Glacial-to-interglacial changes in nitrate supply and consumption in the subarctic North Pacific from microfossil-bound N isotopes at two trophic levels

Abstract

Reduced nitrate supply to the subarctic North Pacific (SNP) surface during the last ice age has been inferred from coupled changes in diatom‐bound δ15N (DB‐δ15N), bulk sedimentary δ15N, and biogenic fluxes. However, the reliability of bulk sedimentary and DB‐δ15N has been questioned, and a previously reported δ15N minimum during Heinrich Stadial 1 (HS1) has proven difficult to explain. In a core from the western SNP, we report the foraminifera‐bound δ15N (FB‐δ15N) in Neogloboquadrina pachyderma and Globigerina bulloides, comparing them with DB‐δ15N in the same core over the past 25 kyr. The δ15N of all recorders is higher during the Last Glacial Maximum (LGM) than in the Holocene, indicating more complete nitrate consumption. N. pachyderma FB‐δ15N is similar to DB‐δ15N in the Holocene but 2.2‰ higher during the LGM. This difference suggests a greater sensitivity of FB‐δ15N to changes in summertime nitrate drawdown and δ15N rise, consistent with a lag of the foraminifera relative to diatoms in reaching their summertime production peak in this highly seasonal environment. Unlike DB‐δ15N, FB‐δ15N does not decrease from the LGM into HS1, which supports a previous suggestion that the HS1 DB‐δ15N minimum is due to contamination by sponge spicules. FB‐δ15N drops in the latter half of the Bølling/Allerød warm period and rises briefly in the Younger Dryas cold period, followed by a decline into the mid‐Holocene. The FB‐δ15N records suggest that the coupling among cold climate, reduced nitrate supply, and more complete nitrate consumption that characterized the LGM also applied to the deglacial cold events.