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Snowball Earth

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Zheng-Xiang Li
added 11 research items
Neoproterozoic (Sinian) sediments are exceptionally well preserved in the Three Gorges region (western Hubei Province) of the South China block. We report new paleomagnetic results, obtained independently by two separate laboratories, from a total of 157 samples of the 748±12 Ma, basal Sinian Liantuo Formation at its type locality. Detailed thermal demagnetization procedures and least-squares line analyses reveal three distinct magnetic components among the suite of samples. Two overprint components can be distinguished from each other by their laboratory unblocking temperatures. The first to be removed (‘C’), always annihilated below 600°C, is common throughout the dataset but is amenable to least-squares line-fitting in only 37 samples. It yields a pole which in present coordinates resembles Mesozoic overprints identified from previous studies in the Three Gorges region (75.7°N, 174.3°E, dp=6.0°, dm=8.3°, Q=4). The higher unblocking-temperature overprint (‘B’), always subsidiary to the ‘A’ component, is more prevalent than ‘C’ and was identified by line-fitting in 67 samples. The ‘B’ direction is very steep and generates a paleopole whose in situ coordinates do not resemble the Mesozoic–Cenozoic apparent polar wander path for South China, and whose tilt-corrected coordinates (20.3°N, 106.2°E, dp=7.2°, dm=7.3°, Q=5) bear no resemblance to any reliable Phanerozoic paleopoles from the South China block. The steep ‘B’ direction, if an unbiased representative of an ancient geomagnetic dipole field, was probably acquired some time in the 200 m.y. interval between deposition of the Liantuo Formation at ∼750 Ma and Cambrian time. The most stable component is a two-polarity remanence, removed at temperatures predominantly >630°C, which we infer to reside in hematite. A change in polarity of this component occupies a similar stratigraphic position (within 5 cm) among three outcrops separated by ∼100 m lateral distance. We calculate a mean paleomagnetic pole from each of the laboratories' datasets and combine these with a previously determined pole from correlative rocks in Yunnan [‘N1’ of Zhang and Piper, Precambrian Res. 85 (1997) 173–199], to obtain an overall weighted mean paleomagnetic pole (04.4°N, 161.1°E, A95=12.9°, Q=7) for the South China block at 748±12 Ma. The combined ‘Z1’ pole is considered to be primary based on its thermal stability, its magnetostratigraphic consistency, and a soft-sediment fold test determined by previous work. Results from individual sampling areas constrain the depositional paleolatitude of the Liantuo Formation and equivalent Sinian rocks to 30–40°. This result applies to one or both of the stratigraphically adjacent Chang'an and Nantuo glacial deposits; unfortunately, it is neither high nor low enough to refute any of the conceptual models for the enigmatic Neoproterozoic glaciations. The new basal Sinian paleopole, in the context of recent paleomagnetic and geochronological results from Australia, suggests that the Nantuo glaciation is pre-Marinoan. The new ‘Z1’ pole may also provide constraints on the various proposed reconstructions of South China's position in Rodinia. In particular, a paleoposition adjacent to northwestern Australia at ∼750 Ma requires a specific relative orientation between the two blocks. Likewise, if Rodinia were still intact by 750 Ma, South China may have lain between Australia and Laurentia only in an orientation different from that originally proposed in the ‘missing link’ hypothesis. As a final alternative, the new paleomagnetic data could be used to position South China, Australia, and Laurentia in an immediately post-Rodinian paleogeography around the nascent Pacific Ocean.
An interpreted primary magnetic remanence that passed a fold test has been revealed from six sampling sites in the pink ‘cap dolomite’ of the Neoproterozoic Walsh Tillite, southern Kimberley, northwestern Australia. A palaeomagnetic pole (WTC) at (21.5°S, 282.4°E) with dp=12.2° and dm=15.4°, based on results of the five best sites, gives a palaeolatitude of 45±12° for the sampling region, and ca 25±12° for the Adelaide Fold Belt where two Neoproterozoic glaciations occur. This palaeopole falls 58° away from the Marinoan glaciation (the Elatina Formation) pole, and the palaeolatitude for the region studied is around 30° higher than that predicted by the Marinoan and younger Neoproterozoic poles from Australia. If the WTC pole is accepted as a palaeopole at the end of the Walsh glaciation, the Walsh Tillite is thus likely to be of Sturtian (ca 770–750 Ma) rather than Marinoan age. A comparison of the revised late Mesoproterozoic to Neoproterozoic apparent polar wander path (APWP) for East Gondwana with that of Laurentia suggests that supercontinent Rodinia broke apart by about 750 Ma, after the Sturtian glaciation. The new data do not discriminate between the snowball Earth model and the high-obliquity Earth model for the Neoproterozoic time. The revised APWP suggests a ‘stop-and-go’ fashion of movement for East Gondwana during the Neoproterozoic.
We report here new geochronological and paleomagnetic data from the 802±10 Ma Xiaofeng dykes in South China. Together with existing data, these results suggest that Rodinia probably spread from the equator to the polar region at ca. 800 Ma, followed by a rapid ca. 90° rotation around an axis near Greenland that brought the entire supercontinent to a low-latitude position by ca. 750 Ma. We propose that it was the initiation of a mantle superplume under the polar end of Rodinia that triggered an episode of true polar wander (TPW) which brought the entire supercontinent into equatorial latitudes. An unusually extensive emerged land area at the equator increased both atmospheric CO2 drawdown and global albedo, which, along with waning plume volcanism led directly to the low-latitude Sturtian glaciation at ca. 750–720 Ma.
Ross N Mitchell
added 3 research items
[1] While cap dolostones are integral to the provocative Snowball Earth hypothesis, the current depositional model does not account for multiple geological observations. Here we propose a model that rationalises paleomagnetic, sequence‐stratigraphic and sedimentological data and supports rapid deglaciation with protracted cap dolostone precipitation. The Snowball Earth hypothesis posits that a runaway ice‐albedo can explain the climate paradox of Neoproterozoic glacial deposits occurring at low palaeolatitudes. This scenario invokes volcanic degassing to increase atmospheric greenhouse gases to a critical threshold that overcomes the albedo effect and brings the planet back from the ice‐covered state. Once this occurs, Earth should shift rapidly from a snowball to an extreme greenhouse. However, cap dolostone units overlying glacial sediments, typically interpreted as transgressive deposits, exhibit multiple magnetic reversals indicating they accumulated in >105 years. By reviewing modern post‐glacial systems, sequence stratigraphic concepts and principles of sedimentology, we suggest that cap dolostones are not restricted to the transgression but rather represent sediment starvation following a major landward shoreline migration associated with the demise of Snowball Earth. Thus, the duration in which cap dolostone accumulated is not directly coupled to the timescale of the Snowball Earth deglaciation.
Estimated at ~58 Myr in duration, the Sturtian snowball Earth (ca. 717‐659 Ma) is one of the longest‐known glaciations in Earth history. Surprisingly few uncontroversial lines of evidence for glacial incisions associated with such a protracted event exist. We report here multiple lines of geologic field evidence for deep but variable glacial erosion during the Sturtian glaciation. One incision, on the scale of several kilometers, represents the deepest incision documented for snowball Earth; another much more modest glacial valley, however, suggests an erosion rate similar to sluggish Quaternary glaciers. The heterogeneity in snowball glacial incisions reported here and elsewhere were likely influenced by actively extending horst‐and‐graben topography associated with the breakup of supercontinent Rodinia. This article is protected by copyright. All rights reserved. [[Free online access at https://rdcu.be/bCUw6]]