Hopanoid hydrocarbons and perylene in Lake Biwa (Japan) sediments: Environmental control on their abundance and molecular composition

We examined the vertical distribution of hopanoid hydrocarbons and perylene in sedimentary sections from Lake Biwa (Japan) and discuss the results with respect to paleoenvironmental information from around the lake region over the last 120kyr.Moretene concentration was high above the sections where fall-out events of tephra (tephra AT) occurred ca. 30kyr before present (BP). This feature suggests that the tephra fall-out events created O2 deficient conditions, resulting in high activity of methane producing bacteria, and of methanotrophic moretene-producing bacteria becoming active above the tephra layer. Five sections at 10–20kyr, ca. 35kyr, ca. 70kyr, ca. 90kyr and ca.110kyr BP exhibited peaks in the ratio of 17α, 21β-22R-homohopane to 17β, 21β-22R-homohopane (hopC31αβ/ββ). The n-C23/n-C31 alkane ratio in these sections was 0.6±0.1 (n=49), very low if compared with typical characteristics of n-alkanes for a Sphagnum peat environment (n-C23/n-C31 ca. 3.6; Bingham et al., 2010). We conclude that Sphagnum peat may not have been an important contributor of n-alkanes over the past 120kyr. Comparison with previous pollen records for the lake region suggests that high hopC31αβ/ββ values may be caused by production of acidic soil from expansion of temperate conifer (e.g. Cryptomeria) forest under cold and wet climate conditions. The abundance of perylene relative to sediment or total organic carbon (TOC) was high in the sections at ca. 20kyr, ca. 70kyr and ca. 90kyr BP. These sections were approximately in accord with those having a maximum in hopC31αβ/ββ. This concordance indicates that the climatic conditions suitable for the production of perylene quinone in soil surrounding the lake coincided with those for a high hopC31αβ/ββ ratio. Thus, hopC31αβ/ββ and perylene/TOC might be useful as proxies of soil acidity, temperature and humidity.