A TALE OF TWO LAKES: DOMINANCE OF CLIMATIC VS. HYDROLOGIC RESPONSE RECORDED IN LAKE SEDIMENTS OF BRUSH AND KETTLE LAKE, MONTANA-NORTH DAKOTA, NORTHERN GREAT PLAINS

Brush Lake, MT, and Kettle Lake, ND, lie within 40 km of each other in one of the driest parts of the Northern Great Plains, and are exposed to virtually identical climate conditions. Both contain excellent Holocene sedimentary paleoclimate records. Yet both have fundamentally differing hydrogeology, lake size, and water chemistry. They constitute a good test of the hypothesis that hydrologically-distinct lakes within the same climate can produce similar estimates of paleoclimate from the lake sediment record. Investigations at both lakes are ongoing, yet have yielded intriguing preliminary results. Brush Lake is large (78 ha), deep (17 m), and relatively saline (6000-7000 mg/L TDS). Kettle lake is, in comparison, small (5 ha), shallow (9 m), and dilute (2200 mg/L TDS). Differences in water chemistry point to a substantial difference in the inflow/outflow ratio of groundwater, the result of a coarse-grained esker channel intersecting the north side of Kettle Lake and providing easy outgress as well as ingress for seepage. Nonetheless, both lakes have a similar rhythmitic lake sediment record, with laminations almost certainly varves over long periods of the Holocene. Thickness of summer layers containing authigenic aragonite is a proxy for groundwater influx rate, e.g. recharge. Laminations are nearly perennial through the Holocene in Kettle but intermittent in Brush, pointing to changes in oxic status of benthic conditions caused by hydroclimatic events. In Brush lake, sediment focusing and, in some localities, unconformities are thought to be significant features reflecting major drought events as well as ongoing hydrologic processes. While both periodicity and timing of climatic events are clearly expressed in both lakes in similar (not identical) fashion, local hydrologic processes individual to both lakes have influenced their paleorecords and need to be considered in making prudent interpretations.