[M-IS06] Biogeochemistry

[MIS06-01] Seasonal variations in surface water nitrogen cycle in the western subarctic North Pacific revealed by nitrogen isotope ratios of amino acid from the settling particles

*Chisato Yoshikawa¹, Nanako O. Ogawa¹, Yoshito Chikaraishi¹, Tetsuichi Fujiki¹, Naomi Harada¹, Makio Honda¹, Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Marine nitrogen cycle, Nitrogen isotope, Marine ecosystem model

The nitrogen isotopic composition (δ¹⁵N) of nutrients in the surface ocean transfer to the δ¹⁵N of phytoplankton, settling particles, and eventually benthic sediment. The δ¹⁵N of settling particles and sediment can therefore be used to trace the past nitrogenous nutrient environment. Previous observations have revealed that there is an inverse relationship between seasonal δ¹⁵N and flux of settling particles. In winter at high latitudes, the settling particles have high δ¹⁵N and low flux as compared with other seasons. In contrast, the surface water nitrate in winter has the lowest δ¹⁵N in a year due to convective mixing. The winter settling particles should also have the lowest δ¹⁵N, if winter phytoplankton assimilates only nitrate. Previous studies pointed out three reasons why δ¹⁵N of settling particles from autumn to winter increases despite the decrease in δ¹⁵N of surface nitrate: (1) the increase in contribution of zooplankton, which have a δ¹⁵N about 3 ‰ higher than that of phytoplankton; (2) the contribution of old particles most degraded with isotope fractionation in a year; and (3) the winter phytoplankton assimilates not only nitrate but also ammonium, which has higher δ¹⁵N than nitrate due to nitrification. In this study, to clarify the reason we applied the compound-specific stable isotope analysis of amino acid and a marine nitrogen isotope model. Sediment trap experiment was conducted at 1000 m depth at station K2 (47°N, 160°E) from June 2014 to July 2015. The bulk δ¹⁵N was determined by a sensitivity-improved EA/IRMS. The δ¹⁵N of glutamic acid and phenylalanine were determined by GC/C/IRMS. The bulk δ¹⁵N show relatively low values around 2‰ from July to August and increases to 5‰ from September to June, which is a typical seasonal variation observed at high latitudes. Surprisingly, the apparent trophic positions of settling particles estimated from the δ¹⁵N of glutamic acid and phenylalanine are 2.0±0.1 both in summer and winter. This is the first evidence that the winter high-δ¹⁵N of settling particles does not reflect the increase in contribution of zooplankton. Our model result suggested that the winter high-δ¹⁵N value of settling particles mainly reflects the winter high-δ¹⁵N of ammonium due to nitrification, which was strongly supported by the nitrogen isotopic compositions of amino acids.