The ocean's acid test is not a test at all? Some personal experience sharing

Last Thursday, I went scuba diving for the first time ever in my life at the Fish Eye Marine Park in Guam. Guam is an island located in the western Pacific Ocean. Scuba diving is a truely amazing experience and I strongly urge you to try if you haven't done it before! Having submerged under the sea for about 45 minutes, I saw plentiful of beautiful tropical fishes and different kinds of spectacular coral reefs, felt like swimming with the fish in those aquariums!

After this scuba diving experience, I was thinking to myself maybe ocean acidification is not actually that bad. Or it hasn't had that much of an impact on those marine life yet, not for those in Guam at least! I am now more convinced that ocean acidification is a highly localised problem. As mentioned in my previous posts, ocean acidity varies with latitude and time. There are still plenty of precious corals and fish out there in the world's oceans! Perhaps we should stay a bit more optimistic towards our oceans future but be cautious at the same time to protect these beautiful sea life.

The Ocean’s Acid Test might just be another scary story?

It seems that most of the scientific literatures and media reports I’ve been looking at so far in this blog share the common believe and even try to persuade us that Ocean Acidification is going to be another global environmental disaster by the end of this century, caused by anthropogenic CO₂ increase. Have you been wondering if this is actually true or not? How reliable are these scientific experiments and findings? Or, they are just another ‘scary story’ or ‘horror film’ portrayed by the scientists and media, like climate change after all?

In this post, I’m not trying to dispute the scientific evidences and media reports that I’ve reviewed in this blog. Instead, I wish to provide a holistic view of this ocean acidification phenomenon and avoid leading you to any particular views e.g. ‘ocean acidification is detrimental’.

The first critique that I want to look into is a report produced by the Science and Public Policy Institute (SPPI), published on the 5^th January 2011. It mainly challenges the video ‘Acid Test:  The Global Challenge of Ocean Acidification’ produced by the National Resources Defense Council (NRDC) as a new propaganda film, which I’ve posted in my first entry. Through reviewing over 100 scientific literatures, the SPPI criticizes strongly of the NRDC documentary ‘reveals nothing of substance’ (Knappenberger, 2010), which has provided no empirical evidence of this ocean acidification catastrophic theory proposed by most of the scientists. They further argued that ocean acidification is impossible to cause catastrophic disruption to the world’s marine ecosystems; in the past geologic era, levels of CO₂ are 20 times higher than today has promoted coral development (SPPI, 2010). Ridgwell has further supported this argument that ‘ocean pH in the past was indeed lower than now during the Cretaceous, and probably lower than anything we will manage in the future’ (Ridley, 2011). In addition, ocean acidification induced by CO2 increase is actually beneficial to most marine calcifiers, especially coccolithophores and coral reef building (SPPI, 2010).

Matt Ridley, who is also an ‘ocean acidification critic’ made similar arguments of this ocean’s acid test challenge. In the article ‘The Threat of Ocean Acidification is greatly exaggerated’, Ridley responds to his critique on his book ‘The Rational Optimist’ in the New Scientist. In his book, he strongly emphasized that ‘ocean acidification may not be the widespread problem conjured into the 21^st Century’, which is completely opposite to projections made by Orr et al. (2005) and Hoegh-Guldberg et al. (2007) that I have previously reviewed in this blog. Ridley has also made the following main arguments:

1.   Ocean pH is not actually turning acidic, but less basic only. The current average ocean pH is about 8.1, which is alkaline and well above neutral (pH=7).

2.   Even ocean pH do experience significant temporal (daily) and spatial variation, some marine organisms, e.g. a four-decade-old-mussels learned to cope with this acidity fluctuation.

3.  There is no evidence of coral danger ‘may be extreme’ e.g. coral bleaching depends more on the rate of change than the absolute temperature (even his critic Hoegh-Guldberg accepts this point). It is therefore even more unconvincing to assume that a 0.1 unit of pH drop since the Industrial Revolution will cause a catastrophic effect on marine ecosystem (SPPI, 2010)

4.  Most literatures disregard the fact that the corals are able to recover quickly from episodic bleaching. In fact, corals are now more resilient and adaptive to thermal stresses.

5.  Ocean acidification increases bicarbonate ion concentration which promotes biological precipitation of carbonate ions by marine organisms (calcification) e.g. some coccolithophores deposit carbonate shells easier at slightly lower pH

Both the SPPI and Matt Ridley’s skepticism of the ocean’s acid test are logical and convincing. However, as Ridley said in his critique, views and opinion about the ocean acidification phenomenon depends on different interpretation of the scientific evidences. In other words, it also depends on the assumptions made about this phenomenon. And of course, assumptions differ among different scientists and many other literature authors, so assumptions can always be challenged and changed constantly. Although both critics argued that there is currently no empirical evidence to support the dominant ocean acidification theory, none of them were able to provide solid evidence and projections on the impacts of ocean acidification either. Since the ocean is an open system and constantly subject to climatic and other environmental changes, there are still significant levels of uncertainty in both the optimistic and pessimistic views about the oceans. Therefore, it is very hard to draw a conclusion of whether ocean acidification is beneficial, detrimental, or simply having no effect on the oceans.

However, in the environmental justice point of view, anthropogenic impacts on the world’s oceans should never be ignored. Because of the definite high uncertainties of the oceans future, the oceans should therefore be even more closely monitored to produce more certain projections, thus leading us to make sensible decisions about the future. 

A summary of the Acid Test so far

Today, I found this fairly recent video about the current scientific work and concerns of the ocean acidification produced by Cassandra Brooks from the Pacific Coast Science and Learning Centre. It summarizes what I've been posting in my blog entries so far very nicely, analysing the ocean acidification problem from the past to the present. Towards the end of this video, some interesting questions were posted in regards to the future of our oceans which I found it quite thought-provoking. For instance, the latest hatchery and oyster growing strategies of pumping seawater only during times with low CO2, but these strategies might be rather short term. Or, maybe ocean acidification is not as bad as it sounds!As said in this video, marine organisms might be able to adapt the acidifying ocean. But again, we don't know whether they will be able to based on our current scientific knowledge. The situation is simply too uncertain.

Indeed, there are many ocean acidification critics out there who simply think that all these are all exaggerated. So, before drawing any conclusions, I would like to explore critiques of this ocean acidification phenomenon in the following week. Let's see who to believe!

Iron Fertilisation: A way out of the Acid Test?

Having seen so much destructive impacts of the ocean’s acid test so far, is there a way to mitigate the situation? The answer is YES, there is. One of the most well-known geoengineering solution to mitigate ocean acidification is Ocean iron fertilisation (OIF), which is originated from the ‘Iron hypothesis’ proposed by an oceanographer John Martin in 1990.

How does ocean iron fertilisation work?

Martin’s Iron hypothesis is based on his research on the last glacial-interglacial CO₂ changes. He argued that the high concentrations of atmospheric CO₂ during the last and Holocene interglacial, i.e. 280ppm pre-industrial level was due to the high deficiency of the element Iron in the Southern Ocean, hence reducing the potential CO₂ uptake by phytoplankton in the surface ocean via the ‘biological pump’.  In contrast, during the Last Glacial Maximum (LGM), atmospheric dust iron supplies were 50 times higher than that of the last interglacial period. This iron enrichment had enhanced phytoplankton growth, which had utilised large amount of upwelled macronutrients to stimulate the overall productivity in the Southern Ocean, leading to the total decrease in CO₂ during the LGM.

The current geoengineering technology, ocean iron fertilisation is therefore based on the principle of using iron to enhance the productivity of the ocean, so as to increase the anthropogenic CO₂ sequestration in the ocean interior. In order to investigate the effects, efficiency and feasibility of OIF, a number of small-scale field OIF experiments and modeling studies have been conducted in different high-nutrient, low-chlorophyll (NHLC) regions across the world’s oceans, including the large parts of the Southern Ocean, the eastern equatorial Pacific and part of the North Pacific (Cao and Caldeira, 2010).

Figure 1 Effects of Ocean Iron Fertilisation 

Figure 1 illustrates one of those OIF field experiments employed in the northeast Pacific Ocean. Red and yellow colour represents regions with high concentration of chlorophyll a, thus a high biomass of phytoplankton and vice versa for blue colour. Area (a) has not been enriched with iron while area (b) has. It is clearly shown that with iron fertilisation, biotic community in the ocean has shifted from cyanobacteria-dominated to diatom-based i.e. larger phytoplankton productivity (Armbrust, 2009).   

How effective and feasible is OIF?

The effectiveness and feasibility of this OIF technology is largely questioned by the scientific community. For instance, the above OIF experiment at NE Pacific Ocean has only generated a slight increase in organic carbon storage in deep water despite of the expected enhanced phytoplankton bloom, with most of the organic carbon consumed and recycled in the surface ocean (Armbrust, 2009). Modeling studies produced similar conclusions and projections with the use of OIF. According to the model simulations by Cao and Caldeira (2010), a globally sustained OIF could not diminish atmospheric CO₂ concentrations to below 833ppm or reduce the mean surface ocean pH change to less than 0.38 units; compared to 965ppm with 0.44 units reduction in pH under the IPCC A2 emission scenario (Figure 2). Ironically, it has been reported that iron fertilisation cannot even mitigate ocean acidification but acidify the deep ocean further. As OIF stimulates carbon sequestration at the deep ocean, it is very likely that deep ocean pH will be further reduced with this addition of carbon. This can pose undesirable acidic environment for marine organisms in the deep ocean to survive, although its extent and impacts are still not very well known.

Figure 2 Minor effects of OIF in ocean acidification mitigation

The associated environmental risks are even more alarming to scientists, especially the impacts on marine ecosystems. Silver et al. (2010) reported that high levels of amino acid neurotoxin domoic acid (DA) have been produced historically through the iron enrichment experiments. The high levels of DA have been released by the increase in a diatom genus Pseudo-nitzschia. Pseudo-nitzschia spp. are present in coastal harmful algal blooms (HABs) worldwide and are recognised as contaminants to a wide range of fauna – from invertebrates to marine birds and animals (Silver et al., 2010).

In addition, Denman (2010) have highlighted the possible side effects associated with this OIF technology. He suggested that the increase in remineralisation triggered by iron fertilisation results in increased denitrification and the production of nitrogen dioxide (N₂O), the third most anthropogenic greenhouse gases. Furthermore, models have projected a decrease in dissolved oxygen or even anoxic in wide areas of subsurface ocean caused by increased remineralisation during OIF, making marine organisms, especially calcifiers harder to adapt to the changed environment. This increase in remineralisation process also reduces available macronutrients returning to the surface ocean, which causes a reduction in productivity i.e. biological carbon exports and the complexity of marine ecosystems. As illustrated above, OIF tends to favour the growth of diatoms, which eventually dominates that of all other phytoplankton groups, e.g. one of the dimothylsulfonioproprionate (DMSP)-producing phytoplankton species. Several OIF field experiments have shown a short-term increase in surface ocean DMS concentrations, followed by a decrease relative to the concentration outside the experimental area. Dimethylsulfide (DMS) is an important gas to stimulate the formation of cloud condensation nuclei. Therefore, a reduction in DMSP-producing phytoplankton might cause undesirable climatic impacts.

Despite of all the above negative consequences and concerns of OIF, it might still be considered as a viable mitigation measure in the long run. A striking research produced by Blain et al. (2007) have concluded that the efficiency of natural iron fertilisation was at least ten times higher than previous estimates from short-term phytoplankton blooms induced by the short-term iron-addition experiments. However, it has been stressed that the ocean natural system is very sensitive to iron, in which the addition of iron e.g. from dust  has to be occurred slowly and continuously as supposed to adding in large amounts purposely in order to achieve effective carbon sequestration.  The effectiveness of OIF is also highly constrained by the availability of macronutrients in local and surrounding water to support remineralisation.

To conclude, in a very large extent, ocean iron fertilisation is not a viable approach in mitigating ocean acidification; it is both economically and environmentally unsound. Deep ocean carbon sequestration using OIF technology not only causes undesirable environmental consequences, it also acts as a catalyst to encourage more anthropogenic CO₂ emissions in the form of carbon credits (Cao and Calderia, 2010). How would you weigh up the benefits of removing 32ppm of CO₂ and reducing a maximum of 0.06 unit of ocean pH in 100 years with the costs of potential marine ecosystems lost and increased N₂O production? I believe that the answer is very apparent here. 

The UN Climate Talk agreement: Can the oceans be relieved from the acid test challenge?

Followed by the failures of the Kyoto Protocol to call upon a legally-binding agreement for countries to reduce carbon emissions, the UN Climate Talk this year finally ends with some results today. You can click here for the Guardian news link. 

Seventeenth session of the Conference of the Parties (COP 17) has been taken place since the 28^th November at Durban, South Africa and finally comes to an end two days ago, with a commitment by all countries to accept binding emission cuts by 2020. A new climate fund is also agreed to be set up, carbon markets will be expanded and countries will be able to earn money by protecting forests. Hopefully, this deal will be able to force countries to cut their emissions enough to stay under a 2^oC temperature increase, the threshold of ‘dangerous climate change’.

In no doubt, this climate talk is a great step forward towards a legally-binding joint action of all countries to cut their carbon emissions. By adopting a lower IPCC emission scenario, it might increase levels of certainty in future climate projections. Hopefully, the oceans will be able to absorb and buffer upon this lower and constant anthropogenic CO2 without dramatic changes in pH and marine ecosystems. Nevertheless, neutralisation and CO₂ buffering in the ocean is a very long process, in a timescale of hundreds and thousands of years. The effects of this climate deal are therefore yet to be seen.  

Model the ocean’s future

Previously, we’ve looked at how the Paleocene-Eocene Thermal Maximum provides us an analogue to the future of our oceans, which projects a similar or even worse marine mass extinction in the future. This week, I’m going to explore what do climate and oceanic models say about our ocean’s destiny in the next 100 years.

Ocean-carbon cycle model projections

Orr et al. (2005) have used 13 coupled ocean-carbon cycle models to simulate changes in surface ocean pH, carbonate ion concentration [CO₃^2-] and calcium carbonate saturation under the Intergovernmental Panel on Climate Change (IPCC) IS92a ‘business-as-usual’ scenario over the 21^st century, which are adopted by the IPCC 4^th Assessment Report (IPCC AR4) Chapter 10.4.1-2. Their multi-model projections indicated large decrease in oceanic pH and carbonate ion concentrations across the world’s ocean during the 21^st Century, driven by direct geochemical effect of increasing anthropogenic CO₂ emissions only (Figure 1).  Currently, surface ocean pH is already 0.1 unit lower than pre-industrial values. It is further projected for another 0.3-0.4 units decrease by the end of this century under the IPCC IS92a scenario, which is equivalent to a 100-150% increase in [H⁺] (Orr et al., 2005). Their results also highlighted similar latitudinal variability pattern; surface water pH and [CO₃^2-] are higher in high latitudes and decrease significantly towards mid and low latitudes. However, according to their model projections, latitudinal differences in [CO₃^2-] between mid and high latitudes will be reduced towards 2100 under the IS92a scenario (Figure 1c). Carbonate ion concentrations i.e. aragonite saturation state will begin to decline and the undersaturation of aragonite is projected to extent throughout the entire Southern Ocean and into the subarctic Pacific Ocean by 2100 (Figure 2).

Figure 1 Projected (a) atmospheric CO2, (b) ocean surface pH and (c) [CO₃^2-] during 21st Century (Orr et al., 2005)



Figure 2 Projected levels of aragonite saturation (%) over the 21st Century (IPCC, 2007)

The authors also found that this aragonite undersaturation in the Southern Ocean could threaten high-latitude ecosystems within decades, not centuries as previous studies suggested; it is likely to be first detected during winter. A more recent paper by McNeil and Matear (2008) further estimated that this Southern Ocean wintertime aragonite undersaturation will occur by the year 2030 and no later than 2038 with atmospheric CO2 of ~450ppmv under the IPCC IS92a scenario.

Coupled atmosphere ocean-climate model projections

In order to simulate the effects of climate variability and climate change (changes in temperature and ocean circulation etc.) on ocean carbonate ion concentrations on top of the direct geochemical effects, Orr et al. (2005) have also analysed three coupled atmosphere ocean-climate models. Surprisingly, their analysis suggested that physical climate change alone will not alter high-latitude surface [CO₃^2-] substantially during the 21^st Century. This argument is also reaffirmed by the coupled climate/ocean-carbon cycle model results generated by Cao and Caldeira (2008). It is largely due to the air-sea CO2 exchange mostly compensates for changes in surface dissolved inorganic carbon (DIC) caused by changes in marine productivity and circulation.

However, high-latitude subsurface [CO₃^2-] is projected to be decreased significantly during the 21^st Century based on all the three coupled model results, with small uncertainties in changes in temperature, ocean stratification and marine biological production and re-mineralization (IPCC, 2007; Orr et al., 2005). Orr et al. (2005) further suggested that the biggest uncertainty comes from atmospheric CO2 trajectories, which is the only means to limit further decline in ocean [CO₃^2-].  Figure 3a showed the atmospheric CO₂ projections under the six IPCC emission scenarios, which exhibits high levels of uncertainty, especially towards Year 2100. As surface ocean [CO₃^2-] is highly dependent upon atmospheric CO₂, high levels of uncertainty in atmospheric CO₂ trajectories have a significant impact on predicting global pH changes as well as the timing and extent of aragonite undersaturation (Figure 3b and c).

Figure 3 Model projections of  (a) Atmospheric CO2, (b) Global Ocean pH and (c) Southern Ocean Saturation under six IPCC emission scenarios (IPCC, 2007)

It is concluded that the increased anthropogenic CO₂ emissions has a significant impact on ocean carbonate chemistry, i.e. ocean acidification, based on both ocean-carbon cycle models and coupled climate/ocean-carbon cycle models. Despite of the large uncertainty in atmospheric CO₂ level projections, the changes in oceanic carbonate ion concentrations is closely linked with atmospheric CO₂. Although more research is required to provide more reliable projections of future atmospheric CO₂, it is clear that the impact of ocean acidification will be more severe in the future under ‘business-as-usual’ scenario. In the following weeks, I will be moving on to discuss ways to mitigate this ocean’s acid test challenge. Stay Tuned!

The Acid Test Today

Having looked at the ocean’s past, now it’s time for us to review the ocean’s acid test today. I would like to begin with a recent news headline on the Alaska Public. 

Alaska, situated at the northwest edge of North America, is the largest state of the U.S.A. which possess plentiful of natural resources. Fishery is one of the very important industries in Alaska, not only as an important food source for the population, but also for the regional economy via exports.  With today’s critical acid test challenge, it is expected to hit Alaska’s fisheries and the economy severely! However, this conclusion was based on discrete data collected by university researchers and fishermen from time to time when they sample on their boats. It is therefore difficult to construct a continuous seawater pH trend based on those measurements.

Assistant Professor of Chemical Oceanography at the UAF School of Fisheries and Ocean Sciences, Dr. Jeremy Mathis proposed the use of buoy network can help getting instantaneous ocean pH data all the time to fill in the missing data due to human constraints. It also provides important data source for us to understand seasonal and inter-annual pH trends over a longer timescale, which is vital for determining the overall acidic trend and future predictions.

Currently, only two buoys are at work in Alaska, one at the Resurrection Bay and another one to the west of Bristol Bay. The monitoring network will be able to expand with $2.7 million of budget allocation.  These buoys are projected to provide important data for quantifying the cost of ocean acidification and subsequent budgets allocate to prevent a catastrophe on Alaska’s fisheries.

Since ocean acidification is a global problem, monitoring pH of the Alaska region only is insufficient to provide significant data and insights to help prevent and mitigate this acid test crisis on Alaska’s fisheries and other marine resources. In addition, as seen from my previous posts, ocean acidification exhibits great latitudinal variability across the world’s ocean. With the interconnected ocean currents, the scale of influence of this ocean acid test on Alaska’s fisheries is certainly beyond its own state. In order to effectively mitigate the situation in Alaska, the buoy monitoring network should expand beyond the Alaska region and across the world’s major oceans to monitor the effects of this global change.   

What are the implications of the PETM for us today and the future?

First of all, this abrupt geological event seems analogous to the present day ‘Anthropocene’ in terms of its rapid release of a large amount of CO₂. The atmospheric and surface ocean response during the PETM has shown great similarity to the recent observed changes in response to anthropogenic release of CO₂ today. During the PETM, ~2000 x 10⁹ metric tons of carbon was released in less than 10,000 years. This might seem rapid, but compare with the anthropogenic emissions, this is far too modest.  Over 297 billion metric tonnes of carbon has been released from mankind in 250 years only since the Industrial revolution in AD1750. Although Ruddiman (2003) argued that the anthropogenic greenhouse gas emission has begun 8000 years ago, it is still a much shorter timespan than that of the PETM. Therefore, many scientists have projected a similar or even worse catastrophic marine mass extinction than the PETM due to the rapid rate of current warming and acidification of the ocean. According to a recent model simulation by Ridgwell and Schmidt (2010), a much more severe deep ocean carbonate undersaturation and rapid environmental changes than that of the PETM are projected by the end of the century, which puts further challenges to marine calcifiers, including surface water phytoplankton.

Secondly, based on the recovery phase of the PETM, it seems that the natural buffer system of the ocean - CaCO₃ deposition via calcareous phytoplankton and the weathering of silicate rocks to restore the carbonate ion level in the deep ocean, might give us signs of hope to save the ocean from this acid test challenge. However, don’t forget that this recovery from the PETM took more than 100,000 years to complete! With the rate of anthropogenic CO₂ increase within 250-300 years only, it is certainly incommensurable with the PETM. The natural oceanic system is simply too slow to react to compensate for what we’ve done.

Although the Paleocene-Ecocene Thermal Maximum event might not be big and dramatic enough to be the current analogue of today’s Anthropogenic Era, it has certainly enhanced our knowledge and understanding of the resilience of our ocean’s natural buffer system. It is important to make use of this past event to produce accurate models for future predictions and subsequent geoengineering solution to mitigate the current situation.  

Paleocene-Eocene Thermal Maximum: Part II

As promised last week, I’ll be moving on to discuss the implications of the Paleocene-Eocene Thermal Maximum. But before I move on, I would like to thank Daniel and Emily for their comments and interests for my last entry, especially Emily, who would like to know about the cause of the methane hydrates dissociation which gives rise to the PETM at the first place! To be honest, I’ve never thought about that myself either! So thank you for suggesting this good blog topic for me. Now, I’m going to share with you of what I found about this ‘methane hydrate dissociation hypothesis’. Hope that answers your query!

In fact, the cause of the carbon isotope excursion (CIE) i.e. the dramatic decrease in global δ¹³C during the PETM is still not very clear. Although the ‘methane hydrate dissociation hypothesis’ is still the most dominant explanation as the cause of the PETM, it is still subject to a great debate among the scientific community, often due to model uncertainties and inadequate data resolution. It is believed that the massive methane release was caused by a change in deepwater source regions, which increased water temperatures rapidly enough to trigger a massive thermal dissociation of the methane hydrate gas reservoir underneath the seafloor. Katz et al. (2001) modelled changes in heat flow on gas hydrate reservoir stability through time, together with seismic data and comparison with other published isotopic records, have neither confirmed nor refuted thermal dissociation as the trigger for the PETM methane release. According to the published isotopic records, rapid δ¹⁸O decrease (indicating rapid warming) did not precede a rapid δ¹³C decrease (indicating CH₄ release) by at least 2000-4000 years, which is their modelled minimum time lag required for such a large amount of methane release (~2000Gt of C) via ocean mixing and the change of deepwater source region in order to trigger a global CIE i.e. PETM. However, new recent high-resolution stable isotopic records based on the planktonic and benthic foraminiferal shells analysis revealed that the onset of the CIE was geologically instantaneous and was preceded by a brief period of gradual surface-water warming (Thomas et al., 2011), which supports the thermal dissociation hypothesis of methane hydrates.  

In addition, neither isotopic comparisons nor their heat flow model indicated a sufficient change in deepwater source region to trigger such a rapid thermal dissociation (Katz et al., 2001). However, Tripati and Elderfield (2005) proved that change in ocean circulation has indeed triggered the destabilisation of methane hydrates in deep sea sediments, based on their seawater temperature and salinity reconstruction from benthic foraminifera (δ¹⁸O record) to infer changes in deepwater source regions from the Late Paleocene (>55.60Ma) to Early Eocene (<55.25Ma). A warming of immediate waters before the CIE is detected (Figure 1), triggered by the downwelling in North Pacific and reduced Southern Ocean convection. This further supports the thermal dissociation of methane hydrates being the driver for the onset of the PETM.

Figure 1 Benthic foraminifera Mg/Ca record indicates a warming of immediate waters across the PETM (Tripati and Elderfield, 2005) 

  

Katz et al. (2001) have also argued that the paleobathymetric distribution of the methane release sites is inconsistent with the broader depth range as predicted for thermal dissociation. Therefore, they proposed an alternative hypothesis that mechanical dissociation i.e. continental slope failures and seafloor erosion would also be responsible for the destabilisation of methane hydrates reservoir in the seafloor. However, if this alternative hypothesis stands, the methane release may have caused, rather than the result of the transient warming during the PETM; the methane releases from the seafloor will be oxidised in the ocean and then escaped to the atmosphere. Therefore, it implies that this increased atmospheric CH₄ and/or CO₂ was the cause of the warming. Higgins and Schrag (2006) further supported that the methane hydrates hypothesis alone is not sufficient to account for such a vast amount of carbon release; an oxidation of at least 5000GtC of organic carbon is a more plausible reason to account for the observed climatic changes of the PETM.

Nevertheless, to a large extent, methane hydrate dissociation is responsible for the onset of the PETM. However, significant levels of uncertainties still exist in the available proxy records, for example, the lack of benthic individuals available for analysis during the decline and extinction of benthic foraminifera (Thomas et al., 2011). This hinders the ability to resolve the initial sequence, timing and duration of the events, which is very important to verify methane hydrates dissociation (thermal or mechanical) as a cause of the PETM and other hypotheses.  

The Big Event: Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) is perhaps the most important geological event in the Cenozoic Era which has drawn the greatest scientific attention. Scientists consider this event to be the closest analogue of the recent anthropogenic rise of atmospheric carbon dioxide at present, which provides important implications for future projections.

What happened during the PETM?

The Paleocene-Eocene Thermal Maximum, occurred around 55 million years ago, is one of the most abrupt global warming events ever recorded in the last 65 million years of Earth’s history. In less than 10,000 years, global sea surface and deep sea temperatures rose by 5-9^oC. This abrupt oceanic warming was mainly caused by the rapid release of carbon (~2000 x 10⁹ metric tonnes) from deep ocean floor in the form of methane hydrates. This evident release of carbon was supported by a coeval global carbon isotope excursion (CIE), where the marine and terrestrial carbon isotope values (δ¹³C) decreased by 3 to 8 per mil, indicating a large input of carbon into the ocean and the atmosphere. The released methane was rapidly oxidised to carbon dioxide, which has significantly lowered seawater pH and its carbonate ion concentration via dissolution of this CO₂. These ocean chemistry changes have caused a rapid shoaling of the calcite compensation depth (CCD) or the calcite saturation horizon of more than 2km in less than 10,000 years, marked by a prominent transition from carbonate-rich to clay layer in the South Atlantic deep-sea sections (Zachos et al., 2005).

The abrupt transition from carbonate to clay layer at the P-E boundary (Zachos et al., 2005)

What were the impacts?

The PETM has triggered a series of biological responses within the plankton assemblages. The most catastrophic biological response throughout the PETM was the mass extinction of benthic foraminifera, with 30-50% of benthic foraminiferal species became extinct (Gibbs et al 2006). It was likely to be caused by the excessive carbonate undersaturation in the deep ocean at the Paleocene-Eocene (P-E) boundary, which prohibited the calcification of the benthic foraminifera. In addition, benthic foraminifera were used to living under the stable condition of the deep ocean, which lowered their ability to cope with such rapid environmental changes.  Therefore, in actual fact, most of the plankton species, which are more adapted to changing environmental condition in surface water survived throughout the event. Gibbs et al. (2006a) presented some interesting evolutionary changes of calcareous nannoplankton from their paleontological records. Instead of a mass extinction, their findings show a prominent increase in origination and extinction of calcareous nannoplankton during the first 70ky of the PETM, with a rate of 1.6 and 1.7% per 10ky respectively compared to 0.5% and 0.1% per 10ky in the pre-event background interval. In contrast with the destiny of the benthic foraminifera species, the excessive carbonate undersaturation during the PETM did not affect the survivorship of most calcareous nannoplankton taxa. Instead, it was concluded that the rate of environmental changes such as temperature, salinity, nutrient availability was the main driver for this evolutionary turnover pattern.

Recovery stage of the PETM

The ocean carbonate chemistry started to recover within 110 to 210ky after the onset of the PETM. Signs of recovery include the gradual recovery of the CCD i.e. carbonate content and δ¹³C (Gibbs et al., 2006a). The PETM recovery is believed to be attributed to the negative feedback mechanism of the oceanic system, in which increased calcareous phytoplankton productivity with intensified chemical weathering of silicate rocks (Zachos et al., 2005) and nutrient run-off was largely responsible for the sequestration of carbon (Gibbs et al., 2006b).

Source

Whoops! Just realised that I’ve written so much about this big event in this post. In case too much information has already been overwhelming you, I’ll carry on talking about the important implications of this big event in my next post instead. Meanwhile, hang on to your thoughts about this big event to be the closest analogue of today’s anthropogenic era and we shall carry on our discussion next week!

No more real undersea films?

In fact, the ocean's acid test not only poses challenges to marine ecosystems, it also put on stresses to the undersea film-making industry. Today, I found this interesting video from the ScienCentral News. A group of environmental scientists and oceanographers from the U.S. presented more alarming facts about the ocean's acid test. They have found out that ocean acidification is worse in coastal regions due to natural processes. In the West Coast of North America, seasonal winds caused water upwelled deeper in the ocean, bringing more naturally occuring CO2 with it. Its impact is much more dramatic than ever expected!

With seawater pH expected to decrease further in a much faster pace, will we still get to see all those beautiful coral reefs and colourful fish in real life?Or we will only get to see the 'lively' ocean portrayed by filmmakers?

What do corals tell us about the oceans?

Speaking about evidences for ocean acidification last week, corals are also one of the few proxies used for past seawater pH reconstruction. Reef-building corals are very sensitive to pH changes in the ocean. Their skeleton materials are built out of aragonite, which is a more soluble form of calcium carbonate (Cao and Caldeira, 2008). As seawater pH fluctuates, carbonate ion concentration in seawater, hence calcification rate of corals also changes accordingly. This therefore suggests that the changes in calcium carbonate response in the ocean can be monitored from these reef-building corals.

The tendency of a coral structure to develop or dissolve is strongly dependent upon the saturation state (Ω) of a particular mineral phase, which is largely controlled by carbonate ion concentration in seawater. Corals tend to develop where aragonite saturation Ω>3.3, with rates of calcification process exceed rates of bio-erosion (Pelejero et al., 2010). With today’s aragonite saturation value of 3.3 of most areas of the ocean, carbonate accumulation or coral building decreases and increasingly confines to areas with Ω>3.3 (Hoegh-Guldberg et al., 2007). Aragonite saturation levels are expected to drop below 3.3 for all oceans in the world by the end of this century (See diagram below).

Aragonite Saturation Levels over time

How closely does coral calcification relate to ocean acidification?

It is often easy to draw the conclusion of coral dissolution is caused by ocean acidification. In fact, there is not always a clear cut. Doney et al. (2007) argued that coral calcification records rarely established links with ocean acidification directly due to its naturally high variability, which is difficult to detect the acidifying signal. The high variability of ocean pH is illustrated from the δ¹¹B  record of the long-living coral Porites from Flinders Reef in the western Coral Sea of the southwestern Pacific, where there is no significant decreasing trend in δ¹¹B, i.e. pH over the last 300 years. The only dominant feature found in the record is the Interdecadal Oscillation pH, with pH values fluctuate between 7.9 and 8.2 units, synchronises with the Interdecadal Pacific Oscillation (Pelejero et al., 2005).

However, a more recent study by Wei et al. (2009) has provided detailed evidence of ocean acidification through the extensive studies on corals of the Great Barrier Reef. δ¹¹B isotope composition record of the extracted Porites coral reflects a decreasing seawater pH trend of 0.2-0.4 units over the last 200 years, despite of the interdecadal variability.

You might wonder that the timescale for coral studies is relatively short i.e. the past 200-300 years.  A latest study done by Douville et al., (2010) has lengthened the timescale of pH reconstruction based on Porites corals through the Holocene to the Last Glacial Period.  The ‘δ¹¹B-pH’ technique is applied on both modern and ancient Porites corals. Their results indicate that the ancient sea surface water pH in the Holocene is 8.20-8.26 units and has reached 8.30 at the end of the last glacial period. These values are much higher than present day values. This ancient coral reconstruction also shows a drop in pH of 0.2 units before and after the abrupt cooling event, Younger Dryas 20.7kyr BP.

Unfortunately, that is only one of the very few studies with extensive improvement in using ancient/fossil corals for a better temporal resolution in past pH reconstruction. It is still subject to high levels of uncertainties. For instance, fossil corals are hard to obtain and preserve. Accurate reconstruction would require high spatial and temporal resolution coral data to determine overall changes in ocean chemistry. 

Understanding the Ocean’s Past: Finding the evidence

Knowing the ocean’s history is vital as it provides us an analogue, if not, deeper understanding of today’s ocean. With this, past environment reconstruction often comes into play using various paleo-records. In my last post, I mentioned about corals being the potential proxy for past ocean chemistry reconstruction.  In fact, after doing more research about the subject this week, corals are actually of a low potential to document chemical changes of the ocean in the past. There is still a lack of evidence proofing the decline in coral calcification is directly related to ocean acidification. My apologies here! Nevertheless, corals are important marine carbonate organisms in marine ecosystems and biodiversity, which I’ll come back to that later on. As for now, I’m going to introduce some common proxies used in palaeoceanography to reconstruct past seawater chemistry.  

Boron Isotope (δ¹¹B­) proxy: The paleo-seawater pH meter

The chemical element, Boron, exists in two molecular species in the ocean: boric acid B(OH)₃ and borate ion B(OH)₄^-. The proportion of the two species varies strongly with oceanic acidity. As calcifying organisms incorporate boron in their structures, the charged borate species, B(OH)₄^- is predominantly incorporated into marine carbonates, substituting HCO₃^- or CO₃^2-. In other words, boron isotopic composition of marine carbonates will be changed accordingly. Hence, by calculating the changes in boron isotope (δ¹¹B­) ratio of marine carbonates, oceanic pH can be inferred. The figure below shows the concentration and isotopic composition of the two boron species with seawater pH. 

   

Image Source

Foraminifera are commonly used in the analysis of boron isotopic ratios as a proxy. It is a calcareous plankton species with shells made of calcium carbonate, exists in both planktonic and benthic form. Foraminifera samples are often taken from deep-sea sediment cores for ancient oceanic pH reconstruction.

However, δ¹¹B is not always a perfect proxy for past pH reconstruction, especially in the context of deep time scales, i.e. beyond 10 to 20 million years. Over this timescale,  δ¹¹B  of seawater cannot be considered as constant due to the residence time of boron in seawater (Pelejero et al., 2010). Furthermore, the concentration of boron in foraminifera is often low (~10ppm), hence measuring its boron isotopic composition can be very difficult (Rae et al., 2011).

Boron/Calcium Ratio of benthic foraminifera : deep water carbonate saturation

Carbonate ion (CO₃^2-) concentration is another important component to understand the ocean carbonate chemistry. It is highly correlated with atmospheric CO₂ (pCO₂). When CO₂ dissolves in seawater, it reduces the available carbonate ion in surface water via the release of protons. This directly decreases the amount of carbonate precipitated at the ocean seafloor. Yu and Elderfield (2007) have successfully reconstructed past deep water carbonate using the measurements of B/Ca ratio of four benthic foraminifera species, in which a strong linear correlation between B/Ca and deep water CO₃^2- is shown.

In recent years, several scientists have attempted to reconstruct past ocean pH using B/Ca shell ratio of marine organisms. Yu et al. (2007) have proved that B/Ca measurements of planktonic foraminifera is a promising proxy for detecting variations in past ocean pH and pCO₂. However, it is not necessarily the case for other calcifying organisms. For example, B/Ca ratio of a California mussel species does not strongly correlate with its seawater pH but largely due to its specific biological control (McCoy et al., 2007). 

Ice core records for atmospheric CO2

And of course, a well documentation of atmospheric CO₂ in the past is essential as it is the main driver of the ocean’s chemical changes. Past CO₂ concentration in the atmosphere can be reconstructed from the composition of air bubbles trapped in ice cores, mostly taken from Greenland or Antarctica. For studying past oceanic chemistry changes,  Antarctic ice cores are common proxies used as it can be dated back to 800,000 years ago (glacial-interglacial timescale). Pelejero et al. (2010) have illustrated that atmospheric CO₂ and ocean surface pH almost synchronise with each other over the last 800,000 years, shown in the diagram below.

 

I have only listed a few common proxies for past ocean chemistry reconstruction in here, but there are a lot more out there worth to explore! As each of the proxies are subject to uncertainties and constraints, analysis using multiple proxies is often a common practice to encounter for spatial and temporal constraints for a better past reconstruction. Next week, I’ll further explore what these proxies actually tell us about the oceans, focusing on the abrupt ocean acidification event at the Paleocene-Eocene Thermal Maximum (PETM) 55Mya. 

Coral Reefs under the Acid Test Challenge

This photograph illustrates that coral reefs are the greatest victims of ocean acidification indeed!The picture on the left shows a healthy coral reef with living Acropora palmata (an important reef-building coral) under good water quality condition. In contrast, the other picture shows degraded coral reefs with dead  Acropora palmata, suffered from poor water quality caused by ocean acidification. This 'coral bleaching' is becoming more prominant in Puerto Rico, other Caribbean and western tropical Atlantic Ocean regions (Moyer, 2009)

The Acid Test

So, what is this Ocean’s Acid Test all about? Obviously, it refers to the ocean acidification phenomenon. In order to explore this global phenomenon further, we need to first understand the mechanism and chemistry of the ocean.

Source: http://na.oceana.org/en/our-work/climate-energy/ocean-acidification/learn-act/what-is-ocean-acidification

The above diagram gives a nice summary of a chain of chemical reactions happening in the ocean with the dissolved CO2 coming from the atmosphere. Like carbonated fizzy drinks, when more CO2 dissolves in the water, it becomes more acidic. When more atmospheric carbon dioxide is in contact with surface water of the ocean, more is dissolved to form carbonic acid. This carbonic acid dissociates to form bicarbonate ion and hydrogen ion (proton) which therefore decreases the seawater pH. These protons are likely to aggregate with carbonate ion present in seawater to form bicarbonate ion, which decreases the amount of carbonate ion available in seawater.  As most marine organisms, especially coccolithophorids, pteropods and foraminiferans require carbonate ions to build their calcium carbonate shells and skeletons, the decrease in carbonate ion makes them harder to calcify or even unable to survive. Therefore, less carbon can be deposited at the bottom of the ocean in the form of calcium carbonate via this carbonate pump. 

In fact, our ocean has been able to buffer upon this constant increase in acidity for the last 420,000 years at least (Hoegh-Guldberg et al., 2007). This natural mechanism is mainly driven by the biological and carbonate pump to transport carbon from the surface waters to the bottom of the ocean. Hence, the ocean’s deep water is rich in carbonate ion which is able to neutralise its natural acidity through the mixing of surface and deep waters via turbulences and ocean currents (Rahmstorf and Richardson, 2008).

However, this neutralisation process takes over 100,000 years! Since Industrial Revolution (1750), atmospheric CO2 concentration has increased from the pre-industrial level of 285ppm to the present level of 389ppm today in less than 300 years. Ocean pH has also decreased by 0.1 unit from a level of about 8.2 (Fenchell, 2011). According to the past geological records, our world’s ocean has never absorbed such vast amount of CO2 in the atmosphere and experienced this sharp decrease in pH in such a short timescale, which I am going to discuss in the following weeks.

Many scientists have projected a further decrease of 0.1 pH unit of our ocean by the end of this century based on the current anthropogenic CO2 emissions. Undoubtedly, this acid test is indeed our oceans’ biggest global challenge ever in history. Calcifying marine organisms and the coral communities are the first biggest victims. Next week, I will be looking at the effects of acidified oceans on these marine organisms and how corals can be used as an important proxy to understand the past ocean chemistry. Stay tuned!

Reference:

Rahmstorf, S. and K. Richardson (2008) Our Threatened Ocean, Haus Publishing: London 

Introduction

Welcome to my blog for GEOG3057: Global Environmental Change. In here, I am going to investigate one of the world’s most threatening global environmental problems – ocean acidification, another problem caused by anthropogenic carbon dioxide emissions. It seems that people tend to concern themselves with the Earth’s atmosphere only as the victim of burning fossil fuels, but often neglecting the ocean. In actual fact, the ocean is one of the major carbon sinks of our Earth, absorbing a quarter of the Earth’s CO2 emissions. However, scientists have discovered that with the ever-increasing level of CO2 in the atmosphere, the ocean has become more and more acidic which severely threatens marine life.  Some researchers even predict that if we continue to emit CO2 in the same way as we do now, ocean acidity is going to double by the end of this century. By then, nearly all marine life will be destroyed!

In the following 10 weeks, I will present to you my investigation on the problem of ocean acidification by analysing the ocean chemistry in the past, so as to understand and to predict our ocean’s destiny in the future. To start with, I would like to share with you a documentary produced by the Natural Resources Defense Council (NRDC) to present to you this most alarming global environmental problem from a group of ocean scientists.