Assessment of the nutrient diol index (NDI) as a sea surface nutrient proxy using sinking particles in the East Sea

Abstract

The Nutrient Diol Index (NDI) has been proposed as a paleo sea surface nutrient proxy. However, the NDI's broad applicability needs to be further assessed by examining its temporal variability in association with modern sea surface nutrient concentrations. This study is the first report to evaluate the NDI with respect to sinking particles in the East Sea. The NDI values showed seasonal variations resembling those of the surface phosphate concentrations in the study area. However, the calibration based on the sinking particles at 1000 m water depth (NDI = 1.134 × [PO4−] + 0.115, R2 = 0.59, n = 26) deviated from the newly compiled global surface sediment dataset (NDI = 0.392 × [PO4−] + 0.023; R2 = 0.58, n = 566). This local calibration predicted the phosphate concentrations in surface sediments in the East Sea better than the global calibration. Our study suggests that the difference of the calibration slope between the East Sea trap dataset and the global surface sediment dataset might be associated with different 1,14-diol producers and their sensitivity to nutrient concentrations in the East Sea. Accordingly, the NDI should be further assessed in various oceanic settings before it is routinely used to reconstruct sea surface nutrient conditions.