preferentially incorporate the lighter 12C isotope. Thus ocean-dwelling photosynthesizers, both protists and algae, tend to be very slightly depleted in 13C, relative to the abundance found in the primary volcanic sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower 13C/12C ratio within organic remains, and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will remain very slightly, but measurably, depleted in 13C. Silicate weathering, an inorganic process by which carbon dioxide is drawn out of the atmosphere and deposited in rock, also fractionates carbon. The emplacement of several large igneous provinces shortly before the Cryogenian and the subsequent chemical weathering of the enormous continental flood basalts created by them, aided by the breakup of Rodinia that exposed many of these flood basalts to warmer, moister conditions closer to the coast and accelerated chemical weathering, is also believed to have caused a major positive shift in carbon isotopic ratios and contributed to the beginning of the Sturtian glaciation. During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of 13C to 12C. Close analysis of the timing of 13C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic. Banded iron formations Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert. In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of Earth's atmosphere during the Palaeoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and precipitated out as iron oxide. The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today's atmosphere is oxygen-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits. For such iron-rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron (as ferrous oxide) could accumulate before it met an oxidant that would precipitate it as ferric oxide. For the ocean to become anoxic it must