Award: OCE-1834475

The uptake of anthropogenic CO2 by the ocean will eventually be mitigated by the dissolution of CaCO3 on the sea floor. Dissolution is an important component of the carbon cycle in models used for climate projections though the relative importance of where it occurs (water column versus sediments) and the rates and processes involved are not fully understood. This ambitious field study is designed to advance our knowledge of the important factors that control carbonate dissolution/preservation in deep ocean sediments. With respect to Broader Impact activities, we always consider the effort to train students as future geochemists/geologist/oceanographers to be a paramount objective and significant outcome of this work. Of the USC students involved in this project, ? were women, one a person of color. A grad student at USC who participated in this project attended the DISCO 2022 meeting and another grad student from USC applied for an NSF graduate fellowship and has yet to hear the outcome. We also had a person from a minority group on board our 2020 trial cruise to film and create an animation short highlighting our work. He has an extensive Instagram network of followers and was posting aspects of our work to this audience, who otherwise may have no idea about scientific research. We partnered with University of Costa Rica and brought along a marine biologist on our trip. She had access to all sediments and other samples. Important Research Accomplishments and Findings include: -A new in situ pore water sampler was built, tested and deployed successfully 10 times -In situ pore waters deviate from pore water collected from recovered cores in significant ways, in situ Alk gradients are steeper and concentrations higher, dissolved Si profiles also deviate (in situ ? ex situ) below 20 cm -13DIC profiles show lighter isotopes downcore, and a Keeling plot interpretation shows the fractional contribution of carbonate dissolution and organic carbon respiration -carbonate dissolution is occurring in sediments underlying waters that are at omega=1, calcite saturation -carbonate dissolution is occurring within the top 5 cm of the sediment column -biogenic Si dissolution is very consistent throughout the study region -gravity cores were obtained from a transect up the Cocos Ridge which provides a unique repository of sediments for paleoceanographic analysis, cores have been XRF scanned. -analysis of water column 13DIC shows evidence of particle dissolution shallow in the water, < 200 m -multicore sediments sectioned at 1cm resolution are being picked for foraminifera identification and geochemical analysis In summary, we document CaCO3 dissolution occurring in sediment pore water even when the overlying water column is at saturation with respect to calcite. We show that respiration of organic carbon by oxygen is responsible for driving dissolution. Sediments throughout the ocean are buffering ocean pH by neutralizing H2CO3 via dissolution with CaCO3 even when pore waters are supersaturated, localized zones of undersaturation exist. Last Modified: 01/30/2023 Submitted by: William M Berelson