De Falco, G., Antonioli, F., Fontolan, G., Lo Presti, V., Simeone, S., Tonielli, R., 2015. Early cementation and accommodation space dictate the evolution of an overstepping barrier system during the Holocene. Marine Geology 369, 52-66. doi:10.1016/j.margeo.2015.08.002
Coastal barriers, narrow, sandy stretches of islands that parallel the mainland, are very attractive locations for beach goers and wildlife alike. Chances are, if you have been to the beach along the United States east coast (Fig 1), you have set foot on one of the many coastal barriers. In fact, approximately 10% of the world’s coastlines are sheltered by coastal barriers. The sandy beaches are great for sunbathing and beach sports. On the exposed side of the barrier, the occasional rough surf attracts surfers and other thrill seekers. A short walk across the barrier reveals a quiescent lagoon and tidal marsh, serving as a nursery for fish species, and providing a more tranquil bathing experience.
Strong coastal storms such as hurricanes often serve as a harsh reminder that coastal barriers are ever changing landforms. The low elevations of coastal barriers leave them prone to overwash by storm surge and large waves, sometimes leaving a barrier cut in two, separated by a breachway. The sensitivity of coastal barriers to large storms and the importance of barriers to societies has led scientists to study how barriers evolve and what conditions must exist for their formation.
The formation of barriers depends upon several conditions:
- Prior to formation, barriers require a substrate, or a lower surface along which they will form. The slope of a substrate dipping in the offshore direction between 0.05 and 0.8 degrees is necessary for barriers to begin to form (Fig 2).
- An adequate supply of sediment such as sand and gravel must be continuously available to build the barrier.
- A barrier requires accommodation space, which is the physical space and the characteristics of the space that separate the forming barrier from the mainland.
- 4. Although somewhat counterintuitive, barriers require wave action and sea level rise to form. However, if sea level is rising too quickly relative to the supply of sediment, or the accommodation space is very flat, the barrier may drown in place rather than migrated towards the mainland.
Over time, barriers form and migrate towards the mainland as a natural response to sea level rise. Sea level has risen and fallen many times in geological history from natural processes such as glaciation and tectonic plate movements. With an adequate supply of sediment and modest rates of sea level rise, barriers have formed many times in the geological past. The current rise in global sea level is being forced by human induced climate change, and we can expect barriers to respond through retreat. Although barrier retreat is a natural occurrence, it is in direct conflict with communities that have been built on seemingly stable land. Following the last glacial period, the rate of global sea level rise was high at first, eventually slowing to modern times. This epoch of time known as the Holocene saw the formation of many barriers that remained fairly stable. With increasing rates of sea level rise due to human induced climate change, barriers are beginning to retreat at alarming rates; water front properties are rapidly becoming under water properties.
There are two modes of barrier retreat:
- The dominant mode of barrier retreat is called the rollover model, in which a barrier migrates continuously towards the mainland with no offshore preservation of the relict barrier positions.
- The more extreme case is the overstepping model in which there is drowning of the barrier and formation of a new barrier at a location closer to the mainland. Depending on the rate of sea level rise and the composition of the sediment, there may be partial preservation of the barrier’s previous location.
The featured study focuses on the now submerged barrier complex at the entrance of the Gulf of Oristano, Sardinia, Italy (Fig 3). The results of the study contribute to the understanding of how coastal barriers may respond to future sea level rise by modeling the evolution of a now drowned coastal barrier complex that was subjected to conditions analogous to what is predicted over the upcoming centuries.
This study relied on several different techniques to characterize the presently submerged barrier complex. To characterize the seafloor, the authors used available Multibeam echosounder data (MBES) to recreate the topography of the seafloor (Fig 3). Two types of sub-bottom profilers were used to take cross-sections below the seafloor to understand the geology of the submerged barriers. Vibrocores, sediment cores that are collected by vibrating a rigid tube through the seafloor, as well as sediment grab samples were collected to better characterize the surficial and sub-seafloor geology. Additionally, divers and Remotely Operated Vechicles (ROV) were used to photograph the preserved barriers.
The authors created a model of the formation and evolution of the now submerged and partially preserved barrier complex at the entrance of the Gulf of Oristano, Sardinia, Italy. The analysis of sub-bottom profiler data reveals that an ancient offshore valley, cut by the Tirso River when sea level was much lower during glacial times (Fig 4A). According to a published regional sea level curve (a plot of how sea level changes over time), sea level began to rise as the Earth came out of the last glacial period, at regional rates of 10 to 15 mm per year (or about a half inch per year). The rising sea filled the ancient river valley with sediment. All the ingredients for a barrier were present. With a substrate slope of 0.3-0.4 degrees, adequate sediment supply from the Tirso River and rising sea level, the barrier was formed.
Typically with a seabed slope of 0.3 to 0.4 degrees and relatively slow sea level rise, coastal barriers would follow a steady retreat towards the mainland rather than the overstepping and partial preservation that is observed in this study. The authors report that the preservation and overstepping was due to an early cementation of the barrier to the substrate upon which it formed. This essentially locks the barrier in place and inhibits the more typical retreat response to sea level rise. As sea level continued to rise, the Tirso River valley, described by the authors as “a conduit for sediment transport”, shifted landward of the barrier complex (Fig 4B). This starved the barriers of sediment, which is critical for the continuous build up of coastal barriers. Despite a relatively modest rate of sea level rise, which would suggest a gradual retreat of the barriers, the cementation or locking of the barrier to the substrate and the reduction of sediment forced the coastal barriers in to the overstepping form of retreat, leading to partial preservation of the ancient barriers (Fig 4C-D). The authors report that the entire evolution of this barrier complex, from formation to drowning probably took about 1000 years. Each individual overstepped barrier likely lasted only a few hundred years at most.
It should come as no surprise that coastal barriers are very valuable to the economies of coastal municipalities. A short visit to the bustling barriers of New Jersey shore all but confirms this. Some barriers are reserved as state and federal parks as well as wildlife sanctuaries, providing nesting grounds and undisturbed habitats to many threatened and endangered species. The significance of coastal barriers to both human interests and wildlife alike necessitate close study and protection. Studies have reported that barriers are not permanent fixtures, even on human time scales. They are prone to being entirely washed over during strong storms, retreating during high rates of sea level rise, or under certain circumstances, being completely drowned in place.
The rate of sea level rise that was responsible for the overstepping response of the now submerged barrier complex at the entrance of the Gulf of Oristano are very similar to the projected rates of sea level rise as reported in the most recent IPCC publication. The overstepping of coastal barriers was previously considered a response to very high rates of sea level rise, whereas this study confirms that overstepping can occur at rates we are likely to experience within several decades. Efforts to protect and stabilize coastal barriers are typically guided by a rollover model of barrier response to sea level rise, which may not be appropriate for all communities on barrier islands. This study suggests that preexisting geological factors may overpower the influence of modest rates of sea level rise on how coastal barriers retreat. For communities adapting to higher rates of sea level rise, the unique geology of each barrier must be considered with the reality that an overstepping mode of retreat may be in their future.
What do you think? Are our coastal barrier communities fully considering a future of rising sea level? Comment below!