Award: OCE-1834492

As oceanographers and geochemists we know what will happen to fossil fuel CO2 in the atmosphere. Over tens of thousands of years it will be taken up by the ocean and then neutralized by the reaction with CaCO3 sediments at the seafloor. This ‘carbonate compensation’ process has worked to keep earth’s climate equitable for billions of years, but the seafloor chemistry is not well understood. As part of our efforts to better understand the dissolution rate of CaCO3 in the ocean, we built a new seafloor lander to sample water from the upper 20-30 centimeters of sediment that would not have the pressure artifacts that have influenced previous efforts of water sampling from sediments on board a ship after the mud is returned to the surface. Our new SIPR device filters extracted pore waters at the seafloor and stores them in glass tubes. Our chemical profiles from the Cocos Ridge off of Costa Rica show artifact free data when compared to sampling cores of the same mud but sampled on board the ship. Comparison of the alkalinity and dissolved inorganic carbon profiles shows that the artifact is due to CaCO3 precipitation during the depressurization of the samples still in contact with sediment as they were returned to the ship. Using isotope labels on CaCO3 we put in the SIPR, we measured the in situ dissolution rate of CaCO3 in pore waters for the first time. Preliminary results show that the rate law as a function of offset from equilibrium has the same shape in pore waters as it does in the water column and in the lab. Our full data set shows that the production of acid by respiring organic carbon in the sediment is an important part of the CaCO3 dissolution process that buffers CO2. This respiration driven dissolution has been hypothesized since 1982 but conclusively demonstrated in our work for the first time. The rapid increase in alkalinity as a function of depth into the pore waters shows a faster than expected timescale of CO2 buffering by ocean sediments, likely due to the respiration driven dissolution itself and will cause us to rethink how quickly the climate system recovers from perturbations to the level of CO2 in the atmosphere. This project supported the PhD work of two female graduate students and the post-doc work of one female scientist. We exposed a new generation of marine scientists to research at sea on two cruises, one off the coast of Southern California and the other to the Cocos Ridge with a Costa Rican scientist onboard as an observer and new collaborator. The project created a new movie by African American animator Jerome Braxton that is in its final stages of post-production. It is based on his experience of being at sea and probing the chemical processes at the bottom of the ocean. Last Modified: 01/20/2023 Submitted by: Jess F Adkins