Biomarkers Record Ecological Responses to Environmental Crises During the Early Mesozoic

Abstract

The Mesozoic was crucial interval of marine algal evolution that recorded in molecular fossils, particularly lipid biomarkers. Algal biomarker turnover is significant at climatic/biotic perturbations. This thesis delves into three critical intervals: the End-Permian Mass Extinction (EPME) and Early Triassic, the Late Triassic Carnian Pluvial Episode (CPE), and the Toarcian Oceanic Anoxic Event (T-OAE). It explores these periods via the abundances, distributions, and isotopic compositions of bacterial and eukaryotic algal biomarkers. The goal is to gain insights into algal and bacterial ecosystems responded to environmental changes.
Throughout these study intervals, algal turnovers, as expressed by sterane distributions or sterane to hopane ratios, coincide with climatic/biotic crises. Concurrent changes are also observed in (methanotrophic/green-sulphur/cyano-) bacterial communities. In the Early Triassic, sterane and hopane analyses suggest an altered photoautotroph community in post-EPME successions, indicating a complicated reorganization of algal communities rather than a simple productivity change. During the CPE, a significant δ2H shift in leaf-wax n-alkanes corroborates previous descriptions of "mega-monsoon", reflecting hydroclimatic fluctuations, and steranes and hopanes suggest consequent impacts on algal and bacterial ecosystems. In the T-OAE, the Ordos Basin records 13C-depleted bacterial lipids (hopanoids), indicating an increased aerobic methane consumption during the carbon cycle perturbation of T-OAE. Collectively, these studies suggest a link between Mesozoic biotic/climatic perturbations and concurrent algal turnovers.
These findings highlight the intricate nature of Mesozoic climatic/biotic crises. Changes in algal or bacterial lipids, their distributions, abundances, and δ13C and/or δ2H signatures contribute to a more comprehensive understanding of the environmental dynamics during this critical phase in Earth's history. While C28/C29 ratios coevally pronounce the events, their variant values among sections suggest temporary and subtle reorganizations in algal ecosystems, masking long-term evolutionary signals. Consequently, the thesis results confirm the contributions of Mesozoic climatic/biotic crises to algal evolution, yet the mechanisms remain unclear.

Date of Award	8 Jul 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	B D A Naafs (Supervisor) & Rich D Pancost (Supervisor)