1,721,258 research outputs found
Towards an integrated global stratigraphy for the Cenomanian-Turonian Oceanic Anoxic Event 2 : preliminary results from the EU C/T-Network project
No full textWalsworth-Bell Ben, Petrizzo Maria-Rose, Tsikos Harilaos, Erba Elisabetta, Jenkyns Hugh, Premoli-Silva Isabella. Towards an integrated global stratigraphy for the Cenomanian-Turonian Oceanic Anoxic Event 2 : preliminary results from the EU C/T-Network project. In: Documents des Laboratoires de Géologie, Lyon, n°156, 2002. STRATI 2002. 3ème congrès français de stratigraphie. Lyon, 8-10 juillet 2002. p. 234
Stratigraphy, paleoceanography, and evolution of Cretaceous Pacific guyots: Relics from a greenhouse earth
No full textMany guyots in the north Pacific are built of drowned Cretaceous shallow-water carbonates that rest on edifice basalt Dating of these limestones, using strontium- and carbon-isotope stratigraphy, illustrates a number of events in the evolution of these carbonate platforms: local deposition of marine black shales during the early Aptian oceanic anoxic event; synchronous development of oolitic deposits during the Aptian; and drowning at different times during the Cretaceous (and Tertiary). Dating the youngest levels of these platform carbonate shows that the shallow-water systems drowned sequentially in the order in which plate-tectonic movement transported them into low latitudes south of the Equator (paleolatitude ∼0°-10° south). The chemistry of peri-equatorial waters, rich in upwelled nutrients and carbon dioxide, may have been a contributory factor to the suppression of carbonate precipitation on these platforms. However, oceanic anoxic events, thought to reflect high nutrient availability and increased productivity of planktonic organic-walled and siliceous microfossils, did not occasion platform drowning. Neither is there any evidence that relative sealevel changes were the primary cause of platform drowning, which is consistent with the established resilience of shallow-water carbonate systems when influenced by such phenomena. Comparisons with paleotemperature data show that platform drowning took place closer to the Equator during cooler intervals, such as the early Albian and Maastrichtian, and farther south of the Equator during warmer periods such as Albian-Cenomaniain boundary time and the mid-Eocene. Initiation of one carbonate platform relatively close to the Equator, at paleolatitudes more northerly than those where others drowned, took place during the cool early mid-Aptian. These correlations are in accord with an interpretation that excess warmth in shallow peri-equatorial waters proved inimical for many carbonate-secreting organisms living on the platforms, allowing subsidence or eustatic sealevel rise to outpace sedimentation and guyots to form. A parallel may be drawn with the recent phenomenon of coral and foraminiferal bleaching, whereby photosynthetic symbionts succumb to prolonged high temperatures (>30°C) and the host organism dies. The fact that most Cretaceous guyots reside in the north Pacific may not be solely related to the age-distribution pattern of ocean floor but to their having run the gauntlet of a difficult and dangerous passage across the Equator. North Pacific guyots are relics from the Cretaceous (and Eocene) "greenhouse" Earth
The paradox of drowned carbonate platforms and the origin of Cretaceous Pacific guyots
No full textGeochemical, stratigraphic and palaeolatitudinal data from deep boreholes drilled through Pacific guyots—flat-topped seamounts—help to explain the drowning of these Cretaceous shallow-water carbonate platforms that once thrived through the accumulation of biogenic and inorganic calcium carbonate sediment in mid-oceanic regions. The platforms drowned sequentially over a 60-million-year interval while they were being transported northward by Pacific plate motion through a narrow equatorial zone (∼0–10° S). Such platforms were apparently resistant to the effects of Cretaceous oceanic anoxic events. Although the mechanism responsible for drowning remains unknown, the tropics have not always been the refuge for atolls that they are toda
Carbon-isotope anomalies and demise of carbonate platforms in the Sinemurian (Early Jurassic) of the Tethyan region: Evidence from the Southern Alps (Northern Italy)
Get PDFDespite its global impact on ecosystems, the Triassic/Jurassic boundary event had only a modest effect on the carbonate depositional systems of the Southern Alps, whereas a fundamental reorganization of the same palaeogeographic area took place during the Sinemurian Age. This paper investigates whether or not the well-documented demise of Sinemurian carbonate platforms in the Tethyan region was a response to a global event by examination of carbon-isotope anomalies in successions of different facies that record this interval of time. A chemostratigraphic transect from LakeGardauptotheeasternItalianborderisillustrated byfourstratigraphicsections;high-resolution (20 cm over key intervals) chemostratigraphic sampling allowed detection of a major negative δ13C anomaly of 1.5‰, preceded by a positive excursion, both in shallow- and deep-water successions, over the stratigraphical range of the ammonite genus Arnioceras. A comparison with sections from the UK suggests that the positive excursion belongs to the turneri Zone and the succeeding negative excursion falls within the obtusum Zone. In the deep-water Belluno Basin, the negative anomaly occurs in a biogenic chert-rich unit recording the onset of mesotrophic conditions in the basin. In the platform-carbonate successions, this major negative carbon-isotope excursion is developed within a calcarenitic unit corresponding to the lowest occurrence of the foraminifer Paleomayncina termieri. Thisevidencefordeepeningandtransgressionacrossthecarbonateplatformsuggestspre-conditioning for drowning. Hence, rather than tectonic subsidence alone, environmental factors may have aided the demise of Tethyan carbonate platforms during the Early Jurassic Sinemurian Age
Chemostratigraphy of Jurassic-cretaceous Italian carbonate platforms
Full text linkSamples of shallow-water carbonates were collected from Jurassic and Cretaceous Italian carbonate platforms and subjected to petrographic, diagenetic and chemostratigraphic analyses (87Sr/86Sr, δ13Ccarb, δ13Corg, δ18O). In general, the new chemostratigraphic data generated reflect trends established by previous work, some of which has been carried out on biostratigraphically calibrated reference sections. Consequently, chemostratigraphic correlations (87Sr/86Sr, δ13Ccarb) of isotope profiles taken from platform carbonates with well-dated reference sections have allowed the application of high-resolution dating frameworks to the biostratigraphically poorly constrained carbonate platforms. The increased resolution in dating of the Italian carbonate platforms has, furthermore allowed a detailed investigation into the facies response of these carbonate platforms to major geological events. In particular, platform responses to oceanic anoxic events and other periods of major perturbation in the global carbon cycle are analysed (early Toarcian, Aalenian-Bajocian, Oxfordian-Tithonian, Valanginian-Hauterivian, Aptian-Albian, Cenomanian-Turonian, Coniacian-Santonian). Lower Jurassic levels of the Trento Platform record platform devastation in the early Toarcian synchronous with a major negative δ13Ccarb excursion, followed by platform recovery synchronous with a pronounced δ13Ccarb positive excursion and return to background values. The Campania-Lucania Platform shows negligible response to the oceanographic events of the early Toarcian even though the characteristic carbon-isotope profile is readily identifiable. The Trento Platform drowned at approximately the Aalenian-Bajocian Stage boundary, synchronously with a reproducible negative followed by positive δ13Ccarb excursion, whereas the Campania-Lucania Platform underwent a facies transition from oolite to cyclically bedded micrite. The Friuli Platform showed negligible depositional response to the carbon-cycle perturbations of the Kimmeridgian-Tithonian, Valanginian-Hauterivian, Aptian-Albian and Cenomanian- Santonian (as registered in the δ13Ccarb record). The Campania-Lucania Platform registered flooding and increased levels of organic-matter preservation coincident with pronounced positive δ13Ccarb excursions at Cenomanian-Turonian and Coniacian-Santonian levels. Observations on the responses of carbonate platforms to oceanographic conditions during periods of global carbon burial lead to the conclusion that temperature excess is a hitherto neglected control on global carbonate accumulation rates
The structural and sedimentological evolution of the Lagonegro zone, Southern Italy
Full text linkThe Lagonegro Zone comprises three structural units which outcrop within the pile of decollement nappes forming the southern Italian Appenines. Stratigraphic, structural and sedimentological evidence suggests that the zone represents the relics of a Mesozoic basin that developed amidst the carbonate platforms bordering the southern margin of Tethys. Mapping has confirmed that the Lagonegro I, Lagonegro II and Monte Foraporta Units are separated by thrusts and are stacked in ascending order. Middle Triassic fine-grained terrigenous clastic sediments and neritic limestone olistoliths of Lagonegro Unit II record the disintegration of a young carbonate platform under the influence of extensional tectonics. Deposition and redeposition of hemi-pelagic lime-mud washed from the adjacent shallow-water platforms during the Late Triassic is manifested in both Lagonegro units by the cherty limestones of the Sirino Formation; a gradual transition from calcareous to siliceous deposition at the top of the formation reflects the subsidence of the basin-floor beneath the Calcite Compensation Depth. Contrasting patterns of siliceous and calciclastic sedimentation developed during the Jurassic, probably imposed by renewed extensional tectonics; coeval calciclastic deposition in a small, and at times anoxic marginal basin is recorded in the Monte Foraporta Unit. Dolomitisation of basinal carbonate rocks took place during the Jurassic due to mixing of saline pore-fluids with meteoric water recharged from the adjacent carbonate platforms. Deposition of terrigenous shales below the Calcite Compensation Depth occurred during the Lower Cretaceous, but calcareous sedimentation was resumed in the Late Cretaceous and Palaeogene. The progressive deepening of the basin documented by these facies transitions is attributed to regional subsidence, caused by crustal extension and attenuation, and relative accretion of the surrounding platforms; comparable basal stratigraphies in several ophiolitic zones of the Alpine-Mediterranean region suggest that many Tethyan 'oceanic' basins may have initially developed in a similar manner
Stepwise extinction of larger foraminifers at the Cenomanian-Turonian boundary: A shallow-water perspective on nutrient fluctuations during Oceanic Anoxic Event 2 (Bonarelli Event)
No full textA two-step pattern in the extinction of larger foraminifers is recorded in the upper Cenomanian shallow-water carbonates of the southern Apennines (Italy). The first step eliminated the alveolinids, the most extreme oligotrophs, and reduced dramatically the diversity of larger foraminifers. The second step wiped out the few survivors, seemingly able to tolerate mesotrophic conditions, leaving a disaster fauna dominated by small heterotrophs. This pattern of extinction parallels the ecological succession of shallow-water benthic foraminiferal assemblages along a gradient of increasing nutrient availability. High-resolution carbon isotope stratigraphy shows that the extinction of alveolinids was contemporaneous with the extinction of rotaliporid planktic foraminifers, the drowning of certain Tethyan carbonate platforms, and an episode of thermal instability recorded in sea-surface temperature in the open ocean. Ocean stratification, during the first phase of Oceanic Anoxic Event 2, would have promoted oligotrophic conditions in surface tropical waters and maximum diversity of larger foraminifers. Following this, ocean overturning caused by surface-water cooling is credited with delivering to shallow-water environments the excess nutrient loads previously stored at depth, triggering the environmental changes leading to stepwise extinction of larger foraminifers. © 2008 The Geological Society of America
Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)
Full text linkAn abrupt rise in temperature, forced by a massive input of CO2 into the atmosphere, is commonly invoked as the main trigger for Oceanic Anoxic Events (OAEs). Global warming initiated a cascade of palaeoenvironmental perturbations starting with increased continental weathering and an accelerated hydrological cycle that delivered higher loads of nutrients to coastal areas, stimulating biological productivity. The end-result was widespread anoxia and deposition of black shales: the hallmarks of OAEs. In order to assess the role of weathering as both an OAE initiator and terminator (via CO2 sequestration) during the Early Aptian OAE 1a (Selli Event, ∼120 Ma) the isotopic ratio of lithium isotopes was analysed in three sections of shallow-marine carbonates from the Pacific and Tethyan realms and one basinal pelagic section from the Tethyan domain. Because the isotopic composition of lithium in seawater is largely controlled by continental silicate weathering and high- and low-temperature alteration of basaltic material, a shift to lighter δ7Li values is expected to characterize OAEs. The studied sections illustrate this phenomenon: δ7Li values decrease to a minimum coincident with the negative carbon-isotope excursion that effectively records the onset of OAE 1a. A second negative δ7Li excursion occurs coeval with the minimum in strontium isotopes after the event. The striking similarity to the strontium-isotope record argues for a common driver. The formation and destruction (weathering) of an oceanic LIP could account for the parallel trend in both isotope systems. The double-spike in lithium isotopes is probably related to a change in weathering congruencies. Such a chemostratigraphy is consistent with the hypothesis that an increase in silicate weathering, in conjunction with organic-carbon burial, led to drawdown of atmospheric CO2 during the early Aptian OAE 1a
Palaeoceanography and sedimentology of a mid-Cretaceous Greensand
Full text linkThe Upper Albian (mid-Cretaceous) sediments of the Anglo-Paris Basin display a range of condensation phenomena, including glauconitic and phosphoritic sands, glauconitic intraformational pebble beds, and mineralised nodular hardgrounds. These are interpreted as the result of sedimentation rate fluctuations controlled by small-scale relative sea-level changes of at least regional nature. The regional signal was modified by local tectonics, including occasional synsedimentary faulting and the development of thinned successions and complex condensation horizons over broad submarine highs. Candidate sequence boundaries, marine flooding surfaces and systems tracts are proposed. Episodic, high-energy storm events record a spectrum of storm intensities and periodicities in the Upper Greensand Formation. Storm processes interacted with early submarine lithification to produce a suite of pebble-shell beds, simple and amalgamated coquinas, and storm-scoured hardgrounds. Analysis of shell bed fabrics and taphonomy indicates the important role of high-energy storms in generating a distinct event stratigraphy. Stable isotope analysis has detected a positive secular change in carbon-isotopic ratios within carbonates of dispar zone age, resolved as two smaller positive shifts across nodular hardgrounds. This correlates with the development of organic-rich sediments in a range of settings world-wide. Analytical problems, including silica diagenesis, have been addressed by a series of parallel control studies. Geochemical and petrographic analysis has confirmed the widespread development of glauconitic minerals throughout the Upper Greensand, and has been used to investigate grain evolution in a range of lithologies. A review of the global development of the Cretaceous glauconitic facies has been used to compile depositional models for a range of settings. Volumetric calculations of Cretaceous glauconite production rates have shown secular changes, with peak Aptian to Santonian values linked to transgressions across the broad shelf seas which developed globally during this time of rising sea-levels and sea-level highstand
A multiproxy geochemical record of the early Aptian Selli event (OAE1a) from the platform carbonates of southern Italy
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