Climate projections for the coastal southwestern United States predict a change in the frequency and magnitude of precipitation, perhaps with an overall drying. Despite the uncertainty in model projections, it is clear that any change to the region's winter dominated, "feast or famine" hydroclimatology will have consequences for the highly-populous, water-stressed region. This proposal develops a decadal to subcentennial scale, continuous sedimentary archive from Lake Elsinore, California using physical (grain size), biological (pollen), chemical (C:N), and isotopic (leaf wax hydrogen isotopes) analyses. Characterized by exceptional resolution and archive continuity, this unique terrestrial site in the coastal southwest offers an unparalleled opportunity to evaluate hydrologic and ecologic change across abrupt climatic transitions during the late Glacial period (9-33ka). The cores will be characterized using a diverse, multi-proxy approach to capture large magnitude, late Glacial transitions, including leaf wax D/H shifts of >100 per mil, grain size evidence for variable run-off, and palynological evidence for changes in catchment vegetation. Lake Elsinore's exceptional archive is characterized by continuous deposition, high sedimentation rates, and the capacity for very high resolution radiocarbon age control using discrete organic material. Beyond representing a high quality archive of past terrestrial environmental change, Lake Elsinore sediments will enable proposed correlations with published marine reconstructions including local and distal marine records of Pacific sea surface temperatures (SSTs), as well as Atlantic SSTs and evidence for Meridional Overturning Circulation in order to assess the driving causes of the region's hydrologic and ecologic change. The team will focus on global abrupt climatic transitions including Heinrich events 1-3, and the transitions into and out of the Bolling-Allerod and the Younger Dryas. Although these transitions differ from the projected anthropogenic climate change, they offer large magnitude, abrupt transitions capable of providing maximum insights into forcing-response relationships. The working hypothesis is that hydrologic changes and ecologic shifts are closely in step and responding to local and global abrupt climatic transitions in the late Glacial to early Holocene. The questions are how much and how fast are these local responses demonstrated, and what might be the implications for the future. The lead investigator's institution, Cal State Fullerton, reaches a large number of minority, and especially Hispanic students. The proposal engages two female co-investigators, one of whom is an early career Assistant professor. The proposed study will provide training and research opportunities for graduate and undergraduate research at three institutions, with a focus on building the analytical and interpretative skill sets. This research will also provide the first terrestrially-based, decadal to subcentennial scale record of how the vegetation of the coastal southwest responds to abrupt climatic transitions. The development of a baseline understanding of the causes, responses, and recovery of past hydrologic and ecologic change is of interest beyond the paleoclimate field with implications for ecologists, conservationists, and policy decisions pertaining to the California Floristic Biodiversity Hotspot.
Intellectual Merits: Lake Elsinore is a large lowland lake in southern California, inland from the Los Angeles metropolitan area. A 30m sediment core recovered from the lake reaches an age of 33,000 years based on radiocarbon dating of wood fragments in the sediments. We have reconstructed past climate change from the lake’s sediments by analyzing 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 identify the origins of the storms that brought the water. Through comparison to collaborators proxy records of runoff amount (by grain size) we can also infer whether these storm track shifts were associated with wetter or drier conditions. This lake sediment core thus provides insights into the 33,000 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 shifts than recorded in historical archives. These events were associated with storm track shifts in part associated with the steering of winds by the Laurentide ice sheet over Canada during the last glacial. However, more relevant to today’s climate, changing ocean circulation and temperature patterns in the Pacific and Atlantic can create variations almost as large as the difference between glacial and non-glaciated climate for California’s water supply. While coincidence in time alone cannot identify causative linkages, cause and effect can be explicitly tested in climate models and thus we include comparison to model experiments to establish the ocean circulation and temperature processes responsible for the observed variability in the sediment core record. Broader Impacts: This research involved the efforts of faculty, graduate students and undergraduate students at the University of Southern California; collaborative research with faculty and undergraduate students at the California State University at Fullerton; collaborative research with faculty and staff at the Lamont Doherty Earth Observatory of Columbia University and international collaborative research with a researcher at the University of New South Wales, Australia. This project succeeded in supporting participation of women in geosciences research, with over 50% of the research team being female. Samples have been shared with researchers at other institutions including University of California Los Angeles and data have been archived at NOAA. Research findings have been disseminated at scientific conferences in the USA and Europe, published as a master’s thesis at the University of Southern California and published in the scientific literature. The PI has been active in communicating the long term climate history of the region to provide context for the present California drought in print, TV and radio, in particular reaching millions of people in the greater LA area through repeat interviews airing on local drive time radio and international audiences through the BBC.
