A palaeo-seismological investigation of speleothems from Bohemia Cave, Northwest Nelson, New Zealand

Abstract

This study aimed to determine whether speleothems (cave formations such as stalagmites) record earthquake events and, if so, whether speleothems could be used to produce palaeoseismic records. Two contrasting methodologies were tested in Bohemia Cave in Northwest Nelson, New Zealand, which is a study site selected for its close proximity to the Alpine Fault and associated earthquake shaking, and for its abundant speleothems displaying disturbance. It is also located in close proximity to the epicentre of the 1929 M 7.8 Murchison earthquake. The first methodology involved determining whether speleothem deposits on boulders preserve the timing of seismically induced boulder disturbances (e.g., rotations or emplacement [i.e., rockfalls]). Uranium–thorium (U–Th) ages were obtained for stalagmite tips and bases (i.e., onset and cessation of speleothem growth ) on six boulders to determine the timing of their disturbances. It was found that the chronology of boulder disturbances could be determined using the speleothems, but their ages largely predate the current New (~4 kyr) Zealand palaeo-seismic record, although four growth cessation ages align with known palaeo-earthquakes. The second methodology involved determining whether physical and geochemical perturbations in individual stalagmites reflect earthquake events. U–Th ages of inferred perturbation horizons were acquired for two whole stalagmites. This study focused on an upper (youngest) section of a Site 4 stalagmite, which was found to partially record the New Zealand palaeo-seismic record. Twenty physical perturbation horizons were identified that are largely short-lived aragonite–calcite or calcite–aragonite transitions, which are also evident from changes in U, Sr, and Ba concentrations due to the different partitioning of these elements between aragonite–fluid and calcite–fluid. The onset ages of these horizons determined by U–Th dating exhibit a >72% match (e.g., 14 out of 20) with known Alpine Fault earthquakes (within error) since 4.5 ka. These horizons exhibit an average recurrence interval of ca. 267 yr, which is consistent with the latest Alpine Fault recurrence interval estimate of 249 ± 58 yr. Both of the studied stalagmites also stopped growing at the time (CE 1926 +21/–19 and CE 1927.6 ± 2.3) of the M 7.8 Murchison Earthquake in 1929. It is proposed that switching between mineralogical phases is caused by seismic rock fracturing altering aquifer throughflow rates, which reduced or increased residence times, and may have also caused a switch between prior calcite and aragonite precipitation. A comparison of boulder disturbance studies undertaken in low (Waitomo, New Zealand) versus high (Bohemia Cave) seismicity regions suggests that this methodology is better suited to low seismicity areas. However, the methodology of dating horizons of physical and chemical change with individual stalagmites has significant potential for extending the Alpine Fault palaeo-seismic record beyond 4.5 ka, and may be applicable to other sites in New Zealand and worldwide.