Biogeochemical changes within the Benguela Current upwelling system during the Matuyama Diatom Maximum: Nitrogen isotope evidence from Ocean Drilling Program Sites 1082 and 1084

Abstract

The Matuyama Diatom Maximum (MDM) is a time of peak opal accumulation from 2.6 to ∼2.0 Ma within the Benguela Current upwelling system that was initiated by increased influence of Southern Ocean water on the eastern South Atlantic. We measured opal, total organic carbon (TOC), and CaCO3 fluxes and C and N stable isotopes in sediments deposited from 2.4 to 1.95 Ma at Sites 1082 and 1084 to explore the biogeochemical dynamics within the Benguela region. The infusion of Southern Ocean water delivered dissolved nutrients and Southern Ocean flora and fauna, resulting in local opal accumulation increasing up to 8 g/cm2ky and the production of diatom mats. Some δ15N measurements of diatom‐bound organic matter indicate that the mats grew within the Benguela region. The bulk sediment δ15N records are taken to reflect changes in the δ15N of nitrate in the incoming water, where lower values at 2.4 Ma reflect less nitrate utilization in the Antarctic. A long‐term increase in relative nitrate uptake in the Southern Ocean is evidenced by the gradual increase in δ15N toward 1.9 Ma.