Contributions from Earth’s to

Louis A. Derry1 and Oliver A. Chadwick2

oils are mixtures of material derived from substrate , Marine and aerosols con- tribute significantly to . 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 and of the Airborne and mineral particles act as condensation nuclei for , Critical Zone. Some of the best-studied examples are from ocean islands and the 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 flux greater than 10% of the global erated by evaporation of sea-spray flux, deposition from the atmosphere plays an important role producing 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 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 selectively removes fine particles from the their radiative properties and their role in acidification, but soil surface, resulting in local 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 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 and 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 . 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 -derived silicate and mineral aerosols can particles or dissolved salts derived from wind erosion of greatly modify the composition of precipitation (, 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 (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 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 depletes mobile constituents derived from weathered (0.3 ka) to highly weathered surfaces 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. 30% of 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 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). The balance of substrate-derived versus atmospherically derived alkaline earth elements is strongly climate depend- ent 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 FIGURE 1 Cation concentration ratios relative to chloride in precip- itation normalized to seawater. The data are from 12 U.S. values, while soil developed at higher MAP approach the coastal or island sites and in most cases are five-year means of annual atmospheric input value. The data from Hawaiian precipitation-weighted chemistry. At all sites Ca is considerably enriched over what is predicted from a sea salt model, while Mg is often slightly chronosequence and climosequence studies illustrate the depleted. K is typically enriched, while Na is indistinguishable from sea evolution of the weathering–atmospheric deposition system salt composition. The deviations from sea salt composition are mostly and demonstrate that under the right conditions, a timescale the result of dissolution of mineral aerosols, of which carbonate of <104 years is sufficient to develop a soil in which nearly is the most important source. all the Sr is derived from atmospheric sources, producing a

E L E M E N T S 334 OCTOBER 2007 87 86 FIGURE 3 Effect of surface erosion on foliar Sr/ Sr and phospho- rous levels, Hawai‘i. Samples of native O’hia (Metrosideros polymorpha) 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 FIGURE 2 Integrated wet deposition flux (ions delivered in the fog plus rain) (solid lines) for Ca and K calculated for the 0.703, while atmospheric sources are mostly sea salt (0.709) with some dust Kilauea volcano area compared to substrate inventory calculated over (near 0.720). FIGURE MODIFIED FROM THE ORIGINAL IN PORDER ET AL. (2005) 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 small amounts of eolian material can be incorporated into rate of weathering loss; for Ca the timescale is 12,000–70,000 years. existing soils. If their compositions are similar, it may be DEPOSITION DATA FROM CARILLO ET AL. (2002) hard to quantify the degree to which mineral aerosols have contributed (Ruhe and Olson 1980). are the other major source of mineral aerosols, particularly vegetation- soil with isotopic signatures markedly different from those free ephemeral channels and dry beds (FIG. 4). The total of the original substrate. Thus, weathering-rate calculations global dust flux is estimated to be near 1800 Tg y-1 (Mahowald based on cation budgets alone can be substantially in error et al. 2005). To put this figure into perspective, dust pro- if atmospheric deposition is not taken into account, some- vides 14% of the annual global sediment flux to the oceans. times a difficult task. Holocene desertification has increased dust flux from regions such as North Africa, while production has declined Marine aerosols can also be important contributors to markedly since the last glacial maximum. Overall, Holocene ecosystems in continental interiors, especially when soils dust fluxes are probably no more than one-third as high as have long residence times and weatherable minerals are the dust fluxes during Quaternary glacial intervals. However depleted. For example, Quade et al. (1995) demonstrated human influences have increased dust fluxes over Holocene that marine Sr provides the dominant isotopic signature to background values in the last several hundred years. soils hundreds of kilometers into the interior of . It is also true that in continental interiors, terrestrial mineral mineral aerosols have both local and regional effects. aerosols may be dissolved and the soluble components may Blocky lava flows or gravelly alluvial fans have rough sur- subsequently be deposited. In the deserts of the southwestern faces with large voids that trap eolian materials, which, U.S., 87Sr/86Sr ratios have been used to demonstrate the because of their high surface area, dominate soil behavior dominance of rainwater-delivered Sr and Ca in the forma- (Reheis et al. 1995). For example, eolian deposits composed tion of calcretes (Capo and Chadwick 1999; Naiman et al. of , , sulfates, and accumulate in the 2000). For the most part, the dissolved Sr was derived from lavas of the Cima volcanic field, which lies 15 km from a that crops out throughout the region. Thus, over dry lake bed in the Mojave desert (Dohrenwend et al. 1986). continents, rain is a complex mixture of marine- and land- These lava flows form a chronosequence, where over time, derived contributions, where the latter arises from dissolu- , clay, , and accumulate in the void spaces tion of mineral aerosols during atmospheric transport. between lava fragments, but the lava shows virtually no signs of weathering. Coarse-grained alluvial piedmonts and MINERAL AEROSOL DEPOSITION coalescent alluvial fans follow similar patterns where the fine-grained eolian components accumulate, weather, and Mineral aerosols are derived from wind erosion of soil and dominate the overall chemical and mineralogical properties sediment. A major source is outwash areas near . of the resulting soils (McFadden et al. 1987). Reynolds et al. There, freshly ground rock is carried by rivers onto broad, (2006) demonstrated that eolian input to soils on the Colorado sparsely vegetated floodplains where it is easily entrained Plateau contributes significantly to the nutrients available by wind. Over thousands of years, thick deposits of loess for plant growth on otherwise nutrient-poor quartz - entirely derived from atmospherically transported particles stone substrates. In temperate deserts of the world, can build up (Pye 1996). At the edges of these deposits, Quaternary climate fluctuations were important in driving

E L E M E N T S 335 OCTOBER 2007 FIGURE 4 A NASA SeaWiFS image collected on August 19, 2004. and eastern portions of the continent. Interestingly, recent Dust from North Africa often blows across the Mediter- changes in land-use practices such as increased road paving ranean Sea into southern Europe. An uncropped version of this image and others showing dust storms originating in desert regions can be have led to reduction in carbonate dust even as SO2 emis- found at http://oceancolor.gsfc.nasa.gov/cgi/image_archive.cgi?c=DUST. sions have declined. The result has been lower net improve- ment in the acidity of rain in the northeastern U.S. than was predicted based on the Clean Air Act (Hedin and Likens soil development through interglacial provision of high 1996). Mineral aerosols are an important transport pathway dust input to surrounding and through glacial for (P), which can be a limiting nutrient in tropical moisture that promoted weathering of the fine-grained ecosystems. Aerosol fluxes of P over intercontinental dis- accumulations. Smaller climate fluctuations may also con- tances have been shown to be important for sustaining pri- tribute to dust generation. In arid regions, relatively modest mary production in the Amazon basin (Swap et al. 1992) shifts in local precipitation can induce changes in vegetation and Hawai‘i (Chadwick et al. 1999). Okin et al. (2004) mod- cover, which in turn control dust generation (Pelletier 2007). eled turnover rates of P provided by mineral aerosols on a range of substrates globally and showed that Amazon At continental scales, mineral aerosols can control rainwater ecosystems had high turnover rates and hence were sensi- composition, hence levels of acidification and soil properties tive to external augmentation of nutrients. (Muhs et al. 2007; McTainsh and Strong 2007). Throughout much of the 20th century, dust derived Quantifying mineral aerosol input to soils is difficult because from arid western regions of North America helped to neu- standard mineralogical methods may not be sufficiently tralize atmospheric acidity in the industrialized midwestern sensitive. A number of studies have used the contrasting

E L E M E N T S 336 OCTOBER 2007 FIGURE 5 Mixing diagram for Sr and Nd isotope ratios in bulk Hawaiian soil samples. Data from dust-impacted hori- zons at Laupahoehoe and Kohala plot on a mixing hyperbola between depleted soil and Asian dust. Mineral aerosol addition initially shifts 87 86 143 144 Sr/ Sr but has less impact on εNd values ( Nd/ Nd normalized relative to a chondritic meteorite reference, CHUR), because of the low Sr/Nd ratio in weathered basalt. Increasing addition of mineral aerosol impacts εNd 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, irre- spective 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) chemical and isotopic compositions of volcanic substrates and major aerosol sources to elucidate the impact of dust climatic fluctuations or anthropogenic activity. Human activ- deposition on . The Hawaiian Islands again ities have greatly perturbed dust production in many serve as an example of the uses and limitations of a tracer regions. Large variations in mineral aerosol fluxes occurred approach to quantifying mineral aerosol input to the globally during the Pleistocene, when many weathering weathering zone. Mineral aerosols produced in central Asia profiles developed. Consequently, data on modern aerosol are transported by prevailing winds to the archipelago, but fluxes, even when available, are unlikely to be representa- because the transport distance is long, deposition flux is tive of the integrated flux history of any particular site low, and only because of contrasting and trace (Reheis et al. 1995). Uncertainties with quantitative appli- element geochemistry can the eolian component be recog- cation of modern flux data make tracer-based methods a nized in the soils. Marine in the north Pacific possible useful alternative. In many of the examples we record the long-term mean composition of dust delivered to have cited, the contrasts in mineral composition, trace ele- the region. The exotic material is quite similar in composi- ment patterns, and isotopic signatures were used to demon- tion to its presumed central-Asian loess source and not far strate continental dust source in volcanic terrains, because from estimates of the mean composition of the upper con- the end members can be distinguished. However, quantify- tinental crust. The aerosols are largely composed of illite ing atmospheric inputs in more typical continental settings and quartz, which can be identified in the Hawaiian soils. may be difficult, because less contrast may exist between Similarly, geochemical analyses of Hawaiian soil regularly aerosol and local bedrock composition. reveal rare earth element and Sr, Nd, and lead (Pb) isotope concentrations that could not have been derived either Uncertainty in flux and composition of atmospheric inputs from Hawaiian rocks or marine aerosols (Monastra et al. to the Critical Zone may compromise our ability to draw 2004 and references therein). The data suggest input with a conclusions about substrate weathering rates and the source composition close to that of North Pacific pelagic clay of solutes in watersheds. For example, Blum and Erel (1997) derived from Asian dust (FIG. 5). Once again, the geochem- interpreted changes in weathering rate from soil ical impact of mineral aerosol deposition depends on site exchange 87Sr/86Sr data along a chronosequence from gla- history. In soils that have experienced considerable weath- cial moraines in Wyoming, but Dahms et al. (1997) pro- ering loss, the original inventory of whole-rock Sr, Nd, and posed that the isotopic shift could be a result of Pb is often strongly depleted. Consequently the addition of incorporation of non-radiogenic dust transported from a a small quantity of exotic mineral aerosol can dominate the nearby volcanic complex. Recent work on a granitoid isotopic and signature of a soil. The Hawaiian weathering profile at Luquillo, Puerto Rico, suggests that Islands are remote and have low mineral aerosol deposition, the majority of the radiogenic Sr in the stream is actually and timescales of at least 104 years may be necessary before derived from weathering of eolian material, not weathering the eolian component becomes significant. In contrast, of biotite as might be expected (Pett-Ridge 2007a). At soils from Mount Cameroon, West Africa, proximal to the Luquillo, dust is geochemically important despite rapid ero- Sahara, show recognizable shifts in Sr, Nd, and Pb isotope sion and land surface turnover. Another complication arises ratios after only a few thousand years (Dia et al. 2006). High in the use of “immobile element” normalization to estimate rainfall there promotes rapid leaching of primary elements, mass transfer in a soil system (Brimhall et al. 1988). If the and high rates of aerosol deposition from relatively element of choice [typically (Ti), zirconium (Zr), proximal Saharan sources deliver material with an exotic niobium (Nb), or thorium (Th)] is also contributed by an signature. eolian source, uncertainty is introduced into elemental mass transfer calculations. The atmospheric transport of CONCLUSIONS (U) and thorium can also significantly complicate the use of U-series tracers for determining weathering Alkaline earth cation budgets and soil mineral composition timescales, because the common assumption that bedrock are a function of the rates of aerosol supply and the inven- weathering is the sole source of soil U-series radionuclides tory of primary constituents. Mineral aerosol deposition may not be valid (Pett-Ridge et al. 2007b). rates vary over several orders of magnitude, depending on proximity to source and transport pathways. Temporal vari- ations are also large, whether a result of seasonal or longer

E L E M E N T S 337 OCTOBER 2007 More effort, including the application of newer tracers, chemically and biologically derived material can be trans- such as (Hf) and its isotopes, and microanalysis of ported over long distances by the atmosphere has, in a accessory minerals, will improve our ability to determine sense, made the world a smaller place. The realization that mineral aerosol input to the Critical Zone and define land-surface processes on one continent can influence bio- aerosol influence on weathering processes, biogeochemical geochemistry on another challenges the Critical Zone cycling, and watershed budgets. The inherently open-sys- research community to think beyond the plot or watershed tem aspect of the Critical Zone presents a real challenge for scale to the global scale, but much of the data needed to assess mass-balance studies, but careful investigation with new the degree of connectivity of biogeochemical processes and old tools holds great promise. The recognition that geo- across large scales still reside in the soils themselves. !

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