Description
To better understand the timing of Lake Cahuilla’s six most recent lake highstands, I conducted a study to analyze the potential of using radiocarbon derived from freshwater mollusks to directly date the lakes. Conventional methods for dating lakes in the past, such as dating from charcoal, does not represent the timing of inundation, but instead represents the timing of dry periods between lakes. 27 Physa humerosa and 9 Anodonta dejecta shells were collected from a 3.0-meter-wide and 2.0-meter-deep excavation of an arroyo wall along the southern shoreline of Lake Cahuilla (Carrizo South Shoreline) and from a former collection site at Carrizo Wash located in Imperial County, California. These specimens, along with 59 others from previous Lake Cahuilla studies, were analyzed for radiocarbon and calibrated into calendar dates using OxCal software. The results were used to calculate 11 potential reservoir-corrections (?R) necessary to adjust lake dates to appropriate ages as determined by a recently developed Lake Cahuilla chronologic model by Rockwell. Two corrections, 379±32 years B.P. and 361±18 years B.P. were developed for Lake 1. A single correction of 247±44 years B.P. was determined for Lake 2. Two corrections, 640±95 years B.P. and 603±62 years B.P., were used for Lake 3. Two corrections, 192±16 years B.P. and 343±14 years B.P. were used for Lake 4. Two corrections, 246±41 years B.P. and 475±59 years B.P. were developed for Lake 5. Lastly, one correction of 376±10 years B.P. was calculated for Lake 6. From this data, I concluded that a reservoir-correction average was not adequate for adjusting shell dates for Lake Cahuilla. Also during the process, evidence showed that many of the shells were reworked, and that the ages of Lakes 4 and 5 at the Carrizo Wash site are not consistent with ages determined by other Lake Cahuilla sites. Lastly, three Lake Cahuilla highstand chronologic models were constructed using non-reworked Physa specimens. The models suggest that using lake-specific reservoir-corrections is the most accurate way to date Lake Cahuilla highstands, and that a global Lake Cahuilla correction (ΔRaverage = 410±134) is not likely to be accurate using the current dataset.
