Contributions from Earth’s Atmosphere to Soil Louis A. Derry1 and Oliver A. Chadwick2 oils are mixtures of material derived from substrate weathering, plant Marine and mineral aerosols con- tribute significantly to soils. The decomposition, and solute and particulate deposition from the atmosphere. chemical and physical state of SThe relative contribution from each source varies widely among soil types inorganic constituents in the atmos- and environments. Atmospheric deposition of marine and mineral aerosols phere is complex and dynamic. can have a major impact on the geochemistry and biogeochemistry of the Airborne salt and mineral particles act as condensation nuclei for water, Critical Zone. Some of the best-studied examples are from ocean islands and the solid particles can undergo because of the strong geochemical contrast between bedrock and atmospheric repeated cycles of hydration and sources, but for the most part continental areas are more severely impacted dehydration during transport. For example, marine aerosols are gen- by atmospheric deposition. With dust flux greater than 10% of the global erated by evaporation of sea-spray river sediment flux, deposition from the atmosphere plays an important role producing salts that may be found in the biogeochemistry of soils worldwide. in various states of hydration and/or dissolution. Mineral aerosols are KEYWORDS: mineral aerosol, marine aerosol, ecosystems, Critical Zone, dust derived from fine mineral particles entrained by wind, and their com- position may subsequently be mod- INTRODUCTION ified by reaction during atmospheric transport. While the Soil is an important contributor to global biogeochemical colloquial term “dust” is commonly applied, mineral aerosols cycles and acts as an open chemical and physical system include both discrete mineral grains and hydrated aerosols subject to element losses and gains (Brantley et al. 2007 this developed around partially or completely reacted particles. issue). The transformation of geological substrate into soil Hydration cycles induce repeated pH changes, which promote involves material input and output in widely varying pro- dissolution of primary minerals in a manner analogous to portions depending on the environment; these losses and terrestrial weathering processes (Spokes et al. 1994). As a gains can occur simultaneously. Mineral weathering pro- consequence of this dynamic nature, the term “aerosol” more duces solutes that may be exported, resulting in mass loss. accurately describes the chemical state and reactivity of Plants take up and recycle mineral components (e.g. K, Ca, atmospheric “salts” and “dust.” Volcanic and anthropogenic Si) that may be subsequently removed by water or wind. aerosols, especially sulfate aerosols, are important for both Wind erosion selectively removes fine particles from the their radiative properties and their role in acidification, but soil surface, resulting in local landscape deflation and addi- we do not consider these in detail here. tion elsewhere. Soil gains mass through several other processes. For example, as soil develops, it may gain signif- MARINE AEROSOL DEPOSITION icant amounts of atmospheric carbon (C) and nitrogen (N), Since most atmospheric water vapor is derived from the which are reduced by biological activity and incorporated ocean, marine aerosols are a major source of solutes in the as dead biomass. The C and N are present as soil organic atmosphere. Major ions in marine aerosols (Na+, K+, Mg++, ++ = - matter and are reused by organisms, and eventually leached Ca , SO4 , Cl ) are initially present in ratios similar to those into rivers and groundwater or released back into the in their parent seawater. Sulfate is an exception. It is often atmosphere. Mass gains resulting from atmospheric trans- much more abundant in marine aerosols than predicted port and deposition of solutes and minerals contribute sig- from a sea-water source. “Non–sea salt sulfate” in marine air nificantly to both the physical structure of soil and its is mostly derived from the oxidation of dimethyl sulfide nutrient status. A full accounting of chemical loss and gain (DMS), produced by phytoplankton. As air masses originat- to soil is daunting because of the many possible sources of ing over the oceans with an aerosol load derived from sea losses and gains. In this paper, we narrow the focus to addi- salt are transported across continental regions, reaction with tions of inorganic elements that are transported as mineral land-derived silicate and carbonate mineral aerosols can particles or dissolved salts derived from wind erosion of greatly modify the composition of precipitation (rain, snow, continental surfaces, or as salts derived from the oceans. and fog). Large-scale spatial patterns in the composition of precipitation demonstrate the importance of mineral dissolu- tion during atmospheric transport. As expected, precipitation 1 Cornell University, Department of Earth & Atmospheric Sciences composition in coastal regions is typically closest to sea salt Ithaca, NY 14853-1506, USA composition, but there are systematic and important differ- E-mail: [email protected] ences. A compilation of element data from 12 United States 2 University of California, Department of Geography NADP (National Atmospheric Deposition Program, http:// Santa Barbara, CA 93106-4060, USA nadp.sws.uiuc.edu) coastal sites, which we would expect to E-mail: [email protected] E L E M E N T S , V O L . 3 , P P . 3 3 3 – 3 3 8 333 OCTOBER 2007 be dominated by marine precipitation, demonstrates that Strontium (Sr) isotope data have been increasingly used to even in coastal regions rainwater composition deviates sig- quantify atmosphere- and substrate-derived alkaline earth nificantly from that of nominal sea salt. Relative to chlo- cations in soils and plants. Sr behaves in a similar, though ride, Ca is strongly enriched, Mg can be slightly depleted, not identical, way to Ca, and there is often sufficient differ- and K is enriched; only Na does not typically differ signifi- ence between atmospheric and weathering sources to make cantly from its proportion in sea salt (FIG. 1). While in gen- the 87Sr/86Sr ratio a useful tracer. In the case of polyminer- eral data from Atlantic coastal stations show the greatest alic rocks, it can be difficult to constrain the 87Sr/86Sr ratio deviations from sea salt composition, presumably because of the weathering “end member,” because different miner- air that has traversed the continental land mass is more fre- als can evolve to different 87Sr/86Sr over time and they may quently sampled, even isolated island stations such as in weather at different rates (Blum and Erel 1997). In relatively Samoa and the Virgin Islands show the same pattern, young volcanic rocks, substrate heterogeneity is much less although rainwater at the island sites is closer to sea salt in a complication and the contrast between atmospheric and composition. An obvious conclusion is that knowledge of substrate sources is usually large. These factors have made the local precipitation composition is essential for making Hawaiian Islands an excellent site to investigate the incor- meaningful estimates of the composition of atmospheric poration of atmospheric input into soils and vegetation. wet deposition (rain, snow or fog) at a given locale. The Sr isotope composition of Hawaiian basalts is mostly It may come as a surprise, but the solute flux from precipi- uniform and near 0.703, while 87Sr/86Sr for marine aerosols tation can add substantial mass to soils. In fact, there are is near 0.709, providing distinct end members. The sub- many situations where mass addition is significant, particu- strates of a chronosequence of soils, developed on shield larly when primary minerals have been depleted by weath- topography under uniform, present-day, mean annual pre- ering. In the absence of erosion, mineral dissolution and cipitation (MAP) of 250 cm yr-1, range from young, little- element leaching depletes mobile constituents derived from weathered basalt (0.3 ka) to highly weathered surfaces rock substrate (Brantley et al. 2007), and their replenishment 4100 ka old. Weathering releases cations from the substrate, by atmospheric deposition leads to dominance of exter- and by 20 ka leaching has depleted the initial rock inven- nally sourced ions in near-surface soil horizons. An exam- tory of Ca, Mg, K, and Na such that the 87Sr/86Sr ratio for ple from Kilauea volcano, Hawai‘i, is illustrated in FIGURE 2. both plant-available cations and those held in the bulk On Kilauea, fog accounts for 88% of the Ca and 68% of K mineral soil closely approaches that of marine aerosol derived from the combination of rain and fog water input (Kennedy et al. 1998; Kurtz et al. 2001). Thus in spite of to the ecosystems growing on the volcano (Carillo et al. high initial inventories in the basalt, the isotopic data con- 2002). Over a timescale of 103–104 years, wet deposition firm the prediction arising from FIGURE 2, that over a 104- (fog plus rain) of Ca and K exceeds the total inventory of year timescale in this humid environment, atmospheric those elements in the top meter of basaltic substrate. Mg deposition becomes the dominant source of alkaline earth and Na behave similarly. Atmospheric fluxes dominate even plant nutrients. Even when soil minerals contain large more when weathering loss from the basalt is taken into amounts of rock-derived plant nutrients, the alkali and account. Thus, on the geologically short timescale of 104 years, alkaline earth ions sorbed to mineral surfaces, and hence a soil can have acquired more atmosphere-derived alkaline readily available to plants, may be derived from atmospheric earth and alkali cations (Ca2+, Mg2+, K+, and Na+) than even sources rather than the substrate minerals. For instance, the complete mineral weathering can provide, and on longer 87Sr/86Sr from plant tissues growing on young (<150 yr) timescales, the flux from atmospheric deposition greatly substrates on Mauna Loa, Hawai‘i, demonstrates that up to exceeds the weathering flux.