Isotopic evidence for Holocene climate change in the northern Rockies from a goethite-rich ferricrete chronosequence

Climate proxy records often show regional heterogeneity due to the complex interplay of large-scale atmospheric circulation patterns and local topographic features. This is especially prevalent in the mountainous regions of the western United States where the interaction of two seasonally distinct weather systems can result in locally abrupt climate boundaries. Here we establish oxygen isotope ratios of goethite from ferricrete chronosequences formed by natural acid rock drainage as a new source of spatially small-scale Holocene climate information. Systematic study of these deposits may prove useful in understanding regional variations in response to global Holocene climate change. isotope ratios of goethites from the mountainous northeastern Yellowstone region show an increase of ∼3‰ over the past 9000 years. We attribute this isotopic change to a local increase of isotopically heavier summer precipitation since the early Holocene, possibly augmented by seasonal temperature differences brought on by orbitally forced insolation changes over the past 9000 years. Our interpretation is compatible with palynology studies as well as Holocene climate simulations, which indicate a decrease in monsoon intensity from 9000 years B.P. to the present. Unlike the southwestern United States, this change may have resulted in wetter summers in our study area.