Carbonate Cycles Research Articles

Coccolithophores: an environmentally significant and understudied phytoplankton group in the Indian Ocean

Coccolithophores are unique primary producers in the ocean with the ability to calcify. They are known to produce calcareous scales, which form the significant part of calcite oozes or chalk deposits on the seafloor. Coccolithophores are very noteworthy and they are explored to a great extent as nannofossils to reconstruct the past climate. Calcite plates in coccolithophores make them a vital tool in global climate change studies specifically with ocean acidification. These microscopic plants are the major contributor of the carbonate rain that controls the inorganic carbon pump in the ocean, which in turn influences both carbon and carbonate cycles. The emergence of advanced techniques enables us to study the biological aspects of this pelagic calcifier with improved precision. But still, they are understudied world over compared to any other phytoplankton groups. The northern Indian Ocean, being landlocked in three sides and vulnerable to climate change and ocean acidification, severely lacks focused studies on coccolithophores, though the US JGOFS in the 1990s have outlined the ecological significance of coccolithophores in the Arabian Sea. This paper reviews and outlines our understanding of coccolithophores as well as the nix in the northern Indian Ocean.

Coccolithophore productivity at the western Iberian Margin during the Middle Pleistocene (310–455 ka) – evidence from coccolith Sr∕Ca data

Abstract. Coccolithophores contribute significantly to marine primary productivity and play a unique role in ocean biogeochemistry by using carbon for photosynthesis (soft-tissue pump) and for calcification (carbonate counter pump). Despite the importance of including coccolithophores in Earth system models to allow better predictions of the climate system's responses to planetary change, the reconstruction of coccolithophore productivity mostly relied on proxies dependent on accumulation and sedimentation rates and preservation conditions. In this study we used an independent proxy, based on the coccolith fraction (CF) Sr∕Ca ratio, to reconstruct coccolithophore productivity. We studied the marine sediment core MD03-2699 from the western Iberian margin (IbM), concentrating on glacial–interglacial cycles of Marine Isotopic Stage (MIS) 12 to MIS 9. We found that IbM coccolithophore productivity was controlled by changes in the oceanographic conditions, such as in sea surface temperature (SST) and nutrient availability, and by competition with other phytoplankton groups. Long-term coccolithophore productivity was primarily affected by variations in the dominant surface water mass. Polar and subpolar surface waters during glacial substages were associated with decreased coccolithophore productivity, with the strongest productivity minima concomitant with Heinrich-type events (HtEs). Subtropical, nutrient-poorer waters, increased terrigenous input, and moderate to strong upwelling during the deglaciation and early MIS11 are hypothesized to have attributed a competitive advantage to diatoms to the detriment of coccolithophores, resulting in intermediate coccolithophore productivity levels. During the progression towards full glacial conditions an increasing presence of nutrient-richer waters, related to the growing influence of transitional surface waters and/or intensified upwelling, probably stimulated coccolithophore productivity to maxima following the rapid depletion of silica by diatoms. We present conceptual models of the carbon and carbonate cycle components for the IbM in different time slices that might serve as a basis for further investigation and modelling experiments.

Elucidation of the Losses in Cycling Lithium-Metal Anodes in Carbonate-Based Electrolytes

Because of their higher energy density, compared to lithium-ion batteries, rechargeable lithium-metal batteries have been considered one of the most attractive next-generation energy-storage systems. Uneven deposition of lithium during charge results mainly from two processes. At the peak of lithium dendrites (or lithium hump) a fresh SEI is formed. This freshly formed SEI has a higher concentration of defects, thus higher lithium-ion conductivity. Another mechanism is the preferential lithium-ion conduction at the grain boundaries (GB) in the SEI, at which the concentration of lithium-ion defects is higher than in the bulk of the crystals. During discharge (lithium dissolution), dead lithium, (lithium particles that are electrically disconnected from the current collector) is formed. In this work we studied the effects of several parameters, in carbonate-based electrolytes, on the properties of the SEI and on capacity losses. The effects of vinyl carbonate (VC) and fluoroethylene carbonate (FEC) additives, current density and cycle number on the total capacity loss (QTL, Q deposition—Q dissolution), the capacity needed to repair the SEI after dissolution of lithium (QSEI repair), two types of dead lithium, roundtrip coulombic efficiency and on the correlation among them, will be discussed. Elucidation of these phenomena will lead to the improvement of the lithium deposition/dissolution (charge and discharge) processes in lithium-metal rechargeable batteries.

The palaeoshoreline of Early Miocene formations (Euphrates and Jeribe) at the periphery of the Zagros Foreland Basin, Sulaimani Governorate, Kurdistan Region, NE Iraq

A sedimentological investigation of the Early Miocene deposits at the periphery of the Zagros Foreland Basin, Kurdistan Region, around Qishlagh-Sargrma and Darbandikhan, reveals the presence of the Euphrates and Jeribe Formations in this area. A carbonate-dominated unit, comprising four regressive carbonate cycles, at the base of the Fatha Formation, has been investigated to characterize the depositional environments and stratigraphic context of these Early Miocene deposits. Outcrop and thin-section analyses of the carbonate succession reveal a gently inclined, carbonate-dominated ramp based on the depositional lithofacies and microfacies. Eight carbonate microfacies were identified and interpreted to have been deposited in a shallow marine environment. The Euphrates Formation passes up from deposits of restricted lagoon to shoal depositional environments, while the Jeribe Formation was deposited in a hypersaline lagoon to a restricted lagoon. The Early Miocene index fossil Borelis melo melo was identified in the deposits of the Euphrates Formation. Stratigraphic correlation of the studied sections allows the development of a revised palaeogeography for the Early Miocene deposits in the Kurdistan Region.

Interplay of Autogenic and Allogenic Processes On the Formation of Shallow Carbonate Cycles in a Synrift Setting (Lower Pliensbachian, Traras Mountains, NW Algeria)

AbstractMeter-scale shallowing-upward cycles are recorded in many carbonate successions around the world. It is often difficult to recognize whether they represent autocycles, formed through intrinsic controls, or allocycles, resulting from orbital forcing or tectonic movements, or both. Here, we review the criteria used in the identification of the two types of cyclicity and apply them to two newly described lower Pliensbachian outcrops in the Traras Mountains, northwestern Algeria. Throughout the investigation of six sections, the deposits are suggested to have formed in intertidal–supratidal to shallow subtidal environments on a tropical ramp in the Western Tethys. In this area, shallowing-upward small-scale peritidal and subtidal cycles have been shown to be, and are assumed to be, ordered. The carbon isotope data mirror the recorded cycles and indicate different lengths of subaerial exposures. These cycles, in a developed within synrift setting, have been interpreted as produced mainly by autocyclic processes, but interacting with allocyclic factors. Peritidal cycles are thought to be generated by progradation of intertidal and supratidal flats into lagoonal sediments, while subtidal cycles are interpreted to have been controlled by lateral migration of shoals. The impact of the minor fluctuations of eustatic sea level is weakly marked, and only long subaerial exposure can reveal the contribution of these fluctuations to the formation of the recorded cycles. Tectonic movements resulting from spreading of the Tethys are interpreted to have controlled cycle distribution and thickness at a regional scale. However, synsedimentary tectonic features are rare in the studied area; this suggests that sediment transport would control the thickness and duration of cycles instead of the rate at which accommodation was created.

Controls On Sedimentation and Cyclicity of the Boquillas and Equivalent Eagle Ford Formation from Detailed Outcrop Studies of Western and Central Texas, U.S.A.

AbstractFacies of the Boquillas and equivalent Eagle Ford Formation can be assigned to two associations: a hydrodynamically recycled association and a pelagic association. The hydrodynamically recycled association is composed of lenticular to continuous cross-laminated lime packstone–grainstone, heterolithic facies, and intraclastic grainstone conglomerate with intercalated calcareous mudrock. Sharp bedding contacts, abundant scours, ripple cross-lamination, oscillatory-current indicators, hummocky cross-bedding, and rounded clasts indicate recycling of the bottom sediments by storms and bottom currents above storm wave base (SWB). The pelagic association is composed of continuous lime packstone to grainstone and chalk beds with intercalated calcareous mudrock. Gradational contacts and planar lamination indicate dominant deposition by pelagic rain, but scour surfaces, coarse-fine alternations, and ripple cross-laminae indicate the influence of bottom currents below SWB.Vertical shifts between the hydrodynamically recycled association, interpreted to represent storm agitation of the seafloor during falls in sea level, alternate with the pelagic association and define four sequence boundaries and three depositional sequences with a lower-order periodicity (third-order periodicity, ca. average 2.4 My). Sequence boundaries correspond approximately to member boundaries recognized by gamma-ray profiles. Spectral analysis reveals higher-frequency mudrock–carbonate cycles prominent in the pelagic association, likely resulting from Milankovitch climate forcing of planktonic carbonate productivity versus clay flux. Comparison of gamma-corrected spectra with absolute age constraints reveals that the Ernst Tinaja section (in Big Bend National Park) is less than 28% complete. This is interpreted to result from abundant depositional hiatuses and erosional gaps that occur at the bed scale and at member boundaries within and bounding the Eagle Ford Formation.We demonstrate that deep-marine pelagic deposits in epicratonic settings may be subject to a complex interaction of depositional processes including pelagic rain and recycling by bottom currents and wave-generated currents modulated by low-frequency sea-level fluctuation and high-frequency fluctuations in sediment supply.

Local drivers of the seasonal carbonate cycle across four contrasting coastal systems

Four contrasting coastal systems in Ireland, each with shellfish production activities, were studied to provide a first evaluation of the spatial and seasonal influences on the local carbonate system. The study sites included; (1) a coastal system with sandstone bedrock and minimal freshwater sources, (2) an estuarine system with a catchment limestone bedrock, (3) an estuarine system with a catchment granite bedrock, and (4) a karst groundwater-fed estuary. The type of bedrock was the dominant control on regional carbonate chemistry, where the calcium carbonate catchment bedrock was a strong source of both dissolved inorganic carbon and total alkalinity input in the two limestone regions, which are supersaturated with respect to atmospheric CO2 throughout the year. Primary production played an important role in the non-limestone regions, where both systems were CO2-undersaturated during productive months. Minimum aragonite saturation (Ωaragonite) was observed at all sites during winter when productivity is lowest; surface winter Ωaragonite is <1.5 close to the mussel farms in Kinvara Bay and Bantry Bay. Ωaragonite was recalculated to account for the higher calcium concentrations in the River Suir from limestone dissolution, which increases Ωaragonite by ∼0.5 in the mid estuary and >2 in the inner estuary. The substrate-to-inhibitor ratio (SIR), an alternative indicator of ecosystem vulnerability to acidification, was positively correlated to Ωaragonite in all systems, however with more variability in the two limestone regions. Results highlight challenges of assessing local ecosystem vulnerability to future acidification and the importance of understanding the local spatio-temporal biogeochemistry.

Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

AbstractPalygorskite is a fibrous, magnesium‐bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolution–precipitation reaction or solid‐state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene–Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre‐scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg2+ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co‐occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form.

Detecting the calcium carbonate saturation state under the stress of ocean acidification using saturometry technique

CO2-induced ocean acidification lowers the degree of carbonate saturation state of the seawater, which affects calcification of marine organisms and influences the marine carbonate cycle, thus have negative impacts on the entire marine biogeochemical system. This study seeks to develop a rapid technique to detect carbonate saturation state of seawater based on conductivity changes. A series of batch and flow-through experiments were conducted using various CaCO3 materials. Results show that the conductivity ratios of seawater with and without carbonate addition increase generally with decreasing carbonate saturation states (Ω). The relationship between conductivity ratio and log10Ω apparently follows a linear trend when Ω < 1. It suggests that conductivity measurements can be used to indicate carbonate saturation state of seawater. It is expected to be deployed on CTD instrument to produce depth profiles of seawater carbonate saturation state and will be of great help to future studies on ocean acidification.

CO2 Capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers

Summary Limiting global temperature rises is increasingly dependent on the development of energy-efficient carbon-capture methods. Here, we report a simple CO2-separation cycle using an aqueous bis(iminoguanidine) (BIG) sorbent that reacts with CO2 and crystallizes into an insoluble bicarbonate salt. X-ray diffraction analysis of the bicarbonate crystals revealed “anti-electrostatic” hydrogen-bonded (HCO3−)2 dimers, stabilized by guanidinium cations and water. Mild heating of the crystals releases the CO2 and regenerates the BIG sorbent quantitatively, thereby closing the CO2-separation cycle. Experimental and computational investigations support a CO2-release mechanism consisting of surface-initiated low-barrier proton transfer from guanidinium groups to bicarbonate anions with the formation of carbonic acid dimers, followed by CO2 and H2O release in the rate-limiting step, with a measured activation energy of 102 ± 12 kJ/mol. The minimum energy required for sorbent regeneration is 151.5 kJ/mol CO2, which is 24% lower than the regeneration energy of monoethanolamine, a benchmark industrial sorbent.

Soil inorganic carbon in mangroves of tropical China: patterns and implications.

Soil inorganic carbon (IC) is neglected in most blue carbon studies despite the globally significant role of the calcium carbonate cycle in ocean C balance and climate change. We sampled soils to 1 m depth from seven mangrove reserves in Hainan Island, China. Only 45 out of 509 samples were rich in IC (greater than 10 mg cm-3). Most of the IC-rich samples were found at the outer part of Qinglan Bay, which is adjacent to the largest coral reef zone of Hainan Island. Soil IC concentration ranged from 0 to 66 g kg-1 (or 0-67 mg cm-3), accounting for 0-92% of total C. IC concentration increased with soil depth where it was abundant. Soil pH was low (2.36-6.59) in IC-depleted soils, but increased to 5.67-7.99 in IC-rich soils. Soil total C stock and IC stock in mangroves of Hainan amounted to 0.76×106 and 0.12×106 Mg, respectively, with IC accounting for 16% of total C. Our study finds that carbonate concentrations can be high in mangrove soils but their spatial distribution indicates they are largely allochthonous in origin. Evidence of carbonate dissolution in mangroves suggests mangroves may increase total alkalinity to buffer acidification in seawater.

Calcium and strontium isotope systematics in the lagoon-estuarine environments of South Australia: Implications for water source mixing, carbonate fluxes and fish migration

This study uses Ca and Sr isotopes (δ44/40Ca and 87Sr/86Sr), coupled with elemental ratios, to better understand the water source apportionment and carbonate output in the Coorong, Lower Lakes and Murray Mouth Estuary, which represents the terminus of Australia’s longest river system. The geochemistry of waters in the Coorong (i.e., North and South Lagoon) can be explained by mixing of three major components, including: (i) the Southern Ocean seawater, (ii) local freshwaters, and (iii) hypersaline lagoon waters, the latter significantly modified by ongoing evaporation and carbonate formation. The Sr and Ca isotope composition of the North Lagoon is indistinguishable from that of the Southern Ocean (i.e., normal salinity of ∼35 PSU), with the exception of transient freshwater input events that can temporarily lower the salinity to brackish levels. Interestingly, our results from the hypersaline South Lagoon (salinity up to ∼120 PSU) confirmed that the latter is highly evaporated brackish water (with ≥40% contribution from continent-derived waters), which has been additionally modified by in-situ carbonate precipitation. Importantly, our Ca isotope and elemental constraints showed that about 15–17% of the dissolved Ca2+ in the South Lagoon has been removed as CaCO3 (primarily as aragonite). This in turn has implications for the local carbonate cycle and blue carbon studies, suggesting that the South Lagoon acts as a net sink for the dissolved inorganic carbon (DIC).Ca isotope data from the otoliths of smallmouth hardyhead fish species (Atherinosoma microstoma) collected in the Coorong indicate that δ44/40Ca is primarily controlled by biological processes (i.e., kinetic isotope fractionation effects related to growth rate), rather than by the Ca isotope composition of local lagoon waters. As to 87Sr/86Sr in otoliths, the latter confirmed the importance of continent-derived water sources in the Coorong, recorded over the life span of the fish.Overall, with suitable fossil carbonate archives (e.g., bivalve shells, foraminifera), our calibration of 87Sr/86Sr and δ44/40Ca in the modern hydrological system, with respect to a large salinity gradient (ranging from fresh to hypersaline, i.e., 0 to ∼120 PSU), implies potential future applications of these isotope tracers in carbonate-producing coastal systems, which include (i) tracing and apportioning different water sources, (ii) quantifying local carbonate outputs, and (iii) reconstructing paleo-salinity changes.

Origin and implications of orbital-induced sedimentary cyclicity in Pliocene well-logs of the Western Mediterranean

The climatic origin of astronomically induced sedimentary cycles in the Mediterranean and adjacent areas during the late Neogene and Quaternary remains puzzling; as cycles have been linked to concomitant but seasonally opposite changes in African summer monsoon precipitation (Eastern Mediterranean sapropels) and Atlantic regulated winter-precipitation (carbonate cycles on the Atlantic side of the Mediterranean). Particularly, little is known about the cyclic sedimentation on orbital time scales in the Western Mediterranean, with the prime exception of the Messinian sapropels from the Sorbas basin (southern Spain).Here we show that regular alternations in Pliocene downhole logs from the industrial drill-site Muchamiel-1, located along the Balearic Promontory in the Western Mediterranean, are related to eccentricity (bundles) and to obliquity and precession cycles (basic meter-scale alternations). We establish an astronomically based age model for the interval between 5.33 and 2.8 Ma, by first correlating cycle bundles to eccentricity and then the basic dominantly precession-related cycles to the 65°N summer insolation of La2004. The striking bed-to-bed similarities between the Muchamiel-1 well-logs and other records from both the Atlantic margin and the Central Mediterranean suggest that the same climatic forcing was responsible for the formation of carbonate cycles across the Western Mediterranean and adjacent Atlantic. We conclude that formation of alternating carbonate-rich/carbonate-poor beds was controlled by Western Mediterranean cyclogenetic mechanisms as well as by peri-Mediterranean precipitation associated with changes in the North Atlantic System (NAS). These findings highlight the importance of peri-Mediterranean precipitation on the sedimentary cyclicity by dictating terrigenous (clay) supply and potentially on the hydrology of the basin by providing additional freshwater required for sapropel formation. Consequently, cyclic sedimentation in the Mediterranean results from the combined effect of precipitation changes driven by (i) the North African monsoon, (ii) the Atlantic system, and (iii) intrabasinal Mediterranean atmospheric dynamics.

A pronounced fall in the CaCO3 saturation state and the total alkalinity of the surface ocean during the Mid Mesozoic

Geochemical models suggest that the calcium carbonate saturation state (ΩCaCO3) and total alkalinity (TA) of the ocean fluctuated considerably in the geologic past, under the influence of the continental weathering HCO3– source and the biological CaCO3 sink. The mid-Mesozoic is a key period in this respect because the advent of planktic calcifying organisms introduced the calcium carbonate compensation mechanism, buffering oceanic ΩCaCO3 and TA with respect to perturbations of the carbonate cycle. As a result, ΩCaCO3 and TA are thought to have fallen drastically in the mid-Mesozoic, and, excluding short-lived perturbations of the carbon cycle, to have remained similar to present values ever since. This important idea, put forward using geochemical models, is tested here using the geological record of marine calcified cyanobacteria. It is shown that under the most conservative of assumptions oceanic ΩCaCO3 must have been ≥6–10, and TA ≥3 mM, in the Permo-Triassic, prior to the advent of planktic calcifying organisms. It is likely, though, that oceanic ΩCaCO3 (≥10–16) and TA (≥4–6 mM) were greater than these minimum estimates during most of this time interval. In the Late Cretaceous, once the pelagic CaCO3 sink was well in place, oceanic ΩCaCO3 (~4) and TA (~2 mM) had fallen to values compatible with those of the modern ocean. This analysis implies a large fall in oceanic ΩCaCO3 and TA between 180 and 90 Myr, a time interval that encompasses the advent of planktic calcifying organisms and the introduction of the carbonate compensation mechanism. This cyanobacteria-based reconstruction strengthens the claim that pelagic calcification revolutionised the carbonate cycle. It also highlights, without implying a cause-effect link, that biocalcifying organisms evolved in an ocean with an elevated HCO3−/H+ ratio, a condition known to favour biological calcification in laboratory experiments.

From Eggshells to Quicklime: Using Carbonate Cycle as an Integrating Concept To Introduce Students to Materials Analysis by TGA and FTIR

In this paper, we report an experiment intended as a tool to introduce chemistry and materials science students to several chemical concepts through the determination of the best experimental conditions for the formation of calcium oxide (lime) from a source of high content of calcium carbonate (eggshells). Thermal gravimetric analysis (TGA), infrared spectroscopy (FTIR), and qualitative tests were used to optimize the experimental conditions of calcination temperature and time for the formation of calcium oxide.

The Ecology, Biogeochemistry, and Optical Properties of Coccolithophores.

Coccolithophores are major contributors to phytoplankton communities and ocean biogeochemistry and are strong modulators of the optical field in the sea. New discoveries are changing paradigms about these calcifiers. A new role for silicon in coccolithophore calcification is coupling carbonate and silicon cycles. Phosphorus and iron play key roles in regulating coccolithophore growth. Comparing molecular phylogenies with coccolith morphometrics is forcing the reconciliation of biological and geological observations. Mixotrophy may be a possible life strategy for deep-dwelling species, which has ramifications for biological pump and alkalinity pump paradigms. Climate, ocean temperatures, and pH appear to be affecting coccolithophores in unexpected ways. Global calcification is approximately 1-3% of primary productivity and affects CO2 budgets. New measurements of the backscattering cross section of coccolithophores have improved satellite-based algorithms and their application in case I and case II optical waters. Remote sensing has allowed the detection of basin-scale coccolithophore features in the Southern Ocean.

The roles of endolithic fungi in bioerosion and disease in marine ecosystems. I. General concepts

ABSTRACTEndolithic true fungi and fungus-like microorganisms penetrate calcareous substrates formed by living organisms, cause significant bioerosion and are involved in diseases of many host animals in marine ecosystems. A theoretical interactive model for the ecology of reef-building corals is proposed in this review. This model includes five principle partners that exist in a dynamic equilibrium: polyps of a colonial coelenterate, endosymbiotic zooxanthellae, endolithic algae (that penetrate coral skeletons), endolithic fungi (that attack the endolithic algae, the zooxanthellae and the polyps) and prokaryotic and eukaryotic microorganisms (which live in the coral mucus). Endolithic fungi and fungus-like boring microorganisms are important components of the marine calcium carbonate cycle because they actively contribute to the biodegradation of shells of animals composed of calcium carbonate and calcareous geological substrates.

Does compaction-induced subsidence control accommodation space at the top of prograding carbonate platforms? Constraints from the numerical modelling of the Triassic Esino Limestone (Southern Alps, Italy)

The demise of the high-relief, steep-slope, prograding Ladinian-Early Carnian carbonate platforms of the Esino Limestone (Central Southern Alps of Italy) is marked by subaerial exposure of the platform top associated with different erosional (mainly karst-related), depositional and diagenetic processes (Calcare Rosso). The exposure-related deposits consist of three major facies associations: 1) residual soils with thin lenses of conglomerates with black pebbles, and, locally, weathered vulcanites; 2) chaotic breccia lenses irregularly distributed in the uppermost part of the Esino Limestone carbonate platform, interpreted as collapse breccias in karstic setting: 3) inter-supratidal carbonate cycles with dissolution and development of paleosols and tepee structures.Facies distribution follows the sub-environments of the underlying Esino Limestone. Facies 1 and 2 typically characterize the core of the platform, covering the underlying inner platform facies. Facies 3 instead develops toward the edge of the platform, above reef-upper slope facies of the prograding facies of the Esino Limestone. The thickness of facies 3 decreases toward the core of the platform. Facies distribution reflects differences in the accommodation space and sedimentary processes from the rim (highest accommodation, favouring the deposition of peritidal-supratidal carbonates) to the core (reduced accommodation, causing pedogenesis and karstification) of the carbonate system.The observed thickness changes may be controlled by different factors: 1) syndepositional tectonics, 2) subsidence induced by magmatic activity or 3) differential subsidence controlled by the stratigraphic architecture of the Esino Limestone platform and adjoining basins. As evidence of tectonics was not observed and the presence of volcanic bodies is only documented tens of km away from the study area, the scenario involving the creation of accommodation space by compaction of the basinal sediments (resedimented, fine-grained calciturbidites) during the progradation of the carbonate platform is here investigated. Numerical modelling was performed to verify the compatibility of compaction-induced subsidence with the observed depositional architecture. The models were built to simulate the architectural evolution of the platform by progressively adding layers from deepest to shallowest, while compacting the underlying sediments, in order to evaluate compaction-induced subsidence (and accommodation space for the Calcare Rosso) after the deposition of the youngest platform strata. Modelling results allow us to conclude that the wedge geometry of the Calcare Rosso, deposited on top of the extinct Esino carbonate platform, can be explained by subsidence controlled by compaction of the basinal sediments present below the early-cemented, fast prograding platform slope deposits.

Ichnocoenoses and taphocoenoses of posidoniid-bearing marl-limestone rhythmites and event beds, Toarcian-Aalenian, Northern Apennines, Italy

The Posidoniid-bearing rhythmic deposits, Toarcian to Aalenian (Early/Middle Jurassic) in age, of the Umbria-Marche basins (Fiuminata Colle Corno and Valdorbia type section) consist of hundred couplets, each composed by two semicouplets, a calcium carbonate-poor bed and a calcium carbonate-rich bed, respectively (CacO3 between 10% to 85%), where event tempestite beds randomly occurs. The aim of this work is to show ichnocoenoses and taphocoenoses as well as the faunistic variations of each semicouplet and event beds. Each couplet is ∼30cm thick in the Toarcian Rosso Ammonitico and up to 40cm in the Aalenian Calcari e Marne a Posidonia Formation, where the sedimentation rate increased. Here, couplets were deposited in the dysaerobic/aerobic transition zone. The clay-rich portion contains abundant thin-shelled poikilaerobic bivalves and benthic foraminifers (whithout radiolarians) and shows oxygen-poor ichnocoenosis with Chondrites, while the taphocoenosis exhibits convex-upward disposition and disarticulated bivalve shells. Conversely, the limestone semicouplet is rich in radiolarians and thin-shelled bivalves, usually more burrowed (ichnocoenosis including Chondrites and few Planolites and Thalassinoides) than the former, with grouping, dispersion, biogenic fragmentation and orientation-reorientation. In all marl-limestone semicouplets, foraminifers and radiolarians exhibit a periodic inverse correlation in abundance, while thin-shelled bivalves are always abundant. Rhythmically disposed within the rhythmites are also event beds indicating an occasional oxygen-rich substrate, fine-grained calcarenitic tempestites with hummocky cross-stratification, and winnowed bed (thin shelled bivalve concentrations). These deposits are intensely bioturbated (ichnocoenosis including Skolithos, Planolites, Trypanites, Chondrites, and Thalassinoides). Taphocoenoses include fragmentation, grouping, surficial burrowing, biogenic reorientation, and deep burrowing. Ichnological and taphonomical features indicate high physical and biogenical reworking affecting sediments. The Jurassic depocenters of Valdorbia and Fiuminata that received rhythmical dilution (clay) or productivity (carbonate) cycles during Toarcian and Aalenian time can therefore strongly contribute to the study of ichnological and taphonomical variations in rhythmic conditions.

Mixed carbonate–siliciclastic sedimentation in forebulge grabens: An example from the Ediacaran Bambuí Group, São Francisco Basin, Brazil

Mixed carbonate–siliciclastic successions have been described in multiple Phanerozoic sedimentary settings recording the dynamic interplay of tectonics, eustasy, climate, in situ carbonate production, and variations in siliciclastic sediment supply. The Ediacaran Bambuí 1st-order sequence (i.e., Bambuí Group) covers most of the intracratonic São Francisco basin (southeast Brazil) and encompasses thick packages of carbonate and fine- to coarse-grained siliciclastic strata. Recording a marine foreland basin stage that developed in the São Francisco plate during the Neoproterozoic–early Paleozoic West Gondwana assembly, the Ediacaran deposits unconformably overlie Archean–Paleoproterozoic metamorphic assemblages of the Sete Lagoas basement high and fill a series of kilometer-long grabens in the southern São Francisco basin. Seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the Sete Lagoas high, in the early evolutionary stages of the Bambuí basin cycle. Based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns, we recognized two transgressive–regressive 2nd-order sequences preserved within one of the focused grabens: (i) Sequence 1 includes the glaciogenic deposits of the basal Carrancas Formation that grade upward into the carbonate ramp successions of the Sete Lagoas Formation; (ii) Sequence 2 contains the siliciclastic-dominated and deep water to deltaic strata of the Serra de Santa Helena Formation and passes upward into peritidal carbonates of the Lagoa do Jacaré Formation. These sedimentary successions encompass suites of retrogradational, aggradational, and progradational lower-rank cycles and are bounded by erosional surfaces. Regional seismic interpretation, well data, and the available literature indicate that most of these deposits and their correlatives are continuous over most of the Sete Lagoas basement high, showing remarkable variations within the studied forebulge grabens. Our analysis demonstrates that the depositional architecture of Sequence 1 and Sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes. The latter are mainly recorded by the lower second-order sequence (Sequence 1), which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late Neoproterozoic. An additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses. As indicated by lower- to higher-rank cycles, it has been responsible for the deposition of thicker transgressive to early regressive successions, a few gravitational flow deposits, and the development of the deltaic to shallow marine carbonate cycles of Sequence 2. The overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the Phanerozoic were also present in the late Neoproterozoic. The close association between organic-rich post-glacial shales, potential reservoir facies, and rift-related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns.