engleski

Evidence of sapropel S1 formation from Holocene lacustrine sequences in Lake Vrana in Dalmatia (Croatia)

The sediment core from Lake Vrana (1000-500 cm) on the eastern Adriatic coast was analyzed using palynological and geochemical approach to detect environmental changes during the Holocene and to correlate the undated sequence from 1000-810 cm. The palynological record indicates that the deepest sediments in Lake Vrana correspond to Late Pleistocene to Holocene transition (1000-967 cm). After that, the Holocene warming is evident and the observed dark layers are identified as Holocene sapropel layers S1, known in the eastern Mediterranean (ca. 10.8 to 6.1 cal kyr BP). Sapropel S1a in Lake Vrana sediment core is evident from 936 to 808 cm and it ended at approx. ~9.1 cal kyr BP. It is formed in a very shallow freshwater ecosystem, dominated by cyanobacteria Gleocapsa and terrigenous detritus as a consequence of runoff. Periodically anoxic condition occurred. The geochemical signature of sapropel S1a from Lake Vrana is consistent with post-depositional oxidation due to transition to the bottom water oxygenation evidenced in elevated Mn and Mn peak in the at the end of sapropel S1a deposition. Sapropel S1b, deposited between 7.9-7.2 cal kyr BP, is characterized by amorphous organic matter. The two sapropels are separated by the horizon at approx. ~8.4 cal kyr BP, corresponding to the drier climate conditions, indicated by the presence of Graminae, Ericaceae, Juniperus and Pistacia. This is considered to reflect regional climatic cooling associated with reduced levels of precipitation that interrupted a period of generally higher precipitation. Based on geochemical data, TOC and N the sapropelic layers terminated at approx. at 7.2 cal kyr BP when phytoplankton became the main source of organic matter (C/N <10) in the lake. The new records allow reconstruction of paleoenvironmental changes on land, which give insight into the processes and climate conditions that led to the formation of S1. The evidence indicates that: (i) S1 sapropel in Lake Vrana sediments is organic-poor ; (ii) the S1 period in the study area is divisible into two sub-phases (S1a and S1b). The key factor that initiated the formation of S1 was increased discharge of freshwater into the lake basin during warmer and wetter climate in Early Holocene in Lake Vrana.

sapropel S1 ; palynology ; Holocene ; Lake Vrana ; Dalmatia ; Croatia

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