How rock weathering sets Earth’s thermostat

Since its formation 4.5 billion years ago our blue planet contains liquid water at its surface. If all water were frozen as on our cold neighbour Mars, or if all water had disappeared into space as on hot Venus there would be no life on Earth. We owe Earth’s habitability to the weathering feedback, balancing volcanic CO2 additions from Earth’s interior by CO2 drawdown through silicate weathering, stabilising the greenhouse effect.

The isotope ratios in seafloor sediment show that this balance has shifted in the last 15 million years. According to 18O/16O it got colder, eventually leading to the northern hemisphere glaciation. 11B/10B discloses that atmospheric CO2 decreased, resulting in cooling by a reduced greenhouse effect. The cause of decreasing CO2 is subject of a lively debate. One view holds that the rise, erosion, and increased weathering of mountain belts increased CO2 drawdown. The other suggests constant weathering fluxes,  as indicated by the ratio of the cosmogenic isotope 10Be rained out from the atmosphere to the stable isotope 9Be released from rocks by weathering (10Be/9Be). With a land surface that got increasingly more “weathering reactive” by geologic reworking, atmospheric CO2 concentration would decrease even if CO2 input and drawdown flux remained constant and balanced. Stable 7Li/6Li indeed confirms such change in the properties of the land surface in the past 15 million years.

Today a dramatic shift is imminent, because humans are massively interfering with this delicate balance through industrial CO2 emissions, an experiment that better be discontinued fast.