Shinya Iwasaki

Calcium carbonate (CaCO₃) dissolution at the Southern Ocean seafloor has hypothetically contributed to lowering the atmospheric carbon dioxide concentration by increasing ocean alkalinity during glacial periods. We present new CaCO₃ burial and dissolution records from two sediment cores obtained off the Chilean margin in the subantarctic SE Pacific and covering the past 140 kyr since Marine Isotope Stage (MIS) 6. These records include CaCO₃ contents and mass accumulation rates, and microfossil-based analysis results, including fragmentation ratios, sieve-based weights (SBWs), and ultrastructural observations of planktic foraminiferal tests. Our bulk CaCO₃-based analyses and Globorotalia inflata SBWs revealed three major CaCO₃ dissolution events during colder stages of MIS 5d and 5b and at the MIS 5/4 boundary that are traceable events in the eastern South Pacific along the Chilean margin and in the Drake Passage. Furthermore, CaCO₃ burial exhibited pronounced glacial/interglacial fluctuations, with almost no burial during glacials (MIS 6, 4, 3, and 2) and recovery during interglacials (MIS 5e and 1) and early glacials (MIS 5d–a). This pattern agrees with previous observations over a wide area of the Southern Ocean, except in the deep Cape Basin > 4600 m in the South Atlantic Ocean. Considering that our sites were located upstream of the Drake Passage, the Circumpolar Deep Water, which was influenced by carbon-rich Pacific Deep Water, likely propagated from the subantarctic eastern Pacific to the South Atlantic at least at depths of ~ 3000 to ~ 4000 m and decreased CaCO₃ burial during glacials. These findings supported the importance of carbonate compensation in the Southern Ocean for the carbon cycle on the glacial/interglacial timescale.

The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean-current system and affects global ocean circulation, climate and Antarctic ice-sheet stability^1–3. Today, ACC dynamics are controlled by atmospheric forcing, oceanic density gradients and eddy activity⁴. Whereas palaeoceanographic reconstructions exhibit regional heterogeneity in ACC position and strength over Pleistocene glacial–interglacial cycles^5–8, the long-term evolution of the ACC is poorly known. Here we document changes in ACC strength from sediment cores in the Pacific Southern Ocean. We find no linear long-term trend in ACC flow since 5.3 million years ago (Ma), in contrast to global cooling⁹ and increasing global ice volume¹⁰. Instead, we observe a reversal on a million-year timescale, from increasing ACC strength during Pliocene global cooling to a subsequent decrease with further Early Pleistocene cooling. This shift in the ACC regime coincided with a Southern Ocean reconfiguration that altered the sensitivity of the ACC to atmospheric and oceanic forcings^11–13. We find ACC strength changes to be closely linked to 400,000-year eccentricity cycles, probably originating from modulation of precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability¹⁴. A persistent link between weaker ACC flow, equatorward-shifted opal deposition and reduced atmospheric CO₂ during glacial periods first emerged during the Mid-Pleistocene Transition (MPT). The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, providing evidence of potentially increasing ACC flow with future climate warming.

The quantitative and objective characterization of dissolution intensity in fossil planktonic foraminiferal shells could be used to reconstruct past changes in bottom water carbonate ion concentration. Among proxies measuring the degree of dissolution of planktonic foraminiferal shells, X‐ray micro‐Computed Tomography (CT) based characterization of apparent shell density appears to have good potential to facilitate quantitative reconstruction of carbonate chemistry. However, unlike the well‐established benthic foraminiferal B/Ca ratio‐based proxy, only a regional calibration of the CT‐based proxy exists based on a limited number of data points covering mainly low‐saturation state waters. Here we determined by CT‐based proxy the shell dissolution intensity of planktonic foraminifera Globigerina bulloides, Globorotalia inflata, Globigerinoides ruber, and Trilobatus sacculifer from a collection of core top samples in the Southern Atlantic covering higher saturation states and assessed the reliability of CT‐based proxy. We observed that the CT‐based proxy is generally controlled by deep‐water Δ[] like the B/Ca proxy, but its effective range of Δ[] is between −20 and 10 µmolkg⁻¹. In this range, the CT‐based proxy appears directly and strongly related to deep‐water Δ[], whereas we note that in some settings, there appears to be a secondary influence on B/Ca which we suggest may be due to elevated alkalinity from carbonate dissolution in sediments. On the other hand, the CT‐based proxy is affected by supralysoclinal dissolution in areas with high productivity. Like the B/Ca proxy, the CT‐based proxy requires species‐specific calibration, but the effect of species‐specific shell difference in susceptibility to dissolution on the proxy is small.

Data report: calibration of XRF scanning CaCO3 estimates for the upper 30 m along the Site U1543 splice, International Ocean Discovery Program Expedition 383

Terrestrial glacial records from the Patagonian Andes and New Zealand Alps document quasi-synchronous southern hemisphere-wide glacier advances during the late Quaternary. Although detailed, these records are inherently incomplete. Here, we provide a continuous marine record of Patagonian Ice Sheet (PIS) extent over a complete glacial/interglacial cycle back to the previous glacial (~140 ka). Sediment core MR16-09 PC03, located at 46° S and ~150 km offshore Chile, received high terrestrial sediment input when the PIS advanced westward. We use biomarkers, foraminiferal oxygen isotopes, and major element data for reconstructing terrestrial sediment and freshwater input related to PIS variations. Our sediment record documents four major intervals of general PIS advance, during Marine Isotope Stage (MIS) 6 (135 – 140 ka), late MIS 5 (~85 – 95 ka), MIS 4 (~60 – 70 ka) and late MIS 3 to MIS 2 (~18 – 40 ka). These higher terrigenous input intervals occur during sea-level low stands when the western PIS covered most of the Chilean fjords which today retain glaciofluvial sediments. Superimposed, high amplitude phases of enhanced sediment supply reoccur at millennial time scales and reflect marine-based phases of the PIS with direct sediment discharge to the continental slope. We assign the late MIS 3 to MIS 2 phases and, by inference, also older advances to Antarctic stadials, consistent with the terrestrial record. Concomitantly, we observe enhanced meltwater release and conclude that the temperature-controlled PIS advances during southern hemisphere cold phases were likely reinforced by higher precipitation through enhanced westerly winds at the northwestern margin of the PIS.

The quantitative and objective characterization of dissolution intensity in fossil planktonic foraminiferal shells could be used to reconstruct past changes in bottom water carbonate ion concentration. Among proxies measuring the degree of dissolution of planktonic foraminiferal shells, X-ray micro-Computed Tomography (CT) based characterization of apparent shell density appears to have good potential to facilitate quantitative reconstruction of carbonate chemistry. However, unlike the well-established benthic foraminiferal B/Ca ratio-based proxy, only a regional calibration of the CT-based proxy exists based on a limited number of data points covering mainly low-saturation state waters. Here we determined by CT-based proxy the shell dissolution intensity of planktonic foraminifera Globigerina bulloides, Globorotalia inflata, Globigerinoides ruber, and Trilobatus sacculifer from a collection of core top samples in the Southern Atlantic covering higher saturation states, and assessed the characteristics and reliability of CT-based proxy. We observed that the CT-based proxy is generally controlled by deep-water Δ[CO32–] like the B/Ca proxy, but its effective range of Δ[CO32–] is between –20 to 10 µmolkg–1. In this range, the CT-based proxy appears directly and strongly related to deep-water Δ[CO32–], whereas the B/Ca of benthic foraminifera appears to be affected by porewater saturation in carbonate-rich substrates. On the other hand, the CT-based proxy is affected by supralysoclinal dissolution in areas with high productivity. Like the B/Ca proxy, the CT-based proxy requires species-specific calibration, but the effect of species-specific shell difference in susceptibility to dissolution on the proxy is small.

Southern Ocean deep-water circulation plays a vital role in the global carbon cycle. On geological time scales, upwelling along the Chilean margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we develop an X-ray Micro-Computer-Tomography method to assess foraminiferal test dissolution as proxy for paleo-carbonate ion concentrations ([CO₃²⁻]). Our subantarctic Southeast Pacific sediment core depth transect shows significant deep-water [CO₃²⁻] variations during the Last Glacial Maximum and Deglaciation (10-22 ka BP). We provide evidence for an increase in [CO₃²⁻] during the early-deglacial period (15-19 ka BP) in Lower Circumpolar Deepwater. The export of such low-carbon deep-water from the Pacific to the Atlantic contributed to significantly lowered carbon storage within the Southern Ocean, highlighting the importance of a dynamic Pacific-Southern Ocean deep-water reconfiguration for shaping late-glacial oceanic carbon storage, and subsequent deglacial oceanic-atmospheric CO₂ transfer.

Southern Ocean deep-water circulation plays an important role in the global carbon cycle. On geological time-scales, upwelling along the Chilean continental margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we use a new X-ray Micro-Computer-Tomography method to assess foraminiferal test dissolution as proxy for paleo-carbonate ion concentrations ([CO₃²⁻]). Our subantarctic Southeast Pacific sediment core depth transect shows significant deep-water [CO₃²⁻] variations during the Last Glacial Maximum and Deglaciation (10 – 22 ka BP). We provide evidence for an increase in [CO₃²⁻] during the early deglacial period (15-19 ka BP), followed by a ca. 40 µmol kg⁻¹ reduction in Lower Circumpolar Deepwater (CDW). This decreased Pacific to Atlantic export of low-carbon CDW contributed to significantly lowered carbon storage within the Southern Ocean, highlighting the importance of a dynamic Pacific–Southern Ocean deep-water reconfiguration for shaping late-glacial oceanic carbon storage, and subsequent deglacial oceanic-atmospheric CO₂ transfer.