www.acsprf.org

Our ACS-PRF grant spanned three years (Fall 2008 to Summer 2011) and consisted of three parts. Part I (Year 1) entailed the acquisition and interpretation of over 75km of seismic reflection data from Lake Elsinore. Part II (Year 2) used the seismic reflection data to determine a core site. Once determined based on sediment thickness and assumed continuity, a 21m sediment core was collected beginning 9m below the sediment-water interface and ending 30m below the sediment-water interface. Part III (Year 3) involved the analysis and interpretation of the sediment core. To date, this three year project has generated 7 abstracts presented at scientific meetings (AGU, ACS-WRM, PACLIM, GSA) and two papers in preparation. The project has involved three female students and produced 2 MSc and 1 BSc theses. Our major results to date include several interesting observations detailed below.

First, the seismic reflection data reveal a significant lake level lowstand between ~1,800 and ~3,000 calendar years before present (cy BP). Independently dated sediment archives from several lakes in the region support this lowstand (i.e., extended drought) interpretation. What remains unknown is the reason for this period of sustained drought; however, future research intends to investigate this question.

Second, radiocarbon dates from the Lake Elsinore sediment core indicate that the core bottom at 30m is ~33kyrs. This result, in the context of all 21 radiocarbon dates, implies continuous and rapid sedimentation over the period 9,000 (9m) to 33,000 (30m) cy BP. Both continuity of the sediment record and high sedimentation rates are critical for developing a high-resolution paleoclimatic archive.

Third, various sedimentological analyses of the core sediments reveal dramatic climatic variability during the late-Glacial to Holocene in Southern California (9-33kyrs). Importantly, the new Lake Elsinore core represents the first terrestrial record from the coastal southwest United States to detail the late-Glacial to Holocene interval. This interval represents a period of dramatic, global climatic change. Our results indicate a three-tier decrease in climate wetness from 18-9kyrs. The three tiers are divided by rapid step-decreases in percent sand - a proxy for runoff and thus climate wetness. These rapid sand decreases occur at centennial-to-subcentennial scales illustrating the abruptness of past climatic change in the coastal southwest United States.

Fourth, the abrupt sand decreases between 18-9kyrs are correlative to hemispheric temperature records from the Greenland ice cores as well as to various sea surface temperature records from both the tropical and extra-tropical Pacific and Atlantic basins. This apparent correlativity indicates that the climate of the coastal southwest United States was responding to the same forcings driving hemispheric abrupt climatic change during the late-Glacial to Holocene. Of particular interest, we observe an interesting, but complex, response in our study region to abrupt changes in North Atlantic meridional overturning circulation (MOC). This observation is relevant to Earth’s modern climate that, with continued warming, could cause changes to MOC dynamics. Understanding past relationships can shed light on the dynamics of future potential relationships, especially in the water-poor, overpopulated coastal southwest.

Fifth, Lake Elsinore, in general, was wet during the last Glacial. At times, the lake environment favored the formation and preservation of laminae indicating a probable deep, anoxic basin. During peak glacial conditions, organic matter accumulation exceeded 10 to 20% at times. This observation suggests that pull-apart basins, such as Lake Elsinore, with thick sediment packages that span several glacial-to-interglacial cycles could be significant repositories for sequestered organic carbon and thus hydrocarbon formation. Gravimetric analysis indicates that the Lake Elsinore basin is up to 1000m in depth. At glacial sedimentation rates, the basin could contain over 1.4 million years of sediment and its sequestered organic carbon. These results suggest that small pull-apart basins such as Lake Elsinore could represent viable hydrocarbon sources in the geological record.

Finally, there remains much to be done. ACS funding for this project represents critical initial results. Using these results, and soon to be published data, myself and two colleagues submitted a NSF grant to continue our investigation of the new core using a variety of physical, chemical, and biological analyses. Without the initial work funded by ACS, the Lake Elsinore project would not exist.