Insight into calcareous nannofossil palaeoecology at the beginning of the Messinian Salinity Crisis in the classical section of Alba (NW Italy)

In the last decade a renewed interest on the onset and developement of the Messinian Salinity Crisis (MSC) lead to a general revision of several classic sections in the mediterranean area (see review in Rouchy & Caruso, 2006). One of the classical land based Messinian section, the composite Alba section of Sturani (1973), has been recently revisited. New stratigraphical, paleontological, sedimentological, geochemical and petrographical data were collected in the Tortonian-Messinian sediments cropping out along the Tanaro river, near Verduno, in order to reexamine the onset and development of the MSC in this critical area of the Tertiary Piedmont Basin (BTP). This work only deals with calcareous nannofossils (CN) data, while integrated biostratigraphical, palaeoecological, sedimentological and petrographical analyses are in progress. The sampled section consists of five main lithologic units: 1) hemipelagic laminated marly clay up to 37 m thick, 2) three primary selenitic gypsum beds intercalated with thinnely laminated marly clays (14 m), 3) finely laminated fine to coarse grained primary “balatino” gypsum (7 m), 4) thinnely bedded marly clays intercalated with coarse grained diagenetic gypsum beds (34m), 5) marly clays of the Lago-Mare facies, with roots traces, 6) marly clays of Pliocene age, with abundant marine fossils. Unit 1 only contains well to moderately preserved CN assemblage, whose composition reflects the progressive deterioration of the marine environment prior to the onset of the MSC (unit 2). The occurrence of Messinian marker species, such as Amaurolithus primus/delicatus, Amaurolithus tricorniculatus identifies the MNN11c subzones of Raffi et al. (2003). The absence of Nicklithus amplificus, whose occurrenceis restricted to the lower part of the MNN11c subzone, could indicate the the uppermost MNN11c subzone above the LO of Nicklithus amplificus or, alternatively, could be due to unfavorable paleoenvironmental conditions. The scattered occurrence of Reticulofenestra rotaria, usually recorded only in MNN11b, could be ascribed to reworking; as strongly supported by the occurrence of high percentages of Cretaceous, Paleogene and Neogene CN. The topmost 3 m of Unit 1 are barren of CN. The relative abundance of stress tolerant CN species supports a pulsating environment with different characteristics of the water column. According to recent studies on the ecological preferences of several Messinian CN species (Flores et al., 2005; Kowenhoven et al., 2006; Wade & Bown, 2006) we can reconstruct the change in the environment during the successive phases of the onset of the MSC. In general, in the upper 14 meters of Unit 1 “normal” marine assemblages alternate with several strongly oligotypic CN assemblages, with changing composition at different stratigraphic levels. In particular, 12 meters below the first selenitic gypsum bed, the relative abundance of “small” Reticulofenestra (R. minuta and R. haqi combined, up to 70% of the total assemblage) suggests episodes of eutrophic environment and brackish to hipersaline surface waters. An abundant peak of Sphenolithus abies (up to 60% of the total assemblage) and high relative abundance of Helicosphaera carteri (up to 20% of the total assemblage) are recorded 8 meters below Unit 2. The concomitant abundance of both species in our material suggests a shallow water environment (supported by H. carteri), reach in nutrient and with fluctuating, possibly high, salinity. In fact S. abies is interpreted as a schizohaline species and H. carteri as tolerating high salinity waters (Kowenhoven et al., 2006). Another peak of “small” Reticulofenestra is recorded 3 metres below the selenitic gypsum bed, and supports the hypothesis of eutrophic surface waters and increasing salinity. The occurrence of discrete oligotypic CN assemblages, in a succession of “normal” Messinian CN assemblages, suggests the presence of surface waters relatively rich in nutrients and progressively increasing in salinity towards the top of Unit 1. The presence of shallow-waters markers support the idea of a decreasing depth of the basin during the the pre-evaporitic phase, and the development of a progressively restricted environment. Messinian Cn assemblages also testifiy that this trend towards increasing salinity and more restricted environment was interrupted by several episodes of re-establishment of salt concentration close to the global ocean values during the pre-evaporitic phase. The occurrence of euryhaline fishes (Aphanius sp.) just 1.5 m below the selenitic gypsum beds of Unit 2 also testifies that the increase in salinity started before the precipitation of gypsum, during the pre-evaporitic phase.