The dissolution of marine calcium carbonate (CaCO₃) is a key geochemical buffer against ocean acidification, releasing alkalinity that neutralises anthropogenic CO₂ over centuries to millennia. Yet the mechanisms and rates of CaCO₃ dissolution in the abyssal (3 - 6 km) and hadal (> 6 km) realms remain poorly constrained, because the extreme hydrostatic pressures involved – up to 1000 bar – make both in-situ measurement and laboratory replication technically challenging. As part of the Deep-C ERC project, we have developed a new generation of instrumented high-pressure reactors operating up to 1000 bar and 2-30°C, with optical sensors for continuous, in-chamber monitoring of pH and dissolved oxygen. We are now using this system to characterise CaCO₃ dissolution under deep-sea pressure conditions along two complementary lines. First, to establish the abiotic baseline, we are constraining the pressure dependence of CaCO₃ solubility: using CO₂ dissociation constants consistent with modern cold-ocean data, to determine the apparent solubility product and dissolution rate of CaCO₃ across the 0-1000 bar range, from which a pressure-corrected saturation state (W) can be computed. Second, to isolate the microbial signal, by tracking the characteristic shifts in total alkalinity (TA) and pH, and hence in W, as well as O₂, that accompany respiration-driven dissolution; thereby separating microbially driven from purely pressure-driven dissolution. Establishing this pressure dependency is a prerequisite for improving how deep-sea CaCO₃ dissolution feedbacks are represented in global ocean biogeochemical models.

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