Adsorption of Th and Pa onto particles and the effect of organic compounds in natural seawater

摘要

^(231)Pa and ^(230)Th are two crucial isotopes in the ongoing GEOTRACES Project.However,the controversy on ^(231)Pa/^(230)Th proxy pertaining to archiving ocean circulation or recording paleoproductivity,is still unresolved,partly owing to the unclear understanding of fractionation between ^(231)Pa and ^(230)Th during adsorption.In this study,controlled experiments were conducted to examine the adsorption of ^(234)Th and ^(233)Pa onto biogenic particles(SiO_(2) and CaCO_(3)),authigenic minerals(MnO_(2) and Fe_(2)O_(3)),and lithogenic minerals(kaolinite,attapulgite,montmorillonite,and aluminum oxyhydroxides),and the role of organic compounds in regulating the adsorption of ^(234)Th and ^(233)Pa in natural seawater was evaluated.The distribution coefficients(K d,presented as logK_(d))varied from 3.56 to 6.05 and from 3.27 to 5.82 for ^(234)Th and ^(233)Pa,respectively.Fe_(2)O_(3) is the strongest sorbent for both ^(234)Th and ^(233)Pa.Most of the particles showed comparable logK_(d) values for either ^(234)Th(~4.8)or ^(233)Pa(~3.9)in the presence of dextran,indicating that the adsorption of Th and Pa is likely controlled by organic coating on particle surfaces.The fractionation factors(F Th/Pa)of SiO_(2)(3±1)and CaCO_(3)(33±1)suggest in situ observed preferential scavenging of ^(230)Th to ^(231)Pa in the surface water of low-to mid-latitude regions and the nearly equal removal in the Antarctic Ocean where biogenic silica dominates the particle regime.The F Th/Pa values of the lithogenic and biogenic particles indicate that ^(230)Th is scavenged prior to ^(231)Pa in the particle-scarce ocean interior.The equal scavenging of ^(230)Th and ^(231)Pa at the ocean margins and the ridge crests is dominated by high particle fl uxes instead of particle composition control.These results imply that ^(230)Th/^(231)Pa can be used as different proxies in different oceanic settings.