Sea of Marmara

Deep under the sea, a giant crack runs through the rock. It is called a fault.

The ground on each side slowly slides past the other. Deep holes formed in the sea floor along it.

The Sea of Marmara sits on the North Anatolian Fault, a crack where two plates of Earth's crust, the Eurasian plate and the Anatolian plate, slide sideways past each other. Its most active branch cuts across the sea floor from east to west.

Along this fault, three deep basins, each about 1,250 meters deep, have formed between higher ridges. Their steep sides are marked by underwater canyons and the scars of landslides. Strong earthquakes have happened along the fault in the past, so scientists study the layers of mud on the sea floor to learn its history.

Çağatay and colleagues describe the sea as lying on a major continental transform boundary, the North Anatolian Fault. Around the sea the fault splits into branches across a zone about 100 km wide; the most active, the Main Marmara Fault, cuts the floor east to west and takes about 75% of the roughly 24 mm a year of right-lateral motion between the Eurasian and Anatolian plates.

The deep subbasins are described as transtensional and hold 5 to 6 km of sediment. Slopes up to 29 degrees on the north side carry submarine canyons and landslide scars. Layers of mud shaken loose by past earthquakes record the fault's history, which the team traced back more than 5,000 years.

Long ago, this sea was a lake. Then sea water flowed back in.

Now light, less salty water floats on top. Heavy, salty water stays below.

During the last ice age, the Sea of Marmara was a lake of fresh to slightly salty water. About 12,550 years ago, it became a sea again.

Today, water flows both ways through the straits. Less salty water from the Black Sea runs along the top, while saltier water from the Mediterranean flows in underneath. A lasting boundary between the two layers sits about 25 meters below the surface. Rivers from the south add more fresh water.

The sea was a fresh to brackish lake during the last glacial period; marine conditions returned about 12,550 years ago (dated 12.55 ka BP), and before that the lake level was set mainly by the Dardanelles sill, about 85 m deep just before the reconnection.

Today a two-way exchange carries Black Sea water of about 18 psu (practical salinity units) in the upper current and Mediterranean water of about 38.5 psu in the lower current, with a permanent pycnocline at about 25 m. A 35-year computer model (1987 to 2021) by Saçu, Şen and Erdik found a strong westward flow in the central basin and a statistically significant decline in the upper-layer flow, linked to a shrinking sea-level difference between the Black Sea and the Aegean, potentially driven by more evaporation from the Black Sea.

Tiny living things called plankton float in the sea. Sometimes so many grow that the water changes color.

Dolphins and porpoises swim in the Sea of Marmara too.

NASA calls the Sea of Marmara a rich soup of nutrients and life. Its fresher surface water and nutrients from rivers help tiny floating organisms called phytoplankton grow. In May 2015, a satellite photographed swirls of them. Scientists found mostly two kinds of dinoflagellates, Prorocentrum micans and Noctiluca scintillans, which can change the color of the water.

Bigger animals live here too. In April 2015, Istanbul University scientists watched a young harbour porpoise swimming with a group of bottlenose dolphins in the Marmara Sea.

NASA's Earth Observatory describes a phytoplankton bloom imaged by Landsat 8 on 17 May 2015: swirling filaments that trace eddies and currents, driven by water rushing through the straits, a jagged coastline and a tectonically fractured sea floor. Sampling by Baris Salihoglu and Ahsen Yuksek showed mostly Prorocentrum micans and Noctiluca scintillans; Prorocentrum bloomed first and Noctiluca later dominated. Noctiluca can eat other phytoplankton.

Tonay, Dede and Amaha Öztürk (2017) reported a juvenile harbour porpoise in a non-aggressive interaction with bottlenose dolphins in the Marmara Sea on 25 April 2015. Mucilage, a slimy material, has also formed in the sea: Doğan and colleagues sampled mucilaginous aggregates at six sites during an event from autumn 2021 to summer 2022 and found a wide range of plankton in them.

If you see dolphins, watch them from far away. Never feed them.

Swim only with a grown-up. If water pulls you out, swim along the beach.

NOAA, the United States ocean agency, gives advice that helps anywhere: watch dolphins and porpoises from a safe, respectful distance, never feed them, and never try to touch them or swim with them.

NOAA also warns about rip currents, narrow channels of fast-moving water near the shore. If one catches you, don't fight it. Swim parallel to the shore, then swim back to land at an angle.

NOAA Fisheries advises viewing wild marine mammals from a safe, respectful distance, at least 50 yards (about 46 m) from dolphins and porpoises, never feeding them, and limiting viewing to 30 minutes or less, because a continued presence can stress the animals. These are United States guidelines, but the reasons behind them apply to any sea.

For swimmers, NOAA's National Ocean Service explains that rip currents can move at up to eight feet (about 2.4 m) per second, faster than an Olympic swimmer. Swimmers who fight one by heading straight to shore tire quickly; the advice is to swim parallel to the shore and return at an angle.