Estimating Time-since-Death from the Free Amino Acid Content of the Mollusc Shell: A Measure of Time Averaging in Modern Death Assemblages? Description of the Technique

The concentration of free amino acids (FAA) increases in three species of bivalves, Tagelus plebeius, Rangia cuneata, and Phacoides pectinatus, taken from ever increasing depth in the upper 72 cm of a core from a mesohaline, sandy-bottom site in Copano Bay, Texas. Analysis of museum shells of known age indicates that [FAA]* increases with shell age (time-since-death) as the proteinaceous matrix gradually breaks down. Estimation of relative age (one shell older than another) in shells less than a few hundred years old is readily accomplished using a measure of [FAA]. The estimation of absolute age (actual time-since-death) depends on an independent calibration. Museum shells of known age offer one possible method. The glycinelalanine ratio decreases with time either because the rate of alanine production increases or because glycine diffuses out of shells more rapidly. Hence [glycine] is a useful estimator of age for relatively young shells and [alanine] or [glutamic acid] for older shells. Age estimates from calibration curves using museum specimens of T. plebeius and R. cuneata and independent estimates based on sediment accumulation rates agree for shells taken from the Copano Bay site. Measuring [FAA] provides a method to appraise the degree of time averaging in modern death assemblages because the relationship of shell age and burial depth can be determined. [FAA] changed little in shells below 72 cm in the core indicating that an increase in time averaging may have occurred below that depth, whereas, in the upper 72 cm, changes in death assemblage