Distribution of long chain alkyl diols along a south-north transect of the northwestern Pacific region: Insights into a paleo sea surface nutrient proxy

Long chain alkyl diols (LCDs) have previously been used to develop several diol indices as a proxy for upwelling or stratification conditions and sea surface temperatures. However, there is still a lack of data on the distribution of LCDs in the Pacific Ocean, which calls into question to what extent these diol indices can serve as a proxy in the Pacific Ocean. Hence, we investigated the distribution of LCDs in suspended particulate matter (SPM) samples collected in July 2015 along a south-north transect ranging from the East Sea of Korea to the Bering Sea in the northwestern Pacific region. Our results showed that both saturated and unsaturated C-28 and C-30 1,14-diols were dominant in most SPM samples investigated. The principal component analysis (PCA) results of the SPM data revealed that the C-28:1 and C-30:1 1,14-diols were positively associated with nutrient concentrations. However, only the C-28:1 1,14-diol in addition to the C-28 1,14-diol was positively associated with nutrient concentrations in the global surface sediment data set previously published, while the C-30:1 1,14-diol was related to sea surface temperature (SST). Based on these observations, we defined a new diol index, i.e., the nutrient diol index (NDI), which included the C-28 1,14-diol as well as the C-28:1 1,14-diol in the numerator and all the 1,13-, 1,14-, and 1,15-diols, excluding the C-32 1,15-diol, in the denominator. The NDI was positively associated with the ocean atlas values of surface water phosphate and nitrate concentrations for the SPM and surface sediment data sets. Accordingly, the results of our study showed that the NDI has potential as a quantitative nutrient proxy for estimating sea surface phosphate (or nitrate) changes. Further studies should be undertaken in order to assess the applicability of the NDI as a paleo-nutrient proxy for confidently reconstructing surface water nutrient variations in the past, by analyzing more SPM, surface sediments, and sediment cores collected from various oceanic settings. (C) 2018 Elsevier Ltd. All rights reserved.We are grateful for constructive comments on an earlier version of this paper by the Associate Editor and two reviewers, Sebastian Rampen and David Naafs. We appreciate our discussions with Seung-Il Nam and Hirofumi Asahi, and the sampling support of Sun Yong Ha at KOPRI. We would like to thank Solbin Kim for her analytical support at HYU. We would also like to thank the captain and crew of R/V ARAON for their safe and skillful operation of the ship during the cruise. This study was supported by grants from the Korea Polar Research Institute - South Korea (KOPRI, PE16490), the National Research Foundation of Korea grant funded by the Ministry of Science and ICT - South Korea (2015M1A5A1037243, PN17090), and partly by the K-AOOS 'Korea-Arctic Ocean Observing System (PM17040)' project funded by the Ministry of Oceans and Fisheries - South Korea