PALEOCLIMATIC IMPLICATIONS FOR TOTAL ORGANIC CONTENT AND PARTICLE SIZE FROM A 12 METER BOG CORE FROM CUMBRES BOG, SOUTHERN COLORADO

The San Juan Mountains of southern Colorado may contain an important terrestrial Holocene climate record in an alpine area where landscapes tend to be responsive to climate change at millennial scales. We are examining paleoclimate-landscape interactions in the Conejos River Valley in southern Colorado where we have mapped surficial features and extracted a 12 meter core from Cumbres Bog, which lies above 3,100 meters along the Colorado – New Mexico border. The bog appears to be a stagnant ice feature created during the retreat of the San Juan Ice Cap at the end of the Last Glacial Maximum. The top three meters of the core consist of unconsolidated modern organic materials including gyttja and algae. Very low density and extreme saturation prevented the recovery of this material. The fourth and fifth meters consist of moderately consolidated fibrous organics which were recovered and placed in bags due to their poor consolidation. Sediment from 6 to 9.5 meters depth is characterized by fine grained, fibrous organics that alternate with silt- and clay-dominated units. Sediment from 9.5 to 11.5 meters depth is finely laminated mud with varying amounts of clay and sand. The bottom .5 meters is made up of rhythmically bedded clay and coarse sands which are interpreted to be varves. Below the varves, the corer was refused in what was thought to be glacial gravels.

The bottom 7 meters of the core were digitized with a high resolution core scanner, described, and measured for magnetic susceptibility. The core was also sampled for C 14 dating and sub-sampled for other analyses. Here we focus on our initial analysis of organic content and grain size. Total organic content was examined by loss on ignition (LOI) and grain size was determined using a laser particle counter (LPC) and pipette methods. Initial results indicate that organic content increases through time while particle size fines. The overall trend is consistent with a shallowing kettle pond, however laminated muds and rhythmic bedding indicate possible variability in climate over short time scales.