Statistical Analysis of ITRAX XRF Data to Identify Marine Incursion, Sediment Source, and Saltwater Leaching in Tsunami Deposits

Abstract:

Geochemical investigation of fine-grained tsunami sediments has found that a characteristic salinity signature can be commonly found in tsunami deposits and underlying soils following an event. We extend this method to assess historic and paleotsunami deposits and investigate a wider range of particle sizes, with the aim of identifying whether salinity signatures are present and determining the source material of the deposits. Geochemical and mineralogical investigation of seven short cores was undertaken at four sites in New Zealand, where historic and/or palaeotsunami deposits were present as sand, silt or gravel layers intercalated between soils. Geochemical signatures were measured using a high-resolution ITRAX X-ray fluorescence (XRF) core scanner and results were analysed using Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA). We were able to explore the signatures of historical events, verify that prehistoric deposits were laid down by tsunamis, and compare these sediments with the background depositional environment. A t-test of means was applied for salinity marker elements (S, Cl, Br) in the soils immediately above and below tsunami sand layers, to test for evidence of saltwater leaching. The dominant mineralogy of the sediments was determined using portable X-ray diffraction and the data used to consider source material and interpret the corresponding XRF data. Geochemical signatures were found to be site specific, depending primarily on the composition of the material. PCA and HCA results clearly distinguished the signature of the tsunami deposits from the background material at each individual site and were able to confirm or deny palaeodeposits as tsunami-related, by comparison with the signatures of known events in the same core or nearby.