OCR Text |
Show Cupressaceae ( cold climate) from 3580- 3320 Ka ( 1576- 1454 m). In the South Rozel well, the same transition occurs between 3790- 3580 Ka, ( 2200 - 2100 m). This event corresponds to the marine record for cooling and increased ice volume 3.8 - 3.6 Ma ( Tiedmann et al, 1994). During the 2.6 Ma event, the pollen diagrams from the East Gunnison and Indian Cove wells show reduced percentages of Chenopodiaceae-> A/ 7? aranf/ 7i/ s, and increased Pinus, Artemisia, Abies, and Cupressaceae ( Fig. 1). However, the effect on the upland taxa is less pronounced than during the 3.7 or 0.75 Ka events. In all Great Salt wells investigated ( Moutoux & Davis, 1995; Moutoux, 1995; Moutoux et al, in press), the 2.6 Ma event coincides with the beginning higher water levels ( high percentages of wetland taxa - Cyperaceae, Typha, etc.). In the marine record, this event marks the beginning of the Pleistocene, with increasing 5 180 values and a greater amplitude of glacial - interglacial fluctuations ( Tiedmann et al, 1994). The 0.75 Ma event is the most distinctive transition of the Great Salt Lake pollen record ( Moutoux & Davis, 1995; Moutoux, 1995; Moutoux et al, in press). Prior to 750 Ka, Chenopodiaceae-. A/ 77ara/ 7f/ 7L/ s pollen exceeds 50% of the pollen assemblage; thereafter, Artemisia + Pinus predominate, Cupressaceae and Other Compositae increase, but Gramineae decreases ( Fig. 1). This event appears to be a general downward displacement of vegetation zones by ca. 500 m, so that halophytic steppe ( with high Chenopodiaceae-/\ A77ara/ 7# 7i/ s %) is replaced by sagebrush steppe in the intermountain basins. The climate reconstruction for Great Salt Lake wells indicates maximum temperature and minimum moisture during the Pliocene, with cooling and increasing moisture thereafter. This trend differs from that of earlier GSL publications ( Moutoux & Davis, 1995; Moutoux, 1995) due to the incorporation of tectonic subsidence in the climate estimates. The pronounced temperature peak ( 5 ° C greater than contemporary value) at 3.8 Ma coincides with the Chenopodiaceae- y4mara/ 7# 7i7S maximum, preceding the 3.7 Ma event ( Fig. 2). It should be considered tentative because the climate reconstruction for that time includes only the Bridge Well. The peak is followed by climatic oscillations, the last " warm" one ( ca. 3.0 Ma) contemporaneous with global warmth reflected in Q_ Precip. ^ V J* E T^>*^ V^_^/ ^ ' ' ~^ X-/ V H 10 ~ Great Salt Lake, Amoco Pollen Spls 5000 4000 1000 3000 2000 AGE ( Ka) Figure 2. Climate reconstruction for Great Salt Lake Wells. j- 700 - 600^ - 500^ - 400 d - 300 " o - P < D A200 £ L100 |