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Researchers reconstruct high-latitude ocean acidification using coralline algae

9 March 2015

Dr Federica Ragazzola from the School of Earth Sciences, University of Bristol, is among a team of scientists who have published the first record of ocean acidification for the high-latitude oceans. The study, published in the Proceedings of the National Academy of Sciences (PNAS), used the calcium carbonate structure made by a particular type of marine organism, the coralline algae, to reconstruct changes in ocean acidity and temperature in the North Pacific Ocean.

Ocean acidification due to man-made carbon dioxide emissions is an area of intense research. Whilst instrumental records of ocean acidity do exist, they have been missing from the cold, high-latitude oceans to the north and south. To better understand the impacts of ocean acidification on the ecosystems of these areas it is important to know the amount of expected acidification, as well as the short-term variability.

Where measurements of ocean acidity are lacking, scientists can learn something by using the chemical composition of the element boron. Each boron atom has a certain mass, and the amount of light boron atoms compared to heavy boron atoms within the shell or skeleton of a marine organism is related to the acidity of the water in which the organism grew.

The organism used in this study - the coralline algae - is long lived and produces a hard structure of calcium carbonate containing year-long growth bands, similar to the rings in trees. A novel technique using a laser allowed the team to measure the boron composition within each growth band. They found a long term acidifying trend in the North Pacific Ocean, in line with predictions based on man-made carbon dioxide emissions. By carefully analysing each growth band the team were also able to examine the annual cycle, which showed changes in acidity that were greater than the long term trend. The seasonal cycle they argue, is caused by the yearly growth of kelp near the sample location which takes in carbon dioxide, making the waters less acidic each spring. By combining these findings with estimates of temperature made using the concentration of magnesium in the calcium carbonate, the authors were also able to suggest that the kelp is growing at earlier and earlier times of the year due to ocean warming.

While some aspects of the study require further testing, it offers a new and intriguing tool for scientists seeking to study the effects of ocean warming and acidification on marine ecosystems.

 

Fietzke J. et al., 2015, Century-scale trends and seasonality in pH and temperature for shallow zones of the Bering Sea. PNAS, doi:10.1073/pnas.1419216112.

Further information

For further information please contact Dr. Federica Ragazzola

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