A 6000 year tropical cyclone record from Western Australia
Research highlights
► We provide the first long-term tropical cyclone record from the Indian Ocean. ► Severe tropical cyclones have struck Shark Bay Western Australia every 200 years. ► There was a major gap in tropical cyclone activity between 5,400 and 3,700 cal BP. ► The 1700 year gap was likely due to a much drier climatic regime or mega-drought.
Introduction
The number and types of palaeo-tropical cyclone records from around the globe are progressively increasing. To date there are two main types – the high resolution isotope records (Miller et al., 2006, Frappier et al., 2007; Nott et al., 2007) and the lower resolution but longer, more geographically widespread, sedimentary records from the Atlantic Basin and Gulf of Mexico (Liu and Fearn, 1993, Liu and Fearn, 2000, Donnelly et al., 2001, Donnelly and Woodruff, 2007, Nyberg et al., 2007, Woodruff et al., 2008), and Pacific Basin (Hayne and Chappell, 2001, Nott and Hayne, 2001; Nott et al., 2009).
The advantage of the isotope records is their high resolution (annual) and ability to identify a broad range of tropical cyclone intensities and frequencies over centennial to millennial time scales. Their disadvantage, to date, is they have a limited spatial coverage because of the fewer opportunities to uncover these records from limestone terrains (stalagmites) and/or suitable trees for ring records. In contrast, the sedimentary records are more numerous and provide a broader geographic coverage. However, for the main part the sedimentary records tend to be self-censoring as they usually record only the most extreme events (Nott, 2004). An issue for both types of records is that too few sites have been examined in any one region to enable a robust long-term regional tropical cyclone climatology to be developed. When this can be achieved these long-term records hold considerable promise for use within risk assessments of this hazard for coastal communities. These palaeo-records will also provide a framework in which the natural variability of tropical cyclones and anthropogenically-induced changes in climate systems can be made.
To this end we present the first palaeo-tropical cyclone record from the Indian Ocean region. At Shark Bay, being the western most point of the Australian mainland, the tropical cyclone record is represented by numerous shore parallel ridges made entirely of one species of cockle shell (Fragum eragatum). These shells are not only species-specific but comprise virtually identical size due to the nature of the predator/prey relationships induced by the hypersaline marine environment at this location. In contrast the Pleistocene shell ridges, lying landward of, and in places stratigraphically below, the Holocene sequence are composed of a number of shell species of varying sizes suggesting that this hypersaline environment was not present prior to the Holocene marine transgression. Hence these shell ridges not only provide a record of tropical cyclone activity here over the latter half of the Holocene and Last Interglacial but they also record distinct changes in the local marine environment.
The aim of this study was to determine the intensity and characteristics of the tropical cyclones that would be required to generate a marine inundation capable of reaching the crests of, and hence deposit, these shell ridges. A further aim was to ascertain a chronology of the ridges, which in turn might provide an insight into the late Holocene climate of this region.
Section snippets
Regional setting
The Shark Bay marine environment is a world heritage listed feature because it is one of the few locations worldwide that contains living stromatolites and is also home to one of the world’s largest area of sea grass meadows. This is at least in part due to the nature of the coastal configuration which has assisted in generating a hypersaline marine environment. Shark Bay is characterised by two elongate bays approximately 120 km long and 30–40 km wide, separated by narrow promontories that
Geology
The Shark Bay region forms part of the Carnarvon Basin, a predominantly Silurian to Lower Carboniferous sequence of marine sediments. Overlying these sediments is a Cretaceous limestone (Calcilutite) which in turn is overlain by a Tertiary Calcarenite. The Peron Sandstone overlies these strata and forms the immediate rock type around Hamelin Pool. To the east this sandstone is overlain by the Tamala Limestone, which developed from calcareous rich sand blown from the exposed sea floor during sea
Climate
The region around Shark Bay is one of the driest coastal areas of Australia. Average annual rainfall is 211 mm with the winter months (June–August) receiving the highest average monthly rainfall (highest is 47.1 mm in June and lowest is 2.3 mm in December) (BOM, 2010). The winter-dominated rainfall regime suggests that Shark Bay falls into the Mediterranean climate typical of south-west Western Australia. Rainfall here is typically generated by sub-polar lows and associated cold fronts tracking
Methods
The methods for this study rest on the assumption that tropical cyclone induced surge and associated waves were responsible for depositing the shell ridges. This is reasonable because the northward opening elongate nature of this bay (Hamelin Pool) provides considerable shelter from swells and waves generated by sub-polar low-pressure systems and cold fronts which generally occur to the south of this location. Also the shallow waters associated with the Faure Sill require hurricane force winds
Surveying
The three shore normal transects surveyed are shown in Fig. 3, Fig. 4, Fig. 5. There are 26 Holocene ridges paralleling shore on the eastern side of the ridge plain complex. There are 17 ridges paralleling shore at the western end of Hamelin Pool. The vast majority of ridge crests sit between 4 and 6 m above Australian Height Datum (AHD).
Chronology
A total of 68 radiocarbon analyses were undertaken from across the three transects. All results, as seen in Table 1, have been marine reservoir corrected,
Discussion
The Holocene beach ridges at Hamelin Pool are unusual in that they are all virtually composed of shells of a single species and size. These ridges are not cheniers because the intervening swales are also composed of the same shells. They are more appropriately referred to as beach ridges because the entire sedimentary unit, including swales and crests, is made of the same material. The first two ridges in transect 1 also contain medium- to coarse-grained sand mixed with the shells but this sand
Conclusion
The shell ridge sequence at Hamelin Pool, Shark Bay, Western Australia provides the first long-term record of tropical cyclones from the Indian Ocean region. These ridges were most likely deposited by waves associated with marine inundations generated by moderately intense tropical cyclones. The ridges standing between 3 and 6 m above mean sea level were deposited by marine inundations generated by category 2–4 tropical cyclones. The average interval between deposition of these ridges is
Acknowledgements
Thanks go to Jordahna Haig and Louise Collins for help with field work. Thanks also to Phillip Playford for valuable suggestions to help improve this manuscript.
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