CLIMATE HISTORY OF THE PAST 140,000 YEARS IN THE NORTHERN ROCKY MOUNTAINS BASED ON LOESS DEPOSITION AND SOIL DEVELOPMENT IN SOUTHERN JACKSON HOLE, WYOMING

Although loess is extensive in North America, exposures that give detailed climate records of the last interglacial-glacial cycle are rare. We excavated a stratigraphic section of 8 alternating intervals of loess deposition and soil development on an outwash terrace above the Snake River, in southern Jackson Hole, Wyoming. The terrace is inside terminal moraines of the greater Yellowstone glacial system that correlate with the ~140 ka Bull Lake glaciation. Our chronology is based on 6 stratigraphically concordant TL ages, Be-10 profiles, soil development, and correlation to dated glacial events. We interpret the following sequence of 8 events, from oldest to youngest (dating uncertainty ~10-20%). (1, 2, 3) Three intervals, from ~140-75 ka, of alternating loess deposition (~0.4 m each) and soils with Bt horizons but little carbonate additions; the most pronounced weathering took place in the oldest soil (1). (4) Deposition of 1 m of sandy loess ~75-65 ka, followed by development of mollic A and Bt horizons but no Bk. (5) Minor loess and soil formation ~55 ka. (6) Deposition of 2.2 m of loess ~55-48 ka, followed by soil development (Bt/Bk horizons). (7) Deposition of 1 m of loess ~40-35 ka, followed by weak soil development (no Bk). (8) Deposition of 2.5 m of loess ~27-12 ka followed by development of the modern soil (A/Bt/Bk). This record suggests the following climate history and correlation with marine isotope stages (MIS). Deposition of outwash and thin loess took place during a cold period (MIS 6), followed by a complex interglacial period (MIS 5) with both loess deposition (MIS 5d and 5b) and soil development (MIS 5e, 5c, 5a) but soil formation under conditions more humid than present. Loess deposition occurred during a cold period equivalent to MIS 4, and was followed by soil development under a relatively humid climate. During MIS 3, major and minor loess deposition occurred, each followed by soil development; the earlier period of soil development (Bk) occurred under semiarid conditions, and the later one did not. The cold, last-glacial period (MIS 2) saw loess deposition, followed by Holocene soil development under a semiarid climate (MIS 1). This area of the northern Rocky Mountains has alternated between both warm (interglacial/interstadial) and cold (glacial/stadial) climates; warmer climates have alternated between humid and semiarid.