Onset Of Oceanic Anoxic Event 2 Research Articles

Spatial heterogeneity in benthic foraminiferal assemblages tracks regional impacts of paleoenvironmental change across Cretaceous OAE2

AbstractThe impact of global climate events on local ecosystems can vary spatially. Understanding this potential heterogeneity can illuminate which environments will be most impacted and the proximal drivers of ecosystem responses. Cenomanian–Turonian marine deposits of the Western Interior Seaway (WIS) record paleoceanographic changes associated with the Greenhorn transgression and the onset of Oceanic Anoxic Event 2 (OAE2). They provide an ideal setting to study basin-wide paleoecological responses during a global perturbation. Here, we integrate benthic foraminiferal assemblages from before, during, and after OAE2 via multivariate ordination analysis to examine spatial patterns in faunal responses across the western United States on a common scale and to interrogate a previously defined faunal marker commonly used for basin-wide correlation, the Benthonic Zone (BZ). We identify oxygenation and organic matter quality as primary and secondary controls of faunal variation across the 10 stratigraphic records and use this variation to infer paleoenvironmental changes. Stratigraphic trends reveal, in contrast to previous studies, deoxygenation at the onset of OAE2. They also reveal temporal patterns in oxygenation and productivity consistent with the gradual northward migration of a southern water mass into the WIS. This spatial heterogeneity hinders the use of the BZ as a temporal marker, because assemblages change in response to diachronous environmental change, and thus timing of the BZ with respect to OAE2 is not consistent across the basin. Our study demonstrates that regional processes can overshadow ecosystem responses to global events and underscores the importance of considering how changes in the position of water masses impact the expression of global biogeochemical perturbations.

Biostratigraphic and chemostratigraphic record of the Cenomanian/Turonian boundary interval of the Tellian Atlas (Algeria)

A multi-proxy approach integrating data on planktonic foraminifera, calcareous nannofossils, δ13Ccarb, δ18Ocarb and the coiling direction of Muricohedbergella delrioensis was carried out for two sections in the Tellian Atlas (Algeria) both to establish a precise stratigraphic scheme and to precise the climatic events.The Sour el Ghozlane section offers a good record of the Oceanic Anoxic Event 2 (OAE2) in the southern margin of the Tethys, with organic-rich sediments and the continuous presence of planktonic foraminifera, nannofossils and other microfossils. The Cenomanian/Turonian boundary (CTB) in the Sour el Ghozlane section lies in the interval limited at the base by the last occurrence of Helenea chiastia and the first occurrence of Quadrum gartneri respectively located immediately below and above the last peak (C) of the carbon isotope excursion. However, in the Sidi Nadji section, the CTB interval is extremely thin; the last occurrences of Rotalipora cushmani and Thalmanninella greenhornensis coincide with the last occurrences of Corollithion kennedyi, Helenea chiastia, Axopodorhabdus albianus and Rhagodiscus asper and occur in the same level, therefore a hiatus is suspected.Sour el Ghozlane and Sidi Nadji are so far the southernmost sections where the percentage of sinistral M. delrioensis has been investigated. High proportion of left-coiled morphotypes among M. delrioensis indicate transient cooling episodes, including the Plenus Cold Event at the onset of OAE2 and before the warm conditions at the CTB characterized by the dominance of dextral forms.

CO2-induced climate forcing on the fire record during the initiation of Cretaceous oceanic anoxic event 2

Abstract Cretaceous oceanic anoxic event 2 (OAE2) is thought to have been contemporary with extensive volcanism and the release of large quantities of volcanic CO2 capable of triggering marine anoxia through a series of biogeochemical feedbacks. High-resolution reconstructions of atmospheric CO2 concentrations across the initiation of OAE2 suggest that there were also two distinct pulses of CO2 drawdown coeval with increased organic carbon burial. These fluctuations in CO2 likely led to significant climatic changes, including fluctuations in temperatures and the hydrological cycle. Paleofire proxy records suggest that wildfire was a common occurrence throughout the Cretaceous Period, likely fueled by the estimated high atmospheric O2 concentrations at this time. However, over geological time scales, the likelihood and behavior of fire are also controlled by other factors such as climate, implying that CO2-driven climate changes should also be observable in the fossil charcoal record. We tested this hypothesis and present a high-resolution study of fire history through the use of fossil charcoal abundances across the OAE2 onset, and we compared our records to the estimated CO2 fluctuations published from the same study sites. Our study illustrates that inferred wildfire activity appears to relate to changes in CO2 occurring across the onset of OAE2, where periods of CO2 drawdown may have enabled an increase in fire activity through suppression of the hydrological cycle. Our study provides further insight into the relationships between rapid changes in the carbon cycle, climate, and wildfire activity, illustrating that CO2 and climate changes related to inferred wildfire activity can be detected despite the estimated high Cretaceous atmospheric O2 concentrations.

New insights into Cenomanian paleoceanography and climate evolution from the Tarfaya Basin, southern Morocco

A 325 m long continuous succession of uppermost Albian to lower Turonian pelagic (outer shelf) deposits was recovered from a new drill site in the central part of the Tarfaya Basin (southern Morocco). Natural gamma ray wireline logging, carbonate and organic carbon content, bulk carbonate and organic carbon stable isotopes and X-ray fluorescence (XRF)-scanner derived elemental distribution data in combination with planktonic foraminiferal biostratigraphy indicate complete recovery of the Cenomanian Stage. This exceptional sediment archive allows to identify orbitally driven cyclic sedimentation patterns and to evaluate the pacing of climatic events and regional environmental change across the Albian-Cenomanian boundary (ACB), the mid-Cenomanian Event (MCE) and Oceanic Anoxic Event 2 (OAE2) in the latest Cenomanian. The deposition of organic-rich sediments in the Tarfaya Basin, likely driven by upwelling of nutrient-rich water masses, started during the latest Albian and intensified in two major steps following the MCE and the onset of OAE2. The duration and structure of the MCE and OAE2 carbon isotope excursions exhibit striking similarities, suggesting common driving mechanisms and climate-carbon cycle feedbacks. Both events were also associated with eustatic sea level falls, expressed as prominent sequence boundaries in the Tarfaya Basin. Based on the 405 kyr signal imprinted on the Natural Gamma Ray (NGR) and XRF-scanner derived Log(Zr/Rb) records, we estimate the duration of the Cenomanian Stage to be 4.8 ± 0.2 Myr.

Water-mass evolution in the Cretaceous Western Interior Seaway of North America and equatorial Atlantic

Abstract. The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian–Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9–93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north–south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas (∼ 90–98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal–austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early–middle Cenomanian (98–95 Ma) followed by a major re-organization during the latest Cenomanian–Turonian (95–94 Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation.

Biostratigraphy of the Cenomanian–turonian Eagle Ford Shale of South Texas

AbstractThe Cenomanian–Turonian Eagle Ford Shale of south Texas occupies an important gateway between the Western Interior Seaway (WIS) of North America and the Gulf of Mexico. While the Eagle Ford north of the San Marcos Arch and its stratigraphic equivalents to the east of the Sabine Arch are shallow-water sediments dominated by terrigenous clastics, the more distal localities in south Texas are dominated by hemi-pelagic carbonates draped over an Early Cretaceous carbonate platform, called the Comanche Platform, and adjacent submarine plateaus and basins. This region was strongly affected by major oceanographic changes during the Cenomanian-Turonian, particularly a significant transgression that drove localized upwelling and organic matter burial in the Lower Eagle Ford prior to the global Oceanic Anoxic Event 2 (OAE2). These pre-OAE2 organic-rich shales are the basis of Eagle Ford shale gas play, which has spurred commercial and academic research into many aspects of the geology of the Eagle Ford Group. Much of this research has been fairly locally focused, and little effort has been made to understand the timing of events across the platform. We compared new data from three study sites across south Texas—Lozier Canyon in Terrell Co.; Bouldin Creek in Travis Co., near the San Marcos Arch in the center of the Comanche Platform; and Swift Energy's Fasken Core in Webb Co., off the platform on the Rio Grande Submarine Plateau—as well as published data from near Big Bend National Park on the western margin, and from Atacosta and Karnes counties on the eastern margin. Using these data we document the occurrence of key foraminiferal species across the platform and present a regional biostratigraphic scheme incorporating five global planktic foraminiferal zones (and contemporaneous occurrences that may serve as proxies for the zonal markers, which tend to be rare in Texas) and four local origination or acme events that serve as useful secondary markers. The succession of events is: 1) highest occurrence (HO) Favusella washitensis, 2) lowest occurrence (LO) Rotalipora cushmani, 3) “Benthonic Zone”, 4) HO R. cushmani and/or Thalmaninella greenhornensis, 5) “Heterohelix shift”, 6) LO “Anomalina W”, 7) LO Helvetoglobotruncana helvetica, 8) HO H. sp., and 9) LO Dicarinella concavata. Overall, we show that lithologic and geochemical trends through most of the Eagle Ford, particularly the oxygenation at the onset of OAE2 and the concurrent shift to more carbonate-rich lithologies, are synchronous across the Comanche Platform. However, the transition from the Eagle Ford Group to the Austin Chalk varies in age. While Austin Chalk deposition began in the middle Turonian Marginotruncana schneegansi Zone on the Rio Grande Submarine Plateau, a transgressive surface on the Comanche Platform (known as the “Rubble Zone” in central Texas) represents a condensed interval at the top of the Eagle Ford that ends in the upper Turonian D. concavata Zone. This is part of a transgressive disconformity that extends north through the WIS, where it is associated with the Juana Lopez Calcarenite.

Calcareous nannofossil paleoecology of the mid-Cretaceous Western Interior Seaway and evidence of oligotrophic surface waters during OAE2

Analysis of calcareous nannofossils, total organic carbon(TOC) values, and carbon isotope records from outcrop and drill cores in the central (Colorado and Kansas) and southern (Texas) Western Interior Basin (WIB) indicates predominantly oligotrophic surface water conditions during the late Cenomanian–Turonian Oceanic Anoxic Event 2(OAE2). High-resolution bio-chemostratigraphy of this interval allows for detailed correlation of changes in nannofossil assemblages and TOC across the WIB and aids comparison with other global records. Prior to and at the onset of OAE2, the A interval in the δ13C positive excursion identified by Pratt and Threlkeld (1984), mesotrophic to eutrophic nannofossil taxa Biscutum and Zeugrhabdotus are relatively high in abundance and foraminiferal assemblages record a high diversity “benthonic zone”. TOC sharply declines at the start of OAE2 despite evidence of high surface productivity and benthic diversity, implying that the seafloor was briefly oxygenated during a period of enhanced mixing in the expanding Western Interior Seaway. During the latter two thirds of OAE2 (intervals B and C in the excursion) TOC periodically increases, suggesting that anoxic conditions returned at the substrate, despite persistently low abundances of Biscutum and Zeugrhabdotus, implying oligotrophic surface water conditions. The divergence of these two proxies suggests a highly stratified water column in the WIB, not higher productivity, which resulted in anoxic conditions during OAE2.If productivity was not the primary cause of increased carbon burial in the WIB it is probable that increased terrestrial runoff and incursion of warm, saline Tethyan waters from the south led to salinity stratification. In the absence of mixing, warm and saline bottom waters would have been isolated from freshened Boreal currents and become stagnant, allowing for preservation of greater amounts of organic material in the substrate. Nannofossil records from Cuba, KS are an exception, and reveal relatively high productivity throughout, suggesting that occasional mixing of nutrients to the surface may have continued along the eastern margin of the central WIB.

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

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