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.