A LACUSTRINE SEDIMENTARY RECORD OF THE HOLOCENE CLIMATIC OPTIMUM IN THE UINTA MOUNTAINS, NORTHEASTERN UTAH

A sediment core retrieved from Water Lily Lake, a closed basin at 2855 m asl, provides information about Holocene paleoclimate in the southern Uinta Mountains. Samples from the core, retrieved in March 2003, were analyzed for water content, %LOI, clay mineralogy (through XRD), and grain size distribution. AMS dates of 2510 ± 140 (2870 to 2310 cal yr BP) on a Salix bud-scale and 6160 ± 160 (7410 to 6670 cal yr BP) on an Abies needle provide preliminary age control. Linear interpolation (assuming 0 years at the top) suggests a sedimentation rate of ~0.3 mm/yr. Water content declines steadily from 90% to 70% downward through the core, with spikes to < 55%. %LOI is more variable (cv 0.14 vs. 0.06), and ranges from 40% to < 20%. Mean grain size ranges from 8 to 51 μm, with an overall mean of 20 μm (fine silt). The bulk mineral assemblage is dominated by quartz, kaolin, muscovite, and feldspar, with minor pyrite, dolomite, and chlorite. Mica and kaolin dominate the < 2 μm size fraction. A major change in sedimentary properties occurs at the 200-cm level, tentatively dated to between 4900 and 4200 cal yr BP. Sediment from below 200 cm is significantly (p=0.004) coarser (mean of 23 vs. 18 μm), with more variable fluctuations in grain size (sd 37 vs. 24 μm). %LOI is also significantly (p < 0.001) lower below 200 cm (30% vs. 33%). Dolomite is present only below 200 cm, and one sample from near the bottom of the core (ca. 7000 years BP) contains pyrophyllite, the nearest reported occurrence of which is ~100 km to the west. The convergence of evidence indicates that the lower part of the core was deposited during the Holocene Climatic Optimum, when warm, dry, conditions lowered the water level in the closed basin, allowing dolomite precipitation or diagenesis. Clastic input was higher, coarser, and more variable, suggesting a decrease of vegetation on surrounding slopes, and dust inputs were increased. This period ended locally ca. 4500 years BP, when near-modern conditions were reached.