2 km and lower [CO32−] by ~ 20 μmol kg− 1 at sites deeper than 3.5 km, producing an 800 m glacial shoaling of calcite saturation horizon. Comparison with benthic foraminiferal δ13C and ɛNd of Fe–Mn oxide leachates shows that the deep glacial waters with low-[CO32−] are consistent with a Southern Ocean source, while those with high-[CO32−] but distinct δ13C chemistry were supplied by two endemic sources, one being the Norwegian–Greenland Sea (NGS). Our carbonate ion results suggest that the glacial boundary between north and south sourced deep waters is at ~2.8 km, significantly deeper than ~ 2.3 km estimated from benthic foraminiferal δ13C. Weakened surface compensation inflow to the NGS promoted cooling and continental ice growth at north high-latitude, and a deeper boundary may reduce atmospheric CO2 sequestration in the deep Atlantic, implying a greater role of other parts of the ocean.

While ocean circulation is driven by the formation of deep water in the North Atlantic and the Circum-Antarctic, the role of southern-sourced deep water formation in climate change is poorly understood. Here we address the balance of northern- and southern-sourced waters in the South Atlantic through the last glacial period using neodymium isotope ratios of authigenic ferromanganese oxides in thirteen deep sea cores from throughout the South Atlantic. The data indicate that northern-sourced water did not reach the Southern Ocean during the late glacial, and was replaced by southern-derived intermediate and deep waters. The high-resolution neodymium isotope record (~ 300 yr sample spacing) from two spliced deep Cape Basin sites indicates that over the last glacial period northern-sourced water mass export to the Southern Ocean was stronger during the major Greenland millennial warming intervals (and Southern Hemisphere cool periods), and particularly during the major interstadials 8, 12, and 14. Northern-sourced water mass export was weaker during Greenland stadials and reached minima during Heinrich Events. The benthic foraminiferal carbon isotopes in the same Cape Basin core reflect a partial control by Southern Hemisphere climate changes and indicate that deep water formation and ventilation occurred in the Southern Ocean during major Greenland cooling intervals (stadials). Together, neodymium isotopes and benthic carbon isotopes provide new information about water mass sourcing and circulation in deep Southern Ocean waters during rapid glacial climate changes. Combining carbon and neodymium isotopes can be used to monitor the relative proportion of northern- and southern-sourced waters in the Cape Basin to gain insight into the processes which control the carbon isotopic composition of deep waters. In this study we show that deep water formation and circulation was more important than biological productivity and nutrient regeneration changes for controlling the carbon isotope chemistry of Antarctic Bottom Water during millennial-scale glacial climate cycles. This observation also lends support to the hypothesis that ocean circulation is linked to interhemispheric climate changes on short timescales, and that ventilation in the glacial ocean rapidly switched between the northern and Southern Hemisphere on millennial timescales.

International audience; We use reductive sediment leaching to extract lead (Pb) from the authigenic fraction of marine sediments and reconstruct the Pb isotope evolution of the deep central Indian Ocean over the past 250 thousand years at ∼3 kyr resolution. Temporal variations define a binary mixing line that is consistent with data from ferromanganese nodules and which records mixing between two well-defined endmembers through time. The unradiogenic endmember appears to represent a widely-distributed Pb source, from mid-ocean ridges or possibly volcanic aerosols, while the radiogenic endmember coincides with the composition of Ganges–Brahmaputra river sediments that are indicative of the Himalayan weathering inputs. Glacial–interglacial Pb isotope variations are striking and can be explained by an enhancement of Himalayan contributions by two to three times during interglacial periods, indicating that climate modulates the supply of dissolved elements to the ocean. While these changes could accurately record variations in the continental chemical weathering flux in response to warmer and wetter conditions during interglacials, the relative proportions of Pb derived from the Ganges and Brahmaputra appear to have been constant through time. This observation may point towards particulate-dissolved interactions in the estuary or pro-delta as a buffer of short timescale variability in the composition (and potentially flux) of the fluvial inputs. In addition, the changes are recorded at 3800 m water depth, and with the lack of deep water formation in the Bay of Bengal, a mechanism to transfer such a signature into the deep ocean could either be reversible scavenging of dissolved Pb inputs and/or boundary exchange on the deep sea fan. Unless the mechanism transferring the Pb isotope signature into the deep ocean was itself highly sensitive to global climate cycles, and with the absence of a precessional signal in our Pb isotope data, we suggest that the Indian climate and its influence on basin-scale chemical weathering were strongly modulated by glacial versus interglacial boundary conditions.

The Black Sea was an inland lake during the last ice age and its sediments are an excellent potential source of information on Eurasian climate change, showing linkages between regionally and globally recognized millennial-scale climate events of the last deglaciation. Here, we detail changes from the last glacial maximum (LGM) through the transition to an anoxic marginal sea using isotopic (strontium and oxygen) and trace element (Sr/Ca) ratios in carbonate shells, which record changing input sources and hydrologic conditions in the basin and surrounding region. Sr isotope records show two prominent peaks between ∼18 and 16 ka BP cal, reflecting anomalous sedimentation associated with meltwater from disintegrating Eurasian ice sheets that brought Black Sea level to its spill point. Following a sharp drop in Sr isotope ratios back toward glacial values, two stages of inorganic calcite precipitation accompanied increasing oxygen isotope ratios and steady Sr isotope ratios. These calcite peaks are separated by an interval in which the geochemical proxies trend back toward glacial values. The observed changes reflect negative water balance and lake level decline during relatively warm periods (Bolling–Allerod and Preboreal) and increasing river input/less evaporation, resulting in higher lake levels, during the intervening cold period (the Younger Dryas). A final shift to marine values in Sr and oxygen isotope ratios at 9.4 ka BP cal corresponds to connection with the global ocean, and marks the onset of sedimentation on the Black Sea continental shelf. This date for the marine incursion is earlier than previously suggested based on the appearance of euryhaline fauna and the onset of sapropel formation in the deep basin.

Evidence from high-sedimentation-rate South Atlantic deep-sea cores indicates that global and Southern Ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice-sheet growth and retreat, respectively. No consistent lead-lag relationships are observed during abrupt millennial warming events during the last ice age, allowing for the possibility that ocean circulation triggered some millenial climate changes. At the major glacial-interglacial transitions, the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets.

Neodymium isotope ratios in the authigenic ferromanganese oxide component in a southeastern Atlantic core reveal a history of the global overturning circulation intensity through the last deglaciation. From a minimum during the Last Glacial Maximum (LGM), North Atlantic Deep Water (NADW) began to strengthen between 18 and 17 kyr cal. BP, approximately 2000–3000 years before the Bolling warming. It exhibits a gradually increasing baseline intensity that plateaus in the early Holocene, tracks increasing Northern Hemisphere insolation and parallels atmospheric CO2 concentration. Millennial-scale fluctuations are superimposed on this baseline NADW increase, corresponding to events in Northern Hemisphere climate records. The millennial excursions show sharp increases associated with the pre-Bolling retreat of continental glaciers and the Bolling warming itself, a decrease with Allerod cooling, and a recovery through the Younger Dryas and early Holocene. The data confirm a close link between deep-ocean circulation and North Atlantic climate changes. There is no clear linkage with meltwater pulses, indicating that rapid meltwater additions did not trigger observable mode changes in NADW production. However, rapid changes in North Atlantic sea ice cover show a strong relationship with the millennial perturbations of NADW flux to the deep Southern Ocean. The data indicate that the baseline intensity of NADW reaching the Cape Basin is paced by Northern Hemisphere insolation and that North Atlantic sea ice coverage acts as a major amplifying link between the oceans and the atmosphere. D 2004 Elsevier B.V. All rights reserved.

International audience; Neodymium isotopic ratios in marine environments have been used as a tracer of water masses and exchange processes between dissolved and particulate phases. The interest in this tracer has been growing with improvement of our knowledge on its chemical behaviour in the modern ocean and the identification of sedimentary phases that preserve past seawater epsilon(Nd) values. In the last few decades the Nd isotopic composition measured on Fe-Mn crusts, sediment leachates, bulk carbonate fraction, corals and fish teeth have been increasingly interpreted in the context of understanding the role of the ocean in paleoclimate changes. In particular, calcareous foraminiferal tests (shells) have acquired increasing attention as an archive of seawater Nd isotopic signatures, because it allows continuous high-resolution records to be measured and directly compared to other proxies including stable isotopes and trace metals. The main challenge of interpreting the Nd isotopic composition of foraminifera is determining the origin of the Nd preserved within them. In this review, we present an overview of methodological progress including that of bulk foraminifera and microanalyses within foraminiferal tests, as well as geochemical meaning of extracted Nd concentrations and isotopic compositions. The growing body of evidence suggests that Nd isotopic signatures of sedimentary planktonic foraminifera correspond to bottom water values rather than surface water ones. The Nd-rich phases associated with sedimentary foraminifera are adhesive nano-scale particles of Mn and Fe oxides and hydroxides, and Mn-rich carbonates formed within layers of foraminiferal calcite. Mechanical cleaning to remove clay minerals is likely to be sufficient in most cases to reconstruct past bottom water circulations. Unresolved issues include the potential influence of pore water Nd on epsilon(Nd) values extracted from sedimentary foraminiferal tests under different sedimentalogical and oceanographic conditions. (c) 2013 Elsevier Ltd. All rights reserved.

High-resolution Hf isotopic records are presented for hydrogenetic Fe-Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ~4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (~15 Ma) it reached +1. It then shifted gradually back to an ε(Hf) value of +3 at 4 Ma, since when it has decreased rapidly to about -1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ~2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ~2 ε(Hf) units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ~5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved using other isotope systems. (C) 2000 Elsevier Science B.V. All rights reserved.

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 (eNd) 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. Sequential acid-reductive leaching experiments at pH ∼2–5 on deep sea sediments from the western Indian Ocean enable us to investigate natural boundary exchange processes over a timescale appropriate to laboratory experiments. We provide evidence that both the dissolution of solid phases and exchange processes influence the eNd of leachates, which suggests that both processes may contribute to boundary exchange. We use major element and rare earth element (REE) data to investigate the pools of Nd that are accessed and demonstrate that sediment leachate eNd values cannot always be explained by admixture between an authigenic component and the bulk detrital component. For example, in core WIND 24B, acid-reductive leaching generates eNd values between −11 and −6 as a function of solution/solid ratios and leaching times, whereas the authigenic components have eNd ≈ −11 and the bulk detrital component has eNd ≈ −15. We infer that leaching in the Mascarene Basin accesses authigenic components and a minor radiogenic volcanic component that is more reactive than Madagascan-derived clays. The preferential mobilisation of such a minor component demonstrates that the Nd released by boundary exchange could often have a significantly different eNd composition than the bulk detrital sediment. These experiments further demonstrate certain limitations on the use of acid-reductive leaching to extract the eNd composition of the authigenic fraction of bulk deep sea sediments. For example, the detrital component may contain a reactive fraction which is also acid-extractible, while the incongruent nature of this dissolution suggests that it is often inappropriate to use the bulk detrital sediment elemental chemistry and/or eNd composition when assessing possible detrital contamination of leachates. Based on the highly systematic controls observed, and evidence from REE patterns on the phases extracted, we suggest two approaches that lead to the most reliable extraction of the authigenic eNd component and good agreement with foraminiferal-based approaches; either (i) leaching of sediments without a prior decarbonation step, or (ii) the use of short leaching times and low solution/solid ratios throughout.

Neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses. A new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material. However, the exact mechanism by which the Nd isotope signal from bottom waters becomes associated with planktonic foraminifera, the spatial distribution of rare earth element (REE) concentrations within the shell, and the possible mobility of REE ions during changing redox conditions, have not been fully investigated. Here we present REE concentration and Nd isotope data from mixed species of planktonic foraminifera taken from plankton tows, sediment traps and a sediment core from the NW Atlantic. We used multiple geochemical techniques to evaluate how, where and when REEs become associated with planktonic foraminifera as they settle through the water column, reside at the surface and are buried in the sediment. Analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938 m water depth indicate that only ∼20% of their associated Nd is biogenically incorporated into the calcite structure. The remaining 80% is associated with authigenic metal oxides and organic matter, which form in the water column, and remain extraneous to the carbonate structure. Remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites, allowing the Nd isotope ratio to undergo partial equilibration with the ambient seawater, as the foraminifera fall through the water column. Analyses of fossil foraminifera shells from sediment cores show that their REE concentrations increase by up to 10-fold at the sediment–water interface, and acquire an isotopic signature of bottom water. Adsorption and complexation of REE3+ ions between the inner layers of calcite contributes significantly to elevated REE concentrations in foraminifera. The most likely source of REE ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters. As planktonic foraminifera are buried below the sediment–water interface redox-sensitive ion concentrations are adjusted within the shells depending on the pore-water oxygen concentration. The concentration of ions which are passively redox sensitive, such as REE3+ ions, is also controlled to some extent by this process. We infer that (a) the Nd isotope signature of bottom water is preserved in planktonic foraminifera and (b) that it relies on the limited mobility of particle reactive REE3+ ions, aided in some environments by micron-scale precipitation of MnCO3. This study indicates that there may be sedimentary environments under which the bottom water Nd isotope signature is not preserved by planktonic foraminifera. Tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes.

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 ɛNd values that are significantly higher than seawater, indicating that volcanic material is easily leached by acid-reductive methods. The ɛNd 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 ɛNd 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.

The Depths of the Changes Over the course of the past glacial cycle, there have been two major types of rapid, large climate warming events: shorter-lived warm intervals lasting on the order of 1000 years and the last glacial-interglacial transition. Although both involved dramatic changes in large-scale ocean circulation, the extent to which those changes were similar is unclear. Roberts et al. (p. 75 ) analyzed the neodymium isotopic composition of the Fe-Mn oxide coatings of planktonic foraminifera and reconstructed patterns of Atlantic Ocean circulation during Heinrich event 1, a rapid global climate fluctuation about 14,000 years ago involving the destruction of Northern Hemisphere ice shelves and the last deglaciation. While both the source of deep water and the whole-ocean overturning rate shifted rapidly and synchronously during the last deglacial transition, only upper ocean circulation strength was affected during Heinrich event 1.

More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150-400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1°C over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process becomes widespread along Arctic continental margins, tens of Teragrams of methane per year could be released into the ocean.

The compilation of changes in the magnetic properties at various sites distributed along the path of the deepwater mass in North Atlantic shows large-amplitude short-term fluctuations reflecting changes in the strength of the North Atlantic Deep Water (NADW). These changes, which suggest a two-mode deep glacial circulation dynamic, are perfectly concurrent with air temperature changes over Greenland. They also share a similar pattern with those reported in the Nd isotope ratios from the deep Cape Basin during the same time period. Greenland interstadials were accompanied by increased flow speed of NADW and relatively more NADW reaching the Southern Ocean, while during Greenland stadials and Heinrich events, both the North Atlantic flow speed of NADW and its presence in the South Atlantic were reduced. It is demonstrated that both proxies are tracing the same water mass, and their reliability for monitoring changes in the deepwater circulation is therefore clearly established. After using the climatically independent geomagnetic assisted stratigraphy to put the Northern and Southern Hemisphere records on the same age scale, the South Atlantic record appears to lag changes in North Atlantic flow speeds by approximately 860 ± 220 years during the most prominent and best defined cycles (interstadials 12 and 8). Although future work is needed, this significant offset provides a first observation and tentative quantification of the time needed for glacial northern component water to mix downward and to flow from the North to the South Atlantic.

[1] Mineralogical and isotopic composition (Sr and Nd) of six dust samples, obtained from six widely spread ice-coring sites in Greenland, were analyzed in order to investigate the regional geographic variability of dust provenance. We show that long-range transport from eastern Asian deserts provides mineral dust with essentially the same composition to all elevated interior sites (Dye 3, Site A, GRIP, and NorthGRIP), while most material deposited at sites located closer to the edge and at lower altitude (Hans Tausen and Renland) derives from proximal source regions. No contribution from other sources is apparent at any of the interior sites from the mineralogical and isotopic composition of the dust samples, each of which represents several decades of dust deposition during the 17th–18th century. These results provide additional evidence that African and North American deserts do not play a significant role in the dust deposited over Greenland, which has implications for ice core record interpretation and atmospheric dust transport model validation.

Paleoceanographic reconstructions rely on chemical proxies which are controlled by physical, chemical, and biological marine parameters. The accurate interpretation of proxy records relies on the integrity of proxy-environmental relationships through time, and under changing conditions. In this study we closely examine paleo controls on authigenic Nd isotope records from five cores in the northern NE Atlantic, approximating a depth-transect, allowing spatial and temporal relationships to be reconstructed. We compare our Nd isotope records with other paleocirculation proxies, and consider the sedimentalogical controls on Nd isotope signals, by comparing ice-rafted detritus lithology and counts, detrital sediment chemistry and redox sensitive element concentrations measured on foraminifera authigenic coatings. With this suite of geochemical and sedimentalogical data we show that Nd isotope records in the northern NE Atlantic were labeled by radiogenic sediments, however this modification did not occur in the pore-waters of each core, but instead likely reflects changes in the Nd isotopic composition of deep-waters caused by the input of ice-rafted sediment during Heinrich events and the last glacial maximum. This study has implications for understanding how localized changes in the Nd isotope signal can set a watermass end-member composition, decoupling chemical proxy-circulation relationships locally, but providing a signal which can be potentially traced along the deep-water flowpath. Such scenarios must be considered in future interpretations of glacial Nd isotope records taken from within the ice-rafted detritus belt and downstream along watermass flowpaths.

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.

Highlights • First deep water Nd and Pb isotope records from the glacial–deglacial Nordic Seas. • Deglacial changed by up to six units from unradiogenic LGM background. • Glacial–deglacial Nd and Pb isotope changes controlled by local weathering input. • Pb isotopes suggest synchronous postglacial weathering fluxes around the North Atlantic. • Holocene Nd isotope evolution dominated by re-invigorated water mass mixing. Abstract Neodymium (Nd) isotopes are a powerful proxy tool for reconstructing past changes in water mass mixing, but reliable application of this proxy requires constraints on past changes of source water compositions. A key region of the global deep water circulation system are the Nordic Seas, which provide dense waters fundamental for the formation of North Atlantic Deep Water (NADW). Yet, the Nd isotope evolution of past deep waters in the Nordic Seas is so far poorly constrained. Here we present the first reconstructions of seawater Nd isotope compositions extracted from marine sediments at two locations in the central and northern Nordic Seas covering the period from the last glacial to the present. Further insights into past changes in sediment provenance, weathering inputs and water mass mixing are provided by complementary seawater and detrital strontium (Sr) and lead (Pb) isotope compositions. Our new data reveal that changes in source and magnitude of weathering inputs from the Scandinavian and Svalbard–Barents Ice Sheets (SIS and SBIS, respectively) controlled the Nd and Pb isotope composition of the Nordic Seas' deep waters during the last glacial period. During the Last Glacial Maximum (LGM), deep waters showed distinctly unradiogenic Nd and radiogenic Pb isotope signatures most likely driven by weathering inputs of the SBIS. In contrast, the deglaciation was characterized by enhanced SBIS ice dynamics and/or meltwater release delivering sediments from the distal Eurasian shelves to the Norwegian Sea. Pulses of volcanogenic sediment supply changed the deep water Nd isotope composition during Heinrich Stadials 1, 2 and the Bølling period. As such, the glacial–deglacial Nd and Pb isotope evolution was markedly different in the Nordic Seas, the North Atlantic and the Arctic Oceans, respectively. During the Holocene, Pb isotopes indicate synchronized weathering fluxes around the North Atlantic, whereas the influence of local weathering input on the Nd isotope evolution of the deep Nordic Seas ceased. Instead, the Holocene Nd isotope signal has been driven by the strength of the advection/convection of water masses in the study area. These new constraints on changes in the Nordic Seas provide important endmember information for Nd isotope based reconstructions of NADW export downstream in the Atlantic Ocean.

The deglacial transition from the last glacial maximum at $$\sim $$ 20 kiloyears before present (ka) to the Holocene (11.7 ka to Present) was interrupted by millennial-scale cold reversals, viz., Antarctic Cold Reversal ( $$\sim $$ 14.5–12.8 ka) and Greenland Younger Dryas ( $$\sim $$ 12.8–11.8 ka) which had different timings and extent of cooling in each hemisphere. The cause of this synchronously initiated, but different hemispheric cooling during these cold reversals (Antarctic Cold Reversal $$\sim $$ 3 $$^{\circ }\hbox {C}$$ and Younger Dryas $$\sim $$ 10 $$^{\circ }\hbox {C}$$ ) is elusive because $$\hbox {CO}_{2}$$ , the fundamental forcing for deglaciation, and Atlantic meridional overturning circulation, the driver of antiphased bipolar climate response, both fail to explain this asymmetry. We use centennial-resolution records of the local surface water $$\delta ^{18}\hbox {O}$$ of the Eastern Arabian Sea, which constitutes a proxy for the precipitation associated with the Indian Summer Monsoon, and other tropical precipitation records to deduce the role of tropical forcing in the polar cold reversals. We hypothesize a mechanism for tropical forcing, via the Indian Summer Monsoons, of the polar cold reversals by migration of the Inter-Tropical Convergence Zone and the associated cross-equatorial heat transport.

Ocean circulation critically affects the global climate and atmospheric carbon dioxide through redistribution of heat and carbon in the Earth system. Despite intensive research, the nature of past ocean circulation changes remains elusive. Here we present deep-water carbonate ion concentration reconstructions for widely distributed locations in the Atlantic Ocean, where low carbonate ion concentrations indicate carbon-rich waters. These data show a low-carbonate-ion water mass that extended northward up to about 20° S in the South Atlantic at 3–4 km depth during the Last Glacial Maximum. In combination with radiocarbon ages, neodymium isotopes and carbon isotopes, we conclude that this low-carbonate-ion signal reflects a widespread expansion of carbon-rich Pacific deep waters into the South Atlantic, revealing a glacial deep Atlantic circulation scheme different than commonly considered. Comparison of high-resolution carbonate ion records from different water depths in the South Atlantic indicates that this Pacific deep-water expansion developed from approximately 38,000 to 28,000 years ago. We infer that its associated carbon sequestration may have contributed critically to the contemporaneous decline in atmospheric carbon dioxide, thereby helping to initiate the glacial maximum. Carbon-rich Pacific deep water extended into the South Atlantic some 38,000 to 28,000 years ago, potentially contributing to a reduction in atmospheric carbon dioxide and the onset of the Last Glacial Maximum, according to deep-water carbonate chemistry reconstructions.