cap carbonates. Firstly, the high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic, and dissolve any carbonates contained within—starkly at odds with the deposition of cap carbonates. Further, the thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin. An alternative mechanism, which may have produced the Doushantuo cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48 ‰— values—as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments. Changing acidity Isotopes of the element boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation. This may indicate a buildup of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form carbonic acid. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation. Space dust Earth's surface is very depleted in the element iridium, which primarily resides in Earth's core. The only significant source of the element at the surface is cosmic particles that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An iridium anomaly has been discovered at the base of the cap carbonate formations, and has been used to suggest that the glacial episode lasted for at least 3 million years, but this does not necessarily imply a global extent to the glaciation; indeed, a similar anomaly could be explained by the impact of a large meteorite. Cyclic climate fluctuations Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to Pleistocene ice age cycles than to a completely frozen Earth. In addition, glacial sediments of the