High sensitivity determination of stable isotopes in nitrate and their seasonal variations in Suruga Bay

Abstract

Nitrogen and oxygen stable isotope ratios (D¹⁵N, D¹⁸O) are used as robust tracers for the nitrogen cycle in the ocean. Using a highly sensitive quantification method for stable isotopes allow us to determine the proportion of atmospheric nitrate in total oceanic nitrate from d (15,18) value. An extraction device capable of introducing the entire sample was constructed, which enabled the introduction of four times the amount of the conventional method, and the measurement accuracy was achieved at 0.3 µM, which is the same as the conventional method. Using this method, nitrate and their isotopes were measured every two months at seven layers up to 200 m depth in Suruga Bay. The results showed that d (15,18) values at less than 50 m were lower than those in the subsurface layer from February to June, and that atmospheric nitrate contributed 1-3% to the water depth of 100 -150 m during the same period.