Determination of Mg isotopes in chlorophyll a for marine bulk phytoplankton from the northwestern Pacific Ocean

We present Mg isotope data of chlorophyll a extracted from marine phytoplankton samples. Thirty‐eight samples of phytoplankton were collected at three different regions (inner bay and the Kuroshio and Oyashio regions) from the northwestern Pacific Ocean. Even though the dominant class of phytoplankton samples is mainly diatom, large variation (2.37‰) of δ 26 Mg values was observed. Variation of δ 26 Mg values in marine phytoplankton collected in the Oyashio region during a spring bloom was smaller than in the Kuroshio region (2.37‰) and within a bay (2.03‰). There is no clear relationship between δ 26 Mg values and other oceanographic parameters such as temperature, salinity, chlorophyll a concentration, and nutrients. We conclude that interspecies variability of diatoms is the most likely cause of variability in δ 26 Mg values. Further experiments can allow us to explore which oceanographic parameters control Mg isotopic composition of chlorophyll a . More Mg isotope data are necessary to assess the usage of Mg isotopes of chlorophyll as a potential oceanographic indicator preserved not only in marine phytoplankton but also in other photosynthetic organisms.