Nutrient conditions in the subpolar North Atlantic during the last glacial period reconstructed from foraminifera-bound nitrogen isotopes

Abstract

Surface nitrate concentration is a potentially useful diagnostic in reconstructing the past circulation of high‐latitude North Atlantic waters. Moreover, nutrient consumption in the North Atlantic surface impacts the atmospheric concentration of carbon dioxide. To reconstruct nutrient conditions in the subpolar North Atlantic region during the last ice age, a record of foraminifera‐bound δ15N was measured in Neogloboquadrina pachyderma (sin.) from core V28‐73 south of Iceland (57.2°N, 20.9°W). Foraminifera‐bound δ15N is up to 2‰ lower during the last ice age than during the Holocene, suggesting as much as ~25% less complete nitrate consumption during the former. This is consistent with stronger light limitation associated with a deeper summer surface mixed layer, perhaps related to the formation of Glacial North Atlantic Intermediate Water previously suggested to have occurred near the core site. However, three single‐point maxima in δ15N in the glacial section and the sharp deglacial δ15N rise coincide with Heinrich event layers. This suggests that increased water column stratification during Heinrich events, presumably due to surface freshening, reduced the nutrient supply from below and led to nearly complete nitrate consumption in the summertime mixed layer. The Heinrich layers in V28‐73 are not accompanied by δ18O minima in either N. pachyderma (sin.) or Globigerinoides bulloides, which we tentatively attribute to extreme mixed‐layer shoaling. The reconstructed subpolar North Atlantic upper water column changes—both glacial/interglacial and millennial—are inverse to those inferred for the Antarctic.