Steryl ethers in a Valanginian claystone: Molecular evidence for cooler waters in the central Pacific during the Early Cretaceous? Simon C. Brassell ⁎ Biogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, IN 47405-1403, USA abstract article info Article history: Received 5 September 2008 Received in revised form 2 August 2009 Accepted 13 August 2009 Available online 27 August 2009 Keywords: Oceanic anoxic events Upwelling Paleoenvironment Biomarkers Biogeochemistry Zooplankton Coring on Shatsky Rise by Ocean Drilling Project Leg 198 recovered sediments rich in organic matter (>2.0% C org ) 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 δ 13 C isotopic excursion and with changes in plankton assemblages, including a demise in nannoconids. The δ 13 C excursion has been previously reported in the Pacific but the occurrence of Valanginian sediments rich in organic matter (> 1% C org ) 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The first well documented global episode of enhanced burial and preservation of organic matter (OM) during the Cretaceous occurs in the early Aptian (e.g. Schlanger and Jenkyns, 1976; Arthur and Schlanger, 1979; Jenkyns, 1980; Arthur et al., 1987, 1990; Bralower et al., 1994; Leckie et al., 2002), hence its designation as oceanic anoxic event OAE- 1a (Arthur et al., 1990; Bralower et al., 1994). It is also characterized by changes in plankton assemblages and its onset marked by a negative δ 13 C excursion in carbonates best documented in the Tethyan realm (Menegatti et al., 1998; Hochuli et al., 1999; Bellanca et al., 2002; Bersezio et al., 2002; Danelian et al., 2004; Erba and Tremolada, 2004 Heimhofer et al., 2004; Herrle et al., 2004), but also manifest in the Tethys–Atlantic seaway (de Gea et al., 2003), in the Atlantic (Herrle et al., 2004; Bralower et al., 1999), and in terrestrial sequences (Gröcke et al., 1999; Ando et al., 2002). The worldwide extent (Leckie et al., 2002) for these changes in the global carbon cycle is related to emplacement of the Ontong–Java Plateau (Larson and Erba, 1999; Weissert and Erba, 2004) and potentially methane release from hydrates (Jahren et al., 2001; Beerling et al., 2002; Jenkyns, 2003). A comparable episode of OM sequestration in the Jurassic is observed in the Toarcian (Beerling et al., 2002; Jenkyns, 2003) and an earlier Cretaceous event with similarities to OAE-1a occurs in the Valanginian, which has been named the Weissert OAE (Erba et al., 2004). A positive δ 13 C excursion in carbonates (Lini et al., 1992; Weissert et al., 1998; Erba and Tremolada, 2004; Weissert and Erba, 2004; Erba et al., 2004) accompanies this event, which also shows a decline in nannoconids (Erba and Tremolada, 2004). Records of this event are best expressed in the Tethyan realm (Lini et al., 1992; Channell et al., 1993; Weissert et al., 1998; Gröcke et al., 2003; Erba et al., 2004), including OM-rich (>1.0% C org ) intervals in the Vocontian Basin (Hennig et al., 1999) and the Southern Alps (Bersezio et al., 2002); they also occur in the Pacific(Erba et al., 2004). In the Pacific, an OM-rich sediment corresponding to this event was first recovered from Shatsky Rise (Core 1213B-15R-1; 2.54% C org ; Brassell et al., 2004) during drilling by the Ocean Drilling Program (ODP) Leg 198 (Bralower et al., 2002), prompting investigation of the characteristics and sources of its OM that can provide evidence helpful in assessing its depositional environment. The comparatively shallow depth of burial (324 mbsf) of this interval and the resultant immaturity of its OM facilitate molecular analysis of Palaeogeography, Palaeoclimatology, Palaeoecology 282 (2009) 45–57 ⁎ Fax: +1 812 855 7961. E-mail address: simon@indiana.edu. 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.08.009 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo