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.     

Three approaches to radiocarbon calibration of amino acid racemization in Mulinia lateralis from the Holocene of the Chesapeake Bay,USA

A radiocarbon-calibrated aminochronology, based on the bivalve Mulinia lateralis, is presented for Chesapeake Bay core MD03-2661, a 25 m piston core drilled near Kent Island (38°53.21′N; 76°23.89′W) during the 2003 USGS Marion-Dufresne cruise. Three separate approaches were used to calibrate amino acid racemization (AAR) data for aspartic acid with radiocarbon data. For the first approach, a direct or paired analysis calibration incorporated eight articulated specimens, thereby allowing for the application of AAR and radiocarbon analysis of the same specimen and effectively eliminating both intrashell variability and time averaging as factors in the calibration. A second direct approach relied on valves that were bilaterally split to facilitate both AAR and radiocarbon dating, thus effectively eliminating time averaging effects from this calibration. For the third indirect approach, nine independent radiocarbon dates were combined with 129 Asx D/L ratios from the same core depths to produce an indirect calibration model, from which intershell variability and time averaging could be estimated. Variability in AAR ratios was recognized from a myriad of sources, including analytical error, intrashell variability, inherent variability, time averaging, and contamination. The majority of this variability was controlled for through experimental design or by the application of these three independent calibration approaches. The direct calibration of articulated shells and the indirect calibration yielded virtually identical age models, well within their respective 95% confidence intervals. This study establishes an aminostratigraphic reference section for the Holocene record of the Chesapeake Bay and demonstrates the usefulness of multiple calibration approaches and the potential utility of AAR for future studies of sedimentary processes and chronologies in the bay.     

Long-term preservation of amino acid racemization interlaboratory comparison samples

This report summarizes the present status and future preservation of four Interlaboratory Comparison Samples (ILC) used in the Amino Acid Racemization (AAR) community over the past 40 years. The ILC samples are now housed in the archival collection of Lamont-Doherty Earth Observatory.     

The effects of the geothermal gradient on amino acid racemization

The “second region” of the kinetic curve for the racemization (epimerization) of isoleucine in foraminifera is defined from previously published data corrected for the thermal history experienced by the samples. These kinetic parameters are applied to racemization data from Deep Sea Drilling Project Sites 332A and 333 from the Deep Drill Valley, Mid-Atlantic Ridge, and Site 148 from the Aves Ridge, in order to determine whether there are any observable effects due to the geothermal gradient. The data for Site 148 clearly show an increase of temperature with depth. The data for the two sites of the Mid-Atlantic Ridge do not show this clear relationship. When bottom water temperature variations are taken into consideration, the effects of the geothermal gradient become apparent. Since the degree of racemization is dependent upon both age and temperature, a knowledge of the age of a sample places constraints upon its thermal history, and hence on the heat flow at the location since deposition of the sample. The crude heat flow models thus developed are compatible with present geologic and geophysical information. It appears probable that detailed heat flow models may be developed by improving analytical precision.     

Results from an amino acid racemization inter-laboratory proficiency study; design and performance evaluation

It is nearly thirty years since the last inter-laboratory study was carried out for amino acid racemization (AAR) analysis using powdered fossil material (Wehmiller 1984). Since then there have been major changes in sample preparation and instrumentation, and it was considered timely to coordinate a new inter-laboratory study in support of current methodologies. In 2010, two such studies were undertaken. The first of these, coordinated by Wehmiller (this edition), used homogeneous hydrolysates of Pleistocene mollusc and eggshell materials and focused on the agreement of analytical measurements between laboratories, without interference from differing sample preparation procedures. The second (this study) was designed specifically as a proficiency test. Unlike previous inter-laboratory comparisons that have focussed on precision estimates, the purpose of this study was to carry out an evaluation of measurement bias by comparing the measurement results of laboratories carrying out their routine methods, including extraction, against the consensus values. Participants were sent one dried sample of a mixed amino acid standards solution and five homogeneous powders: two Pleistocene mollusc test materials prepared from material (ILC-A) supplied and used by Wehmiller in previous inter-laboratory studies (1984; and this edition), one Pleistocene opercula test material from the terrestrial gastropod, Bithynia tentaculata, and two heat-treated modern ostrich eggshell test materials. Results from this study demonstrate that whilst individual laboratory precision may be excellent, suggesting good control of random error influences (less than 1% for replicate measurements by some individual laboratories), agreement between methods, or even between laboratories carrying out the same method, may be very different. Trueness evaluation (determined as the relative percentage bias) reveals the extent of the disagreement reflected by the inter-laboratory variability. Individual laboratory D/L value biases of 10–30% or more when compared to the consensus values are not uncommon. We demonstrate why bias contributions should also be included in AAR uncertainty estimation and recommend that the preparation of defined reference materials are seen as a priority in order to control and correct for systematic error influences in the analytical system.     

Temperature calibration of amino acid racemization age implications for the Yuha skeleton

D/L of aspartic acid ranged from 0.52 to 0.56 for femur samples of the Yuha skeleton. Subsurface temperature measurements made at the burial site indicate average annual temperature is 18°C and diagenetic temperature is 21.6°C. These data and a relation derived for the dependence of the aspartic acid rate constant on diagenetic temperature indicate an age of 23,600. The result is consistent with¹⁴C and²³⁰Th dating of calcrete found coating the bones.     

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.     

A chronological framework for the Clyde Foreland Formation,Eastern Canadian Arctic,derived from amino acid racemization and cosmogenic radionuclides

The most extensive terrestrial outcrops of glacial and glaciomarine deposits in the Eastern Canadian Arctic are exposed in sea cliffs along the Clyde Foreland and Qivitu Peninsula of Baffin Island. Collectively known as the Clyde Foreland Formation (CFF), these stacked deposits record at least seven glacial advances. Despite having been the focus of numerous investigations spanning nearly 50 years, no numerical chronological framework for the age of the deposits has been established. Previous studies relied on biostratigraphy and amino acid racemization (AAR) geochronology and postulated that the oldest units were Late Pliocene to Mid-Pleistocene in age. In this paper, we use a cosmogenic radionuclide isochron approach to determine a minimum age for the burial of a paleosol preserved within the CFF. Abundant palynomorphs in the paleosol are dominated by cool-climate taxa. Combining the paleosol burial age with a compilation of published and new CFF AAR data for marine bivalves Hiatella arctica and Mya truncata, we statistically define seven CFF aminozones and develop a piecewise isoleucine AAR calibration model for Baffin Island. From this, we estimate the minimum age of each aminozone, although the propagation of errors through all calculations produces large uncertainties for each age estimate. The youngest three CFF units, known as the Kogalu, Kuvinilk, and Cape Christian members, were most likely deposited during glaciations in the Mid- to Late-Pleistocene. The paleosol formed prior to 1.15 ± 0.20 Ma, and the underlying aminozones represent sedimentation during Early Pleistocene or latest Pliocene glaciations and record early advances of Laurentide ice across Baffin Island.     

Identifying outliers and assessing the accuracy of amino acid racemization measurements for geochronology: II. Data screening

Amino acid racemization (AAR) is a cost-effective method for dating the large numbers of specimens required for time-averaging studies. Because the aim of time-averaging studies is to determine the structure of the age distribution, any data screening must be done cautiously and systematically. Methods to quantitatively assess the quality of AAR data and to identify aberrant specimens are under-developed. Here we examine a variety of screening criteria for identifying outliers and determining the suitability of specimens for numerical dating including: high serine concentrations (modern contamination), covariance of aspartic acid (Asp) and glutamic acid (Glu) concentrations (diagenetic influences), replication of measurements (specimen heterogeneity), and the relation between Asp and Glu d/l values (internal consistency). This study is based on AAR analyses of 481 late Holocene shells of four molluscan taxa (Ethalia, Natica, Tellina, and Turbo) collected from shallow sediment cores from the central Great Barrier Reef. Different outliers are flagged by the different screening criteria, and 6% of specimens were found to be unsuitable for time-averaging analyses based on screening the raw AAR data. We recommend a hybrid approach for identifying outliers and specimens for numerical dating.     

Constraining Holocene sea levels using U‐Th ages of phreatic overgrowths on speleothems from coastal caves in Mallorca (Western Mediterranean)

Phreatic overgrowths on speleothems (POS) are carbonate formations deposited at the water table of caves in unique karstic coastal settings having morphologies that can be directly related to sea level at the time of formation. The U‐Th ages of calcite and aragonite overgrowths collected from the modern water table in coastal caves on Mallorca (Cova de Cala Varques A and Cova des Pas de Vallgornera) were determined using high‐precision MC‐ICPMS techniques. U‐Th ages indicate that phreatic carbonate deposition occurred between ca 2·8 and at least 0·6 ka BP and are in accord with an archeologically estimated age of 3·7–3·0 ka BP for a drowned prehistoric construction at a depth of 1 m below current sea level in a cave from the same area. Speleothem δ¹³C and δ¹⁸O and chemical composition of cave pools provide supportive evidence that POS reflect mixing between seawater and brackish water table. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Radium-226 chronology of Holocene travertine from Esanatoglia (Central Italy)

Here we investigate the feasibility of using alpha-spectrometric ²²⁶Ra methodology to date low U freshwater carbonate deposits (<0.1 μg g⁻¹) by analysing Holocene freshwater travertine deposits from Esanatoglia, Umbro-Marchean Apennines, Central Italy. Previously, such methods have been tested on hydrothermal deposits, which are characterised by high Ra and U. We calculate a ²²⁶Ra age estimate of 2.9±0.4 ka (1σ) for Holocene travertine that is in reasonable agreement with archaeological evidence by assuming that the ²²⁶Ra initial activity of the Holocene travertine is comparable with that of present-day carbonate deposits in the same area. We also investigate the use of Ba to normalise ²²⁶Ra activities. Comparison of age estimates based on Ra methods with ages derived from alpha-spectrometric ²³⁰Th/²³⁴U isochron techniques for the same co-eval sub-samples indicates that, where only alpha-spectrometric methods are available, the former are likely to be much more useful for Holocene travertine material, especially if there is a significant detrital component.     

Evaluating Quaternary dating methods: Radiocarbon,U-series,luminescence, and amino acid racemization dates of a late Pleistocene emu egg

A whole emu egg, with infilling sediment believed to be coeval with egg laying and burial, was found in late Pleistocene lunette sediments near Lake Eyre, central Australia. The stratigraphic context and initial amino acid racemization (AAR) results suggested an age between 25 ka and 35 ka, ideal for a multiple cross-dating comparison. The sediment infilling the egg provided material for luminescence dating that minimized problems of association. Age estimations from AAR, ¹⁴C and U series methods were obtained from the eggshell and optically stimulated luminescence (OSL) of the infilling sediment. All methods agreed within their respective dating uncertainties confirming the utility of all four methods. They indicate an age for the emu egg of 31.24 ± 0.34 ka.     

A computer program (WDTSRP) designed for computation of sand drift potential (DP) and plotting sand roses

Wind Data Tabulator and Sand Rose Plotter (WDTSRP) is an interactive developed computer program accessible for estimating sand transport potential by winds in barren sandy deserts. The Fryberger (1979) formula for determining sand drift potential (DP) was adopted to create and develop the computer program. WDTSRP is capable of working out weighting factors (WFs), frequency of wind speed occurrence (t), drift potential (DP), resultant drift potential (RDP) and directional variability of winds (DV) and of plotting sand roses. The developed computer program is built up of a simplified system driven by a group of options and dialogue boxes that allow users to input and handle data easily and systematically. Copyright © 2006 John Wiley & Sons, Ltd.     

Holocene recurrent explosive activity at Chimborazo volcano (Ecuador)

Ice-capped Chimborazo is one of the large composite Ecuadorian volcanoes whose recent eruptive activity is poorly known. This paper presents the characteristics and the ages of a newly discovered Holocene sequence of pyroclastic deposits on the east and north sides of the cone. Lying upon a moraine of the Late-Glacial period, the most complete section of ~ 4.5 m in thickness is located 5 km from the present summit crater. It consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils.     

中国东部沙漠/沙地全新世地层序列及其古环境

通过对比中国东部沙漠/沙地典型地层序列的年代和古环境代用指标,文章揭示了中国东部沙漠-沙地全新世以来的地表环境变化特征及其对古气候变化的响应.风沙-古土壤沉积序列中一系列的古环境代用指标和光释光测年数据显示:(1)库布齐沙漠大规模现代沙丘景观形成于全新世时期;并在4~2ka期间普遍发育古土壤,指示了该时段区域降水量或有效湿度增加并促进了植被生长,风沙活动减弱,进而导致有机质累积、土壤发育;最近2ka沙丘大规模的扩张除了受自然因素影响,还与人类活动密不可分,丰富的沙源和人类活动共同促成了该沙漠现代景观的形成.(2)浑善达克沙地现代沙丘景观可能形成于12ka左右,而在此之前浑善达克沙地西部应是一个大湖;在9.6~3ka期间,浑善达克沙地比现代明显湿润,普遍发育古土壤,然而风沙活动并未完全停止;全新世湿润期在该沙地可能具有穿时性,沙地东部气候条件的转好早于西部.(3)呼伦贝尔沙地的古土壤早在14.5ka便开始发育,可能一直延续到最近2ka.中国东部各个沙地的古土壤发育具有高度的空间异质性,甚至各个沙地内部不同地点的古土壤发育时间也不一致,但是总体上中全新世(尤其是在7.5~3.5ka)各个沙地趋于固定,风沙活动强度显著减弱.近十年来中国东部沙地的古环境记录数量迅速增长,但是已经发表的数据数量与中国东部沙地广大的空间范围仍然不匹配,要全面理解中国北方风沙活动与气候变化的关系还需对更多的风沙沉积剖面进行深入研究.     

Holocene aeolian stratigraphic sequences in the eastern portion of the desert belt (sand seas and sandy lands) in northern China and their palaeoenvironmental implications

This paper presents the environmental history and its responses to palaeoclimatic changes since the start of the Holocene in the eastern portion of the desert belt(sand seas and sandy lands) in northern China by comparing the aeolian sandpalaeosol sequences and their palaeoclimatic proxies. The optically stimulated luminescence(OSL) ages of the aeolian sandpalaeosol sedimentary sequences and a series of palaeoenvironmental proxies show that:(1) The large-scale dune landscape currently in the Kubuqi Sand Sea was formed during the Holocene in general; and the palaeosol was generally developed during the period of 4–2 ka, indicating conditions favorable for vegetation growth, soil development, and organic carbon accumulation due to increased precipitation or effective moisture and weakened aeolian activities; the large-scale expansion of dunes in the recent 2 ka is closely linked to human activities. The variable discharge of the Yellow River with diversions for irrigation may have resulted in a more consistent supply of aeolian particles for dune field expansion.(2) The dune landscape of the Hunshandake Sandy Land was likely formed around 12 ka, and before this, the western part of the Hunshandake Sandy Land would have been covered by a single large lake; it was obviously wetter than today in the sandy land during the period of 9.6–3 ka and the palaeosols were developed at the same time. But the aeolian activities have not been completely dormant in this long-lasting wetter epoch; because the Holocene wetter period was likely time-transgressive across the region.(3) The palaeosol of the Hulunbuir Sandy Land began to develop as early as 14.5 ka, probably continuing until the last 2 ka. The palaeosol development of various dune fields in the eastern portion of the desert belt(sand seas and sandy lands) in northern China is spatially heterogeneous, and even the palaeosol development time in different locations within each sandy land is inconsistent. During the middle Holocene(especially the 7.5–3.5 ka), all the sandy lands were stabilized in general and the intensity of aeolian activities was significantly weakened. The number of palaeoenvironmental records in the eastern portion of the desert belt(sand seas and sandy lands) in northern China has increased rapidly in the past decade, but the amount of published data still does not match the vast extent of the dune fields. It does require much more in-depth palaeoenvironmental studies for a full understanding of the relationship between aeolian activities and climate change in northern China.     

Quartz OSL dating of late Holocene beach ridges from the Magdalen Islands (Quebec,Canada)

Quartz optically stimulated luminescence (OSL) dating has been applied to sandy beach ridge systems from the Magdalen Islands in the center of the Gulf of St. Lawrence (Quebec, Canada) to provide the first chronological framework for these features. Nineteen beach ridges (22 samples) from four different sites throughout the archipelago were investigated. At one of the sites, samples were taken at 9 m and 7.5 m depth using a vibracore. The quartz is dominated by the fast OSL component and a single-aliquot regenerative-dose (SAR) protocol was used to measure the equivalent doses; a low preheat (180°C/10 s) was chosen to avoid the influence of thermal transfer. The average dose recovery ratio of all samples is 1.02 ± 0.02 (n = 130) suggesting that the SAR protocol works satisfactorily on this material. The OSL ages are internally consistent and supported by independent age control (radiocarbon). The OSL ages indicate that the ridges were built between 2.6 ± 0.2 ka and 0.40 ± 0.10 ka, i.e. during a period of sea level rise. This rise eroded adjacent sandstone cliffs, which contributed a significant sediment supply to the littoral drift and beaches. Some low-lying coasts in the archipelago are still prograding, despite a relative sea level increase of ∼1.6 mm/a over the last 600 years. The late Holocene ages obtained in this study indicate that these processes have been active for at least the past two thousand years. This study demonstrates for the first time that OSL dating using quartz has great potential in this area, and is an appropriate method for establishing precise chronologies for coastal sediments in this region of the Gulf of St. Lawrence.