Preliminary Paleoclimatic Implications from a Shallowing Kettle Pond: 12 Meters of Data from Cumbres Bog in the Southern San Juan Mountains of Colorado

The San Juan Mountains of southern Colorado may contain an important terrestrial Holocene climate record in an alpine area where landscapes may respond to climate change at millennial scales. We are examining paleoclimate-landscape interactions in the southern San Juan Mountains where we have mapped surficial features and extracted a 12 meter core from Cumbres Bog, which lies above 3,100. 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 five meters of the core were unconsolidated modern organic material including peat and where not recovered intact. 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 0.5 meters is made up of rhythmically bedded clay and coarse sands which are interpreted to be seasonal layers below which the corer was refused in (glacial?) gravels.

The bottom 7 meters of the core were digitized with a high resolution core scanner, described, and measured for magnetic susceptibility. Four radiocarbon dates were taken throughout the core and, while two dates are inverted, the results suggest that the basin began filling immediately after local deglaciation (~15 kya). Overall sedimentation rate appears to be relatively constant at ~0.4 m/ky although material below our lowest date likely accumulated at a higher rate. An increase in organic content towards the top of the core combined with decreasing magnetic susceptibility is consistent with a shallowing kettle pond. Future research will focus on paleoecological indicators in the core and their more detailed implications for past climates in the southern San Juan Mountains.