Regional Holocene climate and landscape changes recorded in the large subarctic lake Torneträsk, N Fennoscandia
Understanding the response of sensitive Arctic and subarctic landscapes to climate change is essential to determine the risks of ongoing and projected climate warming. However, these responses will not be uniform in terms of timing and magnitude across the landscape because of site-specific differences in ecosystem susceptibility to climate forcing. Here we present a multi-proxy analysis of a sediment record from the 330-km2 lake Tornetrask to assess the sensitivity of the Fennoscandian subarctic landscape to climate change over the past ~ 9500 years. By comparing responses of this large-lake system to past climatic and environmental changes with those in small lakes in its catchment, we assessed when the magnitude of change was sufficient to affect an entire region rather than only specific sub-catchments that may be more sensitive to localized environmental changes such as, e.g., tree-line dynamics. Our results show three periods of regional landscape alteration with distinct change in sediment composition: i) landscape development following deglaciation and through the Holocene Thermal Maximum, ~ 9500–3400 cal yr BP; ii) increased soil erosion during the Little Ice Age (LIA); and iii) rapid change during the past century coincident with ongoing climate change. The gradual landscape development led to successive changes in the lake sediment composition over several millennia, whereas climate cooling during the late Holocene caused a rather abrupt shift occurring within ~ 100 years. However, this shift at the onset of the LIA (~ 750 cal yr BP) occurred > 2000 years later than the first indications for climate cooling recorded in small lakes in the Tornetrask catchment, suggesting that a critical ecosystem threshold was not crossed until the LIA. In contrast, the ongoing response to recent climate change was immediate, emphasizing the unprecedented scale of ongoing climate changes in subarctic Fennoscandia.