Bitter Springs Research Articles

Persistent late Tonian shallow marine anoxia and euxinia

The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) is commonly associated with the oxygenation of the Earth’s ocean–atmosphere system, yet the timing and duration of these redox changes remain uncertain. In particular, it is unclear how shallow marine environments, which were likely crucial habitats for early eukaryotic organisms, responded to Neoproterozoic oxygenation. This study uses trace and rare earth element geochemistry to determine shallow marine redox conditions during the deposition of the late Tonian (ca. 760 Ma) Devede Formation, northern Namibia. In this unit, although carbonate phases (microbialite, former aragonite and high Mg calcite marine cements, and primary dolomite marine cements) generally exhibit no Ce/Ce* anomaly, rare negative values (up to 0.55) are consistent with short-lived periods of oxygenation. In contrast, the dominantly positive Eu/Eu* anomalies of the same phases suggest that basinal conditions were anoxic. These phases also exhibit low concentrations of redox-sensitive (U, V, Mo) and chalcophile (Co, Cu, Cd, Zn, Pb) elements, suggesting that euxinic conditions were predominant during the deposition of the Devede Formation. Integration of this data (Ce/Ce* and Eu/Eu*) with that of analogous late Tonian (ca. 840 to 731 Ma) carbonate strata reveals that shallow marine settings were characterised by persistent anoxia and euxinia, suggesting that any potential increase in atmospheric O2 ca. 800 Ma (e.g. the Bitter Springs Anomaly) was insufficient to facilitate resilient ocean oxygenation. These findings suggest that the late Tonian oceans were likely challenging environments for complex (e.g. eukaryotic) life, and add to a growing body of evidence that the spatially variable redox conditions of late Proterozoic shallow marine settings likely reflect the complex nature of the Neoproterozoic Oxygenation Event.

A Bayesian Approach to Inferring Depositional Ages Applied to a Late Tonian Reference Section in Svalbard

Increasing application of high precision uranium-lead (U-Pb) and rhenium-osmium (Re-Os) geochronology to the ancient geological record has resulted in massive improvement in age control and calibration of key Proterozoic stratigraphic successions and events. Nevertheless, some successions and time intervals remain poorly dated. Insufficient age constraints are particularly problematic for successions that are otherwise rich in geochemical, fossil, or other data with high potential to illuminate our understanding of Proterozoic Earth history. The latter Tonian succession in northeastern Svalbard is one such example. The ca. 820–740 Ma Akademikerbreen and lowermost Polarisbreen groups contain important microfossils and well-established carbon- and strontium-isotopic records, but they remain poorly dated. Here we use radioisotopic dates correlated from other Tonian successions across the globe using carbon isotope chemostratigraphy to calibrate a Tonian composite section in Svalbard by integrating Bayesian inference with a simple 1D thermal subsidence model. This approach allows us to assign realistic ages and uncertainties to all stratigraphic heights in a Akademikerbreen-lower Polarisbreen composite reference section. For example, the Bayesian age-height model yields ages for the onset and end of the Bitter Springs negative carbon isotope anomaly of 808.7 +3.3/−3.5 Ma and 801.9 +3.2/−3.3 Ma, respectively, and a total duration of 6.9 ± 0.2 Ma. These age and duration estimates can be applied to calibrate other Tonian successions that capture the Bitter Springs anomaly assuming that this anomaly is globally correlative.

Groundwater sources for the Mataranka Springs (Northern Territory, Australia)

The Mataranka Springs Complex is the headwater of the iconic Roper River of northern Australia. Using environmental tracers measured in springs and nearby boreholes, the origin of groundwater contributing to the springs was evaluated to help assess the impact of proposed groundwater extraction in the Cambrian Limestone Aquifer (CLA) for irrigation agriculture and for hydraulic fracturing in the Beetaloo Sub-basin (an anticipated world-class unconventional gas reserve). Major ions, Sr, 87Sr/86Sr, δ18O-H2O, δ2H-H2O, 3H, 14C-DIC were consistent with regional groundwater from the Daly and Georgina basins of the CLA as the sources of water sustaining the major springs (Rainbow and Bitter) and one of the minor springs (Warloch Pond). However, 3H = 0.34 TU in another minor spring (Fig Tree) indicated an additional contribution from a young (probably local) source. High concentrations of radiogenic 4He (> 10–7 cm3 STP g–1) at Rainbow Spring, Bitter Spring and in nearby groundwater also indicated an input of deeper, older groundwater. The presence of older groundwater within the CLA demonstrates the need for an appropriate baseline characterisation of the vertical exchange of groundwater in Beetaloo Sub-basin ahead of unconventional gas resource development.

A Consistently High‐Latitude South China From 820 to 780 Ma: Implications for Exclusion From Rodinia and the Feasibility of Large‐Scale True Polar Wander

AbstractThe Tonian supercontinent Rodinia is hypothesized to have included almost all Proterozoic continental blocks. Competing models variably place South China at the core or periphery of Rodinia or separated from it entirely. Tonian paleogeographic models also vary in whether they incorporate hypothesized large and rapid oscillatory true polar wander associated with the ca. 810–795 Ma Bitter Springs Stage. Here, we present paleomagnetic data paired with U‐Pb chemical abrasion isotope dilution thermal ionization mass spectrometry zircon geochronology from the Tonian Xiajiang Group in South China to establish the craton's position and test the Bitter Springs Stage true polar wander hypothesis. Fine‐grained siliciclastic sediments and ashes of the Xiajiang Group post‐date the Jiangnan Orogeny, which united the Yangtze and Cathaysia blocks. A U‐Pb zircon date of 815.73 ± 0.18 Ma from a tuff near the base of the Xiajiang Group constrains the Jiangnan Orogeny to have ended between ca. 830 and 816 Ma. The paleomagnetic and geochronologic data constrain South China to high latitudes ca. 813 Ma and indicate a relatively stable high‐latitude position from ca. 821 to 805 Ma. These high‐latitude constraints either connect the craton to Rodinia along its periphery or disconnect it from the supercontinent entirely. The difference in pole position between the pre‐Bitter Springs Stage Xiajiang Group pole and the syn‐Bitter Springs Stage Madiyi Formation pole is significantly less than that predicted for the Bitter Springs Stage true polar wander hypothesis. If this pole difference is interpreted as true polar wander superimposed upon differential plate motion, it requires South China to have been separate from Rodinia.

The Neoproterozoic Gillen Formation, Amadeus Basin, central Australia: an intra-salt petroleum system and viable exploration target?

The Gillen Formation, the basal component of the lower Neoproterozoic Bitter Springs Group, occurs throughout the Amadeus Basin of central Australia and comprises a basal sequence of black shales and stromatolitic dolostones overlain by an evaporitic sequence of a thick halite sandwiched between two anhydrite/dolomite units. These Gillen lithologies include potential source, reservoir and seal units for petroleum, suggesting an analogy could be drawn with the prolific evaporitic Ara Group Petroleum System of South Oman. Although well data penetrating the Gillen Formation are very limited, organic-rich shales have been penetrated in both the north and south of the basin, while both well and seismic data indicate the ubiquitous presence of sealing evaporites across the basin. The Ara Group Petroleum System spans the Neoproterozoic–Cambrian boundary and is blessed with carbonate reservoirs composed of metazoan ‘reefs’ comprising Cloudina and Namacalathus, which provide an open framework porosity approaching 50%. By contrast, the Gillen Formation was deposited during the Tonian Period of the Neoproterozoic, some 300–350 million years older than the Ara Group, and has reservoir facies limited to stromatolitic carbonates with fenestral to vuggy porosities that are typically less than 15%. Nevertheless, seismic data reveal numerous intra-evaporite opportunities within the Gillen Formation that, seismically, are virtually indistinguishable from those being exploited in South Oman, suggesting that a Gillen Intra-Salt Petroleum System is a viable exploration target throughout a majority of the Amadeus Basin.

Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as δ53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth’s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of δ53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, δ53Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ53Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ53Cr values in skeletal carbonates, do not induce variability in δ53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the δ53Cr values of the waters from which they precipitated. However, it remains unclear if these δ53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.

High-resolution correlation between contrasting early Tonian carbonate successions in NW Canada highlights pronounced global carbon isotope variations

Carbonate successions older than the 800 Ma Bitter Springs isotope anomaly are sparse and this time is consequently poorly understood. The Shaler and Mackenzie Mountains supergroups of northwestern Canada, however, contain thick ca. 1050–750 Ma carbonate successions recording both deep- and shallow-water depositional environments that are interpreted to have been deposited in two separate, extensive epicratonic basins on the Rodinia supercontinent. When the contrasting sedimentology (benthic- versus pelagic-dominated carbonate production) and basin dynamics of two correlative carbonate successions from these basins are accounted for, a precise regional correlation can be made using sequence stratigraphic and carbon isotope data. This paper presents a regionally correlatable carbon isotope profile, containing substantial excursions between −2 and 7‰vpdb, that can be used to extend the global curve into early Neoproterozoic time prior to the Bitter Springs anomaly.

Revisiting the depositional environment of the Neoproterozoic Callanna Group, South Australia

The Callanna Group was deposited around 800 million years ago (Ma) during an interval in Earth’s history that saw a transition towards a more oxygenated atmosphere, increasing biodiversity among eukaryotic microfossils and climatic perturbations culminating in low-latitude glaciations. Previous researchers proposed that the Callanna basin was lacustrine and highly alkaline, which could provide important new insights into environmental cause-effect relationships at this time. To further interrogate these records, we examined standard biogeochemical proxies, including organic carbon and nitrogen isotopes, iron speciation, metal abundances and carbonate-associated sulfate. Much of the primary information has been lost because the rocks of the Callanna Group have experienced extensive metamorphism up to amphibolite facies and are altered by modern weathering. However, relics of these proxies, combined with sedimentological features, preserve evidence of redox stratification within this basin. Furthermore, our observations, in particular weakly fractionated nitrogen isotopes and abundant gypsum pseudomorphs, are incompatible with the interpretation of high alkalinity. The high salt content and occurrences of tidal indicators are most parsimoniously explained by frequent incursions of seawater. Thus, the Callanna Group cannot speak straightforwardly to environmental conditions in non-marine habitats at this time. Lastly, the absence of a large carbon isotope anomaly indicates that these rocks do not correlate with the Bitter Springs Formation.

Comparative multi-scale analysis of filamentous microfossils from the c. 850 Ma Bitter Springs Group and filaments from the c. 3460 Ma Apex chert

Filamentous microfossils belonging to Cephalophytarion from the 850 Ma Bitter Springs Group have previously been used as key analogues in support of a biological interpretation for filamentous objects from the 3460 Ma Apex chert. Here we provide a new perspective on this interpretation by combining Raman data with correlative electron microscopy data from both Cephalophytarion and Apex specimens. We show that, when analysed at high spatial resolution, the Apex filaments bear no morphological resemblance to the younger Bitter Springs microfossils. Cephalophytarion filaments are shown to be cylindrical, comprising chains of box-like cells of approximately constant dimensions with lateral kerogenous walls and transverse kerogenous septa. They exhibit taphonomic shrinkage and folding, possess fine cylindrical sheaths and are permineralized by sub-micrometric quartz grains. They fulfil all established biogenicity criteria for trichomic microfossils. In contrast, Apex filaments do not possess lateral cell walls, are not cylindrical in nature, and vary considerably in diameter along their length. Their kerogenous carbon does not have a cell-like distribution and their chemistry is consistent with an origin as exfoliated phyllosilicate grains. This work demonstrates the importance of high-resolution data when interpreting the microstructure, and origins, of putative Precambrian microfossils.

Linking the Bitter Springs carbon isotope anomaly and early Neoproterozoic oxygenation through I/[Ca + Mg] ratios

The ca. 810 Ma Bitter Springs carbon isotope anomaly is an abrupt and long-lived (5–10 Myr.) departure from the positive carbon isotope values (δ13Ccarb) that otherwise characterize early-middle Neoproterozoic carbonate rocks. The onset of this isotope anomaly is manifested by a globally expressed ~8‰ negative shift in δ13C values in shallow marine carbonate strata. Given increasing evidence that metazoan diversification began well before the Ediacaran Period, the Bitter Springs anomaly is a logical interval to explore the potential relationship between biological innovation, perturbations to the carbon cycle, and oxygenation. The iodine-to-calcium+magnesium ratio (I/[Ca + Mg]) in marine carbonates is a sensitive proxy for seawater redox conditions that is increasingly being applied to reconstruct the oxygenation of the global surface ocean through Earth's history. We report I/[Ca + Mg] ratios, along with carbon and oxygen isotope ratios and major and minor element concentrations from carbonate sections spanning the Bitter Springs anomaly in Svalbard, East Greenland, and the Mackenzie Mountains (northwestern Canada). The results from Svalbard and Greenland collectively show two prominent features. The early stage of the Bitter Springs anomaly is characterized by a negative δ13C shift coupled to low I/[Ca + Mg] ratios compared to carbonates pre- and post-dating the Bitter Springs anomaly. The last stage of the Bitter Springs anomaly displays a positive excursion in I/[Ca + Mg], with the highest values at ~7 μmol/mol yet documented in rocks older than 580 Ma. In contrast, carbonates from the Mackenzie Mountains are uniformly low and display no variation in I/[Ca + Mg] across the Bitter Springs anomaly, which we interpret to be the consequence of alteration of primary signatures during diagenesis. The observed geochemical variations in our Svalbard and Greenland datasets are interpreted to be the result of a shift from pre-Bitter Springs ocean conditions, defined by well oxygenated surface waters and anoxic-ferruginous bottom waters, to syn-Bitter Springs ocean conditions, characterized by expanded euxinia.

Geochemistry of carbonate formations of the Chhattisgarh Supergroup, central India: implications for Mesoproterozoic global events

The Chhattisgarh Supergroup is one of the major Proterozoic marine sedimentary sequences of India. It consists of largely undeformed and unmetamorphosed siliciclastic, volcaniclastic, and carbonate formations deposited in two sub-basins, Hirri and Bharadwar, separated by an Archean greenstone belt. In spite of its apparent importance for Mesoproterozoic oceanic records, very few geochemical studies have been carried in the basin. Here, we present results of our high resolution geochemical and C–O–Sr isotopic studies in two carbonate formations of the supergroup: the Charmuria and the Chandi. We observe elevated δ13C values increasing from 2.6‰ to 3.6‰ in these formations, which is consistent with the globally reported late Mesoproterozoic values. Such consistently positive δ13C values are attributed to increased organic carbon burial in the basin margins during the deposition of these carbonates. Based on the principles of δ13C isotope stratigraphy, we suggest a depositional age between 1.0 and 1.2 Ga for these carbonates which form the upper part of the supergroup. The lowest 87Sr/86Sr ratios obtained from the Charmuria and Chandi formations, 0.70723 and 0.70816, respectively, are more radiogenic than the contemporaneous seawater, suggesting that the Sr isotopic system of the formations are altered. Based on the similarity in the δ13C values, we stratigraphically correlate the carbonate formations of the Raipur Group in both the Hirri and Bharadwar sub-basins. We also present a compilation of available δ13C and 87Sr/86Sr records from all the Proterozoic sedimentary successions of India and compare it with the global datasets. We find that while the Indian basins possess records of the Bitter Springs and Shuram δ13C anomalies, they lack evidence for the other major global events of the Proterozoic.

Changes of aliphatic C-H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy.

Aliphatic C-H bonds are one of the major organic signatures detected in Proterozoic organic microfossils, and their origin is a topic of interest. To investigate the influence of the presence of silica on the thermal alteration of aliphatic C-H bonds in prokaryotic cells during diagenesis, cyanobacteria Synechocystis sp. PCC6803 were heated at temperatures of 250-450°C. Changes in the infrared (IR) signals were monitored by micro-Fourier transform infrared (FTIR) spectroscopy. Micro-FTIR shows that absorbances at 2,925cm-1 band (aliphatic CH2 ) and 2,960cm-1 band (aliphatic CH3 ) decrease during heating, indicating loss of the C-H bonds, which was delayed by the presence of silica. A theoretical approach using solid-state kinetics indicates that the most probable process for the aliphatic C-H decrease is three-dimensional diffusion of alteration products under both non-embedded and silica-embedded conditions. The extrapolation of the experimental results obtained at 250-450°C to lower temperatures implies that the rate constant for CH3 (kCH3 ) is similar to or lower than that for CH2 (kCH2 ; i.e., CH3 decreases at a similar rate or more slowly than CH2 ). The peak height ratio of 2,960cm-1 band (CH3 )/2,925cm-1 band (CH2 ; R3/2 values) either increased or remained constant during the heating. These results reveal that the presence of silica does affect the decreasing rate of the aliphatic C-H bonds in cyanobacteria during thermal maturation, but that it does not significantly decrease the R3/2 values. Meanwhile, studies of microfossils suggest that the R3/2 values of Proterozoic prokaryotic fossils from the Bitter Springs Group and Gunflint Formation have decreased during fossilization, which is inconsistent with the prediction from our experimental results that R3/2 values did not decrease after silicification. Some process other than thermal degradation, possibly preservation of specific classes of biomolecules with low R3/2 values, might have occurred during fossilization.

Stratigraphy and geochemistry of the Balwan Limestone, Vindhyan Supergroup, India: Evidence for the Bitter Springs δ13C anomaly

The Vindhyan Supergroup of India, deposited in an intracratonic basin, is one of the important Proterozoic marine successions of the world that contains some of the most controversial Precambrian fossil discoveries. Despite their importance, the chronology of the host strata and global correlation of the events that occurred within the basin remain equivocal. Here, we present results of a detailed geological, geochemical and isotopic (Sr-C-O) study of the Balwan Limestone, the youngest carbonate formation of the supergroup, exposed only in the western sector of the basin. Our results suggest that the Vindhyan Basin had become a structurally controlled marginal sea towards the end of its existence and that the limestone was deposited in a subtidal environment that had strong depositional currents. We find evidence for a strong storm event or a tsunami during its deposition. Near primary 87Sr/86Sr of 0.70676 at the top and 207Pb-206Pb age of 866 ± 90 Ma (Gopalan et al., 2013) of this ∼120 m thick formation suggest its deposition during Late Tonian. δ13C stratigraphy reveals the presence of the globally synchronous Bitter Springs anomaly (∼12‰ shift) in the formation, the first such report from India.

Exploration of the south-eastern part of the Frontier Amadeus Basin, Northern Territory, Australia

The Neoproterozoic to Late Paleozoic-aged Amadeus Basin is a large (~170 000 km2) east–west-trending basin, bounded to the south by the Musgrave Province and to the north by the Arunta Block of the Northern Territory. Commercial oil and gas production is established in the northern part of the basin but the southern part is still a frontier exploration area. Vintage and new seismic reflection data have been used with well data along the south-eastern Amadeus Basin to construct a new structural and depositional model. Three major phases of deformation controlling deposition have been identified. The first phase is characterised by a SW–NE trending structural fabric and is thought to be older than the deposition of the first sediments identified above basement (Heavitree and Bitter Springs formations). The second phase corresponds to the Petermann Orogeny (580–540 Ma) and trends in a NW–SE orientation. The third phase is the Alice Springs Orogeny (450–300 Ma) and is oriented W–E to WNW–ESE in this part of the basin. This tectono-stratigraphic model involving three distinct phases of deformation potentially explains several critical observations: the lack of Heavitree reservoir at Mt Kitty-1, limited salt movements before the Petermann Orogeny (~300 Ma after its deposition) and salt-involved structures that can be either capped by the Petermann Unconformity and overlying Cambrian to Devonian sediments, or can reach the present day surface. Finally, this model, along with availability of good quality seismic data, opens new perspectives for the hydrocarbon exploration of the Amadeus Basin. Each of the tectonic phases impacts the primary petroleum system and underpins play-based exploration.

Iodine proxy evidence for increased ocean oxygenation during the Bitter Springs Anomaly

Iodine proxy evidence for increased ocean oxygenation during the Bitter Springs Anomaly

Vase-shaped microfossil biostratigraphy with new data from Tasmania, Svalbard, Greenland, Sweden and the Yukon

The recent revision of the Cryogenian lower boundary from 850 Ma to 720 Ma has nearly doubled the duration of the Tonian Period to 280 Myrs, an interval longer than the Mesozoic and Cenozoic eras combined. Although subdivision of the Tonian will likely emphasize chemostratigraphy, the relatively rich fossil record of this time period may also be useful. In particular, vase-shaped microfossils (VSMs) occur globally and often abundantly in a variety of lithofacies, exhibit a variety of preservational modes, and are stratigraphically constrained by the two major carbon isotopic events of the Tonian Period, the Bitter Springs and Islay anomalies. VSMs have been used as informal, grade-level indicators of late Tonian age (e.g. ca. 800–700 Ma) for several decades but here, based on new data from an assemblage in northwestern Tasmania and restudy of samples from Svalbard, Greenland, Sweden and the Yukon, we report a species-level faunal assemblage indicative of the interval ca. 789 to 729 Ma. Species-level VSM biostratigraphy presents an opportunity for potential biostratigraphic subdivision of the lengthy Tonian Period and is particularly useful in successions where chemostratigraphy and radiometric age constraints are absent.Eight taxa are described from shales and early diagenetic cherts of the Black River Dolomite, Togari Group, Tasmania, including seven previously named species (Bombycion micron, Bonniea dacruchares, Bonniea pytinaia, Cycliocyrillium simplex, Cycliocyrillium torquata, Melanocyrillium hexodiadema and Palaeoarcella athanata), and one newly described genus and species Pakupaku kabin n. gen et n. sp. Study of additional units resulted in new recognition of B. pytinaia, C. simplex and C. torquata in the Draken Formation, Spitsbergen, Svalbard, B. dacruchares and P. athanata in the Ryssö Formation, Nordaustlandet, Svalbard, B. dacruchares, B. pytinaia, C. torquata and P. athanata in Bed 18 of the Eleonore Bay Group, Greenland, and B. dacruchares, B. pytinaia and Bombycion micron in the Visingsö Group, Sweden.

Paleoecology of Neoproterozoic hypersaline environments: Biomarker evidence for haloarchaea, methanogens, and cyanobacteria.

While numerous studies have examined modern hypersaline ecosystems, their equivalents in the geologic past, particularly in the Precambrian, are poorly understood. In this study, biomarkers from ~820 million year (Ma)-old evaporites from the Gillen Formation of the mid-Neoproterozoic Bitter Springs Group, central Australia, are investigated to elucidate the antiquity and paleoecology of halophiles. The sediments were composed of alternating laminae of dolomitized microbial mats and up to 90% anhydrite. Solvent extraction of these samples yielded thermally well-preserved hydrocarbon biomarkers. The regularly branched C25 isoprenoid 2,6,10,14,18-pentamethylicosane, the tail-to-tail linked C30 isoprenoid squalane, and breakdown products of the head-to-head linked C40 isoprenoid biphytane, were particularly abundant in the most anhydrite-rich sediments and mark the oldest current evidence for halophilic archaea. Linear correlations between isoprenoid concentrations (normalized to n-alkanes) and the anhydrite/dolomite ratio reveal microbial consortia that fluctuated with changing salinity levels. Halophilic archaea were the dominant organisms during periods of high salinity and gypsum precipitation, while bacteria were prevalent during stages of carbonate formation. The irregularly branched C25 isoprenoid 2,6,10,15,19-pentamethylicosane (PMI), with a central tail-to-tail link, was also abundant during periods of elevated salinity, highlighting the activity of methanogens. By contrast, the irregularly branched C20 isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane) was more common in dolomite-rich facies, revealing that an alternate group of archaea was active during less saline periods. Elevated concentrations of isotopically depleted heptadecane (n-C17 ) revealed the presence of cyanobacteria under all salinity regimes. The combination of biomarkers in the mid-Neoproterozoic Gillen Formation resembles lipid compositions from modern hypersaline cyanobacterial mats, pointing to a community composition that remained broadly constant since at least the Neoproterozoic. However, as a major contrast to most modern hypersaline environments, the Gillen evaporites did not yield any evidence for algae or other eukaryotes.

Neoproterozoic carbon isotope stratigraphy of the Amadeus Basin, central Australia

Neoproterozoic shallow-marine and carbonate- rich strata of the northern Amadeus Basin, central Australia, comprise a geochemical record of Neoproterozoic seawater that facilitates correlation with other basins in Australia and worldwide. This study includes new carbon and oxygen isotope measurements of Cryogenian and Ediacaran carbonate in the northern Amadeus Basin that, in combination with previous data from Tonian strata of the Bitter Springs Formation, form a composite Amadeus Basin Neoproterozoic δC record. Key features of the Cryogenian-Ediacaran portion of this record include (1) negative δC excursions in cap carbonates overlying Cryogenian glacigenic deposits of the Areyonga and Olympic Formations (i.e., the Rasthof and Maieberg excursions, respectively); (2) a mid-Cryogenian negative excursion in the Ringwood Member of the Aralka Formation (seemingly correlative with the Taishir excursion of SW Mongolia); (3) a subsequent mid-Cryogenian positive excursion (the Keele Peak excursion) in the Limbla Member of the Aralka Formation; and (4) relatively low-magnitude excursions in the Ediacaran Julie Formation. In comparison with other Neoproterozoic stratigraphic successions, key portions of the overall Neoproterozoic δC record that appear to be absent from the Amadeus Basin based on existing data include large-magnitude negative excursions (Islay and Trezona excursions) that immediately preceded basaland terminal-Cryogenian (i.e., Sturtian and Marinoan, respectively) glaciations. These missing δC excursions are associated with significant unconformities in the Amadeus Basin. δC and δO from carbonate clasts in diamictite of the Areyonga and Olympic Formations suggest that deposition of Islay- and Trezona-equivalent strata was limited or nonexistent in the Amadeus Basin, as opposed to the possibility that these strata were deposited but subsequently removed by glacial erosion. Our new Amadeus Basin results refine Cryogenian and early Ediacaran correlations among the Amadeus Basin, Adelaide rift complex, and Namibia. The lower part of the Ringwood Member, which was deposited during early stages of the Cryogenian nonglacial interlude, includes two negative δC excursions that are also evident in previous results from the Tapley Hill Formation of the Adelaide rift complex. The upper part of the Ringwood Member contains abundant stromatolites and seems to correlate with Cryogenian stromatolite reefs of the Balcanoona Formation in the Adelaide rift complex. The overlying Limbla Member in the Amadeus Basin correlates with the Yankaninna, Etina, Amberoona, and Enorama Formations of the Adelaide rift complex, as well as carbonate from other continents that record the Keele Peak excursion. δC from the early Ediacaran Olympic cap of the Amadeus Basin is (1) comparable to previous results from the Maieberg and Elandshoek Formations in Namibia; and (2) consistent with diachronous deposition of Marinoan cap dolostone in the Amadeus Basin. Additionally, new δC from the Julie Formation of the Amadeus Basin is similar to previous results from the late Ediacaran Kuibis Subgroup in southern Namibia. These correlations underscore that Ediacaran strata of Namibia, as well as the Cryogenian nonglacial interlude strata of the Adelaide rift complex, are roughly an order of magnitude thicker than stratigraphic counterparts in the Amadeus Basin. The wide disparity in sedimentation rates implied by this finding is likely attributable to fundamental differences in the Neoproterozoic tectonic settings of the three locations.

Unradiogenic strontium and moderate-amplitude carbon isotope variations in early Tonian seawater after the assembly of Rodinia and before the Bitter Springs Excursion

Neoproterozoic sedimentary records reveal a poorly documented, ∼150m.y. time interval, between ∼1000 and 850Ma, that limit our understanding of C and Sr isotope variations in seawater and thus operation of the biogeochemical carbon cycle, changes in surface redox state, and fluctuations in continental vs. hydrothermal fluxes to the oceans following assembly of the supercontinent Rodinia. Carbon and Sr isotope data for carbonates from the Karatau Group of the Southern Ural Mountains in Russia provides a record partially covering the younger portion of this time interval. The lower part of the Karatau Group (LKG) contains well-preserved carbonate strata of the Katav, Inzer, and Min’yar formations that are focus of this study. Pb-Pb isochron ages for carbonates from the Inzer and Min’yar formations are 844±24 and 820±77Ma, respectively, establishing an early Tonian age. Carbon isotope data for unaltered carbonates in the LKG show a range of moderately positive to negative values from −2.8 to +5.9‰, with the majority below +3.0‰. 87Sr/86Sr values range from 0.70522 to 0.70534 in the Lower Inzer Member, increasing in the overlying Upper Inzer Member and Min’yar Formation, from 0.70555 to 0.70600. The Sr isotope range of the Lower Inzer Member is similar to that typical for the 1.03–0.95Ga seawater (0.70519–0.70554), suggesting that the Sr isotope composition of seawater in the aftermath of Rodinian assembly was unradiogenic for 150m.y. This pattern indicates that supercontinent-scale orogenic events do not result in an enhanced, long-term flux of radiogenic Sr from continents once the supercontinent assembled. To account for the prolonged period characterized by unradiogenic Sr seawater composition and moderate-amplitude C isotope variations, we suggest a supercontinent configuration ringed by continental arcs and with predominantly internal runoff. In this model, accompanying supercontinent blanketing resulted in thermal perturbation in the mantle, emplacement of multiple Large Igneous Provinces, and high continental freeboard. Weathering of juvenile arcs would have provided unradiogenic Sr and a high flux of sediments to the oceans, enhancing organic carbon burial and progressive oxygenation of surface environments and contributing to instability in the biogeochemical carbon cycle. Mass anomalies induced by protracted mantle plume activity led to True Polar Wander and brought the supercontinent, covered with juvenile mafic volcanics and segmented by failed rift systems, to low latitudes thus enhancing chemical weathering, atmospheric CO2 consumption, and eventually ushering in Snowball Earth conditions. Comparison with the records for early Paleoproterozoic strata indicates that a similar sequence of events bracketed both ends of the Proterozoic, highlighting fundamental relationships among plate tectonics, mantle dynamics, biogeochemical carbon cycling, surface oxygenation, and climate change.

Neoproterozoic evaporites and their role in carbon isotope chemostratigraphy (Amadeus Basin, Australia)

The Tonian Bitter Springs Group (∼830–750Ma), within the Amadeus Basin in central Australia consists of thick halite and sulphate evaporite accumulations. The deposition of halite occurred in shallow marine, lagoon (salina) environment (Gillen Formation), and developed into sulphate-dominated supratidal sabkha during sea level regression (Johnnys Creek Formation). The overall regression was interrupted by a transgressive phase lasting at least 20Ma and leading to deposition of basin-wide stromatolitic dolostone (Loves Creek Formation). The salinity and high evaporation is reflected in positive δ13C in carbonates (+4 to +6‰ VPDB) of the evaporitic units, while the shallow marine incursion of the Loves Creek Formation (−2‰ δ13C) show typical marine carbonate isotopic values. The Bitter Springs Group chemostratigraphy has been correlated globally and the negative excursion was named previously after this unit (Bitter Springs Stage anomaly). However, the mechanism of evaporation-driven fractionation of δ13C is different from the commonly proposed inorganic-organic carbon fractionation, and challenges the views on interpreting global chemostratigraphic anomalies or excursion and their cause.Evaporite basins covered vast regions worldwide prior to the Sturtian glaciation, e.g. the Australian evaporites would have covered a third of the continent. The light surface of evaporites and associated carbonates would have had a high albedo and effectively cause less surface heat absorption. This subsequently would have triggered temperature decrease on a continental and possibly global scale. This paper hypothesises that the deposition of evaporites worldwide would have contributed to global cooling starting ∼100Ma prior to Snowball Earth and would have played an important role in global glaciation.