The Global Calcium Cycle

Sources of Calcium to Terrestrial and Marine Ecosystems

Ultimately, calcium inputs to global ecosystems come from the chemical weathering of calcium-containing minerals (Figure 1). Weathering of silicate, carbonate, phosphate, and sulfate minerals in rocks, sediments, and soils releases calcium ions into solution. The two most important of these weathering reactions can be generalized, for calcium-bearing silicates, as

3H2O + 2CO2 + CaSiO3 ! Ca2+ + 2HCO3- + Si(OH)4 and for carbonates as

The activities of plants are known to increase mineral weathering rates, and studies have suggested that in soils, plants involved in symbiotic relationships with mycorrhi-zal fungi might also directly access mineral-bound calcium before it enters the soil solution.

Calcium is also deposited into Earth surface ecosystems via wet and dry deposition. Calcium from sea salt and soil dust present as atmospheric particulate matter can be deposited by both these mechanisms. Anthropogenic contributions of calcium to atmospheric deposition come from biomass and fuel burning and the manufacturing of cement. The overall amount and relative contribution from each atmospheric source to an area varies seasonally and with factors such as the proximity of the area to the ocean.

Calcium is stored in biological materials, and calcium can be recycled in ecosystems via the breakdown of organic matter and biominerals containing calcium. Large amounts of calcium are generally exported from terrestrial ecosystems into the ocean through ground-water and surface waters. These ground- and surface waters supply 2-3 x 1013 mol Ca yr-1 to the oceans versus the 0.3 x 1013 mol Cayr-1 supplied strictly to the oceans through deep-sea hydrothermal vents.

Figure 1 Schematic representation of the calcium cycle. Calcium is liberated in continents by the weathering of calcium-containing minerals, common constituents of most rocks. Calcium pools in terrestrial ecosystems include reservoirs in soils and soil minerals, organisms, and decomposing organic matter. Riverine and groundwater inputs transfer calcium in its ionic form from continents to the oceans. Hydrothermal vents also serve as calcium inputs in the oceans. In marine ecosystems, calcium ions are abundant in seawater. Marine organisms use calcium to make shells and hard parts. Calcium is removed from the oceans primarily by the sedimentation of these calcified organisms. Atmospheric deposition, both wet and dry, contributes calcium throughout both terrestrial and marine ecosystems. Anthropogenic effects including acid deposition, changes in land use (e.g., desertification), harvesting, and increased atmospheric carbon dioxide concentrations influence this natural calcium cycle.

Figure 1 Schematic representation of the calcium cycle. Calcium is liberated in continents by the weathering of calcium-containing minerals, common constituents of most rocks. Calcium pools in terrestrial ecosystems include reservoirs in soils and soil minerals, organisms, and decomposing organic matter. Riverine and groundwater inputs transfer calcium in its ionic form from continents to the oceans. Hydrothermal vents also serve as calcium inputs in the oceans. In marine ecosystems, calcium ions are abundant in seawater. Marine organisms use calcium to make shells and hard parts. Calcium is removed from the oceans primarily by the sedimentation of these calcified organisms. Atmospheric deposition, both wet and dry, contributes calcium throughout both terrestrial and marine ecosystems. Anthropogenic effects including acid deposition, changes in land use (e.g., desertification), harvesting, and increased atmospheric carbon dioxide concentrations influence this natural calcium cycle.

The Production and Solubility of Calcium Biominerals in the Ocean

Approximately balancing the input of 2—3 x 1013mol Cayr-1 to the ocean is the output of calcium as calcium biominerals formed by marine organisms. In shallow waters, aragonitic corals dominate the production of these minerals, providing 20% of the total output of calcium from the ocean. Calcitic foraminiferans and coc-colithophores and aragonitic pteropods form the majority of the output sedimenting to the deep sea, with foramini-ferans comprising somewhere between 20% and 60% of the total calcium output from the oceans and pteropods contributing only a few percent of it.

Despite the heavy biological usage of calcium, the relatively high abundance of Ca2+ in seawater means that Ca2+ concentrations are relatively invariant and never limiting to the growth of organisms. However, the concentration of Ca2+ together with the carbonate ion (CO^-) concentration defines the saturation state of seawater with respect to calcite and aragonite. The saturation state of seawater is an integral part of the global Ca cycle and has profound implications for biota producing Ca biominerals.

Both the production of calcium carbonate biominerals and their dissolution can be summarized by the simple, reversible reaction

The product of the calcium ion and carbonate ion concentrations in seawater (i.e., [Ca2+] x [CO^- ]) determines whether conditions are supersaturated or undersaturated with respect to the carbonate mineral in question. When the ion activity product is higher than the saturating value, conditions are favorable for mineral formation and dissolution does not occur. The lower the ion activity product below the saturating value, the more difficult it is to precipitate the minerals and the quicker the dissolution of the mineral.

Because the concentration of Ca2+ is relatively invariant in the oceans, it is the variability in the carbonate ion concentration in seawater (along with temperature) that affects biological calcification and the dissolution of carbonate minerals. Carbonate ion concentrations drop with pH, as the increasing H+ concentration favors the protonation of CO2- to form bicarbonate ion (HCO-). Aragonite and calcite also become increasingly more soluble at cooler temperatures. Thus warm, tropical surface waters with their relatively low pH are supersaturated with respect to both aragonite and calcite. Cooler waters require greater carbonate ion concentrations to sustain saturating conditions, making polar waters less saturated than tropical waters. The pH of cold deep waters is relatively low due to the addition of CO2 from the decay of sinking organic matter, and so these waters contain lower concentrations of carbonate ion and may be undersatu-rated with respect to both calcite and aragonite. As a result, production of massive, shallow water reefs by corals occurs only in the tropics, polar waters favor noncalcareous phytoplankton, and carbonate sediments do not accumulate below depths of several thousand meters.

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