Geochronology最新文献

{"title":"The closure temperature(s) of zircon Raman dating","authors":"B. Härtel, R. Jonckheere, B. Wauschkuhn, L. Ratschbacher","doi":"10.5194/gchron-2020-39","DOIUrl":"https://doi.org/10.5194/gchron-2020-39","url":null,"abstract":"Abstract. Zircon Raman dating based on irradiation damage\u0000is a debated concept but not an established geo-/thermochronological\u0000method. One issue is the temperature range of radiation-damage annealing\u0000over geological timescales. We conducted isochronal and isothermal annealing\u0000experiments on radiation-damaged zircons between 500 and 1000 ∘C\u0000for durations between 10 min and 5 d to describe the annealing\u0000kinetics. We measured the widths (Γ) and positions (ω) of\u0000the ν1(SiO4), ν2(SiO4), and ν3(SiO4) internal Raman bands, and the external rotation Raman\u0000band at ∼974, 438, 1008, and 356 cm−1 after each\u0000annealing step. We fitted a Johnson–Mehl–Avrami–Kolmogorov and a distributed\u0000activation energy model to the fractional annealing data, calculated from\u0000the widths of the ν2(SiO4), ν3(SiO4), and\u0000external rotation bands. From the kinetic models, we determined closure\u0000temperatures Tc for damage accumulation for each Raman band. Tc\u0000ranges from 330 to 370 ∘C for the internal ν2(SiO4)\u0000and ν3(SiO4) bands; the external rotation band is more\u0000sensitive to thermal annealing (Tc∼260 to\u0000310 ∘C). Our estimates are in general agreement with previous\u0000ones, but more geological evidence is needed to validate the results. The\u0000Tc difference for the different Raman bands offers the prospect of a\u0000multi-closure-temperature zircon Raman thermochronometer.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":"259-272"},"PeriodicalIF":0.0,"publicationDate":"2020-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82387726","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":"On the treatment of discordant detrital zircon U–Pb data","authors":"P. Vermeesch","doi":"10.5194/gchron-2020-38","DOIUrl":"https://doi.org/10.5194/gchron-2020-38","url":null,"abstract":"Abstract. Zircon U–Pb geochronology is a staple of crustal evolution studies and sedimentary provenance analysis. Constructing (detrital) U–Pb age spectra is straightforward for concordant 206Pb/238U and 207Pb/206Pb compositions. But unfortunately, many U–Pb datasets contain a significant proportion of discordant analyses. This paper investigates two decisions that must be made when analysing such discordant U–Pb data. First, the analyst must choose whether to use the 206Pb/238U or the 207Pb/206Pb date. The 206Pb/238U method is more precise for young samples, whereas the 207Pb/206Pb method is better suited for old samples. However there is no agreement which “cutoff” should be used to switch between the two. This subjective decision can be avoided by using single-grain concordia ages. These represent a kind of weighted mean between the 206Pb/238U and 207Pb/206Pb methods, which offers better precision than either of the latter two methods. A second subjective decision is how to define the discordance cutoff between “good” and “bad” data. Discordance is usually defined as (1) the relative age difference between the 206Pb/238U and 207Pb/206Pb dates. However, this paper shows that several other definitions are possible as well, including (2) the absolute age difference; (3) the common-Pb fraction according to the Stacey–Kramers mantle evolution model; (4) the p value of concordance; (5) the perpendicular log ratio (or “Aitchison”) distance to the concordia line; and (6) the log ratio distance to the maximum likelihood composition on the concordia line. Applying these six discordance filters to a 70 869-grain dataset of zircon U–Pb compositions reveals that (i) the relative age discordance filter tends to suppress the young age components in U–Pb age spectra, whilst inflating the older age components; (ii) the Stacey–Kramers discordance filter is more likely to reject old grains and less likely to reject young ones; (iii) the p-value-based discordance filter has the undesirable effect of biasing the results towards the least precise measurements; (iv) the log-ratio-based discordance filters are strictest for Proterozoic grains and more lenient for Phanerozoic and Archaean age components; (v) of all the methods, the log ratio distance to the concordia composition produces the best results, in the sense that it produces age spectra that most closely match those of the unfiltered data: it sharpens age spectra but does not change their shape. The popular relative age definition fares the worst according to this criterion. All the methods presented in this paper have been implemented in the IsoplotR toolbox for geochronology.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"76 1 1","pages":"247-257"},"PeriodicalIF":0.0,"publicationDate":"2020-12-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"72672528","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":"Luminescence properties and dating of glacial to periglacial sediments from northern Switzerland","authors":"D. Mueller, F. Preusser, Marius W. Buechi, Lukas Gegg, G. Deplazes","doi":"10.5194/gchron-2-305-2020","DOIUrl":"https://doi.org/10.5194/gchron-2-305-2020","url":null,"abstract":"Abstract. Luminescence dating has become a pillar of the understanding of\u0000Pleistocene glacial advances in the northern foreland of the Swiss Alps.\u0000However, both quartz and feldspar from the region are equally challenging as\u0000dosimeters with anomalous fading and partial bleaching being some of the\u0000obstacles to overcome for the establishment of decisive chronologies. In\u0000this study, luminescence properties of coarse- and fine-grained quartz,\u0000feldspar, and polymineral fractions of eight samples from a palaeovalley,\u0000Rinikerfeld in northern Switzerland, are systematically assessed. Standard\u0000performance tests are conducted on all four fractions. Deconvolution of\u0000luminescence signals of the quartz fractions is implemented and shows the\u0000dominance of stable fast components. Reader-specific low preheat\u0000temperatures are investigated on the infrared stimulated luminescence (IRSL)\u0000signal of feldspar. Thermal stability of this signal is found for low\u0000preheats, and thermal quenching could be excluded for higher preheats.\u0000However, anomalous fading is observed in the feldspar and polymineral IRSL\u0000signals and two correction approaches are applied. For one approach, fading\u0000corrected coarse-grained feldspar ages are consistent with those derived\u0000from quartz. In general, coarse-grained quartz and feldspar, as well as the\u0000fine-grained polymineral fraction of one sample, are in chrono-stratigraphic\u0000agreement and present negligible evidence for partial bleaching. However,\u0000ages derived from fine-grained quartz are found to underestimate those of\u0000the coarse-grained quartz fractions. Hence, the impact of alpha efficiency\u0000and water content on the dose rate and thus the ages are assessed. A\u0000finite explanation for the observed discrepancies remains lacking, but this\u0000systematic investigation of different luminescence signals allows for the\u0000establishment of a chronology for the palaeovalley fill dating back to at\u0000least Marine Isotope Stage 6 (MIS 6).\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"77 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77706729","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 multiple geological events using in situ Rb–Sr geochronology: implications for metallogenesis at Tropicana, Western Australia","authors":"H. Olierook, K. Rankenburg, S. Ulrich, C. Kirkland, N. Evans, Stephen Brown, B. McInnes, A. Prent, J. Gillespie, B. McDonald, Miles Darragh","doi":"10.5194/GCHRON-2-283-2020","DOIUrl":"https://doi.org/10.5194/GCHRON-2-283-2020","url":null,"abstract":"Abstract. Dating multiple geological events in single samples using\u0000thermochronology and geochronology is relatively common, but it is only with\u0000the recent advent of triple quadrupole laser ablation inductively coupled\u0000plasma mass spectrometry (LA-ICP-MS) that in situ rubidium–strontium (Rb–Sr) dating has\u0000become a more commonly applied and powerful tool to date K-rich or\u0000Rb-bearing minerals. Here, we date two generations of mineral assemblages in\u0000individual thin sections using the in situ Rb–Sr method. Two distinct mineral\u0000assemblages, both probably associated with Au mineralization, are identified\u0000in samples from the Tropicana gold mine in the Albany–Fraser Orogen,\u0000Western Australia. For Rb–Sr purposes, the key dateable minerals are two\u0000generations of biotite as well as additional phengite associated with the younger\u0000assemblage. Our results reveal that the first, coarse-grained generation of\u0000biotite grains records a minimum age of 2535±18 Ma, coeval with\u0000previous 40Ar∕39Ar biotite, rhenium–osmium (Re–Os) pyrite and uranium–lead (U–Pb) rutile results.\u0000The second, fine-grained and recrystallized generation of biotite grains\u0000record an age of 1207±12 Ma across all samples. Phengite and\u0000muscovite yielded broadly similar results at ca. 1.2 Ga, but data are\u0000overdispersed for a single coeval population of phengite and show elevated\u0000age uncertainties for muscovite. We propose that the ca. 2530 Ma age\u0000recorded by various geochronometers represents cooling and exhumation and\u0000that the age of ca. 1210 Ma is related to major shearing associated with the\u0000regional deformation as part of Stage II of the Albany–Fraser Orogeny.\u0000This is the first time that an age of ca. 1210 Ma has been identified in the\u0000Tropicana Zone, which may have ramifications for constraining the timing of\u0000mineralization in the region. The in situ Rb–Sr technique is currently the only\u0000tool capable of resolving both geological events in these rocks.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-10-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80198589","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":"Confined fission track revelation in apatite: how it works and why it matters","authors":"R. Ketcham, M. Tamer","doi":"10.5194/gchron-2020-31","DOIUrl":"https://doi.org/10.5194/gchron-2020-31","url":null,"abstract":"Abstract. We present a new model for the etching and revelation of confined fission tracks in apatite, based on step etching measurements that demonstrate variable along-track etching velocity, vT(x). We define two end-member model forms: Constant-core, with a central zone of constant etching rate that then falls off toward track tips; and Linear, in which etching rates fall linearly from the midpoint to the tips. Based on these, we construct a characterization of confined track revelation that encompasses all of the relevant processes, including penetration and thickening of semi-tracks from the polished grain surface, intersection of confined tracks, and analyst selection of which tracks to measure and which to bypass. Both model forms are able to fit step-etching data from five sets of paired experiments of fossil tracks and unannealed and annealed induced tracks, supporting the correctness of our approach and providing a series of insights into the theory and practice of fission-track thermochronology. Etching rates for annealed induced tracks are much faster than those for unannealed induced and spontaneous tracks, impacting the relative efficiency of both confined track length and density measurements, and suggesting that high-temperature laboratory annealing may induce a transformation in track cores that does not occur at geological conditions of partial annealing. The model quantifies how variation in analyst selection criteria, summarized as the ratio of along-track to bulk etching velocity at the track tip (vT/vB), likely plays a first-order role in the reproducibility of confined length measurements. It also shows that a large proportion of tracks that are intersected are not measured, indicating that length biasing is an insufficient statistical model for predicting the relative probability of detection of different track populations. The vT(x) model provides an approach to both optimizing etching conditions and linking track length measurements across etching protocols.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"17 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86643039","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":"Development of a multi-method chronology spanning the Last Glacial Interval from Orakei maar lake, Auckland, New Zealand","authors":"Leonie Peti, K. Fitzsimmons, J. Hopkins, A. Nilsson, T. Fujioka, D. Fink, Charles Mifsud, M. Christl, R. Muscheler, P. Augustinus","doi":"10.5194/gchron-2020-23","DOIUrl":"https://doi.org/10.5194/gchron-2020-23","url":null,"abstract":"Abstract. Northern New Zealand is an important location for understanding Last Glacial Interval (LGI) palaeoclimate dynamics, since it is\u0000influenced by both tropical and polar climate systems which have varied in\u0000relative strength and timing. Sediments from the Auckland Volcanic Field\u0000maar lakes preserve records of such large-scale climatic influences on\u0000regional palaeo-environment changes, as well as past volcanic eruptions. The\u0000sediment sequence infilling Orakei maar lake is continuous, laminated, and\u0000rapidly deposited, and it provides a high-resolution (sedimentation rate above\u0000∼ 1 m kyr−1) archive from which to investigate the dynamic nature\u0000of the northern New Zealand climate system over the LGI. Here we present the\u0000chronological framework for the Orakei maar sediment sequence. Our\u0000chronology was developed using Bayesian age modelling of combined\u0000radiocarbon ages, tephrochronology of known-age rhyolitic tephra marker\u0000layers, 40Ar∕39Ar-dated eruption age of a local basaltic volcano, luminescence dating (using post-infrared–infrared stimulated luminescence,\u0000or pIR-IRSL), and the timing of the Laschamp palaeomagnetic excursion. We\u0000have integrated our absolute chronology with tuning of the relative\u0000palaeo-intensity record of the Earth's magnetic field to a global reference\u0000curve (PISO-1500). The maar-forming phreatomagmatic eruption of the Orakei\u0000maar is now dated to > 132 305 years (95 % confidence range:\u0000131 430 to 133 180 years). Our new chronology facilitates high-resolution\u0000palaeo-environmental reconstruction for northern New Zealand spanning the last\u0000ca. 130 000 years for the first time as most NZ records that span all or\u0000parts of the LGI are fragmentary, low-resolution, and poorly dated. Providing\u0000this chronological framework for LGI climate events inferred from the Orakei\u0000sequence is of paramount importance in the context of identification of\u0000leads and lags in different components of the Southern Hemisphere climate\u0000system as well as identification of Northern Hemisphere climate signals.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"4 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82112111","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":"Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology","authors":"G. Balco, B. DeJong, J. Ridge, P. Bierman, D. Rood","doi":"10.5194/gchron-2020-16","DOIUrl":"https://doi.org/10.5194/gchron-2020-16","url":null,"abstract":"Abstract. We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10Be fallout, but 9Be and 10Be concentrations considered together show that the majority of 10Be is present in glacial sediment when it enters the lake, and only a minority of total 10Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10Be fallout that have similar properties as coeval 10Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record 10Be fallout variations. We attempted to match centennial-period variations in reconstructed 10Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core 10Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not 10Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate 10Be me","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"100 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81703679","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":"Delayed and rapid deglaciation of alpine valleys in the Sawatch Range, southern Rocky Mountains, USA","authors":"J. Tulenko, W. Caffee, A. Schweinsberg, J. Briner, E. Leonard","doi":"10.5194/gchron-2020-13","DOIUrl":"https://doi.org/10.5194/gchron-2020-13","url":null,"abstract":"Abstract. We quantify retreat rates for three alpine glaciers in the Sawatch Range of\u0000the southern Rocky Mountains following the Last Glacial Maximum using\u000010Be ages from ice-sculpted, valley-floor bedrock transects and\u0000statistical analysis via the BACON program in R. Glacier retreat in the\u0000Sawatch Range from at (100 %) or near (∼83 %) Last\u0000Glacial Maximum extents initiated between 16.0 and 15.6 ka and was complete\u0000by 14.2–13.7 ka at rates ranging between 35.6 and 6.8 m a−1.\u0000Deglaciation in the Sawatch Range commenced ∼2–3 kyr later\u0000than the onset of rising global CO2 and prior to rising temperatures\u0000observed in the North Atlantic region at the Heinrich Stadial 1–Bølling\u0000transition. However, deglaciation in the Sawatch Range approximately aligns\u0000with the timing of Great Basin pluvial lake lowering. Recent data–modeling\u0000comparison efforts highlight the influence of the large North American ice\u0000sheets on climate in the western United States, and we hypothesize that\u0000recession of the North American ice sheets may have influenced the timing\u0000and rate of deglaciation in the Sawatch Range. While we cannot definitively\u0000argue for exclusively North Atlantic forcing or North American ice sheet\u0000forcing, our data demonstrate the importance of regional forcing mechanisms\u0000for past climate records.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"20 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-05-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84659498","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":"Unifying the U–Pb and Th–Pb methods: joint isochron regression and common Pb correction","authors":"P. Vermeesch","doi":"10.5194/gchron-2-119-2020","DOIUrl":"https://doi.org/10.5194/gchron-2-119-2020","url":null,"abstract":"Abstract. The actinide elements U and Th undergo radioactive decay to three\u0000isotopes of Pb, forming the basis of three coupled geochronometers.\u0000The 206Pb ∕238U and\u0000207Pb ∕235U decay systems are\u0000routinely combined to improve accuracy. Joint consideration with the\u0000208Pb ∕232Th decay system is less\u0000common. This paper aims to change this. Co-measured\u0000208Pb ∕232Th is particularly\u0000useful for discordant samples containing variable amounts of\u0000non-radiogenic (“common”) Pb. The paper presents a maximum likelihood algorithm for joint isochron\u0000regression of the 206Pb ∕238Pb, 207Pb ∕235Pb and\u0000208Pb ∕232Th chronometers. Given\u0000a set of cogenetic samples, this total-Pb/U-Th algorithm\u0000estimates the common Pb composition and concordia intercept age.\u0000U–Th–Pb data can be visualised on a conventional Wetherill or\u0000Tera–Wasserburg concordia diagram, or on a\u0000208Pb ∕232Th vs.\u0000206Pb ∕238U plot. Alternatively,\u0000the results of the new discordia regression algorithm can also be\u0000visualised as a 208Pbc ∕206Pb vs. 238U ∕206Pb or\u0000208Pbc ∕207Pb\u0000vs. 235U ∕206Pb isochron, where\u0000208Pbc represents the common\u0000208Pb component. In its most general form, the\u0000total-Pb/U-Th algorithm accounts for the uncertainties of all\u0000isotopic ratios involved, including the\u0000232Th ∕238U ratio, as well as the\u0000systematic uncertainties associated with the decay constants and the\u0000238U ∕235U ratio. However,\u0000numerical stability is greatly improved when the dependency on the\u0000232Th ∕238U-ratio uncertainty is\u0000dropped. For detrital minerals, it is generally not safe to assume a shared\u0000common Pb composition and concordia intercept age. In this case, the\u0000total-Pb/U-Th regression method must be modified by tying it to a\u0000terrestrial Pb evolution model. Thus, also detrital common Pb\u0000correction can be formulated in a maximum likelihood sense. The new method was applied to three published datasets, including\u0000low Th∕U carbonates, high Th∕U allanites and overdispersed\u0000monazites. The carbonate example illustrates how the total-Pb/U-Th\u0000method achieves a more precise common Pb correction than a\u0000conventional 207Pb-based approach does. The allanite\u0000sample shows the significant gain in both precision and accuracy\u0000that is made when the Th–Pb decay system is jointly considered with\u0000the U–Pb system. Finally, the monazite example is used to illustrate\u0000how the total-Pb/U-Th regression algorithm can be modified to\u0000include an overdispersion parameter. All the parameters in the discordia regression method (including the\u0000age and the overdispersion parameter) are strictly positive\u0000quantities that exhibit skewed error distributions near zero. This\u0000skewness can be accounted for using the profile log-likelihood\u0000method or by recasting the regression algorithm in terms of\u0000logarithmic quantities. Both approaches yield realistic asymmetric\u0000confidence intervals for the model parameters. The new algorithm is\u0000flexible enough that it can accommodate disequilibrium corrections\u0000and intersample error correlations when these are provided by the\u0000user. All","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"38 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-05-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91316300","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":"Miniature radiocarbon measurements ( <  150 µg C) from sediments of Lake Żabińskie, Poland: effect of precision and dating density on age–depth models","authors":"P. Zander, S. Szidat, D. Kaufman, Maurycy Żarczyński, Anna I. Poraj-Górska, P. Boltshauser-Kaltenrieder, M. Grosjean","doi":"10.5194/GCHRON-2-63-2020","DOIUrl":"https://doi.org/10.5194/GCHRON-2-63-2020","url":null,"abstract":"Abstract. The recent development of the MIni CArbon DAting System (MICADAS) allows\u0000researchers to obtain radiocarbon (14C) ages from a variety of samples\u0000with miniature amounts of carbon (<150 µg C) by using a gas\u0000ion source input that bypasses the graphitization step used for conventional\u000014C dating with accelerator mass spectrometry (AMS). The ability to\u0000measure smaller samples, at reduced cost compared with graphitized samples,\u0000allows for greater dating density of sediments with low macrofossil\u0000concentrations. In this study, we use a section of varved sediments from\u0000Lake Żabińskie, NE Poland, as a case study to assess the usefulness\u0000of miniature samples from terrestrial plant macrofossils for dating lake\u0000sediments. Radiocarbon samples analyzed using gas-source techniques were\u0000measured from the same depths as larger graphitized samples to compare the\u0000reliability and precision of the two techniques directly. We find that the\u0000analytical precision of gas-source measurements decreases as sample mass\u0000decreases but is comparable with graphitized samples of a similar size\u0000(approximately 150 µg C). For samples larger than 40 µg C and\u0000younger than 6000 BP, the uncalibrated 1σ age uncertainty is\u0000consistently less than 150 years (±0.010 F14C). The reliability\u0000of 14C ages from both techniques is assessed via comparison with a\u0000best-age estimate for the sediment sequence, which is the result of an OxCal\u0000V sequence that integrates varve counts with 14C ages. No bias is\u0000evident in the ages produced by either gas-source input or graphitization.\u0000None of the 14C ages in our dataset are clear outliers; the 95 %\u0000confidence intervals of all 48 calibrated 14C ages overlap with the\u0000median best-age estimate. The effects of sample mass (which defines the\u0000expected analytical age uncertainty) and dating density on age–depth models\u0000are evaluated via simulated sets of 14C ages that are used as inputs\u0000for OxCal P-sequence age–depth models. Nine different sampling scenarios\u0000were simulated in which the mass of 14C samples and the number of\u0000samples were manipulated. The simulated age–depth models suggest that the\u0000lower analytical precision associated with miniature samples can be\u0000compensated for by increased dating density. The data presented in this\u0000paper can improve sampling strategies and can inform expectations of age\u0000uncertainty from miniature radiocarbon samples as well as age–depth model\u0000outcomes for lacustrine sediments.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"117 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2020-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73749468","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}