Biologically induced initiation of Neoproterozoic snowball-Earth events

Abstract

The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric CO(2) greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate- or iron-reducing bacteria. In both cases, this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO(2) concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased CN of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth.

Fig. 1.

Time-dependent results of scenario #1, a net aerobic remineralization of organic matter leading to a negative δ¹³C isotopic excursion. Time advances from left to right. (A) Prescribed carbon fluxes; the only flux prescribed to change in this scenario is the decreasing oxygenic production F_prod forcing the isotopic signal. (*) Anaerobic remineralization fluxes are plotted shifted and are identically zero in this scenario. (B) Dissolved organic and inorganic carbon masses (solid lines) and δ¹³C isotopic compositions (dashed). (C) Atmospheric pCO₂ (ppm) showing a large increase, due to the increase in dissolved inorganic carbon concentration, and a decrease in ocean pH.

Fig. 2.

Schematic of the biologically induced snowball initiation scenarios. The right side represents the sulfate reduction plus pyrite formation pathway, which increases alkalinity and therefore reduces pCO₂; the left side represents the iron reduction path, which leads to the deposition of siderite and reduction in the size of the DIC pool, and therefore, again, to the reduction of pCO₂. Note the shoaling of the (dashed blue) interface between the anoxic and oxic ocean depth ranges, in response to the enhanced export production, potentially leading to a reduction of both oxygenic production and aerobic remineralization. These scenarios are demonstrated using a single box model described in the text.

Fig. 3.

Time-dependent model scenario (#2) of snowball initiation. (A) Prescribed carbon fluxes, due to oxygenic production (F_prod), aerobic remineralization (F_remin,O2), input from volcanoes and carbonate weathering (F_in, multiplied by 23), sulfate remineralization (F_remin,SO4) and iron reduction (F_remin,Fe). (*) The last two fluxes, representing anaerobic remineralization, are plotted shifted and their initial value before the anomaly is zero. (B) Carbonate speciation. (C) Organic and inorganic carbon masses (solid lines) and δ¹³C isotopic composition (dash). (D) Decrease in sulfate concentration [ΔSO₄(T), blue], or, alternatively, the required sulfate flux into the ocean (green, solid) for the sulfate reduction scenario. The present-day riverine flux is shown by the green, dashed line. (E) Implied iron fluxes of Fe³⁺ for iron reduction part of the scenario (red, with present-day flux into the ocean shown in red dashed line), and the implied flux of Fe²⁺ for the sulfate reduction and pyrite formation (blue). (F) Organic and inorganic burial rates. (G) Ca²⁺ and the weathering fraction α_wthr (SI Appendix). (H) Organic burial fraction f and non-steady-state fraction f^δ defined in the text. (I) Atmospheric CO₂ and ocean pH.