A PALEOENVIRONMENTAL RECORD OF THE LATE PLEISTOCENE AND HOLOCENE INFERRED FROM ORGANIC CARBON ANALYSIS OF GLACIAL LAKES SEDIMENTS IN THE ALBION RANGE, IDAHO

Bottom sediments from lakes in the Albion Range of south-central Idaho provide a record of late Pleistocene to Holocene geomorphic and climatic change. During the late Pleistocene, cirque and small valley glaciers occupied valley heads surrounding Cache Pk. in the southern Albion Range and Mt. Harrison in the north. Moraines and stagnation topography formed during ice recession impound small lakes. We cored several of these lakes and analyzed cores from three; Independence Lakes I and II below Cache Pk. and Lake Cleveland below Mt. Harrison. Analysis involved stratigraphic description, identification of tephra layers, measurement of organic carbon content (OC), particle size, and magnetic susceptibility, and macrofossil AMS radiocarbon dating.

The three cores have very similar stratigraphy. Each bottomed in hard sediment that we interpret as glacial till or outwash. This layer is overlain by laminated, organic-deficient, silts, overlain in turn by massive, much more organic-rich, silts and clays. Within the upper organic-rich unit, OC is highest near the base and lower toward the top. The sequence is best dated in the Independence Lake I core, which is radiocarbon dated at 6 levels and contains two dated tephras. In that core an AMS age of 13020±50 14C yr. BP from the laminated silts provides a minimum age for deglaciation. The transition from basal silts to the overlying organic sediment began after 12820 ± 12 14C yr. BP and was completed by 9925 ± 40 14C yr. BP. This change probably reflects the final disappearance of ice from the drainages and/or an abrupt change to warmer conditions, with increased organic productivity and landscape stabilization by vegetation, at the beginning of the Holocene. OC remained very high until sometime after 8330 ± 60 14C yr. BP, then dropped significantly before deposition of Mazama Ash ca 6800 14C yr. BP. This drop likely marks the end of an early Holocene Altithermal period. Although glaciers did not reform, the change in OC in all three lakes suggests a mid-Holocene transition to cooler and/or wetter conditions, leading to increased periglacial activity and decreased organic productivity. At Lake Cleveland there is evidence of a further small drop in OC near the core top, possibly related to further cooling during the “Little Ice Age”. This change is not evident in the Independence Lakes cores.