Sulfate (SO2-) and NO« anthropogenically emitted to the atmosphere, are oxidized and hydrolyzed to form sulfuric (H2SO4) and nitric (HNO3) acids which are then introduced to ecosystems through precipitation or condensation of water vapor on foliage. These acids drive terrestrial ecosystems toward or into calcium limitation. Contact between acid fog and foliage leaches calcium directly out of leaf membranes, causing tissue damage and calcium depletion in plants. The deposition of extra anions (SO4-and NO-) causes the increased formation of calcium salts, such as CaSO4, in soils and these neutral compounds are easily leached from soils. Acid deposition also increases the concentration of hydrogen ions (H+) in soil solutions. These excess protons, in turn, compete with other cations (especially Ca2+) for space on the negatively charged surfaces (cation exchange sites) of soil particles. Calcium ions that are not bound to particles are easily removed from the soil and lost to the ecosystem by leaching.
As the concentration of H+ rises in a soil, weathering rates of aluminosilicates and other minerals increase, inflating concentrations of total soluble Al and soluble free Al3+. This leads to the increasing displacement of Ca2+ and other cations from the cation-exchange sites of soils. Eventually, the concentration of H+ reaches a point where the protons outcompete Al3+. The aluminum ions then become mobile in the soil solution where they may directly interact with plants. This compounds the problem of the depletion of soil Ca2+, because, by damaging root tissue and displacing Ca2+ from exchange sites on the xylem walls of plants, Al3+ diminishes the ability of plants to take up calcium.
Acid deposition also affects terrestrial bodies of water. The main source of calcium for streams and lakes is runoff and groundwater; as the minerals and soils surrounding these bodies of water become increasingly depleted in Ca2+, so does the water. A lowering of calcium ion concentrations affects the acid-neutralizing capacity (ANC) of waters, resulting in greater pH fluctuations with changes in proton fluxes and increasing the effects of acidic deposition. The runoff will also contain increasing concentrations of Al3+, potentially delivering lethal doses for some aquatic organisms and severely impacting or destroying local food webs.
Anthropogenic Impacts on the Atmospheric Dry Deposition of Calcium
As noted above, calcium is added to both terrestrial and marine ecosystems via the deposition of particulate matter from the atmosphere. Humans make significant contribution to these fluxes via industry (e.g., the manufacturing of cement) and biomass and fuel burning. An equally large, and in some localities much larger, anthropogenic influence is found in the abundance of dust. Land-use changes that result in desertification and dry soils at construction sites increase the amount of Ca-containing dust in the atmosphere. These inputs may sometimes temporarily offset the effects of acid deposition, but when these inputs are stopped, calcium depletion resumes.
Effects of Increased Carbon Dioxide on Terrestrial Ecosystems
Higher atmospheric concentrations of carbon dioxide eventually lead to higher mineral weathering rates, resulting in the enhanced leaching of calcium and other elements from soils. Such changes in weathering rates are not balanced by the natural recharge rate of soil calcium. Increased carbon dioxide concentrations also increase plant growth rates. Relieved of a carbon dioxide limitation, the plants will grow until they are limited by some other nutrient, often calcium. The increased plant growth could translate into a faster biological cycling of calcium, a process that has unpredictable results.
Carbon Dioxide, Calcium Biominerals, and Marine Ecology
Anthropogenic effects on the marine calcium cycle primarily occur through the acidification of seawater by carbon dioxide. The resultant lowering of pH and carbonate ion concentrations decreases the saturation state of seawater with respect to calcite and aragonite, thus making it more difficult for marine organisms to produce and maintain calcium biominerals.
Since the beginning of the industrial revolution, CO2 concentrations in the atmosphere have increased by
90 matm (i.e., by more than a third) and show no signs of slowing down. The CO2 added to atmosphere will eventually be absorbed by the ocean, acidifying it. The pH of surface waters has already dropped by 0.1 units (a significant amount), and within 40 years carbonate ion concentrations below aragonite saturation will begin to occur in polar waters, spreading eventually into lower latitudes. If CO2 emissions continue unabated, the pH of the ocean will eventually sink to levels lower than it has been for hundreds of millions of years.
A drop in ocean pH has implications for the existence and ecology of coral reefs because a decrease in the saturation state of seawater with respect to calcium biominerals will have a corresponding decrease in the rate of calcification of coral reefs. Calcification rates in the tropics have already dropped by 10% since the beginning of the industrial revolution and a doubling of atmospheric CO2 from pre-industrial levels could diminish coral calcification rates by as much as 50%. Such decrease in calcification rates will result in reefs shrinking in size and structural integrity, as reef size and strength result from the balance struck between calcium carbonate production and erosion. A decrease in the areal extent of coral reefs in turn, diminishes the habitat and food available for the hundreds of thousands of species of organisms that dwell within coral reef ecosystems. This is true for both the familiar warm water coral reefs of the tropics whose productive ecosystems are an important resource for subsistence fishers and billion-dollar tourism economies alike, and the deeper-dwelling, cold-water coral reef ecosystems that provide habitat and nursing grounds for numerous species including commercially important fish like rockfish and orange roughy.
The reduced saturation state of seawater with respect to calcium biominerals may also affect the production and maintenance of shells and exoskeletons of organisms like mollusks, echinoderms, and crustaceans. The first impacts will be seen in polar ecosystems whose cold waters may become undersaturated with respect to aragonite at the doubling of pre-industrial CO2 expected by 2050. Experiments and material collected in sediment traps have shown that the shells of the aragonitic pteropod mollusks become rapidly pitted and begin to dissolve upon exposure to undersaturated waters. Even the shells of live pteropods begin to dissolve under conditions equivalent to those expected for polar waters at the end of this century. Pteropods should not survive if they cannot maintain their shells, and their disappearance from polar waters would have a significant impact on polar ecosystems. Pteropods are important prey for many zooplankton, fish, and baleen whales and their fecal pellets and mucous feeding webs are important vectors for the sinking of organic matter to deep-sea ecosystems and sediments.
The lowering of the saturation state of seawater for calcium carbonate minerals will also make it thermody-namically less favorable for the calcitic plankton, foraminiferans, and coccolithophores to biomineralize. If the lowering of carbonate ion concentrations means that the high internal pHs required for calcite precipitation take more energy to maintain, these organisms will have a lesser portion of their total energy budget available for growth and reproduction. Although, experimentally, the response of coccolithophore species to increased acidification is mixed, an acidification-driven shift in the phytoplankton toward noncalcareous forms such as diatoms would have an impact on the cycling of CO2, nutrients, and alkalinity in the ocean by altering efficiency of the biological pumping of particulate organic matter and biominerals into the deep sea. If the ratio of particulate organic carbon to calcium carbonate sinking into the deep sea were to increase due to the lesser production of calcium biominerals by foraminiferans and coccolithophores, the biological pump would be more effective at sequestering CO2 in the deep sea, lowering atmospheric concentrations at the expense of more quickly lowering the pH of the deep sea.
Changes in the calcium cycle due to pollution and other anthropogenic influences and their impact on ecosystems make unraveling and quantifying the fluxes of calcium through ecosystems a pressing concern. Three types of tools have been employed, trace element ratios, such as Sr/Ca, ratios of nonradioactive isotopes of strontium or calcium (e.g., 87Sr/86Sr, 44Ca/42Ca, 44Ca/40Ca), and, less commonly, studies of artificially enriched stable (e.g., 48Ca) and radioactive (e.g., 45Ca) isotopes. Such tracers reflect sources of Ca to ecosystems and, when reconstructed from the wood of long-lived trees, may serve as a means of reconstructing the acidification of environments over the past century or so.
The use of Sr/Ca in terrestrial ecosystems takes advantage of the fact that different minerals serving as sources of Ca2+ contain different Sr/Ca signatures. Studies employing this method rely on the assumption that Sr2+ and Ca2+ ions are not fractionated as they cycle through ecosystems because of their comparable charge and size. The Sr/Ca of different calcium reservoirs (e.g., soil waters, soils, and vegetation) should thus reflect it of their sources of calcium. Complicating the use of Sr/Ca, however, are data suggesting that Sr/Ca is fractionated during uptake by and internal cycling within plants and the fact that all Sr/Ca inputs to ecosystems (specifically, mineral pools) have not been identified.
Strontium and calcium isotopic signatures may also usefully identify the sources of Ca to terrestrial environments. Strontium isotopes, which have the advantage of not being biologically fractionated, are used based on the assumption that Sr and Ca in terrestrial environments have been derived from the same sources. The source of calcium to plants, for example, is identified from their 87Sr/86Sr because it reflects the bulk 87Sr/86Sr of the materials from which the strontium came. Calcium isotopes provide a more direct way of investigating Ca cycling through ecosystems. Solutions enriched in the stable calcium isotope with the lowest natural abundance (48Ca) or a radioactive isotope of calcium ( Ca) have been released to study the movement of calcium through the environment. Natural abundances of Ca isotopes provide a way to directly study the calcium cycle in ecosystems. Such work is in its infancy and studies are underway to characterize the Ca isotopic composition of minerals, natural waters, and vegetation and to define Ca isotopic fractionation during weathering, soft tissue formation, biomineralization, and between different plant tissues. Such studies pave the way for this new tracer to be universally applied.
In marine systems, such trace elements and isotopic systems are not as useful for tracking anthropogenic changes to the calcium cycle as direct measurements of pH, alkalinity, and calcification rates. Reconstructions of the depth distributions of calcite sediments and the calcium isotopic composition of marine sediments, however, help to identify past perturbations in the calcium cycle and their links to climatic and ecological events.
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