The use of oxygen isotope variation in shells of estuarine mollusks as a quantitative record of seasonal and annual Colorado River discharge

We describe a new method for the calculation of river flow that uses the oxygen isotope composition of bivalve mollusk shells that grew in the river-water/seawater mixing zone of the Colorado River estuary. Sclerochronological techniques are used to identify tidally-induced, fortnight-scale bundles of daily growth increments within shell cross-sections. These fortnightly markers are used to establish a chronology for samples taken for O analysis. A composite seasonal O profile derived from five shells that grew in the absence of river-water flow is used as a baseline against which profiles of river-influenced shells are compared. Because this comparison is between matched fortnights within a year, the temperature of shell growth is likely to be very similar. The difference in O between the river-influenced shell and the “no-flow” composite shell therefore represents the change in the O of the water due to the presence of river water in the mixing zone. The river water end-member is also determined within a fortnightly context so that the change in the O of mixing-zone water can be used to calculate the relative proportions of seawater and fresh-water. The fresh-water end-member is calculated from the O of bivalves alive prior to the emplacement of dams and water diversions on the Colorado River. The marine end-member is based on direct measurements of the O of northern Gulf of California water during times of no Colorado River flow. The system has been calibrated to absolute flow amounts using recent releases of known volume and rate. Copyright © 2004 Elsevier Ltd