Did ocean acidification cause the world's largest mass extinction? (Permian-triassic boundary)

This week I am going to look back at the biggest extinction event in the history of the earth and see if ocean acidification played a significant role.  If it did, it intensifies the need for a rapid response to the current crisis.  The PTB occurred 251.4myaBP and saw the loss of over 90% of all marine species, it was also the only extinction event which caused the widespread loss of insects (Bowring et al. 1998, Erwiin, 1994).  Volcanic eruptions and ocean anoxia were both dominant during the PTB and both have been put forward as a possible cause.  Montenegro et al. (2011) have recently created a model to try and assess the role of ocean acidification in the PTB mass extinctions.  The article deduces that the high levels of carbon dioxide during the PTB are enough to make the oceans acidic and undersaturated in aragonite.  This is thought to have been biologically significant and caused widespread coral loss.  More work needs to be done in order to further assess the role of ocean acidification in the PTB mass extinction as the Monetnergo et al. (2011) model is the first to include an accurate portrayal of ocean circulation and seafloor bathymetry.  As ocean acidification is likely to have played a key role in the greatest mass extinction ever, it could suggest that we are on the brink of a 6^th mass extinction, the effects of which could be catastrophic. 

Bowring, S. Erwin, D. Jin, Y. Martin, M. Davidek, K. Wang, W. (1998). ‘U/pb zircon geochronology and tempo of the end-Permian mass extinction’, Science, 280, 1039–1045.

Erwin, D. (1994). ‘The Permo-Triassic extinction’, Nature, 367, 231–235

Monetnergo, A. Spence, P. Meissner, K. Eby, M. (2011). ‘climate simulations of the Permian-traissic boundary: ocean acidification and the extinction event’, Paeleoceanography, 26, 3.

The future of our oceans

Ocean pH is expected to drop by about 0.4 by the end of the century resulting in a 60% fall in calcium carbonate levels.  To prevent our oceans becoming undersaturated in aragonite it is believed that we shouldn’t let the pH drop by any more than 0.2 units in comparison with the pre industrial levels.  The change in ocean pH is almost entirely dependent on atmospheric CO_s levels, if they continue to increase, the oceans will continue to acidify.  It will be hard to reverse the acidifying trend by the end of the century as ocean chemistry works on much much larger timescales (Raven et al. 2005).  The figure below shows that with all scenarios of atmospheric CO₂ levels ocean pH falls, if the projected CO₂ levels reach around 950ppm then the ocean pH will fall to about 7.75 and the southern ocean will only be about 70% saturated in aragonite.  If the pH falls this low, it will result in a tripling in the number of H⁺ ions since the industrial revolution.  This rate of change is faster than anything seen over the last few hundred thousand years and at least 100 times higher than anything seen since then (Raven et al. 2005). 

 Figure 1.  Atmospheric CO₂ concentrations, global ocean pH and the surface saturation state of aragonite for IPCC emission scenarios for 2000-2100. (IPCC, 2007).

References:

IPCC (2007). ‘Climate change 2007: the physical science basis.

Raven , J. Caldeira, K. ELderfield, H. Hoehg-Guldberg, O. Liss, P. Riebesell, U. Shepherd, J. Turley, C. Watson, A. (2005). ‘Ocean acidification due to increasing atmospheric carbon dioxide’, The Royal Society: London. 

http://www.guardian.co.uk/environment/interactive/2009/dec/10/acidification-oceans-copenhagen?INTCMP=SRCH

This interactive link nicely shows how aragonite concentrations have changed globally and also how this is effecting certain marine species.  The dark orange colours in the Arctic are quite scary as it shows just how much aragonite is thought to have been lost between 1865 and 2095, with some areas seeing a decrease of up to 75%.  I think the impact that ocean acidification is having on Whales and Clown fish is quite surprising, and unlike most other marine animals it is not caused by a reduction in aragonite and calcium carbonate.  Acidic oceans are more 'noisy' as sound can travel further, this is what is causing Whale numbers to fall as they cant communicate or navigate as easily.  The problem for clown fish is that the acidifying oceans are affecting their ability to smell the particular species of anemone that they use as a habitat.  If the pH of oceans continues to fall it is likely that they (and other fish species) will lose their sense of smell altogether.  This proves that ocean acidification is having a more devestating impact on ocean ecosystems than you would initially think.

How effective are boron isotopes in reconstructing past ocean pH?

Pagani et al. (2005) deduced that the boron isotope technique needs to be greatly improved in order to accurately reconstruct long term changes in ocean pH.   The boron isotope pH model assumes that the boron isotopic composition of carbonates is the same as that of the boron isotopic concentration borate in solution.   An accurate and precise isotopic fractionation index is required in order to see how pH affects the boron isotopic composition of borate.  There is much uncertainty surrounding the value used for the isotope fractionation index, theoretically the value should be 0.981 but research suggests that a value of 0.974 should be used instead (Kakihana et al. 1977).  Small changes in this isotopic fractionation index cause large changes in the resulting pH and so, future work needs to focus on determining the correct value so that more accurate and realistic ocean pH’s can be reconstructed.  Kasemann et al. (2009) believe that even when a more robust fractionation index is used there are still some reconstructions of cenezoic ocean pH which are too high.  This could be caused by the assumptions of the foraminiferal vital effects in dissolved organic carbon.  It has also been proposed that there is much difficulty in determining the boron isotope compositions of materials and many different values can be obtained from the same sample depending on the analytical techniques that have been used.  Respiration and photosynthesis are thought to alter the pH in the calcifying part of marine organisms which can alter the boron isotopic composition of the carbonate, resulting in a reconstructed pH value which is not a true representation of the ocean pH at the time (Rink et al. 1998).  It is also believed that post depositional dissolution can alter the boron isotope composition, resulting in inaccurate reconstructions (Kasemann et al. 2009).  Despite these problems, Boron isotopes are still a very effective method for reconstructing ocean acidity but improvements are required in order to improve the accuracy of results, especially over long timescales.

References

Kakihana, M. Kotaka, S. Satoh, M. Nomura, M. Okamoto, M. (1977). ‘Fundamental studies on the ion-exchange separation of boron isotopes’, Bull. Chem. Soc. J., 50, 158-163.

Kasemann, S. Schmidt, D. Bijma, J. Foster, G. (2009). ‘In situ boron isotope analysis in marine carbonates and its application for foraminifera and palaeo-pH’, Chemical Geology, 260, 1-2, 138-147.

Pagani, M. Lemarchand, D. Spivack, A. Gaillardet, J. (2005). ‘A critical evaluation of the boron isotope-pH proxy: the accuracy of ancient pH estimates’, Geochimica et Cosmochimica Acta, 69, 4, 953-961.

Rink, S.  Kuhl, M. Bijma, J. Spero, H. (1998). ‘Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa’, Marine Biology, 131, 4, 583-595.

Recontructing ocean pH using Coral

δ¹¹B levels preserved in coral skeletons can be used to look at recent changes in ocean pH.  Boron isotopes occur as either Borate ion or Boric acid, the levels of which are affected by ocean pH (Wei et al. 2009).  Borate ions are exclusively incorporated into marine carbonate, whilst Boric acid is not involved.  This means that the relative abundance of Borate ion is controlled by the pH of the seawater at the time of calcification (Vengosh et al. 1991).  Wei et al. (2009) looked at 200 years worth of δ¹¹B data (with a precision of +- 0.02 pH) and deduced that between 1940 and 1998 the Great Barrier Reef saw multiple fluctuations in δ¹¹B which correspond to changes of 0.5pH.

References

Vangosh, A. Starinsky, A. Chivas, A. McCulloch, M. (1991). ‘coprecipitation and isotopic fractionation of boron in modern biogenic carbonates’, Geochim. Cosmochim. Acta, 55, 2901-2910.

Wei, G. McCulloch, M. Mortimer, G. Deng, W. Xie, L. (2009).  ‘Evidence for ocean acidification in the Great Barrier Reef of Australia’, Geochim. Chosmochim. Acta. 73, 2332-2346.