Emergence of the inflection point on racemization rate constants for D- and L-amino acids in the early stages of terrestrial diagenesis

The racemization rate constants of chiral amino acids such as asparagine and aspartate (Asx, kASX), glutamine and glutamate (Glx, kGLX) and alanine (Ala, kALA) were determined for terrestrial sediments from Rikubetsu, Hokkaido, Japan. The racemization rate constants, plotted in the form ln [(1 + D/L)/(1 − D/L)] versus sediment age, demonstrate that the initial rate constant was based on a fast racemization for L-form amino acids in the terrestrial sediment. The racemization reaction for labile organic matter on the surface could be markedly affected by the rapid hydrolysis: for example, the initial racemization rate constant kASX 1 was 1.1 × 10−4 year−1 (r = 0.98) until about 2,200 year BP. After the inflection point, the rate constant kASX 2 was 2.4 × 10−5 year−1 (r = 0.93) in the refractory organic matter. The racemization rate constant in the oceanic sediment showed a one order of magnitude difference from the terrestrial sediment in time scale to reach the inflection point. This study clearly demonstrates that the rate of increase in the extent of racemization of amino acids in sediments is discontinuous and related to the degree of oxygenation (and by extension, microbial activity).