P Bioavailability in Dust

Primary productivity of continental and marine ecosystems is often limited or co-limited by phosphorus. Because riverine inputs of P preferentially accumulate along the continental margins, airborne sources of P are particularly important for supporting primary production in oligotrophic areas of the open ocean. The major forms of P in soil-derived dust is apatite group and, to a lesser extent, P bound to iron (hydr)oxide minerals. These minerals are highly insoluble in oceanic surface waters. Combined with the short transit times of mineral aerosols through the photic zone, this implies that the main source of bioavailable P in atmospheric deposition is water soluble P produced during airborne processing of soil-derived dust. Mahowald and co-workers estimate that globally about 17% of total atmospheric P deposited at the sea surface is water soluble. The soluble fraction, however, is highly variable, with values ranging between 7 and 100%.

We hypothesize, based on preliminary data, that acid processing of mineral aerosols in the atmosphere is a major pathway for the production of water soluble P. The main acids in the atmosphere, H2SO4 and HNO3, are generated by oxidation of sulfur and nitrogen gases emitted by biogenic, volcanic and anthropogenic sources. The acids condense onto mineral aerosols causing in many cases pH to drop to low values and increasing the solubilities of apatite and iron (hydr)oxide minerals by several orders of magnitude. The resulting production of soluble phosphate is analogous to that invoked to explain the presence of soluble iron and trace metals in atmospheric dust.  Our hypothesis is supported by the soluble ion compositions and reconstructed pH values for dry deposition samples collected over a 5-year period at Finokalia, Crete which shows a clear increase in LIP/Ca ratio in samples which had been in contact with more acid in the atmosphere (Figure 1). In addition we also observed a tenfold increase in soluble phosphorus when Saharan soil and dust (SD) were acidified in laboratory atmospheric simulation experiments with most of the apatite and iron bound-P being converted into LIP in a process analogous to tooth decay after eating too many sweets.

It is therefore hypothesized that acidic conditions (anthropogenic and/or natural) in the atmosphere can increase the amount of bioavailable P which is supplied to the surface ocean thus stimulating carbon uptake and/or nitrogen fixation depending on the location.

An area where we expect this process to be particularly important is the eastern Mediterranean (EM).  It has been shown that primary productivity is P limited in the EM and any addition of P will have an immediate and proportional affect on C uptake. It is also an area where SD from NAfrica meets polluted air from S.Europe , the tailpipe of Europe. We therefore propose initially to concentrate our studies on EMS in general using data and samples provided by supporting colleagues in Crete.