PALEOSOLS, PALEOCLIMATE, AND IMPLICATIONS: USING QUATERNARY DATING METHODS TO DETERMINE RATES AND TIMING OF SOIL FORMATION AND LOESS DEPOSITION IN THE SNAKE RIVER PLAIN, IDAHO

The Loess-Paleosol sequences are preserved during the Late Quaternary period on the Snake River Plain. Loess-Paleosol sequences cover ~400 miles east-west extended Snake River Plain in Southern Idaho. The thickness of these Late Quaternary Loess-Paleosol sequences varied across the Snake River Plain. They preserved the glacial-interglacial cycles from the Late Pleistocene to the Holocene and provide evidence of past climate. The Late Pleistocene period is characterized by glacial outwash deposits and loess deposition on the eastern Snake River Plain whereas, on the western Snake River Plain, loess accumulated ~50 ka ago over 2 Ma old basalt.

The Kimberly site is a semi-arid dryland ecosystem on the boundary between the western and eastern Snake River Plain in south-central Idaho. Two major loess units separated by buried A-horizon have been identified in the Kimberly area. Loess locally is ~4.5 m thick at places farther away from the Snake River Canyon, soils become very thin to ~0.5 m towards the Canyon. Using optically stimulated luminescence (OSL), it is determined that loess (Unit-1) in this area was deposited between 7–20 ka, and paleosols (Unit-2) are 33–50 ka old. Unit-1 loess deposition occurred from Last Glacial Maximum (LGM) to the Holocene which corresponds to MIS-2 and MIS-1 respectively. Unit-2 represents loess deposition during the Late Pleistocene period which corresponds to MIS-3. The deposition rate for Unit-1 is 11 to 18 cm/ka and the deposition rate for Unit-2 (paleosol) is 6-7 cm/ka. Radiocarbon dating of carbonates from 4 soil profiles show that soils have been developed in the area between ca.10 ka cal BP and ca. 38 ka cal BP.

Stable isotopes of δ¹⁸O and δ¹³C of pedogenic carbonates have been used to determine paleoenvironmental conditions on the Snake River Plain. During MIS-2 (~15-18 ka), loess deposition in Kimberly is marked from 90 to 170 cm depth where a sharp increase can be seen in δ¹³C (peak of −0.66 ‰ at 15.46 ka) and δ¹⁸O (peak of −7.57‰ at 15.46 ka). The shift from the low δ¹⁸O to high δ¹⁸O could be aridity, the drier conditions cause more evaporation and increase the δ¹⁸O of pedogenic carbonates. Aridity will also increase the penetration of atmospheric CO₂ because low soil moisture results low respiration rate, so the diffusion of atmospheric CO₂ controls concentration of soil CO₂. Therefore, we can see a stronger atmospheric δ¹³C signal as enriched δ¹³C values.