Discovery of a unique submarine hydrothermal system between shallow photic and deep dark sites around the Greek island of Milos (Aegean Sea, Greece)

On the Greek island Milos and in shallow water at its coast, many spots with hydrothermal activity have been found and studied in the past. The M192 cruise in August 2023 with the German research vessel METEOR followed the idea that these systems may continue along a tran­sect from shal­low, nearshore, photic to the deeper, off­shore, aphotic zone around the island, accompanied by changes in terms of environmental parameters. 

Volcanism along the Hellenic volcanic arc started during the Early to Middle Pliocene, while the last eruption occurred in 1950 (Nea Kammeni volcano). The intense seismic activity in the area is associated with important geothermal gas venting, with the major systems being found in relatively shallow waters (1–500m depth) at Methana, Milos, Santorini (Kolumbo submarine volcano), Kos and Nisyros.

Systematic bathymetry and water column acoustic survey work with METEOR's multibeam with the autonomous underwater vehicle (AUV) MARUM-SEAL on the M192 cruise revealed several previously uncharted hydrothermal vent fields offshore Milos. They are located in the southeast extending from the bays Kiriaki to Paleochori and Thiorychia, as well as in an area northwest of Milos, offshore the bay of Vani. The distribution of the hydrothermal vents seems to be tectonically controlled and follow the prominent faults that have been mapped on Milos.

The areal extents of venting were identified by echosounding using the acoustic anomaly the presence of gas bubbles causes in the water column. But selected hydrothermal vents were furthermore visually observed and sampled using the remotely operated vehicle (ROV) MARUM-SQUID. These individual vents revealed pronounced differences; whereas the shallower vents (around 100 m water depth) were noticed as white patches (of sulfur-oxidizing bacteria) on the sandy seafloor with diffuse venting comparable to the shallow vents close to the coast, the deeper vents (around 200 m water depth) featured remarkable chimney structures sometimes several meters in height that are covered with white biofilms and vent fluids reaching temperatures up to 180 °C. Sampled fluids showed mildly reducing and slightly acidic (pH between 5.0 and 7.9) conditions and were rich in dissolved hydrogen sulfide and dissolved metals. These signals extended up to about 10 m into the water column, as recorded by CTD-rosette water sampler stations.

To date, shal­low-wa­ter and deep-sea hy­dro­thermal sys­tems have been treated as in­de­pend­ent, seem­ingly un­re­lated en­tit­ies; the results of the M192 expedition presented here are the first foray into re­mov­ing this ar­bit­rary bound­ary.