FISH LAKE, UTAH – GEOLOGY, GEOCHEMISTRY AND PRELIMINARY CORING AS PROOF-OF-CONCEPT FOR A LONG CORE ATTEMPT

Fish Lake is located on the Fish Lake Plateau in central Utah. The Lake occupies a NE-striking tectonic graben; one of a suite of grabens on the Plateau that cut 21-26 Ma volcanic rocks. Nearby Fish Lake Hightop (3545 m) was glaciated with an ice field and outlet glaciers. Exposure ages indicate moraine deposition during Pinedale (15-23 ka) and Bull Lake (130-150 ka) times. One outlet glacier at Pelican Canyon deposited moraines and outwash into the lake but the main basin of the lake was never glaciated. Fish Lake sits at 2696 m elevation and is over 10 km² in area. The Lake drains to the NE into the Fremont River via Lake Creek. Prior to Bull Lake time the extent and volume of Fish Lake may have been smaller and less. Seasonal temperature and dissolved oxygen profiles indicate that Fish Lake is dimictic and oligotrophic. Lake waters are fairly dilute although they are concentrated 1.5 to 3× relative to inflowing spring and creek waters. The stable isotopic composition of the lake suggests considerable evaporative enrichment during the 20–35 year residence time. Fish Lake has a mean depth of 27 m and a maximum depth of 37.2 m. The lake bottom slopes from NW to SE with the deepest part near the SE wall, matching the topographic expression of the graben. Gravity measurements indicate that lake sediments thicken toward the SE side of the lake and the thickest sediment package is modeled to be between 210 and 240 m. In Feb. 2014 we collected cores from Fish Lake using a UWITECH coring system in 30.5 m of water. A composite 11.2-m-long core was constructed from overlapping 2 m drives taken in triplicate to ensure total recovery and good preservation. Twelve ¹⁴C ages and 3 tephra layers define the age model. The oldest ¹⁴C age of 32.3±4.2 cal ka B.P. is from 10.6 m. Core lithology, CT scans, and magnetic susceptibility (ms) reveal three sediment packages: an organic-rich, low ms Holocene to post-glacial section, a fine-grained, minerogenic glacial section with high ms, and a short section of inferred pre-LGM sediment with intermediate composition. Extrapolating the age model to the maximum estimated sediment thicknesses suggest sediments may be older than 600-900 ka. Thus Fish Lake is an ideal candidate for long core retrieval as it likely contains paleoclimatic records extending over multiple glacial cycles.