T129 Microbial Mediation of Authigenic Minerals: From Precipitation to Dissolution

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room 211AB

IRON MINERAL FORMATION IN MICROBIAL MATS FORMED FROM SHIELD BRINES ALONG AN OXIDATION-REDUCTION GRADIENT

Paper number: T129-4
Program: Technical Sessions
Time: 2:10 PM – 2:25 PM

Authors

Abstract

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.