The Caspian Sea, spanning the border between Europe and Asia, is the world’s largest enclosed body of water. Despite its designation as a sea, it is technically a saltwater lake. Its waters maintain an average salinity of approximately 12 parts per thousand (ppt), which is notably saltier than typical freshwater but only about a third of the average oceanic salinity of 35 ppt. This moderate saltiness exists despite constant and substantial inflow from major freshwater rivers. The answer lies in a combination of its deep geological past and its present-day hydrological balance.
Geological Identity as a Landlocked Basin
The foundation of the Caspian Sea’s salinity is rooted in its origin as a remnant of an ancient, much larger ocean. Geologically, it is the last major vestige of the Paratethys Sea, separated from the prehistoric Tethys Ocean millions of years ago. Through tectonic uplift and global sea-level fluctuations, the Caspian basin became completely isolated from the world’s oceans in the Late Miocene period, approximately 5.5 million years ago.
This complete separation is the single most important factor determining its current salt content. The basin is classified as endorheic, meaning it has no natural outlet to the sea. This lack of an outflow prevents the flushing of accumulated minerals and salts into the larger ocean system. Over geological timescales, the salts that entered the basin have been continually trapped, slowly building up the moderate salinity observed today.
The Mechanism of Salt Concentration
The process of salt accumulation is a continuous cycle driven by water input and output. The Caspian Sea is fed by over 130 rivers, with the Volga River supplying roughly 80% of the total water inflow. While river water is considered fresh, it is not chemically pure and constantly carries trace amounts of dissolved salts and minerals leached from the rocks and soil of its vast catchment area.
These incoming dissolved solids, introduced at a relatively low concentration, are subjected to the region’s arid climate and the sea’s immense surface area. The Caspian Sea loses a substantial volume of water each year through evaporation, estimated to be between 350 and 375 cubic kilometers. This large-scale natural process is the main mechanism for water loss from the closed basin.
When water transforms into vapor, the dissolved mineral content—the salt—is incapable of evaporating with it. Consequently, the water leaves, but the salt stays behind, concentrating the remaining water. Over thousands of years, this continuous cycle of river input bringing in small amounts of salt and evaporation removing only the pure water has led to the persistent and noticeable salinity of the Caspian Sea.
Salinity Variation Across the Basin
The salinity of the Caspian Sea displays a distinct gradient from north to south. The northern part of the basin, which is the shallowest section, receives the overwhelming majority of freshwater inflow from the Volga and Ural rivers. As a result, the water near the river deltas is almost fresh, with salinity levels dropping to as low as 0.05 to 1 ppt.
Moving southward, away from the influence of the major river inflows, the concentration of salt progressively increases. In the deeper central and southern parts of the sea, salinity stabilizes closer to the basin-wide average of 12 ppt. This southern area receives less freshwater input and is subject to the full effects of evaporation, maintaining a more consistent salt concentration.
The most extreme example of salt concentration is the Kara-Bogaz Gol bay, a large, shallow lagoon on the eastern coast of Turkmenistan. Narrowly connected to the main body of the Caspian Sea, this bay experiences incredibly intense rates of evaporation. Water flows into the bay and evaporates almost completely, leaving behind a highly concentrated brine. The salinity here can reach exceptional levels, exceeding 200 ppt and sometimes even 340 ppt, making it one of the most saline bodies of water on Earth.