Fixed Intertidal Biological Indicators and Holocene Sea-level on the Great Barrier Reef Coast

Abstract

Mid-to-late-Holocene sea-level behaviour for the Great Barrier Reef coast has been contentious for nearly a century. While evidence of emergence in some areas of this region is extensive, debate continues as to its origin. This study located and examined relic formations of intertidal assemblages preserved in growth position on the rocky headlands and islands of the Great Barrier Reef coast. The first evidence of emergence from new locations along 1200 km of coast and offshore islands, from Lizard Island (Lat. 14.45°S) to Great Keppel Island (Lat. 23.35°S), is presented. Time-elevation data from these locations supports a model of a 1.5 – 1.8 m higher than present mid-Holocene sea-level with a sharp fall between 3800 and 3600 cal. BP to approximately 1.2 m above present. A further rapid fluctuation may have occurred between 3000 and 2700 cal. BP. Sea-level then remained between 1.0 and 1.4 m above present until after 900 cal. BP when it began to fall to present. The disappearance or invasion of species recorded within these assemblages can provide further clues to past environments. Some species of barnacles have specific environmental requirements and their presence within a relic assemblage may provide evidence of past sea-surface and/or air temperature, salinity, or tidal range. In particular, a species at the northern or southern limit of its range may be a sensitive indicator of change. Changes in species dominance appear in the sub-fossil record just prior to 3800 cal. BP and again after 2900 cal. BP, suggesting changing environmental conditions. In particular, a cooling just prior to 3800 cal. BP is suggested.