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

摘要

The sediment core from Lake Vrana (between 1000 and 500 cm) on the eastern Adriatic coast was analyzed using palynological and geochemical approaches to detect environmental changes during the Holocene. The deepest sediments (between 1000 and 967 cm) in Lake Vrana could correspond to Late Pleistocene to Holocene transition. Although this interval is not dated, it is indicated by the palynological record. After that, the Holocene warming is evident and the observed dark layers are identified as Holocene sapropel S1, known in the eastern Mediterranean (10.8-6.1 cal kyr BP). Sapropelic layer S1a in Lake Vrana sediment core is evident between 905 and 790 cm and it ended at 7.9 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 sapropelic layer 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 at the end of sapropelic layer S1a deposition. Sapropelic layer S1b, deposited between 7.4 and 6.4 cal kyr BP, is characterized by amorphous organic matter. The two sapropelic layers are separated by the horizon at 7.9-7.4 cal kyr BP, corresponding to the drier climate conditions, indicated by the presence of Poaceae, Ericaceae, Juniperus and Pistacia. This horizon reflects regional climatic cooling associated with reduced levels of precipitation that interrupted a period of generally higher precipitation. Based on geochemical data, Total organic carbon (TOC) and Nitrogen (N), the sapropelic layers are characterized by elevated values comparing to the rest of the core.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 sapropelic layer in Lake Vrana sediments is an organic-poor deposit; (ii) the S1 period in the study area can be 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 Vrana basin during warmer and wetter climate in the Early Holocene.