Organic Geochemistry of Ağaçbaşı Yayla Peat Deposits, Köprübaşı/Trabzon, NE Turkey

Abstract

Peat deposits, which outcrop in the southern Ağaçbaşı region in Trabzon, Northern Turkey, were studied by using chemical, organic geochemical, petrographic and palynological characteristics of peats. Palynological data indicate that the peats were deposited in terrestrial or lacustrine environments, comprising organic matter with an average composition of 70% woody, 19.3% herbaceous and 10.7% amorphous material. The peats are Holocene-recent in age. The peat organic matter that has been interpreted as immature based on the SCI (spore color index) analyses suggests that the peat organic matter was immature.Rock Eval analyses show that the average total organic carbon (TOC) content of the peat is 42%, and the average HI value is 315mgHC/gTOC, which is very high for peat. The high OI values (avg. 134mgCO2/gTOC) point that the depositional environment of peats were deposits under oxic or suboxic conditions.The selected sample (ABY-13) with the highest extractable yield has TAR (Terrigenous/Aquatic Ratio) and CPI (Carbon Preference Index) index values of 2.4 and 3.4, respectively. These values reflect high levels of n-alkanes, supporting a terrestrial organic matter input. The Paq (aquatic plant n-alkane proxy) ratio (0.6) indicates that there was a contribution from mixtures of submerged/floating, emergent and terrestrial plant mixtures. The Pwax (vascular plant n-alkane proxy) ratio (0.6) is a sign of moist and wetter conditions. The TOC, ACL, Qwood/grass:C27/C31, Qwood/plant: (C27+C29)/(C27+C29+C31) and Qgrass/plant:C31/(C27+C29+C31) values indicate that woody plants were the dominant vegetation in the peat mire was woody plants, and the climate was relatively wet and cold.According to the m/z 191 and m/z 217 fragments tracking from GC–MS chromatogram data obtained using biomarker analyses, the lack of C34 and C35 homohopanes was interpreted as that organic matter was deposited under oxic or suboxic conditions. Moretane/hopane, Tm/Ts and Tm/C30 pentacyclic triterpane ratios also point indicate oxic and acidic conditions during the formation of the peat. The absence of 17α(H)-28,30-bisnorhopane from the m/z 191 chromatograms support the interpretation that the Ağaçbaşı Yayla peat was deposited in a terrestrial or lacustrine environment under oxic or suboxic conditions. The dominance of C29 steranes indicates significant terrestrial organic matter input. Using sterane carbon number distributions the extract of the peat sample plots in the terrigenous region. Additionally, C30 steranes, which imply marine environments, were not detected. The Ts/(Ts+Tm), ratios also suggest that the peat contained immature organic matter.