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Micro-organisms have long been implicated in the construction of stromatolites. Yet, establishing a microbial role in modern stromatolite growth via molecular analysis is not always straightforward because DNA in stromatolites can have multiple origins. For example, the genomic material could represent the microbes responsible for the construction of the stromatolite (i.e., "builders"), microbes that inhabited the structure after it was built (i.e., "tenants"), or microbes/organic matter that were passively incorporated after construction from the water column or later diagenetic fluids (i.e., "squatters"). Disentangling the role of micro-organisms in stromatolite construction, already difficult in modern systems, becomes more difficult as organic signatures degrade, and their context is obscured. To evaluate our ability to accurately decipher the role of micro-organisms in stromatolite formation in geologically recent settings, 16/18S SSU rRNA gene sequences were analyzed from three systems where the context of growth was well understood: (a) an actively growing stromatolite from a silicic hot spring in Yellowstone National Park, Wyoming, where the construction of the structure is controlled by cyanobacteria; (b) a mixed carbonate and silica precipitate from Little Hot Creek, a hot spring in the Long Valley Caldera of California that has both abiogenic and biogenic components to accretion; and (c) a near-modern lacustrine carbonate stromatolite from Walker Lake, Nevada that is likely abiogenic. In all cases, the largest percentage of recovered DNA sequences, especially when focused on the deeper portions of the structures, belonged to either the tenant or squatter communities, not the actual builders. Once removed from their environmental context, correct interpretation of biology's role in stromatolite morphogenesis was difficult. Because high-throughput genomic analysis may easily lead to incorrect assumptions even in these modern and near-modern structures, caution must be exercised when interpreting micro-organismal involvement in the construction of accretionary structures throughout the rock record.

The Hartbeesfontein Basin contains the most extensive deposits of Archean lacustrine stromatolites on the Kaapvaal Craton, with stromatolitic facies occurring over ∼100 km 2 in beds up to 7 m thick. Stromatolitic dolostones and cherts both preserve evidence of microbial processes. Dolomitic stromatolites have grumelous microspar textures between organic-rich laminae, suggestive of carbonate precipitation within microbial mats. Stromatolitic laminae within chert preserve detrital material beyond the angle of repose, indicating the trapping and binding of grains by microbial mats. Stromatolitic cherts also preserve fenestral textures and filamentous microfossils. Many fenestrae have rounded shapes surrounded by filamentous laminae and appear to have formed in situ within stromatolite fabrics before lithification. Fenestrae within stromatolitic chert resemble “hourglass-associated fenestrae” noted from recent silica stromatolites from Yellowstone National Park, and are interpreted to originate from gas bubbles forming within stromatolite-building mats. The preservation of delicate structures in Hartbeesfontein stromatolitic chert (e.g., filamentous microfossils and gas-related fenestrae) implies rapid lithification of microbial mats, while the mm to cm scale of fenestrae indicate equally rapid rates of microbial gas production. Textural and mineralogical evidence associated with gas-related fenestrae support the presence of oxygenic photosynthesis, which in turn strengthens previous hypotheses on Archean lakes as potential oxygen oases before the Great Oxidation Event.

Lipid biomarkers preserved in ancient rocks have the potential to reveal much about ancient ecosystems. However, establishing that the compounds of interest are syngenetic has proven to be an analytically challenging task. Traditional biomarker analyses rely on extraction of large quantities of powdered rock, making the association of molecules with sedimentary fabrics difficult, if not impossible. As an alternative approach, here we show that monoclonal antibodies that bind specifically to geolipids can be used as molecular probes for in situ detection and localization of such compounds. Monoclonal antibodies that bind to squalene and cross-react with the biomarker squalane were evaluated for labeling sediment-associated hydrocarbons. The anti-squalene antibodies were shown by dot immunoblotting with composed standards to cross-react also with other isoprenoids, such as phytol and its diagenetic products, suggesting reactivity towards acyclic isoprenoids. Then, the anti-squalene antibodies were shown to react with naturally occurring crude oils and, via an immunofluorescence-labeling approach, to bind to isolated organic-rich laminae in rocks from the Eocene Green River Formation known to contain squalane among other linear isoprenoids. These results suggest that squalane, or structurally similar organic biomarkers that cross-react with the antibodies, are confined to discrete organo-sedimentary fabrics within those rocks, providing evidence for their syngeneity. Depending on the specificity and sensitivity of the antibody/geolipid pair, an in situ antibody detection approach may be useful for establishing biomarker syngeneity in older rocks.

Microbialites have long been a focus of study in geobiology because they are macroscopic sedimentary records of the activities of microscopic organisms. However, abiotic processes can result in microbialite-like morphologies. Developing robust tools for substantiating the biogenicity of putative microbialites remains an important challenge. Here, we report a new potential biosignature based on the detrital magnetic mineral component present in nearly all sedimentary rocks. Detrital grains falling onto a hard, abiogenic, chemically precipitated structure would be expected to roll off surfaces at high incline angles. Thus, the distribution of grains in an abiogenic microbialite should exhibit a dependence on the dip angle along laminae. In contrast, a microbialite formed by the active trapping and binding of detrital grains by microorganisms could exhibit a distribution of detrital grains significantly less dependent on the dip angle of the laminae. However, given that most ancient stromatolites are micritic (composed of carbonate mud), tracking detrital grains vs. precipitated carbonate is not straightforward. Recent advances in our ability to measure miniscule magnetic fields open up the possibility to map magnetic susceptibility as a tracer of detrital grains in stromatolites. In abiogenic carbonate precipitation experiments, magnetic susceptibility fell to zero when the growth surface was inclined above 30° (the angle at which grains rolled off). In cyanobacterial mat experiments, even vertically inclined mats held magnetic material. The results indicate that cyanobacterial mats trap and bind small grains more readily than abiogenic carbonate precipitates alone. A variety of stromatolites of known and unknown biogenicity were then analyzed. Tested stromatolites span many different ages (Eocene to Holocene) and depositional environments (hot springs, lakes), and compositional forms (micritic, sparry crusts, etc.). The results were consistent with the laboratory results. The results of these experiments suggest that magnetic susceptibility shows promise as a new biosignature in the study of putative microbialites.

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (delta Li-7) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the delta Li-7 and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals). Aragonitic mollusks have the lowest delta Li-7, ranging from +16 to +22%, echinoderms have constant delta Li-7 of about +24%, brachiopods have delta Li-7 of +25 to +28%, and finally calcitic mollusks have the largest range and highest delta Li-7 values, ranging from +25% to +40%. Measured brachiopods have similar delta Li-7 compared to inorganic calcite precipitated from seawater (delta Li-7 of +27 to +29%), indicating minimum influence of vital effects, as also observed for other isotope systems and making them a potentially viable proxy of past seawater composition. Calcitic mollusks, on the contrary, are not a good archive for seawater paleo-delta Li-7 because many samples have significantly higher delta Li-7 values than inorganic calcite and display large interspecies variability, which suggests large vital effects. In addition, we observe very large intra-shell variability, in particular for mixed calcite-aragonite shells (over 20% variability), but also in mono-mineralic shells (up to 12% variability). Aragonitic bivalves have less variable delta Li-7 (7% variability) compared to calcitic mollusks, but with significantly lower delta Li-7 compared to inorganic aragonite, indicating the existence of vital effects. Bivalves grown at various temperatures show that temperature has only a minor influence on fractionation of Li isotopes during shell precipitation. Interestingly, we observe a strong correlation (R-2 = 0.83) between the Li/Mg ratio in bivalve Mytilus edulis and temperature, with potential implications for paleo-temperature reconstructions. Finally, we observe a negative correlation between the delta Li-7 and both the Li/Ca and Mg/Ca ratio of calcite mollusks, which we relate to biomineralization processes. To explain this correlation, we propose preferential removal of 6 Li from the calcification site of calcite mollusks by physiological processes corresponding to the regulation of the amount of Mg in the calcifying medium. We calculate that up to 80% of the initial Li within the calcification site is removed by this process, leading to high delta Li-7 and low Li/Ca in some calcite mollusk specimens. Collectively, these results suggest that Mg (and thus [Li]) is strongly biologically controlled within the calcifying medium of calcite mollusks. Overall, the results of this study show that brachiopods are likely to be suitable targets for future work on the determination of paleo-seawater Li isotope composition-an emerging proxy for past weathering and hydrothermal processes.

Hot spring environments can create physical and chemical gradients favorable for unique microbial life. They can also include authigenic mineral precipitates that may preserve signs of biological activity on Earth and possibly other planets. The abiogenic or biogenic origins of such precipitates can be difficult to discern, therefore a better understanding of mineral formation processes is critical for the accurate interpretation of biosignatures from hot springs. Little Hot Creek (LHC) is a hot spring complex located in the Long Valley Caldera, California, that contains mineral precipitates composed of a carbonate base (largely submerged) topped by amorphous silica (largely emergent). The precipitates occur in close association with microbial mats and biofilms. Geological, geochemical, and microbiological data are consistent with mineral formation via degassing and evaporation rather than direct microbial involvement. However, the microfabric of the silica portion is stromatolitic in nature (i.e., wavy and finely laminated), suggesting that abiogenic mineralization has the potential to preserve textural biosignatures. Although geochemical and petrographic evidence suggests the calcite base was precipitated via abiogenic processes, endolithic microbial communities modified the structure of the calcite crystals, producing a textural biosignature. Our results reveal that even when mineral precipitation is largely abiogenic, the potential to preserve biosignatures in hot spring settings is high. The features found in the LHC structures may provide insight into the biogenicity of ancient Earth and extraterrestrial rocks. Fil: Kraus, Emily A.. Colorado School Of Mines; Estados Unidos Fil: Beeler, Scott R.. Washington University in St. Louis; Estados Unidos Fil: Mors, Rodolfo Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Floyd, James G.. Oklahoma State University; Estados Unidos Fil: Stamps, Blake W.. Colorado School Of Mines; Estados Unidos Fil: Nunn, Heather S.. Oklahoma State University; Estados Unidos Fil: Stevenson, Bradley S.. Oklahoma State University; Estados Unidos Fil: Johnson, Hope A.. University of California; Estados Unidos Fil: Shapiro, Russell S.. University of California; Estados Unidos Fil: Loyd, Sean J.. University of California; Estados Unidos Fil: Spear, John R.. Colorado School Of Mines; Estados Unidos Fil: Corsetti, Frank A.. University of Southern California; Estados Unidos

Microbial mats are found in a variety of modern environments, with evidence for their presence as old as the Archean. There is much debate about the rates and conditions of processes that eventually lithify and preserve mats as microbialites. Here, we apply novel tracer experiments to quantify both mat biomass addition and the formation of CaCO3. Microbial mats from Little Hot Creek (LHC), California, contain calcium carbonate that formed within multiple mat layers, and thus constitute a good test case to investigate the relationship between the rate of microbial mat growth and carbonate precipitation. The laminated LHC mats were divided into four layers via color and fabric, and waters within and above the mat were collected to determine their carbonate saturation states. Samples of the microbial mat were also collected for 16S rRNA analysis of microbial communities in each layer. Rates of carbonate precipitation and carbon fixation were measured in the laboratory by incubating homogenized samples from each mat layer with δ13C-labeled HCO3- for 24 h. Comparing these rates with those from experimental controls, poisoned with NaN3 and HgCl2, allowed for differences in biogenic and abiogenic precipitation to be determined. Carbon fixation rates were highest in the top layer of the mat (0.17% new organic carbon/day), which also contained the most phototrophs. Isotope-labeled carbonate was precipitated in all four layers of living and poisoned mat samples. In the top layer, the precipitation rate in living mat samples was negligible although abiotic precipitation occurred. In contrast, the bottom three layers exhibited biologically enhanced carbonate precipitation. The lack of correlation between rates of carbon fixation and biogenic carbonate precipitation suggests that processes other than autotrophy may play more significant roles in the preservation of mats as microbialites.

Mayall and Wright question interpretations in our microfacies analysis of the Cotham Marble microbialites (Ibarra et al. 2014) in which we primarily highlight previously overlooked aspects of Cotham Marble microbialite formation. They are specifically unconvinced about the Cotham Marble’s potential relevance to the end-Triassic mass extinction and our interpretation that Microtubus is not integral to the formation of the dendrolitic microbialite phases. Here we address Mayall and Wright’s comments under the same headings in which they present them.

One of the most severe extinction events in Earth history, the Triassic–Jurassic extinction, struck against a backdrop of radical increases in atmospheric CO2and supercontinent breakup. This juxtaposition of first-order geophysical and biotic changes produced excellent case studies in Earth-Life Transitions. Recent recognition of a worldwide “carbonate gap” following the extinction has focused attention on causes, often invoked as eustacy or ocean acidification, but the ecology of the extinction aftermath remains poorly understood. Results from paleoecological studies on three separate Triassic–Jurassic records are presented and incorporated into regional depositional models. Examination of the Penarth Group of Great Britain reveals a widespread, laterally homogenous, level-bottom microbial stromatolite regime across the innermost ramp. The Sunrise Formation in Nevada, USA, was deposited during a biosiliceous (“glass”) regime dominated by demosponges across the inner ramp that lasted at least two million years. Investigations of the Pucará group in the central Andes of Peru revealed a demosponge-dominated level-bottom glass ramp with many similarities to the Nevada deposits, but offering broader regional extent and variation in recorded depositional settings. This suite of studies demonstrates state-shifts in marine ecological systems that also profoundly altered regional sedimentation regimes. The sponge-dominated systems produced glass ramp conditions instead of carbonate ramps, and indicate the importance of marine silica concentrations. The post-extinction changes in regional marine ecology demonstrate connectivity to changes in global climate and terrigenous weathering driven by global-scale geophysical processes.

Thick cherts and cherty dolomites in the basal Jurassic Aramachay Formation of Peru preserve a thriving continental shelf community dominated by siliceous sponges that followed the end-Triassic collapse of metazoan-rich carbonate accumulation. Similar Hettangian and Sineumurian deposits from Nevada, U.S.A., Austria, and Morocco suggest that an Early Jurassic siliceous sponge takeover was a widespread phenomenon that persisted for ~ 2 m.y. until metazoan-driven carbonate sedimentation recovered. The post-extinction dominance of siliceous sponges likely resulted from the confluence of metazoan carbonate reef collapse (removal of incumbents) and geochemical conditions that fostered the success of the siliceous sponge-dominated ecosystem. Simple mass balance calculations suggest the siliceous sponge takeover was likely permitted by an increased silica flux as a consequence of weathering Central Atlantic Magmatic Province (CAMP) basalts. The CAMP basalts alone could supply all the silica needed to sustain the sponge takeover, although contributions were also likely from increased hot-climate weathering of other silicates and possible reductions in dissolved silica demand by radiolarians. Detailed sedimentological, fossil, and microfacies analyses were conducted at six field sites across a shallow shelf system recorded in the central Peruvian Andes (Yauli Dome), focusing on the metazoan contribution to sedimentation. Sedimentary structures at all six sites demonstrated on-shelf deposition, similar to the underlying upper Triassic Chambara Formation (in contrast to the black shale-rich facies of the Aramachay Formation in other areas of Peru). Examination of up to 147 m of cherty dolomite from the Aramachay Formation revealed a siliceous sponge-dominated ecosystem, including sponge body fossils, compressed in situ sponge materials, and abundant transported spiculite sediments. Siliceous sponges, mostly demosponges and rare hexactinellids, account for the chert lithology and apparently dominated the local ecology for approximately two million years. The role of metazoan biocalcifiers in sediment production and ecological structure was profoundly reduced compared to the under- and overlying formations, representing a clear ecological state shift from pre-extinction carbonate to post-extinction siliceous dominated ecosystems before the carbonate system recovered ~ 2 m.y. after the extinction.

Paleoecological consequences of the global Triassic–Jurassic mass extinction (201.3 Ma) are poorly understood. Fossiliferous marine boundary records are rare, commonly condensed, and typically reveal facies changes previously attributed to eustacy. Sedimentology and biofacies analyses from stratigraphically expanded successions of the lowest Jurassic strata, New York Canyon, Nevada, were investigated with high-resolution paleoenvironmental observations, fossil surveys, and microfacies analysis. Following the collapse of the uppermost Triassic carbonate ramp, the lowest Jurassic Ferguson Hill Member of the Sunrise Formation records a midshelf habitat dominated by previously unrecognized siliceous sponges for approximately two million years. In addition, the earliest Jurassic strata from the Pucara Group, central Peruvian Andes, were examined and record a more greatly expanded stratigraphic succession of facies across the inner to middle shelf. Like Nevada, the lowest Jurassic Aramachay Formation is replete with intense concentrations of siliceous sponges. The revelation of widespread, ecologically dominant siliceous sponges has been overlooked despite detailed biofacies studies in both depositional systems. Sponges expanded across shallow environments with sparse benthic biocalcifier populations, and were likely aided by increased ocean silica concentrations from the weathering of the Central Atlantic Magmatic Province. Facies changes previously attributed to sea-level change are thus interpreted to result from the collapse of the carbonate factory concomitant with the mass extinction, with transition to an alternate state dominated by siliceous sponges before a return to carbonate platform development in the Sinemurian. Our study highlights the need to separate biofacies from paleoenvironmental analysis during mass extinction times when nonactualistic assemblages may dominate and deviate from expected environments (e.g., siliceous sponges as indicators of deep paleoenvironments).

Ancient putative microbial structures that appear in the rock record commonly serve as evidence of early life on Earth, but the details of their formation remain unclear. The study of modern microbial mat structures can help inform the properties of their ancient counterparts, but modern mineralizing mat systems with morphological similarity to ancient structures are rare. Here, we characterize partially lithified microbial mats containing cm-scale dendrolitic coniform structures from a geothermal pool (“Cone Pool”) at Little Hot Creek, California, that if fully lithified, would resemble ancient dendrolitic structures known from the rock record. Light and electron microscopy revealed that the cm-scale ‘dendrolitic cones’ were comprised of intertwined microbial filaments and grains of calcium carbonate. The degree of mineralization (carbonate content) increased with depth in the dendrolitic cones. Sequencing of 16S rRNA gene libraries revealed that the dendrolitic cone tips were enriched in OTUs most closely related to the genera Phormidium, Leptolyngbya, and Leptospira, whereas mats at the base and adjacent to the dendrolitic cones were enriched in Synechococcus. We hypothesize that the consumption of nutrients during autotrophic and heterotrophic growth may promote movement of microbes along diffusive nutrient gradients, and thus microbialite growth. Hour-glass shaped filamentous structures present in the dendrolitic cones may have formed around photosynthetically-produced oxygen bubbles—suggesting that mineralization occurs rapidly and on timescales of the lifetime of a bubble. The dendrolitic-conical structures in Cone Pool constitute a modern analog of incipient microbialite formation by filamentous microbiota that are morphologically distinct from any structure described previously. Thus, we provide a new model system to address how microbial mats may be preserved over geological timescales., Paleobiology: modern microbes may tell an ancient tale Microbial mats currently thriving in a hot pool in California may help explain the origin of fossilized evidence of early life on Earth. Modern microbial mats that are structurally similar to microbial fossil mats are rare. John Spear at the Colorado School of Mines, with co-workers from elsewhere in the USA and in Chile, examined the microbial mats growing in a geothermal pool at Little Hot Creek in California. Light microscopy and electron microscopy identified crucial fine structure similarities with branching mat structures in the fossil record. The researchers developed hypotheses to explain the influence of nutrient flow on the growth and movement of the microbes in the mats. These living mats are a useful model system to help researchers understand how ancient microbial mats formed and were preserved over geological timescales.

The Luning and Gabbs formations in west-central Nevada, USA represent a Late Triassic shallow marine sedimentary succession with global significance (the Gabbs Formation was a candidate for the basal Jurassic GSSP). Typically, the Norian–Rhaetian stage boundary is placed at the contact between the formations, and the Rhaetian–Hettangian boundary (the Triassic–Jurassic boundary) is within the Muller Canyon Member of the Gabbs Formation. However, the use of different biostratigraphical index-species schemes in Norian–Rhaetian successions between Tethys and Panthalassa, the two largest ocean basins at the time, makes precise correlation problematic. Here, we compare 87Sr/86Sr measurements of well-preserved carbonate shell material from Nevada to the well-known and biostratigraphically constrained 87Sr/86Sr record from Tethys, where a negative excursion in 87Sr/86Sr is noted across the Norian–Rhaetian boundary. Our new 87Sr/86Sr data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in 87Sr/86Sr within the Nun Mine Member of the Gabbs Formation, suggesting the position of the Norian–Rhaetian boundary is higher in the succession, and not between the Luning and Gabbs formations as previously defined. Relating the stage boundary using global isotopic signals is a useful tool for biostratigraphical correlation of successions between Tethys and Panthalassa, and for estimating the rate of faunal turnover at the Norian–Rhaetian stage boundary in comparison with the succeeding Late Triassic mass extinction. If correct, this biostratigraphical–chemostratigraphical correlation suggests that the current index groups for the Panthalassic stage boundary should be changed.

A remarkably aerially extensive (∼2,000 km2) unit of carbonate microbialites occurs in many Triassic–Jurassic boundary interval outcrops of the southwestern United Kingdom and captures petrographic evidence that could link them to the end-Triassic extinction event. The bioherms—known regionally as the Cotham Marble—occur as discrete ∼20-cm-thick, decimeter- to meter-scale mounds, and display at least five growth phases that alternate between laminated and dendritic mesofabrics. Cross sections parallel to bedding through the dendritic phases expose a reticulate dendritic framework separated by polygonal spaces (∼1–3 cm diameter), characteristic of “tubestone” microbialites. Microscopically, the dendrolites contain evenly distributed rod to filamentous putative microfossils (∼2 µm diameter and ∼10 µm in length) in a matrix of micrite and contain higher total organic carbon than the surrounding matrix. Round to ellipsoidal spar-filled regions (∼200 µm in diameter) within the dendrolites (previously ...

A previously undescribed succession of currently-inactive spring-associated carbonates located near Zaca Lake, Southern California, was investigated in order to determine the nature of deposition (ambient temperature or hydrothermal water, as both are found within the region). The carbonate deposits are up to ~ 1 m thick and formed discontinuously for over 200 m in a narrow valley between two ridges that drain Miocene Monterey Formation bedrock. Depositional facies along the presently dry fluvial path include barrage deposits, narrow fluvial channels, and cascade deposits. The carbonates are mesoscopically banded and contain ubiquitous micro- to macrophyte calcite encrusted fabrics. All of the depositional facies contain alternating bands (~ .05 mm to 5 mm thick) of dark brown and light brown isopachous calcite; the dark brown bands are composed of dense isopachous bladed calcite, whereas the light brown bands are composed of bundles of calcite tubules interpreted as the biosignature of the desmid microalgae Oocardium stratum . Oxygen isotope thermometry utilizing modern water δ 18 O values from the piped spring reveal depositional water temperature estimates that collectively range from ~ 11 to 16 °C. Stable isotope carbon values exhibit a mean δ 13 C value of − 9.01 ± 0.62‰ (1σ, n = 27). Our petrographic and geochemical data demonstrate that (1) inactive carbonates were likely sourced from ambient temperature water with a strong soil-zone δ 13 C signal, (2) the Oocardium calcite biosignature can be used to infer depositional temperature and flow conditions, and (3) the occurrence of extensive carbonates (especially the presence of a perched cascade deposit) indicate the carbonates formed when conditions were much wetter.

The eruption of the Central Atlantic Magmatic Province (CAMP)—the largest igneous province known—has been linked to the end-Triassic mass extinction event, however reconciling the response of the biosphere (at local and nonlocal scales) to potential CAMP-induced geochemical excursions has remained challenging. Here we present a combined sedimentary and biological response to an ecosystem collapse in Triassic-Jurassic strata of the southwest United Kingdom (SW UK) expressed as widely distributed carbonate microbialites and associated biogeochemical facies. The microbialites (1) occur at the same stratigraphic level as the mass extinction extinction, (2) host a negative isotope excursion in δ13Corg found in other successions around the world and (3) co-occur with an acme of prasinophyte algae ‘disaster taxa’ also dominant in Triassic-Jurassic boundary strata of other European sections. Although the duration of microbialite deposition is uncertain, it is likely that they formed rapidly (perhaps fewer than ten thousand years), thus providing a high-resolution glimpse into the initial carbon isotopic perturbation coincident with the end-Triassic mass extinction. These findings indicate microbialites from the SW UK capture a nonlocal biosedimentary response to the cascading effects of massive volcanism and add to the current understanding of paleoecology in the aftermath of the end-Triassic extinction.

Ocean acidification associated with emplacement of the Central Atlantic Magmatic Province (CAMP) has been hypothesized as a kill mechanism for the end-Triassic mass extinction, but few direct proxies for ancient ocean acidity are available. This paper describes a new proxy that uses the presence of fossil corals and coral reefs to determine aragonite saturation state (Ω Arag ). Modern scleractinian corals struggle to biomineralize a skeleton below an Ω Arag of 2 and modern shallow water coral reefs are typically only found in areas with source water of Ω Arag > 3; so when corals or coral reefs are preserved in the fossil record, these ocean saturation states can be inferred. Atmospheric pCO 2 reconstructions are combined with the coral Ω Arag limitations to calculate the total dissolved inorganic carbon (DIC) in the Late Triassic ocean, which is a measure of the buffering capacity or ocean sensitivity to acidification. Once DIC is known, the severity of an acidification due to a carbon dioxide injection can be determined, for example the Triassic–Jurassic (T–J) event. Our results suggest that if Late Triassic DIC values were low to moderate (2000–3000 μmol/kg), the T–J pCO 2 increases would have depressed saturation state to the point where coral biomineralization would have been extremely challenging (Ω Arag 2 elevations recorded in stomatal and pedogenic proxies are not sufficient to cause complete carbonate undersaturation, modeled scenarios for CAMP-related T–J pCO 2 increases (within error of pedogenic pCO 2 proxies) suggest that aragonite undersaturation is plausible and in extreme cases calcite undersaturation is achievable. Thus, a short but extreme acidification event could occur and would satisfactorily explain the significant extinction of calcareous organisms, the Early Hettangian coral gap, and the T–J carbonate crisis.

Stratigraphic carbonate-carbon isotope trends are similar for correlative Ediacaran and Cambrian carbonates of Sonora, Mexico, and Death Valley, California. In contrast, the sulfur isotope compositions of both carbonate-associated sulfate (CAS) and pyrite in the two regions exhibit unique trends with high degrees of stratigraphic variability. We have established that the sulfur records are coeval using δ13C chemostratigraphy and biostratigraphic markers, where present. Over short stratigraphic intervals, δ34SCAS variability is consistent with regionally low marine sulfate concentrations during this period. Values of Δ34S (δ34SCAS–δ34Spyr) range from −5.8‰ to +27.1‰ and average ∼+11‰, consistent with limited net fractionation during bacterial sulfate reduction, which is additional evidence for low sulfate concentrations. Modeling based on these regional sulfur isotope trends suggests sustained low sulfate conditions throughout the Neoproterozoic and well into the Cambrian, with concentrations of ∼2 mM or lower. When all of the available sulfate proxy data from our work and previously published studies are considered, most Neoproterozoic and Cambrian successions exhibit trends consistent with low seawater sulfate. The persistent and complete disagreement in δ34Ssulfate among multiple basins was briefly interrupted ∼580 million years ago, coincident with the onset of the Wonoka-Shuram carbon isotope anomaly and again near the termination of Series 3 of the Cambrian—characteristics generally unrecognized in older rock units. During these two intervals, similar stratigraphic trends in δ34SCAS are recorded globally, whereas absolute values remain distinct among individual basins. However, these periods of broad isotopic trend agreement coincide with large-magnitude sulfur isotope excursions, which also point to low seawater sulfate concentrations. Therefore, although brief intervals of isotopic homogeneity exist, the Neoproterozoic and Cambrian ocean must have been dominated by low sulfate throughout. Ultimately, the recognition of persistently low sulfate well into the Paleozoic raises questions about the relationships between sulfate concentration in seawater and its primary controls, including ocean oxygenation and its influence on metazoan evolution.

Carbonate concretions can form as a result of organic matter degradation within sediments. However, the ability to determine specific processes and timing relationships to particular concretions has remained elusive. Previously employed proxies (e.g., carbon and oxygen isotopes) cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen, nitrate, metal oxides, and sulfate as electron acceptors, in addition to degradation by thermal decarboxylation. Here, we employ concentrations of carbonate-associated sulfate (CAS) and δ34SCAS (along with more traditional approaches) to determine the specific nature of concretion authigenesis within the Miocene Monterey Formation. Integrated geochemical analyses reveal that at least three specific organo-diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample-site specific. One calcitic concretion from the Phosphatic Shale Member at Naples Beach yields δ34SCAS values near Miocene seawater sulfate (∼+22‰ VCDT), abundant CAS (ca. 1000 ppm), depleted δ13Ccarb (∼−11‰ VPDB), and very low concentrations of Fe (ca. 700 ppm) and Mn (ca. 15 ppm)—characteristics most consistent with shallow formation in association with organic matter degradation by nitrate, iron-oxides and/or minor sulfate reduction. Cemented concretionary layers of the Phosphatic Shale Member at Shell Beach display elevated δ34SCAS (up to ∼+37‰), CAS concentrations of ∼600 ppm, mildly depleted δ13Ccarb (∼−6‰), moderate amounts of Mn (ca. 250 ppm), and relatively low Fe (ca. 1700 ppm), indicative of formation in sediments dominated by sulfate reduction. Finally, concretions within a siliceous host at Montana de Oro and Naples Beach show minimal CAS concentrations, positive δ13C values, and the highest concentrations of Fe (ca. 11,300 ppm) and Mn (ca. 440 ppm), consistent with formation in sediments experiencing methanogenesis in a highly reducing environment. This study highlights the promise in combining CAS analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity.

Dolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay-fraction dolomite ooze. Based on d 18 O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na-SO4-dominated brine contains up to 500 mm dissolved inorganic carbon whereas Mg 2+ and Ca 2+ concentrations are ca 1 and 0AE 3m m, respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca 2+ (below 0AE01 mm). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca 2+ and Mg 2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing d 13 C values near to equilibrium with atmospheric CO2 (ca 3&) contrasts the formation of isotopically light (organically derived), hard-lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C-rich sediments.

Integrated Ocean Drilling Program (IODP) Expedition #310 recovered 632 meters of Post Last-Glacial-Max (Post-LGM) and late Pleistocene coral reef framework rocks (37 holes at 22 sites) from the current fore-reef slopes surrounding the island of Tahiti. The cores were collected primarily to constrain the timing and amplitude of rising sea levels associated with the end of the last global glaciation (10–18,000 YBP), to identify the climatic and environmental changes during that period, and to evaluate the reef response to those combined changes. The cores, which were collected in three regions (Faaa, Tiarei, and Maraa), are remarkable in that more than 60% of the total core volume is associated with microbialites rather than macroscopic coral or coralline algae assemblages. We present here paleomagnetic data from 316 oriented paleomagnetic samples collected from the Post-LGM framework rocks of the Maraa region (12 holes at 6 sites). 96% of our samples were recovered from microbialites that have filled the primary cavities of the coral/coralgal framework. All samples appear to accurately record geomagnetic field variability at Tahiti (average inclination = −30.6°, alpha 95 ~ 2.9°, site axial dipole inclination = −32.6°) and yet the samples are composed of ~ 99% precipitated calcium carbonate. Our rock magnetic data indicate that the NRM resides almost entirely in detrital titanomagnetite grains derived from the Tahiti volcanic edifice. We present magnetic and microscopic evidence that the natural remanent magnetization in the carbonate rocks was locked in through mediation of organic biofilms (mostly bacterial) that held the grains in place until they were trapped by carbonate precipitation.

Closed-basin alkaline lakes record climate change particularly well because they generally contain a sedimentary record that is high in carbonate mineral content from which climate proxies can be determined. Various approaches are used to estimate paleo-lake level and volume (δ18O, dating of “shoreline” tufas, biotic proxies, etc.), yet all carry certain caveats that limit their usefulness. Ultimately, the relationship between the chemistry of the lake, the volume of the lake, and the response of the proxy will determine how well a proxy serves a paleolimnologic purpose. Here, we discuss the use of carbonate-associated sulfate (CAS), the sulfate contained within the lattice of carbonate minerals that precipitate in lake water, as a proxy for lake water chemistry and by extension, lake volume. Walker Lake, an alkaline closed-basin lake in western Nevada, has experienced a well-documented lake-level decline since 1880 and provides a test case for CAS as a lake-level proxy. By extracting the CAS from sedimentary carbonate and tufas that have been age dated, we can relate these values to lake sulfate content based on historical or other proxy data. We confirm that CAS tracks lake sulfate. Our study of sedimentary carbonates demonstrates that CAS is a linear function of lake sulfate through a range of 10–25 mM, which corresponds to a change in lake level of 30 m. As confirmation of the CAS technique, we analyzed a stromatolitic tufa dated using AMS 14C. The CAS trend in the stromatolite suggested that it grew during a lake-level decline, a result consistent with other proxy data. Finally, laboratory experiments were conducted that demonstrate CAS is monotonically correlated with sulfate concentration and that precipitation kinetics are not likely a major control on CAS in alkaline lakes, but that ionic strength of the solution exerts a strong control on CAS.

Cm-sized carbonate seafloor fans occur in the Neoproterozoic Rainstorm Member of the Johnnie Formation, Death Valley, USA. The fans formed in a mixed carbonate-clastic succession near storm wave base at the base of parasequences on a storm-dominated ramp. Petrographic observations indicate that the fans were originally precipitated as aragonite and later inverted to calcite during diagenesis. Although not directly dated, the Rainstorm Member preserves a large magnitude negative carbon isotopic anomaly (down to −11‰ PDB) tentatively correlated to the largest known carbon isotope excursion found in many stratigraphic successions around the world between 585 Ma and 550 Ma. Thus, the age distribution of seafloor aragonite fans in Neoproterozoic strata appears more widespread than previously thought, occurring in strata significantly younger than the last widespread Neoproterozoic glaciation. Rainstorm Member carbonate fans and oolitic units (representing time-correlative shallower environments) record similar carbon isotope ratios during the negative carbon isotopic anomaly. The carbon isotopic homogeneity displayed between fans and other carbonate sediments implies that waters across the shelf were well-mixed rather than stratified during the late Neoproterozoic isotopic anomaly. In addition, the similarity of carbon isotope ratios shared among fans along a stratigraphic horizon (on a m- to cm-scales) suggests that the local source of alkalinity required for fan growth was derived from a well-mixed reservoir, likely seawater, rather than local diagenetic fluids. Increased alkalinity and the presence of inhibitors to carbonate nucleation (perhaps Fe2+ under anoxic conditions) likely fostered precipitation of aragonite crystal fans on the seafloor.

An integrated stable isotope, elemental and petrographic analysis of Early Triassic (Spathian) carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in δ34S composition of carbonate associated sulfate (CAS). The variations in the δ34S of CAS are strongly correlated with the Ca/Mg composition of carbonates, suggesting that the variations are driven by the degree of dolomitization. The δ34S of dolostones and evaporites are similar to one another and exhibit lower δ34S values than limestones from all localities. Three hypotheses may explain the differences in δ34S between proximal dolostones/evaporites and inner/middle shelf limestones: (1) limestones experienced anaerobic sulfate reduction and subsequent incorporation of 34S-enriched sulfate into CAS during diagenesis, while dolostones did not—this is unlikely because of the lack of correlation between δ34SCAS and TOC, as well as other indicators of diagenesis, (2) dolomitization controlled the δ34SCAS in proximal paleoenvironments, where the source of the 34S depleted fluids was either continentally-derived or the result of Rayleigh distillation during evaporite formation, and (3) a δ34S depth gradient existed during the Early Triassic such that limestones formed in distal waters are more enriched in 34S versus evaporites and dolostones formed in proximal settings—we do not favor this hypothesis because the strong correlation between Ca/Mg and δ34SCAS implies that dolomitization controls the δ34SCAS in these samples. Results from subtidal, well-preserved (non-dolomitized) limestones suggest that the δ34S of Spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone.

The sulfur isotopic composition of carbonate associated sulfate (CAS) has been used to investigate the geochemistry of ancient seawater sulfate. However, few studies have quantified the reliability of δ34S of CAS as a seawater sulfate proxy, especially with respect to later diagenetic overprinting. Pyrite, which typically has depleted δ34S values due to authigenic fractionation associated with bacterial sulfate reduction, is a common constituent of marine sedimentary rocks. The oxidation of pyrite, whether during diagenesis or sample preparation, could thus adversely influence the sulfur isotopic composition of CAS. Here, we report the results of CAS extractions using HCl and acetic acid with samples spiked with varying amounts of pyrite. The results show a very strong linear relationship between the abundance of fine-grained pyrite added to the sample and the resultant abundance and δ34S value of CAS. This data represents the first unequivocal evidence that pyrite is oxidized during the CAS extraction process. Our mixing models indicate that in samples with much less than 1 wt.% pyrite and relatively high δ34Spyrite values, the isotopic offset imparted by oxidation of pyrite should be much less than − 4‰. A wealth of literature exists on the oxidation of pyrite by Fe3+ and we believe this mechanism drives the oxidation of pyrite during CAS extraction, during which the oxygen used to form sulfate is taken from H2O, not O2. Consequently, extracting CAS under anaerobic conditions would only slow, but not halt, the oxidation of pyrite. Future studies of CAS should attempt to quantify pyrite abundance and isotopic composition.

Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death Valley, California record a remarkable negative carbon isotope anomaly – to a nadir of near − 11‰ – that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate δ 34 S values. The carbonates, including the laterally extensive Johnnie Oolite, were deposited during marine flooding atop a sequence boundary best observed in cratonward sections. The 12 C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O 2 , which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. Similar carbon isotope anomalies are recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian–Cambrian boundary in Oman, India, China, Australia, and Namibia, supporting the view that the oxygenation event was global. Thus, the biogeochemical anomaly post-dates the last widely recognized Neoproterozoic ice age, but is nonetheless associated with sea level drawdown that is likely to be eustatic in origin. The radiation of a high diversity microbiota and the first appearance of large metazoan fossils (Ediacaran animals) follow this carbon isotope anomaly, suggesting that a critical threshold with respect to atmospheric O 2 had been crossed by this time. A negative excursion of similar magnitude occurs in overlying strata at the Precambrian–Cambrian boundary, which likely reflects similar processes.

© 2015 Elsevier B.V. The Wonoka-Shuram Anomaly represents the largest negative carbon isotope excursion recognized in the geologic record and is associated with the emergence and diversification of metazoan life ca. 580 million years ago (Ma). The origin of the anomaly is highly debated, with interpretations ranging from primary to diagenetic, each having unique and potentially transformative implications for early life. Here, we apply carbonate clumped isotope thermometry to three sections expressing the anomaly in order to constrain mineral formation temperatures and thus directly calculate water oxygen isotope compositions (δ18Ow) with which carbonate minerals equilibrated. With δ18Ow known, it is possible to address previous hypotheses for the origin of the anomaly. In each section, precipitation temperatures correlate positively with reconstructed δ18Ow. Previous hypotheses, based on the covariance of δ18Ocarb vs. δ13Ccarb (uncorrected for temperature effects), suggested a meteoric diagenetic origin for the anomaly. However, reconstructed δ18Ow values do not covary with carbon isotope compositions (δ13Ccarb) within anomaly facies. Rather, the oxygen isotope and temperature data are consistent with carbonate recrystallization and equilibration under increasingly rock-buffered conditions. Based on simple modeling and comparison to modern formation fluids, recrystallization may have occurred in an environment far removed from the initial depositional or early diagenetic regime. In addition, although clumped isotope temperatures vary significantly and reach elevated values consistent with burial diagenesis, it is unclear to what degree, if at all, carbon isotope values were reset during recrystallization. Ultimately, these new data indicate that Wonoka-Shuram-aged carbonates experienced equilibration with fluids under increasingly closed-system conditions. The clumped isotope data do not provide a means to distinguish previous hypotheses outright, but provide additional context for the evaluation of geochemical signatures within these ancient carbonate rocks.

Changes in dolomite fabric within the Neoproterozoic–Cambrian stratigraphic succession in Death Valley, California preserve evidence for a first-order biogeochemical change in early diagenetic environments through the Neoproterozoic–Cambrian transition. Oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time. Two key generalizations can be made: (1) late Neoproterozoic dolomitization in Death Valley occurred very early in the paragenetic sequence, was volumetrically significant, and preserved fine detail of the original aragonite or calcite precursor; and (2) Early Cambrian dolomitization in Death Valley occurred late in the paragenetic sequence with burial, was commonly incomplete, and obliterated details of the calcite or aragonite precursor; early diagenetic dolomitization of any kind was absent. The dolomitization fabrics in Death Valley parallel the presumed pattern of seawater Mg/Ca with respect to mimetic dolomitization in the Paleozoic (low Mg/Ca, little/no mimetic dolomitization) and Cenozoic (higher Mg/Ca, more common mimetic dolomitization). However, the volume of Neoproterozoic mimetic dolomitization is immense compared to the Cenozoic examples. We speculate that a fundamental shift in the early diagenetic environment occurred in late Neoproterozoic time, perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment-capping microbial mats. An oscillatory trend in the fabric and volume of dolomite through the Phanerozoic has been suggested, but the termination of volumetrically significant mimetic dolomitization across the Neoproterozoic–Cambrian transition, a critical time in Earth history for many reasons, dwarfs the oscillatory trends noted in the Phanerozoic.

Sedimentary systems, as well as the earth's biota, changed dramatically following the end-Permian mass extinction. This global sedimentological transformation is reflected in the widespread occurrence of flat-pebble conglomerates, subtidal wrinkle structures, microbialites, and carbonate seafloor fans in various Lower Triassic sections. As a case study, a facies analysis of the Lower Triassic Virgin Limestone Member of the Moenkopi Formation and the Middle and Upper Members of the Union Wash Formation, southwestern United States was conducted and all unusual facies were documented within a palaeoenvironmental framework. The occurrence of the flat-pebble conglomerates and wrinkle structures is significant because these reflect a long-term reduction in vertical bioturbation. In the case of the subtidal microbialites and carbonate seafloor fans, their formation is linked to an increase in alkalinity. The presence of these facies in the uppermost Lower Triassic strata of the southwestern United States indicates that reduced levels of infaunal bioturbation as well as (and perhaps a consequence of) unusual ocean chemistry extended for 5–6 million years after the end-Permian mass extinction, suggesting that depositional environments as well as the biota were significantly affected by the extinction event.

Macroscopic seafloor-precipitated crystal fans are rare in post-Paleoproterozoic strata, but they are more common in the unusual carbonates that cap certain Neoproterozoic glacial strata. The uppermost carbonate and marble unit of the Scout Mountain Member of the Neoproterozoic Pocatello Formation (younger than 667±5 Ma) contains seafloor fans, records negative δ 13 C values, and lies within a transgressive sequence, all features consistent with such post-glacial carbonate units. It does not, however, rest directly upon a known glaciogenic deposit or sequence boundary. Instead, it occurs within a cyclic upward-fining succession that overlies an erosional contact on the pink dolostone post-glacial carbonate atop the late Sturtian Scout Mountain Member diamictites. It is possible that the carbonate unit in question is a cap carbonate and thus requires recognition of an additional glaciation recorded in the Pocatello Formation. Alternatively, it may imply that means independent of glaciation, such as overturn of an anoxic ocean, foster the deposition of cap-like carbonates independent of post-glacial processes. Thus, Neoproterozoic carbonate units with unusual lithofacies and negative carbon isotope values need not indicate the aftermath of low-latitude glaciation. Additionally, the radiometrically calibrated δ 13 C record of the Pocatello Formation demonstrates that the interval from 700 to 580 Ma has more variation than previous compilations would suggest.

A previously undescribed carbonate-shrub facies has been discovered in the lower member of the Noonday Dolomite—a Neoproterozoic post-glacial cap carbonate. This unusual facies consists of centimeter-scale structures composed of micrite and encased in early-marine cement, and exhibits an overall clotted, mottled appearance. Shrub architecture is characterized by occurrence of a central stalk with diverging branches that are composed of micrite leaves. A combination of biological, environmental, and diagenetic influences contributed to the growth and present appearance of Noonday Dolomite carbonate shrubs. Comparison with modern and ancient known abiogenic and biogenic shrub-like structures indicates that microbial communities were most likely responsible for at least localizing and initiating calcium carbonate growth in the Noonday Dolomite shrubs, although no undisputable microbial fossils have yet been discovered. Diagenetic processes may have obliterated Noonday Dolomite shrub microstructure a...

The Ordovician Meiklejohn Peak mud-mound in western Nevada contains sedimentary structures (e.g., stromatactis and “zebra-rock”) that resemble features found in modern methane hydrate deposits. However, the δ13C of the mud-mound carbonates (−1 to 1‰ PDB) does not suggest interaction with δ13C-depleted methane. Consequently, Krause (Sedimentary Geology 145 (2001) 189) proposed that the Meiklejohn structures might have been produced by the formation and dissociation of CO2 gas hydrate rather than methane gas hydrate. Structures in Neoproterozoic cap carbonates, sheet cracks and tepee-like structures have also been suggested to represent gas hydrate interaction with carbonate sediments (Geology 29 (2001) 443), but, similarly, δ13C values do not suggest direct interaction with methane. The objective of this contribution is to see if the presence of CO2 gas hydrate is plausible for either setting. To test the feasibility of CO2 gas hydrate involvement in the formation of these structures, necessary concentrations of TCO2 were calculated based on required phase equilibria using the quadruple points Q1 (−1.73 °C, 10.2 atm) and Q2 (10.2 °C, 44.5 atm) from the phase diagram for CO2 gas hydrate (Miller, S.L., 1974. The Nature and Occurrence of Clathrate Hydrates. In: Kaplan, I.R. (Ed.), Natural Gases in Marine Sediments. Plenum, New York, NY, pp. 151–178). A limit for Ca+2 was established based on the lack of evidence for gypsum formation in this environment and the assumption that sulfate was up to 100% of its present oceanic value; the calculated TCO2 concentration, however, is relatively insensitive to the concentration of SO4−2 used. The lowest permissible value of pCO2 and TCO2 were found at Q1, where TCO2 must be at least 300 times the present value. These conditions are implausible for the formation of the Meiklejohn Peak mud-mound. Although some models for Neoproterozoic “Snowball Earth” require highly elevated atmospheric CO2, these models predict pCO2 that is still too low to permit the formation of CO2 gas hydrates in carbonate sediments. In addition, if the required conditions could be achieved during “Snowball Earth”, one would expect to find the associated structures only at the base of the section, while they are commonly observed meters to tens of meters above the base. Therefore, we conclude that it is unlikely these structures were produced by the formation and dissociation of CO2 gas hydrate.

A thin carbonate unit associated with a Sturtian-age (≈750–700 million years ago) glaciogenic diamictite of the Neoproterozoic Kingston Peak Formation, eastern California, contains microfossil evidence of a once-thriving prokaryotic and eukaryotic microbial community (preserved in chert and carbonate). Stratiform stromatolites, oncoids, and rare columnar stromatolites also occur. The microbial fossils, which include putative autotrophic and heterotrophic eukaryotes, are similar to those found in chert in the underlying preglacial units. They indicate that microbial life adapted to shallow-water carbonate environments did not suffer the significant extinction postulated for this phase of low-latitude glaciation and that trophic complexity survived through snowball Earth times.

Seafloor-precipitated calcium carbonate fans, exceedingly rare in post-Paleoproterozoic time, make a dramatic reappearance in the post-glacial cap carbonates associated with Neoproterozoic low latitude glaciation (snowball Earth). Their presence is commonly interpreted to indicate elevated seawater alkalinity; the source of the anomalous alkalinity has been a critical, much-debated component in competing snowball Earth hypotheses. Two new Neoproterozoic seafloor fan occurrences have been reported recently in the Western United States (Death Valley and southeastern Idaho). Each were deposited during transgression and record negative δ 13 C values as do all known cap carbonates, but they lack a known underlying glacial deposit and do not necessarily rest on the transgressive surface. It is possible, but not likely, that the units represent post-glacial cap carbonates without a preserved/discovered underlying glaciogenic unit. More likely, processes independent of glaciation may cause negative δ 13 C excursions and cap carbonate-like facies and caution must be exercised when interpreting the meaning of seafloor precipitates in association with snowball Earth events.

Marine phosphate-rich sedimentary deposits (phosphorites) are important geological reservoirs for the biologically essential nutrient phosphorous. Phosphorites first appear in abundance approximately 600 million years ago, but their proliferation at that time is poorly understood. Recent marine phosphorites spatially correlate with the habitats of vacuolated sulfide-oxidizing bacteria that store polyphosphates under oxic conditions to be utilized under sulfidic conditions. Hydrolysis of the stored polyphosphate results in the rapid precipitation of the phosphate-rich mineral apatite—providing a mechanism to explain the association between modern phosphorites and these bacteria. Whether sulfur bacteria were important to the formation of ancient phosphorites has been unresolved. Here, we present the remains of modern sulfide-oxidizing bacteria that are partially encrusted in apatite, providing evidence that bacterially mediated phosphogenesis can rapidly permineralize sulfide-oxidizing bacteria and perhaps other types of organic remains. We also describe filamentous microfossils that resemble modern sulfide-oxidizing bacteria from two major phosphogenic episodes in the geologic record. These microfossils contain sulfur-rich inclusions that may represent relict sulfur globules, a diagnostic feature of modern sulfide-oxidizing bacteria. These findings suggest that sulfur bacteria, which are known to mediate the precipitation of apatite in modern sediments, were also present in certain phosphogenic settings for at least the last 600 million years. If polyphosphate-utilizing sulfide-oxidizing bacteria also played a role in the formation of ancient phosphorites, their requirements for oxygen, or oxygen-requiring metabolites such as nitrate, might explain the temporal correlation between the first appearance of globally distributed marine phosphorites and increasing oxygenation of Neoproterozoic oceans.

Stromatolites are commonly interpreted as evidence of ancient microbial life, yet stromatolite morphogenesis is poorly understood. We apply radiometric tracer and dating techniques, molecular analyses and growth experiments to investigate siliceous stromatolite morphogenesis in Obsidian Pool Prime (OPP), a hot spring in Yellowstone National Park. We examine rates of stromatolite growth and the environmental and/or biologic conditions that affect lamination formation and preservation, both difficult features to constrain in ancient examples. The "main body" of the stromatolite is composed of finely laminated, porous, light-dark couplets of erect (surface normal) and reclining (surface parallel) silicified filamentous bacteria, interrupted by a less-distinct, well-cemented "drape" lamination. Results from dating studies indicate a growth rate of 1-5 cm year(-1) ; however, growth is punctuated. (14)C as a tracer demonstrates that stromatolite cyanobacterial communities fix CO(2) derived from two sources, vent water (radiocarbon dead) and the atmosphere (modern (14)C). The drape facies contained a greater proportion of atmospheric CO(2) and more robust silica cementation (vs. the main body facies), which we interpret as formation when spring level was lower. Systematic changes in lamination style are likely related to environmental forcing and larger scale features (tectonic, climatic). Although the OPP stromatolites are composed of silica and most ancient forms are carbonate, their fine lamination texture requires early lithification. Without early lithification, whether silica or carbonate, it is unlikely that a finely laminated structure representing an ancient microbial mat would be preserved. In OPP, lithification on the nearly diurnal time scale is likely related to temperature control on silica solubility.

The formation of spherical mineral aggregates is commonly observed in nature and well-known from carbonates formed in marine and hypersaline environments and based on their internal structure called ooids, pellets, pisolites, etc. (e.g. Bathurst 1967; Donahue 1969; Davies et al. 1978; Tucker 1984; Tucker and Wright 1990)

The Neoproterozoic succession in the Death Valley region contains a physical- and chemostratigraphic record of glaciation. Direct evidence for glaciation includes dropstones, glacially influenced diamictites, and cap carbonates in the Neoproterozoic Kingston Peak Formation. Within this formation, glacially influenced deposits and cap carbonates occur at two distinct horizons, suggesting at least two glacial episodes. Cap-like carbonates and sequence boundaries elsewhere in the succession may indicate additional glacial intervals. The basal Beck Spring Dolomite has fades and isotopic characteristics commonly associated with cap carbonates, and the rest of the succession is punctuated by numerous sequence boundaries including a prominent incised horizon in the uppermost Johnnie Formation. This horizon is locally overlain by carbonate with cap-like facies and isotopic characteristics. Together, these observations indicate at least two and possibly four distinct Neoproterozoic glaciations.

The Neoproterozoic (c.a. 1000–540 Ma) was a time of great change. Eukaryotes radiated through the ocean, while metazoans originated and diversified. The rock record preserves multiple negative excursions of over 15% in the carbonate ^(13)C/^(12)C, at least three of which are associated with global low-latitude glaciations.

We formed calcite crusts in the presence and absence of the heterotrophic bacterium Desulfovibrio desulfuricans strain G20 to investigate microbial morphological signatures in fast-accreting carbonate precipitates. Submicrometer- to micrometer-sized pores (micropores) were present and ubiquitous in the G20 crusts but absent in abiotically precipitated crusts. Bacterial micropores resemble inclusions under transmitted light, but have distinct size, biological shapes and patterns (swirling or dendritic) and are distributed differently from common fluid inclusions. We observed similar porosity in both modern and ancient carbonate crusts of putative biotic origin. Our experiments support the microbial origin of micropores and help define specific criteria whereby to recognize these features as biosignatures in the rock record.

Comparison of secular variations in carbon-isotopic compositions of carbonates from the NeoproterozoicCambrian succession in the Whiteinyo region of eastern California with similarly-aged sequences worldwide suggests that, in California, a hiatus potentially encompassing the latest Neoproterozoic and much of the Tommotian may be present. The hiatus is found between the carbonate units in the Lower Deep Spring Formation, suggesting that the lowermost Deep Spring, Reed, and Wyman formations are Neoproterozoic in age, and the remaining Deep Spring Formation is uppermost Tommotian or Atdabanian in age. It is unclear where the Neoproterozoic-Cambrian boundary may lie, since chemostratigraphic data is not available for the boundary in New foundland type section, but it is most-likely encompassed within the inferred hiatus