Onset Of OAE Research Articles

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.

Exploring the paleoceanographic changes registered by planktonic foraminifera across the Cenomanian-Turonian boundary interval and Oceanic Anoxic Event 2 at southern high latitudes in the Mentelle Basin (SE Indian Ocean)

Planktonic foraminiferal population dynamics and benthic foraminiferal and radiolaria distributions combined with δ13C and δ18O measurements of both bulk carbonate and foraminifera provide clues concerning the paleoceanographic changes across the Cenomanian-Turonian boundary interval and the Oceanic Anoxic Event 2 (OAE 2) at southern high latitudes. Samples analyzed are from Integrated Ocean Discovery Program (IODP) Expedition 369 Site U1516 in the Mentelle Basin (eastern flank of the Naturaliste Plateau, Indian Ocean, SW Australia). Site U1516 was located at 60°–62°S paleolatitude during the mid-Cretaceous, and it is the first high latitude locality in the Southern Hemisphere where planktonic foraminifera are consistently recorded across the OAE 2 interval and its associated positive δ13C excursion. The sedimentary record at Site U1516 consists of a sequence of alternating black, dark greenish gray, and light greenish gray claystone in the Cenomanian that grade to white and light gray calcareous chalk interbedded with chert in the Turonian. The correlation between the δ13C and δ18O profiles at Site U1516 and the European reference section at Eastbourne (England) coupled with the integrated calcareous plankton biostratigraphy and stable isotopic data at Site U1516, indicate that a complete record of OAE 2 at Site U1516 was recovered. Below and in the lower part of OAE 2, the planktonic foraminiferal assemblages are dominated by small-sized (125–38 μm) opportunistic species of Microhedbergella and radiolaria indicating a dominantly eutrophic regime. Above the onset of OAE 2, a trough in the δ13C profile (Plenus Carbon Isotope Event: P-CIE) coinciding with a δ18O increase may correspond to the Plenus Cold Event as observed at low latitudes, although no evidence of cooling is registered in the microfossil assemblages. At Site U1516, the middle part of OAE 2 at the initiation of the plateau phase of the δ13C profile is masked by absence of carbonate, by the highest TOC values, and high biogenic silica (dominance of radiolaria) indicating this interval corresponded to a time of highly stressed eutrophic conditions with possible shoaling of the Carbonate Compensation Depth (CCD). Above this interval, bulk isotopic results yield lower δ13C values, and the CaCO3 increases are associated with the presence of even smaller-sized Microhedbergella showing cyclic fluctuations in absolute abundances with benthic foraminifera indicating dominantly eutrophic conditions likely affected by upwelling of nutrient-rich and δ13C-depleted intermediate water masses. Toward the top of OAE 2 and across the Cenomanian-Turonian boundary interval, the planktonic foraminiferal assemblages show changes in composition (e.g., Microhedbergella is replaced by Muricohedbergella), species occupying relatively deep ecological niches appear and an overall increase in diversity is observed. These features coupled with the foraminiferal species-specific δ13C and δ18O patterns reveal that Site U1516 occupied a paleoceanographic setting still affected by eutrophy likely related to enhanced input of nutrients but with episodes of stability with ecological/thermal separation in the surface waters. This interval also records the highest sea surface water paleotemperatures values estimated as 20°–23°C based on δ18O values of foraminiferal test and assuming seawater δ18O values of −1‰V-SMOW. Mesotrophic to oligotrophic conditions persisted after the OAE 2 and throughout the Turonian as evidenced by a diverse planktonic foraminiferal assemblage with different species occupying separate ecological niches in the mixed layer and thermocline.

Carbon Cycling During Oceanic Anoxic Event 2: Compound‐Specific Carbon Isotope Evidence From the Western Interior Seaway

AbstractThe Cenomanian‐Turonian Oceanic Anoxic Event 2 (OAE2, ∼94 Ma) was a period of widespread ocean deoxygenation and marine organic carbon burial. Increased CO2and nutrient flux before OAE2 are generally considered the main drivers of ocean anoxia and carbon burial, though some evidence has suggested multiple phases of CO2input and burial throughout the event. To test hypotheses about the nature of carbon cycle perturbations before and during OAE2, we analyzed stable carbon isotopes in marine‐derived and terrestrial‐derived biomarkers from an expanded sedimentary record of OAE2 from the western margin of the Western Interior Seaway. Biomarker carbon isotope data were used to estimate the concentration and carbon isotopic composition of aqueous and atmospheric CO2, as well as changes in marine productivity, through OAE2. While complicated by biological and environmental uncertainties, ourpCO2reconstructions generally agree with estimates from other locations and proxies around the world. High‐resolution sampling revealed several short‐lived carbon cycle perturbations before and during OAE2 that may reflect fluctuations in the global carbon cycle, including several pulses of isotopically light carbon before OAE2, during the onset of OAE2, and during the Plenus event. This study provides new constraints on carbon cycle dynamics during OAE2 that highlight the complex nature of the event and its causes.

A new view on the evolution of seawater molybdenum inventories before and during the Cretaceous Oceanic Anoxic Event 2

Extreme periods of global warming in Earth's history have been strongly associated with declining oxygen concentrations in the oceans, a scenario not unlike the evolution observed in the oceans in recent times. One of the most intense Phanerozoic deoxygenation events (so called Ocean Anoxic Events (OAE)) is the late Cretaceous OAE2 approximately 94 Ma ago. Although several studies have investigated the evolution of redox sensitive geochemical proxies during the OAE2 event, geochemical records that span the time period leading up to OAE2, in particular for molybdenum (Mo) and its isotopes are rare. Here, we investigate Mo cycling in the Tarfaya upwelling system in the Cretaceous proto-North Atlantic Ocean before and during OAE 2 throughout a 5 Ma record. The observed changes in sedimentary Mo isotope compositions can be explained by a mode of Mo cycling similar to that in modern oxygen minimum zones. Based on this interpretation we estimate the pre-OAE2 Mo isotope composition of seawater to be 1.6‰ indicating that late Cretaceous pre-OAE2 seawater had a lighter Mo isotope composition than the modern ocean. We suggest that Mo burial in oxic sediments was likely diminished under conditions of globally reduced oxygen concentrations. This allows us to model the Mo isotope composition of late Cretaceous pre-OAE2 seawater without lowering of the seawater Mo inventory (e.g., −55% oxic sink, + 40% anoxic sink). Previous estimations of the Mo seawater isotope composition during OAE2 are close to the signal observed pre-OAE here. The Mo isotope variation associated with the onset of OAE2 is therefore small, which is unexpected because strong expansion of anoxic sedimentation in OAE2 is indicated by e.g. sulfur isotopes. A further decrease of Mn burial rates in the open ocean during OAE2 could “buffer” the Mo seawater isotope signal during OAE2, to account for the small offset. Our findings emphasize that changes to the relative size of not only the anoxic sink but also the oxic sink are important considerations when interpreting paleo-Mo isotope data.

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.

Obliquity pacing of the hydrological cycle during the Oceanic Anoxic Event 2

The Oceanic Anoxic Event 2 (OAE2, ca. ∼93.5 Ma) represents a major phase of environmental change during the Mesozoic, and is associated with a pronounced positive excursion in the carbon-isotope record. Short-term climate oscillations within the OAE2 are recorded as Milankovitch cycles, which have been used to establish a precise temporal framework for the OAE2. However, few studies discuss the sedimentary expression of Milankovitch cycles during the OAE2, and its paleoenvironmental implications. Here we present carbonate and organic-carbon isotope data from a biostratigraphically well-dated, organic-rich OAE2 interval in a sedimentary succession outcropping in the Briançonnais Domain at Roter Sattel (Fribourg Prealps, Switzerland). We sampled the OAE2 interval (4.28 m) for proxies of detrital sediment quantification (Al, Ti, magnetic susceptibility) at ultra-high resolution (1 cm). Time-series analysis of multiple detrital proxies permits the construction of an orbital timescale for the OAE2 based, for the first time, on the stable 173 kyr (s3–s6) obliquity modulation cycle. The resulting OAE2 orbital timescale at Roter Sattel is consistent with previous timescales obtained in the Western Interior Basin, and in southern Tibet (China). Our cyclostratigraphic results show an unusually strong obliquity signal during the initiation of OAE2. Previous studies have demonstrated that the onset of OAE2 was associated with magmatic pulses coupled with increases in atmospheric pCO2, followed by an overall, gradual drawdown of CO2. Accordingly, we suggest that detrital input during the OAE2 was the result of intensified continental weathering related to magmatic activity, the substantially release of greenhouse gases, and an accelerated hydrological cycle modulated by obliquity cycles.

Sedimentary Mercury Enrichments as a Marker for Submarine Large Igneous Province Volcanism? Evidence From the Mid‐Cenomanian Event and Oceanic Anoxic Event 2 (Late Cretaceous)

AbstractOceanic Anoxic Event 2 (OAE 2), during the Cenomanian‐Turonian transition (∼94 Ma), was the largest perturbation of the global carbon cycle in the mid‐Cretaceous and can be recognized by a positive carbon‐isotope excursion in sedimentary strata. Although OAE 2 has been linked to large‐scale volcanism, several large igneous provinces (LIPs) were active at this time (e.g., Caribbean, High Arctic, Madagascan, Ontong‐Java) and little clear evidence links OAE 2 to a specific LIP. The Mid‐Cenomanian Event (MCE, ∼96 Ma), identified by a small, 1‰ positive carbon‐isotope excursion, is often referred to as a prelude to OAE 2. However, no underlying cause has yet been demonstrated and its relationship to OAE 2 is poorly constrained. Here we report sedimentary mercury (Hg) concentration data from four sites, three from the southern margin of the Western Interior Seaway and one from Demerara Rise, in the equatorial proto‐North Atlantic Ocean. We find that, in both areas, increases in mercury concentrations and Hg/TOC ratios coincide with the MCE and the OAE 2. However, the increases found in these sites are of a lower magnitude than those found in records of many other Mesozoic events, possibly characteristic of a marine rather than atmospheric dispersal of mercury for both events. Combined, the new mercury data presented here are consistent with an initial magmatic pulse at the time of the MCE, with a second, greater pulse at the onset of OAE 2, possibly related to the emplacement of LIPs in the Pacific Ocean and/or the High Arctic.

Empirical links between trace metal cycling and marine microbial ecology during a large perturbation to Earth's carbon cycle

Understanding the global redox state of the oceans and its cause-and-effect relationship with periods of widespread organic-carbon deposition is vital to interpretations of Earth's climatic and biotic feedbacks during periods of expanded oceanic oxygen deficiency. Here, we present a compilation of new and published data from an organic-rich locality within the proto-North Atlantic Ocean during the Cenomanian–Turonian boundary event that shows a dramatic drawdown of redox-sensitive trace elements. Iron geochemistry independently suggests euxinic deposition (i.e., anoxic and sulfidic bottom waters) for the entire section, thus confirming its potential as an archive of global marine metal inventories. In particular, depleted molybdenum (Mo) and vanadium (V) concentrations effectively record the global expansion of euxinic and oxygen-deficient but non-sulfidic waters, respectively. The V drawdown precedes the OAE, fingerprinting an expansion of oxygen deficiency prior to an expansion of euxinia. Molybdenum drawdown, in contrast, is delayed with respect to V and coincides with the onset of OAE2. Parallel lipid biomarker analyses provide evidence for significant and progressive reorganization of marine microbial ecology during the OAE in this region of the proto-North Atlantic, with the smallest relative eukaryotic contributions to total primary production occurring during metal-depleted intervals. This relationship may be related to decreasing supplies of enzymatically important trace elements. Similarly, box modeling suggests that oceanic drawdown of Mo may have approached levels capable of affecting marine nitrogen fixation. Predictions of possible nitrogen stress on eukaryotic production, locally and globally, are consistent with the low observed levels of Mo and a rise in 2-methylhopane index values during the peak of the OAE. At the same time, the environmental challenge presented by low dissolved oxygen and euxinia coincides with increased turnover rates of radiolarian clades, calcareous nanofossils, and foraminifera, suggesting that the temporal patterns of anoxia/euxinia and associated nutrient limitation may have contributed to the fabric of OAE2-related turnover.

Iron limitation in the Western Interior Seaway during the Late Cretaceous OAE 3 and its role in phosphorus recycling and enhanced organic matter preservation

The sedimentary record of the Coniacian–Santonian Oceanic Anoxic Event 3 (OAE 3) in the North American Western Interior Seaway is characterized by a prolonged period of enhanced organic carbon (OC) burial. This study investigates the role of Fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black shale deposition within the Coniacian–Santonian-aged Niobrara Formation in the USGS #1 Portland core. Iron speciation results indicate the development of a reactive Fe limitation coeval with reduced bioturbation and increased organic matter preservation, suggesting that decreased sulfide buffering by reactive Fe may have promoted enhanced organic matter preservation at the onset of OAE 3. An Fe limitation would also provide a feedback mechanism to sustain elevated primary productivity through enhanced phosphorus recycling. Additionally our results demonstrate inconsistencies between Fe-based and trace metal redox reconstructions. Iron indices from the Portland core indicate a single stepwise change, whereas the trace metal redox proxies indicate fluctuating redox conditions during and after OAE 3. Using Fe speciation to reconstruct past redox conditions may be complicated by a number of factors, including Fe sequestration in diagenetic carbonate phases and efficient sedimentary pyrite formation in a system with limited Fe supply and high levels of export production.

Basin-scale controls on the molybdenum-isotope composition of seawater during Oceanic Anoxic Event 2 (Late Cretaceous)

It is well established that the burial of organic carbon in marine sediments increased dramatically at a global scale at the Cenomanian–Turonian boundary (Oceanic Anoxic Event 2: OAE-2, ∼94Myr ago, Late Cretaceous). Many localities containing chemostratigraphic expressions of this event are not, however, enriched in organic carbon, and point to a heterogeneous set of oceanographic and environmental processes operating in different ocean basins. These processes are difficult to reconstruct because of the uneven geographical distribution of sites recording OAE-2, thus limiting our understanding of the causes and palaeoceanographic consequences of the environmental changes that occurred at this time. A new, highly resolved molybdenum-isotope dataset is presented from the Cape Verde Basin (southern proto-North Atlantic Ocean) and a lower resolution record from the Tarfaya Basin, Morocco. The new data reveal periodic oscillations in the Mo-isotope composition of proto-North Atlantic Ocean sediments, from which coupled changes in the dissolved sulphide concentration and Mo inventories of the basin seawater can be inferred. The cyclic variations in sedimentary Mo-isotope compositions can be hypothetically linked to regional changes in the depth of the chemocline, and in the rate of seawater exchange between basinal waters and global seawater. The new data suggest that a global seawater Mo-isotope composition of ∼1.2‰ was reached very soon after the onset of OAE-2, implying a rapid expansion of marine deoxygenation coeval with, or slightly preceding, enhanced global rates of organic-carbon burial. During OAE-2, the modelled flux of Mo into anoxic sediments is likely to have been ∼60–125 times greater than at the present day, although the spatial extent of anoxia is unlikely to have been greater than 10% of the total seafloor.

Paleocirculation and foraminiferal assemblages of the Cenomanian–Turonian Bridge Creek Limestone bedding couplets: Productivity vs. dilution during OAE2

The limestone–marlstone (or limestone–calcareous shale) bedding couplets of the lower Bridge Creek Member of the Greenhorn Formation coincide with Oceanic Anoxic Event 2 and the Cenomanian–Turonian stage boundary at 93.9 Ma, and are characterized by fluctuations in microfossil and macrofossil biofacies, and organic carbon. Since G.K. Gilbert (1895), these strongly alternating lithofacies have been attributed to climate and/or productivity cycles. Heretofore, only the calcareous shale and marlstone parts of the Bridge Creek bedding couplets have been quantitatively analyzed for planktic and benthic foraminiferal assemblages. In this study, foraminiferal assemblages extracted from the hard limestone beds are comparable with the muddier lithologies thereby allowing a quantitative evaluation of the foraminiferal response to cyclically changing conditions in the U.S. Western Interior Sea (WIS) that resulted in the deposition of these lithologic couplets. The results reveal a modest cyclical response of foraminiferal assemblages extracted from limestone beds compared to adjacent calcareous shale or marlstone. These include the absence of planktic planispiral morphotypes (Globigerinelloides), increase in the proportion of planktic biserial and triserial morphotypes (Heterohelix and Guembelitria, respectively), and an increase in the proportion of benthics relative to total foraminifera (decrease in percent planktics) in the limestone beds. Such conditions suggest that the limestones may have been more productive than the adjacent shales and marlstones. Reduced surface salinity and greater stratification of the upper water column may have also contributed to the differences in assemblages preserved in the marlstones and calcareous shales. The onset of OAE 2 in the late Cenomanian is marked by an abrupt benthic oxygenation event (‘Benthonic Zone’) as Tethyan waters were drawn well north into the WIS, and cool Boreal waters spread across northwest Europe, known as the Plenus Cold Event. At this time, the WIS became an important ocean gateway for surface ocean circulation with rising sea level that helped facilitate the development and spread of OAE 2. A cyclonic (counterclockwise) gyre circulation in the WIS during deposition of the lower part of the Bridge Creek was driven by the difference between precipitation in the north and evaporation in the south. The gyre is represented by two modes, strong and weak, responsible for deposition of the limestone and marlstone, respectively. For the middle and upper parts of the studied section representing the plateau of OAE 2 and subsequent peak transgression of the WIS, the counterclockwise gyre was driven less by E-P gradient but by the amount of surface runoff from both margins of the WIS with deposition of limestone beds during the wetter (strong) phase and marlstones during the drier (weak) phase. Highest levels of TOC redevelop after OAE 2 in the early Turonian with the incursion or development of an oxygen minimum zone at the time of peak transgression.

Planktonic foraminiferal biostratigraphy and assemblage composition across the Cenomanian–Turonian boundary interval at Clot Chevalier (Vocontian Basin, SE France)

The Cenomanian–Turonian boundary interval is generally considered a critical time for planktonic foraminifera due to the environmental perturbations associated with Oceanic Anoxic Event 2. However, only the rotaliporids became extinct at the onset of the event, whilst several lineages evolved and/or diversified. This remarkable morphologic plasticity is often overlooked in the literature, partly because a number of stratigraphic sections have only been studied in thin-section due to the degree of lithification of the samples. Improved documentation of the morphological variability of planktonic foraminifera and better defined species concepts are required in order to improve biostratigraphy, particularly as Helvetoglobotruncana helvetica is an unreliable marker for the base of the Turonian. At the same time, detailed study of the planktonic foraminiferal response to OAE 2 demands a more profound knowledge of the assemblage composition.We present new biostratigraphic, taxonomic, and quantitative data for planktonic foraminiferal species from the Clot Chevalier section (Vocontian Basin, SE France), with the aim of (1) providing a detailed biostratigraphic analysis of the section, (2) documenting the morphological plasticity of specimens in this time interval and stabilizing species concepts, and (3) identifying promising markers to improve the resolution of the present biozonation and allow regional correlation. Samples were processed with acetic acid to extract isolated planktonic foraminifera. Assemblages were assigned to the upper Cenomanian Rotalipora cushmani Zone and to the uppermost Cenomanian–lowermost Turonian Whiteinella archaeocretacea Zone. Planktonic foraminiferal bioevents and assemblage composition identified at Clot Chevalier are compared with the well-studied Pont d'Issole section located ca. 15 km to the NE, highlighting similarities and differences in the species occurrences that may complicate the stratigraphic correlation between the two sections.The results of our study support the validity and common occurrence of species that have been misidentified and/or overlooked in the literature (i.e., Dicarinella roddai, Praeglobotruncana oraviensis, Marginotruncana caronae) and indicate that primitive marginotruncanids evolved before the onset of OAE 2, although species diversification occurred only after the event. Moreover, we believe that the first appearance of P. oraviensis might represent a promising bioevent for approximating the Cenomanian/Turonian boundary, after calibration with bio- and chemostratigraphically well-constrained sections. Finally, we describe three new trochospiral species, named “Pseudoclavihedbergella” chevaliensis, Praeglobotruncana pseudoalgeriana and Praeglobotruncana clotensis.

Ca isotope stratigraphy across the Cenomanian–Turonian OAE 2: Links between volcanism, seawater geochemistry, and the carbonate fractionation factor

The Ca isotope composition of marine carbonate rocks offers potential to reconstruct drivers of environmental change in the geologic past. This study reports new, high-precision Ca isotope records (δCa44/40; 2σSD=±0.04‰) for three sections spanning a major perturbation to the Cretaceous ocean-climate system known as Ocean Anoxic Event 2 (OAE 2): central Colorado, USA (Portland #1 core), southeastern France (Pont d'Issole), and Hokkaido, Japan (Oyubari, Yezo Group). In addition, we generated new data for selected samples from Eastbourne, England (English Chalk), where a previous Ca isotope study was completed using different methodology (Blättler et al., 2011). Strata of the Yezo Group contain little carbonate (∼1 wt.% on average) and accordingly did not yield a clear δCa44/40 signal. The Portland core and the Pont d'Issole section display comparable δCa44/40 values, which increase by ∼0.10–0.15‰ at the onset of OAE 2 and then decrease to near-initial values across the event. The Eastbourne δCa44/40 values are higher than previously reported. They are also higher than the δCa44/40 values for the Portland core and the Pont d'Issole section but define a similar pattern. According to a numerical model of the marine Ca cycle, elevated hydrothermal inputs have little impact on seawater δCa44/40 values. Elevated riverine (chemical weathering) inputs produce a transient negative isotope excursion, which significantly differs from the positive isotope excursions observed in the Portland, Pont d'Issole, and Eastbourne records. A decrease in the magnitude of the carbonate fractionation factor provides the best explanation for a positive shift in δCa44/40 values, especially given the rapid nature of the excursion. Because a decrease in the fractionation factor corresponds to an increase in the Ca/CO3 ratio of seawater, we tentatively attribute the positive Ca isotope excursion to transient ocean acidification, i.e., a reduction in the concentration of CO32− during CO2 uptake. Recent studies utilizing a variety of isotope proxies, e.g., Nd, Os, and Pb, implicate eruption of the Caribbean Large Igneous Province as a likely source of increased CO2. Moreover, integration of C, Ca, and Os isotope data reveals new information about the timing of events during the onset of OAE 2.

Heterogeneous oxygenation states in the Atlantic and Tethys oceans during Oceanic Anoxic Event 2

The Cenomanian–Turonian boundary (ca. 93.5 Ma) is marked by an episode of profound environmental change, including a major perturbation of the carbon cycle and an Oceanic Anoxic Event (OAE-2). Here, we present molybdenum (Mo) isotope variations within the OAE-2 interval for four sections from the western Tethys (Furlo and La Contessa) and the North-Atlantic (ODP site 1276 and DSDP site 367). The main target of this study is to investigate the extent of reducing conditions (truly global in extent or restricted to poorly-ventilated restricted deep basins), with particular reference to the relationship between the change in the oxygenation state of the ocean and the link to global perturbations of the carbon cycle recorded in carbon isotopes.All four sections show fluctuations in the redox sensitive trace metal (RSTE) distribution, suggesting rapid variations in local redox conditions, ranging from anoxic to euxinic. The RSTE enrichment factors (EFs) also suggest different depositional conditions and paleoceanographic processes in the western Tethys versus the North Atlantic. Whereas the North Atlantic sites show evidence of weak watermass restriction associated with the action of a particulate shuttle within the water column, the EFs of the Tethyan sections are characteristic of unrestricted marine systems. Mo isotopes show surprisingly negative values through the Tethyan sections. At the onset of OAE-2, an increasing trend in δMo98 is observed, with values ranging from −0.6 to 0.6‰. During the second half of OAE-2, the δMo98 curve shows a progressive shift towards more negative values. In the North Atlantic, δMo98 signatures from ODP site 1276 show a similar behaviour as observed in the western Tethys. At DSDP site 367, Mo isotopes are generally heavier during OAE-2, fluctuating around an average value of 1.1‰. This is consistent with fully euxinic conditions and the black shales deposited may have recorded the seawater signature during OAE-2. The Mo isotope signature is lighter than the modern ocean value (2.3‰) and suggests a higher proportion of bottom water euxinia (∼10%). Although this estimate is greater than the euxinic fraction in the modern ocean, our results challenge the suggestion of a globally euxinic deep waters during OAE-2. The variations in Mo/TOC ratios within the North Atlantic and Tethys oceans are in good agreement with a general drawdown of Mo within the OAE interval, suggesting a general increase in anoxic conditions during OAE-2. Our results highlight the role of local parameters in the development of euxinia, even during period of widespread anoxia such as OAEs.

Marine 187Os/188Os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during Oceanic Anoxic Event 2

High-resolution osmium (Os) isotope stratigraphy across the Cenomanian–Turonian Boundary Interval from 6 sections for four transcontinental settings has produced a record of seawater chemistry that demonstrates regional variability as a function of terrestrial and hydrothermal inputs, revealing the impact of palaeoenvironmental processes. In every section the 187Os/188Os profiles show a comparable trend; radiogenic values in the lead up to Oceanic Anoxic Event 2 (OAE 2); an abrupt unradiogenic trend at the onset of OAE 2; an unradiogenic interval during the first part of OAE 2; and a return to radiogenic values towards the end of the event, above the Cenomanian–Turonian boundary. The unradiogenic trend in 187Os/188Os is synchronous in all sections. Previous work suggests that activity of the Caribbean LIP (Large Igneous Province) was the source of unradiogenic Os across the OAE 2 and possibly an instigator of anoxia in the oceans. Here we assess this hypothesis and consider the influence of activity from other LIPs; such as the High Arctic LIP.A brief shift to high radiogenic 187Os/188Os values occurred in the Western Interior Seaway before the onset of OAE 2. We evaluate this trend and suggest that a combination of factors collectively played critical roles in the initiation of OAE 2; differential input of nutrients from continental and volcanogenic sources, coupled with efficient palaeocirculation of the global ocean and epeiric seas, enhanced productivity due to higher nutrient availability, which permitted penecontemporaneous transport of continental and LIP-derived nutrients to trans-equatorial basins.

A perturbed hydrological cycle during Oceanic Anoxic Event 2

Research Article| February 01, 2014 A perturbed hydrological cycle during Oceanic Anoxic Event 2 Niels A.G.M. van Helmond; Niels A.G.M. van Helmond * 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands *E-mail: n.vanhelmond@uu.nl. Search for other works by this author on: GSW Google Scholar Appy Sluijs; Appy Sluijs 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Gert-Jan Reichart; Gert-Jan Reichart 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Search for other works by this author on: GSW Google Scholar Jaap S. Sinninghe Damsté; Jaap S. Sinninghe Damsté 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Search for other works by this author on: GSW Google Scholar Caroline P. Slomp; Caroline P. Slomp 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Henk Brinkhuis Henk Brinkhuis 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Search for other works by this author on: GSW Google Scholar Author and Article Information Niels A.G.M. van Helmond * 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Appy Sluijs 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Gert-Jan Reichart 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Jaap S. Sinninghe Damsté 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Caroline P. Slomp 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Henk Brinkhuis 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands *E-mail: n.vanhelmond@uu.nl. Publisher: Geological Society of America Received: 11 Jul 2013 Revision Received: 19 Oct 2013 Accepted: 22 Oct 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (2): 123–126. https://doi.org/10.1130/G34929.1 Article history Received: 11 Jul 2013 Revision Received: 19 Oct 2013 Accepted: 22 Oct 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Niels A.G.M. van Helmond, Appy Sluijs, Gert-Jan Reichart, Jaap S. Sinninghe Damsté, Caroline P. Slomp, Henk Brinkhuis; A perturbed hydrological cycle during Oceanic Anoxic Event 2. Geology 2014;; 42 (2): 123–126. doi: https://doi.org/10.1130/G34929.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Late Cretaceous Oceanic Anoxic Event 2 (OAE2; ca. 94 Ma) was one of the largest global carbon cycle perturbations during the Phanerozoic. OAE2 represents an important, although extreme, case study for modern trends because widespread anoxia and enhanced organic carbon burial during OAE2 were linked to exceptionally warm climates and high atmospheric CO2 concentrations. However, the consequences of this warmth for the hydrological cycle remain poorly understood, hampering our understanding of its impact on biogeochemical cycles during this greenhouse episode. Here we show evidence for changes in the hydrological cycle during OAE2 based on combined geochemical and palynological data for the stratigraphically expanded coastal OAE2 succession at Bass River, located on the New Jersey shelf (Ocean Drilling Program Leg 174AX), eastern United States. Paleothermometry, based on TEX86, indicates sea-surface warming at the onset of OAE2 and a subsequent pronounced cooling event. Palynological data show that these changes in temperature were associated with strong variations in precipitation and runoff. We suggest that an acceleration of the hydrological cycle during OAE2 played a key role in supplying nutrients to coastal waters and enhancing stratification, thus contributing to the development of ocean anoxia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence for oxic conditions during oceanic anoxic event 2 in the northern Tethyan pelagic realm

Upper Cenomanian pelagic sediments from the northern Alpine Helvetic fold-and-thrust belt (northern Tethyan margin) coeval with Oceanic Anoxic Event (OAE) 2 are characterized by the temporal persistence of micrite sedimentation and lack of organic carbon-rich layers. We studied an expanded section in the Chrummflueschlucht (east of Euthal, Switzerland), which encompasses the OAE 2 time interval. In order to identify the paleoceanographic and paleoenvironmental conditions during OAE 2 in this part of the northern Tethyan margin, and more specifically to trace eventual changes in nutrient levels and oxic conditions, we investigated the biostratigraphy (planktonic foraminifera), the bulk-rock mineralogy, and measured stable carbon- and oxygen-isotopes, total phosphorus (P) and redox-sensitive trace-element (RSTE) contents. We were able to determine – with some remaining uncertainties – the different planktonic foraminiferal biozones characteristic of the Cenomanian–Turonian boundary interval ( Rotalipora cushmani, Whiteinella archaeocretacea and Helvetoglobotruncana helvetica zones). In the lower part of the section ( R. cushmani total range zone), the bulk-rock δ 13C record shows a long-term increase. Within sediments attributed to the W. archaeocretacea partial range zone, δ 13C values reach a maximum of 3.3‰ (peak “a”). In the following the values decrease and increase again to arrive at a plateau with high δ 13C values of around 3.1‰, which ends with a peak of 3.3‰ (peak “c”). At the top of the section, in sediments belonging to the H. helvetica total range zone, δ 13C values decrease to post-OAE values of around 2.2‰. The last occurrence of R. cushmani is observed just above the positive δ 13C shift characterizing OAE 2. P contents display small variations along the section with a long-term decreasing trend towards the top. Before the OAE 2 interval, P values show higher values and relatively good covariation with detrital input, indicating higher nutrient input before OAE 2. In sediments corresponding to the onset of the δ 13C positive excursion, P content is marked by a sharp peak probably linked to a slowdown in sedimentation rates and/or the presence of a small hiatus, as is shown by the presence of glauconite and phosphatic grains. In the interval corresponding to OAE 2, P values remain low and increase slightly at the end of the positive shift in the δ 13C record (in the H. helvetica total range zone). The average contents of RSTE (U, V, As, Co, Mo and Mn) remain low throughout the section and appreciable RSTE enrichments have not been observed for the sedimentary interval corresponding to OAE 2. No correlation is observed with stratigraphic trends in RSTE contents in organic-rich deeper-water sections. The presence of double-keeled planktonic foraminifera species during most of the Cenomanian/Turonian boundary event is another evidence of relatively well-oxygenated conditions in this part of the northern Tethyan outer shelf. Our results show that the Chrummflueschlucht section corresponds to one of the most complete section for the Cenomanian–Turonian boundary interval known from the Helvetic realm even if a small hiatus may be present at the onset of the δ 13C record (peak “a”). The evolution of P contents suggests an increase in input of this nutritive element at the onset of OAE2. However, the trends in RSTE contents and the planktonic foraminifera assemblages show that the Helvetic realm has not been affected by strongly depleted oxygen conditions during OAE 2.

Phosphorus cycling from the margin to abyssal depths in the proto-Atlantic during oceanic anoxic event 2

Phosphorus (P) burial during the Cenomanian/Turonian oceanic anoxic event (OAE 2, ∼ 93.55 Ma) was studied using sediment records from four sites in the proto-Atlantic Ocean. The sites represent a variety of depositional settings, from the continental margin and slope to the abyssal plain. Strong links between changing marine conditions during OAE 2 and P cycling were observed in black shale sections on the continental margin and slope. Sedimentary P contents and P MARs increased around the onset of OAE 2, which may reflect enhanced productivity and P burial, partly in the form of biogenic apatite (fish debris). During the main stage of OAE 2, phosphorus was efficiently regenerated from the sediment, resulting in P-depleted sediments and high organic C (C org)/total P ratios. The increased availability of dissolved P may have helped to sustain high productivity during OAE 2. The termination of OAE 2 was associated with a strong recovery of P burial efficiency in the sediment. Sedimentary records from the two deep sites do not reveal such close links between different stages of OAE 2 and the characteristics of P cycling. Here, OAE 2 was a period with maximum deposition of relatively thin black shale intervals that alternate with more oxic deep sea sediments. The black shale layers record enhanced P regeneration from organic matter and, in some cases, P enrichment relative to the oxic sediments. Detailed chemical P fractionation was performed on sediment from all sites, but the results were found to mostly reflect diagenesis and sample handling artifacts. In these ancient sediments, total P concentrations and the C org/total P ratio appear to be the most robust indicators of P cycling and regeneration.

Cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode

Oceanic anoxic events (OAEs) were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen-deficient bottom waters. OAE2, occurring at the Cenomanian/Turonian boundary (about 93.5 Myr ago), is the most widespread and best defined OAE of the mid-Cretaceous. Although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios, the actual trigger mechanism, corresponding closely to the onset of these episodes of increased carbon sequestration, has not been clearly identified. It has been postulated that large-scale magmatic activity initially triggered OAE2 (refs 4, 5), but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of OAE2 has not yet been found. Here we report seawater osmium isotope ratios in organic-rich sediments from two distant sites. We find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of OAE2. Using a simple two-component mixing equation, we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin, a approximately 30-50-fold increase relative to pre-OAE conditions. Furthermore, the magmatic osmium isotope signal appears slightly before the OAE2-as indicated by carbon isotope ratios-suggesting a time-lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial, which may reflect the reaction time of the global ocean system. Our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of OAE2, which may have triggered the subsequent deposition of large amounts of organic matter.

Oceanic anoxic cycles? Orbital prelude to the Bonarelli Level (OAE 2)

The prelude to the second global oceanic–anoxic event of the Cretaceous (OAE 2), as recorded in the classic sections of the Scaglia Bianca Formation in the Umbria–Marches region of Italy, is characterized by an oscillation between black to gray, organic-rich cherts and white pelagic limestones. Spectral analysis of these lithologic alternations reveals a robust set of cycles whose periods are remarkably similar to the orbital cycles calculated by Laskar et al. [Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A. C. M., Levrard, B., 2004, A long term numerical solution for the insolation quantities of the Earth. Astronomy and Astrophysics, 428, 261–285.]. Bandpass filtering shows that the 140–80 kyr signal approximates the envelope of the 26–16 kyr signal, just as expected if these components represent responses to orbital eccentricity and precession. Matching the presumed eccentricity signal to the theoretical eccentricity of Laskar et al. yields an age for the base of the Bonarelli Level (onset of OAE 2 in Italy) of 94.21 Ma (93.72 Ma provides an alternative), consistent with radiometric constraints from a correlative section in the western United States. Given the strong orbital control on the cyclic deposition of the organic-rich precursors of OAE 2 within the Scaglia Bianca Formation, we hypothesize that the timing of OAE 2 itself is similarly controlled by astronomical variations. Indeed, the occurrence of OAE 2 coincides with a protracted period of unusually low insolation variation, owing to simultaneous nodes in all orbital parameters. We speculate that other OAEs similarly coincided with insolation nodes and dampened seasonality. From this perspective, some (but perhaps not all) of the OAEs may be periodic events reflecting cycles of oceanic anoxia that predictably punctuated a ~50 Myr period of Cretaceous climate.