Arguments
Cracking the mystery of the corrosive ocean
Posted on 25 June 2015 by Guest Author
This is a re-post from ClimateSight
Around 55 million years ago, an abrupt global warming event triggered a highly corrosive deep-water current to flow through the North Atlantic Ocean. This process, suggested by new climate model simulations, resolves a long-standing mystery regarding ocean acidification in the deep past.
The rise of CO2 that led to this dramatic acidification occurred during the Paleocene-Eocene Thermal Maximum (PETM), a period when global temperatures rose by around 5°C over several thousand years and one of the largest-ever mass extinctions in the deep ocean occurred.
The PETM, 55 million years ago, is the most recent analogue to present-day climate change that researchers can find. Similarly to the warming we are experiencing today, the PETM warming was a result of increases in atmospheric CO2. The source of this CO2 is unclear, but the most likely explanations include methane released from the seafloor and/or burning peat.
During the PETM, like today, emissions of CO2 were partially absorbed by the ocean. By studying sediment records of the resulting ocean acidification, researchers can estimate the amount of CO2 responsible for warming. However, one of the great mysteries of the PETM has been why ocean acidification was not evenly spread throughout the world’s oceans but was so much worse in the Atlantic than anywhere else.
This pattern has also made it difficult for researchers to assess exactly how much CO2 was added to the atmosphere, causing the 5°C rise in temperatures. This is important for climate researchers as the size of the PETM carbon release goes to the heart of the question of how sensitive global temperatures are to greenhouse gas emissions.
Solving the mystery of these remarkably different patterns of sediment dissolution in different oceans is a vital key to understanding the rapid warming of this period and what it means for our current climate.
A study recently published in Nature Geoscience shows that my co-authors Katrin Meissner, Tim Bralower and I may have cracked this long-standing mystery and revealed the mechanism that led to this uneven ocean acidification.
We now suspect that atmospheric CO2 was not the only contributing factor to the remarkably corrosive Atlantic Ocean during the PETM. Using global climate model simulations that replicated the ocean basins and landmasses of this period, it appears that changes in ocean circulation due to warming played a key role.
55 million years ago, the ocean floor looked quite different than it does today. In particular, there was a ridge on the seafloor between the North and South Atlantic, near the equator. This ridge completely isolated the deep North Atlantic from other oceans, like a giant bathtub on the ocean floor.
In our simulations this “bathtub” was filled with corrosive water, which could easily dissolve calcium carbonate. This corrosive water originated in the Arctic Ocean and sank to the bottom of the Atlantic after mixing with dense salty water from the Tethys Ocean (the precursor to today’s Mediterranean, Black, and Caspian Seas).
Our simulations then reproduced the effects of the PETM as the surface of the Earth warmed in response to increases in CO2. The deep ocean, including the corrosive bottom water, gradually warmed in response. As it warmed it became less dense. Eventually the surface water became denser than the warming deep water and started to sink, causing the corrosive deep water mass to spill over the ridge – overflowing the “giant bath tub”.
The corrosive water then spread southward through the Atlantic, eastward through the Southern Ocean, and into the Pacific, dissolving sediments as it went. It became more diluted as it travelled and so the most severe effects were felt in the South Atlantic. This pattern agrees with sediment records, which show close to 100% dissolution of calcium carbonate in the South Atlantic.
If the acidification event occurred in this manner it has important implications for how strongly the Earth might warm in response to increases in atmospheric CO2.
If the high amount of acidification seen in the Atlantic Ocean had been caused by atmospheric CO2 alone, that would suggest a huge amount of CO2 had to go into the atmosphere to cause 5°C warming. If this were the case, it would mean our climate was not very sensitive to CO2.
But our findings suggest other factors made the Atlantic far more corrosive than the rest of the world’s oceans. This means that sediments in the Atlantic Ocean are not representative of worldwide CO2 concentrations during the PETM.
Comparing computer simulations with reconstructed ocean warming and sediment dissolution during the event, we could narrow our estimate of CO2 release during the event to 7,000 – 10,000 GtC. This is probably similar to the CO2 increase that will occur in the next few centuries if we burn most of the fossil fuels in the ground.
To give this some context, today we are emitting CO2 into the atmosphere at least 10 times faster than than the natural CO2 emissions that caused the PETM. Should we continue to burn fossil fuels at the current rate, we are likely to see the same temperature increase in the space of a few hundred years that took a few thousand years 55 million years ago.
This is an order of magnitude faster and it is likely the impacts from such a dramatic change will be considerably stronger.
Written with the help of my co-authors Katrin and Tim, as well as our lab’s communications manager Alvin Stone.
Thank Alvin interesting and worrying clearly.
Some thoughts,
There was no ice on the planet in the PETM before the CO2 injection, having no ice will ~half the equilibirium climate sensitivity compared to today; as we have ice melting.
GEOLOGIC CONSTRAINTS ON THE GLACIAL AMPLIFICATION OF PHANEROZOIC CLIMATE SENSITIVITY
JEFFREY PARK* and DANA L. ROYER**
[American Journal of Science, Vol. 311, January, 2011, P. 1–26, DOI 10.2475/01.2011.01],
The PETM also had a mucher higher starting CO2 ppm, therefore in terms of actual change in heating forcing the addition of CO2 then has much less of an effective than now. We are starting at a very low CO2 level in comparison to then and thereforein terms of doubling a 7000-10,000Gt means alot more, and therefore much larger effective heating forcing will result for the same addition of a specific CO2 amount.
Therefore to compare in terms heating disturbance rate therefore, although a 10x faster actual CO2 injection than nature has ever managed is concerning, the above factors will influence things in a way to make it even faster than the 10x, al least 20x due to the ice albedo effect and more again due to lower starting CO2.
Food for thought.
However also possible reassuring that the 5C CO2 induced temperature rise didn't produce the deep sea die off without the addition of the corrosive water, which shouldn't occur this time...is that right?
What is missing here is both the starting CO2 concentration and how much it increased in the <10,000 year time frame. I suppose I can look it up in other sources, but still....
With respect to the increase in temperature, would the doubling effect of CO2 be more important than the starting concentration?
I don't understand the physics here. Most heat enters the ocean from above, so how can the deep ocean become warmer than the top? Unless the world cools, of course, but in that case I'd expect the oceans to shrink rather than overflow the bathtub. It's not like the isolated north Atlantic could be affected by a La Niña-like circulation.
Does it have something to do with the relative densities of sea water and soda water?
Treesong - true, it's not clear in this post, but my interpretation is that the upper North Atlantic Ocean became denser as a result of surface warming and thus higher evaporation, leaving behind saltier, denser, water in the upper ocean. Eventually, the gradual warming in the deep ocean and the increased salinity (density) in the upper ocean reached a critical point whereupon the surface water began to sink. This denser surface water displaced the corrosive deep water - causing it to spill over the sill into the rest of the ocean.
Hi and sorry I am late responding to these comments! Kaitlin here, lead author of the study...
ranyl: You're right, this corrosive water spilling over the sill couldn't happen again today, because none of the deep ocean basins today are isolated like the deep North Atlantic was during the PETM. Ocean acidification coming from the surface is still a major concern though.
knaugle: The starting concentration is 1680 ppm followed by an instantaneous carbon release of 7000 GtC. We also simulated similar behaviour in simulations with starting CO2 ranging from 840 to 2520 ppm, and carbon releases from 3000 to 10,000 GtC (including a gradual carbon release).
RickG: This model simulates present-day surface air temperatures to be about 14 C, more like 13 C for preindustrial (see Eby et al. 2013, doi:10.5194/cp-9-1111-2013). Our PETM simulations started around 24 C, followed by a warming of about 4 C. However, it's not the absolute temperature which matters for ecosystems as much as the rate and magnitude of change.
Treesong2: The deep ocean stays cooler than the surface ocean throughout the entire simulation. However, density depends not just on temperature but also on salinity. The surface and deep North Atlantic had approximately equal contributions of warming to decreased density (although the change happened faster in the surface). But in the surface ocean this was partially offset by increased salinity (due to increased evaporation). So the deep North Atlantic experienced a greater drop in density than the surface did, and eventually in some locations it was less dense than at the surface, even though it was still cooler.
Rob Painting - correct :-)
"But our findings suggest other factors made the Atlantic far more corrosive than the rest of the world’s oceans. This means that sediments in the Atlantic Ocean are not representative of worldwide CO2 concentrations during the PETM."
"Corrosive" - I don't understand, the oceans are alkaline, if they are moving towards the acid side of the PH scale, wouldn't they first have to become more neutral to get there? Aqueous solutions are corrosive at either end of the PH scale but the middle of the scale is the least corrosive. Are you claiming that the oceans actually crossed into being on the acid side of the PH scale like fresh water?
Sinbod... No, "acidification" does not require that you be on the acid side of the scale. It merely requires that you're moving the pH in that direction.
Sinbod:
To follow up on Rob Honeycutt's comment, you are still allowed to say "I'm going south for a holiday this winter", even if you are only going from New York to Miami. You don't have to pass into the southern hemisphere before you are "going south".
My point isn't about the term acidification, it's about claiming that a slightly less alkaline ocean is somehow going to be more corrosive. If PH is heading towards neutral that doesnt make sense. In order for the oceans to become more corrosive, the PH level would have to go either somewhat higher a lot lower (at least below 7) - in 300 million years the oceans have been alkaline. So what gives with the corrosive statement.
[JH] Pease read the intermediate version of the SkS rebuttal article, Ocean acidification: global warming's evil twin.
Sinbod,
The pH at which water is corrosive depends on the reactivity of the material. For sodium, pH 14 water is severely corrosive.
For this discussion the question is at what pH is sea water corrosive to calcium carbonate. It turns out that pH is about pH 8.0. Since the current average pH of the ocean is only slightly above that, it only requires a small amount of acidification to affect shell forming animals.
In high latitudes, carbon dioxide is more soluble in the ocean becasue the temperature of the water is lower. In some of these locations the ocean is already becoming corrosive to shell forming animals. Oregon and Washington have observed die offs of oyster larve from acidified sea water.
Thanks, the relativity of the material, understood.
Note, I live in the northwest. I was concerned about the shell fish issue however when I read the acidification paper in detail it was clear that the results were ambiguous with some species unchanged, some worse and some better off. I also reviewed the oyster issue which was a problem with parasites... Have you read the details?
Since there are fresh water shellfish, is 8 some kind magic Ph bad spot for a specific type of shellfish?
Note, not denying anything - just like to understand what on the surface seems contradictory.
[Rob P] - The die-off of larval oysters in the hatcheries was due to the intake of corrosive (carbonate undersaturated) sea water. See my SkS post: Corrosive Seawater, Not Low pH, Implicated As Cause of Oyster Deaths.
It's not pH per se that is the issue, as the oceans will remain alkaline, but rather the decline in carbonate ion abundance that results when more carbon dioxide is dissolved into the oceans. As carbonate ion abundance decreases so does the carbonate saturation state. When undersaturation is reached seawater becomes physically corrosive to calcium carbonate forms.
Sinbod @12, generating shells requires energy. How much energy will depend on the availability of the base materials in the environment, and the corrosivity of the environment to the shell. Because in most circumstances these will be fairly stable features, animals with shells will evolve specific mechanisms to generate shells in their particular environments, tending towards the least metabolic cost pathway available. As a result, we would not expect corrosivity to be an equal factor across species regardless of the background pH in their normal environment. Further, some species may be robust to change in pH within a range because that range of pH is the normal in their environment. Further, some species to be more robust to change in pH outside the normal range in their environment, not due to evolved capability but just by chance, depending on the specific mechanism of shell generation they use.
The issue, therefore, is not whether there is some magic number which is corrosive for shell fish. Rather, for all shell fish there will be a pH range (different for different species) which represents the mean and deviation normally experienced in their environment, and pushing the pH range below the range of variability will be deleterious to the species. At a minimum level it will be harmful by either/or increasing the metabolic cost of maintaining the shell or thining the shell resulting in it being less protection against predators. At the high end, the species will be unable to grow shells at all. The greater the decrease in pH the greater the risk of a high end response. Further, the more rapid the change in pH (and hence the less time for an evolved response), the greater the risk of a high end response.
Finally, as I understand it, the real issue is aroganite saturation, rather than the pH itself, with pH being a good proxy for aragonite saturation (but chemistry is not my strong suite so don't quote me on that).
Sinbod,
Googling "Washington oysters pH" gives many hits on problems with pH in the Washington oyster aquaculture program. Most are newspapers but this one is from NOAA. When I Googled "Washington Oysters parasite" I got no hits. Can you cite a reference for your claim that the problem was parasites?
There are several articles that detail acidic water killing the oyster spat. They monitor the pH of the incoming water at the hatcheries and have resolved the issue for the present. Washington is first affected because upwelling water there is from further north and is not very old. Others will be affected as the acidic water moves south on currents. As CO2 concentrations in the air increase, pH will decrease more and more.
As Tom said, some species will be affected earlier than others. These oysters are vulerable when they are very small. Other species will be affected differently (although young animals are often less resilient than older animals).
Freah water species have evolved to survive in lower pH. The biggest problem is not the absolute pH but the change from what the animals are adapted to. It is very damaging for most animals to have change from optimal conditions.
Excellent explanation. Various species of organisms have there own sweet spot and anything outside that range is potentially quite harmful to development. In theory stomach bacteria would find a neutral Ph environment quite distasteful. Is it fair to say that anything outside the prefered Ph level is "corrosive"?
[Rob P] - No. See my previous comment. Whether or not seawater is corrosive to calcium carbonate shells and skeletons depends on the calcium carbonate saturation state which decreases with geologically-rapid injections of carbon dioxide into the atmosphere.
On the left-hand column of the page is a series called OA not OK. It was written by experts in this field and goes into quite some detail. The answer to many common myths about ocean acidification can be found there.
Sinbod,
The issue here is calcium carbonate dissolving due to lower pH. An increase in pH, while it would be harmful to animals, would not dissolve the shells of shellfish and corals. Therefor an increase in pH would not be corrosive in the way acidic water is. I think corrosive is being used because it is a description of the action of the acidic water.
Sinbod @15:
Probably not. Low pH (or insufficient aragonite) will result in more difficulty building and maintaining shells. High pH will probably result in either wasted metabolic energy (ie, the shell building process is not as efficient as it could be in the conditions), and/or excess calcium deposition. The later may deform shells or result in depositions in otherwise harmfull locations (ie, the mollusc equivalent of gout or kidney stones). Both the metabolic inefficiency and excess deposition may make species vulnerable to displacement by invasive species better adapted to the new conditions. Other than that, however, neither will cause short term problems SFAIK.
I do wonder if a quick lesson in what pH actually measures would be worthwhile. There is no "neutral" pH as such. Water is famously H2O. However, the hydrogen in H2O isn't as sticky as it could be so pure water also contains H1O and H3O. pH is actually a measure of the stickiness of hydrogen within a solution. (It's actually hydrogen ions. There's an electrical charge involved +H.) In pure water one-in-10million mollecules are H30, a ratio which can also be expressed as 1:107. This 7 is the measure of pH, the equilibrium stickiness of hydrogen in pure water. Change the solution by adding stuff with different stickiness and the stickiness will obviously change.
So something with a low pH has more +H flying about which makes it more acidic than something with a high pH where the hydrogen is stickier so doesn't fly about so much. But with high pH there is still some unstuck +H. Thus there is no "neutral" as such. The popular idea that alkali is the opposite of acid is a bit of a nonsense. But explaining that is a whole lot more complicated.
@18,
I -for one- appreciate this insight into the phenomenon!
@16,
Colourful Language is what haunts this 24-7 Hollywood-inspired world of buy-buy-buy-NOW!!
@17, possibly worth knowing en masse to prevent the resource bottlenecks of panic that are predicted!
The definition of "corrosive" means that calcium carbonate (CaCO3) ions are undersaturated in the ocean, so sedimentary CaCO3 will keep dissolving until saturation is achieved. It doesn't necessarily have anything to do with pH.