Punctuated transgression (?): Comment on Oliver, T.S.N., Donaldson, P., Sharples, C., Roach, M., and Woodroffe, C.D. “Punctuated progradation of the Seven Mile Beach Holocene barrier system, southeastern Tasmania”

Sea-level rise is known to have emplaced incipient sandy barriers during post-glacial marine transgression and these same barriers are forecast to erode or transgress as rising seas accelerate with global warming. In order for a barrier to prograded over the intervening millennia, a positive sediment budget must exist such that sand supplied to the coast is ample enough to fill the existing accommodation space (embayment) or a drop in sea-level forced seaward progradation. Storms have long been understood to repeatedly punctuate progradation over days to decades. Oliver et al. (2017a), however, present two periods of perceived pauses in progradation lasting centuries and millennia with each hiatus considered to have resulted from changes in sediment supply. Despite the sizable dataset, the there are gaps in the stratigraphy and chronology of barrier formation during these two specific time periods in question (~6500 to 3500 years ago and 500 years ago to present). Additionally, the morphology representing barrier evolution during ~6.5-3.5 ka is obscured by the selected rendering of the Light Detection And Ranging (LiDAR) data and breaks in the topographic profiles extracted from the LiDAR. Reanalysis of this data within the context of existing literature indicate that seaward progradation slowed, not paused, between ~6.5 to 3.5 ka. While this is not surprising for a site with no active riverine or longshore sediment source, the rapid progradation that follows is. This transition and the timing do make sense within the context of a fall from a mid-Holocene highstand, which is debated in this far-field region of Australia. Furthermore, it is apparent that the last few centuries do not represent a hiatus in barrier deposition, but rather a termination in progradation and transition to erosion and transgression. The barrier is not likely to resume prograding seaward, but continue to erode and transgress in the future due to the forecast acceleration in sea-level rise and increase in storm intensity associated with global warming.