Use of long chain diols for developing a new paleo sea surface nutrient proxy

Abstract

The nutrient is the important factor to control the primary productivity and the useful indicator of the paleo productivities and water masses in the marine environments. In several nutrient proxies were proposed for the marine environment and paleo-oceanographic studies, such as trace metals (eg. Cd and Zn) and carbon isotope analysis for benthic planktonic foraminifera and nitrogen stable isotope for organic matter. These tracers useful to understand of the deep-water masses movements or nitrate utilization, not sea surface nutrient conditions. Long chain alkyl diols (hereafter referred as diols) were widely observed in the various environments from the global marine sediments. Especially, saturated and mono-unsaturated C28 and C30 1,14-diols are produced by marine planktonic diatoms, genus Proboscia which is dominant phytoplankton in nutrient-rich and upwelling area, highly productive marine environments. Recently, several potential proxies for the upwelling and high nutrient condition were suggested, however, diol indices did not work as the quantitative indicator for upwelling and/or nutrient conditions. To develop a paleo nutrient proxy, the spatial distributions of diols were investigated from the sea surface suspended particulate matters during transection from the East Sea to the Bering Sea including the north west Pacific Ocean where has the high nutrients concentration. It was founded that the different behavior between C28 1,14-diol and C30 1,14-diol based on the principal component analysis, and this differences also observed in the reported global surface sediment dataset. Based on this result, for the first time, this thesis proposed a novel paleo sea surface nutrient proxy, nutrient diol index (NDI) which included the C28 1,14-diol as well as the C28:1 1,14-diol in the numerator and all the 1,13-, 1,14-, and 1,15-diols, excluding the C32 1,15-diol, in the denominator. The NDI was shown the positive correlation with the surface water phosphate and nitrate concentrations from ocean atlas values for the global surface sediment data sets. This new proxy was tested for the validation and assessment in the modern marine environments using one-year record using time series sediment trap in the East Sea. The NDI values showed the sensitive variations against with in-situ nutrient concentrations by measured and estimated nutrient concentrations by the atlas dataset. Moreover, the NDI-derived nitrate and phosphate concentrations for the core-top sediment were similar with the ranges of the observational data when considering the variations of the observational data during ~20 years and the estimation error ranges. The reconstruction of the nutrients concentration and paleo sea surface temperature were performed by analyzing a box core (ES14-BC03) collected in the southwestern continental slope of the East Sea over the last 240 yrs. The reconstructed nutrients and temperatures were slightly varied through time. There was the period (1988−1911) which had higher nutrients concentrations and lower temperatures, it suggests that stronger upwelling intensity at the study site. And these results indicate that the NDI might be useful to reconstruct the nutritional conditions from the sediment core for paleo marine environmental studies. And the diols and alkenones were determined to the sediment core (ES14-GC01) collected in the Ulleung Basin which covered last 40 kyr BP. Interestingly, the NDI-derived nutrient concentrations in the Ulleung Basin, reconstructed sea surface salinity in the Okinawa Trough and alkenone and biogenic opal production in the Tsugaru Strait were indicated the similar patterns of variations through time during glacial−interglacial period. It is interpreted that the changes in nutrient concentration in the East Sea by the freshwater influx, changes in salinity of Okinawa Trough by the strengthen of the Kuroshio Current, and changes in the productivity of Tsugaru Strait by the Oyashio Current were inter-linked through the paleo current changes by the global sea level changes during glacial−interglacial period. In conclusion, this thesis suggests the new paleo-sea surface nutrient proxy for the first time. NDI was confirmed that is sensitive to the changes of nutritional conditions in modern marine environments. Moreover, NDI can support the information of paleo production as well as nutrient concentration of the sea surface. And the multi proxy study with NDI and paleothermometer will be helpful to understand water mass movement of the sea surface, the paleo surface currents, in the paleo environment reconstruction.