Volcanic Carbon DioxideTimothy Casey B.Sc. (Hons.)
Revision 2 ISO:2011-Dec-11
A brief survey of the literature concerning volcanogenic carbon dioxide emission finds that estimates of subaerial emission totals fail to account for the diversity of volcanic emissions and are unprepared for individual outliers that dominate known volcanic emissions. Deepening the apparent mystery of total volcanogenic CO2 emission, there is no magic fingerprint with which to identify industrially produced CO2 as there is insufficient data to distinguish the effects of volcanic CO2 from fossil fuel CO2 in the atmosphere. Molar ratios of O2 consumed to CO2 produced are, moreover, of little use due to the abundance of processes (eg. weathering, corrosion, etc) other than volcanic CO2 emission and fossil fuel consumption that are, to date, unquantified. Furthermore, the discovery of a surprising number of submarine volcanoes highlights the underestimation of global volcanism and provides a loose basis for an estimate that may partly explain ocean acidification and rising atmospheric carbon dioxide levels observed last century, as well as shedding much needed light on intensified polar spring melts. Based on this brief literature survey, we may conclude that volcanic CO2 emissions are much higher than previously estimated, and as volcanic CO2 contributions are effectively indistinguishable from industrial CO2 contributions, we cannot glibly assume that the increase of atmospheric CO2 is exclusively anthropogenic.
1.0 Introduction: How Volcanoes make the Carbon Budget Holier than Thou
If we neglect to ask how the greenhouse effect of various gases is quantified in terms of real, measurable thermodynamic properties, the idea of anthropogenic global warming may well survive long enough for us to ask how the carbon budget establishes that observed increases in CO2 (Keeling et al., 2005) could not be caused by anything other than human activity. Plimer (2001), Wishart (2009), and Plimer (2009) point out that an enormous and unmeasured amount of CO2 degases from volcanoes. This is not such a silly idea given that the source chemistry for lavas contains a surprising amount of carbon dioxide. Along with H2O, CO2 is one of the lightest volatiles (materials of relatively low melting point), found in the mantle (Wilson, 1989). The fluid nature of the aesthenosphere, or upper mantle of the earth, ensures that lighter volatiles are fractionated, buoyed towards the surface, and either extruded or outgassed into the atmosphere via volcanoes and faults. The "solid earth", a term popular amongst climatologists, is a deceptive misnomer as the aesthenosphere is a deeply convecting fluid upon which flexible sheets of crust (i.e. plates) float. This deeply convecting fluid tears these delicate plates apart at rift zones and crushes them together like the bonnet of a wrecked car at convergence zones. Mountains rise out of fold belts resulting from the crumpling of plates, and where differences in plate buoyancy allow, one plate rides over another, forcing the other plate to follow the convection current into the aesthenosphere. Furthermore, this liquid aesthenosphere, which continues to create new crust at rifting zones such as the mid oceanic ridges, melts down subducting crust as the residue of this crust is drawn deeper into the mantle. While volatiles trapped in the remaining crustal residue are ultimately assimilated into the mantle, lighter volatiles from the crustal melt are fractionated and float up towards the surface to feed plate margin volcanoes. Volatiles, such as CO2, are more prone to outgassing at the surface via tectonic and volcanic activity because of the fluid nature of the earth.
1.1 The Importance of CO2 in Volcanic Emissions
The importance of juvenile (erupted and passively emitted) volcanic CO2 is due to the fact that carbon, and particularly carbon dioxide has a strong presence in mantle fluids, so much so that it is a more abundant volcanic gas than SO2 (Wilson, p. 181; Perfit et al., 1980). According to Symonds et al. (1994) CO2 is the second most abundantly emitted volcanic gas next to steam. Although you might imagine that there is no air in the mantle, the chemical conditions favour oxidation, and shortages of oxygen ions are rare enough to ensure a strong presence of CO2 (Schneider & Eggler, 1986). Oxidation of subducted carbon sources such as kerogen, coal, petroleum, oil shales, carbonaceous shales, carbonates, etc. into CO2 and H2O makes volcanic CO2 quite variable in back arc and continental margin volcanoes, where these volatile gases can be surprisingly abundant (eg. Vulcano & Mount Etna). Subduction isn't the only way CO2 enters magma. At continental rift zones, where an entire continent is being pulled apart by divergent mantle convection, magma rising to fill the rift is enriched in CO2 from deep mantle sources (Wilson, 1989, p. 333). Oldoinyo Lengai is an example of a continental rift zone volcano, which has above average CO2 outgassing at 2.64 megatons of CO2 or 720 KtC per annum (Koepenick et al., 1996).
If volcanoes produce more CO2 than industry when they are not erupting, then variations in volcanic activity may go a long way towards explaining the present rise in CO2.
1.2 The Location of CO2 Monitoring Station in regions enriched by volcanic CO2
Volcanic CO2 emission raises some serious doubts concerning the anthropogenic origins of the rising atmospheric CO2 trend. In fact, the location of key CO2 measuring stations (Keeling et al., 2005; Monroe, 2007) in the vicinity of volcanoes and other CO2 sources may well result in the measurement of magmatic CO2 rather than a representative sample of the Troposphere. For example, Cape Kumukahi is located in a volcanically active province in Eastern Hawaii, while Mauna Loa Observatory is on Mauna Loa, an active volcano - both observatories within 50km of the highly active Kilauea and its permanent 3.2 MtCO2pa plume. Samoa is within 50 km of the active volcanoes Savai'i and/or Upolo, while Kermandec Island observatory is located within 10 km of the active Raoul Island volcano.
Observatories located within active volcanic provinces are not the only problem. There is also the problem of pressure systems carrying volcanic plumes several hundred kilometers to station locations. For example, the observatory in New Zealand, located somewhere along the 41st parallel, is within 250 km of Tanaki and the entire North Island active volcanic province. Low pressure system centres approaching and high pressure system centres departing the Cook Strait will displace volcanic plumes from the North island to the South Island.
Another class of problem for monitoring stations plagues "Christmas Island", which is actually Kiribati Island (02º00'N, 157º20'W) where the Clipperton Fracture Zone (Taylor, 2006) crosses the Christmas Ridge and is nowhere near Christmas Island (10º29'S, 105º38'E; located on the other side of Australia, 10,000 km due west of Kiribati). Christmas Ridge is formed in a concentration of Pacific Seamounts. Extraordinary numbers of seamounts are volcanically active (Hillier & Watts, 2007). Moreover, active fracture zones also offer a preferred escape route for magmatic CO2, as this CO2 also finds its way into aquifers (eg. Giggenbach et al., 1991), which can be cut by fracture zones that consequently provide a path to the surface (Morner & Etiope, 2002). This may raise doubts concerning measurements taken at the La Jolla observatory, which is located near the focal point of a radial fault zone extending seaward from the San Andreas Fault (see imagery sourced to SIO, NOAA, USN, NGA, & GEBCO by Europa Technologies & Inegi, for Google Earth).
Amundsen Scott South Pole Station appears to be well separated by 1300 km from the volcanic lineation extending along Antarctica's Pacific Coast (From the Ross Shelf to the Antarctic Peninsula), However, Antarctic volcanoes are not nearly as well mapped as those in more populated regions, such as Japan. In any case, the strong circumpolar winds that delay mixing will inevitably concentrate Antarctica's volcanic CO2 emissions over the Antarctic continent, including Amundsen Station. The same potential problem exists with the observatory at Alert in Northern Canada, because it is located inside the circumpolar wind zone along with the Arctic Rift and thousands of venting seamounts along key parts of the Northwest Passage.
That leaves us with Point Barrow, arguably the only CO2 monitoring station whose CO2 measurements are unlikely to be influenced by magmatic gas plumes. However, the Canada Basin, extending seaward from Point Barrow, is also referred to as "the Hidden Ocean" because of poor access, which consequently leaves us with very little information about the sea floor in this region. The high probability of active seamounts in the vicinity of Point Barrow has not been ruled out, and in view of the fact that the other observatories probably experience significant skew due to magmatic CO2, it would not be unreasonable to remain skeptical until this possibility has been ruled out.
This question of volcanic skew in CO2 measurements has been raised a number of times, in addition to other more serious allegations (Bacastow, 1981; Jaworowski et al., 1992; Segalstad, 1996).
2.0 Calculated Estimates: Glorified Guesswork
The estimation of worldwide volcanic CO2 emission is undermined by a severe shortage of data. To make matters worse, the reported output of any individual volcano is itself an estimate based on limited rather than complete measurement. One may reasonably assume that in each case, such estimates are based on a representative and statistically significant quantity of empirical measurements. Then we read statements, such as this one courtesy of the USGS (2010):
Scientists have calculated that volcanoes emit between about 130-230 million tonnes (145-255 million tons) of CO2 into the atmosphere every year (Gerlach, 1991). This estimate includes both subaerial and submarine volcanoes, about in equal amounts.
In point of fact, the total worldwide estimate of roughly 55 MtCpa is by one researcher, rather than "scientists" in general. More importantly, this estimate by Gerlach (1991) is based on emission measurements taken from only seven subaerial volcanoes and three hydrothermal vent sites. Yet the USGS glibly claims that Gerlach's estimate includes both subaerial and submarine volcanoes in roughly equal amounts. Given the more than 3 million volcanoes worldwide indicated by the work of Hillier & Watts (2007), one might be prone to wonder about the statistical significance of Gerlach's seven subaerial volcanoes and three hydrothermal vent sites. If the statement of the USGS concerning volcanic CO2 is any indication of the reliability of expert consensus, it would seem that verifiable facts are eminently more trustworthy than professional opinion.
This is not an isolated case. Kerrick (2001) takes a grand total of 19 subaerial volcanoes, which on p. 568 is described as only 10% of "more than 100 subaerial volcanoes". It is interesting to observe that Kerrick (2001) leaves out some of the more notable volcanoes (eg. Tambora, Krakatoa, Mauna Loa, Pinatubo, El Chichon, Katmai, Vesuvius, Agung, Toba, etc.). Nevertheless, despite these omissions Kerrick calculates 2.0-2.5 x 1012 mol of annual CO2 emissions from all subaerial volcanoes, which is understated on the assumption that the sample is from the most active volcanic demographic. This is in spite of the fact that eight of the world's ten most active volcanoes are omitted from Kerrick's study (Klyuchevskoy Karymsky, Shishaldin, Colima, Soufriere Hills, Pacaya, Santa Maria, Guagua Pichincha, & Mount Mayon). At 44.01g/mol, 2.0-2.5 x 1012 mol of CO2 amounts to a total of 24-30 MtCpa - less than 0.05% of total industrial emissions (7.8 GtCpa according to IPCC, 2007). My main criticism of Kerrick's guess is that it putatively covers only 10% of a highly variable phenomenon on land, and with the cursory dismissal of mid oceanic ridge emissions, ignores all other forms of submarine volcanism altogether. If we take the Smithsonian Institute's list of more than 1000 potentially active subaerial volcanoes worldwide, Kerrick's 10% is reduced to 1-3%.
According to Batiza (1982), Pacific mid-plate seamounts number between 22,000 and 55,000, of which 2,000 are active volcanoes. However, none of the more than 2,000 active submarine volcanoes have been discussed in Kerrick (2001). Furthermore, Kerrick (2001) justifies the omission of mid oceanic ridge emissions by claiming that mid oceanic ridges discharge less CO2 than is consumed by mid oceanic ridge hydrothermal carbonate systems. In point of fact, CO2 escapes carbonate formation in these hydrothermal vent systems in such quantities that, under special conditions, it accumulates in submarine lakes of liquid CO2 (Sakai, 1990; Lupton et al., 2006; Inagaki et al., 2006). Although these lakes are prevented from escaping directly to the surface or into solution in the ocean, there is nothing to prevent superheated CO2 that fails to condense from dissolving into the seawater or otherwise making its way to the surface. It is a fact that a significant amount of mid oceanic ridge emissions are not sequestered by hydrothermal processes; a fact which is neglected by Kerrick (2001), who contends that mid oceanic ridges may be a net sink for CO2. This may well sound reasonable except for the rather small detail that seawater in the vicinity of hydrothermal vent systems is saturated with CO2 (Sakai, 1990) and as seawater elsewhere is not saturated with CO2, it stands to reason that this saturation is sourced to the hydrothermal vent system. If the vent system consumed more CO2 than it emitted, the seawater in the vicinity of hydrothermal vent systems would be CO2 depleted.
Morner & Etiope (2002) published a somewhat more representative estimate of subaerial volcanogenic CO2 output based on a more comprehensive selection and found as a bare minimum that subaerial volcanogenic CO2 emission is on the order of 163MtCpa. Morner & Etiope (2002) also provide a much better explanation of how CO2 is cycled through the mantle and the lithosphere. However, this still does not account for active volcanic emissions and remains vulnerable to eruptive variability. Based on data reproduced in Shinohara (2008), there were on average about five subaerial volcanic eruptions every year producing an average of 300KtSpa (kilotons of sulphur per year) from 1979-1989. Shinohara (2008) also presents molar ratios of CO2, SO2, & H2S from which, via the same academic daring as Gerlach (1991) and Kerrick (2001), we might derive an average ratio of 3.673 mol carbon for every mol of sulphur in gaseous volcanic emissions. That would loosely translate to 1.376KtC for every 1.000KtS. This gives us a figure of around 2MtCpa for minor volcanic activity based on SO2 emission events reported in Shinohara (2008). However, applying the same statistical assumption to some of the more notable eruptions of recent history, contrasted with one or two slightly older examples, gives us the following estimates:
|Year||Volcano||Mean Sulphurous Output||Source||Est. Carbon output during year(s) of eruption|
|1883AD||Krakatoa||38 MtSO2pa||Shinohara (2008)||26.14 MtCpa|
|1815AD||Tambora||70 MtSO2pa||Shinohara (2008)||48.16 MtCpa|
|1783AD||Laki||130 MtSO2pa||Shinohara (2008)||89.44 MtCpa|
|1600AD||Huaynaputina||48 MtSO2pa||Shinohara (2008)||33.02 MtCpa|
|1452AD||Kuwae||150 MtH2SO4pa||Witter & Self (2007)||67.40 MtCpa|
|934AD||Eldja||110 MtSO2||Shinohara (2008)||75.68 MtCpa|
|1645BC||Minoa||125 MtSO2pa||Shinohara (2008)||86.00 MtCpa|
|circa 71,000BP||Toba||1100 MtH2SO4pa||Zielenski et al. (1996)||494.24 MtCpa|
Notice how all but one of the individual annual volcanogenic carbon outputs, estimated above, dwarf the global subaerial volcanogenic carbon outputs estimated by both Gerlach (1991) & Kerrick (2001). Even the Morner & Etiope (2002) subaerial estimate (163 MtCpa) is shaken by most of these figures and dwarfed by one. If this is not enough evidence of just how unreliable volcanic emission estimates can be, let us take a closer look at my 89 MtCpa estimate for the 1783AD Laki eruption. Consider the difference it makes if, instead of using the average ratio by weight for carbon and sulphur emissions I derived from Shinohara (2008), we take the ratio we use for the Laki estimate from more direct observations. Agustsdottir & Brantley (1994) studied emissions from Grimsvotn, from which Laki extends as a fissure, and found that Grimsvotn outgasses 53 KtCpa for 5.3 KtSpa. In other words, the weight of carbon emitted at Grimsvotn is ten times that of the sulphur emitted there. This would extend to Laki, which shares the same source, and is described by Agustsdottir & Brantley (1994) as a fairly stable ratio. By this ratio, Laki's 130 Mt of sulphur dioxide in 1783AD translates to an emission of 650 MtCpa that year. This demonstrates just how much uncertainty is involved when trying to audit the volcanic contribution to the "carbon budget".
As you can see, volcanic systems are diverse and unpredictable. They cannot be statistically second-guessed for the same reason that lottery numbers cannot be statistically second-guessed. This in itself raises serious doubt concerning the reliability of volcanic carbon dioxide emission estimates. This is especially problematic when significant elements of the estimates, such as passive submarine volcanic emission, all active volcanic emission, and at least 96% of passive subaerial emissions, are based on statistical assumptions rather than on any actual measurement.
3.0 Abusing Doctor Suess: Pulling the Cat out of the Hat
So far, the evidence presents the rather tantalizing implication that volcanogenic CO2 emission is a significant if not dominant contributor to atmospheric CO2 levels. The next logical step for those trying to prove that the CO2 rise is anthropogenic is to find a signature to fingerprint anthropogenic CO2 as separate from all other sources of CO2. The research of one Harmon Craig, first submitted for publication on ISO:1953-Apr-20, found that 13C & 14C are enriched in carbonates. Harmon Craig discusses the carbon dating errors that can be introduced by natural isotopic fractionation, along with other processes (Craig, 1954). While Rankama (1954), suggests that 13C depletion is characteristic of biogenic sources, Craig (1954) goes so far as to suggest the use of 13C as a tracer for 14C. This becomes the subject of research by Hans E. Suess into the contamination of 14C dates by variations in normal atmospheric 14C, which quantified the effect of processes discussed by Craig (1954). Part of Suess' explanation of his own results was seized upon as a way to fingerprint fossil fuel CO2 because fossil fuels, being too old to contain measurable amounts of this cosmogenic isotope, will deplete atmospheric concentrations of 14C when burned. In Cleveland & Morris (2006, p. 427) Hans Suess and the Suess Effect, used to account for contamination of radiocarbon dates by various phenomena, are given the following entries:
Suess, Hans 1909-1993, U.S Chemist who developed an improved method of carbon-14 dating and used it to document that the burning of fossil fuels had a profound influence on the earth's stocks and flows of carbon. (Fossil fuels are so ancient that they contain no C-14.)
Suess Effect Climate Change. a relative change in the ratio of C-14/C or C- 13/C for a carbon pool reservoir; this indicates the addition of fossil fuel CO2 to the atmosphere.
However, this is only half of the explanation offered by Suess. In Suess (1955, p. 415) we read:
The decrease can be attributed to the introduction of a certain amount of C14-free CO2 into the atmosphere by artificial coal and oil combustion and to the rate of isotopic exchange between atmospheric CO2 and the bicarbonate dissolved in the oceans.
As you can see, Suess himself puts the Suess Effect down to more than just fossil fuel consumption. Yet, the exclusion of other processes, such as isotopic exchange and volcanic input, are hardly surprising given the assumption that fossil fuels are the only cause of 14C depletion. This assumption has quite some history in the literature. According to Tans et al (1979):
THE dilution of the atmospheric 14CO2 concentration by large amounts of fossil-fuel derived CO2 which do not contain any 14C is commonly called the Suess effect. Its magnitude can be calculated with the same geochemical models as the global carbon cycle that also predict the future rise of atmospheric CO2 to be caused by the combustion of fossil fuels.
Keeling (1979) concurs with a bizarre emphasis on "formulating models rather than surveying and interpreting data". This reflects the rather general attitude, amongst anthropogenic global warming proponents, that the Suess Effect fingerprints the rising atmospheric carbon dioxide as the exclusive product of fossil fuel combustion. Does such a narrow interpretation concur with the original author's idea? Suess (1955), who first proposes the idea that fossil fuels may contaminate the carbon isotope reservoir with adverse effects on carbon dating methods, estimates that fossil fuel CO2 accounted for less than 1% of carbon isotope reservoir contamination.
The smaller effects noted in the other three trees indicate relatively large local variations of CO2 in the atmosphere derived from industrial coal combustion, and that the worldwide contamination of the earth's atmosphere with artificial CO2 probably amounts to less than 1 percent.
While, superficially, this may be interpreted as either 1% of contamination or 1% of total atmospheric carbon, the apparently "smaller effects" of "large local variations" in atmospheric CO2 due to industry shows that something other than industrial CO2 accounts for the bulk of the effect. Suess' next statement further clarifies this point:
Hence the rate by which this CO2 exchanges and is absorbed by the oceans must be greater than previously assumed.
It does not necessarily follow from a 1% contamination of total atmospheric carbon that other processes are at work. Only if industrial CO2 provides 1% of the 14C contamination does it necessarily follow that, hence, another process must play a greater role. In other words, Suess acknowledged that other sources of contamination played a much larger role (Suess, 1955, p.416), but authors, such as Fergussen (1958), Keeling (1979), Tans (1979), Cleveland & Morris (2006), ignored this rather important point. Moreover, insistent on correcting the "misleading" arguments of Durkin (2007) in their 2007 glossy handout, Climate Change Controversies: a simple guide, the Royal Society gets its name plastered to this evident faux pas:
In contrast to this natural process, we know that the recent steep increase in the level of carbon dioxide – some 30 per cent in the last 100 years – is not the result of natural factors. This is because, by chemical analysis, we can tell that the majority of this carbon dioxide has come from the burning of fossil fuels.
Aside from the fact that isotopic analysis is not chemical analysis, I would go so far as to suggest that the same basin sediment kerogen (the carbon source for oil) in addition, no doubt, to some petroleum reservoirs have been subducted and are a major source for volcanic CO2 emissions at continental margins. Due to the fact that the subduction zone is where crustal material enters the mantle, subducted carbon reservoirs would represent the youngest magmatic source of CO2. Given the confirmed presence of carbon and particularly CO2 enriched fluids in magma and lava (Wilson, 1989), one may well ask if it would not be so irrational to suppose that volcanogenic carbon released at continental margins (closest to the subduction zone) is very old; far too old in fact to contain any measurable amount of 14C. Moreover, mantle carbon and CO2 is vastly older still, as only longer lived cosmogenic isotopes such as 10Be can be used to confirm the speed of mantle convection. In fact, Clark & Fritz (1997) document that there is no volcanic emission of 14C.
The misuse of the Suess Effect as a fossil fuel fingerprint instead of an empirical standard for the correction of carbon dating contamination, lead to an initially idiosyncratic expansion of this concept by Keeling (1979), who sought to include 13C depletion of vegetation and its effect on the atmosphere. The atmosphere is enriched in 13CO2 by the process of photosynthesis, which favours the assimilation of 12C into plant tissue during growth (Furquhar et al., 1989). This is used to differentiate between terrestrial and oceanic CO2 sources (Keeling et al., 2005), and the concept, proposed by Craig (1954), is actually older than Suess' original research. Moreover, plant based fossil fuel derivatives are therefore considered to be 13C depleted. Following this line of logic, fossil fuel emissions, being derived from plants, should be 13CO2 depleted as well. However, when the Keeling (1979) article expanded its internal definition of the Suess Effect to include this observation, it was once again to the exclusion of volcanic influence.
In point of fact, magmatic carbon is, for the most part, 13C depleted. This is solidly confirmed by numerous studies of deep mantle rocks (Deines et al., 1987; Pineau & Mathez, 1990; Cartigny et al., 1997; Zheng et al., 1998; Puustinen & Karhu, 1999; Ishikawa & Marayuma, 2001; Schultz et al., 2004; Cartigny et al., 2009; Statchel & Harris, 2009) as well as mid-oceanic ridge outgassing (De Marais & Moore, 1984). Moreover, 13C depletion of volcanic emissions is so well known that Korte and Kozur (2010) explore volcanism, amongst other possible causes, in search of an explanation for atmospheric depletion of 13C across the Permian-Triassic boundary. Although many significant carbonates are not 13C depleted, they are eventually subducted along with organic carbon sources depleted in 13C. Nevertheless, the emissions of continental margin and back arc volcanoes that source a significant proportion of their carbon from subducted volatiles, remain 13C depleted (eg. Giggenbach et al., 1991; Sano et al., 1995; Hernández et al., 2001). Thus, as plants continue to enrich the atmosphere in 13C while supplying the 13C depleted kerogen that is subducted into the mantle, volatiles failing to return to the surface may cause the mantle to become increasingly 13C depleted over time. Moreover, the significant proportion of volcanic carbon dioxide that diffuses through the soil (Gerlach, 1991) has its carbon isotope chemistry further contaminated by 13 depleted biogenic soil carbon (Hernández et al., 2001).
Both tectonic and volcanic CO2 are magmatic and depleted in both 13C & 14C. In the absence of statistically significant isotope determinations for each volcanic province contributing to the atmosphere, this makes CO2 contributions of volcanic origin isotopically indistinguishable from those of fossil fuel consumption. It is therefore unsurprising to find that Segalstad (1998) points out that 96% of atmospheric CO2 is isotopically indistinguishable from volcanic degassing. So much for the Royal Society's unexplained "chemical analysis". If you believe that we know enough about volcanic gas compositions to distinguish them chemically from fossil fuel combustion, you have indeed been mislead. As we shall see, the number of active volcanoes is unknown, never mind a tally of gas signatures belonging to every active volcano. We have barely scratched the surface and as such, there is no magic fingerprint that can distinguish between anthropogenic and volcanogenic sources of CO2.
4.0 The Rise and Fall of Oxygen
Manning et al. (1999) find, as an average at La Jolla, that 1.3 mol of O2 are consumed for every mol of CO2 produced. They point out that if all atmospheric CO2 was produced by the combustion of fossil fuels, this result would be 1.44 mol of O2 consumed for every mol of CO2 produced. Cellular respiration as a simplified reaction is as follows:
C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O
Photosynthesis does not throw out the balance of cellular respiration following the same molar ratios of CO2 and O2 in play:
6 CO2 (g) + 6 H2O → C6H12O6 (aq) + 6 O2 (g)
As you can see, the net effect of respiration is to lower the number of mols of O2 consumed for every mol of CO2 produced with no skew introduced by photosynthetic consumption of CO2. Volcanoes, once again ignored by Manning et al (1999), produce CO2 freely without any directly observed O2 consumption, although it remains possible that volcanic activity may well consume significant amounts of O2. This could explain the mystery of the loss of half the atmosphere's oxygen 250 million years ago; a mystery that remains unsolved (see Berner et al, 2003). As we can't clearly identify exactly what amount of atmospheric O2 is consumed by volcanic processes (eg. oxidation of H2S to H2O, SO2, H2SO4) for every mol of volcanogenic CO2 released to the atmosphere, we can only guess that volcanogenic emissions reduce this ratio towards a figure substantially less than unity. The argument is therefore made that because we don't see a significantly lower ratio, volcanogenic CO2 cannot possibly be very much. This however, is a deduction from a guess, and clearly neglects common oxidation reactions that consume O2 without producing any CO2, such as some forms of corrosion, combustion of certain volcanic volatiles, and weathering. For example:
4 Fe + 3 O2 → 2 Fe2O3
H2S + 2 O2 → H2SO4
2 H2S + 3 O2 → 2 H2O + 2 SO2
4 FeSiO3 + 2 O2 + 2 H2O → 4 FeO(OH) + 4 SiO2
Weathering and the successive oxidation of elements like iron from minerals such as pyroxenes present a major example of how oxygen is consumed without producing carbon dioxide, because carbon is not the only element on the planet that preferentially combines with oxygen. Such reactions drive the number of mol of O2 consumed per mol of CO2 produced higher. As you can see, it is not only fossil fuels that drive this ratio in this direction, and it is a simpler matter to more comprehensively measure volcanic CO2 output to determine whether volcanoes are indeed a significant CO2 contributor.
5.0 Plimer Strikes Again: 139,000 Intraplate Volcanoes Leaking CO2 into the Ocean
Until reading Hillier & Watts (2007), I would have estimated that the oceans, occupying twice the surface area of land, would have twice the number of volcanoes. In fact the number of submarine volcanoes is very much higher than twice the number of subaerial volcanoes. Given the update of Werner & Brantley (2003), which raises the estimate of subaerial volcanogenic CO2 from 27±3 MtCpa to 78±6 MtCpa, this would seem to imply roughly 200 MtCpa from submarine volcanogenic CO2 and brings the total estimate of volcanic CO2 in line with the bare minimum determined by Morner & Etiope (2002). Plimer (2001; 2009) & Wishart (2009) maintain that the amount of CO2 from volcanoes is enormous, and without estimating an amount suggests that it dwarfs anthropogenic contributions. If we take the updated estimate, correct the conservative bias, and extend to submarine environments we still wind up with a figure around 1.5 GtCpa for total passive volcanic emissions (excluding imponderables such as mid oceanic ridge emissions) and that is still only 20% of the 7.8 GtCpa attributed to anthropogenic CO2 emissions by the IPCC. As it turns out, there is a lot more to the distribution of volcanoes across different tectonic settings, and Plimer (2009) omits the rather small detail of a 2007 paper presenting primary evidence that underpins his claim in spectacular fashion.
Hillier & Watts (2007) surveyed 201,055 submarine volcanoes estimating that a total of 3,477,403 submarine volcanoes exist worldwide. According to the observations of Batiza (1982), we may infer that at least 4% of seamounts are active volcanoes. We can expect a higher percentage in the case of the count taken by Hillier & Watts (2007) because it includes smaller, younger seamounts; a higher proportion of which will be active. Nevertheless, in the spirit of caution and based on our minimum inference of 4% seamount activity from Batiza's observations, I estimate 139,096 active submarine volcanoes worldwide. If we are to assume, in the absence of other emission figures for mid oceanic plate volcanoes, that Kilauea is a typical mid oceanic plate volcano with a typical mid oceanic emission of 870 KtCpa (Kerrick, 2001), then we might estimate a total submarine volcanogenic CO2 output of 121 GtCpa. Even if we assume, as Kerrick (2001) and Gerlach (1991) did, that we've only noticed the most significant outgassing and curb our estimate accordingly, we still have 24.2 GtCpa of submarine volcanic origin.
If guesses of this order are anywhere near the ballpark, then we can take it that either what has been absorbing all this extra CO2 is not absorbing as much or there has been some variation to volcanic output over the past 500 years or so. Both are normal assumptions given the variable state of the natural environment, and considering that vegetation consumed something on the order of 38GtCpa more in 1850 than today (see my Deforestation article for the quick and dirty calculation), it is hardly surprising that we were missing a large natural CO2 source in the carbon budget. The other possibility is that both Werner et al (2000: approx. 38 KtCpa) and Werner & Brantley (2003: approx. 4000 KtCpa) are correct, which could imply that volcanogenic CO2 emissions are increasing. This certainly would explain steadily rising CO2 observed at stations in regions most affected by volcanic emissions, it could partly explain the recent increase in ocean acidification discussed by Archer (2009, pp. 114-124), and further it would explain the more intense Spring melting centred on the Pacific Coast of Antarctica and along the Gakkel Ridge under the Arctic ice cap.
6.0 Conclusion: Three Million Volcanoes "Can't be Wrong"
The second most erupted gas on the planet next to steam has a significant magmatic source in which it is preferentially fractionated towards the surface. On the scale of atmospheric composition, the isotopic composition of volcanogenic CO2 is effectively indistinguishable from fossil fuel CO2 due to the complete lack of statistically significant carbon isotope determinations for each of the contributing volcanic and tectonic provinces. Moreover, molar oxidation estimates cannot be used to constrain volcanogenic CO2 output because such estimates neglect the fact that carbon is not the only abundant element on the planet that preferentially combines with oxygen. It is only through emission monitoring taken in statistically significant empirical samples for each volcanic province that we may calculate a scientific estimate of total worldwide volcanic CO2 emission and perhaps, with statistically significant carbon isotope data for each volcanic province, we may one day be able to distinguish volcanic and industrial CO2 contributions in the atmosphere.
Eruptions and volcanic geochemistry are highly variable and so too are volcanic emissions. The lack of any sizeable volcanic eruptions (on the scale of Krakatoa, Tambora, Laki, Huaynaputina, Kuwae, Eldja, etc.) in the 20th Century confirms the volcanic quiescence of this time. Perhaps the reduction of frequency and amount of SO2 ejected into the stratosphere may explain the slight upward trend of atmospheric temperature last century. Perhaps the simplest explanation for the last century's volcanic quiescence is a greater and more consistent release of volcanic gases in passive emissions whose sub-surface accumulation would have otherwise resulted in the buildup of pressure in magma chambers, and consequently much more violent eruptions.
Irrespective that some authors may neglect to allow for significant volcanogenic CO2 input to the atmosphere, volcanoes represent an enormous CO2 source that is mostly submarine. Furthermore, volcanic activity beneath both ice caps and localized to the regions of most intense melting has demonstrated an obvious cause of stronger Spring melts at the Poles. It is evident from the observations of Sohn et al. (2008) & Reves-Sohn et al. (2008) that the Northwest Passage was opened up by powerful volcanic activity under the Arctic Ice along the Gakkel Ridge, while West Antarctic melting (as opposed to thickening of ice throughout the rest of Antarctica) can be explained by recent volcanic activity beneath the ice (Corr & Vaughan, 2008). Moreover, there are simply too many volcanoes to deny that the atmospheric concentration of the most erupted gas next to water is predominantly controlled by the balance or lack thereof between volcanic activity and photosynthesis. Furthermore, there is simply no established volcanic CO2 fingerprint by which we may distinguish atmospheric proportions of anthropogenic and volcanogenic contributions. This leaves us with no empirical method by which we may attribute the 20th century rise in CO2 to human energy consumption.
7.0 Postscript: Science Overtakes Philosophy
This has been a wild ride upon ricketty speculations about a subject, of which, we know far too little. This year has seen a couple of publications which shine a spotlight on the great divide between officially sanctioned conjecture and the facts determined from actual research. Moreover, it is these two publications which establish the profoundly fraudulent nature of any further claims that "the science is settled". Speaking for the United States Geological Survey, Gerlach (2011) continues to underestimate the statistical limitations of his conclusion. In fact he had this to say:
"Humans currently live in a time of volcanic quiescence [Plimer, 2009, pp. 149, 211, 225]. But if the Earth’s volcanoes were emitting more CO2 than present-day human activities, volcanic quiescence would be a rare experience."
Sic. (The incorrect use of square brackets, in this quote, is not mine. This error is either Gerlach's or, worse still, his editor's)
Gerlach, in this statement, seems to have forgotten the elementary role of volatiles in volcanic eruptions. In fact, volatiles play a pivotal role in determining the violence and explosivity of volcanic eruptions. This is because it is the accumulation of volatiles, such as water vapour and carbon dioxide, which dicates the amount of internal pressure and this, in turn, dicates the explosivity of a volcanic eruption. The only reason we are experiencing a period of volcanic quiescence is because volatiles, such as carbon dioxide, are being released to the atmosphere instead of being bottled up for an eruption. Volcanic quiescence, thus, dictates greater volcanic emission of carbon dioxide - and it is only when that emission is hampered by conditions that we enter a period of more "active" or, rather, violent volcanism. As we have seen, Gerlach (2011) says precisely the opposite.
It seems that Gerlach (2011) drew his interpretation (quoted above) from a preference for the "global" "magmatic" carbon dioxide emission estimate of Marty and Tolstikhin (1998) which was devoloped from the generalisation of isotope ratios across provinces of varied geochemistry. This multimodal generalisation, as I have shown in the example of Laki (Section 2, above), can be spectacularly inaccurate. Gerlach reports this figure in the following contrastive statement:
"The projected 2010 anthropogenic CO2 emission rate of 35 gigatons per year is 135 times greater than the 0.26-gigaton- per-year preferred estimate for volcanoes."
In the units I am using here, that translates to a "preferred" estimate of worldwide volcanic carbon emission at 0.071 GtCpa. At this point, I think it worth contrasting this with a quote from Cardellini et al. (2011) who are actually engaged in some real research:
"Quantitative estimates provided a regional CO2 flux of about 9 Gt/y affecting the region (62000 km2), an amount globally relevant, being ~ 10% of the present-day global CO2 discharge from subaerial volcanoes."
That 9GtCO2pa translates to 2.45 GtCpa for just one region, which is more than 34 times the latest personally "preferred" "global" estimate offered by Gerlach (2011). One may well be keen to ask how it is possible that anyone would prefer to propagate a "global" estimate which is almost 35 times smaller than only one of the many provincial figures that must be summed in order to arrive at a worldwide estimate in the first place?
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