Dating the Kawakawa/Oruanui eruption: Comment on “Optical luminescence dating of a loess section containing a critical tephra marker horizon,SW North Island of New Zealand” by R. Grapes et al. |
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Authors: | David J Lowe CJN Wilson RM Newnham AG Hogg |
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Institution: | Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand;School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand;School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, UK;Waikato Radiocarbon Dating Laboratory, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand |
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Abstract: | An IRSL age of 17.0 ± 2.2 ka (and a “mean age” of ca. 19 ka) reported by Grapes et al. Grapes, R., Rieser, U., Wang, N. Optical luminescence dating of a loess section containing a critical tephra marker horizon, SW North Island of New Zealand. Quaternary Geochronology 5(2-3), 164–169.] for the Kawakawa/Oruanui tephra, and other ages associated with a loess section in New Zealand are untenable: age data presented are inconsistent, no formal statistical treatments or error determinations were undertaken in age analysis, and the ages proposed are seriously at odds with multiple radiocarbon age determinations on tephra sequences bracketing the Kawakawa/Oruanui tephra and with palaeoenvironmental evidence elsewhere for the time period concerned. We suggest that the bulk polymineral IRSL ages on the tephra and encapsulating loess deposits were underestimated in part because of contamination of the loess by the integration of younger materials during slow deposition and continuous modification by upbuilding pedogenesis. Single-grain luminescence assays may reveal such contamination. A 14C-based age of ca. 27 ± 1 ka cal BP (2σ), reported in 2008, currently remains the best estimate for the age of eruption of the Kawakawa/Oruanui tephra. |
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