UC Riverside

The Silurian is the shortest period of the Phanerozoic (~443.1-419 Ma) yet contains the highest frequency of repeated global carbon perturbation events associated with marine biotic crises for any similar time interval in Earth history. A largely unexplored and unknown aspect of Silurian marine ecosystems is the composition of the microbial communities that underpin marine food webs and mediate biogeochemical cycles. In order to characterize the marine microbial ecology and ocean chemistry throughout this time interval, I performed detailed lipid biomarker and stable isotope (δ13Ccarbonate, δ13Corganic, δ15Ntotal) investigations of Silurian age near-shore and distal marine paleoenvironments across a range of spatial and temporal scales.

First, I present detailed lipid biomarker and stable isotope stratigraphic records that were obtained for a 132-meter long drill core from Gotland, Sweden spanning the Llandovery-Wenlock boundary that were deposited in an open marine shelf setting. The expanded succession captures the full Ireviken Biogeochemical Event (IBE), including the faunal extinction events and the complete stable carbon isotope excursion. Our results demonstrate a distinct time series of events across the IBE which indicate a protracted deoxygenation of the local shelf setting due to the expansion of offshore oxygen minimum zones, fueled mainly by increased green algal productivity. Second, I present multiple lipid biomarker proxies obtained from a 400-meter long drill core from Podillya, Ukraine, deposited in a shallow-marine, reef-rimmed lagoonal tropical setting. The paleoenvironmental conditions favored sustenance of bacteria over algae in this nutrient-depleted, marine reefal habitat.

An important feature of the Silurian marine environment, and more broadly the early Paleozoic, was diminished dissolved oxidant availability in the oceans which induced mutually reinforcing effects that promoted enhanced methane oxidation and methanogenesis in marine waters and shallow sediments through the diagenesis of organic matter. Finally, I present the lipid biomarker assemblages from a large suite of over 580 well-preserved sedimentary rocks from over 20 localities and a series of simulations with an Earth system model (cGENIE) spanning the Phanerozoic Eon. Our data indicates high aerobic methane oxidation rates in the surface ocean during the Ordovician-Middle Devonian interval and a protracted ventilation of early Paleozoic oceans.