Early Miocene repetitive vegetation and climatic changes in the lacustrine deposits of the Rubielos de Mora Basin (Teruel, NE Spain

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Early Miocene repetitive vegetation and climatic changes in the lacustrine deposits of the Rubielos de Mora Basin (Teruel, NE Spain
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  Early Miocene repetitive vegetation and climatic changes in thelacustrine deposits of the Rubielos de Mora Basin (Teruel, NE Spain) Gonzalo Jiménez-Moreno a,b,c, ⁎ , Séverine Fauquette d ,Jean-Pierre Suc a  , Hayfaa Abdul Aziz e a   Laboratoire PaléoEnvironnements et PaléobioSphère (UMR CNRS 5125), Université Claude Bernard- Lyon 1, Bâtiment Géode, 27-43 Boulevard du 11 Novembre, 69622 Villeurbanne Cedex, France  b  Departamento de Estratigrafía y Paleontología, Universidad de Granada, Avda. Fuente Nueva S/N, 18002 Granada, Spain c  Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, NM 87131, USA d  Institut des Sciences de l'Evolution de Montpellier, Equipe Paléoenvironnements (UMR CNRS 5554),Université Montpellier 2 (Case Courrier 061), Place Eugéne Bataillon, 34095 Montpellier Cedex 05, France e  Paleomagnetic Laboratory  “   Fort Hoofddijk  ”  , Faculty of Earth Sciences,Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands Received 10 October 2006; received in revised form 6 March 2007; accepted 7 March 2007 Abstract Pollen analysis of a Lower Miocene succession from the Rubielos de Mora Basin (NE Spain) has been carried out with the aimof reconstructing the flora, vegetation and climatic changes. Previous paleobotanical studies on these sedimentary rocks andadjacent areas interpreted very diverse climates for the Early Miocene: from humid temperate to dry subtropical. In this study, arich thermophilous pollen spectrum and a diverse subarid flora including  Nitraria , Caesalpiniaceae,  Ephedra  and  Acacia ,indicative of a dry subtropical climate are identified. On the other hand, mesothermic taxa with high water requirements are alsoabundant. Therefore, the pollen assemblages evidence the juxtaposition of very contrasted environments: the presence of subdesertic taxa, typical of plants growing in the lowlands and conditioned by a long warm, dry season, together with others withvery high water requirements, needing constant water. This can be explained by the presence of the Rubielos de Mora Lake providing local conditions for developing riparian forests. The vegetation was clearly controlled by the water availability under asubtropical and dry-seasonal climate. Pollen changes along the succession, which coincide with sedimentological changes, arerelated to climatic variations. Alternation in pollen taxa (thermophilous – dry vs. mesothermic – riparian) reflects the influence of thecyclicity of temperature and precipitation on the lake level and vegetation.© 2007 Elsevier B.V. All rights reserved.  Keywords:  Palynology; Plants; Paleoclimate; Early Miocene; Southwestern Europe 1. Introduction The lacustrine sediments in the Rubielos de MoraBasin show a distinct oxic – anoxic cyclicity together with finer scale cyclicity within the laminated anoxicsediments (rhythmites) (Anadón et al., 1988a). They bear a very well-preserved and rich fossil fauna, which Palaeogeography, Palaeoclimatology, Palaeoecology 250 (2007) 101 – 113www.elsevier.com/locate/palaeo ⁎  Corresponding author. Present address: Center for EnvironmentalSciences & Education, Box 5694, Northern Arizona University,Flagstaff,AZ86011,USA.Tel.:+15052808764;fax:+15052778843.  E-mail addresses:  Gonzalo.Jimenez-Moreno@NAU.EDU,gonzaloj@ugr.es (G. Jiménez-Moreno).0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2007.03.004  include rodents, amphibians, crustaceans, insects andarachnida that have been extensively studied (Crusa-font-Pairó et al., 1966; de Bruijn and Moltzer, 1974;Martínez-Delclòs et al., 1991; Montoya et al., 1996;Montoya, 2002; Peñalver et al., 2002; Peñalver andMartínez-Delclòs, 2003). The laminated lacustrinemudstones also consist of abundant vegetal remains.Preliminary studies of the micro- and macrofloras have been carried out in several outcrops throughout the basin(Fernández Marrón and Alvarez Ramis, 1988; AlvarezRamis and Fernández Marrón, 1994; Barrón andSansisteban, 1999; Roiron et al., 1999; Rubio et al.,2003). They show vegetation dominated by mesother-mic and riparian plants, thus they interpret a humidtemperate climate during the Early Miocene for NESpain. This interpretation conflicts with other studies onmicrofloras and macrofloras from adjacent areas whereseveral very warm and subdesertic taxa of the familyCaesalpiniaceae (  Banksia ,  Caesalpinia ,  Cassia ), Mimo-saceae (  Acacia ,  Mimosa ) and Proteaceae ( Grevillea ,  Protea ) have been identified (Bessedik, 1985; Sanz deSiria Catalán, 1993). Therefore, the Early Miocenevegetation and climate of northeastern Spain still remainunclear and need to be clarified.A 386.5 m long core (Rubielos de Mora-1) wasdrilled in the western part of the basin, which allowedfor high-resolution sampling for pollen analysis of thecomplete sedimentary sequence. Botanical taxonomyand quantitative analysis of the pollen data have beenused here to reconstruct the flora, vegetation, andclimate in NE Spain during the Early Miocene. Thisstudy will also determine whether the cyclicity previ-ously described in sedimentological studies is mainlycaused by climatic changes. 2. Rubielos de Mora Basin The Rubielos de Mora Basin is located in the easternIberian Chain, in the southernmost part of the linkingzone between this chain and the Catalan Coastal Ranges(Fig. 1). The Mesozoic cover of this area was affected by Neogene normal faults (Fig. 1). The approximately10 km long and 3 km wide basin has an ENE – WSWorientation and displays structural asymmetry (Fig. 1),which is the result of enhanced sudsidence in thesouthern part of the margin. The subsidence led to thedevelopment of a relatively deep lacustrine systemwhere anoxic bottom water conditions prevailed, whichfavored the preservation of fossils and organic-richsediments (Anadón et al., 1989). The presence of twomammal sites in the upper part of the sedimentarysequence permitted the dating of the basin-fill as EarlyMiocene (Ramblian – Aragonian boundary) (de Bruijnand Moltzer, 1974; Crusafont-Pairó et al., 1966;Montoya et al., 1996). One of the fossil sites, themammal locality of Rubielos de Mora-2 of localRamblianage, islocatedclose tothestudieddrilling site.The basin-fill comprises an 800 m thick sequence of fluvial and lacustrine sediments (Fig. 1) (Anadón et al., 1988b). Three stratigraphic units have been distin-guished and described by Anadón et al. (1988a,b, 1989):Lowerunit.Itismadeupofa300mthicksequenceof sandstones interbedded with mudstones and some Fig. 1. Location of the Rubielosde Mora Basin and main structuralunits in NE Spain (left). Tripartite lithostratigraphic subdivisionof the Rubielos deMora Basin fill into lower, middle and upper units (modified from Anadón et al., 1988b). The approximate stratigraphic position of the two mammallocalities within the upper unit is indicated (bones).102  G. Jiménez-Moreno et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 250 (2007) 101  –  113  conglomeratesofreddish – yellowishcolor.Theyhavea fluvial srcin and discordantly overlay Cretaceoussediments ( “ Weald ”  and  “ Utrillas ”  facies).Middleunit.Itcomprisesa100mthickunitofmainlylacustrine sediments. Lignites are abundant in theeastern part of the basin, while fluviatile sedimentsdominate in the western part. This unit represents anearlystageinthedevelopmentofthelakeinthebasin.Upper unit. This unit is up to 400 m thick and mainlyconsists of open lacustrine sediments. In the western part,lacustrinefaciesarecharacterizedbyoxic – anoxiccyclical sequences of massive mudstones (oxic facies)anddarklaminated mudstones (anoxic facies) (Fig.2).Eachcycleisabout0.6 – 9mthickandistheproductof the alternating periods between anoxic and oxic lake bottom conditions (Anadón et al., 1988a) (Fig. 2). The laminated (anoxic) facies, comprising sandymudstones, mudstones with bioclastic lamination, oil-shales, rhythmites and marls, were deposited during highlake level stages,whenthe lake was permanentlystratified(meromicticlake)andthelakebottomwasanoxic(Fig.2).At that time lake sediments were produced at a very slowrate and contained a high abundance of organic matter.Changes in the lake level, mainly produced by cyclicalwater influxes, would produce the generation of rhyth-mites, which are characterized by the alternation of  precipitated calcareous (mainly calcite and aragonite) andterrigenous(marls)laminae(Anadónetal.,1988a)(Fig.2). Subsequent drop in the water level and oxycline weremost likely due to an increase in evaporation related to awarming of the climate. This period is marked by green-grey massive claystones (marls) rich in carbonateswhich are mainly low in magnesium calcite anddolomite. Evaporative concentration was probablylinked to the increase in the evaporation whichgenerated alkaline conditions in the lake waters(Anadón et al., 1988a). Bioturbation by roots is commonwithin these marls. Lack of evaporitic saline or desiccation features imply permanent lake conditions.A significant water influx started a new depositionalcycle in which the lake-water volume was increased andtraction currents produced a new detrital/terrigenousfacies (Fig. 2). During this lacustrine transgression, thewater in the lake was again stratified and resultant laminated facies developed, etc.The mega-sequential sedimentological organizationin this basin, characterized by the superposition of thethree units, indicates the steps in the development of alake. The deepening trend was mainly influenced bysynsedimentary tectonics (Anadón et al., 1989). Anadón et al. (1988a) interpreted the two observed cycles withinthe upper unit as a product of climatic changes, forced by precession (oxic – anoxic cycles) and climate season-ality (rhythmites), independent of tectonics. 3. Materials and methods 3.1. Borehole Rubielos de Mora-1 The 386.5 m long core was drilled close to the villageof Rubielos de Mora (40° 10 ′ 60N; 0° 39 ′ 0W) (Fig. 1).The three main lithological units, as described byAnadón et al. (1988a), were drilled (see description Fig. 2. Main sedimentological features and interpretation of an ideal cycle between anoxic and oxic bottom conditions in the western part of theRubielos de Mora Basin (modified from Anadón et al., 1988a).103 G. Jiménez-Moreno et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 250 (2007) 101  –  113  above and Fig. 3). The borehole is located in the western part of the asymmetric basin where the upper unit ischaracterized by an abundance of laminated to massivelacustrine mudstone cycles (Fig. 3). 3.2. Pollen sampling and processing  The palynomorph-rich samples (80 in total) of thisstudy come principally from the lacustrine facies of theupper unit (Fig. 3). Conglomerates, sandstones or reddish claystones from the lower and middle unitswere not sampled because palynomorphs do not usually preserve under oxidizing conditions.In the chemical treatment, only 15 – 20 g of sediment were used. The samples were processed with cold HCl(35%) and HF (70%), removing carbonates andsilicates. Separation of the palynomophs from theremaining residue was carried out using ZnCl 2  (densi-ty=2). Sieving was done using a 10  μ m nylon sieve.The pollen residue, together with glycerin, was preparedon slides. A transmitted light microscope, using ×250and ×1000 (oil immersion) magnifications, was used for identification and counting of palynomorphs. Eventhough palynomorphs are very abundant in the sedi-ments, spores have not been considered due to their poor  presence.The identification of the pollen grains was done bycomparing the fossils with their present-day relativesusing the pollen identification key of  Faegri and Iversen(1989), the photograph collection stored in the laboratoryof Lyon, several pollen atlases (China, Taiwan, Africa, North America, Mediterranean Region, etc.) and thePhotopal website (http://medias.obs-mip.fr/photopal). Aminimum of 150 terrestrial pollen grains,  Pinus  andindeterminable Pinaceae excluded, were counted in eachanalyzed sample (Cour, 1974).A detailed pollen diagram (Fig. 4) and two standardsynthetic diagrams (Suc, 1984) with, and without   Pinus  and indeterminable Pinaceae, have been plotted(Fig. 5). In these latter diagrams, taxa have been grou- ped into 9 different groups based on ecological criteria( Nix, 1982; Appendix A) in order to clearly visualizethe composition of the past vegetation. Complete pollendata will be available on the web in the  “ CenozoicPollen and Climatic values ”  database (CPC) (http://cpc.mediasfrance.org). 3.3. Temperature reconstruction Temperatures were reconstructed from pollen datausing the  “ Climatic Amplitude Method ”  (Fauquetteet al., 1998a,b). This method has been applied to many Fig. 3. Geological log of the studied part of the Rubielos de Mora-1core showing the three main depositional units from Anadón et al.(1988b). Positions of the studied samples are indicated by crosses.104  G. Jiménez-Moreno et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 250 (2007) 101  –  113  Fig. 4. Detailed pollen diagram, without   Pinus  and indeterminable Pinaceae, of the studied part of the core Rubielos de Mora-1. Pollen zones A, B and C are shown. Black dots indicate percentageslower than 1%. 1   0   5    G . J   i   m é   n ez -M or  e n o e t   a l    . /   P  a l    a e o  g e o  gr  a  p h   y ,P  a l    a e o c l    i   m a t   o l    o  g  y ,P  a l    a e o e c o l    o  g  y2  5  0   (  2  0  0  7   )  1  0 1  – 1 1  3 
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