engleski

Palaeoecology and climate change during the Karpatian/Badenian transition in conjuction with local to global events

Miocene sediments from the Medvednica Mt. belong to the North Croatian Basin of SW Central Paratethys. The marine succession of the Karpatian/Badenian transition near Čučerje village and from nearby outcrops at Bokanjščica stream, expose ca. 90 m thick marly deposits with intercalations of clastic sediments containing rich and very well preserved microfaunal assemblages (planktonic and benthic foraminifera, ostracods) with calcareous nannofossil. Karpatian The last appearance of Helicosphaera ampliaperta BRAMLETTE & WILCOXON is noticed where Sphenolithus heteromorphus DEFLANDRE Paracme ends and represents a very useful correlation tool defining the Subzone MNN4b (MNN4c recently) within Chron C5Br in Langhian successions of the Mediterranean estimated at 15.527 Ma. This zone is correlated with the NN4 Zone in Karpatian successions of the Paratethys. The basal part of the interval is characterised by the domination of smaller to medium coccoliths of Coccolithus pelagicus (WALLICH) SCHILLER, typical for cold, eutrophic waters. Helicosphaera carteri (WALLICH) KAMPTNER flux could be a consequence of lower surface temperatures (ca. 15°C). Similar abundances of H. carteri are associated with today's known La Niña episodes in the tropics while it has been recognized here during the MCO (ca. 17-15 Ma). Nevertheless, planktonic foraminiferal assemblage consists of numerous specimens of the cool surface water indicators such as Tenuitella munda (JENKINS), Tenuitella angustiumbilicata (BOLLI) and Globigerinita glutinata (EGGER). This fits well with already noticed larger climate instability during the early stage of the MCO. An increase in Si component was likely a direct effect of the cooler temperatures favouring siliceous organisms during the long-term global climate cooling (Mi2 event) at ca. 16 Ma. This is evident especially in the upper part of interval, where silicoflagellate, diatom and radiolarians implying to intensified upwelling caused by a shift in the local current system during the global cooling. Nevertheless, larger temperate forms of C. pelagicus, known from upwelling regions where temperature ranges from 13-16°C, dominates in assemblage. However, skeletons of siliceous organisms are mostly broken probably because of the unstable shallower environment of coastal turbulent, nutrient enriched water. It is detected in assemblage with abundant Pontosphaera multipora (KAMPTNER, ex DEFLANDRE in DEFLANDRE & FERT) ROTH, which is indicative for the shallow marine nearshore environment with normal to slightly hyposaline water due to the seasonally driven convergence of different water bodies. Frequent occurrence of benthic foraminiferal infaunal genera Bolivina, Bulimina and Melonis indicate a low oxygen but organic carbon reach sea bottom. Badenian The regular occurrence of coccolith S. heteromorphus together with close records of Helicosphaera. waltrans THEODORIDIS corresponding to the Mediterranean Subzone MNN5a. In the Paratethys, it is detected in the lower part of the Helicosphaera waltrans horizon of early Badenian. In early Badenian small Reticulofenestra species dominates in assemblage with abundant H. carteri, common Reticulofenestra haqii BACKMAN and sphenoliths. Similar nannoflora assemblage is recorded in Messinian, in shallow, hypersaline environment with periodic influxes of fresh, nutrient rich water. This resulted in an extreme environment with highly fluctuating salinity levels and elevated abundances of Reticulofenestra minuta ROTH. The increasement of C. pelagicus was also noticed probably as a result of the aforementioned convergence of different water bodies. Planktonic foraminiferal association is characterised by prevailens of warm water Cassigerinella chipolensis (CUSHMAN & PONTON) (50%) over cool water Turborotalita quinqueloba (NATLAND) (10%). Warmer climate and MCO was reestablished in the early Badenian but in more fluctuating environment. Discovery of very rare Indo-Pacific ostracod Paijenborchella iocosa KINGMA indicates the establishment of connections between the Paratethys, Mediterranean and Indo-Pacific Ocean. This is in accordance with the Badenian transgression that began at 15.4 Ma and recognised above the Lan2/Ser1 sequence boundary. Reach and highly diversified benthic foraminiferal assemblage, dominated by infaunal organic carbon-preferring genera such as Bulimina, and low-oxygen adapted taxa like Bolivina, Uvigerina and Pappina, indicate middle shelf environment where Bolivina dominates, while relatively abundant dinocyst association (Systematophora placacantha (DEFLANDRE & COOKSON) DAVEY et al., Lingulodinium machaerophorum (DEFLANDRE & COOKSON) WALL, Melitasphaeridium choanophorum (DEFLANDRE & COOKSON) HARLAND & HILL, Spiniferites sp., Lejeunacysta sp.) indicate somewhat deeper and distal marine environment.

microfossils ; Karpatian/Badenian transition ; SW Central Paratethys

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