Tropical Pacific Forcing of Late-Holocene Hydrologic Variability in the Coastal Southwest United States

Abstract:

Change in water availability is of great concern in the coastal southwest United States (_CSWUS). Reconstructing the history of water pre–1800 AD requires the use of proxy data. Lakes provide long-lived, high-resolution terrestrial archives of past hydrologic change, and their sediments contain a variety of proxies. This study presents geochemical, sedimentological, and biological data from Zaca Lake, CA (Santa Barbara County) used to reconstruct a 3000 year history of winter season moisture source (dD_wax) and catchment run-off (125–2000 mm sand) at decadal resolution. Vegetative response to hydrologic change is also investigated using pollen. Here we show that winter season moisture source and run-off are highly variable over the past 3000 years; superimposed are regime shifts between wetter or drier conditions that persist on average over multiple centuries. Moisture source and run-off do not consistently covary indicating multiple atmospheric circulation modes where wetter/drier conditions prevail. Grain-size analysis reveals two intervals of multi-century drought with less run-off that pre-date the “epic droughts” as identified by Cook et al. (2004). A well-defined wet period with more run-off is identified during the Little Ice Age. Notably, the grain size data show strong coherence with western North American percent drought area indices for the past 1000 years. As a result, our data extend the history of drought and pluvials back to 3000 calendar years BP in the _CSWUS. Comparison to tropical Pacific proxies confirms the long-term relationship between El Niño and enhanced run-off in the _CSWUS. Our results demonstrate the long-term importance of the tropical Pacific to the _CSWUS winter season hydroclimate.