Holocene temperatures in southwestern Greenland controlled by topography, ice sheet proximity and oceanic conditions

The Holocene thermal maximum (HTM), a period during the early and middle-Holocene when Greenland likely experienced warmer than pre-industrial climate, provides an ideal opportunity to test the sensitivity of the Greenland Ice Sheet to prolonged warmer-than-preindustrial conditions. However, available climate reconstructions from the region provide a controversial picture of the HTM—several reconstructions show an earlier HTM between the early- to middle-Holocene, while others show a delayed HTM between the middle to late Holocene. This discrepancy may be due to either the seasonal sensitivity of the proxies or to spatio-temporal climate variations. Here we generate five new Holocene branched glycerol dialkyl glycerol tetraether (brGDGT)-inferred ice-free season lake water temperature timeseries from lakes along a latitudinal transect in southwestern Greenland, yielding a total of seven Holocene brGDGT timeseries in this region. Lake model simulations suggest minimal intra-lake variation in both the seasonal production window of brGDGTs and the sensitivity of studied lakes to air temperature changes, suggesting regional climate as a primary mechanism influencing these timeseries. Five of the brGDGT timeseries suggest a thermal maximum between approx. 7 and 5 ka, following the peak summer solar insolation and in agreement with many regional reconstructions. A coastal site that is influenced by ocean-atmosphere heat exchange experienced a thermal maximum between approx. 5 to 3 ka, coinciding with nearby sea surface temperature reconstructions. A site far from both the coast and the Greenland Ice Sheet suggests peak warmth in the early Holocene. This suggests that local variations in temperature, influenced by the proximity to the ice sheet and ocean, caused the discrepancies in the Holocene temperature reconstructions in proxy timeseries in southwestern Greenland. Further investigations quantifying seasonal sensitivity and local effects (e.g., site-specific systematics, and proximity to ice sheet and ocean) may reveal similarities among proxy timeseries.