Early Cretaceous life and the environment were strongly influenced by the accelerated break up of Pangaea, which was associated with the formation of a multitude of rift basins, intensified ...spreading, and important volcanic activity on land and in the sea. These processes likely interacted with greenhouse conditions, and Early Cretaceous climate oscillated between “normal” greenhouse, predominantly arid conditions, and intensified greenhouse, predominantly humid conditions. Arid conditions were important during the latest Jurassic and early Berriasian, the late Barremian, and partly also during the late Aptian. Humid conditions were particularly intense and widespread during shorter episodes of environmental change (EECs): the Valanginian Weissert, the latest Hauterivian Faraoni, the latest Barremian–earliest Aptian Taxy, the early Aptian Selli, the early late Aptian Fallot and the late Aptian–early Albian Paquier episodes. Arid conditions were associated with evaporation, low biogeochemical weathering rates, low nutrient fluxes, and partly stratified oceans, leading to oxygen depletion and enhanced preservation of laminated, organic-rich mud (LOM). Humid conditions enabled elevated biogeochemical weathering rates and nutrient fluxes, important runoff and the buildup of freshwater lids in proximal basins, intensified oceanic and atmospheric circulation, widespread upwelling and phosphogenesis, important primary productivity and enhanced preservation of LOM in expanded oxygen-minimum zones. The transition of arid to humid climates may have been associated with the net transfer of water to the continent owing to the infill of dried-out groundwater reservoirs in internally drained inland basins. This resulted in shorter-term sea-level fall, which was followed by sea-level rise. These sea-level changes and the influx of freshwater into the ocean may have influenced oxygen-isotope signatures. Climate change preceding and during the Early Cretaceous EECs may have been rapid, but in general, the EECs had a “pre”-history, during which the stage was set for environmental change. Negative feedback on the climate through increased marine LOM preservation was unlikely, because of the low overall organic-carbon accumulation rates during these episodes. Life and climate co-evolved during the Early Cretaceous. Arid conditions may have affected continental life, such as across the Tithonian/Berriasian boundary. Humid conditions and the corresponding tendency to develop dys- to anaerobic conditions in deeper ocean waters led to phases of accelerated extinction in oceans, but may have led to more luxuriant vegetation cover on continents, such as during the Valanginian, to the benefit of herbivores. During Early Cretaceous EECs, reef systems and carbonate platforms in general were particularly vulnerable. They were the first to disappear and the last to recover, often only after several million years.
The Frasnian-Famennian boundary records one of the most catastrophic mass extinctions of the Phanerozoic Eon. Several possible causes for this extinction have been suggested, including ...extra-terrestrial impacts and large-scale volcanism. However, linking the extinction with these potential causes is hindered by the lack of precise dating of either the extinction or volcanic/impact events. In this study, a bentonite layer in uppermost-Frasnian sediments from Steinbruch Schmidt (Germany) is re-analysed using CA-ID-TIMS U-Pb zircon geochronology in order to constrain the date of the Frasnian-Famennian extinction. A new age of 372.36 ± 0.053 Ma is determined for this bentonite, confirming a date no older than 372.4 Ma for the Frasnian-Famennian boundary, which can be further constrained to 371.93-371.78 Ma using a pre-existing Late Devonian age model. This age is consistent with previous dates, but is significantly more precise. When compared with published ages of the Siljan impact crater and basalts produced by large-scale volcanism, there is no apparent correlation between the extinction and either phenomenon, not clearly supporting them as a direct cause for the Frasnian-Famennian event. This result highlights an urgent need for further Late Devonian geochronological and chemostratigraphic work to better understand the cause(s) of this extinction.
The Pliensbachian and Toarcian (Early Jurassic) ages are characterised by several, relatively short-lived carbon cycle perturbations, climate change and faunal turnover. The cause(s) of biotic and ...abiotic disturbances remain unclear but most probably involved increased magmatic activity in the Karoo–Ferrar large igneous province. The Toarcian oceanic anoxic event (T-OAE) might represent the most extreme of these events, and as such, is becoming increasingly well documented worldwide. So far, other critical time intervals of the Pliensbachian–Toarcian have received considerably less attention. Here, the effects of the Middle Toarcian Variabilis event on the neritic–epeiric realm are explored making use of three well-exposed and extended stratigraphic sections in the Central High Atlas, Morocco. The carbon and oxygen isotopic compositions of 112 bulk micrite samples were analysed and placed against 39 data points from carefully screened brachiopod valves in order to differentiate between palaeo-environmental and diagenetic patterns. Additionally, the phosphorus concentrations of 109 micrite samples were determined to evaluate the P-cycling. In all studied sections, an upper middle Toarcian major change from carbonate- to clastics-dominated sedimentation is recorded, pointing to a first-order carbonate production crisis. Our results reveal that these major sedimentological patterns coincide with an increase of oxygen-isotope ratios as well as a decrease of phosphorous accumulation rates. This suggests that the late middle Toarcian carbonate ramp crisis was related to a transient cooling event, potentially triggered by pulsed massive SO4 exhalation events in the context of the Karoo large igneous province. Short-term cooling was likely amplified by the drawdown of atmospheric CO2 levels related to the coeval decline of neritic carbonate precipitation and the warm water mass circulation disruption between the Tethys and the continental shelf. The data shown here provide the first evidence for coupled changes in carbon cycling, continental weathering and neritic systems in the aftermath of the T-OAE.
•A carbonate crisis is highlighted during the middle Toarcian in W Tethys.•Oxygen isotopes of brachiopod valves are used to reconstruct seawater temperature.•The carbonate crisis is coeval to a mass extinction and a transient cooling event.•Phosphorus analyses document enhanced nutrient levels during the Late Toarcian.
Anomalously high rates of continental weathering have frequently been proposed as a key stimulus for the development of widespread marine anoxia during a number of Late Devonian environmental and ...biospheric crises, which included a major mass extinction during the Frasnian–Famennian transition (marked by the Upper and Lower Kellwasser horizons). Here, this model is investigated by presenting the first stratigraphic record of osmium-isotope trends (187Os/188Os) in upper Devonian strata from the Kowala Quarry (Holy Cross Mountains, Poland). Changes in reconstructed 187Os/188Os seawater values to more radiogenic compositions are documented at the base of both the Lower (~0.42 to ~0.83) and Upper (~0.31 to ~0.81) Kellwasser horizons characteristic of the Frasnian–Famennian transition, and additionally within upper Famennian shales that record a more minor environmental perturbation known as the Annulata Event (~0.20 to ~0.53). These shifts indicate the occurrence of extremely enhanced continental weathering rates at the onsets of the Kellwasser crises and during the later Annulata Event. The similarity of 187Os/188Os values in this study from Frasnian–Famennian boundary and lower Famennian strata (between 0.4 and 0.5) to those from North American stratigraphic equivalents suggests that the 187Os/188Os values record global trends. These findings support a causal relationship between increased continental weathering (and thus, nutrient supply to the marine shelf) and the environmental perturbations that occurred during numerous Late Devonian events, including both of the biospherically catastrophic Kellwasser crises as well as other, less severe, oceanic anoxic events.
•First stratigraphic record of Late Devonian osmium-isotope trends.•Record includes the Frasnian–Famennian extinction and later Annulata anoxic event.•Enhanced continental weathering rates shown to occur during all the studied crises.•Supports the runoff of terrestrial nutrients as a trigger of Late Devonian anoxia.
The area of the western Jura Mountains constitutes the former Jura-Burgundy threshold between the Tethys Ocean and the epicontinental Paris Basin Sea. During the Barremian, the area was covered by a ...shallow-water Urgonian carbonate platform. Tectonic processes influenced the architecture of the Urgonian platform and were notably responsible for the formation of fault-related depressions on top of the Urgonian series, which were subsequently transformed into incised valleys and then to marine depocenters. Their sedimentary infills are mostly represented by the Perte-du-Rhône Formation and record stepwise environmental change on the innermost platform, which was strongly influenced by the nearby landmass. From the latest Barremian onward, sedimentation progressively changed from: i) predominantly oncoidal and detrital charophyte-rich marly limestones; to ii) heterozoan marly limestones and marls; followed by iii) detrital sediments interspersed with numerous phosphatic conglomerates. These changes are linked with the onset of the latest Barremian Taxy Episode and with the following oceanic anoxic episodes (OAE's) 1a to d in the neighbouring basins. The sedimentary changes are associated with sedimentary sequences which formed during 2nd-order transgressions, and which express the effect of sea level fall followed by drowning events. The presence of a rich ammonite-fauna allowed a precise biostratigraphical dating of these events during the latest Barremian to Cenomanian time interval.
•Precise documentation and age model for uppermost Barremian-Cenomanian platform series in the Jura Mountains•Episode of environmental change from the latest Barremian onwards linked to the onset of the Taxy Episode.•Since the late early Aptian, the Jura Domain was affected by a succession of drowning events and OAEs.•Stepwise changes of the depositional environments from oligotrophic to mesotrophic conditions•Carbonate platform progressively drowned and became covered by pelagic sediments.
The Late Devonian was marked by repeated faunal crises and episodes of geographically widespread marine anoxia, and featured one of the ‘Big Five’ mass extinctions of the Phanerozoic Aeon during the ...Frasnian–Famennian transition. However, the processes responsible for causing the numerous anoxic events remain unclear. This study highlights the occurrence of disturbances to the phosphorus cycle during several Late Devonian crises by investigating sedimentary concentrations of the element (Ptot) as a tracer of nutrient influx, as well as its ratio with total organic carbon (TOC) to infer the recycling of the element from marine sediments. Increased TOC/Ptot ratios in the Frasnian–Famennian Lower and Upper Kellwasser horizons and upper Famennian Annulata and Hangenberg levels suggest that such nutrient recycling occurred across extensive areas of the marine shelf in Laurentia and both Rheic Ocean margins at those times, helping to sustain reducing conditions in those environments. Elevated Ptot values in the Upper Kellwasser, Annulata, and Hangenberg levels are consistent with an enhanced nutrient influx as the initial trigger for the anoxia. Correlation of phosphorus trends with other geochemical indicators of weathering/detrital influx (osmium-isotope, silicon/aluminum, and titanium/aluminium ratios) support a scenario in which terrestrial runoff provided these nutrients both to marine shelves and the oceanic inventory. Upwelling of oceanic deep-water bodies may have then brought the phosphorus to areas that had not featured major direct inputs of terrigenous material. The exception is the Lower Kellwasser Event, during which there was no increase in phosphorus delivery to marine areas and no evidence for terrestrial influx at the studied sections, invoking a different mechanism for the development of water-column anoxia. Clearly, the Late Devonian marine realm was unusually susceptible to becoming anoxic through various possible triggers, including nutrient influx from land and/or deep-water upwelling, and the recycling of phosphorus from newly deposited sediments.
•Global scale phosphorus cycle disturbance during the Frasnian–Famennian extinction.•Phosphorus recycling from sediments helped to sustain marine anoxia/euxinia.•Similar phosphorus cycle perturbations during the Annulata and Hangenberg events.•Anoxia likely initiated by influx of terrestrial phosphorus from enhanced weathering.
A stratigraphic and depositional model, constrained by biostratigraphy, geochemistry, total phosphorus contents, and bulk-rock mineralogy, is proposed for lower Aptian sediments from the Languedoc ...platform in Ardèche, SE France. The upper lower Aptian is documented by the Chabert Formation (upper Deshayesites forbesi Zone to upper Dufrenoyia dufrenoyi Subzone), deposited on a discontinuity surface on top of the Urgonian platform, recording a first emersion phase and consecutive drowning event. The Chabert Formation starts with the marly Violette Member, which passes into crinoidal limestone of the Rocherenard Member. The top of this member is associated with a second discontinuity, recording a further drowning phase, which is followed by the deposition of the glauconitic and partly phosphatic Picourel Member (upper Deshayesites grandis to upper Dufrenoyia dufrenoyi Subzone). A third erosive phase is documented by a phosphatic conglomerate (upper Dufrenoyia furcata Zone), which represents a lag deposit derived from underlying sediments. The formation of this conglomerate was associated with a substantial emersion phase. This emersion was followed by a drowning event reworking the phosphatic conglomerate into the base of the upper Aptian black marls (Frayol Formation). The carbon-isotope record shows a negative excursion which coincides with the onset of the early Aptian oceanic anoxic Selli episode (OAE 1a) in the middle/upper part of the Deshayesites forbesi Zone. Emersion phases were an important factor implied in the formation of the sequence boundaries, which were transformed into drowning unconformities during subsequent phases of significant transgressions. These phases were associated with the installation of higher trophic levels, transforming or impeding carbonate production. The first drowning phase preceded the onset of the Selli episode, suggesting that rapid sea-level change and associated environmental change were already an important element of the early Aptian before the major phase of environmental change during the Selli episode.
•Precise documentation and age model for lower Aptian platform succession in Ardèche•Progressive demise of the platform in three steps near and during the OAE1a•At least two of the three drowning steps preceded by platform emersion•Rapid and high-amplitude sea-level change affected platform evolution.•The drowning phases started before the onset of the Selli anoxic episode.
Phosphorus (in the form of phosphate) is an essential nutrient and energy carrier on many different levels of life, and a key element in mediating between living and lifeless parts of the biosphere. ...One of the most important aspects of the phosphorus cycle is its vital role in governing productivity, thereby interacting with the exogenic part of the carbon cycle, which, in turn, is important in regulating Earth's climate.
Phosphorus is a prime element to be traced in Earth's history, because it allows for the reconstruction of long-term feedback mechanisms between climate, environment and ecology, and of global change as such. Marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle, because, in the case of ubiquity, their origin may result from a general acceleration of the global phosphorus cycle. Sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox-driven phosphate desorption from iron and manganese oxyhydroxides. Dissolved sea-water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds. The main locus of phosphogenesis is near the sediment-water interface, but phosphogenesis also occurs at greater sediment depths. Current-induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits, which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime. In a sequence-stratigraphic context, phosphate deposits preferentially occur along marine or maximum flooding surfaces. Consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits.
(Bio-)chemical weathering on continents represents the most significant source of bioavailable phosphorus. This implies that long-term changes in marine phosphorus levels — and with these changes in marine ecology, productivity rates and ratios of exported carbonate carbon and organic carbon — are a response to changes in continental weathering rates. A compilation of marine sedimentary phosphorus burial rates for the last 160 Myr suggests that natural variations have occurred that span one order of magnitude. For the late Jurassic, Cretaceous and most of the Paleogene, the phosphorus cycle appears to have been accelerated in times of climate warming, which was most likely due to the spreading of zones of humid climate and more intense continental weathering. In the Neogene, the phosphorus cycle appears to have responded to changes in glacially induced weathering. This suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution. Integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions, as well as to improve our understanding of periods of enhanced phosphate accumulation, periods which were usually characterized by steep gradients in the development of climate and environment.
With regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere, the present-day input rates of phosphate into the world's oceans should be of great concern. They are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale.
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