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Elements; October 2007; v. 3; no. 5; p. 333-338; DOI: 10.2113/gselements.3.5.333
© 2007 Mineralogical Society of America
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Contributions from Earth's Atmosphere to Soil

Louis A. Derry1 and Oliver A. Chadwick2

1 Cornell University, Department of Earth & Atmospheric Sciences
Ithaca, NY 14853-1506, USA
E-mail: lad9{at}cornell.edu
2 University of California, Department of Geography
Santa Barbara, CA 93106-4060, USA
E-mail: oac{at}geog.ucsb.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MARINE AEROSOL DEPOSITION
 MINERAL AEROSOL DEPOSITION
 CONCLUSIONS
 REFERENCES
 
Soils are mixtures of material derived from substrate weathering, plant decomposition, and solute and particulate deposition from the atmosphere. The relative contribution from each source varies widely among soil types and environments. Atmospheric deposition of marine and mineral aerosols can have a major impact on the geochemistry and biogeochemistry of the Critical Zone. Some of the best-studied examples are from ocean islands because of the strong geochemical contrast between bedrock and atmospheric sources, but for the most part continental areas are more severely impacted by atmospheric deposition. With dust flux greater than 10% of the global river sediment flux, deposition from the atmosphere plays an important role in the biogeochemistry of soils worldwide.

KEYWORDS: mineral aerosol, marine aerosol, ecosystems, Critical Zone, dust


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MARINE AEROSOL DEPOSITION
 MINERAL AEROSOL DEPOSITION
 CONCLUSIONS
 REFERENCES
 
Soil is an important contributor to global biogeochemical cycles and acts as an open chemical and physical system subject to element losses and gains (Brantley et al. 2007 this issue). The transformation of geological substrate into soil involves material input and output in widely varying proportions depending on the environment; these losses and gains can occur simultaneously. Mineral weathering produces solutes that may be exported, resulting in mass loss. Plants take up and recycle mineral components (e.g. K, Ca, Si) that may be subsequently removed by water or wind. Wind erosion selectively removes fine particles from the soil surface, resulting in local landscape deflation and addition elsewhere. Soil gains mass through several other processes. For example, as soil develops, it may gain significant amounts of atmospheric carbon (C) and nitrogen (N), which are reduced by biological activity and incorporated as dead biomass. The C and N are present as soil organic matter and are reused by organisms, and eventually leached into rivers and groundwater or released back into the atmosphere. Mass gains resulting from atmospheric transport and deposition of solutes and minerals contribute significantly to both the physical structure of soil and its nutrient status. A full accounting of chemical loss and gain to soil is daunting because of the many possible sources of losses and gains. In this paper, we narrow the focus to additions of inorganic elements that are transported as mineral particles or dissolved salts derived from wind erosion of continental surfaces, or as salts derived from the oceans.


Figure 1

Marine and mineral aerosols contribute significantly to soils. The chemical and physical state of inorganic constituents in the atmosphere is complex and dynamic. Airborne salt and mineral particles act as condensation nuclei for water, and the solid particles can undergo repeated cycles of hydration and dehydration during transport. For example, marine aerosols are generated by evaporation of sea-spray producing salts that may be found in various states of hydration and/or dissolution. Mineral aerosols are derived from fine mineral particles entrained by wind, and their composition may subsequently be modified by reaction during atmospheric transport. While the colloquial term "dust" is commonly applied, mineral aerosols include both discrete mineral grains and hydrated aerosols developed around partially or completely reacted particles. Hydration cycles induce repeated pH changes, which promote dissolution of primary minerals in a manner analogous to terrestrial weathering processes (Spokes et al. 1994). As a consequence of this dynamic nature, the term "aerosol" more accurately describes the chemical state and reactivity of atmospheric "salts" and "dust." Volcanic and anthropogenic aerosols, especially sulfate aerosols, are important for both their radiative properties and their role in acidification, but we do not consider these in detail here.


   MARINE AEROSOL DEPOSITION
TOP
 ABSTRACT
 INTRODUCTION
 MARINE AEROSOL DEPOSITION
MINERAL AEROSOL DEPOSITION
 CONCLUSIONS
 REFERENCES
 
Since most atmospheric water vapor is derived from the ocean, marine aerosols are a major source of solutes in the atmosphere. Major ions in marine aerosols (Na+, K+, Mg++, Ca++, SO4 =, Cl-) are initially present in ratios similar to those in their parent seawater. Sulfate is an exception. It is often much more abundant in marine aerosols than predicted from a sea-water source. "Non-sea salt sulfate" in marine air is mostly derived from the oxidation of dimethyl sulfide (DMS), produced by phytoplankton. As air masses originating over the oceans with an aerosol load derived from sea salt are transported across continental regions, reaction with land-derived silicate and carbonate mineral aerosols can greatly modify the composition of precipitation (rain, snow, and fog). Large-scale spatial patterns in the composition of precipitation demonstrate the importance of mineral dissolution during atmospheric transport. As expected, precipitation composition in coastal regions is typically closest to sea salt composition, but there are systematic and important differences. A compilation of element data from 12 United States NADP (National Atmospheric Deposition Program, http://nadp.sws.uiuc.edu) coastal sites, which we would expect to be dominated by marine precipitation, demonstrates that even in coastal regions rainwater composition deviates significantly from that of nominal sea salt. Relative to chloride, Ca is strongly enriched, Mg can be slightly depleted, and K is enriched; only Na does not typically differ significantly from its proportion in sea salt (FIG. 1). While in general data from Atlantic coastal stations show the greatest deviations from sea salt composition, presumably because air that has traversed the continental land mass is more frequently sampled, even isolated island stations such as in Samoa and the Virgin Islands show the same pattern, although rainwater at the island sites is closer to sea salt in composition. An obvious conclusion is that knowledge of local precipitation composition is essential for making meaningful estimates of the composition of atmospheric wet deposition (rain, snow or fog) at a given locale.


Figure 2
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  		FIGURE 1 Cation concentration ratios relative to chloride in precipitation normalized to seawater. The data are from 12 U.S. coastal or island sites and in most cases are five-year means of annual precipitation-weighted chemistry. At all sites Ca is considerably enriched over what is predicted from a sea salt model, while Mg is often slightly depleted. K is typically enriched, while Na is indistinguishable from sea salt composition. The deviations from sea salt composition are mostly the result of dissolution of mineral aerosols, of which calcium carbonate is the most important source.

 

It may come as a surprise, but the solute flux from precipitation can add substantial mass to soils. In fact, there are many situations where mass addition is significant, particularly when primary minerals have been depleted by weathering. In the absence of erosion, mineral dissolution and element leaching depletes mobile constituents derived from rock substrate (Brantley et al. 2007), and their replenishment by atmospheric deposition leads to dominance of externally sourced ions in near-surface soil horizons. An example from Kilauea volcano, Hawai'i, is illustrated in FIGURE 2. On Kilauea, fog accounts for 88% of the Ca and 68% of K derived from the combination of rain and fog water input to the ecosystems growing on the volcano (Carillo et al. 2002). Over a timescale of 103-104 years, wet deposition (fog plus rain) of Ca and K exceeds the total inventory of those elements in the top meter of basaltic substrate. Mg and Na behave similarly. Atmospheric fluxes dominate even more when weathering loss from the basalt is taken into account. Thus, on the geologically short timescale of 104 years, a soil can have acquired more atmosphere-derived alkaline earth and alkali cations (Ca2+, Mg2+, K+, and Na+) than even complete mineral weathering can provide, and on longer timescales, the flux from atmospheric deposition greatly exceeds the weathering flux.


Figure 3
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  		FIGURE 2 Integrated wet deposition flux (ions delivered in the fog plus rain) (solid lines) for Ca and K calculated for the Kilauea volcano area compared to substrate inventory calculated over the top meter. Dashed lines show substrate inventory assuming no weathering losses. Dotted lines show substrate inventory assuming quasi-first-order losses with a time constant of 7000 years for the first 104 years and declining losses after that time. Wet deposition of K exceeds the basalt inventory in ca. 2300-3300 years, depending on the rate of weathering loss; for Ca the timescale is 12,000-70,000 years. DEPOSITION DATA FROM CARILLO ET AL. (2002)

 
Strontium (Sr) isotope data have been increasingly used to quantify atmosphere- and substrate-derived alkaline earth cations in soils and plants. Sr behaves in a similar, though not identical, way to Ca, and there is often sufficient difference between atmospheric and weathering sources to make the 87Sr/86Sr ratio a useful tracer. In the case of polymineralic rocks, it can be difficult to constrain the 87Sr/86Sr ratio of the weathering "end member," because different minerals can evolve to different 87Sr/86Sr over time and they may weather at different rates (Blum and Erel 1997). In relatively young volcanic rocks, substrate heterogeneity is much less a complication and the contrast between atmospheric and substrate sources is usually large. These factors have made the Hawaiian Islands an excellent site to investigate the incorporation of atmospheric input into soils and vegetation.

The Sr isotope composition of Hawaiian basalts is mostly uniform and near 0.703, while 87Sr/86Sr for marine aerosols is near 0.709, providing distinct end members. The substrates of a chronosequence of soils, developed on shield topography under uniform, present-day, mean annual precipitation (MAP) of 250 cm yr-1, range from young, little-weathered basalt (0.3 ka) to highly weathered surfaces 4100 ka old. Weathering releases cations from the substrate, and by 20 ka leaching has depleted the initial rock inventory of Ca, Mg, K, and Na such that the 87Sr/86Sr ratio for both plant-available cations and those held in the bulk mineral soil closely approaches that of marine aerosol (Kennedy et al. 1998; Kurtz et al. 2001). Thus in spite of high initial inventories in the basalt, the isotopic data confirm the prediction arising from FIGURE 2, that over a 104-year timescale in this humid environment, atmospheric deposition becomes the dominant source of alkaline earth plant nutrients. Even when soil minerals contain large amounts of rock-derived plant nutrients, the alkali and alkaline earth ions sorbed to mineral surfaces, and hence readily available to plants, may be derived from atmospheric sources rather than the substrate minerals. For instance, the 87Sr/86Sr from plant tissues growing on young (<150 yr) substrates on Mauna Loa, Hawai'i, demonstrates that up to 30% of leaf Sr was derived from atmospheric sources, despite conditions that should lead to relatively high weathering rates (warm, wet climate and fresh basalt flows) (Vitousek et al. 1999). Over longer timescales, sampling of plants in rainforests on the Hawaiian Islands demonstrates that after about 150 ky, nearly all plant Sr is derived from marine aerosols (87Sr/86Sr values near 0.709). Derivation from weathering is inhibited because a depleted soil zone forms on shield volcano surfaces (Chadwick et al. 1999). By contrast, erosion of old shield surfaces exhumes unweathered rock and drives plant Sr back toward lava values (FIGURE 3; Porder et al. 2005).


Figure 4
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  		FIGURE 3 Effect of surface erosion on foliar 87Sr/86Sr and phosphorous levels, Hawai'i. Samples of native O'hia (Metrosideros polymorpha) leaves from locations where erosion has cut deeply into the original constructional volcanic shield surface have more "basaltic" Sr and higher P levels than leaves in trees growing on undisturbed, highly weathered shield surfaces. Erosion makes more basalt-derived Sr and P available. Note reversal of Sr axis. The value of 87Sr/86Sr in basalt is near 0.703, while atmospheric sources are mostly sea salt (0.709) with some dust (near 0.720). FIGURE MODIFIED FROM THE ORIGINAL IN PORDER ET AL. (2005)

 
The balance of substrate-derived versus atmospherically derived alkaline earth elements is strongly climate dependent as demonstrated by a study along a rainfall gradient on a 170 ka old Hawaiian substrate on Kohala Mountain (Stewart et al. 2001). As rainfall increases, the supply of basalt-derived Sr first increases as weathering rates increase then decreases because the primary mineral reservoir has been exhausted. The flux of atmospherically derived Sr increases linearly with precipitation, and at a point near 200 cm MAP the atmospheric flux of Sr exceeds weathering flux. Soils developed under lower MAP retain essentially basaltic Sr values, while soil developed at higher MAP approach the atmospheric input value. The data from Hawaiian chronosequence and climosequence studies illustrate the evolution of the weathering-atmospheric deposition system and demonstrate that under the right conditions, a timescale of <104 years is sufficient to develop a soil in which nearly all the Sr is derived from atmospheric sources, producing a soil with isotopic signatures markedly different from those of the original substrate. Thus, weathering-rate calculations based on cation budgets alone can be substantially in error if atmospheric deposition is not taken into account, sometimes a difficult task.

Marine aerosols can also be important contributors to ecosystems in continental interiors, especially when soils have long residence times and weatherable minerals are depleted. For example, Quade et al. (1995) demonstrated that marine Sr provides the dominant isotopic signature to soils hundreds of kilometers into the interior of Australia. It is also true that in continental interiors, terrestrial mineral aerosols may be dissolved and the soluble components may subsequently be deposited. In the deserts of the southwestern U.S., 87Sr/86Sr ratios have been used to demonstrate the dominance of rainwater-delivered Sr and Ca in the formation of calcretes (Capo and Chadwick 1999; Naiman et al. 2000). For the most part, the dissolved Sr was derived from limestone that crops out throughout the region. Thus, over continents, rain is a complex mixture of marine- and land-derived contributions, where the latter arises from dissolution of mineral aerosols during atmospheric transport.


   MINERAL AEROSOL DEPOSITION
TOP
 ABSTRACT
 INTRODUCTION
 MARINE AEROSOL DEPOSITION
 MINERAL AEROSOL DEPOSITION
CONCLUSIONS
 REFERENCES
 
Mineral aerosols are derived from wind erosion of soil and sediment. A major source is outwash areas near glaciers. There, freshly ground rock is carried by rivers onto broad, sparsely vegetated floodplains where it is easily entrained by wind. Over thousands of years, thick deposits of loess entirely derived from atmospherically transported particles can build up (Pye 1996). At the edges of these deposits, small amounts of eolian material can be incorporated into existing soils. If their compositions are similar, it may be hard to quantify the degree to which mineral aerosols have contributed (Ruhe and Olson 1980). Deserts are the other major source of mineral aerosols, particularly vegetation-free ephemeral channels and dry lake beds (FIG. 4). The total global dust flux is estimated to be near 1800 Tg y-1 (Mahowald et al. 2005). To put this figure into perspective, dust provides 14% of the annual global sediment flux to the oceans. Holocene desertification has increased dust flux from regions such as North Africa, while loess production has declined markedly since the last glacial maximum. Overall, Holocene dust fluxes are probably no more than one-third as high as the dust fluxes during Quaternary glacial intervals. However human influences have increased dust fluxes over Holocene background values in the last several hundred years.


Figure 5
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  		FIGURE 4 A NASA SeaWiFS image collected on August 19, 2004. Dust from North Africa often blows across the Mediterranean Sea into southern Europe. An uncropped version of this image and others showing dust storms originating in desert regions can be found at http://oceancolor.gsfc.nasa.gov/cgi/image_archive.cgi?c=DUST.

 
Desert mineral aerosols have both local and regional effects. Blocky lava flows or gravelly alluvial fans have rough surfaces with large voids that trap eolian materials, which, because of their high surface area, dominate soil behavior (Reheis et al. 1995). For example, eolian deposits composed of quartz, mica, sulfates, and carbonates accumulate in the lavas of the Cima volcanic field, which lies 15 km from a dry lake bed in the Mojave desert (Dohrenwend et al. 1986). These lava flows form a chronosequence, where over time, silt, clay, calcite, and gypsum accumulate in the void spaces between lava fragments, but the lava shows virtually no signs of weathering. Coarse-grained alluvial piedmonts and coalescent alluvial fans follow similar patterns where the fine-grained eolian components accumulate, weather, and dominate the overall chemical and mineralogical properties of the resulting soils (McFadden et al. 1987). Reynolds et al. (2006) demonstrated that eolian input to soils on the Colorado Plateau contributes significantly to the nutrients available for plant growth on otherwise nutrient-poor quartz sandstone substrates. In temperate deserts of the world, Quaternary climate fluctuations were important in driving soil development through interglacial provision of high dust input to surrounding landscapes and through glacial moisture that promoted weathering of the fine-grained accumulations. Smaller climate fluctuations may also contribute to dust generation. In arid regions, relatively modest shifts in local precipitation can induce changes in vegetation cover, which in turn control dust generation (Pelletier 2007).

At continental scales, mineral aerosols can control rainwater composition, hence levels of acidification and soil properties (Muhs et al. 2007; McTainsh and Strong 2007). Throughout much of the 20th century, calcium carbonate dust derived from arid western regions of North America helped to neutralize atmospheric acidity in the industrialized midwestern and eastern portions of the continent. Interestingly, recent changes in land-use practices such as increased road paving have led to reduction in carbonate dust even as SO2 emissions have declined. The result has been lower net improvement in the acidity of rain in the northeastern U.S. than was predicted based on the Clean Air Act (Hedin and Likens 1996). Mineral aerosols are an important transport pathway for phosphorus (P), which can be a limiting nutrient in tropical ecosystems. Aerosol fluxes of P over intercontinental distances have been shown to be important for sustaining primary production in the Amazon basin (Swap et al. 1992) and Hawai'i (Chadwick et al. 1999). Okin et al. (2004) modeled turnover rates of P provided by mineral aerosols on a range of substrates globally and showed that Amazon ecosystems had high turnover rates and hence were sensitive to external augmentation of nutrients.

Quantifying mineral aerosol input to soils is difficult because standard mineralogical methods may not be sufficiently sensitive. A number of studies have used the contrasting chemical and isotopic compositions of volcanic substrates and major aerosol sources to elucidate the impact of dust deposition on soil chemistry. The Hawaiian Islands again serve as an example of the uses and limitations of a tracer approach to quantifying mineral aerosol input to the weathering zone. Mineral aerosols produced in central Asia are transported by prevailing winds to the archipelago, but because the transport distance is long, deposition flux is low, and only because of contrasting mineralogy and trace element geochemistry can the eolian component be recognized in the soils. Marine sediments in the north Pacific record the long-term mean composition of dust delivered to the region. The exotic material is quite similar in composition to its presumed central-Asian loess source and not far from estimates of the mean composition of the upper continental crust. The aerosols are largely composed of illite and quartz, which can be identified in the Hawaiian soils. Similarly, geochemical analyses of Hawaiian soil regularly reveal rare earth element and Sr, Nd, and lead (Pb) isotope concentrations that could not have been derived either from Hawaiian rocks or marine aerosols (Monastra et al. 2004 and references therein). The data suggest input with a composition close to that of North Pacific pelagic clay derived from Asian dust (FIG. 5). Once again, the geochemical impact of mineral aerosol deposition depends on site history. In soils that have experienced considerable weathering loss, the original inventory of whole-rock Sr, Nd, and Pb is often strongly depleted. Consequently the addition of a small quantity of exotic mineral aerosol can dominate the isotopic and trace element signature of a soil. The Hawaiian Islands are remote and have low mineral aerosol deposition, and timescales of at least 104 years may be necessary before the eolian component becomes significant. In contrast, soils from Mount Cameroon, West Africa, proximal to the Sahara, show recognizable shifts in Sr, Nd, and Pb isotope ratios after only a few thousand years (Dia et al. 2006). High rainfall there promotes rapid leaching of primary elements, and high rates of aerosol deposition from relatively proximal Saharan sources deliver material with an exotic signature.


Figure 6
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  		FIGURE 5 Mixing diagram for Sr and Nd isotope ratios in bulk Hawaiian soil samples. Data from dust-impacted horizons at Laupahoehoe and Kohala plot on a mixing hyperbola between depleted soil and Asian dust. Mineral aerosol addition initially shifts 87Sr/86Sr but has less impact on {epsilon}Nd values (143Nd/144Nd normalized relative to a chondritic meteorite reference, CHUR), because of the low Sr/Nd ratio in weathered basalt. Increasing addition of mineral aerosol impacts {epsilon}Nd values as the input of dust Nd gradually over-whelms the depleted reservoir of basaltic Nd. Over time, the largest input of Sr to the soils is from marine arosol, so 87Sr/86Sr ratios in near-surface weathered basalt tend to approach the seawater value, irrespective of dust or basaltic inputs. However, the impact of mineral or marine aerosol deposition on deep soil horizons is negligible. FIGURE MODIFIED FROM THE ORIGINAL IN KURTZ ET AL. (2001)

 


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MARINE AEROSOL DEPOSITION
 MINERAL AEROSOL DEPOSITION
 CONCLUSIONS
REFERENCES
 
Alkaline earth cation budgets and soil mineral composition are a function of the rates of aerosol supply and the inventory of primary constituents. Mineral aerosol deposition rates vary over several orders of magnitude, depending on proximity to source and transport pathways. Temporal variations are also large, whether a result of seasonal or longer climatic fluctuations or anthropogenic activity. Human activities have greatly perturbed dust production in many regions. Large variations in mineral aerosol fluxes occurred globally during the Pleistocene, when many weathering profiles developed. Consequently, data on modern aerosol fluxes, even when available, are unlikely to be representative of the integrated flux history of any particular site (Reheis et al. 1995). Uncertainties with quantitative application of modern flux data make tracer-based methods a possible useful alternative. In many of the examples we have cited, the contrasts in mineral composition, trace element patterns, and isotopic signatures were used to demonstrate continental dust source in volcanic terrains, because the end members can be distinguished. However, quantifying atmospheric inputs in more typical continental settings may be difficult, because less contrast may exist between aerosol and local bedrock composition.

Uncertainty in flux and composition of atmospheric inputs to the Critical Zone may compromise our ability to draw conclusions about substrate weathering rates and the source of solutes in watersheds. For example, Blum and Erel (1997) interpreted changes in biotite weathering rate from soil exchange 87Sr/86Sr data along a chronosequence from glacial moraines in Wyoming, but Dahms et al. (1997) proposed that the isotopic shift could be a result of incorporation of non-radiogenic dust transported from a nearby volcanic complex. Recent work on a granitoid weathering profile at Luquillo, Puerto Rico, suggests that the majority of the radiogenic Sr in the stream is actually derived from weathering of eolian material, not weathering of biotite as might be expected (Pett-Ridge 2007a). At Luquillo, dust is geochemically important despite rapid erosion and land surface turnover. Another complication arises in the use of "immobile element" normalization to estimate mass transfer in a soil system (Brimhall et al. 1988). If the element of choice [typically titanium (Ti), zirconium (Zr), niobium (Nb), or thorium (Th)] is also contributed by an eolian source, uncertainty is introduced into elemental mass transfer calculations. The atmospheric transport of uranium (U) and thorium can also significantly complicate the use of U-series tracers for determining weathering timescales, because the common assumption that bedrock weathering is the sole source of soil U-series radionuclides may not be valid (Pett-Ridge et al. 2007b).

More effort, including the application of newer tracers, such as hafnium (Hf) and its isotopes, and microanalysis of accessory minerals, will improve our ability to determine mineral aerosol input to the Critical Zone and define aerosol influence on weathering processes, biogeochemical cycling, and watershed budgets. The inherently open-system aspect of the Critical Zone presents a real challenge for mass-balance studies, but careful investigation with new and old tools holds great promise. The recognition that geochemically and biologically derived material can be transported over long distances by the atmosphere has, in a sense, made the world a smaller place. The realization that land-surface processes on one continent can influence biogeochemistry on another challenges the Critical Zone research community to think beyond the plot or watershed scale to the global scale, but much of the data needed to assess the degree of connectivity of biogeochemical processes across large scales still reside in the soils themselves.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MARINE AEROSOL DEPOSITION
 MINERAL AEROSOL DEPOSITION
 CONCLUSIONS
 REFERENCES
 

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