Northwestern Tethys Research Articles

Palaeobiogeographic implications of Late Bajocian–Late Callovian (Middle Jurassic) dinoflagellate cysts from the Central Alborz Mountains, northern Iran

The Dalichai Formation with an age of Late Bajocian–Late Callovian was sampled in Central Alborz Mountains of northern Iran and studied for palynological, palaeobiogeographical and palynocorrelation purposes. Palynological studies revealed diverse and well-preserved dinoflagellate cyst assemblages and lead to identification of three zones i.e., Cribroperidinium crispum (Late Bajocian), Dichadogonyaulax sellwoodii (Bathonian to Early Callovian) and Ctenidodinium continuum (Early to Middle Callovian) Zones. Subzone a of the D. sellwoodii Zone (Early to Middle Bathonian) was also differentiated. This biozonation corresponds to those recognised in Northwest Europe. Furthermore, the ammonoid families recorded including Phylloceratidae, Oppeliidae, Reineckeiidae, Perisphinctidae, Haploceratidae, Parkinsoniidae and Sphaeroceratidae, which confirm the Late Bajocian to Late Callovian age, are quite similar to those of Northwest Europe and the northwestern Tethys. The close similarities of the dinoflagellate cyst assemblages and ammonite fauna of northern Iran with those of Northwest Europe and the northwestern Tethys during the Middle Jurassic indicate direct but episodic marine connection and faunal exchange between the two areas.

A new terrestrial palynoflora from the Palaeocene/Eocene boundary in the northwestern Tethyan realm (St. Pankraz, Austria)

Inner neritic marly claystone of the South-Helvetic thrust unit (Eastern Alps) at St. Pankraz (Salzburg, Austria) yields high percentages of the dinoflagellate genus Apectodinium including the species Apectodinium augustum. The range of this species is almost exclusively restricted to the negative carbon isotope excursion (CIE) at the base of the Eocene. The associated terrestrial palynoflora consists of common Normapolles s.l. and post- Normapolles types with accessory mesothermal and megathermal families such as Anacardiaceae, Arecaceae, Euphorbiaceae s.l., Hamamelidaceae, Icacinaceae, Malvaceae (subfamilies of Helicteroideae, Sterculoideae, Tiloideae), Rutaceae, Sapotaceae and Schizaeales. This composition is similar to the Upper Thanetian palynological record of the same sedimentary succession. It suggests a warm and humid climate in the northwestern Tethyan realm during the Late Thanetian and Early Ypresian. Three new taxa probably have their lowermost occurrence in the basal Eocene. However, neither a major floral change nor the establishment of fully tropical conditions are indicated for the Palaeocene–Eocene transition.

Ammonite paleobiogeography during the Pliensbachian–Toarcian crisis (Early Jurassic) reflecting paleoclimate, eustasy, and extinctions

The Pliensbachian–Toarcian crisis (Early Jurassic) is one of the major Mesozoic paleoecological disturbances when ca. 20% of marine and continental families went extinct. Contemporaneously, profound paleobiogeographical changes occurred in most oceanic domains including a disruption of ammonite provincialism during the Early Toarcian. Here, we quantitatively reappraise the structure and evolution of paleobiogeographical patterns displayed by ammonite faunas before, during, and after the biological crisis, over a time-interval including 13 biochronozones. The high-resolution study presented here involves the use of hierarchical Cluster Analyses, non-metric Multi-Dimensional Scaling methods, and Bootstrap Spanning Network approaches that we apply to a large database including 772 ammonite species from 16 northwestern Tethyan and Arctic basins. Our results confirm a robust faunal dichotomy between Euro-Boreal and Mediterranean areas throughout the Pliensbachian, with the first emergence of an Arctic biome during the cooling regressive event of the Spinatum Zone. Whatever its complexity, Pliensbachian provincialism could be directly linked to paleogeographical barriers and to latitudinal paleoclimatic and paleoecological contrasts. During the Early Toarcian, this pattern was progressively lost, with northward expansions of Mediterranean ammonites during the Tenuicostatum Zone, followed by a strong interprovincial mixing during the Falciferum Zone. This faunal homogenization results from the combination of several parameters including a major sea-level rise facilitating basinal connections, a global warming event stretching the spatial range limits of southern taxa, and a mass extinction preferentially removing endemic species. Ammonite provincialism, although slightly different, was progressively re-established during the cooling regressive trend of the Middle Toarcian. These results therefore suggest a paramount influence of paleoclimatic, eustatic, and extinction constraints on the paleobiogeography of Early Jurassic ammonites, even if some threshold effects or independent biological factors may sporadically complicate the patterns.

Middle Triassic horseshoe crab reproduction areas on intertidal flats of Europe with evidence of predation by archosaurs

A systematically excavated track site in a 243.5 Myr old Middle Triassic (Karlstadt Formation, Pelsonian, middle Anisian) intertidal carbonate mud-flat palaeoenvironment at Bernburg (Saxony-Anhalt, central Germany) has revealed extensive horseshoe crab trackways attributable to the KouphichniumNopsca, 1923 ichnogenus. The exposed track bed of a Germanic Basin-wide spanned intertidal megatrack site is a mud-cracked biolaminate surface on which detailed tracks have been preserved because of rapid drying and cementation as a result of high temperatures, followed by rapid covering with a protective layer of arenitic storm or tsunami sediments. The different trackway types and their orientations have allowed a tidal sequence to be reconstructed, with the initial appearance of swimming horseshoe crabs followed by half-swimming/half-hopping limulids under the shallowest water conditions. The Bernburg trackways, which have mapped lengths of up to 40 m, were all produced by adult animals and exhibit a variety of shapes and patterns that reflect a range of subaquatic locomotion behaviour more typical of mating than of feeding activities. The closest match to the proportions and dimensions of the horseshoe crab tracks at Bernburg is provided by the largest known Middle Triassic limulid Tachypleus gadeai, which is known from the north-western Tethys in Spain. The horseshoe crab body fossils recognized in the German Mesozoic intertidal zones, instead, are from juveniles. The uniformly adult size indicated by the trackways therefore suggests that they may record the oldest intertidal reproductive zones of horseshoe crabs known from anywhere in the world, with the track-makers having possibly migrated thousands of kilometres from shallow marine areas of the north-western Tethys to reproduce in the intertidal palaeoenvironments of the Germanic Basin. Chirotherium trackways of large thecodont archosaurs also appeared on these flats where they appear to have fed on the limulids. With the tidal ebb, smaller reptiles such as Macrocnemus (Rhynchosauroides trackways) appeared on the dry intertidal flats, probably feeding on marine organisms and possibly also on horseshoe crab eggs.

Climate and sea-level variations along the northwestern Tethyan margin during the Valanginian C-isotope excursion: Mineralogical evidence from the Vocontian Basin (SE France)

A high resolution mineralogical study (bulk-rock and clay-fraction) was carried out upon the hemipelagic strata of the Angles section (Vocontian Basin, SE France) in which the Valanginian positive C-isotope excursion occurs. To investigate sea-level fluctuations and climate change respectively, a Detrital Index (DI: (phyllosilicates and quartz)/calcite) and a Weathering Index (WI: kaolinite/(illite + chlorite)) were established and compared to second-order sea-level fluctuations. In addition, the mineralogical data were compared with the High Nutrient Index (HNI, based on calcareous nannofossil taxa) data obtained by Duchamp-Alphonse et al. (2007), in order to assess the link between the hydrolysis conditions recorded on the surrounding continents and the trophic conditions inferred for the Vocontian Basin. It appears that the mineralogical distribution along the northwestern Tethyan margin is mainly influenced by sea-level changes during the Early Valanginian ( Pertransiens to Stephanophorus ammonite Zones) and by climate variations from the late Early Valanginian to the base of the Hauterivian (top of the Stephanophorus to the Radiatus ammonite Zones). The sea-level fall observed in the Pertransiens ammonite Zone (Early Valanginian) is well expressed by an increase in detrital inputs (an increase in the DI) associated with a more proximal source and a shallower marine environment, whereas the sea-level rise recorded in the Stephanophorus ammonite Zone corresponds to a decrease in detrital influx (a decrease in the DI) as the source becomes more distal and the environment deeper. Interpretation of both DI and WI, indicates that the positive C-isotope excursion (top of the Stephanophorus to the Verrucosum ammonite Zones) is associated with an increase of detrital inputs under a stable, warm and humid climate, probably related to greenhouse conditions, the strongest hydrolysis conditions being reached at the maximum of the positive C-isotope excursion. From the Verrucosum ammonite Zone to the base of the Hauterivian ( Radiatus ammonite Zone) climatic conditions evolved from weak hydrolysis conditions and, most likely, a cooler climate (resulting in a decrease in detrital inputs) to a seasonal climate in which more humid seasons alternated with more arid ones. The comparison of the WI to the HNI shows that the nutrification recorded at the Angles section from the top of the Stephanophorus to the Radiatus ammonite Zones (including the positive C-isotope shift), is associated with climatic changes in the source areas. At that time, increased nutrient inputs were generally triggered by increased weathering processes in the source areas due to acceleration in the hydrological cycle under greenhouse conditions. This scenario accords with the widely questioned palaeoenvironmental model proposed by Lini et al., (1992) and suggests that increasing greenhouse conditions are the main factor that drove the palaeoenvironmental changes observed in the hemipelagic realm of the Vocontian Basin, during the Valanginian positive C-isotope shift. This high-resolution mineralogical study highlights short-term climatic changes during the Valanginian, probably associated to rapid changes in the C-cycle. Coeval Massive Paraña–Etendeka flood basalt eruptions may explain such rapid perturbations.

Fossil middle triassic “sea cows” – placodont reptiles as macroalgae feeders along the north-western Tethys coastline with Pangaea and in the Germanic basin

The descriptions of fossil Triassic marine pla- codonts as durophagous reptiles are revised through comparisons with the sirenia and basal proboscidean mammal and palaeoenvironment analyses. The jaws of placodonts are conver- gent with those of Halitherium/Dugong or Mo- eritherium in their general function. Whereas Halitherium possessed a horny oral pad and counterpart and a special rasp-like tongue to grind seagrass, as does the modern Dugong, placodonts had large teeth that covered their jaws to form a similar grinding pad. The sirenia also lost their anterior teeth during many Millions of years and built a horny pad instead and specialized tongue to fed mainly on seagrass, whereas placodonts had only macroalgae availa- ble. Indirect evidence for Triassic macroalgae is provided by benthic palaeocommunities from different layers and extended European regions in the Germanic Basin. Studies of tooth wear stages for Placodus indicate that anterior teeth may have been used in a similar manner to the procumbent front teeth of modern Dugong. Paraplacodus and Placodus seem to have used these teeth as spatulas to dig out seaplants. Cyamodus and other placodonts such as Placochelys had smaller or reduced anterior teeth. The scarcity of highly worn palatine or maxillary and lower jaw dentary Placodus or Cyamodus teeth (less then 0.5%) suggests that they had a relatively soft diet. The seaplants would only have been squeezed in a similar feeding strategy to that of modern Dugong feeding on seagrass without jaw rotation and grinding. The phylogenetic trend in tooth reduction within the placodonts Paraplacodus, Placodus, especially in Cyamodus but also Placochelys, and Henodus within 11 My appears to have been a result of this plant-feeding adaptation and may even explain the origin or at least close relationship of the earliest Upper Triassic turtles as toothless algae and jellyfish feeders, in terms of the long-term convergent development with the sirens.

Foraminifera and their associations of a possibly Rhaetian section of the Nayband Formation in central Iran, northeast of Esfahan

The following paper describes the foraminiferal fauna and associated faunal assemblages of the bedded and reef carbonates of the Upper Triassic (most probably Rhaetian) Nayband Formation, which are exposed in a section south of the small town of Bagher-Abad, northeast of Esfahan. Foraminifers are extremely rare in sponge- or coral-dominated bioconstructions and in the bedded carbonates of the Nayband Formation in central Iran. Some carbonate beds are composed of bioclastic wackstone/packstone. These are exposed in the solenoporacean horizon at the uppermost part of the section. Here, the aulotortid- and trocholinid-type foraminifers are relatively abundant. The following foraminiferal taxa with different abundances were found within the carbonates of the investigated section: Trocholina umbo Frentzen, T. turris Frentzen, T. gracilis Blau, Aulotortustumidus (Kristan-Tollmann), Aulotortustenuis (Kristan), Aulotortus friedli (Kristan-Tollmann), Coronipora etrusca (Pirini), Semiinvoluta clari Kristan, Turrispirillina? liciavariabilis Blau, Galeanella? laticarinata Al-Shaibani, Carter and Zaninetti, Ophthalmidium sp., Agathammina sp., “Sigmoilina” schaeferae Zaninetti, Planiinvolutacarinata Leischner, Planiinvoluta sp., Nubecularia sp., Endothyra sp., Paeolituonella sp. and some sessile agglutinated and nodosariid types. All mentioned taxa are very rare, except the involutinid and trocholinid types. The following species are described as new: Trocholina blaui nov. sp., Spirilina? iranica nov. sp., and Coroniporaserraforma nov. sp. Trocholina blaui is usually attached to solenoporacean thalli. Four foraminiferal associations, which are named after the occurrence of the abundant species, were distinguished as Aulotortus tumidus association, Aulotortus friedli association, Trocholina umbo association, and Trocholina blaui association. Aulotortid types and Trocholina umbo were found within the bioclastic wackstone/packstone carbonates. Trocholina blaui is abundant in solenoporacean framestones. The foraminiferal association of investigated carbonates contains a mixed fauna, known from Upper Triassic–Liassic in the Tethyan realm. Carbonates of the whole investigated section are dated—due to occurrence of the genus Aulotortus, with species A. tumidus (Kristan-Tollmann), A. tenuis (Kristan), and A. friedli (Kristan-Tollmann)—as Upper Triassic (most probably Rhaetian). The “typical” foraminifers occurring in the reef biotopes in the northwestern Tethys are either missing or extremely rare in the Iranian bioconstructions.

The missing link in the evolutionary origin of the foraminiferal genus Hantkenina and the problem of the lower-middle Eocene boundary

The initial appearance of the planktonic foraminiferal genus Hantkenina has been used for about fifty years to recognize the base of the Lutetian and middle Eocene. However, probably as a result of incomplete stratigraphic records, discrepant ranges of Hantkenina have been reported by various investigators at many Eocene sections. Here we report the first complete evolutionary transition from Clavigerinella to Hantkenina , from the northwestern Tethyan deep-water section at Holzhausl (Salzburg, Austria). A newly discovered species, Hantkenina nov. sp., is the link between Clavigerinella caucasica and Hantkenina mexicana . This finding unequivocally heralds the initial entry of Hantkenina , which is correlated to the upper part of calcareous nannoplankton Subzone NP15b ( Sullivania gigas Subzone), to be defined. This indicates a mismatch of ∼4.5 m.y. between the base of the Lutetian at the type locality, which has been placed within Subzone NP14b, and the first appearance datum of Hantkenina . Consequently, the first occurrence of Hantkenina can no longer be used as a marker for the base of the middle Eocene.

The Middle Carnian Wet Intermezzo of the Stuttgart Formation (Schilfsandstein), Germanic Basin

The Middle Carnian Wet Intermezzo (MCWI) of the Stuttgart Formation (Schilfsandstein) and age-equivalent strata of the northwestern Tethys occurred entirely within equivalents of the upper subzone of the Austrotrachyceras austriacum ammonoid zone of the late Julian. Its duration is estimated to be only about 0.7–0.8 myr. In both the Germanic Basin and the northwestern Tethys, the warm climate during the MCWI was characterized by a rate of precipitation that exceeded somewhat the evaporation but was not so great as to be pluvial. The MCWI was related to the atmospheric circulation of a megamonsoonal system that was characterized by strong, moisture-laden, northwesterly flowing trade winds that rose as they reached the estimated 2000–3000 m high eastern shoulder uplift of a huge rift causing them to drop an extraordinary amount of rain. This eastern shoulder uplift lay within the Caledonides of modern day western Scandinavia. This region only, between 30 and 50°N palaeolatitude, had a truly pluvial climate, and the huge amounts of fresh water dropped there transported large amounts of siliciclastics from this rift-shoulder uplift southward into the Germanic Basin. Before deposition of the siliciclastics an early Julian eustatic sea-level fall caused widespread erosion in the Germanic Basin. In the later part of late Julian, a transgression from the eastern gates with the concurrent strong fresh water influx from the north flooded the centre of the northern Germanic Basin with a shallow brackish sea in which the Osterhagen Horizon (Basisschichten) of the lower Stuttgart Formation was deposited. In the upper Osterhagen Horizon the salinity rapidly decreased from mesohaline through mio- and oligohaline to fresh water levels. In southern Germany the Basisschichten formed entirely within fresh water or very low salinity brackish environments . Later, a slight subsidence of the southwestern Germanic Basin shifted the main outflow of fresh water toward the southwestern end of the basin, creating local brackish conditions in the northern marginal part of the northwestern Tethys (e.g. in the Lunz Beds). During this time interval, tidal influence also can be found in the Stuttgart Formation deposited in palaeoestuaries of the southwestern Germanic Basin adjacent to the Tethys. The very strong fresh water influx from the north, however, prevented these estuaries from developing a strongly elevated salt content, so that only fresh water to oligohaline brackish faunas are found there such as the Eberstadt bivalve–conchostracan fauna. At the base of the Tuvalian, the megamonsoonal system either disintegrated or else the trade winds shifted their principal flow direction. This caused the climate within the Germanic Basin and in the nearby northwestern Tethys Sea to become arid again as it had been before the deposition of the Stuttgart Formation. These changes in the atmospheric circulation patterns also terminated the pluvial climatic regime to the north along the Scandinavian eastern rift-shoulder uplift. This in turn ended the transport of huge amounts of siliciclastics from this region southward into the Germanic Basin and ended the deposition of the Schilfsandstein. After this, the hypersaline sabkha and playa sedimentation of the Weser Formation began, which was accompanied by some minor marine ingressions in the southwestern Germanic Basin.

Late Santonian bioevents in the Schattau section, Gosau Group of Austria – implications for the Santonian–Campanian boundary stratigraphy

The Schattau section in the Gosau Group (Northern Calcareous Alps) records a late Santonian interval in the northwestern Tethys. The crinoid Marsupites laevigatus, which indicates the lower part of the latest Santonian Marsupites zone, has been found within a Micraster bed in the Bibereck Formation. Co-occurring ammonites, such as Boehmoceras arculus and Placenticeras maherndli, indicate a late Santonian age. The upper Santonian zonal markers Placenticeras paraplanum and Cordiceramus muelleri muelleri, which occur below the Micraster bed, in the Sandkalkbank Member (Hochmoos Formation), indicate the upper Santonian paraplanum Zone. Within the Micraster bed, the planktonic foraminifera Globotruncanita cf. elevata and Globotruncanita stuartiformis have their first (local) appearance. Dicarinella asymetrica is still present up to the top of the section, indicating the asymetrica–elevata Concurrent Range Zone. Nannofossils give evidence for the CC17 and UC12-13 standard zones. Strontium and carbon isotope data, albeit interpreted with caution, allow some further correlations of the section. This indicates that Globotruncanita elevata and Globotruncanita stuartiformis have their first occurrences in the late Santonian. The first occurrence of the nannofossil Broinsonia parca parca, which defines the base of the CC18/UC14 standard zones, was found in neighbouring sections and is an early Campanian event.

Early Cretaceous (late Berriasian to early Aptian) palaeoceanographic change along the northwestern Tethyan margin (Vocontian Trough, southeastern France): δ 13C, δ 18O and Sr-isotope belemnite and whole-rock records

Stable carbon, oxygen, and strontium isotope records were obtained from uppermost Hauterivian to lowermost Aptian belemnite rostra, which were collected in well-dated sections from the Vocontian Trough (southeastern France). This data set complements previously published belemnite-isotope records from the uppermost Berriasian-Hauterivian interval from the same basin. The belemnite carbon and oxygen isotope record is compared to the carbonate bulk-rock isotope record from the same sections, and from additional Italian sections. With regards to their long-term trends, both belemnite and whole-rock δ 18O records are well correlated, except for the uppermost Hauterivian-lower Barremian interval, within which they deviate. This discrepancy is interpreted to be linked to the latest Hauterivian Faraoni oceanic anoxic event and its early Barremian aftermath. The Faraoni level is characterized by enhanced sea-water stratification, probably induced by the onset of a warmer and more humid climate along the northern Tethyan margin. The early Barremian was characterized by stronger vertical sea-water mixing reflected by a decrease in density contrast between sea-surface and deeper waters. The belemnite oxygen isotope record shows a more stable evolution with smaller fluctuations than its bulk-rock counterpart, which indicates that deeper water masses were not as much subjected to density fluctuations as sea-surface water. The comparison of belemnite and bulk-rock carbon isotope records allows observing the impact of regional influence exerted by platform carbonate ooze shedding on the carbon cycle. Discrepancies in the two records are observed during time of photozoan carbonate platform growth. The strontium isotopic record shows a gradual increase from the uppermost Berriasian to the uppermost lower Barremian followed by a rapid decrease until the uppermost Barremian and a renewed small increase within the lowermost Aptian. The major inflection point in the uppermost lower Barremian appears to predate the onset in the formation of the Ontong-Java volcanic plateau.

The Cretaceous-Paleogene (K/Pg) boundary at Gams, Austria: Nannoplankton stratigraphy and geochemistry of a bathyal northwestern Tethyan setting

In the area east of Gams (Styria, Austria), the Cretaceous/Paleogene boundary has been recognized in a newly discovered section, which comprises the upper part of the Cretaceous Nephrolithus frequens Zone (CC26) and the lower part of the Paleocene Markalius inversus Zone (NP1). The 6.5m long section is part of the Nierental Formation of the Gosau Group of the Northern Calcareous Alps. The boundary is characterized by (1) an enrichment of the contents of the siderophile elements Ir, Co, Ni, and Cr compared to the background and continental crustal values, (2) a sudden decrease of carbon and oxygen isotope values, (3) a sudden decrease of carbonate content, and (4) an acme of the calcareous dinoflagellate cyst Operculodinella operculata, which is succeeded by an acme of the small coccolith species Neobiscutum parvulum. The Neobiscutum acme is associated with a positive excursion of δ18O indicating a transient cooling of ocean surface waters due to short-lived changes in the configuration of ocean circulation after the impact.

Late Albian paleoceanography of the western subtropical North Atlantic

A late Albian–early Cenomanian record (∼103.3 to 99.0 Ma), including organic‐rich deposits and a δ13C increase associated with oceanic anoxic event 1d (OAE 1d), is described from Ocean Drilling Program sites 1050 and 1052 in the subtropical Atlantic. Foraminifera are well preserved at these sites. Paleotemperatures estimated from benthic δ18O values average ∼14°C for middle bathyal Site 1050 and ∼17°C for upper bathyal Site 1052, whereas surface temperatures are estimated to have ranged from 26°C to 31°C at both sites. Among planktonic foraminifera, there is a steady balance of speciation and extinction with no discrete time of major faunal turnover. OAE 1d is recognized on the basis of a 1.2‰ δ13C increase (∼100.0–99.6 Ma), which is similar in age and magnitude to δ13C excursions documented in the North Atlantic and western Tethys. Organic‐rich “black shales” are present throughout the studied interval at both sites. However, deposition of individual black shale beds was not synchronous between sites, and most of the black shale was deposited before the OAE 1d δ13C increase. A similar pattern is observed at the other sites where OAE 1d has been recognized indicating that the site(s) of excess organic carbon burial that could have caused the δ13C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (δ13C) rather than lithostratigraphically defined.

Plate tectonic trigger of changes in pCO 2 and climate in the Oxfordian (Late Jurassic): Carbon isotope and modeling evidence

The transition from the Middle to the Late Jurassic was characterized by significant changes in oceanography and climate and by changes in global carbon cycle as shown in the C-isotope record. A prominent mid-Oxfordian positive excursion in bulk carbonate carbon isotope values ( δ 13C carb) with an amplitude of more than 1‰ has been documented from many sections in the Northern Tethys realm. In this study we present new bulk organic matter C-isotope data ( δ 13C org) from northwestern Tethys that do not record the mid-Oxfordian positive excursion in carbonate carbon. On the contrary, δ 13C org decreases during the interval of the most rapid increase in δ 13C carb. We demonstrate that this decrease is not due to a changing marine–terrestrial organic carbon partitioning but that the contrasting isotope trends record peculiar environmental and climate changes which occurred near the beginning of the Late Jurassic. Using a simple carbon cycle model we show that an increase in atmospheric pCO 2 starting at modern levels could be the cause of contrasting trends in δ 13C carb and δ 13C org. We suggest that a reorganisation of ocean currents related to the opening and/or widening of the Tethys–Atlantic–Pacific seaway, and a massive spread of shallow-sea carbonate production led to higher pCO 2. Model simulations indicate that this increase in pCO 2 may have triggered changes in the biological carbon pump and in organic carbon burial that can explain the Middle Oxfordian C-isotope record.

Geodynamic control on carbonate diagenesis: Petrographic and isotopic investigation of the Upper Jurassic formations of the Paris Basin (France)

The Oxfordian and Kimmeridgian carbonates of the eastern edge of the Paris Basin display poor reservoir properties ( Φ < 15% and K < 0.1 md). The petrographic investigation and the resultant paragenetic sequence show that this is due to extensive precipitation of 2 types of almost synchronous blocky LMC (Low Magnesian Calcite) cements during mesogenesis (burial). Stable isotope investigation of these non-recrystallized cements, through both microdrilling sampling and in-situ SIMS (Secondary Ion Mass Spectrometry) analysis, shows that they originated from slightly buffered meteoric fluids (− 3‰ SMOW to − 8‰ SMOW) flowing through the Malm carbonate aquifers of the eastern edge of the Paris Basin. After a review of the post-Upper Jurassic history of the Paris Basin, it appears that meteoric water supplies leading to such cementations were associated with the evolution of the basin's geodynamic setting, and notably with the LCU (Late Cimmerian Unconformity) major event which corresponds to the uplift of the London–Brabant massif. Denudation of the Oxfordian and Kimmeridgian carbonates on the southern flank of the London–Brabant massif, to the north of the study area, then lead to lateral recharging of the associated aquifers with meteoric fluids. This scenario is similar to the one proposed to explain cementations of the Jurassic carbonates in the Wessex–Weald Basins which emphasizes the main role played by the Cretaceous unconformities over carbonate diagenesis at the scale of the northwestern Tethys margin.

A Tethys-wide mid-Carnian (Upper Triassic) carbonate productivity crisis: Evidence for the Alpine Reingraben Event from Spiti (Indian Himalaya)?

A significant mid-Carnian lithological change from carbonatic to siliciclastic sedimentation is described from the Spiti Himalaya, N India. The recorded sudden increase of terrigenous influx occurs at the boundary from the Chomule to the Rama Formation; it is dated within the late Lower Carnian (uppermost aonoides Zone) and isochronous all over the Tethys Himalaya. Bio- and chemostratigraphic data point to a synchrony with changes in carbonate productivity known from on area of wide extent along the NW Tethys: The ‘Reingraben Event’ of the Northern Calcareous Alps (Austria) defines a regional decline in carbonate productivity related to both a platform demise and enhanced siliciclastic input. Laminated shales found in a distal part of the Spiti basin document the onset of anoxic conditions within the deeper shelf synchronous to the ‘Reingraben Event’ in Austria. High-resolution biostratigraphy and identical δ 13C carb-curves are therefore in support of a direct link between the change in Spiti and the Reingraben Event. Our data, combined with a comprehensive review of previously postulated scenarios regarding the ‘Reingraben Event’, suggest linking of climatic change with siliciclastic input and a carbonate productivity crisis, initiated by onset of a relatively humid, megamonsoonal late Lower Carnian “atmospheric system” that affected both the southern and northwestern Tethys and point to a supra-regional trigger of the Alpine ‘Reingraben Event’.

High-frequency palaeoenvironmental fluctuations recorded in Jurassic coral- and sponge-microbialite bioconstructions

During the Jurassic, coral and sponge reefs were particularly widespread along the northwestern Tethys. These bioconstructions occurred from shallow- to deep-shelf settings, in pure carbonate, carbonate-dominated or clay-rich mixed carbonate-siliciclastic sedimentary contexts. According to the depositional environment, the amount of microbialites strongly fluctuates, to form up to 70% of the reef volume. This study focuses on bioconstructions where microbialites constitute at least 15% of the reef volume. Analysis of the close relationships between the reef structures and the stacking pattern of laterally-deposited sediments reveals analogous architectures and growth modes between coral- and sponge-microbialite bioconstructions. Firstly, corals or sponges erected a primary framework more or less projected above the sea floor (constratal or superstratal growth fabrics). Contemporaneously, a thin microbialite layer encrusted the local dead parts of corals or sponges. In a second stage, microbialites largely developed and entirely covered the reef surface, becoming the main reef builders. The whole development of coral- and sponge-microbialite reefs corresponds to a ‘low-frequency reef-growth phase’. These bioconstructions also display surfaces of reef growth interruption delimiting ‘medium-frequency reef-growth phases’. They include several ‘high-frequency reef-growth phases’ (or elementary units) corresponding to coral or sponge frameworks plus microbialite crusts. At the scale of the elementary unit, microbialites are interpreted to reflect nutrient-richer conditions and developed to the detriment of phototrophic-, mixed- or heterotrophic-dominated assemblages of corals or sponges. The development of Jurassic coral and sponge bioconstructions was punctuated by high-frequency (millennial time-scale) ecological crises during which microbialites occurred at the reef surface. Such ecological shifts from skeletal reef metazoans (i.e. corals or sponges) to microbialite crusts were recorded whatever the palaeogeographical position (shallow-proximal platform or deep-distal shelf) along the northwestern Tethys margin. Three main orders of climatic oscillations, correlated to the ‘high-, medium- and low-frequency reef-growth phases’, could have directly regulated the amount of terrigenous materials and nutrients delivered to the ocean, which in turn controlled platform carbonate production and accumulation, as well as reef development.

Fertilization of the northwestern Tethys (Vocontian basin, SE France) during the Valanginian carbon isotope perturbation: Evidence from calcareous nannofossils and trace element data

A high resolution calcareous nannofossil study associated with a geochemical analysis (major, trace elements, and carbon and oxygen isotope stratigraphies) was carried out in the Angles section (hemipelagic setting of the Vocontian basin, SE France) during the Valanginian positive carbon isotope excursion. The behaviour of calcareous nannofossil taxa in relation to fertility conditions was studied to elaborate new nutrient indices in this environment: a high nutrient index based on Biscutum spp., Discorhabdus rotatorius, Zeugrhabdotus fissus, (high fertility indicators) and Watznaueria barnesae (low fertility indicator); and a medium nutrient index based on Lithraphidites carniolensis (medium fertility indicator) and W. barnesae (low fertility indicator). These two indices show a major fertilization from the Stephanophorus ammonite Zone to the Trinodosum ammonite Zone, with a maximum during the positive carbon isotope excursion. Since high values of the nutrient indices are in phase with high values of chemical elements related to terrigenous material and low values of the coccolith total abundance, it is proposed that pulses of detrital inputs into the basin triggered the nutrification which, in turn, caused a biocalcification crisis of the calcareous nannofossils. Nutrification is also responsible for the reef demise in the surrounded platforms, as indicated by the increased Sr/Ca seawater ratio at that time. The intensification of the Paranà–Etendeka volcanic activity, triggering CO 2 excess in the atmosphere, is probably responsible for an acceleration of the hydrological cycle, the increased weathering, and the subsequent higher terrigenous and nutrient transfer from continents to the Vocontian basin. In such a scenario, nutrification is a dominant factor controlling neritic and hemipelagic biocalcification. However, one cannot exclude that the global increase of atmospheric CO 2 could generate chemical changes of the sea-surface waters, acting with the nutrification, to modify the biocalcification of the carbonate producers.

Rare earth and trace elements of microbialites in Upper Jurassic coral- and sponge-microbialite reefs

Major, trace (Y, Zr, Pb, Th and U), rare earth element (REE) and Sm–Nd isotope analyses were carried out on microbialites and detrital carbonate sediments collected in Upper Jurassic coral- and sponge-microbialite reef settings. Selected bioconstructions developed in various palaeogeographic settings and depositional environments on northwestern Tethys and eastern Atlantic margins: pure carbonate lagoon, carbonate-dominated deep shelf to epicontinental basin and mixed carbonate-siliciclastic shallow ramp and lagoon. The proximity to terrigenous sources directly controlled REE patterns of carbonate samples. Lateral and intra-reef sediments, and to a lesser degree stromatolitic microbial crusts, are more susceptible to contamination by terrigenous material than are microbialites of thrombolitic fabric. In mixed carbonate-siliciclastic settings, microbialites display a flat shale-normalized REE pattern. In pure carbonate lagoonal settings, microbialites have shale-normalized REE patterns similar to those of modern seawaters with a negative Ce anomaly [(Ce/Ce ⁎) up to 0.14], positive La and Gd anomalies, and light REE depletion relative to heavy REE. These data imply that similar REE chemistry than modern seawater already occurred on the northwestern Tethys margin at the Upper Jurassic. Strong negative Ce anomalies (< 0.4) imply microbialite formation in well-oxygenated seawaters both in coral reefs of pure carbonate lagoonal environments (Pagny-sur-Meuse) and sponge bioherms of deep-shelf settings (Plettenberg). Absence of positive Ce anomalies rules out an influence of alkaline and high-pH seawaters. Thus, REE patterns and Nd isotopic signatures, which are in perfect agreement with existing results for Tethysian seawater, confirm that microbialites can be used as proxies of palaeoenvironmental seawater chemical characteristics.

Discussion on subsidence history of the north Indian continental margin, Zanskar–Ladakh Himalaya, NW India Journal , Vol. 162, 2005, pp. 135–146

E. Garzanti, D. Sciunnach & M. Gaetani write: In their recent paper, Corfield et al . (2005) attempted a subsidence analysis of the northwestern Tethys Himalaya stratigraphic succession, and discussed specifically the implications for the obduction of a section of oceanic lithosphere (Spontang Ophiolite) on top of the Indian passive continental margin (Zanskar Shelf). The emplacement age of the Spontang Ophiolite represents a crucial problem in Himalayan geology, which has been lively debated for two decades (e.g., Searle 1986; Kelemen et al. 1988; Garzanti et al . 1987; Guillot et al . 2003). In a series of papers, Mike Searle and co-workers have long proposed and supported the idea that the Spontang Ophiolite was obducted in the Late Cretaceous, making implicit or explicit correlation with the well-studied Semail Ophiolite of Oman (e.g, Searle 1983; 1986; Pedersen et al . 2001; Corfield et al . 2001, 2005). Semail and Spontang, however, are two very different ophiolite complexes. The Semail crust has a supra-subduction geochemical signature and was generated in the mid-Cretaceous (Searle & Cox 1999). Instead, the Spontang basalts have a MORB-like signature and were generated in the mid-Jurassic (Pedersen et al . 2001), similar rather to the other Oman ophiolite exposed on Masirah Island (Gnos et al . 1997). The analogy with the Semail Ophiolite obduction, associated with Late Cretaceous nappe stacking and high-pressure metamorphism of the underthrusted Arabian margin (Searle & Cox 1999), led Searle (2001) to reject the geochronological evidence for the Eocene age of high-pressure metamorphism of Indian-margin rocks (Kaghan and Tso–Morari eclogites; Tonarini et al . 1993; De Sigoyer et al . 2000). Because of the Eocene age of the eclogites, and of the fact that the Spontang Ophiolite lies tectonically on top of Indian margin sediments as …