For approximately the first 2 billion years of Earth history, atmospheric oxygen levels were extremely low. It wasn't until at least half a billion years after the evolution of oxygenic photosynthesis, perhaps as early as 3 billion years ago, that oxygen rose to appreciable levels during the Great Oxidation event. Shortly after, marine carbonates experienced a large positive spike in carbon isotope ratios known as the Lomagundi event. The mechanisms responsible for the Great Oxidation and Lomagundi events remain debated. Using a carbon-oxygen box model which tracks surface and interior C fluxes and reservoirs while also tracking C isotopes and atmospheric oxygen levels we demonstrate that about 2.5 billion years ago a tectonic transition resulting in increased volcanic CO 2 emissions could have led to increased deposition of both carbonates and organic carbon via enhanced weathering and nutrient delivery to oceans. Increased burial of carbonates and organic carbon would have allowed accumulation of atmospheric oxygen while also increasing delivery of carbon to subduction zones. Coupled with preferential release of Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

Multiple hypotheses/models have been put forward regarding the cooling history of the Earth. The search for life beyond Earth has brought these models into a new light as they connect to one of the two energy sources life can tap. The ability to discriminate between different Earth cooling models, and the utility of adopting such models to aid in the assessment of planetary habitability, has been hampered by a lack of uncertainty analysis. This motivates a layered uncertainty analysis for a range of thermal history models that have been applied to the Earth. The analysis evaluates coupled model input, initial condition, and structural uncertainty. Layered model uncertainty, together with data uncertainty and multiple working hypotheses (another form of uncertainty), means that results must be evaluated in a probabilistic sense even if the models are deterministic. For the Earth's cooling history uncertainty leads to ambiguity -multiple models, based on different hypotheses, can match data constraints. This has implications for using such models to forecast conditions for exoplanets that share Earth characteristics but are older than the Earth, i.e., it has implications for modeling the long-term life potential of terrestrial planets. Even for the most Earth-like planet we know of, the Earth itself, model uncertainty and ambiguity leads to large forecast spreads. Given that this comes from the planet with the most data constraints we should expect larger spreads for models of terrestrial planets in general. The layered uncertainty approach can be expanded by coupling planetary cooling models to climate models and propagating uncertainty between them to assess habitability from a probabilistic versus a binary view.

Thermal history models, historically used to understand Earth's geologic history, are being coupled to climate models to map conditions that allow planets to maintain life. However, the lack of structural uncertainty assessment has blurred guidelines for how thermal history models can be used toward this end. Structural uncertainty is intrinsic to the modeling process. Model structure refers to the cause and effect relations that define a model and are assumed to adequately represent a particular real world system. Intrinsic/structural uncertainty is different from input and parameter uncertainties (which are often evaluated for thermal history models). A full uncertainty assessment requires that input/parametric and intrinsic/structural uncertainty be evaluated (one is not a substitute for the other). We quantify the intrinsic uncertainty for several parameterized thermal history models (a subclass of planetary models). We use single perturbation analysis to determine the reactance time of different models. This provides a metric for how long it takes low‐amplitude, unmodeled effects to decay or grow. Reactance time is shown to scale inversely with the strength of the dominant model feedback (negative or positive). A perturbed physics analysis is then used to determine uncertainty shadows for model outputs. This provides probability distributions for model predictions. It also tests the structural stability of a model (do model predictions remain qualitatively similar, and within assumed model limits, in the face of intrinsic uncertainty?). Once intrinsic uncertainty is accounted for, model outputs/predictions and comparisons to observational data should be treated in a probabilistic way.

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Paleo-temperature data indicates that the Earth's mantle has not cooled at a constant rate. The data show slow cooling from 3.8 to 2.5 Ga followed by more rapid cooling until the present. This has been argued to indicate a transition from a single plate mode to a plate tectonics. However, a tectonic change may not be necessary. Multistage cooling can result from deep water cycling coupled to mantle convection. Melting and volcanism removes water from the mantle (degassing). Dehydration tends to stiffen the mantle, which slows convective vigor and plate velocities causing mantle heating. Higher mantle temperature tends to lower mantle viscosity and increase plate velocities. If these two tendencies are in balance, then mantle cooling can be weak. Breaking this balance, via a switch to net mantle rehydration, can cool the mantle more rapidly. We use coupled water cycling and mantle convection models to test the viability of this hypothesis. Within model and data uncertainty, the hypothesis that deep water cycling can lead to a multi-stage Earth cooling is consistent with data constraints. Probability distributions, for successful models, indicate that plate and plate margin strength play a minor role for resisting plate motions relative to the resistance from interior mantle viscosity. Plain Language Summary The Earth's internal energy drives geologic and tectonic activity. How the Earth's interior has cooled over time provides constraints on how this energy has been tapped. Observational constraints indicate that Earth cooling was multistaged with slow interior cooling followed by an acceleration in cooling rate. We use thermal history models to test the hypothesis that this transition is due to a change in the nature of water cycling between surface and interior reservoirs. The models indicate that water cycling, coupled to plate tectonics, can lead to multistage Earth cooling that is consistent with observational constraints. This hypothesis requires no change in the tectonic mode of the Earth-it is consistent with the idea that plate tectonics has operated over geologic history.