Historical Tropical Cyclones and the 1883 Krakatoa Tsunami Inferred from Geological Archives of the Ashburton Delta (NW Australia)

Abstract:

The NW Australian coast is impacted by 1–2 tropical cyclones (TCs) per year, and ten historical tsunamis have been recorded since 1858, including the 1883 Krakatoa tsunami. However, no sedimentary evidence on this particular event has been presented yet, and little is known about the geological imprint of (pre)historic TCs and tsunamis in NW Australia. High resolution geo-bioarchives, such as speleothems, promise to provide detailed information on occurrence patterns of past TCs for the era prior to historical documentation. Likewise, past TCs and tsunamis may be inferred from geomorphic and sedimentary archives, i.e. in the form of particular landforms (beach ridges, washover fans), deposits (washover sediments in lagoons) or erosional features. Notwithstanding, the identification and interpretation of event deposits in geological archives is associated with site- and archive-specific difficulties, and both event types may produce identical sedimentological characteristics. The differentiation between tsunami and storm in the geological record thus remains challenging, notably where modern deposits and/or historical reports on the event are absent.

Here we present a sedimentary record from the Ashburton Delta (WA). It spans the last ~150 years and stores deposits of historical TCs and the 1883 Krakatoa tsunami. It is shown how optically stimulated luminescence (OSL) can be a key in linking such a record to historical events. The study aims at (i) providing a detailed characterization of the event deposits; (ii) distinguishing between TC and tsunami deposits based on multi-proxy sediment analyses; (iii) establishing a robust chronostratigraphy by OSL, and assigning layers to known TCs and tsunamis; and (iv) demonstrating how narrow the window of opportunity for recording coastal events may be if the sediment archives are located in dynamic sedimentary environments. Finally, our results contribute to the data pool of TC and tsunami deposits in Holocene stratigraphies.