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Last glacial atmospheric CO2 decline due to widespread Pacific deep-water expansion

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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.
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1Research School of Earth Sciences, The Australian National University, Canberra, Australian Capital Territory, Australia. 2State Key Laboratory of Loess
and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China. 3Climate Change Research Centre, University of
New South Wales, Sydney, New South Wales, Australia. 4CAS Center for Excellence in Quaternary Science and Global Change, Xi’an, China. 5Open Studio
for Oceanic–Continental Climate and Environment Changes, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China.
6Lamont–Doherty Earth Observatory, Columbia University, New York, NY, USA. 7State Key Laboratory of Marine Geology, Tongji University, Shanghai,
China. 8Department of Earth Sciences, University of Cambridge, Cambridge, UK. 9Ocean and Earth Science, National Oceanography Centre, University
of Southampton, Southampton, UK. 10Centro de Investigación Mariña, GEOMA, Palaeoclimatology Lab, Universidade de Vigo, Vigo, Spain.
Ocean circulation and the carbon cycle are intricately linked,
thus ocean circulation reconstructions can provide impor-
tant insights into the mechanisms of past atmospheric CO2
changes. Circulation in the deep (more than ~2.5 km) Atlantic dur-
ing the Last Glacial Maximum (LGM; 18–22 thousand years ago
(ka)) is traditionally viewed as following a mixing model between
deep waters formed in the basin’s polar regions, without much con-
tribution from waters from other oceans14. Using this long-held
ocean circulation model, however, it is difficult to explain the
observed older radiocarbon (14C) ages and more radiogenic neo-
dymium isotopic (εNd) signatures at ~3.8 km than at ~5 km in the
LGM South Atlantic5,6 (Fig. 1). Burke etal.7 showed that sluggish
recirculation of southern-sourced waters combined with reduced
mixing with 14C-rich northern-sourced waters can contribute to old
14C ages at ~3.8 km, in the absence of interocean water-mass inter-
actions. Yet, additional mechanisms are probably needed to fully
explain the depth structure and large magnitude of 14C age changes,
along with the more radiogenic εNd signal observed at 3.8 km
(Fig. 1). Pacific Deep Water (PDW) can notably affect deglacial εNd
signatures in the Drake Passage (Southern Ocean)8, but their role in
the deep South Atlantic during the LGM remains unexplored. PDW
stores a large amount of respired carbon9,10, thus temporal changes
in its volumetric extent would have important implications for past
atmospheric CO2 levels.
Deep-water carbonate ion concentrations ([CO32–]) can provide
critical information about past deep ocean circulation and dissolved
inorganic carbon (DIC) changes. In the modern Atlantic, contrast-
ing [CO32–] signatures between water masses reflect ocean circula-
tion patterns11 (Fig. 2). Also, past DIC changes may be quantified
from [CO32–] reconstructions12. Here we present deep-water [CO32–]
reconstructions for extensive locations in the Atlantic to deci-
pher the role of ocean circulation in the glacial atmospheric CO2
decrease. We focus on deep South Atlantic hydrography, which
remains incompletely understood despite intensive studies58,1316.
First meridional [CO32–] transect for the LGM Atlantic
We have reconstructed deep-water [CO32–] using benthic B/Ca for
the Holocene (0–5 ka) and LGM samples from 41 cores (Fig. 2 and
Extended Data Figs. 1–3). Five cores at 3.0–4.2 km and an abyssal
core at ~5 km from the South Atlantic were chosen to investigate the
reasons for the 14C and εNd anomalies at 3.8 km water depth (Figs. 1
and 2a). Thirty additional cores from widely spread locations (1.1–
4.7 km, 36° S to 62° N) in the Atlantic and five cores at 3–4 km from
the equatorial Pacific provide a broader context of water-mass sig-
natures. Benthic B/Ca is converted into deep-water [CO32–] using
species-specific global core-top calibrations17. The uncertainty asso-
ciated with [CO32–] reconstructions is ~5 μmol kg–1 (ref. 17). Detailed
information about the samples and analytical methods along with
new (n = 173 samples) and compiled (n = 260 samples) data is given
in Methods and Supplementary Tables 1–7.
Figure 2c shows the first meridional [CO32–] transect for the
deep Atlantic during the LGM (Methods). Given the locations of
the studied cores, this transect mainly reflects [CO32–] distributions
for eastern Atlantic basins. Future work is needed to investigate the
extent of zonal homogeneity in the LGM Atlantic. Above ~2.5 km,
the [CO32–] of glacial North Atlantic waters reached up to ~140 μmol
kg–1, which is ~20 μmol kg–1 higher than in modern North Atlantic
Deep Water (NADW)11. These waters likely represent the previously
Last glacial atmospheric CO2 decline due to
widespread Pacific deep-water expansion
J. Yu 1,2 ✉ , L. Menviel 3, Z. D. Jin 2,4,5, R. F. Anderson 6, Z. Jian7, A. M. Piotrowski8, X. Ma2,
E. J. Rohling 1,9, F. Zhang 2,4, G. Marino1,10 and J. F. McManus6
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 pres-
ent 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 radio-
carbon 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 differ-
ent 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.
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... It is widely accepted that the magnitude of cooling and continental ice expansion at the LGM was amplified by the reduction in atmospheric CO 2 (e.g. Shakun et al., 2012;Marcott et al., 2014), and that more CO 2 was sequestered within the ocean at that time than is the case today (Yu et al., 2020;Curry and Oppo, 2005;Gebbie et al., 2015). ...
... There is also evidence for a shutdown in the export of AABW from the Weddell Sea region, which would further shift deep water ε Nd towards more radiogenic values (Huang et al., 2020). This reduction in NCW is consistent with evidence from benthic foraminiferal d 13 C (McCorkle et al., 1998;Hodell et al., 2003;Lund et al., 2015;Curry and Oppo, 2005;Gebbie et al., 2015;Sikes et al., 2017), d 18 O (Sikes 2017), and Cd/Ca (Marchitto et al., 1998;Umling et al., 2019), reconstructed seawater carbonate ion concentrations (Yu et al., , 2020 and south Atlantic and Southern Ocean sediment distributions during the LGM (Diekmann et al., 1999). ...
... The radiogenic ε Nd data of core TT1811-34GGC indicate that the lower circulation cell was made up of a greater proportion of recirculated PDW throughout the last glacial period, not just during the LGM (Wilson et al., 2020;Yu et al., 2020). The offset in ε Nd between our core T1811-34GGC and ODP Site 1088 suggests the inability of NCW from the South Atlantic to mix into the main body of LCDW across the entire last glacial period, even at times when AMOC may have been relatively strong (Bohm et al., 2015). ...
The chain of events surrounding the initiation and intensification of the last glacial cycle remain relatively poorly understood. In particular, the role of Southern Ocean paleocirculation changes is poorly constrained, in part, owing to a paucity of sedimentary records from this region. In this study we present multiproxy data e including neodymium isotope and sortable silt measurements e for paleocirculation changes within the deep (3167 m water depth) Indian sector of the Southern Ocean from a new sediment core, TT1811-34GGC (41.718?S, 80.163?E). We find a tight coupling between circulation changes, Antarctic climate, and atmospheric CO2 concentrations throughout the last 118,000 years, even during the initial stages of glacial inception of Marine Isotope Stage (MIS) 5.4 to 5.1. We find that periods of cooling correspond to reductions in the entrainment of North Atlantic-sourced waters within the deep Southern Ocean, as evidenced by more radiogenic neodymium isotope values of deep water bathing our core site. Cooling also corresponds to generally slower bottom water flow speeds, as indicated by finer sortable silt size fractions. A reduction in entrainment of North-Atlantic sourced waters occurred during MIS 5.4e5.1, when Atlantic circulation was strong, suggesting a Southern hemisphere control on paleocirculation changes at that time. We hypothesise that expanded Southern Ocean sea-ice during MIS 5.4 increased the density of the deep Southern Ocean, reducing the ability of Atlantic-sourced waters to mix into Lower Circumpolar Deep Water. This led to an expanded contribution of Pacific Deep Water within the lower circulation cell and increased stratification within the deep Southern Ocean. These paleocirculation changes can help account for the reduction in atmospheric CO2 across the MIS 5.5 to 5.4 transition, and in doing so help explain the chain of events surrounding the decent into the last glacial period.
... However, more records from the Southern Indian Ocean, particularly along these two separate pathways and water column data are required to validate the above hypothesis. On the contrary, similar values with more radiogenic glacial ε Nd (ε Nd ¼ À6.8 ± 0.4) in South-East Atlantic , equatorial Indian Ocean (Piotrowski et al., 2009), and Arabian Sea (present study) suggest reduction in the export/shallowing of NADW (Rutberg et al., 2000;Yu et al., 2020). ...
... To quantify the fractional contributions of NADW and AABW during G-I periods in the Arabian Sea, we performed a Nd isotope mass balance calculation following the method of Rahaman et al. (2020) with the assumptions that Nd isotope behaves quasiconservatively and the mixing of the water masses at this core site is binary. Since the isotopic values of the end-members have changed during the G-I cycles (Yu et al., 2020), we have calculated water mass fractions separately for the interglacial and glacial stages. The late Holocene period (0e5 ka) is taken as a representative of the warm interglacials and the last glacial maximum (LGM) (18e22 ka) as a representative of the cold glacials. ...
... The endmember details of water masses are given in Table S5. The Holocene ε Nd and [Nd] end-member values of AABW and NADW, À8.0 ± 1.0, 25.1 pmol/kg, and À13.5 ± 0.5, 17.5 pmol/kg, respectively, were taken from published literature (Howe et al., 2016;Yu et al., 2020). However, the glacial NADW (GNADW) ε Nd end member value is not well constrained because of the large variability (À10.5 to À13.9) observed in the North Atlantic during the glacial periods (Bohm et al., 2015). ...
Global overturning circulation plays a vital role in atmospheric CO2 and climate variability during glacial-interglacial (G-I) cycles; however, the exact mechanism remains elusive due to inadequate knowledge on past deep water circulation in the global ocean. Since no deep water is formed in the northern Indian Ocean, it ventilates from the south and acts only as a host for deep water circulation. Absence of any active deep water formation makes the northern Indian Ocean an ideal location to assess the extent of southern source waters and its role on past CO2 variability during the G-I climate cycles. This study provides the first record of deep water circulation in the Arabian Sea, the northwestern Indian Ocean, during the past 136 ka based on authigenic Nd isotope record (εNd). The Arabian Sea εNd record shows large variability ranging from −8.8 to −6.5 with more radiogenic values during the glacial stages (MIS 2 & 6) and less radiogenic values during the interglacial stages (MIS 1 & 5) indicating changes in water mass sources. The observation of more radiogenic εNd values similar to the glacial Antarctic Bottom Water (AABW) indicates enhanced flow of AABW (95–100%) and substantial reduction and/or almost complete retreat of North Atlantic Deep Water (NADW, 0–5%) during the glacials, whereas less radiogenic values indicate enhanced flow of NADW (∼20–40%) during the interglacials. The Arabian Sea εNd record followed exactly similar pattern to that of the equatorial Indian Ocean (EIO). However, amplitude of their variations differed significantly during the interglacials (MIS 1 & 5); the Arabian Sea εNd values were more radiogenic than the EIO. This suggests that during the interglacials, the Arabian Sea received more fraction of AABW through the western pathway, whereas the EIO received more fraction of NADW through the central pathway. This highlights differences in deep water exports from the Southern Ocean to the Arabian Sea and the EIO during the interglacials whereas export of similar water masses and its uniform distribution up to the northern Indian Ocean during the glacials. Our findings of significant G-I changes in AABW and NADW exports to the Indian Ocean and intra-basinal differences in their distribution have important implications for regional biogeochemical processes, paleo-redox conditions in the water column, carbon sink (organic and inorganic) and atmospheric CO2 variability during the G-I climate transitions.
... Buoyancy fluxes associated with sea ice production and melt play a primary role in Southern Ocean circulation (Abernathey et al., 2016;Pellichero et al., 2018), and changes in sea ice extent can potentially influence the basin-wide organization of water masses (Ferrari et al., 2014). While we do not see evidence for a dramatic shoaling of NADW at any time over the last glacial cycle, increased mid to deep gradients in εNd and δ 13 C could indicate the presence of more southern-sourced deep water, potentially with a larger contribution from the Pacific (as was observed at the LGM by Yu et al., 2020). Increased sea ice can also influence δ 13 C values through its influence on the regional extent of air-sea gas exchange (Broecker & Maier-Reimer, 1992;Lynch-Stieglitz & Fairbanks, 1994;Lynch-Stieglitz et al., 1995;Marchitto & Broecker, 2006). ...
... Using the B/Ca proxy for carbonate ion, Yu et al. (2020) also found evidence for a greater fraction of Pacific-sourced water in the deep Cape Basin. While they argue that this results from a shoaling of NADW, which our data do not support, we similarly do find evidence for a larger fraction of PDW relative to NADW at deep Sites RC11-83/TN057-21. ...
Full-text available
A common conception of the deep ocean during ice age episodes is that the upper circulation cell in the Atlantic was shoaled at the Last Glacial Maximum compared to today, and that this configuration facilitated enhanced carbon storage in the deep ocean, contributing to glacial CO2 draw‐down. Here, we test this notion in the far South Atlantic, investigating changes in glacial circulation structure using paired neodymium and benthic carbon isotope measurements from International Ocean Discovery Program Site U1479, at 2,615 m water depth in the Cape Basin. We infer changes in circulation structure across the last glacial cycle by aligning our site with other existing carbon and neodymium isotope records from the Cape Basin, examining vertical isotope gradients, while determining the relative timing of inferred circulation changes at different depths. We find that Site U1479 had the most negative neodymium isotopic composition across the last glacial cycle among the analyzed sites, indicating that this depth was most strongly influenced by North Atlantic Deep Water (NADW) in both interglacial and glacial intervals. This observation precludes a hypothesized dramatic shoaling of NADW above ∼2,000 m. Our evidence, however, indicates greater stratification between mid‐depth and abyssal sites throughout the last glacial cycle, conditions that developed in Marine Isotope Stage 5. These conditions still may have contributed to glacial carbon storage in the deep ocean, despite little change in the mid‐depth ocean structure.
... . It has been proposed that deep-water, formed due to increased surface salinity driven by subdued freshwater flux during the last deglacial cold events, may have played a key role in global heat distribution and carbon cycles (Okazaki et al., 2010;Rae et al., 2014Rae et al., , 2020Yu et al., 2020). However, some studies argue that during the deglacial cold events, only enhanced intermediate-water ventilation occurred above a depth of ∼2,000 m in the North Pacific driven by the active polynya formation and brine rejection during sea ice formation in the Okhotsk Sea and/or western Bering Sea, while the deep ocean remained isolated (Gong et al., 2019;Jaccard & Galbraith, 2013;Max et al., 2014;Ohkushi et al., 2003). ...
... Previous studies suggested that North Pacific deep-water could form during the last deglacial abrupt cold events due to increased surface salinity driven by subdued local freshwater flux (Okazaki et al., 2010;Rae et al., 2014Rae et al., , 2020Yu et al., 2020). Precipitation in the North Pacific mid-latitudes is mainly shaped by the storm tracks, the position of which can be affected by the equator-to-pole temperature gradient (Brayshaw et al., 2008;Shaw et al., 2016). ...
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A multi-proxy study including organic carbon and bulk nitrogen isotopes along with major and trace element concentrations in sediments from Integrated Ocean Drilling Program (IODP) Sites U1425 and U1430 in the Japan Sea has been conducted in order to trace deep-water evolution in the Japan Sea and the North Pacific since the late Miocene. The high total organic carbon (TOC) flux, as well as other published geochemical and sedimentary evidence, indicates the occurrence of anoxic deep-water in the Japan Sea before ~7.4 Ma. The low nitrogen isotope values probably suggest nearly complete denitrification. In contrast, the sharply enhanced biological production but decreased burial of organic matter during ~7.4−4 Ma, as shown by high enrichment factor of Ba (BaEF) values, together with low TOC flux, highlights enhanced deep-water oxygenation in the Japan Sea during that time. We suggest that deep-water formation in the North Pacific ventilated the deep Japan Sea via northern deep seaways before the sea became semi-closed in the early Pliocene. The synchronously increased equator-to-pole temperature gradients driven by late Miocene global cooling may have caused southward shift of mid-latitude storm tracks, coupled with the weakened East Asian summer monsoon and moisture transport, leading to decreased precipitation in mid-latitude regions. The potential increases in surface salinity in the North Pacific may have broken the ocean stratification and favored deep-water formation, and further caused deep-water ventilation in the Japan Sea.
... The Pacific Ocean, which is the biggest marine carbon reservoir at the end of the global ocean conveyor belt, hosts the oldest and carbon-rich deep water in the global ocean owing to accumulated organic matter remineralization (Yu et al., 2020). The corrosive Pacific deep water (Sexton and Barker, 2012) results in shallower preservation depth of calcite (rarer and more soluble aragonite is excluded in our analysis), which hereafter is the only carbonate mineral considered in this article, in the Pacific sediments than that in the Atlantic and Indian ocean sediments (Berger et al., 1976;Biscaye et al., 1976;Kolla et al., 1976). ...
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Distribution of calcium carbonate (CaCO3) in marine sediment has been studied over the last century, and influence by multiple factors with regard to dissolution and dilution of sedimentary CaCO3 has long been established. There is still lack of quantification on the influence of those factors, so it remains elusive to determine which specific process is driving the down-core variation of CaCO3 content (wtCaCO3%) records. Here, based on a newly compiled CaCO3 data set and a carbonate model, depth-profiles of sedimentary wtCaCO3% from the West Pacific Ocean can be well illustrated, and influence from different factors on their distribution features can be quantified. The deep ocean circulation is found to largely shape the inter-basin disparity in sedimentary wtCaCO3% distribution between the equatorial regions (e.g., the Western Equatorial Pacific Ocean and the Central Pacific Ocean) and the north–west regions (the Philippine Sea and the Northwest Pacific Ocean) in our study region. Moreover, the slow carbonate dissolution rate in the deep Central Pacific Ocean guarantees better accumulation of CaCO3 at depth compared to that in other regions. However, enhanced dilution by non-carbonate materials of sedimentary CaCO3 on a topographic complex can potentially obstruct the dissolution profiles constituted by sedimentary wtCaCO3% in the pelagic ocean. The aforementioned assertion suggests that changes of wtCaCO3% accumulation in marine sediment in the West Pacific Ocean can be used to dictate past changes of the deep ocean circulation (2,500 to 3,000 m) in this area but constraint on the non-carbonate flux, especially on the topographic complex, should be necessary.
... 2− ], with highest values during the LGM as seen in many studies for middepth records from the North Atlantic Ocean (Yu et al., 2008(Yu et al., , 2010(Yu et al., , 2020. The δ 13 C in the infaunal species O. umbonatus is on average ∼1‰ lower during the last glacial (∼40-19 ka) compared to the Holocene (Figure 4e). ...
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The Fram Strait is the only deep gateway between the Arctic Ocean and the Nordic Seas and thus is a key area to study past changes in ocean circulation and the marine carbon cycle. Here, we study deep ocean temperature, δ¹⁸O, carbonate chemistry (i.e., carbonate ion concentration [CO3²⁻]), and nutrient content in the Fram Strait during the late glacial (35,000–19,000 years BP) and the Holocene based on benthic foraminiferal geochemistry and carbon cycle modeling. Our results indicate a thickening of Atlantic water penetrating into the northern Nordic Seas, forming a subsurface Atlantic intermediate water layer reaching to at least ∼2,600 m water depth during most of the late glacial period. The recirculating Atlantic layer was characterized by relatively high [CO3²⁻] and low δ¹³C during the late glacial, and provides evidence for a Nordic Seas source to the glacial North Atlantic intermediate water flowing at 2,000–3,000 m water depth, most likely via the Denmark Strait. In addition, we discuss evidence for enhanced terrestrial carbon input to the Nordic Seas at ∼23.5 ka. Comparing our δ¹³C and qualitative [CO3²⁻] records with results of carbon cycle box modeling suggests that the total terrestrial CO2 release during this carbon input event was low, slow, or directly to the atmosphere.
... A comparison of our ACC strength record to a high resolution carbonate saturation reconstruction from the Subantarctic South Atlantic 49 indicates a correspondence of stronger ACC with reduced carbonate saturation during the major Antarctic warm intervals (Fig. 4b-d). These changes in inflow of Pacific-type deep-water masses likely influenced the carbonate chemistry in the subantarctic South Atlantic competing with North Atlantic sourced-deep waters at millennial time-scales 49,55 . The contribution of Antarctic Bottom Water circulation to the modern ACC is modest, but potentially increased during glacial periods when expanded sea ice favored its production and increased its salinity 12,40 . ...
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The Antarctic Circumpolar Current (ACC) plays a crucial role in global ocean circulation by fostering deep-water upwelling and formation of new water masses. On geological time-scales, ACC variations are poorly constrained beyond the last glacial. Here, we reconstruct changes in ACC strength in the central Drake Passage in vicinity of the modern Polar Front over a complete glacial-interglacial cycle (i.e., the past 140,000 years), based on sediment grain-size and geochemical characteristics. We found significant glacial-interglacial changes of ACC flow speed, with weakened current strength during glacials and a stronger circulation in interglacials. Superimposed on these orbital-scale changes are high-amplitude millennial-scale fluctuations, with ACC strength maxima correlating with diatom-based Antarctic winter sea-ice minima, particularly during full glacial conditions. We infer that the ACC is closely linked to Southern Hemisphere millennial-scale climate oscillations, amplified through Antarctic sea ice extent changes. These strong ACC variations modulated Pacific-Atlantic water exchange via the “cold water route” and potentially affected the Atlantic Meridional Overturning Circulation and marine carbon storage. How the Antarctic Circumpolar Current (ACC) changed on glacial-interglacial time scales is not well known. Here, the authors present a 140,000 year long sediment record from the Drake passage and show both glacial-interglacial as well as millennial-scale variability which are linked to Atlantic variability and marine carbon storage.
... 2− ] and neodymium isotopes (Yu et al., 2020). Our δ 14 R record, taken together with published records, suggests that the deep Southern Ocean became more isolated from the upper ocean and the atmosphere during the LGM. ...
Full-text available
Processes underlying changes in the oceanic carbon storage during the Last Glacial Maximum and the subsequent deglaciation are not fully understood. Here, we present a new high‐resolution radiocarbon reconstruction (expressed as δ¹⁴R) at the depth of the modern Lower Circumpolar Deep Water from the Pacific Sector of the Southern Ocean. Our record shows δ¹⁴R increases during Heinrich Stadial 1 and the Younger Dryas that agree with the deep‐to‐shallow transfer of old carbon in the Southern Ocean during these two periods. Our record also shows, for the first time, a clear ∼80‰ decline in δ¹⁴R during the Antarctic Cold Reversal (ACR), indicating the development of poorly ventilated conditions in the deep Southwest Pacific. These conditions are consistent with the increased Southern Ocean sea‐ice and associated stratification between Upper and Lower Circumpolar Deep Waters. This enhanced stratification in the deep South Pacific possibly facilitated greater carbon storage in the ocean interior during the ACR, effectively limiting oceanic CO2 release and contributing to the atmospheric CO2 plateau as observed in ice cores at that time.
The ocean plays a critical role in the global climate by modulating atmospheric CO2 via carbon exchange between the atmosphere and the ocean, a process tightly linked to ocean circulation changes. Ocean circulation transports atmospheric CO2 to the deep ocean via deep water formation and then transfers it with respired CO2, which regulate CaCO3 accumulation in marine sediments on seafloor. Compared to studies on the Atlantic and Pacific, much less has been done on past circulation changes in the Indian Ocean. This is partly due to the lack of systematic investigation on sub-basinal scales calcium carbonate (CaCO3) distributions and limited understanding of their controlling mechanisms in the Indian Ocean. Based on an extensive data compilation and a simple carbonate accumulation model, here we show that CaCO3 distributions in various basins of the Indian Ocean are mainly controlled by ocean circulation changes with productivity only playing a secondary role. Comparison of CaCO3 distributions between the Last Glacial Maximum (LGM) and the late Holocene suggests little change in deep water acidity between these times. If the source-water acidity remains unchanged through time, this finding suggests a vigorous ventilation of the deep Indian Ocean during the LGM. Given that the glacial oceans were likely more alkaline, the stability of deep Indian acidity must suggest greater storage of carbon (mainly as bicarbonate ion) during the LGM. Additionally, we find a very corrosive LGM deep water in the Southwest Indian Basin which may be attributed to intrusion of the Pacific Deep Water.
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The transfer of vast amounts of carbon from a deep oceanic reservoir to the atmosphere is considered to be a dominant driver of the deglacial rise in atmospheric CO 2 . Paleoceanographic reconstructions reveal evidence for the existence of CO 2 -rich waters in the mid to deep Southern Ocean. These water masses ventilate to the atmosphere south of the Polar Front, releasing CO 2 prior to the formation and subduction of intermediate-waters. Changes in the amount of CO 2 in the sea water directly affect the oceanic carbon chemistry system. Here we present B/Ca ratios, a proxy for delta carbonate ion concentrations Δ[CO 3 ²⁻ ], and stable isotopes (δ ¹³ C) from benthic foraminifera from a sediment core bathed in Antarctic Intermediate Water (AAIW), offshore New Zealand in the Southwest Pacific. We find two transient intervals of rising [CO 3 ²⁻ ] and δ ¹³ C that that are consistent with the release of CO 2 via the Southern Ocean. These intervals coincide with the two pulses in rising atmospheric CO 2 at ~ 17.5–14.3 ka and 12.9–11.1 ka. Our results lend support for the release of sequestered CO 2 from the deep ocean to surface and atmospheric reservoirs during the last deglaciation, although further work is required to pin down the detailed carbon transfer pathways.
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The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atlantic Deep Water (NADW), a major component of the global overturning ocean circulation, was significantly more radiogenic than modern during the Last Glacial Maximum (LGM), and shifted towards modern values during the deglaciation. We propose that weathering contributions of unradiogenic Nd modulated by the North American Ice Sheet dominated the evolution of the NADW Nd isotope endmember. If water mass mixing dominated the distribution of deep glacial Atlantic Nd isotopes, our results would imply a larger fraction of NADW in the deep Atlantic during the LGM and deglaciation than reconstructed with a constant northern endmember. The Nd isotope composition of seawater has been used to reconstruct past changes in the various contributions of different water masses to the deep ocean, with the isotope signatures of endmember water masses generally assumed to have been stable during the Quaternary. Here, the authors show that deep water produced in the North Atlantic had a significantly more radiogenic Nd signature during the Last Glacial Maximum compared to today.
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During the Last Glacial Maximum (LGM; ~20,000 years ago), the global ocean sequestered a large amount of carbon lost from the atmosphere and terrestrial biosphere. Suppressed CO2 outgassing from the Southern Ocean is the prevailing explanation for this carbon sequestration. By contrast, the North Atlantic Ocean-a major conduit for atmospheric CO2 transport to the ocean interior via the overturning circulation-has received much less attention. Here we demonstrate that North Atlantic carbon pump efficiency during the LGM was almost doubled relative to the Holocene. This is based on a novel proxy approach to estimate air-sea CO2 exchange signals using combined carbonate ion and nutrient reconstructions for multiple sediment cores from the North Atlantic. Our data indicate that in tandem with Southern Ocean processes, enhanced North Atlantic CO2 absorption contributed to lowering ice-age atmospheric CO2.
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Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO 2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000–28,000 years BP) to have been within the range of 20–50 μmol/kg, much less than the modern value of ~168 μmol/kg. We further demonstrate that reduced oxygen levels characterized the water column below a depth of ~1,000 m. Converting the ice age oxygen level to an equivalent concentration of respiratory CO 2 , and extrapolating globally, we estimate that deep-sea CO 2 storage during the last ice age exceeded modern values by as much as 850 Pg C, sufficient to balance the loss of carbon from the atmosphere (~200 Pg C) and from the terrestrial biosphere (~300–600 Pg C). In addition, recognizing the enhanced preservation of organic matter in ice age sediments of the deep Pacific Ocean helps reconcile previously unexplained inconsistencies among different geochemical and micropaleontological proxy records used to assess past changes in biological productivity of the ocean.
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The origin and carbon content of the deep water mass that fills the North Atlantic Basin, either Antarctic Bottom Water (AABW) or North Atlantic Deep Water (NADW) is suggested to influence the partitioning of CO2 between the ocean and atmosphere on glacial–interglacial timescales. Fluctuations in the strength of Atlantic meridional overturning circulation (AMOC) have also been shown to play a key role in global and regional climate change on timescales from annual to millennial. The North Atlantic is an important and well-studied ocean basin but many proxy records tracing ocean circulation in this region over the last glacial cycle are challenging to interpret. Here we present new B/Ca-[CO3²⁻] and boron isotope-pH data from sites spanning the North Atlantic Ocean from 2200 to 3900 m and covering the last 130 kyr from both sides of the Mid-Atlantic Ridge. These data allow us to explore the potential of the boron-based proxies as tracers of ocean water masses to ultimately identify the changing nature of Atlantic circulation over the last 130 kyr. This possibility arises because the B/Ca and boron isotope proxies are directly and quantitatively linked to the ocean carbonate system acting as semi-conservative tracers in the modern ocean. Yet the utility of this approach has yet to be demonstrated on glacial–interglacial timescales when various processes may alter the state of the deep ocean carbonate system. We demonstrate that the deep (∼3400 m) North Atlantic Ocean exhibits considerable variability in terms of its carbonate chemistry through the entirety of the last glacial cycle. Our new data confirm that the last interglacial marine isotope stage (MIS) 5e has a similar deep-water geometry to the Holocene, in terms of the carbonate system. In combination with benthic foraminiferal δ¹³C and a consideration of the [CO3²⁻] of contemporaneous southern sourced water, we infer that AABW influences the eastern abyssal North Atlantic throughout the whole of the last glacial (MIS2 through 4) whereas, only in the coldest stages (MIS2 and MIS4) of the last glacial cycle was AABW an important contributor to our deep sites in both North Atlantic basins. Taken together, our carbonate system depth profiles reveal a pattern of changing stratification within the North Atlantic that bears strong similarities to the atmospheric CO2 record, evidencing the important role played by ocean water mass geometry and the deep ocean carbonate system in driving changes in atmospheric CO2 on these timescales.
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Circulation changes have been suggested to play an important role in the sequestration of atmospheric CO2 in the glacial ocean. However, previous studies have resulted in contradictory results regarding the strength of the Atlantic Meridional Overturning Circulation (AMOC) and three-dimensional, quantitative reconstructions of the glacial ocean constrained by multiple proxies remain scarce. Here we simulate the modern and glacial ocean using a coupled physical-biogeochemical, global, three-dimensional model constrained simultaneously by δ13C , radiocarbon, and δ15N to explore the effects of AMOC differences and Southern Ocean iron fertilization on the distributions of these isotopes and ocean carbon storage. We show that δ13C and radiocarbon datasparsely sampled at the locations of existing glacial sediment cores can be used to reconstruct the modern AMOC accurately. Applying this method to the glacial ocean we find that a surprisingly weak (6-9 Sv or about half of today's) and shallow AMOC maximizes carbon storage and best reproduces the sediment isotope data. Increasing the atmospheric soluble iron flux in the model's Southern Ocean intensifies export production, carbon storage, and further improves agreement with δ13C and δ15N reconstructions. Our best fitting simulation is a significant improvement compared with previous studies, and suggests that both circulation and export production changes were necessary to maximize carbon storage in the glacial ocean.
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As the largest reservoir of carbon exchanging with the atmosphere on glacial–interglacial timescales, the deep ocean has been implicated as the likely location of carbon sequestration during Pleistocene glaciations. Despite strong theoretical underpinning for this expectation, radiocarbon data on watermass ventilation ages conflict, and proxy interpretations disagree about the depth, origin and even existence of the respired carbon pool. Because any change in the storage of respiratory carbon is accompanied by corresponding changes in dissolved oxygen concentrations, proxy data reflecting oxygenation are valuable in addressing these apparent inconsistencies. Here, we present a record of redox-sensitive uranium from the central equatorial Pacific Ocean to identify intervals associated with respiratory carbon storage over the past 350 kyr, providing evidence for repeated carbon storage over the last three glacial cycles. We also synthesise our data with previous work and propose an internally consistent picture of glacial carbon storage and equatorial Pacific Ocean watermass structure.
The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end-member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air-sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high-nutrient AAIW with nutrient-depleted, carbonate-rich waters sourced from the region of the modern-day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.
Micropaleontological and geochemical analyses reveal distinct millennial-scale increases in carbonate preservation in the deep Southeast Atlantic (Cape Basin) during strong and prolonged Greenland interstadials that are superimposed on long-term (orbital-scale) changes in carbonate burial. These data suggest carbonate oversaturation of the deep Atlantic and a strengthened Atlantic Meridional Overturning Circulation (AMOC) during the most intense Greenland interstadials. However, proxy evidence from outside the Cape Basin indicates that AMOC changes also occurred during weaker and shorter Greenland interstadials. Here we revisit the link between AMOC dynamics and carbonate saturation in the deep Cape Basin over the last 400 kyr (sediment cores TN057-21, TN057-10, and Ocean Drilling Program Site 1089) by reconstructing centennial changes in carbonate preservation using millimeter-scale X-ray fluorescence (XRF) scanning data. We observe close agreement between variations in XRF Ca/Ti, sedimentary carbonate content, and foraminiferal shell fragmentation, reflecting a common control primarily through changing deep water carbonate saturation. We suggest that the high-frequency (suborbital) component of the XRF Ca/Ti records indicates the fast and recurrent redistribution of carbonate ions in the Atlantic basin via the AMOC during both long/strong and short/weak North Atlantic climate anomalies. In contrast, the low-frequency (orbital) XRF Ca/Ti component is interpreted to reflect slow adjustments through carbonate compensation and/or changes in the deep ocean respired carbon content. Our findings emphasize the recurrent influence of rapid AMOC variations on the marine carbonate system during past glacial periods, providing a mechanism for transferring the impacts of North Atlantic climate anomalies to the global carbon cycle via the Southern Ocean.
A growing body of evidence suggests that respired carbon was stored in mid-depth waters (∼1–3 km) during the last glacial maximum (LGM) and released to the atmosphere from upwelling regions during deglaciation. Decreased ventilation, enhanced productivity, and enhanced carbonate dissolution are among the mechanisms that have been cited as possible drivers of glacial CO2 drawdown. However, the relative importance of each of these mechanisms is poorly understood. New approaches to quantitatively constrain bottom water carbonate chemistry and oxygenation provide methods for estimating historic changes in respired carbon storage. While increased CO2 drawdown during the LGM should have resulted in decreased oxygenation and a shift in dissolved inorganic carbon (DIC) speciation towards lower carbonate ion concentrations, this is complicated by the interplay of carbonate compensation, export productivity, and circulation. To disentangle these processes, we use a multiproxy approach that includes boron to calcium (B/Ca) ratios of the benthic foraminifera Cibicidoides wuellerstorfi to reconstruct deep-water carbonate ion concentrations ([CO3²⁻]) and the uranium to calcium (U/Ca) ratio of foraminiferal coatings in combination with benthic foraminiferal carbon isotopes to reconstruct changes in bottom water oxygen concentrations ([O2]) and organic carbon export. Our records indicate that LGM [CO3²⁻] and [O2] was reduced at mid water depths of the eastern equatorial Pacific (EEP), consistent with increased respired carbon storage. Furthermore, our results suggest enhanced mixing of lower Circumpolar Deep Water (LCDW) to EEP mid water depths and provide evidence for the importance of circulation for oceanic-atmospheric CO2 exchange.
Stratification of the deep Southern Ocean during the Last Glacial Maximum is thought to have facilitated carbon storage and subsequent release during the deglaciation as stratification broke down, contributing to atmospheric CO2 rise. Here, we present neodymium isotope evidence from deep to abyssal waters in the South Pacific that confirms stratification of the deepwater column during the Last Glacial Maximum. The results indicate a glacial northward expansion of Ross Sea Bottom Water and a Southern Hemisphere climate trigger for the deglacial breakup of deep stratification. It highlights the important role of abyssal waters in sustaining a deep glacial carbon reservoir and Southern Hemisphere climate change as a prerequisite for the destabilization of the water column and hence the deglacial release of sequestered CO2 through upwelling.