Identifying Tsunami Deposits in the Absence of Sand

Abstract:

Tsunamis originating in South America are known to have affected Lyttelton Harbour, New Zealand, in the recent past. However, while there was clear evidence of inundation, in particular in 1960 and 2010, there was no report of any sand left behind. Our study aimed to search for evidence of these small historical tsunamis, in the absence of any visible sand layer. Shallow trenches revealed discontinuous layers of small grey mud clasts at various depths across the study area, most likely transported from the nearby harbour. The origin of these mud clast layers was investigated using sedimentological, geochemical and diatom analyses complemented by dating from ¹³⁷Cs activity profiles and historical data. Subtle variations consistent with inclusions of marine mud such as a decrease in organic content and magnetic susceptibility and increases in geochemical markers (e.g. Ca, K, K/Rb, Si/Rb and Sr/Rb) were found in the sediment profile. Variations in diatom assemblages suggesting a marine influence were also recorded at similar depths, aligning with layers of mud clasts. Based on ¹³⁷Cs activity profiles and historical data, these deposits were attributed to the 1960 Chile and 1964 Alaska tsunamis, and we also found evidence for an older deposit, possibly associated with the 1868 Arica tsunami. Sedimentary evidence for the 2010 Chile tsunami was not found at the study site, but our study revealed spatial variations in Ca, Cl and Sr concentrations that were inversely related to Ti concentrations. This was used to identify the landward extent of inundation by the 2010 event. We show that a broad multi-proxy analysis can distinguish even the subtle signatures of an inconspicuous deposit laid down by a small tsunami.