The radiocarbon signatures of the western Sargasso Sea and the atmosphere as a function of time over the past 200 years are known. These can be combined with seawater dissolved inorganic carbon (DIC) radiocarbon data from the Gulf of Maine and the northern Middle Atlantic Bight (MAB), including Long Island Sound (LIS), for 1983 and pre-1955 shell analyses for radiocarbon, to set the amount of Antarctic Intermediate Water (AAIW) required to balance the 14C budget in the northern MAB. Approximately 40% of the water entering the Middle Atlantic Bight from the north must be AAIW, the other 60% being Sargasso Sea water. Contemporary water from LIS, a part of northern MAB, can be explained as a mixture of Sargasso Sea water and Gulf of Maine water but at times in the past more low-14C water (AAIW) was added to this mixture as recorded in shells from LIS. This implies variations in upwelling rates over time in the region of the Middle Atlantic Bight.