How much “bark” was in the arc? The question of CO2 contribution from magmatic arcs,
especially continental arcs that poise platform carbonates in the paths of ascending magmas (Lee et al.,
2013), is important given the power of tectonically outgassed CO2 to modulate Earth’s climate
(e.g., Royer et al., 2004; Lee et al., 2013; McKenzie et al., 2016). CO2 fluxes from
continental arcs are the cumulative expression of magmatism, contact metamorphism and assimilation of
sedimentary rocks by magmas, and fluid flow through the crustal column. Because of its connection to
magmatic and hydrothermal systems (e.g., Baumgartner and Ferry, 1991), metamorphic CO2
production in continental arcs remains a challenge to quantify and has thus been on the periphery of
most studies. The movement of CO2 during metamorphism is further complicated by metamorphic
reaction progress, fluid availability, geothermal gradients, and chemical potentials. Nonetheless, the
strides made in studies of continental arcs position us to advance our understanding of metamorphic
decarbonation through geologic time, its role in the carbon cycle, and its influence on past climates.
Maps of fossilized magmatic systems and experiments replicating sub-arc and lower crustal environments
have been employed to estimate CO2 fluxes from continental arcs. In general, these studies
establish upper and lower estimates for CO2 fluxes from continental arcs, but questions
remain regarding the proportion of CO2 produced via metamorphism. For example, estimates of
area addition rates of magma through geologic time proxy for magma fluxes (Cao et al., 2017;
Ratschbacher et al., 2019), which are critical parameters that set the tempo and duration of metamorphic
decarbonation (e.g., Cathles et al., 1997). Without information regarding the rocks in which the magma
intrudes, only magmatic CO2 fluxes from continental arcs can be approximated. Experiments
replicating sub-arc and lower crustal conditions show that carbonate rock can be almost wholly
decarbonated (Carter and Dasgupta, 2016), which has been corroborated by observations of extremely low
13C/12C ratios of calc-silicate xenoliths from the Merapi volcano (Whitley et al.,
2019). The degree to which continental arc magmas completely decarbonate their host rocks is unknown,
but given the relatively open-system nature of continental arcs, these findings likely reflect upper
limits for decarbonation rates.
The geochemical and isotopic composition of volcanic emissions from active continental arcs reveal
CO2 generated by metamorphism. A global compilation of CO2/ST
measurements shows that arcs where magma intrudes platform carbonates often produce large CO2
fluxes (Aiuppa et al., 2019). Moreover, the isotopic composition of volcanic emissions from these
continental arcs further suggests input of sedimentary carbon (Mason et al., 2017). Despite these
advancements, measurement uncertainty in these data hampers a quantitative assessment of the metamorphic
proportion of continental arc CO2 outputs. By focusing on active systems, this approach
cannot convey how continental arc magmatism and concomitant CO2 fluxes have changed through
Numerical models have been useful in understanding metamorphism in continental arcs. Studies have
typically scaled observations, such as changes in the length of continental arcs through time, to fluxes
of metamorphic CO2 (e.g., Mills et al., 2019; Wong et al., 2019). Although these methods
provide meaningful boundary estimates, they do not fully consider the thermodynamics of reactive
transport. Other studies have used numerical models of open-system heat and mass transfer (e.g., Nabelek
et al., 2014; Chu et al., 2019), providing accurate flux estimates. The drawback of these models,
however, is that they involve geologic specificity that belies a broad representation of metamorphism in
continental arcs. To predict how metamorphic decarbonation has varied through geologic time, a balance
between these common approaches needs to be found.
In this paper, we show that sedimentary, igneous, and metamorphic rock evidence can be used to quantify
the rates of metamorphic decarbonation in continental arcs through the Phanerozoic. Metamorphic rocks in
continental arcs can directly trace decarbonation rates, but the reactive transport processes involved
in their formation is not simple. We thus review common rocks that form through metamorphic
decarbonation in the shallow crust, the reactions and conditions that generate them, and the
CO2 amounts that they can release as a byproduct of their formation. Additionally, through
numerical modeling, we demonstrate that the volume fraction of sedimentary rock that undergoes
decarbonation can be related to the relative volumes of sedimentary rock and magma in continental arcs.
This finding is validated against the well-characterized rock record of the Cretaceous Sierra Nevada
batholith (SNB). When compiled stratigraphic sections of North America and arc magma fluxes through the
Phanerozoic are imposed in our model, we predict how fluxes of CO2 from metamorphic
decarbonation changed through geologic time.
Field Observations of Decarbonation and Re-Carbonation in the Rock Record
There is abundant rock-hosted evidence for CO2 liberation, transport, and immobilization in
exhumed arc crust within circum-Pacific batholiths, including the SNB. Whereas the isolated screens and
roof pendants of metamorphic rocks appear as slivers in granitoid plutons, they are volumetrically
underrepresented at Earth’s surface due to erosion, overprinting by younger intrusions, and/or downward
transport to the sub-arc during pluton emplacement (e.g., Ducea et al., 2015). These rocks show abundant
evidence that carbonate-bearing rocks spanned from upper crustal contact aureoles to lower crustal
granulite facies domains (e.g., Kerrick, 1977; Newberry and Einaudi, 1981). The capacities of these
pendants to produce CO2 are tied to their protoliths, fluid budgets, and reaction progresses
Skarn rocks, composed of varying proportions of garnet, pyroxene ± wollastonite, are synonymous
decarbonation (Fig. 1B) and are often associated with economic base and precious metal deposits. Skarns
epitomize optimal conditions for releasing CO2 where infiltration of water-rich fluid
maintains high chemical potentials, driving decarbonation locally to completion (e.g., Chu et al., 2019,
and references therein). For example, a cubic meter of garnetite skarn signifies 1.01–1.05 metric tons
of CO2 released from calcite (Lee and Lackey, 2015).
Arc decarbonation. (A) Schematic representation of plutons intruding carbonate-bearing crust at various
depths in a magmatic arc (not to scale); (B) 30-cm-wide outcrop of garnet, clinopyroxene, and
wollastonite (white) typical of Sierra Nevada batholith skarn; (C) 20-cm-wide slab of
garnet-wollastonite-diopside calc-silicate rock with folding of original sedimentary structures; (D)
calc-silicate with garnet (red) showing traces of Al-rich domains in garnet-wollastonite calc-silicate
(coin is 24 mm across); (E) laminated carbonate typical of rocks metamorphosed to form C and D (hammer
is 28 cm long); (F) cartoon depicting metamorphic decarbonation, common metamorphic rock types, their
yields; (G) retrograde calcite deposited in 1-cm-wide cavity within garnet
Skarns often form at shallow crustal depths (3–5 km) and along the margins of granitoid rocks that
intruded into carbonates. Any carbonate that was not converted to skarn coarsens into marble.
Marbles are more abundant than skarns (Fig. 2B) and can appear to be relatively unaffected by
metamorphic decarbonation. Yet, small amounts of reaction progress are enabled by trace quantities
(<5 modal %) of quartz present, producing considerable amounts of CO2 (~32–46 kg
CO2 per cubic meter of rock; Ferry, 1989). Further, if marble bodies abut water-rich
metapelitic units, CO2 can diffuse out of the marble and thus export nontrivial amounts of
CO2 (e.g., Vidale and Hewitt, 1973; Ague, 2000).
(A) Predicted volume fraction of aureole as a function of volume fraction of intrusion. Error is 1σ. (B)
Skarn-marble area distributions for a corridor of the Sierra Nevada batholith (SNB). (C) Surface area
addition rates for intrusions and metamorphic pendants in the SNB through the 40 m.y. of elevated
Calc-silicate rocks, with white, green, or red laminations inherited from sedimentary
laminations, are also composed of microcrystalline wollastonite, pyroxene, and garnet (Fig. 1C–1E).
Whereas skarns see copious CO2 release by fluid infiltration and metasomatism of originally
pure carbonate rocks, calc-silicates release similar amounts of CO2 because interbedded
layers of carbonate, silica, and clay minerals predispose mixed carbonate-siliciclastic rocks to fully
decarbonate (Fig. 1F). Water-rich fluids are still necessary to fully decarbonate these rocks, but their
laminated character can cause a positive feedback that enables near-complete decarbonation. The
enhancement of permeability during decarbonation promotes CO2 transport, enabling further
decarbonation (e.g., Zhang et al., 2000).
Even if thermodynamic conditions enable decarbonation to proceed, not all CO2 produced makes
it out of the crust. CO2 is most often immobilized when low crustal permeability inhibits
fluid flow or magma production rates decrease. Secondary carbonate deposition represents CO2
immobilization in arcs, often occurring away from hotter, deeper areas of the arc crust, and down
temperature gradients. Examples include calcite veins that cross-cut skarn rocks and precipitate in vugs
and brittle fractures (Fig. 1G). Other silicates that form when CO2 is mineralized, including
retrograde serpentine and tremolite, occur appreciably (up to 2 wt%), even when crustal permeability is
high enough to promote continued CO2 removal (Nabelek et al., 2014). Veins and deposits at
shallower levels in the crust are further evidence that CO2 can be re-precipitated. Even
granitoids in arc crust are noted to contain regular but small amounts of calcite (0.2–1.0 wt%; White et
al., 2005). Overall, these observations suggest that seemingly trace amounts of retrograde
CO2 mineralization can manifest in large masses of CO2 left behind after prograde
The journey of an individual molecule of CO2 can be complicated by a series of prograde and
retrograde reactions at different times and locations in the arc crust. Yet, at a fundamental level, the
amount of metamorphically derived CO2 that exits from continental arcs directly relates to
the composition of rocks that comprise the arc and the amount of magma that is emplaced over a given
time. These underlying principles, while still considering the intricacies of metamorphic decarbonation
and its geologic record, motivate our model design.
Analogue Model for Metamorphic Decarbonation
The basis of the analogue model is to determine the volume of sedimentary rock that undergoes
decarbonation in a continental arc. The model simulates two-dimensional fluid flow, heat transfer, and
fluid-rock oxygen isotope exchange after the emplacement of an intrusion as a proxy for metamorphic
decarbonation (further model setup and assumptions are described in Ramos et al., 2018, and in the
Supplementary Information1). The δ18O values of carbonates decrease during
progressive decarbonation (e.g., Bowman, 1998). Therefore, in each simulation, we track the changes in
host-rock δ18O values during hydrothermal fluid flow to highlight areas around a magma body
that meet likely conditions for decarbonation. Once hydrothermal activity has ceased, the
δ18O values of the host rock define a volume of rock that undergoes decarbonation, which we
term the aureole volume.
Our numerical model considers effects of crustal permeability and magma volume on aureole volumes. The
model domain remains constant across each simulation (i.e., Vhost rock +
Vintrusion = constant; Fig. DR3, see footnote 1). A series of model runs predicts aureole
volumes as a function of intrusion volume and crustal permeability where the largest volumes of
decarbonated host rock (Vaureole) are at intermediate relative intrusion volumes (Fig. 2A).
Effectively, as magma volumes exceed the volume of host rock in an arc, the aureole volume diminishes,
concomitant to a diminished aureole decarbonation flux. This result counters common thought, where
continental arc flare-ups (i.e., times of maximum intrusion volume) are thought to be times of maximum
CO2 output from arcs (Lee and Lackey, 2015).
The mineralogy of the host rocks in which the magma intrudes controls the magnitude of the decarbonation
flux it produces. We thus amass magma addition rates and sedimentary rock information—including rock
types, depositional ages, and stratigraphic thicknesses—for the SNB and the entirety of North America.
Details about how we compare sedimentary and magma volumes through time are given in the Supplementary
Information, but in short, our model predicts a metamorphic CO2 flux—which includes
CO2 produced via metamorphism in the aureole and by assimilation of host rock in the
intrusion—based off this volume comparison. Independent of the model, we compute a CO2 flux
for the Cretaceous SNB by scaling the area distribution of metamorphic pendants and skarns within a
portion of the SNB to amounts of produced CO2 along the entire arc. This estimate is compared
to our model prediction to assess its utility in estimating CO2 fluxes.
Ground-Truthing the Analogue Model with the Geologic Record from the Cretaceous SNB
The area distribution of skarns (Fig. 2B) varies considerably along the SNB corridor we examined, with
some exposures containing <10 m2 of skarn and others containing >1 km2. The skarn area
is generally dwarfed by the marble area and only comprises 4% of the total mapped area. If we assume a
maintained skarn-marble ratio within pendants along the entire SNB, an average carbonate fraction in
sedimentary rocks in the arc of 20%, and skarn occurrence over 7 km of depth in the SNB, we compute a
total skarn volume in the SNB of 19,000 km3. This volume, if decarbonated over a 40 m.y. time
interval, produces an average CO2 flux of ~1 Mt/yr. This value, which excludes CO2
from calc-silicates and marbles and fluxes from magma degassing and assimilation, is fivefold less than
measurements of modern global continental arcs that intersect platform carbonates (5 Mt/yr; Aiuppa et
al., 2019) and nearly two orders of magnitude less than previous estimates for net CO2 fluxes
from all Cretaceous continental arcs (Lee et al., 2013; Lee and Lackey, 2015). This disparity can
largely be attributed to the sparse distribution of metamorphic pendants in the SNB and the difficulty
of computing assimilation fluxes from the geologic record. Thus, this skarn CO2 flux is
considered a minimum estimate for metamorphic CO2 fluxes from the Cretaceous SNB.
When sedimentary rock volumes and proportions from SNB-specific sites (Fig. DR2, see footnote 1) are
compared with granitoid volumes emplaced in North America from 125 to 85 Ma (Cao et al., 2017), we
predict the net metamorphic CO2 flux from North American arcs to be 32.3 ± 28.4 Mt/yr during
the Cretaceous, with 13% of the flux deriving from assimilated wall rock and 87% coming from
decarbonation in the aureole (see Supplementary Information for further details on the flux
calculation). Western SNB rocks, typified by the Triassic–Jurassic Kings Sequence (Fig. DR2), which
contains both mixed carbonate-siliciclastic rocks and platform carbonates, contribute 59% of all
generated CO2. Paleozoic sections such as the Morrison block in the eastern SNB, which are
composed predominantly of siliciclastic rocks and 23% carbonate, contribute 41%. Notably, this net flux
agrees within 2σ error of the net decarbonation estimate from (1) SNB skarn outcrops (1 Mt/yr) and when
(2) North American sedimentary rock information is used (40 Mt/yr) instead of SNB-specific stratigraphic
sections (Fig. 3C at ca. 100 Ma time marker). Although location-specific geology will always yield more
accurate flux estimates, these findings support the utility of North American sedimentary rocks as a
globally representative archive of sedimentary rock types.
Phanerozoic estimates of rock volumes and decarbonation fluxes. All errors are 1 standard deviation
(error bars in B and shaded volumes in C and D). (A) Cumulative volume estimates for all rock types.
Rates of deposition (in units of volume/time) can be gleaned by the negative slope of the lines. (B)
Volume fractions of intrusions and aureoles. (C) Metamorphic decarbonation fluxes, including the Sierra
Nevada batholith (SNB)-specific flux. (D) Atmospheric p
estimates from the
GEOCARB model from 570 to 420 Ma (Royer et al., 2004) and from measurements 420 Ma–present (Foster et
Phanerozoic Metamorphic Decarbonation Rates
The variational growth rate of sedimentary rock and granitoid volumes underpins the changes in
decarbonation rates in continental arcs through time (negative slope of lines in Fig. 3A). Once
corrected for erosion (assuming an erosional half-life of 400 m.y. sensu Cao et al., 2017), sedimentary
rocks from North America, granitoids (from Cao et al., 2017), and their volumetric distributions grow
unsteadily. Cambrian through Devonian time (542–400 Ma) is marked by similar rates of growth of
different rock types, highlighting the voluminous deposition of carbonate throughout the Phanerozoic.
The volume of mixed carbonate-siliciclastic rocks surpasses that of pure carbonate by latest
Pennsylvanian (ca. 300 Ma) when the growth rate of siliciclastic rocks increases well beyond all other
rock types. Sediment deposition rates plateau in the Triassic (245–206 Ma) after the assembly of Pangea
(Cao et al., 2017) and subsequently increase upon its breakup in the Jurassic (ca. 180 Ma). Carbonate
and mixed carbonate-siliciclastic rocks grow in volume in the Cretaceous but are dwarfed by increases in
the siliciclastic deposition rate. These trajectories of growth remain constant through the Cenozoic.
Change in area addition rates of granitoid is out of phase with the deposition of sedimentary rocks (Fig.
3A). Globally, granitoid volumes grow at a roughly consistent rate until the breakup of Pangea,
whereupon their cumulative volumes grow rapidly. Continental arc activity in North America is quiescent
through much of the Paleozoic but quickly grows throughout the Mesozoic, punctuated by volumetrically
significant emplacement of granitoids and coinciding with the formation of the large metamorphic
pendants of the SNB. Juxtaposing increases in sediment deposition rates in the Cenozoic, area addition
rates of granitoid decrease by threefold (fig. 4C in Cao et al., 2017).
Changes in the size of igneous and sedimentary rock volumes manifest in changes in metamorphic
decarbonation rates through time, where gradual decreases in FCO2 are
the Cambrian toward the present (Fig. 3C). The confluence of high granitoid area addition rates and high
proportions of carbonate rocks produces the highest metamorphic decarbonation fluxes, between the
Cambrian and Silurian (540–420 Ma), where fluxes oscillate between 120 and 140 Mt/yr, or almost 2–3
times the current flux of CO2 from all volcanoes (Fischer et al., 2019). This maximum net
CO2 flux contrasts with the minima in the Cenozoic (66–5 Ma), where granitoid area addition
rates decrease by threefold, and <20% of all sediments undergo decarbonation (Fig. 3B).
Decarbonation within the aureole produces significantly more CO2 than from assimilation of
host rock by the emplaced magma, even when the volume fraction of the granitoid exceeds that of the
aureole (Fig. 3B). Less decarbonation is predicted when the volume fraction of granitoids is highest
(225 and 180 Ma). This marked decrease in net decarbonation fluxes underscores the propensity for
metasomatized sedimentary rocks to produce more CO2 than their assimilated counterparts.
Nonetheless, all assimilated CO2 fluxes are appreciable and within error of previous
degassing estimates (Ratschbacher et al., 2019).
Metamorphic Decarbonation in the Geologic Carbon Cycle
The simplest way to assess the role of metamorphic decarbonation at continental arcs in the geologic
carbon cycle is to compare its magnitude to those of other “endogenic” CO2 fluxes, which are
fluxes from the solid Earth (endogenic system) to the hydrosphere, biosphere, and atmosphere (exogenic
system). From our model, the range of global metamorphic CO2 fluxes we predict through the
Phanerozoic (27–129 Mt/yr; Fig. 3C) is similar in magnitude to all other endogenic CO2 fluxes
(Table 1). The similarity in the range of the fluxes underscores the likelihood of the geologic carbon
cycle maintaining an equilibrium state over million-year time-scales (Berner and Caldeira, 1997).
Endogenic CO2 fluxes should change, however, as paleogeography, hypsometry, sea level, and
the thermal states of Earth’s crust and mantle change (e.g., Kelemen and Manning, 2015; Lee et al.,
2018). How endogenic CO2 fluxes temporally change, concomitant with other changes in the
Earth system, remains an open question.
As an integrative climate metric, atmospheric pCO2 is influenced by all fluxes of
CO2 between the atmosphere and solid Earth, which makes it challenging to determine the
dominant CO2 fluxes through geologic time. While endogenic fluxes establish base-level
climate states, atmospheric pCO2 is also influenced by silicate weathering, organic
carbon burial, oxidation of organic matter, and the paleogeography of crustal material (e.g., Kump et
al., 2000; Macdonald et al., 2019). Most tectonic fluxes appear weakly correlated with
pCO2 from 200 Ma to present (Wong et al., 2019). From our predictions, we find that
the connection between metamorphic CO2 fluxes from continental arcs and atmospheric
pCO2 is tenuous (Fig. 3D). Beyond the similar decreases from the Cambrian toward the
present and the shared relative maxima prior to the Devonian, atmospheric pCO2 and
metamorphic CO2 fluxes appear disconnected and cannot be wholly compared without knowledge of
Nonetheless, times where the correlation between metamorphic CO2 fluxes and atmospheric
pCO2 are weakest can be leveraged to explore the operation of other Earth system
processes. For example, between 320 and 270 Ma during icehouse conditions in the Permian, metamorphic
CO2 fluxes remain high while atmospheric pCO2 is low. This time interval
also coincides with the waning stages of Pangea formation. Despite elevated metamorphic fluxes, could
atmospheric pCO2 have remained low because generation of relief during
supercontinent assembly enhanced silicate weathering (e.g., West et al., 2005)? Instead, could there
have been prolonged organic carbon burial as equatorial regions remained hot and humid and forests
proliferated (Ronov, 1982)? For a contrasting example, in Permian–Triassic time after Pangea’s assembly,
atmospheric pCO2 increases while metamorphic CO2 fluxes drop by a factor
of 2. Does atmospheric pCO2 increase because the aridification of continental
interiors inhibits silicate weathering? If so, can modest CO2 outputs from continental arcs
with diminished silicate weathering fluxes be enough to increase atmospheric pCO2 by
a factor of 2, or are other endogenic fluxes necessary, such as organic carbon oxidation or continental
rifting (e.g., Lee et al., 2016)? Between these contrasting scenarios, the unifying question concerns
the thresholds at which metamorphic CO2 fluxes can be attributed to the development of past
climates, if at all.
Of all time periods on Earth, the Cretaceous period likely represents a time in which enhanced
continental arc metamorphism promoted a hothouse climate. The emergence of deep-water calcifiers in the
Triassic (e.g., Ridgwell and Zeebe, 2005), increases in granitoid addition rates, doubling in length of
continental arcs that intersect crustal carbonates (Lee et al., 2013), and increased evidence of skarn
formation within circum-Pacific batholiths (e.g., Lee and Lackey, 2015) support the plausibility of
elevated metamorphic CO2 fluxes contributing to hothouse climate conditions in the
Cretaceous. Our model predicts maximum values for aureole volume fractions during the Mesozoic (Fig.
3B), purporting an increased proportion of aureole decarbonation. The average metamorphic CO2
flux from arcs during the Cretaceous exceeds estimates for mid-ocean ridge CO2 fluxes (60
Mt/yr; Wong et al., 2019). Unless CO2 fluxes from continental rifts or oxidation of organic
matter were significant in magnitude during the Cretaceous, continental arc metamorphism likely
contributed the largest fraction of CO2 of all endogenic fluxes. With further quantifications
of endogenic CO2 fluxes and their variation through time, benchmarked against known climatic
changes, the role of tectonic outgassing in the evolution of Earth’s climate will become increasingly
J. Muller and J. Ryan-Davis helped with digitizing maps for area calculations. We thank C.-T.A. Lee for
comments on an early draft of the paper, two anonymous reviewers for thorough suggestions, and M. Ducea
for careful editorial handling. All MATLAB codes used in the analogue model and for figure generation
can be found in the GitHub repositories of the first author: https://github.com/ejramos/skarn_model and https://github.com/ejramos/Phanerozoic_decarbonation. National Science
Foundation grant NSF-OCE-1338842 awarded to Lackey, Barnes, and others as part of the Frontiers in Earth
Systems Dynamics program supported this work.
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