Modern sediment records of stanol to sterol ratios in Lake Suigetsu, Japan: An indicator of variable lacustrine redox conditions

We tested the usefulness of the 5α(H)-stanol/Δ5-sterol ratios obtained by the tetramethylammonium hydroxide (TMAH) thermochemolysis gas chromatography-mass spectrometry (GC-MS) method used to reconstruct redox events in marine sediments as a redox tracer in lake sediments using cores from Lake Suigetsu, Japan. The lake was previously a freshwater lake and is now brackish owing to excavation works conducted during the 17th century. Other redox tracer analyses (e.g., based on farnesol) have shown that C26(nor-24)Δ22/C26(nor-24)Δ5,22 (24-nordehydrocholestanol/24-nordehydrocholesterol), C28(24Me)Δ22/C28(24Me)Δ5,22 (diatomstanol/diatomsterol), and C27Δ22/C27Δ5,22 (22-dehydrocholestanol/22-dehydrocholesterol) ratios have risen (∼0.53) following strong anoxic conditions that formed in Lake Suigetsu (A.D. 1848 and 1935). This study is the first to show that 5α(H)-stanol/Δ5-sterol ratios determined using the TMAH method could be useful as a redox tracer in lacustrine environments. In particular, the ratio of diatomsterol, which is detected in both brackish and fresh water in relatively high abundance, has the potential to be a very useful tracer of redox events. On the other hand, the other 5α(H)-stanol/Δ5-sterol ratios (C27Δ0/C27Δ5 [cholestanol/cholesterol], C28(24Me)Δ0/C28(24Me)Δ5 [campestanol/campesterol], and C29(24Et)Δ0/C29(24Et)Δ5 [sitostanol/sitosterol] ratios) showed no increasing trends, suggesting the influence of terrestrial inputs from degraded products.