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.