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John B Hunt

Bio: John B Hunt is an academic researcher from Rothamsted Research. The author has contributed to research in topics: Tephra & Tephrochronology. The author has an hindex of 15, co-authored 23 publications receiving 1259 citations. Previous affiliations of John B Hunt include University of Gloucestershire & University of Edinburgh.

Papers
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Journal ArticleDOI
TL;DR: In this article, it is suggested that the "black box" approach to geochemical analysis should be avoided by the consideration of several factors, such as the quality of the geochemical data of avoidably low quality, and little attention appears to have been directed to modification of EPMA methods.
Abstract: Distal tephrochronological studies are reliant upon precise and accurate geochemical quantitation. This quantitation can involve difficulties not often encountered in more conventional petrology. Solving these problems is of particular importance in the Holocene (and Late Quaternary) of northwest Europe, where numerous tephra layers of Icelandic origin have been detected in peat bogs and lake and marine sediments. Electron probe microanalysis (EPMA) has proved the most applicable analytical technique when insufficient quantities of tephra are available for bulk analysis. Standard EPMA techniques require fine tuning to the tephra problem as the geochemical instability of glass is a common feature.In many previous studies, geochemical data of avoidably low quality have been produced, and little attention appears to have been directed to modification of EPMA methods. It is suggested that the 'black box' approach to geochemical analysis should be avoided by the consideration of several factors. These include ...

192 citations

Journal ArticleDOI
TL;DR: In this paper, a new palaeohydrological transfer function based on sampling raised mires from across Europe is presented, which relates modern assemblage composition to water table and moisture content, applied to fossil sequences.
Abstract: Proxy climate data can be obtained from reconstructions of hydrological changes on ombrotrophic (rain-fed) peatlands using biological indicators, such as testate amoebae. Reconstruc- tions are based on transfer functions, relating modern assemblage composition to water table and moisture content, applied to fossil sequences. Existing transfer functions in Europe and elsewhere are limited geographically and there are often problems with missing or poor analogues. This paper presents a new palaeohydrological transfer function based on sampling raised mires from across Europe. Relationships between assemblages and hydrological variables are described using ordination analyses. Transfer functions are developed for depth to water table (n ¼ 119) and moisture content (n ¼ 132) with root mean squared errors (RMSEP) of 5.6 cm and 2.7% respectively. Both transfer functions have an r 2 of 0.71, based on 'leave one out' cross-validation. Comparisons with an existing transfer function for Britain show that the European transfer function performs well in inferring measured water tables in Britain but that the British data cannot be used to infer water tables for other European sites with confidence. Several of the key missing and poor analogue taxa problems encountered in previous transfer functions are solved. The new transfer function will be an important tool in developing peat-based palaeoclimatic reconstructions for European sites. Copyright 2006 John Wiley & Sons, Ltd.

189 citations

Journal ArticleDOI
TL;DR: In this article, the authors used terrestrial and marine AMS 14C dates from the time of deposition of the Icelandic Vedde Ash to examine the marine 14C reservoir age, which changed from its modem North Atlantic value of ca. 400 yr to ca. 700 yr during the Younger Dryas climatic event.
Abstract: Increased marine 14C reservoir ages from the surface water of the North Atlantic are documented for the Younger Dryas period. We use terrestrial and marine AMS 14C dates from the time of deposition of the Icelandic Vedde Ash to examine the marine 14C reservoir age. This changed from its modem North Atlantic value of ca. 400 yr to ca. 700 yr during the Younger Dryas climatic event. The increased marine reservoir age has implications for both comparing climatic time series dated by 14C and understanding palaeoceanographic changes that generated the increase.

182 citations

Journal ArticleDOI
TL;DR: In this article, the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe were discussed, and it was shown that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls.
Abstract: This paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe. Application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of Quaternary chronology associated with records of proxy-environmental or proxy-climatic change. The geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto (or hidden) tephras in areas far removed from tephra sources. These tephras are dominated by glass shards, which, in many distal environments, may be either small in size (μm) or may be highly pumiceous with low glass:void ratios and thin (<10 μm) shard walls. We demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls. This approach distorts the geochemical data, creating analytical differences that may generate inappropriate tephrogeochemical fingerprints. Additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron-sized crystalline phases such as feldspar. Copyright © 2001 John Wiley & Sons, Ltd.

118 citations

Journal ArticleDOI
TL;DR: In this article, the performance of seven northern European electron microprobe centres are compared, using a geochemically homogeneous obsidian secondary standard, and the results reveal that a number of tephra-oriented probe centres could benefit from exercises similar to the programme outlined within.

114 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, Heaton, AG Hogg, KA Hughen, KF Kaiser, B Kromer, SW Manning, RW Reimer, DA Richards, JR Southon, S Talamo, CSM Turney, J van der Plicht, CE Weyhenmeyer
Abstract: Additional co-authors: TJ Heaton, AG Hogg, KA Hughen, KF Kaiser, B Kromer, SW Manning, RW Reimer, DA Richards, JR Southon, S Talamo, CSM Turney, J van der Plicht, CE Weyhenmeyer

13,605 citations

Journal ArticleDOI
TL;DR: In this article, the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP.
Abstract: Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

2,800 citations

Journal ArticleDOI
TL;DR: A detailed (ca. 100 yr resolution) and well-dated (18 AMS ^(14)C dates to 23 cal. ka BP) record of latest Pleistocene-Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate as discussed by the authors.

1,499 citations

Journal ArticleDOI
TL;DR: In this paper, a new radiocarbon calibration curve, IntCal04 and Marine04, has been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98.
Abstract: New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0-26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0-10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-res- olution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue). ABSTRACT. New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0-26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0-10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-res- olution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue).

1,205 citations

Journal ArticleDOI
TL;DR: The Greenland Ice Core Chronology 2005 (GICC05) as discussed by the authors is a time scale based on annual layer counting of high-resolution records from Greenland ice cores, which continuously covers the past 60 ka.
Abstract: . The Greenland Ice Core Chronology 2005 (GICC05) is a time scale based on annual layer counting of high-resolution records from Greenland ice cores. Whereas the Holocene part of the time scale is based on various records from the DYE-3, the GRIP, and the NorthGRIP ice cores, the glacial part is solely based on NorthGRIP records. Here we present an 18 ka extension of the time scale such that GICC05 continuously covers the past 60 ka. The new section of the time scale places the onset of Greenland Interstadial 12 (GI-12) at 46.9±1.0 ka b2k (before year AD 2000), the North Atlantic Ash Zone II layer in GI-15 at 55.4±1.2 ka b2k, and the onset of GI-17 at 59.4±1.3 ka b2k. The error estimates are derived from the accumulated number of uncertain annual layers. In the 40–60 ka interval, the new time scale has a discrepancy with the Meese-Sowers GISP2 time scale of up to 2.4 ka. Assuming that the Greenland climatic events are synchronous with those seen in the Chinese Hulu Cave speleothem record, GICC05 compares well to the time scale of that record with absolute age differences of less than 800 years throughout the 60 ka period. The new time scale is generally in close agreement with other independently dated records and reference horizons, such as the Laschamp geomagnetic excursion, the French Villars Cave and the Austrian Kleegruben Cave speleothem records, suggesting high accuracy of both event durations and absolute age estimates.

965 citations