Inter-tidal fixed indicators of former Holocene sea levels in Australia: a summary of sites and a review of methods and models

The reconstruction of mid-late Holocene marine environments using the calcareous remains of inter-tidal fixed biological indicators as sea level proxies is reviewed. Useful palaeo-environmental information can be obtained from the fossil evidence of successive invasions and disappearances of tropical and temperate species within the sub-fossil inter-tidal biological sequences. Examples illustrating spatially synchronous late Holocene sea-level fluctuations are described from 3000 km of north–south coastline along eastern Australia. The method uses tightly constrained species (such as the tubeworm 'Galeolaria caespitosa') which allows for the precise measurement of relative sea-level fluctuations by using the differential between emerged sub-fossil encrustations and current distributions of the same species. The error range can be defined according to the extent of preservation of the relic distributions, particularly the extent that diagnostic boundaries between species limited to different inter-tidal zones have been preserved. The rapidity of sea-level fluctuations is inferred by the preservation from erosion of emerged shell layers, as well as discontinuities in the relic biological sequences, and the relative preservation of fragile species. This apparent rapidity of sea-level fluctuations, as well as evidence of near-uniform synchronous sea-level heights along the north–south span of the entire Australian coast, and equally consistent variations Australia-wide in Spencer Gulf and Rottnest Island, raises questions about the predicted poleward decline of sea-level curves derived from hydro-isostatic rebound modelling.