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Abstract:
In the modern eastern tropical North Pacific (ETNP), high surface productivity and long residence times of mid-depth waters give rise to one of the world’s largest oxygen deficient zones (ODZs), allowing for water column denitrification, a major mechanism of ocean nitrogen (N) loss. The history of water column denitrification can provide insight into past changes in ocean circulation and carbon cycling, with implications for atmospheric CO2. Given that water column denitrification acts to elevate the 15N/14N (δ15N) of nitrate available for primary production above ODZs, past changes in water column denitrification should be reflected in the δ15N of the buried products of surface productivity. In sediment records from the ETNP, bulk sedimentary N from the last glacial maximum (LGM) largely shows lower δ15N values relative to the Holocene, which has been interpreted to reflect reduced water column denitrification during the LGM. However, diagenesis and terrigenous N input may alter the bulk sediment N isotope signal. In contrast to the bulk sediment records, in the adjacent eastern equatorial Pacific (EEP), records of planktonic foraminifera-bound N δ15N (FB-δ15N) indicate no δ15N difference between the LGM and the Holocene. The FB-δ15N records may better reflect the history of water column denitrification in the region, suggesting similar water column denitrification rates during the LGM and Holocene. However, in the EEP, the degree of nitrate consumption by phytoplankton in the surface is a confounding variable. Here, we report a FB-δ15N record for ETNP core ODP Site 1241 extending back to the last ice age. ODP Site 1241 is from a region of nearly nutrient-free surface waters, removing the degree of phytoplankton nitrate consumption as a potential variable. Unlike the bulk sediment records but similar to the EEP FB-δ15N records, our ETNP FB-δ15N record suggests no δ15N difference between the LGM and Holocene. Thus, the new data constitute evidence for similar rates of ETNP water column denitrification between the LGM and the Holocene.
