More than just a dating technique: determining the integrity of lake sediment records through tephrochronology

The reliability, accuracy and validity of palaeoenvironmental reconstructions hinge upon the establishment of robust chronological control. Lake sedimentary sequences can be problematic to date for reasons including carbon reservoir effects, sediment disturbance and the in-wash of eroded organic material. Such records do, however, capture run-off from the surrounding area, making them ideal for a host of palaeoenvironmental methodologies, including geochemistry, sedaDNA and lipid biomarker analysis. Here we consider opportunities and challenges presented by sedimentary records from small lakes on islands sandwiched between Ireland and Scotland around the Sea of Moyle, and demonstrate important insights into the integrity of the sediment through the application of tephrochronology. The lakes offer possibilities to reconstruct a history of changing landscapes, environment and population history on the islands, providing an important basis for understanding the sustainability and resilience of complex socioecological systems over the Late Holocene. Initial 14C rangefinder dates from the sequences revealed age-reversals and off-sets that posed barriers for establishing the age of the sediment. The application of tephrochronology presents a solution to the chronology, yet one that is at odds with a series of 210Pb age estimates for the top of the core. The tephrostratigraphic record brings to light, however, systemic sediment disturbance through the sequences that highlight issues with establishing robust age control. We consider the implications of the tephra profile for understanding the palaeoenvironmental reconstructions from the lake sediment, and evaluate from the proxy records, including sedaDNA, whether the sediment mixing is clearly evident in the absence of the tephrostratigraphic data.