Laptev Sea, marginal sea of the Arctic Ocean off the coast of Northern Siberia (Russia), bounded by the Taymyr Peninsula (Poluostrov) and the islands of Severnaya Zemlya on the west and by the New Siberian Islands and Kotelny Island on the east. It is connected in the west with the Kara Sea and in the east with the East Siberian Sea. Formerly called the Siberian Sea, it was renamed in 1935 after Khariton and Dmitry Laptev, the brothers who first mapped its shores (1735–40). Its area is about 276,000 square miles (714,000 square km), the average depth 1,896 feet (578 m), and the greatest depth 9,774 feet (2,980 m).
Large bays cut into the shore, and numerous rivers, the largest being the Lena, flow into the sea. Several rivers form extensive deltas. Dozens of islands, primarily in the west, vary in landscape and origin.
Ancient rivers and glaciers were important in forming the bottom relief and shoreline. The floor of the sea is a gently sloping plain, breaking off abruptly toward the Arctic Ocean. The bottom of the deepwater part is covered with silt, the shallower areas with sand and silt. In the east, under a thin layer of deposits, occurs a layer of very old “relic” ice. As regards salinity, the thawing of ice and the inflow of fresh river water might have resulted in a layer of fresh water 53 inches (135 cm) thick. In winter the salinity in the southeastern part of the sea is 20–25 parts per thousand, in the northern part up to 34, and in summer the salinity falls to 5–10 parts per thousand in the southeast and 30–32 in the north.
Air temperature below 32° F (0° C) occurs in the north during about 11 months and in the south 9 months. The average temperature in January is -24° to -29° F (-31° to -34° C), the minimum about -58° F (-50° C). In July the average temperature in the north is just above the freezing point, in the south about 43° F (6° C), with a maximum of 50° F (10° C). On the shores the maximum can reach 75° F (24° C). In winter there are frequent gales, blizzards, and snowstorms; in the summer, snow squalls and fogs.
For most of the year the sea is covered with ice. In the winter the temperature of the water under the ice is 30.6° F (-0.8° C) in the southeast and 28.8° F (-1.8° C) in the north; in the deep regions it is from 29.1° to 28.9° F (-1.6° to -1.7° C). In the summer, in ice-free regions, a thin layer of water warms to above the freezing point.
At the mouths of the rivers people live by catching salmon and other fish. Mammals include various seals, walrus, and polar bear. The Laptev Sea is on the Northern Sea Route, with Tiksi the main port. Timber, building materials, and furs are the primary cargoes.