Back to Table of Contents

43135-AC2
Biogeochemical Profiles of Climatic and Environmental Changes Inducing Deposition of Organic-Rich Sediments during the Early Aptian

Simon C. Brassell, Indiana University (Bloomington)

A critical challenge for organic geochemical studies of 'oceanic anoxic events' such as the Early Aptian Oceanic Anoxic Event (OAE1a) is to determine temporal changes in environmental variables such as oxygenation levels, primary production, and biota, associated with enhanced preservation of organic matter. At Shatsky Rise (ODP Site 1207), which lay in the central tropical Pacific Ocean 120 Myr ago, the temperature record reconstructed using the TEX₈₆ molecular proxy exhibits fluctuations of 4-6°C that suggest tropical climatic instability during OAE1a. Investigation of biomarker distributions through this sediment interval indicates changes in the sources of organic matter that appear to vary with the observed temperature shifts. Specifically, there appear to be differences in the proportions of 2-methylhopanes relative to hopanes between the warm and cooler time intervals. 2-methylhopanoids, which have been widely reported in sedimentary rocks, provide a signature for cyanobacterial contributions, especially when their occurrence is coupled with low δ¹⁵N values (< -2‰) consistent with N₂fixation. Thus, the abundance of 2-methylhopanoids in OAE1a sediments from Shatsky Rise reflects substantial cyanobacterial contributions to OM, recorded by high values for 2-MHI (up to 45%; ratio of 2-Mehopanes to hopanes), which is substantiated by the low δ¹⁵N values indicative of N₂ fixation by cyanobacteria occur the OAE1a interval. However, 2-MHI values increase during time intervals characterized by cooler sea surface temperatures and oxygenated waters, indicating that environmental changes during OAE1a likely affected cyanobacterial populations. In modern marine environments higher temperatures favor filamentous non-heterocystous cyanobacteria as the dominant N₂-fixing organisms, and exclude heterocystous species, whereas unicellular cyanobacteria are favored by low pO₂. Thus, 2-MHI variations during OAE1a suggest that N₂-fixing cyanobacterial populations changed during these episodes of perturbation of the carbon cycle. The environmental coupling of 2-methylhopanoids, cyanobacteria speciation, and O₂ levels during OAEs also bears on their relationships at other times throughout geologic history, especially during evolutionary diversification of cyanobacteria on the early Earth. The proposition that 2-MH originated in heterocystous cyanobacteria that perform N₂ fixation helps explain their occurrence in the Late Archean during a global-scale expansion of oxygenated habitats, and their scarcity in anoxic Paleoproterozoic sequences. The presence of isorenieratane in OAE sequences that contain 2-MH suggests changes in populations of green sulfur bacteria and cyanobcteria as environmental conditions alternately favored anoxygenic and oxygenic photosynthesis, respectively. Perhaps changes in phytoplankton communities during OAE may re-enact the dynamic interchange of these organisms that first accompanied the transition to an oxygenated world. This interpretation requires biosynthesis of 2-MH by N₂-fixing heterocystous cyanobacteria prior to the initial accumulation of atmospheric O₂, and the fact that these heterocystous cyanobacterial groups are phylogenetically more evolved members of the clade also argues against the interpretation that specific groups of cyanobacteria post-date the Great Oxygenation Event.

Back to top