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Keywords:

  • Double diffuse sedimentation;
  • plume;
  • sediment cores;
  • thermal convection;
  • turbidity current

Abstract

A hydrothermal plume forms in Lake Banyoles, NE Spain, as a result of convection above a springwater-fed suspension cloud ponded on the lake floor. The plume propagates upwards reaching a level of neutral buoyancy from where a turbidity current spreads out laterally. Two-dimensional temperature and particle concentration measurements show the fate of the hydrothermal plume and its associated turbidity current and reveal its seasonal development. Silt particles transported by the plume have been used as tracers to determine the maximum and equilibrium heights of the plume. When the lake is stratified, the vertical transport of sediment is confined to the lake hypolimnion, as the thermocline limits the vertical propagation of the plume. In contrast, when the lake water column is mixed, the plume reaches the surface of the lake. The field measurements have been compared with models for thermal convection from finite isolated sources. Measurements of the flow velocity at the source of the hydrothermal plume (i.e. the rim current velocity) indicate that cold hypolimnetic water is entrained by the plume. Sedimentation rates measured from sediment traps at the zone where the turbidity current develops vary between 10 and 25 g m−2 day−1, and result from continuous silt particle sedimentation from the turbidity current. Sedimentation rates in traps are higher for stations situated close to the source than those further away (<5 g m−2 day−1). Moreover, the results demonstrate that double diffusive sedimentation from the turbidity current was dominant over grain-by-grain settling, causing a mixed distribution of sediments in the region where the turbidity current spreads. The deposition of silt particles could explain the occurrence of silt layers interbedded with biocalcarenites in the littoral zones of the lake and the stratigraphy identified by seismic profiles and cores taken from the lake floor.