Century-long acidification reveals possible consequences of coral reef sediment dissolution

Coarse permeable carbonate sediments play a key role as biocatalytical filters in element cycling on coral reefs, but are subjected to increased dissolution due to ocean acidification (OA). We investigated coral reef sediment properties and remineralization rates along a pH gradient in an area of volcanic CO₂ seeping within a fringing coral reef (Papua New Guinea). In coarse carbonate-rich sediments of the reference site (water column pH_T = 8.1) in-situ microprofiles showed a buffered porewater pH of 7.7 to 7.9. In contrast, sites with diffuse CO₂ seeping (water column pH_T 8.0 to 7.7) experienced porewater pH of less than 6 to 7. At the seep sites, the sediments were almost free of carbonates and were dominated by silicates. We found that this resulted in reduced grain sizes leading to decreased permeability and oxygen penetration into the sediment. Areal oxygen consumption and sulfate reduction rates declined at the seep sites. The pattern in oxygen consumption could be explained by oxygen limitation due to lower permeability. However, sulfate reduction was never limited by electron acceptor, indicating that the seep site sediments were limited in electron donors. In line with lower process rates, abundances of microorganisms and meiofauna declined at the seep sites. Our findings suggest that an enhanced dissolution of carbonate sediments due to OA could impact their biocatalytical filtration function. This could slow down the intense element cycling in coral reefs and other coastal carbonate environments, with consequences for ecosystem productivity and functioning.