Hydrothermal influence on nearshore sediments of Kos Island, Aegean Sea

Abstract

The Kos-Nisyros volcanic centre is a long-active, Plio-Pleistocene magmatic system in the subduction zone along the easternmost edge of the active Hellenic volcanic arc in the Aegean Sea. Although today there are signs of relative quiescence in volcanic activity, active onshore fumaroles and shallow-sea hydrothermal vents persist on, amongst others, the island of Kos. The present study explores the large-scale imprint of hydrothermally sourced heavy metals and nutrients on the island’s coastal marine environment, based on geochemical data collected in September 2007 from hydrothermal waters and surficial nearshore sediments (<10 m water depth) at several vents in the Bros Thermi and Kephalos Bay hydrothermal fields. There were strong spatiotemporal fluctuations in gas and water flow rates at the vent sites—for example, the gas/water flux ratio varied by as much as two orders of magnitude (0.03–1.30) at Kephalos Bay vents over a 2-day measurement period on 15–16 September 2007. Confirming earlier work, hydrothermal waters had higher levels of F, Ca and Sr, and lower levels of Na, Mg and SO4 compared with ambient seawater. Moreover, there was novel evidence of strong, widespread Zn and also Mn enhancement. Cluster and factor analyses of surficial sediment data from the Bros Thermi vents (fine (mud) fractions, <63 μm, 6N HCl dissolution) revealed distinct groups of elements such as Cu+Pb+Zn and Mn+Mg. Spatial distribution maps showed similarities in the dispersion of Cu and Pb, and of Ca and Ba. The distribution of HCl-soluble material is the inverse mirror image of the distribution of the residual (silicate and Al-silicate) phase. These findings considerably expand on existing knowledge that the coastal zone of Kos is severely influenced by ongoing submarine hydrothermal activity, and confirm that shallow-water sediment Fe, Mn, Zn and Pb levels are substantially higher than those of other islands along the Hellenic volcanic arc, and even exceed those of some deep-water hydrothermal vents in other world regions. Evidently, there may be significant metallic sulphide deposits of hydrothermal origin at depth beneath Kos.