Moderate greenhouse climate and rapid carbonate formation after Marinoan snowball Earth

The Neoproterozoic Era (1000–541 million years ago) witnessed extreme climatic events, including the Sturtian and Marinoan snowball Earths. The Marinoan snowball Earth's deglaciation, triggered by rising atmospheric CO2, transitioned the planet into a much warmer supergreenhouse climate. This rapid climatic shift significantly impacted early life forms that survived both the glaciation and subsequent warming. Understanding the severity and duration of this supergreenhouse climate is crucial for reconstructing the evolution of life during this period. However, significant uncertainties exist regarding initial conditions at the start of deglaciation, particularly concerning atmospheric and oceanic carbon reservoirs. Existing literature generally agrees on a slow, millions-of-years-long decline in atmospheric CO2 driven by continental weathering. This study challenges this view by investigating the previously unquantified role of faster oceanic carbon cycle processes operating on timescales of less than ten thousand years. By using the ICON-ESM Earth system model, we simulate the first five thousand years after the Marinoan snowball Earth deglaciation, explicitly resolving the co-evolution of climate, ocean, and carbon cycle, and considering uncertainties in initial conditions. This allows us to assess the plausibility of a moderate and short-lived supergreenhouse climate compared to the traditionally assumed long and intense supergreenhouse.

Previous research on the aftermath of snowball Earth events has predominantly focused on the long-term effects of continental weathering on atmospheric CO2 drawdown. Studies like Higgins & Schrag (2003) and Le Hir et al. (2008) explored the role of continental weathering in regulating the post-snowball climate, suggesting a slow decline over millions of years. However, these studies generally neglected the potentially rapid dynamics of the ocean's carbon cycle in influencing atmospheric CO2. Hoffman et al. (2017) provided a comprehensive overview of snowball Earth climate dynamics and geobiology, highlighting the challenges in understanding the post-snowball climate evolution. Ramme & Marotzke (2022) investigated climate and ocean circulation after a Marinoan snowball Earth, setting the stage for the current research by using the ICON-ESM model but without a detailed quantification of the fast oceanic carbon cycle processes.

This study employed the icosahedral nonhydrostatic Earth system model ICON-ESM, a coupled model encompassing atmosphere, ocean, and biogeochemistry components. ICON-ESM includes an interactive carbon cycle, treating carbon as a mass-conserving tracer across all components. The model was configured with a coarse resolution adapted to Marinoan snowball Earth conditions. The ocean biogeochemistry model HAMOCC within ICON-ESM was utilized, describing the dynamics of 17 prognostic state variables, including total alkalinity (TA) and dissolved inorganic carbon (DIC), essential for carbonate chemistry calculations. To adapt the model to Neoproterozoic conditions, several modifications were made: shell production by phytoplankton was deactivated, phytoplankton growth temperature dependence was adjusted to avoid unrealistically high growth rates, iron input was increased to account for high dust production on bare continents, and atmospheric oxygen was set at 50% of present-day levels. Inorganic carbonate precipitation was incorporated into the model, representing the formation of carbonates from supersaturated waters, with a rate dependent on the saturation state exceeding a threshold value. Several ICON-ESM simulations were designed to explore various scenarios by adjusting initial conditions including meltwater inflow (500-1500m sea-level equivalent), initial atmospheric CO2 (10^4-10^5 ppm), initial ocean carbon reservoir states (equilibrium or disequilibrium with atmosphere), and initial ocean alkalinity (variable). Box model calculations were performed to supplement the ICON-ESM simulations, extending the analysis across a wider range of initial conditions. The specific experiments varied in initial ocean alkalinity and dissolved inorganic carbon, aiming to isolate the effects of different carbon cycle processes, including meltwater dilution, ocean warming, biological carbon pump reactivation, atmosphere-ocean equilibration, and carbonate formation. Inorganic carbonate formation was calculated using a rate constant and threshold saturation. Idealised carbonate chemistry calculations were employed to extend the ICON-ESM simulations across a larger range of ocean chemistry conditions and initial atmospheric CO2 concentrations, reconstructing the potential amount of carbonate precipitated and related CO2 outgassing. The simulations were initiated from a pre-snowball state, transitioned into a snowball state through prescribed negative CO2 emissions, and then deglaciation was triggered by reducing snow albedo.

The study identified and quantified five key fast carbon cycle processes influencing atmospheric CO2 during the initial few thousand years post-snowball: 1) **Meltwater inflow dilution:** Dilution of sub-snowball brine by meltwater reduced DIC and pCO2, leading to CO2 uptake from the atmosphere (10-15% reduction). 2) **Ocean warming:** Reduced CO2 solubility at higher temperatures caused CO2 outgassing, partially offset by the reactivation of the solubility pump (25% increase). 3) **Biological carbon pump (BCP):** Reactivation of the BCP led to CO2 uptake, but its magnitude and timing were uncertain (0-2000 ppm reduction). 4) **Atmosphere-ocean equilibration:** If a disequilibrium existed during the snowball state, subsequent equilibration led to CO2 uptake, with a maximum potential of 80% reduction depending on initial conditions. 5) **Inorganic carbonate formation:** Warming and biological activity increased carbonate saturation, leading to inorganic carbonate precipitation and CO2 outgassing. This process's magnitude depended heavily on ocean chemistry (0-70,000 ppm increase). The model simulations revealed a range of possible supergreenhouse climate evolutions. The reservoir equilibration effect and carbonate formation were identified as primary drivers shaping the climatic evolution. Rapid carbonate formation was found to be most impactful at high ocean alkalinities, leading to significant CO2 outgassing; conversely, a substantial atmospheric CO2 reduction was observed at low ocean alkalinities, particularly if the ocean was initially depleted in carbon. This explains the variability of post-snowball climate evolution depending on initial conditions. The study also found that the rapid carbonate formation mechanism could explain the rapid accumulation of Marinoan cap dolostones, given sufficient initial ocean alkalinity. Carbonate deposition in the model primarily occurred in the tropics and subtropics within the first 2000 years post-deglaciation.

The findings challenge the established paradigm of a long and intense supergreenhouse climate following snowball Earth events. While scenarios with prolonged high atmospheric CO2 concentrations are plausible, the study highlights the possibility of a moderate and potentially short-lived supergreenhouse, contingent on initial ocean alkalinity and atmospheric CO2 levels. If the Marinoan cap dolostones truly formed within a few thousand years, low ocean alkalinity would be improbable due to the resulting ocean acidity. Moreover, geochemical proxies suggesting maximum CO2 concentrations around 10^4 ppm shortly after the snowball Earth support scenarios with moderate supergreenhouse climates, which would avoid excessive atmospheric CO2 increase from carbonate formation. The three scenarios illustrated—stable supergreenhouse, intensifying supergreenhouse, and moderate/rapidly declining supergreenhouse—reflect the range of plausible post-snowball climate pathways. The moderate supergreenhouse scenario is consistent with both rapid carbonate formation and lower atmospheric CO2 concentrations shortly after deglaciation, implying less severe conditions for early life.

This study significantly advances our understanding of post-snowball Earth climate evolution by quantifying the previously neglected influence of the rapid ocean carbon cycle. Three plausible scenarios for the supergreenhouse climate evolution are proposed, with a moderate and possibly short-lived supergreenhouse being equally likely as the traditional long and intense scenario. The rapid formation of carbonates from pre-existing ocean alkalinity is presented as a plausible mechanism for the origin of Marinoan cap dolostones. Future research should focus on further refining the estimates of initial conditions, particularly ocean alkalinity and carbon inventory, during and immediately after the snowball Earth period using geochemical proxies and improved modeling techniques. This research suggests the need for a reevaluation of the traditional understanding of the post-snowball Earth supergreenhouse climate.

The model used, while sophisticated, is a simplification of a complex Earth system. Uncertainties remain regarding the exact initial conditions, particularly the ocean's chemical composition and carbon reservoir size at the onset of deglaciation. The model's representation of meltwater inflow and the dynamics of the biological carbon pump could be further refined. The model simplifies the regional variations in carbonate precipitation, potentially affecting the precise timing and location of carbonate deposition, thus requiring caution when relating the results directly to geological observations.