Oxygen level,primary productivity,and water turbulence during the OAE2 interval of Zagros Basin (SW Iran): Benthic foraminiferal variations in the carbonate microfacies

Cenomanian/Turonian boundary (upper Sarvak Formation) benthic foraminiferal assemblages were analyzed to reconstruct oxygen level, primary productivity, and water turbulence in the Izeh Zone, Zagros Basin. The interplay between environmental perturbations during the Oceanic Anoxic Event 2 (OAE2) and regional tectonic activities in the Zagros Basin resulted in formation of various benthic foraminiferal assemblages in the study section. The OAE2 interval at the region of study starts with extinction of rotaliporids at the onset of δ¹³C positive excursion (peak “a”), which is associated with population of infaunal benthic foraminifera (especially Bolivina alata). The following interval at the onset of Whiteinella archaeocretacea Biozone is characterized by the total absence of benthic taxa and dominance of planoheterohelicids (“Heterohelix shift”) in the black shale strata, indicating expansion of oxygen minimum zone and unhospitable conditions for both benthic and planktic foraminifera. The upper part of OAE2 interval (including δ¹³C peaks “b” and “c”) coincides with harbinger of Neo-Tethys closure in the Arabian Plate, causing a compressional tectonic regime, and creation of uplifted terrains in the basin. The relative sea level started to locally fall in this succession, which was accompanied by a better ventilation of seafloor, lower TOC contents, and reappearance of benthic foraminifera.     

Stratigraphy of the Cenomanian–Turonian Oceanic Anoxic Event OAE2 in shallow shelf sequences of NE Egypt

Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ¹³C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ¹³C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ¹³C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.     

西藏特提斯喜马拉雅海相白垩纪—古近纪生物地层与重大地质事件研究进展

地质历史时期曾发过许多对生命的演化进程造成过重大影响与制约的全球性地质事件,白垩纪—古近纪就是一个重大地质事件频发的时期。随着冈瓦纳大陆在中生代时期的解体,全球海陆格局发生了根本的变化,地球的表层和岩石圈层均发生了重大的改变,由此引发了构造运动空前活跃,发生过诸如大洋缺氧事件(OAE)、大洋富氧事件(CORBs)、白垩纪/古近纪(K/Pg)生物大灭绝事件、古新世—始新世极热(PETM)事件、印度-亚洲板块碰撞、新特提斯洋的演化及最终消亡等一系列的全球性重大地质事件。对这些重大地质事件的研究,有助于加深我们对古海洋、古地理、古环境的认识。尝试追踪和捕捉这些重大地质事件,恢复和重建古地理,其基础是建立精确的年代地层格架。西藏南部保存了中国最为完整的海相白垩纪—古近纪沉积,完整地记录了上述的全球性重大地质事件,通过对札达、岗巴、定日、亚东、江孜、萨嘎和吉隆等地区高分辨率浮游和底栖有孔虫、介形虫、钙质超微化石和放射虫生物地层学研究,可直接约束全球性重大地质事件发生的时间,并为重建新特提斯洋古海洋环境和古地理提供证据。此外,在重大地质环境突变期间生物的演化过程,也可为探明极端环境变化发生时期气候-环境-生物之间的协同演化关系提供证据。本文系统总结了课题组为主的近年来对藏南白垩纪—古近纪海相地层中微体古生物学的研究成果及重要进展,并对未来研究提出展望。     

Stratigraphy of the Cenomanian–Turonian Oceanic Anoxic Event OAE2 in shallow shelf sequences of NE Egypt

Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ¹³C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ¹³C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ¹³C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.     

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.     

The upper Cenomanian–lower Turonian carbonate platform of the Preafrican Trough,Morocco: Biostratigraphic,paleoecological and paleobiogeographical distribution of ostracods

The upper Cenomanian–lower Turonian paleoenvironments of the Preafrican Trough carbonate platform is characterized by analyzing the structure of the ostracod assemblages and the information provided by other groups, and also by linking together the paleontological and geochemical data (detrital influx-redox-paleoproductivity proxies, δ¹³C curve). Two different domains (eastern and western) can be recognized on the platform during the late Cenomanian, before the onset of the OAE2. The western domain corresponds to a low-energy environment developed on a mid and/or outer ramp with hypoxic waters, low detrital influx and low paleoproductivity. The paleoecological assemblages show limited specific diversity but variable density. The ostracods are opportunistic and unspecialized (r strategists), being associated with Buliminidae, surface and intermediate-water planktonic foraminifera, and fishes. The eastern domain corresponds to an inner ramp and/or peritidal environment with oxic waters, low detrital influx and low paleoproductivity, developed in a higher energy environment with paleoecological assemblages showing high diversity but variable density. The ostracods are more specialized (K strategists), being represented by diverse and constant assemblages associated with diversified benthic foraminifera, calcareous sponges and echinoderms, as well as intermediate- and deep-water planktonic foraminifera. The onset of the OAE2 has no influence on the western ostracod assemblages, but leads to the decline of the ostracod fauna and the disappearance of the deep-water planktonic foraminifera in the eastern domain. During the early Turonian, after the OAE2, the platform becomes an outer ramp with increased paleoproductivity, but is associated with a decrease of taxonomic diversity in hypoxic waters. The ostracods are very sparse and unspecialized, associated with siliceous sponges, Buliminidae, surface-living planktonic foraminifera, fishes and pelagic crinoids. Marine paleobiogeographic communication is relatively easy across the carbonate platforms between the Preafrican Trough and other Moroccan regions, as well as between Morocco and different parts of the South Tethyan and East Atlantic margins belonging to the Cenomanian–Turonian South Tethyan Ostracod Province (STOP). Thirteen new species are described: Cytherella tazzouguertensis n. sp., Bairdiacypris chaabetensis n. sp., Bythocypris amelkisensis n. sp., Pontocypris tadighoustensis n. sp., Procytherura? elongatissima n. sp., Loxoconcha akrabouensis n. sp., Hemiparacytheridea sagittaemucronata n. sp., Rehacythereis errachidiaensis n. sp., Rehacythereis zizensis n. sp., Veenia (Nigeria) tardaensis n. sp., Veenia (Nigeria) mediacostarobusta n. sp., Xestoleberis? preafricanensis n. sp., and Xestoleberis circinatus n. sp.     

Variation on Foraminiferal Composition in Cretaceous Black-Gray-Red Bed Sequence of Southern Tibet, China

An Upper Cretaceous black-gray-red bed sequence was deposited in the Tethys-Himalayan Sea where abundant foraminifera, especially planktons, were yielded. In the shallow shelf to the upper slope on the north margin of Indian plate was recorded an extinction-recovery-radiation cycle of foraminiferal fauna highly sensitive to paleoceanographical changes. The black unit, consisting of the Late Cenomanian-earliest Turonian beds, displays a major extinction, with keeled planktonic and many benthic species as the principal victims at the end of the Cenomanian when existed only low diversity, sin-face water-dwelling foraminifera. The gray unit spans a long-term recovery interval from the Turonian to the early Santonian with keeled planktonic foraminifera returning stepwise to the water colunm. The planktonic biota in the red unit, extremely abundant, indicate a biotic radiation during the Late Santonian and the Early Campanian, implying that the high oxygen levels had returned to all the oceanic depth levels,and that the water stratification disappeared, followed by the radiation of all depth-dwellers. The variation on foraminiferal faunas from the whole sequence refers to the extreme warm climate that appeared in the Middle Cretaceous and to the declined temperature toward the late epoch. Substantial deposits for this warming and cooling zones represent the black shales in the Middle Cretaceous and the red beds in the later period of the southern Tibet. The change in the foraminiferal composition corresponded to the formation of dysaerobic facies and to the development of high-oxidized circumstances.     

Repeated anoxia–extinction episodes progressing from slope to shelf during the latest Cenomanian

Oceanic Anoxic Event 2 (OAE 2) during the Cenomanian–Turonian (C/T) transition caused stepwise marine extinctions. Using organic compounds, stable carbon and oxygen isotopes, and foraminifera from three depth-transect sections in northern Spain, this study revealed repeated anoxic/euxinic events coinciding with warming and stepwise extinctions of planktonic and/or benthic foraminifera within intermediate to surface waters in the proto-North Atlantic during the C/T transition. Those short-duration euxinic events occurred four times: at 93.95 Ma, marked by the extinction of Rotalipora greenhornensis; at 93.90 Ma, marked by the extinction of Rotalipora cushmani; at the mid-time maximum of the plateau of the δ¹³C of carbonates (93.70 Ma); and at the time of the C/T boundary (93.55 Ma). Furthermore, the main benthic foraminiferal extinctions occurred during the first and second euxinic events in the upper slope, during the second and third euxinic events in the outer to middle shelf, and during the third and fourth events in the middle shelf. The main euxinic events in each section also showed a progression to the shallow shelf. The main anoxia–extinction events occurred in the upper slope and outer shelf then moved to the middle shelf. The shallowest section had relatively weak anoxia and a proportionally low extinction rate. These new findings indicate that foraminiferal extinctions started from the intermediate water and the continental slope and then moved to the continental shelf. This was the result of the repeated progression of euxinic–anoxic water from the upper slope to the middle shelf on the eastern continental margin of the proto-North Atlantic four times during a 400 kyr period, to the end of the Cenomanian.     

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.     

Cenomanian–Turonian carbon isotope stratigraphy of the Western Canadian Sedimentary Basin

The Cenomanian–Turonian boundary was characterized by distinctive positive carbon isotope excursions that were related to the formation of widespread oceanic anoxia. High-resolution geochemical proxies (TOC, CaCO₃, δ¹³C_org, and δ¹³C_carb) obtained from bulk rock, planktic foraminifers, and inoceramids from four marine marlstone-dominated stratigraphic sections in the Western Canada Sedimentary Basin (WCSB) were used to establish a regional carbon isotope stratigraphic framework and to investigate paleoenvironmental variability in four different depositional settings. Compared to background δ¹³C_org, (<−27‰) and δ¹³C_carb (<2‰) values which were correlative to stable isotope excursions during Oceanic Anoxic Event (OAE) II worldwide, the δ¹³C_org (>24‰), and δ¹³C_carb (>4‰) derived from inoceramid prisms in the studied sections within WCSB, were elevated during the Late Cenomanian–Early Turonian. During this interval, TOC and CaCO₃ values which increased sporadically to >40% and 7%, respectively, were not consistent enough to be used for stratigraphic correlations. Based on the δ¹³C_org excursions, two bentonite beds were regionally correlated across this portion of the Western Interior Seaway (WIS). The eruption associated with the “Red” bentonite occurred approximately coeval with the maximum δ¹³C_org-excursion during OAE II in the Neocardioceras juddii Zone, whereas the “Blue” bentonite coincides with the termination of OAE II in the latest Watinoceras devonense zone. During the Late Cenomanian–Early Turonian in the WCSB, benthic foraminifers were sparse or totally absent, indicating the existence of fully anoxic bottom-water conditions. Planktic foraminifera were common in the well-oxygenated surface waters. A benthic oxic zone characterized by several agglutinated species occurs in the eastern part of the WSCB at the beginning of OAE II in the Sciponoceras gracile zone. The termination of the OAE II in the WCSB coincides with the first occurrence of small ammonites (Subprionocyclus sp.) in the western part of the basin.     

Palaeoenvironmental reconstruction of the Oligocene Afales Basin, Ithaki island, western Greece

Assemblages of benthic foraminifera from one clastic succession in the Afales Basin (Ithaki Island, western Greece) were investigated to reconstruct palaeoenvironmental conditions during the Oligocene. The section consists of alternating hemipelagic marls and detrital deposits, designated as flysch-like beds, attributed to biostratigraphic Zones P20 and P21. Planktic percentages are mostly high (66–80%). Benthic foraminiferal assemblages comprise calcareous and agglutinated taxa (up to 15%). The prevalence of epifaunal foraminifera indicates good ventilation of the bottom water resulting from basin morphology, which enabled the undisturbed flow of water throughout the basin. Palaeodepth estimates imply bathyal deposition, from about 800 to 1200 m deep. The benthic foraminiferal fauna is of high diversity along the section, as is expected in deep marine environments. The abundances of the most common foraminiferal taxa (Cibicidoides spp., Oridorsalis umbonatus, Gyroidinoides spp., Stilostomella spp., Nodosariidae, Nuttallides umbonifera) are quite variable and imply generally oligotrophic to mesotrophic environmental conditions with variable organic flux.     

对西藏岗巴上白垩统的新认识

在前人研究的基础上对岗巴地区上白垩统的划分做了厘定 ,进一步建立了晚白垩世的 1 2个浮游有孔虫化石带。根据岩性特征及化石带的研究 ,上白垩统被划分为赛诺曼期至土仑早期的冷青热组 ;土仑中期至三冬期的岗巴村口组和康潘期至马斯特里赫特期的宗山组。该区赛诺曼期与土仑期的界线位于冷青热组上部 ,以 H elvetoglobotruncana praehelvetica的初现为标志。     

Stratigraphy and paleobathymetric interpretation of the Cretaceous Karai shale formation of Uttatur Group, TamilNadu, India

The Karai shale Formation of the Uttatur Group is exposed in a bad land area at the western margin of the Cauvery Basin. This shale has been investigated based on foraminiferal fauna and clay minerals. The foraminiferal assemblages obtained contain predominantly calcareous benthic foraminifera, rare planktic and arenaceous foraminifera. The planktic foraminiferal index taxa Planomalina buxtorfi, Rotalipora reicheli, Praeglobotruncana stephani, and Hedbergella portsdownensis suggest the late Albian to middle Turonian age. The benthic assemblage dominated by Lenticulina, Gavelinella, Osangularia and Quadrimorphina, suggests an outer neritic (100–200 m) environment. The clay mineral content dominated by kaolinite-illite-montmorillonite indicates that the Karai shale was formed from weathering of igneous rocks.     

Oceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco

Profound biotic changes accompanied the late Cenomanian δ¹³C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid δ¹³C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding δ¹³C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in diversity to just two species, the dominant (80–90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.     

Turonian cephalopods (ammonoids and a nautiloid) from the Wadi Daya Formation of the Talerzha Basin (South Riffian Ridges Domain,northern Morocco)

The Wadi Daya Formation, or the Calcaires crayeux of the older literature, attains a thickness of 10–40 m in the Talerhza Basin of the South Riffian Ridges. Previously, this unit was first dated as “Vraconian” (i.e., late upper Albian), but then reinterpreted as Cenomanian-Turonian and Cenomanian-Coniacian on the basis of foraminiferal and ostracod assemblages, respectively. Here, we record for the first time in the South Riffian Ridges, some typically Turonian ammonoids and a nautiloid species, namely Romaniceras (Yubariceras) cf. ornatissimum (Stoliczka), Spathites (Jeanrogericeras) cf. reveliereanus (Courtiller), Neoptychites cephalotus (Courtiller), Pachydesmoceras linderi (de Grossouvre), Lewesiceras peramplum (Mantell) and Angulithes galea (Fritsch, in Fritsch & Schlönbach). These species are herein described and illustrated. In view of these data, the underlying Marnes et marno-calcaires jaunes Formation, formerly dated as “Vraconian”, could in fact be of a middle to late Cenomanian date, in accordance with the age assignment based on planktonic foraminifera. Deposition of the overlying Marnes jaunes Formation, previously dated as Cenomanian-“Senonian”, probably started during the latest Turonian or earliest Coniacian.     

从有孔虫分析西藏南部白垩纪海平面升降

白垩纪是地质历史中的最大海侵时期。海平面的升降改变了海洋的物理、化学及生物因素,从而影响了有孔虫的演化与发展。根据对有孔虫丰度、分异度及演化类型的研究,认为西藏白垩纪最大海侵时期形成于赛诺曼期与土仑期的界线附近。此后海平面总的处于下降趋势,只是在康尼亚克期至三冬期海平面又有回升。马斯特里赫特末期有孔虫的大量灭绝反映了白垩纪与第三纪界线事件的影响。     

北京顺义早更新世有孔虫动物群的古生态及古环境

1975年我们报道了泥河湾盆地怀来及蔚县境内发现半咸水有孔虫化石,当时认为它们的层位可能属第四系上部,后来进一步工作证实应是泥河湾组上部沉积。1977年我们对华北平原第四纪海进海退现象作了初步报道,认为华北平原在早更新世时发生过海进(渤海海进),并引用陈方吉在北京东南发现海相介形虫资料,说明早更新世渤海海进时海水曾经到达这一地区。1978年李鼎容等在北京市顺义县早更新世地层中发现大量有孔虫化石。进一步证实北京地区早更新世确曾发生过海进,使我们解释怀来及蔚县所发现的半咸水有孔虫来源,有更多证据。这一发现,对于北京地区第四纪地层划分对比、新构造运动判别和古地理恢复有重大意义。     

Integrated foraminifera and δ<Superscript>13</Superscript>C stratigraphy across the Cenomanian–Turonian event interval in the eastern Baltic (Lithuania)

The Cenomanian–Turonian transition marks one of the most important extinction episodes of the Mesozoic era. This extinction event was associated with the development of widespread oceanic anoxia and pronounced stable carbon isotopic excursion. Despite its importance, the effects of the perturbation on higher latitude biotas, and from the Baltic region in particular, are currently underexplored. Therefore, in this contribution we present the fossil record of a foraminifera succession integrated with δ¹³C trends from two deep cores: Bliūdsukiai-19 from western Lithuania and Balta?i?k?-267 from southern Lithuania. Two foraminiferal zones were distinguished: Rotalipora cushmani from the upper Cenomanian and Whiteinella archaeocretacea from the boundary strata between the Cenomanian and Turonian in the Balta?i?k?-267 core section, and a W. archaeocretacea Zone in the Bliūdsukiai-19 core section. A chemostratigraphical analysis of the stable carbon isotopes revealed a positive Cenomanian–Turonian δ¹³C anomaly, with maximum values reaching 3.57‰ in the upper part of the Bliūdsukiai-19 core section. A non-metric multidimensional scaling analysis of the foraminifera communities revealed that the major changes in their assemblages were strongly temporally organized and associated with the changes in the stable carbon isotopic ratios. This fact points to the significant effects of the C–T extinction event on the northern Neotethys paleocommunities.     

The biotic crisis across the Oceanic Anoxic Event 2: Palaeoenvironmental inferences based on foraminifera and geochemical proxies from the South Iberian Palaeomargin

Open marine sediments deposited during the Cenomanian–Turonian transition are well exposed in the Spanish Baños de la Hedionda section (Betic Cordillera, South Iberian Palaeomargin). Analysis of foraminiferal assemblages and geochemical proxies allow inferences on the impact of the Oceanic Anoxic Event 2 (OAE2) in this area of the western Tethys. Three main intervals have been identified corresponding to different lithological units and biozones. (1) The top of the Capas Blancas Member (Rotalipora cushmani Biozone) represents the pre-extinction phase with diverse foraminiferal assemblages and well developed water-column tiering, well-oxygenated, oligotrophic deep-waters and oxygenated to poorly oxygenated, mesotrophic surface-waters. Foraminiferal opportunist species point to a minor event with dysoxic conditions preceding the OAE2. (2) The black radiolaritic shales (Whiteinella archaeocretacea Biozone) consist of a foraminiferal-barren interval, except for the lowermost centimetres where planktic surface-dweller opportunists are common. Redox sensitive elements (Cr/Al, V/Al, U/Th, Mo_EF, Mo_aut, U_EF and U_aut) and increased TOC values reflect oxygen depleted conditions related to the OAE2. The increase in P/Ti values at the base of this stratigraphic interval indicates an abrupt increase in productivity. High concentrations of radiolarians are congruent with high surface productivity probably related to changes in oceanic circulation and enhanced upwelling currents, as well as subsequent shallowing of the oxygen-minimum zone. The increase in Mo_EF and Mo_aut towards the top of the black radiolaritic shales indicates temporal euxinic conditions. (3) A slow, bottom-up recovery of foraminiferal assemblages is inferred at the base of the Boquerón Member (Helvetoglobotruncana helvetica Biozone), with seafloor recolonization by benthic foraminifera being recorded previous to the water column colonization by planktic forms, mainly by intermediate-dwellers typical of mesotrophic waters. The subsequent proliferation of surface-dweller opportunists and deep-dweller opportunists adapted to mesotrophic to eutrophic conditions, and the decrease in planktic foraminiferal diversity, may indicate the persistence of poorly oxygenated conditions in the water column towards the lower-middle part of the H. helvetica Biozone.     

Geochemical Record of the Cenomanian–Turonian Anoxic Event in Tunisia: Is it Correlative and Isochronous to the Biotic Signal?

In Tunisia, five Bahloul spaced sections, Bargou, Jerisa, Guern Halfaya, Kherij and Gafsa were analyzed for biostratigraphy (foraminifera and radiolarians) and major and trace elements. This high-resolution biostratigraphic and chemostratigraphic integrated analyses for the Late Cenomanian–Early Turonian Bahloul Formation provide new insight into the palaeoceanographic evolution of the southern Tethyan margin. Relative low abundance of related terrigenous Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba, Cu, and Ni (organic matter related trace elements) suggests that the Bahloul, deposited during a relatively short period (0.5 Ma), was of relatively elevated primary productivity and minimal detrital input. While higher D* values concurrent with lower Ti/Al ratios are interpreted as caused by enhanced fluvial material contribution, due to more humid climate during the OAE-2. Enhanced humidity triggered probably fluvial influxes, resulted in a sluggish water circulation and consequent anoxic/euxinic conditions favoring the preservation of organic matter at the bottom. Enrichments in redox-sensitive trace metals U, V, and Mo in the Bahloul Formation deposits and redox indices, such as V/(V+Ni), U/Th, V/Cr, and Ni/Co, indicate that oxygen-restricted conditions prevailed during the Late Cenomanian to earliest Turonian times and correlate well with relative abundances of some foraminiferal and radiolarian paleo-environmental relevant indicators. High Baxs values and Uauth may indicate anoxic conditions at least at the water–sediment interface during the Bahloul Formation deposition and provide information about low to moderate sulfate-reduction reactions.