Loess studies in Aotearoa New Zealand

Abstract

Loess in Aotearoa New Zealand (ANZ) has been studied since its first documented recognition (on Banks Peninsula) in 1878 by Julius von Haast. A decade later, John Hardcastle revealed that southern ANZ loess was both glacial in origin and contained signals of past climates. As a fine-grained aeolian deposit dominated by quartz and feldspar (± mica), it is derived mainly from greywacke and schist rocks shattered by frost cracking at high elevations, fluvially comminuted then deflated from aggrading floodplains, and from exposed continental shelves during cold periods. Such quartzo-feldspathic loess predominates in eastern and southern parts of both South and North islands and in Westland and Tasman. In addition, subsurface tephric loess prevails in central-western North Island. Unlike many deposits overseas, ANZ loess generally lacks secondary calcium carbonate, is denser (with lower macro- and mesoporosity, higher clay content) and, although less prone to collapse, is susceptible to accelerated erosion including tunnel gullying and landsliding. Mean rates of accumulation in the Last Glacial Maximum were ~3−25 mm/century; mass accumulation rates were generally 70−150 g cm ⁻² yr ⁻¹ but locally could be as low as 20 g cm ⁻² yr ⁻¹ and as high as 360 g cm ⁻² yr ⁻¹ . Studies mapping and characterizing loess have been driven in part by cognizance of its importance as a widespread parent material (~60%−70% of ANZ’s soils contain loess) for arable/pastoral soils underpinning ANZ’s predominantly agronomy-based economy. A key feature of ANZ loess studies has been the recognition of developmental upbuilding pedogenesis involving syn-depositional alteration of loess as it accumulates in cold and/or cool (stadial) periods, and stronger alteration (forming more-developed soils, which subsequently become recognizable as buried soil stratigraphic units or paleosols) during minimal accumulation in interglacial and/or warm (interstadial) periods. Multisequal loess-paleosol successions are the result. ANZ loess-paleosol successions have been dated and correlated via a range of dating and age-equivalent techniques including tephrochronology and paleomagnetism. Most loess is ≤500 kyrs old, but occurrences as old as Pliocene (Otago) and c. 1.7–1.0 Ma (Waikato, Wairarapa) are recognized. Loess chronostratigraphy has enabled loess and associated buried soils/paleosols to be correlated to the Quaternary marine oxygen isotope records, which, alongside mapping efforts, have revealed landscape responses to climate and tectonic controls particularly along the eastern regions of ANZ bordering the Hikurangi Subduction Margin.