A 13,000 YEAR MULTI-PROXY CLIMATE RECORD FROM THE FISH LAKE PLATEAU OF UTAH WITH EMPHASIS ON THE CONDITIONS LEADING TO LARGE MASS MOVEMENTS

Younger Dryas to earliest Holocene mega-landslides (>10 km²) in east-central Utah required unusually wet conditions to drive movement of masses this large. The chaotic topography of landslides permits the recovery of sediment from small lakes, ponds, or fens from swales between hummocks. Such sediments preserve archives of past climate variation since landslide formation, as well as climate states at times of movement. An integrated geophysical, geochemical, and micro-paleontological investigation characterized the timing of mass movement and established the subsequent climate history of the region.

High resolution P-wave surveys of fen deposits ensured that core was obtained to the base of the wetlands. ¹⁴C ages of basal sediment from four cores showed two episodes of movement: 12.8 to 12.5 ka and ~10.5 ka. The earlier ages indicate that maximum effective precipitation states were reached during the Younger Dryas, driving mass wasting. Later, during early Holocene time, colder winters followed by warmer summers combined with vigorous monsoons drove movement as rapid spring snow melt was followed by wet summers.

These fens comprise excellent climate archives, where detailed climate proxy data were extracted from one core. Perpetually saturated subsurface conditions favor excellent pollen preservation. Diatoms record large variations in precipitation as the wetland switched from fen (intermittent standing water/wet soil) to pond (permanent standing water) states. This mode switching is revealed in the presence of abundant aerophilic diatoms during dry climate states, as well as diatoms that thrive only in standing water during wet episodes.

In detail, a wet episode (pond state) prevailed from 11.5 to 10 ka after which the climate became much drier (fen state) until 6 ka due to weakening of the North American Monsoon. Between 6 and 2.5 ka, volatile climate states appear as suggested by the presence of both aerophilic and standing water diatoms. After 2.5 to 2.0 ka, reduced insolation produced cooler summers and wet winters (pond state) consistent with neoglacial conditions observed elsewhere in western North America. In particular, diatoms are particularly good moisture proxies in features likely to have switched between aerophilic wet ground and standing water conditions in response to a dynamic climate.