Constraining timing and origin of unusual carbon cycle dynamics in the terminal Proterozoic and middle Paleozoic Eons

Abstract

The global carbon cycle plays a critical role in maintaining an equable climate on Earth. Thus, studying its operation in deep time is a cornerstone in our understanding of the co-evolution of life and the surface environment. This thesis focuses on two time periods of non-uniformitarian change in the Earth system: (1) the Ediacaran Period (635 - 541 Ma), when animals first appeared in abundance in the rock record, and (2) the Silurian-Devonian transition (~420 Ma), when land plants began to establish a terrestrial biosphere.

Broadly coeval with these biotic innovations, measurements of &delta;¹³C on shallow marine carbonates provide evidence for large perturbations to the global carbon cycle. In the Ediacaran, the event is known as the ''Shuram'' excursion, with an implied perturbation that strains current paradigms to describe the carbon cycle. The meaning of Ediacaran &delta;¹³C records occupies the first three chapters of this thesis, which focus on the Wonoka Formation, a South Australian carbonate succession that hosts the putative Shuram excursion. Field relationships and development of both traditional (&delta;¹³C, &delta;<super>18</super>O, trace element abundances) and non-traditional ( &delta;<super>44</super>Ca, &delta;<super>26</super>Mg) geochemical datasets document the consistent basinal expression of chemostratigraphic signals. Results further demand that the unprecedented signals are products of the surface environment, and require explanatory models to explain their unique occurrence in the Ediacaran.

The final chapter focuses on the Silurian-Devonian, and couples a field-based geochemical dataset with U-Pb dating on zircon from the Helderberg Group of North America. The developed absolute age model allows the carbon fluxes and reservoir sizes needed to drive the associated positive &delta;¹³C excursion to be quantified. Results indicate that established models used to interpret carbon isotopic records can explain the chemostratigraphic observations, as long as the ocean carbon reservoir remained below 2× its modern size.

Although the following chapters cover diverse events in Earth history, they are united by a common approach. All research presented seeks a multi-disciplinary approach -- combining original field observations with the development of diverse stable and radio-isotopic datasets -- to constrain better the origin and timing of unusual carbon cycle dynamics in the deep past.