Seasonal Variations in the Siliciclastic Fluxes to the Western Philippine Sea and Their Impacts on Seawater epsilon(Nd) Values Inferred From 1Year of In Situ Observations Above Benham Rise

Abstract

Plain Language Summary The Sr and Nd isotopic compositions of siliciclastic sediments and the Nd isotopic compositions of planktonic foraminifera are investigated in sediment trap samples collected in 2015 at water depths of 500 and 2,800m to (1) track the seasonal changes in the sources and transport patterns of siliciclastic sediments to Benham Rise, (2) constrain the Nd isotopic compositions of the planktonic foraminifera throughout the water column, and (3) assess the influences of lithogenic inputs on the Nd isotopic compositions of the water masses. We demonstrate that volcanic matter and eolian dust are derived primarily from Luzon Island and the Ordos Desert, respectively. In addition, we show that reduced precipitation over Luzon Island and the weakened East Asian winter monsoon intensity result in mass fluxes from Luzon Island and the eastern Asian deserts to the sea, respectively, that are weaker in the winter than in the spring. Furthermore, the epsilon(Nd) values of the foraminifera collected at a water depth of 500m change slightly in 2015, suggesting negligible impacts of lithogenic Nd inputs characterized by significant seasonal flux variations. In contrast, the epsilon(Nd) values of the planktonic foraminifera collected at a water depth of 2,800m are systematically more radiogenic and appear to display seasonal variability. Such results suggest the rapid modification of epsilon(Nd) values during the settling of planktonic foraminifera by the precipitation of Mn coatings derived from water masses at deposition depths, with greater contributions of colder water masses originating from the volcanic Luzon Island margin during the winter. Based on the first report of Sr-Nd isotopic compositions of siliciclastic sediments and Nd isotopic compositions of planktonic foraminifera in sediment trap samples collected from the western Philippine Sea in 2015 at water depths of 500 and 2,800m together with the relevant current information, we try to recover the potential linkages between the East Asian winter monsoon strength, the East Asian summer monsoon (precipitation) intensity, and the siliciclastic sediment source-to-sink processes as well as the influences of lithogenic inputs on the Nd isotopic compositions of the water masses at the northern margin of the western Pacific Warm Pool in different seasons of 2015. On the one hand, we quantitatively estimate the eolian dust sources to the study area. On the other hand, we find the rapid changes in planktonic foraminiferal epsilon(Nd) values during settling in the western Philippine Sea. Such results are critical for effectively interpreting the siliciclastic sediment source-to-sink processes and marine circulation as well as the behind controlling mechanisms at low latitudes during the geological past.