School of Geography - Research Publications

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Author: Bajo, Petra × Author: Hellstrom, John × Author: Woodhead, Jonathan ×

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    Persistent influence of obliquity on ice age terminations since the Middle Pleistocene transition
    Bajo, P ; Drysdale, RN ; Woodhead, JD ; Hellstrom, JC ; Hodell, D ; Ferretti, P ; Voelker, AHL ; Zanchetta, G ; Rodrigues, T ; Wolff, E ; Tyler, J ; Frisia, S ; Spotl, C ; Fallick, AE (AMER ASSOC ADVANCEMENT SCIENCE, 2020-03-13)
    Radiometric dating of glacial terminations over the past 640,000 years suggests pacing by Earth's climatic precession, with each glacial-interglacial period spanning four or five cycles of ~20,000 years. However, the lack of firm age estimates for older Pleistocene terminations confounds attempts to test the persistence of precession forcing. We combine an Italian speleothem record anchored by a uranium-lead chronology with North Atlantic ocean data to show that the first two deglaciations of the so-called 100,000-year world are separated by two obliquity cycles, with each termination starting at the same high phase of obliquity, but at opposing phases of precession. An assessment of 11 radiometrically dated terminations spanning the past million years suggests that obliquity exerted a persistent influence on not only their initiation but also their duration.
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    "Cryptic" diagenesis and its implications for speleothem geochronologies
    Bajo, P ; Hellstrom, J ; Frisia, S ; Drysdale, R ; Black, J ; Woodhead, J ; Borsato, A ; Zanchetta, G ; Wallace, MW ; Regattieri, E ; Haese, R (Elsevier, 2016-09-15)
    Speleothems are usually considered as one of the most amenable palaeoclimate archives for U-series dating. A number of studies in recent years, however, report cases of diagenetic alteration which compromises the use of U-series systematics in speleothems, resulting in inaccurate U-Th ages. Here we present the results of a high-resolution U-Th dating study of a stalagmite (CC26) from Corchia Cave in Italy where we document a number of departures from an otherwise well-defined age-depth model, and explore potential causes for these outliers. Unlike examples illustrated in previous studies, CC26 contains no visible evidence of neomorphism, and appears, at least superficially, ideally suited to dating. Good reproducibility obtained between multi-aliquot U-Th analyses removes any possibility of analytical issues contributing to these outliers. Furthermore, replicate analyses of samples from the same stratigraphic layer yielded ages in stratigraphic sequence, implying very localized open-system behavior. Uranium loss is suggested as a causative mechanism on account of the fact that all the outliers are older than their assumed true age. A limited number of micro-voids were observed under micro-CT analyses, and it is proposed that these were pathways for U loss. Uranium-loss modelling allows us to constrain the possible timing of diagenetic alteration and indicates that the precursor for the outlier with the largest age discrepancy (309%) must have been aragonite. This study indicates that visibly unaltered speleothems may still contain small domains that have experienced post-depositional alteration. Such “cryptic” diagenesis, as recorded in this stalagmite, has implications for the constancy of accuracy of the U-series dating technique, and suggests a need for careful examination of speleothems prior to dating, particularly in low-resolution U-Th studies.