The biological carbon pump (BCP) stores ∼1,700 Pg C from the atmosphere in the ocean interior, but the magnitude and direction of future changes in carbon sequestration by the BCP are uncertain. We quantify global trends in export production, sinking organic carbon fluxes, and sequestered carbon in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) future projections, finding a consistent 19 to 48 Pg C increase in carbon sequestration over the 21st century for the SSP3-7.0 scenario, equivalent to 5 to 17% of the total increase of carbon in the ocean by 2100. This is in contrast to a global decrease in export production of –0.15 to –1.44 Pg C y –1 . However, there is significant uncertainty in the modeled future fluxes of organic carbon to the deep ocean associated with a range of different processes resolved across models. We demonstrate that organic carbon fluxes at 1,000 m are a good predictor of long-term carbon sequestration and suggest this is an important metric of the BCP that should be prioritized in future model studies.

The rise in atmospheric CO 2 during the last deglaciation (18-11.5 ka) occurred primarily during Henrich Stadial 1 (HS1; 18-14.7 ka) and the Younger Dryas (YD; 13-1.5 ka) intervals that were associated with high-latitude cooling in the northern hemisphere and warming in the south. These CO 2 increases, each of >40 ppm, were separated by a ∼1,500-year interval known as the Antarctic Cold Reversal (ACR), where the deglacial rise in CO 2 stalled and Antarctic temperatures cooled (Figure 1a; Bereiter et al., 2014;Blunier et al., 1997;Parrenin et al., 2013). The effects of the ACR have been documented in paleo-temperature records as far afield as 40°S, thus this enigmatic pause in the deglaciation and the associated changes in the Southern Ocean have drawn much attention (Pedro et al., 2016;Putnam et al., 2010).The Southern Ocean surrounding Antarctica is a region where carbon-rich and nutrient-rich deep waters are upwelled (Figure 2), with the strength and position of the upwelling controlled by sea ice and wind stress (Ferrari et al., 2014;Menviel et al., 2018). Expansive sea ice during the Last Glacial Maximum (LGM) likely presented a physical barrier to air-sea gas exchange in the Southern Ocean (Stephens & Keeling, 2000) and changed the large scale geometry of the overturning circulation (Ferrari et al., 2014), giving rise to poorly oxygenated bottom waters (Jaccard et al., 2016), and greater carbon storage at depth (Rae et al., 2018). As Antarctica warmed during HS1 and the YD and sea ice retreated, the westerly winds shifted southward causing intensified upwelling in the Antarctic Zone, stimulating high primary productivity (Figure 1f), and releasing these deep carbon stores (Anderson et al., 2009). By contrast the ACR cooling was associated with a resurgence of Antarctic sea ice extent as evidenced by concentrations of wind-blown sea-salt within Antarctic ice cores (ssNa; Figure 1a; Buizert et al., 2015), a resurgence of Fragilariopsis curta and F. cylindrus abundance within Antarctic Zone diatom assemblages (Figure 1e; Bianchi & Gersonde, 2004), and climate modeling (Lowry et al., 2019). Furthermore, enhanced sea ice would have shifted the position of the westerlies northward (McCulloch et al., 2000). Correspondingly, there is some evidence of higher primary

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<p>Long-lived colonial cold-water corals have the potential to provide robust continuous archives of environmental change. These high-resolution records of the subsurface ocean are particularly valuable, especially at understudied intermediate water depths. Yet, to understand the anthropogenic impacts on the sub-surface ocean and better predict future changes, it is critical to establish the natural variation of temperature and circulation of the ocean system prior to the Industrial Revolution.</p><p>Here we combine temperature proxy and radiocarbon data from specimens of two taxa of cold-water coral that grew in intermediate water depths (~1500 m) in the tropical North Atlantic. In 2013, specimens of the bamboo coral <em>Lepidisis spp. </em>and scleractinian coral<em> Enallopsammia rostrata</em> were collected from sites currently situated in the boundary of North Atlantic Deep Water and Antarctic Intermediate Water to reconstruct the temperature and circulation history of the region. We demonstrate that bamboo corals can be used to reconstruct ambient seawater radiocarbon content when independently dated by organic node annual band counting. Radiocarbon was also analysed in <em>Enallopsammia rostrata</em> to develop age models for both the radial section and from discrete corallites (polyps) along a branch. Dating results show that this coral is about 500 years old, allowing us to generate a temperature record as far back as the Little Ice Age. Trace metal ratios were analysed along the growth axis of the coral, and the Li/Mg ratio was used as a temperature proxy. We find that the Li/Mg derived temperature of the most recent polyps is consistent with modern ambient temperature. The overall temperature record shows a general increasing trend since the Little Ice Age, while the radiocarbon record indicates no significant change until the late 20<sup>th</sup> century. Combining these records allows us to reconstruct potential ocean circulation changes in the central tropical North Atlantic over last 500 years.</p>