PRELIMINARY PALEOCLIMATE IMPLICATIONS FROM A 7 METER SUB-ALPINE BOG CORE IN SOUTHERN COLORADO, USA

High resolution terrestrial climate records from alpine areas can provide critical information about past regional climates of the Holocene. Here we present multiple proxies from a core extracted from Cumbres Bog, located above 3,100 meters in the southern San Juan Mountains of southern Colorado. The surface of the bog consists of 4 meters of peat which floats ~1 meter above the modern sedimentary surface at the bottom of the bog. The core contains approximately 7 meters of sediment. The sediment consists of dark green organic rich, laminated silts and clays grading into massive grey and pink sandy clays. A radiocarbon date (18,303 cal yr) sampled ~50 cm above the gravel basal layer, implies that standing water likely first appeared at Cumbres Pass immediately after deglaciation at the terminus.

We examined multiple core properties commonly used as proxies in paleoclimate reconstruction. Pollen samples were extracted and examined at 10 cm intervals. Additionally, the core was scanned and magnetic susceptibility and density was measured at 0.5 cm intervals for its entire length. The core was sampled at 2 cm increments for particle size examination with a laser particle counter and organic content testing using loss on ignition. Finally, eight samples for radiocarbon dating were taken from the core resulting an age model.

Radiocarbon ages are roughly linear with depth and range from 1,437 cal yr BP 68 cm from the top to the basal dates. Preliminary examination using the constructed age model shows that the rapid change from clastic dominated sediment to organic dominated sediment (40% to 70%) occurred ~ 11,500 cal yr BP. This change in sediment type corresponds with a significant change in observed pollen abundances and coincides with the end of the Younger Dryas chron. Magnetic susceptibility, which varies between -5 and 5 SI units in the Holocene, increases with depth to more than 200 in the Late Pleistocene. Pollen data show significant climate variability during the late Pleistocene and Holocene. A significant cooling is observed on the vegetation coinciding with the time and duration of the YD. Several changes on the pollen record are also observed during the Holocene, which could be related to millennial-scale climate variability. Future research will focus on the impact of climate change on landscape evolution in alpine areas.