BIOGEOCHEMICAL WEATHERING OF SERPENTINITE IN BATCH AND FLOW-THROUGH DISSOLUTION EXPERIMENTS

Biological processes, specifically mediated by microbial communities, are closely tied to mineral weathering and element cycling. Serpentinites exert strong ecological controls on the biota that develop on them. The bulk chemistry of serpentinite is high in trace metals such as Cr, Ni, Co, and Cu, and has a very high Mg:Ca ratio. This causes an extreme and stressful environment for plants and microbes, that has been extensively studied by ecologists. However, the extent to which these organisms influence serpentine soil development is not understood. In order to address this question, we are conducting laboratory studies to examine how iron-oxidizing bacteria impact weathering rates of serpentine minerals.

We hypothesize that iron-oxidizing bacteria enhance serpentine mineral dissolution, contributing to early serpentinite weathering reactions in bedrock. To test this, crushed lizardite grains were reacted under both biotic and abiotic batch conditions. Biotic reactors contained Acidithiobacillus ferrooxidans, a species common in acid mine drainage. Experiments were each conducted for 2 weeks over a pH range of 1.5 to 4.0. Preliminary results suggest that the bacteria do not have a significant effect on magnesium release.

Additionally, whole rock dissolution experiments are being conducted in mixed flow-through reactors using crushed serpentinite from Deer Isle, Maine. These reactors, which run for several weeks, more closely simulate a natural environment but are more complicated due to the presence of several mafic minerals. These experiments allow us to regulate pH, prevent bacteria from being enveloped in waste products, and provide a higher iron content to the bacteria, which may not be satisfied by the lizardite.

Samples from all experiments will be analyzed for a suite of major nutrients and trace metals using ICP-MS. Magnesium and silicon release rates will be used to calculate dissolution rates of lizardite in both biotic and abiotic experiments. Iron-oxide precipitates will be analyzed with SEM-EDS. Measured dissolution rates in the presence of microorganisms will be compared to abiotic dissolution rates.