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1.
《Marine pollution bulletin》2010,60(8-12):221-233
This study assessed foraminiferal assemblages in Biscayne Bay, Florida, a heavily utilized estuary, interpreting changes over the past 65 years and providing a baseline for future comparisons. Analyses of foraminiferal data at the genus level revealed three distinct biotopes. The assemblage from the northern bay was characterized by stress-tolerant taxa, especially Ammonia, present in low abundances (∼2.0 × 103 foraminifers/gram) though relatively high diversity (∼19 genera/sample). The southwestern margin of the bay was dominated by Ammonia and Quinqueloculina, an assemblage characterized by the lowest diversities (∼12 genera/sample) and highest abundances (∼1.1 × 104 foraminifers/gram), influenced by both reduced salinity and elevated organic-carbon concentrations. A diverse assemblage of smaller miliolids and rotaliids (∼26 genera/sample) characterized the open-bay assemblage, which also had a significant component (∼10%) of taxa that host algal endosymbionts. In the past 65 years, populations of symbiont-bearing taxa, which are indicators of normal-marine conditions, have decreased while stress-tolerant taxa, especially Ammonia spp., have increased in predominance.  相似文献   

2.
Amino acid racemization (AAR) is widely used as a cost-effective method to date molluscs in time-averaging and taphonomic studies, but it has not been attempted for echinoderms despite their paleobiological importance and distinct biomineralization. Here we demonstrate the applicability of AAR geochronology for dating Holocene Peronella peronii (Echinodermata: Echinoidea) collected from Sydney Harbour (Australia). Using standard HPLC methods we determined the extent of AAR in 74 Peronella tests and performed replicate analyses on 23 tests. Replicate analyses from the outer edge of 23 tests spanning the observed range of D/L values yielded median coefficients of variation <4% for Asp, Glu, and Phe D/L values, which is comparable to the analytical precision. Correlations between THAA D/L values across 178 independently prepared subsamples of 74 individuals are also very high (Spearman ρ ≥ 0.95) for these three amino acids. The ages of 20 individuals spanning the observed range of D/L values were determined using 14C analyses, and Bayesian model averaging was used to determine the best AAR age model. Only three models fit to Glu D/L contributed to the final averaged age model. Modeled ages ranged from 14 to 5496 years, and the median 95% confidence interval for the 54 AAR ages was ±29% of the modelled age. In comparison, the median 2σ confidence interval for nine graphite target 14C ages was ±8% of the median age estimate and the median 2σ confidence interval for 20 carbonate target 14C ages was ±26% of the median age estimate. Overall Peronella yield high-quality D/L values and appear to be a good target for AAR geochronology.  相似文献   

3.
Effects on sediments of fish farming activity near Vrgada Island was analysed through living and total foraminiferal assemblages and concentration of major, minor and trace elements from three sediment cores. Elemental concentrations of sediments are in accordance with carbonate characteristics of the surrounding area and show mostly natural element variations between sampling locations and throughout the cores, with no significant increases due to fish farming activity. Only phosphorus concentration shows elevate values below the fish cage, assigned to fish pellets. Foraminiferal communities are dominated by epifaunal and stress tolerant species, while diversity indices point to normal marine conditions. The type of substrate and phosphorus content in sediments principally influence foraminiferal community composition, while other elemental concentrations have no perceptible effect on the assemblages. Some foraminiferal species Ammoniatepida, Ammoniabeccarii, Elphidiumcrispum, Elphidiummacellum and genus Haynesina are confirmed to be tolerant to elevated nutrient (phosphorus) content, while Ammonia parkinsoniana shows sensitivity to pollution. Postmortem processes cause decrease of foraminiferal density and species richness with core depth. All results point to negligible influence of fish farming and relatively stable environmental conditions at all sampling locations.  相似文献   

4.
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.  相似文献   

5.
Several analytical techniques have been used in amino acid geochronology to measure the relative abundances of D- and L-enantiomers. During the past two decades, reverse-phase (RP) liquid chromatography has become most common, whereas ion-exchange (IE) liquid chromatography was widely used prior to the mid-1990s. This study is based on intra-lab paired analyses of RP and IE liquid chromatography to mathematically convert A/I (allo-isoleucine:isoleucine) values determined with IE to corresponding D/L values for comparison with new RP results. Pooled results of 340 paired IE and RP analyses show that A/I values can be confidently converted to equivalent D/L values for aspartic acid, glutamic acid, alanine, valine and isoleucine, with 2σ uncertainties of ±24–41% depending on the amino acid. For specific sample types including gastropod, bivalve, eggshell and whole rock, A/I can readily be converted to D/L aspartic acid and D/L glutamic acid with 2σ uncertainties of ±20% and 26%, respectively, using sample-type-specific relations. A/I is also reliably converted to D/L valine and D/L isoleucine with 2σ uncertainties of ±31 and 35%, respectively. Regression equations are provided to convert A/I to an equivalent D/L value for five amino acids, thereby enabling the large literature base of AAR results from IE chromatography to be compared and integrated with AAR results from RP chromatography.  相似文献   

6.
Well‐sorted detrital limestone is one of the typical lithofacies of the latest interval of the Pleistocene Ryukyu Group, which is exposed in the Ryukyu Archipelago in southwestern Japan. The depositional environments of the limestone are interpreted to be extremely shallow and to include back‐reef lagoons or moats and subaerial sand dunes. However, detailed micropaleontological analyses have not been performed on this limestone. In this study, the interpretation of the depositional environments and paleo‐water depths was improved by quantitative examination of foraminiferal assemblages for the well‐sorted detrital limestone of the Minatogawa Formation in the southern part of Okinawa Island. Thin sections of limestone collected from the Minatogawa (Horikawa) quarry were subjected to sedimentological and foraminiferal analyses. Comparison with modern foraminiferal distribution within the Ryukyu Archipelago indicates that back‐reef and fore‐reef dwelling foraminifers characterize the fossil assemblages from the well‐sorted detrital limestone (bioclastic grainstone). Three ratios of indicator foraminiferal taxa (ratios of back‐reef to fore‐reef taxa, planktonic foraminifers to Amphistegina lobifera and Amphistegina lessonii, and Calcarina gaudichaudii to other Calcarina species), as well as multivariate analyses suggest that the well‐sorted detrital limestone was deposited in fore‐reef setting shallower than 40 m in water depth. A comparable depth range was reconstructed from the coral assemblage in the associated coral limestone, suggesting that the Minatogawa Formation was deposited in a gently inclined ramp setting with patch reefs and/or fringing reefs. Stratigraphic changes in paleo‐water depth, together with evidence of several unconformities associated with paleosol layers suggest that there were repeated transgressions and regressions, with an amplitude up to several tens of meters, when the Minatogawa Formation was deposited.  相似文献   

7.
Katsumi  Ueno  Satoe  Tsutsumi 《Island Arc》2009,18(1):69-93
This paper deals with a Lopingian (Late Permian) foraminiferal faunal succession of the Shifodong Formation in the Changning–Menglian Belt, West Yunnan, Southwest China, which has been geologically interpreted as one of the closed remnants in East Asia of the Paleo‐Tethys Ocean. The Shifodong Formation is the uppermost stratigraphic unit in thick Carboniferous–Permian carbonates of the belt. These carbonates rest upon bases consisting of oceanic island basalt and are widely accepted as having a Paleo‐Tethyan mid‐oceanic (seamount‐ or oceanic plateau‐top) origin. Sixteen taxa of fusuline foraminifers and 37 taxa of smaller (non‐fusuline) foraminifers are recognized from the type section of the Shifodong Formation located in the Gengma area of the northern part of the Changning–Menglian Belt. Based on their stratigraphic distribution, three fusuline zones can be established in this section: they are, in ascending order, the Codonofusiella cf. C. kwangsiana Zone, Palaeofusulina minima Zone, and Palaeofusulina sinensis Zone. These three biozones are respectively referable to the Wuchiapingian, early Changhsingian, and late Changhsingian, of which the Wuchiapingian is first recognized in this study in the Changning–Menglian mid‐oceanic carbonates. The present study clearly demonstrates that the foraminiferal fauna in a Paleo‐Tethyan pelagic shallow‐marine environment still maintained high faunal diversity throughout the almost entire Lopingian, although the very latest Permian fauna in the upper part of the Palaeofusulina sinensis Zone of the Shifodong section records a sudden decrease in both faunal diversity and abundance. Moreover, the Shifodong faunas are comparable in diversity with those observed in circum‐Tethyan shelves such as South China. The present Paleo‐Tethyan mid‐oceanic foraminiferal faunas are definitely more diversified than coeval mid‐oceanic Panthalassan faunas, which are typically represented by those from the Kamura Limestone in a Jurassic accretionary complex of Southwest Japan. It is suggestive that the Paleo‐Tethyan mid‐oceanic buildups presumably supplied a peculiarly hospitable habitat for foraminiferal faunal development in a pelagic paleo‐equatorial condition.  相似文献   

8.
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.  相似文献   

9.
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.  相似文献   

10.
The field of amino acid racemization (AAR) geochronology had its beginnings in the 1960's with the analysis of Quaternary mollusks of known relative age from United States Atlantic coast sites. Subsequent AAR studies of sites from Florida, California and then the entire western and eastern coasts of the U.S. have documented two important concepts: 1) in both uplifted marine terrace and subsurface sections, it can be demonstrated that D/L values increase with increasing geologic age; 2) D/L values in samples of equal age increase with decreasing latitude (increasing temperature). These two north–south coastlines with broad latitude ranges (Pacific sites span more than 20°, Atlantic sites more than 15°) provide an ideal, rare framework to test these principles, and the contrast in thermal histories of these two coasts (one maritime, the other more continental) adds additional insights into issues of aminozone correlation over latitude ranges as small as 2°. The trend of D/L values vs. latitude (isochrons) is established using calibrations based primarily on U–Th coral dating, the best of these calibrated trends seen for multiple sites on the Pacific coast. Pacific coast isochrons follow smooth trends and have been used to create and evaluate kinetic models using the modern latitudinal temperature gradient for comparison. Atlantic coast isochrons are more difficult to reconcile with the modern temperature gradient and available U–Th coral ages, suggesting either complex effective temperatures or unknown diagenetic effects on the AAR results. In addition to these studies that used local or regional field studies to test and evaluate AAR methods, relative or numerical ages based on AAR studies on both coasts have contributed to research on coastal uplift or subsidence rates, coastal stratigraphy and sea-level history, age mixing, and diagenesis of carbonate fossils.  相似文献   

11.
Isotopic analyses have been made on five species of planktonic foraminifera collected in two deployments of PARFLUX Mark II time-series sediment traps in the Panama Basin. The automated sampling system on the traps provided 4 one-month samples from 29 July to 16 November 1979 and 6 two-month samples from December 1979 to November 1980.The δ18O values of Globigerinoides ruber and Globigerinoides sacculifer in this region are primarily affected by a low-salinity surface layer that forms during the early winter. These species each show a 1‰ total range in δ18O. The δ18O values of the colder-water species Globorotalia menardii, Neogloboquadrina dutertrei, and Globorotalia theyeri show smaller seasonal changes in δ18O. The δ13C values of G. ruber and G. sacculifer exhibit small seasonal changes (0.35 and 0.4‰ respectively) despite large seasonal changes in surface water productivity. The colder-water species exhibit only slightly larger changes in δ13C (up to 0.55‰) throughout the year. All colder-water species exhibit minimum or near minimum δ13C values during February and March, which is the period of maximum upwelling and primary productivity. Seasonal variations in the flux of foraminifera in the water column at this location will have only a small effect on the isotopic composition of the sediment assemblage; extreme values of δ18O and δ13C do not occur during the periods which are associated with the high flux of foraminiferal tests.  相似文献   

12.
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.  相似文献   

13.
Kaoru  Sugihara  Naoto  Masunaga  Kazuhiko  Fujita 《Island Arc》2006,15(4):437-454
Abstract The taxonomic diversity of hermatypic corals decreases with increasing latitude, which correlates with sea‐surface temperatures. However, little is known about latitudinal changes in the taxonomic diversity and biogeographic patterns of larger benthic foraminifera, although their physiological requirements are similar to those of hermatypic corals because of their symbiotic relationships with microalgae. The present study examined how the abundance and taxonomic composition of larger foraminiferal assemblages in shallow‐water reef sediments change with latitude along the Ryukyu Islands (Ryukyus), which are located near the northern limit of coral‐reef distributions in the western Pacific Ocean. Three islands from different latitudes in the Ryukyus were selected to investigate latitudinal changes in larger foraminiferal assemblages: Ishigaki Island (24°20′N, 124°10′E), Kudaka Island (26°09′N, 127°54′E) and Tane‐ga‐shima Island (30°20′N, 131°E). Four sediment samples were taken at each of three topographic sites (beach, shallow lagoon and reef crest) on the reef flat of each island. Foraminiferal tests of a 2.0‐ to 0.5‐mm size fraction were selected, identified and counted. The variations in foraminiferal abundance in reef sediments from three latitudinally different islands exhibit two contrasting trends along reef flats: a shoreward decrease on Ishigaki and Tane‐ga‐shima Islands and a shoreward increase on Kudaka Island. A total of 25, 24 and 13 foraminiferal taxa were identified in Ishigaki, Kudaka and Tane‐ga‐shima Islands, respectively. Baculogypsina sphaerulata, Neorotalia calcar and Amphistegina spp. were dominant (i.e. >3% of foraminiferal assemblages) in the three islands. Calcarina gaudichaudii and Calcarina hispida were common on Ishigaki and Kudaka Islands but were absent on Tane‐ga‐shima Island. Larger foraminiferal assemblages from three different reef‐flat environments on Ishigaki Island can be distinguished, whereas those from the three environments on Kudaka and Tane‐ga‐shima Islands are similar in composition. These latitudinal changes in larger foraminiferal assemblages in reef sediments may possibly be caused by variations in the topography of reef flats, distributions and standing crops of living foraminifers on reef flats, and the northern limit of some calcarinid species in the northern Ryukyus.  相似文献   

14.
The inter- and intra-crystalline fractions of Patella vulgata limpets recovered from archaeological sites in Northern Spain (covering Neolithic, Mesolithic, Magdalenian, Solutrean, and Aurignacian periods) were examined for amino acid composition and racemisation over time. The calcitic apex and rim areas of the shells were found to probably be composed of similar proteins, as the D/L values and amino acids were comparable and varied in the same way with increasing age; however, the mineral structures present in these areas differed. The aragonitic intermediate part of the shell showed a distinctly different amino acid composition and mineral structure. The main protein leaching from the inter-crystalline fraction occurred within the first 6000 yr after the death of the organism. In contrast, the intra-crystalline fraction — comprised of a different protein composition than the inter-crystalline fraction — appeared to behave as a closed system for at least 34 ka, as reflected by the lack of a significant decrease in the amino acid content; however, changes in the amino acid percentages occurred during this period. The concentration of aspartic acid remained almost constant with age both in inter- and intra-crystalline proteins, and its contribution to the total amino acid content increased with age at the expense of other amino acids such as glutamic acid, serine, glycine and alanine. Temperature is thought to play a key role in the amino acid racemisation of P. vulgata and could explain why in the localities belonging to the Gravettian and Solutrean period, which formed during relatively cold conditions, D/L values were similar to those detected in shells from sites formed during the Magdalenian.  相似文献   

15.
Sediments deposited during glacial-interglacial cycles through the Early to Mid-Pleistocene in the North Sea are chronologically poorly constrained. To contribute to the chronology of these units, amino acid racemization (AAR) and strontium (Sr) isotope analyses have been performed on samples from four shallow borings and one oil well along a transect in the northern North Sea. D/L Asp (aspartic acid) values obtained through reverse-phase liquid chromatography in the benthic foraminiferal species Elphidium excavatum is focused on because of consistent results and a good stratigraphic distribution of this benthic species. For the Early Pleistocene, an age model for the well 16/1–8, from the central part of the northern North Sea based on Sr ages allows for dating of the prograding wedges filling the pre-Quaternary central basin. A regional calibration curve for the racemization of Asp in Elphidium excavatum is developed using published ages of radiocarbon-dated samples and samples associated with the previously identified Bruhnes/Matuyama (B/M) paleomagnetic boundary and a Sr age from this study. Based on all the available geochronological evidence, samples were assigned to marine oxygen isotope stages (MIS) with uncertainties on the order of 10–70 ka.Sr ages suggest a hiatus of <2 million years (Ma) possibly due to non-deposition or low sedimentation between the Utsira Formation (Pliocene) and the Early Pleistocene. An increase in sedimentation rates around 1.5 ± 0.07 Ma (∼MIS 51) may partly be due to sediment supply from rivers from the south-east and partly due to the extension of ice sheet around 1.36 ± 0.07 Ma from the Norwegian coast to the central North Sea. A possible basin-wide glaciation occurred around 1.1 Ma (∼ MIS 32) (upper regional unconformity/top of unit Q4 in this study), resulting in erosion and regional unconformity. Two interglacials in the Norwegian Channel have been dated: the Radøy Interglacial to 1.07 ± 0.01 Ma (possibly MIS 31, the ‘super interglacial’), and the Norwegian Trench Interglacial to 0.50 ± 0.02 Ma (possibly MIS 13). A massive till unit identified at the same stratigraphic level in all shallow borings may partly represent an extensive MIS 12 glaciation. This study shows that the combined use of amino acid racemization data and Sr isotope chronology can refine the chronological ambiguities of Quaternary North Sea sediments related partly to the impact of glacial processes.  相似文献   

16.
Hiroki Hayashi 《Island Arc》2004,13(1):318-331
Abstract The present paper describes the general outline of Neogene paleoceanographic changes in the northwestern Pacific by means of planktonic foraminiferal assemblages. Planktonic foraminiferal fossils occur commonly in the upper Miocene to lower Pleistocene sediments of Hole 1151A, Ocean Drilling Program Leg 186 in the forearc basin off northeast Japan, with the exception of 11 barren intervals. These barren intervals are explained as a result of dissolution under organic decomposing processes. Three assemblages of planktonic foraminifers were identified by Q‐mode cluster analysis. The succession of the assemblages can be divided into four paleoceanographic stages: (i) warm‐temperate Tortonian; (ii) cold‐temperate Messinian to lower Pliocene; (iii) warm climatic optimum in the middle part of the Pliocene; and (iv) strong glacial–interglacial oscillation of the upper Pliocene to the lower Pleistocene. Three short warming events—namely, the late Miocene climatic optimum 3, the Miocene–Pliocene boundary and the middle Pliocene events—and a short cooling event of the late Miocene could be determined in the studied section of Site 1151.  相似文献   

17.
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.  相似文献   

18.
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.  相似文献   

19.
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.  相似文献   

20.
Ritsuo Nomura 《Island Arc》2021,30(1):e12421
The lower part of the Josoji Formation, Shimane Peninsula, contains clues for figuring out changes in deep-water characteristics during the opening of the Japan Sea. The foraminiferal assemblage includes early to middle Miocene biostratigraphic index taxa such as planktonic foraminiferal Globorotalia zealandica and Globorotaloides suteri. The occurrence of these two species, together with the absence of praeorbulinids, suggests that the lower part of the Josoji Formation is assigned to the top of planktonic foraminiferal Zone N7/M4 (16.39 Ma). The benthic foraminiferal assemblage, which is characterized by Cyclammina cancellata and Martinottiella communis, clearly suggests that the lower Josoji Formation was deposited at bathyal depths, and that it developed in association with the abrupt appearance of deep-sea calcareous forms. Such bathyal taxa are the main constituents of the Spirosigmoilinella compressa–Globobulimina auriculata Zone of the Josoji Formation and also of the Gyrodina–Gyroidinoides Zone at Ocean Drilling Program Site 797 in the Japan Sea. The base of these benthic foraminiferal zones can be correlated with the base of the nannofossil Sphenolithus heteromorphus Base Zone (= CNM6/CN3); thus, its estimated age is 17.65 Ma. This biostratigraphic information suggests that the lower Josoji Formation was deposited from shortly before 17.65–16.39 Ma in upper limit age. Evidence that fresh to brackish and shallow-water basins formed in the rifting interval of 20–18 Ma in the Japan Sea borderland suggests that the abrupt appearance of deep-sea calcareous foraminifera occurred about 1 my earlier in this area than in other sedimentary basins and suggests that a significant paleoceanographic change occurred in the proto-Japan Sea at 17.65 Ma.  相似文献   

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