Conclusions

So, this blog has tried to explore the issue of ocean acidification. It has been somewhat jumbled and so today I am going to try and bring it all together. 11bn tonnes of carbon dioxide have been absorbed by our oceans in the last 200 years, causing a 30% increase in ocean acidity.  CO₂ dissolves in sea water and forms carbonic acid, this dissociates into HCO₃^- and H⁺, this H⁺ is what’s making our oceans more acidic. I think the most important thing that I have learned from this is that the problem is not that by the end of the century our oceans will be more acidic than they have been in the past 30 million years; it is the rate of the change. This rate is currently 100 times greater than anything we have experienced over the last 650,000 years.  Currently it is expected that by 2100 the pH will have fallen by 0.5 to 7.7.  If oceans acidify slowly, marine species have time to adapt.  At the current rate of change however, species are caught by surprise resulting in many becoming extinct.  The greatest threat is to calcifying organisms due to the shallowing of the aragonite and calcium carbonate saturation horizon, calcification can only occur above this horizon, with dissolution occurring below it. Once corals and other calcium carbonate based species are lost, wide scale extinctions will occur along the food chain.  At the PTB it is thought that the loss of coral and other such marine species was the start of the 5^th mass extinction, this suggests that the losses we are seeing today could cause the initiation of the 6^th mass extinction.  The problem is very real and we can already see the consequences in many of the world’s coral reefs, namely the Great Barrier Reef, and it is believed that all of the world’s reefs will have been lost within 40 years.  I thought it was interesting that even large marine mammals are being affected, and in ways that you would not immediately think of, for example clown fish are finding it much harder to smell the specific anemone that they use as a habitat due to the acidic waters.  The increase in size of lobsters in acidic conditions shows that some species can thrive in these conditions; however they may still be at threat due to changes in the food web.  Boron isotopes work by looking at the ratio of borate ion to boric acid.  The levels of which are affected by ocean pH as borate ion is preferentially incorporated into marine carbonates.  They have been quite successful in helping to reconstruct past pH but in the future further work needs to be done in order to reduce the uncertainty surrounding the isotope fractionation index and to improve the analytical techniques that are used. Towards the end of the blog I have briefly considered if there is any ‘quick-fix’ to improve the health of our oceans, if we are to stop our oceans from becoming undersaturated in aragonite the ocean pH mustn’t fall by more than 0.2 (relative to preindustrial levels).  Iron fertilisation is one of the main theories that has been proposed to combat ocean acidification and it is thought that by dumping large amounts of iron into HNLC areas of the ocean the productivity of phytoplankton will be greatly increased, resulting in algal blooms which will draw large amounts of carbon dioxide out of the atmosphere.  There is much uncertainty surrounding the potential success of iron fertilisation, it is thought that the drop in CO₂ is only found in the surface waters and many studies suggest that only a 10ppm reduction in CO₂ would occur.  There are also possible negative ecological effects that iron fertilisation could have and microbial shifts could cause an increase in other, more harmful, greenhouse gases (For example, methane and DMS).

If I had to try and sum up this blog, the 4 key findings would be:

-          Ocean acidification is a real problem that can’t be ignored, the effects on calcifying organisms is evident and the 30% increase in ocean acidity since preindustrial times is worrying, to say the least.

-          The use of boron isotopes as a proxy needs to be improved further (in terms of the fractionation index and analytical techniques) to increase our understanding of past ocean acidification in order to allow us to see how the current situation varies from past natural variability.

-          Calcifying organisms are most at threat; however some species can thrive in acidic conditions.  More work needs be done to see how the food chain will be affected.

-          Iron fertilisation and other combating schemes need to be improved or drastic cuts in atmospheric carbon dioxide levels need to occur in order to try and slow the rate of change. The unsuccessful Durban climate convention suggests that the latter is unlikely in the near future.

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.