The Marine Carbon System and Ocean Acidification during Phanerozoic Time

Abstract

The global CO₂-carbonic acid-carbonate system of seawater, although certainly a well-researched topic of interest in the past, has risen to the fore in recent years because of the environmental issue of ocean acidification (often simply termed OA). Despite much previous research, there remain pressing questions about how this most important chemical system of seawater operated at the various time scales of the deep time of the Phanerozoic Eon (the past 545 Ma of Earth’s history), interglacial-glacial time, and the Anthropocene (the time of strong human influence on the behaviour of the system) into the future of the planet. One difficulty in any analysis is that the behaviour of the marine carbon system is not only controlled by internal processes in the ocean, but it is intimately linked to the domains of the atmosphere, continental landscape, and marine carbonate sediments.

For the deep-time behaviour of the system, there exists a strong coupling between the states of various material reservoirs resulting in an homeostatic and self-regulating system. As a working hypothesis, the coupling produces two dominant chemostatic modes: (Mode I), a state of elevated atmospheric CO₂, warm climate, and depressed seawater Mg∕Ca and SO₄∕Ca mol ratios, pH (extended geologic periods of ocean acidification), and carbonate saturation states, and elevated Sr concentrations, with calcite and dolomite as dominant minerals found in marine carbonate sediments (Hothouses, the calcite-dolomite seas), and (Mode II), a state of depressed atmospheric CO₂, cool climate, and elevated seawater Mg∕Ca and SO₄/Ca ratios, pH, and carbonate saturation states, and low Sr concentrations, with aragonite and high magnesian calcites as dominant minerals found in marine carbonate sediments (Icehouses, the aragonite seas).

Investigation of the impacts of deglaciation and anthropogenic inputs on the CO₂–H₂O–CaCO₃ system in global coastal ocean waters from the Last Glacial Maximum (LGM: the last great continental glaciation of the Pleistocene Epoch, 18,000 year BP) to the year 2100 shows that with rising sea level, atmospheric CO₂, and temperature, the carbonate system of coastal ocean water changed and will continue to change significantly. We find that 6,000 Gt of C were emitted as CO₂ to the atmosphere from the growing coastal ocean from the Last Glacial Maximum to late preindustrial time because of net heterotrophy (state of gross respiration exceeding gross photosynthesis) and net calcification processes. Shallow-water carbonate accumulation alone from the Last Glacial Maximum to late preindustrial time could account for ~24 ppmv of the ~100 ppmv rise in atmospheric CO₂, lending some support to the ‘‘coral reef hypothesis’’. In addition, the global coastal ocean is now, or soon will be, a sink of atmospheric CO₂, rather than a source. The pHT (pH values on the total proton scale) of global coastal seawater has decreased from ~8.35 to ~8.18 and the CO₃^2- ion concentration declined by ~19% from the Last Glacial Maximum to late preindustrial time. In comparison, the decrease in coastal water pHT from the year 1900 to 2000 was ~8.18 to ~8.08 and is projected to decrease further from about ~8.08 to ~7.85 between 2000 and 2100. During these 200 years, the CO₃^2- ion concentration will fall by ~ 45%. This decadal rate of decline of the CO₃^2- ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene!

In terms of the modern problem of ocean acidification and its effects, the “other CO₂ problem”, we emphasise that most experimental work on a variety of calcifying organisms has shown that under increased atmospheric CO₂ levels (which attempt to mimic those of the future), and hence decreased seawater CO₃^2- ion concentration and carbonate saturation state, most calcifying organisms will not calcify as rapidly as they do under present-day CO₂ levels. In addition, we conclude that dissolution of the highly reactive carbonate phases, particularly the biogenic and cementing magnesian calcite phases, on reefs will not be sufficient to alter significantly future changes in seawater pH and lead to a buffering of the CO₂-carbonic acid system in waters bathing reefs and other carbonate ecosystems on timescales of decades to centuries. Because of decreased calcification rates and increased dissolution rates in a future higher CO₂, warmer world with seas of lower pH and carbonate saturation state, the rate of accretion of carbonate structures is likely to slow and dissolution may even exceed calcification. The potential of increasing nutrient and organic carbon inputs from land, occurrences of mass bleaching events, and increasing intensity (and perhaps frequency of hurricanes and cyclones as a result of sea surface warming) will only complicate matters more. This composite of stresses will have severe consequences for the ecosystem services that reefs perform, including acting as a fishery, a barrier to storm surges, a source of carbonate sediment to maintain beaches, and an environment of aesthetic appeal to tourist and local populations. It seems obvious that increasing rates of dissolution and bioerosion owing to ocean acidification will result in a progressively increasing calcium carbonate (CaCO₃) deficit in the CaCO₃ budget for many coral reef environments. The major questions that require answers are: will this deficit occur and when and to what extent will the destructive processes exceed the constructive processes?

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The Marine Carbon System and Ocean Acidification during Phanerozoic Time
Fred T. Mackenzie¹ and Andreas J. Andersson²

Responses of the Emiliania huxleyi Proteome to Ocean Acidification

A new  study, published in PLOS ONE  thie month investigates how a strain of the coccolithophore Emiliania huxleyi might respond if all fossil fuels are burned by the year 2100 – predicted to drive up atmospheric CO₂ levels to over four times the present day. Specimens grown under this high CO₂ scenario were compared with specimens grown under present day CO₂ levels.

Below,  the press release published at the NOCS website.

Press Release: Marine algae show resilience to carbon dioxide emissions

A type of marine algae could become bigger as increasing carbon dioxide emissions are absorbed by the oceans, according to research led by scientists based at the National Oceanography Centre, Southampton (NOCS).

Example of coccoliths derived at the end of the experiment

Coccolithophores are microscopic algae that form the base of marine food chains. They secrete calcite shells which eventually sink to the seafloor and form sediments, drawing down and locking away carbon in rocks. Because of their calcitic shells, some species have been shown to be sensitive to ocean acidification, which occurs when increasing amounts of atmospheric CO₂ are absorbed by the ocean, increasing seawater acidity.

But these findings suggest that not all coccolithophore species respond to ocean acidification in the same way.

“Contrary to many studies, we see that this species of coccolithophore gets bigger and possesses more calcite under worst-case scenario CO₂ levels for the year 2100,” says Dr Bethan Jones, lead author and former researcher at University of Southampton Ocean and Earth Science, which is based at NOCS. “They do not simply dissolve away under high CO₂ and elevated acidity.”

However, the researchers also observed that cells grew more slowly under the high CO₂ scenario, which could be a sign of stress.

The researchers also tested for changes in protein abundance – using a technique developed by the collaborating institutes – as well as other biochemical characteristics. They detected very few differences between the two scenarios, indicating that apart from growth, this strain of coccolithophore does not seem to be particularly affected by ocean acidification.

Co-author Professor Iglesias-Rodriguez, formerly at University of Southampton Ocean and Earth Science, says: “This study suggests that this strain of Emiliania huxleyi possesses some resilience to tolerate future CO₂ scenarios, although the observed decline in growth rate may be an overriding factor affecting the success of this ecotype in future oceans. This is because if other species are able to grow faster under high CO₂, they may ‘outgrow’ this type of coccolithophore.

“Given that chalk production by calcifiers is the largest carbon reservoir on Earth – locking away atmospheric CO₂ in ocean sediments – understanding how coccolithophores respond to climate change is a first step in developing models to predict their fate under climate pressure such as ocean acidification.”

The team used a technique called ‘shotgun proteomics’, optimised for marine microbiological research at the University of Southampton’s Centre for Proteomic Research, to detect changes in proteins under the different CO₂ scenarios.

The collaborative study involved researchers at University of Southampton Ocean and Earth Science (which is based at NOCS), University of Southampton Institute for Life Sciences, University of Southampton Centre for Proteomic Research, University of Cambridge, University College London and Xi’an Jiaotong-Liverpool University, China.

A fantastic Ocean and Earth Day at National Oceanography Centre, Southampton

Discovering the world under the microscope

Another very successful Ocean and Earth Open Day held at NOC, Southampton on Saturday 23 March, 2013. A great turnout on a very cold day, we had over 3000 visitors and the feedback was extremely positive.

The Descent into the Icehouse  (@IntotheIcehouse) participated in the event with a poster display and information material about our research work.  We had a great attendance; our stand was full most of the time and it was a rewarding and exciting experience for both us and the visitors. It was fantastic to see the excitement in the children’s faces discovering through a microscope the tiny world of microfossils and especially those of foraminifera, a group of single-celled animals that construct shells of almost infinite variety and in most geographical regions.

The study of fossil foraminifera has many applications beyond expanding our knowledge of the evolution and diversity of life. Fossil foraminifera are useful in paleoecology, paleobiogeography, as well as, oil exploration.

The children and their parents had a great time exploring and discovering the wonders of nature and a whole world under the microscope.

Our OED day would not happen without the help of our scientists and volunteers

… and without their enthusiasm, dedication, determination and sheer hard work, this day could not be such a success.  I’d like to take this opportunity to say again a very big THANK YOU.

More photos and a video will be made available from NOC online soon, so look out for another post in the next few weeks.