AN ALTERNATIVE EXPLANATION TO THE INITIAL CARBON ISOTOPE EXCURSION AT ST. AUDRIE’S BAY DURING THE END-TRIASSIC MASS EXTINCTION AS INDICATED BY BIOMARKER ANALYSIS

The end-Triassic marks one of the largest of six recognised mass extinctions. One of the most studied sections is at St. Audrie’s Bay, UK (SAB) where an iconic initial and main negative excursion in organic carbon isotopes (δ¹³C_org) were identified across this event. Similar isotopic patterns are observed across other widespread geographic locations with differing magnitudes and timings. These initial excursions have been largely attributed to the dissociation of methane clathrates very depleted in ¹³C. Recent investigations into the SAB show sedimentological and biotic evidence of a marine to fresh water transition with periodic exposure during the initial excursion. Three other investigated UK sections presented here (Whitehead, Lavernock, and St. Mary’s Well Bay) also show an initial δ¹³C_org excursion of similar timing and magnitude with evidence of similar environmental change, indicating the SAB δ¹³C_org and correlated oligohaline record is characteristic of a very large area. We also present a biomarker record of ecological and redox changes at SAB that suggests instead the initial excursion were the result of local biotic community changes resulting from a dramatic relative sea level drop and influx of fresh water. During the initial excursion the relative biomarker abundance of green algae increase whilst those of red algae, chrysophytes, and prasinophytes decrease. This period also sees reduction in C₂₈ 28, 30 bisnorhopanes and an increase in the Homohopane Index indicating a change in microbial communities as a result of a drop in water depth and increases in microbial activity, respectively. We also find greater abundance of hopanes (largely produced by bacteria) compared to steranes (predominantly attributed to eukaryotes) in what has already been determined a sub-oxic to aerobic environment indicating greater bacterial activity in a lacustrine environment. A simple interpretation of this data indicates the initial excursion is the result of bacterial community changes with increased bacterial activity, due to the efficiency of bacteria fractionation compared to eukaryotes. These recent results demonstrate the need for further multi-proxy biomarker investigations into other end-Triassic mass extinction, particularly those that demonstrate isotopic patterns very similar to SAB.