Shallow-water Carbonate Sediments Research Articles

Laser In Situ U–Pb Isotope Dating of Carbonate Rocks in Weijia Guyot in the Western Pacific Ocean and Its Geological Significance

Shallow-water carbonate rocks constitute a crucial component of large guyots, arising in distinct environments and harboring valuable insights into the evolutionary stages of seamount islands as well as the tectonic conditions of the underlying oceanic plate. Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) was used to conduct in situ U–Pb isotope dating of carbonate minerals with low uranium content collected from Weijia Guyot. This dating approach yielded crucial evidence for the vertical development of the seamount. Our study indicates that shallow-water carbonate rocks in Weijia Guyot had a temporal range between 91 My and 137 My. The carbonate rocks underwent two growth phases, Hauterivian to Barremian and Cenomanian to Turonian, with a hiatus of approximately 20 My. Since the Hauterivian age, the shield volcano of Weijia Guyot is essentially complete, with its seamount top exposed at or near sea level and receiving its first stage of shallow-water carbonate sedimentation. Based on the dating of both shallow-water carbonate rocks and hawaiite within the Weijia Guyot, it is inferred that approximately 10 My elapsed from shield-building volcanism to late alkalic volcanism. During the Turonian age, the main reason for the second phase of shallow-water carbonate rocks in the seamounts was the regional tectonic uplift triggered by the drift of the Weijia Guyot along with the Pacific Plate toward the Society hotspot.

Development and chronology of the Late Jurassic shallow-water carbonate deposits of the Holy Cross Mountains area, central Poland

The Late Jurassic shallow-water carbonates with intervening clayey-marly deeper-water deposits in the Holy Cross Mts. area formed over large bank of the elevated part of the Northern Tethyan Shelf during about 12 myr. They comprise three main successions (I, II and III) deposited partly in different environmental conditions, controlled by tectonic and climatic factors, and still preserved in the north-eastern margin, the north-western margin and the south-western margin of the Holy Cross Mountains. The history of sedimentation is presented according to the concept of the large tectono-stratigraphic units COK, LUK and KVB, which owe their origin to variable rates of tectonic subsidence, as introduced by Kutek (1994) for the area of central Poland. The studied deposits of the COK megasequence corresponding to the Upper Oxfordian and the Lower Kimmeridgian up to the Hypselocyclum Zone consist of coral limestones, various grained (including oolitic) limestones, and micritic limestones formed over the gradually enlarging shallow-water carbonate platform of the Holy Cross Mts. This platform was subsequently subdivided into two elevated areas, separated by a depressed zone in the middle, bounded by the Nowe Miasto–Iłża–Bałtów Fault Zone in the north-east and the Holy Cross Fault System in the south. The younger megasequence LUK with it strongly transgressive character marks the successive stages of the marine transgression which entered the central, lowered part of the area of the Holy Cross Mts. from the west, where it appeared already in the early Hypselocyclum Chron. It successively spread across the Holy Cross Mts. area towards the north-east and south bringing everywhere the deposition of various oyster lumachelles and marls with ammonites at the end of the Hypselocyclum Chron and during the Divisum Chron of the Early Kimmeridgian to the Acanthicum/Mutabilis Chron of the earliest Late Kimmeridgian. The following megasequence KVB is represented by the detrital lumachelles and chalky limestones with nereineids of the Eudoxus Chron of the Late Kimmeridgian marking the development of still younger shallow-water carbonate platform in the uplifted areas in the north-eastern and possibly the south-western margins of the mountains, allegedly subdivided by a deeper area of sedimentation of marly deposits. The youngest Late Jurassic deposits of the Holy Cross Mts., are very fragmentarily preserved, mostly because of Early Cretaceous uplift and erosion. They suggest an initial episode of complete drowning of the carbonate platform which became covered by marly deposits during the Early Tithonian, and the subsequent restoration of shallow-water carbonate sedimentation at the end of the Early Tithonian.

Diagenetic effects on strontium isotopes in shallow-water carbonate sediments from Middle Miocene to Recent

Seawater 87Sr/86Sr ratios have been used as a proxy for continental weathering flux and the partial pressure of atmospheric CO2 (pCO2) in Earth's past, but the fidelity of 87Sr/86Sr records in marine carbonates under diagenesis remains controversial. To investigate this issue, we undertook an integrated chemostratigraphic study of 87Sr/86Sr, δ13C, and δ18O in bulk carbonate from three cores of the South China Sea region: lithified bulk carbonate of Late Miocene to Holocene age in the 607-m-long Xike-1 core (0–11,942 kyr B.P.) from the Paracel (Chinese: Xisha) Islands, and unlithified carbonate sediment in two shallow (<5-m deep) pushcores from the Spratly (Chinese: Nansha) Islands, Jiuzhang A (0–680 yr B.P.) and Jiuzhang B (0–350 yr B.P.). The carbonate sediment of the Jiuzhang cores yielded 87Sr/86Sr signatures matching those of modern seawater, confirming that carbonate sediments can be faithful archives of contemporaneous oceanic Sr-isotopic compositions. Four diagenetic zones were identified in the Xike-1 core based on δ13C and δ18O patterns: meteoric (Unit I, 0–161 m), mixed-diagenetic (Unit II, 161–215 m), and marine diagenetic zones (Unit III, 215–577 m), along with a second meteoric zone (Unit IV, 577–607 m). The bulk‑carbonate 87Sr/86Sr profile was not strongly altered by dolomitization or other processes in any of the diagenetic zones (although minor effects were observed in the marine zone). This study concludes that marine carbonates are likely to retain primary seawater Sr isotopic signatures even after undergoing multiple stages of diagenesis. This study provides detailed 87Sr/86Sr secular variations and continental weathering history since 12 Ma.

Climato-tectonic evolution of siliciclastic sandstones on Puerto Rico: from lithic arenites to quartz-arenitic sands in an oceanic island-arc setting

Abstract Siliciclastic sandstone composition on the island of Puerto Rico, part of the Greater Antilles, was influenced by both tectonic setting and climate. Cretaceous through Eocene volcanic and plutonic rocks on Puerto Rico formed in an oceanic-arc setting. Sandstones deposited during arc volcanism are quartz-poor lithic and feldspatholithic arenites, whose geochemistry largely matches that of the oceanic-arc volcanic and plutonic rocks on Puerto Rico. After cessation of volcanism on Puerto Rico during collision of the Greater Antilles island arc with the Bahamas Bank, an Oligocene through Holocene overlap assemblage was deposited unconformably above the arc deposits in the North Coast and South Coast basins of Puerto Rico. The overlap assemblage consists of some siliciclastic material, but largely shallow-water carbonate deposits. Siliciclastic deposition in the overlap assemblage reflects the volcanic-arc source, plus quartz derived from plutons exposed during uplift. In the Pleistocene and Holocene (and possibly earlier), poorly consolidated quartz-arenitic sands with SiO2 values from 92 to 98% were deposited in the overlap assemblage. On geochemical tectonic-affinity diagrams, the lithic sandstones plot, as expected, in or near the fields for oceanic-arc provenance. However, the quartz-arenitic sands plot incorrectly in the passive-margin field when considering major elements, and correctly in or near the oceanic-arc field when considering trace elements. Trace elements in the quartz-arenitic sands are largely found in refractory minerals, whereas major elements in feldspars and lithic fragments are effectively removed by intense tropical weathering. Hence, future use of tectonic-affinity diagrams should rely on trace-element geochemistry. Sandstones derived from the Sierra Nevada continental arc in California have QFL quartz fractions below 60%, even though the source magmatic arc has significant modal quartz. This observation suggests a “climate cap,” which does not “allow” formation of more quartzose sediments. In contrast, tropical weathering on Puerto Rico removes this climate cap allowing the creation of quartz-arenitic sands from a source rock containing limited quartz. It is remarkable that quartz-arenitic sands occur on Puerto Rico, sourced from a provenance area containing extremely limited quantities of modal quartz (estimated at less than 5%) in an oceanic-arc environment bounded by two active subduction zones. Quartz-arenitic sands and sandstones are not uniquely continental or of cratonal origin; chemical weathering is fundamentally important for the origin of first-cycle quartz-arenitic sands.

Availability and type of energy regulate the global distribution of neritic carbonates

The study of carbonate rocks is primarily reliant on microfacies analysis, which is strongly based on the comparison with modern allochem assemblages. Despite the existence of several models aimed at comprehensively explaining, on the bases of abiotic factors, the distribution of carbonate-producing organisms, a global, quantitative and standardized overview of the composition of shallow-water carbonate sediments is still missing. Aiming to address this gap in knowledge, the current study provides a global database of the available quantitative data on neritic carbonate sediments. This is paired with satellite-based observations for the abiotic parameters. The results highlight a non-linear, multi-variable, dependence in the distribution of allochems and suggest that depth, temperature, and trophic state are, to a certain extent, interchangeable. The implication of which is a level of non-uniqueness for paleoenvironmental interpretation. The resulting distribution is rather continuous and stretches along an energy gradient. A gradient extending from solar energy, with autotrophs and symbiont-bearing organisms to chemical energy with heterotrophs. Further, quantitative data from modern oceans are still required to disentangle the remaining elements of uncertainty.

A 13 million-year record of Li isotope compositions in island carbonates: Constraints on bulk inorganic carbonate as a global seawater Li isotope archive

Lithium isotope (δ7Li) variations in marine carbonate sediments and rocks have been widely used to reconstruct global seawater Li isotope compositions and then trace the paleo-weathering processes. However, there are still debates on whether ancient carbonates can faithfully document original seawater δ7Li signals, as δ7Li values of shallow-water carbonates are tightly related to carbonate mineralogy and diagenetic alteration. In this study, we present high-resolution δ7Li, trace element and carbonate mineralogy data of shallow-water carbonate deposits from two shallow drillcores (Jiuzhang A and B) and a deep drillcore (XK-1) in the South China Sea. We compare these new δ7Li data to the ca. 13 Myr history of seawater δ7Li evolution from the middle Miocene to Pleistocene, in order to better constrain the effects of carbonate mineralogy and early diagenesis on δ7Li values of bulk inorganic carbonates. We observe that the δ7Li values of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores (24.6% ± 1.6%, n = 37, 1σ) are significantly lower than modern seawater δ7Li values (Δ7Liprimary-seawater = ∼–6.0%). In contrast, marine diagenetic carbonates in the XK-1 drillcore exhibit δ7Li values of 28.7% ± 0.7% (n = 54, 1σ), approaching coeval seawater values (Δ7Limarine diagenetic-seawater = ∼–2.0%) and much higher than those of primary carbonate deposits. Meteoric diagenetic carbonates in the XK-1 drillcore show δ7Li values of 22.4% ± 1.6% (n = 46, 1σ), close to that of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores, but significantly lower than coeval seawater δ7Li values (Δ7Limeteoric diagenetic-seawater = –5.0% to –9‰ for modern and Miocene seawater). Such δ7Li variations in shallow-water carbonates are attributed to variations in carbonate mineralogy (i.e., aragonite, high-Mg calcite and low-Mg calcite) and diagenetic regimes (fluid- or sediment-buffered conditions of marine and meteoric diagenesis). By comparing δ7Li data from the South China Sea to those from the Bahamas, we suggest strongly fluid-buffered conditions for Li isotopes during marine diagenesis, resulting in δ7Li values of marine limestones and dolostones approaching ambient seawater δ7Li signals. In contrast, meteoric diagenetic carbonates, likely marked by sediment-buffered conditions, may inherit the original δ7Li signals of primary carbonate deposits. Hence, full considerations of the carbonate mineralogy and diagenesis facilitate a better use of bulk inorganic carbonate-archived δ7Li to reconstruct paleo-weathering evolution in deep time lacking the skeletal fossil records.

From shallow-water carbonate ramp to hemipelagic deep-marine carbonate deposition: Part 1. General characteristics, microfacies and depositional history of the Middle to Late Anisian Bulog sedimentary succession in the Inner Dinarides (SW Serbia)

The opening of the Neo?Tethys started in the Middle Anisian and is recorded in the drowning succession of the shallow?water Ravni/Steinalm Carbonate Ramp and the subsequent deposition of deep?marine limestones, e.g., the red nodular limestones of the Bulog Group and equivalents. In the Inner Dinarides of southwest Serbia the continental break?up of the Neo? Tethys Ocean is characterized by the formation of a horst?and?graben topography. The change from deposition of shallow?water carbonates formed in an epicontinental sea (graben stage of the Neo?Tethys Wilson Cycle) to red nodular deep?marine limestones (Bulog Limestone) in the late Pelsonian (Middle Anisian) is relatively abrupt due to the rapid decrease of carbonate production. The deeper?water Bulog Limestone, deposited in the early stage of the passive continental margin evolution of the western Neo?Tethys can be dated by conodonts and in rare cases also by ammonoids quite exactly, and therefore it is possible to reconstruct the Pelsonian to Illyrian sedimentological evolution precisely: 1) The late Pelsonian is characterized by the drowning of the shallow?water Ravni Carbonate Ramp and a rapid deepening of the depositional realm. Extension led to the formation of neptunian dikes in the shallow?water Ravni Formation, filled with deeper?marine red micrite, and the formation of a horst?and?graben morphology. Whereas some of the horsts uplifted and emerged in the grabens near to the newly formed escarpments thick breccia successions were deposited with a fining?upward trend during the early?middle Illyrian. On top of other horsts, the grabens, or the newly formed gentle slopes red nodular limestones were deposited. In cases layers with enriched ammonoids formed (Fossillagerst?tten). 2) The early?middle Illyrian ongoing subsidence resulted in the deposition of more and more condensed red nodular limestones with hardground formation. 3) Around the middle/late Illyrian boundary a new pulse of tectonic motions resulted in the tilting of blocks, the formation of new escarpments and again mobilization of mass transport deposits. In addition, a second generation of neptunian dikes was formed. They crosscut the late Pelsonian to middle Illyrian Bulog Limestone, the Pelsonian Ravni Formation, and the older generation of neptunian dikes in the shallow?water Ravni Formation. All formerly emerged horsts flooded and red nodular limestones were deposited on the karstified shallow?water Ravni Formation after a gap. This second pulse of tectonic motions is related to the widespread volcanism in the Dinarides as visible in the appearance of mm?sized biotite clasts in the late Illyrian Bulog Limestone. In contrast to the Outer Dinarides, where thick volcanics are intercalated in the Illyrian sedimentary succession, in the Bulog Limestone successions of the Inner Dinarides volcanics are missing. In general the Pelsonian?Illyrian sedimentary succession is characterized by a stepwise deepening of the depositional realm.

Stratigraphy and facies of the Apennine Carbonate Platform (southern Italy): the record of Mesozoic OAEs and Miocene transgression

The Apennine Carbonate Platform of southern Italy witnesses nearly 150 Myr (Late Triassic to Late Cretaceous) of shallow-water carbonate sedimentation in the subtropical central Tethys. During this field trip, you will have a look at the stratigraphy and facies across some key intervals of global palaeoenvironmental perturbation and at some important stratigraphic discontinuities in the history of the platform.During the first day, you will visit three localities of the Matese Mts. At San Lorenzello you will look at Milankovitch cyclicity expressed in Lower Cretaceous (Valanginian-Hauterivian) peritidal carbonates. At Pietraroja you will have a look at the foramol facies of the Cusano formation, marking the Early Miocene transgression on top of the eroded Cretaceous substrate. Moving north, at the Regiapiana you will see the abandoned mines that exploited the mid-Cretaceous karst bauxites, which mark a prolonged subaerial exposure. Moreover, you will walk through the Middle Miocene synorogenic history of the platform, from its exposure and erosion in the forebulge to its incipient flexural subsidence, highlighted by the Lower Miocene carbonates of the Cusano formation, to its drowning below the photic zone, marked by a phosphatic hardground overlain by the Orbulina marls of the Longano formation.During the second day, you will head south toward Salerno. In a quarry at Mercato San Severino, you will look at the record of the early Toarcian oceanic anoxic event, marked by the abrupt demise of lithiotid bivalves and dasycladalean algae, the major carbonate producers of the Lower Jurassic carbonate platforms. At the base of Mt Tobenna, you will look at the Aptian Orbitolina level: a marker bed whose palaeoenvironmental meaning has been long debated. From Mt Tobenna you will move south toward Monteforte Cilento where you will look at the record of the Cenomanian-Turonian OAE2 in the Apennine Carbonate Platform.

Holocene Lacustrine Abiotic Aragonitic Ooids from the Western Qaidam Basin, Qinghai-Tibetan Plateau

Carbonate ooids are a significant component of shallow water carbonate deposits in the present and geologic past, yet their origin and formation mechanism have been the subject of continuing debate. This study focuses on the well-preserved Holocene aragonitic ooids collected from the west Qaidam Basin, Qinghai-Tibetan Plateau (QTP). The mineralogical and chemical compositions, and stable (δ13C and δ18O), and radiocarbon isotopes of the ooids were analyzed to investigate their formation and develop a depositional model. The ooids formed approximately 5377±61 cal BP, and their cortices were composed of microcrystalline aragonite, with most nuclei being quartz grains. Stable carbon and oxygen isotopes indicate that authigenic aragonite precipitation is driven by evaporation and associated degassing of CO2 under turbulence conditions in a shallow alkaline lakes. Furthermore, eletron microscopy showed no presence of microfossils in ooid cortices or other evidence of microbial activity. Therefore, we propose that aragonite precipitation during ooid formation is most likely induced abiotically by increasing alkalinity due to evapoconcentration of lake waters based on an absence of an efficient carbonate-inducing metabolic pathway. New observations and detailed analyses of aragonitic ooid samples in the Qaidam Basin provide an improved understanding of the origin and formation processes of carbonate ooid in modern environment and the geologic past.

Constraining diagenesis within shallow water carbonate environments: Insights from clumped and sulfur isotopes

The geochemical records of shallow-water carbonate sediments can be influenced by both meteoric and marine diagenesis, where the contribution from each can be difficult to constrain using traditional δ18O and δ13C values. Here sediment cores from Enewetak Atoll have been utilized as a case study, in which clumped isotope-derived temperatures and calculated δ18Ofluid values are combined with the δ34S values of the carbonate-associated sulfate, to differentiate the types of diagenetic alteration which have influenced the geochemical record, thus providing further insights into the extent and timing of carbonate alteration. While sediments at Enewetak Atoll have undergone repeated periods of subaerial exposure, calculated δ18Ofluid values indicate that the amount of meteoric alteration is reliant on the availability of meteoric fluids, as the presence of exposure surfaces alone does not imply that extensive diagenetic alteration has occurred. Additionally, in the deepest zone of the Enewetak cores, it could be determined that the sediments were initially exposed to meteoric fluids, resulting in partial alteration, and later affected by burial and circulation of deep ocean water, resulting in recrystallization within the section, uniformly overprinting the temperature and δ18Ofluid values. The results of this study indicate that by utilizing systematics such as δ34SCAS, ∆47, and δ18Ofluid values, in addition to the more traditional proxies, a detailed understanding of the diagenetic processes which have altered shallow water sediments can be better constrained, allowing for more accurate interpretations of ancient sedimentary records.

238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (δ238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent of global oceanic anoxia in deep time. Multiple studies have examined whether primary calcium carbonates can directly capture seawater δ238U and whether bulk measurements of recent and ancient carbonates preserve seawater U isotope signatures. Here we assess the role of diagenesis in altering δ238U signatures in carbonates sediments that have a primary calcitic mineralogy at the Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid global warming and oceanic deoxygenation at ∼56 million years ago.Although primary abiotic and biogenic calcium carbonates (aragonite and calcite) can directly capture seawater δ238U with small offsets (<0.1‰) relative to modern seawater, diagenetic alteration of Bahamian shallow-water platform carbonate sediments that have a predominantly primary aragonitic mineralogy resulted in significantly larger offsets (up to 0.6‰). Since U concentration in primary aragonite is at least one order of magnitude higher than primary calcite (>1 ppm vs. <0.1 ppm), δ238U in calcite should be even more susceptible to diagenesis than that in aragonite.We find strong evidence of this effect in analysis of δ238U in PETM shallow-water carbonate sediments from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations in bulk carbonate δ238U from −0.69 to +0.71‰ around the PETM boundary but consistently heavier δ238U (between −0.14 and +0.47‰) than modern seawater outside of this interval. The significantly lighter δ238U values than modern seawater were interpreted to result from the operation of a Mn oxide shuttle. The heavier δ238U values are most likely caused by authigenic reductive accumulation of U(IV) in pore waters below the sediment-water interface. We found that carbonate δ238U values higher than modern seawater tend to increase with increasing U/Ca. This relationship is well-explained by an authigenic reductive accumulation model that simply assumes addition to primary calcite during diagenesis of calcitic cements containing isotopically heavier U(IV).Our work confirms expectations that δ238U in primary calcite is more susceptible to the amount of diagenetic cementation compared to primary aragonite, and that variations of δ238U in carbonate sediments with a primary calcitic mineralogy would more dominantly reflect the local redox state of depositional and early diagenetic environments. It is essential to identify the original carbonate mineralogy, the diagenetic history, and constrain the redox state of local deposition environments of sedimentary carbonate rocks when applying bulk carbonate δ238U as a global proxy for oceanic anoxia in deep time.

A new methodology using seismic data to differentiate shallow-water carbonate deposits from similar, noncarbonate structures

A new methodology using seismic data to differentiate shallow-water carbonate deposits from similar, noncarbonate structures

The Late Jurassic–Palaeogene Carbonate Platforms in the Outer Western Carpathian Tethys—A Regional Overview

The present work focuses on palaeogeographic reconstruction of shallow-water carbonate deposition in the Outer Western Carpathian Tethys. Platform deposits are preserved only as a component of turbidites and olistostromes, and reconstructions of these platforms are based on clastic material redistributed into slopes and deep basins and occurring among the Outer Carpathian nappes. Similar platforms were also present on the Tethys margins. These reconstructions were performed using the global models of plate tectonics. Several ridges covered by carbonate platforms developed in that area during the latest Jurassic–Palaeogene times. Three main shallow-water facies associations—Štramberk, Urgonian, and Lithothamnion–bryozoan—could be distinguished. The Tithonian–lowermost Cretaceous Štramberk facies is related to early, synrift–postrift stage of the development of the Silesian Domain. Facies that are diversified, narrow, shallow-water platforms, rich in corals, sponges, green algae, echinoderms, foraminifera, microencrusters, and microbes are typical of this stage. The Urgonian facies developed mainly on the south margin of the Outer Carpathian basins and is characterised by organodetritic limestones built of bivalves (including rudists), larger benthic foraminifera, crinoids, echinoids, and corals. Since the Paleocene, in all the Western Outer Carpathian sedimentary areas, Lithothamnion–bryozoan facies developed and adapted to unstable conditions. Algae–bryozoan covers originating on the siliciclastic substrate are typical of these facies. This type of deposition was preserved practically until the final stage in the evolution of the Outer Carpathian basins.

Middle Triassic carbonates of Eastern Iberia (Western Tethyan Realm): A shallow platform model

Much of the eastern sector of the Mesozoic Iberian basin belongs to the ‘Mediterranean type’ Triassic. This type is characterized by a Middle Triassic stratigraphic record comprised of two carbonate Muschelkalk units (lower and upper) separated by a detrital-evaporite middle Muschelkalk unit. In the present study areas, the two first marine flooding episodes of the Middle Triassic led to the deposition of extensive shallow-water carbonates on a block-controlled epicontinental platform in the lower and upper Muschelkalk units. These carbonates were part of third-order transgressive–regressive sequences. The two carbonate sequences display similar vertical facies and stratigraphic sub-unit arrangements. The base of the two sequences is marked by a (lower) carbonate-marl alternation, which was deposited in mixed tidal flat-to-lagoon settings. In the middle of the sequences, the carbonate character is predominant and facies associations mainly suggest shallow waters and high environmental variability. The change from a transgressive to regressive trend occurs in the middle of the sequences, although the position of the maximum flooding zone can vary from one to another stratigraphic section. The tops of the sequences show two different lithological assemblages, which are coeval and grade laterally to each other. One assemblage is a ‘carbonate terminal complex’ formed by tidal flat and sabkha deposits, which is predominant in the lower Muschelkalk unit. The other assemblage is a lagoon-peritidal (upper) carbonate-marl alternation, predominant in the upper unit. Together, the two sequences suggest a complex platform morphology with a mosaic facies distribution, which evolved from a not very high energy ramp-like profile (transgressive phases, TST) to a lower energy rimmed shelf, ending with tidal/lagoon environments (regressive phases, HST). Facies associations, however, suggest somewhat deeper depositional settings for the upper than lower Muschelkalk unit. Here we compare the two carbonate units in the study areas with those described in the literature from other Triassic basins of Eastern Iberia and the SW Germanic Basin, all forming part of the Peri-Tethys area of the Tethyan Realm.

Geomorphology and Neotectonics of Southwestern Crimea

Abstract—The area of southwestern Crimea includes the ending of the Crimean Mountains that arose during the neotectonic activation at the place of the Cretaceous–Paleogene denudation plain and the adjacent shallow-water carbonate sedimentation basin. The Crimean Mountains are one of the links of the Alpine–Himalayan orogenic belt formed during the collision of the Eurasian, African, and Indo–Australian plates. Their area includes late Cenozoic marine terraces of the complete Mediterranean series and a staircase of Neogene, Paleogene and Cretaceous planation surfaces over them. The planation surfaces of different ages resulted from the successive lowering of the World Ocean level. Their subsequent deformations make it possible to outline the area of the neotectonic uplifting and determine its parameters. The main mechanism of the neotectonic activation was the thrust of the East Black Sea microplate under the Scythian one and the formation of a ramp fold structure. The amplitude of the neotectonic uplifting of southwestern Crimea for the past 2 Myr varies from 0 to 800 m, i.e., is up to 0.04 mm/year. The recent neotectonic structure of the area is formed by the northern flank of the ramp fold; it is a monocline of NW dip consisting of “keys” of NW strike separated by the latest faults with vertical displacements of 10 to 120 m. The uplifting of the area and the lowering of the World Ocean level led to a widespread of denudation surfaces. Their good preservation makes it possible to refine the sequence of neotectonic events, whose first pulses reached the study area in the Oligocene, and the main activation phase began in the Pliocene.

Upper Triassic carbonate-platform facies, Timor-Leste: Foraminiferal indices and regional tectonostratigraphic association

Timor is part of the non-volcanic Outer Banda Arc with chaotic geology in the late Neogene collision zone between Sundaland (Southeast Eurasia) and the Australian continent. Confusion in the distinction between Triassic, Jurassic and Cenozoic shallow-marine carbonate units has led to a lack of appreciation of the widespread extent of Triassic carbonate-platform facies, at least in Timor-Leste. It also resulted in misunderstanding the different tectonostratigraphic/palaeogeographic affinities of the shallow-water limestones. Foraminiferal assemblages are critical in stratigraphic discrimination of these units. This study records for the first time the foraminiferal microfauna of Upper Triassic shallow-water carbonate deposits in Timor. Many of the foraminiferal species are known from North Africa and Europe and date these units to the Carnian–Rhaetian. The Upper Triassic Bandeira Group represents extensive sheet-like carbonate-platform deposits (possibly on horst plateaus as well as shelf areas adjacent land in the intracratonic sea). Contacts between the hard limestone, forming fatus (limestone peaks) and ridges, and friable mud-dominated surrounding units are often obscured under deep tropical soil cover. At all localities the Bandeira Group is closely associated in outcrop with coeval basinal mud-dominated deposits (Babulu and Aitutu groups). With these it forms part of the East Gondwana Interior Rift Association (EGIRA), similar to the classical Dachstein Platform–Hallstatt Basin facies associations found in the European Alps. The basinal units contain turbiditic and debris-slide deposits that include clasts derived from the carbonate platform. Shallow-water carbonate facies attributed to EGIRA, are present throughout the East Gondwana Interior Rift from the Papuan Basin in the north to Exmouth Plateau, part of the Northern Carnarvon Basin, in the south. These were deposited before Gondwanan breakup along this rift system. The Triassic carbonate-platform deposits have a different dominant biogenic mineralogy (aragonite rather than calcite) and a different architecture to the bryozoan-crinoidal carbonate mounds present in the Permian of EGIRA in Timor-Leste. No Lower and Middle Triassic shallow-water carbonate-platform deposits have been recognized in Timor-Leste. The Bandeira Group, confined to the Upper Triassic, belongs to a stratigraphic association (EGIRA; autochthonous) different both in constituent units and outcrop coverage from that of the allochthonous Overthrust Terrane Association to which the Lower Jurassic carbonate-platform deposits (viz. Perdido Group) belong. The Bandeira Group has been confused not only with the Perdido Group but also with other shallow-water limestone units of the Permian and Miocene in some former studies of Timor. The distinction between these units solves a major tectonostratigraphic problem in Timor, and may apply elsewhere along the Outer Banda Arc.

Preliminary exploration of molybdenum isotope fractionation during coprecipitation of molybdate with abiotic and microbial calcite

Molybdenum isotopes in marine carbonates have been proposed to track the redox conditions of oceans through time. Large variations of Mo concentration and isotopic composition were observed in primary biogenic carbonates and shallow-water carbonate sediments from modern oceans. However, it remains unknown whether the abiotic calcium carbonate precipitation process fractionates Mo isotopes and affects Mo isotopic composition in marine carbonates. To better understand carbonate Mo isotope proxy, we conducted abiotic calcite coprecipitation experiment to determine Mo partition coefficient in calcite and Mo isotope fractionation during molybdate incorporation into calcite. We also investigated Mo isotopic compositions in microbial calcite induced by photosynthetic activity of cyanobacteria and diatoms in the surface water of Fayetteville Green Lake (FGL), New York, USA.Our calcite coprecipitation experiment, for the first time, determined the Mo partition coefficient Kd=MoCacalcite/MoO42−CO32−solution = 1.49 × 10−5 in calcite at pH ~8.3. We observed a relatively small Mo isotope fractionation of 0.15 ± 0.06‰ with light Mo isotopes preferentially incorporated into calcite. The Mo isotope fractionation observed in our experiment most likely results from equilibrium isotope fractionation among different Mo species and preferential incorporation of the isotopically light Mo(VI) species (MoO42−) into calcite. Microbial calcite formed in FGL fractionated Mo isotopes in the same direction as abiotic calcite coprecipitation experiment but with significantly larger isotope fractionations (0.82–2.45‰) and significantly higher Mo concentrations (0.04–0.28 ppm vs. the inferred 0.002 ppm Mo from our abiotic calcite coprecipitation experiment). Molybdenum isotopic composition decreased with Mn/Fe ratios from +1.56 to −0.07‰, suggesting that Fe and Mn oxides dominantly control Mo isotope fractionation in these microbial calcite samples. Additionally, isotopically light Mo associated with organic matter (e.g., live or dead microorganisms like cyanobacteria) might also contribute to the light Mo isotopic composition in microbial calcite.The relatively small Mo isotope fractionation (0.15 ± 0.06‰) and Mo partition coefficient (1.49 × 10−5) in our abiotic calcite coprecipitation experiment indicate that calcite precipitation process has negligible effects on Mo isotopic composition in primary biogenic calcite precipitates. In contrast, Mo associated with organic matter, detrital particles, and Fe and Mn oxides, and carbonate diagenesis strongly affect Mo isotopic compositions in calcium carbonate precipitates in nature. Caution should be taken when applying Mo isotopes in ancient carbonates rocks to track seawater redox conditions.

The rudist-rich carbonate units in central Tunisia as markers of Late Cretaceous transgressive events

Rudist-rich carbonates are common in central Tunisia. They constitute frequent intercalations in Upper Cretaceous series, particularly within the Cenomanian-Campanian interval. They are generally transgressive upon shallower deposits, such as Lower Cretaceous siliciclastic sediments, or Upper Cretaceous tidal flat deposits. The sequence stratigraphy analysis of the study carbonate series reveals discontinuities and unconformities that mark the boundaries between the identified sequences. These unconformities, which are mainly represented by oxidized hard-grounds with several borings and micro-karstification features, are in relation with a sea level drop or a tectonic uplift of central Tunisia during the Late Cretaceous. Sea-level changes, tectonic movements, and climatic events appear acting a lot for the growth but also for the demise of Upper Cretaceous rudist and coral communities. Globally, the Upper Aptian-Lower Albian shallow-water carbonate deposits are unconformably overlain by Upper Albian-Cenomanian transgressive deposits, locally rich in organic matter, represented by black-shales. This unconformity, which is identified at least in the whole central Tunisia, consists of a regional sequence boundary that announces typical greenhouse conditions and the beginning of a carbonate-rich sedimentation admitting frequent intercalations of rudist-sponge-coral-rich deposits. Above this main regional unconformity, the Upper Cretaceous series show additional sequence boundaries separating 3rd-order sequences. Overall, six transgressive-regressive sequences were identified within the Cenomanian-Campanian interval. The identification of these 3rd-order sequences including rudist-rich deposits has helped a lot to establish correlations at least at regional, north African (Algeria, Egypt), and Tethyan scales (Italy, Germany and other areas). In central Tunisia, Rudist-coral-rich carbonates appear both during shallowing and at the beginning of deepening stages. However, according to our study, they are obviously more common and well represented during transgressions. In this context, they constitute deepening-upward cycles in transgressive systems tracts. For these reasons, they could be considered as markers of most Late Cretaceous transgressive events, essentially the Cenomanian-Turonian and the Campanian global transgressions. Commonly, the Upper Cenomanian-Lower Turonian transgressive event, for example, is globally marked by the deposition of black-shales. However, in shallower environments, more closed to carbonate platforms and their margins, regionally extended rudist-coral-rich carbonates could be used as markers of such transgressive event.

U-Pb and Re-Os geochronology tracks stratigraphic condensation in the Sturtian snowball Earth aftermath

AbstractThe snowball Earth hypothesis predicts a strong hysteresis resulting in discrete multi-million-year glaciations followed by globally synchronous deglaciation. Here we present new U-Pb zircon and Re-Os sedimentary rock geochronology and Os isotope chemostratigraphy from post-Sturtian sequences in south China to test the synchroneity of deglaciation. High-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon dates refine the minimum age of deglaciation to 660.98 ± 0.74 Ma, which is ∼2 m.y. older than previously reported. We also provide a new maximum age constraint on the onset of the Marinoan glaciation of 657.17 ± 0.78 Ma. A global compilation of new Os isotope chemostratigraphy reveals a large and systematic trend to unradiogenic values over &amp;lt;1 m of stratigraphy. Together, these data indicate that the Mn-carbonates in south China are not cap carbonates that formed as a response to post-snowball alkalinity, but are authigenic carbonates that formed millions of years after deglaciation. Sturtian cap carbonates tend to be absent or more condensed than their younger Marinoan counterparts. We suggest that this reflects a combination of enhanced accommodation space in early Cryogenian underfilled rift basins, stronger hysteresis, larger ice volume, and/or higher CO2 levels needed for deglaciation of the longer Sturtian glaciation. Further, our findings indicate that the apparent diachroneity of deglaciation can be explained readily as a consequence of stratigraphic condensation, itself due to the large post-Sturtian glacioeustatic transgressive sequence that outpaced shallow-water carbonate deposition.

Carbonate factory turnovers influenced by the monsoon (Xisha Islands, South China Sea)

Carbonate factory turnovers are the integrated result of climatic, oceanographic and biological controls. Sedimentological, geochemical and palaeontological data from core XK-1 drilled in the Xisha Islands, South China Sea, reveal that photozoan to heterozoan carbonate factory turnovers during the Early and Middle Miocene in isolated carbonate platforms were mainly caused by changes in the upwelling regime. A heterozoan open bank association thrived in tropical shallow water under conditions of nutrient excess, indicated by high ln(Cu/Ti) and ln(Ba/Ti), and by the increased abundances of rhodoliths and some foraminifer genera. During the Early and Middle Miocene, episodes characterized by a heterozoan carbonate production correlate with times of East Asian Summer Monsoon strengthening, especially from 21.2 to 17.3 Ma and from 15.6 to 11.6 Ma. This study thus provides new insights into the variability of shallow water carbonate deposition in monsoon-influenced areas and also shows that carbonate factory turnovers can contribute to a better understanding of the monsoon9s evolution. Supplementary material: Biostratigraphic and magnetostratigraphic data, coral content, geochemical data and mineral compositions related to this study are available at https://doi.org/10.6084/m9.figshare.c.4507070