A 13 000 year multi-proxy climate record from central Utah (western USA), emphasizing conditions leading to large mass movements

Younger Dryas to earliest Holocene mega-landslides (> 10 km(2)) in the eastern Fish Lake Plateau of central Utah required unusually wet conditions to drive movement. The sediment from abundant small lakes, ponds and espe-cially fens that formed in swales between hummocks on the landslide surfaces are excellent archives of past cli-mate. An integrated geophysical, geochemical and micro-palaeontological investigation characterized fen deposits, determining the timing of mass movement and establishing the subsequent climate history of the region. High-resolution P-(compressional) wave surveys of fen deposits were conducted to image fen-landslide contacts. Past climate states were assessed through loss on ignition, pollen and diatom abundances. Diatoms, in particular, record large variations in precipitation as the present-day wetland switched from fen (intermittent standing water) to pond states in response to variable precipitation. One core was analysed for detailed climate proxies. Awet epi-sode (pond) prevailed from 11.5 to 10 ka after which the climate became much drier (fen) until 6 ka due to weak-ening of the North American Monsoon. After 2.5-2.0 ka, reduced insolation produced cooler summers and wet winters (pond). Only recently (< 500 years) has a fen re-emerged based on direct observation and the disappear-ance of diatoms that require standing water. C-14 ages of basal sediment from four cores show two episodes of movement: 12.8-12.5 and 10.5 ka. The earlier ages indicate that Younger Dryas high effective precipitation caused mass wasting. Later, during early Holocene times, colder winters followed by warmer summers and vigor-ous monsoons drove movement as rapid spring snow-melt was followed by wet summers. In broad terms, this work highlights variable climate conditions that can drive mass movement, as well as the sensitivity of diatom records in fens to effective precipitation.