Discerning diagenetic pathways for discrete sterol precursors

Abstract

Studies of steroidal hydrocarbons in sedimentary sequences have helped elucidate the diagenetic fate of sterols, although discrete aspects of their transformation pathways remain unresolved. Steroidal hydrocarbons are prominent aliphatic hydrocarbons in Campanian to Paleocene sediments (∼78–63 Ma) recovered from the Transkei Basin (Hole U1581B), offshore South Africa, during IODP Expedition 392. Their distributions parallel those observed for other immature marine Cretaceous sediments, comprising C₂₇ to C₂₉ Δ⁴- and Δ⁵-sterenes and Δ^4,22- and Δ^5,22-steradienes, 20S and 20R diasterenes, partially rearranged sterenes (19-nor-5β-methylster-8-enes), and 5α- and 5β-steranes. In the upper Campanian through Danian sediment sequence, the proportion of C₂₈ steradienes (>60%) far exceeds that for sterenes and steranes (∼10–30%) likely reflecting major phytoplankton contributions of their C₂₈ Δ^5,22 sterol precursor. The composition of steroidal hydrocarbons changes systematically with depth matching the succession of proven diagenetic transformations, including progressive increase in 20S/(20S + 20R) diasterenes. The occurrence of partially rearranged sterenes (19-nor-5β-methylster-8-enes) and increasing amounts of diasterenes in the middle Campanian are accompanied by a concomitant decrease in the proportions of sterenes and steradienes. Increasing downcore proportions (∼20%) of C₂₈ steranes and diasterenes indicates a pathway for diagenetic hydrogenation of Δ^4,22- and Δ^5,22-steradienes to Δ⁴- and Δ⁵-sterenes, which are subsequently rearranged to diasterenes or reduced to 5α- and 5β-steranes dependent on the agency of clay catalysis. Hence, contributions from steradienes lead to the diagenetic evolution of the carbon number distributions of steranes and diasterenes as they gradually become an aggregate measure of all sterol precursors.