Funding is provided to develop a high-resolution (decadal or better), multi-proxy climate record spanning the past 4 millennia from the sediments of Zaca Lake, Santa Barbara County, California. The researchers use a suite of approaches including molecular isotopic analyses (i.e. D/H ratios in aquatic and terrestrial biomarkers), biological analyses (including charcoal counts), and physical sediment analyses (including grain size). The goals are to reconstruct the nature and frequency of climate variability (droughts and pluvials), flood event stratigraphy and changes in precipitation and catchment hydrology under changing climate. Correlation of the Zaca Lake record to regional and global climate records and forcings are used to explore the mechanisms driving these changes. In particular, the proximity of Zaca Lake to the Santa Barbara Basin enables comparison of this new high-resolution terrestrial record to a high-resolution marine record within 50km. The broader impacts include cross-institutional collaboration between University of Southern California and California State University at Fullerton (#5 in the nation for the number of bachelor's degrees awarded to minority students), support for a new female researcher, and the training of two graduate students and multiple undergraduate students at both institutions. Zaca Lake provides a rare opportunity in otherwise lake-poor Southern California to reconstruct the frequency and severity of past floods and droughts beyond the historical period, with findings of relevance to hazard and water management decisions in this water-stressed and highly populous region.
Intellectual Merits: Zaca Lake is a rare, natural lake in southern California, close to Santa Barbara. In 2009 we extracted a 9 meter sediment core from the lake. The age of the sediments was found to extend from the present back to 3,000 years based on radiocarbon dating of wood fragments in the sediments. We have reconstructed past climate change from the lake’s sediments by searching in the sediments for the waxy molecules derived from plant leaves. These molecules wash into the lake from the plants and soils and are well preserved in the sediments. We analyze the chemistry of these molecules to reconstruct past climate. Plants drink water from rainfall and that rain contains both regular hydrogen (H2O) and also more rarely, the heavier isotope, deuterium (HDO). The proportion of deuterium in the waxy molecules allows us to reconstruct that in rainfall when the plant was growing. Since the proportion of deuterium varies with climatic processes, such as storm tracks, we are able to reconstruct past wet or dry climate conditions, and the origins of the storms that brought the water. Isotopes in rainfall are useful tracers of weather and climate. In addition to the isotopic method, we also examined various properties of the sediment such as grain size, C:N ratios, and charcoal concentration. Of particular interest is how the proportion of sand in the lake sediment changes over time. The transportation of sand into a lake’s central basin requires a high-energy process such as strong run-off during wet winter seasons. Coupling sand with the leaf wax data, we observe evidence for periods of wet and dry intervals of varied length, in some cases lasting several centuries. This lake sediment core provides insights into the 3,000 year history of wet and dry events in California, allowing us to explore the droughts as well as the extreme wet conditions, each which are hazardous to society in this drought and flood prone semi-arid landscape. We find in the long record that there have been much larger wet and dry extremes than recorded in historical archives. Many of these wet or dry periods lasted for decades, longer than seen in the historical era. Wet intervals are associated with storm tracks originating from the North Pacific allowing us to pinpoint the drivers of floods and drought to conditions to the Pacific Ocean and to demonstrate the magnitude of excursions from what is considered ‘normal’ today. Broader Impacts: This research involved the efforts of faculty, graduate students and undergraduate students at the University of Southern California, and collaborative research with faculty and undergraduate students at the California State University at Fullerton. This project succeeded in supporting participation of a diversity of gender and ethnicities in geosciences research. Samples have been shared with researchers at other institutions on request and data have been archived at NOAA. Research findings have been published in the scientific literature, disseminated at scientific conferences and communicated to the public via the Santa Barbara Independent and the Zaca Lake Retreat.
