Ocean Acidification

Geochemical records preserved in the long-lived carbonate skeleton of corals provide one of the few means to reconstruct changes in seawater pH since the commencement of the industrial era. This information is important in not only determining the response of the surface oceans to ocean acidification from enhanced uptake of CO2, but also to better understand the effects of ocean acidification on carbonate secreting organisms such as corals, whose ability to calcify is highly pH dependent. Here we report an ~200 year 11B isotopic record, extracted from a long-lived Porites coral from the central Great Barrier Reef of Australia. This record covering the period from 1800 to 2004 was sampled at yearly increments from 1940 to the present and 5-year increments prior to 1940. The 11B isotopic compositions reflect variations in seawater pH, and the 13C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, 18O and Mg/Ca data was obtained providing proxies for terrestrial runoff, salinity and temperature changes over the past 200 years in this region. Positive thermal ionization mass spectrometry (PTIMS) method was utilized in order to enable the highest precision and most accurate measurements of 11B values. The internal precision and reproducibility for 11B of our measurements are better than ±0.2‰ (2), which translates to a precision of better than ±0.02 pH units. Our 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. In the periods around 1940 and 1998 there are also rapid oscillations in 11B compositions equivalent changes in pH of almost 0.5 units. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal 11 B 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, the range being dependent on the value assumed for the fractionation factor α(B3-B4) of the boric acid and borate species in seawater. Correlations of 11B with 13C 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 the rapidly increasing levels of atmospheric CO2, mainly from fossil fuel burning. This suggests that the increased levels of anthropogenic CO2 in atmosphere has already caused a significant trend towards acidification in the oceans 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 of Greenhouse gas emissions on coral calcification.

Wei G., McCulloch M. T., Mortimer G., Deng W. & Xie L., 2009. Evidence for ocean acidification in the Great Barrier Reef of Australia. Geochimica et Cosmochimica Acta 73(8): 2332-2346 Article (subscription required).