THE ENVIRONMENTAL, CLIMATIC, AND EVOLUTIONARY LEGACY OF BIOMARKERS IN ANCIENT ORGANIC-RICH SEDIMENTS

The survival and persistence of molecular species in ancient, well-preserved organic-rich marine sediments provides an invaluable resource for recognition and elucidation of the structural characteristics of biomarkers in the geological record. More significantly, the stratigraphic occurrences, distributions, abundances and isotopic compositions of preserved suites of biomarkers all contribute to the understanding of their biological origins and sedimentary fate, which in turn enables their assessment as proxies of environmental, climatic and evolutionary change. Exploration of the diverse assemblage of biomarkers continues to provide new evidence of their varied biological sources and demonstrates the diversity of bacterial constituents, including the role of cyanobacteria among the plankton, as indicated by the prevalence of 2-methylhopanoids in mid-Cretaceous sediments formed during oceanic anoxic events. Environmental changes shown by stratigraphic variations in these compounds attest to fluctuations in plankton assemblages potentially related to nutrient availability. Biomarkers also reflect ocean climate. For example, the occurrence of sterol ethers appears to be restricted to Quaternary and Neogene sediments associated with upwelling systems, which may reflect the environment of occurrence of their, as yet unknown, source organisms. However, extension of the relationship to colder waters to ancient settings, suggests that their apparent prevalence in Oligocene sediments may be interpreted as a response to global cooling. Similarly, perhaps their presence in a late Valanginian organic-rich interval from the mid-Pacific reflects cooler conditions during this interval of the early Cretaceous. Evidence from sediments containing exceptionally well-preserved organic matter also aids extend the temporal ranges of biomarkers and elucidation of the progression of evolutionary changes in molecules over time. Thus, the first occurrence of specific biomarkers may be associated with biosynthetic innovation, such as the recognition of alkenones in lower Aptian now extended to ~120 Ma, and related to evolutionary changes in calcareous nannoplankton.