Geochemical Behaviors Of Th In The South China Sea And The Western North Pacific

Seawater samples were collected in Autumn 2013 from the South China Sea,Summer 2014 from the South China Sea,and Spring 2014 from the western North Pacific for the analysis of Th concentrations with a MC-ICP-MS?Muti-Collector Inductively Coupled Plasma Mass Spectrometry?method.The method is developed for high precision measurement of seawater U and Th concentrations and isotope ratios.The geochemical behaviors of Th in the South China Sea and the western North Pacific were then investigated.In the South China Sea,the contents of dissolved ²³⁰Th ranged from 2.24 to 93.53 fg/kg,while the dissolved 232Th ranged from 113.54 to 921.74 pg/kg.Both of them were higher than the North Pacific,but similar to marginal seas and the Arctic Ocean.Most of the contents of ²³⁰Th in the upper water column are one order of magnitude higher than the open ocean,which may be resulted from the dissolved 30Th supplyment of interstitial water.Here we also present unusual deficiencies of ²³⁰Th in mesopelagic water?500-1000 m?in the South China Sea.Compared to the model-predicted ²³⁰Thxs(total ²³⁰Th-average terrigenous ²³⁰Th)distribution from the RSM line,the measured ²³⁰Thxs showed relatively uniform distribution and is in deficit.The most probable explanation for the ²³⁰Th deficit in the mesopelagic water is an enhanced scavenging effect by the particles supplied from the lateral sediment transport,probably via the South China Sea meridional circulation.In the western North Pacific,the concentrations of dissolved ²³⁰Thxs ranged from 1.29 to 12.27 fg/kg.The measured ²³⁰Thxs agreed with the model-predicted ²³⁰Thxs distribution from the RSM line,and showed liner distribution.The concentrations of dissolved 232Th ranged from 248.24 to 408.46 pg/kg,which were one order of magnitude higher than that of open ocean,the same order with that of the Arctic Ocean.We obtained the residence time?7.2 a?of ²³⁰Th by calculation according to the average sinking rate?S = 400 m/a,Okubo et al.,2012?in the whole water column,while the residence time was 15.6a at 2000m water depth.The short residence time indicated the high eliminating rate in the Pacific coast.Besides,the terrestrial input flux in the western North Pacific was 4.63 g·m-2.a-1 which reflected the significant influence by dust deposition from the coast toward the ocean.The K value in the SCS ranged from 0.06 to 0.11,which is slightly lower than the other seas,possibly caused by particle composition,and need to explore it further.The terrestrial input flux in the SCS is 0.38-0.55 g·m2·a-1 which is within the limits of fitted values from the atmospheric deposition model.However,it may be inapplicable in the case of particular condition such as extreme weather events when calculating terrestrial fluxes in China marginal seas.Therefore,we need more observed values to calculate terrestrial fluxes realistically.