Explore the vast range of titles available.

Neoproterozoic iron formations are exposed in the Wadi Hamama area (Egypt) in the northwestern part of the Arabian–Nubian Shield. Mafic and felsic volcanic and volcaniclastic rocks of an intra-oceanic island-arc setting host multiple, thin iron-formation units. Major element compositions of the iron formation confirm a low detrital input, whereas the rare-earth elements and Y data suggest deposition related to an influx of low-temperature hydrothermal fluids. Unlike most Neoproterozoic banded iron formations, but similar to other iron-formation occurrences from the Arabian–Nubian Shield, the Nd isotopic compositions of the Wadi Hamama iron formations are predominantly mantle-like. SIMS U–Pb zircon ages of the host volcaniclastic units indicate that the age of iron-formation deposition is ca. 695 Ma, which is within the Sturtian epoch that is presumed to be a glacial event of global extent. Nevertheless, there is no robust evidence of any influence of Sturtian glaciation in the Arabian–Nubian Shield. Our results rather suggest that the iron formations in the area may have formed as low-temperature exhalites on the floor of an island-arc basin. The iron formations were deposited during periods of volcanic quiescence, with metals having been derived during low-temperature pervasive hydrothermal alteration of volcanic and volcaniclastic rocks exposed at the seafloor–seawater interface. Precipitation took place due to mixing of metal-bearing hydrothermal fluids and cold, oxygenated seawater. There is no need to invoke possible effects of global glaciation to explain the origin of the Sturtian-aged iron formations in the shield. Our study thus suggests that not all Neoproterozoic iron formations are necessarily linked to glacial events as the Hamama deposit represents a non-Rapitan exhalative iron formation.

The glaciogenic nature of the Yudnamutana Subgroup was first recognized over a century ago, and its global significance was recognized shortly after, with the eventual postulation of a global Sturtian Glaciation and Snowball Earth theory. Much debate on the origin and timing of these rocks, locally and globally, has ensued in the years since. A significant corpus of research on the lithology, sedimentology, geochronology and formal lithostratigraphy of these sequences globally has attempted to resolve many of these debates. In the type area for the Sturtian Glaciation, South Australia's Adelaide Superbasin, the lithostratigraphy and sedimentology are well understood; however, formal stratigraphic nomenclature has remained complicated and contested. Absolute dates on the stratigraphy are also extremely sparse in this area. The result of these longstanding issues has been disagreement as to whether the sedimentary rocks of the Yudnamutana Subgroup are truly correlative throughout South Australia, and if they were deposited in the same time span recently defined for Sturtian glacial rocks globally, c. 717 Ma to c. 660 Ma. This study presents a large detrital zircon study, summarizes and compiles existing global geochronology for the Sturtian Glaciation and revises the formal lithostratigraphic framework of the Yudnamutana Subgroup. We show equivalence of the rocks that comprise the revised Sturt Formation, the main glaciogenic unit of the Yudnamutana Subgroup, and that it was deposited within the time span globally defined for the Sturtian Glaciation.

The Sturtian (720–670 Ma) glacial–interglacial transition period was an important interval for sedimentary manganese metallogenesis, including the Mn oxide deposit in the Otjosondu region in Namibia and Mn carbonate deposits in the Datangpo Formation in the south-eastern Yangtze Platform, South China. During this period, Earth experienced the breakup of Rodinia, the Sturtian glaciation, and the Neoproterozoic oxygenation event. In this study, we investigate scenarios that might have provided geologically and geochemically favorable conditions for Mn metallogenesis. In these scenarios, the global recovery of microorganisms enhanced marine primary productivity and O2 levels of the hydrosphere and atmosphere during the Sturtian glacial–interglacial transition. However, the water column was not completely oxidized, maintaining redox stratification. Transgression–regression cycles or O2-rich downwelling drove the exchange of oxygenated topwater and anoxic deep water in rift-related basins that developed due to Rodinia’s breakup. The coupling of these processes precipitated existing dissolved Mn(II) at the margins of basins (Otjosondu region) or at their centers (Yangtze Platform). In the latter case, precursor Mn oxides were further converted into Mn carbonates via the reduction of Mn oxides coupled with organic matter oxidation during early diagenesis. A brief review of Mn metallogenesis in the geological record reveals that Mn metallogenic processes typically occur under geo-environmental conditions that, in concert, produce favorable conditions for Mn sourcing, concentration, and sedimentation.

The evolution of the paleoclimate and the duration of the two glacial periods during the Cryogenian in the Yangtze Block are of great geological significance for understanding \"Snowball Earth.\" We studied the evolution of the paleoclimate during the Cryogenian, using chemical weathering, and determined the warming time of the interglacial paleoclimate, using zircon U-Pb dating in the Yangtze region. A new laser ablation ICP-MS zircon U-Pb age of 659.9±3.2 Ma is reported from the middle Datangpo Formation. The chemical index of alteration (CIA) for the Cryogenian fine clastic rock samples from a drill core shows that the climate in the provenance area during the Gucheng and Nantuo glacial periods was dry and cold and that the chemical weathering was weak; the average CIA values were 63.3 and 64.2, respectively. In the early stage of the Datangpo period, the climate in the provenance area was still dry and cold, with a CIA average of 60.7, compatible with that in the two glacial periods; in the middle and late stages, the climate was warm and humid, chemical weathering was enhanced, and the average CIA rose to 77.4, compatible with the 76.9 of the Liantuo period before the Cryogenian. The evolution of other chemical weathering (paleoclimate) proxies, such as the chemical index of weathering, the plagioclase index of alteration, and Rb/Sr, is compatible with that of the CIA. The warming time of the interglacial paleoclimate during the Cryogenian in the Yangtze Block was ca. 660 Ma, roughly consistent with the ending time of the Sturtian in Australia and Mongolia, indicating that the lower part of the Datangpo Formation may still represent Sturtian glacial deposits, thus further suggesting the global consistency of the end of the Sturtian.

The Neoproterozoic Dalradian Supergroup is a dominantly siliciclastic metasedimentary succession in the Caledonian orogenic belt of Scotland and Ireland. Despite polyphase deformation and greenschist- to upper amphibolite-facies metamorphism, carbonate units distributed throughout the Dalradian record marked δ13Ccarbonate excursions that can be linked to those associated with key environmental events of Neoproterozoic time. These include: (1) tentative correlation of the Ballachulish Limestone with the c. 800 Ma Bitter Springs anomaly; (2) the presence of the pre-Marinoan Trezona anomaly and 635 Ma Marinoan-equivalent cap carbonate sequence in rocks of the middle Easdale Subgroup; (3) the terminal Proterozoic (c. 600-551 Ma) Wonoka-Shuram anomaly in the Girlsta Limestone on Shetland. These linkages strengthen previously inferred correlations of the Stralinchy-Reelan formations and the Inishowen-Loch na Cille-MacDuff ice-rafted debris beds to the respectively 635 Ma Marinoan and 582 Ma Gaskiers glaciations, and suggest that the oldest Dalradian glacial unit, the Port Askaig Formation, represents one of the c. 750-690 Ma Sturtian glacial episodes. These δ13C data and resulting correlations provide more robust constraints on the geological evolution of the Dalradian Supergroup than anything hitherto available and enhance its utility in helping refine understanding of Neoproterozoic Earth history.

A sample from the top of the Banxi Group (early Neoproterozoic) in Hunan Province (South China) was dated by the SHRIMP zircon U-Pb method. The resulting age of 725±10 Ma probably solves the debate between the synglacial and the preglacial correlation of the Liantuo Formation and indicates that the Liantuo Formation most likely correlates with the preglacial Banxi Group. Since the Banxi (Danzhou) Group is upwardly transitional to the glacial Jiangkou Group, the age becomes a new maximum age constraint on the onset of the Jiangkou glaciation, which therefore most likely correlates with the Sturtian glaciation.

We examine the general conditions that must be satisfied by the configuration of the continents in order that steady state solutions for Neoproterozoic climate exist that are characterized by heavy continental glaciation but for which a substantial area of open water in the equatorial region persists. Such solutions have previously been termed “soft snowball” or “slush ball” to distinguish them from the “hard snowball” solutions that some have suggested be required to fit the observational constraints. It is found that three conditions are critical in this regard: (1) the continental area in high latitudes should be large enough that a massive ice sheet may develop even when pCO2 is relatively high, an ice sheet complex that is subsequently capable of flowing to lower latitude. (2) The continental fragments in low latitude must be connected (or separated only by continental shelves above which water depths are small) to a significant degree with those at higher latitudes. (3) A relatively simple supercontinental outline favors the formation of the “soft snowball” state. Although the latter requirement is important, we have nevertheless found that soft snowball solutions are realized for the realistic Sturtian continental configuration of Li et al. (2008) that existed at ∼720 Ma. However, in order for these states to exist, the positions of the individual continental fragments must be slightly adjusted so as to improve their connectivity. These adjustments are consistent with the error bars on the paleomagnetic inferences of paleolatitude and the even less well constrained paleolongitude. We also demonstrate that “soft snowball” solutions do not exist in models devoid of active continental ice sheets capable of flowing over the landscape. These results for Sturtian conditions extend our previously published results for the Marinoan period during which the supercontinent was centered upon much higher latitudes.

It has been suggested that a negative climate feedback may have operated during the Neoproterozoic Era as a consequence of the existence of a massive oceanic pool of dissolved organic carbon (DOC). As climate cooled so as to induce intense glaciation, the drawdown of oxygen into the Neoproterozoic ocean would have been enhanced because of the temperature dependence of the solubility of oxygen in seawater. Such increasing ventilation would have enhanced DOC remineralization, thus increasing the content of dissolved inorganic carbon (DIC) in the ocean. CO2 concentration in the atmosphere increases rapidly with DIC, thereby inhibiting further climate cooling. The model employed to illustrate the resulting climate dynamical behavior was an idealized one in which stochastic influence was assumed to be absent. However, such influence is expected to exist due to the action of processes that are not explicitly included in the model. Furthermore, the paleogeography assumed for the purpose of the published analyses was more appropriate to the Marinoan glaciation than to the earlier Sturtian event. In this paper, we fully investigate the stability of the system represented by the carbon cycle coupled climate model for both the Marinoan and Sturtian continental configurations and in the presence of stochastic perturbations. It is found that the hysteresis predicted by the ice sheet coupled model is sensitive to both continental configuration and to the strength of the negative feedback which arises due to carbon cycle coupling. Nevertheless, the very low frequency cyclic glaciation process predicted by the initial version of the model is found to persist in the presence of noise of significant amplitude. However, this cyclic behavior may be arrested entirely if the glaciation process were to result in insufficient alkalinity being delivered to the ocean basins. In this case the system would be expected to execute only a single excursion into and escape from the glacial state.

In South China, the Wuqiangxi Formation of the Banxi Group and its equivalents underlie the early Cryogenian (Sturtian) glacial deposits but their thickness varies from <200 m to >2000 m. In the Guzhang section of western Hunan, the Wuqiangxi Formation is only 152 m thick, and an ash bed 58 m below the glacial diamictite yielded a SHRIMP U-Pb age of 809.3±8.4 Ma. In contrast, 90 km south of the Guzhang section towards the basin in Zhijiang area where the Wuqiangxi Formation is 2200 m thick, an age of 725±10 Ma has been reported from the top of this unit, 300 m below the glacial diamictite. These ages provide new evidence for the regional stratigraphic correlation across the Nanhua basin, and suggest unusually large (>2 km) stratigraphic erosion potentially associated with the Sturtian glaciation in South China. The magnitude of erosion may imply significant uplifting and tectonotopography at the onset of the Sturtian glaciation. [PUBLICATION ABSTRACT]

New oxygen and hydrogen isotope ratios of chert from middle, intraformational breccias, and upper breccia members of the Sixtymile Formation（SMF） in eastern Grand Canyon National Park（AZ） yield palaeoclimate estimates between 27 and 33℃. The isotopic compositions of cherts define a domain approximately parallel to the meteoric water line when plotted on a δD–δ-（18）O diagram; these data indicate that meteoric water was involved during formation of the chert. In thin section, the absence of interlocking mega quartz（〉35 lm） and silicafilled fractures and veins, along with preserved micromorphological silica fabrics, suggest that the chert has not been permeated by later hydrothermal fluids. Petrographic observations in thin section such as cyclic silica precipitation phases and glaebular micromorphologic fabrics lend support to the interpretation that meteoric waters were involved during chert precipitation. The post 742 Ma SMF has been correlated with diamictite（transition） beds of the Kingston Peak Formation（CA）, which in turn have been interpreted to have been deposited during the Sturtian Ice Age（-750–700 Ma）. Absence of facetted and striated clasts and other diagnostic glaciogenic features in the SMF,an unconformable contact with the stratigraphically older Chuar Group, coupled with warm palaeotemperature data inferred from stable isotope values of chert, tentatively suggest that deposition of sediment in the SMF likely did not take place during the Sturtian Ice Age.