What do corals tell us about the oceans?

Speaking about evidences for ocean acidification last week, corals are also one of the few proxies used for past seawater pH reconstruction. Reef-building corals are very sensitive to pH changes in the ocean. Their skeleton materials are built out of aragonite, which is a more soluble form of calcium carbonate (Cao and Caldeira, 2008). As seawater pH fluctuates, carbonate ion concentration in seawater, hence calcification rate of corals also changes accordingly. This therefore suggests that the changes in calcium carbonate response in the ocean can be monitored from these reef-building corals.

The tendency of a coral structure to develop or dissolve is strongly dependent upon the saturation state (Ω) of a particular mineral phase, which is largely controlled by carbonate ion concentration in seawater. Corals tend to develop where aragonite saturation Ω>3.3, with rates of calcification process exceed rates of bio-erosion (Pelejero et al., 2010). With today’s aragonite saturation value of 3.3 of most areas of the ocean, carbonate accumulation or coral building decreases and increasingly confines to areas with Ω>3.3 (Hoegh-Guldberg et al., 2007). Aragonite saturation levels are expected to drop below 3.3 for all oceans in the world by the end of this century (See diagram below).

Aragonite Saturation Levels over time

How closely does coral calcification relate to ocean acidification?

It is often easy to draw the conclusion of coral dissolution is caused by ocean acidification. In fact, there is not always a clear cut. Doney et al. (2007) argued that coral calcification records rarely established links with ocean acidification directly due to its naturally high variability, which is difficult to detect the acidifying signal. The high variability of ocean pH is illustrated from the δ¹¹B  record of the long-living coral Porites from Flinders Reef in the western Coral Sea of the southwestern Pacific, where there is no significant decreasing trend in δ¹¹B, i.e. pH over the last 300 years. The only dominant feature found in the record is the Interdecadal Oscillation pH, with pH values fluctuate between 7.9 and 8.2 units, synchronises with the Interdecadal Pacific Oscillation (Pelejero et al., 2005).

However, a more recent study by Wei et al. (2009) has provided detailed evidence of ocean acidification through the extensive studies on corals of the Great Barrier Reef. δ¹¹B isotope composition record of the extracted Porites coral reflects a decreasing seawater pH trend of 0.2-0.4 units over the last 200 years, despite of the interdecadal variability.

You might wonder that the timescale for coral studies is relatively short i.e. the past 200-300 years.  A latest study done by Douville et al., (2010) has lengthened the timescale of pH reconstruction based on Porites corals through the Holocene to the Last Glacial Period.  The ‘δ¹¹B-pH’ technique is applied on both modern and ancient Porites corals. Their results indicate that the ancient sea surface water pH in the Holocene is 8.20-8.26 units and has reached 8.30 at the end of the last glacial period. These values are much higher than present day values. This ancient coral reconstruction also shows a drop in pH of 0.2 units before and after the abrupt cooling event, Younger Dryas 20.7kyr BP.

Unfortunately, that is only one of the very few studies with extensive improvement in using ancient/fossil corals for a better temporal resolution in past pH reconstruction. It is still subject to high levels of uncertainties. For instance, fossil corals are hard to obtain and preserve. Accurate reconstruction would require high spatial and temporal resolution coral data to determine overall changes in ocean chemistry.