The mass balance of dissolved thallium in the oceans

Abstract

This study provides a comprehensive and quantitative discussion of the inventory and mass balance of dissolved thallium (Tl) in the oceans. The calculations are based on a critical evaluation of recently published analytical data. Thallium is estimated to have a mean seawater concentration of 65 pmol/kg and this implies a total oceanic inventory of about 8.76×10 4 Mmol Tl. The five primary sources of dissolved Tl in the oceans are rivers, hydrothermal fluids, eolian contributions from volcanic emanations and mineral aerosols, and benthic fluxes from continental margin sediments. Combined, these sources generate a Tl flux of between 1.4 and 12 Mmol/year, with a best estimate of 5.2 Mmol/year. Based on this, an oceanic residence time of 7–63 ky is calculated for Tl, with a best estimate of 17 ky. The two main oceanic sinks of Tl are scavenging from seawater by the authigenic phases of pelagic clays and uptake of Tl during low-temperature alteration of oceanic crust. These sinks remove about 1.6–7.7 Mmol/year of Tl, with a best estimate of 4.2 Mmol/year. This indicates an oceanic residence time of 11–56 ky, with a best estimate of 21 ky. The mass balance yields input and output fluxes which are identical, within the uncertainties, and this is in accord with a budget that is in steady state.