Drivers of Dissolved Thorium Isotope Distributions Along the Adélie Land Margin, East Antarctica: The Roles of Reversible Scavenging, Conservative Transport, and Lithogenic Supply

Abstract

In this study, we present a mix of full-depth profiles and near-bottom observations of dissolved ²³⁰Th and ²³²Th from the Adélie Land margin in East Antarctica, a key site of Antarctic Bottom Water (AABW) formation. We show that the distribution of dissolved Th isotopes in this region is governed by a combination of reversible scavenging, conservative transport and strong lithogenic inputs, especially on the continental shelf. A model incorporating both reversible scavenging and the upwelling of Circumpolar Deep Water successfully reproduces observed ²³⁰Th_xs distributions in the upper and mid-water column. However, deviations at depth point to additional processes, such as enhanced scavenging near the seafloor, and inputs of ²³²Th from benthic and/or suspended sediment dissolution. Exceptionally high inputs of ²³²Th to bottom waters on the Adélie shelf, likely sourced from glaciogenic sediments near the Mertz Glacier, are observed. This lithogenic inputs impart a distinct elevated ²³²Th/²³⁰Th_xs signature to shelf waters, which appears to be largely removed via scavenging before entering the abyssal ocean within the AABW. In contrast, the upper 200 m of the water column shows offshore transport of shelf-derived ²³²Th within Winter Water (WW). Based on the strong correlation between ²³²Th/²³⁰Th_xs and dissolved manganese (dMn) in Shelf Water samples, we estimate ∼75% of the dMn observed in WW is sourced from the dissolution of shelf sediment. As WW is progressively mixed into overlying surface waters, this source likely acts as a “slow release” pathway of dMn to the euphotic zone, potentially supporting biological productivity.