Subaqueous hot springs in Koycegiz Lake, Dalyan Channel and Fethiye-Gocek Bay (SW Turkey): Locations, chemistry and origins

Avsar, Ozgur
Avşar, Ulaş
Arslan, Sebnem
Kurtulus, Bedri
Niedermann, Samuel
Güleç, Nilgün Türkan
In this study, horizontal temperature measurements along organized grids have been used to detect subaqueous hot springs. The study area, located in the southwest of Turkey and comprised of Koycegiz Lake, Dalyan Channel and Fethiye-Gocek Bay, was scanned by measuring temperatures horizontally, 2-3 m above the bottom of the lake or sea. After analyzing the temperature data along the grids, the locations with anomalous temperature values were detected, and divers headed here for further verification. Accordingly, among these anomalies, the divers confirmed seven of them as subaqueous hot springs. Three of these hot springs are located in the Koycegiz Lake, three of them are located in the Dalyan Channel and one hot spring is located in the Fethiye-Gocek Bay. At the locations where temperature anomalies were detected, the divers collected samples directly from the subaqueous hot spring using a syringe -type sampler. We evaluated these water samples together with samples collected from hot and cold springs on land and from local rivers, lakes and the sea, with an aim to generate a conceptual hydrogeochemical model of the geothermal system in the study area. This model predicts that rainwater precipitating in the highlands percolates through fractures and faults into the deeper parts of the Earth's crust, here it is heated and ascends through the sea bottom via buried faults. Pervious carbonate nappes that are underlain and overlain by impervious rocks create a confined aquifer. The southern boundary of the Carbonate-Marmaris nappes is buried under alluvium and/or sea/lake water bodies and this phenomenon determines whether hot springs occur on land or subaqueous. The chemical and isotopic properties of the hot springs point to seawater mixing at deep levels. Thus, the mixing most probably occurs while the water is ascending through the faults and fractures. The gas geochemistry results reveal that the lowest mantle He contributions occur in the samples from Koycegiz Lake, whereas the highest ones are found in samples from the Dalaman plain. For the first time, we made use of the micro-XRF sediment core scanning (ITRAX Scanner) for exploring the relation between subaqueous geothermal occurrence and chemical properties of the surrounding sediments. The spatial elemental distribution of sea/lake bottom sediments suggests that depending on the surrounding rock units and the temperature of the hot spring, the sediments around the spring can be enriched with certain elements.