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The extent of racemization of aspartic acid (Asp) has been used to estimate the ages of 9 shells of the epifaunal calcitic brachiopod Bouchardia rosea and 9 shells of the infaunal aragonitic bivalve Semele casali. Both taxa were collected concurrently from the same sites at depths of 10 m and 30 m off the coast of Brazil. Asp D/L values show an excellent correlation with radiocarbon age at both sites and for both taxa (r2Site 9 B. rosea = 0.97, r2Site 1 B. rosea = 0.997, r2Site 9 S. casali = 0.9998, r2Site 1 S. casali = 0.93). The Asp ratios plotted against reservoir-corrected AMS radiocarbon ages over the time span of multiple millennia can thus be used to develop reliable and precise geochronologies not only for aragonitic mollusks (widely used for dating previously), but also for calcitic brachiopods. At each collection site, Bouchardia specimens display consistently higher D/L values than specimens of Semele. Thermal differences between sites are also notable and in agreement with theoretical expectations, as extents of racemization for both taxa are greater at the warmer, shallower site than at the cooler, deeper one. In late Holocene marine settings, concurrent time series of aragonitic and calcitic shells can be assembled using Asp racemization dating, and parallel multi-centennial to multi-millennial records can be developed simultaneously for multiple biomineral systems.
To evaluate the potential of using surficial shell accumulations for paleoenvironmental studies, an extensive time series of individually dated specimens of the marine infaunal bivalve mollusk Semele casali was assembled using amino acid racemization (AAR) ratios (n = 270) calibrated against radiocarbon ages (n = 32). The shells were collected from surface sediments at multiple sites across a sediment-starved shelf in the shallow sub-tropical São Paulo Bight (São Paulo State, Brazil). The resulting 14C-calibrated AAR time series, one of the largest AAR datasets compiled to date, ranges from modern to 10,307 cal yr BP, is right skewed, and represents a remarkably complete time series: the completeness of the Holocene record is 66% at 250-yr binning resolution and 81% at 500-yr binning resolution. Extensive time-averaging is observed for all sites across the sampled bathymetric range indicating long water depth-invariant survival of carbonate shells at the sediment surface with low net sedimentation rates. Benthic organisms collected from active depositional surfaces can provide multi-millennial time series of biomineral records and serve as a source of geochemical proxy data for reconstructing environmental and climatic trends throughout the Holocene at centennial resolution. Surface sediments can contain time-rich shell accumulations that record the entire Holocene, not just the present.
The variation in time-averaging between different types of marine skeletal accumulations within a depositional system is not well understood. Here we provide quantitative data on the magnitude of time-averaging and the age structure of the sub-fossil record of two species with divergent physical and ecological characteristics, the brachiopod Bouchardia rosea and the bivalve Semele casali. Material was collected from two sites on a mixed carbonate-siliciclastic shelf off the coast of Brazil where both species are dominant components of the local fauna.
Individual shells (n = 178) were dated using amino acid racemization (aspartic acid) calibrated with 24 AMS radiocarbon dates. Shell ages range from modern to 8118 years b.p. for brachiopods, and modern to 4437 years for bivalves. Significant differences in the shape and central tendency of age-frequency distributions are apparent between each sample. Such differences in time-averaging magnitude confirm the assumption that taphonomic processes are subject to stochastic variation at all spatial and temporal scales. Despite these differences, each sample is temporally incomplete at centennial resolution and three of the four samples have similar right-skewed age-frequency distributions. Simulations of temporal completeness indicate that samples of both species from the shallow site are consistent with a more strongly right-skewed and less-complete age-frequency distribution than those from the deep site.
We conclude that intrinsic characteristics of each species exert less control on the time-averaging signature of these samples than do extrinsic factors such as variation in rates of sedimentation and taphonomic destruction. This suggests that brachiopod-dominated and bivalve-dominated shell accumulations may be more similar in temporal resolution than previously thought, and that the temporal resolution of multi-taxic shell accumulations may depend more on site-to-site differences than on the intrinsic properties of the constituent organisms.
Biological veracity of the sharp diversity increase observed in many analyses of the post-Paleozoic marine fossil record has been debated vigorously in recent years. To assess this question for sample-level (“alpha”) diversity, we used bulk samples of shelly invertebrates, representing three major fossil groups (brachiopods, bivalves, and gastropods), to compare the Jurassic and late Cenozoic sample-level diversity of marine benthos. After restricting the data set to single-bed, whole-fauna, bulk samples (n ≥ 30 specimens) from comparable open marine siliciclastic facies, we were able to retain 427 samples (255 Jurassic and 172 late Cenozoic), with most of those samples originating from our own empirical work.
Regardless of the diversity metric applied, the initial results suggest that standardized sample-level species (or genus) diversity, driven by evenness and/or richness of the most common taxa, increased between the Jurassic and late Cenozoic by at least a factor of 1.6. When the data are partitioned into the three dominant higher taxa, it becomes clear that (1) the bivalves, which dominated the samples for both time intervals, increased in sample-level diversity between the Jurassic and the late Cenozoic by a much smaller factor than the total fauna; (2) the removal of brachiopods, which were a noticeable component of the Jurassic samples, did not significantly affect standardized sample-level diversity estimates; and (3) the gastropods, which were rare in the Jurassic but common in many late Cenozoic samples, contributed notably to the increase in sample-level diversity observed between the two time intervals. Parallel to these changes, the samples revealed secular trends in ecological structure, including Jurassic to late Cenozoic increases in proportion of (1) infauna, (2) mobile forms, and (3) non-suspension-feeding organisms. These trends mostly persist when data are restricted to bivalves.
Supplementary analyses indicate that these patterns cannot be attributed to sampling heterogeneities in paleolatitudinal range, lithology, or paleoenvironment of deposition. Likewise, when data are restricted to samples dominated by species with originally aragonitic shells, the observed temporal changes persist at a comparable magnitude, suggesting that the pervasive loss of aragonite in the older fossil record is unlikely to have been the primary cause of the observed patterns. The comparable ratio of identified to unidentified species and genera, observed when comparing the Jurassic and late Cenozoic samples, indicates that the relatively poorer (mold/cast) preservation of Jurassic aragonite species also is unlikely to have been responsible for the observed patterns. However, the diagenesis-related taphonomic and methodological artifacts cannot be ruled out as an at least partial contributor to the observed post-Paleozoic changes in diversity, taxonomic composition, and ecology (the outcomes of the three tests of the diagenetic bias available to us are incongruent).
The study demonstrates that the post-Paleozoic trends in the sample-level diversity, ecology, and taxonomic structure of common taxa can be replicated across multiple studies. However, the diversity increase estimated here is much less prominent than suggested by many previous analyses. The results also narrow the list of causative explanations down to two testable hypotheses. The first is diagenetic bias—a spurious trend driven by either (a) increasing taphonomic loss of small specimens in the older fossil record or (b) a shift in sampling procedures between predominantly lithified rocks of the Mesozoic and predominately unlithified, and therefore sievable, sediments of the late Cenozoic. The second hypothesis is genuine biological changes—macroevolutionary trends in the structure of marine benthic associations through time, consistent with predictions of several related models such as evolutionary escalation, increased ecospace utilization, and the Mesozoic marine revolution. Future studies should focus on testing these two rival models, a key remaining challenge for identifying the primary causative mechanism for the long-term changes in sample-level diversity, ecology, and taxonomic structure observed in the Phanerozoic marine fossil record.