An Oscillating Holocene Sea-level?: Revisiting Rottnest Island, Western Australia, and the Fairbridge Eustatic Hypothesis

Abstract

There is a similarity in the pattern of Holocene sea-level change at Rottnest Island and a number of other locations in southern Western Australia. Rottnest Island was one of the field study areas that Fairbridge (1961) used to develop his fluctuating Holocene sea-level curve. Eight ¹⁴C ages obtained from fixed shell material of Serpulid tubeworms from tightly constrained inter-tidal zones are presented from Rottnest Island, and thirteen more from a transect extending 1000 km along the coast of southwest Australia. The age-elevation profiles show a consistent mid- to late- Holocene sea-level fluctuation over the entire southwest Australian region, which argues against a strong local tectonic or a broader hydro-isostatic influence. A comparison of the southwest and southeast coasts of Australia, based on fixed inter-tidal biological indicators obtained by similar methods, indicates a broadly similar mid- to late-Holoceneoscillating sea-level curve up to two metres higher-than-present. Peak levels are preceded and followed by sharp ~1 metre falls, which appear to be associated with cooling sea temperatures at ~5200 and ~3800 cal years BP. Both rising sea levels and warmer-than-present sea surface temperatures occur at all locations at ~4200 and ~6500 calyears BP, with a net sea-level fall to the present, but a slight warming beginning at ~1400 cal yr BP. There also appears to be a common pattern of species change suggestive of synchronous climatic variations. These mid-latitude fluctuations and environmental changes on the Australian coastline are similar to some events over the same period in the equivalent far-field locations of southern Brazil and southeast Asia. The synchronicity of these oscillations and their climatic underpinnings, in mid-latitude sites far from the complications of glacial isostasy, is not inconsistent with Fairbridge's (1961) basic thesis of world-wide eustatic Holocene sea-level change.