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Past Events of Ocean Acidification

In the last 200 years the acidity of the ocean has increased by 30%, a faster rate than has been recorded in the last 65mya and could be the cause of future extinctions according to the geological record.

The response of calcifying organisms to predicted future ocean acidification effects has been experimented extensively under laboratory conditions. It has been found that impacts are species specific and had a lesser effect on open ocean ecosystems. However species may be able to evolve through genetic change or adapt to shifts in habitat and neither of these responses can be quantified on an experimental timescale. Therefore the best possibility of testing for ecosystem shifts and adaptation potential as well as the scale of recovery to transient warming and ocean acidification is by interpreting the geological record. The records of times with similar climates as well as ecosystem and continent configuration will give the most comparable analogue for today's ocean acidification.

The Pliocene warmth, 3.3 to 3 million years ago,is the most recent time with carbon dioxide levels as high as today's. During the Pliocene warmth had an increase of global warming of 3 degrees global warming over millennia, making it an ideal analogue for future climate. Ecosystems during this time were also very similar to present, where this climate change led to the poleward migration of organisms however no large-scale extinctions were recorded. This allows us to compare that the relative slow changes in carbon dioxide, temperature and pH over time did not permanently alter ecosystems. Today, however, the rapid increase carbon dioxide levels may lead to more permanent effects.

The late Cretaceous, 99.6 to 65.5 million years ago is an example of a period with levels of carbon dioxide as high as the projections for non-mitigation scenarios. Although carbon dioxide was high for millions of years during this period, the coastline of Britain documents vast carbonate deposition by coccolithophores, which can be affected by ocean acidification. However this may not be relevant to today as the ocean during the cretaceous had approximately two times the Ca concentration of today, meaning that low pH and low carbonate ion concentrations could still allow for the super-saturation of the surface ocean. Also gradual increases of carbon dioxide and decreases in pH on a scale over millions of years would have provided enough time for genetic adaptation to these changes. Therefore it may not be a credible analogue for future ocean acidification.

The end Cretaceous meteorite impact, 65.5 million years ago led to rapid environmental change and a mass-extinction and is also linked to ocean acidification. The absorption of carbon dioxide into the ocean lowered surface water pH over a significantly shorter time than our current change. Planktonic ecosystems showed the largest extinction amongst highly specialised tropical and subtropical species while more opportunistic species in higher latitudes were less affected. Benthic deep-sea species and non-calcifying planktonic organisms showed no extinction. These features are very different from predictions for future impacts of ocean acidification and may mean that the impact of environmental change in the future may come down to highly specialised ecosystems versus more tolerant ecosystems.

Palaeocene-Eocene Thermal Maximum, 55.5 million years ago is currently the most accurate analogue for ocean acidification. Sediments deposited during this time characterize transient climatic warming and associated carbonate dissolution due to a massive input of carbon. The biotic response was very different in the deep sea benthic ecosystem than in the planktonic ecosystem. Planktonic organisms reacted to the environmental change with migration to higher latitudes, whilst benthic organisms in the deep sea, mainly foraminifers, became extinct. The extinction is much larger amongst the calcifying benthic foraminifers than those that built shells from organic material, suggesting that the ability to calcify was severely affected. In addition the shells of surviving benthic foraminifers and ostracods (small crustaceans) were thin-walled which suggests low saturation conditions. The planktonic ecosystem did not experience extinction however many planktonic assemblages, showed major compositional changes in response to changing nutrient and temperature conditions. This shows that adaptation to these environmental changes was possible if the organism had a habitat they migrate with. If the rate of change is comparable to future predictions, future extinction of benthic calcifiers is highly likely.

By use of these examples, it allows us to quantify the extent of ocean acidification, linking rates to the degree of ecosystem change and can therefore quantitatively identify the potential threshold of irreversible damage to ecosystems in response to ocean acidification of today.

Dicynodon Illustration courtesy of John Sibbick.
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