SALINITY TOLERANCE OF THE FRESHWATER MUSSEL ANODONTA DEJECTA LEWIS IN HOLOCENE LAKE CAHUILLA, SOUTHEASTERN CALIFORNIA: A CAUTION IN THE USE OF FOSSIL FRESHWATER MUSSELS AS A FRESHWATER INDICATOR IN STABLE ISOTOPE STUDIES

Fossil freshwater mussels are interpreted to be from fluvial environments in mixed nearshore marine - non-marine settings and can be a convenient freshwater endpoint source in stable isotope studies. However, their salinity tolerance can be fairly broad and an apparently fluvial environment may be brackish-water estuarine. An example this is demonstrated by the salinity tolerance of Anodonta dejecta Lewis from Holocene Lake Cahuilla, southeastern California. The Colorado River filled Lake Cahuilla north of its delta to a highstand 12 m above sea level and depth of 97 m as recently as ~1500 CE. When the Colorado River diverted to the Gulf of California, Lake Cahuilla evaporated over an estimated 60 yr period and progressively increased salinity as the lake level declined. I constructed a paleosalinity model for Lake Cahuilla from published salinity data from the Colorado River and Salton Sea and published interpreted paleosalinities of Lake Cahuilla deposits. Paleosalinity was 0.7 ppt at highstand, increased slowly to 6 ppt at -55 m (55 m below highstand), then increased rapidly to >35 ppt at -85 m (modern Salton Sea surface level). Freshwater mollusc faunas were collected from Lake Cahuilla deposits at 18 localities representing levels from its highstand shoreline to -67 m. Anodonta dejecta was found as articulate valves in life position at 11 of 12 localities -55 m and higher with only reworked fragments found at 6 localities -58 m and lower. Members of the genus Anodonta are found living in lakes at maximum depths of 2-8 m. Because no in situ specimens of Anodonta dejecta were found at localities -58 m and lower, the salinity of Lake Cahuilla must have become lethal to it at a lake surface level above -56 m if it lived in 2 m of water. Using the -55 m locality as the lowest level Anodonta dejecta lived in Lake Cahuilla, lake surface levels would have been -47 m if it lived at 8 m depth and -53 m if it lived at 2 m depth. The paleosalinity range corresponding to these lake surface levels is 4.8-5.9 ppt. Consequently, the maximum salinity tolerance of Anodonta dejecta in Lake Cahuilla overlaps the minimum salinity tolerance of many estuarine bivalve genera. Therefore, as demonstrated here, fossil freshwater mussels may be an unreliable freshwater indicator and their use as a source of stable isotopes representing a freshwater endpoint should be approached with some caution.