USING SOIL EXTRACTABLE IRON TO INTERPRET THE GLACIAL HISTORY OF 4TH OF JULY CIRQUE IN THE FRONT RANGE OF THE COLORADO ROCKY MOUNTAINS

Determining the timing of glacial advance and retreat in the Front Range of Colorado is critical to understanding historical climate variability in the region. Although cosmogenic dating has grown in popularity as a tool for dating moraines, relative soil development can be a useful, low-cost tool in determining relative ages for glacial moraines. In addition, soil geomorphic studies provide additional information about post-glacial moraine erosion and alpine landscape evolution. 4^th of July Cirque, which lies in the Front Range of Colorado, contains two well preserved moraines in a small, south-facing valley. Soils were observed on both the LGM side-moraine (four profiles), and the younger end-moraine (five profiles). Samples were taken from each horizon in each profile for soil iron extraction.

Extractable iron in soils can be used to estimate the age of the soil by either A) determining the total amount of bound iron in the soil or B) comparing the amount of bound iron with the amount of unbound iron in a ratio. The first procedure (Feo) relies on an oxalate extraction solution by which only unbound iron is extracted. The second procedure (Fed) uses a citrate-dithionate extracting solution to remove both bound and unbound iron. Here, we present the results of both types of iron extraction: Feo and Fed. Specifically, we compare the total Fed results from the entire profile to the Feo/Fed ratio and interpret both for age. We also link our results to recent soil iron content studies from the San Juan Mountains of southern Colorado.