ARS | Publication request: WEATHERING RATES OF A FELDSPAR-QUARTZ FRACTION ISOLATED FROM AN ARID ZONE SOIL

Submitted to: Chemical Geology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 1996
Publication Date: N/A

Interpretive Summary: Knowledge of the weathering rates of silicate minerals is necessary for determination of the effect of enhanced atmospheric CO2 on the terrestrial carbon budget. Dissolution of silicate minerals results in a net sink of inorganic carbon. Previous laboratory studies have consisted of obtaining specimen mineral samples, grinding and reacting with water under controlled conditions for up to 3-4 months. In this study we isolated a feldspar mineral fraction from a soil and reacted with water, with and without pretreatment for up to 3-years. Reaction rates decreased with time. After 3 years rates were almost 1,000 times slower than previous published rates. Our rates are also consistent with field estimates of weathering based on mass balance in watershed studies. We conclude that laboratory experiments can be designed to represent "field" conditions but that previous published laboratory rates were not representative of field conditions. Silicate weathering, while important, contributes less to inorganic carbon sequestration than estimates from previous lab studies.

Technical Abstract: Numerous studies have established that laboratory determined weathering rates are several orders of magnitude greater than rates derived from field studies at the catchment or basin scale. The laboratory rates have been determined on specimen minerals or rock isolates which were subsequently crushed. Some samples were pretreated to remove organic coatings and possible carbonate contamination . Using magnetic, density, and size separation, we isolated a feldspar-quartz fraction from an arid zone soil, (classified as Pachappa, Mollic Haploxeralf). Rates determined from reaction after up to 4 years were in the range 0.015 x10-12 mol m-2 s-1, comparable to published field rates when field rates are recalculated for estimated surface roughness. These data are in contrast to faster laboratory determined rates based on crushed or ground samples, or shorter reaction times. Based on laboratory studies with and without various pretreatments,we conclude that surface coatings due to natural weathering are not the explanation for these low reaction rates.