A time in Earth history (~251 Ma) when life was all but snuffed out and from which the creatures most familiar to us eventually emerged is understandably revisited quite often. Causes ranging from impacts (no convincing evidence as yet), through flood-basalt emissions, catastrophic methane release, low atmospheric oxygen to ocean anoxia have all been proposed. Hesitantly, opinion is converging on a climatic crisis of some kind, and indeed the coincidence of both terrestrial and marine faunal and flora extinctions points to climate being the global transmitter of some cause or a coincidence of causes. After the waning of Southern Hemisphere glaciations, the late Permian was warm, even at high latitudes. Until recently, attempts at modelling the end-Permian climate have not been entirely convincing because of limitations in the models themselves. Jeffrey Kiehl and Christine Shields of the US National Center for Atmospheric Research in Colorado have assembled a model that couples land, atmosphere, oceans, sea-ice and palaeogeography for the period (Kiehl, J.T. & Shields, C.A. 2005. Climate simulation of the latest Permian: Implications for mass extinction. Geology, v. 33, p. 757-760).

The critical test for the model is running it with parameters for the near-present, and it performs well. Several lines of evidence point to a much higher CO₂ level in the Permian atmosphere, so this is the main input parameter. The outcome is a world with a mean surface temperature that is 8° C higher than now. Unlike today, there was no geographic hindrance to poleward heat transport, so the high mean temperature is reflected in the summer warmth and humidity of Permian high-latitude land. The sub–tropics on the other hand were scorching (around an average summer minimum of 51° C, 15° C higher than now); a clear contributor to minimising life there. Sea-surface temperatures at high latitudes are higher in the model outcomes, this warmth extending to depths of 3 km. Surprisingly, low-latitude sea temperature emerges as much the same as now. The model also suggests that seawater was saltier than now, and that results in greater uniformity of density with depth and location: a hindrance to bottomward circulation and mixing. There would probably have been no thermohaline circulation worth speaking of. The model helps confirm the likelihood of an oxygen-free lower ocean and little transfer of nutrients. The oceans too would have been inhospitable. A shutdown of biological productivity and therefore carbon burial would have accelerated warming. So, pushing the biosphere into a mass extinction would have been inevitable. The last straw may have been the additional stress of increasing acidity from sulphur dioxide emissions from the Siberian flood basalts.

Milankovich forcing and Early Jurassic methane

Periods of environmental crisis less severe than those leading to mass extinction appear throughout the fossil record. As well as minor extinction peaks they are often signified by departures of carbon-isotope records from long-lasting norms. Such a crisis appears in the d ¹³C record of the Early Jurassic, and is beautifully preserved in about 15 m of black shales on the North Yorkshire coast of England. Geoscientists from the Open University, UK and the University of Cologne, Germany have produced an extremely high-resolution time series of carbon-isotope data from the section (Kemp, D.B. et al. 2005. Astronomical pacing of methane release in the Early Jurassic period. Nature, v. 437, p. 396-399). The quality is sufficiently good to analyse the time series using Fourier analysis that yields the frequencies that contribute to the observed wave-like patterns in the data. Of course, the time in a stratigraphic time series is measured in metres, unless it is possible to calibrate the section by precise radiometric dating. The Yorkshire Jurassic contains only fossils and no dateable horizons, but the fine stratigraphic division based on ammonites is also widespread and calibration is possible from dates obtained elsewhere. The overwhelmingly dominant frequency in the carbon-isotope curve is 1.23 cycles m^-1, which represents 21 ka after the calibration of depth to time. That is the signal of precession of the equinoxes, part of the astronomical forcing bound up in Miliutin Milankovich’s theory of astronomical forcing of climate.

Astronomical pacing turns up throughout the stratigraphic column, wherever sediments are suitable for time-series analysis (steady, unbroken sedimentation), so a precessional signal is no great surprise. The important feature is the profundity of the d ¹³C excursions; a total of –7‰, largely accomplished by three abrupt shifts of –2 to –3‰. The first two coincide with bursts in extinctions. The most likely phenomenon to have produced these shifts is massive release of methane by destabilization of submarine gas hydrates. Emissions seem to have been blurting out on a regular basis as the Earth’s rotational axis precessed like a gyroscope. So, the complete time period was one in which gas hydrate was unstable, probably due to overall warming. Yet something else must have triggered vast releases three times. The Lower Jurassic extinctions link in time with massive magmatism in Southern Africa and Antarctic (the Karoo-Ferrar large igneous province). Perhaps especially large volcanic events there set the stage for large precessional methane releases. An alternative view is that volcanic emissions of CO₂ gradually produced enough widespread warming for the astronomical trigger to cause breakdown of gas hydrate simultaneously over very wide areas of the ocean floor. Other explanations have been suggested for the Lower Jurassic warming and carbon-isotope excursions, such as wildfires, impacts and connections with petroleum maturation and migration. The clear cyclicity rules them out.