Carbonate Shelf Research Articles

Microbial sulphur-cycling and atmospheric signatures in the 2.52 Ga Gamohaan Formation, South Africa

The Late Archean atmosphere is thought to have been largely reducing as suggested by the preservation of large Mass-Independent fractionations of S isotopes (MIF-S) in sedimentary sulphides and sulphates. However, there is a growing body of evidence for transient oxygen accumulation in the Neoarchean oceans, especially from shallow marine environments. Here ∼2.52 Ga carbonates in the upper Campbellrand Group of South Africa are investigated from the shallow marine carbonate shelf Kogelbeen Formation to the deeper water basinal Gamohaan Formation. Previous studies of these rocks have found large positive Δ33S signatures, but with differing interpretations of the controls on, and pathways for, the preservation of Δ33S. Here bulk S isotope measurements from sedimentary sulphides and carbonate associated sulphate (CAS) are reported, combined with in-situ S isotope measurements by Secondary Ion Mass Spectrometry (SIMS) from well-preserved pyritised microbial mat microtextures of the Gamohaan Formation.Large positive Δ33S signatures are observed in sulphide grains of the Gamohaan and Kogelbeen formations (max Δ33SSIMS=12.6‰) confirming the isotopic effects of photolytic reactions in an anoxic atmosphere. Small, early pyrites in fenestrate microbialites record an atmospheric signature (average δ34SSIMS=7.6‰, Δ33SSIMS=8.5‰, Δ36SSIMS=−7.6‰) reflecting the preservation of atmospherically derived Sn particles in the microbial carbonates. Contorted microbial mat layers, also from the carbonate slope, exhibit large mass-dependent fractionations with δ34SSIMS of up to 30.6‰ plotting off the Archean reference array, suggesting oxygenated seawater on the shelf-slope. Pyrite concretions in organic rich carbonates from deeper water facies show S isotope values (average δ34SSIMS=3.1‰, Δ33SSIMS=−0.2‰, Δ36SSIMS=2.5‰) indicative of microbial sulphate reduction (MSR).The CAS data show positive Δ33S with values of up to 8.49‰ and positive δ34S values that are more similar to sulphide data measured from each sample than estimated late-Archean seawater sulphate. This is interpreted to record the early oxidation of sulphides with positive Δ33S by shallow oxygenated waters in the upper diagenetic zone that moved through the sediment and was then incorporated into the platform carbonates. In this way, we hypothesise that positive Δ33S can be inherited by the Neoarchean diagenetic sulphate reservoir and preserved in carbonates.

Paleoenvironmental analyses of coralline red algal assemblages in Lower Miocene carbonate deposits of Central Iran

ABSTRACT This work is based on coralline red algal flora-dominated Lower Miocene carbonates of the Qom Formation from Central Iran Basin. Five assemblages, including corallinoid, lithophylloid, mastophoroid, melobesioid and sporolithoid corresponding to 11 genera of geniculate and non-geniculate, were identified. Also, growth forms and taphonomic characteristics of coralline-algal grains were assessed. The arborescent, layered to foliose lamellae, fruticose, lumpy, warty and encrusting growth forms of studied taxa are observed and five taphonomic traits belonging to both mechanical [abrasion, disarticulation, fragmentation] and biological [encrustation, bioerosion] processes were recognised. The relative abundance of assemblages, growth forms, and taphonomy of red algae visualised at thin-section scale were calculated. Our quantitative analysis highlights mechanical (abrasion > fragmentation) and biological (bioerosion> encrustation) processes. Moreover, layered to foliose and fruticose abundance peaks reveal higher than others. This kind of algal distribution in the examined region indicates that the Qom Formation was deposited on a marine carbonate shelf, which is a deepening depositional system in tropical and subtropical zone. This approach expresses more abundance of red algal assemblages on the deep part of inner and proximal to distal middle shelf settings along with a regime of moderately high towards low hydrodynamic energy and mesophotic – oligophotic conditions.

Sequence Stratigraphy and Sedimentary Environment of the Shiranish Formation, Duhok region, Northern Iraq

The Shiranish Formation is cropped out in several areas in northern Iraq. A stratigraphic and facies study was conducted within the Duhok region to determine the sedimentary environment. Three microfacies, reflecting the various subenvironments within different shelf parts of the deep sea, have been identified within the Shiranish Formation. Four depositional environments are identified: slope, the toe of slope, deep shelf, and deep-sea or cratonic deep basin. The Shiranish Formation in the Duhok region, Northern Iraq, was deposited in an open shelf carbonate platform. The Shiranish Formation sequence is divided into six third-order cycles in the study area. These asymmetrical cycles reflect an imbalance between the relative level of the sea and the production of carbonate, and each one reflects a rise in the sea level following a period of standstill. There is a two-sequence boundary type SB-2 that defines the surface. The Shiranish sequence developed in a high-subsidence area that played the main role in the evolution of the formation. It was deposited on a carbonate platform with high subsidence due to major transgression, wherever the successive sea-level rise and stillstand episodes persist.

Sequence Stratigraphy of Miocene Mixed Carbonate-Siliciclastic in Fold and Thrust Belt System, Banggai Basin

This study was specifically analyzed in the southern part of the Banggai Basin, precisely in the TKA Field. In the Tertiary, regional stratigraphy reveals, the Banggai Sula microplate comprises carbonate shelf and siliciclastic sequence. Several previous studies had been concluding, the tertiary limestone was deposited during the Early Eocene to Middle Miocene. Carbonate formations which has been known by Tomori and Minahaki Formation. Meanwhile, their siliciclastic dominated is Matindok Formation and Celebes Mollase. These sequences formed a distinct stratigraphic style in the geophysical well log and seismic data. Four cycles are distinguished, the lower part of Tomori Formation at the outer shelf and directly overlying by the upper part of inner shelf Tomori Formation. it was superimposed by siliciclastic sediments that have been deposited in the outer litoral during regressive phase. Whereas during rising sea level or highstand, the litoral system migrated to basinward wherein Minahaki Formation was deposited. Sequence stratigraphy provides a reliable methodology for correlate both carbonate and siliciclastic successions. It was analyzed based on 4 wells, 2D, and 3D seismic data which has been available. Sequence stratigraphy analysis using T-R sequence concept by Embry and Johanessen. Trasngressive and regressive phases were obviously determined by lithofacies, coral colony and foram that were recorded on biostratigraphic analyses. In this study, cyclic sea-level change will be recognized as system tract record of mixed carbonate-siliciclastic sequence.

Albian/Cenomanian boundary in the Sergipe-Alagoas Basin: Sea-level changes and paleoecology based on ostracods

This paper presents an analysis of the paleoenvironmental evolution of the Sergipe-Alagoas Basin, Brazil, at the Albian/Cenomanian boundary, based on ostracods obtained from drill core UFRJ-2-LRJ-1-SE. The variability in the composition of the ostracod fauna allowed for the identification of the stratigraphic cycles throughout the studied sequence. A total of 2292 ostracod specimens were obtained from 44 samples. Ostracod abundance, diversity and richness led to the identification of five ostracod biofacies (A, B, C, D, E), each associated with different paleoenvironmental changes. Combined with available lithological data, we suggest the presence of three 4th order transgressive (T) regressive (R) cycles in the studied section, with the maximum flooding surfaces (MFSs) being delineated by the decrease in the relative abundance of ostracods, and the maximum regression surfaces (MRSs) being characterized by the increase in the relative abundance and diversity of ostracods. Furthermore, the T-R1, T-R2, and T-R3 sequences can be correlated with the global eustatic sea level curve. The stratigraphic distribution of the ostracod biofacies suggests that the studied section was deposited in a middle to outer shelf environment. The transition from a carbonate shelf to a slope environment in the Riachuelo Formation is here suggested as being characterized by the presence of Eucytherura amoriensis, which can be used to track these changes in the Sergipe-Alagoas Basin. The similarity of the paleoenvironments and the ostracod fauna of the Sergipe Group and of the Talme Yafe Group from Israel suggests that there is a close relationship between the depositional environments of these two regions.

Ordovician tectonic transition from passive margin into peripheral foreland in the southern Ordos: A diagnostic insight into the closure of Erlangping Ocean between the North Qinling Arc and North China Block

AbstractAchieving a reliable closure time of a back‐arc ocean is an essential aspect in studies on detailed tectonic processes of an active continental margin and arc–continent collision. This is particularly the case for the northern Qinling Orogen, which records the accretion of the North Qinling Arc (NQA) onto the North China Block (NCB) after the Erlangping back‐arc ocean closure. Sedimentological, petrological and geochronological signatures from the Ordovician successions in the southern Ordos reveal a tectonic transition from passive continental margin to peripheral foreland in the southern NCB at the beginning of Katian. Sedimentological and geochronological investigations reveal an abrupt shift of accelerating basin subsidence and deepening at the earliest Katian, separating ca. 300‐m‐thick shallow‐marine carbonate shelf assemblages from overlying ca. 2000‐m‐thick deep‐water carbonate slope and turbidite associations. Zircon age spectra of the Katian turbidites are characterized by early‐Palaeozoic and Neoproterozoic age clusters, which are different from those of the Middle Ordovician quartz arenites sourced merely from the NCB basement. Instead, these age patterns match well with those of the coeval successions in the northern NQA, indicating a spatially linked abyssal deposystem. Stratigraphic architecture deciphers a typical foreland basin geometry, involving, from south to north, northward‐propagating turbiditic wedge, northward‐backstepping carbonate slope and progressively shoaling carbonate platform, embodying foredeep, forebulge and backbulge, respectively. These characteristics of basin‐fill evolution reflect the northward migration of the flexural wave as a dynamic response to the northward expansion of the thickened NQA thrust wedge. Together with the other geological and geochronological data, our new insights indicate a southward subduction polarity of the Erlangping back‐arc oceanic crust followed by its termination at ca. 450 Ma, which was earlier than that of the main Proto‐Tethyan Shangdan Ocean between the NCB and South China Block. Our new data provide an updated view of the complex history of the Proto‐Tethys closure during the Gondwana assembly.

Controls on large-scale architecture and facies distribution of an oolitic-microbial carbonate shelf margin, Nanpanjiang Basin, south China

Carbonate factory types and facies architecture of carbonate depositional systems result from complex interactions among several space- and time-dependent variables, including paleobathymetry, tectonic setting, water chemistry and oceanographic setting, clastic sediment influx, and accommodation. Whereas tropical factories that dominate modern settings have been documented and modelled extensively, microbial-oolitic systems, which are very common throughout the Paleozoic and Mesozoic, have received less attention.Continuous exposures along a platform-to-basin transect of an Early Triassic (Induan) microbial-oolitic carbonate ramp to shelf system in the Nanpanjiang Basin, South China, serves as a viable analogue to similar systems, including the Jurassic Smackover Formation of the Gulf of Mexico, and provide a natural laboratory to test these variables. Carbonate deposition following end-Permian regional drowning and retreat of the Yangtze Platform resulted in low-angle margin-slope systems during the Early Triassic. Lower Triassic carbonate systems display shifts in carbonate factory types, including skeletal, microbial, and abiotic factories, and lateral and vertical architectural variability.We applied stratigraphic forward modelling using DIONISOSFLOW, calibrated with detailed stratigraphic sections and facies mapping, to assess the impact of carbonate factory type, antecedent topography, and variations in accommodation on facies distribution and large-scale geometries of the carbonate shelf margin. Results from model simulations suggest that in the absence of skeletal carbonates in the Lower Triassic, the architecture of the carbonate margin is mainly controlled by rate of microbial growth along the slope.

Early Miocene Intensification of the North African Hydrological Cycle: Multi‐Proxy Evidence From the Shelf Carbonates of Malta

AbstractDuring the Miocene (23.0–5.3 Ma) North Africa experienced both humid and arid intervals, but the underlying cause of these transitions is unknown. Earth's climate was characterized by a unipolar icehouse with a dynamic Antarctic ice sheet, which may have influenced regional hydrology through atmospheric teleconnections. However, the Miocene also witnessed the restriction of the Mesopotamian Seaway, which may have had significant climatic impacts. The Maltese il‐Blata section (Central Mediterranean) comprises Late Oligocene to Early Miocene marine deposits previously used to constrain the timing of the Mesopotamian Seaway restriction using the εNd tracer. The location of this section also makes it sensitive to climatic changes in the North African region, and biogeochemical changes in the central Mediterranean. Here, we present lithological and geochemical records of the il‐Blata section. We find a marked shift in lithology and an increase in sedimentation rate coeval with the Early Miocene (∼19–20 Ma) restriction of the Mesopotamian Seaway. Concomitant changes in bulk sediment CaCO3, Sr/Ca, K/Al, Ti/Al, Zr/Al, and Si/Ti support a major humid climate transition and associated intensification of river systems over western North Africa. We propose that these changes in North African hydroclimate reflect either a tipping point effect in a gradually warming global climate, or are the result of the initial restriction of the Mesopotamian Seaway, perhaps through consequent changes in Atlantic Meridional Overturning Circulation and the West African Monsoon. We also suggest the restriction of the Mesopotamian Seaway inhibited phosphorite deposition at low latitudes.

Depositional Facies Model and Reservoir Quality of Paleogene Limestone in Labengki Island, Southeast Sulawesi

Eastern Indonesia has become an attractive venture for hydrocarbon exploration since 10 years ago. The discovery of hydrocarbon from Miocene carbonate of Tondo Formation has opened a new opportunity and hopes in the Southeast Sulawesi region. Can we find other potential reservoirs in Southeast Sulawesi? In this study, we assess and reconstruct the depositional model and the reservoir quality of the Paleogene Tampakura Formation in Labengki Island, Southeast Sulawesi. This field observation and petrographical study revealed that: (1) Tampakura Formation comprises mainly of grainstone, boundstone and floatstone with minor packstone and dolomitic wackestone/mudstone, (2) Tampakura Formation was deposited mainly in wide carbonate sand shoals and reef margin belt of rimmed carbonate shelf, (3) Boundstone and floatstone facies could be the best reservoir candidate in the region since they show extensive porosities development of cavernous and fracture porosity, (4) Dolomite cementation has deteriorated the reservoir quality of packstone which was deposited in platform interior-restricted marine, (5) Extensive calcite cementation in grainstone facies has reduced the reservoir quality of Tampakura Formation. However, locally, solution enlarged fracture porosity may have enhanced it. We suggest that post collision event of Late Oligocene - Early Miocene between Australian-originated microcontinent and ophiolite complex was highly responsible to create cavernous porosity. The collision resulted in the folding and uplifting of Tampakura Formation to the subaerial exposure. The carbonate strata were exposed to the surface developing a cavernous porosity and potentially becoming the best reservoir candidate for the next exploration target.Keywords: Carbonate facies, Kendari Basin, reservoir quality, Tampakura Formation.

Facies analysis and depositional framework of Late Permian-Jurassic sedimentary successions, Western Salt Range, Pakistan: implications for sequence stratigraphic trends and paleogeography of the Neo-Tethys Sea

Facies analysis and T-R sequence stratigraphic approach of Late Permian to Jurassic sedimentary units in Western Salt Range, Pakistan were accomplished to construe the depositional environments and basin evolution. The analysis affirms the rendition of sequence stratigraphic trends and paleogeography of first megasequence phase of Neo-Tethys Sea. Sedimentological contingents, T-R sequence stratigraphic framework and sea level curve of the strata argue active tectonic’s effects on sedimentation. During Late Permian, the rift related magmatic activities deceased, which imparted to the onset of transgression and deposition of shelf carbonates with retrogradational (transgressive) parasequence sets in Wargal Formation. Respective rifting and tectonic uplift events induced the sea level fall ascertaining the onset of terrigenous shelves and deltaic successions in terminal Permian and Early-Mid Triassic with an enlighten switch from agradational to progradational parasequence sets (regressive parasequences) in Chhidru, Mianwali and Tredian formations. The closure of Paleo-Tethys and emergence of semi-arid hot tropical climate throughout Late-Triassic, led the onset of tidal-lagoonal environments and deposition of retrogradational (transgressive) parasequence sets in Kingriali Formation. During Early Jurassic, a well-known northward drift of Pangaea ensued in global cooling and increased humidity, which consequently stimulated clastic-carbonate sedimentation of Datta and Samana Suk formations with progradational and retrogradational parasequence sets respectively.

Microbiostratigraphy, microfacies analysis and lateral basin evolution of Lower Cretaceous deposits in the south of Kerman region, SE Iran

Detailed microbiostratigraphy and basin evolution of the Lower Cretaceous deposits in the Rayen area, south of Kerman Region, SE Iran are investigated for the first time in two sections. The section no. 1 is 324.6m in thickness and comprises five lithostratigraphic units. The section no. 2 is 218 m in thickness and includes three lithostratigraphic units. The identified fauna and flora include 41 benthic foraminifera and 11 calcareous algae species. The identified assemblage indicates that the marine strata in both sections were deposited during the Barremian to Albian. The microfacies analyses carried out on 22 carbonate and 2 clastic microfacies indicate that the deposits in the section no. 1 were deposited on a homoclinal carbonate ramp, whereas in the section no. 2 they were deposited on a rimmed carbonate shelf. Generally, the Cretaceous deposit in the two studied sections represent different sedimentary models and fossil content indicating different basin evolution histories. The paleogeographic setting of the studied area on the south eastern margin of the Central-East Iranian Microcontinent and the active tectonic history during the Mesozoic suggest that the syndepositional tectonism influenced the basement’s morphology and resulted in changes in the fossil diversity and sedimentary nature of adjacent sedimentary basins.

Porosity prediction based on conventional and geostatistical image computation, Lower to Middle Eocene siliciclastics in the Tuz Gölü Basin (Central Anatolia, Turkey)

The study area is located at the east of Tuz Gölü Basin, one of the sedimentary basins in the Central Anatolia basins between Ş. Koçhisar and Karapınar town. The main subject of study is based on the Boyalı Formation consisting of braided river deltas and submarine fan deposits with shelf carbonates blocks of the Lower to Middle Eocene. Laboratory analysis showed that porosity and permeability values are between % 1.40–19.40 (average % 9.15) and 0.01–43.95 mD (average 5.07 mD), respectively. The conventional method is compared with the geostatistical image analysis method by rock plug samples parallel to layering for porosity prediction. These samples were saturated with blue resin and made thin sections. The boundaries of the blue areas (mean pores) were lined for pore volume calculation to create solid objects. A proposed empirical mathematical method, geostatistical based, was used to gain porosity values to converge to the laboratory values. Also, in the scope of this study, an architectural, mathematical logic called geostatistical first-order logic architecture (GFOLA) was generated. The geostatistical based programme “GeoST Porosity v.1.0” has been developed for numerical analysis. The empirical approach has been used to transform the areal measurement into volumetric measurement, eliminate the conditions that could not be measured in the 3rd dimension, and positively affects the porosity approach ratio, which is the ratio of empirical to laboratory porosity. In the empirical approach, the porosity is the function of grain size (x), sorting (y) and skewness (z) ∅=g(x,y,z). To obtain the deviation from the “ideal condition (μ)”, the yielded result has been modelled as an attenuation relation "eGf" which is a function of grain texture factor (Gf). A correlation graph of the grain texture factor and porosity approach ratio obtained by the empirical approach has been determined to be applied to siliciclastics. For practical purposes, a geostatistical modelling study proved that an empirical approach could be proposed as a better method instead of conventional laboratory porosity measurement.

The role of siliciclastics in carbonate fabric diversity and preservation: A case study from the Neoproterozoic carbonate − siliciclastic Horse Thief Springs Formation, Death Valley

AbstractThe sedimentary record of the Pahrump Group in Death Valley comprises well‐exposed successions of mixed carbonate and siliciclastic deposits. Despite the abundance of studies focussing on the depositional dynamics of mixed carbonate – siliciclastic deposition in the Phanerozoic, the record of similar Proterozoic examples is comparatively sparse. Using high‐resolution stratigraphy and microfacies analyses, this study investigates the Tonian Horse Thief Springs Formation within the Pahrump Group of Death Valley, California, in order to propose first‐order constraints on the interplay between carbonate and siliciclastic deposition in the early Neoproterozoic. The mixed successions are unlike many previously studied examples interpreted as being caused by glacio‐eustatic sea‐level changes, thus arguing for a more relevant tectonic influence on siliciclastic deposition. This interpretation is supported by detailed microfacies analyses of siliciclastic‐rich dolostones, which show abundant soft‐sediment deformation features suggesting sudden pulses of sandstone deposition onto a shallow marine carbonate shelf. The Tonian carbonate factory recovered quickly after being smothered by siliciclastics, particularly due to abundant stromatolite growth. A new relationship between siliciclastic input, carbonate fabric diversity, and carbonate preservation is established based on microfacies analyses, putting forward that siliciclastic deposition had a significant impact on the formation and preservation of later‐stage diagenetic dolomite cements, as well as on stromatolite morphology and carbonate fabric diversity within the microbialites. This study shows how repeated siliciclastic incursions had a significant impact on the Proterozoic carbonate factory of the Horse Thief Springs Formation, as well as on diagenetic modification and preservation of shallow marine carbonates, and establishes previously unexplored relationships between carbonate and siliciclastic strata in the Proterozoic.

Asymmetric abundances of submarine sediment waves around the Azores volcanic islands

Sediment waves are common on the submarine flanks of volcanic islands, but the processes that form them are difficult to decipher from geophysical data alone. Here, we identify downslope-trending trains of sediment waves in multibeam sonar data from five islands of the Azores. Train abundances are derived by counting the train intersections with depth contours around each island, revealing that they are typically twice as common on the north slopes of the islands compared with their south slopes. To provide context, we also characterise the Holocene mobility of sediments on the shelves, ocean density structure (for evaluating internal wave critical gradients) and sediment lithologies in four cores from beneath the slopes, which contain shelf carbonates in some volcaniclastic turbidites. Holocene mobility is assessed using sidescan sonar data, which reveal shelf-crossing sand streams. Using results of a high-resolution wave model, wave-imposed shear stresses are found to have exceeded the stresses at threshold of motion for most of the modern shelf sediments during recent extreme wave conditions.Sediment waves could potentially arise from sedimentary flows, gravity mass movements (shallow slumping) or ocean internal waves. Sedimentary flow origins are favoured by streamwise changes of wavelengths and upslope bedform migrations suggested by seismic reflections. The asymmetry of abundances north and south of each island is also an important clue. For example, it is unlikely that systematically different sediment geotechnical properties occur on the two sides of the islands, hence no reason to expect greater mass movements on northern slopes. Internal waves originate mainly from the south and hence also do not explain the asymmetry. Primary volcaniclastic flows are likely to be equally important on both sides. Sedimentary flows originating from shelf sediment agitated during storms or from failed slope sediment appear to explain best the asymmetry. In that case, prevailing wind and waves approaching the islands from the northwest lead to more frequent sediment gravity flows because they lead to greater flux of sediment from coastal erosion towards northerly outer shelves and upper slopes, where it is prone to slope failure. Offshore-directed near-bed currents originating from northwesterly wind-driven surface currents also transport shelf sediment efficiently to the north, further aided by greater wave agitation on that side. The high abundances of wave trains on the north sides of the islands therefore favours origins involving sedimentary gravity flows associated with wind and wave effects.

The late Aptian–early Albian transgressions in the Chott area, southern Central Tunisia

The detailed sedimentological and biostratigraphic study of the upper Aptian–Albian series in the Chott area, southern Central Tunisia, leads to the identification of several, well-dated depositional sequences. The upper Aptian sequences are marked by very shallow marine, first partly evaporitic, then mainly clastic deposits. The last sequence yielded latest Aptian ammonites (H. jacobi zone). The Aptian–Albian boundary is assumed to be marked by a long-lasting hiatus, and by the abrupt change from clastic to carbonate deposits. The overlying “Knemiceras Beds” express the Albian transgression, and comprise four depositional sequences. New findings of ammonites allow to date these beds to the lower Albian (D. mammillatum superzone), and to correlate the four sequences with those defined in Central Tunisia. Five distinct morphologies of Knemiceras sp. are recognized in the “Knemiceras beds”. Because of their stratigraphic position, the overlying massive shelf carbonates (Radhouane Member) are ascribed to the uppermost lower Albian (T. camatteanum and L. pseudolyelli zones), and correlated with the Allam Limestone of Central Tunisia. This carbonate series is capped by a major discontinuity, correlated with the middle Albian discontinuity known in Central Tunisia. The ammonites and rudists fauna are presented and illustrated.

Early Mesoproterozoic Ca-carbonate precipitates record fluctuations in shallow marine oxygenation

Oxygenation, a key control on marine life, has a long and varied history. This is well seen in the early Mesoproterozoic. During this critical period, midway between the Great Oxidation (GOE) and Neoproterozoic Oxidation (NOE) events, low marine levels of pulsed oxygenation - likely driven by fluctuations in cyanobacterial productivity - alternated with episodic anoxia. However, since baseline oxygen levels were low, deciphering the scale and variability of redox evolution during this prolonged interval is challenging. This in turn has made it difficult to confidently identify key underlying controls on redox evolution. Combining sedimentological and geochemical methods, we studied shallow ∼1.57 Ga shelf carbonates with contrasting depositional fabrics. Carbonate I/(Ca + Mg) data indicate that (i) carbonate mud precipitated in a weakly oxygenated water column, whereas (ii) seafloor crystal fans formed in more poorly oxygenated seawater, containing Fe2+ and Mn2+. We infer that in (i) weakly oxygenated (hypoxic-oxic) conditions reflect increased planktic cyanobacterial productivity that, in the absence of carbonate precipitation inhibitors (Fe2+ and Mn2+), promoted water column carbonate mud (‘whiting’) precipitation. At the same time, aerobic degradation of settling organic matter lowered seafloor CaCO3 saturation. In contrast, in (ii) more poorly oxygenated seawaters containing Fe2+ and Mn2+ inhibited calcite precipitation but permitted aragonite which formed seafloor crystal fans. Together, these contrasting precipitates indicate relatively rapid pulsed redox fluctuations. These were relatively subtle, but sufficient to result in contrasting carbonate sediments, readily recognizable in the field. We hypothesize that cyanobacterial productivity, possibly linked to the availability of nutrients such as phosphate, was a key influence on water column redox cyclicity during the Mesoproterozoic.

Palaeoenvironment, palaeogeography, and palaeoclimatology of the Mississippian carbonate ramp in the Alborz tectonostratigraphic zone: Implications for deciphering the controlling factors on ramp evolution

The Mississippian successions of the Alborz tectonostratigraphic zone have been studied in several sections (Dozdehban, Naserabad, Shahmirzad, and Mighan) extending from north to southeast. The sedimentological data indicate three main facies associations (FA): (FA1) a basin to distal outer ramp of low‐energy, low‐oxygen environment with low bioturbation and less developed in situ fossils; (FA2) a middle ramp facies accompanied by storm‐induced sediments (tempestites), echinoderm‐dominated mud mounds, and high fragmentation of skeletal components; and (FA3) an inner ramp facies, represented by sand shoal, and lagoonal/back‐barrier, mainly characterized by the dominance of oolitic, bioclast, and intraclast grainstones. The sedimentary features imply the formation of the succession on a broad flat‐topped, high‐to‐moderate‐energy shelfal carbonate ramp which had been open to the N/NE, the downslope direction, as revealed by local palaeoenvironmental reconstructions. Analysis of several proxies indicates a subtropical palaeoclimate evidenced by (a) widespread tempestite facies and occurrence of storm‐depositional processes, (b) diverse and abundant pellet‐forming thalassinoideans and burrowing echinoids trace fossil assemblages, (c) geochemical proxy indicating a palaeotemperature of 23°–25°, and (d) a rich subtropical heterozoan fossil association. The data provided here imply the palaeolatitudinal position of the Alborz Mississippian ramp is 30–40°S along the southern margin of the Palaeotethys.

Geochemical Records Reveal Protracted and Differential Marine Redox Change Associated With Late Ordovician Climate and Mass Extinctions

AbstractThe Ordovician (Hirnantian; 445 Ma) hosts the second most severe mass extinction in Earth history, coinciding with Gondwanan glaciation and increased geochemical evidence for marine anoxia. It remains unclear whether cooling, expanded oxygen deficiency, or a combination drove the Late Ordovician Mass Extinction (LOME). Here, we present combined iodine and sulfur isotope geochemical data from three globally distributed carbonate successions to constrain changes in local and global marine redox conditions. Iodine records suggest locally anoxic conditions were potentially pervasive on shallow carbonate shelves, while sulfur isotopes suggest a reduction in global euxinic (anoxic and sulfidic) conditions. Late Katian sulfate‐sulfur isotope data show a large negative excursion that initiated during elevated sea level and continued through peak Hirnantian glaciation. Geochemical box modeling suggests a combination of decreasing pyrite burial and increasing weathering are required to drive the observed negative excursion suggesting a ∼3% decrease of global seafloor euxinia during the Late Ordovician. The sulfur datasets provide further evidence that this trend was followed by increases in euxinia which coincided with eustatic sea‐level rise during subsequent deglaciation in the late Hirnantian. A persistence of shelf anoxia against a backdrop of waning then waxing global euxinia was linked to the two LOME pulses. These results place important constraints on local and global marine redox conditions throughout the Late Ordovician and suggest that non‐sulfidic shelfal anoxia—along with glacioeustatic sea level and climatic cooling—were important environmental stressors that worsened conditions for marine fauna, resulting in the second‐largest mass extinction in Earth history and the only example during an icehouse climate.

Depth-range distribution and diversity patterns of a polychaete (Annelida) community in the continental shelf of the Southern Gulf of Mexico

We tested the mid-domain effect to evaluate the bathymetric patterns of annelid polychaete species richness from the Southern Gulf of Mexico as a possible hypothesis to explain the species richness gradient, exploring the overlapping of species depth ranges towards the middle continental shelf. The bathymetric gradient of the number of species was estimated using the depth ranges of 259 polychaete species and the mid-domain effect was tested using a Monte Carlo simulation program at bands of 10 m depth. The carbonate (234 species) and the terrigenous (121 species) Gulf shelves showed clear differences in species richness which was characterized by a highly significant presence of polychaetes with short depth ranges (<10 m). The distribution of richness could be described as a cubic polynomial curve but the maximum values for all case studied (200, 181 and 84 species, for the whole study area, and the carbonate and terrigenous shelves, respectively) are strongly skewed to shallow waters (40 m). This is not consistent with the peak of diversity at 60–70 m predicted by the model. Therefore, the observed patterns cannot be reproduced by the mid-domain effect, suggesting the existence of non-random factors affecting the species richness gradients in the Gulf. We also found a gradient from west to east in species richness with increasing species richness in the carbonate shelf.

Верхнебашкирские карбонатные отложения в бассейне реки Шаръю (гряда Чернышева): литология, изотопия углерода и кислорода, условия осадконакопления

The Upper Bashkirian deposits in the Srednievorota section (the Sharyu River, the Chernyshev Ridge) are composed of un¬changed limestones. In their composition δ13CPDB = 0.3...3.9‰, δ18ОSMOW = 19.0...24.3‰. Isotopic ratios depend on changes of sedimentation conditions during Late Bashkirian. Formation of studied deposits took place in two stages. At the first stage, during the period of maximum regres¬sion and activation of tectonic mode, rock associations of microbial-pelitomorphic and bioclastic limestones were formed. δ13C gradually increases from 1,4 to 3,1‰, and then decreases to 1,2‰, and δ18О is first equal to 22,3...23,2‰, and then decreases to 20,9‰. At the second stage rock associations of algal, microbial-bioclastic and bioclastic limestones with crinoids and corals were formed on a shallow carbonate shelf during the gradual deepening of the sea. In the isotopic system, there is a gradual increase of average values of δ13C from 2,75 to 3,69‰, and average values of δ18O are equal to 22,18...22,88‰ with deviations of 2–3‰.