Aptian and Albian atmospheric CO2 changes during oceanic anoxic events: Evidence from fossil Ginkgo cuticles in Jilin Province,Northeast China

The Early Cretaceous was a time with super-greenhouse conditions and episodic global oceanic anoxic events. However, relative timing of atmospheric CO₂ emissions and oceanic anoxic events, and their causal relationships remain matters of debate. Using the stomatal index approach, well-preserved fossil cuticles of Ginkgo from the Lower Cretaceous Changcai Formation, eastern Jilin, and from the Lower Cretaceous Yingcheng Formation, central Jilin, Northeast China, were investigated to reconstruct atmospheric CO₂ concentrations during the Aptian and earliest Albian (Early Cretaceous). The results indicate that the CO₂ concentrations reached 1098–1142 ppmv (Carboniferous standardization) or 970–1305 ppmv (regression function) during the Aptian and earliest Albian. Our estimates of palaeoatmospheric CO₂ concentrations during the earliest Albian (OAE 1b) are slightly higher than the data between the early Aptian Selli (OAE 1a) and the middle Aptian Fallot OAEs; this may indicate the absence of any great emissions of CO₂ during the latest Aptian and earliest Albian.     

Paleoceanographic evolution and chronostratigraphy of the Aptian Oceanic Anoxic Event 1a (OAE1a) to oceanic red bed 1 (ORB1) in the Gorgo a Cerbara section (central Italy)

We performed a detailed study of the stratigraphic transition of Oceanic Anoxic Event 1a (OAE1a) to oceanic red bed 1 (ORB1) from the classic Gorgo a Cerbara section in the Umbria region of central Italy. We focused on a 25.5-m-thick stratigraphic succession, from which we analyzed 305 samples for total organic carbon (TOC), CaCO₃, magnetic susceptibility, diffuse reflectance spectrophotometry and the stable carbon and oxygen isotopic composition of both bulk samples and organic matter. In the Gorgo a Cerbara section, the Selli Level of OAE1a (∼1.81 m thick) consists of laminated to bioturbated dark gray to black mudstones and shales with medium to dark gray radiolarian-rich silty to sandy layers and a maximum TOC content of 20.22%. The carbon isotopic values show a negative excursion (C3 stage, ∼0.14 m) at the base of the Selli Level, followed by a stepwise positive excursion (C4–C6 stages, ∼1.67 m) in the upper part of the Selli Level. The transition from OAE1a to ORB1 (∼3.19 m thick) is characterized by bioturbated greenish gray cherty limestones and marly limestones with subordinate marls, corresponding to stable carbon isotopic C7 stage and lasts for ∼0.75 Ma. The ORB1 interval (∼13.15 m) consists of reddish marly claystones, dark-red marlstones, red marly limestones and red calcareous shales which indicate a highly oxic environment. Our results reveal a stepwise transition from a predominantly mesotrophic and dysoxic to anoxic environment at the time that the OAE1a black shales were deposited to an oligotrophic and oxic environment during the transitional interval and finally to highly oxic conditions during the ORB1 interval. The nannoconid crisis occurs at the top of the C2 stage, just 0.34 m below the negative excursion in δ¹³C isotopic values. The massive CaCO₃ dissolution phase occurs 0.25 m above the negative excursion; it persisted for 0.85 Ma and probably resulted from excess CO₂, ocean acidification, and carbonate compensation depth (CCD) shoaling. Deposition of massive black shales occurs at the base of the C6 stage and lasted for 0.4 Ma.     

High fidelity preservation of fossil insects from the Crato Formation (Lower Cretaceous) of Brazil

Fossil insects from the Lower Cretaceous (Aptian) Crato Formation of north-east Brazil are preserved as goethite replacements in laminated limestones of lacustro-lagoonal origin. They display remarkable degrees of morphological detail down to the macromolecular level in some examples. We document the fidelity of preservation and reveal an astonishing variety of morphological detail comparable in some instances with that found in amber inclusions.     

从人地关系的视角看“五帝时代”的历史真实性及其年代

“五帝时代”是中国史前社会演进的一个重要阶段,然而其历史真实性却一直被质疑,其具体年代也存在较大争议。作者从人地关系视角出发,对这一重大历史问题进行了初步的探讨。首先介绍了历史文献记录“五帝时代”和龙山中晚期考古证据所揭示出类似的“万邦”林立、相互竞争的政治景观及其社会含义,分析了“五帝时代”的可能年代,认为二者在地望、政治景观以及年代等方面都可能存在一致性。基于竞争和冲突与气候变化之间可能存在的成因联系,重建了“五帝时代”的气候背景,发现4.5～4.0 ka BP阶段,即龙山中晚时期为一明显的降温期,且此降温阶段可以与世界其他地区古气候记录对比,进一步证明了该降温期的存在及其广域性分布特征。最后从人地关系的视角,论证了该降温阶段气候变化与“万邦”(酋邦社会)形成之间的成因联系,从一个侧面证明了“五帝时代”存在的历史真实性,同时进一步表明“五帝时代”的年代极有可能相当于龙山时代的中晚期,具体年代为4.5～4.0 ka BP。     

Fault-controlled dolostone geometries in a transgressive–regressive sequence stratigraphic framework

This study investigates the geometries of fault-controlled dolostone geobodies and their structural and sequence stratigraphic controls, which provide new insights for the prediction and production of fault-controlled dolomitized hydrocarbon reservoirs. A very thick succession (>1600 m) of Aptian–Albian shallow-marine carbonates of the Benassal Formation that crop out in the Benicàssim area (Maestrat Basin, eastern Spain) is partly replaced by dolomite, resulting in dolostone geometries ranging from massive patches to stratabound bodies. Detailed mapping, systematic logging and correlation were carried out to characterize the structural, sedimentary and sequence stratigraphic framework of the area and to constrain the principal controls on the full-range of dolostone geometries. The results show that carbonate sediments accumulated in a half graben stacked in three transgressive–regressive sequences. Large-scale massive dolostone patches (with up to kilometre extension) formed near large-scale faults indicating that they acted as entry points for warm dolomitizing fluids into the basin. These dolostone patches laterally pass to large stratabound bodies that extend for long distances (at least 7 km) away from feeding faults, forming a continuum. The presence of a regional unconformity and a clastic fine-grain low-permeability unit (Escucha Formation) on top of the Benassal Formation likely constrained the dolomitization fluids to an up to 580 m thick interval below the base of the Escucha Formation. Thus, only limestones within this interval, corresponding to the two uppermost transgressive–regressive sequences, were dolomitized. There is a clear relationship between the stratigraphic framework and the preferred replaced beds. Dolomitization preferentially affected sediments deposited in inner to middle ramp settings with predominant wackestone to packstone textures. Such facies are laterally most abundant in the east of the study area (i.e. basinward) and vertically in layers around the maximum flooding zone of the top sequence, which is preferentially affected by dolomitization.     

Palaeoecology and biostratigraphy of early Cretaceous (Aptian) calcareous nannofossils and the δCcarb isotope record from NE Iran

Upper Barremian-Lower Aptian sediments of the Sarcheshmeh and Sanganeh formations in the Kopet Dagh area, northeast Iran were studied with regard to their calcareous nannofossil content and their δ¹³C_carb signal. The sediments are composed mainly of marlstones, argillaceous limestones and limestones. Based on the occurrence of biostratigraphic index taxa, the calcareous nannofossil zones NC5, NC6 and the NC7A Subzone were recognised. The calcareous nannofossils and the δ¹³C_carb data enable recognition of the early Aptian Oceanic Anoxic Event 1a (OAE 1a). The deposits of the OAE 1a interval are characterised by the rarity of nannoconids and a sharp negative δ¹³C_carb excursion (1.36‰), followed by an abrupt positive δ¹³C_carb excursion of 4-5‰; both events have been recognised elsewhere in OAE 1a deposits in the Tethys. In the OAE 1a interval, the relative abundance of Watznaueria barnesiae/Watznaueria fossacincta is higher (more than 40%) than that of Biscutum spp., Discorhabdus spp. and Zeugrhabdotus spp., which indicates dissolution. In the upper part of the section, the higher relative abundance of mesotrophic and oligotrophic taxa (Watznaueria spp. and nannoconids respectively) and the enhanced relative abundance of eutrophic taxa (Biscutum spp., Discorhabdus spp., Zeugrhabdotus spp.) is indicative of an environment with slightly increased nutrient content. The presence of warm water taxa (Rhagodiscus asper and nannoconids) and the absence of cool water taxa (Repagulum and Crucibiscutum) suggest warm surface-water conditions.     

Calcareous phytoplankton from the Barremian/Aptian boundary interval in NW Germany

Two cored boreholes in the central part of the North West German Basin recovered a unique section of Upper Barremian to Lower Aptian strata. Calcareous nannofossils show a distinctive shift from boreal endemic assemblages in the Barremian to cosmopolitan ones in the Aptian. This onset of new cosmopolitan species (e.g., Chiastozygus litterarius, Flabellites oblongus, Rhagodiscus angustus, Braarudosphaera sp., Eprolithus sp.) is spread over an interval of 25m, starting well below the early Aptian "Fischschiefer", a dark laminated shale rich in organic matter. These changes in the composition of calcareous nannofossils indicate that major palaeoceanographic changes occurred before the deposition of the Fischschiefer.The distribution pattern of calcispheres allows the differentiation of two sedimentary successions, separated by the Fischschiefer. The lower succession, which includes the sediments below the base of the Fischschiefer, indicates a boreal-pelagic environment. The lower part of the upper successions which includes the Fischschiefer, represents warmer inner shelf conditions. This corresponds to the presence of rich Tethyan-derived nannoconid assemblages and the presence of the planktonic foraminifera Hedbergella in the Fischschiefer. The topmost part of the upper succession (middle Aptian, Hedbergella marl) suggests a change to a pelagic warmer water environment. Two new taxa are introduced: the calcareous dinoflagellate cyst Obliquipithonella laqueata n. sp. and the foraminiferan Choanaella fortunate n. gen. n. sp.     

Ostracodes from the Aptian–Santonian of the Santos,Campos and Espírito Santo basins,Brazil

The Lower and Upper Cretaceous deposits from the Brazilian marginal basins present a rich and diversified fauna of ostracodes in continental, marine and mixohaline paleoenvironments. While the Cretaceous ostracodes from the northeastern region have already been the subject of many taxonomic and biostratigraphical studies, data from the southeastern basins are still scarce and restricted to the record of a few species. The present study represents a comprehensive contribution to the systematic knowledge of the Aptian–Santonian ostracodes from the Brazilian southeastern marginal basins. A total of 1045 cutting samples were analyzed from 10 wells drilled offshore in Santos, Campos and Espírito Santo basins. Thirty-eight ostracode taxa were recorded from different depositional environments, including the following six new species: Amphicytherura fragilis sp. nov., Brachycythere multidifferentis sp. nov., Fossocytheridea ballentae sp. nov., Fossocytheridea elegans sp. nov., Nigeroloxoconcha itanhaensis sp. nov. and Paracypris eniotmetos sp. nov.