Abstract: Cyclostratigraphy of the Middle to Upper Ordovician successions of the Armorican Massif (western France) using portable X-ray fluorescence Matthias Sinnesael, Alfredo Loi, Marie-Pierre Dabard, Thijs R. A. Vandenbroucke, and Philippe Claeys Geochronology, 4, 251–267, https://doi.org/10.5194/gchron-4-251-2022, 2022 We used new geochemical measurements to study the expression of astronomical climate cycles recorded in the Ordovician (~ 460 million years ago) geological sections of the Crozon Peninsula (France). This type of geological archive is not often studied in this way, but as they become more important going back in time, a better understanding of their potential astronomical cycles is crucial to advance our knowledge of deep-time climate dynamics and to construct high-resolution timescales. PubDate: Mon, 16 May 2022 09:04:31 +020 DOI: 10.5194/gchron-4-251-2022 2022
Abstract: Technical Note: colab_zirc_dims: a Google-Colab-based Toolset for Automated and Semi-automated Measurement of Mineral Grains in LA-ICP-MS Images Using Deep Learning Models Michael C. Sitar and Ryan J. Leary Geochronology Discuss., https//doi.org/10.5194/gchron-2022-12,2022 Preprint under review for GChron (discussion: open, 0 comments) We developed code to automatically and semi-automatically measure dimensions of detrital mineral grains in reflected light images saved at LA-ICP-MS facilities that use Chromium targeting software. Our code uses trained deep learning models to segment grain-images with greater accuracy than is possible using other image segmentation techniques. We implement our code in user-accessible, ready-to-run Google Colab notebooks, links to which can be found at https://github.com/MCSitar/colab_zirc_dims. PubDate: Thu, 05 May 2022 07:42:43 +020 DOI: 10.5194/gchron-2022-122022
Abstract: A revised alpha-ejection correction calculation for (U-Th)/He thermochronology dates of broken apatite crystals John He and Peter W. Reiners Geochronology Discuss., https//doi.org/10.5194/gchron-2022-11,2022 Preprint under review for GChron (discussion: open, 0 comments) Apatite helium thermochronology is a method used for dating the time at which a rock (and the apatite crystals contained within) cooled below a certain temperature, by measuring radioactive parent isotopes (uranium and thorium) and daughter isotopes (helium). This paper proposes a revision to a commonly used calculation that corrects raw data to account for instances when the analyzed apatite crystals are fragmented. It demonstrates the improved accuracy and precision of the proposed revision. PubDate: Mon, 02 May 2022 07:42:43 +020 DOI: 10.5194/gchron-2022-112022
Abstract: Calcite U–Pb dating of altered ancient oceanic crust in the North Pamir, Central Asia Johannes Rembe, Renjie Zhou, Edward R. Sobel, Jonas Kley, Jie Chen, Jian-Xin Zhao, Yuexing Feng, and Daryl L. Howard Geochronology, 4, 227–250, https://doi.org/10.5194/gchron-4-227-2022, 2022 Calcite is frequently formed during alteration processes in the basaltic, uppermost layer of juvenile oceanic crust. Weathered oceanic basalts are hard to date with conventional radiometric methods. We show in a case study from the North Pamir, Central Asia, that calcite U–Pb age data, supported by geochemistry and petrological microscopy, have potential to date sufficiently old oceanic basalts, if the time span between basalt extrusion and latest calcite precipitation (~ 25 Myr) is considered. PubDate: Thu, 21 Apr 2022 14:22:18 +020 DOI: 10.5194/gchron-4-227-2022 2022
Abstract: Technical note: Quantifying uranium-series disequilibrium in natural samples for dosimetric dating – Part 1: gamma spectrometry Barbara Mauz, Paul J. Nolan, and Peter G. Appleby Geochronology, 4, 213–225, https://doi.org/10.5194/gchron-4-213-2022, 2022 It is of critical importance to dosimetric dating techniques that the quantity of the radiation dose is estimated accurately. Here we describe gamma spectrometry in terms of instrument, measurement procedures, and data analyses required for estimating parent nuclide activities. The description includes analytical procedures required to generate data with sufficient accuracy and precision for samples in secular equilibrium. We also outline procedures required to quantify disequilibrium. PubDate: Thu, 14 Apr 2022 13:13:25 +020 DOI: 10.5194/gchron-4-213-2022 2022
Abstract: Cosmogenic ages indicate no MIS 2 refugia in the Alexander Archipelago, Alaska Caleb K. Walcott, Jason P. Briner, James F. Baichtal, Alia J. Lesnek, and Joseph M. Licciardi Geochronology, 4, 191–211, https://doi.org/10.5194/gchron-4-191-2022, 2022 We present a record of ice retreat from the northern Alexander Archipelago, Alaska. During the last ice age (~ 26 000–19 000 years ago), these islands were covered by the Cordilleran Ice Sheet. We tested whether islands were ice-free during the last ice age for human migrants moving from Asia to the Americas. We found that these islands became ice-free between ~ 15 100 years ago and ~ 16 000 years ago, and thus these islands were not suitable for human habitation during the last ice age. PubDate: Thu, 07 Apr 2022 10:02:26 +020 DOI: 10.5194/gchron-4-191-2022 2022
Abstract: Supergene phases from ferruginous duricrusts: non-destructive microsampling and mineralogy prior to (U-Th)/He geochronological analysis Karina Patricia Prazeres Marques, Thierry Allard, Cécile Gautheron, Benoît Baptiste, Rosella Pinna-Jamme, Guillaume Morin, Ludovic Delbes, and Pablo Vidal-Torrado Geochronology Discuss., https//doi.org/10.5194/gchron-2022-9,2022 Preprint under review for GChron (discussion: open, 0 comments) We proposed a new non-destructive mineralogical methodology on inframilimetric grains that allows to quantify the hematite and goethite content and hematite/goethite ratio of grains prior to (U-Th)/He geochronological analysis. (U-Th)/He data performed on different aliquots with different acquisition time shows no remarkable differences in age, opening a new way to investigate the (U-Th)/He data evolution in supergene lateritic duricrusts. PubDate: Tue, 05 Apr 2022 14:59:49 +020 DOI: 10.5194/gchron-2022-92022
Abstract: How many grains are needed for quantifying catchment erosion from tracer thermochronology' Andrea Madella, Christoph Glotzbach, and Todd A. Ehlers Geochronology, 4, 177–190, https://doi.org/10.5194/gchron-4-177-2022, 2022 Cooling ages date the time at which minerals cross a certain isotherm on the way up to Earth's surface. Such ages can be measured from bedrock material and river sand. If spatial variations in bedrock ages are known in a river catchment, the spatial distribution of erosion can be inferred from the distribution of the ages measured from the river sand grains. Here we develop a new tool to help such analyses, with particular emphasis on quantifying uncertainties due to sample size. PubDate: Thu, 31 Mar 2022 14:59:49 +020 DOI: 10.5194/gchron-4-177-2022 2022
Abstract: Comparison of basin-scale in situ and meteoric 10Be erosion and denudation rates in felsic lithologies across an elevation gradient at the George River, northeast Tasmania, Australia Leah A. VanLandingham, Eric W. Portenga, Edward C. Lefroy, Amanda H. Schmidt, Paul R. Bierman, and Alan J. Hidy Geochronology, 4, 153–176, https://doi.org/10.5194/gchron-4-153-2022, 2022 This study presents erosion rates of the George River and seven of its tributaries in northeast Tasmania, Australia. These erosion rates are the first measures of landscape change over millennial timescales for Tasmania. We demonstrate that erosion is closely linked to a topographic rainfall gradient across George River. Our findings may be useful for efforts to restore ecological health to Georges Bay by determining a pre-disturbance level of erosion and sediment delivery to this estuary. PubDate: Tue, 29 Mar 2022 14:59:49 +020 DOI: 10.5194/gchron-4-153-2022 2022
Abstract: 230Th/U Isochron Dating of Cryogenic Cave Carbonates Paul Töchterle, Simon Steidle, R. Lawrence Edwards, Yuri Dublyansky, Christoph Spötl, Xianglei Li, John Gunn, and Gina E. Moseley Geochronology Discuss., https//doi.org/10.5194/gchron-2022-10,2022 Preprint under review for GChron (discussion: open, 0 comments) Cryogenic cave carbonates (CCCs) provide a marker for past permafrost conditions. Their formation age is determined by 230Th/U dating. However, samples can be contaminated with small amounts of 230Th at formation, which can cause inaccurate ages and requires correction. We analysed multiple CCCs and found that varying degrees of contamination can cause an apparent spread of ages, when really all samples formed within a single freezing event. A statistical correction method is presented. PubDate: Tue, 29 Mar 2022 14:59:49 +020 DOI: 10.5194/gchron-2022-102022
Abstract: Constraining the geothermal parameters of in situ Rb–Sr dating on Proterozoic shales and its subsequent applications Darwinaji Subarkah, Angus Leslie Nixon, Monica Jimenez, Alan Stephen Collins, Morgan Lee Blades, Juraj Farkaš, Sarah Gilbert, and Simon Holford Geochronology Discuss., https//doi.org/10.5194/gchron-2022-8,2022 Preprint under review for GChron (discussion: open, 0 comments) Advancements in technology allow for new techniques to date rocks quicker, cheaper, and with little sample preparation. A unique use of this method is to date carbon-rich siltstones and constrain when these rocks were first deposited. This approach can also time when these sequences are subsequently affected by heat or fluids after they were deposited. This is useful, as the formation of metal-bearing fluids or petroleum source rocks is commonly associated with such processes. PubDate: Wed, 23 Mar 2022 15:49:54 +010 DOI: 10.5194/gchron-2022-82022
Abstract: Short communication: Modeling competing effects of cooling rate, grain size, and radiation damage in low-temperature thermochronometers David M. Whipp, Dawn A. Kellett, Isabelle Coutand, and Richard A. Ketcham Geochronology, 4, 143–152, https://doi.org/10.5194/gchron-4-143-2022, 2022 Multi-thermochronometry, in which methods such as (U-Th)/He dating of zircon and apatite and apatite fission track dating are combined, is used to reconstruct rock thermal histories. Our ability to reconstruct thermal histories and interpret the geological significance of measured ages requires modeling. Here we use forward models to explore effects of grain size and chemistry on cooling ages and closure temperatures for the (U-Th)/He decay systems in apatite and zircon. PubDate: Tue, 22 Mar 2022 15:49:54 +010 DOI: 10.5194/gchron-4-143-2022 2022
Abstract: Technical note: Rapid phase identification of apatite and zircon grains for geochronology using X-ray micro-computed tomography Emily H. G. Cooperdock, Florian Hofmann, Ryley M. Collins, Anahi Carrera, Aya Takase, and Aaron J. Celestian Geochronology Discuss., https//doi.org/10.5194/gchron-2022-7,2022 Preprint under review for GChron (discussion: open, 0 comments) Apatite and zircon are the most widely used minerals for dating rocks, but they can be difficult to identify in crushed rock samples. Incorrect mineral identification results in wasted analytical resources and inaccurate data. We show how X-ray computed tomography can be used to rapidly and accurately distinguish apatite from zircon based on density variations, and provide non-destructive 3D grain-specific size, shape, and inclusion information for improved data quality. PubDate: Fri, 18 Mar 2022 10:48:33 +010 DOI: 10.5194/gchron-2022-72022
Abstract: Cosmogenic nuclide weathering biases: Corrections and potential for denudation and weathering rate measurements Richard F. Ott, Sean F. Gallen, and Darryl E. Granger Geochronology Discuss., https//doi.org/10.5194/gchron-2022-5,2022 Preprint under review for GChron (discussion: open, 0 comments) Cosmogenic nuclides are a tool to quantify denudation – the total removal of mass from near the Earth's surface. Chemical weathering can introduce biases to cosmogenic nuclide based denudation rates measurements. Here, we investigate the effects of weathering on cosmogenic nuclides and develop tools to correct for this influence. Our results highlight which additional measurements are required to determine accurate denudation rates in regions where weathering is not negligable. PubDate: Tue, 15 Mar 2022 11:09:43 +010 DOI: 10.5194/gchron-2022-52022
Abstract: Late Holocene cryptotephra and a provisional 15 000-year Bayesian age model for Cascade Lake, Alaska Lauren J. Davies, Britta J. L. Jensen, and Darrell S. Kaufman Geochronology, 4, 121–141, https://doi.org/10.5194/gchron-4-121-2022, 2022 Subarctic and Arctic lake sediments provide key data to understand natural climate variability and future climate change. However, they can be difficult to date accurately and of limited use without a robust chronology. We use volcanic ash deposits from the last ~4000 BP to identify anomalously old radiocarbon ages at Cascade Lake, Alaska. A provisional ~15 000-year Bayesian age model is produced for the lake, and a new location for ash from five Late Holocene eruptions is reported. PubDate: Fri, 11 Mar 2022 11:09:43 +010 DOI: 10.5194/gchron-4-121-2022 2022
Abstract: An algorithm for U-Pb geochronology by secondary ion mass spectrometry Pieter Vermeesch Geochronology Discuss., https//doi.org/10.5194/gchron-2022-4,2022 Preprint under review for GChron (discussion: open, 0 comments) Secondary Ion Mass Spectrometer (SIMS) is the oldest and most sensitive analytical technique for in-situ U-Pb geochronology. This paper introduces a new algorithm for SIMS data reduction that treats the data as 'compositional data', which means that the relative abundances of 204Pb, 206Pb, 207Pb and 238Pb are processed within a tetrahedral data space or 'simplex'. The new method is implemented in an eponymous computer program that is compatible with the two dominant types of SIMS instruments. PubDate: Fri, 04 Mar 2022 11:03:36 +010 DOI: 10.5194/gchron-2022-42022
Abstract: Short communication: Experimental factors affecting fission-track counts in apatite Carolin Aslanian, Raymond Jonckheere, Bastian Wauschkuhn, and Lothar Ratschbacher Geochronology, 4, 109–119, https://doi.org/10.5194/gchron-4-109-2022, 2022 Fission tracks are damage trails from uranium fission in minerals, whose ages and thermal histories are deduced from their number and length. A mineral is etched for observing the tracks with a microscope. We show that the etching and observation conditions affect the track count and explain it in the framework of a recent etch model. We conclude that established solutions do not secure that the ages and thermal histories inferred from track counts and measurements are accurate. PubDate: Wed, 23 Feb 2022 15:08:30 +010 DOI: 10.5194/gchron-4-109-2022 2022
Abstract: Potential impacts of chemical weathering on feldspar luminescence dating properties Melanie Bartz, Jasquelin Peña, Stéphanie Grand, and Georgina E. King Geochronology Discuss., https//doi.org/10.5194/gchron-2022-3,2022 Preprint under review for GChron (discussion: open, 0 comments) Chemical weathering alters the chemical composition of mineral grains, and it follows that luminescence dating signals may also be progressively modified. We artificially weathered feldspar samples under different chemical conditions to understand the effect of feldspar partial dissolution on their luminescence properties. Only minor changes were observed on luminescence dating properties, implying that chemical alteration of feldspar surfaces may not affect luminescence dating signals. PubDate: Mon, 21 Feb 2022 15:08:30 +010 DOI: 10.5194/gchron-2022-32022
Abstract: Cosmogenic 3He paleothermometry on post-LGM glacial bedrock within the central European Alps Natacha Gribenski, Marissa M. Tremblay, Pierre G. Valla, Greg Balco, Benny Guralnik, and David L. Shuster Geochronology Discuss., https//doi.org/10.5194/gchron-2022-1,2022 Preprint under review for GChron (discussion: open, 0 comments) We apply quartz 3He paleothermometry along two deglaciation profiles in the European Alps to reconstruct temperature evolution since the Last Glacial Maximum. We observe a 3He thermal signal clearly colder than today in all bedrock surface samples exposed prior the Holocene. Current uncertainties in 3He diffusion kinetics do not permit to distinguish if this signal results from Late Pleistocene ambient temperature changes or from recent ground temperature variation due to permafrost degradation. PubDate: Wed, 09 Feb 2022 13:19:38 +010 DOI: 10.5194/gchron-2022-12022
Abstract: A 62 kyr geomagnetic palaeointensity record from the Taymyr Peninsula, Russian Arctic Stephanie Scheidt, Matthias Lenz, Ramon Egli, Dominik Brill, Martin Klug, Karl Fabian, Marlene M. Lenz, Raphael Gromig, Janet Rethemeyer, Bernd Wagner, Grigory Federov, and Martin Melles Geochronology, 4, 87–107, https://doi.org/10.5194/gchron-4-87-2022, 2022 Levinson-Lessing Lake in northern central Siberia provides an exceptional opportunity to study the evolution of the Earth's magnetic field in the Arctic. This is the first study carried out at the lake that focus on the palaeomagnetic record. It presents the relative palaeointensity and palaeosecular variation of the upper 38 m of sediment core Co1401, spanning ~62 kyr. A comparable high-resolution record of this time does not exist in the Eurasian Arctic. PubDate: Fri, 28 Jan 2022 13:19:38 +010 DOI: 10.5194/gchron-4-87-2022 2022
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