Closed-system behaviour of the intra-crystalline fraction of amino acids in mollusc shells

When mollusc shells are analysed conventionally for amino acid geochronology, the entire population of amino acids is included, both inter- and intra-crystalline. This study investigates the utility of removing the amino acids that are most susceptible to environmental effects by isolating the fraction of amino acids encapsulated within mineral crystals of mollusc shells (intra-crystalline fraction). Bleaching, heating and leaching (diffusive loss) experiments were undertaken on modern and fossil Corbicula fluminalis, Margaritifera falcata, Bithynia tentaculata and Valvata piscinalis shells. Exposure of powdered mollusc shells to concentrated NaOCl for 48 h effectively reduced the amino acid content of the four taxa to a residual level, assumed to represent the intra-crystalline fraction. When heated in water at 140 °C for 24 h, only 1% of amino acids were leached from the intra-crystalline fraction of modern shells compared with 40% from whole shell. Free amino acids were more effectively retained in the intra-crystalline fraction, comprising 55% (compared with 18%) of the whole shell after 24 h at 140 °C. For fossil gastropods, the inter-shell variability in D/L values for the intra-crystalline fraction of a single-age population was reduced by 50% compared with conventionally analysed shells. In contrast, analysis of the intra-crystalline fraction of C. fluminalis does not appear to improve the results for this taxon, possibly due to variability in shell ultrastructure. Nonetheless, the intra-crystalline fraction in gastropods approximates a closed system of amino acids and appears to provide a superior subset of amino acids for geochronological applications.     

Intra-crystalline protein diagenesis (IcPD) in Patella vulgata. Part I: Isolation and testing of the closed system

This study successfully isolates a fraction of intra-crystalline proteins from shells of the marine gastropod Patella vulgata and assesses the suitability of these proteins for IcPD (Intra-crystalline Protein Diagenesis) geochronology. We discuss the mineralogical composition of this gastropod, investigated for the first time by X-ray diffraction mapping, and use the results to inform our sampling strategy. The potential of the calcitic rim and of a bulk sample (containing both apex and rim) of the shell to act as stable repositories for the intra-crystalline proteins during diagenesis is examined. The composition and the diagenetic behaviour of the intra-crystalline proteins isolated from different locations within the shell are compared, highlighting the necessity of targeting consistent sampling positions.We induced artificial diagenesis of both intra-crystalline and whole-shell proteins by conducting high-temperature experiments in hydrous environment; this allowed us to quantify the loss of amino acids by leaching and therefore evaluate the open- or closed-system behaviour of the different fractions of proteins. The results obtained provide further confirmation that patterns of diagenesis vary according to the protein sequence, structure, and location within or outside the intra-crystalline fraction. As Patella is frequently found in the fossil record, both in archaeological and geological contexts, the application of IcPD geochronology to this biomineral opens up the possibility to obtain reliable age information from a range of sites in different areas of the world.     

Intra-crystalline protein diagenesis (IcPD) in Patella vulgata. Part II: Breakdown and temperature sensitivity

Artificial diagenesis of the intra-crystalline proteins isolated from Patella vulgata was induced by isothermal heating at 140 °C, 110 °C and 80 °C. Protein breakdown was quantified for multiple amino acids, measuring the extent of peptide bond hydrolysis, amino acid racemisation and decomposition. The patterns of diagenesis are complex; therefore the kinetic parameters of the main reactions were estimated by two different methods: 1) a well-established approach based on fitting mathematical expressions to the experimental data, e.g. first-order rate equations for hydrolysis and power-transformed first-order rate equations for racemisation; and 2) an alternative model-free approach, which was developed by estimating a “scaling” factor for the independent variable (time) which produces the best alignment of the experimental data. This method allows the calculation of the relative reaction rates for the different temperatures of isothermal heating.High-temperature data were compared with the extent of degradation detected in sub-fossil Patella specimens of known age, and we evaluated the ability of kinetic experiments to mimic diagenesis at burial temperature. The results highlighted a difference between patterns of degradation at low and high temperature and therefore we recommend caution for the extrapolation of protein breakdown rates to low burial temperatures for geochronological purposes when relying solely on kinetic data.     

Isolation of the intra-crystalline proteins and kinetic studies in Struthio camelus (ostrich) eggshell for amino acid geochronology

Ostrich eggshell is a favoured substrate for amino acid geochronology, yielding consistent results and thought to approximate a closed system with respect to protein diagenesis. We found that the intra-crystalline fraction in ostrich eggshell is more challenging to isolate than that from mollusc shells, requiring 72 h oxidative treatment with NaOCl, resulting in a loss of up to half of the whole-shell protein. Through high temperature studies we have shown that under continuous leaching conditions approximately 99% of the intra-crystalline amino acids are resistant to leaching. Furthermore, high temperature experiments of the intra-crystalline proteins at pH 5, 7 and 9 have shown that this fraction is unaffected by pH changes over this range. This study confirms that the intra-crystalline protein fraction in OES may approximate a closed system. The intra-crystalline amino acids have been shown to follow predictable patterns of hydrolysis and racemization for all amino acids studied. Most amino acids showed some conformity to pseudo reversible first order reaction kinetics for racemization over limited D/L ranges, but hydrolysis was generally described poorly by first order kinetics. We therefore tried a new scaling method to determine relative reaction rates, estimated by applying scaling factors to overlap the observed rates of racemization (or hydrolysis) at the different temperatures, with the advantage of not forcing a linear relationship with respect to time. Using apparent first order kinetics, constrained power functions and scaling methods, we estimated the Arrhenius parameters (activation energy, E_A and frequency factors, A) for both hydrolysis and racemization. The different methods for the estimation of reaction rates can give significantly different, but equally plausible, activation energies. This study reinforces previous work that indicates we need to understand the underlying mechanisms in order to estimate accurate kinetic parameters.     

Pecten as a new substrate for IcPD dating: The quaternary raised beaches in the Gulf of Corinth,Greece

Intra-crystalline protein diagenesis (IcPD), a recent development of amino acid racemization dating (AAR), is now established as a reliable geochronological tool for the Quaternary. However, extending the method to new biominerals requires extensive testing in order to provide evidence for the closed-system behaviour of the intra-crystalline proteins and to assess the temporal span that can be covered.Here we present results from high-temperature experiments on the IcPD of the bivalve Pecten, demonstrating that a fraction of proteins can be isolated from a bleach-resistant mineral matrix, which effectively operates as a closed system under conditions of accelerated diagenesis in the laboratory. Analyses of Pecten from the well-dated terrace system of the Gulf of Corinth (Greece) provided a pilot test for the integrity of the intra-crystalline fraction in subfossil shells. The small sample sizes in this preliminary study preclude a full assessment of the aminostratigraphic power of Pecten IcPD, but a concordance is observed between the extent of IcPD and sites dating from between MIS 5 and MIS 11.We conclude that Pecten is a potentially good substrate for IcPD dating in the Mediterranean, and that the temporal limit of the technique in this area lies beyond MIS 11.     

Amino acid racemization in four species of ostracodes: Taxonomic,environmental, and microstructural controls

Here we quantified the aspartic acid and glutamic acid racemization rates of the four main ostracode species (Herpetocypris reptans, Candona neglecta, Ilyocypris gibba and Cyprideis torosa) present in several Iberian Peninsula localities covering a wide chronological range (ca. 1 Ma to present). At low D/L values (at Asp D/L < 0.40; and Glu D/L = 0.09–0.18), H. reptans racemized at higher rates than C. neglecta, C. torosa and I. gibba. In contrast, for Asp D/L > 0.4 and Glu D/L > 0.18, H. reptans, C. neglecta and C. torosa showed similar racemization rates. I. gibba exhibited the lowest D/L values in old samples (Middle and Lower Pleistocene). We attribute these differences in amino acid racemization rates mainly to variations in valve protein composition. We found that the microstructure of the valves of each species (size, morphology, and arrangement of crystals) differed, but did not appear to change over time (at least for the last ca. 1 Ma). Such differences may also be linked to the type of proteins involved in the respective calcification processes of these organisms. On the basis of our results, and given that other studies have demonstrated that the majority of inter-crystalline proteins are leached early after death (a few centuries or millennia), we propose that the degradation rates of the most resistant inter- and intra-crystalline proteins in each species differ depending on the protein composition of the valves. Although further research is required, we suggest that amino acid racemization in each ostracode species might be related to valve microstructure.     

Testing the limitations of artificial protein degradation kinetics using known-age massive Porites coral skeletons

High-temperature isothermal heating of biominerals has commonly been used to artificially accelerate protein degradation in order to extrapolate kinetic parameters to the lower temperatures experienced in vivo and in the burial environment. It is not easy to test the accuracy of these simulations due to the difficulty of finding samples of known age held at a known temperature. We compare protein degradation in the intra-crystalline organic matrix of heated (80 °C, 110 °C, and 140 °C) massive Porites sp. coral to that directly measured in the skeleton of colonies growing at ∼26 °C and deposited over the last five centuries. This provides the opportunity to critically evaluate the underlying assumption that high-temperature experiments accurately mimic degradation processes and kinetics occurring in a ‘naturally aged’ biomineral. In all samples the intra-crystalline protein fraction was isolated and the L- and D- concentration of multiple amino acids measured using reverse-phase high-performance liquid chromatography (RP-HPLC). There was no evidence of a failure of the closed system in the high-temperature experiments (assessed by X-ray diffraction, thermogravimetric analyses and determination of leached amino acid concentration). We compared conventional methods for estimation of kinetic parameters with a new ‘model-free’ approach that makes no assumptions regarding the underlying kinetics of the system and uses numerical optimisation to estimate relative rate differences. The ‘model-free’ approach generally produced more reliable estimates of the observed rates of racemization in ‘naturally aged’ coral, although rates of hydrolysis (as estimated from the release of free amino acids) were usually over-estimated. In the amino acids for which we were able to examine both racemization and hydrolysis (aspartic acid/asparagine, glutamic acid/glutamine and alanine), it was clear that hydrolysis was less temperature sensitive than racemization, which may account for the differences in degradation patterns observed between the ‘naturally aged’ coral and high-temperature data. It is clearly important to estimate the individual temperature dependence of each of the parallel reactions.     

Radiocarbon-calibrated multiple amino acid geochronology of Holocene molluscs from Bramble and Rib Reefs (Great Barrier Reef,Australia)

Calibrated amino acid racemisation methods allow paleobiologists to quantify the age distributions of fossil assemblages. Focussing on 110 Scissulina dispar and 110 Liloa sp. specimens collected from Bramble and Rib Reefs (central Great Barrier Reef, Australia), we create calibration curves for seven amino acids for each taxon. Using these curves we calculate seven quasi-independent age estimates for each specimen. We evaluate each calibration curve for consistency and use the weighted mean and uncertainty of the quasi-independent ages as the specimen age for geochronological analyses. We extend the “Y” criterion for screening specimens and describing the precision of an AAR dataset from two amino acids to any number of amino acids. Using weighted mean ages and Y < 0.2 we demonstrate that the top 1.4 m of Bramble and Rib Reefs preserve remarkably well-mixed shell assemblages spanning from living to ∼3400 years old with median ages of 373 and 326 years old, respectively.     

Testing the effect of oxidizing pre-treatments on amino acids in benthic and planktic foraminifera tests

Amino acid racemization (AAR) is a geochronological method that uses the ratio of D- to L-configurations in optically active amino acids from carbonate fossils to determine the time elapsed since the death of an organism. Although AAR techniques have been widely applied to foraminiferal tests, there have been limited dedicated assessments of the potential of isolating a bleach-resistant, intra-crystalline fraction of proteins to improve the reliability of AAR in this biomineral system. In this study, we evaluate the effect of two oxidative pre-treatments (hydrogen peroxide and bleach) on amino acid concentrations and D/L values in sub-modern benthic foraminifers (Ammonia spp. and Haynesina germanica) and well-preserved mid Holocene and mid Pleistocene planktic foraminifers (Pulleniatina obliquiloculata, Globorotalia truncatulinoides, and Globorotalia tumida). The oxidative pre-treatments successfully reduced the amino acid content of the foraminiferal tests to a residual fraction, and with the exception of Ammonia spp., neither pre-treatment substantially affected the relative proportion of individual amino acids. The bleaching pre-treatment does not consistently alter D/L values when compared to peroxide pre-treatment, but it does tend to reduce the subsample variability in D/L values, albeit only to a small degree in an inconsistent fashion. Therefore, we recommend that a relatively weak oxidative pre-treatment with 3% hydrogen peroxide is sufficient for foraminifera-based AAR applications.     

Radiocarbon age anomalies of land snail shells in the Chinese Loess Plateau

In the Chinese Loess Plateau, land snail shells are often the only material available for dating in paleoenvironmental and archaeological research. However, the geochronological suitability of land snail shells is limited because of poor knowledge about their deposition dynamics, particularly with regards to the incorporation of inorganic carbonate and the resulting age anomalies. To evaluate the factors controlling these age anomalies, radiocarbon and stable carbon analyses were carried out on surface soils, as well as the shells and organic bodies of different modern snail species from different ecological habitats. The results showed that all specimens were depleted in ¹⁴C, indicating the influence of inorganic, radiocarbon-free carbonate on the ¹⁴C-activity of the snail shells. The apparent ¹⁴C-deficiencies and the resultant age anomalies of both the Cathaica and the Bradybaena snail shells were within close ranges across the Chinese Loess Plateau, indicating that the shells of these species could, after corrections for radiocarbon anomalies, provide reliable age estimates. The apparent ¹⁴C-deficiencies were closely associated with the ecological habitats of the snails. The shells of the ground-dwelling Bradybaena had the smallest age offsets (533 ± 150 a), followed grass-dwelling Cathaica (1107 ± 138 a) and Cathaica living on trees (1550 ± 345 a). These results suggest that the availability of calcium in the respective ecological habitats is an important factor in explaining the apparent ¹⁴C-deficiencies. The influence of carbonate on the stable carbon isotope composition of shells is overwhelmed by the organic diets of snails, making δ¹³C unsuitable for identifying and correcting shell age anomalies. The radiocarbon activities of surface soils (A_calc) increase with weathering intensity. Thus, a significant uncertainty could be caused by assuming that A_calc is zero when estimating the proportions of different carbon sources in shells, as has been the case in most previous studies.     

Identifying outliers and assessing the accuracy of amino acid racemization measurements for geochronology: I. Age calibration curves

Numerical ages derived from amino acid racemization (AAR) geochronology are typically based on calibration curves that relate the extent of AAR to the age of independently dated specimens. Here, we compare options for developing calibration curves and quantifying age uncertainties using AAR data from 481 late Holocene shells, and AMS ¹⁴C analyses of 36 shells of four molluscan taxa (Ethalia, Natica, Tellina, and Turbo) collected from shallow sediment cores from a back-reef lagoon of the central Great Barrier Reef. The four taxa differed substantially in the quality of their geochronogical results. Explicitly including data from specimens alive at the time of collection improves calibration curves, but weighting numerical ages based on their uncertainty has no effect. Calibration curve statistics do not adequately assess calibration uncertainty. The relation between ages inferred from different amino acids is recommended for identifying aberrant specimens and quantifying the uncertainty of inferred ages. For this study, the AAR ages based on two amino acids (aspartic acid and glutamic acid) exceed 200 yr or 20% of their mean inferred age in 15% of the specimens. Once these were removed, the mean age error (1σ) for individual specimens based on two amino acids analyzed in duplicate subsamples ranged from 53 to 142 yr for Tellina and Turbo, respectively, or about a 30% age error for these relatively young shells. This compares favorably with analytical errors estimated at 50 yr or 5%. The presence of notable outliers undetectable using data from single amino acids emphasizes the importance of analyzing multiple amino acids.     

Variations in racemization/epimerization ratios and amino acid content of Glycymeris shells in raised marine deposits in the Mediterranean

In this study we compare the organic geochemistry of fossil and modern Glycymeris shells. Amino acids were preserved within the shells. The amino acid content of the shells was similar at all the sites studied. Amino acid racemization and epimerization of Glycymeris shells are suitable techniques for dating Pleistocene raised marine deposits. As reported in other studies, we found that isoleucine epimerization analysis has a greater capacity to discriminate between sites of different age than glutamic acid and aspartic acid. However, particular constraints regarding the use of amino acid dating concern intrashell variability, so to avoid divergent results, it is necessary to sample the same part of the shell, namely the complex cross lamellar region near the umbo. The dating of high-energy coastal marine deposits calls for extensive field work in order to ensure the collection of a large number of samples in order to obtain robust results and reject spurious values. Shell accumulations on the shore-line are conditioned by several factors. The high coefficients of variation for epimerization values and their distribution pattern can be attributed mainly to time-linked taphonomical processes (time-averaging) that gave rise to the shell-bearing bed. However, reworking from former highstand sea level deposits, which usually occupy higher topographic levels, is not a common occurrence. Therefore, as a result of time-averaging and post-depositional processes, it is difficult to identify substages in stacked shell beds in raised beach deposits belonging to the same marine oxygen isotope stage by means of amino acid racemization/epimerization.     

A Cerion-based chronostratigraphy and age model from the central Bahama Islands: Amino acid racemization and 14C in land snails and sediments

The taxonomy of the land-snail genus Cerion, and the relevance of hundreds of shell forms to phylogenetic and evolutionary processes has been the subject of much discussion over the past few decades. Fundamental to understanding evolutionary trends is the independent confirmation of the age, biostratigraphic, and chronostratigraphic order of a comprehensive set of fossil forms. Amino acid racemization (AAR) is a geochronological tool applicable to dating living and fossil Cerion shells. The extent of racemization of aspartic acid (Asp D/L) and glutamic acid (Glu D/L) were determined for 507 Cerion shells dating between the early last interglaciation (marine isotope stage (MIS) 5e, c. 125 ka) and the present from the Bahamas. The AAR data based on Cerion support the established morphostratigraphic succession of deposits younger than MIS 5e and provide greater resolution, particularly for biostratigraphic successions of Holocene age. Age models were constructed for Asp and Glu using D/L values paired with ¹⁴C ages and historical collection dates from 23 Cerion shells. In addition to the relative-age time series, the Holocene age models developed from D/L Glu and Asp apply to the last 7 ka and geographically within the central Bahamas. In two case studies, large numbers of AAR-generated ages provide insights into the age structure of accretionary soils and rates and age constraints on evolutionary processes related to Cerion land snails.     

Chronostratigraphy and morphological changes in Cerion land snail shells over the past 130 ka on Long Island,Bahamas

Despite the nearly 600 named species of the land snail Cerion, studies of the geological and paleontological framework of modern species are few. To address this deficiency, the biostratigraphic succession of Cerion was investigated at several areas on Long Island, Bahamas. A chronostratigraphic framework was developed through whole-rock and Cerion land snail aminostratigraphies. About 175 individual Cerion shells from last interglacial and Holocene deposits were age-ranked using stratigraphic position and amino acid racemization (AAR) geochronology. AAR ages were generated using an existing AAR-¹⁴C age model for Cerion from the central Bahamas. The age structure of Cerion fossils in sediments was determined with AAR ages, and the magnitude of “dead carbon” anomalies was evaluated using this chronological approach.Temporal changes in gross shell morphology were examined from four study areas. The last interglacial, marine isotope stage/substage (MIS) 5e (Aminozone E) is characterized by generally large shells and in some cases, bimodal sets of very small (α shells) and very large forms (β shells) coexisting in the same stratigraphic levels (primarily soils), which may encompass the transition from between MIS 5e and 5d/c. Similar bimodality of nearly identical α and β shell forms and sizes is observed at other late MIS 5e sites from the furthest reaches of Great Bahama Bank (including Long, Exumas, Eleuthera, and New Providence Islands). The widespread distribution of α and β forms in soils capping MIS 5e marine and eolian deposits implies that there may have been a synchronous, regional morphological convergence on Great Bahama Bank. None of these forms are observed in Holocene deposits of Aminozone A.The earliest MIS 1 Cerion appear in a oolite deposited 6500 a BP, and are of intermediate size compared to the Pleistocene α and β forms. As MIS 1 progressed, the diversity of shell sizes and shapes increased into modern times. The greater variety of shell forms over the past 1000–2000 a suggests that humans may have played a role in the introduction and redistribution of Cerion across the region. The potential for frequent and widespread human introductions, combined with the propensity of Cerion to hybridise freely may explain the farrago of shell sizes and shapes in the recent snail faunas of Long Island and other Bahama islands.     

Calculation of the reservoir age from organic and carbonate fractions of sediments in the Gulf of Cariaco (Caribbean Sea)

A set of 24 AMS ¹⁴C measurements were performed on different sediment fractions from short gravity cores taken from the Gulf of Cariaco (northeastern Venezuela) in order to construct a detailed sedimentary archive for recent centuries. A local reservoir effect was expected because of i) strong upwelling from the neighboring Cariaco Trough and anoxia and ii) the specific geomorphological setting related to active faults with seeps and diapirism. Measurements were thus performed on different components: sediment bulk organic fraction (SBOF), pteropods shells (surface water), bivalve shells (bottom), and plant fragments. Based on comparisons of the different results, we discuss different age corrections and calibrations, which lead us to consider a negligible local reservoir correction (ΔR) for the bivalve and pteropod shells, which show no reservoir age for carbonate and an absence of water stratification. However, a local 633 ± 64 yr ΔR appears in the organic fraction of the sediment, and its origin is primary related to the upwelling mechanism.     

Amino acid racemization in mono-specific foraminifera from Quaternary deep-sea sediments

The deep-sea environment is among the most stable on Earth, making it well suited for amino acid geochronology. Foraminifera with calcareous tests are distributed across the World Ocean and are often recovered in sufficient abundance from sediment cores to derive robust mean amino acid D/L values of multiple replicates from each stratigraphic level. The extent of racemization (D/L) can be compared with independent age control, which in most cases is based on correlation with global marine oxygen-isotope stages and radiocarbon ages from the same stratigraphic levels. In this study, we report the results of amino acid racemization analysis of multiple foraminifera species from well-dated sediment cores taken from the Pacific, Atlantic, and Arctic oceans. The composite of results analyzed to date (179 samples, each composed of an average of 8.6 subsamples = 1531 analyses) show that D/L values generally increase systematically down core, and are similar for samples of comparable ages from different deep-sea sites. Previously published equations that relate D/L values of aspartic and glutamic acids to post-depositional temperature and sample age for Pulleniatina obliquiloculata generally conform to the D/L trends for species analyzed in this study. Laboratory heating experiments were used to quantify the difference in the rate of racemization between P. obliquiloculata and other taxa. For example, aspartic acid in P. obliquiloculata racemizes an average of 12–16% faster than in the common high-latitude species, Neogloboquadrina pachyderma (s). Apparently, the unexpectedly high D/L values previously reported for N. pachyderma (s) older than 35 ka from the Arctic Ocean cannot be attributed to taxonomic effects.     

Differential concentration of Technetium-99 (99Tc) in common intertidal molluscs with different food habits

Concentration of ⁹⁹Tc has been measured in fucoids and molluscs, sampled in a sheltered intertidal at the southwest coast of Norway from February to November 2006. The concentrations of ⁹⁹Tc in molluscs differed significantly between species. The filtering bivalve Mytilus edulis had the lowest concentrations with averages of 2.3-5.9 Bq kg⁻¹ d.w., while the herbivorous gastropods Littorinalittorina, Littorina obtusata and Patella vulgata had higher concentrations. P. vulgata and L. obtusata had the highest concentrations, 40-47 and 26-30 Bq kg⁻¹ d.w., respectively. L. obtusata has a specialized habit of living, and prefers to feed on fucoids. P. vulgata can graze extensively on the fucoid Ascophyllum nodosum when available. Fucoids are known to have very high uptake of ⁹⁹Tc, and this was also found in the present study. The high ⁹⁹Tc-concentrations of L. obtusata and P. vulgata are most likely due to their habit of feeding on fucoids.     

Calibration of decomposition of serine to alanine in Bithynia opercula as a quantitative dating technique for Middle and Late Pleistocene sites in Britain

The concentration ratio of the amino acids alanine and serine in calcite opercula of freshwater gastropods of genus Bithynia has been assessed as an age proxy for British Middle and Late Pleistocene sites. Due to decomposition of peptide-bound serine within intra-crystalline protein molecules, alanine/serine ratios increase with sample age, enabling resolution of the marine oxygen-isotope stage of almost all sites and substage resolution for some sites. However, slight overlaps are evident in alanine/serine ratios between some sites from late in one temperate stage and early in the next; this effect results from the low decomposition rates during cold stages, combined with an evident slight dependence of decomposition rates on site conditions. The technique, which can be extended to the Early Pleistocene without technical refinements, is amenable to quantitative calibration using a first-order rate equation, making it feasible in future to present results as numerical ages, subject to an assumed temperature history. Most Pleistocene sites investigated are shown using this technique to have the same ages as previously deduced using biostratigraphy. However, the serine–alanine ages obtained preclude glaciation of southeast England during MIS 16 or of central England during MIS 10.     

Large 14C age offsets between the fine fraction and coexisting planktonic foraminifera in shallow Caribbean sediments

Absolute chronologies in paleoceanographic records are often constructed using the ¹⁴C dating of coarse fraction foraminifera (>150 μm). However, due to processes such as changes in sediment sources or abundances, sedimentation rates, bioturbation, reworking, the adsorption of modern carbon, etc., several studies conducted in different environmental settings have shown time-lags between records obtained from various granulometric fractions. In this study, we examined temporal phasing between the coarse foraminifera and fine fractions by studying changes in the abundances of δ¹⁸O, the ¹⁴C ages of the planktonic foraminifera Globigerinoides ruber (G. ruber, 250–350 μm), and the sediment fine fraction (<63 μm) over the last 45 ka in a core obtained from the northern Caribbean Sea. All of the records were found to be in phase during part of the Holocene (at least for the last ≈6 ka). As determined from δ¹⁸O records and ¹⁴C ages, the fine fraction was younger than G. ruber during the Last Deglaciation (of 1.89 ka). The coupling between bioturbation and changes in the fine fraction, and G. ruber abundances, as tested using a numerical model of the bioturbation record within a mixed-layer depth of 8 cm, was sufficient to explain the results. ¹⁴C age discrepancies increased from 5.64 to 8.5 ka during Marine Isotopic Stages (MIS) 2 and 3, respectively. These chronological discrepancies could not be explained by only one process and seemed to result from the interplay between mechanisms: size-differentiated bioturbation (for 1.5 to 2.5 ka), the adsorption of modern atmospheric CO₂ (for 3.04 to 5.92 ka), and variations in sedimentological processes that influenced the fine carbonate fraction. However, even if variations in the mineralogical composition of the fine carbonate fraction were identified using scanning-electron microscopy observations, X-ray diffraction measurements, and geochemical analyses (the mol % MgCO₃ of magnesian calcite and the Sr/Ca ratio of the bulk fine fraction), they can not account for the observed age differences. The results presented for core MD03-2628 extend beyond this case study because they illustrate the need for a detailed characterization of the various size fractions prior to paleoclimate signal interpretations, especially for chronological studies.     

Mathematical expressions used in amino acid racemisation geochronology—A review

A variety of mathematical expressions that describe changes over time (t) in the extent of amino acid racemisation (AAR, expressed as the ratio of d- to l-amino acid isomers or epimers) have been used in Quaternary geochronology. The integrated rate equation was first used to estimate fossil age from D/L but its geochronological utility is disadvantaged by uncertainties regarding the conformity of AAR in fossil protein to apparent reversible first-order kinetics for the entire reaction history. ‘Non-linear’ models have subsequently been used to relate D/L to t. The logarithmic equation successfully applied to Atlantic Coastal Plain research has not achieved widespread application, perhaps due to the regional calibration required if sensitivity to temperature is to be modelled, or the difficulties encountered when extending the model to include fossils with D/L<0.1. Success producing a linear correlation between D/L transformed with a power function and t has seen this approach emerge as one of the most commonly applied in AAR geochronology in recent years. Like parabola curve fitting, which has been applied to trends in D/L versus t in a variety of fossils and geographic settings, power transformations may not be suitable for geochronological modelling during the latter stages of amino acid diagenesis. Several studies have demonstrated the utility of simple and contingent linear equations for relating D/L to t. Future research should aim to reduce reliance on independent calibration and explore the geochronological benefits of AAR in pools other than the total hydrolysable amino acids.