Field work at the Nunnock River site, Australia, 2001

Field work at the Tanana River site, Alaska, 2002

The rates, mechanisms, and products of chemical weathering of minerals in silicate-dominated rocks are of fundamental importance because chemical weathering is a key step in formation of soils and sediments, and a determinant of water chemistry.
 

We began work on weathering by analysis of the chemistry and structural state of reaction products through characterization of the  mineral-reaction product interface  in weathered granite with atomic resolution (e.g., Banfield and Eggleton 1988). This work, like the vast majority of mineralogical and geochemical research on weathering of crustal rocks to date, focused on study of inorganic weathering reactions.

It is clear that microorganisms impact mineral weathering in numerous ways.  In addition to chemical (release of acids, organic complexing agents, etc.) and enzymatic (catalysis of redox reactions) effects, organisms contribute to physical weathering processes.

Our goal is to understand the nature of microbe-mineral interactions that lead to geochemical chanage at the level of pathways and genes.   The first step was to identify the form of the interaction, and the range in magnitude of the various effects.

Thus, ten years ago, we began to study of microbe-mineral interactions by characterization of the intact interface between lichens (symbioses of fungi plus algae and bacteria) and minerals at high resolution (down to the atomic scale; e.g., Barker and Banfield, 1996). Subsequently, laboratory studies were used to quantify the processes inferred to be important. For example, high-resolution imaging and chemical analyses indicate intimate association between mineral surfaces, extracellular polymers, and clays. Dissolution experiments were conducted to measure the pH-dependent effect of model polysaccharides on mineral dissolution rates and secondary products (Welch et al. 1999, Welch et al., 2001). Effects range from suppression of dissolution to up to a thousand times increase in dissolution rates for feldspars. We have also measured the effects of dissolved organic acids and quantified pH gradients in proximity to cell surfaces (Barker et al. 1998) so that experimental rates can be applied to natural systems.

Recently, we have begun to characterize the inorganic and microbial contributions to granite weathering and soil formation.  We are applying molecular biological techniques used in the sulfide  AMD geomicrobiology work to identify organisms within microbial communities in the soil and saprolite (weathered granite with intact granitic texture).  We will also continue to isolate organisms and conduct experiments with these to test hypotheses about the impact of microbes in the profile on weathering reactions and, conversely, mineralogical and geochemical controls on microbial populations.

An important new direction in our work on biogeochemical weathering of granite is the formation of a new collaboration with geomorphologists, including Prof. Bill Dietrich, UC Berkelely.  Our goal is to link their larger scale studies of landscape evolution involving transport models and cosmogenic nuclide dating with geochemical studies of the profile and process occurring within it to quantitatively evaluate the contributions of dissolution in the saprolite and soil to landscape lowering, and to evaluate how these vary with landscape position and curvature.

To date, most of our work has been carried out in southern New South Wales, Australia.  The first site, referred to as the  Bemboka site, located at the top of the escarpment, has been studied by us for many years.  We have characterized the mineralogy, geochemistry, and total microbial population size. The second site, referred to as the Nunnock River site, was studied in terms of curvature, soil thickness, and landscape lowering rate by Heimsath, Dietrich, and colleagues.  It is 3 km from the Bemboka site, and below the top of the escarpment.  Recently, we began a detailed geochemical, mineralogical, and microbiological characterization effort at the Nunnock River site so that both data sets were available from one site.  This involved an extensive sampling effort along the ridge. In addition, we dug a trench through the soil and saprolite to provide access to the saprolite at depth and to improve our understanding of the soil-saprolite sequence across the ridge (see photos above).  Future plans include more extensive surveying to determine the form of the rock-saprolite interface.

Dissolution of insoluble phosphate minerals by microorganisms

We have focused on the impact of microorganisms on the dissolution of phosphate minerals, including apatite and secondary phosphate minerals  that are commonly encountered in saprolite and soil.  This is an important problem because bioavailability of phosphorus limits the productivity of many soils, and "insoluble" secondary phosphates are an important reservoir for phosphate in some cases.  The solubilization of these minerals was demonstrated in the field. (Taunton et al. 2000).  Ongoing experiments are revealing the details of the mechanisms by which microbes access phosphate bound in these phases.

The Tanana River site, south of Fairbanks, Alaska