Amino acid racemization in lacustrine ostracodes, part I: Effect of oxidizing pre-treatments on amino acid composition

Bleaching biominerals reduces the concentration of amino acids to a residual fraction which is thought to be preserved within intra-crystalline proteins. Intra-crystalline amino acids represent a relatively closed-system and may be superior for amino acid geochronological and paleothermometry investigations. Here we quantify the effects of extended bleaching (to 144 h) on amino acid concentrations and d/l values in the valves of seven modern ostracode species. Then we compare the effects of increasingly aggressive pre-treatments (sonication, exposure to 3% hydrogen peroxide for 2 h, and exposure to 10% bleach for 48 h) on amino acid concentrations and d/l values in fossil ostracode (Candona) valves from numerous locations throughout the western United States and northern Mexico. Bleaching modern ostracode valves results in a rapid order-of-magnitude reduction in amino acid concentrations and a slight increase in d/l values. Amino acid concentrations in five of the seven modern species increased slightly after prolonged exposure to bleach, suggesting that bleaching weakens a resistant protein source within the ostracode valves. Increasingly aggressive pre-treatments had virtually no effect on amino acid concentrations, d/l values, or inter-sample variability in fossil Candona valves older than about 1 ka. Candona valves younger than about 1 ka showed a progressive decrease in amino acid concentrations with more aggressive oxidizing pre-treatments. Valves older than 1 ka generally had amino acid concentrations < 10 nM mg^-1. A delayed increase in amino acid concentrations (15-30 nM mg^-1), followed by a gradual decrease to residual concentrations consistently < 10 nM mg^-1, was observed in both outcrop and core samples. We propose that a large amount of weakly bound proteins is leached geologically instantaneously from Candona valves. Long-term (10's of ka) diagenesis gradually weakens and then destroys a secondary pool of tightly bound proteins contained within the valves. Residual amino acid concentrations are likely derived from intra-crystalline proteins and resistant inter-crystalline proteins that are essentially immune to leaching. Candona valves deposited in pluvial Lake Chewaucan and exposed in sediments that crop out along the Ana River, Oregon, have unexpectedly well-preserved proteins and lower d/l values compared to fossil Candona valves of similar age from other locations. The Ana River results argue for developing site-specific calibrations for amino acid geochronology or paleothermometry studies that utilize ostracodes.