Some geologists have argued that anoxia played a role in the first extinction pulse, though this hypothesis is controversial. In the early Hirnantian, shallow-water sediments throughout the world experience a large positive excursion in the δ34S ratio of buried pyrite. This ratio indicates that shallow-water pyrite which formed at the beginning of the glaciation had a decreased proportion of 32S, a common lightweight isotope of sulfur. 32S in the seawater could hypothetically be used up by extensive deep-sea pyrite deposition. The Ordovician ocean also had very low levels of sulfate, a nutrient which would otherwise resupply 32S from the land. Pyrite forms most easily in anoxic and euxinic environments, while better oxygenation encourages the formation of gypsum instead. As a result, anoxia and euxinia would need to be common in the deep sea to produce enough pyrite to shift the δ34S ratio.