Pliensbachian-Toarcian ostracod biogeography in NW Europe: Evidence for water mas s structure evolution

Cannen Arias *

Departamento de Paleontología, Facultad de Ciencias Geológicas, Universidad Complutense de Madrid, Jase Antonio Novais, 2; 28040-Madrid, Spain

Absfract

This paper examines fue role played by palaeoceanographic and c1imatic conditions on the palaeobiogeography of the Pliensbachian Toarcian ostracods (Early Jurassic) in the European Epicontinental Sea (BES). The infIuence of the palaeogeography, ocean currents and sea level, temperature and salinity variations on ostracod abundance, diversity and migration pattems is reconstructed. Ostracod migration follows an antic10cbvise circulation in the eastem side of the EES, with a leading northeast southwest movement, and fue frequent arrival ofTethyan falUlas into fue central and westem parts ofthe EES during the Pliensbachian. A three-fold c1assification ofwater masses based on salinity, temperature, lithological and fossil data is proposed. The repeated inflow ofTethyan ostracods into the EES ended by the earliest Toarcian. This ostracod event is related to the opening ofthe Hispanic Corridor and to the reorganization ofthe surface and deep circulations that may have generated a cold episode at the beginning of the Toarcian.

Keywords: Ostracod; Palaeobiogeography; Pliensbachian-Toarcian bmllldary; Water masses; Deep-water cITculation; European Epicontinental Sea; Hispanic Corridor; Palaeoclimatology

1. Introduction Ostracods are a group of small crustaceans enclosed in a laterally compressed, bivalved ca1careous carapace, The aim of this paper is to describe the general with most of the species approximating to 1 mm in pattem of ostracod migration between the European 1ength (Schram, 1986). They live in al! marmer of Epicontinenta1 Sea (EES) and the Tethys Ocean (Fig. 1) aquatic habitats, reaching their highest diversity in during the Pliensbachian-Toarcian interva1 (Ear1y carbonate envrronments in a mid-shelf wann tropical Jurassic), showing their routes of migration and environment (Varmier et al., 1989). What is especial!y examining the significance of these movements in important is therr almost ubiquitous occurrence in tenns of palaeogeography, distribution of oceanic aquatic habitats of all kinds, combined with therr precise currents, heat fluxes, interchange of water masses, sea envrronmental requrrements. They can be excellent level variations and climate changes. swimmers, but the vast majority Uve close to the sediment-water interface. Sorne authors believe (Van Morkhoven, 1962; Sandberg, 1964; McKenzie, 1973; .. Tel.: +34 91 544 5459; fax: +34 394 48 49. Whatley, 1988; Babinot and Co1in, 1992; Lethiers and E-mail address: [email protected] . Crasquin-So1eau, 1995) that ostracods might have a Fig. 1. Global Early Jurassic palaeogeography and Pliensbachian-Toarcian palaeogeography of the northwest European region (modified from Ziegler, 1988, 1990, 1992; BassouUet et al., 1992; Ziegler et al., 2001; Scotese, 2002). Emergent areas: AM: Annonican Massif; BM: Bohemian Massif; C-S H Corsica-Sardinia High; FC: Flemish Cap; GB: Grand Bank; GH: Grampian High; IBM: Iberian Meseta; 1M: Irish Massif; LBM: London-Brabant Massif; MC: MassifCentral; PB: Porcupine Bank; RFM: Ringk?bing-Fyn-High; RHB: Rockall-Hatlon Bank; RM: RhenishMassif; SP: Shetland Platfonn; VII: Vmdelecian High; and \VII: Welsh Massif. limited trans-oceanic migmtion capacity (eggs, pre-adult geographical analysis of each ostracod species. Each and adult stages would be passively dispersed by means species was plotted on a palaeogeographieal map, using of wind, birds, fish, drifting algae or ocean cunents). one map for each time periodo All of the species placed Sinee the majority of present day ostracod species have together in each palaeogeographical reeonstruetion a high degree of ecological flexibility and lolemnce, allowed the drawing of a general pattem of migration they should have a high ability lo successfully invade for each tempoml interval. Although faunal interchange new envrronments. However, it appears that migration routes have been mainly deduced from the study ofEarly from deep to shallow water seems unlikely lo have Jurassic ostraeods, several other invertebrate fossil oeeurred. Thus, a eareful analysis of ostracod assem­ groups, inc1uding ammonites, bivalves and brachiopods blages can be extremely useful as palaeotemperature have also been eonsidered. and palaeosalinity indicators, and they may also provide valuable data on palaeobathyrnetry (water masses and 3. Geological setting deep-water cireulation). The world palaeogeography of 210-179 million 2. Materials and methods years BP was eharacterized by the presenee of a large landrnass (the Pangaea) centered over the equator, The data used in this analysis are based on 210 sunounded by ahuge worldwide ocean (the Panthalassa Pliensbaehian and Early Toarcian ostraeod species Ocean) and with a wedge-shaped ocean nmning into the (Table 1) taken from 199 published papers. Since the eastem margin (the Tethys Ocean). Pangaea split into number of individual loealities is large, the sites were two eontinents (Laurasia and Gondwana) at the grouped into 15 large geographical areas: C: Grand beginning of the Late Triassic, with an epicontinental Banks, Canada; CI: the Cordillera Ibérica, Spain; DK: sea fonned between 5°N and 25°S of the two land­ Danish Embayrnent, Denmark; FB: the F astnet and masses. This is designated the European Epicontinental North Celtic Basins; GB: British basins; lt: ltaly; M: Sea (EES) (Figs. 1 and 2). Morocco; NEG: Northeast Germany; NWG: Northwest The evolution ofthe Early Jurassic EES was strongly Gennany; PB: the Paris Basin, Franee; Q: Querey, influenced by the evolution of the Central Pangaea and Franee; P: the Lusitanian Basin, Portugal; SWG: by sea level variations. This eoupled action allowed the Southwest Gennany; S: Skcme, Sweden; and Sz: development of new seaways and barriers among the Switzerland (F ig. 2). different basins through the epicontinental sea (Vail Only illustrated taxa have been eonsidered, and the et al., 1977; Hallam, 1978; Kutzbach et al., 1990). The methodology followed is based on the individual palaeo- Late Triassie marked the end of the Pangaea Table 1 Table 1 (continued) List ofPliensbachian and Toarcian ostracod species mentioned in the OstTacod species text Ektyphocythere knitteri (Rlegraf, 1984) Ostracod species Ektyphocythere lanceolata (Boomer, 1988) Acrocythere gassumensis (Michelsen, 1975) Ektyphocythere luxuriosa (Apostolescu, 1959) Acrocythere oeresundensis (Michelsen, 1975) Ektyphocythere multicostata (K.lingler and Neuweiler, 1959) Acrocythere troesteri (Riegrnf, 19&4) Ektyphocythere neumannae (Maupin, 1978) Ambigocythere concentricostata (Herrig, 1985) Ektyphocythere quadrata (Boomer and Lord, 1988) Aphelocythere kuhni (Triebel and Klingler, 1969) Ektyphocythere vitilis forcata (Wienhol.z, 1958) Bairdia kempfi (Ainsworth, 1989) Ektyphocythere aff. Ektyphocythere vitiosa (Apostolescu, 1959) Bairdia cZio (Bizon, 1960) Ektyphocythere vulgaris (Klingler and Neuweiler, 1959) Bairdia crassa (Drexler, 1958) Eucythernra angulocostata (Knitter, 1983) Bairdia donzei (Herrig, 1979a,b) Eucythernra liassica (Bate and Coleman, J975) Bairdia guttulae (Herrig, 1979a,b) Eucythernra tatei (Ainsworth, 1986a, b) Bairdia hahni (lmd and Moorley, 1974) Eucythernra tricostata (Michelsen, 1975) Bairdia michelseni (Herrig, 1979a, b) Eucythernra sp. (Riegraf, 1985) Bairdia molesta (Apostolescu, 1959) F abalacypris symmetrica (Herrig, 1979a,b) Bairdia ohmerti (Knitter, 1983) Gammacythere ubiquita (M:alz and Lord, 1976) Bairdia praehilda (Herrig, 1979a, b) Gramannella apostolescui (Gramann, 1963) Bairdia rostrata (Issler, 1908) Gramannella laevigata (M:ichel.sen, 1975) Bairdia thuringica (Knitter, 1983) Gramannella tatei (Grarnann, 1963) Bairdia? sp. A (Ainsworth, 1986a,b) Gramannicythere aubachensis (Riegraf, 1984) Bairdia sp. A (Ohm, 1986) Gramannicythere bachi (Herrig, 1982a,b) Bairdia eirensis (Ainsworth, 1986a,b) Gramannicythere conmncta (Herrig, 1982a,b) Bairdiacypris anisica brevis (Herrig, 1979a,b) Gramannicythere sp. (Malz and Nagy, 1989) Bairdiacypris dorisae (Knitter, 1983) Henniella ambo (Lord and Moorley, 1974) Bairdiacypris rectangularis (Ains,worth, J986a, b) Henniella cincta (MaLz, 1975) Bairdiacypris triangularis (Ainsworth, 1986a, b) Henniella circumvallata (Dreyer, 1967) Bairdiacypris triasica postera (Herrig, 1979a, b) Henniella comesMaIz, 1975 Bythocypris fabaifórmis (Drexler, 1958) Henniella hyblea (Barbieri, 1%4) Bairdiacypris tumida (Ain.sworth, 1987) H. intercedens (Dreyer, 1967) Cardobairdiafastnetensis (Ainsworth, 1987) Henniella klingleri (1vialz, 1975) Cardobairdia liassica (Drexler, 1958) Hutsonia decorata (Apostolescu et al. , 1961) Cardobairdia toarcensis (Ainsworth, 1986a,b) Hutsonia aff. Hutsonia decorata (Apostolescu et al. , 1961) Cardobairdia Nr. 103 (K.lingler, 1962) Infracytheropteron groissi (Knitter, 1983) Cardobairdia sp. A (Ainsworth, 1987) Infracytheropteron gwashense (Bate and Coleman, 1975) Cardobairdia sp. K (Apostolescu., 1959) Infracytheropteron pulchellum (Michel.sen, 1975) Cristacythere betzi (Klingler and Neuweiler, 1959) Infracytheropteron rarum (Knitter, 1983) Cythere? terquemiana (l ones., 1872) Infracytheropteron riegrafi (Riegraf, 1985) Cytherella demiexensis (Ainsworth, 1989) Infracytheropteron supraliasicum (Herrig, 1981a ,b,c) Cytherella lindseyensis (Lord, 1974a,b,c) Isobythocypris cylindrica (Herrig, J 979a,b) Cytherella praecadomensis (K.nitter and Riegraf, 1984) Isobythocypris dorsoconversa Ainsworth, 1986a,b Cytherella toarcensis (Bizon, 1960) Isobythocypris fabaefonnis (Drexler, 1958) Cytherelloidea anningi (Lord, 1974a,b,c) Isobythocypris ovalis (Bate and Colema.n, 1975) Cytherelloidea cadomensis (Bizon, 1960) Isobythocypris pliensbachiensis (Ainsworth, 1986a,b) Cytherelloidea drexlerae (Field, 1966) Isobythocypris tatei (Coryell, 1963) Cytheropteron alafastigatum (Fischer, 1962) Isobythocypris unispinata (Apostolescu, 1959) Cytheropteron byfieldensis (Boomer and Bodergat, 1992) Kinkelinella costata (Knitter, 1984) Cytheropteron cavatum (Michelsen, 1975) Kinkelinella fischeri (Malz, 1966) Cytheropteron diversum (Herrig, 1969 a, b) Kinkelinella mandelstami (Wienholz, 1958) Cytheropteronfoveolatum (MicheLsen, 1975) Kinkelinella persica (Bate and Cole:man, 1975) Cytheropteron? sp. A (Riegraf, 1985) Kinkelinella procera (Herrig, 1985) Cytheropteron sp. B (Itiegraf, 1985) Kinkelinella sennoisensis (Apostolescu, 1959) Ektyphocythere acuminata (Riegraf, 1984) Kinkelinella tenuicostata (Martin, 1960) Ektyphocythere ambo (Boomer, 1988) Klinglerella elongata (Michelsen, J 975) Ektyphocythere anterocosta (Boomer, 1988) Klinglerella foveolata (Michel.sen, 1975) Ektyphocythere bucki (Bizon, 1960) Klinglerella herrigi (Ain.sworth, 1989) Ektyphocythere champeau (Bizon, 1960) Klinglerella intermedia (K.lingler and Neuv.re iler, 1959) Ektyphocythere debilis (Bate and Coleman, 1975) Klinglerella katsloesensis (Sivhed 1980) Ektyphocythere dharennsourensis (Boutakiout et al., 1982) Klinglerella lacunosa (Ainsworth, J989) Ektyphocythere intrepida (Bate and Coleman, 1975) Klinglerella moorei (Brady, 1872) Table 1 (continued) Table 1 (continued) Ostracod species Ostracod species Klinglerella variabilis (Klingler and Neuweiler, 1959) Praeschuleridea levita (Ainswortb, 1986a,b) Ledahia conviniens (Herrig, 1980) Praeschuleridea magnycourtensis (Apostolescu, 1959) Ledahia septenaria (Gründel, 1964) Praeschuleridea pseudokinkelinella (Bate and Coleman, 1975) Liasina lanceolata (Apostolescu, 1959) Praeschuleridea punctulata (plurnhoff, 1963) Liasina? vestibulifera (Gramann, 1962) Praeschuleridea reticulata (Ainswortb, 1986a,b) Liasina cylindrica (Ainsworth, 1986a,b) Praeschuleridea ventriosa (plurnhoff, 1963) Monoceratina amlingstadtensis (Triebel and Bartenstein, 1938) Praeschuleridea whatleyi (Ainswortb, 1986a,b) Monoceratina mesoliassica (Triebel and Bartenstein, 1938) Procytheridea? jardensis (Maupin, 1978) Monoceratina michelseni (Riegraf, 1984) Procytherura celtica (Ainswortb, 1986a,b) Monoceratina seebergensis (Triebel and Bartenstein, 1938) Procytherura euglyphea (Ainsworth, 1986a,b) Monoceratina stimulea (Schwager, 1866) Procytherura hastata (Bate and Coleman, 1975) Monoceratina striata (Triebel and Bartenstein, 1938) Procytherura mediocostata (Bate and Coleman, 1975) Monoceratina ungulina (Triebel and Bartenstei.n, 1938) Procytherura multicostata (Ainsworth, 1986a,b) Nanacythere (D.)finna (Herrig, 1969a,b) Procytherura suebica (Herrig and Richter, 1990) Nanacythere (D.)fissicosta (Herrig, 1969a,b) Procythernra werneri (Riegraf, 19&4) Nanacythere (G.) minor (Michelsen, 1975) Procythernra? exquisita (Ainsworth, 1986a,b) Nanacythere (N.) simplex (Herrig, 1969a,b) Procythernra? liassica (Ainsworth, 1986a, b) Nanacythere persicaefonnis (Ri.egraf, 1984) Progonoidea reticulate (K.lingler and Neuweiler, 1959) Ogmoconcha amalthei amalthei (Quenstedt, 1967) Pseudohealdia etaulensis (Apostolescu, 1959) Ogmoconcha amalthei rotunda (Dreyer, 196 7) Pseudohealdia gruendeli (Mm, 197 J) Ogmoconcha contractula (Triebel, 194 1) Pseudohealdia transversa ( Gründe ~ 1970) Ogmoconcha convexa (Boomer, 199 1) Pseudohealdia grodidieri (Viaud, 1963) Ogmoconcha dentata (Issler, 1908) Pseudohealdia pseudoespina (Herrig, 1969a,b) Ogmoconcha eocontractula (Park, 1984) Pseudohealdia pseudohealdidae (Gründel, 1964) Ogmoconchella adenticulata (pietzzenuk, 196 1) Pseudohealdia truncata (Malz, 197 1) Ogmoconchella aequalis (Hemg, 1969a,b) Pseudomacrocypris subaequalis (Michelsen, 1975) Ogmoconchella aspinata (Drexler, 1958) Pseudomacrocypris? sp. A (Ainsworth, 1986a,b) Ogmoconchella danica (Michelsen, 1975) Pseudomacrocypris subtriangularis (Michelsen, 1975) Ogmoconchella impressa (Mili, 1975) Trachycythere angusta (Triebel and Klingler, 1969) Ogmoconchella michelseni (Mi.chelsen, 1975) Trachycythere horrida (Triebel and Klingler, 1969) Ogmoconchella mouhersensis (Apostolescu, 1959) Trachycythere tubulosa seratina (Triebel and Klingler, 19 69) Ogmoconchella secunda (Herrig, 1981 a, b,c) Trachycythere tubulosa tubulosa (Triebel and Klingler, 1969) Ogmoconchella propinqua (MaIz, 197 1) Trachycythere veJTUcosa (Triebel and Klingler, 1969) Ogmoconchella sp. A (Lord, 1974a,b,c) Triassocythere multiestriata (Michelsen. 1975) Ogmoconchella sp. B (Apostolescu, 1959) Triassocythere? sp. 4135 (Michelsen, 1975) Olygocythereis? mochrarensis (Boomer, 199 1) Paracypris liassica (Bate and Coleman, 1975) Paracypris redcarensis (BW::e, 1876) Paracypris sp. (Cubaynes, 1986) Paracypris sp. 1 (Exton, 1979) supercontinent. lts break-up cornmenced with the south­ Paracypris sp. 2 (Exton, 1979) ward propagation of the Norwegian-Greenland rift into Paracypris sp. A (Riegrnf, 1985) Paracypris sp. e (Ainsworth, 1986a,b) the Central Atlantic area, and the gradual westward Pleurifera harpa harpa (Klingler and Neuweiler, 1959) propagation ofthe Tethys rift (McHone, 1996, Marzoli et Pleurifera harpa harpoidea (Gramann, 1962) al. , 1999; McHone, 2000; Bartolini and Larson, 2001; Pleurifera vermiculata (Apostolescu, 1959) Mchone, 2002; Hames et al., 2002). Bythe Late Triassic, Polycope minor (Michelsen, 1975) the area between both the continents, along the present Polycope pelta (Fischer, 196 1a) Polycope plumhoffi (Bate and Coleman, 1975) European continent, was the site of a shallow carbonate Polycope cerasia (Blake, 1876) platfonn that became a stable epicontinental sea with Polycope cincinnata (Apostolescu, 1959) restricted basins among an archipelago of islands Polycope decorata (Apostolescu, 1959) (Zieg1er, 1988, 1992). Another consequence of the Polycope tenuireticulata (Herrig, 1981a,b,c) evolution ofthis rifting system was the development of a Praeschuleridea arguta (Ainsworth, 1986a,b) Praeschuleridea aspera (Knitter, 1983) mosaic of structural highs and lows in Westem Tethys, Praeschuleridea bernierensis (Apostolescu, 1959) subsequently covered by small carbonate platfonns, Praeschuleridea costata (Ainsworth, 1986a,b) from the Iberian Massif through lo the Apulia Block Praeschuleridea ellipsoidea (Ainsworth, 1986a,b) (Laubscher and Bemoulli, 1977; Bassoullet et al. , 1992; Praeschuleridea gallemannica (Malz, 1966) Stampfli et al., 1991, 1998; Ziegler et al. , 2001). 4. Palaeobiogeographical analysis lioceratidae) extended northward from the Briaconnais area, Burgundy lo the British basins and subsequently 4.1. Early Pliensbachian to the Cordillera Ibérica, Spain, reaching even Green­ land (Dornmergues, 1982; Phelps, 1985; Dommergues, The analysis of Early Pliensbachian ostracod palaeo­ 1987; Dommergues and Meister, 1992; Cariou and biogeography exhibits two main characteristics: a cIear Hantzpergue, 1997; Dommergues et al. , 1997). southward movement of the EES failllas, and the constant influx of Tethyan fawms mto the central and 4.2. Late Pliensbachian westem parts of the EES during the Early and middle Early Pliensbachian (F ig. 3a, A and B). Ostracods and During the Late Pliensbachian, Tethyan ostracod other fossil groups suggest several routes, which Tethyan faunas continued to enter into the EES, especially fonns could have followed (the Balcony Mounts, the through its southem and westem margins. At the North Austrian Alps, the Sub-Briayonnais and the Upper beginning of the Late Pliensbachian (F ig. 3a, C), the Austroalpine or the Bohemian Massif) through the continuous southwards expansion of ostracod faunas southem EES (Faugeres andMouterde, 1980; Dommer­ from the northem parts ofthe EES lo the Bay ofBiscay gues, 1982; Enay and Mango1d, 1982; Cariou et al., and the Iberian basins is observed in the movements of 1985; Dornrnergues, 1987; Dommergues and Meister, several species, such as Cytherella toarcensis, Ektypho­ 1990, 1991, 1992; Meister and Stampfly, 2000; Arias cythere quadrata, Hermiella ambo, Hermiella comes, and Whatley, 2004). Ledahia septenaria, Liasina vestibulifera, Ogmo­ The existence of these interchange areas is a concha amalthei and Pleurifera harpa. The mid Early consequence of the plate-tectonic history of this area. Pliensbachian represents the maximum Tethyan influ­ Hettangian carbonate p1atfonns (High Middle Atlas, ence in the EES. But by the latest Early Pliensbachian Sabara Atlas, Moroccan Massif; Sub-Belic and Apen­ (Fig. 3a, D) this northward expansion had ended. nines) began lo break-up at the end ofEar1y Sinemurian. It is remarkable that in the latest Pliensbachian many From the Late Sinemurian lo the Ear1y Pliensbachian, the species that originated in the northem Gennan basins marine transgression extended along these platfonns with did not appear in the British and Spanish basins until new seaways being created (Donnnergues, 1979; Enay, later. This interruption may have been due lo the 1980; Dommergues, 1982; Alméras and Moulan, 1982; presence of sorne kind of barrier along the southem Ahnéras and Ehni, 1987, 1993; Ahnéras andFaure, 2000). margin of the London-Brabant Massif that kept the In the Ear1y Pliensbachian, the pattem of ostracod British basins in isolation. Physical baniers, such as the distribution a10ng the eastem coast of the EES illustrates Morton-in-the-Marsh and Market-Weighlon highs, may the long-standing entrance of Tethyan taxa, which have served as temporary barriers to faunal movement expanded from the southwestem Gennan to northem between northem Gennan and the southem British Gennan and Danish basms, then rotating back aroillld to basins (Donovan, 1967; Mégnien, 1980; Dommergues, the Iberian basins. This movement follows an antic­ 1982; Enay and Mangold, 1982; Meister and Stampfly, lockwise circulation throughout the eastem EES. During 2000; Arias and Whatley, 2004). much of the Early Pliensbachian, the ostracod fuuna (e.g. At the end ofthe Late Pliensbachian (Fig. 3a, D), the Bairdia donzei, Bairdia guttulae, Bairdia michelseni, continuing transgression totally connected the Paris Bairdia molesta, Bairdia rostrasta, Henniella interce­ Basin with the southem British and Gennan basins. This dens, Ogmoconchella aff. o. aspinata) moved from the connection can be observed in the study of the northeastem to southwestem areas ofthe EES around the movement of the large cytheroids (Ektyphocythere aff London-Brabant and the Annorican Massif; through E. vitiosa, Gramannella apostolescui, Pseudohealdia the Westem Approaches Trough and the Bay of Biscay pseudohealdidae, Trachycythere tubulosa tubulosa and to the Iberian basins. Direct communication between the Trachycythere verrucosa) that dominated ostracod Paris Basin and the Quercy area is not apparent, possibly assemblages at the end of the Pliensbachian. The due to the closure ofthe "Seuil duPoitou", ashallow zone composition and abundance of Spanish ostracods was joining the Annorican lo the Central Massifs and quite similar lo that ofthe French area, but they have less separating the Aquitaine and Paris Basins (Gabilly, 1976). siruilarity with the coeval assemblages from British, 'When the ostracod results are compared with those Gennan and Danish areas (Arias and Lord, 1999a,b). obtained from other fossil groups, such as arnmonites, we Brachiopods offer supplementary confinnation. The see a close agreement. Early Pliensbachian arnmonites analysis ofthe distribution ofLower Jurassic brachiopod (Liparoceratidae, Acanthopleuroceratidae and Dacty- faunas (Ager, 1956, 1960, 1971, 1973) allowed the Fig. 2. Palaeogeographie position of the selected loealities during the Toarcian (after Zi.egler, 1988; Bassoullet el al., 1992). (1) Northeastern Gennany (NEG): Dobbemn and Eisenach (pietrzenuk, 1961), Brandenburg (Dreyer, 1967); Greifswald (Herrig, 1969a,b); Thüringen (Hemg, 1979a,b; 1980; 1981 a,b,e; 1982a,b); (2) Northwestern Gennany (N\VG): Niedersachten (Triebel and Bartenstein, 1938); Hannover (Triebel and Klingler, 1959); Weseker Sattel (Gr.unann, 1963); Niedersachten (Malz, 1971); Empelde (Ohm, 1986); (3) Southwestern Gennany (SWG): Baden-Württemberg (Knauff, 1954; Klinger and Neuweiler, 1959; Fischer, 1961a,b,c; Malz, 196 1, 1966, 1975; Klingler, 1962; Lord and Morley, 1974; Urlichs, 1977; Knitter, 1983, 1984; Knitter and Ohmert, 1983; Knitter and Rie gra~ 1984; Harloff; 1993); Kalk-Alpen Bayern and North Tyrol (Harloff and Jager, 1994); (4) Grand Banks, NewfmUldland Canada (C): (Exton and Gradstein, 1984); (5) Cordillera Ibérica, Spain (CI): (A.rias, 1995, 1997,2000; Arias and Lord, 1999a,b); (6) Celtie Sea-Fasmet Basin-Porcupine (FB): (Ainsworth, 19S6a,b, 1987, 1989, 1990; Ainsworth and Horton, 1987; Ainsworth et al. , 1987); (7) Danish Embayment, Denmark (Dk): (Miehelsen, 1975); (8) Skane, Southern Sweden (S): (Sivhed, 1980); (9) The ParisBasin, Franee (PB): (Apostolescu, 1959, 1961; Apostolescu et a1., 1961; Bizon and Oertl~ 1961; Cousin and Apostolescu, 1961; Champeau, 196 1; Magné et aL, 1961; Magné and Malmoustier, 196 1; Oertli and Grosdidier, 1961; Magn¿ and Obert, 1966; Maupin, 1978; Bodergat et al., 1985, 1991 ; Dépéche, 1985; Donze, 1985; Riegrnf, 1985; Bodergat and Donze, 1988); (10) Quercy, southern Franee (Q): (Cubaynes and Ruget, 1985; Cubaynes, 1986; Bodergat et al. , 1991, 1998; Andreu et a1. , 1998); (11) Great Britain (GB): Central and southern England (Lord, 197 1a,b, 1972, 1974 a,b,c, 1978, 19 88); Empingham and Upwood (Bate and Coleman, 1975); Ilminster (Boomer, 1991); Dorset and Isle of Wight (Lord and Bown, 1981); Mochras, Wales (Boomer, 1992); (12) Centralltaly (It): Strettura (Lord, 1978, 1988); Umbria (Arias, 1993); (13) Zambuja~ Portugal (P): (Exton, 1979; Exton and Gradstein, 1984; Boomer et a1., 1998) and (14) N\V Switzerland (Sz): (Richter, 1987; Troster, 1987). recognition of a series ofbelts throughout the EES. The central part of the EES became. Two bivalve provinces intermediate belt, regarded as Tethyan facies (character­ are recognized within the Proto-Atlantic: the Boreal ized by the scarce occurrence ofaxinifonn rhynchonel­ Bivalve Province in the north (England, northwestem Uds, and sorne species of Terebratula) would have Germany and Denmark) and the Southem Transitional spread throughout the southem part of Germany, Frunce Province (Spain, France and Portugal). The former and southwestem England. Around it, the outermost belt province is assigned to the Boreal Faunal Realm, the would inc1ude northeast England, from southeast latter one lo the Tethyan Faunal Realm. Gennany to the Carpathian area to the east; and Amrnonites give us additional evidence on the ocean Nonnandyand Causes (Franee), the Iberian Peninsula surface current pattem with the rnigration of sorne and north Morocco to the west. This belt is characterized irnportant taxon, such as the northward rnigration of by the presence of Tetrarhynchia, Gibbirhynchia and Amaltheidae mto the regions previously dorninated by Lobothyris, dentate zeillerids and ribbed spiriferids, and Harpoceratinae, extendmg mto the southwestem Ger­ reflects the initial northward extension followed by the rnan, French and central British basms, as well as the westward extension of the European Epicontinental North Atlantic strait, the periarctic American coast and faunas. The palaeobiogeography of the latest Pliensba­ the northem coast ofSiberia (Enay and Mangold, 1982) chian bivalves (Liu et al. , 1998) reveals how problema­ during the middle-Late Pliensbachian. This movement tical the cornmwlication between the northem and was accornpanied by a southwestward expansion to the southem parts of the EES in the Late Pliensbachian The distribulion of a number of ostracod species (B. (Donnnergues and Mouterde, 1980; Enay and Man­ donzei, e toarcensis, Cytheropteron byfieldensis, lso­ gold, 1982; Dommergues and Mouterde, 1987; Dom­ bythocypris tatei, Kinkelinella sermoisensis, E. bucki, mergues, 1987; Andreu et al, 1998; Meister and Stampfly, Ektyphocythere champeau, E. aff E. vitiosa, 1. septe­ 2000). naria, Monoceratina stimulea and T tubulosa) indicates The other route ofmigration ofTethyan ostracod and the opening of the connection between the Aquitaine other rnvertebrate groups, the Iberian-Moorish Strait, Basin and the Cordillera Ibérica area at the cornmence­ took place aroillld the westem comer of the EES, from ment ofthe Toarcian. This new route closes off one large North Africa lo the Iberian Peninsu1a and the Brilish anliclockwise gyre in central EES. basins by the middle Ear1y Pliensbachian (Elmi et al., Another proposed route for the entrance of the 1974; Faugeres and Mouterde, 1980; Elmi et al., 1982; Tethyan fauna was through the Betic area. Arnmonite Donnnergues, 1982; Enay and Mangold, 1982; Thierry, assemblages from the Cordillera Ibérica clearly belong 1988; Arias, 2000). Ostracod species, such as Ekty­ to the Euroboreal realm, while the Betic assemblages phocythere dharennsourensis, corrobomte the existence show fawml affinities with those described from the of this very active migration route by the Late Mediterranean area (Braga et al., 1982; Goy et al., 1988; Pliensbachian-Early Toarcian (Arias, 2000). The Gómez and Goy, 2002). Therefore, during the Pliens­ ammonites and brachiopods also indicate the existence bachian and Toarcian the fawlal evidence suggests sorne of this route (e.g. the migration of sulcate terebratullids type of barrier (Ruiz-Ortiz et al., 2004) between both from the Rift area and westem part of the Cordillera Cordilleras. Vera (1998) shows, in his evolutionary Bética through western Portugal to southwestern model of the southem Iberian Continental palaeomar­ England and Normandy by the Late Pliensbachian gin, that during the Pliensbachian-Toarcian began a (Ager, 1956). Elmi et al. (1982) described the migration phase of intercontinental rifting which created a mosaic of Moroccan costate terebratullids to Portugal, Spain, of stmctoral highs and lows in the Sub-Belic area, while France and Great Britain (Ager and Walley, 1977; Elmi the pre-Betic area remained as a shallow zone. Elmi et al., 1982). Other examples include the spalial et al. (1982) and Vera (1998) suggested that the southem distribution of the arnmonite genus Hildoceratidae, Iberian palaeomargin acted as a barrier for the entrance described in the Lusitanian Basin, the Cordillera Ibérica, ofTethyan faunas (Elmi et al., 1982; Cariou et al., 1985; Nonnandy and southem England, or the occurrence of Alméras and Elmi, 1987; Donnnergues, 1987). Mediterranean ammonites Reynesocoeloceras and Bou­ During the falciferum zane (Early Toarcian) an liceras (described originally from Madagascar, Kenya, important change is detected in the crrculation model Somalia, Arabia, Morocco) in Portugal and Spain (Fig. 3b, F). The connection between the central and (Mouterde, 1953; Mooterde et al., 1979; Donnnergues westem European basins was weaker than during earlier and Mouterde, 1987; Goy and Martinez, 1990). Hallam periods, while analysis of the ostracod distribution (1971) described the close similarity between Pliensba­ distinguishes an interruption of the cornmwlication chian bivalve assemblages from Morocco, Spain, between British, French and German basins. The England and Greenland. cornmunication between the French and Spanish basins, however, remained active. This observation is supported 4.3. Early Toarcian by the migration of several ostracod species, such as Bairdiacypris dorisae, C. toarcensis, E. bucki, Ekty­ In terms of the ostracod fawm migration, the earliest phocythere anterocosta, lnfracytheropteron gwashense, Toarcian (Fig. 3b, E) maintains a similar distribution K. tenuicostata and M striata. pallem to the Late Pliensbachian, with a southward Preservational bias and incompleteness of the fossil expansion of faunas from the northem part of the EES lo record in the central basins of the EES have been the Iberian basins, with an anticlockwise crrculation in the repeatedly related to early phases of black shale central parts of the EES. This pallem can be observed in deposition (e.g. the Jet Rack in Great Britain, Pasi­ the movement of several ostracod species, such as B. donschiefer in Gennany and Switzerland or Schistes molesta, Ektyphocythere bucki, Ektyphocythere neuman­ Cartons in France), occurring principally in the falci­ nae, G. apostolescui, Kinkelinella tenuicostata, 1. ferum zone, although locally extending into the tenui­ septenaria, 1. vestibulifera, Monoceratina striata, Mo­ castatum zone (Hallam, 198 1; Riegraf, 1984; Riegraf noceratina ungulina, Polycope cincinnata and T tubu­ et al., 1984; Reigraf, 1985; Jenkyns, 1985; Hallam, losa seratina. Only the presence of sorne kind of barrier 1987; Espitalié et al., 1987; Fleet et al., 1987; Jenkys, may have modified this general model. 1988; Ziegler, 1988, 1991). These shales have been explained in tenns of restricted circulation, salinity stra­ disappearance of spiriferids and the development of tification, slow sedimentation rates, anoxic bottom waters very particular faunas (ubiquitous forros belonging to and low productivity; or even the reverse, high sedimen­ the "Spanish facies" described from the French tation rates and high productivity, possible upwelling Provence lo the Atlas Basin). (parrish and Curtís, 1982; Wignall and Meyers, 1990; The reduction in cornmunication among the different Tyson and Pearson, 1991 ; Wignall and Hallam, 1991 ; basins was only transitory, as a re-opening of the seaways Jenkyns and Clayton, 1997; Schmid-R6hl et al. , 2002). between the central and westem European basins took The deposition of the Toarcian black shales is also place in the bifrons zone. Ostracod species, such as, reflected in another fossil group, the brachiopods, with Cytherella praecadomensis, C. toarcensis, Cytheropteron an important crisis (Choffat, 1947; Goy, 1974; Garcia­ alafastigatum, C. byfieldensis, E. bucki, E. champeau, Joral and Goy, 1984; Goy et al. , 1984; Alméras and Ektyphocythere debilis, Eucytherura angulocostata, Eu­ Elmi, 1993; Ahnéras and Faure, 2000) marked by the cytherura tricostata, Kinkelinella costata, K. fisheri, K.

a

Fig. 3. Ostracod migration routes. (a) (A) Early Pliensbachian (jamesoni zone); (B) Early Pliensbachian (ibex and davoei zones); (e) Late Pliensbachian (margaritatus zone); (D) Late Pliensbachian (spinatum zone); and (b) (E) Pliensbachian-Toarcian (tenuicostatum zone); (F) Early Toarcian (jalcifernm zone); (G) Early Toarcian (bifrons zone). Black arrows show the direction of ostracod migration. b

Fig. 3 (continued). persica, K. sermoisensis, Polycope pelta and Praeschu­ distribution patterns of living ostracod commw1ities. leridea pseudokinkelinella track an antic10ckwise gyre These inc1ude physical parameters such as water temper­ migmlion pattem across of lhe EES during lhe bifrons ature, salinity and the nature of substrate, ocean currents zone (Fig. 3b, G). and biological factors. It is, however, difficult to cite any one control as wliversally dominant, for while many 5. Palaeoceanographic control on ostracod workers feel that, in the case of marine ostracods in distribution particular, water temperature is the most important, other would argue that salinity is more fundamental, The majority of marine ostracods are bottom­ while in costal environments, the nature of substrate dwelling fonns, and only a small number occupy lhe may be lhe overriding influence (Cronin and Dowsett, planktonic realm. A wide range of factors govems the 1990). 5.1. Water temperatures: the European Epicantinental (and other invertebrate fossil groups) have ecological Sea water masses limits controlled mainly by temperature and sa1inity. In relation to the first aspect considered, the Early The present-day distribution of ostracod is c10sely Jurassic world has been cornmonly interpreted as related to water temperature, and therefore former essentially one in which low-Iatitude regions were changes in the water temperatures will be reflected in either desert or seasanally summer wet biomes, mid the species composition of fossil ostracod assemblages. latitude regions were winter wet and warm temperate Thus, ocean currents and water mass temperatures (due biomes, while high latitudes were caal temperate to the ability of ocean currents lo modulate the heat biomes (Hallam, 1975; Frakes, 1979; Zieg1er et al., transported from the tropics lo high latitudes) exert 1984; Hallam, 1985; Frakes and Francis, 1988; Zeigler strong controls over the latitudinal distribution ofbenthic et al., 1998; Gibbs et al., 2002; Zeig1er et al. , 2003). But shelf ostracods today, and any variation of the surface this combined faunal, floral and lithological data can be and deep-water crrculation can alter the position of the only used to determine Early Jurassic terrestrial c1imate bOillldaries of therr zoogeographical provinces. Hence, zones (Vakhrameev, 1964; Hallam, 1984, Vakhrameev, the analysis of marine ostracod assemblages can provide 1991 ; Hallam, 1994; Rees et al., 2000; Sellwood et al., a unique source of information on ocean palaeotempera­ 2000; Arias and Whatley, 2004). Another type of tures and, by implication, on changing c1imatic condi­ approximation for describing ocean "biomes" or water tions, enabling researchers to reconstruct changes in the masses (as the marine equivalent for the terrestrial position ofwater masses in the past (Wood and Whatley, biomes) is needed (Ziegler et al. , 1998). Using particular 1994). salinity and temperature data derive from palaeoclimatic Detailed palaeoecologica1 studies have shown that models and palaeodata (Chandler et al., 1992; Frakes many ostracod species are associated with water masses et al., 1992; Chandler, 1994; Arias and Whatley, 2004), that possess distinctive thermal characteristic. The and the abundance and the distribution of the ostracod properties of the water masses in a shallow sea are assemblage, a three-fold c1assification of Early Jurassic constantly changing. Seasonal influences are magnified water masses can be delineated (Fig. 5): by the proximity of land, because it varies the annual range in atmospheric temperatures and the freshwater (i) Tropical water masses. Tropical water masses are supply through rivers and runoff. This makes the limited within the 35° latitude lines and are characterization of water masses more difficult to characterized by having high salinities and determine than in the deep ocean, where most of the temperatures. These conditions allowed abillldant water masses are not in contact with the atmosphere. In botlom productivity and carbonate build-ups in the absence ofthe air-sea contact, the physical proper­ ties ofwater masses can only be transformed when they mix with other water masses of different origins and in consequence, with physical-chemical properties (tem­ perature and salinity). These circumstances complicate the reconstruction of the EES water masses structure. For the study of the water masses in a shallow sea, such as the Early Jurassic EES, the present author, the model developed by Ziegler et al. (1994, 2(03), Scotese (2002) and Winguth et al. (2002) has been adopted. They described the Pennian water masses taking into con­ sideration the marine and terrestrial-sensitive sediment distribution and the present-day relationship of sediments to characteristic c1imate conditions. The EES water masses map was reconstructed by means of the analysis of the marine, nearby terrestrial climate-sensitive sedi­ ments and the palaeobiogeographic distribution of ostracod assemblages and other marine fossils (Fig. 4).

This method is based on the premise that different Eg. 4. Early Jurassie eva1X'rite (e) and eoal (e) distributions (modified sediment types are indicative of very particular marine from Frakes, 1979; Parrish et al, 1982; Hallam, 1985; Br.mdt, 1986; envrronments and on the statement, that ostracod species Frakes et al., 1992; Cllandler, 1994; Scotese· PALEOMAP Project, 2002). areas of good circulation (reef distribution was limited to the northem and westem margins ofthe Tethys Ocean within 5-35°N and S). In relation to the ostracod fawla, smooth-shelled bairdioids (Bairdia, Pontocyprella, lsobythocypris and Bairdiacypris), omamented healdioids, in asso­ ciation with a minor nwnber of small cytheroids dominated the assemblages. (H) Subtropical water masses. This category is characterized by high salinity and warm water temperature conditions occurring within the central and southem parts of the EES, where important evaporite deposits are frequent (eva­ porites dominate the south and west sides of the EES, between 400 N and 40° S of the equator). Most ostracod assemblages described here include a few species of metacopids (Ogmo­ concha, Ogmoconchella) and a larger nwnber of Eg. 5. The ESS water masses in the Early Jurassic. Early Jurassic large and thicker cytheroids (Ektyphocythere, ocean water mas ses have been reconstructed based on marine and Kinkelinella, Gramannella, Praeschuleridea and nearby terrestrial climate-sensitive sediments and the composition and Trachycythere), smal! cytherurids (Cytheropteron, the palaeogeographic distribution ofEES ostracod assemblages. Infracytheropteron, Rutlandella and Eucyther­ ura) and cytherel!oids (Cytherella, Cytherelloi­ and spread out at intermediate depths (subtropical water dea) as a minor component. masses) displacing intermediate water from Antarctica (iH) Temperate water masses. Temperate water masses (temperate water masses) in the north EES, adding dominate the northem part of the EES, reaching additional complexity to the flow as seen in the central latitudes as low as 300 N. This type is defmed by part of the Fig. 5 and making possible the survival of brackish conditions similar to the present Baltic sorne Tethyan forms in the northem parts of the EES. Sea. These areas were also characterized by the During the Pliensbachian-Toarcian transition, however, widespread development of coal deposits aroillld the northward drift of warm water Tethyan ostracods the marginal areas (Ziegler et al. , 1994; Rees et al. , was drastically reduced, indicating an alteration of the 2000; Zeig1er et al. , 2003). Ostracod assemblages water temperature. An important water temperature from these regions are characterized by having a variation, in a marginal sea as the EES was, is generally very low diversity and by comprising mainly the result of a significant change in the ocean's smooth healdioids (Ogmoconcha, Ogmoconch­ circulation or the existence of sorne kind of physical ella and Pseudohealdia), smal! cytheroids (Eu­ or envrronmental barriers between the Tethys Ocean and cytherura, Cytheropteron, Gammacythere and the EES. Lophodentina) and bairdioids (Bairdia, Bairdia­ cypris and Sigilloidea). 5.2. Communication between the Tethys Ocean and the EES: the EES deep-water circulation and the formation The pattem of marine ostracod migration across the 01 block shales EES during the Pliensbachian outlines the distinct water masses. During the Pliensbachian, the migration pattem Understanding the ostracod migration between the is characterized by the continuous arrival of Tethyan EES and the Tethys Ocean requires a fmn comprehen­ ostracod failllas into the central and westem parts and sion of the EES deep-water circulation. The EES may the Boreal ostracod faunas into the northem parts of the be considered a marginal semi-enc1osed sea as a frrst EES. In the EES basins, these movements of ostracod approximation, separated from the open Tethys Ocean assemblages indicate a complicated temperature-salinity by shallow sills, semi-barriers or narrow passages relationship in the EES. The EES deep waters were a (Varos, 1977; Meister and Stampfly, 2000). The complex mixture of waters coming from different areas horizontal density differences between the two water in the ocean. For example, wann, salty water from the masses (the EES and the Tethys Ocean) at the level of Tethyan Ocean (tropical water masses) entered the EES the strait, and the inclination of the sea swface in the connecting area due to the water exchange, are the two (for example, the Briaconnais and Corso-Sardinia factors that determine the type of cornmunication blocks or the Balearic and pre-Betic platforms) might between the marginal and the open sea. have acted as a barrier separating the EES from the Using the temperature and salinity values proposed Tethys Ocean (Alméras and Elmi, 1987). Only a few in Early Jurassic climate models (Chandler et al. , 1992; areas may have temporally worked as entry passage­ Chandler, 1994; Arias and Whatley, 2004), the EES ways (for example, the Lower Austroalpine, South deep waters would comprise three deep-water masses Ca1careous Alps, the Ba1cony Mounts, the Burgundy (Fig. 6): (a) the EES SurJace water, an upper layer area or the Sub-Briaconnais area) along the southem formed in the northem par! of the EES, which would margin of the EES (Donnnergues and Meister, 1991; flow through the Strait driven by an excess of fresh Meister and Stampfly, 2000). The opening of these water input over loss by evaporation; (b) the EES passageways would be responsible for the north-south lntennediate water, which would be composed of one or direction of the boundary during Pliensbachian. maybe two water masses formed in the northem side of The passageways between the sea and the open ocean the basin; and (c) the Tethyan Deep water, characterized were ofien narrow and shallow and so were extremely by its constancy over the westem part of the EES basin, sensitive to rises and falls in eustatic sea level. Low high temperature and high salinity and that penetrated stands reduce open seaways and the spread of the deep into the EES. ostracod failllas; whereas, high sea levels facilitate failllal Two aspects must be firstly considered before exchange, leading to an increase in overall diversity and analyzing the commwlication between water bodies, extensive distribution of the ostracod assemblages. The the existence of sills or sorne sort ofbarrier and the sea­ Early Jurassic was a time of major sea-Ievel transgres­ level changes. The Early Jurassic was a period of sions and regressions (Fig. 7) on bothregional and global tectonic plate reorganization, with the break-up of the scales, with an overall trend towards lower sea-Ievel Pangaea, which involved rifting and the opening and from the Late Triassic (Hesse1bo et al. , 2004) to the c10sing of oceanic seaways, as well as changes in long­ Hettangian, across the Pliensbachian-Toarcian bOillld­ term eustatic sea-levels (Hallam, 1978; Liu et al. , 1998). ary and in the latest Toarcian (Hallam, 1984); and Physical barriers, such as shallow carbonate platforms towards high sea levels during the latest Hettangian, or emerged land belts along the southem par! ofthe EES latest Sinemurian, mid Pliensbachian, mid Toarcian and

o G Tethys The European Ocean Epicontinental Sea River

Fig. 6. Interchange ofwater masses between the EES and the Tethys Ocean. The water flowmg mto the EES carne out ofLaurasian rivers and had a low density. This relatively low salty water mixed with the seawater and flowed mto the Tethys Ocean. The deeper waters were saltier, denser and thus could not flow over the shallow sill mto the Tethys Ocean. Late Toarcian (Vail et al., 1977; Hallam, 1978, 1984; rivers during monsoonal surnmer conditions over the Haq et al., 1987, 1988; Cope, 1988; Hardenbo1 et al., northem margin of the sea is envisaged to increase the 1998; Gómez and Goy, 2002). stratification and reduced the density of the surface As it was discussed in the fonner section, the most layer. These circumstances would have prevented the important change in the ostracod migration pattem freshened water from reaching the deeper layers. The took place during the falciferum zone, Early Toarcian result would have been the establishment of a fresh (Fig. 3b, F), when the connection among British, upper layer that may have formed a cap, which could French and German basins was blocked, with only the have prevented oxygen transfer to the denser saline commwlication between French and Spanish basins water. Water below the halocline would have been remaining active. The existence of these physical renewed only very slowly through mixing across the barriers that ostracods cannot pass through may halocline and by the inflow of oceanic water through the explain the end of the commwlication between central connecting strail. Oxygen at depth could have been EES basins. consumed by similar process (such as the remineraliza­ Nevertheless, there are indications that the ending of tion of nutrients) to those described in the present Black the entronce of the Tethyan faunas and the disruption in Sea. the migration pattem of ostracod failllas in the central But there is another factor to consider: the depth of EES may be due less lo palaeogeographic factors than to the sill. If the sill was not lying low enough below the environmental factors (e.g. changes in the water temp­ level of the less dense top layer, the heavier water of erature or salinity values or a low oxygen contents. The the open ocean could not have flowed illlimpeded development of marine anoxia and the black shales are across the sill, thus the circulation renewal of the deep generally regarded as the proximate cause of the water could not have been initiated. The anoxia could Toarcian losses, with the release of gas hydrates further have taken place when the stroit between the EES and contributing to the high-stress environmental changes. the Tethys was very shallow as a consequence of However, two crucial aspects, the relative timing and the sorne kind of regressive trend or tectoruc reactivation origin of these phenomena, are not completely compre­ of major pre-existing structures related to the opening hended. The monsoonal activity may have been the of the Hispanic Corridor. driving factor for the formation of black shales Additional infonnation about palaeoceanographic deposition. The freshening of the surface waters conditions can be obtained from the isotopic record. resulting from excess rain and freshwater input from Lower Toarcian sequences of carbon-rich black shales +-T R-. myr LATE AALENIAN JURASSIC s: '., - 175 J : - 180 TOARClAN U f-'-- (; - R EAR LY '\ 185 f A PUENSBACHI AN r- . - 190 S ~ - 195 s JURASSIC SINEMU RI AN 1 \= / \'. 200 C ,; HETTANG IAN LONGT{~ - 205 \ SHORT TERM 210

T RIASSIC LATE T RI ASS IC RHAET IAN ¿= - 215 f'.'

Eg. 7. Sea-level cycles (adapted from Vail et al., 1977; Haq et al., 1987, 198&). along the EES show a Ül3C minimum in the uppennost EES. The occurrence ofwann marine ostracod faunas in tenuicostatum and the falciferum zones of the Toarcian the EES during the Pliensbachian could be used lo draw (Jenkyns and Clayton, 1986, 1997; Hesselbo et al., the spreading out of the Tethyan Deep Water. Hence, 2000; McArthur et al. , 2000; Jenkyns et al., 2002; changes in the properties of such deep waters across the Rosales et al. , 2004). Two explanations have been EES basin can be only the result of significant salinity of given for the cause of this negative ül3e excursion: temperature global changes, which could modify the Firstly, by the recycling of remineralized carbon from properties of the water masses. Observational and the deeper parts of an intennittently stratified water modelling evidences and the analysis ofthe sedimentary column up into the photic zone, where it would have and fossil records suggest that the poleward ocean heat been incorporated by photosynthetic phytoplankton transport haves varied as a response to palaeogeographic (Srelen et al. , 2000; Schouten et al. , 2000; R6hl et al., and palaeoceanographic changes (Chand1er et al., 1992; 2001; Schmid-R6hl et al. , 2002). Secondly, anumber of Arias and Whatley, 2004). authors have suggested that the negative ül3e isotope The migration ofwarm Tethyan ostracods inlo the EES exclusion resulted from the rapid release of carbon during the Pliensbachian-Toarcian Boundary (the PTB) is dioxide to the ocean-atmosphere system from dissocia­ indicator that water !empemtures were high enough lo tion of methane hydrate (Hesselbo et al., 2000; pennit their survival in the Boreal EES. The end of the Jenkyns, 2(03). Because biogenic methane is depleted northward migration of the Tethyan fuunas could be in úl3e relative to ú12C its subsequent oxidation to explained in par! by amajor sudden cold even!. Areduction carbon dioxide in the oceans and atmosphere leaves a in ocean heat transport lo the high-latitude EES, associated light carbon isotope signal on ocean dissolved with the reduction, shutdo\vn, or southward shift in the inorganic (DIC) carbon and, shifting their isolopic formation of the EES deep water may be hypothesized lo composition towards more negative values (Küspert, have caused or amplified the abrupt cooling even!. 1982; Jenkyns, 1988; R6hJ et al., 2001 ; Van de Early Jurassic cHmate sllnulations show a Pangaea Schootbrugge et al. , 2005). supercontinent characterized by the development of According lo the EES deep-water structure (Fig. 6), monsoonal crrculation, with low-pressure cells sited the development of tbis negative anomaly are related to over the eastem margin of the EES during the surnmer the fonnerly described palaeoceanographic conditions (Fig. 9A) and over the central par!s ofthe Gondwana in (Le. the combination of a restrictive crrculation together the winter (F ig. 9B). High pressure cells are situated with a significant density stratification in the EES) over the southem Gondwana (far away from the EES) during the tenuicostatum-falciferum transition, which during the surnmer (Fig. 9A), fonning a large belt may explain the low ostracod abillldance and diversity crossing the EES during the winter (Fig. 9B), with one described at the beginning of the falciferum zone high pressure cell over Siberia and another over the (F ig. 8). Nevertheless, the Toarcian Anoxic Event cannot northwestem Gondwana (parrish and Curtis, 1982; explain why the incursions of the Tethyan ostracod Parrish et al. , 1982; Kutzbach, 1985; Robertson and faunas inlo the westem par! of the EES ceased, or why Ogg, 1986; Scotese and Surnmerhayes, 1986; Crowley the drrection of the ostracod migration changed at the et al., 1987; Crowley, 1988; Kutzbach and Gallimore, beginning ofthe Toarcian (Fig. 3a). Another explanation 1989; Chandler et al. , 1992; Fawcett et al., 1994). The can be related to a rapid change of water temperature monsoonal character of the atmospheric crrculation over along the EES. the EES generated the development of weak westerlies over most of the northem par! of the EES and strang 5.3. Suiface oceanic general circulations in the EES: a easterlies over the central parts of the EES during the possible collapse of the deep-water circulation winter. But in the surnmer, westerly winds are replaced by weak easterlies in the northem par! ofthe EES. Bottom temperatures of deep ocean water masses do These winds create Ekman convergence in the mid not show the north-south latitudinal gradient because latitude EES, driving the subtropical gyre. The northem deep-water masses are fonned through density-driven side of the westerlies creates Ekman divergence at the processes (salinity and temperature). Subsequently, the higher latitudes of the EES, driving the subpolar deep-water temperatures reproduce those at the origin of circulation. Consequently, the flow of the trades and the water mass fonnation, with negligible mixing westerlies induces the fonnation of a c10ckwise occurring at water masses bOillldaries. Therefore, once circulation in the EES. The Fig. 10 shows a broad, fonned, deep-water masses hold characteristic tempera­ basin-wide, mid latitude gyre as we expect from ture and salinity signatures that could be traced in the Sverdrup's theory. In the west, a westem boundary s DK NEG NWG SWG Sz PB GB FB el z Sites

Fig. 8. Plot afthe ostracod species diversity in the Pliensbachian to Toarcian successions afthe European Epicontinental Sea. Sites: Z: Zambujal, Portugal; el: the Cordillera Ibérica, North East Spain; FB: the Fasmet Basin, South West Ireland; GB: Central and Southern England and Wales; PB: the Paris Basin, France; Sz: NE Swiss basins; SWG: Southwestern Gennan basins; N\VG: Northwestern Gennan basins; NEG: Northeastern Gennan basins; DK: Danish basins; and S: Swedish basins. Ammonite zones: U)jamesoni zone; (i) ibex zone; and (d) davoei zone afthe Early Pliensbachian; (m) margaritatus zone and (s) spinatum zone afthe Late Pliensbachian; and (t) tenuicostatum zone; (f)falcifernm zone; and (b) bifrons zone afthe Early Toarcian. current, a pre-Gulf Stream, completes the gyre. There­ water circulation. The shut-off of this pre-Gulf Stream fore, tbis subtropical gyre's westem bOillldary current is not only greatly reduced the transport of wann water in a pre-Gulf Stream system, which is off the east coast of the EES, producing a moderate cold northem hemi­ N orthem Pangaea, along the westem margin of the EES. sphere climate, but it may have reinforced the cold water Much of the heat-transported poleward by the oceans is transport from the Arctic Ocean lo the EES, leading to carried by these sorts of mid latitude westem boundary the beginning of a temperature decline episode. In currents in the Northem Hemisphere. As these currents surnmary, if the deep-ocean thennohaline crrculation separate from the coastal bOillldaries, extend eastward was shut off, the influx of wann, salty water from the inlo the EES interior, they flux sorne of their heat lo the Tethys would decrease and the water temperature in the atmosphere and store sorne of their heat in the EES might become cooler, making impossible the subtropical gyre. A weakening of this system would survival of wann ostracod failllas. cause a cooling episode in the EES. In the present paper it is proposed that the opening of 6. Conclusions the Hispanic Corridor could have disrupted the system. The opening of the Hispanic Corridor (a new route The analysis of the Pliensbachian-Toarcian ostra­ through the Caribbean region) by the Late Pliensbachian cod palaeobiogeography has allowed obtaining infor­ (Kocurek and Dott, 1983; Robertson and Ogg, 1986; mation about the surface and deep-water circulations Damborenea, 2000; Aberhan, 2001; Arias, 2006) may and has made it possible to propose a new palaeocea­ have modified this palaeoceanographic system by nographic reconstruction of the Early Jurassic European means ofthe development of a new westward equatorial Epicontinental Sea (EES). The additional infonna­ current from the Tethys to the Panthalassa Oceans, tion from other fossil groups also enhanced the recon­ which would have weakened this "pre-Gulf Stream" and struction of the evolution of the local crrculation in the consequently, would have affected the surface and deep- EES. The Pliensbachian was a period of maximum expansion for ostracod failllas throughout the EES, characterized by a southward migration of the failllas and by the development of an antic10ckwise migration inside the Central European basins. The Early Jurassic ostracod migration pattem is initially explained in the broader context ofthe EES deep oceanic crrculation and in its variations, as a consequence of the palaeogeo­ graphic and palaeoenvrronmental changes. Considering the ostracod migration pattem, the combined faunal and lithological data and the EES and Tethyan salinity and temperature distribution derived from c1imate models, an integrated three-fold classifica-

Eg. 10. Surface currents patterns in the European Epicontinental Sea. The prevailing easterlies in conjilllction with the westerlies create surface current flow within the subtropical gyre. Narrow black arrows show the direction of surface currents.

tion ofwater masses (tropical, subtropical and temperate) has been proposed. In this model the deep-water crrculation would be of an estuarine type, with wann water from the Tethys flowing northwards at depth lo the EES and superficial cold and freshening water flowing out from the EES to the Tethys Ocean. Deep waters are presumed lo have been formed in the smal! basins along the southem margin of the EES due to the high evaporation expected at this latitude (indicated by the presence ofevaporite deposits). As aresult, the deep water filling the ESS was wanner. This assumption is consistent with estimated palaeotemperatures and with the low ternperature gradients between the northem latitudes and the tmpics during the Pliensbachian. This migration pattem changed, however, by the latest tenuicostatum-early serpentinus transition, when the interruption ofthe entmnce ofTethyan fonns into the central EES and the end of the commilllication between the westem and central basins of the EES took place. A possible change ofthe former described EES deep-water circulation is considered for explaining this change. The alteration of the EES deep-water structure due to palaeogeographic (the opening of the Hispanic Corri­ dor) and sea-Ievel variations modified the inflow of Tethyan waters and therefore the stratification of water masses in the EES. This modification of the density Fig. 9. (A) Abnospheric and oceanic cITculation (pliensbachian­ Toarcian) during smnmer in the Northern Hemisphere; (B) during stratification and the resultant anoxia has been sug­ winter in the Northern Hemisphere (adapted from Parrish et al., 1982; gested as a possible mechanism of black shales Scotese and Summerhayes, 1986; Chandler et al., 1992). fonnation and Toarcian ostracod extinction. However, the main disruption of the EES deep-water Ager, DV, 1960. Brachi0IX'd distributions in the European Mesozoic. circulation, in the present paper, is linked to the change Report International Geological Congress, XXI Session, Norgen 22,20-25. of the surface ocean crrculation resulting from the Ager, DV, 1971. Space and time in bracmopod history. In: Middlemiss, opening ofthe Hispanic Corridor. The c10ckwise surface F.A., Rawson, F.A., Newal, G. (Eds.), FalUlal provinces in space and circulation derived from Early Jurassic atmospheric time. Geological Journa~ vol. 4, pp. 95-110. circulation models across the EES changed as a Ager, DV, 1973. Mesozoic Brachiopods. In: Hallam, A. (Ed.), Atlas consequence of the opening of the Hispanic Corridor ofPalaeobiogeography. EIsevier, Amsterdam, pp. 431-436. Ager, D.V., Walley, C.D., 1977. Mesozoic Brachiopod Migrations and during the Toarcian because this event altered the gyral the Opening of the North Atlantic. Palaeogeography, Palaeocli­ circulation with the development of a new westward matology, Palaeoecology, Amsterdam 21 (2),85-99. equatorial surface current from the Tethys to the Ainsworth, N.R., 1986a. Rhaetian, Hettangian and Sinemurian Panthalassa Oceans and the weakened of the pre-Gulf Ostracoda from the Fasmet Basin, offshore Southwest Ireland. Stream cunent along the EES. k; a resul!, the deep­ Bulletin ofthe Geological Survey ofIreland 4,107-150. Ainsworth, N.R, 1986b. Toarcian and Aalenian Ostracoda from the water circulation was disrupted, modifying the inflow of Fasmet Basin, offshore Southwest !reland. Bulletin of the wann Tethyan waters into the EES and lending lo a Geological Survey ofIreland 3, 277-336. radical c1imate change in many areas around the EES. Ainsworth, N.R, 1987. Pliensbachian Ostracoda from the Fasmet The low tempemtures resulting from this event impeded Basin, offshore Southwest Ireland. Bulletin of the Geological the entrance of wann ostracod species from the Tethys Survey of Ireland 4 (1), 41-62. Ainsworth, N.R., 1989. Remarks on the nomenclature oftwo ostracod into the EES at the beginning of Toarcian. species from the Pliensbachian and upper Toarcian-Aalenian of the Fasmet Basin, offshore Southwest Ireland. Bulletin of the Acknowledgments Geological Survey ofIreland 4 (2),165-166. Ainsworth, N.R., Horton, N.F., 1987. Mesozoic micropaleontology of An early version of the manuscript benefited from exploration well Elf 55/30-1 from the Fasmet Basin, offshore southwest Ireland. Journal ofMicropaleontology 5 (1), 19-29. the careful reviews of Professor Robin Whatley (Uni­ Ainsworth, N.R, 'Neill, O., Rutherford, M., Clayton, M.M., Horton, versity ofWales, Aberystwyth) and Professor Pat Wilde G., Pelllley, N.F., 1987. Biostratigraphy ofthe Lower Cretaceous, (University of California, Berkeley), who are both Jurassic and uppennost Triassic of the North Celtic Sea and Fasmet wannly thanked. We thank Dr. Bas van de Schootbrugge Basin. In: Brooks, J., Glellllie, K., Trobnan, M. (Eds.), PetrolelUll (Universitiit Frankfurt am Main) and Professor Antonio Geology of North West Europe. Graham and Trobnan Ltd, London,pp.611-622. Goy (UCM, Mathid) for their stimulating discussions Alméras, Y, Elmi, S., 1987. Evolution des peuplements des and enthusiastic support. 1 would like lo thank Dr. Ian brachiopodes en fonction de l' envITollllent dans le Lias ardéchois. Boomer ofthe University ofNewcastle for their helpful Cahiers Scientifiques de l'Université Catholique de Lyon, Science, discussions and constructive comments and for taking Lyon 1, 21-56. the time to cornment on the several chapters, which Alméras, Y, Elmi, S., 1993. Palaeogeography, physiography, paleoenvITonments and brachiopod commlUlities. Examples of improved the manuscript, and also Dr. Nigel Ainsworth the Liassic brachiopods in the Western Tethys. Palaeogeography, who kindly suggested improvements that did much to Palaeoclimatology, Palaeoecology, Amsterdam 100 (1-2), augment the readability ofthe fmal manuscript. My debt 95-108. to Dm. Alicia Moguilevsky and the Translations Group's Alméras, Y, Faure, P., 2000. Les brachiopodes liassiques del translators and Ernma Benzal for their helpful improve­ pyrénéens, paleontologie, biostratigraphie, paleobiogeographie et paleoenvITollllants. Strata 2 (36), 1-395. ments and English corrections. 1 would like to express Alméras, Y, Moulan, G., 1982. Les Terebratulides liasiques de my thanks to the long-term fmancial support from the Provence. Paleontologie, biostratigraphie, paléoécologie, phylo­ support by the Projects CGL 2005-01765/ BTE and génie. Documents des LaboratoITes de Géologie de la Faculté des CGL-2005-04574/BTE from the Ministerio de Educa­ Sciences de Lyon 86,1-365. ción y Ciencia (Spain) and the Project UCM-CAM-No. Andreu, B., Bodergat, A.M., Bnmel, F., Colin, J.P., Cubaynes, R., 1998. Ostracodes du Carixien supérieur-Domérien (Jurassique 910431 from the Comunidad de Mathid (Spain). inférieur) du Querey, Bassin d' Aquitaine, France. Palaeontogra­ phica A 250,68-122. References Apostolescu, v., 1959. Ostracodes du Lias du bassin de Paris. 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