Cenomanian-Turonian Oceanic Anoxic Event Research Articles

Redox conditions and ecological resilience during Oceanic Anoxic Event 2 in the Western Interior Seaway

Oceanic Anoxic Events (OAEs) are important geological events that may be analogues to future climate-driven deoxygenation of our oceans. Much of the global ocean experienced anoxic conditions during the Cenomanian–Turonian OAE (OAE2; ∼94 Ma), whereas the Western Interior Seaway (WIS) experienced oxygenation at this time. Here, organic geochemical and palynological data generated from Cenomanian–Turonian age sediments from five sites in the WIS are used to investigate changing redox and ecological conditions across differing palaeoenvironments and palaeolatitudes. Heterogeneity across the sites is apparent, but important relationships and trends among oceanographic variables are recognised: 1) Increasing total organic carbon (TOC) and CaCO3 percentages indicate the onset of a sea-level maximum towards the end of OAE2; 2) C28 sterane is shown to be a useful marker for prasinophyte abundance, and concurrent increases in this marker and overall sterane abundance indicate prasinophyte-driven increase in algal productivity in a stratified water column; and 3) sterane ratios can be a more reliable geochemical proxy than redox proxies for assessing the Benthic Oxic Zone. Our redox data do not always follow established trends for the WIS overall, particularly for proximal settings. We therefore surmise that local effects, such as nutrient-driven expansion of the oxygen minimum zone and/or sedimentation-driven anoxia just below the sediment-water interface, have overprinted regional trends.

Isotopic evidence for changes in the mercury and zinc cycles during Oceanic Anoxic Event 2 in the northwestern Tethys, Austria

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) was one of the most extreme carbon cycle and climatic perturbations of the Phanerozoic Eon. Widespread deposition of organic-rich shales during OAE 2 has been attributed to a rapid rise in atmospheric CO2, global heating, and marine anoxia triggered by intense large igneous province (LIP) volcanism. Here, we present new Hg and Zn elemental and isotopic analyses from samples spanning OAE 2 in a hemipelagic section from Rehkogelgraben, Austria, which was part of the north-western Tethys. We compare our data to existing records from a range of sites to constrain the relative timing, magnitude and geographic extent of the perturbation. We find a prominent Hg concentration peak and an overall positive Δ199Hg excursion, with no correlation between Hg content and organic matter (OM), Mn-Fe-oxyhydroxides, and/or clay minerals. We interpret this to indicate a terrestrial volcanic origin of Hg. The Hg excursion is coincident with an osmium (Os) isotope excursion, and together, this supports a global period of intense LIP volcanism. The δ66Zn record from the Rehkogelgraben section decreases abruptly by ~0.5‰ prior to the onset of OAE 2, a change recorded consistently among all reference sections. Combined with the Hg data, we interpret this to result from isotopically light Zn sourced from LIP activity. However, the second negative excursion in δ66Zn during the Plenus Cold Event (PCE), which is recorded in the proto-North Atlantic and adjacent areas and has been attributed to Zn released from OM during re‑oxygenation, is not recorded in this section. We suggest that the cool, oxygenated deep water mass did not invade the Penninic Ocean in the northwestern Tethys. Alternatively, this excursion could be missing in our section due to the presence of carbonate-free sediments during the PCE. After the PCE, the positive excursion in δ66Zn recorded in all sections reveals a recovery of the atmosphere-ocean system. Our findings highlight the significance of spatial and temporal variations in Hg and Zn isotopes during OAE 2.

The Cenomanian-Turonian oceanic anoxic event (OAE-2) and continuous drowning up to the Santonian of the Western Valles-San Luis Potosi Platform, Central to Eastern Mexico: Biostratigraphy, chemostratigraphy and paleoenvironments

The Upper Cretaceous stratigraphic succession in the western part of the Valles-San Luis Potosi Platform (central to eastern Mexico) reflect the interaction between various paleo-oceanographic factors including sea-level change, tectonic factors, and the type and supply of sediments. The El Abra Formation is a shallow-water carbonate deposit that is overlain by the hemipelagic-pelagic Soyatal Formation, which represents the transition to a deeper, eutrophic, open marine environment in the latest Cenomanian-earliest Turonian (Whiteinella archaeocretacea Partial Range Zone). This unit is linked to a sea-level rise that occurred on a global scale and flooded the platform, coinciding with the Oceanic Anoxic Event 2 (OAE 2). This change is well–marked by faunal assemblages with a low diversity, and environmental stress–resistant species as well as values of delta 13C excursion, total organic carbon (TOC) and trace element signatures. These data suggest that in this time the ocean was oxygen deficient. During the early Turonian the platform was completely drowned, which led to re-establishment of the oligotrophic environment. These conditions favored for development of the planktonic foraminifera of the Helvetoglobotruncana helvetica Total Range Zone with the occurrence of keeled forms (k/strategists) even continuing as far as the late Santonian thus in the Soyatal interval two biostratigraphic zones are also recognized: the Dicarinella concavata Interval Zone (late Turonian-late Coniacian) and Dicarinella asymetrica Total Range Zone (early-late Santonian). This indicates that the studied part of the Valles-San Luis Potosi Platform remained flooded during this time, in contrast with Mediterranean Tethys regions where recovery of the shallow-water carbonate platforms occurred in the latest Turonian to mid-Coniacian. The planktonic foraminiferal assemblages of this interval are diverse, composed of large and complex keeled morphotypes, which indicate an open deep–sea environment.

Evidence for changes in sea-surface circulation patterns and ~20° equatorward expansion of the Boreal bioprovince during a cold snap of Oceanic Anoxic Event 2 (Late Cretaceous)

The Plenus Cold Event (PCE) temporarily interrupted the supergreenhouse conditions exacerbated during much of the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE 2). The PCE is coeval to the occurrence of Boreal benthic macroinvertebrates and of the nektonic belemnite Praeactinocamax plenus (after which the event is named) in Europe, and to the re-oxygenation of bottom waters in the Northern Hemisphere. However, its effects on the sea-surface circulation are unknown and evidence for changes in the biogeography of planktonic organisms are limited to the equatorward migration of the dinoflagellate cysts grouped in the Cyclonephelium compactum–membraniphorum morphological plexus. This study presents new planktonic foraminiferal quantitative data from two complete OAE 2 records of the Anglo-Paris (Eastbourne, SE England) and Vocontian (Clot Chevalier, SE France) basins that registered the equatorward pulse of Boreal macroinvertebrates during the PCE and have been extensively studied for bio- and chemostratigraphy. At the onset of OAE 2 (before the PCE), planktonic foraminifera are mainly represented by oligo-mesotrophic Tethyan taxa (rotaliporids and whiteinellids) in both localities, but this assemblage is sharply replaced by cold and meso-to-eutrophic species (praeglobotruncanids, dicarinellids and muricohedbergellids) during the PCE. The cold-water assemblage shows strong affinities with the coeval fauna of the Norwegian Sea and yields the Boreal endemic species Muricohedbergella kyphoma and Praeglobotruncana plenusiensis . This observation combined with previously published data collected in other localities of the Northern Hemisphere and on other fossil groups suggests a ~20° equatorward expansion of the Boreal marine bioprovince during the PCE. Moreover, contrarily to the nektonic belemnitellids that were able to move independently of ocean circulation, planktonic organisms are passively transported by currents and changes in the planktonic foraminiferal assemblages documented in the Anglo-Paris and Vocontian basins have been interpreted to reflect the transition from a dominant influence of warm, saline, and thermally stratified waters carried by the proto-Gulf Stream before the PCE to cold and low-saline Boreal waters originated in the Norwegian Sea during the PCE. We suggest that such changes were forced by the equatorward shift of the proto-Arctic Front (i.e., the boundary between warm saline Tethyan-Atlantic and cold low-saline Boreal waters) from offshore Norway to Southern England. In this southerly position, the proto-Arctic Front represented an oceanographic barrier that limited the influence of the proto-Gulf Stream in the Anglo-Paris Basin, and favored the inflow of Boreal waters from the North to the European epicontinental basins. The sea-surface cooling and equatorward expansion of Boreal planktonic assemblages during the PCE are of the same order of magnitude of those reconstructed between some glacial and interglacial intervals of the Plio-Pleistocene. Despite obvious differences between Cretaceous and Plio-Pleistocene paleogeography and climate dynamics, this study reviews the extent of environmental changes occurred during the PCE, provides evidence for a profound re-organization of the sea-surface circulation patterns and a more comprehensive overview of the equatorward migration of Boreal marine communities in the Northern Hemisphere. • Planktonic foraminiferal quantitative abundances across OAE 2 are provided for Eastbourne and Clot Chevalier. • Rotalipora and Whiteinella dominate the assemblages at the onset of OAE 2 and after the Plenus Cold Event (PCE). • Praeglobotruncana, Dicarinella and Muricohedbergella increase their abundance during the PCE • The assemblage of the PCE interval shows many affinities with the coeval Boreal fauna of the Norwegian Sea. • The biogeography of Boreal taxa and the proto-Arctic Front shifted equatorward of ~20° from the onset of OAE 2 to the PCE. • The shift of the proto-Arctic Front limited the inflow of the proto-Gulf Stream and favored that of Boreal waters in Europe.

Enhanced terrestrial runoff during Oceanic Anoxic Event 2 on the North Carolina Coastal Plain, USA

Abstract. A global increase in the strength of the hydrologic cycle drove an increase in the flux of terrigenous sediments into the ocean during the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE2) and was an important mechanism driving nutrient enrichment and thus organic carbon burial. This global change is primarily known from isotopic records, but global average data do not tell us anything about changes at any particular location. Reconstructions of local terrigenous flux can help us understand the role of regional shifts in precipitation in driving these global trends. The proto-North Atlantic basin was one of the epicenters of enhanced organic carbon burial during OAE2, so constraining terrigenous flux is particularly important in this region; however, few local records exist. Here, we present two new OAE2 records from the Atlantic Coastal Plain of North Carolina, USA, recognized with calcareous nannoplankton biostratigraphy and organic carbon isotopes. We use carbon / nitrogen ratios to constrain the relative contribution of marine and terrestrial organic matter; in both cores we find an elevated contribution from vascular plants beginning just before OAE2 and continuing through the event, indicating a locally strengthened hydrologic cycle. Terrigenous flux decreased during the brief change in carbon isotope values known as the Plenus carbon isotope excursion; it then increased and remained elevated through the latter part of OAE2. Total organic carbon (TOC) values reveal relatively low organic carbon burial in the inner shelf, in contrast to black shales known from the open ocean. Organic carbon content on the shelf appears to increase in the offshore direction, highlighting the need for cores from the middle and outer shelf.

Redox conditions, glacio-eustasy, and the status of the Cenomanian–Turonian Anoxic Event: new evidence from the Upper Cretaceous Chalk of England

The nature of the Cenomanian–Turonian Oceanic Anoxic Event (CTOAE) and its δ13 C Excursion is considered in the light of (1) the stratigraphical framework in which the CTOAE developed in the European shelf seas, (2) conclusions that can be drawn from new detailed investigations of the Chalk succession at three locations in England, at Melton Ross and Flixton in the Northern Province where organic-rich ‘black bands’ are present, and at Dover in the Southern Province (part of the Anglo-Paris Basin) where they are absent, and (3) how these conclusion fit in with the present understanding of the CTOAE. The application of the cerium anomaly method (German and Elderfield 1990) at Dover, Melton Ross and Flixton has allowed the varying palaeoredox conditions in the Chalk Sea and its sediments to be related to the acid insoluble residues, organic carbon, δ18O (calcite), δ13C (calcite), δ13C (organic matter), Fe 2+ and Mn2+ (calcite), and P/TiO2 (acid insoluble residue). This has provided evidence that the initial stages of the δ13C Excursion in England were related to (1) a drop of sea level estimated at between 45 and 85 metres, (2) influxes of terrestrial silicate and organic detritus from adjacent continental sources and the reworking of exposed marine sediments, and (3) the presence of three cold water phases (named the Wood, Jefferies and Black) associated with the appearance of the cold-water pulse fauna during the Plenus Cold Event. Conditions in the water column and in the chalk sediment were different in the two areas. In the Northern Province, cerium-enriched waters and anoxic conditions were widespread; the δ13C pattern reflects the interplay between the development of anoxia in the water column and the preservation of terrestrial and marine organic matter in the black bands; here the CTOAE was short-lived (~0.25 Ma) lasting only the length of the Upper Cenomanian Metoicoceras geslinianum Zone. In the Southern Province, water conditions were oxic and the δ13C Excursion lasted to the top of the Lower Turonian Watinoceras devonense Zone, much longer (~1.05 Ma) than in the Northern Province. These differences are discussed with respect to (1) the Cenomanian–Turonian Anoxic Event (CTAE) hypothesis when the ocean-continent-atmosphere systems were linked, (2) limitations of chemostratigraphic global correlation, and (3) the Cenomanian–Turonian Anoxic Event Recovery (CTOAER), a new term to define the varying lengths of time it took different oceans and seas to recover once the linked ocean-continent-atmosphere system was over. The possibility is considered that glacio- eustasy (the glacial control hypothesis of Jeans et al. 1991) with the waxing and waning of polar ice sheets, in association with the degassing of large igneous provinces, may have set the scene for the development of the Cenomanian–Turonian Anoxic Event (CTAE).

Cenomanian–Turonian oceanic anoxic event (OAE2) imprint on the northwestern part of the Adriatic Carbonate Platform and a coeval intra-platform basin (Istria and Premuda Island, Croatia)

The Cenomanian–Turonian boundary (CTB) on the intra-Tethyan Adriatic Carbonate Platform (AdCP) is generally characterised by a transition between microbially laminated and/or bioclastic limestones to calcisphere-rich massive limestone with bioturbated intervals, organic-rich interbeds, firmgrounds, as well as neptunian dikes, carbonate turbidites, tempestites and slumped structures. Compilation of the results from two study sites in the northwestern part of the AdCP and from previous research (on Istria Peninsula and islands in the Adriatic Sea in Croatia) provides a more complete overview of geological events and paleoenvironmental conditions that transformed the formerly contiguous shallow-marine environments during this time period. For the first time, a comparison between protected inner-platform area (Barban section) and a coeval intra-platform basin (Premuda Island section) during the CTB was made. This study utilized a combination of litho-, bio-, and microfacies studies with SEM, EDS, TOC, δ13C and δ18O stable isotope analyses. The stratigraphic successions start with shallow-marine carbonate deposits of the Milna Formation that is conformably overlain by the drowned-platform deposits of the Sveti Duh Formation on the platform and by the Veli Rat Formation in the contemporaneously developed intraplatform basin. These deposits are in turn overlain by the Gornji Humac Formation, which represents re-establishment of shallow-marine depositional systems on the AdCP, whereas the deeper water environment persisted in the intra-platform basin until the Santonian.Despite diagenetic modifications of shallow-marine carbonate deposits, the results of TOC and stable isotope analyses indicate the influence of global Oceanic Anoxic Event 2 (OAE2). Combination of local and regional synsedimentary tectonics and global Late Cretaceous sea-level changes accompanied by anoxic and hypoxic conditions, extinction of numerous benthic foraminifera, diversification and expansion of planktonic foraminifera and calcareous dinoflagellates, provide new insights into the character of the CTB interval in this part of the Tethyan realm.

Nannoplankton and 13C and 13O stable isotope stratigraphy record of the mid Cretaceous sequences, Zagros Basin (western Iran)

Deposition of black shales and dark argillaceous limestones across the Cenomanian to Turonian interval has been recorded in many parts of the world. In this regard, a stratigraphic section in the Iran's hydrocarbon field in the Kermanshah region in the Zagros Basin has been studied in detail to reconstruct palaeoceanographic conditions under a reliable biostratigraphic framework. Biostratigraphy based on calcareous nannofossils integrated with high resolution carbonate and oxygen stable isotope analyses were carried out on the Cenomanian-early Turonian interval of the upper part of Garau Formation. In this formation, 24 species of calcareous nannofossils were identified in the pelagic marine deposits. Our study allowed the definition of nannoplankton bio zones from the upper part of Eiffellithus turriseiffelii Zone to the lower part of Quadrum gartneri Zone in the mid Cretaceous. This research first recorded the upper part of the Garau Formation at site and it is consistent with the previous bio zones identified in the Tethyan domain in the region. The middle Cretaceous events of calcareous nannofossils proposed here correlate with the isotope global profile for the early Cenomanian-earliest Turonian interval and with those, reported for the southeastern France and south England reference curves, represented by the positive Cenomanian to Turonian δ13C excursion. Thus, the sequences of dark argillaceous limestone rich in organic matter in Kermanshah region can be considered as a continuation of the global trend in the Tethyan domain, where the Cenomanian-Turonian Oceanic Anoxic Event constrained to about 91.5 ± 8.6 millions of years, occurred in an anoxic pelagic sedimentary environment correlates with both, global and local sea-level rises, represented in this study by the characterization of the nannoflora assemblage reflected in a variety of widespread paleoenvironmental changes that coincides with significant shifts in the stable carbon and oxygen isotope signature.

Controls and timing of Cenomanian-Turonian organic enrichment and relationship to the OAE2 event in Morocco, North Africa

The Cenomanian-Turonian Oceanic Anoxic Event (OAE2) is a significant global event that has been linked to major source rock development in marine environments. The aim of this study is to integrate biostratigraphic, sedimentological and geochemical data (including stable carbon isotopes and trace elements) to document the timing and controls of organic matter enrichment across the Cenomanian-Turonian boundary.Morocco has excellent exposures of Tethyan carbonate platform and basinal facies and this paper details a high-resolution analysis of organic-rich Late Cretaceous intervals exposed in the Errachidia-Goulmima Basin, to investigate their relationship to the OAE2. Integrating planktic foraminiferal and ammonite biostratigraphy with carbon isotope correlations indicate that organic-rich black mudstones identified were deposited from latest Cenomanian to early Turonian, suggesting a late-OAE2 to post-OAE2 age.The sedimentology, redox water conditions and palaeoproductivity of the palaeoenvironments were studied to characterise the lithofacies and geochemical signatures, and to analyse the controlling factors for organic–enrichment. In the study area during the lower OAE2 interval, a shallow water mid-ramp environment prevailed, with predominantly bivalve-rich limestone deposition. Subsequent organic-rich mudstone development can be correlated with the latest Cenomanian (late OAE2 interval) and early Turonian (post-OAE interval) transgression, which allowed development of anoxic to euxinic conditions in the outer-ramp environment, characterized by high TOC values, from 2.7 to 17 wt%. Shallow platform locations continued to experience limestone deposition with no organic enrichment.Trace and major element data from the C/T black mudstones show an extremely high concentration in palaeoproductivity-sensitive (P, Zn, Ni and Cu) and redox-sensitive elements (V, Mo, and U). The redox environments evolved from dysoxia to anoxia and sulfidic euxinia, which is interpreted to have provided optimum conditions for organic carbon preservation in the Errachidia area.The results highlight the important interplay of local paleogeography and eustatic sea-level (the late Cenomanian/early Turonian transgressions) as the main controls on organic mudstone deposition, rather than simply global OAE2 control. The development of organic-rich mudstones reflects both increased productivity at this time and enhanced organic matter preservation due to the change in redox bottom water conditions.

Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)

Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin. Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.

Sedimentological, Paleontological and Mineralogical Evidences for Oceanic Anoxic Event-2 (OAE-2) in the Gulneri Formation (Early Turonian), Northeastern Iraq

In the current work, the Gulneri Formation (Early Turonian) is studied in three surface sections (Degala, Dokan and Azmir) from northeastern Iraq. The formation is characterized by grey to black color limestone and marly limestone in the Degala and Dokan sections, whereas in Azmir section, fissile marl and marly limestone with pale to reddish color are dominated in addition to few beds, which occasionally contain fish remains, with distinct gray color reflecting the very few amount of total organic carbon in these beds. Microfacies analysis revealed that the formation consists of three microfacies: mudstone, wackestone and packstone. The predominance of dwarfish planktonic foraminifera (Heterohilex) and thin shell filaments particularly in packstone microfacies represent Heterohelix shift event and filament event respectively. In addition to these events, fish remains, radiolarian pyritization, planktonic foraminiferal chambers elongation and glauconite are all refer to anoxic environmental conditions that may have been coincided with the Global Cenomanian-Turonian Oceanic Anoxic Event (OAE-2).

Zinc isotope evidence for paleoenvironmental changes during Cretaceous Oceanic Anoxic Event 2

AbstractPaleoclimate during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, 94.5–93.9 Ma) was characterized by elevated atmospheric CO2 concentrations and peak global temperatures. In this study, we employ δ66Zn measured on samples spanning OAE 2 in an expanded hemipelagic section in Tibet to trace changes in the major fluxes that influence biogeochemical cycles. The prominent feature of the δ66Zn record in the studied section is a continuous decrease from ∼1‰ at the onset of OAE 2 to a minimum of ∼0.2‰ within the Plenus Cold Event (ca. 94.3 Ma), followed by a stepwise recovery through the upper part of OAE 2. The negative shift in δ66Zn corresponds with higher terrigenous inputs, as revealed by previously published detrital index and TOC/TN (total organic carbon to total nitrogen) ratio records, and covaries with a notable decreasing trend recorded in compiled pCO2 data of different basins. We propose that influx of isotopically light Zn from weathered volcanic rocks associated with submarine large igneous provinces and/or (sub)tropic Indian continental volcanics is likely responsible for the δ66Zn decrease. We infer that the recovery of δ66Zn was caused by continued high primary production and an inevitable decline in the flux of light Zn as volcanic terrains were progressively weathered. The ultimate cessation of OAE 2 may have been a consequence of the same effect, with the nutrient supply from weathering reaching a minimum threshold to maintain productivity-anoxia feedback.

Regional chronostratigraphic synthesis of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) interval, Western Interior Basin (USA): New Re-Os chemostratigraphy and 40Ar/39Ar geochronology

AbstractFluctuations in depositional conditions during the onset of severe climate events in Earth history predispose stratigraphic archives to hiatuses, often hindering complete reconstructions of paleoclimate events and their triggers. Several studies have proposed that a hiatus of unknown duration exists at the base of Oceanic Anoxic Event 2 (OAE2) in the North American Western Interior Basin at the base Turonian global boundary stratotype section and point (GSSP) in Pueblo, Colorado, which potentially influences integrated radioisotopic, biostratigraphic, and astrochronologic age models of the Cenomanian-Turonian boundary interval. To quantify the duration of this hiatus, refine the chronology of OAE2, and assess marine geochemical perturbations associated with the onset of the event, we present new 40Ar/39Ar dates from regional bentonites along with a new proximal-distal chemostratigraphic transect of the epeiric Western Interior Basin (WIB), including initial osmium isotope (Osi) and stable carbon isotope (δ13C) data. The new 40Ar/39Ar age determinations confirm and further constrain previous estimates of Cenomanian-Turonian boundary timing. Further, the regional chemostratigraphic synthesis demonstrates the conformity of the OAE2 successions correlated to Pueblo, shows that the duration of the lag between the onset of the Osi and δ13C excursions is ∼60 k.y., and thus constrains the magnitude of the pre-OAE2 hiatus in Pueblo to less than this value. The new astronomically tuned, conformable Osi record across the onset of OAE2 captures a geologically rapid onset of large igneous province volcanism, consistent with other records, such that the addition of CO2 to the ocean-atmosphere system may have driven changes in marine carbonate chemistry. Additionally, the refined chronostratigraphy of OAE2 and the Cenomanian-Turonian boundary in the central WIB improves correlation with other records, such as those in the Eagle Ford Group, Texas. The correlations highlight that discrepancies among OAE2 age models from globally distributed sections commonly stem from differing definitions of the event and uncertainties associated with astronomical tuning, in addition to stratigraphic preservation.

Organic facies variability and paleoenvironmental changes on the Moroccan Atlantic coast across the Cenomanian—Turonian Oceanic Anoxic Event (OAE2)

The Cenomanian—Turonian boundary is marked by worldwide paleoenvironmental and paleoceanographical changes that are associated with a perturbation of the global carbon cycle and black shale deposition, called the Oceanic Anoxic Event 2 (OAE2). The objective of this study is to precisely characterize the nature of the organic matter, its variability, paleoenvironmental significance and the preservation conditions of two areas located along the northwest African Shelf where large-scale organic carbon burial contributed significantly to the global carbon burial associated with OAE2.With these premises, an organic petrographic and geochemical approach to the Cenomanian—Turonian record of the Moroccan Atlantic coast allowed the definition of the variability of the organic facies and the paleoenvironmental characterization of the upper Cenomanian—lower Turonian sediments of the Agadir section (Agadir Basin) and of the lower Turonian record of the Amma Fatma section (Tarfaya Basin).At Agadir, the marine microplankton dominated assemblages, with variable degree of amorphization, allowed the characterization of a transgressive-regressive cycle during the upper Cenomanian followed, during the lower Turonian, by a drowning event with establishment of a deep platform environment (under reducing conditions, increase in EFU, EFV, EFMo). The base of the succession (base of W. archaeocretacea biozone) and the top of the upper Cenomanian are characterized by two sea-level lowstands, and a high energy oxic environment, that culminate in emersion surfaces (high EFMn and low organic matter preservation).The organic facies in the lower Turonian of Amma Fatma is largely dominated by bacterial material (probably derived from sulfur bacteria), indicating an outer shelf setting, with some degree of restriction, under euxinic conditions (high EFMo) and high primary productivity (high EFCu, EFNi, EFZn).Differences in terms of the redox conditions between the two basins during the lower Turonian were determined, with anoxia prevailing at Agadir, while euxinic conditions are established at Tarfaya. These are explained by different oceanic circulation patterns, with the mixing of Tethyan waters at Agadir. High primary productivity is recorded at both locations, possibly promoted by the occurrence of upwelling currents. This study further corroborates a delay in the anoxia in Agadir, that during the isotopic excursion of OAE2 only reaches slight dysoxia (upper Cenomanian), and the maintenance of reducing conditions in Tarfaya post-OAE2.

Albian (Lower Cretaceous) carbon isotope chemozones, Texas Comanche Shelf and Mexican Chihuahua Trough: Implications for OAEs

Organic-carbon-rich black shale beds that are associated with positive carbon isotope excursions and yield pelagic fossils are evidence of short-term changes in ocean water mass and are important chronostratigraphic correlation tools. Five recognized Cretaceous Oceanic Anoxic Events (OAEs) are well documented in many basins on the Texas Comanche Shelf and the northern Mexico Chihuahua Trough: lower Aptian OAE 1a, Aptian-Albian OAE 1b, upper Albian OAE 1c, uppermost Albian OAE 1d, and Cenomanian-Turonian OAE 2. Two of these chemozones have previously been documented in Mexico and Texas, OAE 1b by a positive δ13C excursion of 2‰ and OAE 1d with a positive δ13C excursion of 1.6–1.9‰. Here we document for the first time OAE 1c in gray marine shale of the Boracho Formation in West Texas by a positive δ13C excursion of 1‰; the Boracho records the late Albian flooding of the Western Interior. On the interior shelf in shallow water carbonates the absence of carbon isotopic excursions suggests that dysaerobic waters did not affect the upper part of the water column. Review of biotic assemblages of each Gulf of Mexico Albian OAE suggests that water depths ranged from at least 300 m to just below storm wave base at about 15–30 m. These Albian OAEs were identified in the Mediterranean area and in the deep Atlantic Ocean and are recognized on the North American cratonic blocks, which supports the hypothesis that ocean/climate conditions favoring low-oxygen water masses were widespread although deposition was in local basins.

Neritic ecosystem response to Oceanic Anoxic Event 2 in the Cretaceous Western Interior Seaway, USA

Cretaceous oceanic anoxic events (OAEs) were periods of geologically short (<1 million years) global change characterized by elevated temperatures, changes in ocean biogeochemistry, ecological turnover, and the global-scale deposition of black shales. After decades of OAE research, the intensity and spatiotemporal heterogeneity of ocean anoxia and its direct effects on marine ecology remain areas of active study. We present high-resolution organic geochemical and foraminiferal records from the western margin of the Western Interior Seaway (WIS) during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2, ~94 Ma) that indicate reorganization of a neritic ecosystem in response to sea-level rise, and dynamic changes in redox conditions that were likely driven by enhanced marine productivity. A significant transgression prior to OAE2 decreased terrestrial organic matter input and led to enhanced productivity, anoxic bottom waters and sediments, and episodic photic zone euxinia. During the middle of OAE2, further enhanced productivity led to the “Heterohelix shift” in planktic foraminifera, the “Gavelinella acme” in benthic foraminifera, and deoxygenation in sediments, bottom waters, and the upper water column. The combined use of high-temporal resolution algal, bacterial, and terrestrial biomarkers, in addition to foraminiferal records, demonstrates the sensitivity of marine autotrophic and heterotrophic communities to sea-level rise and variable water column oxygenation in marginal marine settings during hyperthermal events. Furthermore, our results reveal the temporal and spatial heterogeneity of anoxia during OAE2 in the WIS. This study highlights widespread shallow marine feedbacks during global change that may affect neritic ecosystems under future warming scenarios.

Cenomanian–Turonian drowning of the Arabian Carbonate Platform, the İnişdere section, Adıyaman, SE Turkey

AbstractThe Cenomanian–Turonian carbonate ramp in the Adıyaman Region of SE Turkey (Northern Arabian Platform) records an abrupt shift from benthic carbonate deposits to pelagic deposits near the Cenomanian–Turonian boundary event (CTBE) in the İnişdere stratigraphic section and surrounding borehole sections. A positive δ13C excursion of up to 2.15% is recorded in carbonate and organic carbon deposited around the CTBE and provides evidence of a direct link between the CTBE and oceanic anoxic events and the demise of the shallow carbonate production in the Derdere Formation. The microfacies analyses, biostratigraphic dating and palaeoenvironmental interpretations suggest that the platform was drowned near the CTBE as a result of changing environmental conditions. The microfacies indicating significant deepening show a contemporaneity to equivalent surfaces globally and thus strongly support an isochronous formation of Cenomanian–Turonian facies by eustatic sea-level changes. Anoxia spreading over the platform drastically reduced the carbonate production as observed in the studied sections and, therefore, resulted in a reduction in carbonate accumulation rates. Regional/local subsidence and a coeval sea-level rise during the late Cenomanian to early Turonian interval were the cause of the drowning of the platform, including regional anoxia at the northern Arabian platform linked to the Cenomanian–Turonian oceanic anoxic event (OAE2).

Orbitally driven redox fluctuations during Cretaceous Oceanic Anoxic Event 2 (OAE2) revealed by a new magnetic proxy

Recent studies of the Cretaceous sedimentary records have documented that rapid redox changes between euxinic (sulfidic) and ferruginous conditions occurred during the perturbations to the global carbon cycle associated with oceanic anoxic events. This observation has major implications for the dynamics of and feedbacks on marine primary productivity and organic carbon burial during these events. The high-resolution rock magnetic study of a significantly expanded OAE2 interval in southern Tibet, presented here, provides further insight into the redox dynamics during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) (~94 Ma). The ratio of hard isothermal remanent magnetization to magnetic susceptibility, HIRM/χ, is used as a proxy for bottom-water redox conditions and shows repeated fluctuations between relatively oxic and dysoxic/anoxic conditions in the eastern Tethys of the Southern Hemisphere during OAE2. Spectral analysis of a log (HIRM/χ) time series reveals cyclical variations at orbital frequencies and the subtle cyclic redox changes are found to occur within the obliquity frequency band. Moreover, variability in redox conditions at the onset of OAE2 correlates well with that recently documented for the OAE2 interval at Tarfaya, Morocco, implying that the cyclic redox variations spanned both hemispheres. Notably, the presence of obliquity signals, which dominantly influence high-latitude climate processes, in the tropics and subtropics of both the Northern and Southern Hemispheres further confirms previously hypothesized more vigorous meridional overturning circulation (MOC) during OAE2 and implies the interhemispheric/global nature of this major shift in oceanic circulation.

Nitrogen and carbon cycle perturbations through the Cenomanian-Turonian oceanic anoxic event 2 (~94 Ma) in the Vocontian Basin (SE France)

Oceanic Anoxic Event 2, spanning the Cenomanian-Turonian boundary, was an episode of major perturbations in global biogeochemical cycles, characterized by the widespread burial of organic carbon in both oceans and epicontinental basins. In the Vocontian Basin (SE France), this event was expressed by the deposition of the Niveau Thomel, a succession of interbedded dark shales, grey marls and bioturbated carbonates as observed in the Lambruisse section. Analyses of the total organic carbon (TOC) content, combined with the concentrations of redox-sensitive trace elements, revealed that conditions fluctuated from oxic to suboxic-anoxic during the deposition of the Niveau Thomel. Transient periods of deoxygenation were related to perturbations in nutrient cycling reflected by near-zero to negative δ15N signatures indicative of increased diazotrophy.The development of deoxygenated conditions in the Vocontian Basin was likely preconditioned by hydrographically restricted conditions, as confirmed by Mo vs. U enrichments, reducing bottom-water ventilation associated with water-column stratification. Intervals of reoxygenation were likely linked to climate-induced changes. In particular, a phase of regional or global cooling associated with a drop in pCO2, called the Plenus Cold Event (PCE) resulted in vigorous reoxygenation of the Vocontian Basin.

Vegetation response to exceptional global warmth during Oceanic Anoxic Event 2

The Cenomanian–Turonian Oceanic Anoxic Event (OAE2; ~94.5 million years ago) represents an episode of global-scale marine anoxia and biotic turnover, which corresponds to one of the warmest time intervals in the Phanerozoic. Despite its global significance, information on continental ecosystem response to this greenhouse episode is lacking. Here we present a terrestrial palynological record combined with marine-derived temperature data (TEX86) across an expanded OAE2 section from the Southern Provençal Basin, France. Despite high TEX86-derived temperature estimates reaching up to 38 °C, the continental hinterland did support a diverse vegetation, adapted to persist under elevated temperatures. A transient phase of climatic instability and cooling during OAE2 known as Plenus Cold Event (PCE) is marked by the proliferation of open, savanna-type vegetation rich in angiosperms at the expanse of conifer-dominated forest ecosystems. A rise in early representatives of Normapolles-type pollen during the PCE marks the initial radiation of this important angiosperm group.