Vascular plant biomarker distributions and stable carbon isotopic signatures from the Middle and Upper Jurassic (Callovian-Kimmeridgian) strata of Staffin Bay, Isle of Skye, northwest Scotland

Abstract

The molecular and stable carbon isotopic composition of higher plant biomarkers was investigated in Middle to Upper Jurassic strata of the Isle of Skye, northwest Scotland. Aromatic hydrocarbons diagnostic of vascular plants were detected in each of nineteen sedimentary rock samples from the Early Callovian to Early Kimmeridgian interval, a succession rich in fossil fauna including ammonites that define its constituent chronozones. The higher plant parameter (HPP) and higher plant fingerprint (HPF) calculated from the relative abundance of retene, cadalene and 6-isopropyl-1-isohexyl-2-methylnaphthalene (ip-iHMN) exhibit several large fluctuations throughout the Skye succession studied. These molecular profiles contrast with both (1) the more uniform profiles previously observed in Jurassic successions, putatively of the same age, from other palaeogeographical settings, including the Carnarvon Basin, Western Australia and (2) the steady rise in global sea level during this interval. This suggests that the HPF profiles of Jurassic marine successions may not be reliable indicators of global climate change. Our results indicate that other factors such as local tectonism, resulting in changes to the relief and landscape of the hinterland, likely influenced the palaeoflora and the mode of transport of its detritus into adjacent marine depocentres. However, the Skye succession showed similar d13C profiles of total organic carbon (TOC; comprising mainly fossil wood), the vascular plant biomarker retene and a predominant phytoplanktonic biomarker (phytane). The apparent isotopic relationship between terrigenous and marine-derived biomarkers supports a strong coupling of the atmosphere and ocean. The maximum isotopic excursion occurs in the Cardioceras cordatum ammonite biozone of the Early to Middle Oxfordian, which may be indicative of changes in atmospheric and oceanic levels of CO2.