The sedimentary expression of oceanic anoxic event 1b in the North Atlantic

The Aptian/Albian oceanic anoxic event 1b contains the record of several perturbations in the global carbon cycle and multiple black shale levels, particularly in the Western Tethys. The local lithological expression of an oceanic anoxic event depends on palaeogeographical and depositional setting as well as on regional palaeoclimate. Marine sediments at a particular location may therefore be more or less organic‐rich (or not at all) and they may consist of different lithologies. In most studies, however, much of the lithological variability associated with oceanic anoxic events is left unaccounted for and, thus, the exact processes leading to the enrichment of organic matter in these marine sediments and their subsequent preservation in the geological record are unknown. This study focuses on the local sedimentary processes behind the deposition of organic‐rich sediments at Deep Sea Drilling Project Site 545 and Ocean Drilling Program Sites 1049 and 1276 in the North Atlantic during oceanic anoxic event 1b. Although specifically dealing with the sediments deposited during this particular event at these localities, it is expected that the same processes were responsible for determining the exact sedimentary products at localities in similar settings, as well as during other similar events in the Mesozoic. Here, it is shown that the deposition of organic‐rich sediments during oceanic anoxic event 1b was a consequence of the enhanced productivity favoured by upwelling and by riverine nutrient input, or aeolian fertilization of the euphotic zone depending on geographical location. Slope instability processes resulted in the transfer of part of these organic‐rich sediments from the shelf to deep sea depocentres as mud‐laden organic‐rich turbidites, especially in the northern North Atlantic. The so‐called ‘black shales’ are much more varied than their name implies. The end product of sedimentation during an oceanic anoxic event at a particular location is commonly the result of several equifinal processes acting on a local scale rather than the direct result of basinal or even global mechanisms.     

Report on the 4th International Meeting of the IUGS Lower Cretaceous Ammonite Working Group, the “Kilian Group” (Dijon, France, 30th August 2010)

The 4th Kilian Group meeting (Dijon, France, 30th August 2010) focused on the Aptian and Albian Stages. For the Aptian, a two-fold division of the stage was adopted for the Mediterranean area with a boundary between the Dufrenoyia furcata and Epicheloniceras martini Zones. The main changes to the zonal scheme concern the Lower Aptian with: the introduction of a Deshayesites luppovi Subzone in the upper part of the Deshayesites oglanlensis Zone; the replacement of Deshayesites weissi by Deshayesites forbesi as new index-species of the second interval zone; the introduction of a Roloboceras hambrovi Subzone in the upper part of the D. forbesi Zone; and the subdivision of the D. furcata Zone into the D. furcata and Dufrenoyia dufrenoyi Subzones. For the Albian, the upper part of the Douvilleiceras mammillatum Zone (Lower Albian) is now characterized by a Lyelliceras pseudolyelli Subzone. The main amendments concern the Upper Albian. The base of this substage is defined by the base of the Dipoloceras cristatum Zone. Above it, the Upper Albian zonal scheme comprises in stratigraphic order the Mortoniceras pricei, Mortoniceras inflatum, Mortoniceras fallax, Mortoniceras rostratum, Mortoniceras perinflatum and Arrhaphoceras briacensis Zones.     

Barremian–Albian (Early Cretaceous) ammonite faunas of the Katsuuragawa Basin,southwest Japan

The Lower Cretaceous ammonite fauna of Japan was influenced by the Tethyan, Boreal and North Pacific realms with their oceanic current patterns and ammonite distributions. The hypothesis of oceanic circulation can be utilized to interpret the existence of the “Bering Strait” and the changing position of the “Boreal front,” that is the contact region of warm and cold-water masses. To understand such a system fully, a comprehensive understanding of the geographical distribution of ammonite faunas is required. The occurrence of twenty-five ammonite species, belonging to twenty genera, is confirmed in the Barremian to Albian of Japan. Of these, 24 species are described in this paper, including Barremites macroumbilicus sp. nov. The fauna can be divided into three associations, lower, middle, and upper, indicating late Barremian, late Aptian, and late Albian of the European standard zonation. The faunal characters suggest that the habitats of these ammonite faunas may have changed during later Early Cretaceous, with faunas characterizing three different environments, i.e., nearshore, intermediate, and distal shelf to upper slope setting. Ammonites of the lower association (late Barremian) are related to those of the Tethys, Boreal European, and circum-Pacific regions, and suggest that the Early Cretaceous Katsuuragawa Basin was deposited under the influence of currents from both high latitude and equatorial areas. The occurrence of Crioceratites (Paracrioceras) suggests that the Boreal European elements, including Simbirskites and Crioceratites (Paracrioceras), transited between Northwest Europe and Japan through the Arctic Sea, indicating that the Pacific Ocean was connected with the Arctic Sea at that time. The second association (late Aptian) is composed of ammonites of the Tethyan and circum-Pacific regions. As European Boreal ammonite taxa are absent in this association, it is concluded that the Pacific Ocean probably was not connected with the Arctic Sea at the time. Consequently, the “Boreal Front,” marking the contact between warm and cold water masses, was located at mid-latitude in the “Bering Strait” region during Barremian and subsequently moved northward during Albian. The uppermost ammonite association (late Albian) also consists of Tethyan and circum-Pacific taxa. Desmoceras (Pseudouhligella) poronaicum expanded its range with northward and eastward circulation of oceanic currents, suggesting the current must have represented a warm water-mass from the equator. The species subsequently migrated from the southern Katsuuragawa Basin to the Hokkaido area during late to latest Albian.     

The emerging terrestrial record of Aptian-Albian global change

The integration of terrestrial carbonate δ¹³C chemostratigraphy and radiometric dates is opening a new window into the continental paleoclimate dynamics of the major carbon cycle perturbations of the Aptian-Albian interval. Results published to date by many researchers clearly show that there was a tight temporal coupling between Aptian-Albian marine, atmospheric, and terrestrial carbon pools that now permits refined global chemostratigraphic and chronostratigraphic correlations on time scales of 10⁶ years or less. This development opens new opportunities to explore the Aptian-Albian Earth system by incorporating continental climate change dynamics in a developing global synthesis. In this paper, we present new U–Pb and U–Th/He age dates on a late Albian volcanic ash deposit in a stratigraphic section that fills a previous gap in in the terrestrial δ¹³C record. Here we also present, for the first time, coordinated δ¹³C_carbonate, δ¹⁸O_carbonate, and δ¹³C_organic data from stacked successions of paleosols in Aptian-Albian terrestrial strata of the Cedar Mountain Formation of Utah, USA. From the whole of this record, the late Aptian C10 C-isotope feature is especially noteworthy as an interval of major global change. Coordinated carbonate and organic carbon isotope data from this interval suggest that this positive carbon isotope excursion (CIE) was related to a buildup of atmospheric pCO₂ to a peak level of about 1200 ppmV over a period of several million years duration, above earlier Aptian baseline levels of about 1000 ppmV. The C10 interval was immediately preceded and followed by drawdowns in pCO₂ to levels of about 800 ppmV, and the entirety of the Aptian-Albian record from the Cedar Mountain Formation suggests a long-term fall of pCO₂ levels from about 1000 down to 600 ppmV. We suggest that the late Aptian buildup likely is related to submarine volcanic activity in the Kerguelen Large Igneous Province in the southern Indian Ocean. Strata of the C-10 C-isotope feature are also associated with sedimentary evidence for an aridification event in the leeward rain shadow of the Sevier Mountains. On the basis of diagenetic studies of dolomitized calcretes in this C10 interval, we calculate that the precipitation-evaporation deficit intensified to the extent that 35–50% of the shallow groundwater system was lost to the atmosphere through evaporation.     

High-resolution integrated stratigraphy of the OAE1a and enclosing strata from core drillings in the Bedoulian stratotype (Roquefort-La Bédoule,SE France)

In 2009 two wells were drilled with 100% core recovery at Roquefort-La Bédoule (Bouches-du-Rhône, SE France), the historical Bedoulian stratotype. Here we present holostratigraphic results based on a detailed study of the cored sediments. Our work confirms that the La Bédoule area offers one of the best records for the period spanning the late Bedoulian, the anoxic event OAE1a and the Bedoulian/Gargasian (lower-upper Aptian substages) transition. New data provide a refined succession of micropaleontogical events already well correlated with ammonites from previous fieldwork and, thus, improve the cross-calibration of bioevents with high-resolution isotope stratigraphy. Methods of the quantitative micropalaeontology applied on benthic foraminifera such as tritaxias help testing their probable orbitally triggered cyclicity, which might be used to precise estimates of duration of events such as OAE1a, the Dufrenoya furcata ammonite Zone, the Globigerinelloides ferreolensis planktonic foraminiferal zone and the C7 isotopic stage.The lithologic, biotic and possibly isotopic changes seen at the level of and around bed 170 (top of “Niveau Blanc” sensu auctorum) are strong arguments to use this key-level as the boundary between the two Aptian substages (or stages in an alternative classification) and to support the proposal of La Bédoule as a potential locality for the GSSP of the Gargasian Substage (or of historical Aptian sensu stricto, in the alternative classification).     

Biostratigraphy,palaeoenvironments and sequence stratigraphy of the Aptian sedimentary succession at Jebel Bir Oum Ali (Northern Chain of Chotts,South Tunisia): Comparison with contemporaneous Tethyan series

The Aptian sedimentary succession of the Chott region in southern Tunisia was deposited on the margin of the Saharan shield, and is punctuated by numerous hiatuses that separate seven 3rd-order depositional sequences. Early Aptian deposits correspond to the Berrani Member (early Bedoulian), which was deposited contemporaneously with the large carbonate platforms with rudists that developed under oligo-mesotrophic, tropical environmental conditions on both margins of the Tethys. Late Bedoulian sediments were deposited under mesotrophic conditions characterized by seagrass, algae, abundant orbitolinids and aragonite-producing organisms. The early to late Aptian transition was marked by the temporary disappearance of carbonate platforms and an important renewal of the microfauna, whose tests and skeletons became less and less aragonite-rich and more and more calcite-rich and arenaceous. This episode is reported from both Tethyan margins. The platform was subsequently flooded, and dysoxic environments with annelids marked the end of the early Aptian carbonate platform development. An arid and probably colder episode (earliest late Aptian) induced the deposition of gypsum in an intrashelf basin. Following on top, the return to more humid conditions triggered an enhanced input of detrital material in fluvio-deltaic environments (late Aptian). Finally, the return to oligo-mesotrophic, marine conditions allowed the temporary installation of wide lagoons with rudists in the latest Aptian and probably in the earliest Albian. Long-distance correlations have been established by means of benthic foraminiferal occurrences. They highlight the importance of stratigraphic gaps linked to low sea levels, which have been tentatively estimated.     

Deep incision in an Aptian carbonate succession indicates major sea‐level fall in the Cretaceous

Long‐term relative sea‐level cycles (0·5 to 6 Myr) have yet to be fully understood for the Cretaceous. During the Aptian, in the northern Maestrat Basin (Eastern Iberian Peninsula), fault‐controlled subsidence created depositional space, but eustasy governed changes in depositional trends. Relative sea‐level history was reconstructed by sequence stratigraphic analysis. Two forced regressive stages of relative sea‐level were recognized within three depositional sequences. The first stage is late Early Aptian age (intra Dufrenoyia furcata Zone) and is characterized by foreshore to upper shoreface sedimentary wedges, which occur detached from a highstand carbonate platform, and were deposited above basin marls. The amplitude of relative sea‐level drop was in the order of tens of metres, with a duration of <1 Myr. The second stage of relative sea‐level fall occurred within the Late Aptian and is recorded by an incised valley that, when restored to its pre‐contractional attitude, was >2 km wide and cut ≥115 m down into the underlying Aptian succession. With the subsequent transgression, the incision was backfilled with peritidal to shallow subtidal deposits. The changes in depositional trends, lithofacies evolution and geometric relation of the stratigraphic units characterized are similar to those observed in coeval rocks within the Maestrat Basin, as well as in other correlative basins elsewhere. The pace and magnitude of the two relative sea‐level drops identified fall within the glacio‐eustatic domain. In the Maestrat Basin, terrestrial palynological studies provide evidence that the late Early and Late Aptian climate was cooler than the earliest part of the Early Aptian and the Albian Stage, which were characterized by warmer environmental conditions. The outcrops documented here are significant because they preserve the results of Aptian long‐term sea‐level trends that are often only recognizable on larger scales (i.e. seismic), such as for the Arabian Plate.     

Palaeoenvironmental and palaeoecological change on the northern Tethyan carbonate platform during the Late Barremian to earliest Aptian

A major shift from Urgonian oligotrophic carbonate accumulation to orbitolinid‐rich mixed siliciclastic–carbonate deposition is observed near the Barremian–Aptian boundary in many sections both within and outside the shallow‐marine Tethyan Realm. This important facies change in the Swiss Helvetic Alps is documented here and interpreted in the context of general palaeoenvironmental change. To achieve this, a detailed micropalaeontological, sedimentological, mineralogical and geochemical study has been carried out on six sections across the upper part of the lower Schrattenkalk Member (Late Barremian), the Rawil Member (formerly ‘Lower Orbitolina Beds’, earliest Aptian) and the lowermost part of the upper Schrattenkalk Member (Early Aptian). The sediments of the Rawil Member exhibit inner‐platform facies with rudists, miliolids, orbitolinids and dasycladals to outer‐platform facies characterized by small benthic foraminifera, orbitolinids, crinoids and bryozoans. Stratigraphic trends in microfacies environments and the composition of microfossil assemblages, indicate that the Rawil Member includes a transgressive systems tract and the base of a highstand systems tract which are composed of an increasing number of parasequences in distal directions (five to nine in the sections studied here). The sea‐level rise discerned in the Rawil Member is coeval with increased detrital input and phosphorus burial, with maximum values up to 80 times and 21 times the background values in the subjacent part of the lower Schrattenkalk Member, respectively. Furthermore, the Rawil Member records the appearance of kaolinite, indicating a change towards tropical and more humid climate conditions. This change may have led to an increase in continental weathering rates and an associated increase in detrital and nutrient fluxes towards the ocean. The phase of climate change observed near the Barremian–Aptian boundary may have been triggered by a phase of intensified volcanic activity linked with the onset of the Ontong Java large igneous province and the Rawil Member may be the expression of a precursor episode to Oceanic Anoxic Event 1a in the shallow‐marine environment.     

Geodynamic significance of the Cretaceous pillow basalts from North Anatolian Ophiolitic Mélange Belt (Central Anatolia,Turkey): geochemical and paleontological constraints

The most widespread blocks within the Cretaceous ophiolitic mélange (North Anatolian ophiolitic mélange) in Central Anatolia (Turkey) are pillow basalts, radiolarites, other ophiolitic fragments and Jurassic-Cretaceous carbonate blocks. The pillow basalts crop out as discrete blocks in close relation to radiolarites and ophiolitic units in Cretaceous ophiolitic mélange.

The geochemical results suggest that analyzed pillow basalts are within-plate ocean island alkali basalts. The enrichment of incompatible elements (Nb, Ta, Light REE, Th, U, Cs, Rb, Ba, K) demonstrates the ocean island environment (both tholeiites and alkali basalts) and enriched MORB. Dated calcareous intrafills and biodetrital carbonates reveal an age span of Callovian—Early Aptian. The thin-shelled protoglobigerinids, belonging to the genus Globuligerina, in the calcareous intrafills between pillow basalt lobes indicates a Callovian—Barremian age interval, most probably, Valanginian to Late Barremian. The volcanic and radiolarite detritus-bearing orbitolinid—Baccinella biodetrital carbonates dated as Late Barremian-Early Aptian in age, were probably deposited around atolls and have a close relationship with the ocean island pillow basalts.

The results collectively support the presence of a seamount on the Neo-Tethyan oceanic crust during the Valanginian—Late Barremian and atolls during the Late Barremian-Early Aptian interval. The presence of an oceanic crust older than that seamount along the Northern Branch of Neo-Tethys is conformable with the geodynamic evolution of the Tethys.     

中晚全新世以来玛曲高原的化学风化过程演变

在对玛曲高原东南部具有代表性的黄土-古土壤-风成砂沉积剖面进行常量、微量元素及地球化学风化参数分析的基础上,结合¹⁴C测年结果,探讨了中晚全新世以来玛曲高原的化学风化过程。结果表明:(1)不同地化学元素在剖面中的化学活性表现出明显的差异性,常量元素Ca、Si、Na以强烈迁移淋失为主,Fe、Mg则表现为明显富集;元素活动性及迁移顺序为Ca >Si >Na >Al >K >Fe >Mg;常量元素的迁移特征表明玛曲高原处于脱Ca、Na的初期化学风化阶段;微量元素除Sr迁移淋失显著外,Cu、Mn、Zn均呈现出强富集特征。(2)根据典型地化学元素记录的冷干-凉润-凉干-温湿的气候变化过程,结合化学风化指标将玛曲高原中晚全新世以来的化学风化过程划分为3.8 ka BP以前的风化较弱期、3.8~3.5 ka BP风化较强期、3.5~1.6 ka BP风化减弱期和1.6 ka BP以来的风化增强期4个阶段。