OXYGEN DEFICIENCY IN THE EASTERN TROPICAL NORTH PACIFIC OCEAN OVER GLACIAL CYCLES: EVIDENCE FROM FORAMINIFERA-BOUND NITROGEN ISOTOPES

Abstract

The eastern tropical North Pacific (ETNP) contains the world’s largest oxygen deficient zone (ODZ), which hosts water column denitrification (WCD). WCD acts to elevate the 15N/14N (δ15N) of nitrate used for primary production above ODZs, causing past changes in WCD rates to be reflected in the δ15N of ETNP sediments. In cores from the ETNP, lower δ15N measured in bulk sedimentary N from the last glacial maximum (LGM) relative to the Holocene, has been interpreted to reflect reduced WCD during peak glacial periods. Foraminifera-bound nitrogen isotope records (FB-δ15N) from ETNP core ODP Site 1241 suggest little change in WCD rates between glacial and interglacials, including the Last Glacial Maximum (LGM) and the Holocene. Given the apparent lack of sensitivity of WCD rates to mean global climate, we expand FB-δ15N for ODP Site 1241 from 65 ka to 265 ka in order to determine alternative drivers of past WCD variability. FB-δ15N from Site 1241 lacks a glacial-interglacial difference over multiple glacial cycles, suggesting ODZs are a stable feature of the ETNP. Spectral analysis of FB-δ15N shows strong obliquity and precession signals, with elevated FB-δ15N values corresponding to strong northern hemisphere summer insolation, high obliquity, and a strong summer inter-tropical insolation gradient. We propose multiple mechanisms invoking both zonal and meridional dynamics of the regional wind-driven circulation, which may lead to increased upwelling, biological production, and oxygen deficiency in the ETNP.