Sources of Ca to Watersheds: Explaining the Excess Corey Lawrence Nick Rising Christopher Andersen
Presentation Overview • The Issue: Excess Calcium in Watersheds • Useful Tools: Strontium Isotope System and Elemental Ratios • Papers: • Clow et al., 1997 – Dust • White et al., 1999 – Disseminated Calcite ------------------------------------------ • Interesting Tangent: • Ley et al., 2004 – Extreme Microbes
Calcium in Watersheds • Importance of Ca for forest productivity and watershed alkalinity • Silicate vs. Carbonate Weathering • Excess Ca relative to Mineral Stoichiometry • Short Term • Acidification • Biological Losses • Long Term • Accelerated Silicate Phase Weathering • Selective Leaching of Anthrocite • Disseminated Calcite • Eolian Dust
Introduction to the Strontium Isotope System. Corey Lawrence
87Rb decays to 87Sr through beta decay Rubidium and Strontium behave differently in during melting process leading to segregation during mantle melting. Source: Capo et al., 1998
Strontium Isotope System • Indicator of both age and geochemical origin • Older rocks with same initial Rb/Sr will have higher 87Sr/86Sr than younger ones. • Rocks of a given age composed of different minerals will show differentiation in strontium ratios • The combination of these factors allows strontium to be used a tracer of cation source
Strontium vs. Calcium • Both are alkaline earth elements with +2 valence charge • Strontium • Atomic number = 38 • Ionic Radius = 1.18 angstroms • Calcium • Atomic number = 20 • Ionic Radius = 1.00 angstroms
Critical Assumptions • Strontium is not fractionated by biological or physical mechanisms • Strontium isotopic ratios are constant over time and climate • Congruent weathering • Differences in isotope ratios between sources are large enough to solve mixing model
Strontium 87/Strontium 86 as a Tracer of Mineral Weathering Reactions and Calcium Sources in an Alpine/Subalpine Watershed, Loch Vale, Colorado.(Clow et al., 1997) Nick Rising
Goals • “Our specific objective was to use Sr-87/Sr-86 as a tool to characterize the dominant sources of dissolved calcium in surface waters in Loch Vale.” • Determine sources of excess Ca in watershed.
Geology • 80% Precambrian Gneiss. • 20% Precambrian Silver Plume Granite. • Mineralogy: • Quartz (28-41%) • Plagioclase* (25-30%) • Biotite (6-16%) • Microcline (9-34%) • Sillimanite (0-6%)
Methods • Stream gaging stations on Andrews Creek and Icy Brook used to take water samples. • Eolian Dust samples collected at weather station. • Bedrock, soil and dry deposition samples were also taken from the catchment.
Calcium Sources • Weathering of plagioclase was thought to be the dominant source of dissolved Ca. • Stream water has higher amounts of Ca than plagioclase. • Previous studies have shown that weathering of calcite in bedrock is also a source of Ca. • Dry deposition (dust) may also be a major source.
Sr-87/Sr-86 Variability • Stream water samples have highest Sr-87/Sr-86 ratios. • Springs have the largest range of ratios. • Represent shallow subsurface flow (highly variable). • Soils have lower ratios, but higher than precipitation. • Soil Sr-87/Sr-86 ratios are derived from mixing atmospheric and bedrock sources.
Effects of Dust • Eolian dust increases Ca/Na ratios while keeping Sr ratios steady. • Dust, when combined with precipitation, yield higher Ca/Na ratios with lower Sr-87/Sr-86 ratios. • More important in rain than snow.
Mass Balance equation was used to determine the weathering rates which account for flux into the Andrews Creek subbasin. 42% Plagioclase 38% Calcite 18% Biotite 2% Microcline
Results • Using Sr-isotope mixing equation, 1/5 to 1/3 of annual inputs into streams are the result of dry deposition (dust). • 26% (+/- 7%) from dust. • 23% (+/- 1%) from weathering of plagioclase. • 41 to 59% from the dissolution of calcite in bedrock.
The role of disseminated calcite in the chemical weathering of granitoid rocks (White et al., 1999) • Goals: • Investigate sources for the release of excess Ca by detailing the content and distribution of disseminated calcite in granitoid rocks • Long term experimental weathering studies on both fresh and naturally weathered granitoids. • Use results to interpret observed solute concentrations and weathering fluxes
The Approach • Compare minerology using XRD ,cathode luminescence, and SEM. • Measure solute concentration from laboratory sequential weathering experiment using ICP-MS. • Contrast weathering experiment with observed stream chemistry in sampled watersheds.
Results • There is a range of calcite and CO2 between granitoids • Weathered material contains much less Ca and CO2 than fresh material
Results • Ca, Na, and Si decrease with time during the simulated weathering of fresh material • Na typically lower than Ca in all sites except Rio Icacos. • Comparing Ca/Na ratios suggests stream solutes are a mixture of fresh and weathered material in Loch Vale and Yosemite • Contrasting Ca/Na results in other watersheds suggest must reflect differences in weathering conditions
Results • Decreasing calcium concentrations correlates with decreasing pH • Effluent alkalinities are elevated relative to calcium in Yosemite and Rio Icacos granitiods • Indicates alkalinity is derived from both calcite dissolution and silicate hydrolysis in these systems.
Sr/Ca ratios are higher in the plagioclase than in disseminated calcite. • Effluent Sr/Ca ratio should reflect a mixture of the two sources Elevated Sr/Ca in Rio Icacos suggests nonstoichiometric weathering • 94-80% Ca contribution in Yosemite • 65% Ca contribution in Rio Icacos
Initial effluent from fresh rock is close to Ca saturation • Both fresh and weathered effluent calcite saturation decreases with time
Lack of correlation between the calcite content of the fresh granitoid and the extent of calcium excess in streams reflects differences in natural weathering conditions.
Conclusions • Calcite occurs in granitoid microfractures as disseminated calcite • Calcite is preferentially removed during natural weathering conditions • Calcium excess is related to age of bedrock and weathering conditions • Accessory calcite can contribute to significant proportion of stream Ca in younger basins
Microbial population dynamics in an extreme environment: controlling factors in talus soil at 3750 m in the Colorado Rocky Mountains(Ley et al., 2004) Chris Andersen
Introduction • High elevation talus slopes are extreme environments for life. They contain oligotrophic cold soils, with very little microbial biomass, that are key components of water catchment areas that supply drinking water. • Purpose: • Evaluate the seasonality of Carbon (C) inputs to talus and microclimate characterized by soil moisture and temp. • Determine how these factors correlated with microbial biomass dynamics in vegetated and unvegetated soils.
Approach • Characterize unvegetated vs. vegetated soils • Measure C inputs to soils from eolian dust and measure photosynthetically active radiation (PAR) as a proxy for Photosynthesis • Estimate miomass of two microbial functional using substrate induced respiration method • Glutamate Mineralizers (GM) • General heterotrophs • Largest functional group in tundra soil • Salicylate Mineralizers (SM) • Specialized group of fungi in unvegetated soils
Unvegetated soil coarse textured, very little organic matter low water retention capacity low level of nutrients low levels of microbial biomass. Vegetated soil loamy Texture 8-16% organic matter higher water retention higher microbial biomass Description