Vulnerability of coastlines - How do environmental changes affect coastlines and river deltas? [Past]

Coastal changes have occurred throughout geological time due to tectonic and isostatic processes as well as sea level changes induced (primarily) by climatic changes. During the Quaternary, one of the main controls on coastal evolution was sea-level changes through the exchange of mass between ice sheets and oceans. However, local and regional changes are superimposed on the global signal. These local/regional changes become more important as the temporal scale resolution increases, producing substantial spatial and temporal variability in sea-level changes, even when localities lie close to each other. This becomes even more complex as humans occupied the coastal zone (e.g. Syvitski, this issue).

Furthermore, it seems that the overall warming and sea-level rise of the Holocene was punctuated by climatic events and, apparently, impacted the coastal evolution. In fact, a correlation between marsh evolution and rapid climatic changes (RCCs) in the Delaware Bay has been established at a millennial scale. The idea of coastal evolution linked to climatic changes is supported by stratigraphic sequences occurring simultaneously with RCCs recognized in the western Gulf of Mexico, in the Trinity/Sabine River incised valley system and in northern Spain. Among the RCCs identified during the Holocene, an event at 750-950 AD was characterized by polar cooling, tropical aridity and major atmospheric circulation changes. Although this event was global in scale, records of it are poorly correlated due to its different behavior between regions (Mayewski et al. 2004).

Figure 1: A) Map of the Pamlico estuarine system in North Carolina showing the location of the main active inlets. B) Paleoenvironmental reconstruction of the southern Pamlico Sound region ca. 850 AD (modified from Grand Pre et al. 2011). Barrier island destruction along the southern Outer Banks resulted in a shallow, submarine sand shoal and localized deeper tidal channels over which normal marine waters were advected. The Cape Hatteras region exhibited several inlets with large flood-tide deltas.

Concomitant with these reported RCC events, major coastal geomorphological changes have been identified. For instance, recent work undertaken in the US North Carolina estuaries and barrier islands suggests that the period ca. 750-1400 AD was characterized by a high degree of barrier island segmentation and open marine influence in areas now occupied by the modern estuaries. Figure 1B shows the interpretation of the environmental change that occurred in the southern part of the Pamlico Sound at 850 AD, reflecting the destruction of large segments of the barriers compared to the current situation (Fig. 1A) (Grand Pre et al. 2011). Estuaries along the southern Bay of Biscay reflect similar changes associated with RCCs. These changes might have impacted the tidal frame, currents and sediment transport. In fact, dramatic changes in the tidal frame have been modeled for the Bay of Fundy in response to the catastrophic breakdown of a barrier system (Shaw et al. 2010). Also, tidal changes have been recorded in Delaware Bay over the last 4000 years in response to the change of the basin shape during the late Holocene sea-level rise (Leorri et al. 2011).

Over the Holocene, coastal environments have moved across the landscape. However, accelerated rates of climate change and sea-level rise could affect coastal environments by overcoming the natural mechanisms of self-maintenance. The impact of these changes might be considered significant since there are more than 20,000 km of barrier islands along the world’s open ocean coast, and they represent the front line to impacts of projected climate change. This may alter coastal systems from current conditions in a number of ways by: 1) increasing salt water intrusion landward, producing more rapid salinization; 2) altering the species composition through modified migration and other mechanisms; 3) enhancing tidal erosion, potentially forcing a coastal retreat; and 4) increasing the potential impact of future storms.

In the case of North Carolina, it has been suggested that hurricanes impacted the barrier islands at ca. 850 AD, causing the destruction of large segments of barriers. These barrier destruction events are essentially synchronous with intervals of RCCs at 750-950 AD and are coincident with transgressive surfaces in Delaware Bay, highlighting the importance of environmental changes in coastal evolution and suggesting their potential impact for future coastal evolution.

acknowledgements

NSF (OCE-1130843); MICINN (CGL2009-08840), FCT (PTDC/CTE/105370/2008). IGCP Project 588.

selected references

Full reference list online under: http://pastglobalchanges.org/products/newsletters/ref2012_1.pdf

Grand Pre C et al. (2011) Quaternary Research 76, 319-334

Leorri E, Mulligan R, Mallinson D and Cearreta A (2011) Journal of Integrated Coastal Zone Management 11(3): 307-314

Mayewski PA et al. (2004) Quaternary Research 62: 243-255

Shaw J et al. (2010) Canadian Journal of Earth Science 47: 1079-1091