Silicate Dissolution in Las Pailas Thermal Field: Implications for Microbial Weathering in Acidic Volcanic Hydrothermal Spring Systems

Abstract

A longitudinal field microcosm study was conducted in the Las Pailas hot spring system, located on the SW flank of Rincon de la Vieja, Costa Rica, in order to investigate initial microbial attachment and colonization, as well as chemical (abiotic) and biological silicate weathering under hydrothermal conditions. Solution chemistry was pH = 2.42–3.96, T = 43–89.3°C, Si = 4.45–8.19 mmol L⁻¹, Fe = 1.50–6.95 mmol L⁻¹and PO³⁻ ₄ = below detection limits-4.9 μmol L⁻¹. Microcosms consisted of washed, sonicated primary silicate samples in polycarbonate vessels. The vessels were enclosed either by mesh to observe water/rock/microbial interactions or by 0.2–0.45 μm filters to observe water/rock interactions. Microcosms were incubated for periods of 6 h, 24 h, or 2 mo, fixed in the field, then analyzed in the laboratory. Scanning electron microscopy (SEM) analysis revealed that microbial attachment to mineral samples occurred in as little as 6 h. Microbial colonization and the development of minor etch pits associated with microorganisms occurred within 24 h. The most significant differences in chemical vs. biological weathering were observed after 2 mo. SEM analysis of these incubated surfaces showed that volumetric losses to mineral samples were more than one order of magnitude greater for samples that had been colonized by microorganisms and thus weathered biologically. With time, preferential colonization of anorthoclase mineral samples with Fe-oxides and apatite inclusions occurred. Subsequent weathering, therefore, may be a metabolic strategy by microorganisms to access mineral-bound PO³⁻ ₄, which is otherwise scarce in solution. Results from this study suggest that microorganisms may play a significant role in weathering in some hydrothermal systems.

Keywords:

• microbial weathering
• hydrothermal fluid
• phosphate
• scanning electron microscopy

Acknowledgments

We would like to thank the two anonymous reviewers whose careful revisions improved this work. This work would not be possible without the financial support of the Costa Rica/United States of America Foundation for Cooperation (CRUSA) to (awarded to Jennifer Roberts and Marielos Mora), and KU Geology Associates Grant (awarded to David Fowle), as well as the support of the Costa Rica National Parks Service, which provided access to the site. Microscopy and fieldwork support were provided by a Geological Society of America Student Research Grant, an Ernest Angino Geochemistry Scholarship, and an ExxonMobil Student Research Grant (awarded to Charity Phillips Lander). We thank Victor Day for his technical assistance with XRD analyses and Annette Summers Engel and Geoffrey Lander for consulting on microcosm construction.