HOLOCENE SEA LEVELS AND COASTAL EVOLUTION

I have long been interested in trying to unravel the driving forces that underpin the geomorphological and sedimentary record of coastal deposits. This has involved the study of sand barrier, river delta and coral reef sequences in different areas of the world. It is no surprise that this work has not been without challenges both in collecting and analysing data on the one hand and interpreting the depositional sequences on the other. One can only keep trying!

For purposes of better understanding the role changing sea levels have or have not played in coastal evolution, I have been inspired by recent papers by Fruergaard et al. (2015) and Kinsela et al. (2016). These studies crystallise much of what I have read and personally observed and written about over the years, often in collaboration with many others who have put up with my rants and raves.

The key point is distinguishing between eustatic changes as recorded along a particular coastal stretch and those factors that influence depositional processes and responses in an area. For purposes of this rather simplified exposition I will differentiate three spatial and temporal scales which leave behind a record in coastal landforms and depositional sequences (i.e. in the morphostratigraphic record).

The first or macro scale is represented by inner continental shelf marine “flooding”. This is what is termed the Postglacial Marine Transgression (PMT). Accommodation spaces (compartments or embayments) along a coastline start to get inundated during the early Holocene phase from around 10000 BP. The specific nature of change depends on geomorphic settings (e.g. sand barrier roll-over; estuary valley marine drowning; carbonate platform initial growth). A key characteristic is that shorelines are retreating, or in the words of Fruergaard et al. “relative sea level rise outpaced sediment accumulation”. Drill cores in great delta sequences like the Mississippi show these deposits at depth below later Holocene sediments. On the Danish coast, rates of relative sea level (RSL) rose at around 8mm/yr., a rate linked to marine transgression prior to 7000 BP. But as the rate declined to 4mm or less backbarrier infill deposition occurred. A phase of progradation was documented with the rate of RSL rise at less than 2mm/yr. (c. 5000BP). On the southeast Australian coast, the PMT appears to have slowed down more rapidly; by around 7000 yr. RSL was close to, if not at or above, present sea level, commencing the so-called “stillstand” phase. Buried estuary and backbarrier deposits define this early to mid-Holocene period of the PMT in various coastal embayments on this coast. It is in the period immediately following RSL reaching around its present position in this region that different geomorphic responses emerge depending on sediment availability from onshore and offshore sources; this takes me to the next scale.

Regional or meso scale is where spatial and temporal sequences form within embayments commencing in the mid-Holocene. RSL behaviour (eustatic, isostatic) becomes modulated to varying degrees by other external forces such as regional tectonism/subsidence, wave climate, tidal regime and biogenic productivity interacting with those internal to the accommodation space (river-mouth switching, lagoon entrance closure, flood tidal delta growth affecting tidal prism and levels). This is where an understanding of coastal morphodynamics becomes relevant to the determination of regional RSL history. All these various factors can be seen as “noise” when searching for a eustatic signal along a particular section of coast. But as so neatly explained by Kinsela et al. in examining coupled shoreface-barrier evolution at Tuncurry, a key role is played during this period of mid to late Holocene by shoreface sand supply “driven by the ongoing relaxation of disequilibrium morphology”. This may promote over time shoreline stability and “potentially moderating initial shoreline response to sea-level rise”.

Local or micro scale is where we see evidence for local change in coastal landforms that can be driven by a range of factors both natural and anthropogenic. Here is where it becomes difficult to attribute RSL rise due to climate change from other local factors that may drive the sediment budget of a coastal area. Through our recent national study of sediment compartments (Thom et al., 2018) it is apparent that shoreline behaviour at open coast sites or within estuaries is highly variable. Chris Sharples refers to fast and slow responders, but to what? Human disturbance to sediment availability is evident in many locations both in Australia and overseas. To attribute shoreline erosion to a rise in RSL at rate of 1-2mm/yr. may be questionable especially where beachface accretion takes place during post-storm events. However, in areas of sediment deficit or where tidal levels are accentuated by whatever reason then RSL due to climate change may be a contributing factor but not the only factor.

In essence, current rates of global sea-level rise as recorded around the Australian coast may not be fast enough to induce shoreline change in many sediment compartments. There is a high degree of resilience to recession in such cases. Post-storm recovery and general sediment budget sufficiency could be seen as dampening any morphologic impact of the present rate of sea-level rise. However, if the rate rises to around that of the end of the PMT (say 8mm/yr. or more) then shoreline recession will most likely overwhelm any tendency to recover after extreme storm events. This conclusion reinforces the need for observations like that at Narrabeen and north of Moruya airport and elsewhere to compare with modelled projections.

References

Kinsela, M. Daly and P. Cowell, 2016, Origins of Holocene coastal strandplains in Southeast Australia: Shoreface sand supply driven by disequilibrium morphology. Marine Geology 374, 14-30.

Fruergaard et al., 2015, Stratigraphy, evolution and controls of a Holocene transgressive-regressive barrier island under changing sea level: Danish North Sea coast. Journal Sedimentary Research, 85, 820-844. 103-120.

Thom et al., 2018, National sediment compartment framework for Australian coastal management. Ocean & Coastal Management. 154, 103-120.

FEEDBACK

Hi Bruce- Your blog post is interesting and poses some important and timely questions about “tipping points” that, I think, deserve a fairly thorough literature review if one doesn’t already exist. The question is what is the maximum rate of SLR at which a coast can balance SLR with accretion without simply being drowned.

For coastal marshes, Morris and others found that marsh accretion could pace with SLR up to about 5mm/yr and Turner et al came to a similar conclusion for several river deltas. For the cases of sandy barriers like Moruya, availability of inner shelf sands combined with wave climate would probably be critical determinants of the tipping point.

The present observed rate of global SLR of 3.1mm/yr are still below most reported tipping points. But predictions of future rates of between 8mm/yr and 16mm/yr well exceed all of the estimates that I have seen. I think your NSW cases would make a valuable contribution to the literature, particularly if comparisons are mades with other estimated tipping points from different environments. Some references for marshes and deltas are in ”Coastal Erosion and Land Loss: Causes and Impacts” Chapter 9 of “Tomorrow’s Coasts – Complex and Impermanent”. 

Don Wright

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2020, for correspondence about this blog post please email austcoastsoc@gmail.com

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