Biomarkers and geochemical indicators of Holocene environmental changes in coastal Lake Sarbsko (Poland)

Abstract

Organic-rich samples from the Holocene sedimentary sequence of Lake Sarbsko were investigated to reconstruct variations in the aquatic system in response to environmental change. The aims were to (i) identify organic matter (OM) sources, (ii) examine changes in soluble OM composition in relation to fluctuations in limnological conditions and (iii) validate the sensitivity of organic geochemical indicators in tracing environmental changes. Diatom frustules are abundant in all samples. Alginite macerals and terrigenous organic matter further contribute to the sedimentary OM. Organic petrography, total organic carbon (TOC)/N ratio values and high content of biogenic silica (SiO 2 biogenic ) argue for a predominant algal source of OM. In contrast, the n-alkane distributions, high relative abundance of C 29 steroid hydrocarbons and high content of detrital quartz in the sediments provide evidence for the input of terrigenous material as a major OM source within the basin. The terpenoid biomarker composition indicates a contribution from conifers and angiosperms. From the diterpenoids in the samples a contribution from species of the coniferales family Pinaceae is concluded. TOC/S and Mg/Ca values, as well as steroid/hopanoid concentration ratios, reflect enhanced water salinity in this coastal lake due to ingression of brackish water from the Baltic Sea during the Atlantic and Sub-Atlantic periods. As indicated by the positive relationship between δ 13C of TOC and Mg/Ca, δ 13C values are influenced by changing contribution of marine vs. freshwater phytoplankton. Additional isotopic heterogeneity in δ 13C may result from differences in terrigenous OM supply vs. autochthonous OM production in the lake. The isotopic composition of calcites supports the establishment of freshwater conditions in the lake during the Sub-Boreal period. Pristane/phytane and Fe/Mn ratios are interpreted to reflect differences in the redox conditions. Oxygenation of bottom waters was temporarily variable, with highest levels of oxygen availability during the Sub-Boreal (freshwater lake). High contents of hopanoid hydrocarbons argue for enhanced microbial activity in the water column and sediment. The occurrence of 2β-methyl-hopanes is related to the presence of cyanobacteria or methylotrophic bacteria. The results demonstrate that in coastal basins of the Baltic Sea sedimentary OM content and composition are a function of the degree of isolation from the sea water. Therefore, organic geochemical data of coastal lake and lagoonal sediments are sensitive indicators for tracing environmental changes during the Holocene.