Evaluating the Behavior of ²³⁰ Th and ³ He Constant‐Flux Proxies Across a Carbonate Preservation Gradient in the Cocos Ridge

Abstract

We present a comparison of extraterrestrial ³ He ( ³ He _ET ) and excess ²³⁰ Th ( ²³⁰ Th _xs ) as constant flux proxies across a depth transect on the Cocos Ridge, evaluating their behavior as a function of carbonate dissolution and lateral sediment redistribution. Overall, 60% of paired ³ He _ET ‐ and ²³⁰ Th _xs ‐derived mass accumulation rate (MAR) estimates overlap within 1σ uncertainty, indicating broad agreement between the two systems. However, statistically significant discordances between He‐ and Th‐derived MARs occur in three of four cores, with He‐derived MARs exceeding Th‐derived MARs at the ridge crest (positive ΔMAR values) and falling below Th‐derived MARs at flank sites (negative ΔMAR). The discordance at the ridge crest is consistent with sediment winnowing and preferential removal of ³ He‐bearing particles, whereas the smaller discordances in flank cores are best explained by a complementary focusing signal, likely reflecting redistribution of material removed from the ridge top. Evaluation of the negative ΔMAR offsets demonstrates that ²³⁰ Th _xs is effectively retained during carbonate dissolution, as ΔMAR responses remain small despite strong CaCO ₃ preservation gradients, and theoretical constraints indicate negligible thorium mobility. When applied jointly, the two proxies provide complementary constraints on MARs that would remain unresolved using a single proxy. However, their interpretation remains site‐dependent, requiring careful consideration of sediment composition, transport regime, and analytical limitations. Because sedimentary burial fluxes are calculated from component concentrations and MAR, these proxy‐dependent differences directly affect estimates of carbonate and other elemental burial fluxes, with implications for reconstructing both past and modern marine biogeochemical cycles.