engleski

Evidence of seasonality in Middle Miocene Central Paratethys shallow marine shelf – recorded by benthic foraminifera.

North-western part of Croatia (including Medvednica Mt.) during the Middle Miocene was the south-western margin of the Pannonian Basin System (Central Paratethys). The upper part of the Middle Miocene (Late Badenian or Middle Serravalian Mediterranean substage), is a particularly interesting period in the development of the Paratethys, because the end of the fully marine regime coincided with this period. The lower part of the studied, Vrapce section (SW part of the Medvednica Mt.) consists of variably consolidated greyish-brown siltites. Sediments in the upper part of the section show cyclic alteration of three types of thin laminae (average thickness from 0.5 to -1 cm, occasionally up to 2 cm): grey-coloured calcitic-siltose ; brownish argillaceous- siltose, and greyish-brown siltose laminae. Heterolepa-Bolivina assemblage can be assigned to the spring-early summer period, with pronounced freshwater discharge and spring phytoplankton bloom, and, consequently, oxygen depletion at sea-bottom. Opportunistic foraminiferal taxa increase in number responding to the newly available nutrients. Bolivina-Cassidulina assemblage corresponds to summer-early autumn period. Summer phytoplankton bloom, increased temperature, high organic matter degradation and maximal stratification of water-column cause the dysoxic/anoxic conditions at the sea-bottom. Elphidium-Asterigerinata assemblage reflects the period late autumn-early spring. Re- established vertical water circulation again supplies bottom waters and benthic assemblage with oxygen.

Miocene, benthic foraminifera, Paratethys, Croatia, seasonality

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