Are the phosphate oxygen isotopes of Saharan dust a robust tracer of atmospheric P source?

Abstract

Abstract Aeolian dust is a significant source of phosphorus (P) in various ecosystems. The Saharan desert is the major dust source on earth and it contributes atmospheric P to terrestrial ecosystems which often depend on these inputs. Since estimations on the contribution of dust to the atmospheric P cycle vary between studies, accurate estimations on the aerosol-P inputs to ecosystems can be enhanced by an identification method for dust from the other atmospheric P sources. In the current study, we explored the use of the oxygen stable isotopes in phosphate, δ18OP, as a marker for desert dust P. To characterize the δ18OP in Saharan dust, we analyzed atmospheric particles sampled in Puerto Rico, which has good exposure in summer to African dust with little exposure to wildfires, local, or anthropogenic influence on atmospheric inputs. Additionally, to evaluate possible isotopic fractionations in desert dust sources, which could affect the soil δ18OP before transport, we analyzed the δ18OP in Israeli desert soils that served as an analog. Saharan dust storms sampled at Puerto Rico were found to have δ18OP values of 22.5 ± 0.3‰ in the resin fraction (which represents labile inorganic P), similar to previously reported values of Saharan dust sampled in Israel. The data from Israel desert soils demonstrated that biological activity in desert crusts can have a small effect (with a maximum difference of 3.3‰ between the topsoil and the subsoil) on the δ18OP of desert soils, mainly when developed soil crust is present, as a result of intracellular isotopic equilibration between oxygen in phosphate and soil water. However, biological crusts are not considered a major dust source. Additionally, Saharan dust sampled in Israel and in Puerto Rico had a δ18OP similar to apatite minerals derived from sedimentary phosphorites. Thus, we suggest the dust source had little or no biological activity. Furthermore, the δ18OP in Saharan dust was distinctive from other atmospheric P sources, such as pollen and wildfire ash, and as a result, the δ18OP can be used to distinguish between these atmospheric P main sources.