Scarcity of the C-30 sterane biomarker, 24-n-propylcholestane, in Lower Paleozoic marine paleoenvironments

摘要

24-n-Propylcholestane (24-npc), a C-30 sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae, occurs in certain Neoproterozoic rocks and oils and throughout the Phanerozoic rock record. This broad distribution leads 24-npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils. Here we report two significant hiatuses in the occurrences of 24-npc in the Lower Paleozoic marine rock record: the first in the Middle-Late Cambrian and the second in the Late Ordovician-early Silurian transition for a range of lithofacies (carbonates and siliciclastic rocks), organic carbon contents (both organic-lean and organic-rich), and paleoceanographic environments (shelf and deeper water marine settings) and observed offshore of two paleocontinents, Laurentia and Baltica. The Ordovician-Silurian gap is at least 9 million years, and possibly up to 20 million years, in duration. Robust older occurrences of 24-npc steranes in some Neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of Lower Paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton. Caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of Early Paleozoic time using lipid biomarker assemblages. (C) 2014 Elsevier Ltd. All rights reserved.