Speleothem Archives of New Zealand Volcanism: In Search of theTaupō Eruption of 232 ± 10 CE

Abstract

This study examined whether New Zealand speleothems preserved the 232 ± 10 CE Taupō eruption (VEI 7). A stalagmite, TR-990218B (TR-B), from Te Reinga Cave in the eastern North Island was subjected to various geochemical techniques to search for evidence of the eruption, which deposited ~10-20 cm of volcanogenic material over the cave site. High-precision uranium-series ages revealed a significant increase in growth rate shortly after the eruption (a factor of ~4 for ~35-40 yr). Trace element profiles obtained by laser-ablation-inductively-coupled-plasma-mass-spectrometry exhibited two concentration peaks in S at 214 ± 27 CE (lasting ~7 yr) and 243 ± 23 CE. Field-emission-scanning-electronmicroscopy with energy-dispersive-spectroscopy observations at 214 ± 27 CE revealed particulate/colloidal material with elevated Si, K, Na, Mg, Al, S, Cl, and Ti contents, but not consistent with rhyolitic cryptotephra. High-resolution stable C-O isotopic analyses show decoupling for ~11 yr from 214 ± 27 CE, as compared to other parts of the stalagmite record. These observations strongly suggest TR-B preserved a fingerprint of the Taupō eruption. Growth rate increases can be ascribed to: (1) de-vegetation above the cave site due to ashfall, which perturbed normal evapotranspiration processes, and thus increased water flow into the cave; (2) the enhanced acidity of the percolating groundwater (due to increases in inorganic/organic acids) increasing karst dissolution and subsequent delivery of Ca (for stalagmite growth) into the cave; and (3) regenerating vegetation as new plant root systems and bacterial colonies thrive on surplus nutrients in the soil enhancing carbonic acid production, and thus stalagmite growth. Lower Mg/Ca ratios at this time can also be accounted for by these processes, as a decrease in prior-calcite-precipitation due to evapotranspiration decrease above the cave (i.e., effectively causing wetter conditions) would decrease stalagmite Mg/Ca ratios. Moreover, this process is consistent with stable isotope decoupling, due to vegetation destruction above the cave site. The patterns in S can be explained by volcanically sourced aerosols being deposited above the cave and then by immediate leaching from the volcanic ash deposited above the cave, which persisted for more than ~20 yr due to biogeochemical cycling. In summary, this study suggests that New Zealand speleothems preserve evidence of volcanic eruptions, which can be used to refine eruption ages using U-series dating techniques and also assess the nature and duration of environmental change produced by eruptions.