PhD: Searching for geological evidence for the co-evolution of life and carbonate weathering

The chemical weathering of terrestrial land surfaces occurs at the interface of the hydrosphere, geosphere and biosphere. It is a critical process within both the rock cycle and as a component of the global chemostat, regulating global element cycles, including atmospheric CO2, by mediating the release of solutes. It is known that a variety of modern organisms contribute to weathering both directly (such as plant roots which secrete organic acids and chelates that promote the dissolution of primary minerals) and indirectly (such as animal bioturbation changing the surface area of weathered surfaces). There is strong reason to hypothesise that the initial evolution of these biotic innovations had profound effects on the chemical weathering factory; variably changing and deepening the critical zone of weathering, trapping and binding weathering by-products (muds), assisting the development of the first organic carbon-rich soils and potentially enhancing production of carbonic acid, so promoting rock weathering. A better understanding of the evolution of carbonate weathering will yield profound insights into the role of chemical weathering on our biotic planet.

For the first four billion years of its history, Earth was a fundamentally different planet to that which we know today: the only life on land (if any) was provided by mats of microbes. During the early Palaeozoic, this previously stable Earth state underwent two dramatic and unidirectional biogenic shifts: firstly, the origin and subsequent evolution of land plants with their roots and symbiotic mycorrhizal fungi caused dramatic and irreversible changes to Earth surface processes; secondly, infaunal bioturbation fundamentally reshaped the complexity of the physical boundary between the lithosphere and atmosphere. The global sedimentary-stratigraphic record may yield a physical record of innovation and change in weathering proxies: a focussed interrogation of such promises to reveal new insights into the role of biology in underpinning surface processes in the modern Earth system.

The student will develop and test a hypothesis that from the Palaeozoic onwards life dramatically enhanced the amount of physical (e.g., bioturbation, mechanical rooting effects) and chemical (e.g., vegetation induced or amplified) rock weathering in terrestrial environments. The student will search the global rock record for spatial and temporal trends in development of carbonate features such as karst weathering surfaces, dripstone cements in limestones, and production of carbonate cements in sandstones. The student will use a proven combination of field and laboratory studies to explore the ancient carbonate record, beginning with construction of a database from a literature search, and then using this to select the appropriate global field sites for sedimentological and geochemical investigations. Trends identified from this search will be verified using appropriate statistical and computer modelling techniques.