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Biostratigraphy and paleoecology of Sarmatian sediments based on siliceous microfossils on the S and SW part of Mt Medvednica (SW Paratethys, Croatia)

Almost 14 Ma ago, global cooling and change in the ocean circulation came as a result of the ice formation in the eastern Antarctic. That, in turn, was caused by decreasing amount of CO2 in the atmosphere, due to its trapping in CaC03 and its precipitation in the oceans (Flower & Kennett, 1993). At the end of the Badenian (Badenian/Sarmatian), tectonical activity in the Parathethys became more intensive, producing different development of the Sarmatian deposits in Central and Eastern Paratethys, respectively. In the Sarmatian, laminated sediments (diatomite, tripolite, varves) have been laid down on the SW part of Mt Medvednica with siliceous microplankton (diatoms, silicoflagellates, radiolarians, ebriids, archaeomonas, sponge spicules, and dinoflagellate endoskeleton ; Jurilj, 1957 ; Jerković, 1969 Bajraktarević, 1983 ; Galović, 2001, corresponding to the silicoflagellate zone Dictyocha soljanii and NN7 calcareous nannoplankton zone of the Earla Sarmatian-Volhynian (Bajraktarević, 1984). This research established the existence of the Distephanus slavnicii silicoflagellate zone, Anaulus simplex - Coscinodiscus doljensis diatom zone, and the NN 8 zone at various localities (Markuševec, Podsusedsko Dolje, Kostanjek). This is proved by the following Early Sarmatian assemblage (black - not yet mentioned and red - found for the first time in the investigated area or in some part of Paratethys). The most frequent siliceous microfossils are diatoms: Achnanthes exiqua, A. fimbriata, A. hauckiana var. eliptica, A. lanceolata var. elliptica, A. lanceolata var. lanceolatoides, Actinocyclus karstenii, Amphora binodis var. bigibba, Am. costata, Am. hidasensis, Am. holsatica, Am. laevis var. A., Anaulus simplex, An. minutus, Bacteriastrum furcatum, Chaetoceros cinctus, Ch. compressus, Ch. wighamii, Cocconeis boryana, C. pinnata, C. placentula var. euglypta, C. pseudofluminensis, C. sarmatica, C. scutellum var. inequlepunctata, C. scutellum var. parva, C. scutellum var. pulchra, C. scutellum var. raena, Coscinodiscus plicatus, C. rothii, C. rugulosus, C. sarmaticus, C. stellaris, Cussia aff. paleacea, Cymatosira miocaenica, Cymbella cf. sinuata, Cy. cf. ventricosa, Delphineis surirella, Denticulopsis hustedtii, Dimerogramma boryanum, Dimidiata saccula, Diploneis coffaeiformis, Dip. ovalis, Dip. sejuncta, Dip. sejuncta fo. constricta, Eunotia cf. tenella, Fogedia finmarchica, Fragilaria brevistriata var. fossilis, F. leptostauron, F. martyi, Glyphodesmis driveri, Goniothecium odontella, Grammatophora oceanica var. macilenta, G. oceanica var. oceanica, G. stricta var. biharensis, Gyrosigma distortum var. parkery, Hantzschia cf. virgata, Lyrella praetexta, Mastogloia binotata, M. lacustris, M. pethöi, M. quinquecostata, M. szontaghii, Melosira bituminosa, Navicula arenaria, N. cf. digitoradiata, N. cryptocephala var. intermedia, N. forcipata, N. hennedyi var. furcata, N. lyroides, Neidium cf. iridis var. A, Nitzschia cf. didyma var. A, Ni. frustulum var. obtusa, Ni. frustulum var. subsalina, Ni. sinuata var. tabellaria, Ni. imperforata, Opephora marina, Perissonoë cruciata, Pinnularia viridis, Podosira robusta, Rhaphoneis diamantella, Rh. gratiosa, Rh. intermedia, Rh. ischaboensis, Rh. mertzi, Rh. rhombica, Rhizosolenia oligocaenica, Rhopalodia gibba, Rho. giberula, Staurosira construens, St. construens var. venter, Stictodiscus hungaricus, Surirella biharensis, Thalassionema nitzschioides, Thalassiosira nordenskioldi, Trahineis aspera var. intermedia and Xantiopyxis ovalis.. The silicoflagellate are represented by Dictyocha fibula ausonia, D. brevispina brevispina, Dictyocha pentagona, D. rhombica, D. subclinata, Distephanopsis crux, Dss. crux parvus, Dss. hannai Dss. longispinus, Dss. schauinslandii, Dss. slavnićii, Dss. staurodon, Dss. stradneri, Dss. šoljani, Distephanus crux lockerii, Ds. quinquengellus, Ds. speculum, Ds. speculum elongatus, Mesocena elliptica elliptica, Paramesocena apiculata and P. circulus). The Early Sarmatian assemblage of diatoms contains the following species: Coscinodiscus doljensis, Odontella aurita, Achnanthes baldjikii, Caloneis liber var. zagrebiensis, Cocconeis distans, C. scutellum var. inequlepunctata, C. scutellum var. pulchra, Cymatosira biharensis, Dimidiata saccula, Grammatophora insignis, Mastogloia pethoei, M. szontaghii, Rhopalodia giberula i Thalassionema nitzschioides. The boundary between the lower (Volhynian) and upper (Early Bessarabian) part of the NN8 zone has been proposed on the bases of the last occurrence of silicoflagellates, different assemblage of diatoms (Achnanthes baldjikii, Cocconeis distans, Cocconeis pediculus, Cymatopleura solea, Cymbella cf. ventricosa, Cy. cf. sinuata, Nitzschia frustulum var. obtusa), and the presence of tuff in the sediments. The Sarmatian sediments disconformably overlie the Badenian deposits (Markuševec, Podsusedsko Dolje) due to a regional transgression, cca 12, 5 Ma ago (Kovac et al., 2001). The maximum transgression, based on diatoms, in neritic development, occurs in the Volhynian. The near shore areas of the Sarmatian sea, i.e. the marginal marine zones, where the upwelling occurs, are characterized by the maximum concentration of biogeneous silica (silicoflagellates and diatoms: Chaetoceros, Coscinodiscus curvatulus, Thalassionema nitzschoides...). The upwelling of the cold bottom water enriched the surface layer with the nutrients, witch were used for the assimilation processes by the phytoplankton blooms. In the areas of continental upwelling, where the high biogenic production in the surface water occurs, the most common type of sedimentation is by the "marine snow" (Brown et al, 1989). The above mentioned conditions developed during the middle part of the Sarmatian, when the climate became more temperate (Distephanopsis crux, Dss. longispinus, Dss. schauinslandii, Dss. staurodon, Ds. speculum, Ds. speculum elongatus, Mesocena elliptica elliptica, Paramesocena apiculata and P. circulus) and the marly laminated sediments of the varve type are deposited. The temperature ranges, inferred from diatom's assemblages, vary from 4 - 16°C in colder periods with polihaline (20 - 40‰ ) association and from 15 - 27°C in warmer periods when the thermo- and halocline have been formed in the surface layers and resulted in meso- to-oligohaline conditions , but in amounts lesser than 5% (Aulacoseira islandica, Cocconeis placentula var. euglypta, Diploneis ovalis, Gyrosigma distortum var. parkeri). Silica plays more important role during the warmer periods in the stratified system around the thermohaline levels. Thus it enhances the biochemical cycles, accompanied by the phytoplankton blooms, (Viličić et al., 1996/97 ; Lončar, 1992), which leads to increased eutrophication (Puškarić, 1988). The ebriid species Hermesinum adriaticum indicates stratified systems in the halocline zone (brackish/marine), and defines oxsic/anoxic conditions with developed chemocline (Viličić et al., 1996/97) in the SW part of Mt. Medvednica (Markuševec, Dolje). The species is common in anaerobic/sulfur enriched deeper layer of the Black Sea (Bodeanu, 1969). The input of allochtonous organic matter results in anoxic conditions of the bottom layer, where the temperature is much lower than at the surface. The accumulation and preservation of siliceous phytoplankton are increased in this environment. The diatom (plankton/benthos) ratio indicates oscilations of sea level, with generally regressive trend during the Early Bessarabian. At the end of the Sarmatian the connections with the oceans become weaker, salinity decreased, and the more near shore development become predominant (Kostanjek). A drastic decrease of both individuals and species diversity at the end of the Sarmatian is probably the result of a lower salinity, more near shore influence, and, maybe, a change in the water chemistry that occurs in the marginal seas or in locally enclosed bays.

silicoflagellates; diatoms; paleoecology; Sarmatian Zona

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