Carbon burial in continental and marine settings: Lacustrine and marine records of major environmental change in deep time and their depositional and diagenetic consequences

Thesis

The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) is recognized as possibly the most extreme oceanographic and global climatic change event of the Phanerozoic. The T-OAE was marked by massive carbon release into the ocean–atmosphere system, widespread oceanic de-oxygenation, global organic-matter burial, and a strongly perturbed global carbon cycle. Although effects of the T-OAE are extensively studied in the marine realm, the environmental and climatic response on the continents is effectively unknown. In this thesis, I identify the first lacustrine record of the T-OAE, in the Da'anzhai Member of the Sichuan Basin (China), and demonstrate that a major lake system developed contemporaneously with the T-OAE, likely due to enhanced hydrological cycling under elevated atmospheric pCO₂. Increased lacustrine organic productivity from elevated fluvial nutrient supply resulted in ˜460 Gt of organic carbon burial in the palaeo-Sichuan lake alone, representing a major, but hitherto overlooked, carbon sink and negative feedback in the T-OAE global carbon cycle, and likely other similar events. Algal dominated biomass, deposited under a stratified water column during lake level rise, marks the Da’anzhai Member as potential hydrocarbon source rock. Given the challenges associated with Mesozoic stratigraphic correlation, I further present the first integrated (marine) stratigraphic reference framework for the complete Toarcian Stage from the Llanbedr (Mochras Farm) Borehole (Cardigan Bay Basin, UK), which allows precise correlation between the T-OAE and coeval Karoo-volcanism. I also show that accelerated global weathering rates during the T-OAE resulted in widespread mass transport deposits at different palaeolatitudes and changes in ocean chemistry leading to pronounced marine siderite deposition. Periodic smaller-magnitude carbon cycle perturbations in the Hettangian and Sinemurian were possibly in phase with short (∼100kyr) and long (~405kyr) eccentricity astronomical forcing, with the formation of organic-rich laminated black shales in the Bristol Channel Basin, similar to Mediterranean sapropels in the Pleistocene.

Authors: Xu, W

• DOI: 10.5287/ora-4r6rnrzbg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford