Negative carbon isotope excursions: an interpretive framework

Abstract

Numerous negative carbon isotope excursions (nCIEs) in the geologic record occurring over 10 4 -10 5 years are interpreted as episodes of massive carbon release. nCIEs help to illuminate the connection between past carbon cycling and climate variability. Theoretically, the size of a nCIE can be used to determine the mass of carbon released, provided that the carbon source is known or other environmental changes such as temperature or ocean pH can be constrained. A simple isotopic mass balance equation often serves as a first order estimate for the mass of carbon input, but this approach ignores the effects of negative carbon cycle-climate feedbacks. Here we show, using 432 earth system model simulations, that the mass of carbon release and associated environmental impacts for a nCIE of a given size and carbon source depend on the onset duration of that nCIE: the longer the nCIE onset duration, the greater the required carbon input in order to counterbalance the input of 13 C-enriched carbon through carbonate compensation and weathering feedbacks. On timescales >10 3 years, these feedbacks remove carbon from the atmosphere so that the relative rise in atmospheric CO 2 decreases with the nCIE onset duration. Consequently, the impacts on global temperature, surface ocean pH and saturation state are reduced if the nCIE has a long onset duration. The framework provided here demonstrates how constraints on the total nCIE duration and relative shape-together determining the onset duration-affect the interpretation of sedimentary nCIEs. Finally, we evaluate selected well-studied nCIEs, including the Eocene Thermal Maximum 2 (∼54 Ma), the Paleocene-Eocene Thermal Maximum (∼56 Ma), and the Aptian Oceanic Anoxic Event (∼120 Ma), in the context of our model-based framework and show how modeled environmental changes can be used to narrow down the most likely carbon emissions scenarios.

Volcanically driven lacustrine ecosystem changes during the Carnian Pluvial Episode (Late Triassic)

“…Conservative estimates suggest that Wrangellia emplacement released ∼5 × 10 3 Pg of mantle C (8). Additional C emissions through contact metamorphism are unlikely because of the oceanic site of (46). These model simulations do not include the isotopic effects of concurrent TOC burial (Fig.…”

Inclusion of a suite of weathering tracers in the cGENIE Earth system model – muffin release v.0.9.23

“…These removal fluxes are dependent on the abundance of inorganic carbon and pH (or carbonate saturation) (Hall and Chan, 2004;Marriott et al, 2004) and are not spatially uniform (Hathorne and James, 2006). The residence time of Li in the ocean is estimated to be 0.3-3 Myr (Stoffyn-Egli and Mackenzie, 1984) with more recent estimates closer to 1 Myr (Vigier and Goddéris, 2015). Li has two stable isotopes (7.52 % 6 Li and 92.48 % 7 Li, Penniston-Dorland et al, 2017).…”

Inclusion of a suite of weathering tracers in the cGENIE Earth System Model – muffin release v.0.9.10

“…Since more alkalinity than Ca 2+ is derived from continental weathering in our set-up, the climate-driven increase in weathering results in a transient imbalance between alkalinity and Ca 2+ supplies which does not appear in C cycle simulations without an igneous Ca 2+ source at the seafloor (e.g. Lord et al, 2016;Vervoort et al, 2019). The imbalance is small (10% of the Ca input at its peak) but cumulatively the unbalanced alkalinity supply is large enough to causes a slight reduction of the steady-state marine Ca and Sr reservoirs (by ∼0.5% following the emission of 5000 Pg C).…”