Assessing marine communities and carbon cycling that sustained shallow-marine ecosystems on Silurian, reef-rimmed carbonate platforms

A detailed middle Silurian to Early Devonian biomarker stratigraphic record was obtained from thermally well-preserved strata, sampled from a > 400 m long drill-core 25 from Podillya, Ukraine, deposited in the Podillyan peri-continental basin. These lipid biomarker records provide useful insights into the temporal changes in microbial ecology and ocean chemistry for this ancient, tropical shallow-marine reefal carbonate platform. The paleoenvironmental conditions favored sustenance of bacteria over algae (as indicated by elevated hopane/sterane ratios (average: 10.7, maximum value of 36.9); and appreciable 2α-methylhopanes and 3β-methylhopanes) in this nutrient-depleted, marine reefal habitat. The setting behind the reef tract in a lagoon and muted nutrient supply from the open ocean likely contributed to the persistently low primary productivity and low net biomass over millions of years of deposition. The most prolific algal primary producers were green algae consistent with a C₂₉ sterane dominance. No detectable contributions of 24-n-propylcholestane (24-npc) and/or other C₃₀ regular sterane compounds were found with GC–MRM-MS. Elevated 3β-methylhopane index values (3-MeHI; range: 3–13%, mean: 6.4%) were observed throughout the core interval, which is a characteristic associated with source contributions from methanotrophic bacteria and indicative of an enhanced marine methane cycle. This extends the previous findings of consistently high abundances of 3-methylhopanes for Ordovician marine environments to include a tropical, marine shelf setting persisting through the Silurian Period.