A quantitative, deglacial reconstruction of bottom-water nitrate in the intermediate Pacific using the pore density of the denitrifying benthic foraminifera

The marine nitrogen cycle is being altered on a global scale by increasing anthropogenic production and use of chemical fertilizers. Nitrate (NO₃⁻) is an important macronutrient that is limiting in some marine environments. Modelling studies have predicted increased NO₃⁻ concentrations during glacial periods due to reduced water column denitrification as compared to interglacials. The aim of our study is to provide a widespread, quantitative reconstruction of bottom-water NO₃⁻ concentrations ([NO₃^-]_BW) in the intermediate Pacific covering the last deglaciation. Downcore samples taken from the Eastern Tropical South Pacific in the Gulf of Guayaquil (M77/2-059-1), and Eastern Tropical North Pacific from the Mexican Margin (MAZ-1E-04), Sea of Okhotsk (MDO1-2415), and Gulf of California (DSDP-480) were utilized. We have utilized the pore density of the denitrifying benthic foraminifera Bolivina spissa and Bolivina subadvena as proxy for deglacial [NO₃⁻] concentrations. These shallow infaunal foraminifera species are abundant in oxygen-depleted environments all around the Pacific. They can denitrify and most likely take up NO₃^- as an electron acceptor through the pores, making their pore density an empirical proxy for NO₃^- concentrations. We found that the [NO₃^-]_BW in the Gulf of Guayaquil, Gulf of California, and Mexican Margin were higher during the glacial period than the Holocene. The Gulf of Guayaquil showed a pronounced decrease in [NO₃^-]_BW during the Heinrich Stadial, while the Mexican Margin showed a peak in [NO₃^-]_BW during the entire Younger Dryas. The Sea of Okhotsk core covers only from the Younger Dryas until the middle Holocene, and [NO₃^-]_BW was found to increase from the Younger Dryas to the Holocene. In all cores studied, we also compared past and present [NO₃^-]_BW. Our data shows that both the Gulf of Guayaquil and the Gulf of California had higher [NO₃^-]_BW in the past than today. In contrast, the Mexican Margin and the Sea of Okhotsk had lower [NO₃^-]_BW in the past than today. We speculate that a site-specific balance between reduced water column denitrification, sea-ice retreat, increased nutrient input from continental shelves as a result of sea-level changes, changes in water mass ventilation, a decrease in upwelling productivity, or thermal contrast between land and sea may account for changes in nitrate concentrations during cold-warm periods at the studied sites. A comprehensive understanding about past nutrient cycling under rapidly changing climatic conditions is one prerequisite to predict future changes in marine nutrient budgets in the Eastern Equatorial Pacific.