²³⁰Th-²³⁸U age of Rotoehu Ash and its implications for marine terrace chronology of eastern Bay of Plenty, New Zealand

Abstract

Rotoehu Ash, from the Wairoa River mouth, is dated at 71 000 ± 6000 years by the 23CTfh_ 238 U radioactive disequilibrium system. The age is significantly older than the commonly quoted 14C age of 41 700 ± 3500 years B .P. Marine deposits ofTe Papa Terrace in the Bay of Plenty area, younger than Otamaroa Terrace of the last interglacial maximum (stage 5e), are usually conformably overlain by Rotoehu Ash. Thus, Te Papa Terrace, which emerged prior to the deposition of Rotoehu Ash, was probably formed during a global high sea-level stand at c. 85 ka. Some locations ofTe Papa Terrace, however, may have been cut during an older high sea-level stand at c. 107 ka, because a peat layer occurs between marine deposits ,and Rotoehu Ash. Te Papa Terrace should be subdivided into two terraces which represent two successive relative high sea levels following the major high sea level of th(! last interglacial.

Radiocarbon Dating of Wood Using Different Pretreatment Procedures: Application to the Chronology of Rotoehu Ash, New Zealand

“…More recently, K/Ar dating of lava flows above and below the ash suggested an age of 64 ± 4 ka (Wilson et al 1992), while uranium series (Ota et al 1989) and electron-spin resonance (ESR) (Buhay et al 1992) dating yielded ages of 71 ± 6 ka and 45 ± 8 ka, respectively. Whitehead and Ditchburn (1994) have revised the interpretation of both the ESR and a wider body of 14 C data, and have suggested new values of 42.9 ± 7.8 ka and 35.1 ± 2.8 ka BP, respectively.…”

Letter to the Editor

“…It is evident that the Ota et al (1989) age is likely to be erroneous, but the ESR age is problematic, and the 14 C ages as presented can only be considered minimal ages at best. The database used by Whitehead & Ditchburn (1994) for the I4 C analysis and interpretation was incomplete: previously published infinite ages were not taken into account and neither was the likely contamination (demonstrable for NZ1357) by younger carbon of samples with supposedly finite ages.…”

“…They suggested that the U-Th disequilibrium age (71 ± 6 ka) of Ota et al (1989) is almost certainly incorrect, the true age being much younger, and hence, by implication, that the K-Ar age (64 ± 4 ka) of Wilson et al (1992) is likely to be too old. To support this contention, Whitehead & Ditchburn (1994) pointed firstly to problems with the ESR age (45.2 ± 8.2 ka) of Buhay et al (1992), making it younger by 5% (42.9 ± 7.8 ka).…”

“…Froggatt & Lowe (1990, p. 99) published the following comment about the 71 + 6 ka age of Ota et al (1989): ..."this age should be regarded as provisional because the isochron from which the age is derived (Ota et al 1989, fig. 4) is essentially based on only one data point, that of the whole rock sample.…”

Pleistocene marine terraces in New Zealand: A review

“…Paleomagnetic determinations subsequently demonstrated the speleothems to be reversely magnetised and hence older than 730 000 years (Lyons & Williams 1988). Another example of the application of uranium-series dating in New Zealand is the ~ age of7l 000 ± 6000 years reported by Ota et al (1989) for Rotoehu Ash, which overlies Te Papa and Otamaroa marine terraces in eastern Bay of Plenty. (Ghani 1978) • Ages based on linear kinetic model, calibrated to Rangitawa Pumice.…”