Atmospheric dust is a primary source of iron (Fe) to the open ocean, and its flux is particularly important in the high nutrient, low chlorophyll (HNLC) Southern Ocean where Fe currently limits productivity. Alleviation of this Fe limitation in the Subantarctic Zone of the Atlantic by increased dust-borne Fe supply during glacial periods has been shown to increase primary productivity. However, previous work has found no such increase in productivity in the Pacific sector. In order to constrain the relative importance of Southern Ocean Fe fertilization on glacial-interglacial carbon cycles, records of dust fluxes outside of the Atlantic sector of the Southern Ocean at the Last Glacial Maximum (LGM) are required. Here we use grain size and U-series analyses to reconstruct lithogenic and CaCO 3 fluxes and Nd, Sr, and Pb isotopes to ascertain the provenance of terrigenous material delivered to four deep water cores in the SW Pacific Ocean over the last~30 kyr. We find evidence for an increase in the relative proportion of fine-grained (0.5-12 μm) terrigenous sediment and higher detrital fluxes during the LGM compared to the Holocene. The provenance of the LGM dust varied spatially, with an older, more "continental" signature (low ε Nd , high 87 Sr/ 86 Sr) sourced from Australia in the northern cores, and a younger, more volcanogenic source in the southern cores (high ε Nd , low 87 Sr/ 86 Sr), likely sourced locally from New Zealand. Given this increase in lithogenic flux to the HNLC subantarctic Pacific Southern Ocean during the LGM, factors besides Fe supply must have regulated the biological productivity here.
<p>The leading hypotheses proposed to explain the rise in atmospheric CO2 during the last glacial to interglacial transition proposes enhanced carbon transfer from the intermediate and deep oceans to the atmosphere via the intensification of southern ocean upwelling. To test this scenario, we generated a high resolution record of boron isotopes (d<sup>11</sup>B) and B/Ca (proxies for pH and carbonate ion concentration, respectively) measured on shells of the benthic foraminifera C. wuellestorfi from a marine sedimentary core located at intermediate depth (1536m) on the Chilean margin. Our records confirm the link between changes in ocean circulation and variations in the carbonate chemistry at this site. The data also reveal the increase of intermediate water pH at the very late LGM, before the beginning of the deglaciation and the rise in atmospheric pCO<sub>2</sub>. To account for this observation, we suggest the existence of an early release of carbon from the intermediate ocean to the atmosphere in response to sea ice retreat occurring at the same time. The lack of any clear increase in atmospheric CO2 suggests that this release of intermediate ocean carbon was compensated by enhanced biological pumping.</p>
<p>Vertical and lateral exchanges of heat and carbon make the Southern Ocean a key player in regulating global climate, yet its role in future climate change remains uncertain. To address this knowledge gap, paleoceanographers study the state of the Southern Ocean under past climate states to better understand the processes governing its role in global climate. For instance, the Southern Ocean is widely thought to play a driving role in the atmospheric CO<sub>2</sub> fluctuations of the ice ages, ventilating carbon-rich deep waters to the atmosphere during interglacial periods and limiting this deep-surface exchange during glacial periods. However, direct evidence of these dynamics and of the Southern Ocean&#8217;s overall role in glacial CO<sub>2</sub> draw down remains limited.</p><p>Here we present a suite of geochemical data that provides new insights into Southern Ocean carbon cycling and circulation, evincing deep-ocean carbon storage over the last glacial cycle. Trace element and stable isotope (&#948;<sup>13</sup>C, &#948;<sup>18</sup>O) compositions of foraminiferal calcite from the high-latitude Indian Ocean demonstrate how carbon was sequestered in the deep ocean during glacial intensification and subsequently released to surface waters during deglaciation. These dynamics are captured by geochemical records reflecting temperature, pH, and circulation changes, providing key insights into the processes responsible for this carbon cycling. This observational data provides the foundation for developing a better mechanistic understanding of the Southern Ocean&#8217;s role in past and future climate change, including processes such as advection and mixing, ocean-ice interactions, and productivity.</p>
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