What controls the fate of carbohydrates in meromictic lake sediments throughout the Holocene?

Carbohydrates are a ubiquitous constituent of living organisms and an important contributor to global sedimentary carbon pools. Yet, the factors that control the pool size and degradation of sedimentary carbohydrates are not well understood. Here, we investigated carbohydrate cycling over a complete Holocene sedimentary succession in high-altitude, meromictic Lake Cadagno (Switzerland). This succession includes sedimentary records of a stepwise evolving lake redox history from oxic, hypoxic to euxinic anoxic, as well as intercalated layers of lacustrine and event deposits. Event deposits differ from lacustrine layers in organic carbon content, stable isotope signatures, carbohydrate contents, and carbohydrate macromolecular compositions (pyrolysis gas/mass spectrometry) indicating significant terrestrial inputs. However, past changes in redox conditions, implied by redox-sensitive elements (e.g. Mn, Fe, Mo, and S), are not reflected in carbon isotope and carbohydrate data. Carbohydrate contributions are stable, or even increase, with sediment age and show compound-specific variations. Certain carbohydrate pyrolysis products of likely aquatic origin, such as levoglucosan, increase in percentage with sediment age, whereas others, such as furaldehyde, decrease with age. On the other hand, pyrolysis products that are of likely terrestrial origin, such as 3-acetamido-methylfuran, decrease strongly with age. In contrast to carbohydrate macromolecules, gas chromatography with flame-ionization detection analyses showed no clear changes in total hydrolyzed carbohydrate monomer compositions in relation to sediment age or between aquatic- and terrestrial-dominated layers. Our results indicate that carbohydrate degradation in sediments is mainly controlled at the macromolecular level. Notably, our findings suggest that carbohydrates can be well-preserved over the entire Holocene in lake sediments and that aquatic carbohydrates are in some cases selectively preserved over their terrestrial counterparts in these sediments.