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FA3 (dolomitic floodplain). (A) Transmitted light, stained thin section. Sandy dolocrete (FA3) containing equant nodule cemented by ferroan saddle dolomite (turquoise), with local late calcite (red), interpreted as a fill of a small anhydrite nodule. W2, Backlundtoppen-Kvitfjellet ridge, 74Á7 m. (B) Facies 3S stromatolite with fenestrae. Paired transmitted light (left) and CL (right) images. Brightly luminescing dolomite may be primary or an early replacement of a precursor. W2, Dracoisen 69Á95 m. (C) Paired transmitted light (left) and CL (right) images. Dolocrete showing very fine-grained quartz sand grains floating in dolo(micro-)spar with crystals displaying a common zonation of bright to dull CL. Displacive primary dolomite growth is the preferred interpretation. W2, Ditlovtoppen, 118Á5 m. (D) Paired transmitted light (left) and CL (right) images. Dolocrete, similar to Fig. 10D and F, with sparse floating quartz and feldspar (black and blue, respectively, in CL) and calcite-filled cracks (centre) and pores (base). Uniformly luminescing dolomicrite crystals differ in brightness within nodules presumably forming at different stages. Calcite-filled pores show CL zonation (base) or no CL (cracks, centre). W2, Dracoisen, 82Á9 m.
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The Marinoan panglaciation (ca 650 to 635 Ma) is represented in north-east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn-glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2-rich atmosphere, late in the glaciation, and global climate models facilitate testing of propos...
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... dolomite-cemented layers and associated with calcite-filled frac- tures. These phenomena are found at one hori- zon in member W3 (Fig. 10D), as well as in several locations in member W2 (for example, Fig. 10F). A rarer phenomenon is the presence of equant centimetre-scale cauliflower-shaped pseudomorphs, filled by ferroan saddle dolomite cement ( Fig. 11A) occurring at the top of a con- glomerate-based fining-upward cycle (Fig. ...
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... from FA3 is characteristically bright under CL (Fig. 11B to D). In Facies 3S, dolomi- crite clots are uniformly bright, while adjacent dolomicrospar displays duller growth filling small fenestrae, and larger fenestrae are filled by dolospar with more variable properties (Fig. 11B). Manganese (3000 to 4000 ppm), Fe (10 000 to 15 000 ppm), Na (2000 ppm) and Sr (250 to 350 ppm) concentrations are all ...
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... from FA3 is characteristically bright under CL (Fig. 11B to D). In Facies 3S, dolomi- crite clots are uniformly bright, while adjacent dolomicrospar displays duller growth filling small fenestrae, and larger fenestrae are filled by dolospar with more variable properties (Fig. 11B). Manganese (3000 to 4000 ppm), Fe (10 000 to 15 000 ppm), Na (2000 ppm) and Sr (250 to 350 ppm) concentrations are all unusu- ally high ( Bao et al., 2009), and unpublished electron microsope images and microanalyses from the present authors also show enrichments in many transition metals and rare earths, as well as a consistent ...
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... microsope images and microanalyses from the present authors also show enrichments in many transition metals and rare earths, as well as a consistent chemical zonation within crystals of dolomicrite mosaics. In Facies 3D, a difference in mean CL brightness, and hence timing of growth, is observed between some nodules and surrounding matrix (Fig. 11D), while zonation within individual crystals grow- ing between siliciclastic sand grains is observed locally (Fig. 11C). Sulphate is enriched (4000 ppm); there is no D 17 O anomaly but sul- phate d 18 O is exceptionally high ( Bao et al., 2009;Benn et al., ...
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... rare earths, as well as a consistent chemical zonation within crystals of dolomicrite mosaics. In Facies 3D, a difference in mean CL brightness, and hence timing of growth, is observed between some nodules and surrounding matrix (Fig. 11D), while zonation within individual crystals grow- ing between siliciclastic sand grains is observed locally (Fig. 11C). Sulphate is enriched (4000 ppm); there is no D 17 O anomaly but sul- phate d 18 O is exceptionally high ( Bao et al., 2009;Benn et al., ...
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... Facies 3D, the dolomicritic, syndepositional, passive to displacive growth with nodular struc- ture and cracks is characteristic of calcretes in which precipitation is driven by evaporative concentration at or above a water table. Although rare, the spar-filled pseudomorphs (Fig. 11A), interpreted to have originated as anhydrite ( , attest to eva- porative conditions. The displacive growth, nod- ules and cracks identify these carbonates as alpha calcretes (Wright, 1990), which in Phanerozoic examples tend to occur on non-car- bonate substrates and in more arid conditions than the more common beta calcretes that are ...
Citations
... Purgstaller et al. (2017) confirmed these results and showed that the formation of anhydrous calcium carbonates is controlled mainly by the prevailing physicochemical conditions, such as the Mg/Ca ratio of the aqueous medium and H 2 O availability. It is notable that ikaite is only one-third as dense as calcite and so calcite pseudomorphs would be expected to be initially highly porous, even if the CaCO 3 was precipitated locally (Fairchild et al., 2016). However, no naturally-occurring aragonite or vaterite glendonites have been found as yet. ...
... These pseudomorphs were reported from Cryogenian sediments in Brazil (Olcott et al., 2005, fig. C), north-west Canada (Young, 1976, James et al., 2005 3), Svalbard (Halverson et al., 2004;Halverson, 2011;Fairchild et al., 2016), Scotland (Spencer, 1971 including possible occurrence from permafrost deposits: Dempster and Jess, 2015), and possibly South Africa (Behr et al., 1983). However, the possible South African glendonites have been subsequently reconsidered as replacements of evaporitic minerals. ...
Ikaite is a metastable calcium carbonate hexahydrate (CaCO3⋅6H2O), which naturally occurs in environments characterized by temperatures of 2 to +7 ◦C. It is therefore considered as an indicator of cold water conditions, although its suitability as a paleotemperature indicator is still questioned, especially as under certain laboratory conditions, ikaite was formed at temperatures of up to 35 ◦C. At warmer temperatures, ikaite becomes unstable and quickly decomposes to anhydrous calcium carbonate(s) and water. However, the primary ikaite crystal morphology can occasionally be preserved as a pseudomorph, generally known as ‘glendonite’. Even in early studies, glendonites were considered as indicators of cold climates due to their association with cold water fauna, dropstones and other cold climate indictors. We are using a global glendonite database with more than 900 occurrences to reconstruct their distribution through Phanerozoic time. Based on these reconstructions we observe that the Early Paleozoic glendonites cannot be directly correlated with cooler global climate episodes, but are restricted to the upwelling of cold waters on the wide shelf of the Baltic paleobasin. Carboniferous–Permian occurrences correlate with the Late Paleozoic Ice Age, one of the longest and most prominent glacial events in Earth’s history. Our plate reconstructions suggest that most Late Paleozoic glendonite occurrences are restricted to the high latitudes. Glendonites are absent from Triassic and Upper Cretaceous sediments around the world, which were deposited during periods generally considered as the hottest of the Phanerozoic. However, glendonites are numerous within certain Jurassic and Cretaceous strata deposited during well-known cooling intervals. Our reconstructions suggest that the majority of Mesozoic glendonite occurrences are confined to the polar or near-polar regions. Paleogene–Neogene occurrences are restricted to the middle and high latitudes of the northern hemisphere. In addition to glendonites from marine deposits, a few findings are known from continental environments. Quaternary ikaites and glendonites have a widespread distribution in marine settings close to polar seas (mainly Arctic shelves) or on deep-water continental margins (e.g., Zaire deep fan, Nankai Trough, Sakhalin Island slope), with rare findings in terrestrial environments such as lakes, ice sheets and caves. In contrast to Quaternary occurrences, the older Mesozoic and Paleozoic glendonites are confined to shallow marine shelf environments. This discrepancy is likely to be related to the lower preservation potential of deepwater and terrestrial sediments in deep time, as opposed to sediments deposited at high rates along the margins of epicontinental or marginal seas.
... The zoned crystals are typically more inclusion-rich than the pore-filling cements (Frank et al., 2008;Huggett et al., 2005;Larsen, 1994;Scheller et al., 2022). Although the term "guttulatic" was coined recently, this microfabric had already been described in numerous cold-water marine and lacustrine carbonate strata ranging from Cryogenian to modern in age (Fairchild et al., 2016;Rogov et al., 2021;Scheller et al., 2022;Selleck et al., 2007), many of which were identified as pseudomorphs after ikaite based on the morphology of large cm-scale crystals. The defining characteristics of guttulatic microfabric are closely linked to the paragenesis of ikaite stabilization. ...
Plain Language Summary
Between 717 and 635 million years ago, Earth experienced two dramatic global glacial events, known as “Snowball Earth” glaciations, during which ice covered the oceans all the way to the equator. Geoscientists are still seeking to fully understand what caused these extreme climate events and how life on Earth survived them. Although geochemists have a variety of tools to reconstruct the temperature of ancient oceans, these methods are difficult to apply in rocks this old because primary signals have been too altered. Instead, we looked for a key microscopic fingerprint (“guttulatic microfabric”) of a type of calcium carbonate mineral (“ikaite”) that only forms in cold‐water environments. Previous work had proposed that we might expect to find evidence of this cold‐water carbonate mineral associated with a specific type of sediment called “giant ooids.” We found abundant evidence of guttulatic microfabric in sedimentary rocks containing giant ooids that formed in a low‐latitude shallow marine environment millions of years before the onset of global glaciation. Our observations suggest that Earth’s climate was cold before the onset of global glaciation, which could mean that marine organisms were accustomed to cold conditions well before the Snowball glaciations.
... The carbonate units can be associated with short interglacials, as assumed for the dolomite rhythmite in the upper part of the Nichatka Formation, which shows moderate positive δ 13 C values (+4.6‰) and marine δ 18 O values (30.2‰). Moderately positive δ 13 C values (from -2 to +5‰) were also obtained for the continental carbonates in glacial deposits of the Neoproterozoic Wilsonbreen Formation related to the Marinoan glaciation (Fairchild et al., 2016). At the same time, carbonate rocks of the older Sturtian glaciation are characterized by moderately negative δ 13 C values (from -4 to +1‰) (Hood et al., 2020). ...
New data on the C and O isotope composition in the Neoproterozoic carbonates of the Ballaga-nakh and Dal'nyaya Taiga regional stages (southeastern margin of the Patom paleobasin (hereafter, Basin) are presented. The negative δ 13 С anomaly, established in the upper part of the Ballaganakh regional stage, is comparable (in amplitude and stratigraphic position) to the Trezona anomaly, which preceded the Marinoan glaciation. The carbonate layers in glacial deposits of the Nichatka Formation of the Dal'nyaya Taiga regional stage have moderately positive δ 13 С values. The dolomite unit at the postglacial sequence base is characterized by moderately negative δ 13 С values that are typical for the cap carbonate sequence associated with the end-Marinoan glaciation. The lower part of the postglacial sequence (Barakun Formation) is characterized by a gradual increase of calcite in the carbonate component of rocks and a shift in the isotope composition corresponding to the difference in the fractionation coefficient of stable C and O isotopes for dolomite and calcite. This phenomenon, recorded in the postglacial sequences of Namibia and Canada as well, indicates that the carbonate mineral composition was governed by a global change in water chemistry rather than post-sedimentary alteration. The overlying carbonate rocks of the Dal'nyaya Taiga regional stage in the epiconti-nental part of the Patom Basin (Sen Formation) are depleted in 13 С compared to their shelf analogs in the Ura Uplift. Probably, the carbonates were accumulated asynchronously in the epicontinental and shelf facies of the Patom Basin. The reconstructed trend of variations in the carbon isotope composition for the Dal'nyaya Taiga regional stage is characterized by positive and low-amplitude negative δ 13 С anomalies similar to those in the Doushantuo Formation (China).
... Карбонатные горизонты могут быть связаны с короткими межледниковьями как это предполагается для доломитового ритмита в верхней части ничатской свиты, имеющего умеренные положительные значения δ 13 С (+4.6‰) и близкие к морским значения δ 18 О (30.2‰). Умеренно положительные значения δ 13 С (от -2‰ до +5‰) также получены для континентальных карбонатов ледниковых отложений неопротерозойской формации Вилсонбрин ледниковой эпохи Марино [Fairchild et al., 2016]. В то же время карбонатные отложения более древнего оледенения Стерт характеризуются умеренно-отрицательными значениями δ 13 С (от -4‰ до +1‰) [Hood et al., 2020]. ...
Представлены новые данные об изотопном составе C и О в неопротерозойских карбонатах баллаганахского и дальнетайгинского горизонтов юго-восточной окраины Патомского палеобассейна. В верхней части баллаганахского горизонта установлена отрицательная аномалия δ13С, сопоставимая по амплитуде и стратиграфическому положению с аномалией Трезона, предшествующей оледенению Марино. Карбонатные прослои в составе ледниковых отложений ничатской свиты дальнетайгинского горизонта имеют умеренно положительные значения δ13С, а горизонт доломитов в основании постгляциальной последовательности характеризуется умеренно отрицательными значениями, типичными для венчающих карбонатов, связанных с завершением оледенения Марино. В нижней части постгляциальной последовательностии (баракунская свита) происходит постепенное увеличение доли кальцита в карбонатной составляющей пород и сдвиг изотопного состава, соответствующий разнице коэффициентов фракционирования стабильных изотопов С и О для доломита и кальцита. Этот феномен, фиксируемый также в постгляциальных последовательностях Намибии и Канады, указывает на то, что состав карбонатных минералов определялся глобальным изменением химизма воды, а не постседиментационным замещением. Вышележащие карбонатные отложения дальнетайгинского горизонта в эпиплатформенной части Патомского палеобассейна (сеньская свита) обеднены 13С по сравнению с их шельфовыми аналогами на Уринском поднятии. Высказано предположение, что карбонатонакопление в эпиплатформенных и шельфовых фациальных зонах Патомского палеобассейна происходило несинхронно. Реконструированный тренд вариаций δ13С для дальнетайгинского горизонта характеризуется положительными и малоамплитудными отрицательными аномалиями δ13С, сходными с таковыми в формации Доушаньто Китая.
... The formation of cap carbonates was associated with sea-level rise during the Marinoan deglaciation (Fairchild et al., 2016) and has been interpreted by multiple models, including the upwelling of alkalinityrich deep-ocean seawater (Knoll et al., 1996), postglacial chemical weathering of silicate and carbonate rocks (Hoffman et al., 2017;Myrow et al., 2018), oxidation of methane from gas hydrate destabilization (Jiang et al., 2006;Kennedy et al., 2008), and microbially mediated carbonate precipitation from deglacial meltwater (Shields, 2005). In each of these models, changes in ocean chemistry during the Marinoan deglaciation could provide a favorable environment for cap carbonates deposition, such as high alkalinity (Hoffman, 2011) and enhanced biotic activity (Meyer et al., 2012). ...
What caused ocean/climate changes that drove Marinoan deglaciation, and the subsequent genesis of Ediacaran cap carbonates remains unclear. To address this issue, we examined the Hg records in Ediacaran cap carbonates from shelf to slope depositional settings in exposures from South China. These cap carbonates show higher total Hg (THg) concentrations (4.9 to 405 ppb), most of which are comparable to that observed in carbonates deposited during non-LIPs periods. The lack of THg/TOC anomalies in these cap carbonates suggests that background volcanic activity, rather than a short-term large igneous province event, drove the Marinoan deglaciation. The cap carbonates show positive Δ 199 Hg values (0.18 to 0.34 ‰) in slope settings and slightly negative to slightly positive Δ 199 Hg values (− 0.16 to 0.11 ‰) in shelf settings, suggesting a binary mixing of seawater-and terrestrial-derived Hg in early Ediacaran Ocean. We infer that the accumulation of greenhouse gases, due to ongoing volcanic emissions of CO 2 and enhanced release of gas hydrates, triggered global warming. This warming led to melting of sea ice cover, enhanced terrestrial inputs of alkalis (e.g., K + , Na + , Ca 2+ , Mg 2+), and large-scale dissolution of atmospheric CO 2 into seawater, driving widespread deposition of Ediacaran cap carbonates.
... Mei et al. (2006) Moreover, anaerobic methane oxidation (AMO) increased the concentration of CO 3 2À through sulphate reduction, favouring dolomite formation regardless of the formation depth (Moore et al., 2004). The process was the same with that of the formation of interbedded dolostones from the Sturtian and Marinoan syn-glaciation (Allen & Etienne, 2008;Fairchild et al., 2016;Fleming et al., 2016;Gu et al., 2019;Hood et al., 2022;Zhao et al., 2020). ...
... The Neoproterozoic hard snowball Earth hypothesis first proposed by Hoffman et al. (1998) which envisages a fully frozen Earth for millions of years is facing severe challenges from growing new evidence, such as syn-glacial carbonate precipitation (Allen & Etienne, 2008;Fairchild et al., 2016;Fleming et al., 2016;Gu et al., 2019;Hood et al., 2022;Zhao et al., 2020), paired δ 13 Ccarb and δ 13 Corg values (i.e., Δ 13 Ccarb-org) from post-glacial cap carbonates (Sansjofre et al., 2011), and glacial and nonglacial red beds (Allen & Etienne, 2008;Bai et al., 2020). These indicate an episode of an interglacial period during the Marinoan glacial epoch (Allen & Etienne, 2008;Bai et al., 2020). ...
Cap carbonate of the early Ediacaran period represents an abrupt climate shift from severe Marinoan glaciation to a post‐glacial warm environment. Some consensuses on the distinctive negative excursion of δ ¹³ Ccarb and the origin of its unique sedimentary structures have been reached over the past decades, but certain aspects such as the distribution and redox conditions of cap carbonate still remain controversial. To resolve these issues, we collected detailed samples of the Doushantuo cap carbonate from the Ganlong‐Fanyang section and the Poutoushang reservoir section in the Danzhai County, southeast Guizhou Province, respectively. We conducted carbon and oxygen isotope analysis and trace element analysis in this study. The Ganlong‐Fanyang section was possibly situated in a submarine highland because of its higher carbon isotope than that of the Potoushang section according to the carbon isotopic gradient. Negative carbon isotope excursions at the basal Doushantuo cap carbonate from the two sections correlate well with those from other equivalent cap carbonates and were corresponding to the first one (SN1 or EN1) of the carbon isotopic curve during the Ediacaran period, implying a remarkable global event after the post‐Marinoan glaciation. Geochemical features of trace elements and rare earth elements support an overall oxygen‐rich condition with syn‐depositional or pene‐contemporaneous hydrothermal activities and methane seeps during the deposition of cap carbonate. We reconstruct a new sedimentary model that the Doushantuo cap carbonates were mainly precipitated in relatively shallow‐ to deep‐water shelf environments without disturbance by abundant terrestrial materials, which may be on isolated platforms over the early developed grabens in a deep‐water basinal condition, rather than within the entire oceanic basin.
... L, Liljequisthøgda; Sv, Sveanor (Backaberget on Norsk Polarinstitutt maps); S, Søre Russøya (S), Sk, Skytttelodden. (C) Spitsbergen locations (Fairchild et al., 2016a(Fairchild et al., , 2016bFairchild & Hambrey, 1984;Halverson et al., 2004;Harland et al., 1993 had previously been made on the basis of Sr and C isotope chemostratigraphy coupled with the sharply stratigraphically resolved evidence for glaciation Halverson et al., 2020). Each glacigenic unit is overlain by a cap carbonate and the two caps show distinctly different lithologies and carbon isotope trends that compare respectively with the Sturtian and Marinoan caps as seen worldwide (Fairchild et al., 2016a;Halverson et al., 2004;Hoffman et al., 2012;Kennedy et al., 1998;Xiao & Narbonne, 2020). ...
... It displays interstratified limestone and glacigenic sediments near its top and is succeeded by a thin limestone interpreted as an offshore deposit, typical for a Sturtian cap (Fairchild et al., 2016a). The upper glacigenic unit, the Wilsonbreen Formation (Figure 2), is non-marine (Benn et al., 2015;Fairchild et al., 2016b;Fleming et al., 2016), and is overlain by a cap dolostone which is the subject of this paper ( Figure 3). This is correlated with the Marinoan cap carbonate, the lower boundary of which was used to define the base of the Ediacaran System in South Australia (Knoll et al., 2006). ...
... Burial temperatures of Cryogenian rocks in the study area did not exceed 160°C, based on incomplete solidstate ordering of clumped carbonate isotopes in calcites from Dracoisen (Mackey et al., 2020). Previous authors have argued that carbon isotope values in North-East Svalbard reflect values established during early diagenesis (Fairchild et al., 2016a(Fairchild et al., , 2016bHalverson et al., 2004). A lack of δ 13 C-δ 18 O covariation at any stratigraphic level indicates there is no prima facie argument for any meteoric or burial alteration to δ 13 C. Carbonate platform sediments deeper in the pile also show no evidence of burial resetting of δ 13 C . ...
Two cap carbonates overlying Cryogenian panglacial deposits are found in North‐East Svalbard of which the younger (635 Ma) forms the base of the Ediacaran Period. It is represented by a transgressive succession in which laminated dolostone, typically around 20 m thick (Member D1), is succeeded transitionally by a similar thickness of impure carbonates (Member D2). In Spitsbergen, there is evidence of microbially influenced sediment stabilisation and carbonate precipitation in the lower part of D1, whilst the upper part of D1 and D2 show centimetre‐decimetre‐scale graded units with undulatory lamination interpreted as evidence of storm activity. Carbonate originated as possible freshwater whitings, as well as microbial precipitates. Exhumed and eroded hardgrounds display replacive 10‐30 μm dolomite crystals with cathodoluminescence characteristics consistent with early diagenetic manganese and iron reduction. Regionally, carbon isotope values consistently decrease by around 2‰ from around ‐3‰ over 30 m of section which is both a temporal and a bathymetric signal, but not a global one. An exponential decline in carbonate production predicted by box models is fitted by a semi‐quantitative sedimentation model. A mass‐anomalous 17O depletion in carbonate‐associated sulphate in dolomite, inherited from precursor calcite, decreases from ‐0.6 to ‐0.3‰ in the basal 15 m of section and then approaches background values. The post‐glacial anomalous 17O depletion in carbonate‐associated sulphate and barite elsewhere has been interpreted in terms of ultra‐high pCO2 at the onset of deglaciation. Such anomalies, with larger amplitude, have been reported in Svalbard from underlying lacustrine and tufaceous limestones representing a hyperarid glacial environment. The anomalous sulphate could be produced contemporarily, or the internally drained landscape may have continued to release 17O‐anomalous sulphate as it was transgressed during cap carbonate deposition. The late Cryogenian to earliest Ediacaran record in Svalbard provides the most complete record of the basal 17O‐depletion event in the world.
... Accordingly, cyanobacteria could have played a major role in sustaining life during the glaciations of the Cryogenian period, consistent with the detection of microbialites preserved in a Cryogenian sequence in Yukon, Canada (Macdonald et al., 2018). Moreover, carbonates from Svalbard, deposited during the Marinoan pan-glaciation (∼650 -635 Ma ago), show textures consistent with microbial mat accumulation associated with glacial sedimentation (Fairchild et al., 2016), thus, providing strong parallels between Cryogenian sediments and modern deposits from the McMurdo Dry Valley region. A principal coordinate analysis, based on the Bray-Curtis dissimilarity of 16S rRNA gene bacterial community, revealed distinct assemblages across the different meltwater ponds Frontiers in Microbiology | www.frontiersin.org ...
Persistent cold temperatures, a paucity of nutrients, freeze-thaw cycles, and the strongly seasonal light regime make Antarctica one of Earth's least hospitable surface environments for complex life. Cyanobacteria, however, are well-adapted to such conditions and are often the dominant primary producers in Antarctic inland water environments. In particular, the network of meltwater ponds on the 'dirty ice' of the McMurdo Ice Shelf is an ecosystem with extensive cyanobacteria-dominated microbial mat accumulations. This study investigated intact polar lipids (IPLs), heterocyte glycolipids (HGs), and bacteriohopanepolyols (BHPs) in combination with 16S and 18S rRNA gene diversity in microbial mats of twelve ponds in this unique polar ecosystem. To constrain the effects of nutrient availability, temperature and freeze-thaw cycles on the lipid membrane composition, lipids were compared to stromatolite-forming cyanobacterial mats from ice-covered lakes in the McMurdo Dry Valleys as well as from (sub)tropical regions and hot springs. The 16S rRNA gene compositions of the McMurdo Ice Shelf mats confirm the dominance of Cyanobacteria and Proteobacteria while the 18S rRNA gene composition indicates the presence of Ochrophyta, Chlorophyta, Ciliophora, and other microfauna. IPL analyses revealed a predominantly bacterial community in the meltwater ponds, with archaeal lipids being barely detectable. IPLs are dominated by glycolipids and phospholipids, followed by aminolipids. The high abundance of sugar-bound lipids accords with a predominance of cyanobacterial primary producers. The phosphate-limited samples from the (sub)tropical, hot spring, and Lake Vanda sites revealed a higher abundance of aminolipids compared to those of the nitrogen-limited meltwater ponds, affirming the direct affects that N and P availability Frontiers in Microbiology | www.frontiersin.org 1 June 2022 | Volume 13 | Article 903621 Evans et al. Biomarkers From Antarctic Meltwater Ponds have on IPL compositions. The high abundance of polyunsaturated IPLs in the Antarctic microbial mats suggests that these lipids provide an important mechanism to maintain membrane fluidity in cold environments. High abundances of HG keto-ols and HG keto-diols, produced by heterocytous cyanobacteria, further support these findings and reveal a unique distribution compared to those from warmer climates.
... Many researchers refer to the hypothesis of Snowball Earth, for which the Gowganda Formation is an important area in worldwide correlations (Young, 2013(Young, , 2014. However, the evidence for Snowball Earth scenarios is equivocal, as has been confirmed by detailed dating of outcrops of Neoproterozoic (Le Heron et al., 2020) and Palaeoproterozoic strata, including the Gowganda Formation Young, 2014Young, , 2019, by the geochemical record (Bahlburg & Dobrzinski, 2011;Bristow et al., 2011;Grotzinger et al., 2011;Zimmermann et al., 2011), including (mainly for Neoproterozoic and younger strata) ikaites/glendonites -the minerals that are supposed to be stable only in cold temperatures (Aspler et al., 2001;Hoffman, 2013;Fairchild et al., 2016), but are documented to have formed in temperate and even tropical waters (Popov et al., 2019) and, lastly, by numerous studies of the geology of Precambrian diamictites (e.g., Eyles, 1993;Molén, 2017;Kennedy et al., 2019;Kennedy & Eyles, 2021). ...
During more than a century since its original identification, the Gowganda Formation in Ontario (Canada) has gradually been reinterpreted from representing mainly subglacial tillites to secondary gravity flow and glaciomarine deposits. The main pieces of geological evidence advanced in favour of glaciation in recent articles are outsized clasts that have been interpreted as dropstones and patches of diamictites in a single small-sized area at Cobalt which is still interpreted as displaying subglacial basal tillites. The present research considers field evidence in the Gowganda Formation in the light of more recent work on gravity flows linked to tectonics. Detailed studies have demonstrated that the clasts which are interpreted to be dropstones rarely penetrate laminae and are commonly draped by sediments the appearance of which is similar to lonestones in gravity flows. The “subglacial area” at Cobalt displays evidence of tectonics and gravity flows, which can be traced from the underlying bedrock, and then further in the overlying sequence of diamictites and rhythmites. The sum of geological features displays appearances at odds with a primary glaciogenic origin, and there is no unequivocal evidence present of glaciation. The data indicate deposition by non-glaciogenic gravity flows, including cohesive debris flows for the more compact units, probably triggered by tectonic displacements.
... The record of global Sturtian glaciation is found in South Australia, North-West Canada, Greenland, Namibia, Oman, Scotland, and South China (Fairchild et al, 2016). However, less attention has been paid to distal-or non-glacially influenced deposits (Lang et al., 2018). ...
The Cryogenian Period (720–635 Ma) is wide known by the occurrence of two global glaciations, Sturtian and Marinoan, responsible of dramatic climate and sea level changes as well as seawater oxygenation events. Constrained ⁸⁷Sr/⁸⁶Sr ratios from 0.7064 to 0.7071 and δ¹³C excursion are consistent with Cryogenian cap carbonates as documented in the upper Villa Mónica Formation. This unit constitutes the basal sedimentary cover of the Río de La Plata Craton in Central Argentina which begins its deposition in a shallow marine siliciclastic shelf, turning to a carbonate succession with exceptionally well-preserved stromatolites. The detailed facies analysis added to stromatolite morphologies included in paleoenvironmental models of deposition remain unknown. The later are considered necessary to establish the stratigraphic framework and the regional correlation of the unit across the basin as a piece for the Cryogenian assembly. The entire Villa Mónica Formation in the Tandilia Basin was deposited on a shelf with sedimentary facies variations observed from the shallow margins to the offshore transition zone. The transition from the lower to the upper sections implies the conquer of microbial communities to paleogeographically controlled chemocline fluctuations where the stromatolites evidence the development from a post-glaciation phosphorus-rich ocean. This drastic shift in paleoenvironments represented on the two distinctive marker sections was influenced by changes in climatic and oxygenation conditions. In this context, presumed glacial influence is predictable in a partially ice-covered Cryogenian Earth surface.
