Paleoproterozoic snowball Earth: Extreme climatic and geochemical global change and its biological consequences

Geological, geophysical, and geochemical data support a theory that Earth e xperienced several intervals of intense, global glaciation ("snowball Earth " conditions) during Precambrian time. This snowball model predicts that po stglacial, greenhouse-induced warming would lead to the deposition of bande d iron formations and cap carbonates. Although global glaciation would have drastically curtailed biological productivity, melting of the oceanic ice would also have induced a cyanobacterial bloom, leading to an oxygen spike in the euphotic zone and to the oxidative precipitation of iron and mangane se. A Paleoproterozoic snowball Earth at 2.4 Giga-annum before present (Ga) immediately precedes the Kalahari Manganese Field in southern Africa, sugg esting that this rapid and massive change in global climate was responsible for its deposition. As large quantities of Oz are needed to precipitate th is Mn, photosystem II and oxygen radical protection mechanisms must have ev olved before 2.4 Ga. This geochemical event may have triggered a compensato ry evolutionary branching in the Fe/Mn superoxide dismutase enzyme, providi ng a Paleoproterozoic calibration point for studies of molecular evolution.