Do Darwin points and tectonics hold the answer?

To better asses the future of coral islands within the context of sea level rise an understanding of the mechanics of coral island development is necessary. Charles Darwin although famous for his work on evolution during the 1836 Beagle voyage, also solved the puzzle of atoll formation. R.W.Grigg (1982) references a note Darwin wrote about Tahiti and Moorea “Hence if we imagine such an Island after long successive intervals, to subside a few feet in a manner similar but with a movement opposite to the continent of S.  America; the coral would be continued upwards, rising from the foundation of the encircling reef. In time, the central land would sink beneath the level of the sea and disappear but the coral would have completed its circular wall. Should we not then have a coral Island? Under this view we must look at a Lagoon Island as a monument raised by myriads of tiny architects to mark the spot where a former land lies buried in the depths of the ocean.”

Figure 1 below visualizes this process. Source: The Australian.

However the investigation of academic material by this blog has clearly shown that it is possible for there to be stages after (3.) in Figure 1 above, whereby the Atoll becomes submerged. This submergence has become apart of the de facto rhetoric of the environmentalist movement, suggesting that sea level rise caused by global warming is the sole answer to all atoll submergence. However Grigg (1982) suggested another reason for this submergence is due to underlying tectonics, he studied the Hawaiian Archipelago to test his assertion of the existence of a ‘Darwin Point’. A Darwin point is a latitudinal boundary at which due to the subduction of the underlying plate and unfavorable environmental conditions for Coral to continue to calcify coral islands become submerged. Grigg (1982) detailed the Darwin point in the following sketch shown below in Figure 2.

For the Hawaiian Archipelago Grigg (1982) put the point at 28 degrees North as his results had suggested that at this point calcification rates were not high enough to cope with isostatic and eustatic sea level rise.

Grigg (1982) goes further, suggesting that these Darwin points will occur at other latitudes in different oceans and geographic locations depending on regional climate and tectonic pasts. This could therefore account for a proportion of the 14% of Pacific islands found to have lost land area over recent decades by Webb and Kench (2010). The argument for a large scale loss of low lying coral islands due specifically to global warming induced sea level rise without a role of other mechanisms seems to have been undermined yet again by examination of the scientific evidence.

However to some extent it still appears that human induced sea level rise has a role to play in the submergence of coral islands. By increasing the rate of sea level rise this ‘Darwin Point’ could move progressively south, threatening more coral islands, sooner than otherwise would of been the case. Therefore in the next post, i will attempt to make informed suggestions as to the actions these threatened human populations should take to  mitigate negative impacts.

References:

Grigg, R.W. 1982. Darwin Point: A Thershold for Atoll Formation. Coral Reefs. 1. 29-34.

Webb and Kench. 2010. doi: 10.1016/j.gloplacha.2010.05.003