Early-Middle Miocene paleoenvironmental and paleoclimate changes in the Toplica Basin (Serbia) inferred from plant biomarkers, biochemical and elemental geochemical proxies

Abstract

The study investigates the influence of alluvial-lacustrine processes and paleoclimate variations on the distribution of terpenoids and unsubstituted Polycyclic Aromatic Hydrocarbons (PAHs). The XRF, ICP-MS, Rock-Eval, and organic geochemical analyses were employed to investigate thirty Lower and Middle Miocene sedimentary samples from the Prebreza and Čučale formations, collected from boreholes BL3 and BL5, situated in the central part of the Toplica Basin (Serbia). The development of the studied basin part was influenced by alluvial-lacustrine processes, which affected the type of organic matter (OM) and the paleoenvironment. Sandy silt and gravel layers in the profile of the BL3 borehole indicate the contribution of thicker clasts brought by rivers. In the BL5 borehole, there are fine-grained intrabasinal lacustrine sediments in the lower part, and swamp sediments in the upper part. The lowest total organic carbon (TOC) content is in alluvial sediments of BL3 and some lacustrine sediments of BL5. Based on Hydrogen Index (HI) and C/N ratio, various mixtures of terrigenous and algal organic matter are present in the sediments, while an increase in the proportion of terrestrial organic matter with higher HI (Type II kerogen) is recorded in upper parts of both boreholes, which may be related to paleoclimatic changes. Based on Tmax, the OM is immature and/or in the initial stage of maturity. The presence of plant terpenoids and unsubstituted PAHs, which reflected paleoflora and paleoclimate changes, was associated with the suggestion of predominating Type III kerogen in the studied sediments. Various factors influenced the application of gymnosperms/angiosperms parameters. For instance, the progressive aromatization of triterpenoids occurred in the BL5, whereas the process was hindered in the upper part of the BL3, probably as a result of high sedimentation rates. Based on C-value, Sr/Cu, and Rb/Sr ratios, during the deposition of the Lower and Middle Miocene formations of Čučale and Prebreza, a warm and humid climate prevailed, reflecting the Middle Miocene Climatic Optimum (MMCO). The production of unsubstituted PAHs in the studied samples probably relates to paleo-wildfires, anoxic conditions, or the presence of specific biomass precursors.