Search

The stratigraphic association of late Precambrian glacial deposits with shallow-marine carbonate successions, which are biased toward the warmer parts of the surface ocean, was known in Africa and elsewhere since the 1930's, but geologists under-appreciated it as a paradox in physical climatology. A physical explanation for global freeze-up emerged from one-dimensional energy-balance climate models in 1969, after which it was widely discussed and refined. In 1981, it was suggested that a global freeze-up could be self-terminating after millions of years through slow accumulation of atmospheric CO 2 , a greenhouse gas. The idea of a self-reversing global freeze-up, or 'snowball Earth,' was first applied geologically by Kirschvink (1992) based on paleomagnetic evidence in South Australia. He cited no paper in physical climatology and geologists remained unaware of developments in that science until after 2000. A self-reversing global freeze-up involves strong predictions in duration, synchroneity and termination state that have now been tested in the geological record. Inflections in growth rate and disciplinary diversity of peer-reviewed papers in 1997 implies that climate physics and geology were galvanized to test both the Siderian (ca 2430 Ma) and Cryogenian (717−635 Ma) snowball Earth hypotheses by new field research conducted in South Africa and northern Namibia, respectively. • A Siderian snowball Earth (SBE) was first inferred from African paleomagnetic data. • Tandem Cryogenian SBEs were most forcefully posed by African carbonate stratigraphy. • SBEs have been tested by geochronology, geochemistry, sedimentology & climate models. • SBEs followed supercontinent break-up and flood-basalt volcanism on the paleoequator. • Preexisting polar−alpine biota survived SBEs and were ancestral to later surface life. [ABSTRACT FROM AUTHOR]

The balance of evidence suggests that oxygenic photosynthesis had evolved by 3.0–2.7 Ga, several hundred million years prior to the Great Oxidation ≈2.4 Ga. Previous work has shown that if oxygenic photosynthesis spread globally prior to the Great Oxidation, this could have supported widespread aerobic ecosystems in the surface ocean, without oxidising the atmosphere. Here we use a suite of models to explore the implications for carbon cycling and the Great Oxidation. We find that recycling of oxygen and carbon within early aerobic marine ecosystems would have restricted the balanced fluxes of methane and oxygen escaping from the ocean, lowering the atmospheric concentration of methane in the Great Oxidation transition and its aftermath. This in turn would have minimised any bi-stability of atmospheric oxygen, by weakening a stabilising feedback on oxygen from hydrogen escape to space. The result would have been a more reversible and probably episodic rise of oxygen at the Great Oxidation transition, consistent with existing geochemical evidence. The resulting drop in methane levels to ≈10 ppm is consistent with climate cooling at the time but adds to the puzzle of what kept the rest of the Proterozoic warm. A key test of the scenario of abundant methanotrophy in oxygen oases before the Great Oxidation is its predicted effects on the organic carbon isotope ( δ 13 C org ) record. Our open ocean general circulation model predicts δ C org 13 ≈ − 30 to −45‰ consistent with most data from 2.65 to 2.45 Ga. However, values of δ C org 13 ≈ − 50 ‰ require an extreme scenario such as concentrated methanotroph production where shelf-slope upwelling of methane-rich water met oxic shelf water. [ABSTRACT FROM AUTHOR]

Abstract: Existing correlation schemes between early Paleoproterozoic successions divorce the low-latitude Makganyene glaciation in southern Africa from the Great Oxidation (GO), as recorded by the disappearance of mass-independent fractionation of S-isotopes (MIF-S) in sedimentary sulfide and sulfate minerals. They also suggest a younger age for the GO in southern Africa (~2.3Ga) compared with North America (~2.4Ga), which is physically implausible. A new correlation scheme is proposed in which the Makganyene glaciation is temporally linked to the GO and to the second of three Huronian glaciations in North America, the one with postglacial cap-carbonate. In the new scheme, only three glacial epochs are needed globally, all three are represented in southern Africa, and the second was a circa 2.40Ga snowball Earth coincident with the GO. [Copyright &y& Elsevier]