Wednesday, September 16, 2015

Coral Reefs, Atolls And Sea Level Rise

Will coral reefs and atolls (coral islands) be able to keep pace with the current and projected sea level rise and remain geologically stable in the coming decades and centuries? Will atolls in  the Pacific and Indian Oceans remain habitable?

Regarding  the first question,  I came  across a couple of recent  studies that suggest that reef growth in the Pacific, Indian and Caribbean seas has historically and in the geological  past been able to keep pace with sea level rise of magnitudes equal to or even greater than the current rate of change of sea level.

In a recent issue of  Geology, P.S Kench and colleagues study six time slices of shoreline position of the Funafuti Atoll in the tropical Pacific Ocean and find out that there has been no loss of  island due to erosion by sea level rise. This part of the Pacific has experienced some of the highest measured rates of  sea level rise amounting to about 5 mm per year over the past 60 years. Their analysis showed that reef islands in this group shifted their size, shape and positions in response to sea level rise.

What could be happening? Coral reefs are prolific producers of carbonate skeletal material. As sea level rises, corals grow upwards and outwards from established communities keeping pace with the sea level rise so as to remain in the optimum water depth range. Wave energy keeps breaking down corals and produce carbonate sand which then gets redistributed and deposited in adjacent areas including island beaches. Corals thus form a renewable supply of sediment that balances sediment lost to erosion. Thus coral islands, although may change in shape and position due to changes in depositional locus will not experience any net loss of land.

Studies which go back in geological time also seem to confirm that coral reefs have often extraordinary growth rates that they can sustain for centuries and may keep up with extremely rapid episodes of sea level rise. In a special issue of Sedimentology ( Feb 2015 Open Access) on carbonate response to sea level change, Gilbert F. Camoin and Jody M. Webster document very rapid coral growth rates  using age constrained fossil coral reefs from Barbados in the Caribbean Sea and from atolls in the Pacific and Indian Oceans.

Their results show that following the melting of the global ice caps beginning around twenty thousand years ago, coral reefs kept pace with high rates of sea level rise amounting to 6-10 mm per year and astonishingly in places like Tahiti, for periods of a few  centuries, amounting to 45 mm per year. This very high rate dated to 14.65 k to 14.3 k corresponds to a Melt Water Pulse i.e. an accelerated rise in sea level due to collapse of portions of the ice sheet. Healthy reef growth means a steady supply of sediment to replenish coral island beaches, thus maintaining geological stability through periods of sea level rise.

This suggests that many coral atolls will not simply vanish beneath the waves as sea level rise in the coming centuries, although they will change their shape and positions. The other danger besides sea level rise is the changing chemistry of sea water and other biological changes that might harm coral growth. Sea water acidification may slow down the capacity of corals to build calcium carbonate skeletons, although again, studies on the impact of changing pH on coral growth have shown mixed results, with ill effects on some coral species in some locations, while others seem to have sufficient internal buffering capacity to maintain normal growth patterns. Increasing sea water temperature may also result in a) expulsion of symbiotic algae that corals depend on, thus slowing down their growth and/or b) infection by parasites that might harm the coral animal. So, there is still much to worry about the health of coral ecosystems as the earth warms and ocean temperatures rise.

Now to the second question - will coral atolls remain habitable? Habitations on these islands are built on a foundation of dead coral communities and sand which are not going to be lifted up in response to sea level rise. Although the fringing living reef communities will supply sediment to these islands, powerful storms and high tides will still pose problems. Reefs don't form water tight sea walls around these atolls and tidal surges will bring sea water further inland.

Another problem is the impact of sea level rise on groundwater. Many of these island  communities rely on a thin fresh water aquifer for their water supply. The foundation of these islands is porous Pleistocene limestone. Holocene coral communities and sand is piled up on this Pleistocene foundation to build the island. The fresh water aquifer usually occurs in this Holocene sediment. The pores and fractures in the Pleistocene limestone below the fresh water lens is filled with sea water. The contact between the fresh water aquifer and the underlying sea water aquifer is called the Thurber Discontinuity. The graphic below shows a typical cross section and hydrogeology of a coral atoll.

 Source: Bailey et. al. 2010 adapted from Ayers, J.F.; Vacher, H.L. Hydrogeology of an atoll island: A conceptual model from detailed study of a Micronesian example. Ground Water 1986, 24, 2-15

What will be the impact of sea level rise on this fresh water lens. This is an active area of study and early results seem to suggest a variety of outcomes with small fresh water lenses further diminishing while larger ones persisting. This is a complex topic with a variety of controlling parameters like amount of eustatic sea level rise, island size and shape and island topography which will channel the extent of storm wave washover. As sea level rises over the next few decades and centuries, especially on coral atolls which are experiencing erosion and loss of land, the danger of salinization of the fresh water lens is a real possibility, which will make living on these islands a difficult proposition.

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