Combining seawater Th-232 and Th-230 concentrations to determine dust fluxes to the surface ocean

Aeolian dust is a significant source of iron and other nutrients to the surface ocean, but constraining the size of this flux is difficult. Chemical tracers, such as Al, are useful, but are limited by lack of information about their rate of removal from surface seawater. In this study, we test the use of Th isotopes as an improved tracer of dust input by measuring 232Th and 230Th concentrations of upper-ocean seawater at six stations along a meridional Atlantic section that spans the Saharan dust plume. Open ocean 232Th, like Al, is dominantly derived from dust dissolution, while 230Th is produced in seawater by U decay and provides an assessment of Th removal rates. The highest 232Th concentrations and 232Th/230Th ratios, particularly in the surface waters, are observed at stations expected to be influenced by input of Saharan dust. The pattern of 232Th concentration is in agreement with dissolved Al distributions and SeaWiFS aerosol imagery, supporting the use of 232Th as a proxy for short-term dust addition. Deeper in the water column, 232Th concentration may also reflect longer-term dust inputs. In addition, lateral input of 232Th from Mediterranean Outflow Water is detected at 1000m in the north Atlantic. Surface-water residence times are calculated separately from 230Th and 234Th using a simple scavenging model and values from the two isotopes are generally consistent with one another. Th-230 residence times appear to reflect the longer-term value, however, rather than exhibiting the strong seasonal influence seen for 234Th. These Th residence times allow dust flux to the surface ocean to be calculated using the known concentration of Th in dust, and assumed values for the solubility of dust Th in seawater. Resulting dust fluxes show good agreements with models of dust deposition if Th solubility is 1 to 5% in the north Atlantic (yielding fluxes of 0.4-19.2gm-2yr-1) and 5 to 10% in the south Atlantic (fluxes of 0.2-0.7gm-2yr-1). Despite the need for better constraints on the fractional solubility of dust Th, this study demonstrates the power of combining Th isotope measurements to constrain dust fluxes to the surface ocean. © 2011 Elsevier B.V