THE IMPORTANCE OF CHRONOLOGY IN DISCERNING PATTERNS AND DRIVERS OF PAST CLIMATE CHANGE IN THE WESTERN US

In the search for driving mechanisms of past climate change, spatial patterns are often compared to those of modern observational patterns such as ENSO or the Pacific Decadal Oscillation. However, in order to accurately assess spatial patterns in paleorecords, the chronological control for each record must be considered. For example, Mensing and colleagues (2013; QSR v78 266-282) identified an extreme drought in Stonehouse Meadow (Nevada) between 2800 and 1850 years ago. In that record, the drought was characterized by a change in dominance from arboreal to grass pollen, a decrease in total organic matter, and an increase in preservation of spring snails. Mensing et al. also compiled other records from across the central Great Basin that display a period of aridity at about the same time, and contrasted that pattern with neutral or wetter conditions in coeval records from the northern Great Basin. They noted that this wet northwest/dry southwest pattern is similar to the El Niño dipole pattern, related to large-scale North Pacific circulation patterns. Here we examine the methods and density of dating for robust records of aridity in the Great Basin roughly corresponding to the same period, and assign Bayesian uncertainty envelopes to the corresponding proxy records. We test whether the current chronological control on the records is sufficient to distinguish between a north-south or south-north progression of the drought, a single regional drought, or multiple periods of drought affecting smaller areas, unconnected to a single driving mechanism.