BIOMARKERS CONFIRM CYANOBACTERIA AS A MAJOR SOURCE OF PLANKTONIC ORGANIC MATTER DURING THE EARLY APTIAN

Organic-rich sediments deposited during the early Aptian Oceanic Anoxic Event (OAE1a), one of the global episodes of enhanced carbon sequestration during the mid-Cretaceous, were recovered at Shatsky Rise during ODP Leg 198. They contain exceedingly high organic carbon contents (up to 34.7%) and consist of finely laminated radiolarian claystones (Site 1207), and clayey and radiolarian porcellanites (Site 1213).

Molecular analyses attest to the algal and bacterial origin of the organic matter (OM) and its immaturity, which is indicated by the presence of various functionalized biomarkers, including alkenones, sterones and tocopherols. The occurrence of abundant steroidal components, particularly sterenes and sterones, and of alkenones, suggests that the OM has major contributions from eukaryotic algae, including haptophytes. The individual carbon isotopic compositions of these individual compounds, and of lycopane, all range between -28.4 ‰ and -32.5 ‰; such values are among the lowest ever reported for phytoplankton biomarkers.

Prominent components of bacterial origin within the sediments include suites of 2-methylhopanes and 2-methylhopanones indicative of cyanobacterial contributions to the OM. The carbon isotopic compositions of the hopanes (-28.4 to -29.8 ‰) and 2-methylhopanes (-29.0 to -30.7 ‰) are similar, which suggests that both may derive from cyanobacteria. The values of the 2-methyhopanoid index range from 0 - 39 % for hopanes and 21 - 68% for hopanones. In each sample, the ratios for individual hopanoid homologues are similar. The biosynthesis of 2-methylhopanoids by cyanobacteria is always accompanied by production of hopanoids. Thus, these organisms are clearly significant contributors of hopanoids in the lower Aptian sediments. Therefore it seems likely that cyanobacterial contributions of hopanoids in the sedimentary record may have been underestimated, which has implications for nitrogen cycling through geologic time.