Australia: The Land Where Time Began
In South Australia the older of the 2 Cryogenian events is the Sturtian Glaciation that is composed of the diamictites, siltstones and banded iron formations of the Yudnamutana subgroup. A U-Pb zircon age of 659.7 ± 5.3 Ma (SHRIMP = sensitive high-resolution microprobe) has been determined for the tuffaceous horizon in the Wilyerpa Formation that is situated just above the diamictite of Sturtian (Appila) age, providing a maximum age for the basal layer of the interglacial sediments. There are 2 major sequences that coarsen upwards overlying the diamictites of Sturtian age. The siltstones of the lower Tapley Hill Formation grade upwards into shallow marine sands, oolites and microbial reefs that are part of the Etina Formation and its northern equivalent, the Balcanoona Formation.
A major flooding surface, that marks the base of the Enorama Formation, is overlain by the second coarsening sequence that shallows upwards, culminating in the flake breccias, stromatolite bioherms, bioclastic packstones and oolitic grainstones that are part of the Trezona Formation. An 18 ‰ negative shift in the δ13C of inorganic carbon between the carbonate of the Etina Formation (δ13C = + 8 ‰) and the Trezona Formation. This δ13C excursion is the largest known of in the entire history of the Earth. Just below the Elatina Formation glacial deposits (Marinoan) the interval of very negative δ13C values is called the Trezona Anomaly, that have been linked to the initiation of ice-house conditions, and has also been used to correlate overlying glacial deposits in other parts of the world. The Elatina Formation, as well as its capping dolostone, of the Nuccaleena Formation, have been correlated to Marinoan successions in China and Namibia, zircon-dated by U-Pb isotope dilution thermalisation mass spectrometry (ID-TIMS) giving a date of about 635 Ma, the contact between the 2 formations being regarded as the global boundary stratotype section, and the point for the base of the Ediacaran Period.
Sturtian Glaciation in Svalbard?2
According to the authors2 if the diamictites of the Polarisbreen Group represent the Marinoan Glaciation in Svalbard then the question is raised as to where is evidence of the older Sturtian Global Glaciation in the underlying strata of the Hekla Hoek (Knoll, 2000)? Older glacial deposits have not been found, and the authors1 suggest they may never be found because of the limited exposure area, also suggesting it is not surprising as there is a vast succession of conformable carbonates of shallow water origin that would not provide topography for the nucleation of glaciers. Glacial deposits are present on the upper continental slope in Namibia, though they are largely unknown from the carbonate platform, of Marinoan age, for that reason (Hoffman, et al.,1988b; Hoffman, 2002), being a facies that is not present in Svalbard. Being transgressive, cap carbonates of post-glacial age, usually extend a long way from the margins of the deposits (Hoffman & Schrag, 2002).
In Australia, Canada and Namibia cap carbonates of Sturtian age have characteristic features (Kennedy et al., 1998) which the authors2 suggest should make its recognition possible in Svalbard in the absence of glacial deposits from the Sturtian. Beginning with argillaceous limestone that contain allodapic beds or, less often, sublittoral microbial laminae they are generally dark grey, regressive sequences. Compared with cap carbonates from the Marinoan those from the Sturtian Glacial are condensed and there are no transgressive tracts hosting the declining arm of the δ13C anomaly, resulting in δ13C increasing in line with stratigraphic height from a low point of -2‰ in basal layers to highs of +5‰ (Kennedy et al., 1998; McKirdy et al.. 2001; Yoshioka et al., 2003). It has been speculated that Sturtian seawater didn't reach critical saturation with respect to CaCO3 until following the post-glacial transgression Hoffman & Schrag, 2002), and they also speculated that the weathering flux of alkalinity was lower in the Marinoan as a result of lower availability of carbonate rock on a global scale for submarine dissolution and weathering.
The authors2 argue that the closest analogue beneath the Petrovbreen diamictite to the Rasthof Formation cap carbonate is the lower Russøya Member, though there are negative δ13C values within the Akademikerbreen carbonates (Knoll et al., 1986; Kauffman et al., 1997; Halverson et al., 2003). As in the case of the Rasthof Formation (Hoffman et al., 1998; Kennedy, 1998) it is comprised of a single shoaling-upward half-sequence, in which organic matter is relatively high and begins with argillaceous limestone originating in deep water, that is bluish-grey to black, and in some sections it is intercalated with thin beds of dolomite. Measured section 1, the most carbonate rich, δ13C increases from 1.5‰ to 5‰ in the lower half of the sequence. The δ13C profile matches the profile of the Rasthof Formation, after making allowance for the poor exposure and the lack of carbonate in the lowermost Russøya Member (Yoshioka et al., 2003).
The authors1 suggest that the top of the Dartboard Dolomite should be the glacial surface if the lower Russøya Member is a cap carbonate from the Sturtian, based on the characteristics of other cap carbonates that are equivalent. The Dartboard Dolomite is considered to be as sedimentologically strange as its name, and the contact is poorly exposed. Many of its distinctive features are similar to cap dolostones from the Marinoan. Such features include isopachous sea-floor cements, macropeloids, wave-like bed-forms and sub-littoral bacterial laminae. Transgressive cap dolostones are lacking in most cap carbonates from the Sturtian, and begin at the maximum flooding or later (Hoffman & Schrag, 2002), the authors2 asking why a cap dolostone from the Sturtian that was 13C-enriched should appear in Svalbard, though there is no known equivalent in East Greenland? They suggest it is tempting to speculate that over Svalbard platform waters reached critical levels of oversaturation with respect to the dolomite precursor than occurred elsewhere as a result of the local source of alkalinity, being in this case the Akademikerbreen carbonate platform that was strongly 13C- enriched.
If by analogy to the Marinoan cap dolostones the Dartboard Dolomite were the cap dolostones of the Sturtian, a glacial surface should be at its base. As the Dartboard Dolostone base is as poorly exposed as the top, there is no obvious sequence that can be identified at this level in the stratigraphy. According to the authors2 significant relief on this contact is suggested by the range in facies in the Dartboard Dolomite, as implied by the variable deposition depth from sublittoral (Knoll et al., 1989) to shallow/intermittently exposed. The possibility is left open that the Dartboard Dolomite is not part of the Sturtian cap-carbonate sequence, though lithologically similar to the cap dolostones of Marinoan age, as the top of the Dartboard Dolomite is locally brecciated (Fairchild & Hambrey, 1984), and not present in northeasternmost Spitsbergen.
A profound depositional style change affecting the EGES platform occurred in the uppermost Akademikerbreen Group (Herrington & Fairchild, 1989), whatever the origin and timing of the Dartboard Dolomite. The sudden influx of fine siliciclastic sediment in the Kinnvikka Member, and local deepening on the EGES platform to tectonic uplift (Fairchild & Hambrey, 1995). There is evidence that is highly suggestive of a glaciostatic oscillation, including evidence of high amplitude fluctuations of base level, development in measured section 10 of a pinnacle reef 45 m high pinnacle reefs with multiple aprons of debris, though the authors2 suggest tectonic uplift may also have played a part. A transient landscape disequilibrium that was associated with associated with the deterioration of the climate during the incipient Sturtian Glaciation (Zhang et al., 2001) is suggested to have been a possible alternative explanation for the influx of fine clastic sediment in the Kinnvikka Member.
The authors2 regard the problem of the Sturtian Glaciation on the EGES platform to be unresolved, saying the hypothesis that the Russøya Member, that forms the base of the Polarisbreen Group, is the cap carbonate of a cryptic Sturtian Glaciation, is speculative. They say it cannot be rued out completely that the Petrovbreen Member or the underlying Akademikerbreen Group do not represent the Sturtian Glaciation, though these options are indicated to be lass likely by a separate paper in which global correlations will be developed. Directly west of the Vestfonna Ice Dome on Nordaustlandet is the unexplored exposure of the Kinnvikka-Dartboard-lower Russøya that is potentially the key interval.
According to the authors3 it is possible that the δ13C anomaly might be a record of secondary fluid alteration after the deposition of the carbonate. They have documented the presence of dropstones within the carbonate sediment just below the diamictite of the Marinoan Glacial in South Australia. The soft-sediment deformation of beds below the glacial diamictite, and subglacial erosion of the underlying carbonates, was caused by advancing ice sheets, indicating that the Trezona δ13C values were acquired prior to glaciation. These stratigraphic relationships require the nadir of the Trezona δ13C anomaly to have occurred prior to local glacial advance, and well before the occurrence of the late stage diagenesis burial, though they don't provide a mechanism that would explain the Trezona δ13C anomaly.
Cryogenian glaciation and the onset of carbon-isotope decoupling
It has been found that throughout the history of the Earth perturbations of the global carbon cycle are often linked to changes in palaeogeography, glaciation, oxygenation of the oceans, and biological innovation. During the Ediacaran Period, 635-542 Ma, there was a pronounced carbonate carbon isotope excursion, associated with invariant or decupled organic-isotope values, has been explained by a model that relies on a large reservoir in the ocean of organic carbon. The authors4 have presented carbonate and organic carbon isotope data showing no coupling between 820-760 Ma, and between the Sturtian and Marinoan Glacial events of the Cryogenian Period, about 720-635 Ma. They suggest the growth of the organic carbon pool could possibly be related to deep ocean conditions of iron-rich and sulphate-poor water that are facilitated by an increased Fe:S ratio of the riverine flux following the removal by the Sturtian Glaciation of long-lived regolith.
Paired records of carbonate carbon (δ13Ccarb) and coeval bulk organic carbon (δ13Corg) carbon isotopes are consistent with a model showing that within marine sediments organic carbon is derived and fractionated from contemporaneous dissolved inorganic carbon (DIC), as the authors1 demonstrate, for the time period covering most of the Phanerozoic Eon to the present. In the Ediacaran , 635-542 Ma, carbonate succession records of δ13Ccarb and δ13Corg show δ13Corg that is relatively invariant during larges changes of δ13Ccarb values across the Shuram-Wonoka anomaly, about 580 Ma. This behaviour was the basis for the development and support of a model of the carbon cycle of the Neoproterozoic , 1000-542 Ma, in which invariant δ13Corg result from a very large reservoir in the oceans of dissolved organic carbon (DOC) that was depleted in δ13Corg and particulate organic carbon (POC), or it could have been sourced from a large sedimentary reservoir, that overwhelms the signal from primary biomass that was fractionated from contemporaneous DIC. The authors4 considered the large oceanic reservoir model and use the term DOC to refer collectively to refer to truly dissolved organic carbon as well as colloidal organic carbon in suspension and fine POC.
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