High-resolution biogeochemical study of organic-rich sediments from the Early Aptian oceanic anoxic event at Shatsky Rise, ODP Leg 198.

摘要

The formation of organic-rich sediments during episodes of oxygen-deficient conditions named Oceanic Anoxic Events (OAEs) has long been recognized. Yet, the precise causes and the controls on the development and termination of these OAEs remain enigmatic, mainly because of the limited recovery of their sedimentary records from world's ocean basins. Recent recovery of an Early Aptian (∼120 Myr) pelagic sequence in the northwestern Pacific at Shatsky Rise (ODP Leg 198, Site 1207) corresponding to the first Cretaceous OAE (OAE1a) offered the rare opportunity to examine in detail such an event. A suite of elemental, isotopic and molecular analyses were performed to assess the climatic conditions, sources of organic matter (OM), and characteristics of the depositional environment during the Early Aptian, and ultimately, to determine the mechanism(s) that triggered and sustained OAE1a at Shatsky Rise.; Elevated organic carbon contents (up to ∼40%), and a positive delta 13Corg excursion at the base of the recovered interval, attest to enhanced sequestration of OM linked to increased productivity during OAE1a. Assessment of sea surface temperatures (SSTs) using the TEX86 proxy reveals two cooling episodes of ∼4°C, which reflect significant temperature instability in the tropical Pacific, likely triggered by changes in carbon cycling induced by increased burial of OM. Molecular and isotopic analyses reveal that OM is derived from algal, bacterial and archaeal sources, including dinoflagellates, haptophytes, green sulfur bacteria, cyanobacteria and crenarchaeota. The 2-methylhopanoid index variations together with low delta15N values suggest that N2-fixing cyanobacterial populations changed from heterocystous to unicellular and nonheterocystous species because of climatic and redox changes during OAE1a. Inorganic geochemical and biomarker parameters suggest that the sediments were deposited during a period of high, albeit variable, primary productivity that far exceeded levels before and after OAE1a, and that the depositional environment experienced fluctuating redox conditions, which may have been coupled with changes in SSTs. Elemental enrichments provide support for the interpretation that significant quantities of metals released into the ocean during the emplacement of Ontong-Java and Manihiki igneous provinces caused an increase in biological productivity and ultimately, led to deposition of the organic-rich sediments during the Early Aptian.