Palaeolimnology of Palaeozoic lakes, focussing on a single lake cycle in the Middle Devonian of the Orcadian Basin, Scotland

There are few palaeolimnological studies of Palaeozoic lake deposits using methodologies developed for the study of Quaternary lakes. New research described here indicates the benefits of applying multidisciplinary high-resolution sampling and analysis on the Middle Devonian lake deposits of the Orcadian Basin in order to understand lake palaeoecology and response to water chemistry change and possible climate change. In a continuously sampled 4 m-thick lake cycle in two correlated boreholes in Caithness there are two distinct features. There is a broad trend in δ13C of sedimentary organic matter from high values in the shallow water deposits to lower values in the deeper water deposits, attributed to changing contributions from terrestrial plant material (δ13Corg c. − 28‰) and algal material (δ13Corg c. − 33‰). This variation is consistent with palynological data and is best explained by proximal–distal variation as the lake filled. A smaller but well-correlated up-section decreasing δ13Corg trend occurs over approximately 1 m, starting at the cycle centre where the lake was at its deepest. Since the same feature is seen in two boreholes, we suggest that local effects of migrated bitumen, diagenesis or facies changes are unlikely to be the cause, and that changes in the carbon cycle of the lake are responsible. Explanations for this trend may relate to the source of carbon that algae utilised as the lake changed from a fresher to a more saline environment, and to changes in the carbon isotope composition of the lake water during periods of enhanced productivity.