Postglacial environment reconstruction of the northwestern USA from the lacustrine record: Bells Lake, northern Idaho

Lakes acts as the ubiquitous substitutes of oceans by effectively imprinting the signatures of varying environments in their sediments. A multiproxy sedimentary record from Bells Lake (N Idaho, USA) has been investigated to ascribe the postglacial paleoenvironment of NW USA. It is a 5 m deep, 6.3 ha lateral lake situated on the floodplain of the St. Joe River. It receives its sedimentary input from the central part of the Rocky Mountains. Therefore, perfectly situated for archiving environmental variability related to alpine glacial variability and precipitation fluctuations in relation with millennial-scale latitudinal migrations of the Northern Hemisphere Westerlies (NHW) since the Last Glacial Maximum (LGM). We recovered a continuous 15 m core using a Livingstone corer at the centre of the lake for this study. The core stratigraphy consists of five major units ranging from black organic-rich clay towards the top and clayey sand-silt in the bottom units. The bottom of the core consists of stiff clayey sediments that prevented further penetration and were dated by radiocarbon to 15.2 ka. Therefore, it appears to represent sediments that shortly post-date the last Missoula Flood event. The whole record was framed by seven radiocarbon dates and three tephra isochrons. The record shows that during the early Holocene, an increase in detrital geochemical proxies (Al, K, Fe, and Ti) and a high sedimentation rate (3.72 mm/yr) point towards high terrestrial input in a warm and humid environment, probably inducing high productivity in the lake. These changes likely resulted from intensified weathering conditions and high surface runoff with the latitudinal migration of the NHW, which induced diminishing conditions of the continental alpine glaciers in the Rockies. The Younger Dryas (12.9-11.7 ka) is clearly recorded by several parameters, including paleo-redox proxies (e.g., Mn/Fe), weathering indices (Chemical Index of Alteration), Fe-S plot, a decrease in TOC, and an increase in clay content. This suggests oxic hypolimnion (due to lower lake levels or increased wind strength) and increased fine detrital input (possibly from glacial expansion). Occasional flooding might have been responsible for the deposition of fine sand layers at 11.6-11.2 ka. Following this episode, the 8.2 ka & 4.2 ka events of aridity were also well identified by the sudden drops in the detrital proxies and magnetic susceptibility values, probably pointing to reduced weathering conditions during a short return to a cold and arid phase; later was possibly due to the dramatic warming of North Pacific Ocean might be caused by increased solar irradiance or volcanism disrupting the SST gradient between tropical eastern and the western Pacific Ocean. A thick Mt. Mazama tephra (7.6 ka), a confounding event, is also capsuled in the record likely contributing to the rapid formation of long gun barrel levees that extended into Lake Coeur d'Alene (CDA). A major change in the limnological conditions appear to occur at 6.1 ka and is interpreted as the isolation of Bells Lake basin from the larger Lake CDA, currently occupying the lowlands in the west within the modern mean state Mediterranean type of climate system.