Specific previous interglacial periods showed millennial-scale climate variability that influenced deep water formation and circulation. However, our understanding about the spread of deep waters formed in the North Atlantic throughout the South Atlantic during interglacial periods remains limited. To reconstruct South Atlantic deep water geometry and the Atlantic Meridional Overturning Circulation (AMOC) differences between the Last Interglacial (LIG) and the Holocene, we present new authigenic neodymium isotope (epsilon Nd) and sortable silt (SS) records from the middepth western South Atlantic. The comparison of our results to other Atlantic epsilon Nd records reveals notable variations in the penetration of northern-sourced waters into the South Atlantic between these two periods. Moderate reduction of northern-sourced waters entrainment in the western South Atlantic was observed during the onset of the LIG (ca. 128-124 ka) compared to the Holocene, with a significant decrease in the eastward spread of this water mass. This reduction may be linked to freshwater inputs into the North Atlantic, which reduced water density. Counterintuitively, when the AMOC was expected to reach its full interglacial mode at the end of the LIG (ca. 120-116 ka), more radiogenic epsilon Nd values and lower SS values indicated a shallow AMOC. Increased mixing with southern-sourced waters during the LIG was likely due to decreased deep convection events at the North Atlantic high latitudes due to fresh subsurface water advection. Our findings suggest that the export and mixing ratio between northern- and southern-sourced waters in the South Atlantic are determined by Northern Hemisphere climate during interglacial periods.

The correlation between the amount of organic carbon (OC) and sulfur (S) in sediments has been widely used as a paleosalinity indicator to distinguish between marine and freshwater environments. However, whether the ratio of total OC to total S (TOC/TS) can be used to identify unsteady or dynamic marine environments across sedimentary strata is still contended. An HZW1907 sediment core of 80 m in length was successfully collected in the middle of Hangzhou Bay (HZB), serving as one of the few boreholes that are crucial for the study of geologic and geo-environment changes in the coastal regions of eastern China since the Last Glacial Maximum (LGM). Total OC (TOC), stable carbon isotope, and TS of 82 subsamples from the HZW1907 core were analyzed to reconstruct the history of the shallow water biological pump and sulfur preservation record in the bay since the Late Pleistocene. Our results indicate that the samples had low concentrations of TOC (0.21%) and total nitrogen (TN) (0.02%), high mass ratio of TOC/TN (10.8), low δ13C (−24.9‰), low TS content (0.06%), and a high ratio of TOC/TS (9.1) from 33.6 ka BP to 12.3 ka BP, implying that freshwater organic matter (OM), algae, and C3 plant fragments were the main sources of OM in a relatively cold environment. The abundances of TOC, TN, and TS increased to 0.56%, 0.07%, and 0.4%, respectively, while δ13C (−23.9‰) increased and TOC/TS (2.7) decreased in the Holocene sediments, suggesting that seawater began to influence the composition of the sediments of HZB. Climate warming, which is likely to have impacted the results, was experienced from 12.3 ka BP. An OC isotope mixing model indicated that since the Mid-late Holocene, more than 70% of riverine OM accounted for the total OM. The TOC/TS ratio was identified as an effective indicator of seawater intrusion, with C/S ratios of 1–6 being considered to indicate a “sea–land transitional zone” sedimentary environment, a C/S >6 indicating freshwater, and a C/S<1 indicating normal marine facies. These findings provide crucial evidence for using TOC/TS to distinguish freshwater from marine environments and enhance our understanding of past climate changes. Therefore, these geochemical indicators can be used in conjunction with other sedimentary records to obtain accurate results about sedimentary evolution.

Approximately 1,250,000 to 700,000 years ago, the pacing of glacial-interglacial cycles changed from 41,000 years to 100,000 years, a shift known as the 'Mid-Pleistocene Transition' (MPT). The cause - or causes - of this shift remain uncertain. However, changes in ocean circulation and removal of northern hemisphere regolith have both been proposed as potential triggers. Here, we present continuous, orbitally resolved reconstructions of deep ocean neodymium isotopes from three locations in the equatorial Atlantic, Indian and southwest Pacific oceans, spanning 1.7 million years from the Holocene to before the MPT, to test these two hypotheses. We find that global seawater neodymium isotope variability over glacial-interglacial cycles is controlled by changes in both neodymium input to the North Atlantic and deep ocean mixing. Using this neodymium isotope data, we show that enhanced northern hemisphere regolith removal began approximately 1.45 million years ago, 200,000 years prior to the onset of the MPT and 500,000 years prior to a major expansion in northern hemisphere ice sheets between 900,000 and 870,000 years ago. This ice sheet expansion was immediately preceded by an interval of reduced mixing of Atlantic-sourced waters into the deep southwest Pacific Ocean. We hypothesize that this circulation reorganization reflected increased stratification of the deep Southern Ocean interior, possibly in response to cooling and Antarctic sea ice expansion at this time. Taken together, these data suggest an expansion and/or thickening of northern hemisphere ice sheets during the MPT was facilitated by a combination of reduced northern hemisphere regolith cover alongside Southern Ocean circulation changes. Together, these shifts allowed the build up of larger northern hemisphere ice sheets that were more resistant to deglaciation, facilitating the longer glacial cycles of the post-MPT world.

Deep-sea oxygen concentrations reflect combined effects of air-sea exchange in high-latitude surface waters, ventilation through ocean circulation and the organic carbon remineralization at depth. Reconstruction of past bottom water oxygen (BWO) concentrations has been challenging due to limi-tations of each existing BWO proxy whose fidelity may be complicated by diagenetic or depositional factors. Therefore, evaluations on BWO changes with multi-proxy approach are always preferred. In this study, we exploit the authigenic uranium content on mixed planktonic foraminiferal coatings as a BWO proxy by presenting new foraminiferal U/Ca and U/Mn ratios of the Holocene and last glacial maximum (LGM) sediments from 54 sites throughout the Pacific Ocean, covering a range of modern BWO from 8 to 210 mu mol/kg. Distinct correlation between Holocene foraminiferal U/Ca and U/Mn with BWO is observed in the modern Pacific Ocean, which has not been obscured by other complicating factors such as sedi-mentation rate and organic carbon flux. Based on the comparison of our foraminiferal U/Ca and U/Mn ratios between the Holocene and LGM and existing redox proxy data, we provide new constraints on Equatorial and South Pacific oxygenation changes during the LGM. First, the boundary between better oxygenated upper ocean and less oxygenated deeper ocean in the Eastern Equatorial Pacific was limited to a narrower water depth range between similar to 0.6 and 0.7 km surrounding the Panama Basin. Second, our data imply better oxygenation in the upper and bottom waters of the Pacific Ocean and mid-depth deoxygenation, which contrasts with findings in the deep Atlantic and Indian Oceans. After excluding influences from other factors such as sedimentation rates and productivity, our study demonstrates foraminiferal U/Ca and U/Mn provide a useful proxy for BWO reconstruction in the Pacific, thus helping to constrain the glacial-interglacial oceanic carbon cycle. (c) 2023 Elsevier Ltd. All rights reserved.

The Nordic Seas are the primary location where the warm waters of the North Atlantic Current densify to form North Atlantic Deep Water, which plays a key part in the modern Atlantic Meridional Overturning Circulation. The formation of dense water in the Nordic Seas and Arctic Ocean and resulting ocean circulation changes were probably driven by and contributed to the regional and global climate of the last glacial maximum (LGM). Here we map the source and degree of mixing of deep water in the Nordic Seas and through the Arctic Gateway (Yermak Plateau) over the past 35 thousand years using neodymium isotopes (εNd) measured on authigenic phases in deep-sea sediments with a high spatial and temporal resolution. We find that a large-scale reorganization of deep-water formation in the Nordic Seas took place between the LGM (23–18 thousand years ago) and the rapid climate shift that accompanied the subsequent deglaciation (18–10 thousand years ago). We show that homogeneous εNd signatures across a wide range of sites support LGM deep-water formation in the Nordic Seas. In contrast, during the deglaciation, disparate and spatially variable εNd values are observed leading to the conclusion that deep-water formation may have been reduced during this time. Deep-water formation in the Nordic Seas that helps to drive the Atlantic Meridional Overturning Circulation was vigorous during the last glacial maximum, much as it is today, and declined during deglaciation, according to neodymium isotope records.

Novel N-doped biochar (AKB) nanomaterials derived from raw kelp root were successfully developed at different temperatures (600-1000 degrees C) and were used to explore the adsorption performance of cationic dye from water. Various characterization technologies including scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) patterns, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy were used to analyze the physical and chemical properties of the samples. The porosity analysis demonstrated that AKB carbonized at 600 degrees C (AKB-600) possessed a higher specific surface area (1738 m(2)/g) with a honeycomb-like microstructure. Meanwhile, AKB exhibited abundant functional groups on its surface, such as-OH, C@O and -CONH-from FT-IR and XPS analysis, resulting to the high removal efficiency of dye, up to 94.61 % at 60 min. The adsorption isotherm and kinetics data were more suitable for the Langmuir model and the Pseudo-second-order model. Electrostatic interactions and physical adsorption played dominant roles in the adsorption process, particularly, the nitrogen-containing and conjugated structures also facilitated the actual adsorption. Hence, AKB has enormous practical application potential acting as a high-effective adsorbent in the field of dye wastewater treatment. (C) 2022 Elsevier B.V. All rights reserved.

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 higher 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.

Nickel is abundant in the earth's crust and is as one of the eight major elements that cause soil pollution in the soil in China. Recently, nickel pollution has become an important environmental problem affecting China's economic and social development. In order to practice the scientific concept of "lucid waters and lush mountains are invaluable assets" and understand the pollution degree of nickel from the source, scientific methods have been adopted to improve the pollution in the soils. This study summarizes pollution status of nickel both domestic and international, and takes a contaminated plot in the Longwan District in Wenzhou City of Zhejiang Province as an example to further analyze the source of nickel pollution in the soil, which is of great significance for finding suitable remediation methods. Various remediation technologies for nickel pollution involve physical and chemical remediation technology, plant and microbial remediation technology, particularly, passivator remediation technology with great application potential and commercial prospects is used as a key way for nickel pollution control from soils.

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 degrees 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 radio genic 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. Crown Copyright (C) 2021 Published by Elsevier Ltd. All rights reserved.

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. (C) 2021 Elsevier B.V. All rights reserved.

ABSTRACT: Continental shelves have the potential to remove atmospheric carbon dioxide via the biological pump, burying it in seafloor sediments. The efficiency of marine carbon sequestration changes rapidly due to variations in biological productivity, organic carbon oxidation, and burial rate. Here we present a high temporal resolution record of marine carbon sequestration changes from a western South Atlantic shelf site sensitive to Brazil Current-driven upwelling. The comparison of biological records to rare earth element (REE) patterns from authigenic oxides shows a strong relationship between higher biological productivity and stronger particle reactive element cycling (i.e. REE cycling) during rapid climate change events. This is the first evidence that authigenic oxides archive past changes in upper ocean REE cycling by the exported organic carbon. In addition, our data suggest that Brazil Current-driven upwelling varies on millennial-scales and in time with continental precipitation anomalies as registered in Brazilian speleothems during the Holocene. This indicates an ocean-atmosphere control on the biological pump, most probably related to South American monsoon system variability.

The Antarctic Peninsula's Pacific margin is one of the best studied sectors of the Antarctic continental margin. Since the 1990s, several research cruises have targeted the continental rise with geophysical surveys, conventional coring and deep-sea drilling. The previous studies highlighted the potential of large sediment drifts on the rise as high-resolution palaeoenvironmental archives. However, these studies also suffered from chronological difficulties arising from the lack of calcareous microfossils, with initial results from geomagnetic relative palaeointensity (RPI) dating promising a possible solution. This paper presents data from new sediment cores recovered on cruise JR298 from seven continental rise sites west of the Antarctic Peninsula and in the Bellingshausen Sea with the objectives to (i) seek calcareous foraminifera, especially at shallow drift sites, to constrain RPI-based age models, and (ii) investigate the depositional history at these locations. We present the results of chronological and multiproxy analyses on these cores and two cores previously collected from the study area. We establish new age models for the JR298 records and compare them with published RPI-based age models. In addition, we evaluate the reliability of different palaeoproductivity proxies and infer depositional processes. Planktic foraminifera are present in various core intervals. Although their stable oxygen isotope (delta O-18) ratios, tephrochronological constraints and glacial-interglacial changes in sediment composition provide age models largely consistent with the RPI chronologies, we also observe distinct differences, predominantly in the Bellingshausen Sea cores. Enrichments of solid-phase manganese together with evidence for "burn-down" of organic carbon in late glacial and peak interglacial sediments document non-steady-state diagenesis that may have altered magnetic mineralogy and, thus, RPI proxies. This process may explain discrepancies between RPI-based age models and those derived from delta O-18 data combined with tephrochronology. The data also indicate that organic carbon is a much less reliable productivity proxy than biogenic barium or organically-associated bromine in the investigated sediments. In agreement with previous studies, sediment facies indicate a strong control of deposition on the rise by bottom currents that interacted with detritus supplied by meltwater plumes, gravitational downslope transport processes and pelagic settling of iceberg-rafted debris (IRD) and planktic microfossils. Bottom-current velocities underwent only minor changes over glacial-interglacial cycles at the drift crests, with down-slope deposition only rarely affecting these shallow locations. Maximum concentrations of coarse IRD at the seafloor surfaces of the shallow sites result predominantly from upward pumping caused by extensive bioturbation. This process has to be taken into account when past changes in IRD deposition are inferred from quantifying clasts >1 mm in size. (C) 2021 The Author(s). Published by Elsevier Ltd.

Chemical weathering plays an important role in sequestering atmospheric CO 2 , 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. Here, we present a novel method for tracing the origin of weathered rocks on continents, based on the measurement of REE and Nd isotopes (e Nd ) 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 (De Nd Feox-Det) reflects the relative contribution of sedimentary vs crystalline silicate rocks during weathering. Rivers draining old cratons and volcanic provinces display near-zero De Nd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1), while multi-lithological catchments hosting sedimentary formations yield 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

The oceanic thermocline circulation provides a route of communication between the surface and deep ocean and could have played an important role in the global carbon cycle, but studies on reconstructing past thermocline water properties are limited. Here we explore the potential use of left-coiling Globorotalia truncatulinoides as a recorder of thermocline conditions by measuring the stable oxygen and carbon isotopic compositions of this species from 28 surface sediments in the southwest Pacific near New Zealand. Our data show that G. truncatulinoides (sinistral) calcify mainly in the range of subsurface/thermocline depths in this study region between 100 and 850 m with their carbon isotopes largely corresponding to the surrounding seawater values. To understand the controlling factors of the thermocline δ13C evolution in the South Pacific, a 260 ka downcore δ13C record on this species (δ13CG.trunc) from core site ODP1123 is presented and compared with other δ13C records. The convergence of δ13C from thermocline, upper and lower circumpolar deep waters (UCDW/LCDW) during glacial terminations indicates that the deep ocean is the predominant source of increased atmospheric pCO2 and the δ13C anomalies in the upper ocean and atmosphere during the deglacials. This is evident in both the South Pacific and South Atlantic. A quantitative calculation of predicted surface ocean δ13C based on thermodynamic air-sea equilibrium implies this process has a significant control on the temporal thermocline water δ13C variation over the last glacial-interglacial (G-I) cycle. The lower deglacial δ13CG.trunc values in the South Atlantic compared to the Pacific further suggest a stronger upwelling in the Atlantic sector of the Southern Ocean, indicating that this was a major ventilation route with an important stock of light δ13C from the deep waters of this region. This study demonstrates the deep water influence (via upwelling) and atmospheric carbon isotope imprint (via air-sea exchange) on thermocline water δ13C evolution. It also provides important evidence for the rapid exchange of carbon between the Southern Ocean and atmosphere over multiple G-I cycles.

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 (epsilon(Nd)) 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 Feoxyhydroxide 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 epsilon(Nd) difference between paired Fe-oxide and detrital fractions in river sediments (defined here as AeNd 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 Delta epsilon(Nd) (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 Delta epsilon(Nd) (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 Delta epsilon(Nd) (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, Delta epsilon(Nd) (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.

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.

Changes in deep ocean circulation play a major role in climate change due to heat transportation and large carbon storage in the deep ocean. The South Atlantic Ocean is a key area for monitoring changes in the global overturning circulation intensity. Two proxies which have been used to infer water mass sourcing and mixing proportions are stable carbon isotopic composition ( 13 C/ 12 C, expressed as δ 13 C) and neodymium isotopic composition ( 143 Nd/ 144 Nd, expressed as εNd). To better understand the sensitivity of these two proxies to past changes in ocean circulation, and determine what processes cause their decoupling, their distribution in the modern deep ocean needs to be clarified. Here we present the first profile of dissolved neodymium isotopes alongside carbon isotopes of dissolved inorganic carbon measured on the same seawater samples across a longitudinal section at 40°S in the South Atlantic (GEOTRACES section GA10). The results show the different behaviors of the two elements and their isotopes between the upper ocean (< 2500m depth) and the deep ocean (> 2500m depth). Our data highlight the quasi-conservative behavior and covariation of Nd and C isotopes in the deep ocean, indicating the high reliability of the two proxies for palaeo-ocean circulation in the South Atlantic. We also examine non-conservative behaviors in the upper ocean from a reversible scavenging perspective and discuss the implications regarding changes in vertical transport of geochemical signals with varying particle flux.

The stable isotopic composition of particulate organic carbon (δCPOC) in the surface waters of the global ocean can vary with the aqueous CO2 concentration ([CO2(aq)]) and affects the trophic transfer of carbon isotopes in the marine food web. Other factors such as cell size, growth rate and carbon concentrating mechanisms decouple this observed correlation. Here, the variability in δCPOC is investigated in surface waters across the south subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon isotope fractionation (εp) by phytoplankton and the contrasting mechanisms of carbon uptake in the subantarctic and subtropical water masses. Our results indicate that cell size is the primary determinant of δCPOC across the Atlantic SSTC in summer. Combining cell size estimates with CO2 concentrations, we can accurately estimate εp within the varying surface water masses in this region. We further utilize these results to investigate future changes in εp with increased anthropogenic carbon availability. Our results suggest that smaller cells, which are prevalent in the subtropical ocean, will respond less to increased [CO2(aq)] than the larger cells found south of the SSTC and in the wider Southern Ocean. In the subantarctic water masses, isotopic fractionation during carbon uptake will likely increase, both with increasing CO2 availability to the cell, but also if increased stratification leads to decreases in average community cell size. Coupled with decreasing δ13C of [CO2(aq)] due to anthropogenic CO2 emissions, this change in isotopic fractionation and lowering of δCPOC may propagate through the marine food web, with implications for the use of δCPOC as a tracer of dietary sources in the marine environment.

Abstract. The stable isotopic composition of particulate organic carbon (δ13CPOC) in the surface waters of the global ocean can vary with the aqueous CO2 concentration ([CO2(aq)]) and affects the trophic transfer of carbon isotopes in the marine food web. Other factors such as cell size, growth rate and carbon concentrating mechanisms decouple this observed correlation. Here, the variability in δ13CPOC is investigated in surface waters across the south subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon isotope fractionation (εp) by phytoplankton and the contrasting mechanisms of carbon uptake in the subantarctic and subtropical water masses. Our results indicate that cell size is the primary determinant of δ13CPOC across the Atlantic SSTC in summer. Combining cell size estimates with CO2 concentrations, we can accurately estimate εp within the varying surface water masses in this region. We further utilize these results to investigate future changes in εp with increased anthropogenic carbon availability. Our results suggest that smaller cells, which are prevalent in the subtropical ocean, will respond less to increased [CO2(aq)] than the larger cells found south of the SSTC and in the wider Southern Ocean. In the subantarctic water masses, isotopic fractionation during carbon uptake will likely increase, both with increasing CO2 availability to the cell, but also if increased stratification leads to decreases in average community cell size. Coupled with decreasing δ13C of [CO2(aq)] due to anthropogenic CO2 emissions, this change in isotopic fractionation and lowering of δ13CPOC may propagate through the marine food web, with implications for the use of δ13CPOC as a tracer of dietary sources in the marine environment.