We present a comprehensive data set of organic and inorganic geochemistry from a lower Cretaceous pelagic bedded chert succession of the Shimanto accretionary belt in the Yokonami Peninsula (Kochi, Japan). Based on stable isotopic composition of total organic carbon (δ 13 C org ), in conjunction with radiolarian biostratigraphic data, we propose that a 1.3-m-thick interval within the examined section is correlative with Tethyan Selli Level (Apennines, Italy), a sedimentary expression of oceanic anoxic event (OAE) 1a. Specifically, the δ 13 C org record illustrates a discernible negative shift and subsequent positive excursions upsection, a pattern that resembles the typical δ 13 C org pattern across OAE 1a reported from various sites such as the Mediterranean Tethys and Pacific seamount flanks. Our δ 13 C org record from the deep Pacific basin supports the idea that the δ 13 C variation across OAE 1a was induced by a significant perturbation of global carbon cycle. The slight increase in total organic carbon contents of sediment deposited during OAE 1a suggests slight or no expansion of oxygen-deficient water mass in the overlying water column. Rare earth elements and lead isotopic compositions indicate relatively higher contributions of volcanic or hydrothermally altered components before and after OAE 1a. The volcanic or hydrothermal source may be associated with emplacement of the Ontong Java Plateau during the early Aptian, or tectonically induced hydrothermal alteration associated with the formation of the accretionary complex.

The Early Cretaceous represents a time interval in the greenhouse world that was characterized by dramatic changes in the paleogeography, paleoceanography, and paleoclimate of the Earth system. Furthermore, a striking, prominent feature of the geomagnetic polarity time scale is the ~34 m.y. period of the normal polarity field (Cretaceous Normal Polarity Superchron). Although marine anomalies and paleomagnetic data from deep-sea cores and land sections indicate that a reversed polarity (M0r) with a duration of ~0.4 m.y. occurred before the superchron, incomplete exposure, coupled with gaps in sampling due to the presence of marl layers, has limited the identification of M0r in a number of sections. An integrated multidisciplinary investigation of lower Cretaceous sediments at the base of the Poggio le Guaine (PLG) core (Northern Apennines, central Italy) was carried out to identify the Barremian-Aptian contact, which is defined by the M0r lower boundary. Rock magnetic measurements of the studied interval of the PLG core reveal magnetite as the main magnetic carrier. Paleomagnetic results indicate a short interval characterized by reverse polarity. This interval is in the uppermost part of the Hedbergella excelsa planktonic foraminiferal zone and in the upper part of the Chiastozygus litterarius (NC6) calcareous nannofossil zone. Stable carbon (δ 13 C) and oxygen (δ 18 O) isotopes indicate a chemostratigraphy of the PLG core with the signature of oceanic anoxic event 1a (OAE 1a). The lithological expression of OAE 1a is the organic-rich black shale unit known as the Selli Level. The comparison of our magnetostratigraphic, biostratigraphic, and chemostratigraphic records throughout the Barremian-Aptian boundary with those available from previously investigated oceanic and land-based sites allows recognition of the magnetochron M0r and OAE 1a at PLG for the first time.

Abstract At the Curití Quarry, Eastern Cordillera of Colombia, a 23.35 m section includes a 6.3 m interval of organic-rich marlstones, calcareous clay-shales and clay-shales devoid of benthic fossils and bioturbation, at the base of the Paja Formation. It overlies carbonate ramp deposits of the Rosablanca Formation, with the topmost layer containing reworked and phosphatized assemblages of middle Barremian to lowest Aptian ammonites of the genera Pulchellia , Gerhardtia , Toxancycloceras , Karsteniceras and Prodeshayesites . High-resolution analyses of the section, including total inorganic carbon (TIC, 0.09–50 wt%), total organic carbon (TOC, up to 8.4%) and stable carbon isotope measurements (δ 13 C org between −22.05‰ and −20.47‰) allow the determination of the stratigraphic relationship between the organic-rich level within the Paja Formation and Oceanic Anoxic Event-1a (OAE-1a). The range of values and the overall pattern of the δ 13 C org curve are comparable with the Lower Aptian interval C7. Thus, the organic-rich shale interval of the Paja Formation at the Curití section, showing characteristics of oxygen-depleted conditions, was deposited after OAE-1a, which is known to occur between isotopic levels C3 and C6. Supplementary material: TIC, TOC and Stable Carbon Isotope data at Curiti Section are available at http://www.geolsoc.org.uk/SUP18634

Among various paleoenvironmental proxy records across the lower Aptian Selli Level (Umbria-Marche basin, Italy) and its equivalents (i.e., sedimentary expressions of oceanic anoxic event [OAE] 1a), one very intriguing feature is a prominent negative δ 13 C excursion at the base of this sedimentary unit, generally by as much as a few per mil in marine carbonates. This early Aptian δ 13 C event has received special attention as an important clue to the genesis of OAE 1a, but the exact amplitude of this relatively short lived δ 13 C variation is not precisely constrained, and this fact has been an obstacle in paleoenvironmental modeling. Particularly enigmatic is the large amplitude, by −7‰, in pelagic limestones at the central Pacific Deep Sea Drilling Project (DSDP) Site 463; it may be a primary δ 13 C signal, considering its deposition in a fully open-ocean setting and a conservative burial diagenetic environment. Nevertheless, new Sr isotope data help identify a peculiar diagenetic overprint on the lower Aptian interval of this site. While the majority of examined samples represent slightly shifted, but acceptable, 87 Sr/ 86 Sr ratios for marine carbonates of this age, markedly unradiogenic 87 Sr/ 86 Sr ratios are recorded within and just below the Selli Level–equivalent interval. In particular, extremely unradiogenic 87 Sr/ 86 Sr ratios (0.70623–0.70642) are detected at exactly the same interval as the most negative δ 13 C values and a smectite-rich lithology. It is therefore proposed that hitherto unknown diagenetic process, or processes, under peculiar interstitial-water geochemistry, resulting from volcanic ash alteration, played a role for the paired δ 13 C- 87 Sr/ 86 Sr anomaly at DSDP Site 463. By removing the δ 13 C data from samples that possess the highly unradiogenic 87 Sr/ 86 Sr ratios, the amplitude of early Aptian negative δ 13 C excursion is reevaluated to be −2.7‰ or −3.3‰, facilitating comparison with published δ 13 C records from other sections. The refined Site 463 δ 13 C profile further implies that the early Aptian negative δ 13 C event is better described as a twin excursion.

Abstract Recent ichnological analysis conducted in two sections (Rodiles and Lastres) of the Asturian Basin revealed the presence of Halimedides Lorenz von Liburnau 1902 , which occurs just above the black shales related to the end of the Toarcian Oceanic Anoxic Event (T-OAE). Halimedides is associated with recovery of the trace-maker community after the re-establishment of favourable, oxic, conditions. The appearance of Halimedides after the T-OAE event, previously not registered, supports the close relationship of the trace maker with oxygen conditions, as occurs in other anoxic events including the Cretaceous OAE-1a and OAE-2. Also, a relationship between morphometric and palaeoenvironmental parameters is observed: occurring larger and densely chambered specimens in darker, weakly oxygenated facies, while smaller and sparsely chambered forms are registered in lighter, better oxygenated sediments.

Abstract: The deep-water Pindos Basin was characterized by siliceous sedimentation during Jurassic-Cretaceous times. Cretaceous siliceous oceanic red beds, i.e., red cherts and radiolarites 25 to 100 m in thickness, are present in the Katafito Formation (Valanginian-Coniacian), whereas the overlying Upper Cretaceous Platy Limestone Formation is characterized by red and gray pelagic carbonates and turbidites. The Katafito Formation consists of siliceous red claystones, ribbon radiolarites, cherts to marly claystones with an apparent red-green cyclicity, silicified mud, and radiolarian-sand turbidites. Sedimentation rates vary between 0.5 and 1.5 mm/kyr. In the siliceous CORB succession, five green to black levels have been recognized, composed of greenish claystones, mudstone turbidites, green to black layered or nodular cherts, and distinct black shales. These green to black intervals can be correlated to oceanic anoxic events, namely, the Late Valanginian Weissert Event, a latest Barremian event, and the Early Aptian OAE 1a, the Late Aptian to early Albian OAE 1b, and the Late Albian OAE 1c and OAE 1d events. From the Middle Cenomanian onwards, turbidites and mass-flow deposits increase in number, but red radiolarites remain the normal background sedimentation in the Pindos Basin. The overlying red marlstones and limestones of the basal Platy Limestone Formation, starting diachronously in the Coniacian to Santonian, record a shallowing of the basin to above the calcite compensation depth and again an increase in detrital carbonate input. The siliceous red beds of the Pindos Basin and other basins in the eastern Mediterranean indicate a long-term record of oxic sedimentation during the Early Cretaceous up to the Coniacian that was punctuated only by short-term anoxia during widespread oceanic anoxic events.

The mid-Cretaceous was marked by emplacement of large igneous provinces (LIPs) that formed gigantic oceanic plateaus, affecting ecosystems on a global scale, with biota forced to face excess CO 2 resulting in climate and ocean perturbations. Volcanic phases of the Ontong Java Plateau (OJP) and the southern Kerguelen Plateau (SKP) are radiometrically dated and correlate with paleoenvironmental changes, suggesting causal links between LIPs and ecosystem responses. Aptian biocalcification crises and recoveries are broadly coeval with C, Pb, and Os isotopic anomalies, trace metal influxes, global anoxia, and climate changes. Early Aptian greenhouse or super-greenhouse conditions were followed by prolonged cooling during the late Aptian, when OJP and SKP developed, respectively. Massive volcanism occurring at equatorial versus high paleolatitudes and submarine versus subaerial settings triggered very different climate responses but similar disruptions in the marine carbonate system. Excess CO 2 arguably induced episodic ocean acidification that was detrimental to marine calcifiers, regardless of hot or cool conditions. Global anoxia was reached only under extreme warming, whereas cold conditions kept the oceans well oxygenated even at times of intensified fertility. The environmental disruptions attributed to the OJP did not trigger a mass extinction: rock-forming nannoconids and benthic communities underwent a significant decline during Oceanic Anoxic Event (OAE) 1a, but recovered when paroxysmal volcanism finished. Extinction of many planktonic foraminiferal and nannoplankton taxa, including most nannoconids, and most aragonitic rudists in latest Aptian time was likely triggered by severe ocean acidification. Upgraded dating of paleoceanographic events, improved radiometric ages of the OJP and SKP, and time-scale revision are needed to substantiate the links between magmatism and paleoenvironmental perturbations.