T19 Tropical Small Mountainous River Biogeochemistry: Terrestrial Losses, Internal Processing, Coastal Inputs, and Marine Burial

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room 102BC

CHEMICAL WEATHERING OF SERPENTINITE IN THE KLAMATH MOUNTAINS, CALIFORNIA

Paper number: T19-7
Program: Technical Sessions
Time: 9:35 AM – 9:50 AM

Authors

Abstract

Chemical weathering is an important process that is a critical component in soil formation, global carbon cycles, and controlling the chemistry of natural waters. While many studies have measured the dissolution rates of granite and basalt, relatively few studies have measured the dissolution rates of ultramafic minerals, such as serpentine. Previous laboratory dissolution studies of serpentine minerals have focused mainly on the dissolution of chrysotile and antigorite. Field studies of serpentine alteration have focused on secondary mineral formation, rather than the measurement of a weathering rate. Therefore, it is the goal of this study to characterize weathering of serpentinite from parent rock to soil in the Klamath Mountains of northern California and measure a dissolution rate of serpentine in a natural environment.

Serpentinite-derived soils and serpentinite rock cores were collected in depth profiles from the Trinity ultramafic body of the Klamath Mountains in northern California. Bulk chemical and mineralogical analyses of the serpentine soils and rock were performed using X-ray fluorescence (XRF) and X-ray diffraction (XRD). Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were also used to examine changes in the soil and rock. Biological activity reaction tests (BARTsTM) were conducted to test for the presence of iron-oxidizing bacteria, which may have an impact on serpentine weathering.

XRD results from one of the sampling sites indicate that both the soil and rock core are composed of lizardite, magnetite, and clinochlore; the top-most soil sample also contained quartz. Dust input therefore appears to be minimal, and chemical weathering appears to be dominated by dissolution of the parent minerals. The bulk chemistry of the soil and rock samples, as measured by XRF, indicate depletion of Si, Fe, and Mg with decreasing depth in both the soil and rock core, suggesting that chemical weathering has advanced beneath the soil surface into the rock itself.