Diatoms in Castor Lake (North-Central Washington, USA) – Proxies of Climate and Hydrologic Variation

This research provides a high temporal resolution (1 sample / 2-3 years) record of hydrologic variation for the last 2,000 years using a lake sediment record from Castor Lake, a closed-basin system in Washington, USA. The core was dated using 137Cs, 14C, and tephrochronology. Approximately 600 diatoms were identified and enumerated in 198 samples from a Castor Lake freeze core and Livingstone-piston core. A diatom-inference model for salinity was applied to reconstruct fossil diatom salinity. Diatom-inferred salinity for the last century tracked Palmer Drought Severity Index, indicating diatom community composition tracks effective moisture and can be used to infer past hydrologic change. The 2000-year record of salinity from Castor Lake shows that during the Medieval Climate Anomaly (800-1300 AD) Castor Lake experienced cooler, wetter conditions than during the Little Ice Age (1400-1850 AD), which is opposite to what has been reported for the US southwest. These findings are consistent with a wet Pacific Northwest and a dry southwest recorded during La Niña and cool Pacific Decadal Oscillation (PDO) phases. Although these suggest a link to sea surface temperatures, diatom-inferred salinity was only related to proxy records of the PDO for the period between 1000 and 1500 AD. This indicates that other forcing mechanisms could also be important to controlling effective moisture at Castor Lake