Ammonite-benthic Foraminifera turnovers across the Lower-Middle Jurassic transition in the Lusitanian Basin (Portugal)

This paper describes and characterises the co-occurrence of ammonite and benthic foraminiferal assemblages across the São Gião outcrop (Central Portugal), a reference section for the Lower-Middle Jurassic boundary in the Lusitanian Basin. The upper Toarcian-lower Aalenian marls and marly-limestones in this section provide a precise and detailed ammonite-based biostratigraphic zonation, with a mixed assemblage of northwest European and Mediterranean faunal elements, associated with benthic foraminifera assemblages with northern hemisphere affinities, both correlatable with the Aalenian GSSP at the Fuentelsaz section (Iberian Cordillera, Spain). A total of 447 well-preserved ammonite specimens and 13.116 foraminifera have been studied; no evidence was detected of any taphonomic processes that could have changed the original assemblages. From a biostratigraphic point of view, the ammonite record has enabled four biostratigraphic units to be recognised (the Mactra and Aalensis subzones of the Aalensis Biozone in the upper Toarcian, and the Opalinum and Comptum subzones of the Opalinum Biozone in the lower Aalenian). With regard to the benthic foraminifera, the taxa identified have enabled the Astacolus dorbignyi Zone and 11 bioevents to be identified, most of which representing local biostratigraphic proxies. However, the increase in the relative abundance of Lenticulina exgaleata Dieni from the upper part of the Opalinum Subzone to the lower part of the Comptum Subzone has a regional value. The constant and continuous ammonite record of northwest European taxa, together with typical Mediterranean taxa – namely Grammoceratinae – throughout the section, the high relative abundance of Miliolina representatives – generally interpreted as foraminifers typical of shallow waters – and the absence of foraminiferal forms typical of cool waters, do not support the inference of cool seawater temperatures attributed to the Early Aalenian, or the global character of the “Comptum cooling event”, at least with reference to the Lusitanian Basin.     

Uppermost Pliensbachian-Lowermost Toarcian ostracod assemblages from the Western Tethys: Comparison between Traras Mountains (Algeria), Subbetic (Spain), and Algarve (Portugal)

Major changes in ostracod assemblages have been recorded in the Lower Jurassic of Western Tethys related to biotic crises of the Pliensbachian-Toarcian boundary and the Polymorphum-Levisoni boundary (Toarcian Oceanic Anoxic Event). The study is focused on the ostracods assemblages from Benzerka section (Traras Mountains, Algeria) and their comparison with sections from External Subbetic and Algarve Basin (South Iberian Palaeomargin). Three stratigraphic intervals coincident with the ammonite zones have been differentiated with important fluctuations in the composition of ostracod assemblages. In the ostracod assemblages from the Traras Mountains the turnover related to this benthic biotic crisis is shown by assemblages previous to the biotic crisis (uppermost Pliensbachian) mainly composed by families Healdiidae and Cytherellidae to assemblages just after the T-OAE (upper part of the Levisoni Zone) with assemblages dominated by families Protocytheridae and Polycopidae and the extinction of family Healdiidae (suborder Metacopina). The Toarcian Oceanic Anoxic Event (T-OAE) probably resulted in restricted conditions but a barren interval due to oxygen-depleted conditions in this setting is not recorded. In this context, the change of components of ostracod assemblages looks gradual. In the Southiberian Palaeomargin, represented by the External Subbetic the turnover of ostracods assemblages is also recorded but the families involved are different than in Traras Mountains. Therefore, different phases have been differentiated in the ostracod turnover occurring from Upper Pliensbachian (assemblages dominated by families Pontocyprididae, Healdiidae and Bairdiidae) to end of Lower Toarcian (assemblages dominated by family Cytherellidae and the extinction of family Healdiidae). In the Algarve Basin the families Healdiidae and Bairdiidae were dominant previously to the Toarcian Oceanic Anoxic Event. Differences in the composition of ostracod assemblages between Traras Mountains and External Subbetic may be related to the fragmentation of these palaeomargins related to early stages of rifting in the Western Tethys. Moreover, the incidence of oxygen-depleted conditions was higher in External Subbetic than in Traras Mountains where some ostracod genera survive during the biotic crisis.     

Global radiolarian zonation for the Pliensbachian, Toarcian and Aalenian

Jurassic radiolarians from 220 samples in Queen Charlotte Islands, B.C., Williston Lake, B.C., east-central Oregon, Baja California Sur, southern Spain, Austria, Slovenia, Turkey, Oman, Japan and Argentina were studied in order to construct global zonation for the Pliensbachian, Toarcian and Aalenian stages. Well-preserved faunas from continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this time interval, and all collections are tied to North American ammonite zones or assemblages. Collections from nearly all other areas lack independent dating except for early Toarcian carbon-isotope dating in Slovenia and late Aalenian ammonites in Spain.A database of 197 widely distributed updated taxonomic species was used to construct a Unitary Association (UA) zonation for the interval. A global sequence of 41 UAs was obtained for the top of the Sinemurian to the base of the Bajocian. The first and the last UAs represent the Late Sinemurian and the Early Bajocian respectively. The remaining 39 UAs were merged into nine zones (four Early Pliensbachian, one Late Pliensbachian, one Early Toarcian, one Middle-Late Toarcian, and two Aalenian) according to prominent radiolarian faunal breaks and ammonite data. The new zones are the Canutus tipperi - Katroma clara Zone (latest Sinemurian/earliest Pliensbachian); Zartus mostleri - Pseudoristola megaglobosa, Hsuum mulleri - Trillus elkhornensis and Gigi fustis - Lantus sixi zones (Early Pliensbachian); Eucyrtidiellum nagaiae - Praeparvicingula tlellensis Zone (Late Pliensbachian); Napora relica - Eucyrtidiellum disparile Zone (Early Toarcian); Elodium pessagnoi - Hexasaturnalis hexagonus Zone (Middle and Late Toarcian); Higumastra transversa - Napora nipponica Zone (early Aalenian); and Mirifusus proavus - Transhsuum hisuikyoense Zone (late Aalenian). These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian and the Middle to Upper Jurassic. The new zonation allows high-resolution dating in the studied interval and provides a solid basis for analyzing faunal turnovers and the paleobiogeography of Jurassic radiolarians.     

The Early Jurassic palynostratigraphy of the Lusitanian Basin,western Portugal

A comprehensive investigation of the Early Jurassic stratigraphical palynology of the Lusitanian Basin in western Portugal was undertaken, with most emphasis placed on dinoflagellate cysts. A total of 214 samples from an upper Sinemurian to upper Toarcian composite section based on six successions were examined. The Sinemurian material examined was barren of dinoflagellate cysts; however, the Pliensbachian and Toarcian successions are characterised by relatively low diversities where Luehndea spinosa, Mancodinium semitabulatum, Mendicodinium microscabratum, Nannoceratopsis gracilis, Nannoceratopsis senex, and Scriniocassis priscus are relatively common and biostratigraphically significant. Luehndea spinosa dominates the lowermost Toarcian (Dactylioceras polymorphum ammonite Biozone), and is an index species. At the base of the Hildaites levisoni ammonite Biozone, the effects of the Toarcian Oceanic Anoxic Event (T-OAE) caused Luehndea spinosa to become extinct. At the same time, dinoflagellate cyst abundance and diversity markedly decreased. After the T-OAE, during the middle and late Toarcian, phytoplankton recovery was prolonged and slow in the Lusitanian Basin. The Luehndea spinosa and Mendicodinium microscabratum dinoflagellate cyst biozones are defined, both of which are subdivided into two dinoflagellate cyst subbiozones.     

Paleoceanographic implications of smaller benthic and planktonic foraminifera from the Eocene Pazin Basin (Coastal Dinarides,Croatia)

Summary Smaller benthic and planktonic foraminifera from the clastic sediments of the Pazin Basin (Istria, Croatia) were studied in order to obtain more data about paleoceanographic conditions that existed in the Middle Eocene Dinaric foreland basin. The succession investigated corresponds to the Middle Eocene planktonic foraminiferal zones Globigerapsis kugleri/Morozovella aragonensis (P11), Morozovella lehneri (P12), and Globigerapsis beckmanni (P13). Benthic foraminiferal assemblages from the clastic succession are dominated by epifaunal trochospiral genera suggesting oligotrophic to mesotrophic conditions and moderately oxygenated bottom waters. Planktonic foraminiferal assemblages indicate mesotrophic to eutrophic conditions of the surface waters, with increased eutrophication in the upper part of the section. Water depth, based on the ratio between planktonic and epifaunal benthic foraminifera and on the recognized species of cosmopolitan benthic foraminifera, was estimated to have been between about 900 and 1200 m. The basin was elongated and open to marine currents on both sides allowing good circulation and ventilation of the bottom water.     

Extinction pattern of marine Ostracoda across the Pliensbachian-Toarcian boundary in the Cordillera Ibérica, NE Spain: Causes and consequences

This paper discusses the extinction pattern of the Pliensbachian-Toarcian boundary (PTB) ostracod assemblages at the Almonacid de la Cuba section (Cordillera Ibérica, NE Spain), which has been recently proposed as auxiliary boundary stratotype for the PTB. The ostracod record shows that the main Early Jurassic ostracod extinction event occurred not at the end of the Pliensbachian, but near the top of the Mirabile ammonite Subzone, Tenuicostatum ammonite Zone (Early Toarcian). On the basis of the evaluation of PTB ostracod record, a new causal explanation for the Early Toarcian ostracod turnover is proposed. This paper suggests that a reorganization of surface and deep-water circulations caused by the opening of the Hispanic Corridor could have generated a mild cooling episode, finally affecting the survival of healdioid ostracods.     

Planktonic versus benthic foraminifera response to Milankovitch forcing (Late Jurassic,Betic Cordillera): testing methods for cyclostratigraphic analysis

Foraminiferal assemblages from the Bifurcatus Zone (Oxfordian, Upper Jurassic) are studied in the Navalperal section (Betic Cordillera, southern Spain). A total of 24 sampling stations and around 5,700 specimens of foraminifera were recognized on thin-section analysis and classified into two major categories: planktonic and benthic. The abundance of foraminifera (number of specimens/cm²) was calculated for both categories and the cyclic pattern was studied by spectral analysis, considering the autochthonous and para-autochthonous character of the studied assemblages. The Lomb-Scargle periodogram together with a permutation test were tested for performing the high-resolution spectral analysis, being particularly well suited for working with short time series and uneven sampling. Spectral analysis reveals the influence of orbital-scale Milankovitch cyclicity at the eccentricity, obliquity, and precession bands. Moreover, this incidence is significantly different depending on the analyzed group (benthic versus planktonic). Whereas the long-range eccentricity band is not distinguishable from a trend and the short-range eccentricity band is not statistically significant (at 90% confidence level), the obliquity band is better represented in the planktonic component and the precession band is better developed in the benthic group. Variations in temperature affecting upper waters, determined by obliquity-scale fluctuations, could be responsible for changes in the planktonic foraminiferal assemblage, while changes in nutrient availability and substrate oxygenation, as a consequence of input variations from source areas at the precession-scale cycles, could affect the benthic foraminiferal assemblage.     

Évolution des peuplements de foraminifères pendant la crise toarcienne à l’exemple des données des monts des Ksour (Atlas saharien occidental,Algérie)

The micropalaeontologic analysis of the Lower Toarcian from the Ksour Mountains (western Saharan Atlas, Algeria) allows us to recognize several assemblages of benthic foraminifera and to obtain information about the deposit environment. These assemblages dominated by Nodosariids are organized in four successive stages of settlement (normal, survival, extinction, and repopulation). This distribution gives new information on the environmental conditions and on their effect on the change in the vertical distribution of benthic foraminifera. The quantitative evolution documents a progressive deepening of the environment that is controlled by local tectonic and sedimentary dynamics. The result is an ecosequence depending both on the general sea-level rise and on a progressive confinement of the bottom water. The normal stage occurred at the beginning of the Polymorphum zone; uncoiled and small sized Nodosariids species indicate the opening of the Atlasic furrow. After, during the middle of this zone, these species were associated with Polymorphinids and Ceratobuliminids, which indicate a deep but isolated and confined environment (survival stage). The ecosequence ended with an episode without foraminifera; it is the extinction stage dated in the Polymorphum and Early Levisoni zones. This stage corresponds to an umbilicus setting. Such palaeophysiographic conditions exaggerated the general coeval hypoxy, responsible of the main Toarcian crisis. Then, from the late Levisoni zone to the end of the Middle Toarcian, the ecosequence evolution is inverted, as a consequence of the filling of the deep parts of the umbilicus, which were less partitioned and more oxygenated (repopulation stage).     

Relationships between the early Toarcian anoxic event and organic-walled phytoplankton in central Italy

The integration of palynological and geochemical data from three lower Toarcian successions in central Italy reveals that the composition of organic-walled phytoplankton assemblages were strongly affected by palaeoecological conditions related to bituminous sedimentation which accompanied the global anoxic event. The marked compositional variations of dinoflagellate cysts and prasinophytes, together with geochemical variations, have been linked to changes in surface water habitats during the lower Toarcian transgression. On the basis of the relationships between total organic carbon (TOC) and marine palynomorph assemblage composition, the lower Toarcian evolution of the Umbria-Marche Basin, central Italy, has been divided into four phases. Total organic carbon values rose significantly during the early Toarcian (Lower-middle Dactylioceras tenuicostatum ammonite Zone), and this can be linked to certain dinoflagellate cyst datums, for example the temporary disappearance of Mancodinium semitabulatum and the extinction of Luehndea spinosa. The presence of Umbriadinium mediterraneense and Valvaeodinium spp. accompany these moderately high TOC values. Subsequently, TOC levels increased to over 2% and prasinophytes became abundant in the Middle-upper D. tenuicostatum ammonite Zone. Mancodinium semitabulatum reappeared when TOC values eventually decreased in the Upper D. tenuicostatum ammonite Zone. This analysis has allowed the different sunlight requirements and life strategies of the early Toarcian Tethyan dinoflagellates to be modelled. Due to the cosmopolitan nature of the early Toarcian anoxic event, the principal marine palynological signals observed have been interpreted as sequence stratigraphical and palaeoecological indices. The Transgressive Systems Tract (TST) is accompanied by an increase in dinoflagellate cyst species diversity and a decrease in abundance. The succeeding maximum flooding surface (mfs) corresponds with a prasinophyte acme. During the Highstand Systems Tract (HST), the phytoplankton shows an increase in abundance and a decrease in diversity. The range top of Luehndea spinosa appears to characterise the early Toarcian TST.     

Benthic foraminiferal assemblages of the Middle and Upper Jurassic sediments from the northeastern Alborz and western Koppeh Dagh,Iran: Systematic palaeontology and palaeoecology

This research is focused on a poorly studied Jurassic sequence in the eastern part of Alborz and Western Koppeh Dagh basins. The foraminifera are reported from Callovian (Middle Jurassic) to Tithonian (Upper Jurassic) sediments of the Farsian and Chaman Bid formations. The benthic foraminiferal associations belong to the families Ammodiscidae, Epistominidae, Hauerinidae, Hormosinidae, Ichthyolariidae, Lagenidae, Nodosariidae, Nubeculariidae, Ophthalmidiidae, Polymorphinidae, Rhabdamminidae, Saccamminidae, Spirillinidae, Spirocyclinidae, Textulariidae, Trocholinidae, and Vaginulinidae. In the studied area, the foraminifera are dominated by the families Nodosariidae, Vaginulinidae and Epistominidae. Palaeoecological analysis of the foraminiferal assemblages from the Farsian Formation indicates that these sediments were deposited on the inner to mid shelf environment, with normal salinity (euhaline), and relatively well oxygenated waters, with eutrophic conditions in the Goznawwi area and oligotrophic conditions in the Chaman Bid area. The foraminiferal assemblages in this study are the first record from Jurassic sediments of the eastern part of the Alborz and Koppeh Dagh basins in northern Iran.     

Benthic foraminifera off West Africa (1°N to 32°S): Do live assemblages from the topmost sediment reliably record environmental variability?

Recent benthic foraminifera (> 125 μm) were investigated from multicorer samples on a latitudinal transect of 20 stations between 1°N and 32°S along the upper slope off West Africa. Samples were selected from a narrow water depth interval, between 1200 and 1500 m, so that changes in water masses are minimized, but changes in surface productivity are important and the only significant environmental variable. Live (Rose Bengal stained) benthic foraminifera were counted from the surface sediment down to a maximum of 12 cm. Dead foraminifera were investigated in the top 5 cm of the sediment only. Five live and five dead benthic foraminiferal assemblages were identified using Q-mode principal component analysis, matching distinct primary productivity provinces, characterized by different systems of seasonal and permanent upwelling. Differences in seasonality, quantity, and quality of food supply are the main controlling parameters on species composition and distribution of the benthic foraminiferal faunas.To test the sensitivity of foraminiferal studies based on the uppermost centimeter of sediment only, a comparative Q-mode principal component analysis was conducted on live and dead foraminiferal data from the top 1 cm of sediment. It has been demonstrated that, on the upper slope off West Africa, most of the environmental signals as recorded by species composition and distribution of the “total” live and dead assemblages, i.e., including live and dead foraminifera from the surface sediment down to 12 cm and 5 cm, respectively, can be extracted from the assemblages in the top centimeter of sediment only. On the contrary, subsurface abundance maxima of live foraminifera and dissolution of empty tests strongly bias quantitative approaches based on the calculation of standing stocks and foraminiferal numbers in the topmost centimeter.     

Benthic foraminiferal fauna during the time of the Indian-Asian contact, in southern Balochistan, Pakistan

Cretaceous and early Paleocene benthic foraminifera were studied from one section along the western Gaj River, southern Balochistan, Pakistan, to reconstruct the paleoenvironment of the Tethys Sea during the Indian-Asian contact. We recognize three lithostratigraphic units in ascending order: the Mughal Kot Formation, the Pab Sandstone, and the Jamburo Group. Both the Maastrichtian Mughal Kot Formation, which consists of shale with grey marly limestone, and the Maastrichtian Pab Sandstone, which consists of quartzose sandstone, indicate an open ocean environment as they have diversified planktic and benthic foraminiferal assemblages. The Maastrichtian-Paleocene Jamburo Group, consisting of dark grey, calcareous shale and marlstone with some sulfide grains, is characterized by low diversities of benthic assemblages. The change to the lower diversities may be associated with the development of poor circulation of deeper water that was caused by narrowing of the Tethys Sea.The Trochammina spp. Assemblage from the Jamburo Group, which can be correlated with flysch-type agglutinated foraminiferal assemblages, has a low benthic species diversity, indicating an unfavorable condition for calcareous foraminifera because of the development of oxygen-depleted water. The absolute abundance of agglutinated specimens shows a remarkable change from low numbers in the Maastrichtian to high ones in the Paleocene. The benthic foraminiferal evidence supports the hypothesis that the collision of the Asian and Indian plates occurred near the end of the Cretaceous.     

Benthic foraminiferal morphogroups of mid to outer shelf environments of the Late Jurassic (Prebetic Zone, southern Spain): Characterization of biofacies and environmental significance

Benthic foraminiferal assemblages belonging to a mixed carbonate–siliciclastic shelf succession of the Oxfordian (Upper Jurassic) in the Prebetic, southern Spain, were analyzed. The faunal data, obtained on thin sections of strongly lithified sediments, allowed the detailed differentiation of a foraminiferal morphogroups system to be applied to the interpretation of paleoenvironmental conditions. Eleven morphogroups were differentiated according to shell composition, general morphology, number of chambers and mode of coiling. Paleoenvironmental analysis combines these features with inferred life-style (epifaunal, shallow infaunal and deep infaunal) and feeding strategy (suspension-feeder, deposit-feeder, herbivores, etc.) of the foraminifera. Analogies and differences regarding previously established morphogroup systems could be outlined accordingly.

In the Prebetic Oxfordian, distribution trends of foraminiferal morphogroups allow for a comparison of mid to outer shelf environments occupying the South-Iberian paleomargin, indicating that nutrient supply was the most important factor controlling morphogroup development. The mid shelf settings are dominated by epifaunal active herbivores–phytodetritivores, in shallow to deep substrate position of the redox boundary. The outer shelf is dominated by agglutinated and infaunal detritivore-bacterial scavenger morphogroups revealing a deep substrate position of the redox boundary. Particularly favorable trophic conditions in mid shelf environments are related with higher phytodetritus input from more proximal and shallower shelf areas.     

Benthic foraminiferal assemblages of the Cretaceous platform carbonate succession in the Yavca area (Bolkar Mountains, S Turkey): biostratigraphy and paleoenvironments

The microbiostratigraphic analysis of the three outcrop sections from the Cretaceous inner platform carbonate succession in the Yavca area (Bolkar Mountains) allows to recognize the four local benthic foraminiferal zones. These are: (1) Voloshinoides murgensis and Praechrysalidina infracretacea Cenozone in the Lower Aptian; (2) Pseudorhapydionina dubia and Biconcava bentori Cenozone in the Middle-Upper Cenomanian; (3) Ostracoda and Miliolidae Interval Zone in the probable Turonian, represented by dolomitized limestones without any significant markers; (4) Moncharmontia compressa and Dicyclina schlumbergeri Cenozone in the Coniacian-Santonian. The benthic foraminiferal assemblages correspond to those in other areas of the Mediterranean realm, with the exception of a lack of alveolinids and orbitolinids due to unfavorable environmental conditions (inner platform, restricted shelf). After the regionally well-known emergence during the late Aptian, Albian and early Cenomanian, very shallow subtidal to intertidal conditions were re-established during the middle-late Cenomanian time. The Coniacian-Santonian benthic foraminiferal assemblage shows an increase in diversity and abundance as a result of open marine influence, confirmed by the presence of larger foraminifera (Dicyclina), Rotaliidae and radiolitid fragments. Thaumatoporella and Aeolisaccus-bearing wackestone intercalations still indicate the existence of sporadic restricted environment conditions. The Cretaceous shallow-water platform carbonate succession of the Yavca area is conformably overlain by gray pelagic limestones with calcispheres and planktonic foraminifera. The Campanian flooding of the Bolkar Da? carbonate platform resulted in drowning of the pre-existing biota and facies.     

Danian/Selandian boundary stratigraphy,paleoenvironment and Ostracoda from Sidi Nasseur,Tunisia

Two detailed records (NSF and 05NSC, Sidi Nasseur, Tunisia) across the Danian/Selandian (D/S) boundary were investigated for their micropaleontological content. Calcareous nannofossils and planktic foraminifera provided a biostratigraphic framework. The interval spans part of planktic foraminiferal Zone P2, Subzone P3a and part of Subzone P3b. This corresponds to calcareous nannoplankton Zone NP4. Using a more detailed nannofossil zonation the studied section spans part of Zone NTp6, Zone NTp7a and part of NTp7b. Quantitative ostracod and qualitative benthic foraminiferal data were used to characterize environmental changes across the D/S boundary. The two subsections have yielded a total of 50 ostracod taxa. The ostracod assemblage of the entire section belongs to the Southern Tethyan Type showing subtle but distinct changes up section. Based on statistical analysis of the quantitative ostracod data, faunal changes at a glauconitic maker bed (P3a/P3b boundary) were demonstrated. The local Reticulina proteros assemblage, with the typical species R. proteros, Oertliella vesiculosa and Cytheroptheron lekefense, is gradually replaced by the Protobuntonia nakkadii assemblage, with the typical species Cristaeleberis arabii, Xestoleberis tunisiensis, Cytheropteron sp. and P. nakkadii, across the glauconitic bed. The benthic foraminifera also demonstrated distinct changes at this marker bed. The changes in ostracods and foraminifera are related to changes in paleoproductivity and an overall relative sea-level fall.The lithological and faunal changes at the P3a/P3b zone boundary within the Sidi Nasseur sections seem to correspond to the D/S boundary in the type region in Danmark and are characterized by a significant hiatus, yielding this section not suitable as a GSSP candidate for this boundary.     

Isotopic and distributional evidence of a planktonic habit for the foraminiferal genusStreptochilus Bro¨nnimann and Resig, 1971

Oxygen and carbon isotope analyses show that the biserial foraminiferal genusStreptochilus, which was originally described from pelagic sediments on the Eauripik Rise and Ontong Java Plateau, lived deep in the upper water column within the oxygen minimum layer. The species ofStreptochilus average from 4 to 19% of the foraminiferal assemblages in which benthic forms compose less than 1 or 2%. Specimens ofStreptochilus are selectively dissolved when in contact with the bottom water mass. Their rapid evolutionary turnover of less than a few million years and their wide areal distribution in the equatorial Indo-Pacific are indicative of planktonic foraminifera. Aside from usefulness of the species ofStreptochilus as stratigraphic indices, these Neogene biserial planktonic foraminifera are potential indices of paleoceanographic stratification.     

Toarcian ammonites of the Polymorphum Zone,Amellago area (eastern central High Atlas), Morocco,with a discussion on ammonite turnover events

In the Amellago area (central High Atlas), the widely distributed Toarcian succession consists of marls and marly limestones of the Tagoudite and Agoudim formations. We here describe, for the first time, the ammonites of the Polymorphum Zone from the Tagoudite Formation, including Canavaria cf. rosenbergi Fucini, C. aff. zancleana (Fucini), Dactylioceras (Eodactylites) mirabile (Fucini), D. (E.) aff. mirabile (Fucini), D. (E.) pseudocommune Fucini, D. (E.) simplex (Fucini), D. (Orthodactylites) aff. crosbeyi (Simpson), Lytoceras gr. villae Meneghini, Neolioceratoides cf. hoffmanni (Gemmellaro) and Praepolyplectus sp. This ammonite assemblage is closely correlated with those reported from the Polymorphum Zone of several basins of the Tethyan margins and the Northwest European Subboreal Realm, indicating marine communications between the different Moroccan Jurassic basins and the central Atlantic trough. The overlying Agoudim Formation yields ammonites of the Levisoni Zone in its base and rare ammonites of the Bifrons Zone in the middle part. Four significant changes in the ammonite assemblages during the early-middle Toarcian and their probable causes are also discussed.     

Stable isotopic study of Oligocene-Miocene sediments from DSDP Site 354, Equatorial Atlantic

The oxygen- and carbon-isotope compositions of planktic and benthic foraminifera and calcareous nannofossils from Middle Oligocene-Early Miocene Equatorial Atlantic sediments (DSDP Site 354) indicate two important paleoceanographic changes, in the Late Oligocene (foraminiferal Zone P.21) and in the Early Miocene (foraminiferal Zone N.5). The first change, reflected by a δ¹⁸O increase of 1.45‰ inGlobigerina venezuelana, affected only intermediate pelagic and not surface, deep or bottom waters. The second change affected surface and intermediate waters, whereas deep and bottom waters showed only minor fluctuations. In the case of the former the isotope effect of the moderate ice accumulation on the Antarctic continent is amplified in the Equatorial Atlantic by changes in the circulation pattern. The latter paleoceanographic change, reflected by a significant increase in¹⁸O in both planktic and benthic forms (about 1.0‰ and 0.5‰, respectively), may have been caused by ice volume increase and temperature decrease. Both oxygen- and carbon-isotope compositions indicate a marked depth-habitat stratification for planktic foraminifera and calcareous nannofossils. Three different dwelling groups are recognized: shallowGlobigerinoides, Globoquadrina dehiscens, Globorotalia mayeri and nannofossils; intermediateGlobigerina venezuelana; and deepCatapsydrax dissimilis. The comparison of foraminifera and calcareous nannofossils suggests that the isotopic compositions of nannofossils are generally controlled by the same parameters which control the isotopic composition of shallow-dwelling foraminifera, but the former are more enriched in¹⁸O.     

A Decline in Benthic Foraminifera following the Deepwater Horizon Event in the Northeastern Gulf of Mexico

Sediment cores were collected from three sites (1000–1200 m water depth) in the northeastern Gulf of Mexico from December 2010 to June 2011 to assess changes in benthic foraminiferal density related to the Deepwater Horizon (DWH) event (April-July 2010, 1500 m water depth). Short-lived radioisotope geochronologies (²¹⁰Pb, ²³⁴Th), organic geochemical assessments, and redox metal concentrations were determined to relate changes in sediment accumulation rate, contamination, and redox conditions with benthic foraminiferal density. Cores collected in December 2010 indicated a decline in density (80–93%). This decline was characterized by a decrease in benthic foraminiferal density and benthic foraminiferal accumulation rate (BFAR) in the surface 10 mm relative to the down-core mean in all benthic foraminifera, including the dominant genera (Bulimina spp., Uvigerina spp., and Cibicidoides spp.). Cores collected in February 2011 documented a site-specific response. There was evidence of a recovery in the benthic foraminiferal density and BFAR at the site closest to the wellhead (45 NM, NE). However, the site farther afield (60 NM, NE) recorded a continued decline in benthic foraminiferal density and BFAR down to near-zero values. This decline in benthic foraminiferal density occurred simultaneously with abrupt increases in sedimentary accumulation rates, polycyclic aromatic hydrocarbon (PAH) concentrations, and changes in redox conditions. Persistent reducing conditions (as many as 10 months after the event) in the surface of these core records were a possible cause of the decline. Another possible cause was the increase (2–3 times background) in PAH’s, which are known to cause benthic foraminifera mortality and inhibit reproduction. Records of benthic foraminiferal density coupled with short-lived radionuclide geochronology and organic geochemistry were effective in quantifying the benthic response and will continue to be a valuable tool in determining the long-term effects of the DWH event on a larger spatial scale.     

Glacial Holocene environment of the southeastern Okhotsk Sea: evidence from geochemical and palaeontological data

Environmental conditions and productivity changes in the southeastern Okhotsk Sea have been reconstructed for the last 20 ka using planktonic and benthic foraminiferal oxygen isotope records and calcium carbonate, organic carbon and opal content data from two sediment cores. Species variability in benthic foraminiferal and diatom assemblages provides additional palaeoceanographic evidence. AMS radiocarbon dating of the sediments and oxygen isotope stratigraphy serve as the basis for the age models of the cores for the last 20 ¹⁴C kyr and for correlation between environmental variations in the Okhotsk Sea, and regional and global climate changes. Benthic foraminiferal assemblages in the two cores (depth 1590 and 1175 m) varied with time, so that we could recognise seven zones with different species composition. Changes in the benthic foraminiferal assemblages parallel major environmental and productivity variations. During the last glaciation, fluxes of organic matter to the sea floor showed strong seasonal variations, indicated by the presence of abundant A. weddellensis and infaunal Uvigerina spp. Benthic foraminiferal assemblages changed with warming at 12.5–11 and 10–8 ¹⁴C kyr BP, when productivity blooms and high organic fluxes were coeval with global meltwater pulses 1A and 1B. Younger Dryas cooling caused a decline in productivity (11–10 kyr BP) affecting the benthic faunal community. Subsequent warming triggered intensive diatom production, opal accumulation and a strong oxygen deficiency, causing significant changes in benthic fauna assemblages from 5.26–4.4 kyr BP to present time.