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.

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.

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.

Natural trend or Anthropogenic change?

This blog post will explore some of the past changes in ocean pH to try and evaluate whether the current change is part of a natural trend or solely anthropogenic.  It is, however, hard to compare current CO₂ induced changes with those of the distant past because of continental drift, changes in the elevation and orientation of the continents and the location of mountains.  All of which alter the oceanic and atmospheric circulations (Goodwin et al. 2009). 

It is widely believed that the ocean is currently more acidic than it has been at anytime over the last 2my and δ¹¹B levels show that the predicted pH for 2100 has not been experienced since about 40mya (Pelejero et al. 2010).  Around 55mya, during the Paleocene-Eocene thermal maximum, there was a significant ocean acidification event.  There was a huge carbon input into the ocean, causing a 2km shift of the calcite saturation horizon towards the surface in less than 2ky (Doney et al. 2009).  The effects of which were catastrophic.  For a full review there is an excellent article by Zachos et al. (2008).  There were two epochs during the Cenezoic era which are thought to have seen extensive ocean acidification however, δ¹¹B levels have not yet been able to quantify this.

During the Cretaceous CO₂ levels were extremely high (up to 2000ppm) and it is believed that 100mya ocean pH was around 0.8 units lower than present (Kump et al. 2009).  Diatoms survived the event but calcified species were generally wiped out (Pelejero et al. 2010).  The early Aptian Oceanic Anoxic event happened around 120mya, a series of major eruptions along the Java plateau were thought to release enough CO₂ to cause a 2km shoaling of the calcite saturation zone.  This coincided with the demise of the nanoconids which were one of the most dominant species of the time and were heavily calcified (Kump et al. 2009).

Carbon isotopes suggest that there was a substantial decrease in ocean pH at the Triassic-Jurassic boundary (Hautmann et al. 2008).   Extensive volcanism was thought to release large amounts of CO₂, resulting in a temperature increase which initiated the release of further CO₂ from marine sediments (Palfy, 2003).  The CO₂ levels were high enough to inhibit the precipitation of CaCO₃ resulting in a subsaturated and acidified ocean (Hautmann et al. 2008).  Calacerous phytoplankton suffered very badly, whereas phytoplankton with organic walls actually benefitted from the acidic conditions (Van de Schootbrugge et al. 2007). 

A major ocean acidification event was thought to have occurred during the “super-greenhouse” state which followed the late Neoproterozoic and models have estimated that the pH could have fallen below 6.0 (Le Hir et al. 2008).  However, it is thought that this acidification occurred gradually, over a period of up to 30 my (Ibid.).  Calcifying organisms had not evolved during this time so it is hard to tell the impacts the acidification had on ocean biology and also how long the acidification lasted (Kump et al. 2009).

Several studies have looked back over the last 500my and reinforce the fact that ocean acidification played a major role in 5 mass extinctions, suggesting that the current ocean acidification event could contribute to the 6^th mass extinction.  It is clear that ocean pH has varied hugely in the past, with some acidification events being far more extreme than what we are currently experiencing.  This could suggest that the current acidification is part of a long term trend; however the rate of change suggests that humans are playing a significant role.

References:

Doney, S. Fabry, V. Feely, R. Kleypas, J. (2009). ‘Ocean Acidification: the other CO₂ problem’, Annu. Rev. Marine. Sci., 1, 168-192.

Goodwin, P. Williams, R. Ridgwell, A. Follows, M. (2009). ‘Climate sensitivity to thecarbon cycle modulated by past and future changesin ocean chemistry’, Nature Geoscience, 2,145–150.

Hautmann, M. Benton, M. Tomasovych, A. (2008) ‘ Catastrophic ocean acidification at the Triassic-Jurassic boundary.’, Neues Jahrbuch Geol. Palaontol. Abhand.,  249,  119–127.

Le Hir, G. Ramstein, G. Donnadieu, Y. Goddéris, Y. (2008). ‘Scenario for the evolution of atmospheric pCO2 during a snowball Earth’, Geology, 36,47–50.

Kump, L. Bralower, T. Ridgwell, A. (2009) ‘Ocean acidification in deep time’, Oceanography, 22, 4, 94- 107.

Palfy, J. (2003). ‘Volcanism of the Central Atlantic MagmaticProvince as a potential driving force in the end-Triassic extinction’, Geophysical Monograph, 136:255-267.

Pelejero, C. Calvo, E. Hoegh-Guldberg, O. (2010). ‘Paleo-perspectives on ocean acidification’, Trends in ecology and evolution, 25, 6, 332-344.

van de Schootbrugge, B. Tremolada, F. Rosenthal, Y. Bailey, T. (2007) ‘End-Triassic calcification crisis and blooms of organic-walled ‘disaster species’’, Palaeogeogr. Palaeoclimatol. Palaeoecol.,  244, 126–141.

Zachos J. Dickens G. Zeebe R. (2008) ‘An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics’, Nature, 451, 279–283.