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The human fingerprint in coral

Posted on 11 October 2010 by John Cook

Atmospheric carbon dioxide has risen by nearly 40% since pre-industrial times. Of course, all this extra CO2 coinciding with our dumping of billions of tonnes of CO2 into the air might be pure coincidence. How can we be sure we're responsible? Extra evidence comes from the different types of carbon isotopes found in the air. The most common carbon isotope is carbon-12 (12C) which is found in roughly 99% of the carbon dioxide in the atmosphere. The slightly heavier carbon-13 (13C) makes up most of the rest. Plants prefer carbon-12 over carbon-13. This means the ratio of carbon-13 to carbon-12 is less in plants than it is in the atmosphere. As fossil fuels originally come from plants, it means when we burn fossil fuels, we're releasing more 12C into the atmosphere. If fossil fuel burning is responsible for the rise in atmospheric CO2 levels, we should be seeing the ratio of 13C to 12C decrease.

That is exactly what we observe. Atmospheric measurements of carbon dioxide find the ratio of 13C to12C (otherwise refered to ?13C) has been steadily decreasing over the last few decades (Ghosh 2003). However, this data only goes back to the 1980s. Fortunately, corals provide a window further into the past. In Evidence for ocean acidification in the Great Barrier Reef of Australia (Wei et al 2009), the authors drilled a coral core from Arlington Reef, situated in the middle of the Great Barrier Reef. This enabled them to measure ?13C going back to 1800.

Ratio of Carbon-13 to Carbon-12 in Great Barrier Reef coral
Figure 1: Change in ?13C (13C/12C ratio) in coral from Arlington Reef (in the centre of the Great Barrier Reef).

What they find is the ratio of carbon-13 to carbon-12 is relatively steady over much of the last two centuries. However, it starts to dramatically decrease in the latter half of the 20th Century. Increasing anthropogenic emissions of CO2 not only increased the levels of atmospheric CO2 concentration but also decreased the ?13C composition of the atmosphere. Thus, the decrease in ?13C is attributed to the burning of fossil fuels.

The story doesn't end there. As CO2 increases in ocean waters, seawater pH levels fall. Some key marine organisms, such as calcareous micro-organisms and corals, have difficulty maintaining their external carbonate skeletons when pH levels drop. The coral core extracted from Arlington Reef also provided measurements of boron isotope levels (?11B), which act as a proxy for seawater pH. They found that from 1941 to 2004, there was an overall trend of decreasing ?11B which corresponds to a drop in pH levels. The positive correlation between coral ?11B and ?13C provides strong confirmation that seawater acidification is closely linked to the anthropogenic CO2 emissions from burning of fossil fuels.

Thus fossil fuel burning is a cause of both global warming (which causes coral bleaching) and ocean acidification. To add further insult to injury, the paper also finds that coral bleaching may have the effect of further reducing pH levels. It seems coral reefs just can't catch a break.

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Comments 1 to 17:

  1. I find the y-axis of the graph a little confusing -- it appears to be a negative percentage change, but it isn't clear what the baseline the change is being measured against is. Also, one thing you could make clearer is that I don't think the total amount of C13 in the atmosphere is declining, it is the percentage of all CO2 which is C13 that is declining.
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  2. Eric L- The y-axis scale is given in "delta" units which represent the deviation in parts per thousand of the 13C/12C ratio in a sample - coral calcium carbonate in this case - measured relative to that in the international reporting standard for carbon stable isotopes - PDB, a carbonate fossil (belemnite) in the Pee Dee Formation in South Carolina. A value of -3 indicates that the 13C/12C ratio of the sample is lower than that of PDB by 3 parts per thousand or 0.3%.
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  3. Eric L: the change is per mille. Which is like per-cent, but per thousand rather than per hundred. So it's a fractional change, but to get to per-cent divide the figures by 10. (this sounds small, but it can be a very sensitive test - it's the sort of stuff they use to determine temperatures from ice and sediment cores)
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  4. And I forgot to mention the standard sample, because I didn't konw the coral standard. For ice cores they use standard mean oceanic sea water. Mike's answer should help clear up the coral bit :)
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  5. Some interesting work suggests coral reefs may also be under threat under threat from non-anthropogenic sources of CO2 and other anthropogenic non-CO2 related hazards . The impact of non-anthropogenic CO2 is presented here as a model of the potential impact of anthropogenic ocean acidification. Some reefs, however, seem to have evolved resilience in the face of rises in temperature . It seems some corals have evolved resilience in the face of temperature rises . I note concerns about acidification is presented here as related predominantly to dissolution of calcium carbonate. However, problems with acidification are complex in that acidification impacts more strongly on bleaching and productivity than calcium carbonate formation. I could not find much other material relating to coral resilience to acidity beyond this article in a sceptical source - I did not have the time or inclination to follow up the references in the article but thought I'd include the link for what it's worth. Looking at a peer-reviewed source, it seems that coral symbiota involved in initiation of coral bleaching events show greater capacity to pass on traits associated with resistance to warming than do corals - not a good outlook, if this is correct. At the same time, the appearance of corals in the Ordovician when CO2 levels were much higher (though temperatures lower) may warrant further discussion.
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  6. Some broken links - sorry - too hard and too little time to fix so the comment is somewhat incoherent :-(
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  7. What is the justification for starting the delta 13C change at -1.5% in the graph? My first reaction to this post was, "Something must be missing. What about the CO2 12C/13C makeup in ice core samples?" Two articles, and there are others, shed more light on the subject: Preindustrial People Had Little Effect on Atmospheric Carbon Levels in Science Now and Stable isotope constraints on Holocene carbon cycle changes from an Antarctic ice core in Nature. I still don't understand the justification for starting delta 13C change at -1.5% in the graph. At what point in time was delta 13C change at 0%? Volcanic activity can also modify the 13C/12C ratios and bias the measurements. Does the report note any activity in the vicinity and make appropriate adjustments?
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  8. Perhaps the answer to my previous question in #7 might lie in the International PDB standard for Carbon Dating. The following definition was taken from the "Volcanic activity" reference in #7. Isotope ratios are, usually, expressed in δ-units. In the case of 13C/12C isotope ratio in carbon dioxide, the δ-value is given by: δ13C = (R13sample/R13reference-1), where R13 represents the abundance ratio [13C]/[12C]. Usually expressed in per mill (‰), it is referred to the PDB-standard material (Belemnite of the Pee Dee formation in South Caroline, [13C]/[12C]=11237.2 10-6).
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  9. Now I see that Mike Palin@2 explained it, but it didn't register with me, I guess.
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  10. @ #7 Roger A. Wehage : Perhaps by combining "Plants prefer carbon-12 over carbon-13" plus the food chain starts with "plants" so any byproduct will reflect this preference plus much calcium carbonate in coral reefs being provided by coralline algae, you can find the reason for the value starting below "the measured standard for carbon stable isotopes". I was taught to firstly look for the answers within the system and look for an external event as a last resort. I find this approach not so common every time I read some debate about climate change written in English. Is there something I should know?
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  11. I'll finish off the comment omitting some of the links, which are a bit hard to reconstruct. Basically, corals can adapt to some extent to a drop in pH and still make calcium carbonate but apparently at the cost of metabolic stress, which may in turn lead to less efficient growth and/or reproduction. Some species are much more adaptable than others. Temperature rise can lead to proliferation of algae . Temperature rises may also cause metabolic stress. Corals consequently face a potential double whammy. However, it's important to distinguish between reef damage attributable to drops in pH rises in temperature and damage arising from leaching of fertiliser from agricultural land which can trigger proliferation and expulsion of algae (which give corals their characteristic brown red colouration) thus causing bleaching. Interestingly, although corals first appeared in the Cambrian period, some 542 million years ago, fossils are extremely rare until the Ordovician period, 100 million years later, when Rugose and Tabulate corals became widespread. As readers of this site know well,CO2 was much higher in the late Ordovician although temperatures were much lower because solar output was also lower during these periods. At the same time, given the lower temperatures at the time, ocean pH would have been significantly higher as seas would have been more saturated with CO2. Admittedly, the Rugose and Tabulate corals of the late Ordovician are now long extinct and so may not tell us that much about what will happen with modern coral populations or their capacity to adapt to significant drops in pH.
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    Moderator Response: If you could check your links in your comment at 6 above to see if they were what you intended, I think I fixed them. Previewing comments before submitting catches a lot of issues. :)
  12. Thanks John - great job of fixing links :-)
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  13. headpost: "Plants prefer carbon-12 over carbon-13. This means the ratio of carbon-13 to carbon-12 is less in plants than it is in the atmosphere." Does this preferential uptake of carbon-12 also apply to corals and their algal photosymbionts?
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  14. Roger A. Wehage @ 7, 8, 9 PDB is the standard for reporting the stable isotope ratio 13C/12C. Because it is many millions of years old, it contains no measurable 14C and so has nothing to do with dating. The use of PDB simply allows the 13C/12C ratio of a sample of carbon to be reported more conveniently - using d13C = -1.5 is much simpler than 13C/12C = 0.0112203. The 13C/12C ratio of dissolved carbonate in seawater varies from place to place in the modern ocean and has changed over geologic time as deduced from measurements of fossil carbonate. Because most of the carbon at Earth's surface is in sedimentary rocks, the 13C/12C ratio of the oceans at any time in the past gives geologists a snapshot of the proportions of this carbon that are stored as dead organic matter (lower 13C/12C) and carbonate (higher 13C/12C). Thus, during the so-called Carboniferous era (360-300 million years ago) when much organic matter was buried and evemtually turned into coal, oil and gas, the 13C/12C ratio of the oceans was shifted high (d13C = +4). Now that those 13-depleted hydrocarbons are being burned, the the 13C/12C of carbonate in the oceans is shifting downward. By the way, boron isotope ratios (11B/10B) are also reported in "delta" notation relative to the synthetic boric acid standard NIST 951.
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  15. @13 Charlie A Isotopic fractionation (the preference for C12 over C13) definitely occurs for endosymbionts in corals, as it does for all photosynthesizing organisms. The degree of fractionation can be less if CO2 becomes depleted in the local environment, since you can't prefer one isotope over another if you use all of the available CO2. That doesn't affect the stable isotopic composition of calcareous skeleton however. The processes associated with depositing calcium carbonate do not fractionate nearly to the degree that photosynthesis does.
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  16. @15 Stephen Baines -- thanks, it makes sense that the calcium carbonate formation has less isotopic fractionation. I had seen some papers regarding foraminafera and the problem of assuming stable isotopic ratios in samples from late paleocene. D'Hont et al '94 and Houston & Huber '98, and thought perhaps those processes of localized depletion of C12 might be at work as well as the change in atmospheric isotope ratios.
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  17. Nice writeup on coral bleaching over at ClimateProgress today:
    "The atmospheric levels of CO2 we are already committed to reach, no matter what mitigation is now implemented, have no equal over the entire longevity of the Great Barrier Reef, perhaps 25 million years. And most significantly, the rate of CO2 increase we are now experiencing has no precedent in all known geological history. Reefs are the ocean’s canaries and we must hear their call. This call is not just for themselves, for the other great ecosystems of the ocean stand behind reefs like a row of dominoes. If coral reefs fail, the rest will follow in rapid succession, and the Sixth Mass Extinction will be upon us — and will be of our making." - J.E.N. Veron
    Veron is the former chief scientist of the Australian Institute of Marine Science. He is principal author of 8 monographs and more than 70 scientific articles on the taxonomy, systematics, biogeography, and the fossil record of corals. His books include the three-volume Corals of the World and A Reef in Time: The Great Barrier Reef from Beginning to End (2008). His research has taken him to all the major coral reef regions of the world during 66 expeditions. The Yooper
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