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Micropalaeontological and geochemical record of Holocene salt-marsh and marine stages in submerged sinkholes (Osor, Island of Cres)

Southwestern part of the Island of Cres is dominated by karst relief with many sinkholes. Some sinkholes were drowned during the Holocene sea level rise, while others are not jet inundated by the sea. Therefore, different stages of environmental change can be observed in this area and progression from terrestrial, salt-marsh to marine environment is recognizable. Cores LK-3 and LK-3B were extracted from Sonte embayment that today represents a submerged sinkhole. It is located in Lošinj Channel near Town of Osor, Island of Cres. This embayment is limited by a sill with water depth of about 3 m. Total length of cores LK-3 and LK-3B is 371 cm and they were collected at a water depth of 5 m. In the Adriatic Sea, at the beginning of the Holocene, sea level was 40-50 meters lower than it is today (CORREGGIARI et al., 1996 ; LAMBECK et al., 2004). Hence, the main aim of this study is to reveal the succession of events that occurred during the Holocene in Sonte embayment and to reconstruct environmental variability that is related to water depth fluctuations by sediment core analysis. In this research patterns of environmental change are defined by integrating palaeontological, geochemical and sedimentological data. From Figure 1. it can be observed that there is a general decrease (with some deviations) in colour lightness (L*), Ca concentration, sand fraction and total inorganic carbon (TIC%) with the depth of the core. Magnetic susceptibility (MS2E), silt and clay, nitrogen (N%), carbon (C%), total organic carbon (TOC%), carbon to nitrogen ratio (C/N) and Al concentrations show opposite trend (Fig. 1). Special attention was dedicated to microfossil studies. Foraminiferal species are present throughout the core. In the zone with the highest TOC and the highest C/N ratio the number of foraminiferal individuals tends to reach minimum values. Specimens were picked from the samples and subsequently identified to species level following classification of LOEBLICH & TAPPAN (1987) and CIMERMAN & LANGER (1991). It is possible to recognize few dominant species, such as Elphidium crispum (LINNE), Elphidium macellum (FICHTEL & MOLL), Elphidium cf. E. advenum (CUSHMAN), Ammonia beccarii (LINNE), Ammonia tepida (CUSHMAN), Peneroplis pertusus (FORSKAL), Quinqueloculina seminula (LINNE) and Spiroloculina sp. Results indicate the existence of four different zones of deposition. It is possible to distinguish alteration of saltmarsh and marine sediments. A chronological framework for the core was based on several AMS radiocarbon age determinations. Therefore, it is possible to predict the age of marine intrusion and formation of fully marine conditions. As the sea level rose during the Holocene Sonte embayment was partially inundated and salt- marsh has been formed. This sinkhole was not drowned when the sea level was low and it was separated from the rest of Lošinj Channel with a sill. At 6300 years BP sea level reached so high to fully spill over the sill separating the sinkhole from the sea, pumping seawater into the sinkhole and deposition of marine sediments occurred. In younger deposits we again have development of salt-marsh environment. Formation of the salt- marsh could be a consequence of sea level fluctuations which would cause partial closure of coastal embayment and its isolation from the direct marine influence. The most probable explanation of renewed salt-marsh development could be elevation of the land as a consequence of tectonic activity. As transgression continued, fully marine conditions developed again at 6000 years BP and they still persist nowadays. To conclude, palaeontological, geochemical and sedimentological data provide evidence for palaeoenvironmental development of Sonte embayment. Although our knowledge is still rather incomplete, these data document the existence and alteration of marine and salt- marsh stages in what is today marine embayment.

Adriatic Sea ; core ; sinkholes ; Holocene ; foraminifera ; geochemistry

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