mid-Cretaceous Oceanic Anoxic Events Research Articles

Elevated sea surface temperature and enhanced primary productivity during Ocean Anoxic Event 1d in the eastern Tethys: Calcareous nannofossil evidence from southern Tibet, China

Mid-Cretaceous oceanic anoxic events (OAEs) are characterized by major disturbances to the global carbon cycle and are considered ancient analogs for anthropogenic global warming. Among these events, OAE 1d, occurring around the Albian-Cenomanian boundary, remains understudied in the eastern Tethys region, leaving a critical palaeogeographic gap that hampers a global understanding of its impacts. In this paper, we present a detailed record of calcareous nannofossil assemblages from the lower to middle part of the Lengqingre Formation in the Chaqiela section of Gamba, southern Tibet, China. The study interval contains abundant, moderately to well preserved calcareous nannofossil assemblages, with Watznaueria barnesiae, Discorhabdus ignotus, Biscutum constans, and Zeugrhabdotus erectus as the dominant species constituting >50% of the assemblages. These nannofossil assemblages allow the study interval to be constrained to Upper Cretaceous (UC) Biozones UC0 through UC2, with the Albian-Cenomanian boundary located in the lower part of the Lengqingre Formation, and the middle part assigned to Cenomanian in the study area. As such, the position of OAE 1d in this section is accurately determined. Calculated nutrient and temperature indices based on nannofossil species with preferred ecologies reveal significant palaeoceanographic changes across the OAE 1d. These changes show increased abundance of warm-water taxa (e.g., Rhagodiscus asper and Zeugrhabdotus diplogrammus) and high-productivity taxa (e.g., D. ignotus, B. constans, and Z. erectus), suggesting increased sea surface temperature and enhanced productivity during OAE 1d in the southwestern shelf sea of the eastern Tethys Ocean.

Large igneous province activity drives oceanic anoxic event 2 environmental change across eastern Asia

During mid-Cretaceous Oceanic Anoxic Event 2, significant increase of atmospheric carbon dioxide concentrations from the eruption of the large igneous provinces is hypothesized to have induced a humid climate and an elevation in nutrient runoff from continents to the oceans, resulting in oxygen depletion in the ocean. However, hitherto there is limited insight into the driving factors of Oceanic Anoxic Event 2 from the Pacific and Asian continental margins, even though the former and the latter were the largest ocean and landmass at that time. Here, a multiproxy analysis for the Oceanic Anoxic Event 2 interval of the Yezo Group –deposited on northwestern Pacific along the active Asian continental margin– is interpretated to identify seven volcanic pulses, five of which may have elevated humidity, weathering intensity, and vegetational change in the eastern margin of Asia. Moreover, oxygen depletion occurred simultaneously in the northwest Pacific. Given that these environmental changes in the eastern margin of Asia were penecontemporaneous with the global carbon burial intervals during Oceanic Anoxic Event 2, the elevated nutrient supply from the Asian continental margin to the Pacific Ocean may have, in part, contributed to the worldwide depletion of oxygen of the ocean during Oceanic Anoxic Event 2.

Biogeochemical Cycles during the mid-Cretaceous Oceanic Anoxic Event 2

Biogeochemical Cycles during the mid-Cretaceous Oceanic Anoxic Event 2

Intensive peatland wildfires during the Aptian–Albian oceanic anoxic event 1b: Evidence from borehole SK-2 in the Songliao Basin, NE China

The Cretaceous has been considered a “high-fire” world accompanied by widespread by-products of combustion in the rock record. The mid-Cretaceous oceanic anoxic event 1b (OAE1b) is marked by one of the major perturbations in the global carbon cycle characterized by deposition of organic-rich sediments in both marine and terrestrial settings. However, our understanding is still limited on changes in wildfire activity during OAE1b period. Here, we carried out a comprehensive analysis, including organic carbon isotope (δ13Corg), total organic carbon (TOC), coal petrology, trace elements, and pyrolytic polycyclic aromatic hydrocarbons (pyroPAHs), of coal seams of the middle Aptian to early Albian Shahezi Formation from borehole SK-2 in Songliao Basin, Northeast China. Two negative δ13Corg excursions in the Shahezi Formation can be corresponded with the 113/Jacob and Kilian sub-events of OAE1b. Moreover, the intensive peatland wildfires have been identified during the sub-event periods of OAE1b based on the co-occurrence of high abundance of charcoal and pyroPAHs at that time. In addition, Sr/Ba, Sr/Cu and Sr/Rb ratios demonstrate that enhanced peatland wildfires were controlled by dryer climate conditions owing to episodic northward migration of arid zones in East Asia related with rising global temperature during the sub-events of OAE1b. The climate-driven extensive wildfire activity in the mid-latitude terrestrial ecosystems can be a contributing factor for OAE1b through the increased flux of nutrients fuelling primary producers in the lake and marine environments and leading to more speculative anoxia to allow the deposition of organic-rich sediments. Our results provide essential understanding of the importance of wildfires in driving mechanism of oceanic anoxic events (OAEs) in Earth's history.

Marine paleoecological variations during the mid-Cretaceous oceanic anoxic event 1a in the Vocontian Basin, southeastern France

We analyzed hopanoid and steroid biomarkers as well as marine palynomorphs in sediments from the Goguel level (oceanic anoxic event [OAE] 1a) of the Vocontian Basin in the Sauzeries section, southeastern France, to reconstruct the paleoecological conditions prevailing in sea surface waters. The 2-methylhopanoid index (2-MHI) values are enriched in the upper Goguel level, implying that the producers of these compounds, possibly oligotrophic alphaproteobacteria and/or Synechococcus cyanobacteria, highly contributed to bacterial production under oligotrophic conditions. Palynomorph analysis shows that acritarchs such as sphaeromorphs and pteromorphs are a characteristic group among the marine palynomorphs in the Goguel samples; therefore, the contribution of prasinophyte algae was possibly high among marine primary producers during OAE 1a in the Vocontian Basin. We suggest that prasinophyte algae and 2-methylhopanoid producers flourished as the main primary producers during the deposition of the Goguel level, which corresponds to the lower part of the C6 chemostratigraphic segment of OAE 1a. Moreover, the 2-MHI values increase in the upper half of the Goguel level, implying that the contribution of the 2-methylhopanoid producers was higher than in lower part and/or the producers were severely stressed by nutrient depletion. Low production of 2-methylhopanoid producers and prasinophyte algae under oligotrophic condition suggest that ocean stratification, rather than high productivity, caused anoxia in Vocontian Basin during OAE1a.

Terrestrial and marginal-marine record of the mid-Cretaceous Oceanic Anoxic Event 2 (OAE 2): High-resolution framework, carbon isotopes, CO2 and sea-level change

Oceanic Anoxic Event 2 (OAE2; c. 94.5–93.9 Ma) offers insight into the mechanisms of past climate change linked to organic productivity and carbon sequestration. It has been studied extensively, but the vast majority of data come from marine records, thus providing an incomplete view of past climate dynamics. Here we integrate new high-resolution data and published records on depositional environments, the carbon-isotope composition of bulk organic carbon (δ13Corg) and plant cuticles (δ13Ccut), and stomatal-index values, a proxy for pCO2, in well-preserved terrestrial through marginal-marine archives of the initial phase of OAE2. The study area is located on the western margin of the Western Interior Seaway (southwestern Utah). Age constraints are based on a new U-Pb bentonite age and correlation to an orbitally calibrated interval of the Bridge Creek Limestone. n-Alkane abundance suggests predominance of terrestrial contributions to bulk organic carbon for most samples. Despite similarities between carbon-isotope variations and transgressive-regressive shoreline movements, it is argued that δ13Corg and δ13Ccut are not strongly affected by local variables. A series of negative, ~2‰ carbon-isotope excursions is identified and attributed to changes in the size and isotopic value of the atmospheric CO2 reservoir. The temporal spacing of these anomalies (80–120 kyr) is consistent with changes in insolation modulated by orbital eccentricity. A systematic, phase-shifted relationship between the negative carbon-isotope excursions and transgressive increments further suggests a link between carbon-cycle perturbations and meter-scale sea-level change on the 100-kyr time scale. A conceptual model involving insolation-controlled aquifer charge/discharge and biomass burial/degradation in the monsoonal belt is proposed. The framework presented here is available to facilitate further research on the interplay of terrestrial and oceanic carbon reservoirs during OAE2.

Peter Andreas Hochuli (1946–2018)

Peter Andreas Hochuli (1946–2018)

有機地球化学からみた白亜紀中期,海洋無酸素事変1aおよび1bにおける古生態・古環境変動

有機地球化学からみた白亜紀中期,海洋無酸素事変1aおよび1bにおける古生態・古環境変動

Response of western South American epeiric-neritic ecosystem to middle Cretaceous Oceanic Anoxic Events

Little is known about the impact of the mid-Cretaceous Oceanic Anoxic Events (OAEs) on the neritic carbonate systems in South America. In order to fill this knowledge gap, the present paper reports on the record of environmental changes in the Albian–Turonian neritic carbonates from the western South American domain in Peru. Owing to the very expanded and well-exposed sections in the Oyon region of central Peru, the OAE 1d and 2 intervals were sampled at high temporal resolution for both bulk micrite and bulk organic matter carbon isotopes, allowing us to compare the fingerprint of these two events between the northern and central Peruvian regions. This suggests the installation of two marked depositional modes: 1) the Albian–Turonian formation of a regional facies belt constituted by oyster-rich mixed siliciclastic-carbonate deposition along the western South America platform; 2) a restricted oligotrophic environment, characterized by the mass occurrence of Perouvianella peruviana and associated miliolids in central Peru during the late Cenomanian–Turonian. These observations advocate for the following scenario: Global warming during the late Albian–early Turonian resulted in humid climate on the western platform. This in turn caused enhanced chemical weathering rates on the Brazilian Shield, resulting in high runoff of nutrients onto the western platform. Nutrient runoff promoted the diversification of benthic oyster communities. Due to the uplift of the Marañon Massif and the installation of the Huarmey Trough, central Peru was isolated from the Pacific and from eastern deltaic influx of the Brazilian continental basement, allowing the local development of oligotrophic conditions during OAE 2. Furthermore, an increased influx of argillaceous sediment and reduced carbonate production is recorded in northern Peru at the onset of OAE 2, marked by a prominent negative shift in δ13C. This negative carbon-isotope excursion has also been identified in other sections in the Pacific domain and can be linked to an increase in isotopically light pCO2 induced by the formation of the Caribbean large igneous province.

Marine primary producer community during the mid-Cretaceous oceanic anoxic events (OAEs) 1a, 1b and 1d in the Vocontian Basin (SE France) evaluated from triaromatic steroids in sediments

Desmethyl and A-ring methyl triaromatic (TA) steroids, including dinosteroids, were analyzed in the sediments deposited during the mid-Cretaceous oceanic anoxic events (OAEs) 1a (Goguel), 1b (Jacob, Kilian and Paquier) and 1d (Breistroffer) from the Vocontian Basin, SE France. These TA steroids were identified by GC–MS using non-polar (DB5-HT) and mid-polar (VF-200ms) columns; their peaks are more clearly separated by the mid-polar column. The triaromatic dinosteroid indices (TADS and TADS′) are defined as alternative indicators for reconstructing dinoflagellate production. In these relationships, the TADS′ values tend to increase in the samples that have higher abundances of the co-eluted 3-methyl TA steroids. The TADS values are lower in the Goguel, Jacob and Kilian samples but increase in the Paquier and Breistroffer samples, and are especially consistently higher in the Paquier. These results suggest that dinoflagellates flourished under the stratified and eutrophic conditions in surface waters during the Paquier deposition (OAE1b). Moreover, an inverse correlation between the TADS and C27 TA steroid (TAS) values is noticeable in the samples from the Kilian and Paquier deposits. Thus, the C27 TA steroids in these sediments, which originated from C28 sterols, may be primarily derived from an ecological competitor of dinoflagellates. We propose that their competitors are possibly coccolithophorids and/or prasinophyte algae. During the mid-Cretaceous hothouse period, coccolithophorids/prasinophytes had the potential to flourish under more oligotrophic conditions, whereas dinoflagellates were dominant as primary producers under eutrophic conditions.

Phyletic evolution of the mid-Cretaceous radiolarian genus Turbocapsula from southern Tibet and its applications in zonation

Abstract A radiolarian biostratigraphic study ranging from the latest Barremian to the Aptian of a pelagic sequence (section BB1) near Babazhadong in southern Tibet revealed phyletic relationships of species belonging to the genus Turbocapsula . Three species and two subspecies are included in this genus: Turbocapsula tetras LI & MATSUOKA, n. sp., T. fugitiva O'DOGHERTY, and T. costata ( T. costata (WU) multicostata LI & MATSUOKA, n. ssp. and T. costata (WU) costata ). Turbocapsula fugitiva appears to have arisen from T. tetras by losing the septa between the third and the fourth segments. Turbocapsula fugitiva seems to have given rise to T. costata multicostata following the development of thicker costae between its two to five rows of pores. With the decrease in the number of the costae and the development in the costae, T. costata multicostata evolved into T. costata costata . The phyletic evolution of the genus Turbocapsula has a high stratigraphic value. Based on the phyletic evolution of Aurisaturnalis and Turbocapsula , two radiolarian zones have been defined: the Aurisaturnalis carinatus Zone and the Turbocapsula costata Zone. These zones are defined by evolutionary first appearance bio-horizons (EFABs), which are more reliable than the first and the last occurrence bio-horizons over wide areas. The Turbocapsula costata Zone is subdivided into the Turbocapsula costata multicostata Subzone and the Turbocapsula costata costata Subzone. Based on comparisons with radiolarian biostratigraphic research conducted in other areas, in which age-diagnostic calcareous nannofossils and planktonic foraminifera occur alongside radiolarians, the Aurisaturnalis carinatus Zone and the Turbocapsula costata Zone are assigned to the Lower Hauterivian–Lower Aptian and the Lower Aptian–Lower Albian, respectively. This newly proposed zonation is useful for comparing the stratigraphic relationships of Turbocapsula -bearing horizons in other areas. A zonal scheme like this scheme can be used for identifying certain episodes such as mid-Cretaceous oceanic anoxic events (OAEs) in the pelagic realms. The Early Aptian OAE 1a is located between the first appearance bio-horizon of the genus Turbocapsula and the EFAB of T. costata multicostata .

Origins of acritarchs found in sedimentary rocks deposited during the mid-Cretaceous Oceanic Anoxic Events (OAEs) and their contribution to primary production in the Cretaceous ocean.

Origins of acritarchs found in sedimentary rocks deposited during the mid-Cretaceous Oceanic Anoxic Events (OAEs) and their contribution to primary production in the Cretaceous ocean.

Palynomorph analysis of black shales deposited during the mid-Cretaceous Oceanic Anoxic Events (OAEs): Reconstruction of primary producers

Palynomorph analysis of black shales deposited during the mid-Cretaceous Oceanic Anoxic Events (OAEs): Reconstruction of primary producers

Laboratory experiment for attesting the ''weathering hypothesis'' as a possible cause of the mid-Cretaceous Oceanic Anoxic Events

Laboratory experiment for attesting the ''weathering hypothesis'' as a possible cause of the mid-Cretaceous Oceanic Anoxic Events

Paleoenvironmental change during the mid-Cretaceous Oceanic Anoxic Event 1b

Paleoenvironmental change during the mid-Cretaceous Oceanic Anoxic Event 1b

Paleoenvironmental change across the mid-Cretaceous Oceanic Anoxic Event 2 based on benthic foraminiferal assemblage in the Northwest Pacific.

Paleoenvironmental change across the mid-Cretaceous Oceanic Anoxic Event 2 based on benthic foraminiferal assemblage in the Northwest Pacific.

High-resolution analysis of the mid-Cretaceous Oceanic Anoxic Event 1b in the Vocontian Basin, Southeast France

High-resolution analysis of the mid-Cretaceous Oceanic Anoxic Event 1b in the Vocontian Basin, Southeast France

No evidence for anoxia during the Valanginian carbon isotope event—An organic-geochemical study from the Vocontian Basin, SE France

The Valanginian time interval (Early Cretaceous) is characterized by a positive carbon isotope excursion (CIE) which represents the first of several prominent Cretaceous δ13C anomalies. A combined chemostratigraphic and organic-geochemical approach has been chosen to investigate the composition and distribution of sedimentary organic matter (OM) deposited before the Valanginian CIE, during its onset and plateau-phase. This was done to test whether this CIE is accompanied by changes in marine primary production and/or OM preservation. Biostratigraphically well-calibrated deposits from two hemipelagic sections located in the Vocontian Basin of SE France are used as sedimentary archives. A newly established high-resolution δ13C record covering the composite succession shows a characteristic Valanginian pattern and enables a detailed correlation with existing carbon isotope curves from the northern Tethyan margin. The analyzed solvent extractable fraction of the sedimentary OM is mainly composed of a marine origin with an admixture of land plant material. Variations in specific biomarkers for cyanobacteria (2α-methyl-hopanes), dinoflagellates (dinosterane or 4-desmethyl-23,24-dimethyl steranes) and terrigenous plant-derived OM (odd‐numbered long-chain n-alkanes) as well as the sterane/hopane ratio, the C35 hopane index and the isoprenoids pristane and phytane were investigated. In contrast to the well-studied mid-Cretaceous Oceanic Anoxic Events (OAEs), neither significant OM enrichment nor prominent fluctuations in the selected biomarker abundances can be observed during the build-up phase of the Valanginian CIE. This points to relatively stable marine paleoenvironmental conditions with well-oxygenated bottom waters. Prior to the CIE, four cm-thick, finely laminated, dark layers (known as Barrande layers) with total organic carbon content reaching up to 4% show an exception from the generally stable biomarker pattern. Sedimentological and biomarker evidence support deposition under less oxygenated conditions for the Barrande layers. However, their occurrence clearly predates the onset of the positive δ13Ccarb shift (by about 180kyrs). Contrary to the subsequent mid-Cretaceous CIEs, the occurrence of widespread anoxia associated with the Valanginian CIE cannot be confirmed for the Vocontian Basin.

南フランス，ボコンティアン堆積盆における白亜紀海洋無酸素事変（OAE）2時の海洋生態系変動の復元

南フランス，ボコンティアン堆積盆における白亜紀海洋無酸素事変（OAE）2時の海洋生態系変動の復元

Prevailing oxic environments in the Pacific Ocean during the mid-Cretaceous Oceanic Anoxic Event 2

The occurrence of Oceanic Anoxic Event 2 (OAE2) 94 million years ago is considered to be one of the largest carbon cycle perturbations in the Earth's history. The marked increase in the spatial extent of the anoxic conditions in the world's oceans associated with OAE2 resulted in the mass accumulation of organic-rich sediments. Although extensive oceanographic studies of OAE2 have been undertaken in the Atlantic Ocean, the Tethys Sea, and the epicontinental seas of Europe and America, little is known about OAE2 in the Pacific Ocean. Here, we present high-resolution carbon-isotope and degree of pyritization (DOP) data from marine sequences that formed along the continental margins of North America and Asia below the northeastern and northwestern Pacific Ocean. The predominance of low DOP values in these areas revealed that the continental margins of the Pacific Ocean were oxic for most of the OAE2 interval.