Changes To Barrier Island Systems During The Holocene

Barrier islands systems, briefly described as long and relatively narrow sandy islands parallel to the shore and separated from it by a backbarrier lagoon, represent almost 10% of all continental shorelines. Most of the US eastern coastline features such systems, developing long strings of barrier islands and spits, such as the Outer Banks (North Carolina) or the Outer Barriers (New York). These systems are considered to be very important in ecological terms, developing rich wetland habitats in the sheltered environment behind the barrier islands. They are also considered important in reducing the impact of large storms in the continental shoreline, acting as a buffer region.

Although there is an ongoing discussion as to which theoretical model better explains barrier island systems formation, there is a general agreement about the main controlling factors. Their evolution results from the interaction of several processes, namely sediment type and availability, hydraulic transport mechanisms (wave energy and tidal range), accommodation space controlled by the preexisting geological morphology, and most importantly, the rate of sea level change.

In order to better understand what specific conditions are required for these systems to be formed, and how they later evolve under changing sea level rates, a team of the Marine and Environmental Research Center of the University of Algarve (Portugal) studied the Holocene sedimentary sequence preserved in the Ria Formosa barrier island system. A large subsurface dataset acquired from 191 boreholes and five seismic refraction profiles, was subject to a multi-proxy detailed analysis (mean grain size distribution, organic matter content, color variation, micro- and macrofossil identification and foraminifera assemblages, radiocarbon dating), adding the description of modern-day deposition environments analogs and a 3D paleosurface creation.

Our results showed that the system evolution was largely controlled by the configuration of the inherited accommodation space, over which the available sediment would migrate according to the rate of sea level rise. In Ria Formosa, the Holocene sea level rise rate would change from a rapid rate of 7mm/yr from 10kyr cal BP to 7.25kyr cal BP, to a substantial slow-down of 1.1mm/yr until present. The resulting conceptual model for the origin and Holocene evolution of the Ria Formosa barrier island system explicit three main steps, leading to the present system configuration: a) marine flooding of incised paleovalleys by the rapid transgression in the early-Holocene; b) development of a proto-barrier island chain perched on Pleistocene detritic headlands and steeper interfluve areas during the early to mid-Holocene; c) full development of the barrier islands chain and enclosing of the coastal lagoon, followed by the maturation of the system with subsequent siltation and salt marsh expansion from the mid-Holocene until present.

Recent data regarding global sea level rise in the late 20th to 21st century points to present rates of 3mm/yr, with the Intergovernmental Panel on Climate Change (IPCC) projections of increased acceleration until 2100. Under this sea level rise scenario, similar to what was observed in the early Holocene in Ria Formosa, coastal instability is expected that ultimately will lead to barrier landward migration or in place drowning, depending on sediment availability and coastal configuration.

Figure 1 – Detail of a recovered borehole split in half (credit: Carlos AM de Sousa, as part of the SIHER project)