Paper No. 3
Presentation Time: 2:10 PM
IRON MINERAL FORMATION IN MICROBIAL MATS FORMED FROM SHIELD BRINES ALONG AN OXIDATION-REDUCTION GRADIENT
BRISCOE, Lindsey J., Earth Sciences, University of Minnesota, 108 Pillsbury Hall, 310 Pillsbury Drive SE, Minneapolis, MN 55455, ALEXANDER Jr., E. Calvin, Department of Earth Sciences, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, ALEXANDER, Scott C., Earth Sciences, Univ of Minnesota, 108 Pillsbury Hall, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, BERQUÓ, Thelma, Concordia College, Moorhead, MN 56562, GRALNICK, Jeffrey, Department of Microbiology, Biotechnology Institute, University of Minnesota, 356 Gortner Laboratory, 1479 Gortner Ave, St. Paul, MN 55108, MICHEL, F. Marc, Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24060, MOSKOWITZ, Bruce, Institute for Rock Magnetism, Univ. of Minnesota, Minneapolis, MN 55455, SALOMON, Christine E., Center for Drug Design, University of Minnesota, 7-146 Phillips Wangensteen, 516 Delaware St. SE, Minneapolis, MN 55455 and TONER, Brandy M., Soil, Water, and Climate, University of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, Saint Paul, MN 55104, hatz0006@umn.edu
Iron oxidizing microorganisms occur in diverse geologic settings that share a common characteristic: opposing gradients of reduced iron and oxygen. Iron-rich deposits are important because they have the potential to record biogeochemical information at the time of formation and have great influence on iron cycling in the environment throughout a number of geochemical settings. The goal of this research is to identify the iron minerals forming within geochemical gradients, and determine whether microbial activity alters the resulting deposits. Unraveling the master variables of this type of environment will afford us better understanding of the iron cycling within microbe-rich environments across geochemical gradients in the deep biosphere. The study of present day processes can help us interpret mineral deposits in the geologic record, and better understand biogeochemical conditions in the past.
In this study, we are measuring the products and rates of iron cycling in calcium chloride shield brines. These anoxic ground waters generate iron-rich foams, crusts and siderothems along an anoxic-oxic transition. Geochemistry changes drastically along the shallow brine flow channels that reach ~100m in length providing diverse facies transitions and mineralogy.
The physical and chemical properties of these deposits has been characterized by a suite of tools including magnetism, Mossbauer spectroscopy, X-ray scattering, iron 1s extended X-ray absorption fine structure spectroscopy (EXAFS), and scanning electron microscopy (SEM). SEM of critical point dried samples shows that microbial biomass is a substantial component of the mat at 0.3 m from a borehole where low oxidation-reduction potential is observed. Magnetism analysis reveals a variety of iron oxyhydroxides such as ferrihydrite, goethite and trace magnetite across a continuum of particle sizes (nano- to micro-meter). X-ray scattering results have identified specific formations composed of pure 6-line ferrihydrite. For these samples, Mossbauer spectroscopy was performed at 300K and 4.2K. X-ray scattering also confirmed, along with EXAFS, the presence of akaganeite in all samples, as well as a transition from biogenic iron oxyhydroxide to ferrihydrite/goethite.
