Nutrient utilization and the efficiency of the biological pump during late Pleistocene glacial-interglacial cycles in the subarctic Pacific

Reconstructing nutrient utilization in high-nutrient low-chlorophyll (HNLC) regions is important for understanding the relationship between biological productivity, carbon flux, and the climate system. Our research focuses on the HNLC region in the subarctic Pacific. Previously published nutrient utilization records in this region suggest that stronger stratification drives higher nutrient utilization during glacials. However, these studies have not provided an in-depth examination of how nutrient utilization varies by location within the HNLC region and whether the contributing factors are the same. With a sediment core from the edge of the HNLC region at the Bering Slope, IODP site U1345 (1008m water depth), we investigated changes in nutrient utilization over glacial-interglacial cycles in comparison to records from other parts of the HNLC region. We collected and analyzed δ¹⁵ N , δ¹³ C, and C/N of bulk organic matter, applying an isotope mass mixing model to remove the signal from terrigenous organic matter and estimate the marine phytoplankton end member. Compared to U1342, a site in the middle of the HNLC region, we find that U1345 has a) a larger fraction of terrigenous input, and b) overall higher nutrient utilization. The latter is likely due to increased iron and major nutrient transport to the euphotic zone from weaker stratification and shelf-slope exchange. Though it is hypothesized that stratification in this region strengthened through the late Pleistocene, the absence of a long-term trend at slope sites like U1345 suggests that local processes outweigh long-term region-wide stratification increase. While δ¹⁵N at U1345 is relatively stable through the last 500 ky, δ¹⁵N at U1342 increases and converges with U1345 toward the present. This suggests that the biological pump in the subarctic Pacific has become more efficient through time.