Measurements of pH values in the oceans have started only a few decades ago, and in some areas only a few years ago. What were the pH values that organisms coped with 100, 200, or 1000 years ago? Using laser ablation on samples, scientists from the GEOMAR Helmholtz Centre for Ocean Research Kiel (Germany) have now succeeded together with partners from the Universities of Bristol (England), the University of Toronto (Ontario, Canada) and the University of Maine (Orono, ME) in reconstructing the pH values of the northern Pacific ocean over the past 120 years with a monthly resolution.1
They analyzed samples of a coralline alga using laser-ablation high-resolution 2D images of stable boron isotopes. For their study, the team used samples from specimen of the algae Clathromorphum nereostratum. These algae create massive reefs in the northern Pacific Ocean and in the Bering Sea on the ocean floor, where they form solid calcium carbonate structures. In these formations very fine growth rings are visible to the naked eye. These rings contain information about the environmental conditions over time.
The laser-ablated single sample points about 0.1 mm wide; the boron isotope ratios for the ablated material was analyzed by a mass spectrometer. The relationship between the two isotopes is considered a reliable indicator of the pH value of seawater.
The scientists found that the pH value in the North Pacific has actually been declining since the late 19th century, which means the water is acidifying. At the same time, the monthly resolution of the analysis revealed strong variations in pH within one year, which probably originate from the fact that large kelp forests grow in the region the samples came from. "In spring and summer the kelp consumes large amounts of CO2," says says co-author Federica Ragazzola. "This means there is less carbon dioxide in the water and the pH rises."
1. Jan Fietzke et al., PNAS, Early Edition (2015); http://www.pnas.org/content/early/2015/02/17/1419216112