Characteristics of n-alkanes and hopanes in paleosol layers in the sediment core from Lake Suwa (central Japan)

Abstract

Paleosols are the soils formed on land in the past. Soil profile reflects source materials, microtopography, and climatic and environmental conditions. Although most charactersistics of paleosols have been lost due to erosion and diagenesis, the information for paleoclimatic and paleoenvironmental reconstructions has been recorded in the paleosols. Recently, organic geochemical proxies including biomarkers have been applied to the paleosol study. However, there have been few reports for biomarker variations of paleosols in Japan. The sediment core from Lake Suwa contains some paleosols in lacustrine sediments. The n-alkane proxies such as carbon preference index (CPI) odd to even predominance (OEP) and aquatic plant indicator (Paq) were analyzed. The CPI and OEP values are lower in paleosol layers, but C₃₀ ββ-hopane ratios, a hopanoid biomarker index, are higher. In particular, the CPI values are reversely correlated to the C₃₀ ββ-hopane ratios. From these results, we suggest that n-alkanes with weak odd predominance were presumably derived from plant roots and microbial degradation of long chain, that is odd chain, n-alkanes, and increasing ββ-hopanes, which were fresh biological hopanoids derived from bacteria in the rhizosphere of paleosol layers.