By Scott Johnson, Ars Technica

England’s White Cliffs of Dover are certainly an impressive sight. The sheer cliffs, made of bright white chalk, rise as high as 350 feet above the shoreline.

[partner id="arstechnica" align="right"]Despite the fact that the chalk is over 65 million years old, it may have something to tell us about how the ocean will react to the continued use of fossil fuels.

Chalk is composed of tiny calcite (calcium carbonate) plates called coccoliths. These are sections of the intricate spherical housing secreted by a type of phytoplankton smaller than the width of a hair, known as coccolithophores. The coccoliths in ancient chalk deposits like Dover’s cliffs have maintained their microscopic size, resisting the natural tendency of calcite to partially dissolve over time and recrystallize into larger clumps. This left researchers at the University of Copenhagen pondering if there might be something special about the calcite secreted by coccolithophores.

If that’s the case, understanding the details could help us predict how these phytoplankton will respond to ocean acidification – global warming’s oft-overlooked (but equally ugly) twin. The rising concentration of carbon dioxide in the atmosphere doesn’t just change the climate; it also lowers the pH of ocean water, and that’s bad news for things made of calcite, which may dissolve as the pH drops.

To answer their question, the researchers had to develop a new method to monitor the dissolution of individual coccoliths, requiring a degree of precision far beyond existing techniques. They glued single coccoliths to the tip of a tiny cantilever that oscillated. Imagine a ruler held over the edge of a table and plucked: It will vibrate, but if you attached a marble or a golf ball to the end, it would waggle more slowly. In the experiment, as the coccolith dissolved away, its mass decreased and the frequency of cantilever oscillation (waggling speed) increased. This allowed the researchers to measure mass to within a remarkable one-trillionth of a gram.

The results showed that coccoliths are indeed resistant to dissolution. Inorganic calcite crystals begin dissolving around pH 8.2, but the coccoliths remained intact until about pH 7.8. That’s not a trivial difference when you consider that pH is measured in logarithmic units. For example, a pH of 8 is 10 times as basic as a pH of 7. The research team attributes this resistance to the presence of organic material (from the single-celled phytoplankton that lived inside) which protects the calcite from dissolution.

What does this information tell us? For starters, it explains the microscopic characteristics of chalk. But, more importantly, it helps us predict the effects of ocean acidification more accurately. Some marine plankton and invertebrates build shells from aragonite – a form of calcium carbonate which dissolves more easily than calcite – and these organisms will be the first to feel the effect of increasing ocean acidity. Calcite-secreting organisms which aren’t as resistant as coccolithophores will be next. Near pH 7.8, coccolithophores – and any other groups that stabilize calcite similarly – will be in trouble as well.

Projections vary with scenarios of future emissions, but most put the average ocean pH at 7.8 before the end of this century. Average pH has already decreased by about 0.1 units since preindustrial times to roughly 8.1 – a nearly 30 percent increase in acidity. With regional and seasonal variation, some areas will experience a pH of 7.8 or lower much sooner, most notably the Southern Ocean.

Consideration of this scenario is not just an academic exercise. Phytoplankton form the base of the marine food web, and coccolithophores are one of the most abundant groups. Most plankton groups will be impacted by ocean acidification, which could result in serious ecosystem changes. Like burning the grass in a cow pasture, knocking out phytoplankton ultimately means nobody eats.

On a timescale of millennia, another story becomes significant. Phytoplankton like coccolithophores represent a key piece of the carbon cycle. After taking in carbon dioxide from the atmosphere, they eventually die and sink to the ocean bottom, where many accumulate and are buried as carbonate sediment, locking up that carbon in long-term storage. Disrupting phytoplankton growth inhibits the planet’s natural regulation of greenhouse gases by decreasing its ability to lock up excess carbon in sediment.

Coccolithophores may have a couple tricks up their (microscopic) sleeves that will help them hold out a little longer than some other marine organisms, but chemistry can only be kept at bay for so long. Awareness of just where the danger lies allows for effective monitoring and an accurate appraisal of our proximity to it.

Image: A species of coccolithophore phytoplankton called Emiliania huxleyi. (University of Georgia)

Citation: "Tracking single coccolith dissolution with picogram resolution and implications for CO2 sequestration and ocean acidification." T. Hassenkam, A. Johnsson, K. Bechgaard, and S. L. S. Stipp. Proceedings of the National Academy of Sciences of the United States of America*, Vol. 108, No. 21, Pg. 8571-8576. DOI: 10.1073/pnas.1009447108*

Source: Ars Technica

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