Variability of early diagenesis in lake sediments: Evidence from the sedimentary geolipid record in an isolated tarn

Abstract

A 0.5-m-long core of sediment from Coburn Mountain Pond, Maine, U.S.A., has provided a virtually undisturbed and continuous record of sediment deposition for the last 200 yr in this small, essentially pristine tarn. Organic carbon concentrations, C/N ratios, carbon isotopic compositions and extractable hydrocarbon molecular compositions indicate subtle differences in the sources of organic matter between sediments deposited before and after ca. 1950 but are otherwise constant. In contrast, total concentrations and molecular distributions of n-fatty acids and n-alkanols vary considerably. The fatty acid and alkanol geolipid fractions are both sensitive to early diagenetic alteration, yet the two fractions react differently. Concentrations and compositional parameters such as long/short, even/odd and branched/unbranched ratios fluctuate with sediment depth, and reflect amounts of microbial reworking which have varied repeatedly over the depositional history. n-Alkane distributions are relatively unaffected by the short-term changes and therefore better preserve the history of biolipid sources to the sediments.