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.

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 14 C-based age of ca. 27 ± 1 ka cal BP (2), reported in 2008, currently remains the best estimate for the age of eruption of the Kawakawa/Oruanui tephra.

A revised age for the Kawakawa/Oruanui tephra, a key marker for the Last Glacial Maximum in New Zealand

“…The revised KOT age is w1700 cal years younger at face value than the age of 27,097 AE 957 cal yr BP reported by Lowe et al (2008Lowe et al ( , 2010, following Wilson et al (1988). The revised age remains compatible with tephrostratigraphic constraints from overlying Te Rere and Okareka tephras, dated at 25,170 AE 960 and 21,860 AE 290 cal yr BP, respectively, and underlying Poihipi and Okaia tephras, ca 28,450 AE 960 and 28,620 AE 1430 cal yr BP, respectively (Lowe et al, in this issue).…”

“…Pillans et al, 1993;Wilson, 2001;Lowe et al, 2008Lowe et al, , 2010). An accurate and precise age for this isochron enables meaningful comparisons between sequences containing the tephra and independently-dated records beyond its dispersal, or in localities where it was not deposited or is not preserved.…”

“…Reviews of past dating efforts, and the rationale for the previously accepted age of the KOT, are provided by Froggatt and Lowe (1990) and Lowe et al (2008Lowe et al ( , 2010. Our focus in this short paper is on documenting the new 14 C determinations and the modelling approach used to re-evaluate the age of KOT.…”

Tephrochronology and its application: A review

“…In this processs, spatially-referenced sequences containing or comprising tephra layers are traced from outcrop to outcrop, commonly from proximal to distal regions, using their stratigraphic relationships and salient physical properties including colour and bedding characteristics, or features such as pumice density or colour, the presence of accretionary lapilli, or distinctive phenocrysts in pumices, or free crystals, such as biotite or hornblende visible in the field via a hand lens (examples of these minerals are shown in Section 6.3). As noted earlier, stratigraphically associated palaeoenvironmental and palaeoecological information, such as an interstadial pollen assemblage, or the presence of plant macrofossils that indicate the season of deposition, or archaeological associations, may also help establish correlation (e.g., Kohn et al, 1992;Feibel, 1999;Shane and Sandiford, 2003;Lowe et al, 2010;Reyes et al, 2010).…”

“…Similarly the Kawakawa/Oruanui tephra, erupted from northern New Zealand ca. 27.1 ka cal BP, forms a key isochron linking numerous terrestrial and marine sequences extending more than 1500 km across the New Zealand and southwest Pacific region near the start of marine oxygen isotope (MOI) stage 2 (Newnham et al, 2007a;Lowe et al, 2008aLowe et al, , 2010Holt et al, 2010). At each location, the Kawakawa/Oruanui tephra deposit represents a time span probably of just a few months over which the entire eruption episode took place (Wilson, 2001;Manville and Wilson, 2006), together with the short times needed for atmospheric dispersion and marine settling.…”