Constraints on Geologic Carbon Release at the End of the Last Ice Age From the Planetary Radiocarbon Budget

Abstract:

While a reinvigoration of ocean circulation and CO₂ outgassing is the leading explanation for atmospheric CO₂ rise since the end of the Last Glacial Maximum, there is also evidence for carbon release from volcanic sources within the Pacific Ocean and land carbon sources associated with deglacial permafrost loss. However, the magnitude and carbon cycle impacts of these sources remain poorly constrained due to systematic uncertainties in establishing the stable carbon isotope mass balance (Gebbie et al., 2015; Shackleton, 1977). Here, we invert a carbon cycle model based on observational atmospheric CO₂ and radiocarbon (¹⁴C) mass balance constraints. We find only a relatively small amount of CO₂ release to be consistent with the constraints whereas up to ~1700 Pg C is permissible if released to the ocean in the form of bicarbonate ion. Seawater carbonate chemistry—including pH, carbonate saturation and CO₂ partial pressure—is least impacted by carbon release with an alkalinity-to-carbon ratio (ALK:DIC) equaling that of seawater (~1.1), such that 1700 Pg C release in the form of bicarbonate ion (ALK:DIC = 1) results in as little as <5 ppm simulated CO₂ change.

These results allow for the possibility that volcanic systems released large amounts of carbon during the last deglaciation, as previously suggested from severe deglacial radiocarbon anomalies in the Pacific (Rafter et al., 2019; Ronge et al., 2016; Stott et al., 2009). A large release of ¹⁴C-free geologic carbon can provide local and regional radiocarbon anomalies, and our radiocarbon budget places an upper limit on the amount of carbon released. In this most extreme scenario, the added carbon imparts an atmospheric ∆¹⁴C decline of ~50‰, augmenting the ~200‰ decline due to Earth magnetic field strengthening and ~100‰ decline from faster overturning of the ocean carbon inventory (Hain et al., 2014).This implies that radiocarbon evidence for significant geologic carbon release since the last ice age may not be taken as contributing to deglacial CO₂ rise, unless there is evidence for significant local acidification and corrosion of seafloor sediments. If the geologic carbon cycle is indeed more dynamic than previously thought, we may also need to rethink the approach to estimate the land/ocean carbon repartitioning from the deglacial stable carbon isotope budget