Pelagic Carbonate Research Articles

Hybodont dentition from the Upper Jurassic of Monte Nerone Pelagic Carbonate Platform (Umbria‐Marche Apennine, Italy) and its ecological implications

Here we describe a number of articulated teeth of a hybodont shark from Upper Jurassic deposits of the Monte Nerone Pelagic Carbonate Platform, in the Umbria‐Marche‐Sabina Palaeogeographic Domain (Northern Apennines, Central Italy). The material has been referred to asAsteracanthuscf.A. magnus, a quite well‐known taxon already reported from Middle to Upper Jurassic deposits of Europe. Teeth indicate an extreme crushing feeding behaviour, suggesting as putative prey both infaunal and epifaunal hard‐shelled invertebrates dwelling the sea‐floor, such as large bivalves, brachiopods, gastropods as well as vagile crustaceans. The finding represents, to date, the first formal report of hybodont shark in the Umbria‐Marche‐Sabina Domain, throwing further light on the ecology within Pelagic Carbonate Platform settings, and on the occurrence ofAsteracanthusin the Late Jurassic of the Western Tethys.

Palaeoenvironment reconstruction and planktonic foraminiferal assemblages of Campanian (Cretaceous) carbonate succession, Çayırbağ area (Trabzon, NE Turkey)

The Campanian (Cretaceous) carbonate succession, which belongs to the uppermost part of the Mesozoic sequence in the Eastern Pontides (NE Turkey), is mainly composed of Globotruncana-bearing grey and red pelagic limestones. One of the most typical outcrops of the succession is located in the Cayirbagarea (NE Turkey) and it was studied with a combined sedimentological and palaeontological approach. Three microfacies types were identified and interpreted based on petrographic analysis of the depositional textures and fauna. They are (1) allochthonous bioclastic-calcispheral packstone/wackestone microfacies (MF-1), (2) calcispheral packstone/wackestone microfacies (MF-2), and (3) pelagic foraminiferal mudstone/wackestone microfacies (MF-3). These microfacies types correspond to slope, deep self and basin environments, respectively. Based on the planktonic foraminiferal assemblages, including Globotruncana cf. arca (Cushman), Globotruncana arca (Cushman), Globotruncana sp., Whiteinella sp., Archaeoglobigerina sp., Archaeoglobigerina blowi (Pessagno), Globotruncana lineiana (d’Orbigny), Whiteinella sp., Archaeoglobigerina cretacea (d’Orbigny), the age of the pelagic carbonate succession is Campanian. Microfacies and palaeontological characteristics of the studied section suggest that they were deposited in a slope to basin environment. The progressive deepening of the environment is probably caused by the vertical movement of the syn-sedimentary extensional tectonic regime. Moreover, a carbonate platform environment adjacent to the deep-marine environment is inferred to exist during the earliest Campanian because of the presence of rare allochthonous neritic skeletal components in the lower part of the section.

Origin of red pelagic carbonates as an interplay of global climate and local basin factors: Insight from the Lower Devonian of the Prague Basin, Czech Republic

Red pelagic sediments are relatively common in the Phanerozoic. They are often interpreted as products of sea-bottom oxidation during greenhouse climate showing a conspicuous alternation with black shales and thus carrying important palaeoceanographic information. The Lower Devonian (Pragian) carbonate strata of the Prague Basin, Czech Republic (Praha Formation) contain a marked band of red pelagic carbonate, up to ~15m thick, which can be correlated for several tens of km. We investigated seven sections (17 to 255m thick) of the Prague Basin using the methods of facies analysis, outcrop gamma-ray logging, diffuse reflectance spectroscopy, optical microscopy, element geochemistry, magneto-mineralogy and electron microprobe analysis. The aim was to find the mineral carriers of the red colour, investigate the stratigraphic context of the red carbonates and evaluate the local and global prerequisites for their formation. The red pigmentation represents enrichment by hematite with respect to goethite. Approximately 31% of the total reflectance falling in the red colour band represents a threshold for red coloration. The red pigmentation is carried by submicronic hematite dispersed in argillaceous pelagic calcilutite and/or inside skeletal allochems. Gamma-ray log correlation indicates that the red carbonate band developed in stratigraphic levels with low sedimentation rates, typically from 1 to 7.1mm/kyr, which are comparable to the Mesozoic Rosso Ammonitico facies. The red beds and the whole Praha Formation (Pragian to early Emsian) are characterized by low TOC values (<0.05%) and low U/Th, Mo/Al, V/Al, Zn/Al, Cu/Al and P/Al ratios indicating oligotrophic, highly oxic sea-bottom conditions. This period was characterized by global cooling, a drop in silicate weathering rates and in atmospheric pCO2 levels. The lower Devonian successions of the Prague Basin indicate that switching between two greenhouse climatic modes, colder oligotropic and warmer mesotrophic, may have been responsible for the alternation of red and grey carbonate strata, respectively.

Sponge-microbial build-ups from the lowermost Triassic Chanakhchi section in southern Armenia: Microfacies and stable carbon isotopes

The end-Permian mass extinction was the most severe biotic crisis in Earth's history. In its direct aftermath microbial communities colonized some of the space left vacant after the severe decline of skeletal metazoans. The Permian-Triassic boundary microbialites were peculiarly abundant on low-latitude shallow-marine carbonate shelves of central Tethyan continents. Armenia features particularly well preserved and diverse basal Triassic sponge-microbial build-ups (BTSMBs), which were not studied in detail to date. Here, the Chanakhchi section in southern Armenia is described petrographically and by means of stable isotope analyses. The Armenian BTSMBs formed in a distally open marine setting on a pelagic carbonate ramp in the course of two phases of microbial growth during the Induan (Lower Triassic). The BTSMBs are represented by predominantly thrombolitic but also dendrolitic and digitate stromatolite biostromes and mounds that vary in height between 5cm to 12m. The digitate stromatolites are associated with calcium carbonate crystal fans (CCFs). Microfacies analyses revealed that the BTSMBs exhibit a number of different growth forms and internal fabrics. The formation of CCFs was apparently not devoid of biological influence and took place above the sediment surface. The abundance of sponges in the BTSMBs reveals that ecologically complex metazoan-microbial reefs have been present already early after the end-Permian mass extinction. However, the formation of biostromes and mounds did not depend on sponges or other metazoans. BTSMBs that formed during the second microbial growth phase revealed similar δ13C-values like the surrounding sediment. In contrast, the δ13Cmicrobialite and δ13Csediment values from the BTSMBs and CCFs of the first growth phase show a difference of up to +2.3‰, suggesting a significant influence of photoautotrophy during microbially induced carbonate precipitation.

Calcareous nannofossil fluxes and size fluctuations in the middle Eocene (48–39 Ma) from Ocean Drilling Program (ODP) Site 1209 in the tropical Pacific Ocean

Pelagic carbonates constitute the majority of the ocean sediments, particularly since the Jurassic, and record climatic and oceanic changes. Cenozoic sediments record glacial and interglacial periods, as well as thermal maxima, such as the PETM (Paleocene-Eocene Thermal Maximum) at ca. 56Ma or the MECO (Middle Eocene Climatic Optimum) that dates back to ca. 40Ma. The Paleogene period shows variations in seawater temperature and atmospheric CO2, as well as fluctuations of the carbonate compensation depth (CCD). In order to understand the actions and feedbacks between climate (regulated amongst others by temperature, partial pressure of CO2, and atmospheric belts), CCD fluctuations and pelagic carbonate producers, the fluxes, assemblages and sizes of calcareous nannofossils were studied from sediments dated between 48 and 39Ma, including the MECO interval. Calcareous nannofossils were analysed in from the ODP (Ocean Drilling Project) Leg 198 Site 1209A, Shatsky Rise located in the tropical Pacific. The previously proposed age model for Site 1209 has been slightly modified based on new calcareous nannofossil biostratigraphy recalibrated according to the GTS 2012. In a long-term perspective, calcareous nannofossil assemblages and taxa size show significant changes during the middle Eocene. Our data suggest that relatively oligotrophic conditions occurred between 48 and 45Ma, interrupted by higher trophic conditions between 45 and 39Ma, although oligotrophy occurred again during the MECO. Discoaster spp. show smaller sizes from 48 to 39Ma, mirroring the global temperature decrease. At 40Ma, during the MECO, large-sized reticulofenestrids with a peak of Reticulofenestra umbilicus (>14μm) occurred, whereas smaller but heavily calcified Dictyococcites spp. peak in the aftermath of the MECO. Calcareous nannofossil fluxes show a significant increase with respect to background values during the CAEs (Carbonate Accumulation Event), although they are low during the MECO. Calcareous nannofossil fluxes seem to have had important impact on the oceanic carbon cycles, by creating a carbonate flux to the ocean interior, likely contributing to the CCD deepening.

A Cenozoic record of seawater Mg isotopes in well-preserved fossil corals

Reconstructions of seawater Mg isotopic composition (δ^(26)Mg) can provide novel insights into the processes that control the major ion chemistry of seawater over geologic time scales. A key period of interest is the Cenozoic (ca. 65 Ma to today), during which the Mg/Ca ratio of seawater increased by a factor of 2–3. However, two published records of seawater δ^(26)Mg over the Cenozoic disagree, making it difficult to draw conclusions about mechanisms driving seawater Mg/Ca change over the past 65 m.y. Here we present a new record of seawater δ^(26)Mg from a set of well-preserved fossil corals, ranging in age from Paleocene to Recent. Fossil coral δ^(26)Mg decreases by ∼0.3‰ between the early Cenozoic and the Oligocene, then increases by ∼0.15‰ between the Oligocene and present, in strong agreement with the published record derived from bulk pelagic carbonate. Together with this existing record, our fossil coral data suggest that the rise in [Mg]_(seawater) over the Cenozoic was mainly driven by an increase in Mg silicate weathering or a decline in Mg uptake in marine silicates. In contrast, we suggest that changes in the rate of carbonate weathering and dolomite formation likely played a minor, but not insignificant, role in the global Mg cycle over the Cenozoic.

Stratigraphy, petrogenesis and geodynamic setting of Late Cretaceous volcanism on the SW margin of the Black Sea, Turkey

Abstract The Western Pontide Magmatic Belt consists of two different magmatic series corresponding to two distinct periods of intense volcanism, separated by a pelagic limestone marker horizon resting on a regional unconformity. The first stage of magmatism and associated extensional tectonic regime prevailed in the region between the Middle Turonian and Early Santonian. During the first stage, magmas were derived from a depleted mantle source containing a clear subduction signature. The extrusives intercalated with marine clastic sediments and pelagic carbonates associated with thick debris-flow horizons and olistoliths. Based on geochemistry and depositional features, the first stage is interpreted as an extensional ensialic arc setting developed in response to northwards subduction of the Tethys Ocean beneath the southern margin of Laurasia. During the Late Santonian, the volcanism stopped and the whole region suddenly subsided with the deposition of a thin, but laterally continuous, pelagic limestone horizon. This subsidence may imply the break-up of the Laurasian continental lithosphere and the beginning of oceanic spreading in the Western Black Sea Basin. The intensified extension is interpreted to be linked to the southwards rollback of the subducting slab. During the second stage in the Campanian, magmas were derived from two contrasting mantle sources: (1) a depleted lithospheric mantle enriched by a subduction component; and (2) an enriched asthenospheric mantle which is similar to that of the ocean island basalts (OIB). The depleted lithospheric source may be linked to the subcontinental lithospheric mantle of Laurasia, which was metasomatized by the previous Tethyan subduction event rather than by an active arc magmatism. Lavas derived from the depleted source are abundant throughout the stratigraphic column, whereas those from the enriched source dominate the end of the second stage. The presence of the alkaline lavas may indicate thinning of the lithosphere and upwelling of the asthenospheric mantle in the matured stages of rifting. We argue that the main cause of both rifting and temporal change in magma generation was the steepening and rollback of the northwards subducting slab of the Tethys Ocean. The aforementioned rollback also caused the Istanbul Zone to be moved to the south, and colliding with the Sakarya Zone in the south during the Maastrichtian. Based on geochemical, stratigraphic, palaeontological and sedimentary data, we suggest that the oceanic Western Black Sea Basin opened as an intra-arc basin during Turonian–Santonian time.

Conodont biofacies of a monotonous middle Famennian pelagic carbonate succession (Upper Ballberg Quarry, northern Rhenish Massif)

A succession of mostly thin-bedded, cyclic pelagic limestones is exposed in the abandoned, previously unstudied Upper Ballberg Quarry in the northern Sauerland (northern Rhenish Massif, Germany). Macro- and microfacies analyses show a rather uniform lithology of bioturbated, microsparitic mud-wackestones deposited below storm wave base on an outer shelf submarine rise formed by the drowned Honne Valley Reef Complex. Twelve conodont samples are assigned to the middle Famennian Pa. marginifera marginifera-marginifera utahensis zones (former Lower and Upper marginifera zones), which are difficult to separate in the Rhenish Massif. There are 30 taxa, including Icriodus ballbergensis n. sp., a representative of the I. cornutus Group. A refined conodont biofacies analysis, based on the separation of species groups within genera with potentially different palaeoecology, led to the distinction of five subfacies of the outer shelf Palmatolepid Biofacies. A detailed comparison with the available global quantitative conodont record proves their significance in pelagic settings. The distinctive fluctuations in conodont assemblages despite a monotonous lithology prove that refined conodont biofacies analyses have the potential to reconstruct palaeoenvironmental changes with a much higher precision than microfacies studies.

Influence of upwelling on the sedimentation and biota of the segmented margin of the western Neotethys: a case study from the Middle Triassic of the Balaton Highland (Hungary)

In the Middle Anisian, extensional tectonic movements led to the development of a small isolated carbonate platform in the middle part of the Balaton Highland, Transdanubian Range, Hungary. In the Late Illyrian, a condensed pelagic carbonate succession with phosphorite horizons was formed on the top of the already drowned platform. These strata contain an extraordinarily diverse ammonite fauna. This unit is overlain by radiolarian-rich carbonates, locally with radiolarite interbeds. We suggest that the drowning process and the post-drowning sediment deposition were controlled partly by regional factors, i.e., the onset of opening of the Neotethys Ocean, and partly by local factors such as the bottom topography and related current activity, which may also be connected with the opening of the ocean. The predominance of the radiolarian-rich sediments suggests eutrophic surface water, which may be explained by a monsoon-driven upwelling model. The segmented sea-floor topography together with the high-fertility surface water conditions may have provided favorable habitats for the ammonites, which may have adapted to various ecological conditions, leading to extreme diversification of this group. Since similar Middle to Late Anisian evolution was reported from many other units of the western Neotethys margin, regional factors such as the establishment of an extensional tectonic regime and related marginal basin formation, monsoon-driven upwelling, and related high surface water productivity seem to be of critical importance in controlling the depositional conditions.

Extinction, dissolution, and possible ocean acidification prior to the Cretaceous/Paleogene (K/Pg) boundary in the tropical Pacific

Biotic perturbations and changes in ocean circulation during the Maastrichtian stage of the latest Cretaceous raise questions about whether the biosphere was preconditioned for the end-Cretaceous mass extinction of calcareous plankton. A brief acme of inoceramid clams at ~71Ma on Shatsky Rise in the tropical North Pacific was followed by their extinction during the “mid-Maastrichtian event” at 70.1Ma associated with an abrupt warming of deep waters. This was later followed by an interval of intense dissolution beginning ~67.8Ma at ODP Site 1209 (2387m). The late Maastrichtian dissolution interval was initially gradual, and is characterized by a low planktic/benthic (P/B) ratio, highly fragmented planktic foraminifera, mostly an absence of larger taxa, low abundances of smaller taxa, extremely low planktic foraminiferal numbers, and low planktic foraminiferal and nannofossil species richness. A partial recovery in carbonate preservation and calcareous plankton simple diversity began ~250kyr prior to the K/Pg boundary associated with the incursion of a younger (more enriched δ13C) deep water mass, although total abundances of planktic foraminifera in the sediment remained a tiny fraction of their earlier Maastrichtian values. A second, brief dissolution event occurred ~200kyr before the boundary evidenced by renewed increase in planktic fragmentation, but without a decrease in P/B ratio. Our data show that changing deep water masses, coupled with reduced productivity and associated decrease in pelagic carbonate flux was responsible for the first ~1.6-Myr dissolution interval, while Deccan Traps volcanism (?) may have caused surface ocean acidification ~200kyr prior to the K/Pg mass extinction event.

Seismogenic rotational slumps and translational glides in pelagic deep-water carbonates. Upper Tithonian-Berriasian of Southern Tethyan margin (W Sicily, Italy)

Soft-sediment deformation structures (SSDSs), which reflect sediment mobilization processes, are helpful to identify punctual events of paleoenvironmental stresses. In the upper Tithonian-Berriasian calpionellid pelagic limestone of the Lattimusa Fm. outcropping in the Barracù section (W Sicily), paleoenvironmental restoration reveals the occurrence of a deep-water flat basin, characterised by undeformed planar bedding, laterally passing to a gentle slope where the deformed horizons alternate with undeformed beds. Here, two types of gravity slides have been differentiated on the basis of different kinds of SSDSs, brittle deformation, involved lithofacies, geometry and morphology. Type 1 slump sheets, involving marl-limestone couplets, are characterised by chaotic stratification, truncated beds, recumbent folds, tension gashes, clastic dikes and small-scale listric normal faults and thrusts. They moved downslope as a rotational slump, displaying the typical wedge-shaped morphology, becoming thicker toe-wards. Type 2 slump sheets, involving bed packages of lime mudstone, are characterised by arcuate slump scars developing downwards as listric normal faults, associated with rotational beds and bulging. Displaying tabular geometries, they moved coherently along an overall bedding parallel detachment surface as a translational glide characterised by sediment creep.Regional geology indicates that the basin (Sicanian), bordered by a stepped carbonate platform, was affected by synsedimentary tectonics throughout the Mesozoic. The synsedimentary extensional tectonics that affect the Upper Triassic-Jurassic deposits with steeped normal faults, causing tilt-block and instability of the seafloor through fluidization processes, triggered the rotational slumps of the lower portion of the section. Seismic shocks, induced by outside sector tectonics, like those recorded in the adjacent Busambra stepped carbonate platform margin, triggered the rotational slumps that are not related to local-scale faults. To explain the translational glides of the upper portion of the section, the aforementioned outside sector earthquakes produced instability of the sea-floor through thixotropy of marls. In this case, the driving forces (i.e., gravitational) were favoured by the uplifting trend that is well recorded in the Southern Tethyan continental margin.We suggest that the various early deformations represent rotational slump and sediment creep that may be characteristic for slump structures in pelagic carbonates, depending on the involved lithologies (i.e., paleoenvironmental setting) and the intensity/distance of the trigger mechanism (i.e., earthquake epicentre).

Facies characteristics of Jurassic–Lower Cretaceous successions at Kösrelik–Kösrelikızığı (North of Ankara, Turkey)

Jurassic–Lower Cretaceous rocks in the Kosrelik and Kosrelikizik areas (North of Ankara, Central Turkey) which were deposited in a tectonically controlled basin have outcrops in a very restricted area but they carry very important evidences for elucidating the Late Jurassic paleogeography of the Ankara region subjected to heavy tectonic activities during Hettangian–Oxfordian, especially during Sinemurian. Detailed facies analysis of sequences in the study area indicates that the deposition took place in two different tectonically controlled depositional settings, 3 km apart from each other. At the east of the study area, the succession is represented by Hettangian–Sinemurian aged basin sediments lying on the basement rocks with angular unconformity, but succession at the west comprises sediments of Oxfordian pelagic carbonate platform (PCP) overlying the basement rocks with angular unconformity. The PCP sediments were overlapped by radiolaria bearing pelagic limestone, Oxfordian–early Cretaceous in age, under relatively calm environmental conditions. Anomalies in C and O isotopes, trace elements and rare earth element values coincide with the periods of fast changes in the environmental conditions. The sedimentological and geochemical data indicated that the area was deepened and shallowed several times until early Cretaceous.

Gradients in the carbon isotopic composition of Ordovician shallow water carbonates: A potential pitfall in estimates of ancient CO2 and O2

The carbon isotopic composition of the global dissolved inorganic carbon (DIC) reservoir is best estimated from open ocean pelagic carbonate sediments (δ13Ccarb). However, this is not practical for most of geologic time because seafloor subduction has removed the pre-Jurassic record and these time periods may have lacked planktonic calcifying organisms, and therefore shallow water carbonate platform or periplatform sediments are utilized. Shallow water deposits are susceptible to a wide range of post-depositional alteration processes and syn-depositional controls on δ13Ccarb that include carbonate mineralogy, water mass restriction, and a host of related variables (e.g., pH, temperature, organic decomposition, evaporation and CO2 solubility) that can produce local gradients in DIC. The degree to which shallow water δ13C curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates, but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses (e.g., endemic faunas). Here we integrate strontium isotope (87Sr/86Sr) stratigraphy and biostratigraphy to compare δ13C curves in a case study along a depth transect in Middle–Late Ordovician carbonate platform settings. The restricted tidal flat and more open marine deposits are offset by a maximum of ∼2‰ during sea level drop and ∼0‰ during highstand flooding of the platform. Global carbon cycle models such as GEOCARBSULF use published δ13Ccarb curves to drive organic carbon burial rates under the assumption that δ13Ccarb reflects a global seawater signal. We show here the potential pitfalls of using a published δ13Ccarb curve that violates this global assumption. For the 460 million year Middle–Late Ordovician time bin in GEOCARBSULF, improper usage of our locally depleted δ13C curve to drive global organic carbon burial would result in erroneous atmospheric O2 (∼10.8% O2, equal to about a 30% reduction from what it should be at ∼15.2% O2 using global δ13C) and CO2 (∼400 ppm too high, equal to about a 13% increase from what it should be at 2570 ppm CO2). With detailed sedimentologic analysis, it may be possible to identify and exclude δ13C samples that record the influences of local carbon cycling from global carbon cycle models such as GEOCARBSULF.

Contrasting environmental effects of astronomically driven climate change on three Eocene hemipelagic successions from the Basque–Cantabrian Basin

AbstractSeveral processes can contribute to the formation of hemipelagic limestone–marl alternations as a consequence of astronomically driven climate change. The aim of this study was to decipher which environmental factors governed the formation of three Eocene hemipelagic successions of the Basque–Cantabrian Basin using a comprehensive set of physical and bulk carbonate geochemical data (bed thickness, mineralogy, %CaCO3, δ13C and δ18O). The results show that the significance of several environmental processes varied depending on the palaeogeographic setting and eccentricity‐modulated precessional seasonality. In the Sopelana starved deep‐sea basin, limestones were formed as a consequence of high pelagic carbonate productivity during periods of warm seawater and sluggish circulation, which corresponded with periods of low seasonality (summers at aphelion); conversely, marls accumulated when pelagic carbonate productivity decreased during periods with cooler waters and more vigorous circulation, which occurred when seasonality was higher (summers at perihelion). In the Gorrondatxe submarine fan fringe, marls accumulated when high seasonality produced significant continental rainfall and run‐off, causing the dilution of pelagic carbonate sedimentation with terrigenous supplies. In the Oyambre upper slope, marls also accumulated when seasonality was high, as pelagic carbonate productivity decreased due to both the expansion of low‐salinity waters on the ocean surface and the increase in continentally derived nutrients, which caused detrimental seawater conditions for calcareous plankton. Both in Gorrondatxe and Oyambre, limestones accumulated when boreal summer at aphelion caused low seasonality, which allowed relatively stable conditions to prevail. At minimum eccentricity, when precession‐driven seasonality contrast diminished, changes in pelagic carbonate productivity were significant in the three sections. On the contrary, at maximum eccentricity, when seasonality peaked due to summers occurring at perihelion, the effects of other environmental processes, such as continental and oceanic currents, became influential. However, the influence of these processes minimized when summertime coincided with aphelion at maximum eccentricity and seasonality was weakest.

Maastrichtian-Thanetian planktonic foraminiferal biostratigraphy and remarks on the K-Pg boundary in the southern Kocaeli Peninsula (NW Turkey)

The Kocaeli Peninsula (NW Turkey) provides one of the best exposed deep marine Upper Cretaceous-Palaeocene sections in north-western Anatolia. The biostratigraphic framework from three sections, namely Belen, Bulduk, and Toylar, in the southern part of the Kocaeli Peninsula is established by means of planktonic foraminifera. A very rich planktonic foraminiferal assemblage analysed both in thin sections and washed residues records a biozonation ranging from the Contusotruncana contusa (CF6) Zone (Maastrichtian) to the Globanomalina pseudomenardii (P4) Zone (Thanetian). Although a major part of the biozones in the studied interval is clearly defined, the upper three zones (CF1-3) of the latest Maastrichtian and the P0 and P1a zones of the earliest Palaeocene cannot be recognised. These unrecorded biozones are either completely missing or occurred within a very condensed interval in the studied sections. A hardground layer characterised by oxidation and extensive bioturbation might indicate a possible biostratigraphic gap spanning the CF1-3 zones of the uppermost Maastrichtian in the Belen and Bulduk sections. In the Toylar section, on the other hand, the CF1-3 zones still cannot be detected although a hardground layer is not observed. The biostratigraphic resolution across the Cretaceous-Palaeogene (K-Pg) boundary in the studied sections cannot be improved due to the condensed and well-cemented pelagic carbonates of the boundary interval.

Biogeochemical significance of pelagic ecosystem function: an end-Cretaceous case study.

Pelagic ecosystem function is integral to global biogeochemical cycling, and plays a major role in modulating atmospheric CO2 concentrations (pCO2). Uncertainty as to the effects of human activities on marine ecosystem function hinders projection of future atmospheric pCO2 To this end, events in the geological past can provide informative case studies in the response of ecosystem function to environmental and ecological changes. Around the Cretaceous-Palaeogene (K-Pg) boundary, two such events occurred: Deccan large igneous province (LIP) eruptions and massive bolide impact at the Yucatan Peninsula. Both perturbed the environment, but only the impact coincided with marine mass extinction. As such, we use these events to directly contrast the response of marine biogeochemical cycling to environmental perturbation with and without changes in global species richness. We measure this biogeochemical response using records of deep-sea carbonate preservation. We find that Late Cretaceous Deccan volcanism prompted transient deep-sea carbonate dissolution of a larger magnitude and timescale than predicted by geochemical models. Even so, the effect of volcanism on carbonate preservation was slight compared with bolide impact. Empirical records and geochemical models support a pronounced increase in carbonate saturation state for more than 500 000 years following the mass extinction of pelagic carbonate producers at the K-Pg boundary. These examples highlight the importance of pelagic ecosystems in moderating climate and ocean chemistry.

Response of the Cr isotope proxy to Cretaceous Ocean Anoxic Event 2 in a pelagic carbonate succession from the Western Interior Seaway

Chromium offers a redox sensitive isotopic proxy with potential for tracing past oxygen levels in the oceans. We explore this potential in a pelagic succession of marine carbonate sediment deposited during Cretaceous Ocean Anoxic Event 2 (OAE 2) in the Western Interior Seaway, Colorado (WIS), using the USGS Portland #1 Core. Reduction of Cr(VI) causes light isotopes of Cr to preferentially partition into Cr(III). Because Cr(VI) is the thermodynamically favoured species in oxygenated seawater, and Cr(III) is relatively insoluble under the same conditions, increased removal of Cr(III) into anoxic marine sediment during ocean anoxic events should cause positive shifts in seawater δ53Cr values. This assumes that isotopic fractionation associated with all Cr removal fluxes from the oceans was constant during OAE 2 and that there was no change in Cr input fluxes to the oceans. Here, we report findings that counter this prediction, namely evidence for a negative shift in seawater δ53Cr during OAE 2 of ∼1.1‰ in the WIS. The magnitude of the excursion depends on the speciation of Cr removed in this setting over the duration of OAE 2, as well as the fractionation factor accompanying the removal flux of chromate into carbonate sediment, both of which are uncertain. The δ53Cr excursion reaches values as low −0.09‰ during OAE 2, which is indistinguishable from the high-temperature igneous rock baseline value of −0.124±0.101‰ (2σ). Moreover, the minima in the δ53Cr profile coincides with a peak in Cr concentrations in the study core, as well as anomalous enrichments of other trace metals of basaltic affinity. Building on previous studies linking OAE 2 to a massive episode of submarine volcanic activity, we attribute the decrease in carbonate δ53Cr values during OAE 2 to the expansion of a hydrothermal superplume created during eruptions of the Caribbean Large Igneous Province (LIP). This metal laden plume, enriched in Cr(III) and probably anoxic, moved from deep waters of the eruption site in the eastern Pacific into deep waters of the proto-North Atlantic through an oceanic gateway in the Central Americas. Once inside, anoxic metal-rich waters upwelled against the surrounding continental margins, where isotopically unfractionated Cr(III), whose solubility was improved by organic complexation and/or oxidation to Cr(VI), mixed with the isotopically heavy pool of oceanic chromate in surface waters above the redoxcline. Application of a simple mixing equation shows that hydrothermal Cr contributed >90% of the authigenic Cr in the trace metal enriched interval of the study core. The increased flux of hydrothermal Cr was large enough to mask the expected isotopic response of the ocean Cr cycle to increasing anoxia. We consider this interpretation to be robust, in that it is not substantially affected by uncertainty in the speciation of hydrothermal Cr, nor uncertainty in the fractionation factor (ΔswCaCO3) associated with chromate removal into marine carbonate sediment, which we measured in the modern Caribbean Sea to be −0.46±0.14‰ (2σ).

Calcium isotopes in scleractinian fossil corals since the Mesozoic: Implications for vital effects and biomineralization through time

We present a Cenozoic record of δ^(44/40)Ca from well preserved scleractinian fossil corals, as well as fossil coral δ^(44/40)Ca data from two time periods during the Mesozoic (84 and 160 Ma). To complement the coral data, we also extend existing bulk pelagic carbonate records back to ∼80 Ma. The same fossil corals used for this study were previously shown to be excellently preserved, and to be faithful archives of past seawater Mg/Ca and Sr/Ca since ∼200 Ma (Gothmann et al., 2015). We find that the δ^(44/40)Ca compositions of bulk pelagic carbonates from ODP Site 807 (Ontong Java Plateau) and DSDP Site 516 (Rio Grande Rise) have not varied by more than ∼±0.20‰ over the last ∼80 Myr. In contrast, the δ^(44/40)Ca compositions of Mesozoic and Early Cenozoic fossil corals are ∼1‰ lighter than those of modern corals. The observed change in coral δ^(44/40)Ca does not likely reflect secular variations in seawater δ^(44/40)Ca. Instead, we propose that it reflects a vital effect of calcification – specifically, a sensitivity of coral Ca isotope discrimination to changing seawater [Ca] and/or pH. Support for this hypothesis comes from the presence of an empirical correlation between our coral δ^(44/40)Ca record and records of seawater [Ca] and pH since the Mesozoic (Lowenstein et al., 2003 and Honisch et al., 2012). We explore various mechanisms that could give rise to such a vital effect, including: (1) changes in calcification rate, (2) changes in proton pumping in exchange for Ca^(2+), (3) variable Rayleigh distillation from an isolated calcifying fluid, and (4) changes in the calcium mass balance of the extracellular calcifying fluid (termed here the “leaky Ca model”). We test for the dependence of seawater δ^(44/40)Ca on external seawater [Ca] by measuring the δ^(44/40)Ca of cultured corals grown in seawater solutions with [Ca] ranging from 10 to 15 mmol/kg. Corals grown under elevated [Ca] conditions show a slight, ∼0.15‰ depletion of δ^(44/40)Ca at higher seawater [Ca] – a supportive but not definitive result.

Sediment creep on slopes in pelagic limestones: Upper Jurassic of Northern Calcareous Alps, Austria

Slump structures in Upper Jurassic pelagic limestones of the Northern Calcareous Alps were studied using methods of ductile structural geology. The early deformation observed cannot be explained by conventional models for slumping, commonly describing slump complexes with an extending upper and contracting lower part. Instead, the structures suggest distributed bedding-parallel surficial stretching, and folding localized by roll-over normal faulting. The initially isoclinal and recumbent folds are pervasively overprinted by stretching, causing reorientation of fold axes into the downslope direction. Transport of folds separates antiformal and synformal hinges, and the resulting isolated hinges resemble intrafolial folds in metamorphic terranes. Only folds formed late in the process of lithification allow insight in the early stages of fold evolution. We suggest that this type of early deformation represents sediment creep that may be characteristic for slump structures in pelagic carbonates.

Carbonate sedimentation in an extensional active margin: Cretaceous history of the Haymana region, Pontides

The Haymana region in Central Anatolia is located in the southern part of the Pontides close to the Izmir–Ankara suture. During the Cretaceous, the region formed part of the south-facing active margin of the Eurasia. The area preserves a nearly complete record of the Cretaceous system. Shallow marine carbonates of earliest Cretaceous age are overlain by a 700-m-thick Cretaceous sequence, dominated by deep marine limestones. Three unconformity-bounded pelagic carbonate sequences of Berriasian, Albian–Cenomanian and Turonian–Santonian ages are recognized: Each depositional sequence is preceded by a period of tilting and submarine erosion during the Berriasian, early Albian and late Cenomanian, which corresponds to phases of local extension in the active continental margin. Carbonate breccias mark the base of the sequences and each carbonate sequence steps down on older units. The deep marine carbonate deposition ended in the late Santonian followed by tilting, erosion and folding during the Campanian. Deposition of thick siliciclastic turbidites started in the late Campanian and continued into the Tertiary. Unlike most forearc basins, the Haymana region was a site of deep marine carbonate deposition until the Campanian. This was because the Pontide arc was extensional and the volcanic detritus was trapped in the intra-arc basins and did not reach the forearc or the trench. The extensional nature of the arc is also shown by the opening of the Black Sea as a backarc basin in the Turonian–Santonian. The carbonate sedimentation in an active margin is characterized by synsedimentary vertical displacements, which results in submarine erosion, carbonate breccias and in the lateral discontinuity of the sequences, and differs from blanket like carbonate deposition in the passive margins.