Hydrothermal flux and porewater geochemistry in Paleochori Bay, Milos, Greece

Despite more than 25 years of research, the spatial and temporal variability of hydrothermal venting in Paleochori Bay remains poorly constrained because there are no reliable repeat measurements at discrete locations. Using a georeferenced photographic map of Paleochori Bay, more than 160 porewater samples were collected and the exact GPS coordinates reported. The GPS coordinates of the sampling locations should provide a foundation for future research in Paleochori Bay by enabling reliable repeat measurements. Hydrothermally influenced porewaters had low pH, elevated arsenic (As) concentrations, and had temperatures between 23.6 and 122.4 °C. The samples were divided into two distinct groups, which had either Cl concentrations below or above the reference seawater. The high-Cl fluids were enriched in K, Ca, Sr and Br and depleted in Mg and SO4 compared to seawater or the low-Cl fluids. However, both fluid types were enriched relative to seawater in As and Si by up several orders of magnitude. Oxygen and deuterium isotope analyses indicated a seawater origin for the fluids with a minor contribution from meteoric water (low-Cl fluids) and arc magmatic water (high-Cl fluids). As previously reported, both high- and low-Cl fluids discharge throughout the bay, often within a few meters of each other. However, high-resolution sampling, together with exact spatial information in the form of GPS coordinates, allowed us to determine that the low-Cl fluids discharge across the bay, while the high-Cl fluid discharge in particular regions of the bay. Based on the distribution of the δ2H and Na/K values in porewater and the occurrence of microbial mats, we pinpointed a set of deep-seated faults, which seem to control the distribution of hydrothermal vs. non-hydrothermal areas in the bay. At 54 locations, porewater temperature profiles were obtained to calculate fluid flux from the hydrothermal areas in Paleochori Bay, which was determined to be 11,300 ± 1100 m3/day. This value allowed to further calculate an annual As input of approximately 1.5 × 104 kg into the Mediterranean Sea.