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• W2334437269 abstract "High and low resolution analyses on Calcareous nannoflora were performed in Tortonian-Messinian sediments of the ODP Site 654 immediately prior to the Salinity crisis. The main changes in the assemblage were compared with variations in the association of keel Globorotaliids and calibrated with the magnetostratigraphic interpretations proposed for this interval. Around 6.8 Ma an increase in Geminilithella rotula over G. jafarii was observed immediately prior to the reduction of sinistral keel Globorotaliids (group 1 of Globorotalia menardii). The FOD of Amaurolithus primus coincide with a coiling change from sinistral to dextral in the unkeeled Globorotalia, around 6.5 Ma, whereas the FOD of Amaurolithus delicatus, approximately coinciding with the Tortonian/Messinian boundary, is synchronous with the entry of conic planoconvex Globorotalia (group of G. miotumida/G. conoidea) into the Mediterranean. The estimated age for this event is 6.43 Ma, which slightly predates the FOD of Reticulofenestra rotaria. The variations in the total abundance of nannoliths in the sediments enabled us to recognize 5 intervals. Nannoliths in interval A were probably affected by taphonomic processes in a shallow water environment. The low abundance, local absence of nannoflora in interval B, may be due to the action of bottom currents winnowing fines. Towards the top of this interval the effect of currents decrease, but the assemblages are different from that of the adjacent Atlantic probably due to a restriction in the communication with the open ocean. This may be the reason for the later occurrence of Eu-discoaster quinqueramus and Eu-discoaster berggrenii in the Mediterranean. Interval C assemblages are of Oceanic affinity showing an asterolith association similar to that of the adjacent Atlantic. On the other hand, strong fluctuations have been observed in interval D, which may be related to changes in salinity, or water stratification, after a probable reduction in the exchange of Atlantic/Mediterranean waters prior to the Salinity crisis. The progressive increase of Dictyococcites antarcticus along the hole, was related to the decreasing temperatures in surface waters from the late Tortonian to the early Messinian. INTRODUCTION The latest Miocene, which culminated with the Messinian Salinity crisis, has been the object of numerous stratigraphic (Cita 1982; Cita and Mackenzie 1986, Ryan 1973, Cita and Ryan 1973, Van Couvering et al., Hsii et al. 1973, 1977, 1978, etc.), biostratigraphic (Cita 1979, Rio et al. 1976), isotopic (Loutit and Keigwin 1982), or magnetostratigraphic (Langereis et al. 1984, Channell et al. 1990) studies. Likewise, the problem has been analysed from the paleoceanographic point of view, considering in particular the Atlantic-Mediterranean connection prior to the beginning of the crisis. Besides the above mentioned authors, Feinberg and Lorenz 1970, Benson 1976, Berggren and Haq 1976, Bossio et al. 1976, Sierro 1984, Flores 1985, Hodell et al. (1989), Muller and Hsii 1987, Benson et al. (1991), among others have also worked on this subject. Some of the writers of this report have also studied sediments of the Upper Tortonian-Messinian interval in the Guadalquivir basin (Spain) and DSDP Sites of the Adjacent Atlantic (Flores and Sierro 1987, 1989, Sierro and Flores 1989, Sierro et al., in press.) ODP Hole 654A (Leg 107) was drilled in the western Sardinia margin (50o34.76'/10o41.80') at 2217m depth (text-figure 1). Sediments from Upper Tortonian to Pleistocene were recovered in a continuous section, including deposits related to the Salinity crisis. The calcareous nannoflora have been studied by Muller (1990), who established a biostratigraphic scheme. The planktonic foraminifera have been studied in a previous report by Glacon et al. (1990). The purposes of this study are: 1) the definition of the principal biostratigraphic events of hole 654 A, including the calcareous nannoplankton and planktonic foraminifera analysis and its calibration with the geochronologic scale of Berggren et al. (1985), using the paleomagnetic data of Channell et al. (1990) for this Site; 2) comparison with conventional biostratigraphic scales and with the results of the Guadalquivir basin and NE Atlantic; 3) the paleoecologic characteristics of the region (surface water masses); and 4) the paleoceanographic interpretation related to the Atlantic/Mediterranean communication during the interval immediately before the Sali ity crisis. The cores of ODP Site 654 show a sequence similar to that recorded in the Spanish and Moroccan sections (Civis et al. 1988, Flores and Sierro 1989, Benson et al. 1991). The lithostratigraphic study was carried o t by Borsetti et al. (1990) who, from bottom to top, recognized the following units (text-figure 2): Unit 6, consisting of a conglomerate layered with red beds; overlain by Unit 5, bioturbated glauconitic sands (interpreted as deposited in a shallow water environment); passing to Unit 4, a nannofossil ooze in core section 45R-5 (a change takes place from a shallow water to a deep water environment); overlain by Unit 3, dolomitic dark shales, very poor in calcareous microfossils and nannofossils; continuing with Unit 2, interbedded gypsum and carbonate layers. Only Units 5 and 4 have been studied in this work. Sedimentological characteristics to the interpretation of the micropaleontological data include: 1), the high rate of sedimentation during deposition of the upper part of Unit 4, related to productivity; 2), the low nannofossil concentration at the base of the micropaleontology, vol. 38, no. 3, pp. 279-288, text-figures 1-6, 1992 279 This content downloaded from 207.46.13.193 on Thu, 08 Sep 2016 06:02:17 UTC All use subject to http://about.jstor.org/terms J. A. Flores, E J. Sierro and G. Glacon: Calcareous plankton analysis in the pre-evaporitic sediments of the ODP Site 654" @default.
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• W2334437269 title "Calcareous Plankton Analysis in the Pre-Evaporitic Sediments of the ODP Site 654 (Tyrrhenian Sea, Western Mediterranean)" @default.
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