Do Waitomo speleothems record physical and chemical evidence for earthquakes?

Abstract

Three speleothems from the Bone Passage of Gardners Gut Cave, Waitomo, have been investigated to examine whether physical and chemical changes in speleothems might be caused by shaking due to large megathrust earthquakes originating from the Hikurangi subduction margin. Uranium-series dating by multicollector inductively coupled plasma mass spectrometry was used to date the onset and cessation of stalagmite growth, in order to observe whether the speleothems had shared commonalities of events in their growth history. Seven elements were analysed by laser ablation inductively coupled plasma mass spectrometry to determine if the speleothems contain geochemical anomalies that may have been earthquake-induced. Uranium-series dates show that each of the three speleothems (GG2, GG3 and GG9) have a shared event at ca. 51 ka. GG2 commenced growth with a basal age of ca. 51.1 ka. GG9 recommenced growth after the second observed change in the growth axis at ca. 51.0 ± 0.3ka, and GG3 ceased growth at 51.4 ± 0.3 ka for a duration of ~27 kyr. In total, eight growth boundaries were analysed over the three speleothems to determine whether cessation of growth occurred and to quantify their respective duration. Three types of geochemical anomalies across growth boundaries were observed among the three samples. (1) GG2 exhibits a negative correlation between Mg and Sr across visible growth laminae, caused by incongruent dissolution where Mg is preferentially leached compared to Sr and Ca from the karst. (2) The average concentrations of Mg and Sr increased/decreased following the growth boundary across hiatuses of long durations, likely caused by the recommenced growth during a period of different climatic conditions and/or because the fluid flow path through the overlying limestone lengthened/shortened resulting in increased/decreased leaching of elements. (3) Element ‘spikes’ are observed across the three growth boundaries in GG3, for all elements (except Sr), which indicate the deposition of soil/silicate/clay particles on the stalagmite during periods of non-growth. Through U-series dating and analysis of the geochemical compositions of speleothems, it is evident that additional analysis is required to rule out all other possible causes of physical and chemical changes observed at the speleothem growth boundaries to confidently attribute these changes to an earthquake origin.