Rapid relative sea-level fall along north-eastern Australia between 1200 and 800 cal. yr BP: An appraisal of the oyster evidence

Abstract

A fast-paced post-glacial sea-level rise and subsequent mid-Holocene sea-level highstand are well documented at several far field locations away from the presence of former ice sheets but sea-level development during the late Holocene remains ambiguous. In this study, we present new data from modern and fossil oysters attached to shoreline rocks along the north-eastern Australian coastline that reveal new constraints on the nature and timing of relative sea-level change over the past 2500yr. Surveyed elevations of various contemporary oyster zones contextualize modern oyster growth forms in relation to sea-level datum and build the reference for our fossil oyster data. Based on survey data and field observations we developed a robust set of criteria for measuring fossil oysters to determine their relative sea-level position and constrain the uncertainties associated with these reconstructions. Thick (>10cm) fossil oyster visors above the equivalent modern growth suggest higher relative sea-levels in the past (i.e. >1200cal.yrBP). Radiocarbon analyses of the modern oyster visors suggest continuous lateral accumulation over the past ~800yr which implies relatively stable sea-level over this period. The modern and fossil dataset defines a distinct and rapid relative ~1m sea-level fall between 1200 and 800cal.yrBP. Whether the sea-level fall was stepped or followed a broader smooth/monotonic pattern is unclear. The timing coincides with the initiation of some inshore fringing coral reefs in the Great Barrier Reef region and other major geomorphological changes along the coastal zone. A combination of various factors may have been the driving mechanism behind this relative sea-level fall with rates between 1.0 to 5.2mmyr−1.