Six years ago vast areas of Indonesia caught fire after an unusually dry phase in the El Niño – Southern Oscillation (ENSO). Burning forest and peat deposits swathed a vast area in smoke, but another alarming aspect was the greatest addition of carbon dioxide to the atmosphere in half a century. Such a wildfire on a global scale is thought to have marked the end of the Mesozoic, perhaps triggered by the K-T impact event and encouraged by higher oxygen content in the atmosphere. Present oxygen levels seem to be at a balance that staves off spontaneous combustion of green vegetation, but only a few percent more would render vegetation much more prone to bursting into flame. The end of the Palaeocene involved a sudden global warming that coincides with a decrease in the proportion of 13C in marine carbonates. Since photoynthesis, at the base of the trophic pyramid, favours light 12C, such a negative d13C “spike” is generally ascribed to an unusually high release of organic carbon to the environment. The end-Palaeocene warming may have resulted from a massive release of methane from gas-hydrate buried in shallow seafloor sediments (See Methane hydrate – more evidence for the ‘greenhouse’ time bomb and Plankton and the end of the Palaeocene-Eocene global warming August and October 2000 issues of EPN). However, massive burning of living biomass could also produce the carbon-isotope signal. Telling the two mechanisms apart requires information from other organic-related cycles. One key is comparing the carbon- and sulphur-isotopic records that enables the place in which carbon had been stored geologically. For marine burial, the effect of aerobic bacteria that completely oxidises hydrocarbons back to carbon dioxide and water needs to have been suppressed. Periods of massive marine carbon burial coincide with oceanic anoxia episodes, when anaerobic bacteria beneath the seafloor reduce dissolved sulphate ions to sulphides, thereby depositing lots of iron sulphide (pyrite) in black organic mudrocks. This sequesters sulphur that is depleted in 32S into marine sediments, so that the marine carbon- and sulphur-isotope records fluctuate in a clearly related way. During the Palaeocene this relationship is absent, while overall the carbon isotopes do signify progressive burial of organic carbon. The decoupling of the two cycles points to carbon burial on the continents, forming peat and eventually coal deposits.
Playing games on Snowball Earth
For as long as anyone can remember there has been a parade of geoscientific bandwagons in town. Three of the floats today carry banners saying, “Snowball Earth”, “Climate models” and “continental erosion and CO2 drawdown”. Of course there is serious science aboard each, but they are getting overcrowded, especially as separate bands try to jump from one to another. When it sometimes seems, as now, that the “next Big Thing” is some way off, we get the unseemly spectacle of some bands trying to straddle two or even several of the wagons. Three is quite a feat, yet the 18 March 2004 issue of Nature contains perhaps not a vast human pyramid, but at least a tetrahedron of the genre (Donnadieu, Y. et al. 2004. A “snowball Earth” climate triggered by continental break-up through change in runoff. Nature, v. 428, p. 303-306). From about 1100 to 750 Ma ago, the bulk of continental lithosphere was gathered in a supercontinent known as Rodinia (from the Russian for “Mother Earth”). By analogy with modern Eurasia, and the stratigraphic record from the Phanerozoic Pangaea supercontinent, the centre of Rodinia would almost certainly have been dry, being so far from the ocean. Break-up of that continental mass would also probably have allowed moist maritime air to penetrate over a larger proportion of the fragments. The hypothesis that Donnadieu and colleagues try to test using linked geochemical and climate models is that such a tectonic change would increase continental weathering and reduce the “greenhouse” effect. The weak acid formed by solution of carbon dioxide in rain water can provide hydrogen ions to break down silicate minerals. The reactions contribute bicarbonate and soluble metal ions to surface and subsurface water. Ultimately, both reach the oceans and contribute to its chemistry. If conditions are suitable, calcium ions in particular combine with bicarbonate to precipitate calcium carbonate on the ocean floor, either through the action of organisms or inorganically. The two chemical equilibria involved result in a net burial of one carbon atom out of the two involved in the weathering, thereby drawing down carbon dioxide from the atmosphere. The climate model used in their cyber-experiment resolves the Neoproterozoic Earth into cells that are 10 x 10 degrees (about 100 thousand km2) and considers Rodinia at 800 Ma and the result of its break-up at 750 Ma, the time of the first good evidence for extensive low-latitude glaciation. The results, after some tinkering, suggest that increased continental weathering could have reduced CO2 levels to 250 parts per million. Taking account of a 6% less energetic Sun at the time, this would have produced sufficient cooling for ice caps to exist to sea level at the equator. So, taken at face value, the hypothesis seems plausible. However, there are major snags. First, in a mere 50 million years their model sees continental dispersion on a scale that has not yet happened to Pangaea in about 200 Ma of Phanerozoic time. Second, since continental area remains constant, the proportion of rainfall, and therefore weathering and runoff, involving continental crust also stays fixed. Third, continental weathering refers to the crystalline part of its crust, in which there are unstable minerals, such as feldspars, that can do the chemical trick. We have little idea how much of the continents at that time was veneered by sediments that are the products of earlier chemical weathering, and contribute nothing to the process. Exposing such deep crust depends to a large extent on mountain building, which continental extension does not encourage. Fourth, carbon dioxide is not the only source of hydrogen ions that are involved in weathering, especially as much of it goes on in groundwater – bacterial action and oxidation of iron sulphides create much more acid conditions that rainwater. Fifth, and most important, where is the complementary geochemical evidence? Feldspars of the continental crust, on which the hypothesis mainly rests, have high contents of rubidium compared with their oceanic counterparts, and they are old. Much of Rodinia was underpinned by crust formed as far back as 4 billion years ago. Prolonged decay of 87Rb to radiogenic 87Sr makes the strontium isotopes of continental material very different from those of the ocean floor – it has a much higher 87Sr/86Sr ratio. Since soluble strontium would be released to runoff by continental weathering, that signature makes its way to the ocean and should pop up in marine carbonates. Although the ocean strontium isotopes in the Neoproterozoic did rise a little, it did not peak until the very end. In fact, the details show that the periods around supposed “snowball” conditions involved downturns in radiogenic strontium supply to the oceans. Whatever the model suggests, all that it amounts to is the equivalent of a table-top train set