The emergence of high‐amplitude, low‐frequency glacial‐interglacial cycles during the mid‐Pleistocene climate transition (MPT; 800–1,200 ka) is associated with global cooling. In the eastern equatorial Pacific, sea surface temperatures cooled, and the upwelling‐induced cold tongue expanded significantly during the MPT. Here we use sedimentary records of iron, biogenic silica, and nutrient‐nitrogen consumption to evaluate biogeochemical changes hypothesized to accompany the cold tongue expansion. Our results suggest that the eastern equatorial Pacific of the MPT hosted surface waters with higher nitrate contents and biogenic silica production relative to the last 600 ka. Increased production occurred despite low iron supply. We attribute this to enhanced upwelling and nutrient enrichment of thermocline waters, both likely related to the northward migration of Southern Ocean fronts. The return of these fronts to their southward positions after the MPT may be associated with stronger drawdown of nutrients and, potentially, atmospheric CO2 in the Southern Ocean.

Sedimentary nitrogen isotope (as δ 15 N) records from the Southern Ocean provide critical constraints on surface nutrient consumption in the past and the role of Southern Ocean biophysical changes in setting atmospheric pCO 2. We present a field assessment of how surface nitrate consumption is reflected in δ 15 N values of total nitrogen and diatom-bound nitrogen pools of particles and sediments across the Southern Ocean along 170°W during late austral summer. Mixed layer nitrate δ 15 N values increase northwards associated with greater nitrate drawdown. Particles and sediments are expected to follow this trend. Contrary to expectations, surface ocean particle total nitrogen and diatom-bound δ 15 N values decreased northward during the late summer, likely due to recycling of nitrogen and the assimilation of regenerated ammonium, as well as nitrate. The relationship between δ 15 N values of the total nitrogen and diatom-bound pools remains relatively constant across this Southern Ocean transect, suggesting that the isotopic composition of these two surface ocean nitrogen pools are largely set by the δ 15 N value(s) of the assimilated nutrient(s). Surface sediment δ 15 N values do increase away from the region of maximum biogenic silica deposition, suggesting that the recycled nitrogen isotopic signal observed in late summer particles may not significantly impact the sedimentary record. However, the enrichment in δ 15 N values of the diatom-bound pool is greater than what is expected from progressive utilization of the surface nitrate alone and not yet explained. Plain Language Summary Southern Ocean biology helps to draw carbon dioxide out of the atmosphere. Here, we present a test of how well the diatom-bound nitrogen isotope paleoproxy records past surface ocean nutrient conditions, a critical constraint on the role of this biological carbon pump. Overall, the diatom nitrogen pool records the isotopic composition of their source of nutrients. Contrary to expectations, the nitrogen isotopes of the surface ocean particles showed a northward decrease across the Southern Ocean rather than the expected northward increase estimated from the nitrate pool. This is likely due to nitrate and ammonium uptake during the summer rather than growth on nitrate alone. The sedimentary nitrogen pools on the other hand showed an isotope trend more aligned with expectation, but in the case of the diatom-bound nitrogen, with a steeper gradient. This difference between the water column and the sediments likely stems from sampling during the summer, after the bloom and main sedimentation event, and it suggests that the recycled signal reflects a minor contribution to the overall sedimentary record.

Nitrate is a crucial nutrient that supports and limits marine primary productivity, now and through the past (Falkowski, 1997;Moore et al., 2013). The biological pump, through which carbon and nutrients are exported from the surface to the deep ocean by biological processes, is closely linked to atmospheric CO 2 concentrations and global climate. The Southern Ocean has the world's largest pool of underutilized surface nitrate, which, if it were fully consumed, would reduce atmospheric pCO 2 by more than 40 ppm (Sarmiento & Orr, 1991). Greater relative nitrate utilization in the glacial Southern Ocean due to either a decrease in supply or an increase in demand likely contributed to the observed lowering of CO 2 during glacial intervals (Ai et al., 2020;Kohfield et al., 2013;Robinson et al., 2009;Sigman et al., 2010). Changes to the efficiency of nitrate use in the glacial Southern Ocean are monitored using sedimentary nitrogen isotope measurements as a proxy for the relative consumption of nitrate (