Deep Sea Drilling Research Articles

The structure of the incoherent sediments in the Ita Mai Tai Guyot (Pacific Ocean) based on geoacoustic profiling data

The results of geoacoustic studies carried out by the acoustic MAK-1M system in the southeastern Ita Mai Tai Guyot (Magellan Seamounts, Pacific Ocean) are discussed. The obtained geoacoustic sections characterize the incoherent part of the sedimentary sequence with high resolution. The geoacoustic stratification is well correlative with the deep-sea drilling data: the acoustic units are reliably correlated with the lithostratigraphic units defined in the cores of DSDP holes 200 and 202. Three geoacoustic units are distinguished in the sedimentary section (from the top downward): unit I corresponding in age to the terminal Miocene (Messinian)-Quaternary; unit II correlated with the Late Miocene (Tortonian); unit III accumulated in the Early Eocene-Early Miocene. The acoustic basement is composed of reefogenic limestones that were formed at different stages of the guyot’s development through the Aptian-Turonian to Late Paleocene-Eocene. The sonar images allow the distribution of the incoherent carbonate sediments to be contoured along the periphery of the summit’s plateau.

Passive margins: overview

Abstract Passive margins have been the reliable, accessible mainstay of exploration success worldwide for the last 25 years, and have hosted the spectacularly fast exploitation of deepwater resources (Angola, Nigeria, Brazil, Trinidad, USA Gulf of Mexico, Egypt, Australia and India). Despite, or perhaps because of this, there is still much to learn about the variety of hydrocarbon habitats they present. For example: (1) deep seismic observations and deep sea drilling have revealed more of the diversity of passive margins geodynamics. This liberates explorationists from simple geodynamic models, with consequences not only for new views of thermal history but also for the whole tectonic and stratigraphic evolution. For example, the time significance assigned to the geometries traditionally labelled ‘pre-rift, syn-rift and sag’ may be misleading. This has implications for correlations, the significance assigned to unconformities and sequence boundaries, heat flow and structural history. (2) New deep imaging of the sedimentary sections has revealed mistaken assumptions about the importance of ‘mobile substrate’ in major deltas and allowed the detailed unravelling of salt and shale movement and its implications for reservoir and trap. (3) Depositional models for deepwater reservoirs have increased in predictive capability and modern seismic imaging supports new models for shallow water sequences. (4) Discoveries of very large amounts of dry bacterial methane in stratigraphic traps have challenged old assumptions about prospectivity based on thermally matured source rocks. (5) New engineering and development technologies are opening up the commercialization of remote frontiers. As a consequence there is legitimate scope to re-visit old ‘dogmas’ and to propose that each passive margin segment is best regarded as unique, with analysis and interpretation rooted in observation rather than models (at least while the newly proposed models evolve to stability). Many of these themes were visited in the Passive Margins session of the Seventh Petroleum Geology Conference, held in London in 2009. This paper outlines some of these ideas, and considers how exploration along passive margins in the next decade can use new geoscience thinking.

Pliocene and Pleistocene dinoflagellate cyst and acritarch zonation of DSDP hole 610a, Eastern North Atlantic

A diverse and well‐preserved dinoflagellate cyst and acritarch record, comprising 35 genera and 104 species of dinoflagellate cyst and 14 acritarch taxa, is presented for the Lower Pliocene through Middle Pleistocene (4.00–0.53 Ma) of eastern North Atlantic Deep Sea Drilling Project Hole 610A. The Pliocene samples contain twice as many dinoflagellate cyst taxa than the Quaternary samples but far lower cyst concentrations. The disappearance of taxa during the latest Pliocene is likely to have been associated with global cooling. Eight biozones and four subzones are established, representing a level of detail that surpasses earlier studies. The zones are based mostly on highest occurrences, as these are most prevalent in the Pliocene and Lower Pleistocene. The subzones, based on both the acme and highest persistent occurrence of Habibacysta tectata, appear regionally useful within the Lower Pleistocene. The lowest occurrence of Impagidinium cantabrigiense is a potential marker for the uppermost Gelasian Stage (Lower Pleistocene). While this biozonation is intended primarily for use in the eastern North Atlantic, elements will be applicable across the higher‐latitude North Atlantic where calcareous microfossil zonations have reduced reliability and resolution. Most of the stratigraphically useful taxa are illustrated, and selected species are described informally.

Depth and controls of Ca‐rhodochrosite precipitation in bioturbated sediments of the Eastern Equatorial Pacific, ODP Leg 201, Site 1226 and DSDP Leg 68, Site 503

AbstractThe occurrence of early diagenetic Ca‐rhodochrosite [(Mn,Ca)CO3] is reported in association with ‘griotte’‐type nodular limestones from basinal settings in the geological record; however, without the comparison of analogous modern examples, the controls on precipitation remain speculative. Here the findings of four layers of primary Ca‐rich rhodochrosite recovered from a modern deep‐sea setting in the Eastern Equatorial Pacific, from bioturbated sediments 300 m below sea floor, are reported (Ocean Drilling Program, Leg 201, Site 1226). The mineralogy is similar to cements in burrows recovered during Deep Sea Drilling Project Leg 68 at Eastern Equatorial Pacific Site 503 and from Ca‐rhodochrosite laminae in sediments of the central Baltic Sea. Petrographic relationships and constant oxygen isotopic compositions in the Ca‐rhodochrosite around 5‰ at all depths indicate a shallow burial depth of formation. The onset of 1‰ heavier oxygen isotope composition of Ca‐rhodochrosite at Site 503, about 30 m below the Pliocene/Pleistocene boundary, further suggests that precipitation occurs in the range of 30 m below sea floor. The approximate depth of formation allowed an approximate empirical fractionation factor for marine Ca‐rhodochrosite to be constrained that strongly differs from previously published theoretical values. Based on the approximate precipitation depth, authigenic Ca‐rhodochrosite forms within the SO42−‐reduction zone. Moderately negative δ13C values (around −3‰) and total organic carbon lower than 2 wt% indicate a relatively low contribution of CO32− from organic C mineralization within the expanded redox zonation in the Eastern Equatorial Pacific. It is suggested that the alkalinity is increased by a rise in pH at focused sites of Mn‐reduction coupled with S2− oxidation. High concentrations of Mn‐oxide can accumulate in layers or burrows because of Mn‐cycling in suboxic sediments as suggested for the Baltic Sea Ca‐rhodochrosites. This study demonstrates how early diagenetic precipitates document biogeochemical processes from past diagenetic systems.

Intraspecific morphological variation with biometry of Distephanus speculum (Silicoflagellata)

Living specimens of Distephanus speculum (Silicoflagellata) were collected in waters of <150 m depth in the Pacific Sector of the Southern Ocean (57°31.6′S, 150°02.9′E), South of Tasman Island, and a biometric study has been conducted. The skeleton of D . speculum has a simple morphology, which can be readily expressed quantitatively. Using a new methodology, repeated measurements were obtained and converted to numerical indices. The circumference of the basal rings was the most useful character for subdivision into groups, and using this parameter we recognized two populations in the 620 studied specimens: (1) 500 individuals were placed in Group A, with a basal ring circumference of 53 to 67 µm, and a difference in length of spine axes from 2 to 6 μm; and (2) 120 individuals were placed in Group B (basal ring circumference 80 to 105 µm, difference in length of spine axes 3 to 6 µm). The circumference of the basal ring in Group B is approximately 10 µm smaller than in Distephanus speculum giganteus [Bukry, D., 1979. Comments on opal phytoliths and stratigraphy of Neogene Silicoflagellates and Coccoliths at Deep Sea Drilling Project Site 397 off Northwest Africa. Initial report of the DSDP, Washington, 49:977–1009.], and Distephanus boliviensis [Bukry, D., 1976a. Silicoflagellate and coccolith stratigraphy, Norwegian–Greenland Sea. Initial Report of DSDP LEG 38. Initial report of the DSDP, Washington, 38:843–855.; Bukry, D., 1976b. Silicoflagellate and coccolith stratigraphy, southeastern Pacific Ocean, Deep Sea Drilling Project Leg 34. Initial report of the DSDP, Washington, 34:715–735].

Advances in geochemistry during the last four decades: A personal perspective

Abstract This is the author’s speech at the meeting in Cologne (2007) to celebrate the 40th anniversary of the International Association of Geochemistry and Cosmochemistry, which the author served as the President in 2000 to 2004. The paper narrates the author’s personal involvement in important scientific programs during the last 4 decades, including implementation of isotope techniques, oil-and-gas research, diamond research, deep-sea drilling, space research, molecular biology and the origin of life.

Mid‐Miocene paleoproductivity in the Atlantic Ocean and implications for the global carbon cycle

A prominent, middle Miocene (17.5–13.5 Ma) carbon isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of 13C depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. Here we test the hypothesis that the marine δ13C record reflects a change in productivity with proxy records from three Atlantic Ocean sites (Deep Sea Drilling Program Site 608 and Ocean Drilling Program Sites 925 and 1265). Our multiproxy approach is based on benthic foraminiferal accumulation rates, elemental ratios (Ba/Al and P/Al), the δ13C of bulk sedimentary organic matter, and dissolution indices. We compare these proxies to benthic foraminiferal δ13C values measured on the same samples. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal δ13C excursion. A numerical box model confirms that marine productivity cannot account for the δ13C maximum. The model shows that sequestration of 1.5 × 1018 mol C in the terrestrial realm over a period of 3 Ma leads to a 0.9‰ δ13C increase in the deep ocean, which is near the observed records. Therefore, an increase in continental organic carbon sequestration is the most plausible way to enrich the ocean's carbon pool with 13C, which is consistent with coeval lignite deposits worldwide. The δ13C values of bulk sedimentary organic matter parallel the δ13C of dissolved inorganic carbon as reflected by benthic foraminiferal δ13C values suggesting no significant change in atmospheric pCO2 levels over the investigated period.

Ophiolites in ocean–continent transitions: From the Steinmann Trinity to sea-floor spreading

Before the theory of plate tectonics took hold, there was no coherent model for ocean–continent transitions that included both extant continental margins and fragmentary ancient examples preserved in orogenic belts. Indeed, during the early 1900, two strands of thought developed, one relying on the antiquity and permanence of continents and oceans, advocated by the mainstream of the scientific community and one following mobilist concepts derived from Wegener's hypothesis (1915) of continental drift. As an illustration of the prevailing North-American view, the different composition and thickness of continental and oceanic crust and the resulting isostatic response showed “how improbable it would be to suppose that a continent could founder or go to oceanic depth or that ocean floor at ± 3000 fathoms could ever have been a stable land area since the birth of the oceans” [H.H. Hess, Trans. R. Soc. London, A 222 (1954) 341–348]. Because of the perceived permanence of oceans and continents, mountain chains were thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following “zones of crustal weakness” within them, from which geanticlines and finally mountain belts would develop. This teleological concept, whereby a geosyncline would inevitably evolve into a mountain chain, dominated geological interpretations of orogenic belts for several decades in the mid-twentieth century. However, the concept of permanence of oceans and continents and the concept of the geosyncline had already met with the critiques of Suess and others. As early as 1905, Steinmann considered the association of peridotite, “diabase” (basalt/dolerite) and radiolarite (a typical ocean–continent transition assemblage), present in the Alps and Apennines, as characteristic of the deep-ocean floor and Bailey (1936) placed Steinmann's interpretation into the context of continental drift and orogeny. Indeed, in both authors’ writings, the concept of ophiolites as ocean crust is apparent. Between 1920 and 1930, the stage was thus potentially set for modern mobilist concepts that were, however, to prove attractive to only a small circle of Alpine and peri-Gondwanian geologists. After the Second World War, the 1950s saw the rapid progress of the geophysical and geological exploration of oceans and continental margins that provided the data for a reevaluation of the geosynclinal concept. Actualistic models now equated the former preorogenic miogeosyncline of Stille (1940) and Kay (1951) with passive continental margins [C.L. Drake, M. Ewing, G.H. Sutton, Continental margin and geosynclines: the east coast of North America, north of Cape Hatteras, in: L. Ahrens, et al. (Eds.), Physics and Chemistry of the Earth 3, Pergamon Press, London, 1959, pp. 110–189], the (American version of the) eugeosyncline and its igneous rocks with “collapsing continental rises” [R.S. Dietz, J. Geol. 71 (1963) 314–333] and the ophiolites, the Steinmann Trinity, of the (European) eugeosyncline with fragments of oceanic lithosphere [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620]. The concept of sea-floor spreading [H.H. Hess, History of ocean basins, in: Petrologic Studies: a Volume to Honor A.F. Buddington, Geol. Soc. Am., New York. 1962, pp. 599–620; H.H. Hess, Mid-oceanic ridges and tectonics of the sea-floor, in: W.F. Whittard, R. Bradshaw (Eds), Submarine Geology and Geophysics, Colston Papers 17, Butterworths, London, 1965, pp. 317–333] finally eliminated the weaknesses in Wegener's hypothesis and, with the coming of the “annus mirabilis” of 1968, the concept of the geosyncline could be laid to rest. Ocean–continent transitions of modern oceans, as revealed by seismology and deep-sea drilling, could now be compared with the remnants of their ancient counterparts preserved in the Alps and elsewhere.

Evolution of Middle America and the in situ Caribbean Plate model

Abstract Regional geological data and global analogues suggest Caribbean Plate geology continues that seen along the margin of eastern North America in a more extensional setting, between the diverging Americas. From west to east there are continental masses with Triassic rifts, proximal continental blocks with kilometres-thick Mesozoic carbonates, more distal areas of Palaeozoic horsts flanked by Triassic–Jurassic dipping wedges of sediments, including salt and overlain by Cretaceous basalts, and most distal areas of serpentinized upper mantle. Plate history began along with the Late Triassic formation of the Central Atlantic Magmatic Province and involved Triassic–Jurassic rifting, Jurassic–Early Cenozoic extension and Oligocene–Recent strike–slip. Great extension promoted volcanism, foundering, eastward growth of the plate by backarc spreading and distribution of continental fragments on the plate interior and along its margins. Hydrocarbons probably are present. Caribbean geology has important implications for understanding of oceanic plateaus, intra-oceanic volcanic arcs, the ‘andesite problem’ and genesis of ‘subduction’ HP/LT metamorphic rocks. The model can be tested by re-examination of existing samples and seismic data and by deep sea drilling.

PLIOCENE AND PLEISTOCENE DINOFLAGELLATE CYST AND ACRITARCH ZONATION OF DSDP HOLE 610A, EASTERN NORTH ATLANTIC

A diverse and well-preserved dinoflagellate cyst and acritarch record, comprising 35 genera and 104 species of dinoflagellate cyst and 14 acritarch taxa, is presented for the Lower Pliocene through Middle Pleistocene (4.00–0.53 Ma) of eastern North Atlantic Deep Sea Drilling Project Hole 610A. The Pliocene samples contain twice as many dinoflagellate cyst taxa than the Quaternary samples but far lower cyst concentrations. The disappearance of taxa during the latest Pliocene is likely to have been associated with global cooling. Eight biozones and four subzones are established, representing a level of detail that surpasses earlier studies. The zones are based mostly on highest occurrences, as these are most prevalent in the Pliocene and Lower Pleistocene. The subzones, based on both the acme and highest persistent occurrence of Habibacysta tectata, appear regionally useful within the Lower Pleistocene. The lowest occurrence of Impagidinium cantabrigiense is a potential marker for the uppermost Gelasian Stage (Lower Pleistocene). While this biozonation is intended primarily for use in the eastern North Atlantic, elements will be applicable across the higher-latitude North Atlantic where calcareous microfossil zonations have reduced reliability and resolution. Most of the stratigraphically useful taxa are illustrated, and selected species are described informally.

Deep sea drilling in the oceans: History and potentials (to 40th anniversary of the International Ocean Drilling Program)

Deep sea drilling in the oceans: History and potentials (to 40th anniversary of the International Ocean Drilling Program)

Who lives in the sea floor?

The sediments that blanket the sea floor contain tremendous numbers of microorganisms. This deep marine biosphere, which is probed by deep-sea drilling, is a new frontier for microbiologists and geochemists.

Age calibration of dinoflagellate cyst and acritarch events in the Pliocene–Pleistocene of the eastern North Atlantic (DSDP Hole 610A)

An independently calibrated record of dinoflagellate cyst and acritarch events is presented for the Early Pliocene through Middle Pleistocene (ca. 4.0–0.5 Ma) of eastern North Atlantic Deep Sea Drilling Project (DSDP) Hole 610A. A new age model is established for this hole and tied to marine isotope stratigraphy and magnetostratigraphy back to 3.6 Ma. New data on the range of the calcareous nannofossil Reticulofenestra pseudoumbilicus indicate that the base of this hole is about 1.0 Myr younger than previously thought. A diverse dinoflagellate cyst and acritarch record allows the significant highest and/or lowest occurrences of 19 dinoflagellate cyst and seven acritarch taxa to be recognised in Hole 610A and calibrated to the latest astronomically-tuned Neogene time scale (ATNTS 2004) via our new age model. Comparing records across the North Atlantic and Mediterranean reveals near-synchronous highest occurrences of the dinoflagellate cysts Ataxiodinium confusum (2.63–2.65 Ma), Invertocysta lacrymosa (2.72–2.74 Ma in the eastern and central North Atlantic and Mediterranean) and Impagidinium solidum (ca. 3.15–3.17 Ma), and the acritarch Leiosphaeridia rockhallensis (ca. 3.83–3.88 Ma). Highest occurrences of the dinoflagellate cyst Batiacasphaera minuta/micropapillata (3.83–ca. 3.7 Ma) and acritarch Cymatiosphaera latisepta (2.49–2.63 Ma) also provide useful markers for correlation. A precise stratigraphy for Hole 610A allows us to evaluate the impact of paleoceanographic and climatic events on the dinoflagellate cyst record. Climatic and oceanographic reorganizations associated with the onset of Northern Hemisphere glaciation appear responsible for the disappearance of many species between 2.8 and 2.6 Ma. The lowest occurrence of Impagidinium cantabrigiense (1.86 Ma) in the Olduvai Subchron is one of the few good biostratigraphic markers for the uppermost Gelasian in Hole 610A.

Rapid warming and salinity changes of Cretaceous surface waters in the subtropical North Atlantic

Paleomarine carbon isotopic records reveal occasional and rapid release of greenhouse gases into the atmosphere. The regional consequences on surface ocean properties and time relationships between the atmosphere and the ocean, however, are still far from understood. We observe a sudden -1.5‰ shift in leaf wax n -alkane 13C values at the onset of oceanic anoxic event (OAE) 1b from early Albian sediments off northwest Africa (Deep Sea Drilling Project Site 545) that is best explained by the rapid release of isotopically light carbon into the Cretaceous atmosphere. This -1.5‰ shift in the n -alkanes precedes a negative isotope excursion of similar magnitude observed in marine carbonate, organic matter, and algal steranes. A TEX86-based record of sea surface temperature (SST) confirms almost instantaneous warming by ∼3.5 °C along with the marine isotope shifts at Site 545, paralleled by an ∼2 °C TEX86-SST increase and a freshening of surface water salinity from 43 to 41 at Blake Nose, Ocean Drilling Program Site 1049. Multiproxy evidence indicates that, once established, these warm SSTs prevailed and were stable during OAE 1b, suggesting that the emission of 13C-depleted carbon started abruptly but then continued over tens of thousands of years. The SST cooled by ∼1–2 °C at the end of the event, not reaching pre-excursion levels. The new records provide evidence for a time lag (best estimate ∼1–3 k.y.) between atmospheric and oceanic processes that we interpret as a direct response to changes in the atmospheric greenhouse gas concentrations followed by propagation into the ocean and subsequent heating of surface waters.

NSF Shortens Drilling Season

A funding crunch is forcing the National Science Foundation (NSF) to shorten by 4 months annual deep-sea drilling operations beginning in 2009, according to Steven Bohlen of the Joint Oceanographic Institutions (JOI), the NSF-funded operator of the U.S. drill ship JOIDES Resolution. “Our operating costs are well beyond what we anticipated,” he says, due to the escalating costs of ship fuel, drilling gear, and maintenance. Add in NSF's commitments to support ocean-observing systems and non-drilling-ship operations, and “there are not sufficient funds to support the drill ship for science for the entire year,” says Bohlen. JOI will be pursuing work with petroleum companies and other science agencies that Bohlen hopes will fill in the looming gaps. “We're definitely scrambling here. We're worried,” he says. “Is it a big deal?” says Terry Schaff, director of government relations with Woods Hole Oceanographic Institution in Massachusetts. “It would be good if there was enough funds to run it for a whole year,” he says. But most ships that run U.S. academic oceanographic research run between 250 and 300 days a year, he points out. “Most of the ships haven't run a full year for a while. It's not a terribly unusual situation.”

Aurora Borealis—Development of a New Research Icebreaker with Drililng Capability

No abstract available. doi:10.2204/iodp.sd.5.10.2007

The effect of long-term low-temperature exposure on apatite fission track stability: A natural annealing experiment in the deep ocean

Since studies on deep-sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing. Due to sluggish annealing kinetics, this effect cannot be quantified by laboratory annealing experiments. The unknown amount of low-temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data, particularly for samples which experienced slow cooling in shallow crustal levels. To further elucidate these uncertainties, we studied volcanogenic sediments from five deep-sea drill cores, that were exposed to maximum temperatures between ∼10° and 70 °C over geological time scales of ∼15–120 Ma. Mean track lengths (MTL) and etch pit diameters (Dpar) of all samples were measured, and the chemical composition of each grain analysed for age and track length measurements was determined by electron microprobe analysis. Thermal histories of the sampled sites were independently reconstructed, based on vitrinite reflectance measurements and/or 1D numerical modelling. These reconstructions were used to test the most widely used annealing models for their ability to predict low-temperature annealing. Our results show that long-term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11%. Both chlorine content and Dpar values explain the downhole annealing patterns equally well. Low chlorine apatite from one drill core revealed a systematic relation between Si-content and Dpar value. The question whether Si-substitution in apatite has direct and systematic effects on annealing properties however, cannot be addressed by our data. For samples, which remained at temperatures <30 °C, and which are low in chlorine, the Laslett et al. [Laslett G., Green P., Duddy I. and Gleadow A. (1987) Thermal annealing of fission tracks in apatite. Chem. Geol. 65, 1–13] annealing model predicts MTL up to 0.6 μm longer than those actually measured, whereas for apatites with intermediate to high chlorine content, which experienced temperatures >30 °C, the predictions of the Laslett et al. (1987) model agree with the measured MTL data within error levels. With few exceptions, predictions by the Ketcham et al. [Ketcham R., Donelick R. and Carlson W. (1999) Variability of apatite fission-track annealing kinetics. III: Extrapolation to geological time scales. Am. Mineral. 84/9, 1235–1255] annealing model are consistent with the measured data for samples which remained at temperatures below ∼30 °C. For samples which experienced maximum temperatures between ∼30 and 70 °C, and which are rich in chlorine, the Ketcham et al. (1999) model overestimates track annealing.

Strength and stability analysis of deep sea drilling risers

In order to analyze the influence of vortex-induced vibration on the strength and stability of marine drilling risers, a simplified analytical model was established and a method for calculating vortex-induced dynamic response was developed. A case study indicated that at primary resonance, first-order dynamic displacement response is greater than higher-order mode responses, but the influence of dynamic moment and shearing force of higher-order modes cannot be ignored. The natural frequency of risers decreased sharply with increasing length of the riser, but increased with increasing top tension. Various factors should be considered to avoid vortex-induced vibration in practical application, and the influence of vortex-induced vibration could be estimated by calculating the fatigue lifetime of riser.

Tropical warming and intermittent cooling during the Cenomanian/Turonian oceanic anoxic event 2: Sea surface temperature records from the equatorial Atlantic

Oceanic anoxic event 2 (OAE‐2) occurring during the Cenomanian/Turonian (C/T) transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 Myr. However, the impact of this major perturbation on and interaction with global climate remains unclear. Here we report new high‐resolution records of sea surface temperature (SST) based on TEX86 and δ18O of excellently preserved planktic foraminifera and stable organic carbon isotopes across the C/T transition from black shales located offshore Suriname/French Guiana (Demerara Rise, Ocean Drilling Program Leg 207 Site 1260) and offshore Senegal (Cape Verde Basin, Deep Sea Drilling Project Leg 41 Site 367). At Site 1260, where both SST proxy records can be determined, a good match between conservative SST estimates from TEX86 and δ18O is observed. We find that late Cenomanian SSTs in the equatorial Atlantic Ocean (≥33°C) were substantially warmer than today (∼27°–29°C) and that the onset of OAE‐2 coincided with a rapid shift to an even warmer (∼35°–36°C) regime. Within the early stages of the OAE a marked (∼4°C) cooling to temperatures lower than pre‐OAE conditions is observed. However, well before the termination of OAE‐2 the warm regime was reestablished and persisted into the Turonian. Our findings corroborate the view that the C/T transition represents the onset of the interval of peak Cretaceous warmth. More importantly, they are consistent with the hypotheses that mid‐Cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide (CO2) and that major OAEs were capable of triggering global cooling through the negative feedback effect of organic carbon‐burial‐led CO2 sequestration. Evidently, however, the factors that gave rise to the observed shift to a warmer climate regime at the onset of OAE‐2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 Myr.

Overview of tectonic settings related to the rifting and opening of Mesozoic ocean basins in the Eastern Tethys: Oman, Himalayas and Eastern Mediterranean regions

Abstract A combination of geophysical studies and deep-sea drilling have in the past suggested that orthogonally rifted margins fall into two end-members: volcanic-rifted margins (e.g. eastern Greenland) and non-volcanic rifted margins (e.g. Iberia–Newfoundland conjugate). This paper explores the rifted margins of the Eastern Tethys stretching from the Eastern Mediterranean, through Oman to the Himalayas. Rifting in these area was typically pulsed, extending over more than 50 Ma. The timing of final continental breakup ranged from Late Permian in the east, in Oman and the Himalayas, to latest Triassic–earliest Jurassic in many parts of the Eastern Mediterranean (e.g. Antalya in SW Turkey; Pindos in Greece). Rifting in the Himalayas and Oman gave rise to a proximal to a distal ramp geometry with scatted seamounts (continental fragments and atolls) located adjacent to the rifted margin. The Eastern Mediterranean was palaeogeographically varied, and was characterized by a number of mainly elongate continental fragments (tens to several hundreds of kilometres long by tens of kilometres wide). These microcontinents subdivided the Eastern Tethys in the Eastern Mediterranean region into several small ocean basins, which rifted at more or less the same time in latest Triassic–earliest Jurassic time. All of the rifted margins of the Eastern Tethys are associated with rift-related volcanic rocks. However, with the exception of the Permian Panjal Traps in the Himalayas, the volumes of magma and corresponding thermal doming were less than for the ideal Volcanic-rifted margin (i.e. eastern Greenland). None of the Eastern Tethyan rifted margins show evidence of features characteristic of Non-volcanic rifted margins (e.g. sea-floor serpentinite exhumation), in contrast to the Iberia–Newfoundland conjugate or the Alps. Most of the Eastern Tethyan rifted margins appear to correspond to an ‘intermediate’ type, characterized by pulsed rifting, limited rift volcanism and a narrow continent–ocean transition zone. Such ‘intermediate-type’ rifted margins may remain to be explored in the modern oceans by deep-sea drilling. There is little evidence to support previous suggestions that the Eastern Tethyan rifts can be considered as back-arc basins above either northward- or southward-dipping subduction zones. Here it is suggested that the Eastern Tethys documents a fundamentally different type of rifting from either the ‘Volcanic-related’ or ‘Non-volcanic’ intracontinental rifts known from the Alps or the North Atlantic region. The dominant controls of rifting are seen as the traction of rising asthenosphere on the base of the lithosphere, related deviatoric tensional stresses, inherited and thermally induced weaknesses in the crust, and slab-pull. Specifically, in the Eastern Tethyan region continental breakup was probably triggered by a combination of long-term asthenosphere flow, slab-pull related to subduction beneath Eurasia and melt-induced crustal weakening associated with pulsed rifting or plume effects. Final continental breakup corresponds to a major (‘Cimmerian’) convergent phase along the opposing Eurasia margin, which further supports the role of plate boundary forces in Eastern Tethyan rifting. The early Mesozoic oceanic basins opened, probably associated with northwestward propagation of a spreading centre through the already weakened periphery of Gondwana, adjacent to less deformable Palaeotethyan oceanic crust. After a lengthy period of passive margin subsidence, locally punctuated by crustal extension and related volcanism, or plume effects, the rifted margins were finally tectonically emplaced during mid-Mesozoic, late Mesozoic or early Cenozoic time in different areas.