The roles of marginal seas in the marine biogeochemical cycle and their possible impacts on global carbon budget has attracted increasing attention due to high riverine inputs and strong biological productivity. Contrary to conventional wisdom, quite a few shelf seas that were studied for air-sea CO2 exchange have been found to be net CO2 sinks rather than sources. The East China Sea shelf-pump has been estimated to have a capacity of 20-30 Mt C/yr. However, the estimated organic carbon burial rate in the ECS shelf is less than 10 Mt C/yr. The shelf sea has only limited capacity for storing excess CO2. Apparently, a large fraction of carbon must be exported out of the ECS shelf. One of the possible mechanisms is the cross-shelf export of particulate organic matter. However, the lateral transport flux is difficult to quantify. We have tried to use carbon and nitrogen isotopes as tracers to track down the elusive export flux of organic matter from the shelf to the deep sea. Comparing isotopic characteristics of sedimentary organic matter all over the East China Sea and the adjacent Okinawa Trough, we have found uncanny similarity (d13C=-23 to -21 permil, d15N=3.5 to 4.5 permil) between sediments from the inner shelf near the coast and from the Trough. Because the coastal sea is highly productive, a small fraction of the production may represent a significant amount of carbon export.