An OSL, radiocarbon and tephra isochron-based chronology for Birdlings Flat loess at Ahuriri Quarry, Banks Peninsula, Canterbury, New Zealand

Abstract

OSL, radiocarbon dating of pedogenic carbonate and tephrochronology have been used in an attempt to provide a detailed and reliable chronology for Birdlings Flat loess, a thick, proximal loess found on the lower flanks of Banks Peninsula, Canterbury, New Zealand. In a ca 15 m thick section at Ahuriri Quarry the Kawakawa tephra isochron of 26,500 cal. yr BP is identified at a depth of ca. 1.35 m on the basis of glass counting and electron microprobe fingerprinting. Radiocarbon ages of filamentous or root pseudomorph carbonate increase down section from ca. 10,000 cal. yr BP at 2.80 m to ca. 30,000 14C yr BP at 10.90 m. Ages from carbonate lining cracks are often out of sequence, indicating deep percolation by bypass flow down preferential flow paths. OSL ages show reversals with respect to each other, carbonate radiocarbon ages, and with the position and accepted age of Kawakawa tephra. Coincident radiocarbon and OSL ages at 3.85 m depth indicate that OSL ages are underestimates by at least 20%. Before OSL can be deemed a high accuracy dating method of the quatzo-feldspathic loess of South Island, New Zealand, more research into the causes of age underestimation and age reversals must be carried out. The last major episode of loess accumulation on the flanks of Banks Peninsula in Canterbury began before ca. 30,000 14C yr BP (ca. 35,000 cal. yr BP) and possibly before 43,000 yr based on OSL.