Subduction of sedimentary carbonate in the Mariana trench

Calcium carbonate is a major component of shallow marine sediments but is rarely preserved in deep sea environments below the calcite compensation depth (CCD). In this study, we present evidence of sedimentary carbonate on the subducting Pacific plate of the Mariana Trench at a water depth ranging from 6675 to 10,813 m, far surpassing the local CCD of 4800 m. These deposits consist of well-preserved calcareous nannofossils, planktonic foraminifera, and siliceous radiolarians. Fossil assemblages and isotope stratigraphy analyses constrain the age of carbonate sediments to 17.6–13.5 Ma. We propose that the calcareous sediments originated from rapid deposition and burial on the seafloor above the CCD during the Miocene. Subsequently, they were transported into the hadal zone with the subduction of the Pacific Plate and exposed due to the excavation of extensive subducting-related normal faults within the subducting plate. This study implies that sedimentary carbonate may be a key component of subducting carbon at the hadal subduction zones, which has important implications for evaluating global carbon input fluxes at these zones.