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First stable isotope (δ13C, δ18O) results of Pliocene malacofaunas from sediments of Lake Slavonia (Kravarsko, Croatia)

Kravarsko is a small village located in Vukomeričke gorice, a low hills range situated north of the Kupa River in the area between the towns of Zagreb, Sisak and Karlovac in central Croatia. Geotectonically, the investigated area is located in the southwestern part of the Pannonian Basin. This area is predominantly composed of Pliocene sediments deposited in alluvial and lacustrine freshwater environments of the long- living Lake Slavonia. The thickness of Pliocene deposits in Vukomeričke gorice can reach up to 400 meters (CGS - Department for Geology, 2009). During recent sedimentological studies in the area of Kravarsko several outcrops revealed a rich malacofauna confirming a Pliocene age of the sediments. The small vertebrate remains and ostracode shells were additionally observed in the studied sections. The aim of the present study is the description of sedimentary successions from selected outcrops and the paleoenvironmental evaluation of stable isotope (δ13C, δ18O) signatures of different mollusk species from Lake Slavonia, in older literature also known as Paludina Lake (acc. to a junior synonym of Viviparus ; Harzhauser & Mandic, 2008). Stable isotope analysis was performed on 52 specimens belonging to the gastropod genera Lithoglyphus, Microcolpia, Neumayria, Pisidium, Theodoxus, Valvata and Viviparus. Based on the position within the regional stratigraphic scheme of the Lake Slavonia their relative ages are here provisionally denoted as early, middle and late Pliocene. The samples were collected from four localities (Kra-I, VGK 35, VGK 39, VGK 38). The outcrops Kra-I and VGK 39 are landslides which intersect fossil-rich layers of the lower Pliocene Viviparus fuchsi Zone. Sediments of those two outcrops are mostly clayey to silty in composition, with rare intercalations of sand. Outcrop VGK 35 is an erosional gully with predominantly silty to sandy sediments of the upper Pliocene Viviparus hoernesi Zone. Finally, the point VGK 38 revealed shell material from the middle Pliocene Viviparus dezmanianus Zone embedded however to present time stream sediment. Preliminary results show that early Pliocene δ18O values range from -11.21‰ (VPDB) to -1.87‰, and δ13C values from -11.77‰ (VPDB) to -5.43‰. Middle Pliocene δ18O values range from -10.28‰ to -5.52‰, and δ13C values from -10.41‰ to -6.58‰. Finally, late Pliocene δ18O values range from -11.29‰ to -10.08‰, and δ13C values from -11.69‰ to -7.42‰. While the total ranges of the δ13C record are fairly similar from early to late Pliocene, the δ18O values show a decreasing range of values from early and middle Pliocene and the narrowest range in the late Pliocene. This variability is probably related to different habitat preferences and life- strategies amongst the evaluated gastropod taxa. In the future, our stable isotope data will serve as the basis for paleoenvironmental and paleotemperature reconstructions of Lake Slavonia and for an evaluation of the impact of mollusk life-strategies and habitat preferences on expected isotope signatures (e.g., Harzhauser et al., 2007 ; 2012).

Pliocene; Lake Slavonia; sediments; malacofauna; stable isotopes

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