Work will continue on assessing the quality of the biotic and environmental information that can be preserved in the fossil record. Research will focus on a field area that has proven an especially productive "natural laboratory": the northern Gulf of California. Paleoecological and geochemical approaches will be developed to: (1) Estimate changes in productivity. Changes in productivity will be estimated through the analysis of changes in growth rates and carbon isotope composition in bivalve mollusk shells. Shells dated from before the diminution of the Colorado River's flow (in about 1935) are expected to reflect the higher productivity of the northern Gulf of California that resulted from the river's input of nutrients. Such techniques may be applicable to other areas and to pre-Holocene coastal environments. (2) Establish guidelines for restoration of habitats altered by human activity. Diminution of the River's flow has altered the Delta's salinity, sediments, and fauna. The living shelly fauna will be compared to the shelly remains to document the faunal changes. One species, the bivalve Mulinia coloradoensis is now confined to a relict population in the River's mouth, whereas it formerly occurred in great abundance throughout the Delta. Analysis of oxygen isotopes from dated shells of this species may indicate the minimum influx of river water needed to restore populations of this and its associated species. This technique may be of use in other coastal habitats altered by human activity. (3) Reconstruct the paleohydrology of the Colorado River. Oxygen isotope variation will be used to document the variation in the discharge of the Colorado River during the past 2,000 years. Use of stable isotopes from dated Mulinia coloradoensis shells will permit an independent estimate of river discharge in close proximity to the river's mouth, where other species are rare. Techniques developed for estimating the Colorado's paleohydrology may be used in other settings to provide proxy estimates of virgin river flow during the late Holocene.
