PRECIPITATION AND ENSO RECORDS (1959-2004) BASED ON SHELLS OF FRESHWATER BIVALVE MOLLUSKS (MARGARITIFERA FALCATA) FROM BRITISH COLUMBIA

Precipitation patterns and watershed dynamics in the Pacific Northwest are strongly influenced by climate oscillations including the El Niño-Southern Oscillation (ENSO). Therefore, long-term, high-resolution records of this climate variability are needed to predict future trends. We analyzed variations in annual shell growth and stable isotopes of sixteen live-collected and five dead-collected specimens of the freshwater bivalve, Margaritifera falcata, from an agricultural, suburban setting near Vancouver, BC. The individual series were combined into a single master chronology covering the period of 1959 to 2004. Shell oxygen isotopes imply that local precipitation is the main source for recharging the aquifer (Coghlan Creek). The oxygen isotope range approximates the temperature range during the growing season (April-October) when the water δ¹⁸O composition remained constant. However, shell growth is strongly controlled by local summer precipitation and presumably runoff of nutrient-rich stormwater. Local summer rainfall (June-September) accounts for 44% of the variability in annual shell growth, while winter precipitation and the strength of ENSO during the preceding year have a significant, if weaker, effect. In combination, summer and winter precipitation can explain up to 50% of the variability in annual shell growth. The effect of precipitation and ENSO was further substantiated by spectral analyses. Common spectral density was found at periods of 6.5 to 9 years, particularly between 1985 and 2004. Higher frequency oscillation corresponding to periods of three to five years occurred during the early 1970s, early to mid 1980s and later 1990s. These results suggest that skeletal records of bivalve mollusks provide ideal archives of precipitation and ENSO in areas where other climate proxies such as tree-rings and speleothems may not be available.