The stratigraphic, taphonomic and biologic records from two cores in Copano Bay, Texas were analyzed to determine (1) whether variations in shell content with depth were caused by variations in carbonate preservation, carbonate production or sedimentation rate and (2) the extent to which characteristics of fossil assemblages, such as species composition, numerical abundance, biomass, and trophic and habitat structure, identified similar or different trends. Below the top few cm of the sedimentary column, variations in carbonate content with depth could be attributed to variations in carbonate production. Most biological attributes varied similarly with depth and, hence, time on both long (⪖100 yr) and short (∼10 yr) temporal scales. These variations could not be explained by any taphonomic process, sediment reworking and burial, or sedimentation rate. Despite a vigorous taphonomic milieu, to obtain the shell content of Copano Bay sediments requires the preservation of nearly all carbonate produced. Preservation of a large fraction of the shell carbonate added requires the preservation of most of the relatively large-shelled biota (large species and adults) which retain important evidence of changes in the community's history in this area. The results reemphasize the importance of large individuals and biomass in paleontologic reconstruction and suggest that changes in community productivity, which in paleontologic usage, must be carbonate productivity, are preserved in the fossil record.

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

The death assemblage is an important source of data on the living community prior to sampling and between sampling occasions. Proper use of these data requires a knowledge of how death assemblages form from living communities. A large larval settlement and catastrophic mortality of the clam Mulinia lateralis provided an opportunity to test several hypotheses about death assemblage formation. Individuals of M. lateralis were added to the death assemblage in pulses resulting from episodic mortality, but each pulse rapidly disappeared—half of the shells were gone in 192 days. This loss was size-dependent; smaller shells disappeared at over twice the rate of larger shells. The size-frequency distribution for M. lateralis in the death assemblage was bi-modal because of the simultaneous death of two cohorts of different size frequencies. Individuals present in the death assemblage prior to the new settlement had been locally redistributed by physical processes after death. We hypothesize that larger pulses are preserved preferentially because they remain in the death assemblage longer, and, thus, these shells have a greater chance of being physically reworked beneath the taphonomically-active surface zone by storms.

An understanding of death assemblage formation requires a measurement of time since death of constituent individuals. A new dating technique based on the measurement of the free amino acid content of mollusc shells has been developed which is inexpensive, rapid, and effective in dating time scales of a few decades to a few centuries. Since the breakdown of proteins of the matrix of mollusc shells begins soon after deposition, free amino acids gradually increase with shell age. The measurement of these can be used to determine the relative age among a group of shells. The future use of this technique depends on a clearer understanding of how free amino acid accumulation rate varies with age and species and developing effective calibration methods so that absolute rather than relative ages can be readily obtained. Three species were distributed widely enough for use - Rangia cuneata, Tagelus plebeius, and Phacoides pectinatus. A good relationship between free amino acids and relative age was present in all three species over the entire core; however some species and some amino acid were superior to others. Rangia cuneata produced the best correlation because it is epifaunal and thus died at the sediment surface rather than overmore » an extended depth range and, also perhaps, because amino acid accumulation rates were more linear.« less