BIOGEOCHEMICAL RECORDS OF MARINE REDOX CONDITIONS

The production and survival of biomarkers in marine environments is linked intrinsically with oxygenation levels in both the oceanic water column and underlying sediments, which has prompted efforts to recognize molecular and isotopic characteristics indicative of different depositional conditions, and transitions between them. An understanding of the environmental requirements, including the supply of essential elements, necessary to sustain the source organisms of biomarkers is also critical. Moreover, the temporal distribution of individual biomarkers associated with particular environments reflects evolutionary changes in the source biota favored in such settings. Thus, isorenieratane, which derives from sulfur bacteria and indicates photic zone anoxia, first appears in Proterozoic marine sediments and reoccurs intermittently in Phanerozoic deposits, including Mesozoic Oceanic Anoxic Events (OAE). 2-methylhopanoid biomarkers derived from cyanobacteria are prevalent in the Proterozoic and also occur as prominent constituents in Lower Aptian (OAE1a) coupled with isotopic evidence for nitrogen fixation, and in other sediments. Constituents from Archaea dominate sediments corresponding to Albian OAE1b, whereas the prominence of 28,30-bisnorhopane in Cenomanian/Turonian sediments from both Tethys and Brazilian marginal basins suggests a distinct ecological niche for their bacterial source during OAE2. However, the co-occurrence of biomarkers that reflect euxinic, anoxic, dysoxic, and oxic conditions in individual sediments from OAE attests to the spatial variability and dynamic fluctuations in redox conditions over short, perhaps seasonal, time scales during these episodes.