Who stole my δ14N? Local vs. remote drivers of the South Pacific Oxygen Minimum Zone during the Holocene

The waters underlying the Peru margin upwelling region represent the largest of the world’s three oxygen minimum/denitrification zones. Situated at the interface between the poleward Peru Undercurrent and the equatorward Peru Coastal current, Peru margin upwelling represents the final stages of a ventilation process that begins in the southern high latitudes, extends to the western subtropics, before traveling across the pacific basin to the eastern tropical pacific. Microbial denitrification under these oxygen-depleted conditions represents the major loss of nitrate from the global ocean. During denitrification, light nitrate (14NO3-) is preferentially removed leaving the remaining nitrate pool enriched in heavy nitrate (15NO3-). This heavy nitrate signal is transferred to the surface through upwelling where it is used by biota and eventually transferred to the underlying sediments. Using a multi proxy approach and an annually laminated sediment core from the heart of the Peru margin OMZ we explore the possible sources of nitrogen isotopes across the Holocene. Central to our investigation is understanding the dual contributions of local carbon flux verses remote ventilation of the Peru Undercurrent. Waters at 200 to 800 meters depth are depleted in oxygen through a combination of high respiration rates driven by upwelling- induced surface productivity, and poor ventilation. If the oxygen demand imposed by local respiration dominated the subsurface low oxygen signal we would expect to see a positive correlation between indicators of local surface production (C37total and Biogenic Silica) and (δ15N). Instead we find a strong decoupling between local carbon flux (productivity) and remote ventilation, particularly during the early Holocene. To find the source of this enriched δ15N signal we look to climatic processes originating outside the tropics, including the equatorial undercurrent waters that feed the Peru Undercurrent and the circulation of the south Pacific gyre, which could influence the thermocline density structure and ventilation of the OMZ over time. We posit that on centennial to millennial timescales, the ventilation of the OMZ by remote processes dominates over the oxygen demand imposed by regional productivity to determine the intensity of oxygen depletion and nitrate removal along the Peru margin.