Using Be-7 to quantify the flux of trace elements from the atmosphere to the oceans

Following production by cosmic-ray spallation, tropospheric Be-7 (half-life 53.3d) quickly attaches to aerosols and is deposited to the oceans by wet and dry deposition. Due to its radioactive decay, the inventory of Be-7 in the upper ocean is balanced by its flux from the atmosphere. The ability to derive the atmospheric flux of Be-7 from its ocean inventory provides a key linkage between the atmospheric concentrations of chemical species and their deposition to the ocean. This includes many trace elements and isotopes of interest to the GEOTRACES program such as Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, and others.

The atmospheric flux of trace elements and isotopes (TEIs) to the oceans can be extremely important in marine biogeochemical cycles, especially for bioactive trace elements including V, Mn, Fe, Co, Ni, Cu, and Zn. However, there are large uncertainties associated with converting rainfall and aerosol TEI concentrations into estimates of atmospheric deposition. By measuring ratios of TEIs to Be-7 in rainfall and aerosol samples, the atmospheric flux of TEIs can be calculated: Flux(TEI) = Flux(Be-7) * (TEI/Be-7), where the Flux(Be-7) is calculated from the uper ocean inventory of Be-7. Where the fractional solubility of aerosol TEIs in rainwater or surface seawater has been measured, the same ratio approach can be used to calculate the atmospheric flux of soluble TEIs. Soluble TEIs are expected to be intrinsically bioavailable to phytoplankton, and therefore can play an important role in biogeochemical cycling in the upper ocean.

We will present results from field campaigns in the Sargasso Sea (Bermuda) and the Arctic Ocean to test this approach.