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Astronomical forcing of depositional systems and mollusc radiations in the Miocene Dinaride Lake System

The Dinaride Lake System (DLS) was a long-lived, lacustrine-continental environment which became established during the early Miocene along the Dinaric Land on a paleogeographic barrier between the Paratethys and the proto-Mediterranean seas. Two main lacustrine phases occurred in the DLS. The first phase may coincide with a global mid-Burdigalian sea level lowstand which resulted in the restriction of the Paratethys during the Late Ottnangian. At that time, endemic lake-molluscs can be traced as far as the Bakony Hills in Central Hungary. The second phase, occurred around the Early/Middle Miocene boundary and was restricted to the Dinarides. The Dinaride land mass had at that time become smaller, because its former northern shore was already flooded by the Paratethys Sea. The molluscs of both phases are nearly completely fully endemic, originating from autochthonous speciation and radiation events. In total, about 190 species level taxa, grouped into 36 genera, are described from the DLS. Among these, the most diverse genera are prososthenid (Prososthenia) and melanopsid (Melanopsis) gastropods, followed by dreissenid bivalves (Mytilopsis). Each of these groups comprises more than 30 species and subspecies. First fieldtrips concentrated on the Sutina section in the Sinj Basin (SE Croatia) with a 140-m-thick marl/lignite succession. The Sinj Basin belongs to the southernmost basins of the Miocene Dinaride Lake System, positioned on the paleogeographical high between the Central Paratethys and the proto-Mediterranean Sea. Its sedimentary infill is composed by more than 300 m thick lacustrine sediments comprising carbonates, marls and important lignite deposits. The sediments are highly fossiliferous bearing in part a rich mollusc fauna. Its taxonomic inventory represents one of the best known for the whole Dinaride Lake System. The studied section represents the topmost part of the basin's sedimentary infill and comprises two main transgressive-regressive cycles starting with light carbonates and ending with massive lignitic beds. Based on the sedimentological field research, the deposits are tentatively grouped into six facies. Limestones strongly predominate. Biogenic deposits are represented by lignite intercalations and mollusc coquinas, but the occurrence of calcarenites and marls is very rear. Additionally, pyroclastics are represented by at least one intercalation. The poor content of siliciclastics is explained by very low rate of river supply and the predominantly carbonatic basement formed by Cretaceaous and Eocene limestones. The high percentage of endemism impedes straightforward biostratigraphic correlation with regions. As absolute datings (magnetostratigraphic and radiometric) are still in progress, a first approach is a cyclostratigraphic interpretation to allow an estimation of the relative time which is represented in the section. For this, geophysical data (gamma-logging) have been obtained in the field from the highly rhythmic marl/lignite successions and subjected to spectral analysis. The spectral analysis of gamma-log data proved the significance of 17 m thick sedimentary cycles which have been correspondingly interpreted as being forced by the 100 kyr. eccentricity periods. Hence, the succession could represent the deposition of about 800 ky. resulting in a mean sedimentation rate of about 0.2 mm/yr. Based on this time frame, the evolutionary pulses of the mollusc faunas can be evaluated. As the most eye-catching feature, the melanopsids and prososthenid gastropods display low disparity and low diversity close to the main phases of lignite-formation, which are interpreted to be expressions of the 400 kyr eccentricity band. In the intervening interval, diversification starts and the morphologic disparity increases considerably. A climatic trigger, resulting from astronomical forcing, is therefore a very likely component which influenced the evolution of the lake molluscs. The investigation is part of the Austrian FWF Project P18519-B17: "Mollusc evolution of the Miocene Dinaride Lake System" and "Neogene terrestrial environments of the Pannonian Basin and carst region" supported by Ministry of Science, Education and Sports of the Republic of Croatia.

astronomical forcing; mollusc radiations; Miocene; Dinaride Lake System

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