Date of Award

8-2013

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Amanda Olsen

Second Committee Member

Jean MacRae

Third Committee Member

Andrew Reeve

Abstract

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 conducted a laboratory study to examine how iron-oxidizing bacteria impact weathering rates of a serpentine mineral.

We hypothesized 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 conditions. Biotic reactors contained Acidithiobacillus ferrooxidans, a species common in acid mine drainage. Experiments were conducted at initial pH values ranging from 1.5 to 4.0 at increments of 0.5.

Samples from all experiments were analyzed for a suite of major nutrients and trace metals using ICP-MS. Magnesium release rates were used to calculate dissolution rates of lizardite in both biotic and abiotic batch experiments. Rates were calculated using a method after Hausrath and Brantley (2010), using two weeks of data for each experiment. No significant difference was found between abiotic and biotic dissolution rates. Rates were also calculated using the initial rate method, for the first eight hours of dissolution, when concentration versus time data was linear, and the pH of the solutions had not drifted substantially. During this early dissolution, a significant difference was found between abiotic and biotic rates.

Post-experiment mineral examination using SEM-EDS showed that mineral grains from both biotic and abiotic experiments were coated in iron oxides. Experiments starting at a higher pH were covered more extensively, but the formation of secondary coatings blocked surface sites equally in both abiotic and biotic experiments.

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