Application of the newly developed nutrient diol index (NDI) as a sea surface nutrient proxy in the East Sea for the last 240 years

We assessed the applicability of the nutrient diol index (NDI) as a proxy for sea surface nutrients by analyzing sediments from a box core (ES14-BC03) collected in the southwestern continental slope of the East Sea, or the Japan Sea (hereafter the East Sea). The estimated sedimentation rate based on the 210Pb chronology was 0.15 cm yr−1 over the last 240 yrs. The NDI-derived phosphate (0.69 μmol L−1) and nitrate (8.63 μmol L−1) concentrations for the core-top sediment were within the average phosphate (0.26 ± 0.08 μmol L−1) and nitrate (3.38 ± 2.32 μmol L−1) concentration ranges for the last ∼20 years, accounting for both estimation error and variation in observational data. The distributions of the NDI-diols varied slightly through time, grouping into two distinct clusters (Cluster 1 and Cluster 2) in both principle component analysis (PCA) and hierarchical clustering of principal components (HCPC). Cluster 2 represented a time period with higher relative abundances of the C28 and C30 1,14-diols, resulting in higher NDI-derived nutrient concentrations than those of other periods. Interestingly, during this period, the U 37 K ' -derived sea surface temperatures (SSTs) also decreased. These results indicate that higher surface nutrient conditions and colder SSTs occurred between 1884 CE and 1911 CE, which might be associated with stronger upwelling intensity at the study site. This study is the first application of the NDI to a down-core, demonstrating that the NDI can be a useful proxy that provides helpful information about past sea surface nutrient conditions.