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I have included the full text of Gerlach's 1991 paper concerning volcanic carbon dioxide emissions because so few people who cite Gerlach's work have actually read it. This is hardly surprising, considering that until now, this paper has not been available online. Contrary to the claims of Monbiot, the USGS, and many other authors, Gerlach (1991) includes no measurements of actual submarine volcano emissions, makes no attempt at modal representation, and Gerlach's global volcanic emission estimate is based on measurements taken from only seven subaerial volcanoes (Gerlach, 1991, §4, ¶1) and three hydrothermal vent sites (Gerlach, 1991, §3, ¶3). Although a hydrothermal vent site can be a feature of a volcano, hydrothermal vent site emission and the submarine volcanic emission are two completely different measurements. To his credit, Gerlach (1991, §1, ¶4) points out the fact that the data avilable at the time was woefully inadequate to a global estimate. Although Gerlach (1991, §3, ¶3) does mention some proxy measurements for mid-oceanic ridge degassing, he also demonstrates that these are nonetheless doubtful as the degree of fractionation remains unknown (Gerlach, 1991, §3, ¶4). While he talks about "volcanos of the mid-oceanic ridge system" Gerlach (1991) neither offers nor includes emission estimates of any submarine volcano. Moreover, Gerlach (1991, §3, ¶1) asserts "There are no estimates for off-ridge volcanos ". For more information concerning why I've included Gerlach (1991) among the most misquoted and abused papers in the public domain, see http://geologist-1011.mobi.
Eliminating the first-line indentation of paragraphs.
Altering the page identifier to optimise clarity for online reading.
Setting citation elements within the references to bold for ease of use.
Change incorrect use of square brackets on references to the proper round brackets, because Square brackets are reserved for additional explanatory material added by persons other than the original author.
A list of any obviously unintended errata in the text of Gerlach (1991) can be found at the end of this web page. However, this list excludes any and all scientific assertions that may disagree with current knowledge, because such assertions remain, nonetheless, valid and correct in their historical context. This ensures that the views of the original author remain protected from over-zealous pedantry. Thus you are afforded an uncensored view of science from the perspective of the time and of the original author.
The text provided here is entirely typed in by hand from photocopied materiel. Picking off any typos will, no doubt, be an ongoing endeavour. This web page, like others on this site, is protected by copyright and all rights are reserved. However, the actual text of the Gerlach (1991) paper itself, presented here, is sourced to pages explicitly marked by the publisher as "This page may be freely copied". This permission to copy the source pages freely -i.e. without restriction- appears to me a clear submission of the material therein to the public domain.
The photocopy I've acquired is from an edition with no specific pagination, and is spread over five mysteriously unnumbered pages, instead of the three specified by EOS as, 249, 254, and 255. For this reason, I've assembled a numbered list of sections by heading. The first section, being unnamed, I've designated as the "Introduction". Specific references to the text are thereby made by section (§) and paragraph (¶) number -from the first paragraph in the section-, which exclude headings, tables, figures, and captions.
If I receive the correct page break locations I'll paginate this copy of Gerlach (1991) accodringly.
In an effort to better understand processes that control sources of CO2 in the carbon cycle, the U.S. Global Change Research Program (CEES 1990) identifies improving understanding of both volcanic emissions and natural cources of CO2 in the carbon cycle as priority items for research. To implement these goals, the program plan calls for monitoring CO2 emissions from volcanos.
Without the resupply of CO2 by volcanic and metamorphic degassing, removal of atmospheric CO2 by silicate weathering, carbonate deposition, and burial of organic matter would deplete the CO2 content of the atmosphere in 10,000 years and the atmosphere-ocean system in 500,000 years (Holland, 1978; Berner et al., 1983). The CO2 content of the atmosphere-ocean system has varied in the past, but not at the rate expected if CO2 were removed and not replenished. It is assumed, therefore, that CO2 degassing from the earth's interior restores the deficit from the surficial processes and balances the atmospheric CO2 budget on a time scale of 104-106 yr. Earlier atmospheric balancing calculations imply present-day (pre-industrial) CO2 degassing rates of 6-7 x 1012 mol yr-1 (Holland, 1978; Berner et al., 1983); recent calculations suggest degassing rates may be as high as 11 x 1012 mol yr-1 (Berner, 1990).
Atmospheric balancing calculations have inherent drawbacks, however. They do not distinguish volcanic, metamorphic, and diagenetic sources of CO2 degassing—they give an aggregate CO2 degassing rate obtained for all sources. Since the CO2 obtained in these calculations is the difference between several CO2-producing and CO2-consuming processes affecting the atmospheric CO2 budget, it includes the accumulated error in the rate estimates for each contributing process. To minimize these problems, Berner (1990) suggested basing degassing rates on direct measurements, to the extent possible, in future carbon budget calculations.
In this article, I review the results and implications of past efforts to measure rates of CO2 degassing from volcanos, and I attempt to arrive at an estimate of the global rate of volcanic CO2 degassing. My principle aim, however, is to emphasize unsettled problems requiring further study and uncertainties due to inadequate data. I make a few comparisons between volcanic and anthropogenic CO2 emission rates because of current concern about the buildup of CO2 in the atmosphere.
Most of the data on volcanic CO2 emissions come from active volcanos that are in a state of quiescent degassing, that is, degassing without extrusions of lava or explosive ejections of disrupted and fragmented lava. Data biased in favour of quiescent degassing are not, in my view, a serious limitation. First, the low solubility of CO2 in silicate melts at upper crustal depths, where magmas tend to reside before erupting, causes magmas underlying volcanos to leak CO2 continuously and to become depleted in CO2 by diffusive loss through volcano flanks and by advective loss through fractures feeding hydrothermal fluids and atmospheric plumes (Carbonnelle et al.,1985; Gerlach and Graeber, 1985; Allard et al., 1987; Bottinga and Javoy, 1989; Gerlach 1989a,b). Second, the annual quiescent release of CO2 from all active volcanos appears to be more than an order of magnitude greater than that emitted directly from all forms of erupting lava, as discussed below.
Submarine volcanic systems provide about 80% of the present-day magma supply to the crust (Crisp, 1984). Estimates of CO2 emission rates for submarine volcanos are restricted to volcanos of the mid-oceanic ridge system, which provides about 75% of the present-day magma supply (Crisp, 1984). There are no estimates for off-ridge volcanos or volcanos on back-arc spreading centers.
Several investigators have attempted to constrain the CO2 emission rate of the global mid-oceanic ridge system by calculating the product of oceanic 3He flux and measured CO2/3He ratios of hydrothermal vent fluids and converting the CO2 flux obtained to a mole per year emission rate. These calculations have tended to employ the original oceanic 3He flux of 4 atom cm-2s-1 instead of the corrected value of 3 atom cm-2s-1. (The original 3He flux assumed a mean 3He/4He ratio for injected ridge-crest helium of 11 times the atmospheric value; it was subsequently shown that ridge-crest helium has a ratio 8 times the atmospheric value, thus reducing the oceanic 3He flux proportionately (Welhan and Craig, 1983).) All CO2 emission rate estimates based on this approach and presented below for the mid-oceanic ridge system have been recalculated for the corrected 3He flux.
CO2/3He data are available for hydrothermal vent fluids from only three locations, all in the eastern Pacific: the Galapagos Rift, and 13º and 21ºN on the East Pacific Rise. The CO2 emission rates that have been estimated for the mid-oceanic ridges from the datafor these sites are 0.6 x 1012 mol yr-1 (De Marais and Moore, 1984), 0.75 x 1012 mol yr-1 (Des Marais, 1985), and 0.7 x 1012 mol yr-1 (Gerlach, 1989b). Because vent fluid CO2/3He data are restricted to so few sites, there is concern about just how representative they are of the mid-oceanic ridge system. In an ingenious attempt to obtain more representative data, Des Marais (1985) and Marty and Jambon (1987) used the CO2/3He values of MORB glassesfrom many locations as proxies for the CO2?3He ratios of ridge-crest emissions. This greatly increases the number of CO23He data sets, and leads to CO2 emission rate estimates for the global mid-oceanic ridge system that cluster around 1.5 x 1012 mol yr12 (Marty and Jambon, 1987). This value is about double that obtained from vent fluid because CO2/3He ratios for MORB glasses are about twice those of vent fluids examined so far.
The assumption that the ratio is not affected by fractionation during degassing prior to eruption on the seafloor is a critical issue in the use of MORB glass CO2/3He values as proxies for CO23 in ridge-crest emissions. Pre-eruptive degassing of CO2 and He from MORB magma is expected to be significant (Bottinga and Javoy, 1989; Gerlach, 1989b), and it has been suggested that quiescent degassing from subridge magma chambers may be primarily responsible for ridge-crest CO2 and He emissions (Gerlach, 1989b). Marty and Jambon (1987) argue that because the Henry's law solubility constants for CO2 and He in molten MORB are similar, the CO2/3He ratios for the vapor and melt will be about equal during degassing and that the value of the ratio for MORB glasses is therefore a good predictor of the ratio for ridge emissions. However, a slight difference in CO2 and He solubuilities could, with sufficient degassing, cause enough CO2 and He fractionatin to account for a factor of 2 difference between glass and vent fluid ratios and, thereby, the factor of 2 difference in the calculated CO2 emission rates for ridges. This possibility and the possibility that CO2/3He ratios of vent fluids may themselves be affected by fractionation processes (for example, differential hydrothermal solubilities of CO2 and He, carbon precipitation, etc.) need more study.
In view of the disagreement in results thus far for the mid-oceanic ridge CO2 emission rate, alternative approaches should also be pursued. For example, a mass balance approach based on data for the carbon content of MORBs and the CO2 contentof volcanic gases from transitional basalts of the Afar region suggests a ridge CO2 emission rate in the range 0.2-0.9 x 1012 mol yr-1 (Gerlach, 1898b). Updating this estimate with new data for carbon in MORBs (Kingsly, 1989) gives a range of 0.5-0.9 x 1012 mol yr-1, which agrees with estimates based on the CO2 ratios of hydrothermal vent fluids.
Published rates of CO2 degassing exist for only seven active subaerial volcanos (Table 1, Figure 1): five convergent plate volcanos, an intraplate continental volcano, and an intraplate oceanic island hotspot volcano.
Measurements made on quiescent volcanic plumes provide the basis for most of the CO2 emission rates for the seven volcanos. The quiescent plumes includ examples that preceded the initial explosive episode of an eruption (White Island), examples that followed the initial explosive episodes of an eruption (Mount St. Helens, Redoubt), examples that were present between explosive or dome-building episodes of an eruption (Mount St. Helens, Redoubt), and examples that exhibit long-term stability and continuity during, between, and long afte eruptions (Kilauea, Mount Etna, Vulcano). One emission rate estimate (Augustine) is based on plume measurements during a low level explosive episode.
Fig. 1. CO2 emission rates in log (moles per year) arranged in ascending order for subaerial volcanoes from Table 1. The numbers on the tops of the bars are emission rates in 1012 mol yr-1. The median emission rate used in a calculation described in the text is 0.03 x 1012 mol yr-1 (Kilauea).The plume observations onsist of airbourne MIRAN infrared spectrophotometer measurements of above-background CO2 concentrations, or airbourne COSPEC ultraviolet spectrophotometer measurements of SO2 column abundancescombined with measurements ofthe CO2/SO2 ratio of gases supplying the plume. Most studies neglected the diffusive flux of CO2 through volcano flanks; soil gas surveys carried out at Mount Etna and Vulcano suggest this source can be significant (Table 1).
Continuous, long-term measurements of CO2 emission rates do not exist for any volcano. Most estimates are based on spot measurements. The only record of closely spaced measurements over several (15) months is for Mount Sy. Helens (Harris et al., 1981; Casadavall et al., 1983). The long-term emission rate for Kilauea (0.03 x 1012 mol yr -1) (Gerlach and Graeber, 1985) is based on the CO2 content and average supply rate of magma emplaced in Kilauea's summit chamber from July 1956 to April 1983. Rose et al., (1986) suggest a long-term CO2 emission rate for White Island of approximately 0.01 x 1012 mol yr-1; they consider the larger 0.03 x 1012 mol yr-1 rate in November 1983 (Table 1) to be representative of degassing during periods of new magma emplacement prior to an eruption.
Kilauea Volcano provides an example of simultaneous eruptive and quiescent degassing. Lava production rates combined with estimates of the CO2 content of the erupting lava (Greenland et al., 1985; Gerlach, 1986; K. Hon, personal communication, 1991) give a CO2 emission rate of 0.001-0.003 for the current east rift zoen eruption. Quiescent degassing of Kilauea's summit (Table 1) is therefore at least 10-fold greater than contemporaneous eruptive degassing at the present time. Casadawell et al., (1984) report similar eruptive CO2 emission rates between April 2 and April 16 for the 1984 eruption of Mauna Loa Volcano, Hawaii. Unfortunately, the background quiescent emission rate is not known for Mauna Loa.
Marty et al. (1989) estimated the total output of CO2 from island arc volcanos to be in the range 0.1-0.5 x 1012 mol yr-1. This estimate is based on the global SO2 output from subaerial volcanos of 0.24 x 1012 mol yr-1 (Berresheim and Jaeschke, 1983). It assumes that islandarc volcanos are primarily responsible for the global SO2 output and that the CO2/SO2 ratio for arc emissions is 1.5±1. It is possible in principle to follow this approach in estimating the global CO2 emission rate of all subaerial volcanos from the corresponding global volcanic SO2 output. The difficulty in doing so is that the appropriate global volcanic CO2/SO2 value is unknown. Combining the total CO2 emission rate for Etna (summit plume plus diffusive flank), which is exceptionally large, and on the order of 1 x 1012 mol yr-1 (Table 1), with the global volcanic SO2 output suggests that the global volcanic CO2/SO2 value is at least 4.2. (Williams et al. (1990) calculated a global emission rate of 1.2 x 1012 mol yr-1 from the global volcanic SO2 output and CO2/SO2 data for 30 volcanos suggesting a global volcanic CO2/SO2 value of 5.
Another approach to estimating the global subaerial CO2 emission rate of volcanos is to extrapolate the rates for the volcanos in Table 1 to all active subaerial volcanos. The 5-year running average for the number of active volcanos per year is approximately 60 (Simkin and Siebert, 1984). I base the extrapolation on the median emission rate of the seven volcanos (Figure 1) because the data set is small, and the median, unlike the mean, is less sensitive to outlying data. The median value of 0.03 x 1012 mol yr -1 indicates a global subaerial emission rate of approximately 1.8 x 1012 mol yr-1. Reassuringly, this result is larger than the rate for Mount Etna alone and similar to the estimate of Williams et al. (1990). Applying the same procedure to the median SO2 flux for the same seven volcanos (0.0035 x 1012 mol yr-1) gives a global volcanic SO2 output of 0.21 x 1012 mol yr-1, which agrees well with the 0.24 x 1012 mol yr-1 estimate of Berresheim and Jaeschke (1983).
Table I. CO2 Emission Rates for Subaerial Volcanos.
aAverage emission rate over period of observation.
A, measurement by airborne MIRAN infrared spectrophotometer of CO2 content of volcanic plume.
B, measurement by airborne COSPEC ultraviolet spectrophotometer of SO2 column abundances in volcanic plume coupled with data for CO2/SO2 ratio of plume or high-temperature fumarolle gases supplying plume (corrected for atmospheric contamination).
C, soil gas measurements of diffusive CO2 flux through unvegetated volcano flanks.
D, based on volcanic gas data, volatile concentrations in matrix glasses and glass inclusions, and long-term magma supply rate.
The above estimates for the global CO2 emission rate from subaerial volcanos are based almost entirely on measurements during quiescent degassing. They are about an order of magnitude larger than the estimated annual CO2 emission of 0.15 x 1012 mol yr-1 released from all forms of erupting lava (Leavitt, 1982). Leavitt's estimate is based on a chronology for subaerial eruptions between 1800 and 1969, and it assumes an average eruption volume of 0.1 km3 magma (2.7 g cm3) and a release of 0.12 wt% CO2 during eruption. Taken at face value, this estimate implies the predominance of quiescent CO2 degassing from volcanos, as suggested previously by Rose et al. (1986).
Man's emissions of CO2 from fossil fuel burning, cement production, and gas flaring alone not amount to 500 x 1012 mol yr-1 (Boden et al., 1990). Contributions from man's management of the biosphere (for example, deforestation) are less well known but potentially of the same magnitude. Thus man's activities replenish the atmospheric CO2 deficit by more than 45 times over. They are equivalent in terms of CO2 production to turning on about 17,000 additional Kilauea Volcanos or 350-700 additional mid oceanic ridge systems.
The results reviewed above suggest that constraining the global CO2 emission rate by direct measurement is feasible. Both subaerial and submarine volcanos appear to emit CO2 at global rates on the order of 1-2 x 1012 mol yr-1; thus while the global rates from subaerial and submarine volcanos are uncertain at the present time, a total global estimate of 3-4 x 1012 mol yr-1 seems reasonable and conservative. This estimate for volcano degassing is consistent with estimates of total CO2 degassing 6-10 x 1012 mol yr-1 based on atmospheric CO2 balancing, and it indicates that CO2 emissions from volcanos contribute about 35-65% of the CO2 needed to balance the deficit in the atmosphere-ocean system. Although the present-day global emission rate of CO2 from volcanos is uncertain, anthropogenic emissions clearly overwhelm it by at least 150 times.
The global rate of emission of CO2 from the mid-oceanic ridge system is estimated to be in the range 0.7-1.5 x 1012 mol yr-1. Thus, mid-ocean ridges probably account for less than half of the global volcanic CO2 flux, despite the fact that mid-oceanic ridge magmatism provides over 75% of the present-day magma supply to the crust. Efforts should be made to reduce the uncertainty that exists presently in estimates of CO2 degassing from the global mid-oceanic ridge system, but an equally or more important priority in submarine studies is to begin acquiring data for CO2 emission rates at off-ridge volcanic systems such as submarine hot spot volcanos and back-arc basin spreading centers.
The avaliable data suggest that CO2 emissions from all subaerial volcanos are probably greater than from mid-oceanic ridges. This conclusion is at variance with the widely held view that the ridge system produces orders of magnitude larger emissions than do subaerial volcanos (e.g. CEES, 1990, p. 97). Indeed, the output from Mount Etna alone is about equivalent to that of the entire mid-oceanic ridge system. However, CO2 emission data for subaerial volcanos are sparse, and the global contribution from subaerial volcanos is poorly constrained. Improving the data base for CO2 emissions from subaerial volcanos is the highest priority for future work. The available data suggest that contributions of CO2 in the range 0.01-0.05 x 1012 mol yr-1 can be expected from most active subaerial volcanos (Figure 1). however, alkaline volcanos (for example, Mount Erubus, Nyiragongo) may produce 1-2 orders of magnitude larger contributions, if Mount Etna is any indication. On the otehr hand, Etna's large CO2 output may be augmented by contamination from underlying carbonates (Allard et al., 1987).
Invesigations to date suggest that most of the CO2 emitted by volcanos is released during quiescent degassing instead of eruptive degassing. This proposition needs further investigation, however, and should be tested against more data for quiescent degassing and measurements of CO2 emissions from volcanos during episodes of vigorous eruptive degassing. Techniques are sorely needed for making direct CO2 emission measurements, especially during large explosive eruptions, by remote spectroscopic techniques similar to the widely used COSPEC technique for measuring volcanic SO2 emission rates.
Berner and Lasaga (1989) have characterized the calculation of CO2 degassing from igneous and metamorphic activity as the most vexing problem encountered in modelling the carbon geochemical cycle. In hopes of getting at least a reasonable approximation of CO2 degassing over geologic time, modelers have coupled all degassing to seafloor spreading rates (Berner et al., 1983). This approximation is reasonable for CO2 degassing at mid-oceanic ridges and subduction zones. The adequacy of seafloor spreading rates as a predictor of mid-plate volcano degassing rates is less clear, and it is possible that CO2 degassing at mid-plate volcanos is outside the conceptual framework of the current carbon cycle models. The high CO2 degassing rates for Mount Etna underscore the need to ensure that mid-plate volcano degassing is satisfactorily represented in models of the carbon geochemical cycle.
I thank R. Berner, T. Casadevall, and T. Hinkley for several suggestions that improved this paper. Support from the USGS Global Change and Climate History program is gratefully acknowledged.
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The closing statement of Gerlach (1991: §3, ¶3) states the possibility that CO2/3He ratios for MORB glasses are about twice those of vent fluids examined so far as a fact explaining the elevated estimates of the mid-oceanic ridge systems. As Gerlach (1991: §3, ¶4) goes on to explain, this is only a possibility that is, as yet, not an established fact.
Gerlach (1986), as layed out by EOS, lists page 12177 as 12,177. This could be mistaken for No. 12, p. 177 based on the bibliographic style employed by EOS.

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