Diatom‐bound ¹⁵N/¹⁴N: New support for enhanced nutrient consumption in the ice age subantarctic

Diatom‐bound 15 N/ 14 N 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 δ 15 N of 5 to 6‰ during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2‰. The effect of either diatom assemblage or physiological change on the diatom‐bound 15 N/ 14 N 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 15 N/ 14 N 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 CO 2 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.