Luminescence dating and glacial stratigraphy in Westland, New Zealand

Westland is a key region in New Zealand for Quaternary stratigraphy; however, the regional glacial chronology is largely unsupported by numerical ages because of the unreliability of radiocarbon dating in this high-rainfall region. Therefore, we tested the ability of thermoluminescence (TL) and infrared-stimulated luminescence (IRSL) to produce stratigraphically consistent age estimates in this area. At Blue Spur Road, we obtained a stratigraphically reasonable TL age of 47 ± 7 ka in the second buried E horizon and a perhaps underestimated 53 ± 16 ka in the lower part of the fourth buried Bs horizon. A marine sand underlying Waimea outwash gave an incorrect (underestimated) IRSL age of c. 44 ka. In Saltwater Forest we collected five samples from two loess profiles overlying glacial moraines. Only one TL age (36 ± 3 ka in the lower part of the first Bs horizon) is acceptable. The next lower sample (c. 1.2 m), stratigraphically probably younger than 125 ka, gave a reasonable IRSL age of 89 ± 15 ka. In trench M1/1, on an older moraine, two TL age estimates were stratigraphically reversed, with a third result (87 ± 30 ka in unit L4) being reasonable. In contrast, IRSL produced ages of 65 ± 16 ka (1.1 m) and 145 ± 36 ka (2.0 m) for the upper two samples, removing the TL-age reversal, although the age of the lower sample may be an overestimate. The third IRSL result (36 ± 5 ka, 2.7 m) is much too young, like the corresponding TL result. Thus, although TL is generally unreliable in the highly weathered loess-paleosol sequences of Westland, the IRSL technique offers more promise, perhaps because of IRSL's feldspar specificity and its far greater signal-zeroing efficiency.