  Lakes existed on Mars later than 3.6 billion years ago, according to
sedimentary evidence for deltaic deposition. The observed fluvio-lacustrine
deposits suggest that individual lake-forming climates persisted for at least
several thousand years (assuming dilute flow). But the lake watersheds' little
weathered soils indicate a largely dry climate history, with intermittent
runoff events. Here we show that these observational constraints, while
inconsistent with many previously-proposed triggers for lake-forming climates,
are consistent with a methane burst scenario. In this scenario, chaotic
transitions in mean obliquity drive latitudinal shifts in temperature and ice
loading that destabilize methane clathrate. Using numerical simulations, we
find that outgassed methane can build up to atmospheric levels sufficient for
lake forming climates, for past clathrate hydrate stability zone occupancy
fractions >0.04. Such occupancy fractions are consistent with methane
production by water-rock reactions due to hydrothermal circulation on early
Mars. We further estimate that photochemical destruction of atmospheric methane
curtails the duration of individual lake-forming climates to less than a
million years, consistent with observations. We conclude that methane bursts
represent a potential pathway for intermittent excursions to a warm, wet
climate state on early Mars.
