129I concentrations in two sediment cores from the continental slope near Cape Hatteras were determined with tandem accelerator mass spectrometry (TAMS). The natural 129I/127I ratios in these cores are approximately 10-12. The input of anthropogenic 129I has increased this ratio to values close to 10-10 at the surfacee of the sediments. The observed natural level of 129I is about two orders to magnitude above the current detection limit with TAMS, which opens up to possibility for applications of 129I in the marine system such as dating of sediments and crude oils and tracing of slow water movement in sediments.