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.

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.

What is ocean acidification?

Changes in Ocean Chemistry

Increased levels of atmospheric CO₂ is having two serious impacts on the earth, the first is the well known phenomenon of global warming and the second is the subject of this blog- ocean acidification.  For over 650 000 years prior to the industrial revolution the concentration of CO₂ in the atmosphere was somewhere between 180 and 300ppmv (Siegenthaler et al. 2005).  Due to an increase in anthropogenic emissions, mainly through fossil fuel burning, the atmospheric carbon dioxide levels are now between 380 and 390ppmv resulting in a rate of change that is almost 100 times faster than anything the earth has experienced over the last 650kyr (IPCC, 2007).

Over the last 200 years the oceans have absorbed approximately 1/3 (11bn tonnes) of all the carbon dioxide that has been released into the atmosphere (Sabine et al. 2004). Dissolved organic carbon occurs in the ocean in 3 different forms- bicarbonate ions, aqueous carbon dioxide (including carbonic acid) and carbonate ions (Fabry et al. 2008).  The current pH of the ocean is 8.2 and 88% of the carbon occurs as bicarbonate ions.  Carbon dioxide dissolves in the sea water and forms carbonic acid which quickly dissociates to form H⁺ and HCO₃^- . The H⁺ may react with CO₃^2- to form bicarbonate.   This means that by adding CO₂ to the ocean we are increasing the amount of H₂CO₃, H⁺ and HCO₃^- and as an increase in the concentration of H⁺ ions causes pH to fall we are making the oceans more and more acidic (Ibid.).     Since the pre industrial times the ocean pH has fallen by 0.1, which has caused a 30% increase in acidity (Boyd, 2011).  Over the next century it is believed that the pH will fall by approximately 0.5 units, making the oceans more acidic than they have been over the past 30 million years (Calderia and Wickett, 2003). If the change in CO₂ is gradual a buffer system operates and so, interactions with carbonate minerals increase which helps to reduce the sensitivity of the ocean and stabilises the pH (Caldeira and Wickett, 2003).  If the change in CO₂ occurs over a short timescale (less than 10⁴ years) the oceans don’t have time to shift their equilibrium and so, the pH is altered.    The change in pH is most prominent towards the poles (i.e the North Atlantic and Southern Ocean) as more carbon dioxide is taken up at colder temperatures.

References

Boyd, P. (2011). ‘Beyond ocean acidification’, Nature Geoscience, 4: 273-274.

Calderia, K. and Wickett, M. (2003). ‘Anthropogenic carbon and ocean pH’, Nature, 425: 365.

Fabry, V. Seibel, B. Feely, R. Orr, J. (2008). ‘Impacts of ocean acidification on marine fauna and ecosystem processes’, Journal of marine science, 65, 3: 414-432

Sabine, C. Feely, R. Gruber, N. Key, R. Lee, K. Bullister, L. Wanninkhof, R. Wong, C. Wallace, D. Tilbrook, B. Millero, F. (2004). ‘The oceanic sink for anthropogenic CO₂’, Science, 305, 5682: 367-371.

Siegenthaler, U. Stocker, T. Monnin, E. Luethi, D. Schwander, J. Stauffer, B. Raynaud, D. (2005). ‘Stable carbon cycle-climate relationship during the late Pleistocene’, Science, 310: 1313– 1317.