Nitrate Isotope Distribution in the Subarctic and Subtropical North Pacific

Abstract

AbstractNitrogen and oxygen isotopic compositions of nitrate (δ15NNitrate and δ18ONitrate) were determined along 47°N in the subarctic North Pacific and 149°E in the western North Pacific. In the western subarctic gyre, known as a high‐nutrient, low‐chlorophyll region, the δ15NNitrate and the differences between δ15NNitrate and δ18ONitrate (herein Δ(15–18)) in the intermediate water were significantly lower than in the surrounding area. The 15N‐depleted nitrate was generated by nitrified nitrate from the remineralization of organic matter produced by the partial consumption of the surface nitrate pool. Between the western subarctic gyre and the Alaskan gyre, there was a 0.2‰ increase in δ15NNitrate and a 0.3‰ increase in Δ(15–18) in the intermediate water associated with a 7.2‐μM decrease in the nitrate concentration at the surface. Assuming Rayleigh distillation kinetics, the increase in utilization of the surface nitrate pool between the western subarctic gyre and the Alaskan gyre (from 29% to 85%) corresponds to a change in δ15N of organic matter from 2.6‰ to 4.9‰. The 15N enrichment of intermediate nitrate toward the east resulted from the increase in utilization. Moreover, the 15N‐depleted nitrate in the mixed‐layer water of the western subtropical gyre was affected by N2 fixation in the Kuroshio water, whereas the 15N‐enriched nitrate in the intermediate water at the western margin of North America was affected by water column denitrification. The δ15N sediment record in the western subarctic North Pacific likely reflects past changes in the high‐nutrient, low‐chlorophyll region but may also be controlled by water column denitrification and N2 fixation.