A DETAILED INVESTIGATION OF THALLIUM ISOTOPES UNDER LOW-OXYGEN OPEN OCEAN CONDITIONS

Thallium (Tl) isotopes are a new and novel redox proxy to track the global burial of manganese oxides (Mn-oxides) since Mn-oxide precipitation and burial are directly linked to available oxygen content and are heavily affected by global fluctuations in anoxia. The largest fractionation of Tl isotopes and most variable flux is the adsorption to Mn-oxides during formation and burial, thus the burial flux of Mn-oxides is the most significant sink for changes in the seawater Tl isotope composition. Additionally, the residence time of Tl, 18,500 years, is longer than oceanic mixing time but short compared to other redox proxies. Thus, seawater Tl isotopes are more likely to respond to initial redox perturbation while still representing a globally homogenous value, provided the sedimentary archive is well-connected to the open ocean.

Previous studies indicate that euxinic (anoxic and sulfidic water column) and permanently anoxic with sulfidic sediment (such as Santa Barbara Basin) settings faithfully record seawater Tl values. Pyrites deposited in the sulfidic sediments or water column of anoxic settings adsorb Tl with limited or no net fractionation from local seawater during formation and burial due to the near quantitative removal of Tl from seawater. However, limited data from reduced oxygen environments with unknown sedimentary sulfide content record more positive isotopic compositions than the global seawater value, interpreted as a mixture of Mn-oxide and seawater values. This work provides a detailed analysis of Tl isotopes from sediments in various locations within and near the California and Peru margin oxygen minimum zones (OMZs), with various seafloor oxygen content, to better constrain Tl isotope systematics. Permanently anoxic sediments from within these OMZs have isotopic values that are indistinguishable from the modern global seawater value, indicating that these anoxic but non-euxinic sediments also accurately record seawater values. Sites with more oxic sediments however show significantly more positive isotopic compositions. This work improves the Tl mass balance by better constraining the sinks’ isotopic values and expansion of the application of Tl isotopes to track seawater by using permanently reducing setting’s sediments, being cautious with periodically oxic samples.