Warmer or cooler late Holocene marine palaeoenvironments?: interpreting southeast Australian and Brazilian sea-level changes using fixed biological indicators and their δ18O composition

Relic inter-tidal assemblages of sessile organisms, which are referred to as fixed biological indicators (FBIs), are shown to be useful proxies for late Holocene climatic and environmental changes as well as direct indicators of short term fluctuations in sea-level. A detailed comparison of change in the inter-tidal assemblages of two Southern Hemisphere sites at comparable latitudes (Port Hacking, New South Wales, south–east Australia and the Laguna-Imbituba region of southern Brazil) indicates periods of marked similarities in the timing and nature of changes at ∼5200, ∼3800 cal. years BP and differences after ∼2400 cal. years BP. There is sufficient concurrence of cooling phases and falling sea-levels to strongly suggest that in some cases global rather than regional forcing factors are at work. The value of inter-tidal organisms as indicators of changed environments is developed by identifying the past and present distributions of the common marker species: tubeworms, barnacles and mussels. The δ18O content of these extant specimens at Port Hacking confirmed changes in ocean water temperature and associated sea-level behaviour corresponding to invasion or extinction of temperature-sensitive species. A similar pattern is repeated, although in a less marked fashion, in the Laguna-Imbituba region of southern Brazil.