A Reconstruction of Humidity Changes for the Last 8000 Years, Yukon Territory, Canada

Oxygen isotope data from authigenic carbonate in the sediments from two geographically proximal lakes are used to calculate changes in humidity. Jellybean Lake (60.351 °N, 134.805 °W) is hydrologically open with modern lake water δ¹⁸O similar to rivers, spring water and precipitation (-21 per mil) in the region. Sediment-core carbonate values range from -19 to -21 per mil. Marcella Lake (60.074 °N, 133.898 °W) is hydrologically closed, such that most all of the outflow is via evaporation. Modern lake water in Marcella is -8 per mil and sediment-core carbonate values range from -8.5 to -12 per mil. Because Marcella Lake has been hydrologically closed throughout the Holocene, sediment-core δ¹⁸O values are primarily controlled by changes in regional humidity, which will govern evaporation rates. However, changes in the δ¹⁸O values of input waters, either because of changes in ambient temperature or moisture source regions, or both, may also be responsible. The latter factors can be accounted for by the changes in the δ¹⁸O of hydrologically open Jellybean Lake because the lake water is isotopically equivalent to meteoric precipitation. The 1-cm sampling interval used is equivalent to 20 years time. Chronologies for both lakes are constrained by ²¹⁰Pb, the depth of the White River Ash and radiocarbon ages from terrestrial macrofossils. We calculate the isotopic enrichment in Marcella Lake due to evaporation by correcting the δ18O data with Jellybean Lake δ¹⁸O data. Changes in humidity were determined using a model of a terminal lake at hydrological and isotopic steady state. The δ¹⁸O of the uppermost sediments generate humidity values between 63 and 65%, which are consistent with the average humidity in Teslin, YT of 64.9%. During the last 2000 years, high values of 70% between ~2000 and 1200 cal yr B.P., are followed by a dry cycle between 1200 and 800 cal yr B.P. Intermediate values between 65 and 68% occurred between 800 and 300 cal yr B.P. At 290 cal yr B.P. (1660 AD ), a very large and rapid shift from 70 to 59% occurs over a 20 year period after which values rise steadily to the present day. These quantitative estimates are the first of their kind for this semi-arid region of the Arctic where changes in aridity may be the predominant manifestation of climate change during the Holocene and into the future.