科研之友
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摘要
Amino acids in ostracode valves are abundant and well preserved in Quaternary sediments. Ostracode valves are increasingly useful targets for amino acid geochronologic investigations yet little is known about the influence of taxonomy or environmental variables on ostracode protein diagenesis. This paper documents the effects of taxonomy and elevated environmental pH on the amino acid composition of ostracode valves. In one experiment, modern ostracode valves from the genus Candona (Cypridoidea) and Limnocythere (Cytheroidea) were submerged in pH 7 and pH 10 buffers and heated at 120 degrees C for up to 32 d. The buffer solutions and a subset of the ostracode valves were analyzed for their total hydrolysable amino acid (THAA) D/L. values and concentrations. The remaining valves were analyzed for their free amino acid (FAA) D/L values and concentrations. In a second experiment, modern valves from four species of ostracodes within the Cypridoidea were heated at 110 degrees C for up to 160 d. THAA aspartic acid (Asp) and glutamic acid (Glu) D/L values were monitored. Finally, the amino acid compositions in a suite of fossil Candona valves from outcrops in the western United States, including the problematic Ana River section, are compared across a broader spectrum of amino acid data. A total of 586 samples were analyzed. The results show that both THAA and FAA D/L values and concentrations were higher in ostracode valves heated in pH 10 buffer than in valves heated in pH 7 buffer. THAA D/L values in the buffers were consistently higher than in the associated ostracode valves. Amino acids were readily leached from modern Limnocythere valves but minimal leaching was observed from young fossil Candona valves. The extent of leaching was not pH dependent. The rates and extent of racemization for Asp and Glu were similar in Candona spp. and Limnocythere valves heated in either pH 7 or 10 buffers. The rates and extent of amino acid racemization for alanine, valine, and epimerization of isoleucine were consistently lower in Limnocythere valves, regardless of pH environment. Asp racemized significantly faster in Heterocypris valves than in Candona or Ilyocypris valves. There were no substantial differences in the rate of Glu racemization in Heterocypris, Candona, or Ilyocypris valves, nor in the rate of racemization of Asp or Glu in valves from three species of Candona. The heating experiments demonstrate that environmental pH can have significant impact on the amino acid composition of ostracode valves. Differences in the rates of amino acid racemization were detected at both the family and superfamily level. The Ana River Candona valves are strongly enriched in Asp, Glu, and glycine when compared with other fossil Candona valves. These results are not explained by taxonomic differences or by diagenesis under basic (alkaline) environmental conditions. Instead, they suggest that other environmental variables during valve calcification may be responsible for the unique amino acid signature of the Ana River material.
- 出版日期2011-12
