Steryl ethers in a Valanginian claystone: Molecular evidence for cooler waters in the central Pacific during the Early Cretaceous?

Coring on Shatsky Rise by Ocean Drilling Project Leg 198 recovered sediments rich in organic matter (> 2.0% Corg) from three time intervals: the Early Aptian, corresponding to oceanic anoxic event (OAE 1a), the Valanginian and the Berriasian. Episodes of enhanced sequestration of organic matter during the Valanginian are well documented in the Tethyan realm, notably those associated with a positive δ13C isotopic excursion and with changes in plankton assemblages, including a demise in nannoconids. The δ13C excursion has been previously reported in the Pacific but the occurrence of Valanginian sediments rich in organic matter (> 1% Corg) is unprecedented. A distinctive feature of the biomarker composition of these Valanginian sediments at Shatsky Rise is the presence of a suite of steryl ethers, which extends the temporal record of these compounds from the Neogene to the Early Cretaceous. The biological source of steryl ethers remains enigmatic, although the sedimentary occurrences of these compounds in Quaternary and Neogene environments characterized by cool water, high seasonal productivity, and/or nutrient enrichment by upwelling, in combination with their liquid crystal properties, raise the possibility that they may represent storage lipids, akin to wax esters and triacylglycerols. Prior occurrences of steryl ethers suggest that their presence in the Valanginian may reflect an oceanic environment characterized by upwelling, consistent with the likely position of Shatsky Rise within the equatorial divergence zone of the Pacific during the Early Cretaceous. Alternatively, or in addition, the appearance of these compounds may represent a biological response to cooler temperatures, and thereby augment evidence for global cooling during the Early Valanginian, consistent with independent paleontological evidence for contemporaneous, upwelling-induced increases in productivity in the Western Atlantic.