Carbon cycling in Arctic lakes: sedimentary biomarker reconstructions from Disko Island, West Greenland

A palaeolimnological study of three lakes on Disko Island, West Greenland was conducted across a hydrogeomorphic landscape gradient, to reconstruct principally changes in algal pigments, lipid biomarkers and carbon isotopes (δ13C) to investigate carbon cycling at multiple scales and resolutions. All three records reconstructed recent change on Disko, with records from lake Disko 2 additionally spanning since ~7640 cal. yr BP and lake Disko 4 since ~1260 cal. yr BP. Changes in sedimentary proxies were broadly consistent with the spatially and temporally heterogeneous environmental change known to have occurred across the Arctic over these periods, including recent warming (RW), the Little Ice Age (LIA) and the Medieval Climate Anomaly (MCA). However, the individual lake responses to these changes were highly landscape specific. Changes in algal pigments were linked to variations in the inputs of nutrients and DOC from soil microbial activity, variations in permafrost melt regime and glacier melt. Evidence of disturbance from catchment freeze-thaw processes, glacier inputs and the effects of permafrost melt on algal communities varied between lakes. At the highest position in the landscape gradient, lake Disko 2 had poorly developed soils and lower glacier coverage, with simple algal communities, but pigment and δ13CTOC changes since ~7640 cal. yr BP reflected individualistic responses to overarching drivers, consistent with the current understanding of heterogeneous pan-Arctic environmental change. Replicate cores with proximity to talus and debris flow had differing signatures, highlighting the role of geomorphology. In lake Disko 2 there was some similarity between Greenland Ice Sheet surface area and pigment biomarkers of cryptophytes (alloxanthin), which suggests if locally similar, catchment ice variation may regulate nutrient and DOC release from catchments, stimulating algae. At a mid-elevation position in the catchment, lake Disko 1 meltwater inputs from an upstream retreating glacier (since the end of the LIA) may be stimulating algae through increased nutrients and DOC supply, although soil nutrient cycling and permafrost release present in the catchment U-shaped valley may also be contributing to these increases. At the lowest position in the landscape, lake Disko 4 has a complex and variable pigment response likely influenced by multiple glacier inputs, thicker permafrost, greater vegetation and more developed wetland areas, together regulating the cycling of nutrients and DOC. Higher maximum pigment concentration during the MCA, compared with the LIA reflect the spatially and temporally heterogeneous expression of these events across the Arctic. Despite individuality in catchment filtering, all three lakes on Disko Island were found to have recent increases in algal production, which when combined with recent increases in carbon burial suggests these lakes are becoming increased carbon sinks. Although this increased carbon burial reflects a minor store in lakes compared with release to downstream waterbodies, if upscaled across the Arctic this change could be globally significant and should be integrated in future models. With future warming, catchment processes are likely to play a pronounced role in mediating algal community structure in lakes across the Arctic