Deglacial volcanism and reoxygenation in the aftermath of the Sturtian Snowball Earth

Abstract

The Cryogenian Sturtian and Marinoan Snowball Earth glaciations bracket a nonglacial interval during which Demosponge and green-algal biomarkers first appear. To understand the relationships between environmental perturbations and early animal evolution, we measured sulfur and mercury isotopes from the Datangpo Formation from South China. Hg enrichment with positive Δ¹⁹⁹Hg excursion suggests enhanced volcanism, potentially due to depressurization of terrestrial magma chambers during deglaciation. A thick stratigraphic interval of negative Δ³³S_py indicates that the nonglacial interlude was characterized by low but rising sulfate levels. Model results reveal a mechanism to produce the Δ³³S anomalies down to -0.284‰ through Rayleigh distillation. We propose that extreme temperatures and anoxia contributed to the apparent delay in green algal production in the aftermath of the Sturtian glaciation and the subsequent reoxygenation of the iron-rich and sulfate-depleted ocean paved the way for evolution of animals.

Fig. 1.. Hg content, Hg/TOC, Δ¹⁹⁹Hg, δ²⁰²Hg and δ³⁴S, Δ³³S from drill core ZK102.

Age from (5); ppb, parts per billion.

Fig. 2.. A simplified ocean-atmosphere Hg cycling box-model.

This model is used to simulate the excursions of Hg enrichment and isotope signatures in interval II. The numbers in boxes represent Hg reservoir sizes (Mg), the numbers beside the arrows represent Hg fluxes (Mg year⁻¹), which are differentiated by processes identifiers (volcanic Hg emission, hydrothermal Hg discharges, photochemical atmospheric Hg⁰ oxidation, photochemical atmospheric Hg²⁺ reduction, atmospheric Hg⁰ deposition, atmospheric Hg²⁺ deposition, ocean Hg⁰ evasion, riverine flux, and sediment Hg burial).

Fig. 3.. Simulated Hg enrichment and isotopic excursions in interval II in response to volcanic emissions (F_vol) and atmospheric Hg²⁺ deposition rate coefficient (k_{dep_2}).

Left: The modeled marine Hg enrichment (B), δ²⁰²Hg (C), and Δ¹⁹⁹Hg (D) in response to an increase of volcanic Hg emissions by 3 to 4× in the first 50 ka, and a return of volcanic Hg emissions back to the starting condition in the later 50 ka (A). Right: The modeled marine Hg enrichment (F), δ²⁰²Hg (G), and Δ¹⁹⁹Hg (H) in response to an increase of atmospheric Hg²⁺ deposition rate coefficient by 1 to 2× and volcanic Hg emissions by 3.5× in the first 50 ka, and a return of atmospheric Hg²⁺ deposition rate coefficient and volcanic Hg emissions back to the starting condition in the later 50 ka (E).

Fig. 4.. Cross plot of Δ³³S and Δ³⁶S for intervals III and IV in drill core ZK102.

The black line shows linear regression slope, and the dark gray and light gray areas show 2σ and 3σ confidence intervals, respectively.

Fig. 5.. δ³⁴S and Δ³³S evolution of sulfate during a Rayleigh distillation process.

Starting sulfate Δ³³S = 0‰ and δ³⁴S = +30‰.

Fig. 6.. Cross plot of δ³⁴S and Δ³³S in drill core ZK102 and correlative strata of the MacDonaldryggen Member in Svalbard and the Arena Formation in East Greenland.

S-isotopic data from the MacDonaldryggen Member and the Arena Formation are from (16). The model field for sulfate reduction (black outlined field) represents the field of all possible fractionations, and the Rayleigh-colored field indicates δ³⁴S and Δ³³S evolution from f = 1 to 0.

Fig. 7.. Schematic illustration of volcanic activity, temperature, ocean redox state, sulfate levels, iron, and early metazoan evolution during the Cryogenian Period.

Ocean redox state from (11); Early metazoan evolution from (2, 3); Atmos, atmosphere; Temp, temperature.