Thallium isotope a new tool for tracking the global marine ferromanganese burial

Friday, 19 December 2014
Jeremy D Owens and Sune Nielsen, Woods Hole Oceanographic Institute, Woods Hole, MA, United States
Thallium (Tl) isotopes are an emerging proxy with the potential to track surficial redox processes. To date Tl isotopes have not been extensively explored for the entire range of redox condition in the modern ocean. Thallium has a relatively short residence time, ~20 thousand years, however new and existing seawater data shows that the modern, dominantly oxic, ocean remains conservative with respect to concentration and isotopes. Past studies have shown that there are only two geologic processes that impart a large isotopic fractionation (1) adsorption onto manganese oxides and (2) low temperature oceanic crust alteration. However, for relatively short term redox events it is reasonable to assume that the dominate isotope fractionation process is associated with manganese oxide precipitation. We present new water column and sediment, authigenic and detrital, Tl isotope data from the Black Sea and Cariaco Basins – the largest permanently anoxic basins in the modern ocean. Preliminary authigenic sediment data from these anoxic basins suggest these settings effectively capture the seawater value. Combining new and existing oxic open ocean data along with calculated fluxes constrain the mass balance of Tl, which shows the ability to fingerprint the global extent of manganese oxide burial and thus infer the extent of oxic seafloor burial.

Combining thallium isotopes with other reliable paleoredox proxies such as molybdenum (Mo) isotopes and can help to elucidate global redox events. We have employed a quantitative model to explore the possible utility of multiple isotope systems such as the links between Mo and Tl isotopes. As a case study, we have modeled Tl and Mo isotopes during the Toarcian oceanic anoxic event where thallium isotopes in early diagenetic pyrite and have shown the potential to record excursions in the local and/or possibly global burial of manganese oxides. This modeling exercise has shown that Tl isotopes lend insight into the global marine oxic burial flux of metals during oceanic anoxic events and helps to explain variations in other elemental systems.