Changing tectonic controls on the long‐term carbon cycle from Mesozoic to present

Abstract

Tectonic drivers of degassing and weathering processes are key long-term controls on atmospheric CO 2 . However, there is considerable debate over the changing relative importance of different carbon sources and sinks. Existing geochemical models have tended to rely on indirect methods to derive tectonic drivers, such as inversion of the seawater 87 Sr/ 86 Sr curve to estimate uplift or continental basalt area. Here we use improving geologic data to update the representation of tectonic drivers in the COPSE biogeochemical model. The resulting model distinguishes CO 2 sinks from terrestrial granite weathering, total basalt weathering, and seafloor alteration. It also distinguishes CO 2 sources from subduction zone metamorphism and from igneous intrusions. We reconstruct terrestrial basaltic area from data on the extent of large igneous provinces and use their volume to estimate their contribution to degassing. We adopt a recently published reconstruction of subduction-related degassing, and relate seafloor weathering to ocean crust creation rate. Revised degassing alone tends to produce unrealistically high CO 2 , but this is counteracted by the inclusion of seafloor alteration and global basalt weathering, producing a good overall fit to Mesozoic-Cenozoic proxy CO 2 estimates and a good fit to 87 Sr/ 86 Sr data. The model predicts that seafloor alteration and terrestrial weathering made similar contributions to CO 2 removal through the Triassic and Jurassic, after which terrestrial weathering increased and seafloor weathering declined. We predict that basalts made a greater contribution to silicate weathering than granites through the Mesozoic, before the contribution of basalt weathering declined over the Cenozoic due to decreasing global basaltic area.

The Interplay Between the Eruption and Weathering of Large Igneous Provinces and the Deep‐Time Carbon Cycle

“…It is accepted that the drawdown of CO 2 by LIPs is larger than the initial injection of CO 2 into the atmosphere and that LIPs may become a net CO 2 sink on timescales as short as 1-1.5 Myr after the major CO 2 eruption ceases (Dessert et al, 2001(Dessert et al, , 2003Kent & Muttoni, 2013;Lefebvre et al, 2010;Mills et al, 2014;Schaller et al, 2012). However, these effects have not been investigated on a global scale, and the impact of latitudinal and time-dependent distribution of LIPs has not been considered over the past 400 Ma.…”

Paleoclimate Constraints on Terrestrial Hydroclimate, Silicate Weathering, and the Carbon Cycle

“…Together, k and n modify the strength of the weathering feedback (Schwartzman and Volk, 1989;Mills et al, 2014a;. Globally, although weathering fluxes are determined by runoff because weatherability is modified by the overlying runoff distribution (Fig.…”

“…For example, basaltic catchments weather faster than other silicates and account for approximately 20-35% of the modern global silicate weathering and CO2 consumption flux, while occupying less than 5% of sub-aerial continental area (Gíslason et al, 1996(Gíslason et al, , 2009 have been critically important in controlling past levels of atmospheric CO2 (pCO2) (Li and Elderfield, 2013;Kent and Muttoni, 2013;Molnar and Cronin, 2015;Jagoutz et al, 2016). However, scaling modern observations of weathering fluxes from catchments to both large spatial and temporal scales (e.g., Wallmann, 2001;Lefebvre et al, 2013;Li and Elderfield, 2013;Kent andMuttoni, 2008, 2013;Mills et al, 2014a;Li et al, 2016;Jagoutz et al, 2016;Cox et al, 2016) remains a challenge due to the lack of phenomenological modeling frameworks that predict weathering fluxes at both catchment and global scales under different climate states.…”

“…Earth surface weatherability is often treated as a tunable parameter in biogeochemical models (e.g., Gibbs et al, 1999;Berner, 2006aBerner, , 2008Mills et al, 2014a) because it determines the degree of coupling between climate and weathering. In turn, this coupling is a measure of the strength of the negative feedback between climate/atmospheric pCO 2 and silicate weathering.…”

Validation of the BASALT model for simulating off‐axis hydrothermal circulation in oceanic crust

“…This replacement of phases in the rock, which can persist for tens of millions of years, is responsible for geochemical exchange of major (Ca, Mg, C, Fe, Na, K, Al, and Si) and trace elements between the ocean and the upper oceanic crust [ Elderfield and Schultz , ]. For example, carbon uptake by oceanic crust alteration is thought to be comparable to CO 2 outgassing rates from mid‐ocean ridges [ Alt and Teagle , ; Staudigel et al , ; Gerlach , ], indicating that basalt alteration is important to global cycling of carbon and potentially long‐term climate regulation [ Sleep and Zahnle , ; Coogan and Gillis , ; Mills et al , ; Coogan and Dosso , ].…”