Biomarkers in terrestrial organic matter from the Lower Devonian to Oligocene: Evidence from selected regions of the laurasian supercontinent

Abstract

Biomarkers are chemical fossils which are valuable in organic matter provenance determination, and higher plant specific biomarkers could be utilised to deduce the precursor higher plants sources and their evolution through geological time. Twenty (20) selected rock samples from different areas within the Laurasian Supercontinent, including plant fossils of Lower Devonian to Oligocene Period from Canada, Germany, Northern Ireland, and the United Kingdom were examined for their record of aliphatic and aromatic biomarkers specific to higher plants. This aims to determine land plant-derived molecular compounds and its variations within the studied ages, as well as attribute biomarkers to different plant contributors. Aliphatic land plant biomarkers, including tetracyclic diterpenoids (beyerane, kaurane, phyllocladane) as well as high terrestrial/aquatic ratio (TAR >1) and conventional odd-numbered long-chain n-alkanes, which are derived from land plants were detected. Other aromatic land plant biomarkers, including naphthalenes and cadalene, were also distinguished in the samples. The presence of land plant biomarkers (beyerane, kaurane, phyllocladane, cadalene) and their slight increase in the examined Laurasian Devonian to Triassic samples signify the abundance and diversity of higher plants. Vascular plants, including psilophyton, pertica, leclercqia, drepanophycus, and sawdonia dominated the Lower Devonian flora (in the Rhynie Chert, for example), contributing to the evolution of the terrestrial ecosystem, with the diversity of seed plants, appearance of conifers and tall trees in the Carboniferous. Gymnosperms that appeared during the Upper Carboniferous period became the dominant flora in the Triassic and Jurassic periods, signalled by the abundance and increase in the concentration of higher plant biomarkers. The slight decrease of biomarkers post-Middle Jurassic may reflect organic matter degradation and erosion or mass extinction of plants and possible reorganisation in the plant fossil record marked by the Triassic-Jurassic boundary. Since land-plant derived biomarkers are not expected routinely in marine environments, the detected plant biomarkers in Jurassic marine sediments (Oxfordian and Kimmeridge shales) from England are interpreted to have been caused by run-off of terrigenous carbon from land to the marine environment. The study has enhanced our understanding of the organic matter provenance, terrestrialisation process, and spatiotemporal evolution of higher plants across different geological settings of the studied areas within the Laurasian Supercontinent.