SEDIMENTARY RESPONSE OF AN EPHEMERAL LAKE DURING DEGLACIAL AND HOLOCENE CLIMATE EVOLUTION, SWAN LAKE, SOUTHEAST IDAHO.

Perennial water systems are important ecological resources in the arid western US and provide many ecosystem services. Lakes can record fluctuations in climate (such as temperature and effective moisture) and can offer insight into regional variations in climate patterns. We studied a 7.65-m core from Swan Lake, which is a small lake and wetland complex that seasonally overflows, to examine small scale climate changes in its continuous sedimentary record. Our core is similar to that studied by Bright (1966), which exhibits two primary sedimentary facies, mud and peat, with a time span of 13.5 cal ka. The methodology includes radiocarbon dating, particle size analyses, sediment mineralogy, stable isotopes and clumped isotopes on calcium carbonate. Mud in the lower half of the core is primarily composed of clay and silt-size quartz-feldspar, but it transitions to calcium carbonate mud. Clays change from 660 to 585 cm and 350 to 220 cm in the core with an increase in kaolinite and illite upward. Ostracods in the carbonate muds are fairly uniform, indicating a mixture of wetland and lake environments from the last 5.8 ka. Similarly, summer temperatures obtained using Δ47 measurements show fluctuations in the range of 18.1 to 23.4 oC with carbonate muds generally corresponding to lower temperatures and peats to higher temperatures. We interpret muds as open water, shallow lake conditions and peats as wetlands across the floor of the ephemeral lake. If our interpretations are correct, the time from onset of the Younger Dryas to the end of the early Holocene was effectively dry, with low groundwater discharge. Similar events occurred from 4.7 to 1.0 ka and briefly at ~400 cal yr BP. Warmer climate may lead to effective moisture increase in this part of the eastern Great Basin.