Refining the signature of thallium isotopes in low oxygen marine environments

Abstract

Thallium (Tl) isotopic values (ε²⁰⁵Tl) appear to track changes in marine manganese oxide deposition, with these isotope signatures having been utilized as a proxy for rapid oceanic seafloor (de-)oxygenation events. With a residence time longer than ocean mixing time and the ability to track the deposition of manganese oxides, ε²⁰⁵Tl may effectively record the earliest global transitions in the extent of ocean oxygenation. However, some uncertainty remains for the minor Tl sinks in degree of fractionation from seawater values, if any, with the limited data currently available suggesting at least a 6 epsilon unit range in fractionation from seawater values in low oxygen environments. This study provides Tl data for sediment cores from a range of low-oxygen marine environments. With these data, we identify potential processes that impact the range of Tl isotope variations within the sediments, which are not all due to local Mn oxide cycling. Previous work indicates that euxinic (anoxic and sulfidic water column) conditions and early diagenetic pyrite formed under consistently anoxic sediments record seawater values with no (or not measurable) fractionation during absorption to pyrite. Our new data provide downcore confirmation. Meanwhile, only limited data from ‘suboxic’ environments (those with low oxygen but likely not permanently anoxic conditions) has been analyzed. Thus, isotopic data for suboxic systems is needed to refine the current mass balance. Several sites off the California, Mexico, and Peru coasts with a range of redox states from oxic to perennially anoxic were selected to allow for a comparison across a range of open ocean bottom water conditions. The locations with oxic sediments tend to document more positive values compared to seawater, as expected due to local manganese oxide incorporation. Sediments from more ‘suboxic’ (manganous to ferruginous) sites tend to have invariable downcore geochemical signatures that are between marine inputs (−2) and modern seawater (−6) values, indicating a mixing of ε²⁰⁵Tl signatures from different authigenic phases; however, these are not primarily due to the incorporation of Mn oxides as the concentrations are low and uncorrelated. The anoxic sites record Tl isotope compositions near seawater values, confirming that early diagenetically formed pyrite (and precursor minerals) record seawater ε²⁰⁵Tl signatures under permanent anoxia. Importantly, these permanently anoxic localities have minor Mn contents, which suggest no local Mn oxide Tl isotope signatures. Therefore, unlike the anoxic sediments, the ‘suboxic’ sediments have a Tl isotope value that is slightly offset from seawater without significant Mn oxide deposition, thus suggesting there could be a fractionation for this process. Our observations provide improved constraints on the Tl isotope system, especially on a poorly constrained aspect of the mass balance, which will be important for deep-time applications.