SMALLER, BETTER, MORE: FIVE DECADES OF ADVANCES IN GEOCHEMISTRY. PART II - ORGANIC GEOCHEMISTRY

Many of the discoveries made in geochemistry over the last fifty years have been driven by technological advances that have allowed analysis of smaller samples, attainment of better instrumental precision and accuracy or computational capability, and automation that has provided many more data. These advances occurred during development of revolutionary concepts, such as plate tectonics in geology and the genomics era of biology, which have provided an over-arching framework for interpreting many geochemical studies. The branch of geochemistry that deals with fossilized organic molecules had its origins in elucidating the processes and pathways that led to petroleum formation. As awareness of the richness and diversity of organic compounds that can be preserved in sedimentary rocks grew, this gave way to the broader endeavor of molecular paleobiology. Despite great challenges in tying specific biomolecules to groups of organisms, or to metabolic processes, as well as issues of preservation mechanisms, molecular paleobiology remains a prime approach for studying the history of microorganisms, which have been the dominant life form for most of Earth’s history and yet are rarely preserved in the fossil record. The concept of ‘biomarker chemostratigraphy’ has since emerged as a way of using these molecules to chart evolutionary innovation, mass extinctions and radiations, chemical events in the ocean, and climate change. The biochemical diversity of relatively simple life forms, including bacteria and archaea, affords a wealth of distinctive lipid biomarkers that can be used to evaluate the composition of microbial communities in ways that complement inferences that come from genomic surveys. The stable cores of these molecules inform us about the evolution of metabolisms over Earth history, including oxygenic and anoxygenic photosynthesis, methanogenesis, methanotrophy and sulfate reduction. These records have been tied into stable isotope variations of the light chemical elements (C, H, N, O, S) and provide a broad view of biogeochemical evolution.