Abstract
Carbon is a key control on the surface chemistry and climate of Earth. Significant volumes of carbon are input to the oceans and atmosphere from deep Earth in the form of degassed CO2 and are returned to large carbon reservoirs in the mantle via subduction or burial. Different tectonic settings (e.g., volcanic arcs, mid-ocean ridges, and continental rifts) emit fluxes of CO2 that are temporally and spatially variable, and together they represent a first-order control on carbon outgassing from the deep Earth. A change in the relative importance of different tectonic settings throughout Earth’s history has therefore played a key role in balancing the deep carbon cycle on geological timescales. Over the past 10 years the Deep Carbon Observatory has made enormous progress in constraining estimates of carbon outgassing flux at different tectonic settings. Using plate boundary evolution modeling and our understanding of present-day carbon fluxes, we develop time series of carbon fluxes into and out of the Earth’s interior through the past 200 million years. We highlight the increasing importance of carbonate-intersecting subduction zones over time to carbon outgassing, and the possible dominance of carbon outgassing at continental rift zones, which leads to maxima in outgassing at 130 and 15 Ma. To a first-order, carbon outgassing since 200 Ma may be net positive, averaging ∼50 Mt C yr–1 more than the ingassing flux at subduction zones. Our net outgassing curve is poorly correlated with atmospheric CO2, implying that surface carbon cycling processes play a significant role in modulating carbon concentrations and/or there is a long-term crustal or lithospheric storage of carbon which modulates the outgassing flux. Our results highlight the large uncertainties that exist in reconstructing the corresponding in- and outgassing fluxes of carbon. Our synthesis summarizes our current understanding of fluxes at tectonic settings and their influence on atmospheric CO2, and provides a framework for future research into Earth’s deep carbon cycling, both today and in the past.
Highlights
The deep carbon cycle of the Earth connects the exospheric reservoirs of carbon to those in the crust and mantle (e.g., Dasgupta and Hirschmann, 2010)
The carbon entering the mantle at subduction zones can be subdivided into three constituent components which comprise the total carbon in a subducting oceanic lithosphere slab: the carbon locked within altered oceanic crust, the carbon present within serpentinized mantle, as well as organic and inorganic carbon present in deep-sea sediments (Kelemen and Manning, 2015)
The result of the analysis presented here is that while net outgassing flux has stayed close to zero for the last 200 Myr, for the majority of this time net outgassing of CO2 has been slightly positive, at around 25–75 Mt C yr−1. This result suggests that the carbon released from Earth’s mantle is larger than the carbon returned to the mantle at subduction zones, and that there has been a net outgassing of carbon to the Earth’s surface since 200 Ma. We acknowledge that this first-order net degassing curve does not account for the subduction of organic carbon within sediments, which may constitute a significant proportion of subducting carbon, we note that organic carbon subduction, if similar to the ∼20% of subducted sediment at the present-day (Clift, 2017), would not be sufficient to account for the difference between ingassing and outgassing fluxes
Summary
The deep carbon cycle of the Earth connects the exospheric (atmosphere, hydrosphere, biosphere) reservoirs of carbon to those in the crust and mantle (e.g., Dasgupta and Hirschmann, 2010). We utilize the open access global plate motion model of Müller et al (2016) to analyze plate boundary length (Figure 2), which captures the past 200 million years of plate tectonic evolution based on paleomagnetic data, seafloor isochrons, and other plate motion indicators (e.g., ocean island chains). Alongside this model we use the subduction history data of East et al (2019) to determine subduction flux parameters. Plate boundary lengths and other parameters from these models provide a basis for our tectonic carbon flux estimates in 1 Myr intervals. Uncertainties are determined through standard error propagation of uncertainties within present-day carbon flux measurements (see Table 1) and the standard deviation of computed tectonic parameters
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
