Geochronology最新文献

{"title":"Technical note: A prototype transparent-middle-layer data management and analysis infrastructure for cosmogenic-nuclide exposure dating","authors":"G. Balco","doi":"10.5194/gchron-2020-6","DOIUrl":"https://doi.org/10.5194/gchron-2020-6","url":null,"abstract":"Abstract. Geologic dating methods for the most part do not directly measure ages. Instead, interpreting a geochemical observation as a geologically useful parameter – an age or a rate – requires an interpretive middle layer of calculations and supporting data sets. These are the subject of active research and evolve rapidly, so any synoptic analysis requires repeated recalculation of large numbers of ages from a growing data set of raw observations, using a constantly improving calculation method. Many important applications of geochronology involve regional or global analyses of large and growing data sets, so this characteristic is an obstacle to progress in these applications. This paper describes the ICE-D (Informal Cosmogenic-Nuclide Exposure-age Database) database project, a prototype computational infrastructure for dealing with this obstacle in one geochronological application – cosmogenic-nuclide exposure dating – that aims to enable visualization or analysis of diverse data sets by making middle-layer calculations dynamic and transparent to the user. An important aspect of this concept is that it is designed as a forward-looking research tool rather than a backward-looking archive: only observational data (which do not become obsolete) are stored, and derived data (which become obsolete as soon as the middle-layer calculations are improved) are not stored but instead calculated dynamically at the time data are needed by an analysis application. This minimizes “lock-in” effects associated with archiving derived results subject to rapid obsolescence and allows assimilation of both new observational data and improvements to middle-layer calculations without creating additional overhead at the level of the analysis application.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-03-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74837169","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Robust isochron calculation","authors":"R. Powell, E. Green, Estephany Marillo Sialer, J. Woodhead","doi":"10.5194/gchron-2020-4","DOIUrl":"https://doi.org/10.5194/gchron-2020-4","url":null,"abstract":"Abstract. The standard classical statistics approach to isochron calculation assumes that the distribution of uncertainties on the data arising from isotopic analysis is strictly Gaussian. This effectively excludes datasets that have more scatter from consideration, even though many appear to have age significance. A new approach to isochron calculations is developed in order to circumvent this problem, requiring only that the central part of the uncertainty distribution of the data defines a “spine” in the trend of the data. This central spine can be Gaussian but this is not a requirement. This approach significantly increases the range of datasets from which age information can be extracted but also provides seamless integration with well-behaved datasets and thus all legacy age determinations. The approach is built on the robust statistics of Huber (1981) but using the data uncertainties for the scale of data scatter around the spine rather than a scale derived from the scatter itself, ignoring the data uncertainties. This robust data fitting reliably determines the position of the spine when applied to data with outliers but converges on the classical statistics approach for datasets without outliers. The spine width is determined by a robust measure, the normalised median absolute deviation of the distances of the data points to the centre of the spine, divided by the uncertainties on the distances. A test is provided to ascertain that there is a spine in the data, requiring that the spine width is consistent with the uncertainties expected for Gaussian-distributed data. An iteratively reweighted least squares algorithm is presented to calculate the position of the robust line and its uncertainty, accompanied by an implementation in Python.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"34 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-02-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81058277","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility","authors":"E. Cooperdock, R. Ketcham, D. Stockli","doi":"10.5194/gchron-1-17-2019","DOIUrl":"https://doi.org/10.5194/gchron-1-17-2019","url":null,"abstract":"Abstract. (U–Th) ∕ He thermochronometry relies on the accurate and\u0000precise quantification of individual grain volume and surface area, which\u0000are used to calculate mass, alpha ejection (FT) correction, equivalent\u0000sphere radius (ESR), and ultimately isotope concentrations and age. The vast\u0000majority of studies use 2-D or 3-D microscope dimension measurements and an\u0000idealized grain shape to calculate these parameters, and a long-standing\u0000question is how much uncertainty these assumptions contribute to observed\u0000intra-sample age dispersion and accuracy. Here we compare the results for\u0000volume, surface area, grain mass, ESR, and FT correction derived from\u00002-D microscope and 3-D X-ray computed tomography (CT) length and width data\u0000for > 100 apatite grains. We analyzed apatite grains from two\u0000samples that exhibited a variety of crystal habits, some with inclusions. We\u0000also present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium\u0000(eU). The data illustrate that the 2-D approach systematically overestimates\u0000grain volumes and surface areas by 20 %–25 %, impacting the estimates for\u0000mass, eU, and ESR – important parameters with implications for interpreting\u0000age scatter and inverse modeling. FT factors calculated from 2-D and 3-D\u0000measurements differ by ∼2 %. This variation, however, has\u0000effectively no impact on reducing intra-sample age reproducibility, even on\u0000small aliquot samples (e.g., four grains). We also present a grain-mounting\u0000procedure for X-ray CT scanning that can allow hundreds of grains to be scanned\u0000in a single session and new software capabilities for 3-D FT and\u0000FT-based ESR calculations that are robust for relatively low-resolution\u0000CT data, which together enable efficient and cost-effective CT-based\u0000characterization.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"16 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86643162","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Re-evaluating 14C dating accuracy in deep-sea sediment archives","authors":"B. Lougheed, P. Ascough, A. Dolman, L. Löwemark, B. Metcalfe","doi":"10.31223/osf.io/cwqsk","DOIUrl":"https://doi.org/10.31223/osf.io/cwqsk","url":null,"abstract":"Abstract. The current geochronological state of the art for applying the radiocarbon\u0000(14C) method to deep-sea sediment archives lacks key information on\u0000sediment bioturbation. Here, we apply a sediment accumulation model that\u0000simulates the sedimentation and bioturbation of millions of foraminifera,\u0000whereby realistic 14C activities (i.e. from a 14C calibration\u0000curve) are assigned to each single foraminifera based on its simulation\u0000time step. We find that the normal distribution of 14C age typically\u0000used to represent discrete-depth sediment intervals (based on the reported\u0000laboratory 14C age and measurement error) is unlikely to be a faithful\u0000reflection of the actual 14C age distribution for a specific depth\u0000interval. We also find that this deviation from the actual 14C age\u0000distribution is greatly amplified during the calibration process.\u0000Specifically, we find a systematic underestimation of total geochronological\u0000error in many cases (by up to thousands of years), as well as the generation\u0000of age–depth artefacts in downcore calibrated median age. Even in the case\u0000of “perfect” simulated sediment archive scenarios, whereby sediment\u0000accumulation rate (SAR), bioturbation depth, reservoir age and species\u0000abundance are all kept constant, the 14C measurement and calibration\u0000processes generate temporally dynamic median age–depth artefacts on the\u0000order of hundreds of years – whereby even high SAR scenarios (40 and 60 cm kyr−1) are susceptible. Such age–depth artefacts\u0000can be especially pronounced during periods corresponding to dynamic changes\u0000in the Earth's Δ14C history, when single foraminifera of varying\u000014C activity can be incorporated into single discrete-depth sediment\u0000intervals. For certain lower-SAR scenarios, we find that downcore\u0000discrete-depth true median age can systematically fall outside the calibrated\u0000age range predicted by the 14C measurement and calibration processes,\u0000thus leading to systematically inaccurate age estimations. In short, our\u0000findings suggest the possibility of 14C-derived age–depth artefacts in\u0000the literature. Furthermore, since such age–depth artefacts are likely to\u0000coincide with large-scale changes in global Δ14C, which\u0000themselves can coincide with large-scale changes in global climate (such as\u0000the last deglaciation), 14C-derived age–depth artefacts may have been\u0000previously incorrectly attributed to changes in SAR coinciding with global\u0000climate. Our study highlights the need for the development of improved\u0000deep-sea sediment 14C calibration techniques that include an a priori\u0000representation of bioturbation for multi-specimen samples.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"45 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86586031","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Chlorine-36∕beryllium-10 burial dating of alluvial fan sediments associated with the Mission Creek strand of the San Andreas Fault system, California, USA","authors":"G. Balco, K. Blisniuk, A. Hidy","doi":"10.5194/GCHRON-1-1-2019","DOIUrl":"https://doi.org/10.5194/GCHRON-1-1-2019","url":null,"abstract":"Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"103 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88029728","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Isolation of quartz for cosmogenic in situ 14C analysis","authors":"K. Nichols, B. Goehring","doi":"10.5194/gchron-1-43-2019","DOIUrl":"https://doi.org/10.5194/gchron-1-43-2019","url":null,"abstract":"Abstract. Froth flotation is a commonly used procedure for\u0000separating feldspars and micas from quartz for the preparation of quartz\u0000mineral separates to carry out cosmogenic nuclide analysis. Whilst\u0000extracting carbon from quartz we observed in situ carbon-14 (14C)\u0000concentrations which were anomalously high and in excess of theoretical\u0000geological maximum concentrations. Further etching of sample material\u0000reduced carbon yields and 14C concentrations, yet the latter remained\u0000unrealistically high. When quartz from the original whole rock sample was\u0000isolated in our laboratory, we observed even lower carbon yields and\u0000geologically plausible in situ 14C concentrations. After ruling out\u0000unlikely geological scenarios and systematic measurement issues, we decided\u0000to investigate the quartz isolation procedure as a potential source of\u000014C contamination. We hypothesised that laurylamine (dodecylamine), an\u0000organic compound used as part of the froth flotation procedure, elevates\u000014C concentrations if residual laurylamine is present. We demonstrate\u0000that laurylamine has a 14C modern carbon source and thus has the\u0000potential to influence in situ 14C measurements if present in minute\u0000but measurable quantities. Furthermore, we show that insufficient sample\u0000etching results in contaminant 14C persisting through the step heating of\u0000quartz that is subsequently collected with the in situ component released at\u00001100 ∘C. We demonstrate that froth flotation contaminates in situ\u000014C measurements. We provide guidelines for the preparation of quartz\u0000based on methods developed in our laboratory and demonstrate that all froth-flotation-derived carbon and 14C is removed when applied. We recommend\u0000that the procedures presented be used at a minimum when using froth\u0000flotation to isolate quartz for in situ 14C measurements.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"128 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83964979","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon","authors":"C. Brenhin Keller, P. Boehnke, B. Schoene, T. M. Harrison","doi":"10.5194/gchron-1-85-2019","DOIUrl":"https://doi.org/10.5194/gchron-1-85-2019","url":null,"abstract":"Abstract. The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (<0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ∼50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ∼0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"48 1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-06-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85421428","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}