Estimation of Trace Element Atmospheric Deposition Fluxes to the Atlantic Ocean (> 40°N) During Spring 2014 (GEOVIDE, GEOTRACES GA01)
Rachel Shelley1, Montserrat Roca-Marti2, Maxi Castrillejo3, Pere Masque2,4 and Geraldine Sarthou5, (1)LEMAR/UBO, Brest, France, (2)Universitat Autònoma de Barcelona, Física and Institut de Ciència i Tecnologia Ambientals, Barcelona, Spain, (3)Universitat Autònoma de Barcelona, Department of Physics and Institut de Ciència i Tecnologia Ambientals, Barcelona, Spain, (4)Edith Cowan University, School of Natural Sciences, Joondalup, Australia, (5)LEMAR UMR 6539 CNRS UBO IRD IFREMER, IUEM, Plouzané, France
Abstract:
Atmospheric deposition of aerosols is an important source of trace elements (TE) to the ocean, including both essential elements such as Fe and Zn, as well as others with no known biological role and mainly anthropogenic origins,
e.g. Pb. In particular, the atmospheric flux of Fe is a key component of biogeochemical models. At present, atmospheric TE flux estimates are poorly constrained. The largest sources of uncertainty result from the deposition velocity term (dry deposition) and from the precipitation rate (wet deposition). In this study, we use the radioisotope beryllium-7 (
7Be, T
1/2 = 53 days) in a novel way to estimate TE fluxes, and compare the results with those obtained using the traditional approach. Atmospheric fluxes were derived using data from the French-GEOTRACES cruise (GA01; GEOVIDE) that sailed from Portugal to Greenland and into the Labrador Sea (May/June 2014).
Our study region is one of low atmospheric deposition for the most part, north of the extent of the Saharan dust plume. Indeed, our data suggest very low atmospheric deposition throughout May/June 2014. West of 30°W, our flux estimates show good agreement for Al, P, Ti, Mn, Fe, Sr, Y, Zr, Cd and Pb (examples of flux estimates for Al, Fe and Cd from the traditional and 7Be techniques, respectively, are: 9.1±22 µg m-2 d-1 and 15±5.8 µg m-2 d-1; 8.0±19 µg m-2 d-1 and 7.0±4.0 µg m-2 d-1; 0.0053±0.0055 µg m-2 d-1 and 0.0039 ±0.0028 µg m-2 d-1), but are up to an order of magnitude lower for other elements, such as Co, Ni, Cu, Zn and Mo, using the 7Be technique. East of 30°W, the 7Be-derived fluxes were x2 (Al) to x30 (Zn) lower. The various possible reasons that may explain this offset will be discussed. These include: i) the deposition velocity of aerosols used was too high; ii) the precipitation rates were unrepresentative; iii) the assumption of non-negligible scavenging of 7Be in the water column is not correct; iv) the two techniques provide flux estimates on different timescales.
