A joint survey by the Woods Hole Oceanographic Institution and the U. S. Geological Survey, begun in 1962, aims at the geologic exploration of the Atlantic continental margin. This report describes the distribution of carbonate in the surface sediments between Nova Scotia and Hudson Canyon. Samples of surface sediment on a 10-mile grid reveal that over more than 90% of its area the shelf sediment contains less than 5% calcium carbonate. These low values are due to low production by aquatic organisms, a consequence of low average water temperatures. A few locales on the shelf have high carbonate concentration owing to a prolific sessile or nearly sessile fauna. Seaward of the shelf concentrations of calcium carbonate increase regularly with depth to about 30% on the upper continental rise; this increase reflects a greater relative abundance of planktonic Foraminifera due to a decrease in the rate of deposition of land derived detritus. Limestone outcrops are very rare in the area.