Environmental changes in the lower Mirna River valley (Istria, Croatia) during Upper Holocene

Sedimentological, macro- and micropaleontological analyses on 3 cores down to 13 m depth were carried out in the lower section of the Mirna River valley, in order to study the depositional facies and the environmental evolution. The Holocene marine transgression reached upstream for 7 km from the present-day coast, while in the last 7 ka it was followed by progradation of the Mirna River estuarine delta. The protected coast offered by the lower valley and the strong input of fresh water led to the presence of a brackish microfauna in front of the river mouth. The oldest sediments in the cores were characterized by the dominance of Ammonia beccarii and significant proportions of Elphidium spp. and miliolids suggesting a marine/estuarine origin. The foraminiferal assemblage in the overlying sediments became less diverse, as the relative abundance of Elphidium spp. and miliolids dropped, implying a shift to a transitional environment (inner estuarine/lagoon facies, Bb). Sediments originating in hyposaline marshes (facies Ba) had the lowest foraminiferal species diversity index (A. beccarii predominated over Trochammina inflata and Haynesina sp.). Since late-Antiquity a significant input of alluvial matter led to the deposition of several metres of silty clay sediments which reach >9m thick in the M3 core. The sediment supply has been partly increased by deforestation carried out in the Mirna catchment area. This was particularly extensive from the 15th–19th centuries and fed the fluvial system with large amounts of material, causing the increase of sediment input and rapid progradation of the estuarine (bay-head) delta. This study also highlights the potential role of hand augering in sampling and describing the subsoil for reconstruction of the geomorphological evolution of the area and supporting the study of past relative sea levels, climate changes, and anthropogenic activities that occurred during the Holocene.