mid-Cretaceous Oceanic Anoxic Events Research Articles

Cretaceous oceanic red beds as possible consequence of oceanic anoxic events

Oceanic Anoxic Events (OAEs) represent brief periods of burial of large amounts of organic carbon in the oceans during Cretaceous time. Burial of organic carbon, which preferentially sequestered isotopically lighter carbon during OAEs, resulted in positive δ 13C excursions of 2–3‰ recognizable in global ocean. These deposits are typically dark–grey to black shales, considered to be the result of interaction between extreme warm climate, ocean circulation, high bioproductivity and organic carbon preservation. Less interest has been given to periods between OAEs, when oxic deep sea deposits such as red marls and red shales were deposited during mid- and Late Cretaceous. The latter deposits are associated with very low content of organic carbon and oxic depositional environments which dominated western Tethys in post Turonian time, up to the early Eocene. Feedbacks among geochemical cycles in response to decreasing global temperatures, increasing deep ocean circulation forced by high-latitude deep water formation along the Antarctic margin, and widening and deepening of the interconnections between the oceanic basins may have been responsible for the major paleoceanographic change from deposition of organic carbon-rich black shales during mid-Cretaceous, to world-wide deposition of Cretaceous Oceanic Red Beds (CORBs) in the Late Cretaceous. The presence of CORBs sandwiched between mid-Cretaceous OAEs may reflect major climate and paleoceanographic changes. In a contrast to extremely warm climates during the OAEs, the CORBs suggest cold periods, and therefore oscillating climate shifts, that have seldom been considered during modeling of Cretaceous greenhouse climate and global carbon cycling.

Benthic foraminifera and their role to decipher paleoenvironment during mid-Cretaceous Oceanic Anoxic Events – the “anoxic benthic foraminifera” paradox

Oceanic Anoxic Events (OAEs) are a widespread, recurring and well-studied feature of the mid-Cretaceous greenhouse climate, reflecting severe perturbations in Earth's climate and oceanography. To investigate bottom-water environments before, during and after these OAEs and to understand the dynamic fluctuations of seafloor environments, benthic foraminifera are a commonly employed tool. Here, a review of the response of benthic foraminiferal assemblages to OAEs (i.e., OAEs 1a, 1b, 1d, 2, and 3) will be given. I will discuss in detail how benthic foraminifera can be used to identify the paleoclimatic and paleoceanographic mechanisms responsible for OAE formation. The main feature of benthic foraminiferal assemblages during oceanic anoxic events is a dramatic decrease in both diversities and abundances or their total absence. However, some studies record the permanent occurrence of benthic foraminifera throughout OAEs or short-term repopulation events. These patterns clearly indicate that mid-Cretaceous OAEs are complex events. The paradox of anoxic (and even euxinic) conditions during OAEs coincident with the occurrence of benthic foraminifera (here called the “anoxic benthic foraminifera” paradox) is most readily explained by the sampling (and therefore temporal) resolution of typical foraminiferal studies that have homogenized hundreds to thousands of years in a single sample.

Pyrite oxidation during sample storage determines phosphorus fractionation in carbonate-poor anoxic sediments

We investigated the phosphorus (P) and iron (Fe) fractionation in four cores with anoxic sediments, deposited during the mid-Cretaceous oceanic anoxic event 2 (∼94 Ma) and the Paleocene–Eocene thermal maximum (∼55 Ma), that were exposed to oxygen after core recovery. Surprisingly, P associated with iron oxyhydroxides (Fe-bound P) was a major P phase in these laminated sediments deposited under euxinic conditions. A significant fraction of total Fe was present as (poorly) crystalline ferric Fe. This fraction increased with increasing storage time of the investigated cores. In carbonate-poor samples, Fe-bound P accounted for up to 99% of total P and its abundance correlated with pyrite contents. In samples with higher CaCO 3 contents (>5 wt% in the investigated samples), P was mostly present in authigenic Ca–P minerals, irrespective of pyrite contents. We conclude that the P fractionation in anoxic, carbonate-poor, sediments is strongly affected by pyrite oxidation that occurs when these sediments are exposed to oxygen. Pyrite oxidation produces sulfuric acid and iron oxyhydroxides. The abundance of poorly crystalline Fe oxyhydroxides provides further evidence that these were indeed formed through recent (post-recovery) oxidation rather than in situ tens of millions of years ago. The acid dissolves apatite and the released phosphate is subsequently bound in the freshly formed iron oxyhydroxides. Pyrite oxidation thus leads to a conversion of authigenic Ca–P to Fe-bound P. In more calcareous samples, CaCO 3 can act as an effective buffer against acidic dissolution of Ca–P minerals. The results indicate that shielding of sediments from atmospheric oxygen is vital to preserve the in situ P fractionation and to enable a valid reconstruction of marine phosphorus cycling based on sediment records.

C-isotope stratigraphy and paleoenvironmental changes across OAE2 (mid-Cretaceous) from shallow-water platform carbonates of southern Mexico

The stratigraphic and geochemical record of the mid-Cretaceous (Cenomanian–Turonian) Oceanic Anoxic Event 2 (OAE2) has been studied in numerous Tethyan and proto-Atlantic hemi-pelagic/pelagic successions, but little data comes from nearshore carbonate successions from the proto-Pacific region. Here we present the results of a combined stratigraphic and δ 13C study of C–T platform carbonates from southern Mexico, which were deposited within the proto-Pacific. Two scales of sedimentary cyclicity are recognized. High-frequency peritidal and subtidal cycles (0.4–8 m) display little evidence of cycle-capping subaerial exposure and are not correlative between sections; these relationships suggest that the amplitudes of high-frequency sea-level changes were minimal during the peak mid-Cretaceous greenhouse. Longer-term transgressive-regressive sequences (18–40 + m) are correlated between sections, and using δ 13C trends, can be correlated with sequences developed in northern Europe and India. The Mexican successions were sampled at a high resolution (~ 10 ky) for stable isotopes (inorganic, organic carbon and oxygen), total organic carbon, insoluble residues, and trace metals. The δ 13C carb curve matches global trends (including 6 distinct isotopic stages) permitting identification of OAE2 despite the lack of characteristic anoxic facies. Using the δ 13C carb trends, we tie the previously identified ammonite, planktonic foram, and nannofossil biostratigraphy from England and the Western Interior seaway of Colorado into the Mexican sections. The initiation of OAE2, defined by an abrupt positive 3–4‰ δ 13C shift, coincides with a long-term sea-level rise, though the sedimentary expression of the deepening is no greater than that observed for any of the other sea-level events across the studied interval. OAE2 termination (transition from gradually decreasing to background δ 13C values) is not associated with a particular sea-level trend. Stratigraphic changes in insoluble residues (proxy for continental sediment discharge) across OAE2 are not correlative between sections and do not show consistent systematic relationships with δ 13C or sea-level variations, therefore do not support the hypothesis that OAE2 was associated with increased continental-derived nutrient influx. Two peaks in trace metal concentrations coincide with the abrupt increase in δ 13C ratios (onset of OAE2) and during the transition from elevated-to-decreasing δ 13C values (near the C–T stage boundary). These trends are similar to those recorded in coeval deposits of the Western Interior seaway, and are consistent with the hypothesis that OAE2 development was related to the release of reduced metals during the short-lived (< 1 My) Caribbean oceanic plateau basalt eruption. In this scenario, oxidation of the metals depleted the existing low dissolved-O 2 concentrations and thermally-buoyant plumes of seawater enriched in biolimiting elements mixed with surface waters, stimulated primary productivity, and further reduced O 2 concentrations leading to widespread anoxia and a large positive δ 13C shift.

Mid-Cretaceous oceanic anoxic events in the Pacific Ocean revealed by carbon-isotope stratigraphy of the Calera Limestone, California, USA

The paleoceanographic history of the Pacific Ocean during the mid-Cretaceous is poorly constrained due to the loss of much of the contemporaneous Pacific seafloor to subduction and difficulties in recovering chert-rich sediments by ocean drilling. Pelagic sediments that were originally deposited in the Pacific Ocean but that have been subsequently accreted during subduction potentially provide an alternative paleoceanographic archive. This study presents micropaleontological and carbon-isotope data from the Calera Limestone, part of the classic Franciscan Complex exposed in Permanente Quarry, central California, USA. In the three stratigraphic sections studied, pelagic limestones with low organic-carbon contents are the dominant lithology. However, two stratigraphic intervals are recognized that contain organic carbon, and these date to the early Aptian and late Albian–early Cenomanian. These time intervals correspond to two mid-Cretaceous oceanic anoxic events (OAEs): early Aptian OAE1a (equivalent to the “Selli level”) and late Albian OAE1d (equivalent to the “Breistroffer event”). It is well established that both of these events were associated with significant carbon-isotope excursions, which are also shown to exist in the Calera Limestone. The record of OAE1a from the Calera Limestone complements ocean drilling records by providing further evidence for variability in the sedimentary and stratigraphic record of this event. The carbon-isotope data from the late Albian–early Cenomanian provide the first detailed chemobiostratigraphic record of this period for the Pacific Ocean, confirming that environmental change occurred at this time in the Pacific, possibly related to OAE1d.

Further examples of archaeal-derived hydrocarbons in mid-Cretaceous oceanic anoxic event (OAE) 1b sediments

Biomarker and total organic carbon (TOC) analyses were carried out on laminated black shales and massive marls from the Goguel level (OAE1a) and the Paquier and Kilian levels (OAE1b) of the Vocontian Basin of southeast France. TOC content was high (ca. 3–8%) throughout the Goguel, the Paquier and Kilian levels. Archaeal-derived hydrocarbons such as 2,6,15,19-tetramethylicosane (TMI) and 2,6,10,15,19-pentamethylicosane (PMI) were present in the OAE1b sediments, as reported previously [Kuypers, M.M.M., Blokker, P., Erbacher, J., Kinkel, H., Pancost, R.D., Schouten, S., Sinninghe Damsté, J.S., 2001. Massive expansion of marine archaea during a mid-Cretaceous oceanic anoxic event. Science 293, 92–94]. However, we did not detect these archaeal biomarkers in the Goguel level, confirming that the prevalence of archaea producing TMI and PMI was restricted to OAE1b.

Diazotrophic cyanobacteria as the major photoautotrophs during mid-Cretaceous oceanic anoxic events: Nitrogen and carbon isotopic evidence from sedimentary porphyrin

We determined both the nitrogen and carbon isotopic compositions of nickel-chelated deoxophylloerythroetioporphyrin (Ni DPEP), a major sedimentary porphyrin extracted from the Livello Selli and Livello Bonarelli black shales deposited in the western Tethys Sea during mid-Cretaceous oceanic anoxic events (OAEs). Based on empirical isotopic relationships between the tetrapyrrole nuclei of chlorophylls and photoautotroph cells, we estimate that the mean nitrogen isotopic composition of the entire photoautotrophic communities during these periods ranged from −2‰ to +1‰. This result strongly suggests that N 2-fixation was an important primary process in photoautotrophic production during these OAEs. The estimated carbon isotopic composition of the photoautotrophs was elevated (between −20‰ and −22‰) relative to typical Cretaceous examples, indicating as much as a 5‰ reduction in the magnitude of carbon isotopic fractionation associated with photosynthesis during OAEs in the western Tethys Sea. This anomaly can be well explained if cyanobacteria were the dominant producers because they commonly conduct β-carboxylation and/or active transport of carbon substrates, resulting in reduced carbon isotopic fractionation. We therefore conclude that diazotrophic cyanobacteria were the dominant components of primary production during OAE-1a and OAE-2 in the western Tethys Sea.

Iterative evolution of digitate planktonic foraminifera

Digitate shell morphologies have evolved repeatedly in planktonic foraminifera throughout the Cretaceous and Cenozoic. Digitate species are usually rare in fossil and modern assemblages but show increased abundance and diversity at times during the Cretaceous and middle Eocene. In this paper we discuss the morphology and stratigraphic distribution of digitate planktonic foraminifera and establish the isotopic depth ecology of fossil ones to draw parallels with modern counterparts. δ18O and δ13C values of six extinct and two modern digitate species, from six time slices (Cenomanian, Turonian, Eocene, Miocene, Pleistocene and Holocene) have similar isotopic depth ecologies, consistently registering the most negative δ13C and usually the most positive δ18O compared to coexisting species. These results indicate a similar deep, subthermocline (>150 m) habitat, characterized by lower temperatures, reduced oxygen, and enrichment of dissolved inorganic carbon. This is consistent with water-column plankton studies that provide insight into the depth preferences of the three modern digitate species; in over 70% of observations digitates occurred in nets below 150 m, and down to 2000 m. The correlation between digitate species and subsurface habitats across multiple epochs suggests that elongated chambers were advantageous for survival in a deep mesopelagic habitat, where food is usually scarce. Increased abundance and diversity of digitates in association with some early and mid-Cretaceous oceanic anoxic events, in middle Eocene regions of coastal and equatorial upwelling, and occasionally in some modern upwelling regions, suggests an additional link with episodes of enhanced ocean productivity associated with expansion of the oxygen minimum zone (OMZ). We suggest that the primary function of digitate chambers was as a feeding specialization that increased effective shell size and food gathering efficiency, for survival in a usually food-poor environment, close to the OMZ. Episodes of increased digitate abundance and diversity indicate expansion of the deep-water ecologic opportunity under conditions that were unfavorable to other planktonic species. Our results provide evidence of iterative evolution reflecting common functional constraints on planktonic foraminifera shell morphology within similar subsurface habitats. They also highlight the potential of digitate species to act as indicators of deep watermasses, especially where there was expansion of the OMZ.

The record of mid Cretaceous oceanic anoxic events from the Ionian zone of southern Albania

Along the Sopoti section of Mali Gjere mountain are found two distinct mid-Cretaceous marly-shaly-siliceous intervals that occur near the top of the Vigla Limestone Formation (Aptian–Albian) of the Ionian zone in southern Albania. The lower interval is relatively rich in carbonate content (36% CaCO 3 on average) and radiolaria (10% on average). It contains black shale levels rich in organic matter (up to 8.5% total organic carbon (TOC)) of marine origin, which did not experience any strong thermal maturation as suggested by their very low T max values. The age of the lower interval is latest Barremian to Early Aptian based on its calcareous nannofossil and radiolarian assemblages. It is therefore regarded as the equivalent of the Fourcade Level of Greece, reflecting the Oceanic Anoxic Event 1a (OAE1a). The upper interval is richer in both carbonate content (60% CaCO 3 on average) and radiolaria (20% on average) but it is practically devoid of any preserved organic matter. Its age straddles the Aptian–Albian boundary based on integrated biochronologic data of dinoflagellates, calcareous nannofossils and radiolaria. It is tentatively considered as the sedimentary expression of OAE1b ( sensu Leckie et al., 2002) in the Ionian zone of Albania. The presence of large Assipetra nannoliths in both shaly-siliceous intervals and the relative abundance of radiolaria suggest that their accumulation took place during periods of higher productivity in the Ionian zone of Albania.

Mid-Cretaceous Oceanic Anoxic Events recorded in SE France and central Italy

Mid-Cretaceous Oceanic Anoxic Events recorded in SE France and central Italy

Recent Advances in Research on Terrestrial and Marine Sequences From the Mid-Cretaceous Oceanic Anoxic Events (OAEs)

The Cretaceous period was a time of greenhouse climates characterized by sea-surface temperatures of at least 15°C warmer than today. During the climax of Cretaceous warming, anoxic conditions in the oceans expanded globally, resulting in accumulations of organic-rich sediments (socalled black shales). These events, called Oceanic Anoxic Events (OAEs), occurred at least eight times during the Cretaceous period (Fig. 1). Three important aspects make the study of OAEs so interesting: (a) the OAEs acted as a global thermostat because carbon to a large extent was sequestered in the sediments (black shales) instead of adding to the CO2 content of the atmosphere, (b) the expansion of anoxic conditions in the oceans caused mass extinctions of marine biota, and (c) most hydrocarbon source rocks from the Cretaceous formed under anoxic oceanic environments. The discovery of the Cretaceous OAEs (Schlanger and Jenkyns, 1976) was an important achievement of the Deep Sea Drilling Project (DSDP), and understanding Cretaceous OAEs is one of the major scientifi c objectives of the extreme climate initiative outlined in the Integrated Ocean Drilling Program (IODP) Initial Science Plan. To support this initiative a symposium entitled “Recent Advances in Research on Terrestrial and Marine Sequences from the midCretaceous Oceanic Anoxic Events (OAEs)” was held at Hokkaido University, Sapporo, Japan during 21–23 September 2005. The symposium was hosted by the 21 st Century Center of Excellence (COE) Program for the Neo-Science of Natural History, and was organized by Hisatake Okada (COE program leader) and Hiroshi Nishi. Thirteen talks were given, eight by invited speakers, and eight posters dealt with land and marine OAE sections shown in Figure 2. Discussion of land sections concentrated on two types of regions: (a) those with black shale, such as Umbria-Marche Basin, Western Interior, Northwest German Basin and Vocontian Basin; and (b) those devoid of black shale, such as the AngloParis Basin, Yezo Basin and Great Valley Basin. For regions of the fi rst type, the group discussed results of high-resolution studies on a millimeter to centimeter scale of microfossils and organic and inorganic geochemical analyses. It was demonstrated that the black shales in continental interiors and on the continental margins were deposited during marine transgressions, with individual laminae corresponding to the increased runoff from the land areas. For regions of the second type, attention was paid to the integrated analysis of macro- and microfossil and carbon isotope stratigraphies for identifi cation of the precise horizons of OAE levels. Although fully anoxic environments apparently did not develop in these basins, clear evidence of remarkable faunal turnover, involving various taxa, was shown at the OAE levels. Studies of the marine sections dealt with core data mainly from Blake Nose (ODP Leg 171B) and the Demerara Rise (ODP Leg 207). Both sections have yielded extremely well-preserved calcareous fossils suitable for isotopic geochemical analysis. Oxygen and carbon isotope data on planktic and benthic foraminifers revealed the development of two types of water-column structures during

MICROBIAL SURVIVAL: The Paleome: A Sedimentary Genetic Record of Past Microbial Communities

Molecular genetic methods were used to analyze the remnants of microbial ecosystems contained within an ancient oceanic microbial habitat that was recovered from a continental drilled core of black shale approximately 100 million years in age. Bacterial ribosomal RNA genes were vertically amplified from the six different depths of a black shale core associated with a phosphate-rich stratum, defined as one of the mid-Cretaceous oceanic anoxic events (OAEs). Although the black shale core was recovered from a terrestrial coring effort, the recovered 16S rRNA gene sequences showed affinity to microbial communities previously seen in deep-sea sedimentary environments (i.e., the microbial assemblage was easily recognizable as a marine community). In particular, a number of 16S rRNA gene clones of oceanic sulfate-reducing bacteria within the delta-Proteobacteria predominated at the OAE layer. The recovered bacterial DNA signatures are consistent with the interpretation that the sequences are derived from the past microbial communities buried in either sea-bottom or subseafloor environments during the sedimentation process and, after ceasing growth, preserved until the present.

Massive expansion of marine archaea during a mid-Cretaceous oceanic anoxic event.

Biogeochemical and stable carbon isotopic analysis of black-shale sequences deposited during an Albian oceanic anoxic event (approximately 112 million years ago) indicate that up to 80 weight percent of sedimentary organic carbon is derived from marine, nonthermophilic archaea. The carbon-13 content of archaeal molecular fossils indicates that these archaea were living chemoautotrophically. Their massive expansion may have been a response to the strong stratification of the ocean during this anoxic event. Indeed, the sedimentary record of archaeal membrane lipids suggests that this anoxic event marks a time in Earth history at which certain hyperthermophilic archaea adapted to low-temperature environments.

Quantitative analysis of calcareous microfossils across the Albian–Cenomanian boundary oceanic anoxic event at DSDP Site 547 (North Atlantic)

Nannofossil and planktonic and benthic foraminiferal assemblages were studied quantitatively in an expanded Albian to Cenomanian section at DSDP Site 547 in the eastern North Atlantic. The section spans a 0.5‰ positive δ13C excursion across the Albian–Cenomanian boundary, representing one of the mid-Cretaceous Oceanic Anoxic Events (OAE1d). The base of the carbon excursion at Site 547 coincides with a major faunal turnover in planktonic foraminifera, including extinctions and first appearances of several species as well as geographic entries. In contrast, nannofossil assemblage composition, which reflects conditions in the photic zone, shows little variation related to the carbon excursion, arguing against a noticeable change in surface water productivity during the event. Benthic foraminifera, which represent a paleowaterdepth of around 1km, show that there was no change in environmental conditions at that water depth, and offer no support for increased deep water anoxia. Combined with published data, microfossils at Site 547 suggest that improved preservation of organic matter due to a decrease in oxygen content in a shallow oxygen minimum zone is a more likely cause for the Albian–Cenomanian boundary anoxic event than an increase in productivity.

Correlating environmental changes during early Albian oceanic anoxic event 1B using benthic foraminiferal paleoecology

The nature and consequences of mid-Cretaceous oceanic anoxic events (OAEs) are the subject of ongoing debate, and recent studies have shown that different scenarios are needed to explain each of these events. Nevertheless, similarities between the different OAEs can be observed. Here, we have reconstructed paleoenvironmental changes during the early Albian OAE 1b using benthic foraminiferal distributions and lithologies in three sections from different basins and paleowater depths. Eutrophic conditions, as indicated by the presence of infaunal as well as opportunistic genera such as Gyroidinoides, Pseudobolivina, Pleurostomella and bolivinitids, prevailed before and during the OAE and led to dysoxic to anoxic conditions. Dysoxia was most intense in the bathyal sections but also occurred in the outer shelf where more heterogeneous patterns of foraminiferal distributions are believed to reflect fluctuations of the upper boundary of an oxygen-minimum zone. A change from eutrophic to mesotrophic conditions caused the termination of OAE 1b and opportunistic benthic foraminifera (e.g. Pseudobolivina, Pleurostomella) were the first to subsequently repopulate the bathyal sea floor. Repopulation occurred rapidly in the shallow settings and gradually in the deeper sites, where a normal, diverse pre-event fauna was established a few tens of thousands of years after OAE 1b.

Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the Early Mesozoic time–scale and the long–term behaviour of the planets

During the Late Triassic and Early Jurassic the Newark rift basin of the northeastern US accumulated in excess 5 km of continental, mostly lacustrine strata that show a profound cyclicity previousl...

Foraminiferal recovery after the mid-Cretaceous oceanic anoxic events (OAEs) in the Cauvery Basin, southeast India

Abstract The Cauvery Basin, SE India, is one of the best exposed late Mesozoic to Tertiary basins in India. The study of foraminiferal assemblages from the core samples obtained from two 120m deep wells in the basin records the occurrence of two mid-Cretaceous anoxic events in the basin. Abrupt increases in planktonic: benthonic ratios and reductions in benthonic diversity are recorded in the late Albian and in the late Cenomanian-early Turonian. These events coincide with worldwide oceanic anoxic events (OAEs). The OAEs had an impact on the microfauna of the basin, with approximately 35–45% of the benthonic species not surviving and major morphological changes occurring in the planktonic community. The late Cenomanian-early Turonian event was more significant than the late Albian event, with considerable readjustments occurring in the planktonic foraminifera. The genus Rotalipora disappeared and the genus Hedbergella was largely replaced by Whiteinella. Marginotruncana appeared for the first time and the Dicarinella population expanded considerably. Praeglobotruncana was the most tolerant genus, undergoing least change. The planktonic foraminifera evolved from small, weakly ornamented forms with poorly developed keels, into robust, well ornamented forms with well developed keels. The pattern of evolution of planktonic foraminifera suggests a recolonization of deeper water environments after the late Cenomanian-early Turonian anoxic event.