Tracking the western North American hydrologic dipole during the late Holocene using lake sediments from Abbott Lake, California.

摘要

A multi-proxy study on a sediment core from Abbott Lake (Monterey County, California) is used to reconstruct hydrologic variability over the past 2000 calendar years before present (cal yrs BP). Since Abbott Lake is one of only a few natural, permanent lakes between coastal southern and central California, it occupies an important geographical gap for evaluating the spatiotemporal patterns of hydrologic variability between the southern coastal (e.g. Lake Elsinore and Zaca Lake) and northwestern United States (e.g. Castor-Lime Lakes) sites. A variety of sedimentological analyses are used to infer past hydrologic conditions within the lake and its drainage basin including magnetic susceptibility, total organic matter, total carbonate, grain size, and microfossil identification. Additionally, delta18O and delta13C isotopic analyses were measured on Gyraulus parvus shells from twenty sample depths throughout the core. Six AMS 14C dates on charcoal provide age control. The results indicate that coastal central California was relatively dry (less run-off) during the Medieval Climatic Anomaly and wet (more runoff) during the latter part of the Little Ice Age. Hydrologic variability at Abbott Lake is attributed predominately to the Pacific Decadal Oscillation, a northern Pacific Ocean-atmosphere dynamic that controls the position of the North Pacific winter storm track. Influence of the El Nino Southern Oscillation (ENSO) is less robust at the study site when compared to ENSO paleoreconstructions. To constrain the spatiotemporal variation of the western North American hydrologic dipole, the Abbott Lake record was compared to the Castor-Lime Lakes record in eastern Washington and the Zaca Lake record 79 km northwest of Santa Barbara, California. This analysis indicates the hydrologic dipole was south of Abbott Lake between 34-36° N latitude from 1450 to 650 and 350 to 230 cal yrs BP. Thus, our data reveal a more southerly natural average position of the hydrologic dipole in the late Holocene than that observed in the modern record by Wise (2010). The recent northward shift observed by Wise (2010) from the hydrologic dipole's natural average position may be caused by a poleward shift in the westerlies and expansion of the tropical belt due to anthropogenic warming.