Hugh C. Jenkyns

Cretaceous anoxic events: from continents to oceans

Pelagic Cretaceous sediments, deposited in a range of palaeotectonic and palaeogeographic settings, from continents to oceans, are commonly black and bituminous. 3 particular time-envelopes define the major occurrences of such facies: late Barremian-Aptian-Albian, the Cenomanian-Turonian boundary and, to a lesser extent, the Coniacian-Santonian. These intervals define the duration of so-called Oceanic Anoxic Events during which global marine waters were relatively depleted in oxygen, and deposition of organic matter, derived from both terrestrial and planktonic sources, was widespread. Cretaceous OAEs correlate closely with transgressions, and such a correlation exists throughout the stratigraphical column. Flooding of land-masses is thought to have transported much terrestrial plant material seawards; the progressive increase in shelf-sea area is thought to have stimulated production of marine plankton. Bacterial consumption of this organic matter favoured the development of poorly oxygenated mid-to late Cretaceous waters in which many of the characteristic facies of the Period, including glauconitic sandstones and phosphatic chalks, were deposited.

Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event

In the Jurassic period, the Early Toarcian oceanic anoxic event (about 183 million years ago) is associated with exceptionally high rates of organic-carbon burial, high palaeotemperatures and significant mass extinction. Heavy carbon-isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon, which is isotopically light. In contrast, examples of light carbon-isotope values from marine organic matter of Early Toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12C versus 13 C (refs 1,2,5,6). Here, however, we report carbon-isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic, biospheric and atmospheric carbon reservoirs, and that this occurred despite the enhanced burial of organic carbon. We propose that—as has been suggested for the Late Palaeocene thermal maximum, some 55 million years ago—the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments.

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

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 CO 2 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.

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

Summary 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 δ13C values of up to +4.0‰ or + 5.0‰ in contrast to δ13C values of +2.0‰ to +3.0‰ in limestones directly above and below the boundary. The high δ13C 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 δ13C 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 δ13C ‘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.

Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography

Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios (87Sr/86Sr) and carbon-isotope ratios (δ13Ccarb and δ13Corg) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (δ18O) in carbonate, sulphur-isotope ratios (δ34S) in carbonate-hosted sulphate, nitrogen-isotope ratios (δ15Norg) in organic matter, osmium-isotope ratios (187Os/188Os) in black shales and neodymium-isotope ratios (143Nd/144Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ13C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium-isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon-isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.

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

Summary 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. 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.

Evidence for rapid climate change in the Mesozoic-Palaeogene greenhouse world.

The best–documented example of rapid climate change that characterized the so–called ‘greenhouse world’ took place at the time of the Palaeocene–Eocene boundary: introduction of isotopically light carbon into the ocean–atmosphere system, accompanied by global warming of 5–8 °C across a range of latitudes, took place over a few thousand years. Dissociation, release and oxidation of gas hydrates from continental–margin sites and the consequent rapid global warming from the input of greenhouses gases are generally credited with causing the abrupt negative excursions in carbon– and oxygen–isotope ratios. The isotopic anomalies, as recorded in foraminifera, propagated downwards from the shallowest levels of the ocean, implying that considerable quantities of methane survived upward transit through the water column to oxidize in the atmosphere. In the Mesozoic Era, a number of similar events have been recognized, of which those at the Triassic–Jurassic boundary, in the early Toarcian (Jurassic) and in the early Aptian (Cretaceous) currently carry the best documentation for dramatic rises in temperature. In these three examples, and in other less well–documented cases, the lack of a definitive time–scale for the intervals in question hinders calculation of the rate of environmental change. However, comparison with the Palaeocene–Eocene thermal maximum (PETM) suggests that these older examples could have been similarly rapid. In both the early Toarcian and early Aptian cases, the negative carbon–isotope excursion precedes global excess carbon burial across a range of marine environments, a phenomenon that defines these intervals as oceanic anoxic events (OAEs). Osmium–isotope ratios (187Os/188Os) for both the early Toarcian OAE and the PETM show an excursion to more radiogenic values, demonstrating an increase in weathering and erosion of continental crust consonant with elevated temperatures. The more highly buffered strontium–isotope system (87Sr/86Sr) also shows relatively more radiogenic signatures during the early Toarcian OAE, but the early Aptian and Cenomanian–Turonian OAEs show the reverse effect, implying that increased rates of sea–floor spreading and hydrothermal activity dominated over continental weathering in governing sea–water chemistry. The Cretaceous climatic optimum (late Cenomanian to mid Turonian) also shows evidence for abrupt cooling episodes characterized by episodic invasion of boreal faunas into temperate and subtropical regions and changes in terrestrial vegetation; drawdown of CO2 related to massive marine carbon burial (OAE) may be implicated here. The absence of a pronounced negative carbon–isotope excursion preceding the Cenomanian–Turonian OAE indicates that methane release is not necessarily connected to global deposition of marine organic carbon, but relative thermal maxima are common to all OAEs. ‘Cold snaps’ have also been identified from the Mesozoic record but their duration, causes and effects are poorly documented.

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

Carbon stable-isotope variation through the Cenomanian–Santonian stages is characterized using data for 1769 bulk pelagic carbonate samples collected from seven Chalk successions in England. The sections show consistent stratigraphic trends and δ13C values that provide a basis for highresolution correlation. Positive and negative δ13Cexcursions and inflection points on the isotope profiles are used to define 72 isotope events. Key markers are provided by positive δ13C excursions of up to + 2‰: the Albian/CenomanianBoundary Event; Mid-Cenomanian Event I; theCenomanian/Turonian Boundary Event; the Bridgewick, Hitch Wood and Navigation events of Late Turonian age; and the Santonian/Campanian Boundary Event. Isotope events are isochronous within a framework provided by macrofossil datum levels and bentonite horizons. An age-calibrated composite δ13C reference curve and an isotope event stratigraphy are constructed using data from the English Chalk. The isotope stratigraphy is applied to successions in Germany, France, Spain and Italy. Correlation with pelagic sections at Gubbio, central Italy, demonstrates general agreement between biostratigraphic and chemostratigraphic criteria in the Cenomanian–Turonian stages, confirming established relationships between Tethyan planktonic foraminiferal and Boreal macrofossil biozonations. Correlation of the Coniacian–Santonian stages is less clear cut: magnetostratigraphic evidence for placing the base of Chron 33r near the base of the Upper Santonian is in good agreement with the carbon-isotope correlation, but generates significant anomalies regarding the placement of the Santonian and Campanian stage boundaries with respect to Tethyan planktonic foraminiferal and nannofossil zones. Isotope stratigraphy offers a more reliable criterion for detailed correlation of Cenomanian–Santonian strata than biostratigraphy.With the addition of Campanian δ13C data from one of the English sections, a composite Cenomanian–Campanian age-calibrated reference curve is presented that can be utilized in future chemostratigraphic studies. The Cenomanian–Campanian carbon-isotope curve is remarkably similar in shape to supposedly eustatic sea-level curves: increasing δ13C values accompanying sea-level rise associated with transgression, and falling δ13C values characterizing sea-level fall and regression. The correlation between carbon isotopes and sea-level is explained by variations in epicontinental sea area affecting organic-matter burial fluxes: increasing shallow sea-floor area and increased accommodation space accompanying sea-level rise allowedmore efficient burial ofmarine organic matter, with the preferential removal of 12C from the marine carbon reservoir. During sea-level fall, reduced seafloor area, marine erosion of previously deposited sediments, and exposure of basin margins led to reduced organiccarbon burial fluxes and oxidation of previously deposited organic matter, causing falling δ13C values. Additionally, drowning of carbonate platforms during periods of rapid sea-level rise may have reduced the global inorganic relative to the organic carbon flux, further enhancing δ13C values, while renewed platform growth during late transgressions and highstands prompted increased carbonate deposition. Variations in nutrient supply, changing rates of oceanic turnover, and the sequestration or liberation of methane from gas hydrates may also have played a role in controlling carbon-isotope ratios.

New oxygen isotope evidence for long-term Cretaceous climatic change in the Southern Hemisphere

A new composite δ 18 O record, generated from calcareous fine-fraction and bulk sediments from the Exmouth Plateau, details long-term Cretaceous climatic change at mid-latitudes in the Southern Hemisphere. Assessment of new and previously published δ 18 O data indicates that a mid-Cretaceous global climatic optimum was achieved sometime between the time of the Cenomanian-Turonian boundary and the middle Turonian, when surface-ocean paleotemperatures were the highest of the past 115 m.y. Periods of cooling and warming that reversed the general patterns were superimposed on long-term Aptian-Turonian warming and Turonian-Maastrichtian cooling trends, respectively. Extrapolation of Southern Hemisphere paleotemperature trends to Maastrichtian paleotemperature data from a low-latitude Pacific guyot implies that maximum mid-Cretaceous low-latitude paleotemperatures could have been in excess of 33 °C.

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

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.