Fractionation during sediment winnowing drives divergent mass accumulation rates derived from ²³⁰Th and ³He on the Cocos Ridge

We examine seven multicores obtained from the Cocos Ridge, including three potentially winnowed sites, and investigate how grain size fractionation affects ²³⁰Th_XS- and ³He_ET-derived mass accumulation rates (MARs). We find that bulk sediment ³He_ET-derived MARs are a factor of 1.5–4.1 higher than ²³⁰Th_XS-derived estimates in shallow winnowed sites, and 9–37 % lower than ²³⁰Th_XS-derived estimates at deeper sites. Concentration measurements of ²³⁰Th_XS and ³He_ET on multiple grain size fractions show that at winnowed sites, 41 % of the ³He_ET inventory and only 18 % of the ²³⁰Th_XS inventory is found in the smallest size class, while at deeper sites over 75 % of the total inventories of both proxies are in the < 20 μm fraction. The data demonstrates that ³He_ET is preferentially mobilized during sediment winnowing compared to ²³⁰Th_XS, which drives the observed discordance in proxy-derived MARs at sites that have experienced significant removal of fine-grained sediment.

Comparison of ²³⁰Th_XS and ³He_ET to lithogenic dust proxies indicate that ³He_ET remains unfractionated from dust, regardless of notable variations in winnowing and focusing. Likewise, ²³⁰Th_XS does not significantly fractionate from lithogenic proxies, with potential decoupling only observed in intensely winnowed sites. Biogenic silica exhibits behavior similar to dust. Constituent fluxes show less variability across core sites when using helium-derived MARs rather than thorium-derived MARs, suggesting that ²³⁰Th_XS and biogenic silica are differentially transported in winnowed locations. Conversely, thorium-derived MARs present a CaCO₃ flux profile that is more consistent across the cores, indicating that ²³⁰Th_XS remains largely unfractionated from CaCO₃ during winnowing. Thus, the preferred constant flux proxy in a winnowed site depends on the characteristics of the sedimentary component of interest. Additionally, the absence of fractionation of ³He_ET from terrigenous dust and its associated radiogenic ⁴He suggests that winnowing and focusing will not substantially modify ³He_/⁴He and ³He/(1-carbonate fraction) ratios, tracers used to identify longer term changes in the extraterrestrial ³He flux.