ABSTRACT: Stable isotopes in clam shells can recronstruct the salinity of the Late Holocene Colorado Delta.

Rodriguez, Carlie A.
University of Arizona, Tucson.

26th Annual GeoDaze Geoscience Symposium, Tucson, AZ.

The Colorado River is the most important source of water for the arid southwest but the entire flow of the river is now diverted and dammed for human uses before it can reach the delta. This has changed the environments and biotas of the delta. One organism that was affected by the total diversion of the river water is the bivalve mollusk Mulinia coloradoensis. Mulinia coloradoensis was once an extremely abundant mollusk that lived in the Colorado River Delta area. It was so abundant that the delta's beaches are made up of its shells. However, now it has declined, possibly to the point of extinction. It is likely that M. coloradoensis was tolerant only of the lower salinity conditions created by river water reaching the sea. Now that freshwater no longer reaches the Delta, this area may not be suitable for this mollusk to survive. Understanding the paleoecology of this species is a key to understanding the environments of the Delta before the impact of human activity.

I conducted isotopic analyses on shells of M. coloradoensis and Chione fluctifraga, a species that lives in the Delta today. Each shell was sampled for its d18O value and its d13C value. I sampled various areas of the Delta to determine if there is a difference in the isotopic signature of shells of both these species from the mouth of the river and those sampled farther from the mouth. Since freshwater is isotopically lighter than seawater shells sampled closer to the mouth of the river should have a lighter isotopic signature than those sampled farther from the river mouth. In addition, shells from older mollusks (Mulinia coloradoensis), that lived when the Colorado River still reached the Delta should show a much lighter isotopic signature than shells of the mollusks that lived when the area was no longer influenced by the river water (Chione fluctifraga). Preliminary results show that this may be the case. d18O and d13C values from shells of Mulinia coloradoensis tend to have lighter values than shells of Chione fluctifraga. Furthermore, shells sampled closer to the mouth of the river show lighter values than shells sampled from farther away from the mouth.

Determining the salinity tolerance of Mulinia coloradoensis will help to determine if the change in salinity of the delta area due to the lack of incoming freshwater contributed to the major decline of this mollusk. Did the change in salinity in the Delta cause the extinction of Mulinia coloradoensis? Isotopes will tell.

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