Scale and structure of time-averaging (age mixing) in terrestrial gastropod assemblages from Quaternary eolian deposits of the eastern Canary Islands

Quantitative estimates of time-averaging (age mixing) in gastropod shell accumulations from Quaternary (the late Pleistocene and Holocene) eolian deposits of Canary Islands were obtained by direct dating of individual gastropods obtained from exceptionally well-preserved dune and paleosol shell assemblages. A total of 203 shells of the gastropods Theba geminata and T. arinagae, representing 44 samples (= stratigraphic horizons) from 14 sections, were dated using amino acid (isoleucine) epimerization ratios calibrated with 12 radiocarbon dates. Most samples reveal a substantial variation in shell age that exceeds the error that could be generated by dating imprecision, with the mean within-sample shell age range of 6670 years and the mean standard deviation of 2920 years. Even the most conservative approach (Monte Carlo simulations with a non-sequential Bonferroni correction) indicates that at least 25% of samples must have undergone substantial time-averaging (e.g., age variations within those samples cannot be explained by dating imprecision alone). Samples vary in shell age structure, including both left-skewed (17 out of 44) and right-skewed distributions (26 out of 44) as well as age distributions with a highly variable kurtosis. Dispersion and shape of age distributions of samples do not show any notable correlation with the stratigraphic age of samples, suggesting that the structure and scale of temporal mixing is time invariant. The statistically significant multi-millennial time-averaging observed here is consistent with previous studies of shell accumulations from various depositional settings and reinforces the importance of dating numerous specimens per horizon in geochronological studies. Unlike in the case of marine samples, typified by right-skewed age distributions (attributed to an exponential-like shell loss from older age classes), many of the samples analyzed here displayed left-skewed distributions, suggestive of different dynamics of age mixing in marine versus terrestrial shell accumulations.     

Preliminary environmental reconstructions from late Quaternary pollen and mollusc assemblages at Egg Lagoon,King Island,Bass Strait

Abstract A late Quaternary environmental record is currently being developed from Egg Lagoon, King Island, Bass Strait, a site which is geographically well situated to contribute towards a history of the Bass Strait region. Environmental reconstructions are based on a stratigraphic survey and pollen, charcoal and mollusc analyses of sediment core samples. The recorded stratigraphy includes five sedimentary units representing estuarine-marine, freshwater lake and swamp depositional environments. Amino-acid racemization analyses of marine shells indicate a greater than last interglacial age for the basal estuarine-marine unit, while radiocarbon analyses of organic muds and wood suggest that a substantial section of the overlying freshwater lake and swamp facies is beyond the conventional limit for this technique. Local pollen assemblages represent freshwater lake and swamp plant communities that have varied presumably according to water level changes at the site. Regional pollen assemblages represent terrestrial herbaceous communities, believed to have existed under cooler and drier climates than today, and Eucalyptus- and Phyllocladus-dom'maied forests and woodlands from periods with greater effective precipitation than at present. A sustained increase in charcoal representation dating from at least 39 000 years before the present may indicate an anthro-pogenically induced change in the fire regime, consistent with the earliest dates for human occupation in mainland Tasmania.     

Significance of time-averaging for palaeocommunity analysis

Fürsich, F. T. & Aberhan, M. 1990 04 15: Significance of time-averaging for palaeo community analysis. Lethaia , Vol. 23, pp. 143–152. Oslo. ISSN 0024–1164.
Time-averaging caused by sedimentological, biological, or digenetic factors is widespread in marine environments. It can be recognized using stratigraphic, ecological, taphonomic, or sedimcntologic criteria. Community parameters such as diversity, relative abundance of species, taxonomic and, to a lesser extent, trophic and life habit composition may seriously differ between thanatocoenosis (death assemblage) and time-averaged taphocoenosis, depending on the degree of community stability. Time-averaged community relics have a quality different from that of Recent communities and any comparison between the two has to take this into account. Whereas allochthonous time-averaging, which is strongly related to the energy level, is particularly common in very shallow environments, the importance of autochthonous time-averaging, related in addition to the rate of net sedimentation, increases with distance from the shore and with depth. Short-term time-averaging (in the order of up to several thousand years) prevails in near shore shallow environments, whilst long-term time-averaging (in the order of 10⁴ to 10⁵ years) becomes more important towards lower shelf and deep sea environments.     

Hiatal shell concentrations, sequence analysis, and sealevel history of a Pleistocene coastal alluvial fan, Punta Chueca, Sonora

Richly fossiliferous marine sediments exposed along the Sonoran coastline of the Gulf of California near Punta Chueca provide an excellent setting in which to test (a) the strength of the association of skeletal concentrations with sedimentary hiatuses, (b) the utility of taphonomic evidence for reconstructing detailed histories of those non-depositional episodes, and thus (c) the largely unexploited potential of skeletal concentrations in the identification and interpretation of lithologically obscure unconformities and condensed sequences in shallow marine deposits. Sequence analysis based on discontinuity surfaces is possible in the complex, alluvial fan-to-shallow marine transition at Punta Chueca despite rapid facies changes. Progradation of depositional sequences that contain cobbles reworked from older terrace deposits indicates accumulation during a fall in eustatic sea level. The supratidal to subtidal conglomerates and sands contain a variety of predominantly molluscan shell concentrations that, on the basis of postmortem histories of shells, formed during periods of low net sedimentation (i.e. depositional hiatuses); the majority of these shell beds lie along discontinuity surfaces identified by independent physical stratigraphic evidence. Although not all discontinuity surfaces in the terrace are paved by shell material. and not all relative concentrations of shells indicate distinct discontinuities, the strength of the association between skeletal concentrations and stratigraphic hiatuses reveals the high degree of control on fossil occurrence by sedimentation rates, and indicates that skeletal concentrations can provide good clues to stratigraphically significant surfaces. Moreover, the detailed dynamics of non-depositional episodes are reliably revealed by taphonomic analysis of the associated fossil assemblages, improving interpretations of non-depositional episodes in local sedimentary history.     

Empirical calibrated radiocarbon sampler: a tool for incorporating radiocarbon‐date and calibration error into Bayesian phylogenetic analyses of ancient DNA

Studies of DNA from ancient samples provide a valuable opportunity to gain insight into past evolutionary and demographic processes. Bayesian phylogenetic methods can estimate evolutionary rates and timescales from ancient DNA sequences, with the ages of the samples acting as calibrations for the molecular clock. Sample ages are often estimated using radiocarbon dating, but the associated measurement error is rarely taken into account. In addition, the total uncertainty quantified by converting radiocarbon dates to calendar dates is typically ignored. Here, we present a tool for incorporating both of these sources of uncertainty into Bayesian phylogenetic analyses of ancient DNA. This empirical calibrated radiocarbon sampler (ECRS) integrates the age uncertainty for each ancient sequence over the calibrated probability density function estimated for its radiocarbon date and associated error. We use the ECRS to analyse three ancient DNA data sets. Accounting for radiocarbon‐dating and calibration error appeared to have little impact on estimates of evolutionary rates and related parameters for these data sets. However, analyses of other data sets, particularly those with few or only very old radiocarbon dates, might be more sensitive to using artificially precise sample ages and should benefit from use of the ECRS.     

Fréquence des datations Au 14C de faunes marines post-glaciaires de l'est du Canada et variations paléoclimatiques

A statistical analysis of radiocarbon dates for 365 samples associated with postglacial marine deposits of Eastern Canada and New England, U.S.A., yield a non-random statistical distribution. This distribution pattern is explained by two principal controls (that are partly interdependent): (1) secondary eustatic fluctuations, creating relative stabilizations of the sea level with an uplifting land; and (2) more generally the climatic variations. Other factors involved may include: (1) limitations of ¹⁴C dating method; (2) non-statistical sampling methodology adopted by the Quaternary geologists; (3) lack of intensive study of some of the marine basins; and (4) incidence of rapidly changing paleogeographic events (opening and shoaling of marine basins, glacial re-advances and changes in the ice flow). The statistical appraisal of errors in published ¹⁴C dates and the standard deviation, calculated with sliding means, enable us to screen out the influence of most of the non-climatic factors. With this technique, we are also able to draw a curve based on a general synthesis of all variations in ¹⁴C dates distribution. In this way, the irregularities observed in the synthesis curve are now correlated with paleoclimatic and paleogeographic events that took place in Eastern Canada between 15,000 years B.P. to present. That curve shows a close correlation during the Holocene with other climatic indicators and gives some original informations concerning the period 15,000–10,000 B.P. No attempt has been made to adjust radiocarbon years to calendar years for two reasons: (1) lack of agreement between comparative curves prior to 7,000 B.P.; and (2) lack of information concerning the effect of “old waters” on shell dates.     

Mesoproterozoic surface oxygenation accompanied major sedimentary manganese deposition at 1.4 and 1.1 Ga

Manganese (Mn) oxidation in marine environments requires oxygen (O₂) or other reactive oxygen species in the water column, and widespread Mn oxide deposition in ancient sedimentary rocks has long been used as a proxy for oxidation. The oxygenation of Earth's atmosphere and oceans across the Archean-Proterozoic boundary are associated with massive Mn deposits, whereas the interval from 1.8–1.0 Ga is generally believed to be a time of low atmospheric oxygen with an apparent hiatus in sedimentary Mn deposition. Here, we report geochemical and mineralogical analyses from 1.1 Ga manganiferous marine-shelf siltstones from the Bangemall Supergroup, Western Australia, which underlie recently discovered economically significant manganese deposits. Layers bearing Mn carbonate microspheres, comparable with major global Mn deposits, reveal that intense periods of sedimentary Mn deposition occurred in the late Mesoproterozoic. Iron geochemical data suggest anoxic-ferruginous seafloor conditions at the onset of Mn deposition, followed by oxic conditions in the water column as Mn deposition persisted and eventually ceased. These data imply there was spatially widespread surface oxygenation ~1.1 Ga with sufficiently oxic conditions in shelf environments to oxidize marine Mn(II). Comparable large stratiform Mn carbonate deposits also occur in ~1.4 Ga marine siltstones hosted in underlying sedimentary units. These deposits are greater or at least commensurate in scale (tonnage) to those that followed the major oxygenation transitions from the Neoproterozoic. Such a period of sedimentary manganogenesis is inconsistent with a model of persistently low O₂ throughout the entirety of the Mesoproterozoic and provides robust evidence for dynamic redox changes in the mid to late Mesoproterozoic.     

Taphonomy of Eemian marine molluscs and acorn barnacles from eastern Arkhangelsk region, northern Russia

Marine Eemian deposits along the Pyoza river and its tributary Varchuska, Arkhangelsk region, constitute successions of muddy and sandy facies with rich macrobenthic fauna dominated by bivalves and barnacles. Taphonomic features formed by abrasion, disarticulation, dissolution, fragmentation, bioerosion and encrustation define taphofacies for a palaeoenvironmental model. Five bivalve taphofacies and three barnacle taphofacies could be distinguished. Both bivalves and barnacles are poorly preserved in foreshore/shoreface environments, as the shells were subjected to extensive transportation by currents. The shells were best preserved in offshore environments, where rapid episodic sedimentation enabled within-habitat preservation, in some cases even preservation in life position. Barnacles are absent from the most clay-rich offshore deposits, probably because of clogging of filters by turbidity and lack of suitable substrate. Such dissimilarities suggest that the number and distribution of taphofacies may depend on which fossil groups are used. Interspecific variability may exist within the individual taphofacies. The barnacles, for example, tend to be better preserved than the mussel Mytilus edulis, although both are fixosessile suspension feeders. This indicates that not only life habits but also intrinsic shell properties influence preservation. Thus, taphofacies analyses should combine data on taphonomic features, specific life habit and shell properties to determine overall preservation patterns. In that way, taphofacies analyses may form a powerful tool for palaeoenvironmental analyses of marine deposits.     

Taphonomy of modern and fossil intertidal gastropod associations from Isla Santa Cruz and Isla Santa Fe, Galápagos Islands

Examination of modern gastropod associations from the low intertidal zone of Isla Santa Cruz suggests that fossil rocky intertidal deposits from this tropical locality will be taphonomically compromised in three ways: (1) Marine hermit crabs, by their use of empty gastropod shells, will mix the shells from varying tidal heights and habitats, thus facilitating mixed associations of such shells in the fossil record, (2) encrusting organisms on crab-inhabited shells are abundant, while boring organisms are almost non-existent, indicating possible differences in postmortem shell retention, and (3) intertidal shells are further taphonomically altered by terrestrial hermit crabs, which transport the shells onto land as well as physically modify the shells. Gastropod fossils from beach and terrace deposits on Isla Santa Fe are interpreted to be a mixed assemblage of rocky intertidal assemblage with few shells indicating influence from marine hermit crabs. Modification of the shell by marine and terrestrial hermit crabs was also evident. A unique polish to the shells at one locality is attributed to the marine iguanas and is only found in the terrace site biologically bulldozed by egg-laying iguanas. Few studies exist on modern rocky intertidal associations in the Galápagos, and the fossil record of rocky shores may provide a baseline for future studies in how community structure has changed over since the advent of humans. Galapagos, C oenobita C ompressus , gastropods, humans, Gulf of California, bionts, nutrients.
Sally E. Walker, Department of Geology, The University of Georgia, Athens, Georgia, USA; 8th September, 1994; revised 28th June, 1995.     

Late quaternary variations in oxygen and carbon isotopic compositions in Canadian Arctic marine bivalves

The oxygen and carbon isotopic composition of arctic marine bivalves Mya truncata, Hiatella arctica, and Mytilus edulis are reported on samples from raised marine deposits in Hudson Bay and eastern Baffin Island. The shells range in age from modern, through the Holocene, to “old” marine units. During the Holocene the ¹⁸O/¹⁶O ratio in shells rose to a maximum about 3,500 B.P. which coincides in time with the period of maximum growth rates of bivalves, maximum size and maximum faunal diversity. The change is interpreted to indicate that about 3,500 years ago arctic waters may have reached a salinity ≈ 1–2% greater than present. Comparison of Holocene shell-carbonate isotopic compositions with those from the “old” marine shells (that are characteristically extremely thick) suggests that during the early Wisconsin advance on eastern Baffin Island, surface and near-surface waters were more saline than at present. This may have been related to low meltwater discharge. Paradoxically, positive values of ¹⁸O/¹⁶O and ¹³O/¹²C in marine shells occurred during the Holocene marine optimum and during the early Wisconsin ice advance.     

Diet and mobility in a late neolithic population of coastal oman inferred from radiocarbon dating and stable isotope analysis

In Oman, the presence of highly productive marine environments, coupled with relatively limited land resources, have led to intense exploitation of coastal resources, but the question of the seasonality of occupation of coastal sites remains open. Our aim is to evaluate the contribution of marine resources to the diet of the Neolithic population of Ra's al‐Hamra 5 (RH‐5) to shed new light on its mobility, using stable isotopes and radiocarbon (¹⁴C) dating as dietary tracers. Charcoal, shell, human bone and enamel apatite from eight contemporary graves were sampled. Graves are thought to provide the best chance to obtain marine and terrestrial remains that were contemporary with the human remains in order to calculate the marine reservoir effect (MRE) for this period. Inter‐individual variation in human bone apatite δ¹³C value is small, suggesting a homogenous diet. Bone apatite ¹⁴C ages are very close to the shell ages while enamel is significantly younger and plots near the charcoal ages. Older enamel ages were obtained when a stronger acetic treatment was used, demonstrating that the young ages are due to diagenetic alteration rather than a diachronic change in diet and that only bone apatite retained in vivo dietary signals. Bone ages indicate a heavy reliance on marine resources and it is therefore unlikely that the individuals analyzed here were leaving the coast seasonally, although mobility along the coast cannot be excluded. Am J Phys Anthropol 153:353–364, 2014. © 2013 Wiley Periodicals, Inc.     

The relationship of bionts and taphonomic processes in molluscan taphofacies formation on the continental shelf and slope: eight-year trends: Gulf of Mexico and Bahamas

The Shelf and Slope Experimental Taphonomy Initiative (SSETI) deployed a suite of molluscan species in environments covering a range of depths and sediment types in the Bahamas and on the Gulf of Mexico continental shelf and upper slope for 8 years. Taphonomic state rarely correlated with the distribution of biont guilds among environments. The preservable and nonpreservable biont guilds were also routinely orthogonal. Several coincidences of taphonomic trait and biont guild occurred, including green discoloration that consistently co-occurred with boring algae and bacterial films associated with the development of chalkiness and a soft shell surface. Environments of preservation (EOPs) of disparate taphonomic signature and biont guild complement occur in similar sediment types and environments with similar rates of burial. A paucity of biont coverage is no more a reliable indicator of rapid burial than is a limited degree of shell degradation. The suggestion that preservable bionts might protect shells from taphonomic processes is not well supported. Certain EOP groups are delineated from others most readily by a combination of biont guild and taphonomic trait. Thus, biont guilds augment taphonomic analysis in differentiating EOPs. Shell preservational state, including taphonomic signature and biont coverage, is influenced in a complex way by environment. The analysis confirms an expectation that the diversity of EOPs should be greater in shallow water. Clustering of EOPs reveals that visually distinctive environments may be taphonomically and biotically similar. Visually similar environments may be quite disparate in taphonomic state and biont complement. EOPs grouped by similarity in taphonomic signature and biont coverage very likely define geographically widespread biological and taphonomic regimes, which, however, are everywhere restricted locally in areal dimension.     

Changes in the Radiocarbon Reservoir Age in Lake Xingyun,Southwestern China during the Holocene

Chronology is a necessary component of paleoclimatology. Radiocarbon dating plays a central role in determining the ages of geological samples younger than ca. 50 ka BP. However, there are many limitations for its application, including radiocarbon reservoir effects, which may cause incorrect chronology in many lakes. Here we demonstrate temporal changes in the radiocarbon reservoir age of Lake Xingyun, Southwestern China, where radiocarbon ages based on bulk organic matter have been reported in previous studies. Our new radiocarbon ages, determined from terrestrial plant macrofossils suggest that the radiocarbon reservoir age changed from 960 to 2200 years during the last 8500 cal a BP years. These changes to the reservoir effect were associated with inputs from either pre-aged organic carbon or ¹⁴C-depleted hard water in Lake Xingyun caused by hydrological change in the lake system. The radiocarbon reservoir age may in return be a good indicator for the carbon source in lake ecosystems and depositional environment.     

Late Holocene sea level indicators from twelve atolls in the central and eastern Tuamotus (Pacific Ocean)

New sea-level data for the late Holocene period are reported from twelve atolls of the Tuamotu archipelago: Faaite, Hikueru, Marokau, Hao, Amanu, Tatakoto, Pukarua, Nukutavake, Vairaatea, Tureia, Nukutipipi, and Hereheretue. The data come from coral conglomerate outcrops, coral colonies in growth position, in situ reef framework and marine notches; they give consistent results, and their ages are controlled by 29 radiocarbon datings. The Holocene MSL remained 0.8±0.2 m higher than at present, from before 4000 years BP until at least 1500 years BP, then dropped gradually to the present level. This pattern is very similar to that reported from the northwest Tuamotus, suggesting the absence of measurable differential vertical movements over a distance greater than 1300 km during the late Holocene. Effects of the 1983 cyclones on reef morphology are reported from some atolls and radiocarbon dates of some storm-generated reef blocks are given.     

Stratigraphic context and direct dating of the Neandertal mandible from Cova del Gegant (Sitges, Barcelona)

Stratigraphic study of the Cova del Gegant’s sedimentary fill revealed different cycles of accumulation of typical interior cave and delta facies. A precise chronology for these deposits, the faunal remains and stone tools contained therein was obtained by radiocarbon, U-Th and OSL. Our results indicate that the Upper Pleistocene archaeological sequence dates between 49.3 ± 1.8 ka BP, the U-Th age of the overlying flowstone, and 60.0 ± 3.9 ka BP, the OSL age of the basal deposits. We have also directly dated the site’s Neandertal mandible to 52.3 ± 2.3 ka by U-Th.     

Evaluating marine diets through radiocarbon dating and stable isotope analysis of victims of the AD79 eruption of vesuvius

The stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of bone collagen are frequently used in paleodietary studies to assess the marine contribution to an individual's diet. Surprisingly, the relationship between stable isotope these values characteristics and the percentage of marine foods in diet has never been effectively demonstrated. To clarify this relationship, the stable isotope values and radiocarbon dates of nine humans and one sheep from Herculaneum, all who perished simultaneously during the AD 79 eruption of Vesuvius, were determined. Significant differences were found in the radiocarbon dates which are attributable to the incorporation of “old” carbon from the marine reservoir. The magnitude of the observed differences was linearly correlated with both δ¹³C and δ¹⁵N values allowing the response of each isotope to increasing marine carbon in collagen to be independently verified. Regression analyses showed that for every 1‰ enrichment in δ¹³C and δ¹⁵N, 56 years and 34 years were added to the radiocarbon age, respectively. Predictions of the maximum marine reservoir age differed considerably depending on which stable isotope was considered. This discrepancy is attributed to some degree of macronutrient scrambling whereby nitrogen from marine protein is preferentially incorporated in collagen over marine carbon. It is suggested that the macronutrient scrambling explains the observed relationship between δ¹³C and δ¹⁵N from Roman coastal sites and should be considered when interpreting any diet which is not dominated by protein. Nevertheless, without knowing the degree of macronutrient scrambling in different dietary scenarios, the accuracy of dietary reconstructions is severely compromised. Am J Phys Anthropol 152:345–352, 2013. © 2013 Wiley Periodicals, Inc.     

Microbial communities across nearshore to offshore coastal transects are primarily shaped by distance and temperature

Recent studies have focused on linking marine microbial communities with environmental factors, yet, relatively little is known about the drivers of microbial community patterns across the complex gradients from the nearshore to open ocean. Here, we examine microbial dynamics in 15 five-station transects beginning at the estuarine Piver's Island Coastal Observatory (PICO) time-series site and continuing 87 km across the continental shelf to the oligotrophic waters of the Sargasso Sea. 16S rRNA gene libraries reveal strong clustering by sampling site with distinct nearshore, continental shelf and offshore oceanic communities. Water temperature and distance from shore (which serves as a proxy for gradients in factors such as productivity, terrestrial input and nutrients) both most influence community composition. However, at the phylotype level, modelling shows the distribution of some taxa is linked to temperature, others to distance from shore and some by both factors, highlighting that taxa with distinct environmental preferences underlie apparent clustering by station. Thus, continental margins contain microbial communities that are distinct from those of either the nearshore or the offshore environments and contain mixtures of phylotypes with nearshore or offshore preferences rather than those unique to the shelf environment.     

Depth as an overarching environmental variable modulating preservation potential and temporal resolution of shelly taphofacies

In the marine realm, the interpretation of taphofacies relies heavily on how oceanographic and sedimentary conditions affect the preservation state of fossils. Several taphonomic variables either covary with depth or are directly influenced by depth. Facies‐level factors rather than broad, basin‐scale parameters influence the taphonomic profile of mollusc death assemblages according to actualistic and experimental evidence. To determine the possible relation between depth and the taphonomic conditions of multiple species of bivalve remains, we used seven samples gathered over a comprehensive bathymetric gradient (from 7 to 150 m below mean sea level; topmost 10‐ to 20‐cm layer, roughly corresponding to the taphonomically active zone). We selected samples from predominantly muddy facies on the southern Brazilian shelf (SBS). The taphonomic damage profile (TDP) was measured using site samples based on a standard taphonomic analysis (categorical scoring system) of shells and fragments larger than 4 mm, to identify site damage patterns. Restricting the sedimentary grain size (samples from fine sediments) enabled the determination of the variation in damage with depth among the samples. Constrained analysis of proximities (CAP) revealed that up to 46% of the taphonomic variation observed was related to variation in depth (with approximately 28% unexplained by environmental factors). Part of the unexplained fraction was due to the effect of temporal mixing, which is predictable along large‐scale patches but is inversely linked to the TDP. Our results show that taphonomic analysis, considering large spatial scales in recent environments, can explain the variations present in shell beds that formed during distinct time periods of the evolution of a Quaternary sedimentary basin.     

Taphonomic windows and molluscan preservation

Recent studies on silicified fossil biotas have suggested that substantial skewing of the molluscan record resulted from early aragonite dissolution in mid-outer carbonate ramp settings. If those rare skeletal lagerstätten are representative, then the quality and completeness of the molluscan record are thrown into doubt. Yet database studies suggest that the bivalve fossil record is actually relatively complete. If so, then biodiversity must be captured by other processes that preserved shells vulnerable to early dissolution, and which operated on a relatively high frequency, i.e., less than the species duration for bivalves.Storm beds, shell plasters and submarine hardgrounds are identified as fossil deposits that can preserve the labile aragonitic component of the fauna and thus represent potential taphonomic windows. Many storm event beds include rich accumulations of shelly benthos. Differences between storm bed faunas and those of the background facies could reflect transportation effects. However, some storm bed assemblages are rich in originally aragonitic infaunal bivalves that are not represented in background facies or more proximal shelf equivalents, and here rapid burial and removal of organic matter by winnowing may be the keys to aragonite shell preservation. Despite Palaeozoic to Cenozoic changes in the thickness and frequency of shell beds that reflect the predominant bioclast producers, shallow infaunas are commonly concentrated together with epifauna in such deposits.Some low energy, organic-rich mud-dominated settings are associated with preservation of aragonitic molluscs. Infaunal bivalves are a prominent component of shell plasters or pavements in such settings, linked to episodic bottom water anoxia. Decaying algal blooms drew the redox boundary up above the sediment–water interface, and brought populations of infaunal bivalves to the surface where they died. Isolated from the oxic taphonomically active zone, the shells were not dissolved and were buried as thin shell layers. In similar settings, aragonitic shells were preserved as moulds through early pyritisation, or even through preservation of original shell aragonite.In oxic environments, bioturbational reworking of surface sediment destroyed moulds of aragonitic shells after early dissolution. In some hardgrounds, these delicate moulds were preserved due to synsedimentary cementation, probably using carbonate released by aragonite dissolution. The examples included here come from both intervals of “calcite” and “aragonite” seas, and it is not possible to assess whether the saturation state (with respect to aragonite) of the ambient sea water played a role in the selective removal of aragonitic shells.While taphonomic windows may have captured the diversity of individual groups, it is clear from quantitative data involving skeletal lagerstätten that the scale of loss from early aragonite dissolution has drastically altered the trophic composition of some fossil assemblages commonly used as the basis for reconstructions of past communities.