EVOLUTION OF THE DEVONIAN EARTH SYSTEM THROUGH THE LENS OF GEOCHEMISTRY: FROM CARBON CYCLING AND ANOXIA TO PLATE TECTONICS (Invited Presentation)

The Devonian Period marks a time of radical biologic and geologic change that fundamentally altered our earth system. Marine life radiated such that oceans and land saw the first major expansion of fish and land plants, but later doomed during the Late Devonian mass extinction, possibly by marine anoxia. These anoxic conditions are recorded by perturbations to the carbon cycle. The causes of the anoxic periods continue to be studied by workgroups focusing on details such as the severity of anoxia (sometimes producing euxinic conditions) and where in the world/water column were anoxic conditions locally present. Furthermore, oceans and marine life were perturbed by the radiation of land plants by profoundly changing weathering patterns and enhancing clay formation during soil development. Weathering rates also changed during the uplift and weathering of major mountain building events, such as the Acadian and Antler orogenies affecting Laurentia, to increase the riverine flux of siliciclastics to the ocean.

Traditional geochemical proxies such as carbon (δ¹³C) and sulfur (δ³⁴S) isotopes have been a workhorse for identifying potential periods of anoxia in deep time. Paired positive δ¹³C and δ³⁴S excursions recorded in older Cambrian and Ordovician successions are closely linked to biotic turnover and extinction events. However, the Devonian δ³⁴S record has only recently been the focus of similar detailed study. Curiously, no major δ³⁴S excursions occur alongside δ¹³C excursions during the Devonian mass extinction interval similar to older extinctions, suggesting that either anoxia was not as severe or that the sulfate reservoir was large enough to be buffered from brief anoxic periods. Though evidence for local anoxia exist in the form of changes in ichnofabrics, black shales, and I/Ca ratios of carbonate rocks, it is possible that anoxia was not as severe or global in extent. If weathering rates had increased to bury more organic carbon in nearshore settings, this could mimic the appearance of anoxia via increased δ¹³C values. Ongoing work using Li and Sr isotopes suggests that the radiation of land plants, and later uplift from orogenesis, occurred during these periods of anoxia. This has created a far more complicated story of the relationship extinction, anoxia, and the Devonian carbon cycle than previously thought.