What controls the long-term trajectory of Earth’s climate and ocean chemistry? How do marine sediments regulate – and bear witness to – these changes? My research investigates the diverse and dynamic subseafloor geochemical processes actively interacting with the ocean and long-term changes in climate. In this talk, I will present a hypothesis that invokes changes in reverse weathering on the seafloor to explain the increase in seawater Mg/Ca and global cooling observed over the past 50 million years. This hypothesis inverts the prevailing hypothesis and abundant models that require an increase in silicate weathering as the driver of many elemental and isotopic trends over the Cenozoic. Ongoing work investigating Cenozoic changes in the cycling of iron, an important micronutrient, will also be discussed.