Historic Sea Level Rise

As mentioned in the previous post, the potential response of coral reefs to present day climate change can be estimated through the assessment of the elevation and ages of drowned reefs. These drowned reefs also allow the investigation of historic sea level rise, which will not only improve our understanding of the changes brought about by a changing climate but will also enable more accurate predictions to be made about contemporary sea level rise. The last glacial to inter-glacial transition, which occurred around 13,000 – 10,000¹⁴C years BP, is the most studied due to the rapid climate change that occurred and the fact that it is the most recent transition (Hoek, 2008).




Figure 1: Caribbean sea-level rise during the last de-glaciation, with the shaded areas showing drowned reefs and the white areas indicating catastrophic rise events (CRE) (Source: Blanchon and Shaw, 1995). 

Blanchon and Shaw (1995) use drowned coral reefs in the Caribbean-Atlantic region to study ‘glacio-eustatic sea-level changes’ during the last de-glaciation. They emphasise the benefits of using coral reefs for studying sea level changes, referring to their suitability for radiometric dating and the way in which they maintain themselves at sea level through vertical accretion. In particular they investigate Acropora palmata, since it forms monospecific reefs and is strongly depth restricted, meaning it can be successfully and reliably used to infer the sea level. When sea levels rise faster than this species can grow vertically the reef will exhibit a mixed framework with other corals, eventually this is replaced by deep water corals. An inverse relationship has been uncovered between the rate of sea level rise and the thickness of this mixed framework layer. Figure 1 presents the sea level rise that occurred during the last de-glaciation, reconstructed from drowned reefs. The results indicate the large volumes of meltwater that entered oceans at three main points in time, and that sea level rise occurred rapidly. However, it also suggests that reefs were able to form again following the rapid sea level rise, which is good news considering the threats currently facing these ecosystems.




A similar study was carried out in Hawaii by Webster et al (2004) which attempted to investigate the reason for the drowning of the 150m submerged reef off Hawaii. Through radiometric dating techniques they were able to place the drowning of this reef at around 14,700 years ago which coincides with a sea level rise period known as 'Meltwater Pulse 1a'. The study not only provides evidence for the occurrence and timing of this meltwater phase, but also the intensity of the sea level rise (approximately 35m in 500 years, equivalent to 40-50mm per year). 

 


Figure 2: Position of drowned reefs in Hawaii. Reef 1 was subject to reef drowning following Meltwater Phase 1a (Source: Webster et al, 2004).

While the main process under investigation in studies like these is eustatic sea level rise, an understanding of  isostatic changes is required to infer eustatic sea level rise from the assessment of coral reefs. Stirling et al (1998) highlight the inconsistencies in the inferred timing and duration of the last interglacial as a result of isostatic processes that vary spatially.




From these studies it is evident that historic sea level rise caused by de-glaciation following the last glacial maximum caused the drowning of many coral reefs. However, it is also apparent that coral reefs were able to recolonize shallow regions of the ocean once sea levels were stable, which has positive implications for the consequences of present day anthropogenic changes.