Ocean acidification: Anthropocene versus Eocene.

Ammonia beccarii, Benthic foraminifera. From Wikimedia Commons

Last week a report from the International Programme on the State of the Ocean (IPSO) declared that ocean acidification has reached an unprecedented level in Earth’s history. Since the Industrial Revolution, the anthropogenic release of CO2 into the Earth’s atmosphere has increased a 40%.
Over the period from 1750 to 2000, the oceans have absorbed approximately one-third of the CO2 emitted by humans. The cost of this is the decrease in surface ocean pH, that cause dramatic effects on marine life.

When CO2 dissolves in seawater, it produce carbonic acid. The carbonic acid dissociates in the water releasing hydrogen ions and bicarbonate. The formation of bicarbonate then removes carbonate ions from the water, making them less available for use by organisms.

Other consequences of an increasingly acidic ocean include effects on metal speciation, reduced NH3/NH4+ ratios and alteration of underwater sound absorption.

Geological context for ocean acidification. (A) Candidate ocean acidification events. (B) Ocean surface pH calculated at 20 million year intervals. (C) Major changes in plankton assemblages. From Kump, 2009.

The geological record of ocean acidification may provide valuable insights for the future of Earth’s climate and how marine organisms could adapt to severe conditions.

The closest analog for today conditions is the Palaeocene–Eocene Thermal Maximum (PETM, approx. 56Ma), meaning greater similarities in continental configuration, ecosystem structure and function, and global carbon cycling.

The PETM was a short-lived (~ 200,000 years) global warming event when temperatures increased by 5-9°C. It was marked by the largest deep-sea mass extinction among calcareous benthic foraminifera in the last 93 million years. Similarly, planktonic foraminifer communities at low and high latitudes show reductions in diversity.

Nannofossil abundance changes during the PETM. From Kump, 2009.

The PETM is also associated with dramatic changes among the calcareous plankton,characterized by the appearance of transient nanoplankton taxa of heavily calcified forms of Rhomboaster spp., Discoaster araneus, and D. anartios as well as Coccolithus bownii, a more delicate form.

Because the PETM is considered the best analog to modern global warming, the changes in the assemblage of calcareous nanoplankton during this event could provide vital clues to the potential response of modern nanoplankton to ocean acidification.

Not only the magnitude but also the time scale of the carbon input is critical for its effect on ocean carbonate chemistry. The time scale of the anthropogenic carbon input is so short that the natural capacity of the surface reservoirs to absorb carbon is overwhelmed.

Comparison of the effects of anthropogenic emissions (total of 5000 Pg C over 500 years) and PETM carbon release (3000 Pg C over 6 kyr) on the surface ocean saturation state of calcite. From Zeebe, 2013

So, the anthropogenic carbon input rate is probably greater than during the PETM, causing a more severe decline in ocean pH and saturation state. Also the biotic consequences of the PETM were fairly minor, while the current rate of species extinction is already 100–1000 times higher than would be considered natural. This underlines the urgency for immediate action on global carbon emission reductions.

Trevor Manuel, a South African government minister and co-chair of the Global Ocean Commission stated that “Governments must respond as urgently as they do to national security threats – in the long run, the impacts are just as important”.

References:

Zeebe RE and Zachos JC. 2013 Long-term legacy ofmassive carbon input to the Earth system: Anthropocene versus Eocene. Phil Trans R Soc A 371: 20120006. http://dx.doi.org/10.1098/rsta.2012.0006.

Kump, L.R., T.J. Bralower, and A. Ridgwell. 2009. Ocean acidification in deep time. Oceanography 22(4):94–107, http://dx.doi.org/10.5670/oceanog.2009.100