Pub Date : 2022-07-05DOI: 10.5194/gchron-4-435-2022
M. K. Campbell, P. Bierman, A. Schmidt, Rita Y. Sibello Hernández, Alejandro García-Moya, L. Corbett, A. Hidy, Hector A. Cartas Aguila, Aniel Guillén Arruebarrena, G. Balco, D. Dethier, Marc Caffee
Abstract. We use 25 new measurements of in situ produced cosmogenic 26Al and 10Be in river sand, paired with estimates of dissolved load flux in river water, to characterize the processes and pace of landscape change in central Cuba. Long-term erosion rates inferred from 10Be concentrations in quartz extracted from central Cuban river sand range from 3.4–189 Mg km−2 yr−1 (mean 59, median 45). Dissolved loads (10–176 Mg km−2 yr−1; mean 92, median 97), calculated from stream solute concentrations and modeled runoff, exceed measured cosmogenic-10Be-derived erosion rates in 18 of 23 basins. This disparity mandates that in this environment landscape-scale mass loss is not fully represented by the cosmogenic nuclide measurements. The 26Al / 10Be ratios are lower than expected for steady-state exposure or erosion in 16 of 24 samples. Depressed 26Al / 10Be ratios occur in many of the basins that have the greatest disparity between dissolved loads (high) and erosion rates inferred from cosmogenic nuclide concentrations (low). Depressed 26Al / 10Be ratios are consistent with the presence of a deep, mixed, regolith layer providing extended storage times on slopes and/or burial and extended storage during fluvial transport. River water chemical analyses indicate that many basins with lower 26Al / 10Be ratios and high 10Be concentrations are underlain at least in part by evaporitic rocks that rapidly dissolve. Our data show that when assessing mass loss in humid tropical landscapes, accounting for the contribution of rock dissolution at depth is particularly important. In such warm, wet climates, mineral dissolution can occur many meters below the surface, beyond the penetration depth of most cosmic rays and thus the production of most cosmogenic nuclides. Our data suggest the importance of estimating solute fluxes and measuring paired cosmogenic nuclides to better understand the processes and rates of mass transfer at a basin scale.
{"title":"Cosmogenic nuclide and solute flux data from central Cuban rivers emphasize the importance of both physical and chemical mass loss from tropical landscapes","authors":"M. K. Campbell, P. Bierman, A. Schmidt, Rita Y. Sibello Hernández, Alejandro García-Moya, L. Corbett, A. Hidy, Hector A. Cartas Aguila, Aniel Guillén Arruebarrena, G. Balco, D. Dethier, Marc Caffee","doi":"10.5194/gchron-4-435-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-435-2022","url":null,"abstract":"Abstract. We use 25 new measurements of in situ produced cosmogenic 26Al and 10Be\u0000in river sand, paired with estimates of dissolved load flux in river water,\u0000to characterize the processes and pace of landscape change in central Cuba.\u0000Long-term erosion rates inferred from 10Be concentrations in quartz\u0000extracted from central Cuban river sand range from\u00003.4–189 Mg km−2 yr−1 (mean 59, median 45). Dissolved loads (10–176 Mg km−2 yr−1; mean 92, median 97), calculated from stream solute\u0000concentrations and modeled runoff, exceed measured cosmogenic-10Be-derived erosion rates in 18 of 23 basins. This disparity mandates\u0000that in this environment landscape-scale mass loss is not fully represented\u0000by the cosmogenic nuclide measurements. The 26Al / 10Be ratios are lower than expected for steady-state exposure or erosion in 16 of 24 samples. Depressed 26Al / 10Be ratios occur in many of the basins that have the greatest disparity between dissolved loads (high) and erosion rates inferred from cosmogenic nuclide concentrations (low). Depressed 26Al / 10Be ratios are consistent\u0000with the presence of a deep, mixed, regolith layer providing extended\u0000storage times on slopes and/or burial and extended storage during fluvial\u0000transport. River water chemical analyses indicate that many basins with lower 26Al / 10Be ratios and high 10Be concentrations are underlain at least in part by evaporitic rocks that rapidly dissolve. Our data show that when assessing mass loss in humid tropical landscapes,\u0000accounting for the contribution of rock dissolution at depth is particularly important. In such warm, wet climates, mineral dissolution can occur many meters below the surface, beyond the penetration depth of most cosmic rays and thus the production of most cosmogenic nuclides. Our data suggest the importance of estimating solute fluxes and measuring paired cosmogenic nuclides to better understand the processes and rates of mass transfer at a basin scale.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83018562","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 : 2022-06-08DOI: 10.5194/gchron-4-353-2022
Alexander Simpson, S. Glorie, M. Hand, C. Spandler, Sarah Gilbert, B. Cave
Abstract. The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and 0.6 % obtained from calcite with Lu concentrations as low as ca. 0.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary precipitation ages over a significant amount of geological time, although further research is required to constrain the closure temperature. The in situ approach allows calcite to be rapidly dated while maintaining its petrogenetic context with mineralisation and other associated mineral processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to resolve the timing of complex mineral paragenetic sequences that are a feature of many ancient rock systems.
{"title":"In situ Lu–Hf geochronology of calcite","authors":"Alexander Simpson, S. Glorie, M. Hand, C. Spandler, Sarah Gilbert, B. Cave","doi":"10.5194/gchron-4-353-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-353-2022","url":null,"abstract":"Abstract. The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass\u0000spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating\u0000geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age\u0000between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and\u00000.6 % obtained from calcite with Lu concentrations as low as ca.\u00000.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary\u0000precipitation ages over a significant amount of geological time, although\u0000further research is required to constrain the closure temperature. The\u0000in situ approach allows calcite to be rapidly dated while maintaining its\u0000petrogenetic context with mineralisation and other associated mineral\u0000processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to\u0000resolve the timing of complex mineral paragenetic sequences that are a\u0000feature of many ancient rock systems.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"12 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-06-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87678684","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 : 2022-06-07DOI: 10.5194/gchron-4-339-2022
K. Wilcken, A. Codilean, R. Fülöp, Steven Kotevski, A. H. Rood, D. Rood, A. Seal, K. Simon
Abstract. Accelerator mass spectrometry (AMS) is currently the standard technique to measure cosmogenic 10Be and 26Al concentrations, but the challenge with measuring low nuclide concentrations is to combine high AMS measurement efficiency with low backgrounds. The current standard measurement setup at ANSTO uses the 3+ charge state with Ar stripper gas at 6 MV for Be and 4 MV for Al, achieving ion transmission through the accelerator for 10Be3+ and 26Al3+ of around 35 % and 40 %, respectively. Traditionally, 26Al measurement uncertainties are larger than those for 10Be. Here, however, we show that 26Al can be measured to similar precision as 10Be even for samples with 26Al / 27Al ratios in the range of 10−15, provided that measurement times are sufficiently long. For example, we can achieve uncertainties of 5 % for 26Al / 27Al ratios around 1×10-14, typical for samples of late Holocene age or samples with long burial histories. We also provide empirical functions between the isotope ratio and achievable measurement precision, which allow predictive capabilities for future projects and serve as a benchmark for inter-laboratory comparisons. For the smallest signals, not only is understanding the source of 10Be or 26Al background events required to select the most appropriate blank correction method but also the impact of the data reduction algorithms on the obtained nuclide concentration becomes pronounced. Here we discuss approaches to background correction and recommend quality assurance practices that guide the most appropriate background correction method. Our sensitivity analysis demonstrates a 30 % difference between different background correction methods for samples with 26Al / 27Al ratios below 10−14. Finally, we show that when the measured signal is small and the number of rare isotope counts is also low, differing 26Al or 10Be concentrations may be obtained from the same data if alternate data reduction algorithms are used. Differences in the resulting isotope concentration can be 50 % or more if only very few (≲ 10) counts were recorded or about 30 % if single measurement is shorter than 10 min. Our study presents a comprehensive method for analysis of cosmogenic 10Be and 26Al samples down to isotope concentrations of a few thousand atoms per gram of sample, which opens the door to new and more varied applications of cosmogenic nuclide analysis.
{"title":"Technical note: Accelerator mass spectrometry of <sup>10</sup>Be and <sup>26</sup>Al at low nuclide concentrations","authors":"K. Wilcken, A. Codilean, R. Fülöp, Steven Kotevski, A. H. Rood, D. Rood, A. Seal, K. Simon","doi":"10.5194/gchron-4-339-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-339-2022","url":null,"abstract":"Abstract. Accelerator mass spectrometry (AMS) is currently the standard technique to measure cosmogenic 10Be and 26Al concentrations, but the challenge with measuring low nuclide concentrations is to combine high AMS measurement efficiency with low backgrounds. The current standard measurement setup at ANSTO uses the 3+ charge state with Ar stripper gas at 6 MV for Be and 4 MV for Al, achieving ion transmission through the accelerator for 10Be3+ and 26Al3+ of around 35 % and 40 %, respectively. Traditionally, 26Al measurement uncertainties are larger than those for 10Be. Here, however, we show that 26Al can be measured to similar precision as 10Be even for samples with 26Al / 27Al ratios in the range of 10−15, provided that measurement times are sufficiently long. For example, we can achieve uncertainties of 5 % for 26Al / 27Al ratios around 1×10-14, typical for samples of late Holocene age or samples with long burial histories. We also provide empirical functions between the isotope ratio and achievable measurement precision, which allow predictive capabilities for future projects and serve as a benchmark for inter-laboratory comparisons. For the smallest signals, not only is understanding the source of 10Be or 26Al background events required to select the most appropriate blank correction method but also the impact of the data reduction algorithms on the obtained nuclide concentration becomes pronounced. Here we discuss approaches to background correction and recommend quality assurance practices that guide the most appropriate background correction method. Our sensitivity analysis demonstrates a 30 % difference between different background correction methods for samples with 26Al / 27Al ratios below 10−14. Finally, we show that when the measured signal is small and the number of rare isotope counts is also low, differing 26Al or 10Be concentrations may be obtained from the same data if alternate data reduction algorithms are used. Differences in the resulting isotope concentration can be 50 % or more if only very few (≲ 10) counts were recorded or about 30 % if single measurement is shorter than 10 min. Our study presents a comprehensive method for analysis of cosmogenic 10Be and 26Al samples down to isotope concentrations of a few thousand atoms per gram of sample, which opens the door to new and more varied applications of cosmogenic nuclide analysis.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"24 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-06-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76146142","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 : 2022-05-19DOI: 10.5194/gchron-4-297-2022
N. Mercier, Jean-Michel Galharret, C. Tribolo, S. Kreutzer, Anne Philippe
Abstract. In nature, each mineral grain (quartz or feldspar) receives a dose rate (Dr) specific to its environment. The dose-rate distribution therefore reflects the micro-dosimetric context of grains of similar size. If all the grains were well bleached at deposition, this distribution is assumed to correspond, within uncertainties, with the distribution of equivalent doses (De). The combination of the De and Dr distributions in the De_Dr model proposed here would then allow calculation of the true depositional age. If grains whose De values are not representative of this age (hereafter called “outliers”) are present in the De distribution, this model allows them to be identified before the age is calculated, enabling their exclusion. As the De_Dr approach relies only on the Dr distribution to describe the De distribution, the model avoids any assumption about the shape of the De distribution, which can be difficult to justify. Herein, we outline the mathematical concepts of the De_Dr approach (more details are given in Galharret et al., 2021) and the exploitation of this Bayesian modelling based on an R code available in the R package “Luminescence”. We also present a series of tests using simulated Dr and De distributions with and without outliers and show that the De_Dr approach can be an alternative to available models for interpreting De distributions.
{"title":"Luminescence age calculation through Bayesian convolution of equivalent dose and dose-rate distributions: the <i>D</i><sub>e</sub>_<i>D</i><sub>r</sub> model","authors":"N. Mercier, Jean-Michel Galharret, C. Tribolo, S. Kreutzer, Anne Philippe","doi":"10.5194/gchron-4-297-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-297-2022","url":null,"abstract":"Abstract. In nature, each mineral grain (quartz or feldspar) receives a dose rate (Dr) specific to its environment. The dose-rate distribution therefore reflects the micro-dosimetric context of grains of similar size. If all the grains were well bleached at deposition, this distribution is assumed to correspond, within uncertainties, with the distribution of equivalent doses (De). The combination of the De and Dr distributions in the De_Dr model proposed here\u0000would then allow calculation of the true depositional age. If grains whose\u0000De values are not representative of this age (hereafter called\u0000“outliers”) are present in the De distribution, this model allows them to be identified before the age is calculated, enabling their exclusion. As the De_Dr approach relies only on the Dr distribution to describe the De distribution, the model avoids any assumption about the shape of the De distribution, which can be difficult to justify. Herein, we outline the mathematical concepts of the De_Dr approach (more details are given in Galharret\u0000et al., 2021) and the exploitation of this Bayesian modelling based on an R\u0000code available in the R package “Luminescence”. We also present a series of tests using simulated Dr and De distributions with and without outliers and show that the De_Dr approach can be an alternative to available models for interpreting De distributions.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-05-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90869220","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 : 2022-05-18DOI: 10.5194/gchron-4-269-2022
G. Pfalz, B. Diekmann, J. Freytag, L. Syrykh, D. Subetto, B. Biskaborn
Abstract. Age–depth relationships are the key elements in paleoenvironmental studies to place proxy measurements into a temporal context. However, potential influencing factors of the available radiocarbon data and the associated modeling process can cause serious divergences of age–depth relationships from true chronologies, which is particularly challenging for paleolimnological studies in Arctic regions. This paper provides geoscientists with a tool-assisted approach to compare outputs from age–depth modeling systems and to strengthen the robustness of age–depth relationships. We primarily focused on the development of age determination data from a data collection of high-latitude lake systems (50 to 90∘ N, 55 sediment cores, and a total of 602 dating points). Our approach used five age–depth modeling systems (Bacon, Bchron, clam, hamstr, Undatable) that we linked through a multi-language Jupyter Notebook called LANDO (“Linked age and depth modeling”). Within LANDO we implemented a pipeline from data integration to model comparison to allow users to investigate the outputs of the modeling systems. In this paper, we focused on highlighting three different case studies: comparing multiple modeling systems for one sediment core with a continuously deposited succession of dating points (CS1), for one sediment core with scattered dating points (CS2), and for multiple sediment cores (CS3). For the first case study (CS1), we showed how we facilitate the output data from all modeling systems to create an ensemble age–depth model. In the special case of scattered dating points (CS2), we introduced an adapted method that uses independent proxy data to assess the performance of each modeling system in representing lithological changes. Based on this evaluation, we reproduced the characteristics of an existing age–depth model (Lake Ilirney, EN18208) without removing age determination data. For multiple sediment cores (CS3) we found that when considering the Pleistocene–Holocene transition, the main regime changes in sedimentation rates do not occur synchronously for all lakes. We linked this behavior to the uncertainty within the dating and modeling process, as well as the local variability in catchment settings affecting the accumulation rates of the sediment cores within the collection near the glacial–interglacial transition.
{"title":"Improving age–depth relationships by using the LANDO (“Linked age and depth modeling”) model ensemble","authors":"G. Pfalz, B. Diekmann, J. Freytag, L. Syrykh, D. Subetto, B. Biskaborn","doi":"10.5194/gchron-4-269-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-269-2022","url":null,"abstract":"Abstract. Age–depth relationships are the key elements in paleoenvironmental studies\u0000to place proxy measurements into a temporal context. However, potential\u0000influencing factors of the available radiocarbon data and the associated\u0000modeling process can cause serious divergences of age–depth relationships\u0000from true chronologies, which is particularly challenging for\u0000paleolimnological studies in Arctic regions. This paper provides\u0000geoscientists with a tool-assisted approach to compare outputs from\u0000age–depth modeling systems and to strengthen the robustness of age–depth\u0000relationships. We primarily focused on the development of age determination\u0000data from a data collection of high-latitude lake systems (50 to 90∘ N, 55 sediment cores, and a total of 602 dating points).\u0000Our approach used five age–depth modeling systems (Bacon, Bchron, clam, hamstr, Undatable) that we linked through\u0000a multi-language Jupyter Notebook called LANDO (“Linked age and depth\u0000modeling”). Within LANDO we implemented a pipeline from data\u0000integration to model comparison to allow users to investigate the outputs of the modeling systems. In this paper, we focused on highlighting three\u0000different case studies: comparing multiple modeling systems for one sediment\u0000core with a continuously deposited succession of dating points (CS1), for\u0000one sediment core with scattered dating points (CS2), and for multiple\u0000sediment cores (CS3). For the first case study (CS1), we showed how we\u0000facilitate the output data from all modeling systems to create an ensemble\u0000age–depth model. In the special case of scattered dating points (CS2), we\u0000introduced an adapted method that uses independent proxy data to assess the\u0000performance of each modeling system in representing lithological changes.\u0000Based on this evaluation, we reproduced the characteristics of an existing\u0000age–depth model (Lake Ilirney, EN18208) without removing age determination\u0000data. For multiple sediment cores (CS3) we found that when considering the\u0000Pleistocene–Holocene transition, the main regime changes in sedimentation\u0000rates do not occur synchronously for all lakes. We linked this behavior to\u0000the uncertainty within the dating and modeling process, as well as the local variability in catchment settings affecting the accumulation rates of the sediment cores within the collection near the glacial–interglacial\u0000transition.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"2 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-05-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86881339","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 : 2022-05-16DOI: 10.5194/gchron-4-251-2022
M. Sinnesael, A. Loi, M. Dabard, T. Vandenbroucke, P. Claeys
Abstract. To expand traditional cyclostratigraphic numerical methods beyond their common technical limitations and apply them to truly deep-time archives, we need to reflect on the development of new approaches to sedimentary archives that are not traditionally targeted for cyclostratigraphic analysis but that frequently occur in the impoverished deep-time record. Siliciclastic storm-dominated shelf environments are a good example of such records. Our case study focuses on the Middle to Upper Ordovician siliciclastic successions of the Armorican Massif (western France) that are well-studied examples in terms of sedimentology and sequence stratigraphy. In addition, these sections are protected geological heritage due to the extraordinary quality of the outcrops. We therefore tested the performance of non-destructive high-resolution (centimeter-scale) portable X-ray fluorescence and natural gamma-ray analyses on an outcrop to obtain major and trace element compositions. Despite the challenging outcrop conditions in the tidal beach zone, our geochemical analyses provide useful information regarding general lithology and several specific sedimentary features such as the detection of paleo-placers or the discrimination between different types of diagenetic concretions such as nodules. Secondly, these new high-resolution data are used to experiment with the application of commonly used numerical cyclostratigraphic techniques on this siliciclastic storm-dominated shelf environment, a non-traditional sedimentological setting for cyclostratigraphic analysis. In the parts of the section with a relatively homogeneous lithology, spectral power analyses and bandpass filtering hint towards a potential astronomical imprint of some sedimentary cycles, but this needs further confirmation in the absence of more robust independent age constraints.
{"title":"Cyclostratigraphy of the Middle to Upper Ordovician successions of the Armorican Massif (western France) using portable X-ray fluorescence","authors":"M. Sinnesael, A. Loi, M. Dabard, T. Vandenbroucke, P. Claeys","doi":"10.5194/gchron-4-251-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-251-2022","url":null,"abstract":"Abstract. To expand traditional cyclostratigraphic numerical\u0000methods beyond their common technical limitations and apply them to truly\u0000deep-time archives, we need to reflect on the development of new approaches\u0000to sedimentary archives that are not traditionally targeted for\u0000cyclostratigraphic analysis but that frequently occur in the impoverished\u0000deep-time record. Siliciclastic storm-dominated shelf environments are a\u0000good example of such records. Our case study focuses on the Middle to Upper\u0000Ordovician siliciclastic successions of the Armorican Massif (western\u0000France) that are well-studied examples in terms of sedimentology and sequence\u0000stratigraphy. In addition, these sections are protected geological heritage\u0000due to the extraordinary quality of the outcrops. We therefore tested the\u0000performance of non-destructive high-resolution (centimeter-scale) portable X-ray\u0000fluorescence and natural gamma-ray analyses on an outcrop to obtain major\u0000and trace element compositions. Despite the challenging outcrop conditions\u0000in the tidal beach zone, our geochemical analyses provide useful information\u0000regarding general lithology and several specific sedimentary features such\u0000as the detection of paleo-placers or the discrimination between different\u0000types of diagenetic concretions such as nodules. Secondly, these new\u0000high-resolution data are used to experiment with the application of commonly\u0000used numerical cyclostratigraphic techniques on this siliciclastic\u0000storm-dominated shelf environment, a non-traditional sedimentological\u0000setting for cyclostratigraphic analysis. In the parts of the section with a\u0000relatively homogeneous lithology, spectral power analyses and bandpass\u0000filtering hint towards a potential astronomical imprint of some sedimentary\u0000cycles, but this needs further confirmation in the absence of more robust\u0000independent age constraints.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"11 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80454570","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 : 2022-04-21DOI: 10.5194/gchron-4-227-2022
Johannes Rembe, R. Zhou, E. Sobel, J. Kley, Jie Chen, Jian-xin Zhao, Yue‐xing Feng, D. Howard
Abstract. The North Pamir, part of the western syntax of the India–Asia collision zone, preserves remnants of a poorly understood Paleozoic intra-oceanic subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with U–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Dating of radial fibrous to equant spary calcite yielded three meaningful Visean–Serpukhovian ages. Rare-earth elements and yttrium (REE + Y) data reveal that the basaltic host rock of the calcite and oxidizing seawater are major sources of trace elements during calcite precipitation. U–Pb ages seem to be independent of REE + Y concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors. We challenge the hypothesis that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir.
{"title":"Calcite U–Pb dating of altered ancient oceanic crust in the North Pamir, Central Asia","authors":"Johannes Rembe, R. Zhou, E. Sobel, J. Kley, Jie Chen, Jian-xin Zhao, Yue‐xing Feng, D. Howard","doi":"10.5194/gchron-4-227-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-227-2022","url":null,"abstract":"Abstract. The North Pamir, part of the western syntax of the India–Asia collision zone, preserves remnants of a poorly understood Paleozoic intra-oceanic\u0000subduction zone. To constrain the age of this ancient ocean floor, we analyzed calcite phases in vesicular basalt and basaltic volcanic breccia with\u0000U–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Dating of radial fibrous to equant\u0000spary calcite yielded three meaningful Visean–Serpukhovian ages. Rare-earth elements and yttrium (REE + Y) data reveal that the basaltic host rock of the calcite and oxidizing\u0000seawater are major sources of trace elements during calcite precipitation. U–Pb ages seem to be independent of REE + Y\u0000concentrations. Our results demonstrate the potential of calcite dating to constrain the age of ancient ocean floors. We challenge the hypothesis\u0000that a continuous early Paleozoic Kunlun Terrane extended from northern Tibet into the North Pamir.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"108 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-04-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86256288","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 : 2022-04-07DOI: 10.5194/gchron-4-191-2022
C. Walcott, J. Briner, J. Baichtal, A. Lesnek, J. Licciardi
Abstract. The late-Pleistocene history of the coastal Cordilleran Ice Sheet remains relatively unstudied compared to chronologies of the Laurentide Ice Sheet. Yet accurate reconstructions of Cordilleran Ice Sheet extent and the timing of ice retreat along the Pacific Coast are essential for paleoclimate modeling, assessing meltwater contribution to the North Pacific, and determining the availability of ice-free land along the coastal Cordilleran Ice Sheet margin for human migration from Beringia into the rest of the Americas. To improve the chronology of Cordilleran Ice Sheet history in the Alexander Archipelago, Alaska, we applied 10Be and 36Cl dating to boulders and glacially sculpted bedrock in areas previously hypothesized to have remained ice-free throughout the local Last Glacial Maximum (LLGM; 20–17 ka). Results indicate that these sites, and more generally the coastal northern Alexander Archipelago, became ice-free by 15.1 ± 0.9 ka (n = 12 boulders; 1 SD). We also provide further age constraints on deglaciation along the southern Alexander Archipelago and combine our new ages with data from two previous studies. We determine that ice retreated from the outer coast of the southern Alexander Archipelago at 16.3 ± 0.8 ka (n = 14 boulders; 1 SD). These results collectively indicate that areas above modern sea level that were previously mapped as glacial refugia were covered by ice during the LLGM until between ∼ 16.3 and 15.1 ka. As no evidence was found for ice-free land during the LLGM, our results suggest that previous ice-sheet reconstructions underestimate the regional maximum Cordilleran Ice Sheet extent, and that all ice likely terminated on the continental shelf. Future work should investigate whether presently submerged areas of the continental shelf were ice-free.
摘要与劳伦泰德冰盖的年代学相比，沿海科迪勒兰冰盖的晚更新世历史仍然相对缺乏研究。然而，准确重建科迪勒冰盖的范围和太平洋沿岸冰退缩的时间对于古气候模型，评估融水对北太平洋的贡献，以及确定科迪勒勒冰盖沿海边缘无冰土地的可用性对于人类从白令陆桥迁移到美洲其他地区至关重要。为了改进阿拉斯加亚历山大群岛科迪勒兰冰盖历史的年代学，我们对以前假设在当地末次极大冰期(LLGM; 20-17 ka)期间保持无冰状态的地区的巨石和冰川雕刻基岩进行了10Be和36Cl定年。结果表明，这些遗址，以及更普遍的亚历山大群岛北部沿海地区，在15.1±0.9 ka (n = 12个巨石)之前变得无冰;1 SD)。我们还提供了亚历山大群岛南部冰川消融的进一步年龄限制，并将我们的新年龄与之前两项研究的数据结合起来。我们确定冰在16.3±0.8 ka (n = 14个巨石)从亚历山大群岛南部的外海岸撤退;1 SD)。这些结果共同表明，以前被绘制为冰川避难所的现代海平面以上地区在LLGM期间被冰覆盖，直到约16.3 ka至15.1 ka之间。由于在LLGM期间没有发现无冰陆地的证据，我们的研究结果表明，以前的冰盖重建低估了区域最大的科迪勒冰盖范围，并且所有的冰都可能终止在大陆架上。未来的工作应该调查目前被淹没的大陆架区域是否没有冰。
{"title":"Cosmogenic ages indicate no MIS 2 refugia in the Alexander Archipelago, Alaska","authors":"C. Walcott, J. Briner, J. Baichtal, A. Lesnek, J. Licciardi","doi":"10.5194/gchron-4-191-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-191-2022","url":null,"abstract":"Abstract. The late-Pleistocene history of the coastal Cordilleran Ice Sheet remains\u0000relatively unstudied compared to chronologies of the Laurentide Ice Sheet.\u0000Yet accurate reconstructions of Cordilleran Ice Sheet extent and the timing\u0000of ice retreat along the Pacific Coast are essential for paleoclimate\u0000modeling, assessing meltwater contribution to the North Pacific, and\u0000determining the availability of ice-free land along the coastal Cordilleran\u0000Ice Sheet margin for human migration from Beringia into the rest of the\u0000Americas. To improve the chronology of Cordilleran Ice Sheet history in the\u0000Alexander Archipelago, Alaska, we applied 10Be and 36Cl dating to\u0000boulders and glacially sculpted bedrock in areas previously hypothesized to\u0000have remained ice-free throughout the local Last Glacial Maximum (LLGM;\u000020–17 ka). Results indicate that these sites, and more generally the coastal northern Alexander Archipelago, became ice-free by 15.1 ± 0.9 ka (n = 12 boulders; 1 SD). We also provide further age constraints on deglaciation along the southern Alexander Archipelago and combine our new ages with data from two previous studies. We determine that ice retreated from the outer coast of the southern Alexander Archipelago at 16.3 ± 0.8 ka (n = 14 boulders; 1 SD). These results collectively indicate that\u0000areas above modern sea level that were previously mapped as glacial refugia\u0000were covered by ice during the LLGM until between ∼ 16.3 and\u000015.1 ka. As no evidence was found for ice-free land during the LLGM, our\u0000results suggest that previous ice-sheet reconstructions underestimate the\u0000regional maximum Cordilleran Ice Sheet extent, and that all ice likely\u0000terminated on the continental shelf. Future work should investigate whether\u0000presently submerged areas of the continental shelf were ice-free.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"462 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-04-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77141041","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 : 2022-03-31DOI: 10.5194/gchron-4-177-2022
A. Madella, C. Glotzbach, T. Ehlers
Abstract. Detrital tracer thermochronology utilizes the relationship between bedrock thermochronometric age–elevation profiles and a distribution of detrital grain ages collected from riverine, glacial, or other sediment to study spatial variations in the distribution of catchment erosion. If bedrock ages increase linearly with elevation, spatially uniform erosion is expected to yield a detrital age distribution that mimics the shape of a catchment's hypsometric curve. Alternatively, a mismatch between detrital and hypsometric distributions may indicate spatial variability of sediment production within the source area. For studies seeking to identify the pattern of sediment production, detrital samples rarely exceed 100 grains due to the time and costs related to individual measurements. With sample sizes of this order, detecting the dissimilarity between two detrital age distributions produced by different catchment erosion scenarios can be difficult at a high statistical confidence level. However, there are no established software tools to quantify the uncertainty inherent to detrital tracer thermochronology as a function of sample size and spatial pattern of sediment production. As a result, practitioners are often left wondering “how many grains is enough to detect a certain signal?”. Here, we investigate how sample size affects the uncertainty of detrital age distributions and how such uncertainty affects the ability to infer a pattern of sediment production of the upstream area. We do this using the Kolmogorov–Smirnov statistic as a metric of dissimilarity among distributions. From this, we perform statistical hypothesis testing by means of Monte Carlo sampling. These techniques are implemented in a new tool (ESD_thermotrace) to (i) consistently report the confidence level allowed by the sample size as a function of application-specific variables and given a set of user-defined hypothetical erosion scenarios, (ii) analyze the statistical power to discern each scenario from the uniform erosion hypothesis, and (iii) identify the erosion scenario that is least dissimilar to the observed detrital sample (if available). ESD_thermotrace is made available as a new open-source Python-based script alongside the test data. Testing between different hypothesized erosion scenarios with this tool provides thermochronologists with the minimum sample size (i.e., number of bedrock and detrital grain ages) required to answer their specific scientific question at their desired level of statistical confidence.
{"title":"How many grains are needed for quantifying catchment erosion from tracer thermochronology?","authors":"A. Madella, C. Glotzbach, T. Ehlers","doi":"10.5194/gchron-4-177-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-177-2022","url":null,"abstract":"Abstract. Detrital tracer thermochronology utilizes the relationship between bedrock thermochronometric age–elevation profiles and a distribution of detrital grain ages collected from riverine, glacial, or other sediment to study spatial variations in the distribution of catchment erosion. If bedrock ages increase linearly with elevation, spatially uniform erosion is expected to yield a detrital age distribution that mimics the shape of a catchment's hypsometric curve. Alternatively, a mismatch between\u0000detrital and hypsometric distributions may indicate spatial variability of\u0000sediment production within the source area. For studies seeking to identify\u0000the pattern of sediment production, detrital samples rarely exceed 100\u0000grains due to the time and costs related to individual measurements. With\u0000sample sizes of this order, detecting the dissimilarity between two detrital\u0000age distributions produced by different catchment erosion scenarios can be\u0000difficult at a high statistical confidence level. However, there are no\u0000established software tools to quantify the uncertainty inherent to detrital\u0000tracer thermochronology as a function of sample size and spatial pattern of\u0000sediment production. As a result, practitioners are often left wondering\u0000“how many grains is enough to detect a certain signal?”. Here, we\u0000investigate how sample size affects the uncertainty of detrital age\u0000distributions and how such uncertainty affects the ability to infer a\u0000pattern of sediment production of the upstream area. We do this using the\u0000Kolmogorov–Smirnov statistic as a metric of dissimilarity among\u0000distributions. From this, we perform statistical hypothesis testing by means of Monte Carlo sampling. These techniques are implemented in a new tool (ESD_thermotrace) to (i) consistently report the confidence level allowed by the sample size as a function of application-specific variables and given a set of user-defined hypothetical erosion scenarios, (ii) analyze the statistical power to discern each scenario from the uniform erosion hypothesis, and (iii) identify the erosion scenario that is least dissimilar to the observed detrital sample (if available). ESD_thermotrace is made available as a new open-source Python-based script alongside the test data. Testing between different hypothesized erosion scenarios with this tool provides thermochronologists with the minimum sample size (i.e., number of bedrock and detrital grain ages) required to answer their specific scientific question at their desired level of statistical confidence.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"26 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-03-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77918886","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 : 2022-03-22DOI: 10.5194/gchron-4-143-2022
D. Whipp, D. Kellett, I. Coutand, R. Ketcham
Abstract. Low-temperature multi-thermochronometry, in which the (U-Th) / He and fission track methods are applied to minerals such as zircon and apatite, is a valuable approach for documenting rock cooling histories and relating them to geological processes. Here we explore the behaviors of two of the most commonly applied low-temperature thermochronometers, (U-Th) / He in zircon (ZHe) and apatite (AHe), and directly compare them against the apatite fission track (AFT) thermochronometer for different forward-modeled cooling scenarios. We consider the impacts that common variations in effective spherical radius (ESR) and effective uranium concentration (eU) may have on cooling ages and closure temperatures under a range of different cooling rates. This exercise highlights different scenarios under which typical age relationships between these thermochronometers (ZHe>AFT>AHe) are expected to collapse or invert (either partially or fully). We anticipate that these predictions and the associated software we provide will be a useful tool for teaching, planning low-temperature multi-thermochronometry studies, and for continued exploration of the relative behaviors of these thermochronometers in temperature–time space through forward models.
{"title":"Short communication: Modeling competing effects of cooling rate, grain size, and radiation damage in low-temperature thermochronometers","authors":"D. Whipp, D. Kellett, I. Coutand, R. Ketcham","doi":"10.5194/gchron-4-143-2022","DOIUrl":"https://doi.org/10.5194/gchron-4-143-2022","url":null,"abstract":"Abstract. Low-temperature multi-thermochronometry, in which the\u0000(U-Th) / He and fission track methods are applied to minerals such as zircon\u0000and apatite, is a valuable approach for documenting rock cooling histories\u0000and relating them to geological processes. Here we explore the behaviors of\u0000two of the most commonly applied low-temperature thermochronometers,\u0000(U-Th) / He in zircon (ZHe) and apatite (AHe), and directly compare them against\u0000the apatite fission track (AFT) thermochronometer for different\u0000forward-modeled cooling scenarios. We consider the impacts that common\u0000variations in effective spherical radius (ESR) and effective uranium\u0000concentration (eU) may have on cooling ages and closure temperatures under a\u0000range of different cooling rates. This exercise highlights different\u0000scenarios under which typical age relationships between these\u0000thermochronometers (ZHe>AFT>AHe) are expected to\u0000collapse or invert (either partially or fully). We anticipate that these predictions\u0000and the associated software we provide will be a useful tool for teaching,\u0000planning low-temperature multi-thermochronometry studies, and for continued\u0000exploration of the relative behaviors of these thermochronometers in\u0000temperature–time space through forward models.\u0000","PeriodicalId":12723,"journal":{"name":"Geochronology","volume":"19 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-03-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85454323","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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