Organic matter provenance and paleoenvironment in the Cretaceous on the Manihiki Plateau, South Pacific

Three holes at Site 317 on the High Manihiki Plateau were drilled in 1973 as part of the Deep Sea Drilling Project (DSDP). This plateau is of special interest because of its organic-rich shale formation during its early history, potentially linking paleo-depositional conditions and water depth or land exposure. A multi-tracer approach was used to analyze target organic-rich layers of Hole 317A core 16, Sections 2 and 3 from the early Cretaceous. This included bulk elemental and stable carbon isotope compositions of specific chemical classes (lipids, humic acids, kerogen), as well as individual biomarkers separated by thin layer chromatography and analyzed by gas chromatography-mass spectrometry. The major natural product contributions found in the core sediments were derived from three primary sources: algal (low molecular weight (< C20) n-alkanoic acids, n-alkanols and n-alkanes), bacterial (2-hydroxyalkanoic acids and hopanoids), and terrestrial (high molecular weight (> C22) aliphatic lipids, phytosterols, diterpenoids). Algal- and bacterial-derived compounds were observed at the shallowest layers of the core sample (680.30 m below sea floor), and decreased with increasing core depth. Terrestrial inputs increased down core and peaked between 680.33 and 680.70 m below sea floor. This trend coincides with a thick sapropel layer and agrees with the Oceanic Anoxic Event (OAE) in the early Cretaceous, indicating deposition in a shallow water environment in proximity to land (e.g., atoll).