LINKING GEOMICROBIOLOGY WITH ICHNOLOGY IN MARINE SEDIMENTS

Abstract

In the last few years, the wealth of new information on microbial diversity, metabolic capabilities, and environmental constraints has led to significant insights into the ways in which microbes interact with their local environments. Indeed, microbes are integral to mineral dissolution, sorption and precipitation reactions, aqueous redox processes, and, ultimately, global elemental cycles. In this regard, they have helped shape our planet over the last 4 billion years and made it habitable for higher forms of life. The role of microbial communities in driving the major sedimentary diagenetic reactions is one aspect of geomicrobiology that has received significant attention. Through various chemoheterotrophic pathways, microorganisms are ultimately responsible for the conversion of organic carbon to CO more labile materials are degraded in the shallow subsurface on time scales of days to years; more refractory materials are broken down deeper in the sediment on time scales of hundreds to thousands of years, while the most resistant materials, precursors to fossil fuels, are transformed only on time scales of millions of years. Pore-water and mineralogical changes during diagenesis are also directly related to the bacterial reduction of aqueous species (O 2 ,N O3 ,S O4 2 ,C O 2) or metal oxyhydroxides in the sediment. The terminal electron-accepting process that occurs at any given depth depends on which oxidants are available and, in situations in which multiple electron acceptors are present, as in the uppermost sediment layers, on the free energy yield of the specific reaction. The decomposition of freshly deposited organic material thus proceeds in a continuous sequence of redox reactions, with the most electropositive oxidants being consumed at or near the surface and progressively poorer oxidants being consumed at depth, until the labile organic fraction is