Holocene environmental changes in the alpine zone,northern San Juan Mountains,Colorado: Evidence from bog stratigraphy and palynology

Cores from five high alpine basins in the northern San Juan Mountains show several fluctuations in lithology. Typically, peats are interbedded with coarser clastic sediments or else woody peats alternate with fibrous peat. Twenty ¹⁴C dates provide radiometric control. Sediment rates averaged about 2.5 cm/100 yr but varied at the different sites between 1.19 and 50 cm/100 yr. Rates were lower during the middle of the Holocene. Basal radiocarbon dates indicate that these high (ca. 3600 m a.s.l.) northeasterly facing cirques were icefree by 9000 BP. There is some evidence in the cores for a short climatic reversal sometime between 8000 and 7000 BP. A major change occurred in the high basins very close to 5000 BP and thereafter there are several intervals of increased clastic sedimentation which may be related to Neoglacial climatic fluctuations. Analysis of a 2.15 m core near Hurricane Basin indicates significant fluctuation of pollen and macrofossils occurred during the 9000 ± year record. The Picea/Pinus ratios are used to delimit changes in the apparent elevation of the site: the ratios indicate that a short drop of “treeline” occurred about 8000 BP and then remained near present level until about ≥1800 BP when the apparent elevation of the site rose. Macrofossils indicate that spruce was present in the Hurricane Basin (and others) at specific periods and confirms the general results of the Picea/Pinus ratios. The San Juan Mountains do not possess a glacial Neoglacial record but the stratigraphy of these high cirque basins can be used to define glacial stades (cf. Jardine, 1972). The interpreted climatic response record on vegetation and sediment flux has both similarities and differences from other records in the western mountains of North America.