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Late Holocene high resolution sediment record of environmental change in a karst lake: the Blue Lake (Modro jezero), Croatia

The Red (local name Crveno jezero) and Blue (local name Modro jezero) Lakes near the town Imotski (central Dalmatia, Croatia), only approximately 500 m away from each other, represent impressive karst geomorphological forms in the Dinaric karst, collapsed doline, with extreme dimensions. These systems are very sensitive to regional hydrological balances, resulting in considerable lake level, water chemistry and biological fluctuations in response to change in climate, particularly precipitation. The Blue Lake has dried out 7 times through the 20th century based on historical records. The recent drought period (October 2011 to February 2012) provided access to the lake bottom and possibility of drilling an 8-meter-long sediment core. The Blue Lake sediment core covers the last 2400 years, with the sedimentation rate ranging between 0.2 and 0.5 cm/yr. The sedimentary sequence shows evidence of a shallower phase of the lake from 2400 cal BP, and progressive deepening of the lake from app. 2000 cal BP, reaching its maximum during the Medieval Warm Period (MWP). The shallow-water sequence is interrupted by several deeper-water phases and indicates evolution from a wetland prior to 1960 cal BP (lithological zone A). Within this zone, needle-shaped vivianite-rich (iron phosphate mineral) layers occur, which is stable under anoxic conditions, evident in anomalous P- enrichments in this zone. A deeper carbonate- producing lake was formed from 1960 to 1390 cal BP (zone B) with frequent drought periods. The deepest lake period, with no visible discontinuities in sedimentation, lasted for the next 850 years (zone C, between 1390 and 540 cal BP). During the LIA and modern periods (zones D and E, from 540 to the present) the frequency of drying out of the lake increased but their duration was much shorter without significant organic carbon accumulation.

Karst lake, carbonate sediments, Modro jezero

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