An extraterrestrial trigger for the Early Cretaceous massive volcanism? Evidence from the paleo-Tethys Ocean

The Early Cretaceous Greater Ontong Java Event in the Pacific Ocean may have covered ca. 1% of the Earth's surface with volcanism. It has puzzled scientists trying to explain its origin by several mechanisms possible on Earth, leading others to propose an extraterrestrial trigger to explain this event. A large oceanic extraterrestrial impact causing such voluminous volcanism may have traces of its distal ejecta in sedimentary rocks around the basin, including the paleo-Tethys Ocean which was then contiguous with the Pacific Ocean. The contemporaneous marine sequence at central Italy, containing the sedimentary expression of a global oceanic anoxic event (OAE1a), may have recorded such ocurrence as indicated by two stratigraphic intervals with 187Os/188Os indicative of meteoritic influence. Here we show, for the first time, that platinum group element abundances and inter-element ratios in this paleo-Tethyan marine sequence provide no evidence for an extraterrestrial trigger for the Early Cretaceous massive volcanism

Cretaceous Oceanic Anoxic Events: Causes and Consequences

Organic carbon-rich sediments are globally developed in pelagic sedimentary sequences of Aptian-Albian and Cenomanian-Turonian age. They formed in a variety of paleo-bathymetric settings including oceanic plateaus and basins, continental margins and shelf seas. The widespread nature of these deposits suggests that they were not strictly controlled by local basin geometry but were a product of ″Oceanic Anoxic Events″ . Interpretation of these events as the result of the interplay of two major geologic and climatic factors is given. The Late Cretaceous transgression which increased the area and volume of shallow epicontinental and marginal seas and was accompanied by an increase in the production of organic carbon; and the existence of an equable global climate which reduced the supply of cold oxygenated bottom water to the world ocean. This combination of climatic and hypsographic conditions favoured the formation of an expanded oxygen-minimum layer and where this intersected the sediment-water interface, organic carbon-rich deposits could be formed, these being records of ″Oceanic Anoxic Events″

The Cenomanian-Turonian Oceanic Anoxic Event, II. Palaeoceanographic controls on organic-matter production and preservation

Correlation of the δ 13C spike with the well dated occurrences of strata rich in organic carbon detailed in Schlanger et al. (this volume), indicates that a global episode of intense organic carbon (orgC) burial took place during the latest Cenomanian-earliest Turonian 'Oceanic Anoxic Event' (OAE) (A. plenus through I. labiatus macrofossil zones and upper R. cushmani TRZ through W. archecretacea PRZ foraminiferal zones) over a period of no more than 1 million years (m.y.). The shape of the δ 13C curve indicates that rates of orgC burial gradually increased in the early part of the late Cenomanian, increased more rapidly in the later Cenomanian, and levelled off at peak values in latest Cenomanian-early Turonian time during the maximum rate of orgC burial. The δ 13C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in δ 13C reflects decreasing rates of orgC burial following the Cenomanian-Turonian 'oceanic anoxic event' as well as the probable oxidation and return of significant amounts of orgC to the oceans following regression and re-oxygenation of much of the deeper water masses in contact with the seafloor. The Cenomanian-Turonian OAE coincided with a maximum sea level highstand. We suggest that sea level, which may be responding to some volcano-tectonic event, is the common link and ultimately the driving force for orgC deposition in globally distributed basins under different climatic and ocean circulation regimes. The rate of production of warm, saline deep water may have been proportional to the area of shelf flooding such that the maximum occurred near the Cenomanian-Turonian boundary. As rates of deep-water formation increased, rates of upwelling of deeper oceanic water masses must also have increased thereby increasing sea-surface fertility and productivity. In somewhat restricted higher latitude basins, such as the Cretaceous Interior Seaway of North America, periodic high rates of freshwater runoff coupled with deepening seas during the transgression created periodic salinity stratification, oxygen depletion in bottom waters, and resultant enhanced orgC preservation. The disappearance of some types of keeled planktonic formainifers and ammonites at the Cenomanian-Turonian boundary is probably due to the rather sudden but short-term disappearance of suitable shallow midwater habitats because of widespread severe oxygen depletion in these levels. This interpretation is strengthened by the occurrence of benthic-free zones or depauperate benthic faunas near the Cenomanian-Turonian boundary in many localities. © 1987 The Geological Society

The Cenomanian-Turonian Oceanic Anoxic Event, I. Stratigraphy and distribution of organic carbon-rich beds and the Marine delta 13C excursion.

Marine strata deposited during late Cenomanian and early Turonian time display lithological, faunal, and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient. At, or very close to, the Cenomanian-Turonian boundary, between 90.5 and 91.5Ma ago, oxygen deficiencies were particularly marked over a period of <1Ma. This short-lived episode of oceanic oxygen deficiency has been termed the Cenomanian-Turonian 'Oceanic Anoxic Event' (OAE). The widespread distribution of anoxic sediments deposited synchronously during such a short-lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the Cenomanian-Turonian oxygen-minimum zone related to feedback between sea level rise and regional palaeoceanography. The palaeoceanography of the Cenomanian Turonian OAE is discussed in detail in a companion paper by Arthur et al, 1987.-from Author

The Cenomanian-Turonian Oceanic Anoxic Event, I. Stratigraphy and distribution of organic carbon-rich beds and the marine δ13C excursion

Marine strata deposited during late Cenomanian and early Turonian time display lithological, faunal, and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient. At, or very close to, the Cenomanian-Turonian boundary, between 90.5 and 91.5 million years ago, oxygen deficiencies were particularly marked over a period of less than 1 my. This short-lived episode of oceanic oxygen deficiency has been termed the Cenomanian-Turonian 'Oceanic Anoxic Event' (OAE). Marine sediments deposited during this event are, when compared with most of the Phanerozoic record, uncommonly rich in dark-grey to black, pyritic, laminated shales with total organic carbon contents that range from between 1 and 2% to greater than 20% which is largely of marine planktonic origin. The general lack of bioturbation in these beds is taken to indicate an absence of a burrowing fauna due to anoxic conditions. In coeval pelagic and shelf limestone sections the dark shales may be lacking; in such sections the Cenomanian-Turonian boundary is marked by δ 13C values of up to +4.0‰ or + 5.0‰ in contrast to δ 13C values of +2.0‰ to +3.0‰ in limestones directly above and below the boundary. The high δ 13C values are taken to indicate an enrichment of the global ocean in 13C values as a result of the preferential extraction of 12C by marine plankton, the organic components of which were not recycled back to the oceanic reservoir during this period of enhanced organic-carbon burial. In many basins benthonic foraminiferal faunas are lacking in strata at or near the Cenomanian-Turonian boundary or consist of depauperate agglutinate faunas whereas diverse planktonic foraminiferal faunas and radiolarian remains are locally abundant. These zones free of benthonic foraminifera have been previously interpreted as the result of bottom-water oxygen deficiencies. A correlation between high positive δ 13C values and manganese enrichment in shelf chalks has been pointed out by other workers; data presented here substantiates this correlation. Sediments that display one or more of the above characteristics have been studied and identified from diverse basinal settings such as Pacific Basin mid-ocean plateaus, North American cratonic interior seaways, European shelf and interior seaways, circum-African embayments and seaways, Tethyan margins and the Caribbean region. The oxygen-deficient water masses are proposed to have taken the form of an expanded and intensified oxygenminimum zone. Palaeobathymetric interpretation of strata from European and African shelf sequences and sections in the US Western Interior Basin show that shallow embayments, flooded by the rapid Cenomanian-Turonian transgression, were particularly favourable to deposition of anoxic sediments as were the neighbouring shelves and cratonic shallow seaways. The distribution of carbonaceous black shales and coeval light-coloured to red shallow-water limestones marked by a δ 13C 'spike' indicates that the upper surface of the widespread, intensified Cenomanian-Turonian oceanic oxygen-minimum zone was 100 to 200 metres below the surface of the sea in most areas; the lower surface was probably between 1.5 and 2.5 km below sea level. The main phase of the Cenomanian-Turonian OAE as exemplified by the Bonarelli Horizon in the Italian Apennines and the Black Band of Yorkshire and Humberside in England lasted less than 1 my. In some basins where coastal geometry and wind direction were effective in inducing strong upwelling conditions, the propensity for the deposition of carbon-rich facies increased and such facies were deposited in some predicted upwelling zones prior to and following the Cenomanian-Turonian OAE. However, the widespread distribution of anoxic sediments deposited synchronously during such a short-lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the Cenomanian-Turonian oxygen-minimum zone related to feedback between sea level rise and regional palaeoceanography. The palaeoceanography of the Cenomanian-Turonian OAE is discussed in detail in a companion paper by Arthur et al. 1987. © 1987 The Geological Society

The Cenomanian-Turonian Oceanic Anoxic Event, I. Stratigraphy and distribution of organic carbon-rich beds and the Marine delta 13C excursion.

Marine strata deposited during late Cenomanian and early Turonian time display lithological, faunal, and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient. At, or very close to, the Cenomanian-Turonian boundary, between 90.5 and 91.5Ma ago, oxygen deficiencies were particularly marked over a period of <1Ma. This short-lived episode of oceanic oxygen deficiency has been termed the Cenomanian-Turonian 'Oceanic Anoxic Event' (OAE). The widespread distribution of anoxic sediments deposited synchronously during such a short-lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the Cenomanian-Turonian oxygen-minimum zone related to feedback between sea level rise and regional palaeoceanography. The palaeoceanography of the Cenomanian Turonian OAE is discussed in detail in a companion paper by Arthur et al, 1987.-from Author

Campagne Mesopac: Leve de sites de forages ODP en sismique multitraces dans le bassin de Nauru

The Mesopac Cruise was the first multichannel seismic study of the western basins of the Pacific Plate. It was concentrated in the Nauru Basin and in the western area of the Central Pacific Basin. Profiles were calibrated with drilling results from DSDP Sites 462 and 169. For the first time one could observe reflectors within the Cretaceous volcanic complex down to approximately equals 8.5 seconds. Profiles did not allow direct observation of the top of the oceanic crust. After correcting for the load on top of the oceanic crust, results suggest that it cannot lie much deeper than those reflectors. If the oceanic crust lies only a few hundred meters beneath the 2.4 km thick complex, it would be right on the normal thermal subsidence curve corresponding to a Jurassic age

Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/Turonian boundary

Perhaps the most significant event in the Cretaceous record of the carbon isotope composition of carbonate1,2, other than the 1–2.5 ‰ negative shift in the carbon isotope composition of calcareous plankton at the Cretaceous/Tertiary boundary3, is the rapid global positive excursion of ~2 ‰ (13C enrichment) which took place between ~91.5 Myr and 90.3 Myr (late Cenomanian to earliest Turonian (C/T boundary event))1,4,5. This excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon (TDC) reservoir resulting from an increase in rate of burial of 13C-depleted organic carbon, which coincided with a major global rise in sea level5 during the so-called C/T oceanic anoxic event (OAE)6. Here we present new data, from nine localities, which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the C/T boundary7,8 is nearly synchronous with that for carbonate and is widespread throughout the Tethys and Atlantic basins (Fig. 1), as well as in more high-latitude epicontinental seas. The postulated increase in the rate of burial of organic carbon may have had a significant effect on CO2 and O2 concentrations in the oceans and atmosphere, and consequent effects on global climate and sedimentary facies