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Climate change and calcium • Calcium from geochemical weathering causes climate to become cooler • Calcium from dust does not have a climate change effect Carbon Reservoirs • Reservoirs are: – – – – Atmosphere Terrestrial biosphere Oceans Fossil fuels • Reservoirs or components of reservoirs that release carbon to other reservoirs are called sources • Those that absorb carbon are called sinks • This overall network of storage or absorption and transfer is known as the carbon cycle Silicate Weathering • Precipitation from the atmosphere falls to Earth as bicarbonate acid rain, with a natural acidity • CO2 (gas) + H2O (liquid) --> H2CO3 (carbonic acid) • Carbonic acid weathers the rocks on the Earth's surface, releasing ions of calcium (Ca++) and bicarbonate (HCO-3) into the oceans • An example reaction is shown on the next slide Urey Reaction • Named after Harold Urey, Nobel-prize winning geochemist • This reaction is an example of the conversion of silicates to carbonate • CaSiO3 + CO2 = SiO2 + CaCO3 • Wollasonite + carbon dioxide = quartz + limestone Harold C. Urey, 1893-1981 Carbonate-Silicate Cycle Diagram Precipitation of Limestone • Soluble carbonates may be converted to solid form by inorganic chemical reactions, or by organisms which extract lime and use it to build shells • Limy muds or biochemical shells sink to the ocean floor, where the lime may be converted to limestone • This stores large amounts of carbon on the ocean floor Carbon Sequesterization • Limestone added to the ocean flow sequesters carbon • The oceans become sinks for carbon • The atmosphere, which was the source of the carbon, is a source • Carbon is depleted in the atmosphere Carbonate – Silicate Cycle • Cycle combines the Carbon Cycle with Silicate Weathering • A steady state is reached, where the fluxes into and out of each reservoir are constant • Changes to the system can upset the balance • There have been claims that this cycle provides a self-regulating mechanism for CO2 in the atmosphere Example: Plate Collisions • When India hit Asia, the Himalayan mountains were pushed up • Mountain ranges increase precipitation • The extra rain weathered the newly exposed silicate minerals, and started the process of carbon sequesterization in the ocean • This removed carbon dioxide from the atmosphere Himalayas caused global cooling Ice Age Trigger • Eldridge Moores, 1996 president of the Geological Society of America, has suggested that the uplift of the Himalayas may have triggered the Pleistocene glacial advances Eldridge Moores DUST: MORE AROUND THAN YOU THINK DUST STORM IN FEBRUARY Determination of Sources of Ca to Surface Waters in the Rocky Mountains using mineralogic and Sr-isotopic data David Clow1, Thomas D. Bullen2, Mark W. Williams3, John A. Fitzpatrick2, and Julie K. Sueker4 PROBLEM STATEMENT: Surface water in granitoid terrain in the Rocky Mountains often has higher Ca/Na ratios than can be explained by weathering of major primary minerals in local bedrock. APPROACH • Chemical analysis of calcium cannot discriminate sources of calcium – There is not a unique signature of calcium from chemical analysis • Strontium substitutes for calcium in chemical reactions • Different sources of strontium have different isotopic ratios HYPOTHESES • (1) nonstoichiometric weathering of plagioclase • (2) weathering of eolian carbonates • (3) weathering of trace calcite in granitoid bedrock. METHODS • Rock and stream water samples were collected from sites in the Wind River Range of Wyoming, and the Park and Front Ranges of Colorado. Soil samples were collected from a subset of sites in the Front Range. nonstoichiometric weathering of plagioclase • evaluated by analyzing the zonation characteristics and chemical composition of plagioclase crystals in 40 rock samples collected from 12 recently glaciated basins underlain by Proterozoic granitoid bedrock in the Wind River Range of Wyoming, the Front Range of Colorado, and the Park Range of Colorado. • Thin sections were made from the core of each rock sample, and 10 plagioclase crystals in each thin section were examined using a petrographic microscope. Plagioclase zonation Plagioclase zonation • No zonation of plagioclase found • Reject hypothesis one • Reject Williams et al. 1993 weathering of eolian carbonates • evaluating patterns in 87Sr/86Sr of exchangeable cations in soils collected from three different landscapes: forest, alpine, and near glaciers. These landscapes correspond to progressively younger and fresher soils. weathering of trace calcite in bedrock • tested by measuring the carbonate content of the rock samples used in the thin-section analysis. Cores cut from the interior of the rock samples were crushed into powder, and the carbonate content of the powder was measured by coulometric titration 87Sr/86Sr of exchangeable cations in soil inorganic carbon concentrations (as CaCO3) in rock samples Scatterplot of Ca/Na in stream water against % CaCO3 in rocks Scatterplot of Ca/Na in stream water against fraction of basin area with slope ≥30° 87Sr/86Sr Scatterplot of in stream water against Ca/Na in stream water Modeling strontium content • Multivariate stepwise linear regression was performed – – – – basin slope basin size amount of Pleistocene glacial till granite/gneiss ratio • explanatory variables were normalized to 1 Scatterplot of predicted 87Sr/86Sr against observed 87Sr/86Sr in stream water Ca behavior may be summarized as follows: weathering of trace calcite occurs primarily in areas with steep slopes (≥ 30°) because that is where physical weathering is most active. Eolian deposition of carbonates is important in cirques on the lee (east) side of the continental divide. Small glaciers and perennial snowfields occur in those locations because prevailing westerly winds deposit snow that is carried from the west side of the divide there; eolian deposition probably follows a similar pattern.