A METHANE FUSE FOR THE CAMBRIAN EXPLOSION: CARBON CYCLES AND TRUE POLAR WANDER

In addition to biological diversification, the Cambrian Explosion is characterized by anomalously high rates of apparent plate motion, (True Polar Wander, TPW), and by a series of large perturbations in the carbon isotopes that dwarf any others observed through the past 65 million years. We suggest that these biological, tectonic, and geochemical events are intimately related in the following fashion. First, tropical continental margins and shelf-slopes which formed during fragmentation of Rodinia accumulated massive quantities of light organic carbon during late Neoproterozoic time, as indicated by heavy isotope ratios in inorganic carbon during interglacial intervals. Second, an initial phase of Vendian TPW moved these organic-rich deposits to high latitude, where conditions favored trapping biogenic methane in layers of gas hydrate even as shallow as ~300m, and perhaps in permafrost. Continued sedimentation during late Vendian time added additional hydrate/gas storage volume and stabilized the underlying units until the geothermal gradient moved them out of the clathrate stability field, building up deep reservoirs of highly-pressurized methane. Finally, a burst of TPW brought these deposits back to the Tropics, where they gradually warmed and were subjected to regional-scale thermohaline eddy variation and related sedimentation regime changes. Each reservoir reached a critical failure point independently at times throughout the Cambrian.

By analogy with the late Paleocene, these methane deposits yield transient, greenhouse-induced pulses of global warming when they erupt. As temperature has the most powerful association of any environmental parameter with biological diversity, repeated thermal pulses along with progressive disruption and alteration of global ocean circulation patterns by TPW could cause the increase in diversity that accompanied the radiation of metazoans. We suggest that a methane fuse ignited the Cambrian Evolutionary Explosion.