We examined the radiocarbon age, taphonomic condition and stratigraphic position of shells of the venerid bivalveChionespp. from the tidal flats of Bahia la Choya, Sonora, Mexico. Shells in Bahia la Choya are time-averaged. Thirty shells yielded radiocarbon dates from modern (A.D. 1950 or younger) to 3569 years before present. The median calendar age of inner flat shells is 483 years; the median age of tidal channel shells is 427 years. We interpret such long shell survival to be the result of frequent shallow burial. Such burial retards bioerosion of shells.The taphonomic condition of shells varied with environment. Shells from the surface of the inner flats were better preserved than shells from the tidal channel. Shells are more likely to be physically worn and biologically degraded in the waters of the channel than on the quieter and more frequently exposed inner tidal flat. Taphonomic condition is an unreliable indicator of a shell's time-since-death. Poorly-preserved shells on the inner flats tended to be old, but in general shell condition was much more variable than shell age. A shell's condition is more likely the result of its total residence time on the surface than its time-since-death (surface time plus burial time).Two composite short (44 cm and 50 cm) cores revealed varying degrees of stratigraphic disorder (the departure from perfect correlation between relative stratigraphic position and relative age). One of eight shells in the inner flats core was disordered; four of nine shells in the tidal channel were disordered. The actual age range of surface shells approximates the age range of shells in cores. Stratigraphic disorder is a consequence of both time-averaging and physical and biogenic mixing.Time-averaging controls the degree of precision possible in paleoecological studies. Environmental changes and ecological phenomena occurring within a span of 3500 years would not be recognized in deposits like those of Bahia la Choya. Time-averaging and stratigraphic disorder also constrain the temporal resolution possible in microstratigraphic studies of evolution. The extent of time-averaging and stratigraphic disorder will dictate an appropriate sample interval. In order to prevent temporal overlap between successive samples in deposits like Bahia la Choya, sample spacing should not be less than approximately 0.5 m.
We used radiocarbon ages on dead Holocene shells of the venerid bivalveChionespp. to investigate how time-averaging and taphonomy in shallow marine benthic assemblages vary with sedimentary and tectonic setting. We compared shells collected from the sediment surface in five depositional environments from two regions of the Gulf of California, Mexico: Bahía Concepción, a young faulted rift basin with high rates of terrigenous and carbonate sedimentation; and Bahía la Choya, an intertidal system along a sediment-starved shelf. Frequency distributions of shell ages in all environments form a hollow curve, with a mode at young ages and a long tail toward older ages. This pattern suggests that shells are added to the taphonomically active zone (TAZ) at roughly constant rates (via continuous shell deaths), and removed from the TAZ at random, either through destruction or by achieving final burial. Shell half-lives (the amount of time to remove half the shells from the TAZ) provide a comparative measure of time-averaging. Time-averaging varies with sedimentary and tectonic setting. The lowest amounts of time-averaging (shell half-lives of 90 to 165 years) occur in Bahía Concepción, where rapid rates of terrigenous sedimentation (on fan-deltas) and carbonate sedimentation (in pocket bays) bury shells rapidly. Time-averaging is higher in the sediment-starved environments of Bahía la Choya (shell half-lives of 285 to 550 years). The highest amounts of time-averaging occur the inner tidal flats of Bahía la Choya (shell half-life of 550 years). Here the conjunction of low sedimentation rates with low rates of shell destruction (due to periodic tidal emergence) permits shells to persist in the TAZ for very long time spans.There is no systematic relationship between a shell's age and its taphonomic condition (taphonomic grade) in any environment, probably because of the complex and random nature of burial-exhumation in the TAZ. Agevariancetends to increase with increasing taphonomic alteration: highly altered shells range in age from young to several thousand years old, while less altered shells are mostly young. The correspondence between time-averaging and the taphonomic condition of entire shell assemblages is also weak, but might be resolved with further study.These results provide quantitative data on time-averaging in benthic assemblages as a function of sedimentary and tectonic setting, and suggest some guidelines for facies appropriate for particular studies. Shallow marine rift basins like Bahía Concepción can potentially contain within-horizon fossil assemblages representing time spans of only a few hundred years—time resolution often beyond reach in paleontology. In contrast, sediment-starved shelf habitats like Bahía la Choya are unlikely to yield assemblages with time resolution finer than several thousands of years.
The development of taphonomic approaches to facies analysis requires a foundation in facies‐based actualistic studies. Modern intertidal and shallow shelf environments at Provincetown Harbor. northern Cape Cod, Massachusetts (USA) provide an opportunity to compare pattcrns and controlling factors in molluscan biofacies and taphofacies distributions. Variation in faunal composition, ecologic variables, and taphonomic attributes of molluscan death assemblages produce distinct patterns of environmental zonation: (1) Faunal composition (biofacies) primarily tracks variation in substrate type among environments (sand, rock, peat, and Zostera marina beds). (2) Ecologic variables (equitability, infauna: epifauna ratio, gastropod:bivalve ratio, and predation on M. mercenaria) appear to reflect tidal exposure time. (3) Taphononic attributes (fragmentation, abrasionm, corrosion, bioerosion, and encrustation) of the common bivalve M. mercenaria track environmental energy, in particular its effects on the stability and reworking of hardparts at the sediment surface. Shells in different environments proceed along different taphonomic pathways ‐ the order of acquisition of taphonomic features by hardparts. An encrustation/bioerosion‐dominated pathway characterizes low energy environments; the upper intertidal and deeper subtidal. An abrasion‐dominated pathway characterizes the high energy lower intertidal and shallow subtidal. Contrasting pathways produce distinct proportions of taphonomic attributes in time‐averaged samples; proportions that delineate taphofacies. Integrated taxonomic, ecologic and taphonomic data provide a more complete picture of environmental processes than any approach alone. Taphonomic data not only furnish information not readily provided by other approaches, but free paleoecology from the constraints of taxonomic uniformitarianism. □Taphonomy, comparative taphonomy, taphofacies, biofacies, cluster analysis, multidimensional scaling, taphonomic pathways, Recent, actualism, intertidal, molluses.
Neogene rift basin deposits in the Gulf of California region record early phases in the tectonic evolution of the modern gulf. Potential Holocene analogs to these deposits occur today in Bahía Concepción, a rift basin forming a shallow marine bay on the eastern coast of Baja California Sur at ~26.5°North latitude. Bedrock geology and geomorphology suggest Bahía Concepción formed by drowning of an asymmetric graben during the Holocene transgression. A large normal fault zone bounds the bay's east side, associated with a 30-km-long shoreline bajada backed by small, steep drainages. A smaller fault zone probably bounds the bay's west side. The west side has larger drainages with gentler gradients and is characterized by rocky shorelines, pocket bays, mangrove swamps, and few exposed alluvial fans. Holocene sediments of the basin accumulate in alluvial fan, coastal interfan flat, mangrove swamp, fandelta, pocket bay, nearshore shelf, and offshore shelf environments.Three major types of marine sediment dominate the bay. Green clastic mud (with variable mollusc shell content) dominates offshore below 20 m. Volcaniclastic sand (with variable mollusc shell and calcareous algae content) dominates the shallow fandeltas and nearshore shelf on the bay's east and south sides. Carbonate sand (both mollusc and calcalgal origin) dominates in shallow pocket bays and adjacent to rocky shorelines on the bay's west side. Comparison of drainage profiles from the east and west sides of the bay suggests that the restriction of carbonate sediments to the west side is due to trapping of terrigenous sediment in the large western drainages during Holocene sea-level rise.Variation in species composition and taphonomy of molluscan assemblages in the bay is correlated with water depth. Cluster analysis on molluscan shell samples identifies four overlapping, depth-related biofacies: mangrove swamp (intertidal), shallow nearshore (1-to 5-m depth), deeper nearshore (5-to 12-m depth), and offshore (below 20 m). Taphonomic analysis demonstrates depth zonation in both the abundance and condition of mollusc shells. Shell abundance is greatest in intertidal mangrove
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