In modern stromatolites, mineralization results from a complex interplay between microbial metabolisms, the organic matrix, and environmental parameters. Here, we combined biogeochemical, ...mineralogical, and microscopic analyses with measurements of metabolic activity to characterize the mineralization processes and products in an emergent (<18 months) hypersaline microbial mat. While the nucleation of Mg silicates is ubiquitous in the mat, the initial formation of a Ca‐Mg carbonate lamina depends on (i) the creation of a high‐pH interface combined with a major change in properties of the exopolymeric substances at the interface of the oxygenic and anoxygenic photoautotrophic layers and (ii) the synergy between two major players of sulfur cycle, purple sulfur bacteria, and sulfate‐reducing bacteria. The repetition of this process over time combined with upward growth of the mat is a possible pathway leading to the formation of a stromatolite.
In the aftermath of the end‐Permian mass extinction, Early Triassic sediments record some of the largest Phanerozoic carbon isotopic excursions. Among them, a global Smithian‐negative carbonate ...carbon isotope excursion has been identified, followed by an abrupt increase across the Smithian–Spathian boundary (SSB; ~250.8 Myr ago). This chemostratigraphic evolution is associated with palaeontological evidence that indicate a major collapse of terrestrial and marine ecosystems during the Late Smithian. It is commonly assumed that Smithian and Spathian isotopic variations are intimately linked to major perturbations in the exogenic carbon reservoir. We present paired carbon isotopes measurements from the Thaynes Group (Utah, USA) to evaluate the extent to which the Early Triassic isotopic perturbations reflect changes in the exogenic carbon cycle. The δ13Ccarb variations obtained here reproduce the known Smithian δ13Ccarb‐negative excursion. However, the δ13C signal of the bulk organic matter is invariant across the SSB and variations in the δ34S signal of sedimentary sulphides are interpreted here to reflect the intensity of sediment remobilization. We argue that Middle to Late Smithian δ13Ccarb signal in the shallow marine environments of the Thaynes Group does not reflect secular evolution of the exogenic carbon cycle but rather physicochemical conditions at the sediment–water interface leading to authigenic carbonate formation during early diagenetic processes.
Evidence of several major unconformities in the Lower Palaeozoic succession in Iran lead to question the role of tectonic/eustatism/climate in terms of their formation. The studied Palaeozoic ...succession in the Kuh-e Surmeh Anticline is characterized by the preservation of two thin Ordovician and Lower Permian Formations separated by a large hiatus encompassing the Upper Ordovician up to the lowermost Permian. The Ordovician sequences were deposited in shallow shoreface to lower offshore environments and the Lower Permian corresponds to a wave-dominated estuarine system evolving to a delta system. These mainly clastic successions represent good reservoirs in the regional Palaeozoic petroleum system of the Iranian plate separated by a major unconformity. The local erosion of the Zakeen Fm., observed in the neighbouring areas close to the Kuh-e Surmeh anticline, can be used to specify the role of the different controlling factors. We show that the influence of regional tectonics (Hercynian Orogeny/extensional deformation) and climate (Hirnantian and Carboniferous glaciations) related to the late Ordovician and Late Carboniferous/Early Permian succession cannot be ruled out from the possible candidates participating in erosion, but local diapir doming seems to better explain part of this local intense erosion.
The first step of ice-sheet build-up on Antarctica at the Eocene-Oligocene boundary was followed by a phase of climate instability culminating during the Miocene with a warming event called the ...Middle Miocene Climate Optimum (MMCO), that ended with a marked cooling phase identified as the Middle Miocene Climate Transition (MMCT). While numerous benthic foraminifera δ18O and Mg/Ca data have been used to capture the global climate evolution during this interval of climate and ice-sheet instability, geochemical records from shallow-water carbonates still remain scarce. Yet such records are crucial to capture the diversity of regional environmental responses to global climate changes, and thus to better understand the behavior of our climate system during this critical interval. In this work, we test the potential of a multi-proxy approach (δ18O, clumped isotope (Δ47), strontium isotopes (87Sr/86Sr)) applied to bivalves recovered from the Liguro-Provencal Basin and Rhodano-Provencal basin (Northwestern of the Mediterranean Sea) to reconstruct the evolution of shallow seawater temperature and salinity in this region over the latest Oligocene to Middle Miocene interval (~10 Myrs). Our results highlight a local cooling in the northwestern Mediterranean Sea during the MMCO that contrasts with the warming observed in other regions, with seawater temperatures inferred from Δ47 analyses in the 13–18 °C range. These cool seawater temperatures recorded in the studied region during the MMCO are much cooler than those recorded in the open oceans, but are in agreement with the proliferation of bryozoan observed in the Castillon-Du-Gard area during this interval. Low bivalve δ18O values (−3.24‰ in average) are recorded during this episode are interpreted to reflect enhanced freshwater inputs, lowering local salinity and seawater δ18O. Such enhanced freshwater inputs point to a phase of enhanced hydrological cycle in the studied region, possibly linked to the global mid-Miocene warming event. Bivalve 87Sr/86Sr data mostly fall within the global seawater Sr isotope reference curve, but remain compatible with enhanced freshwater inputs during the Aquitanian and during the Langhian, as these inputs were probably not important enough to induce a large departure from the seawater curve at a regional scale.
•Coupled Δ47 - δ18O - 87Sr/86Sr analyses of bivalve shells decipher temperature and salinity•Cool seawater temperatures in the northwestern Mediterranean Sea during the MMCO•Enhanced hydrological cycle in western Europe during the MMCO
The Early Triassic is generally portrayed as a time of various, high ecological stresses leading to a delayed biotic recovery after the devastating end-Permian mass extinction. This interval is ...notably characterized by repeated biotic crises (e.g., during the late Smithian), large-scale fluctuations of the global carbon, nitrogen and sulfur cycles as well as harsh marine conditions including a combination of ocean acidification, anoxia, extreme seawater temperatures and shifting productivity. Observations from different paleolatitudes suggest that sulfidic (H2S-rich) conditions may have developed widely during the Early Triassic, possibly reaching up to ultra-shallow environments in some places. However, the existence and the spatio-temporal extent of such redox swings remain poorly constrained. In order to explore Early Triassic paleoceanographic redox changes and their potential influences on the biotic recovery, we analyzed multiple sulfur isotopes (32S, 33S, 34S, and 36S) of sedimentary pyrite and carbonate associated sulfate (δ34SCAS) from the Mineral Mountains section, Utah. Sediments from this section were mainly deposited in shallow waters and span the Smithian and lower Spathian. We report a 68‰ range of variations in δ34Spy associated with Δ33Spy varying from −0.01‰ to +0.12‰, whereas the δ34SCAS varies between +19.5‰ and + 34.8‰. We interpret the observed signal of multiple sulfur isotopes as reflecting the operation of pore-water synsedimentary microbial sulfate reduction in open system with respect to sulfates before the late Smithian, evolving to a closed system, sulfate limited, Rayleigh-type distillation across the Smithian/Spathian boundary (SSB) and immediately after the SSB. We argue that this marked change is driven by the effectiveness of the connection between the sedimentary pore waters and the overlying water column, which is, in this case, controlled by the local sedimentological conditions such as the bioturbation intensity and the sedimentation rate. Therefore, our results suggest that changes in the sulfur cycle before and across the SSB at Mineral Mountains is probably a local consequence of the loss of the mixed sedimentary layer during the late Smithian extinction event, as opposed to reflecting the development of a lethal anoxic ocean at the global scale.
Extracellular polymeric substances (EPSs) are an important organic carbon reservoir in many pelagic and benthic environments. The production of EPS
is intimately associated with the growth of phyto- ...and picoplankton. EPS plays a critical role in carbonate precipitation through the binding of
cations and by acting as a nucleation site for minerals. Large-scale episodes of fine-grained calcium carbonate precipitation in the water column
(whiting events) have been linked to cyanobacterial blooms, including of Synechococcus spp. The mechanisms that trigger these precipitation
events are still debated. We pose that the cyanobacterial EPS, produced during exponential and stationary growth phases, plays a critical role in the
formation of whitings. The aim of this study was to investigate the production of EPS during a 2-month cyanobacterial growth, mimicking a
bloom. The production and characteristics of EPS were examined in different growth stages of Synechococcus spp. using various techniques
such as Fourier transform infrared (FT-IR) spectroscopy as well as colorimetric and sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE) assays. We further evaluated the potential role of EPS in carbonate precipitation through
in vitro-forced precipitation experiments. EPS produced during the early and late stationary
phase contained a larger amount of negatively charged groups than present in EPS produced during the exponential phase. Consequently, a higher
Ca2+-binding affinity of the stationary-phase EPS led to the formation of a larger amount of smaller carbonate minerals
(< 50 µm) compared to crystals formed in exponential-phase EPS, which were less abundant and larger (> 50 µm). These
findings were used to establish a conceptual model for picoplankton-bloom-mediated CaCO3 precipitation that can explain the role of EPS in
whitings.
In continental volcanic settings, abundant carbonate precipitation can occur with atypical facies compared to marine settings. The (bio-)chemical processes responsible for their development and early ...diagenesis are typically complex and not fully understood. In the Bolivian Altiplano, Laguna Pastos Grandes hosts a 40-km2 carbonate platform with a great diversity of facies and provides an ideal natural laboratory to understand the processes responsible for the precipitation of carbonates in a continental province dominated by volcanism. In order to trace the origin of both water and solutes in the lagoon, the major element and stable isotope compositions (δ2H-δ18O, δ37Cl, δ7Li, δ11B and 87Sr/86Sr) of the spring and stream waters were characterized, as well as the stable isotope compositions (δ13C, δ15N) and noble gas isotope ratios of hydrothermal gases associated with spring waters. The results show that thermal springs discharging on the carbonate platform are close to saturation with calcite. PHREEQC modeling, together with fluid geochemistry and temperature estimated from a combination of geothermometers, indicate that Ca in these springs is inherited from the alteration of the volcanic bedrock by aqueous fluids heated at ∼225 °C and enriched in magmatic mantle-derived CO2. Our results clearly show that the main driver for the precipitation of modern carbonates in Laguna Pastos Grandes is the deeply sourced CO2, which boosts the alteration of volcanic rocks at depth.
A major volcanic episode is recorded across the Neoproterozoic-Cambrian transition in the Moroccan Anti-Atlas. Several volcanic cones are still preserved in the El Graara massif, laterally ...correlatable with volcanic flows dated as Early Cambrian (U/Pb date of 534
±
10
Ma). Volcanic ashes and flows are interbedded with the uppermost part of the Adoudou dolostones, whereas the best-preserved volcano (the Boho Jbel) is onlapped by the overlying Lie-de-vin strata. Available petro-geochemical data from the Boho volcano suggest an alkaline magmatism probably derived from low-grade melting of a garnet–lherzolite mantle source, followed by fractional crystallization. The silica-undersaturated basaltic liquid evolved to form oversaturated rocks: the fractionation of a ferromagnesian phase with high-Ca and low-Al contents is suggested as the main process to cut across the critical plane of silica undersaturation in this geochemical series. Although the Boho geochemical patterns are similar to those of some rift emplacements, more data from other coeval magmatic eruptions are necessary to constrain their geodynamic setting.
Erosion of the Boho volcano favoured formation of a slope-apron composed of four sedimentary facies belts: chaotic megabreccia (related to downslope mass movements of rigid blocks), amalgamated breccia sheets (emplaced by viscous debris flows), a heterogeneous terrigenous belt (representing offshore substrates interrupted by channels intersected by cross-bedded shoals), and variegated shales and stromatolitic dolostones (typical of the Lie-de-vin Formation). Sharp changes in sedimentation rate were associated with modifications in paleorelief sloping and transport mechanisms from subaerial (?) rock fall at the foot of the cone escarpment to sheet-like debris flow on the slopes, and the replacement by sedimentation under wave and storm influence. The presence of an active carbonate productivity, recorded in the primary porosities of the Boho slope-apron, is suggested by widespread development of a robust, coelobiontic, microbial carbonate factory resilient to poisoning by terrigenous influx. Diagenesis in these deposits includes marine, meteoric and deep-burial cementation of calcite, dolomite, iron oxides, quartz, feldspar, and celestine, the latter indicating precipitation from hypersaline pore fluids.
In the carbonate platforms of the western Gondwana margin, the extinction recorded at the Lower-Middle Cambrian boundary is accompanied by a profound change in the style of carbonate deposition. The ...Lancara Formation of the Esla nappe (Cantabrian Mountains, northern Spain) contains a distinct sedimentary turnover due to a combination of tectonism, eustatic fluctuations, and immigration and colonization of new benthic communities, such as the youngest archaeocyathan assemblage of the entire Iberian Peninsula. During latest Early Cambrian times, a regressive trend is recorded in the Lancara Formation. This regression was recorded on a peritidal-dominant, homoclinal ramp that is topped by a tectonically induced discontinuity (D1). The latter surface marks the beginning of a last prograding, regressive tendency recorded on an intra-shelf ramp with ooidal/bioclastic shoals protecting archaeocyathan-microbial patch reefs. The overlying discontinuity (D2) corresponds to a major erosive unconformity, which coincides with the Lower-Middle Cambrian boundary in the Cantabrian Mountains. The subsequent, long-term, earliest Middle Cambrian rise in relative sea-level allowed deposition of low-relief, bioclastic shoals bearing a diverse and cosmopolitan assemblage of benthic fauna. Finally, the previous evolution is bounded by a third discontinuity (D3), which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognised throughout southwestern Europe. Two associations of calcimicrobes occur in the latest Early Cambrian regressive trend of the L a ncara Formation: (i) Proaulopora and Subtiflora are identified in peritidal, high-energy settings, lacking self-supported structures, whereas (ii) intergrowths of Epiphyton, Renalcis and Girvanella encrusted branching colonies and solitary archaeocyaths in protected (back-shoal) patch reefs. The latest Early Cambrian regression is correlated in southwestern Europe in both siliciclastic (Iberian Chains and Ossa-Morena) and carbonate-dominant platforms (Cantabrian Mountains, Montagne Noire and Sardinia). Its tops are recognised as diachronous unconformities ranging in age from early Bilbilian to the Bilbilian-Leonian or Lower-Middle Cambrian boundary.
Lower Cambrian evaporites and carbonates are reported from nearly all the platforms of the western Gondwana margin, which comprises the Souss, Ossa–Morena, Cantabro-Iberian, Armorican and Montagne ...Noire–Sardinian Basins. Both lithologies were deposited in climatically restricted belts and their changing palaeogeographic distributions, according to recent biostratigraphic correlations, are used to infer the latitudinal motion of this margin. As a result, the time span involved in the Cordubian–Ovetian–Marianian interval (8–10
m.y.) requires a relative high rate of drifting, which supports the high apparent polar wander path rates (defined by palaeomagnetic data) proposed for Early Palaeozoic times. The varied and abundant relics of primary to early diagenetic evaporites (gypsum, anhydrite and halite) demonstrate that extensive evaporitic conditions were associated with carbonate and mixed platform systems in an Early Cambrian arid subtropical belt. Evaporites were originally more abundant than suggested by the reported remains because the deposits have undergone a multistep diagenesis that erased most of the former morphologies. Some petrographic criteria are proposed for recognizing silica pseudomorphs after evaporites, such as the development of ‘chicken-wire’ and enterolithic structures, and the presence of lenticular to lozenge-shaped crystals of gypsum and anhydrite relics.
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