Diatom-bound 15N/14N:: New support for enhanced nutrient consumption in the ice age subantarctic -: art. no. PA3003

Diatom-bound N-15/N-14 was used to reconstruct the glacial nutrient status of the Subantarctic Zone in the Southern Ocean. Down-core records from both the Pacific and Indian sectors show delta N-15 of 5 to 6 parts per thousand during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2 parts per thousand. The effect of either diatom assemblage or physiological change on the diatom-bound N-15/N-14 is unknown and cannot yet be ruled out as a possible explanation for the observed change. However, the consistency between Indian and Pacific sector records and with other paleoceanographic data suggests that the glacial-interglacial difference in diatom-bound N-15/N-14 was driven by higher consumption of nitrate in the subantarctic surface during the last ice age. Such a change in nutrient consumption may have resulted from atmospheric iron fertilization and/or decreased glacial mixed layer depths associated with sea ice melting. Enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of CO2 in the ice age atmosphere. It also would have reduced the preformed nutrient content of the low-latitude thermocline, leading to decreases in low-latitude productivity, suboxia, and denitrification.