Barrier islands are common along many wave-dominated shorelines. The US East coast is one example. Here, these islands are popular locations for residents and vacationers, supporting local economies and bringing in tourism as well. Unfortunately, these locations are also prone to rapid change and destruction (Hurricane Sandy, anyone?). As storm strength increases and sea level rises, barrier islands will shift and the sand and dunes will move landward. How can we predict how these near-shore landforms will behave in the future? As always, we can look to the past.
Barrier islands are not only important economic and recreational areas – some ancient barrier-island deposits house petroleum reservoirs – they are also an important component in the Earth’s stratigraphic record. As sediments shift and dunes move over thousands of years of sea and storm change, the sediments are preserved in layers that can be studied by modern-day scientists.
Previous studies have shown that barrier-island formation and evolution are closely tied to changes in sea level, sediment supply, and the intensity of storms. A more recent study published in the Journal of Sedimentary Research reconstructs the formation of a wave-dominated barrier island off the coast of Denmark, and assesses the effects of sea-level changes on the island’s sedimentary evolution during the Holocene (12k years ago – present).
Many marine shorelines go through transgressive-regressive cycles. Transgression is where sea level rises relative to the island and the shoreline moves to higher ground, causing floods. A regression is where sea level falls relative to the land and exposes former sea floor. As with past glacial-interglacial cycles, these cycles can be preserved in sediments over thousands of years.
Lead author Mikkel Fruergaard of the University of Copenhagen wanted to investigate how specific rates of sea level change and sediment supply affected the transgressive-regressive cycle of the Holocene deposits on the Danish barrier island Fanø, situated in the southeastern North Sea.
The study was conducted with percussion sediment coring along a cross profile along the shoreline of the island (Figure 1). Additionally, the study uses a new dataset of core well data, ground-penetrating radar (GPR) data, and high-resolution optically stimulated luminescence (OSL) dating to reconstruct the internal structure and evolution of the island. The OSL chronology obtained in this study is one of the most detailed ever constructed for a barrier-island.
As expected, the results show that the shoreline has transgressed and regressed many many times during the last 12 thousand years. Not only did the shoreline shift, but it shifted dramatically, sometimes a few kilometers. Imagine your favorite beach, perhaps somewhere along the pristine, flat east coast. Now, imagine that all being under water, the shoreline located several miles inland. That has happened many, many times over the Holocene.
Additionally, with the new, more precise data set, the researchers were able to note at what rates of sea level rise or fall they occurred at, and how varying rates can cause very different structural responses for a single barrier island. For Fanø island, it appears that a transgressive situation is reached for a sea-level rise rate higher than about 2 mm/yr. Considering that the current global average sea level rise has been estimated around ~2.6 mm/yr, and has been accelerating, this gives reason for concern.
However, specific regions respond differently. As the study shows, the conclusion remains that barrier-island evolution is controlled not only by rates of sea-level rise, but also by a combination of changes in sea-level rise, storm regime, and sediment supply. The transgressive-regressive sediment record of Fanø confirms that controls on barrier-island evolution are complex.
Hurricane Sandy, among many other intense storms, highlighted the vulnerability of these types of coastal systems. Combined with increasing rates of sea level rise, barrier islands across the globe are at risk of transgressing and shifting shorelines, disrupting local populations, economy and infrastructure. This study was a step closer to being able to predict how these environments may change in the future in response to our changing climate.