Equatorial Pacific bulk sediment δ15N supports a secular increase in Southern Ocean nitrate utilization after the mid-Pleistocene Transition

Abstract

In much of the equatorial Pacific, surface nitrate is consistently available due to upwelling of sub-Antarctic Mode Water-derived nitrate from the thermocline. However, in the low-latitude western boundary current (WBC) region, surface nitrate is limited due to reduced upwelling and less nitrate-replete thermocline water masses. In order to explore South Pacific WBC paleoceanography via upper ocean nitrate dynamics, we present a new bulk sediment δ15N record from within this WBC region at International Ocean Discovery Program Site U1486 (2°22′S, 144°36′E) in the Bismarck Sea north of New Guinea. This record spans from 1420 ka to recent – surpassing nearby sediment δ15N reconstructions by over a million years and allowing for direct comparison to low-latitude Pacific sediment core δ15N records of similar length. Core-top and down-core bulk sediment δ15N, TOC/TN and δ13Corg values indicate minimal or no terrestrial influence on the organic fraction at Site U1486, which is consistent with previous studies of modern processes in the region. After comparison, differences in orbital variability and secular trends throughout the Middle and Late Pleistocene suggest that nitrate dynamics along the equator and in the WBCs were relatively disparate. We observe that δ15N at Site U1486 is consistent with patterns of eastern Pacific denitrification, while we suggest increasing δ15N after the mid-Pleistocene Transition (MPT; ∼1250–700 ka) at Sites 806 (0°19′N, 110°30′W) and 849 (0°19′N, 160°E) is linked to increasing Southern Ocean nitrate utilization. Enhanced nitrate utilization is a key indicator of the strengthened biological pump that contributed to a reduction in atmospheric pCO2 during the last glacial. Therefore, a post-MPT increase in nitrate utilization may bolster the role of the Southern Ocean biological pump in driving the deeper and longer glacial periods of the 100-kyr world.