MILLENNIAL SCALE TERRESTRIAL LANDSCAPE EVOLUTION INFERRED FROM CARBON ISOTOPE RECORDS OF A MARL LAKE IN WESTERN IRELAND

A 7.6-m lake sediment core from a marl lake in the Burren region of western Ireland, Lough Inchiquin, records changes in the terrestrial landscape from 16,800 to 5,540 cal yrs B.P. Carbon isotope values of calcite and bulk organic material were determined along with total organic carbon, total nitrogen, C/N ratios, total organic matter, and total calcite. Carbon isotope variations of 12‰ in lacustrine marl and organic matter are too large to be wholly attributed to adjustments in the internal C-cycling processes within the lake itself (e.g., productivity, respiration, and organic carbon burial) thus, an external influence in the form of the C-weathering flux to the lake must be considered. In a watershed composed of limestone bedrock, as in western Ireland, variations in the C isotope value of the DIC weathering flux will track regional landscape evolution from barren limestone bedrock to a forested ecosystem. Carbon isotope values of DIC recorded by calcite are therefore influenced by two isotopically distinct sources of carbon: terrestrial organic material (-27.1 to -31.2‰VPDB) via soil organic matter oxidation and weathered limestone bedrock (+3.4‰VPDB). We find that millennial scale variability in carbon isotopes in Lough Inchiquin sediments are primarily controlled by changes (vegetation, soil) in the terrestrial environment, which may in turn be related to changes in climate (temperature, moisture source). These findings have important implications for developing paleoclimate records from lacustrine sediments and suggest that by selecting appropriate lakes, it is possible to reconstruct paleoclimate records of terrestrial landscape evolution where other terrestrial records are not available.