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 experienced a warmer climate than pre-industrial times, provides an ideal opportunity to test the sensitivity of the Greenland Ice Sheet to prolonged warmth. However, available climate reconstructions from Greenland and surrounding regions provide a contrasting 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 seasonal sensitivity of the proxies or to spatio-temporal climate variations. Here we generate five new Holocene timeseries of branched glycerol dialkyl glycerol tetraether (brGDGT)-inferred mean air temperature for months above freezing from lakes along a latitudinal transect in southwestern Greenland, yielding a total of seven Holocene brGDGT-inferred time series in this region. Lake model sensitivity tests indicate minimal intra-lake variation in both the seasonal production window of brGDGT and the sensitivity to air temperature changes. Moreover, higher caldarchaeol-to-crenarchaeol ratios during the Early Holocene indicate anoxic or suboxic conditions in all lakes; however, these conditions do not show a consistent relationship with reconstructed temperatures, indicating that regional climate is the primary driver of the reconstructed temperature time series. Six of the brGDGT time series suggest a temperature maximum between approx. 7.5 and 5 ka, following peak summer insolation and in agreement with many regional reconstructions. A site far from both the coast and the Greenland Ice Sheet suggests peak warmth in the Early Holocene. These results, based on a single proxy, suggest that southwestern Greenland experienced similar Holocene summer temperature trends, with local temperature variations caused by proximity to the ice sheet and the ocean. Further investigations quantifying seasonal sensitivity between proxies and local effects (e.g., site-specific systematics and proximity to the ice sheet and the ocean) may help reconcile differences and refine Holocene climate reconstructions.