INTRUSIVE VOLCANIC ACTIVITY CAUSING INITIAL EOCENE GLOBAL WARMING BY EXPLOSIVE RELEASE OF LARGE VOLUMES OF GREENHOUSE GASES FROM METAMORPHIC AUREOLES

Intense volcanic intrusive activity have likely caused global warming by release of large volumes of greenhouse gases from organic-rich sedimentary basins several times in Earth history, e.g., during the initial Eocene (55 Ma), the Toarchian (183 Ma), and at the Permian/Triassic boundary (250 Ma).

Emplacement of hot (>1000 oC) basaltic magma into a sedimentary basin will cause heating and metamorphic reactions in a thermal aureole around the intrusion. The width of the temperature aureole around a thick (>50 m) sheet-like intrusion in a low-permeability host rock is typically of similar magnitude as the intrusion itself. However, the nature of the metamorphic reactions do strongly depend on the host rock lithology and the composition of the pore fluids, in addition to variables such as temperature, pressure and time. Large volumes of methane and other greenhouse gases will be produced if magma is intruded into organic-rich sedimentary rocks. The gas production will lead to a local pressure build-up, possibly causing hydrothermal eruptions and release of greenhouse gases to the atmosphere.

Recent seismic mapping shows that Paleogene sills are present in a region of >85,000 km2 in Cretaceous shale-rich basins in the Norwegian Sea. We have estimated that the total production potential of carbon gases in the intruded region is 500-2500 Gt carbon, being sufficient to cause or trigger the Initial Eocene Thermal Maxima (IETM). More than 800 hydrothermal vent complexes have been identified in the same region, having typical diameters of 1 to 5 km. In total, 2-3000 vent complexes are likely present in this region. These cylindrical structures likely represent the transport conduits for the greenhouse gases formed in the aureoles as they connect the sills with the Top Paleocene paleosurface. New analyses of samples from a commercial borehole penetrating one of the vent complexes reveal a strong thermal impact in the chimney and a biostratigraphic age of the vent complex corresponding to the Paleocene/Eocene boundary (formed within the A. Augustum zone). More than 95% of the vent complexes terminate at the same stratigraphic level, showing that the intense part of the intrusive episode was short lived. An improved constraint on the timing of the IETM and the vent complex formation can likely be obtained by scientific drilling.