BRUSH LAKE, MONTANA: A LARGE EVAPORATIVE GROUNDWATER-DOMINATED LAKE WITH A COMPLEX SEDIMENT AND PALEOCLIMATIC RECORD

Brush Lake, near the MT-ND and US-CAN borders, is ca. 130 ha. in area and 22 m at its deepest point. It lacks surface water inflow or outflow and has salinity 6 g/L, pH>10; its Holocene sediment contains both marl and tufa. Datasets to date include: water geochemistry, limnology, and mineralogy of tufa and lakebed marl (Donovan, 1989-92); five 16- to 17.5-m long cores from its deep hole (Grimm and Donovan, 1999, 2003); short cores from shallow water (Grimm, Mueller, and Fritz, 2005); analyses of marl in these cores for pollen, charcoal, mineralogy, LOI, Mg/Ca, and d¹⁸O (Grimm, Brown, Donovan, Baker, Mueller, Fritz); digital scans of split long cores (Grimm and Mueller, 2004); high-res seismic profiles (Baker and Fritz, 2007); and 14 AMS C¹⁴ dates from long cores. Like Kettle Lake 30 km to the east, Brush marl contains both detrital and endogenic (aragonite) fractions; the latter concentration is proportional to time-varying groundwater inflow and records wet and dry periods at sub-decadal scale. While the thickness of marl is similar between the two lakes, Brush has much more soft-sediment deformation in deeper sediments, including a large slump block up to 5 m thick whose base is >7000 BP. A composite core of the 5 long cores shows an ~7.5 m thick upper section (above the slump), a lower section ~3.7 m long (below it), and the slump itself. The basal block consists of sand/gravel and abundant wood; the overlying block has ~0.5 m of thick-bedded sediment grading upwards into finely laminated, probably varved sediment. The fault block itself is folded convolutedly. The date just below the slump is 7295±35 ¹⁴C yr BP with about 600 years of sediment missing, similar to the estimated age of less extensive faulting in Kettle Lake. The core alternates between massive and laminated marl; we interpret the laminated layers were deposited during meromictic high-salinity phases and the massive layers during more humid ones. The Holocene fire (charcoal) record exhibits marked variability, similar to Kettle Lake. Perplexingly, the most recent ~2000 years of sediment is missing from the long cores, with a ¹⁴C date of 1880±35 ¹⁴C yr BP at 10 cm depth; the top of each core lacks Salsola pollen (marker of NGP European settlement). We believe that Brush Lake contains a unique complex paleorecord of climate and lake sedimentary events that requires dissemination.