A multi-proxy approach to reconstructing Holocene climate variability at Kurupa River Valley, Arctic Alaska

Lake sediments and glacier extents were used to reconstruct late Holocene climate changes from Kurupa Lake (N 68.35°, W 154.61°; 920 m asl) in the north-central Brooks Range, Arctic Alaska. The lake is fed by several tributaries, including meltwater from eight rapidly disappearing cirque glaciers. An age-depth model for the 622-cm-long sediment core was developed using five radiocarbon (14C) ages and a 239+240Pu profile. Sedimentary chlorin (chlorophyll a and derivatives) content was analyzed at high resolution (0.2 cm = ~2 yr intervals) with visible reflectance spectroscopy for the past 5.7 kyr. Sedimentary bulk density was analyzed with gamma-ray attenuation (1.0 cm = ~10 yr intervals). High-resolution measurements of chlorin content from the last 122 yr (top 16 cm of surface sediment) were correlated with a suite of instrumental climate records from Barrow and Bettles, National Center for Environmental Prediction reanalysis data (grid = N°68 to N°70 × W°156 to W°154) and with published climate indices for the Arctic (e.g., Arctic Oscillation, North Atlantic Oscillation). Chlorin content correlates most strongly to average June + September temperature (r = 0.68) and a linear inference model was developed using this regression to quantitatively reconstruct summer temperatures for the past 5.7 kyr. The temperature reconstruction (50-year smoothed) shows minimum summer temperature (5.6 ± 0.9°C) at 650 AD and maximum summer temperature (10.7 ± 0.9°C) 2.9 ka. Fluctuations in bulk density over the past 5.7 kyr were interpreted to reflect changes in up-valley glacier extent. The accumulation-area ratio (AAR) method was used to estimate former equilibrium-line altitudes (ELAs) for the maximum Neoglacial and modern extents of eight cirque glaciers in the study area. Independently dated moraines enable the history of glacier fluctuations to be linked to the lake sediment sequence (r2 = 0.74), and this integrated approach provides a 5.7 kyr quantitative record of centennial-scale ELA variability for Kurupa River valley. The reconstruction supports the previously established onset of Neoglaciation around 4.2 ka, as well as maximum and minimum glacier extents around 700 AD and the present, respectively. The calculated ELA lowering at 700 AD and the LIA, relative to 1982, was 63 and 55 m, respectively.