Abstract

The drawdown of CO2 via the temperature-dependent weathering of silicate minerals is canonically thought to be one of the key processes acting to maintain Earth’s climate within narrow bounds. However, the climatic responsiveness of weathering on multi-million-year timescales, to my knowledge, remains to be demonstrated. If no such relationship exists, or is weaker than typically presumed, previously unexplored factors may be important in driving long-term carbon cycle changes. One such parameter may be the calcium concentration of seawater, via the effect that it has on CaCO3 preservation in the ocean, explored here using a new, near-continuous Cenozoic record of seawater major ion chemistry. The Cenozoic [Ca2+sw] record closely corresponds to the timing of atmospheric CO2 changes, potentially implying a common driver. Testing the causality of this relationship using a carbon cycle box model demonstrates that whether or not this is the case depends to a large degree on the slope of the relationship between climate and silicate weathering, with a shallow slope implying that [Ca2+sw] is likely to drive the system. Given the recently identified major change in the Neogene global sea floor spreading rate, this finding potentially shifts the key driver of long-term climate from the terrestrial to marine realm, and at the very least, highlights the need to determine whether silicate weathering is responsive to climate change on geologic timescales before the long-term drivers of CO2 can be determined.