We are analyzing stable isotope variation within dated marine mollusk shells to reconstruct the seasonal variation in flow of the Colorado River for the past 1,000 years. The accretionary growth of molluscan shells allows us to sample monthly increments from two years of growth within a single shell. Seasonal river discharge has been measured directly for less than 100 years, and has been affected dramatically by diversion and impoundment of river water since 1930.
We collect live and subfossil shells of the bivalve Chione fluctifraga from the Colorado Delta; subfossils are dated using a radiocarbon- calibrated, amino acid chronology. Within shell variation in 18O is a function of both variation in temperature and the mixing of marine and river water. Two modern live-collected shells range from +0.6 to -2.3 and -1.2 to -2.4 o/oo; this variation is a function of temperature variation because river water no longer reaches the delta.Two old shells, one collected in 1884 and the other dated at A.D. ~1275, range from -3.5 to - 5.7 and -0.03 to -5.8 o/oo, respectively. The greater variation of the old shells is a consequence of the influx of Colorado River water. Assuming a 4 o/oo seasonal variation in virgin Colorado River water, there is a four- fold seasonal variation in discharge recorded in the A.D. ~1275 shell. This is less than the average eight-fold difference measured directly between 1905 and 1915. Sampling methods probably cause us to underestimate the actual intra-shell variation in isotopic composition.
Analyses of shells back to A.D. 1000 will provide a long term record of the Colorado River's annual flow and its seasonal variation.
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