Understanding the nature of atmospheric acid processing of mineral dusts in supplying bioavailable phosphorus to the oceans

Abstract

Acidification of airborne dust particles can dramatically increase the amount of bioavailable phosphorus (P) deposited on the surface ocean. Experiments were conducted to simulate atmospheric processes and determine the dissolution behavior of P compounds in dust and dust precursor soils. Acid dissolution occurs rapidly (seconds to minutes) and is controlled by the amount of H + ions present. For H + < 10 −4 mol/g of dust, 1-10% of the total P is dissolved, largely as a result of dissolution of surface-bound forms. At H + > 10 −4 mol/g of dust, the amount of P (and calcium) released has a direct proportionality to the amount of H + consumed until all inorganic P minerals are exhausted and the final pH remains acidic. Once dissolved, P will stay in solution due to slow precipitation kinetics. Dissolution of apatite-P (Ap-P), the major mineral phase in dust (79-96%), occurs whether calcium carbonate (calcite) is present or not, although the increase in dissolved P is greater if calcite is absent or if the particles are externally mixed. The system was modeled adequately as a simple mixture of Ap-P and calcite. P dissolves readily by acid processes in the atmosphere in contrast to iron, which dissolves more slowly and is subject to reprecipitation at cloud water pH. We show that acidification can increase bioavailable P deposition over large areas of the globe, and may explain much of the previously observed patterns of variability in leachable P in oceanic areas where primary productivity is limited by this nutrient (e.g., Mediterranean).atmospheric processing | ocean macronutrients | desert dusts

The Effect of Atmospheric Acid Processing on the Global Deposition of Bioavailable Phosphorus From Dust

“…As observed by Stockdale et al (2016) mineral dust contains Ap-P and Lab-P that both contribute toward the total fraction of inorganic mineral-borne phosphorus that is potentially bioavailable in marine surface waters. Although measurements reported by Stockdale et al (2016) find Lab-P present in much smaller quantities than Ap-P, the total mass of Lab-P is directly bioavailable in the surface waters, whereas the Ap-P requires the presence of acid to yield a bioavailable form of P, hereafter referred to as Acid-P. The differences in immediate bioavailability of the two components result in distinct regional differences that can be seen in Figures 3a and 3b.…”

“…Observations suggest that the percentage of TP in mineral dust that is deposited in a bioavailable form (Bio-P) is spatially variable, ranging from <10 to >80% (Baker, French, et al, 2006;Markaki et al, 2003;Vet et al, 2014;Zamora et al, 2013), and may increase with distance from the dust source (Baker, French, et al, 2006). Recent ambient observations and laboratory experiments (Nenes et al, 2011;Srinivas & Sarin, 2015;Stockdale et al, 2016) provide support for the hypothesis that atmospheric acidification and subsequent dissolution are a primary process for producing Bio-P in mineral dust. Increasing the amount of leachable, or bioavailable, P by acid processes in the atmosphere will have a direct effect of increasing phytoplankton biomass and hence carbon uptake in a variety of ocean systems (e.g., Jickells & Moore, 2015;Mahowald et al, 2008).…”

“…As the dissolution of Ap-P occurs at a faster rate when CaCO 3 is not present, if Ap-P and CaCO 3 were exclusively present on different particles (i.e., externally mixed), then it is possible that more Acid-P would be produced. As discussed by Stockdale et al (2016) it is hypothesized that many particles would contain both Ap-P and CaCO 3 ; however, there is likely a degree of variation between particles with some containing more or less of each mineral component.…”

Understanding the unique biogeochemistry of the Mediterranean Sea: Insights from a coupled phosphorus and nitrogen model

“…In a similar fashion, the inputs of the various P and N species from the EMS are computed by multiplying the EMIW concentrations by the inflow via the Strait of Sicily. [McGill, 1961;Banoub and Williams, 1972;Millero et al, 1978;Copin-Montegut and Copin-Montegut, 1983;Coste et al, 1984;Béthoux et al, 1992;Pujo-Pay et al, 1995;Béthoux et al, 1998;Doval et al, 1999;Moutin and Raimbault, 2002;Lucea et al, 2003;Aminot and Kérouel, 2004;Schroeder et al, 2010;Pujo-Pay et al, 2011;Tanhua et al, 2013] Input Fluxes Atlantic surface water P and N concentrations [Coste et al, 1984;La Corre et al, 1984;Coste et al, 1988;Gómez et al, 2000;Doval et al, 2001;Dafner et al, 2003;Macias et al, 2007;Huertas et al, 2012;Schroeder et al, 2012;Ramirez-Romero et al, 2014] Atmospheric deposition [Migon et al, 1989;Loÿe-Pilot et al, 1990;Bergametti et al, 1992;LeBolloch and Guerzoni, 1995;Guerzoni et al, 1999;Migon and Sandroni, 1999;Migon et al, 2001;Sandroni et al, 2007;Markaki et al, 2010;Nenes et al, 2011;Powley et al, 2014;Van Cappellen et al, 2014;Stockdale et al, 2016] Submarine groundwater discharge [Dowling et al, 2003;…”

Atmospheric Deposition Effects on Plankton Communities in the Eastern Mediterranean: A Mesocosm Experimental Approach

“…Mineral dust can also provide an important source of P, both as labile phosphate and also as mineral apatite originating mainly from deserts (Mahowald et al, 2008). Under natural conditions, insoluble mineral apatite may drop through the surface layers without interacting with the biota but after acidification processes in the atmosphere, mainly interaction with pollutants such as NOx and SOx, it is converted into bioavailable phosphate (Nenes et al, 2011;Stockdale et al, 2016). Atmospheric input also represents a large and increasing source of anthropogenic inorganic N (mainly as NO x from industrial sources and cars and NH 3 from agricultural sources as well as natural inputs from lightning and other sources).…”