Unradiogenic reactive phase controls the ENd of authigenic phosphates in East Antarctic margin sediment

Determining past changes in ocean circulation on the Antarctic margin is important for understanding the interactions between climate, circulation, and ice sheet retreat. However, the application of neodymium isotopes (epsilon(Nd)), a well-known proxy of ocean circulation, is limited on the Antarctic margin, due to the lack of carbonate preservation and inconsistency in authigenic epsilon(Nd) leached from bulk sediment. Here we assess the use of the epsilon(Nd) proxy along the continental rise of Wilkes Land, by combining analyses of seawater with phases extracted using a 10-second reductive leach in co-located surface sediments. Dissolved seawater epsilon(Nd) values displayed the following water mass signatures; epsilon(Nd) = -8.8 +/- 0.2 (n = 1) for Antarctic Surface Water; epsilon(Nd) = -9.7 +/- 0.2 (n = 2) for Winter Water; and epsilon(Nd) = -8.7 +/- 0.3 (n = 6) for modified Circumpolar Deep Water. The sediment leachate did not reproduce a bottom water Nd signature and yielded a very variable epsilon(Nd), ranging from -10.4 to -14.4. In contrast, the bulk detrital sediment eNd fell within a narrow range of -13.1 +/- 0.5 (n = 29). Examination of elemental ratios and rare earth element (REE) anomalies indicates that the leaching procedure extracts exclusively authigenic phases. The strong relationship found between phosphorus (P) and REE suggests that a P-associated phase is the main REE host-phase leached, as opposed to ferromanganese oxyhydroxides. The leached europium anomalies and epsilon(Nd) indicate that the extracted Nd signature is influenced by two end members, one with a seawater-like epsilon(Nd) (similar to-8) and one more negative than the bulk detritus (epsilon(Nd) similar to-14). Finally, the decoupling between [Nd] and epsilon(Nd), as well the distinct middle-REE enrichments measured in the leach implies the authigenic epsilon(Nd) is controlled by diagenesis. We thus infer that the Nd signature of the leached phosphates is derived from porewater and influenced by the presence of a reactive detrital component in the sediment. Unradiogenic reactive sedimentary phases which easily dissolve into porewaters are likely sourced from the subglacial erosion of ancient metasediments and crustal remnants on the Wilkes Land coast. These new epsilon(Nd) data contribute to the evaluation of the epsilon(Nd) proxy at complex, dynamic and under-studied interfaces such as the East Antarctic margin. However, further work is still needed to develop reliable proxies of past Antarctic water masses. (C) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

Determining past changes in ocean circulation on the Antarctic margin is important, but the current proxy methods are limited. This study evaluates the use of neodymium isotopes as a proxy for ocean circulation along the Wilkes Land continental rise. The results show the potential of seawater neodymium isotopes as water mass signatures, but further research is needed to develop reliable proxies for Antarctic water masses.