Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth

Abstract

A large perturbation in atmospheric CO2 and O2 or bioproductivity will result in a drastic pulse of (17)O change in atmospheric O2, as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphere-atmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO2 concentrations proposed by the "snowball' Earth hypothesis produce a typical MOSD duration of less than 10(6) y and a magnitude of (17)O depletion reaching approximately -35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO2 and low O2 concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate (17)O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean-atmosphere exchange only if partial pressure of atmospheric O2 is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event.

Keywords: O3; non–mass-dependent; photochemical reaction; postglacial cap carbonate; stratosphere.

Fig. 1.

Δ¹⁷O_O2 varies with ρ (≡ pO₂/pCO₂) value and RT (residence time, or pO₂/F_op) at steady state (calculated using Eq. 1). PV, present-day value of O₂ residence time; 1 PV = 1,244 y. γ (≡ F_stO2/O₂) and θ (the fraction of steady-state O₂ within the O₂–CO₂–O₃ photochemical reaction network in stratosphere) are assumed to be 0.1321 and 0.017, respectively.

Fig. 2.

Dynamic evolution of Δ¹⁷O_O2 under different initial conditions. All results are obtained from our typical model run, except the one in which F_op is reduced by 80%. Initial values for isolated snowball: pCO₂ = 0.1 bar (100,000 ppmv), pO₂ = ∼0 bar, and Δ¹⁷O_O2 = 0; for crevassed snowball: pCO₂ = 0.1 bar, pO₂ = 0.002 bar, and Δ¹⁷O_O2 = −26‰; for “normal” (nonsnowball): pCO₂ = 3,200 ppmv (0.0032 bar), pO₂ = 0.01 bar, and Δ¹⁷O_O2 = −5‰; for normal but F_op reduced by 80%: pCO₂ = 3,200 ppmv, pO₂ = 0.01 bar, and Δ¹⁷O_O2 = −17‰.

Fig. 3.

Linear relationship between steady-state δ¹⁷O_CO2-O2 and δ¹⁸O_CO2-O2 in O₂–CO₂–O₃ reaction systems. Data are from laboratory experiments (41).

Fig. 4.

Dependence of our model on k_ow (A), k_sw (B), and k_ob (C). k_ow, k_sw, and k_ob are at 1 in the baseline model. The initial values for pO₂, pCO₂, and Δ¹⁷O_O2 are set to ∼0 bar, 0.1 bar, and 0, respectively, for all tests.