Freshwater Bivalve Morphometrics as a Means of Investigating Hydrological Conditions During the Hypsithermal Climatic Interval

Mississippi State University, Mississippi State MS

Investigators

Abstract

With National Science Foundation support, Dr. Evan Peacock will undertake a pilot study aimed at developing a new method for reconstructing past river conditions. The method involves measuring changes in the size and shape of freshwater mussel shells over time. Freshwater mussels display a considerable amount of variation in shell characteristics. Upstream forms are typically small and have pronounced shell sculpture in the form of ridges, knobs, pustules, and other traits. The same species become larger and less sculptured with downstream distance. This is a direct response to environmental conditions, as larger size and a smoother shell allow better anchoring in stronger currents. Presumably, significant changes in climate should have altered stream conditions in the past. Those changed conditions should be reflected in changes in mussel shells from archaeological sites. The Hypsithermal climatic interval was a warm, dry period that lasted from approximately 7,000 to 5,000 years ago. Its effects on prehistoric cultures in the southeastern United States are currently poorly understood. Some researchers believe that streams were lower due to decreased precipitation, meaning that riverine resources such as mussel beds became more available for exploitation. Other researchers believe that dryness led to decreased vegetation on uplands, leading to increased erosion that smothered mussel beds and covered gravel beds from which rock for stone tools had been taken. To investigate the effect of the Hypsithermal on Southeastern rivers, Dr. Peacock will examine mussel shells from the Vaughn Mound, a prehistoric site on the Tombigbee River in eastern Mississippi. A Middle Archaic stratum has yielded shell from the Hypsithermal period, while a Late Woodland stratum provided shell from about 1,200 years ago. Valves from the two distinct cultural layers will be compared in terms of size and sculpture. If the river was significantly lower during the Hypsithermal, the shells should be smaller and more sculptured than during the Late Woodland period when essentially modern climatic conditions prevailed. As an additional test of past hydrological conditions, shell chemical analysis will be conducted using an inductively coupled plasma mass spectrometer. With increased stream flow, elements such as calcium and strontium become diluted in waterways: such changes are discernible in shells, as mussels uptake elements in proportion to what is in the waterways. If this pilot study is successful, it will establish a method that can be used worldwide to investigate past climatic effects on the landscape. Many collections exist that could be analyzed in the manner described. More accurate assessments of past climatic effects will have direct implications for better understanding past settlement patterns, subsistence strategies, raw material procurement, and a host of other issues. New methods such this will help in mapping out the long-term effects of climate change on culture.