Tracing Southern Ocean nitrate consumption from nitrogen isotopes in foraminifera over the past 1.5 million years

Abstract

Over the last 1.5 million years, Earth's cyclic glaciations have become longer in duration and more intense. The subantarctic Southern Ocean is considered an important region for explaining these glacial cycles through the control of biological productivity on atmospheric CO2. Over the last glacial cycle (the last 120,000 years), intervals of increased dust-borne iron flux correlate with increased biological productivity and a greater degree of nitrate consumption in the subantarctic Atlantic Ocean, which contributed lower glacial atmospheric CO2 levels. However, it is not yet clear how such processes may have affected atmospheric CO2 outside of the subantarctic Atlantic, or prior to the last glacial cycle. Here, I analyze the degree of nitrate consumption in the subantarctic South Pacific Ocean during higher frequency, lower amplitude glacial cycles, before the mid-Pleistocene transition (MPT), by measuring nitrogen isotopes in the organic matter bound in planktonic foraminifera. Using these data, I show that increases in biological nitrate consumption during glacial periods before the MPT have similar amplitudes to the last glacial cycle in the subantarctic South Pacific. Additionally, evidence for lower biological productivity during glacial periods before and after the MPT indicates that increases in the dust-borne iron flux did not drive increases in biological productivity at this site. Instead, increased surface ocean stratification during glacial periods may better explain the changes in biological nitrate consumption at this site.