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Recrystallization of Biogenic Aragonite Shells from Archaeological Contexts and Implications for Paleoenvironmental Reconstruction
Date Permissions Signed
Date of Award
Master of Arts (MA)
Campbell, Sarah K.
Koetje, Todd A.
Archaeologists do not consistently test the mineralogy of archaeological bivalve shells before conducting isotope analysis for paleoenvironmental reconstruction. This is a problem because if biogenic aragonite is heated, or cooked, it can recrystallize into calcite, and can become depleted of its heavy oxygen and carbon isotopes. Oxygen isotope ratios are recorded in the growth rings of bivalve shells as they grow, and reflect the temperature conditions of the ambient ocean water. Thus, ancient bivalve shells are used to reconstruct paleoenvironments. I have conducted an experiment to demonstrate the importance of testing the mineralogy of archaeological shells before isotope analysis. I collected modern specimens of four different bivalve taxa and separated them into left and right valves. I heated the left valves in an oven at three controlled temperatures, and I retained the right valves as unheated controls. I conducted mineralogy and isotope analysis on all valves and compared the left and rights of each individual. I found that valves heated to about 400˚C for one hour had recrystallized into calcite and had significantly altered oxygen isotope ratios. This happened before the shell appeared charred. I also conducted mineralogy testing on seven archaeological shell fragments from the Tse-Whit-Zen Village Site (45CA523). Of the seven archaeological specimens, one was entirely calcite, and two had traces of calcite in them. The two fragments containing minor amounts of calcite did not show any visible signs that they were altered or otherwise different from fragments that were entirely aragonite. It is unlikely that archaeologists will know if shells are heated unless mineralogy testing is conducted.
Western Washington University
Subject – LCSH
Bivalves, Fossil--Effect of heat on; Kitchen-middens--Effect of heat on; Calcite crystals; Mineralogy in archaeology; Isotope dilution analysis; Archaeology--Methodology
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Larsen, Susan C., "Recrystallization of Biogenic Aragonite Shells from Archaeological Contexts and Implications for Paleoenvironmental Reconstruction" (2015). WWU Graduate School Collection. 419.