Holocene environmental record from lake sediments in the Bokanjačko blato karst polje (Dalmatia, Croatia)

Abstract

The Bokanjačko blato karst polje (field) is a typical depression along the eastern Adriatic coast that is filled with lake sediments. This large-scale structure is also an hydrologically important region as wells in southern part of the karst polje comprise the water supply for surrounding towns and villages. The evolution of this polje, however, remains poorly understood. A 7.8 m long sediment core from the south-eastern part of the karst polje is utilized in this study to enable a multiproxy approach to the reconstruction of depositional processes that have occurred over the last 10.3 ka cal BP. The sedimentological, mineralogical and geochemical characteristics, as well as palynological analyses of this core indicate that the whole sediment sequence consists of lake sediments alongside a variable amount of carbonates and silicates. The lake existed for approximately 10,300 years before it was drained for agricultural purposes in the 1960s. Depositional changes are also evident throughout the core; between 10.3 and 4.2 ka cal BP, a higher values of magnetic susceptibility (MS) and concentrations of lithogenic elements indicate increased erosion and input of material from the catchment and therefore wetter climatic condition. Deposition of siliciclastic detrital material gradually decreased from 4.2 ka cal BP onwards, replaced by carbonate sedimentation alongside minimal input of the former. The carbonate-based sandy silt sedimentation remains until hydromelioration of the depression. The results of this study show that the lake level gradually increased between 4.2 and 2.9 ka cal BP, evidenced by a higher carbonate content and a lower proportion of detrital material, reaching a maximum between 2.9 and 1.6 ka cal BP. Data also show that at 1.6 ka cal BP, the lake level was likely lowered due to higher nutrient content, including sulfur (S) and phosphorus (P), as well as organic compound levels, prior to human intervention and hydromelioration for agriculture. High concentrations of the nutrient elements, lead (Pb), copper (Cu), and zinc (Zn) in the upper 40 cm of this sequence are indicative of the influence of agricultural activities across the karst polje as well as anthropogenic atmospheric pollution. Geochemical and sedimentological proxies illustrate centennial-scale variability in carbonate deposition and erosional runoff processes comparable with the natural climatic and anthropogenic factors that are known to influence the evolution of Holocene karst poljes across the central Mediterranean.