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Ocean Acidification in the Great Barrier Reef


Malcolm McCulloch1, Gangjian Wei2 and Graham Mortimer1

1  Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
2 Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

Figure 1.

 

Over the past century atmospheric CO2 has risen by over 30%, from pre-industrial values of ~280 ppm to present-day levels of over 380 ppm, and is continuing to rise at an unprecedented rate of ~2 ppm per year. If unabated, this rate of increase will result in a doubling of CO2 by sometime later this century. Unlike the atmosphere where CO2 causes warming through its strong physical interaction with infrared radiation, in the oceans it is a highly reactive species causing a major perturbation to the chemistry of surface waters. This perturbation arises from dissolution of CO2 in surface waters resulting in an increase in the concentration of carbonic acid, and a reduction in seawater pH or what has become known as 'ocean acidification'. This in turn is leading to an overall decrease in the carbonate ion concentration, the key component controlling calcification in marine organisms.

Ocean acidification is thus of major concern not only because it will ultimately lead to dissolution of calcium carbonate organisms as aragonite undersaturation is approached, but also because the rate of coral calcification appears to be directly proportional to the degree of carbonate ion concentration, even in oversaturated conditions. Thus coral reefs will be at risk as calcification decreases while bio-erosion and chemical dissolution increases. Unfortunately, very little is known about the regional variability of ocean acidification on decadal to centennial time scales, especially since the industrial era.

Our current knowledge of ocean acidification is mainly dependent on model calculations and unlike other key climatic indices, such as temperature and salinity, seawater pH has seldom been recorded in marine observations due to the non-routine nature of the measurements. Accordingly, long-term continuous seawater pH records are scarce, with records of several decades now only becoming available from the off-shore sites of Hawaii and Bermuda. This lack of knowledge hinders attempts to properly evaluate not only the current status of ocean acidification, but importantly future trends and likely impacts on calcification of marine biota.

Boron isotope systematics in marine carbonate provide an alternative solution acting as a potential long-term proxy for seawater pH, due to an isotopic fractionation between the boric acid and borate ion species, with their relative proportions being controlled by seawater pH. However in order to utilize this system, high precision measurements of B isotopic compositions are needed to determine the relatively small shifts in seawater pH that are predicted since the commencement of the industrial era. We have pioneered this approach using B measurements in the carbonate skeleton of long-lived (~200 year) Porites corals from the Great Barrier Reef.

Our initial results indicate that the long-term pre-industrial variation of seawater pH in this region is partially related to the decadal-interdecadal variability of atmospheric and oceanic anomalies in the Pacific. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal δ11B compositions to loss of zooxanthellate symbionts. Importantly, from the 1940's to the present-day, there is a general overall trend of ocean acidification with pH decreasing by about 0.2 to 0.3 units. Correlations of δ11B with δ13C during this interval indicate that the increasing trend towards ocean acidification over the past 60 years in this region is the result of enhanced dissolution of CO2 in surface waters from fossil fuel burning at a significantly larger than anticipated from model calculations.

This suggests that the increased levels of anthropogenic CO2 in atmosphere has already caused a marked trend towards acidification in the coral reefs during the past decades. Observations of surprisingly large decreases in pH across important carbonate producing regions, such as the Great Barrier Reef of Australia, raise serious concerns about the impact that Greenhouse gas emissions may already be having on coral calcification in the Great Barrier Reef.