Lake sediment evidence for late Holocene climate change and landscape erosion in western Iceland

Ecosystem variability must be assessed over a range of timescales in order to fully understand natural ecosystem processes. Long-term climate change, at millennial and centennial scales, is a major driver of natural ecosystem variability, but identifying evidence of past climate change is frequently confounded by human-induced impacts on the ecosystem. Iceland is a location where it is possible to separate natural from anthropogenic change in environmental archives, as the date of settlement is accepted to be around AD 874, prior to which the island was free from proven human impacts. We used a lake sediment core from Breieavatn, near Reykholt, a major farm of the Norse period in western Iceland, to examine landscape development. A change in pollen concentration in the sediments, especially the decline in Betula, indicated initial landscape degradation immediately post-settlement, whereas the chironomid fauna and reconstructed temperatures were relatively complacent during this period. The pollen evidence is corroborated by ¹⁴C analyses, which indicate an increase in older carbon entering the lake, inferred to have been caused by increased erosion following settlement. Further decreases in Betula pollen occurred around AD 1300, pre-dating a drop in chironomid-inferred temperatures (CI-T) of ~1°C over 100–200 years. The CI-T reconstruction also shows a significant cooling after ~AD 1800, likely indicative of the coldest phase of the Little Ice Age. The evidence suggests that the chironomid record was relatively unaffected by the increased landscape degradation and hence reveals a temperature reconstruction independent of human impact.