[1] Mid-Cretaceous (Barremian-Turonian) plankton preserved in deep-sea marl, organic-rich shale, and pelagic carbonate hold an important record of how the marine biosphere responded to short- and long-term changes in the ocean-climate system. Oceanic anoxic events (OAEs) were short-lived episodes of organic carbon burial that are distinguished by their widespread distribution as discrete beds of black shale and/or pronounced carbon isotopic excursions. OAE1a in the early Aptian (∼120.5 Ma) and OAE2 at the Cenomanian/Turonian boundary (∼93.5 Ma) were global in their distribution and associated with heightened marine productivity. OAE1b spans the Aptian/Albian boundary (∼113–109 Ma) and represents a protracted interval of dysoxia with multiple discrete black shales across parts of Tethys (including Mexico), while OAE1d developed across eastern and western Tethys and in other locales during the latest Albian (∼99.5 Ma). Mineralized plankton experienced accelerated rates of speciation and extinction at or near the major Cretaceous OAEs, and strontium isotopic evidence suggests a possible link to times of rapid oceanic plateau formation and/or increased rates of ridge crest volcanism. Elevated levels of trace metals in OAE1a and OAE2 strata suggest that marine productivity may have been facilitated by increased availability of dissolved iron. The association of plankton turnover and carbon isotopic excursions with each of the major OAEs, despite the variable geographic distribution of black shale accumulation, points to widespread changes in the ocean-climate system. Ocean crust production and hydrothermal activity increased in the late Aptian. Faster spreading rates [and/or increased ridge length] drove a long-term (Albian–early Turonian) rise in sea level and CO2-induced global warming. Changes in ocean circulation, water column stratification, and nutrient partitioning lead to a reorganization of plankton community structure and widespread carbonate (chalk) deposition during the Late Cretaceous. We conclude that there were important linkages between submarine volcanism, plankton evolution, and the cycling of carbon through the marine biosphere.

pdf/oceanic-anoxic-events-and-plankton-evolution-biotic-response-plitxfx522.pdf

Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous

A detailed carbon- and oxygen-isotope stratigraphy has been generated from Upper Cretaceous coastal Chalk sections in southern England (East Kent; Culver Cliff, Isle of Wight; Eastbourne and Seaford Head, Sussex; Norfolk Coast) and the British Geological Survey (BGS) Trunch borehole, Norfolk. Data are also presented from a section through the Scaglia facies exposed near Gubbio, Italian Apennines. Wherever possible the sampling interval has been one metre or less. Both the Chalk and Scaglia carbon-isotopic curves show minor positive excursions in the mid-Cenomanian, mid- and high Turonian, basal Coniacian and highest Santonian-lowest Campanian; there is a negative excursion high in the Campanian in Chalk sections that span that interval. The well-documented Cenomanian-Turonian boundary 'spike' is also well displayed, as is a broad positive excursion centred on the upper Coniacian. A number of these positive excursions correlate with records of organic-carbon-rich deposition in the Atlantic Ocean and elsewhere. The remarkable similarity in the carbon-isotope curves from England and Italy enables cross-referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the Turonian, Coniacian and Santonian stages. The oxygen-isotope values of the various Chalk sections, although showing different absolute values that are presumably diagenesis-dependent, show nonetheless a consistent trend. The East Kent section, which is very poorly lithified, indicates a warming up to the Cenomanian-Turonian boundary interval, then cooling thereafter. Regional organic-carbon burial, documented for this period, is credited with causing drawdown of CO2 and initiating climatic deterioration (inverse greenhouse effect). Data from other parts of the world are consistent with the hypothesis that the Cenomanian-Turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth.

/pdf/carbon-and-oxygen-isotope-stratigraphy-of-the-english-chalk-2soto6wbc9.pdf

Carbon- and oxygen-isotope stratigraphy of the English Chalk and Italian Scaglia and its palaeoclimatic significance

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 Authors

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

Correlation of the δ13C 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 δ13C 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 δ13C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in δ13C 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.

/pdf/the-cenomanian-turonian-oceanic-anoxic-event-ii-3190mefjsl.pdf

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

The Cretaceous is characterized by unusually widespread distribution of “black shales”-- sequences of variable lithology containing numerous beds with organic-carbon (OC) contents in excess of 1 percent by weight-- in both deep- and shallow-marine settings. General time envelopes of globally important organic-carbon burial during the Aptian-Albian, at the Cenomanian-Turonian boundary, and to a lesser extent in the Coniacian-Santonian, have been termed “Oceanic Anoxic Events (OAEs)”. The available stratigraphic and organic geochemical data and secular trends in the carbon isotopic composition of marine carbonates suggest that the timing of OC burial is broadly synchronous during these episodes and that the mass of OC buried during each is considerable. There may be other important episodes of more widespread OC burial, as yet poorly documented, such as the Valanginian- Hauterivian.

Stratigraphy, Geochemistry, and Paleoceanography of Organic Carbon-Rich Cretaceous Sequences

Summary A survey of DSDP black shales from the North Atlantic has been carried out in order to study the evolution of Cretaceous palaeoenvironments in the region. The study involved a reappraisal of the biostratigraphy of the Cretaceous formations and considered accumulation rate, mineralogical and organic geochemical data. Special attention was paid to distinguishing between redeposited and autochthonous sediments. Three main phases of deposition are recognized which are separated by two unconformities: The Blake-Bahama phase (Valanginian-early Aptian (Bedoulian)) corresponds to relatively uniform depositional conditions over the whole of the North Atlantic with periods of anoxia. The clay mineral assemblages reflect different sedimentary influxes from the continents into the eastern and western parts of the basin which were separated by the mid-ocean ridge. The transition to the Hatteras phase (Event E1) is marked by a drastic fall in sediment accumulation rate and by a rapid rise in the CCD but not by any appreciable change in clay mineral fluxes. The Hatteras phase (late Aptian-late Cenomanian) is characterized by distinctly diverse depositional conditions. In the southeastern part of the basin anoxic conditions were persistent, while in the northeast along the European margin there is no, or only very rare, evidence of anoxia. In the west, periods of oxia and anoxia alternated. Over the mid-ocean ridge conditions were oxic. A ‘barrier’ in the region of the present-day Bermuda Rise separated two areas of distinct clay mineral deposition. The transition to the Plantagenet phase (Event E2) corresponds to low sedimentation rates. It was immediately preceded by, or was coincident with, deposition of a black shale rich in marine organic matter. The Plantagenet phase (Turonian-Senonian) was a period of uniform sedimentation under oxic conditions. Possible causes of these changes in sedimentation are outlined.

Organic-rich sediments and palaeoenvironmental reconstructions of the Cretaceous North Atlantic

Une etude chronostratigraphique, sedimentologique et geochimique des black-shales recoupes par les forages DSDP dans l'Atlantique Nord, a ete menee avec la double preoccupation de reconstituer, d'une part les milieux de depot par l'etude des sequences elementaires et d'identifier, d'autre part, les unites stratigraphiques majeures et les discontinuites qui les separent. Les types de series illustrees dans ce texte a titre d'exemple sont les suivantes : I. Series a matiere organique marine conservee. - Series avec facies autochtones et resedimentes comportant tous de la matiere organique marine. - Series avec sediments autochtones a matiere organique marine mais avec sediments redeposes a matiere organique residuelle. Dans ces deux cas, la presence de matiere organique marine dans les sediments autochtones montre que le milieu de depot etait reduit (type euxinique). I I. Series a matiere organique terrigene ou residuelle. - Argilites noires ou alternance d'argilites noires et d'argilites vertes ou encore alternance d'argilites et de marnes noires. - Alternances de calcaires blancs (a matiere organique indeterminee) et de marnes silteuses (a matiere organique terrigene). La reconstitution du milieu de sedimentation n'implique pas necessairement la presence d'un milieu reducteur au lieu du depot, la matiere organique terrigene ayant une meilleure resistance a l'oxydation que la matiere organique marine, surtout avec un taux d'accumulation assez eleve. III. Series a facies alternant, dont l'un a matiere organique marine et l'autre a. matiere organique detritique residuelle. - Alternances d'argilites noires (a matiere organique marine) et verdâtres (a matiere organique indifferenciee). - Sequences avec, de bas en haut, marnes sombres laminees (matiere organique terrigene et marine associees), argilites noires (matiere organique terrigene) et argilites vertes (matiere organique residuelle). Plusieurs hypotheses sont avancees pour justifier de l'existence de telles recurrences. Les grands ensembles stratigraphiques reconnus peuvent se decrire ainsi A) Ensemble inferieur ou formation de Blake Bahama alternance de calcaires micritiques blancs a radiolaires et de calcaires argilo-silteux sombres (Valanginien-Bedoulien). B) Ensemble moyen ou formation de Hatteras : alternance d'argilites noires ou verdâtres, avec ou sans resediments carbonates (sommet Aptien superieur a Cenomanien moyen ou superieur. C) Ensemble superieur ou formation de Plantagenet argilites versicolores pour les sites profonds et craies, claires ou versicolores elles aussi, pour les autres sites (Turono-Senonien). L'ensemble inferieur correspond a des conditions de sedimentation assez homogenes d'un bord a l'autre du bassin vis-a-vis des conditions redox, avec des periodes de confinement periodique probable. Les parageneses argileuses montrent la differenciation entre un bassin oriental et un bassin occidental separes par la ride mediane, illustrant l'influence des continents sur les apports sedimentaires. Le passage a l'ensemble moyen (evenement E1) est marque par une remontee rapide de la CCD et souvent par une lacune, mais non par une variation sensible du spectre des mineraux argileux. La paleogeographie lors du depot de l'ensemble moyen est franchement diversifiee, avec (1) un bassin oriental situe entre la ride, les Canaries et l'Afrique, caracterise par le maintien des conditions euxiniques, (2) un bassin occidental, entre la ride et la marge americaine avec des alternances possibles de phases de confinement et de phases de circulation, (3) un bassin Nord, entre la ride et les marges marocaine et europeenne, sans indication positive de confinement, (4) la ride elle-meme ou le milieu etait plus nettement oxyde. Les smectites dominent largement sur les autres mineraux pendant cette periode ; toutefois l'etude attentive des parageneses argileuses suggere que la barriere des influences entre continents americain et africain coincidait avec une dorsale situee a l'ouest de la ride mediane et passant par les Bermudes. L'etude des parageneses mineralogiques completes suggere une plus grande ouverture dans le bassin americain situe a l'ouest de cette dorsale, vis-a-vis d'un confinement plus marque, observe a l'Est. Entre les ensembles moyen et superieur (evenement E² ), une nouvelle lacune frequente ou (et) une condensation sedimentaire sont soulignees par une accumulation d'argilites noires a matiere organique marine, repondant a une phase de stagnation generalisee a l'ensemble du bassin. Celle-ci persistera jusqu'au Coniacien dans un fond de bassin compris entre la ride mediane, les Canaries et la marge africaine. Une uniformisation des depots intervient le plus souvent apres l'evenement E². Elle conduit a des sediments oxydes, carbonates ou non suivant la profondeur, avec des parageneses mineralogiques comparables d'un puits a l'autre, temoignant d'une reprise, nuancee mais generale, du detritisme terrigene. Ce renouvellement se produit d'abord a l'Ouest de la dorsale au Sud des le Vraconnien (donc avant E²); ensuite a l'Est, en progressant a partir (superieur ?) au Nord jusqu'au Campanien Sud.

Les formations d'âge Crétacé de l'Atlantique Nord et leur matière organique : paléogéographie et milieux de dépôt

L'analyse chimique de 154 echantillons d'âge Aptien a Turonien, pour l'essentiel des argilites, en provenance des sites DSDP-IPOD 386 (situe sur la ride des Bermudes) et 391 (localise au pied de la marge americaine), met en evidence, d'une maniere independante des lithofacies et des compositions mineralogiques, un certain nombre d'influences enregistrees par les sediments : - influences du potentiel d'oxydo-reduction : enrichissements en Fe et Mn des depots oxydes ; en V, Ni, Co, Zn, Cu des sediments reduits, et notamment des argilites pyriteuses et tres carbonees deposees pendant l'evenement E2a la limite Cenomanien-Turonien ; - influences du detritisme terrigene, avec trois indices particulierement interessants : les comportements compares des alcalins (Na et K), la teneur en Ga des smectites, et le rapport D = AI/(AI + Fe + Mn) ; - influences du volcanisme, soit sous forme de pyroclastes (enrichissement en Ti et Cr), soit sous forme ionique par l'intermediaire de solutions interstitielles circulantes (enrichissement en Fe, Mn, Ba, Ni, Co, Cu, Pb) ; - influence du taux de sedimentation, mise en evidence grâce a Cr. Les resultats obtenus apportent des precisions interessantes sur l'evolution dans l'espace et dans le temps des milieux de depot et de diagenese. Etant donne, par ailleurs, que les sediments etudies se sont deposes dans des conditions de milieux deja connues grâce aux approches de la sedimentologie, de la mineralogie et de la geochimie organique, la reconnaissance des caracteristiques geochimiques de tels milieux pourra probablement aider a la reconstitution des conditions de depot pour des series d'âges et de facies comparables, mais transformees par un metamorphisme isochimique, telles les schistes lustres alpins.

Géochimie minérale de sédiments à faciès black-shales, Crétacé moyen de l'Atlantique Nord, sites DSDP 386 et 391

Organic geochemistry of ODP Leg 107 sediments

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Organic-rich sediments and palaeoenvironmental reconstructions of the Cretaceous North Atlantic

Les formations d'âge Crétacé de l'Atlantique Nord et leur matière organique : paléogéographie et milieux de dépôt

Géochimie minérale de sédiments à faciès black-shales, Crétacé moyen de l'Atlantique Nord, sites DSDP 386 et 391

Organic geochemistry of ODP Leg 107 sediments