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The last deglaciation (~20–10 kyr BP) was characterized by a major shift in Earth's climate state, when the global mean surface temperature rose ~4 °C and the concentration of atmospheric CO₂increased ~80 ppmv. Model simulations suggest that the initial 30 ppmv rise in atmospheric CO₂may have been driven by reduced efficiency of the biological pump or enhanced upwelling of carbon‐rich waters from the abyssal ocean. Here we evaluate these hypotheses using benthic foraminiferal B/Ca (a proxy for deep water [CO₃²⁻]) from a core collected at 1,100‐m water depth in the Southwest Atlantic. Our results imply that [CO₃²⁻] increased by 22 ± 2 μmol/kg early in Heinrich Stadial 1, or a decrease in ΣCO₂of approximately 40 μmol/kg, assuming there were no significant changes in alkalinity. Our data imply that remineralized phosphate declined by approximately 0.3 μmol/kg during Heinrich Stadial 1, equivalent to 40% of the modern remineralized signal at this location. Because tracer inversion results indicate remineralized phosphate at the core site reflects the integrated effect of export production in the sub‐Antarctic, our results imply that biological productivity in the Atlantic sector of the Southern Ocean was reduced early in the deglaciation, contributing to the initial rise in atmospheric CO₂.

 

Decades of observations show that the world's oceans have been losing oxygen, with far‐reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary δ¹⁵N. We combine two δ¹⁵N records from the Santa Barbara Basin (SBB), a 24‐year‐long, biweekly sediment trap time series, and a 114‐year, high‐resolution sediment core together spanning the years 1892–2017. These records allow for the examination of δ¹⁵N variability on seasonal to centennial timescales. Seasonal variability in SBB δ¹⁵N is consistent in timing with the poleward advection of a high δ¹⁵N signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Niño events result in variable δ¹⁵N signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary δ¹⁵N. Seasonal and interannual variability in sediment trap δ¹³C_orgis consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary δ¹⁵N in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in δ¹⁵N reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional δ¹⁵N records our results further suggest that SBB is uniquely situated to record long‐term change in the Eastern Tropical North Pacific.

 

The prevailing hypothesis for lower atmospheric carbon dioxide (CO 2 ) concentrations during glacial periods is an increased efficiency of the ocean’s biological pump. However, tests of this and other hypotheses have been hampered by the difficulty to accurately quantify ocean carbon components. Here, we use an observationally constrained earth system model to precisely quantify these components and the role that different processes play in simulated glacial-interglacial CO 2 variations. We find that air-sea disequilibrium greatly amplifies the effects of cooler temperatures and iron fertilization on glacial ocean carbon storage even as the efficiency of the soft-tissue biological pump decreases. These two processes, which have previously been regarded as minor, explain most of our simulated glacial CO 2 drawdown, while ocean circulation and sea ice extent, hitherto considered dominant, emerge as relatively small contributors.