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

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.