Evidence that low‐temperature oceanic hydrothermal systems play an important role in the silicate‐carbonate weathering cycle and long‐term climate regulation

The feedbacks between changes in atmospheric CO 2 levels, climate, and CO 2 drawdown into rocks are incompletely understood. In particular, the role of the upper oceanic crust in this long‐term carbon cycling is debated. Here, a simple model for the precipitation of calcite in the upper oceanic crust is developed with the aim of understanding why Late Mesozoic upper oceanic crust contains several times higher CO 2 concentrations (~2.5 wt%) than Cenozoic upper oceanic crust (~0.5 wt%). The modeling shows that neither heating of seawater, nor leaching of Ca from the rock with charge balance maintained by Mg uptake by the rock, can lead to >0.2 wt% CO 2 uptake by the oceanic crust. Alkalinity production during fluid‐rock reaction in the crust allows substantially more CO 2 to be taken up by the crust in calcite, and is consistent with changes in the major element composition of Late Mesozoic upper oceanic crust due to hydrothermal alteration. The higher CO 2 content of Late Mesozoic than Cenozoic upper oceanic crust thus requires greater alkalinity production by fluid‐rock reactions in the Late Mesozoic. This may have been due to higher bottom water temperature and/or seawater having a different composition leading to different secondary minerals forming in the Late Mesozoic. Irrespective of the mechanism, the negative feedback on atmospheric CO 2 levels provided by enhanced hydrothermal CO 2 consumption in the Late Mesozoic was of similar magnitude to that from continental weathering.