Land plant evolution decreased, rather than increased, weathering rates

Abstract

The repeated evolution of trees is widely thought to have enhanced the capacity of silicate weathering via the impact of deep rooting. However, land plants are also responsible for wetland assembly and organic carbon burial. The total burial output of carbon via both organic and inorganic deposition must balance input to the exogenic system from volcanic outgassing on million-year time scales. Increased partitioning of carbon burial toward organic carbon and away from inorganic carbon reduces the marine carbonate burial flux, necessitating a lowered total flux of alkalinity to the oceans to maintain mass balance in the Earth’s surface carbon cycle. This flux includes the nutrient delivery from the terrestrial vegetation implicated as a driver of marine evolution, extinction, and environmental change including anoxia and black shale formation. Here, the burial of terrestrial organic carbon, first substantially in the Devonian and continuing through to the present, is argued to require a reduction in silicate weathering rates when compared to earlier times, given the independence of volcanic outgassing from weathering on short time scales. Land plants still may cause reductions in steady-state atmospheric CO2 levels, but via increasing the silicate weathering feedback strength, not silicate weathering rates. The mass-balance constraints on the long-term carbon cycle provide a mechanism for linking how land plant evolution simultaneously increased nutrient recycling and weathering efficiency of the Earth’s surface.

Influence of temporally varying weatherability on CO<sub>2</sub>-climate coupling and ecosystem change in the late Paleozoic

“…2b; Fielding et al, 2008;Isbell et al, 2012;Montañez and Poulsen, 2013;Soreghan et al, 2019), as well as with a large-magnitude eustatic fall archived in paleotropical successions worldwide (Koch and Frank, 2011;Eros et al, 2012). Widespread glacial expansion temporally linked to this interval of lowest overall pCO 2 argues for CO 2 as the primary driver of glaciation rather than recently proposed mechanisms, such as the influence of the closing of the Precaspian Isthmus (Davydov, 2018) or a decrease in the radiative forcing resulting from increased atmospheric aerosols by explosive volcanism at this time (Soreghan et al, 2019). The very low greenhouse radiative forcing associated with this low CO 2 interval would have been amplified by 2.5 % lower solar luminosity (Crowley and Baum, 1992), reduced transmission of shortwave radiation (Poulsen et al, 2015) by the high pO 2 atmosphere of the early Permian (Krause et al, 2018;, and by increased atmospheric aerosols at this time (Soreghan et al, 2019).…”

Intensified Ocean Deoxygenation During the end Devonian Mass Extinction

“…The end Devonian climatic cooling would be the consequence of long term terrestrialization in Devonian, particularly in Late Devonian (e.g., Algeo et al, ; Algeo & Scheckler, ). The development of trees and seed plants with spectacular root system might have enhanced rhizoturbation and chemical weathering (Berner, ), although recent work provide contrasting interpretation (D'Antonio et al, ). These inferences are supported by the substantial long‐term increase in seawater 87 Sr/ 86 Sr from ~0.70778 to ~0.70824 in Late Devonian (McArthur et al, ).…”

Influence of temporally varying weatherability on CO<sub>2</sub>–climate coupling and ecosystem change in the late Paleozoic

“…and references within).Additionally, the burial of substantial organic matter as peat in swamp environments prone to preservation (ultimately as coal) during the Pennsylvanian would have partitioned global CO 2 consumption between silicate weathering and organic carbon burial, further driving a lower steady-state pCO 2(D'Antonio et al, 2019;Ibarra et al, 2019). Our modeling, however, assumes a constant pre-Hercynian solid Earth degassing through the study interval and does not account for increased magmatic CO 2 during Hercynian arc-continent collision and potential widespread eruptive volcanism in the late Carboniferous(Soreghan et al, 2019), both of which could have increased steady-state CO 2 .Short-term fluctuations in pCO 2 superimposed on the 40-Myr record and confirmed as statistically significant (99.9 to 100% of estimates;Fig.…”