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 relative sea-level highstand in tectonically stable and remote (far-field) locations from major ice sheets. The east coast of Australia provides an excellent arena in which to investigate changes in relative sea level during the Holocene. Considerable debate surrounds both the peak level and timing of the east coast highstand. The southeast Australian site of Bulli Beach provides the earliest evidence for the establishment of a 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 detailed morpho- and chronostratigraphic study at Bulli Beach to better constrain the timing 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 provide improved dating of the earliest evidence for a highstand at 6,880±50 cal BP, approximately a millennium later than previously reported. Our results from Bulli now closely align with other sea-level reconstructions along the east coast of Australia, and provide evidence for a synchronous relative sea-level highstand that extends from the Gulf of Carpentaria to Tasmania. Our refined age appears to be coincident with major ice mass loss from Northern Hemisphere and Antarctic ice sheets, supporting previous studies that suggest these may have played a role in the relative sea-level highstand. Further work is now needed to investigate the environmental impacts of regional sea levels, and refine the timing of the subsequent sea-level fall in the Holocene and its influence on coastal evolution.
Highlights
Whilst global sea level has risen through the twentieth century and is expected to increase into the future, considerable uncertainties surround the timing, magnitude and impact of projected change [1, 2]
Reconstructing past sea level can help constrain uncertainties surrounding the rate of change, magnitude and impacts of projected increases through the 21st century
Of significance is the Redating the earliest evidence of the mid-Holocene highstand in Australia and global implications mid-Holocene sea-level highstand (+1 m present mean sea level (PMSL)) which potentially provides an analogue for 21st century warming projected to rise to similar elevations
Summary
Whilst global sea level has risen through the twentieth century and is expected to increase into the future, considerable uncertainties surround the timing, magnitude and impact of projected change [1, 2]. The historic record can be extended back millennia by exploiting natural archives sensitive to sea-level change, including coastal sedimentary and geomorphological features [12,13,14,15], there is an urgent need for a greater network of sites in time and space [16] This is so given the non-linear nature of coastal inundation as a result of sea-level rise [17,18,19] and the major associated environmental and socio-economic impacts projected for the 21st century [20,21,22]. Previous work has argued the onset commenced between approximately 8,000 to 7,000 years ago [39, 42, 43], but reconstructions of sea-levels (using a variety of intertidal deposits including estuarine archives and sub-fossil mangroves) suggest significant spatial and temporal variability [38, 39, 44]
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