Page 8 - i1052-5173-30-5

Figure 3. 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) Atmo-
spheric pCO estimates from the GEOCARB model from 570 to 420 Ma (Royer et al., 2004) and from measurements 420 Ma–present (Foster et al., 2017).
2
through much of the Paleozoic but quickly Decarbonation within the aureole pro- Phanerozoic (27–129 Mt/yr; Fig. 3C) is simi-
grows throughout the Mesozoic, punctuated duces significantly more CO than from lar in magnitude to all other endogenic CO
2
2
by volumetrically significant emplacement of assimilation of host rock by the emplaced fluxes (Table 1). The similarity in the range
granitoids and coinciding with the formation magma, even when the volume fraction of of the fluxes underscores the likelihood of
of the large metamorphic pendants of the the granitoid exceeds that of the aureole (Fig. the geologic carbon cycle maintaining an
SNB. Juxtaposing increases in sediment 3B). Less decarbonation is predicted when equilibrium state over million-year time-
deposition rates in the Cenozoic, area addition the volume fraction of granitoids is highest scales (Berner and Caldeira, 1997). Endo-
rates of granitoid decrease by threefold (fig. (225 and 180 Ma). This marked decrease in genic CO fluxes should change, however, as
2
4C in Cao et al., 2017). net decarbonation fluxes underscores the paleogeography, hypsometry, sea level, and
Changes in the size of igneous and sedi- propensity for metasomatized sedimentary the thermal states of Earth’s crust and mantle
mentary rock volumes manifest in changes rocks to produce more CO than their assimi- change (e.g., Kelemen and Manning, 2015;
2
in metamorphic decarbonation rates through lated counterparts. Nonetheless, all assimi- Lee et al., 2018). How endogenic CO fluxes
2
time, where gradual decreases in F CO are lated CO fluxes are appreciable and within temporally change, concomitant with other
2
2
predicted from the Cambrian toward the error of previous degassing estimates changes in the Earth system, remains an
present (Fig. 3C). The confluence of high (Ratschbacher et al., 2019). open question.
granitoid area addition rates and high pro- As an integrative climate metric, atmo-
portions of carbonate rocks produces the METAMORPHIC DECARBONATION spheric pCO is influenced by all fluxes of
2
highest metamorphic decarbonation fluxes, IN THE GEOLOGIC CARBON CYCLE CO between the atmosphere and solid
2
between the Cambrian and Silurian (540– The simplest way to assess the role of met- Earth, which makes it challenging to deter-
420 Ma), where fluxes oscillate between 120 amorphic decarbonation at continental arcs mine the dominant CO fluxes through geo-
2
and 140 Mt/yr, or almost 2–3 times the cur- in the geologic carbon cycle is to compare its logic time. While endogenic fluxes estab-
rent flux of CO from all volcanoes (Fischer magnitude to those of other “endogenic” CO lish base-level climate states, atmospheric
2
2
et al., 2019). This maximum net CO flux fluxes, which are fluxes from the solid Earth pCO is also influenced by silicate weather-
2
2
contrasts with the minima in the Cenozoic (endogenic system) to the hydrosphere, bio- ing, organic carbon burial, oxidation of
(66–5 Ma), where granitoid area addition sphere, and atmosphere (exogenic system). organic matter, and the paleogeography of
rates decrease by threefold, and <20% of all From our model, the range of global meta- crustal material (e.g., Kump et al., 2000;
sediments undergo decarbonation (Fig. 3B). morphic CO fluxes we predict through the Macdonald et al., 2019). Most tectonic
2
8 GSA Today | May 2020