John A. Fitzpatrick

The effect of temperature on experimental and natural chemical weathering rates of granitoid rocks

The effects of climatic temperature variations (5–35°C) on chemical weathering are investigated both experimentally using flow-through columns containing fresh and weathered granitoid rocks and for natural granitoid weathering in watersheds based on annual solute discharge. Although experimental Na and Si effluent concentrations are significantly higher in the fresh relative to the weathered granitoids, the proportional increases in concentration with increasing temperature are similar. Si and Na exhibit comparable average apparent activation energies (Ea) of 56 and 61 kJ/mol, respectively, which are similar to those reported for experimental feldspar dissolution measured over larger temperature ranges. A coupled temperature–precipitation model, using an expanded database for solute discharge fluxes from a global distribution of 86 granitoid watersheds, produces an apparent activation energy for Si (51 kJ/mol), which is also comparable to those derived from the experimental study. This correlation reinforces evidence that temperature does significantly impact natural silicate weathering rates. Effluent K concentrations in the column study are elevated with respect to other cations compared to watershed discharge due to the rapid oxidation/dissolution of biotite. K concentrations are less sensitive to temperature, resulting in a lower average Ea value (27 kJ/mol) indicative of K loss from lower energy interlayer sites in biotite. At lower temperatures, initial cation release from biotite is significantly faster than cation release from plagioclase. This agrees with reported higher K/Na ratios in cold glacial watersheds relative to warmer temperate environments. Increased release of less radiogenic Sr from plagioclase relative to biotite at increasing temperature produces corresponding decreases in 87Sr/86Sr ratios in the column effluents. A simple mixing calculation using effluent K/Na ratios, Sr concentrations and 87Sr/86Sr ratios for biotite and plagioclase approximates stoichiometric cation ratios from biotite/plagioclase dissolution at warmer temperatures (35°C), but progressively overestimates the relative proportion of biotite with decreasing temperature. Ca, Mg, and Sr concentrations closely correlate, exhibit no consistent trends with temperature, and are controlled by trace amounts of calcite or exchange within weathered biotite. The inability of the watershed model to differentiate a climate signal for such species correlates with the lower temperature dependence observed in the experimental studies.

U-series dating of impure carbonates: An isochron technique using total-sample dissolution

Abstract U-series dating is a well-established technique for age determination of Late Quaternary carbonates. Materials of sufficient purity for nominal dating, however, are not as common as materials with mechanically inseparable aluminosilicate detritus. Detritus contaminates the sample with extraneous Th. We propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution (TSD). Experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age. Results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures. The effects of correcting via leachate-residue pairs and isochron plots were assessed. Isochrons using TSD gave best results, followed by isochron plots of leachates only.

The Solubility and Stabilization of Ikaite (CaCO3·6H2O) from 0° to 25°C: Environmental and Paleoclimatic Implications for Thinolite Tufa

We determined the solubility of ikaite from 0° to 25°C to model its saturation state in natural waters and test the hypothesis that it is the precursor of the calcite pseudomorphs in thinolite tufa of Quaternary Lake Lahontan. Reversible solubility at buffered

Ikaite precipitation by mixing of shoreline springs and lake water, Mono Lake, California, USA

P_{CO_{2}}

Forest calcium depletion and biotic retention along a soil nitrogen gradient

yields the following expression for the dissolution constant of ikaite:

Solute profiles in soils, weathering gradients and exchange equilibrium/disequilibrium

log K_{ikaite} = 0.15981 - 2011.1/T

U-series Dating of Authigenic Gypsum and Anhydrite: A new Sample-Dissolution Technique Using Cation-Exchange Resin

. Where

Aleutian Arc Fluid Geochemical Data

T = ^{\circ}K

Chemical weathering of a marine terrace chronosequence, Santa Cruz, California. Part II: Solute profiles, gradients and the comparisons of contemporary and long-term weathering rates

, and

Climatic Oscillations 10,000-155,000 yr B.P. at Owens Lake, California Reflected in Glacial Rock Flour Abundance and Lake Salinity in Core OL-92

0^{\circ}C \leq t \leq 25^{\circ}C