HYDROTHERMAL VENTING OF GREENHOUSE GASES TRIGGERING EARLY JURASSIC GLOBAL WARMING

Abstract

The climate change in the Toarcian (Early Jurassic) was characterized by a major perturbation of the global carbon cycle. The event lasted for approximately 200,000 years and was manifested by a global warming of ~ 6 °C, anoxic conditions in the oceans, and extinction of marine species. The triggering mechanisms for the perturbation and environmental change are however strongly debated. Here, we present evidence for a rapid formation and transport of greenhouse gases from the deep sedimentary reservoirs in the Karoo Basin, South Africa. Magmatic sills were emplaced during the initial stages of formation of the Early Jurassic Karoo Large Igneous Province, and had a profound influence on the fate of light elements in the organic-rich sedimentary host rocks. Total organic carbon contents and vitrinite reflectivity data from contact aureoles around the sills show that organic carbon was lost from the country rocks during heating. We present data from a new type of geological structures, termed breccia pipes, rooted in the aureoles within the shale of the Western Karoo Basin. The breccia pipes are cylindrical structures up to 150 meters in diameter and are mainly comprised of brecciated and baked black shale. Thousands of breccia pipes were formed due to gas pressure build-up during metamorphism of the shales, resulting in venting of greenhouse gases to the Toarcian atmosphere. Mass balance calculations constrained by new aureole data show that up to 1800 Gt of CO2 was formed from organic material in the western Karoo Basin. About 15 times this amount of CO2 (27,400 Gt) may have formed in the entire basin during the intrusive event. U-Pb dating of zircons from a sill related to many of the pipes demonstrates that the magma was emplaced 182.5 ± 0.4 million years ago. This supports a causal relationship between the intrusive volcanism, the gas venting, and the Toarcian global warming. © 2007 Elsevier B.V. All rights reserved.