Prolonged early to middle Holocene drought in the Pacific Northwest inferred from lacustrine carbonate oxygen isotope values and sedimentology

Abstract

Sediment-based lake-level reconstructions and the oxygen isotopic composition of lacustrine carbonate (δ18OCO3) provide insight on hydroclimate conditions during the early Holocene in western North America and the Pacific Northwest. There are relatively few such records with decadal temporal resolution, and changes in the isotopic composition of meteoric water potentially obscure the hydroclimate signal (precipitation-evaporation) in lacustrine δ18OCO3 data. Here, we present a Holocene-length drought index developed by combining the δ18OCO3 record from Castor Lake, a closed-basin system located in north-central Washington (USA), with the δ18OCO3 record from Lime Lake, which has an open-basin configuration and is located in northeastern Washington. The lake sediment records are dated using primarily a combination of radiocarbon measurements on terrestrial macrofossils, Pb-210, Cs-137, and tephrochronology. The δ18OCO3 drought index, along with changes in Castor Lake sediment facies, suggest periods of prolonged drought in the Pacific Northwest between 11,200 and 6200 cal yr BP, a finding that is supported by other lacustrine proxy records of lake level and temperature change. Early Holocene drought likely resulted from higher than present summer isolation that promoted longer, hotter summers and greater evaporation. Lower than present winter insolation at this time likely altered the location and/or strength of the westerlies, reducing cold season precipitation/snowpack and intensifying the dry conditions. This study highlights the sensitivity of hydroclimate in an already drought- and fire-prone region to the magnitude and duration of summer warmth and precipitation and snowpack levels during the winter.