The molecular record of Cryogenian sponges – a response to Antcliffe (2013)

MOLECULAR fossils or lipid biomarkers, which are preserved in ancient sedimentary rocks and petroleum that have undergone a mild thermal history, have been studied for over five decades (e.g. Eglinton et al. 1964, 1966; Blumer and Snyder 1965) and can yield valuable and unique insights concerning past ocean redox structure, plankton successions and biospheric evolution (e.g. Brocks and Pearson 2005; Peters et al. 2005; Knoll et al. 2007; Briggs and Summons 2014, for recent reviews). Currently, the earliest compelling hydrocarbon biomarker record for Metazoa comes from a ~100 million year record reported by Love et al. (2009) of distinctive Neoproterozoic– Cambrian steroids known informally as 24-isopropylcholestanes (24-ipc), which are derived from sterols produced in abundance by demosponges (McCaffrey et al. 1994). The most extensively documented record for these compounds extends to interglacial sedimentary rocks of the Cryogenian period (713–635 Ma) in a thermally immature succession of Neoproterozoic – early Cambrian marine sedimentary rocks from the Huqf Supergroup of the South Oman Salt Basin (SOSB). The concentration of biomarkers varies in different sedimentary rocks due to lithological and thermal maturity controls, so as well as reporting absolute abundances of 24-ipc steranes detected in each rock sample, Love et al. (2009) normalized their data and documented changes in the ratio of 24-ipc to the structurally similar (see Fig. 1), but analytically resolvable, sterane isomer 24-n-propylcholestane (24-npc), which originates principally from the sterols of marine pelagophyte algae (Giner et al. 2009). 24-npc is typically the dominant C30 sterane detected in marine Phanerozoic oils and rocks from all lithologies (Moldowan 1984; Moldowan et al. 1985, 1990), and there is no evidence that 24-npc steranes can be significantly converted into 24-ipc steranes, or vice versa, either during low temperature diagenesis or higher temperature burial maturation (e.g. Moldowan et al. 1990; Summons et al. 1992; McCaffrey et al. 1994; Zhang et al. 2000). Demosponges, particularly those from the order Halichondrida (Love et al. 2013), are the only living taxon known to produce their major sterols with a 24-ipc carbon skeleton. 24-ipc-related compounds can constitute >99% of the total sterols in membrane lipids in the demosponge Pseudaxynissa (Hofheinz and Oesterhelt 1979), and 24-ipc is among the major sterols in other genera such as Topsentia, Collocalypta, Halichondria and Epipolasis. Love et al. (2009) studied 64 source rock samples encompassing carbonates, siltstones and shales, of appropriate thermal maturity for biomarker preservation (verified independently by Rock-Eval pyrolysis screening). This set of samples provided stratigraphic coverage of all formations of the Neoproterozoic–Cambrian Huqf Supergroup available in drill core. In a related study, Grosjean et al. (2009) verified the anomalously high relative abundances of 24-ipc in a large suite of SOSB rocks as well as in petroleum samples produced from various SOSB formations, while Kelly et al. (2011) found similar patterns of 24-ipc steranes in Neoproterozoic oils from east Siberia, including some oils potentially sourced from putative Cryogenian source rocks. Anomalously high abundances of 24-ipc steranes are also characteristic features of certain Neoproterozoic – early Cambrian oils and rock extracts from India (Peters et al. 2005), Pakistan (Grantham et al. 1987) and Australia (McCaffrey et al. 1994). These rocks and oils possess unusual biomarker characteristics which are also prominent in SOSB oils and rocks (e.g. C29 sterane dominance in the C27–C30 range, high 24-ipc/npc ratio, presence of an unusual mid-chain methylalkane series) and which are only commonly detected together in Neoproterozoic – early Cambrian samples as noted previously (Peters et al. 1995, 2005; Love et al. 2009; Kelly et al. 2011). The SOSB biomarker