Determining Holocene sea-level changes on an exposed coastline: a case study of the Coffs Harbour area mid-north NSW

Abstract

The mid to late Holocene sea-level history of the Australian region provokes intense ongoing debate. Has the sea-level been static to smoothly falling or was it characterised by rapid fluctuations, possibly containing a eustatic signal? A range of evidence types has been utilised by researchers, on all sides of this debate, to establish, develop and provide evidence for their positions. In this study, the relic crusts of the tubeworm 'Galeolaria caespitosa', a fixed intertidal biological indicator (FIBI), have been sought out, surveyed and radiocarbon dated. 'G. caespitosa' has a limited growth range in the intertidal zone and when the vertical difference between the relic crusts and current 'G. caespitosa' is measured a sea-level variation with a small vertical error can be established. The sites described in this study are from the Coffs Harbour area of the NSW mid north coast. Three sites were identified; the first on Mutton Bird Island, the second on Coffs Harbour's southern headland and a third on Sawtell's northern headland. Relic crusts were surveyed and samples removed and dated from two levels on Mutton Bird Island and single levels at the other two sites. Six samples in total were dated with the higher site at Mutton Bird Island yielding two samples ~2.0m above current 'G. caespitosa' with ages of 3875 Cal. yr BP and 3626 Cal. yr BP (Midpoints of age range). A sample from the southern headland of Coffs Harbour was elevated 1.60m above current 'G. caespitosa' and yielded an age of 3509 Cal. yr BP. The lower site on Mutton Bird Island contained two discrete relic crusts of 'G. caespitosa', with horizontal separation of 1.0m, 1.44m above current crevice 'G. caespitosa'. The most northerly crust was dated at 1590 Cal. yr BP and the southerly at 1845 Cal. yr BP. The sample from the Sawtell site was dated at 2535 Cal. yr BP and was 1.75m above the highest current living crevice 'G. caespitosa'. Some characteristics of the sea-level history in this area are suggested by the nature of these relic crusts. Barnacles, both underlying and overlying the 'G. caespitosa' crusts, signify a sea-level that has been both rising and falling. The existence and perseverance of the crusts indicates that they have been stranded by a sea-level that has fallen rapidly. The results of this study challenge the underlying tenants of the hydro-isostatic model while providing support for a fluctuating sea level model.