Redating the earliest evidence of the mid-Holocene relative sea-level highstand in Australia and implications for global sea-level rise

Abstract

Reconstructing past sea levels can help constrain uncertainties surrounding the rate of change, magnitude, and impacts of the projected increase through the 21st century. Of significance is the mid-Holocene sea-level highstand in tectonically stable and remote (far field) locations from major ice sheets. Considerable debate surrounds both the peak level and timing of the onset. The east coast of Australia provides an excellent arena in which to investigate changes in sea level during the Holocene. An east Australian site known as Bulli Beach provides the earliest evidence for the establishment of this highstand in the Southern Hemisphere, although questions have been raised about the pretreatment and type of material that was radiocarbon dated for the development of the regional sea level curve. Here we undertake a multidisciplinary study at Bulli Beach to constrain the onset of the Holocene highstand in eastern Australia. In contrast to wood and charcoal samples that may provide anomalously old ages, probably due to inbuilt age, we find that short-lived terrestrial plant macrofossils provide a robust chronological framework. Bayesian modelling of the ages provides the most precise dating of the highstand which was established at 6,880±50 calendar 40 years ago. Our results are consistent with a growing body of evidence extending from the Gulf of Carpentaria to Tasmania that suggest a synchronous onset, independent of isostatic changes across eastern Australia, and coherent with major ice mass loss from Antarctica. Further work is now needed to refine the structure of the sea-level highstand, the timing of the sea-level fall in the late-Holocene and their impacts on coastal evolution.