Coda of the snowball: combined U-Pb LA-ICPMS dating of calcite-after-aragonite crystal fans and clumped isotope thermometry of Ediacaran cap carbonates

Microfacies, diagenesis and hydrocarbon potential of the Neoproterozoic cap carbonate of the southern Amazon Craton

<p>Fans and hemispheres comprised of crystals of calcium carbonate that directly grew on the seafloor are an intriguing feature of ancient carbonate environments. Calcium carbonate fans from across Earth history are typically made up of crystals of neomorphosed calcium carbonate that maintain an acicular morphology that is pseudohexagonal in cross – section with blunt terminations, pointing to an aragonite precursor. Crystal fans may occur as isolated bodies, or may form larger aggregates that are sometimes associated with microbialites, and form larger, reef – like structures. Some of the first occurrences of these features are within Neoarchean carbonates, when the crystals fans grew to impressive sizes, with lengths of over 1 m, formed layers of marine cement that reached thicknesses of up to several meters, are laterally continuous over 10s of km or more, and formed across carbonate platform settings from high energy subtidal settings to lower intertidal environments. Crystalline carbonate fans become less common and smaller (cm – scale) in the Paleoproterozoic, and nearly disappear prior to the Neoproterozoic, when they are associated with cap carbonates, and are primarily found in deeper water or outer shelf settings with low sedimentation rates. Seafloor cements are rare during the Phanerozoic, and are typically limited to small geographic areas with unusual sedimentary conditions, or are found in void spaces, where seawater chemistry was able to undergo modifications that would allow precipitation of cement fans. The exception is during the interval of time that followed the Permian – Triassic mass extinction, when small, cm – scale fans and hemispheres are found in Lower Triassic and lowermost Middle Triassic rocks. These cement fans occur in a variety of settings, although they are typically found in deeper water environments. Calcium carbonate fans that formed following the Permian – Triassic extinction may have microbial remains preserved within the cements, and are frequently found in close lateral or stratigraphic association with microbialites. However, some examples of post – extinction carbonate fans appear to have formed abiotically, without any microbial influence. Overall, crystalline calcium carbonate fans signal high levels of calcium carbonate supersaturation in ancient oceans. The initial decline in seafloor cement growth from the Neoarchean into the Proterozoic may have been the result of accelerated micrite production, while Neoproterozoic calcium carbonate fan growth is associated with glacial decay and retreat. Lower Triassic seafloor cements are likely the result of stratification and stagnation of the deep oceans that led to enhanced alkalinity. Calcium carbonate crystal fans are an intriguing feature of ancient carbonates that signal depositional systems and ocean chemistry that is much different from modern ocean, and provide a fascinating glimpse into non – uniformitarian sedimentary environments.</p>

Neoproterozoic cap carbonates are characterized by “anomalous” sedimentary features, including sea-floor precipitates, represented by aragonite pseudomorph cement crusts and crystal fans. These features are found in restricted periods of geological history, and their generation in the context of the Neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation. New sedimentary and geochemical results on well-preserved Neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the Sete Lagoas Formation (Bambui Group, central Brazil) are reported. The micrite-settling facies shows the highest abundance of crystal fans, arranged in centimeter-scale layers interfingered with the micrite matrix; this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters. A numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pCO2. In these conditions, the Mg/Ca ratio of the Neoproterozoic seawater is calculated at ca. 1.2. Alkalinity oversaturation is monitored by a variable evaporation degree. Aragonite formation is kinetically favored, and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pCO2, hence alkalinity, is higher. These modeling conditions do not preclude the contribution of incomplete organic-matter degradation to alkalinity as suggested by negative carbon isotope signatures. In addition, the large number of aragonite–micrite pairs suggests a seasonal or paleoclimatic forcing. Other cases of abiotic aragonite precipitation through time are also briefly examined by comparison.

Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were precipitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern seawater, with a weaker hydrothermal signature than Archaean–Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the meltdown of ice-sheets was rapid, cap dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap dolostones, either as digitate seafloor cement associated with Fe-dolomite at the top of the cap dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like breccias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phosphorite formed during a Neoproterozoic deglaciation.

Phosphogenesis, aragonite fan formation and seafloor environments following the Marinoan glaciation

The Mirassol d'Oeste Formation (cap dolomites) and the base of the Guia Formation (cap limestones) represent the cap carbonates of the Neoproterozoic Araras Group, located along the southern border of Amazonian Craton, central Brazil. Petrographical and microfacies descriptions together with geochemical and mineralogical data of the top of the Mirassol d'Oeste and the base of the Guia formations in the Tangará da Serra area allowed us to identify the main diagenetic features (neomorphism, chemical compaction, and fractures) and to evaluate the terrigenous contribution. Dolomitization has been documented in a limestone horizon of the lower part of Guia Formation through petrography, X-ray diffractometry, and geochemistry. Despite diagenetic alteration, sedimentary structures of the Tangará da Serra cap carbonate are well preserved (laminar bedding and crystal fans). Pb–Pb dating of the Guia limestones yielded an age of 622 ± 33 million years. This early Ediacaran age is considered as the deposition age and constitutes further evidence that the cap carbonates of the Araras Group are related to the Marinoan glaciation, which took place at the end of the Cryogenian. The assessment of the primary 87Sr/86Sr signature of the Guia limestones has been improved by sequential leaching of diluted acetic acid, which allowed the elimination of highly radiogenic Sr contributions due to terrigenous grains. The 87Sr/86Sr ratios of 0.70709–0.70729 are regarded as the primary Sr isotopic signature when the carbonates precipitated. The terrigenous contribution can explain the discrepancy with the more radiogenic 87Sr/86Sr ratios previously obtained on the cap carbonate. At a global scale, the Sr isotopic signature around 0.7071–0.7073 of the cap carbonates of the Araras Group is compatible with Sr marine evolution curves at the end of the Marinoan glaciation. However, such low Sr values suggest that the abrupt rise could have happened asynchronously and heterogeneously in the early Ediacaran oceans.

Mercury (Hg), a highly volatile metal, is capable of tracing volcanism through geological history as LIP events transiently emit large amounts of Hg. There are two indicators that make Hg a unique tool for geochemistry, the Hg to total organic carbon ratio (Hg/TOC) and mass-independent fractionation (Hg-MIF, defined as Δ199Hg). Owing to the affinity of Hg to organic matter, anomalous high Hg/TOC ratios in sediments can better reveal large volcanic eruptions. The anomaly of Hg-MIF is mainly observed in Hg photoreactions, providing a fingerprints of specific reaction pathways of Hg. Volcanic Hg usually has Δ199Hg ~ 0, but photochemical processes in the surface environment can alter this signal, resulting in positive Δ199Hg in marine systems (e.g., seawater and marine sediments) and negative Δ199Hg in terrestrial systems (e.g., soil and vegetation). Here, we examined the Hg records in Ediacaran cap carbonates in South China and Upper Cretaceous oceanic red beds (ORBs) in southern Tibet and the North Atlantic, to obtain their sedimentary material sources and the cause of the termination of Marinoan glaciation and Cretaceous oceanic anoxic events. (1) The cap carbonates show higher Hg concentrations (4.9 to 405 ppb), most of which are comparable to that observed in carbonates deposited during non-LIPs periods. The lack of Hg/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 Δ199Hg values (0.18 to 0.34 ‰) in slope settings and slightly negative to slightly positive Δ199Hg 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 CO2 and enhanced release of gas hydrates, triggered global warming. This warming led to melting of sea ice cover, enhanced terrestrial inputs, and large-scale dissolution of atmospheric CO2 into seawater, driving widespread deposition of Ediacaran cap carbonates. (2) In southern Tibet and the North Atlantic, black/gray shales (typical deposition of oceanic anoxic events) show much higher Hg concentrations and Hg/TOC values than ORBs, indicating enhanced Hg flux to global oceans during time of black/gray shale deposition. Black/gray shales show lower Fe3+/Fe2+ and positive Δ199Hg, suggesting a significant input of Hg into the anoxic/dysoxic ocean via atmospheric deposition. The isotope values are consistent with a volcanic source for this excess Hg. ORBs show high Fe3+/Fe2+ and negative shifts of Δ199Hg, suggesting that the dominant source of Hg into the oxic oceans was via terrestrial runoff. These results suggest that volcanism was an important driver of the climate/ocean dynamics during the Late Cretaceous. To sum up, in addition to indicating short-strong volcanic activities, Hg can also trace the source of sedimentary materials under weak magmatism. Moreover, Hg offers a more accurate depiction of the interactions and exchanges among the Earth’s atmosphere-ocean-land system.

CO2 buildup drove global warming, the Marinoan deglaciation, and the genesis of the Ediacaran cap carbonates

Marinoan glaciation in east central Brazil

Tubestone microbialite association in the Ediacaran cap carbonates in the southern Paraguay Fold Belt (SW Brazil): Geobiological and stratigraphic implications for a Marinoan cap carbonate

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The Neoproterozoic Era includes some of the most largest ice ages in the geological history. The exact number of glaciations is unknown, though there were at least two events of global glaciation. Neoproterozoic glacial deposits in the Kuruktag Mountain, Xinjiang, western China have proven that there had occurred three discrete Neoproterozoic glaciations. Diamictite units occurred in the Bayisi, Tereeken, and Hankalchough formations, carbonate units were recognized among the diamictites and immediately overlied the Bayisi, Tereeken and Hankalchough diamictites. Carbonates at the top of the Bayisi Formation are characterized by the dolo-sility stones with negative δ13C values ranging from −4.10‰ to −8.17‰ (PDB), comparable to the Sturtian cap carbonates that overlie the Sturtian glacial deposits from other Neoproterozoic sequences. Carbonates overlying the Tereeken Formation are characterized by the pinkish cap dolostones (ca. 10 m thick) with negative δ13C values ranging from −2.58‰ to −4.77‰ (PDB), comparable to the Marinoan cap carbonates. The cap is also characterized by tepee-like structures, barite precipitates and pseudomorphous aragonite crystal fan limestones. Carbonates at the top of the Hankalchough Formation are characterized by subaerial exposure crust (vadose pisolite structure, calcareous crust structure) dolostones with negative δ13C values ranging from −4.56‰ to −11.45‰ (PDB) and the calcareous crust dolostones, implying that the Hankalchough cap carbonates differ from either the Sturtian or Marinoan cap carbonates in sedimentary environment and carbon isotopic composition. In addition, it is suggested the Hankalchough glaciation belongs to a terrestrial glaciation and it is the third largest glaciation during the Neoproterozoic period on the Tarim platform.

Synsedimentary deformation and the paleoseismic record in Marinoan cap carbonate of the southern Amazon Craton, Brazil

Micro porosity associated with micro-rhombic calcite provides volumetrically the largest pore volume in Middle Eastern Cretaceous carbonate reservoirs. With progressing reservoir development and depletion of macro-pore systems the low permeability micro pores have ever increasing importance as host to remaining hydrocarbons and for reservoir performance. A new technique is presented – Clumped Isotope Thermometry – which yields significant new insights on burial depth, timing of diagenesis and the nature of the diagenetic fluids. This in turn helps to better understand the widespread distribution of micro porosity in Abu Dhabi reservoirs. The genesis of micro-rhombic calcite has previously been placed into the shallow burial realm under the influence of meteoric or marine pore waters (Budd, 1989; Moshier, 1989) using oxygen stable isotopes and petrographic observations. However, the interpretation of oxygen isotope data is limited by the fact that the isotopic composition of calcite minerals is determined by two independent factors: the isotopic composition and the temperature of the precipitating water. The new analytical technique of clumped isotopes has established a way to independently determine one of the variables (temperature) thus allowing the other (water composition) to be fixed. The new technique has been applied to 15 samples from three cores of a giant Abu Dhabi reservoir. Samples are from the Barremian Thamama B reservoir unit. Stable isotope data exhibit a normal range for Thamama B calcite (δ13C: 2.8‰ to 3.9‰; δ18O: 4.6‰ to 5.9‰). Clumped isotope analyses indicate a most likely precipitation temperature of 65°C to 72°C and an isotopic composition of precipitating waters of 3.3‰ to 4.9‰ δ18O. Using a well calibrated basin model the temperature range places the depth of diagenesis between 0.65 km and 1.1 km and the timing of diagenesis into the Late Cretaceous (Turonian to Campanian). This coincides with the subsidence of the eastern Arabian plate margin caused by the opduction of oceanic crust (Semail ophiolite and associated marine sediments). The heavy oxygen isotopic composition of the diagenetic water of 3.3‰ to 4.9‰ δ18O corroborates a burial setting for diagenesis. Results imply that diagenesis was driven by a regional east to west burial flow system driven by compaction and convection with likely significant implications for the regional distribution of porosity & permeability.

Research Article| June 01, 2011 Formation of dolomite at 40–80 °C in the Latemar carbonate buildup, Dolomites, Italy, from clumped isotope thermometry John M. Ferry; John M. Ferry 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA Search for other works by this author on: GSW Google Scholar Benjamin H. Passey; Benjamin H. Passey 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA Search for other works by this author on: GSW Google Scholar Crisogono Vasconcelos; Crisogono Vasconcelos 2Geological Institute, ETH-Zürich, CH-8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar John M. Eiler John M. Eiler 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Geology (2011) 39 (6): 571–574. https://doi.org/10.1130/G31845.1 Article history received: 22 Oct 2010 rev-recd: 04 Feb 2011 accepted: 09 Feb 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation John M. Ferry, Benjamin H. Passey, Crisogono Vasconcelos, John M. Eiler; Formation of dolomite at 40–80 °C in the Latemar carbonate buildup, Dolomites, Italy, from clumped isotope thermometry. Geology 2011;; 39 (6): 571–574. doi: https://doi.org/10.1130/G31845.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The temperature of formation of replacement dolomite and δ18O(H2O) of dolomitizing fluid in the Latemar carbonate buildup, Dolomites, Italy, were estimated independently from carbonate clumped isotope thermometry. Dolomite formed at 42–72 ± 9–11 °C (±2 standard deviations, SD) from fluid with δ18O(H2O) that averages –0.3‰ ± 3.3‰ (Vienna standard mean ocean water; ±2 SD). The estimated temperature and δ18O(H2O) are similar to those of modern diffuse flow fluids at mid-ocean ridges, the kind of fluid that has been proposed previously as the dolomitizing fluid in the Latemar buildup, based on the trace element compositions of dolomite. Calcite in limestone preserves original δ18O, but records clumped isotope temperatures, 44−76 ± 9−11 °C (±2 SD), that are higher than those at which the limestone formed. Temperature recorded by calcite, but not δ18O, was likely reset during dolomitization. Clumped isotope thermometry has great potential for application to studies of burial and diagenesis by retrieving independent estimates of temperature and δ18O(H2O) with uncertainties as low as ±5 °C (±2 standard errors, SE) and ± 0.75‰ (±2 SE), respectively, from a single stable isotope analysis of a carbonate mineral. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.