Evidence for ocean acidification in the Great Barrier Reef of Australia

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Wei, Gangjian, McCulloch, Malcolm, Mortimer, Graham, Deng, Wenfeng, Xie, Luhua
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Pergamon-Elsevier Ltd
• Subjects: Keywords: acidification; calcification; carbon; coral reef; hydrogeochemistry; ionization; isotopic composition; mass spectrometry; pH; seawater; Australasia; Australia; Coral Sea; Great Barrier Reef; Pacific Ocean; Queensland; Anthozoa; Porites; Pacific; Ocean acidification
• Online Access: http://hdl.handle.net/1885/54914; https://doi.org/10.1016/j.gca.2009.02.009; https://openresearch-repository.anu.edu.au/bitstream/1885/54914/5/GeCA73_2332.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/54914/7/01_Wei_Evidence_for_ocean_2009.pdf.jpg

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 δ11B 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 δ11B isotopic compositions reflect variations in seawater pH, and the δ13C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, δ18O 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 δ11B values. The internal precision and reproducibility for δ11B of our measurements are better than ±0.2‰ (2σ), 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 δ11B compositions equivalent changes in pH of almost 0.5 U. 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 1940s to the present-day, there is a general overall ...