Biogeophysics: The Effects of Microbial Processes on Geophysical Properties of the Shallow Subsurface

The geologic record suggests the presence of microbes on Earth as early as the Precambrian (Hall-Stoodley et al., 2004). Microbes are involved in practically every aspect of earth evolution. The term microbe is a general descriptor for tiny organisms that individually are too small to be seen with the unaided eye. Microbes may include bacteria, archaea, fungi, and protists. Viruses are also included as a major type of microbe, although there is some debate whether viruses can be classified as living organisms. The role microbes play in altering environmental systems is well documented in many biogeochemical studies. Notable is the role of microbes in water-rock interactions (Chapelle and Bradley, 1997). Field observations and laboratory experiments suggest that bacteria can accelerate silicate weathering either by direct contact with minerals or by producing organic and inorganic acids that enhance the dissolution of silicates (Heibert and Bennett, 1992). Thus, microbes are able to directly alter mineral surface chemistry and pore water chemistry over short to geologic time scales. Microbial induced changes in water-rock-regolith environments over variable time scales cause changes in the physical properties of these environments that may be detected and measured using geophysical methodologies