MOLECULAR ISOTOPIC AND BIOCHEMICAL-EVIDENCE OF THE ORIGIN AND DIAGENESIS OF SHELL ORGANIC MATERIAL

Molecular data from shell matrix proteins provide new possibilities fo r delineating biomolecular changes in information-rich fossils. Protei n separations for ancient and modern Polinices duplicatus and Mercenar ia campechiensis suggest degradation of the hydrophilic fraction and a n increase in the charge of the hydrophobic fraction with time. A comp arison of aspartic acid concentrations between the fossil and modern s hells indicates a decrease (17.8 nmol/mg) in the hydrophilic fraction and an increase (0.5 nmol/mg) in the hydrophobic fraction. Considerati on of this trend and the observation that delta(13)C values of amino a cids from the hydrophobic fraction of the modern shell are higher than those of the fossil may indicate that some of the C-13-depleted mater ial from the hydrophilic fraction was reincorporated into the hydropho bic fraction during diagenesis. Loss of isotopically depleted products from the modern hydrophobic fraction is also likely to have taken pla ce. Large differences (8 parts per thousand or more) in delta(13)C val ues of corresponding amino acids between hydrophobic and hydrophilic f ractions of the modern shell likely reflect variations in the extent o f fractionation within unique pathways of synthesis. The ability to ad dress longstanding questions regarding the history and origins of anci ent organic material from fossils will require new or unique combinati ons of approaches such as those presented here.