Pub Date : 2023-07-19DOI: 10.5194/gchron-5-323-2023
P. Vermeesch, Yuntao Tian, J. Schwanethal, Y. Buret
Abstract. In situ U–Th–He geochronology is a potentially disruptive technique that combines laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with laser microprobe noble gas mass spectrometry. Despite its potential to revolutionize (detrital) thermochronology, in situ U–Th–He dating is not widely used due to persistent analytical challenges. A major issue is that current in situ U–Th–He dating approaches require that the U, Th, and He measurements are expressed in units of molar concentration, in contrast with conventional methods, which use units of molar abundance. Whereas molar abundances can be reliably determined by isotope dilution, accurate concentration measurements are not so easy to obtain. In the absence of matrix-matched U–Th concentration standards and accurate He ablation pit measurements, the required molar concentration calculations introduce an uncertainty that is higher than the conventional method, an uncertainty that is itself difficult to accurately quantify. We present a solution to this problem by using proton-induced 3He as a proxy for ablation pit volume and by pairing samples with a standard of known U–Th–He age. Thus, the U–Th–He age equation can be solved using relative rather than absolute concentration measurements. Pilot experiments show that the new method produces accurate results. However, it is prone to overdispersion, which is attributed to gradients in the proton fluence. These gradients can be measured, and their effect can be removed by fixing the geometry of the sample and the standard during the proton irradiation.
{"title":"Technical note: In situ U–Th–He dating by 4He ∕ 3He laser microprobe analysis","authors":"P. Vermeesch, Yuntao Tian, J. Schwanethal, Y. Buret","doi":"10.5194/gchron-5-323-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-323-2023","url":null,"abstract":"Abstract. In situ U–Th–He geochronology is a potentially disruptive technique\u0000that combines laser ablation inductively coupled plasma mass\u0000spectrometry (LA-ICP-MS) with laser microprobe noble gas mass\u0000spectrometry. Despite its potential to revolutionize (detrital)\u0000thermochronology, in situ U–Th–He dating is not widely used due to\u0000persistent analytical challenges. A major issue is that current\u0000in situ U–Th–He dating approaches require that the U, Th, and He\u0000measurements are expressed in units of molar concentration, in\u0000contrast with conventional methods, which use units of molar\u0000abundance. Whereas molar abundances can be reliably determined by\u0000isotope dilution, accurate concentration measurements are not so\u0000easy to obtain. In the absence of matrix-matched U–Th concentration\u0000standards and accurate He ablation pit measurements, the required\u0000molar concentration calculations introduce an uncertainty that is\u0000higher than the conventional method, an uncertainty that is itself\u0000difficult to accurately quantify. We present a solution to this\u0000problem by using proton-induced 3He as a proxy for\u0000ablation pit volume and by pairing samples with a standard of known\u0000U–Th–He age. Thus, the U–Th–He age equation can be solved using\u0000relative rather than absolute concentration measurements. Pilot\u0000experiments show that the new method produces accurate results.\u0000However, it is prone to overdispersion, which is attributed to\u0000gradients in the proton fluence. These gradients can be measured, and\u0000their effect can be removed by fixing the geometry of the sample and\u0000the standard during the proton irradiation.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"7 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81103614","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}
Pub Date : 2023-07-17DOI: 10.5194/gchron-5-301-2023
A. Balter-Kennedy, J. Schaefer, R. Schwartz, J. Lamp, Laura Penrose, J. Middleton, Jeanene P. Hanley, B. Tibari, P. Blard, G. Winckler, A. Hidy, G. Balco
Abstract. Here, we present cosmogenic-10Be and cosmogenic-3He data from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock core from the McMurdo Dry Valleys, Antarctica, with the goal of refining the laboratory methods for extracting beryllium from pyroxene, further estimating the 10Be production rate in pyroxene and demonstrating the applicability of 10Be–3He in mafic rock. The ability to routinely measure cosmogenic 10Be in pyroxene will open new opportunities for quantifying exposure durations and Earth surface processes in mafic rocks. We describe scalable laboratory methods for isolating beryllium from pyroxene, which include a simple hydrofluoric acid leaching procedure for removing meteoric 10Be and the addition of a pH 8 precipitation step to reduce the cation load prior to ion exchange chromatography. 10Be measurements in pyroxene from the surface samples have apparent 3He exposure ages of 1–6 Myr. We estimate a spallation production rate for 10Be in pyroxene, referenced to 3He, of 3.6 ± 0.2 atoms g−1 yr−1. 10Be and 3He measurements in the bedrock core yield initial estimates for parameters associated with 10Be and 3He production by negative-muon capture (f10∗=0.00183 and f3∗fCfD=0.00337). Next, we demonstrate that the 10Be–3He pair in pyroxene can be used to simultaneously resolve erosion rates and exposure ages, finding that the measured cosmogenic-nuclide concentrations in our surface samples are best explained by 2–8 Myr of exposure at erosion rates of 0–35 cm Myr−1. Finally, given the low 10Be in our laboratory blanks (average of 5.7 × 103 atoms), the reported measurement precision, and our estimated production rate, it should be possible to measure 2 g samples with 10Be concentrations of 6 × 104 and 1.5 × 104 atoms g−1 with 5 % and 15 % uncertainty, respectively. With this level of precision, Last Glacial Maximum to Late Holocene surfaces can now be dated with 10Be in pyroxene. Application of 10Be in pyroxene, alone or in combination with 3He, will expand possibilities for investigating glacial histories and landscape change in mafic rock.
{"title":"Cosmogenic 10Be in pyroxene: laboratory progress, production rate systematics, and application of the 10Be–3He nuclide pair in the Antarctic Dry Valleys","authors":"A. Balter-Kennedy, J. Schaefer, R. Schwartz, J. Lamp, Laura Penrose, J. Middleton, Jeanene P. Hanley, B. Tibari, P. Blard, G. Winckler, A. Hidy, G. Balco","doi":"10.5194/gchron-5-301-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-301-2023","url":null,"abstract":"Abstract. Here, we present cosmogenic-10Be and cosmogenic-3He data\u0000from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock core\u0000from the McMurdo Dry Valleys, Antarctica, with the goal of refining the\u0000laboratory methods for extracting beryllium from pyroxene, further\u0000estimating the 10Be production rate in pyroxene and demonstrating the\u0000applicability of 10Be–3He in mafic rock. The ability to routinely\u0000measure cosmogenic 10Be in pyroxene will open new opportunities for\u0000quantifying exposure durations and Earth surface processes in mafic rocks.\u0000We describe scalable laboratory methods for isolating beryllium from\u0000pyroxene, which include a simple hydrofluoric acid leaching procedure for\u0000removing meteoric 10Be and the addition of a pH 8 precipitation step\u0000to reduce the cation load prior to ion exchange chromatography. 10Be\u0000measurements in pyroxene from the surface samples have apparent 3He\u0000exposure ages of 1–6 Myr. We estimate a spallation production rate for\u000010Be in pyroxene, referenced to 3He, of 3.6 ± 0.2 atoms g−1 yr−1. 10Be and 3He measurements in the bedrock core\u0000yield initial estimates for parameters associated with 10Be and\u00003He production by negative-muon capture (f10∗=0.00183\u0000and f3∗fCfD=0.00337). Next, we demonstrate that the 10Be–3He pair in pyroxene can be\u0000used to simultaneously resolve erosion rates and exposure ages, finding that\u0000the measured cosmogenic-nuclide concentrations in our surface samples are\u0000best explained by 2–8 Myr of exposure at erosion rates of 0–35 cm Myr−1. Finally, given the low 10Be in our laboratory blanks\u0000(average of 5.7 × 103 atoms), the reported measurement precision, and\u0000our estimated production rate, it should be possible to measure 2 g samples\u0000with 10Be concentrations of 6 × 104 and 1.5 × 104 atoms g−1 with 5 % and 15 % uncertainty, respectively. With\u0000this level of precision, Last Glacial Maximum to Late Holocene surfaces can\u0000now be dated with 10Be in pyroxene. Application of 10Be in\u0000pyroxene, alone or in combination with 3He, will expand possibilities\u0000for investigating glacial histories and landscape change in mafic rock.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"22 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-07-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88707555","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}
Pub Date : 2023-06-19DOI: 10.5194/gchron-5-285-2023
Gabriel West, D. Kaufman, M. Jakobsson, M. O’Regan
Abstract. We report the results of amino acid racemization (AAR) analyses of aspartic acid (Asp) and glutamic acid (Glu) in the planktic Neogloboquadrina pachyderma, and the benthic Cibicidoides wuellerstorfi, foraminifera species collected from sediment cores from the Arctic Ocean. The cores were retrieved at various deep-sea sites of the Arctic, which cover a large geographical area from the Greenland and Iceland seas (GIS) to the Alpha and Lomonosov ridges in the central Arctic Ocean. Age models for the investigated sediments were developed by multiple dating and correlation techniques, including oxygen isotope stratigraphy, magnetostratigraphy, biostratigraphy, lithostratigraphy, and cyclostratigraphy. The extent of racemization (D/L values) was determined on 95 samples (1028 subsamples) and shows a progressive increase downcore for both foraminifera species. Differences in the rates of racemization between the species were established by analysing specimens of both species from the same stratigraphic levels (n=21). Aspartic acid (Asp) and glutamic acid (Glu) racemize on average 16 ± 2 % and 23 ± 3 % faster, respectively, in C. wuellerstorfi than in N. pachyderma. The D/L values increase with sample age in nearly all cases, with a trend that follows a simple power function. Scatter around least-squares regression fits are larger for samples from the central Arctic Ocean than for those from the Nordic Seas. Calibrating the rate of racemization in C. wuellerstorfi using independently dated samples from the Greenland and Iceland seas for the past 400 ka enables estimation of sample ages from the central Arctic Ocean, where bottom water temperatures are presently relatively similar. The resulting ages are older than expected when considering the existing age models for the central Arctic Ocean cores. These results confirm that the differences are not due to taxonomic effects on AAR and further warrant a critical evaluation of existing Arctic Ocean age models. A better understanding of temperature histories at the investigated sites, and other environmental factors that may influence racemization rates in central Arctic Ocean sediments, is also needed.
{"title":"Amino acid racemization in Neogloboquadrina pachyderma and Cibicidoides wuellerstorfi from the Arctic Ocean and its implications for age models","authors":"Gabriel West, D. Kaufman, M. Jakobsson, M. O’Regan","doi":"10.5194/gchron-5-285-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-285-2023","url":null,"abstract":"Abstract. We report the results of amino acid racemization (AAR) analyses of aspartic acid (Asp)\u0000and glutamic acid (Glu) in the planktic Neogloboquadrina pachyderma, and the benthic Cibicidoides wuellerstorfi, foraminifera species collected from sediment cores from the Arctic Ocean. The cores were retrieved at various deep-sea sites of the Arctic, which cover a large geographical area from the Greenland and Iceland seas (GIS) to the Alpha and Lomonosov ridges in the central Arctic Ocean. Age models for the investigated sediments were developed by multiple dating and correlation techniques, including oxygen isotope stratigraphy, magnetostratigraphy, biostratigraphy, lithostratigraphy, and cyclostratigraphy. The extent of racemization (D/L values) was determined on 95 samples (1028 subsamples) and shows a progressive increase downcore for both foraminifera species. Differences in the rates of racemization between the species were established by analysing specimens of both species from the same stratigraphic levels (n=21). Aspartic acid (Asp) and glutamic acid (Glu) racemize on average 16 ± 2 % and 23 ± 3 % faster, respectively, in C. wuellerstorfi than in N. pachyderma. The D/L values increase with sample age in nearly all cases, with a trend that follows a simple power function. Scatter around least-squares regression fits are larger for samples from the central Arctic Ocean than for those from the Nordic Seas. Calibrating the rate of racemization in C. wuellerstorfi using independently dated samples from the Greenland and Iceland seas for the past 400 ka enables estimation of sample ages from the central Arctic Ocean, where bottom water temperatures are presently relatively similar. The resulting ages are older than expected when considering the existing age models for the central Arctic Ocean cores. These results confirm that the differences are not due to taxonomic effects on AAR and further warrant a critical evaluation of existing Arctic Ocean age models. A better understanding of temperature histories at the investigated sites, and other environmental factors that may influence racemization rates in central Arctic Ocean sediments, is also needed.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"58 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80443233","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}
Pub Date : 2023-06-06DOI: 10.5194/gchron-5-271-2023
S. Kreutzer, Steve Grehl, Michael Höhne, Oliver Simmank, K. Dornich, Grzegorz Adamiec, Christoph Burow, H. Roberts, G. Duller
Abstract. The concept of open data has become the modern science meme, and major funding bodies and publishers support open data. On a daily basis, however, the open data mandate frequently encounters technical obstacles, such as a lack of a suitable data format for data sharing and long-term data preservation. Such issues are often community-specific and best addressed through community-tailored solutions. In Quaternary sciences, luminescence dating is widely used for constraining the timing of event-based processes (e.g. sediment transport). Every luminescence dating study produces a vast body of primary data that usually remains inaccessible and incompatible with future studies or adjacent scientific disciplines. To facilitate data exchange and long-term data preservation (in short, open data) in luminescence dating studies, we propose a new XML-based structured data format called XLUM. The format applies a hierarchical data storage concept consisting of a root node (node 0), a sample (node 1), a sequence (node 2), a record (node 3), and a curve (node 4). The curve level holds information on the technical component (e.g. photomultiplier, thermocouple). A finite number of curves represent a record (e.g. an optically stimulated luminescence curve). Records are part of a sequence measured for a particular sample. This design concept allows the user to retain information on a technical component level from the measurement process. The additional storage of related metadata fosters future data mining projects on large datasets. The XML-based format is less memory-efficient than binary formats; however, its focus is data exchange, preservation, and hence XLUM long-term format stability by design. XLUM is inherently stable to future updates and backwards-compatible. We support XLUM through a new R package xlum, facilitating the conversion of different formats into the new XLUM format. XLUM is licensed under the MIT licence and hence available for free to be used in open- and closed-source commercial and non-commercial software and research projects.
{"title":"XLUM: an open data format for exchange and long-term preservation of luminescence data","authors":"S. Kreutzer, Steve Grehl, Michael Höhne, Oliver Simmank, K. Dornich, Grzegorz Adamiec, Christoph Burow, H. Roberts, G. Duller","doi":"10.5194/gchron-5-271-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-271-2023","url":null,"abstract":"Abstract. The concept of open data has become the modern science meme, and major funding bodies and publishers support open data. On a daily basis, however, the\u0000open data mandate frequently encounters technical obstacles, such as a lack of a suitable data format for data sharing and long-term data\u0000preservation. Such issues are often community-specific and best addressed through community-tailored solutions. In Quaternary sciences, luminescence\u0000dating is widely used for constraining the timing of event-based processes (e.g. sediment transport). Every luminescence dating study produces a\u0000vast body of primary data that usually remains inaccessible and incompatible with future studies or adjacent scientific disciplines. To facilitate\u0000data exchange and long-term data preservation (in short, open data) in luminescence dating studies, we propose a new XML-based structured data\u0000format called XLUM. The format applies a hierarchical data storage concept consisting of a root node (node 0), a sample (node 1), a sequence\u0000(node 2), a record (node 3), and a curve (node 4). The curve level holds information on the technical component (e.g. photomultiplier,\u0000thermocouple). A finite number of curves represent a record (e.g. an optically stimulated luminescence curve). Records are part of a sequence\u0000measured for a particular sample. This design concept allows the user to retain information on a technical component level from the measurement\u0000process. The additional storage of related metadata fosters future data mining projects on large datasets. The XML-based format is less\u0000memory-efficient than binary formats; however, its focus is data exchange, preservation, and hence XLUM long-term format stability by\u0000design. XLUM is inherently stable to future updates and backwards-compatible. We support XLUM through a new R package xlum,\u0000facilitating the conversion of different formats into the new XLUM format. XLUM is licensed under the MIT licence and hence available\u0000for free to be used in open- and closed-source commercial and non-commercial software and research projects.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"8 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-06-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74466350","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}
Pub Date : 2023-05-17DOI: 10.5194/gchron-5-263-2023
A. Lipp, P. Vermeesch
Abstract. Distributional data such as detrital age populations or grain size distributions are common in the geological sciences. As analytical techniques become more sophisticated, increasingly large amounts of distributional data are being gathered. These advances require quantitative and objective methods, such as multidimensional scaling (MDS), to analyse large numbers of samples. Crucial to such methods is choosing a sensible measure of dissimilarity between samples. At present, the Kolmogorov–Smirnov (KS) statistic is the most widely used of these dissimilarity measures. However, the KS statistic has some limitations such as high sensitivity to differences between the modes of two distributions and insensitivity to their tails. Here, we propose the Wasserstein-2 distance (W2) as an additional and alternative metric for use in geochronology. Whereas the KS distance is defined as the maximum vertical distance between two empirical cumulative distribution functions, the W2 distance is a function of the horizontal distances (i.e. age differences) between observations. Using a variety of synthetic and real datasets, we explore scenarios where the W2 may provide greater geological insight than the KS statistic. We find that in cases where absolute time differences are not relevant (e.g. mixing of known, discrete age peaks), the KS statistic can be more intuitive. However, in scenarios where absolute age differences are important (e.g. temporally and/or spatially evolving sources, thermochronology, and overcoming laboratory biases), W2 is preferable. The W2 distance has been added to the R package, IsoplotR, for immediate use in detrital geochronology and other applications. The W2 distance can be generalized to multiple dimensions, which opens opportunities beyond distributional data.
{"title":"Short communication: The Wasserstein distance as a dissimilarity metric for comparing detrital age spectra and other geological distributions","authors":"A. Lipp, P. Vermeesch","doi":"10.5194/gchron-5-263-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-263-2023","url":null,"abstract":"Abstract. Distributional data such as detrital age populations or grain size distributions are common in the geological sciences. As analytical techniques become more sophisticated, increasingly large amounts of distributional data are being gathered. These advances require quantitative and objective methods, such as multidimensional scaling (MDS), to analyse large numbers of samples. Crucial to such methods is choosing a sensible measure of dissimilarity between samples. At present, the Kolmogorov–Smirnov (KS) statistic is the most widely used of these dissimilarity measures. However, the KS statistic has some limitations such as high sensitivity to differences between the modes of two distributions and insensitivity to their tails. Here, we propose the Wasserstein-2 distance (W2) as an additional and alternative metric for use in geochronology. Whereas the KS distance is defined as the maximum vertical distance between two empirical cumulative distribution functions, the W2 distance is a function of the horizontal distances (i.e. age differences) between observations. Using a variety of synthetic and real datasets, we explore scenarios where the W2 may provide greater geological insight than the KS statistic. We find that in cases where absolute time differences are not relevant (e.g. mixing of known, discrete age peaks), the KS statistic can be more intuitive. However, in scenarios where absolute age differences are important (e.g. temporally and/or spatially evolving sources, thermochronology, and overcoming laboratory biases), W2 is preferable. The W2 distance has been added to the R package, IsoplotR, for immediate use in detrital geochronology and other applications. The W2 distance can be generalized to multiple dimensions, which opens opportunities beyond distributional data.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"9 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74105078","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}
Pub Date : 2023-05-04DOI: 10.5194/gchron-5-241-2023
W. M. van der Meij, A. Temme, S. Binnie, T. Reimann
Abstract. Understanding long-term soil and landscape evolution can help us understand the threats to current-day soils, landscapes and their functions. The temporal evolution of soils and landscapes can be studied using geochronometers, such as optically stimulated luminescence (OSL) particle ages or radionuclide inventories. Also, soil–landscape evolution models (SLEMs) can be used to study the spatial and temporal evolution of soils and landscapes through numerical modelling of the processes responsible for the evolution. SLEMs and geochronometers have been combined in the past, but often these couplings focus on a single geochronometer, are designed for specific idealized landscape positions, or do not consider multiple transport processes or post-depositional mixing processes that can disturb the geochronometers in sedimentary archives. We present ChronoLorica, a coupling of the soil–landscape evolution model Lorica with a geochronological module. The module traces spatiotemporal patterns of particle ages, analogous to OSL ages, and radionuclide inventories during the simulations of soil and landscape evolution. The geochronological module opens rich possibilities for data-based calibration of simulated model processes, which include natural processes, such as bioturbation and soil creep, as well as anthropogenic processes, such as tillage. Moreover, ChronoLorica can be applied to transient landscapes that are subject to complex, non-linear boundary conditions, such as land use intensification, and processes of post-depositional disturbance which often result in complex geo-archives. In this contribution, we illustrate the model functionality and applicability by simulating soil and landscape evolution along a two-dimensional hillslope. We show how the model simulates the development of the following three geochronometers: OSL particle ages, meteoric 10Be inventories and in situ 10Be inventories. The results are compared with field observations from comparable landscapes. We also discuss the limitations of the model and highlight its potential applications in pedogenical, geomorphological or geological studies.
{"title":"ChronoLorica: introduction of a soil–landscape evolution model combined with geochronometers","authors":"W. M. van der Meij, A. Temme, S. Binnie, T. Reimann","doi":"10.5194/gchron-5-241-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-241-2023","url":null,"abstract":"Abstract. Understanding long-term soil and landscape evolution can help us understand\u0000the threats to current-day soils, landscapes and their functions. The\u0000temporal evolution of soils and landscapes can be studied using\u0000geochronometers, such as optically stimulated luminescence (OSL) particle ages or radionuclide inventories.\u0000Also, soil–landscape evolution models (SLEMs) can be used to study the\u0000spatial and temporal evolution of soils and landscapes through numerical\u0000modelling of the processes responsible for the evolution. SLEMs and\u0000geochronometers have been combined in the past, but often these couplings\u0000focus on a single geochronometer, are designed for specific idealized\u0000landscape positions, or do not consider multiple transport processes or\u0000post-depositional mixing processes that can disturb the geochronometers in\u0000sedimentary archives. We present ChronoLorica, a coupling of the soil–landscape evolution model Lorica\u0000with a geochronological module. The module traces spatiotemporal patterns of\u0000particle ages, analogous to OSL ages, and radionuclide inventories during\u0000the simulations of soil and landscape evolution. The geochronological module\u0000opens rich possibilities for data-based calibration of simulated model\u0000processes, which include natural processes, such as bioturbation and soil\u0000creep, as well as anthropogenic processes, such as tillage. Moreover,\u0000ChronoLorica can be applied to transient landscapes that are subject to\u0000complex, non-linear boundary conditions, such as land use intensification,\u0000and processes of post-depositional disturbance which often result in complex\u0000geo-archives. In this contribution, we illustrate the model functionality and\u0000applicability by simulating soil and landscape evolution along a\u0000two-dimensional hillslope. We show how the model simulates the development\u0000of the following three geochronometers: OSL particle ages, meteoric 10Be inventories\u0000and in situ 10Be inventories. The results are compared with field\u0000observations from comparable landscapes. We also discuss the limitations of\u0000the model and highlight its potential applications in pedogenical,\u0000geomorphological or geological studies.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"89 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-05-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"79397594","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}
Pub Date : 2023-05-04DOI: 10.5194/gchron-5-229-2023
A. Monteath, Matthew S. M. Bolton, J. Harvey, M. Seidenkrantz, C. Pearce, B. Jensen
Abstract. Radiocarbon dating marine sediments is complicated by the strongly heterogeneous age of ocean waters. Tephrochronology provides a well-established method to constrain the age of local radiocarbon reservoirs and more accurately calibrate dates. Numerous ultra-distal cryptotephra deposits (non-visible volcanic ash more than 3000 km from source) have been identified in peatlands and lake sediments across north-eastern North America and correlated with volcanic arcs in the Pacific north-west. Previously, however, these isochrons have not been identified in sediments from the north-west Atlantic Ocean. In this study, we report the presence of two ultra-distal cryptotephra deposits; Mazama Ash and White River Ash eastern lobe (WRAe), in Placentia Bay, North Atlantic Ocean. We use these well-dated isochrons to constrain the local marine radiocarbon reservoir offset (ΔR) and develop a robust Bayesian age–depth model with a ΔR that varies through time. Our results indicate that the marine radiocarbon offset in Placentia Bay was -126±151 years (relative to the Marine20 calibration curve) at the time of Mazama Ash deposition (7572 ± 18 yr BP) and −396 ± 144 years at the time of WRAe deposition (1098–1097 yr BP). Changes in ΔR appear to coincide with inferred shifts in relative influences of the inner Labrador Current and the Slopewater Current in the bay. An important conclusion is that single-offset models of ΔR are easiest to apply and often hard to disprove. However, such models may oversimplify reservoir effects in a core, even over relatively short timescales. Acknowledging potentially varying offsets is critical when ocean circulation and ventilation characteristics have differed over time. The addition of tephra isochrons permits the calculation of semi-independent reservoir corrections and verification of the single ΔR model.
摘要海洋沉积物的放射性碳定年由于海水的强烈不均匀年龄而变得复杂。温度年代学提供了一种完善的方法来限制当地放射性碳储层的年龄，并更准确地校准日期。在北美东北部的泥炭地和湖泊沉积物中发现了许多超远端隐火山灰沉积物(距离源头超过3000公里的不可见火山灰)，并与太平洋西北部的火山弧相关联。然而，在此之前，这些等时线并没有在西北大西洋的沉积物中被发现。在这项研究中，我们报告了两个超远端隐肾沉积物的存在;北大西洋普拉森西亚湾的马扎马灰和白河灰东叶(WRAe)。我们使用这些年代确定的等时线来约束当地海洋放射性碳储层偏移(ΔR)，并利用aΔR建立了一个稳健的贝叶斯年龄-深度模型，该模型随时间变化。结果表明，在Mazama Ash沉积(7572±18 yr BP)和wrae沉积(1098 ~ 1097 yr BP)期间，Placentia Bay的海洋放射性碳补偿分别为-126±151年和- 396±144年(相对于Marine20校准曲线)。ΔR的变化似乎与推断出的拉布拉多内流和海湾内坡面水流相对影响的变化相吻合。一个重要的结论是，ΔR的单次补偿模型最容易应用，而且往往很难反驳。然而，这种模型可能过于简化岩心中的储层效应，甚至过于短的时间尺度。当海洋环流和通风特征随时间变化时，承认潜在的不同抵消是至关重要的。tephra等时线的加入允许计算半独立的储层改正和验证singleΔR模型。
{"title":"Ultra-distal tephra deposits and Bayesian modelling constrain a variable marine radiocarbon offset in Placentia Bay, Newfoundland","authors":"A. Monteath, Matthew S. M. Bolton, J. Harvey, M. Seidenkrantz, C. Pearce, B. Jensen","doi":"10.5194/gchron-5-229-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-229-2023","url":null,"abstract":"Abstract. Radiocarbon dating marine sediments is complicated by the\u0000strongly heterogeneous age of ocean waters. Tephrochronology provides a\u0000well-established method to constrain the age of local radiocarbon reservoirs\u0000and more accurately calibrate dates. Numerous ultra-distal cryptotephra\u0000deposits (non-visible volcanic ash more than 3000 km from source) have\u0000been identified in peatlands and lake sediments across north-eastern North\u0000America and correlated with volcanic arcs in the Pacific north-west.\u0000Previously, however, these isochrons have not been identified in sediments\u0000from the north-west Atlantic Ocean. In this study, we report the presence of\u0000two ultra-distal cryptotephra deposits; Mazama Ash and White River Ash\u0000eastern lobe (WRAe), in Placentia Bay, North Atlantic Ocean. We use these\u0000well-dated isochrons to constrain the local marine radiocarbon reservoir\u0000offset (ΔR) and develop a robust Bayesian age–depth model with a\u0000ΔR that varies through time. Our results indicate that the marine\u0000radiocarbon offset in Placentia Bay was -126±151 years (relative to\u0000the Marine20 calibration curve) at the time of Mazama Ash deposition\u0000(7572 ± 18 yr BP) and −396 ± 144 years at the time of WRAe\u0000deposition (1098–1097 yr BP). Changes in ΔR appear to coincide with\u0000inferred shifts in relative influences of the inner Labrador Current and the\u0000Slopewater Current in the bay. An important conclusion is that single-offset\u0000models of ΔR are easiest to apply and often hard to disprove.\u0000However, such models may oversimplify reservoir effects in a core, even over\u0000relatively short timescales. Acknowledging potentially varying offsets is\u0000critical when ocean circulation and ventilation characteristics have\u0000differed over time. The addition of tephra isochrons permits the calculation\u0000of semi-independent reservoir corrections and verification of the single\u0000ΔR model.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"60 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-05-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86186347","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}
Pub Date : 2023-05-02DOI: 10.5194/gchron-5-197-2023
Spencer D. Zeigler, J. Metcalf, R. Flowers
Abstract. Apatite (U–Th) / He (AHe) dating generally assumes that grains can be accurately and precisely modeled as geometrically perfect hexagonal prisms or ellipsoids in order to compute the apatite volume (V), alpha-ejection corrections (FT), equivalent spherical radius (RFT), effective uranium concentration (eU), and corrected (U–Th) / He date. It is well-known that this assumption is not true. In this work, we present a set of corrections and uncertainties for V, FT, and RFT aimed (1) at “undoing” the systematic deviation from the idealized geometry and (2) at quantifying the contribution of geometric uncertainty to the total uncertainty budget for eU and AHe dates. These corrections and uncertainties can be easily integrated into existing laboratory workflows at no added cost, can be routinely applied to all dated apatite, and can even be retroactively applied to published data. To quantify the degree to which real apatite deviates from geometric models, we selected 264 grains that span the full spectrum of commonly analyzed morphologies, measured their dimensions using standard 2D microscopy methods, and then acquired 3D scans of the same grains using high-resolution computed tomography (CT). We then compared our apatite 2D length, maximum width, and minimum width measurements with those determined by CT, as well as the V, FT, and RFT values calculated from 2D microscopy measurements with those from the “real” 3D measurements. While our 2D length and maximum width measurements match the 3D values well, the 2D minimum width values systematically underestimate the 3D values and have high scatter. We therefore use only the 2D length and maximum width measurements to compute V, FT, and RFT. With this approach, apatite V, FT, and RFT values are all consistently overestimated by the 2D microscopy method, requiring correction factors of 0.74–0.83 (or 17 %–26 %), 0.91–0.99 (or 1 %–9 %), and 0.85–0.93 (or 7 %–15 %), respectively. The 1σ uncertainties in V, FT, and RFT are 20 %–23 %, 1 %–6 %, and 6 %–10 %, respectively. The primary control on the magnitude of the corrections and uncertainties is grain geometry, with grain size exerting additional control on FT uncertainty. Application of these corrections and uncertainties to a real dataset (N=24 AHe analyses) yields 1σ analytical and geometric uncertainties of 15 %–16 % in eU and 3 %–7 % in the corrected date. These geometric corrections and uncertainties are substantial and should not be ignored when reporting, plotting, and interpreting AHe datasets. The Geometric Correction Method (GCM) presented here provides a simple and practical tool for deriving more accurate FT and eU values and for incorporating this oft neglected geometric uncertainty into AHe dates.
{"title":"A practical method for assigning uncertainty and improving the accuracy of alpha-ejection corrections and eU concentrations in apatite (U–Th) ∕ He chronology","authors":"Spencer D. Zeigler, J. Metcalf, R. Flowers","doi":"10.5194/gchron-5-197-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-197-2023","url":null,"abstract":"Abstract. Apatite (U–Th) / He (AHe) dating generally assumes that grains can\u0000be accurately and precisely modeled as geometrically perfect hexagonal\u0000prisms or ellipsoids in order to compute the apatite volume (V),\u0000alpha-ejection corrections (FT), equivalent spherical radius\u0000(RFT), effective uranium concentration (eU), and corrected (U–Th) / He\u0000date. It is well-known that this assumption is not true. In this work, we\u0000present a set of corrections and uncertainties for V, FT, and RFT\u0000aimed (1) at “undoing” the systematic deviation from the idealized\u0000geometry and (2) at quantifying the contribution of geometric uncertainty to\u0000the total uncertainty budget for eU and AHe dates. These corrections and\u0000uncertainties can be easily integrated into existing laboratory workflows at\u0000no added cost, can be routinely applied to all dated apatite, and can even\u0000be retroactively applied to published data. To quantify the degree to which\u0000real apatite deviates from geometric models, we selected 264 grains that span\u0000the full spectrum of commonly analyzed morphologies, measured their\u0000dimensions using standard 2D microscopy methods, and then acquired 3D scans\u0000of the same grains using high-resolution computed tomography (CT). We then\u0000compared our apatite 2D length, maximum width, and minimum width\u0000measurements with those determined by CT, as well as the V, FT, and\u0000RFT values calculated from 2D microscopy measurements with those from\u0000the “real” 3D measurements. While our 2D length and maximum width\u0000measurements match the 3D values well, the 2D minimum width values\u0000systematically underestimate the 3D values and have high scatter. We\u0000therefore use only the 2D length and maximum width measurements to compute\u0000V, FT, and RFT. With this approach, apatite V, FT, and\u0000RFT values are all consistently overestimated by the 2D microscopy\u0000method, requiring correction factors of 0.74–0.83 (or 17 %–26 %), 0.91–0.99\u0000(or 1 %–9 %), and 0.85–0.93 (or 7 %–15 %), respectively. The 1σ\u0000uncertainties in V, FT, and RFT are 20 %–23 %, 1 %–6 %, and\u00006 %–10 %, respectively. The primary control on the magnitude of the\u0000corrections and uncertainties is grain geometry, with grain size exerting\u0000additional control on FT uncertainty. Application of these corrections\u0000and uncertainties to a real dataset (N=24 AHe analyses) yields 1σ\u0000analytical and geometric uncertainties of 15 %–16 % in eU and 3 %–7 % in the\u0000corrected date. These geometric corrections and uncertainties are\u0000substantial and should not be ignored when reporting, plotting, and\u0000interpreting AHe datasets. The Geometric Correction Method (GCM) presented\u0000here provides a simple and practical tool for deriving more accurate FT\u0000and eU values and for incorporating this oft neglected geometric\u0000uncertainty into AHe dates.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"436 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-05-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77011673","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}
Pub Date : 2023-04-20DOI: 10.5194/gchron-5-181-2023
T. Pollard, J. Woodhead, J. Hellstrom, J. Engel, R. Powell, R. Drysdale
Abstract. Initial radioactive disequilibrium amongst intermediate nuclides of the U decay chains can have a significant impact on the accuracy of U–Pb ages, especially in young samples. For samples that can reasonably be assumed to have attained radioactive equilibrium at the time of analysis, a relatively straightforward correction may be applied. However, in younger materials where this assumption is unreasonable, it is necessary to replace the familiar U–Pb age equations with more complete expressions that account for growth and decay of intermediate nuclides through time. DQPB is software for calculating U–Pb ages while accounting for the effects of radioactive disequilibrium among intermediate nuclides of the U decay chains. The software is written in Python and distributed as both a pure Python package and a stand-alone graphical user interface (GUI) application that integrates with standard Microsoft Excel spreadsheets. The software implements disequilibrium U–Pb equations to compute ages using various approaches, including concordia intercept ages on a Tera–Wasserburg diagram, U–Pb isochron ages, Pb*/U ages based on single aliquots, and 207Pb-corrected ages. While these age-calculation approaches are tailored toward young samples that cannot reasonably be assumed to have attained radioactive equilibrium at the time of analysis, they may also be applied to older materials where disequilibrium is no longer analytically resolvable. The software allows users to implement a variety of regression algorithms based on both classical and robust statistical approaches, compute weighted average ages and construct customisable, publication-ready plots of U–Pb age data. The regression and weighted average algorithms implemented in DQPB may also be applicable to other (i.e. non-U–Pb) geochronological datasets.
{"title":"DQPB: software for calculating disequilibrium U–Pb ages","authors":"T. Pollard, J. Woodhead, J. Hellstrom, J. Engel, R. Powell, R. Drysdale","doi":"10.5194/gchron-5-181-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-181-2023","url":null,"abstract":"Abstract. Initial radioactive disequilibrium amongst intermediate nuclides of the U decay chains can have a significant impact on the accuracy of\u0000U–Pb ages, especially in young samples. For samples that can reasonably be assumed to have attained radioactive equilibrium at the\u0000time of analysis, a relatively straightforward correction may be applied. However, in younger materials where this assumption is unreasonable, it is necessary to replace the familiar U–Pb age equations with more complete expressions that account for growth and decay of intermediate nuclides through time. DQPB is software for calculating U–Pb ages while accounting for the effects of radioactive disequilibrium among intermediate nuclides of the U decay chains. The software is written in Python and distributed as both a pure Python package and a stand-alone graphical user interface (GUI) application that integrates with standard Microsoft Excel spreadsheets. The software implements disequilibrium\u0000U–Pb equations to compute ages using various approaches, including concordia intercept ages on a Tera–Wasserburg diagram,\u0000U–Pb isochron ages, Pb*/U ages based on single aliquots, and 207Pb-corrected ages. While these age-calculation\u0000approaches are tailored toward young samples that cannot reasonably be assumed to have attained radioactive equilibrium at the time of analysis,\u0000they may also be applied to older materials where disequilibrium is no longer analytically resolvable. The software allows users to implement a\u0000variety of regression algorithms based on both classical and robust statistical approaches, compute weighted average ages and construct\u0000customisable, publication-ready plots of U–Pb age data. The regression and weighted average algorithms implemented in DQPB may also be applicable to other (i.e. non-U–Pb) geochronological datasets.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"17 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86896793","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}
Pub Date : 2023-04-12DOI: 10.5194/gchron-5-153-2023
Jack Muston, Marnie Forster, Davood Vasegh, Conrad Alderton, Shawn Crispin, Gordon Lister
Abstract. The Martabe gold deposits in Sumatra formed in a shallow crustal epithermal environment associated with intermediate mafic intrusions adjacent to an active right-lateral wrench system. Gas/fluid temperatures reached 200–350 ∘C. The structural geology suggests episodic switches in stress orientations during a Plio-Pleistocene seismotectonic evolution. Different mineralisation events may have been associated with oscillations in this earthquake cycle, so samples containing alunite were collected for 40Ar / 39Ar geochronology to constrain the timing. 39Ar diffusion experiments were performed to constrain variation in argon retentivity. The age spectra were produced by incremental step-heating with heating times chosen so similar percentages of 39Ar gas release occurred during as many steps as possible. This ensured the detail necessary for analysis of the complex morphology of these spectra by applying the method of asymptotes and limits, which enabled recognition of different growth events of alunite in overprinting fluid systems. It was possible to provide estimates as to the frequency of individual events and their duration. The heating schedule also ensured that Arrhenius data populated the inverse temperature axis with sufficient detail to allow modelling. Activation energies were between 370–660 kJ mol−1. Application of Dodson's recursion determined closure temperatures that range from 400–560 ∘C for a cooling rate of 100 ∘C Ma−1. Such estimates are higher than any temperature to be expected in the natural system, giving confidence that the ages represent the timing of growth during periods of active fluid movement and alteration: a hypothesis confirmed by modelling age spectra using the MacArgon program. We conclude that gold in the Purnama pit resulted from overprinting fluid rock interactions during very short mineralisation episodes at ∼2.25 and ∼2.00 Ma.
{"title":"Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite","authors":"Jack Muston, Marnie Forster, Davood Vasegh, Conrad Alderton, Shawn Crispin, Gordon Lister","doi":"10.5194/gchron-5-153-2023","DOIUrl":"https://doi.org/10.5194/gchron-5-153-2023","url":null,"abstract":"Abstract. The Martabe gold deposits in Sumatra formed in a shallow crustal epithermal environment associated with intermediate mafic intrusions adjacent to an active right-lateral wrench system. Gas/fluid temperatures reached 200–350 ∘C. The structural geology suggests episodic switches in stress orientations during a Plio-Pleistocene seismotectonic evolution. Different mineralisation events may have been associated with oscillations in this earthquake cycle, so samples containing alunite were collected for 40Ar / 39Ar geochronology to constrain the timing. 39Ar diffusion experiments were performed to constrain variation in argon retentivity. The age spectra were produced by incremental step-heating with heating times chosen so similar percentages of 39Ar gas release occurred during as many steps as possible. This ensured the detail necessary for analysis of the complex morphology of these spectra by applying the method of asymptotes and limits, which enabled recognition of different growth events of alunite in overprinting fluid systems. It was possible to provide estimates as to the frequency of individual events and their duration. The heating schedule also ensured that Arrhenius data populated the inverse temperature axis with sufficient detail to allow modelling. Activation energies were between 370–660 kJ mol−1. Application of Dodson's recursion determined closure temperatures that range from 400–560 ∘C for a cooling rate of 100 ∘C Ma−1. Such estimates are higher than any temperature to be expected in the natural system, giving confidence that the ages represent the timing of growth during periods of active fluid movement and alteration: a hypothesis confirmed by modelling age spectra using the MacArgon program. We conclude that gold in the Purnama pit resulted from overprinting fluid rock interactions during very short mineralisation episodes at ∼2.25 and ∼2.00 Ma.","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134950572","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}
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