  The methane cycle is a key component of the Earth system that links planetary
climate, biological metabolism, and the global biogeochemical cycles of carbon,
oxygen, sulfur, and hydrogen. However, currently lacking is a numerical model
capable of simulating a diversity of environments in the ocean where methane
can be produced and destroyed, and with the flexibility to be able to explore
not only relatively recent perturbations to Earth's methane cycle but also to
probe methane cycling and associated climate impacts under the reducing
conditions characteristic of most of Earth history and likely widespread on
other Earth-like planets. Here, we present an expansion of the ocean-atmosphere
methane cycle in the intermediate-complexity Earth system model cGENIE,
including parameterized atmospheric photochemistry and schemes for microbial
methanogenesis, aerobic methanotrophy, and anaerobic oxidation of methane. We
describe the model framework, compare model parameterizations against modern
observations, and illustrate the flexibility of the model through a series of
example simulations. Though we make no attempt to rigorously tune default model
parameters, we find that simulated atmospheric methane levels and marine
dissolved methane distributions are generally in good agreement with empirical
constraints for the modern and recent Earth. Finally, we illustrate the model's
utility in understanding the time-dependent behavior of the methane cycle
resulting from transient carbon injection into the atmosphere, and present
model ensembles that examine the effects of oceanic chemistry and the
thermodynamics of microbial metabolism on steady-state atmospheric methane
abundance.
