HOBART LAKE: A NEW DIATOM RECORD FROM SOUTHWESTERN OREGON, USA

The few late Pleistocene and Holocene climate records from southwestern Oregon focus on vegetation and fire history. In addition to the terrestrial records, there are limited speleothem and coastal marine records. Hobart Lake (41.09935^o N, 122.4810^o W, 1458 m, 3.2 ha, ~11.5 m deep) is a landslide-formed lake located near Ashland, Oregon, and contains the first Holocene diatom record of paleolimnological variability in the Siskiyou Mountains. A recent study of the lake (White et al., 2015) using physical properties (magnetic susceptibility, loss-on-ignition), pollen, and charcoal found the early and middle Holocene in the region was warmer and drier than at present, and that following ~3,500 cal yr BP, temperatures decreased and precipitation increased. This shift corresponds with the establishment of the modern oceanographic regime along the northern California-Oregon margin.

Planktic diatoms (Aulacoseira spp. and Stephanodiscus spp.) dominate the early and middle Holocene, with Stephanodiscus spp. comprising as much as 90% of the flora during the early part of this interval, indicating an abundance of nutrients. Planktic taxa disappear at ~3,500 years ago and are replaced by small fragilarioid species (Pseudostaurosira sp., Staurosira spp., and Staurosirella spp.). This change in the flora suggests increased regional precipitation that resulted in the formation of a shallow shelf around the margin of the lake. During the transition from the Medieval Climate Anomaly to the cooler and wetter Little Ice Age (~AD 1250), this flora was replaced by one dominated by a diverse benthic flora, which indicates an increase in aquatic macrophytes, lower nutrient levels, and more acidic waters . Biogenic silica (opal) concentrations mimic the changes in the diatom flora, indicating these different diatom groups drove primary productivity largely through bottom-up ecological forcing in response to Holocene climate variability. Prior to 3,800 cal yr BP, the values fluctuate between 10 and 50%. Between 3,800 and about 800 years ago the values stabilize, generally above 30 wt %. During the last 800 years, the values are generally below 30 wt. %.

White, A., Briles, C., and Whitlock, C., 2015, Postglacial vegetation and fire history of the southern Cascade Range, Oregon: Quaternary Research, v. 84, p. 348-357.