Biogeochemical Weathering of Serpentinites: An Examination of the First Reactions Controlling Serpentine Soil Formation.

Serpentinites, perhaps more than any other rock type, control the composition and evolution of the biota that develop on them.  The bulk chemistry of serpentinite rocks, high in Mg and trace elements and low in nutrients such as Ca, K, P, and N, creates an extreme and stressful environment for biota.  Since the role of parent material in soil formation is extremely important in young soils, and porosity generation is a crucial process in converting rock into a soil that can support life, incipient weathering of serpentinite rock likely has a strong effect on the development of serpentine ecosystems.  We are therefore examining the first reactions controlling weathering of serpentinite rocks and formation of serpentine soils using a combination of field, laboratory and modeling approaches.

            We are analyzing profiles from bedrock to soil surface at two locations, the Klamath Mountains, CA, and Pine Hill, Maine.  Results suggest that the relative dissolution of minerals within the profiles follows the order pyroxene > Fe-rich serpentine> Mg-rich serpentine > Al-rich serpentine.  The first mineral to dissolve within rocks, increasing porosity and therefore allowing additional weathering to occur, has been termed the “profile-controlling mineral”.  Even if present in small concentrations, therefore, this mineral may play a vital role in soil formation.  Fe-oxidizing bacteria have been detected within both the soils and weathered rock at the Klamath Mountains site, and experiments are ongoing to determine the effect of Fe-oxidizing bacteria on serpentine mineral weathering.  Laboratory dissolution experiments of serpentine minerals with organic acids suggest the importance of a trace element biosignature of biological interaction consisting of enhanced release of Fe, Al, Ni, Cr, Ti, and Co.  Preliminary reactive transport modeling to quantitatively interpret the processes controlling biogeochemical weathering of serpentinites from bedrock to soil surface suggest the preferential depletion of pyroxene within the weathering profile.