Holocene timberline and palaeoclimate of the northern Uinta Mountains, northeastern Utah, USA

Abstract

Clastic and organic sediments exposed in two stream cutbanks above modern timberline in the headwaters of the Henrys Fork drainage record multiple episodes of fluvial, lacustrine and wetland deposition. The location of the upper Henrys Fork at the boundary between modern summer-wet/winter-dry and summer dry/winter-wet precipitation regimes suggests that changes in vegetation during the Holocene were due primarily to variations in growing-season temperature. A radiocarbon date of 9310 6 70 BP on a Salix fragment from the base of one exposure indicates that the upper reaches of the Henrys Fork were vegetated by riparian willows by the early Holocene. Four other dates on wood and bulk organics ranging up to 4070 6 70 BP indicate that deposition continued through the middle Holocene. High Picea/Pinus ratios and high percentages of Artemisia pollen suggest that an open Picea parkland was established at timberline by 9.5 ka cal. BP, in response to mean annual and July temperatures ~1.0°C greater than at present. Continued warmth through the middle Holocene allowed Pinus to expand upwards into the spruce parkland by 7.5 ka cal. BP. A period of maximum warmth was reached between 6.5 and 5.4 ka cal. BP, and near-modern conditions prevailed over the final ~1000 years of the record (until 3.8 ka cal. BP).