Project 2

Volcanism and hydrothermalism in and around the Atlantis II Deep

German coordinator: Prof. Colin Devey

Saudi coordinator: Prof. Ali Basaham

German scientists: Dr. Dieter Garbe-Schönberg, Dr. Sven Petersen, Prof. Dr. Lars Rüpke

Saudi scientists: Dr. Rashad A. Bantan

The Red Sea, and particularly the brine-filled Deeps of the Jeddah Transect, are an ideal natural laboratory to test models of hydrothermal circulation. Although the exit points of such circulation (black smokers) are well known, the entry points of the water (“recharge zone”) are still unknown and difficult to determine. In the Red Sea, the natural evaporite layers present close to the axis provide a natural tracer (salt, leading to the formation of unique highly saline brines when dissolved in seawater) for this inflow. It is known that the Atlantis II Deep is filled with brine and that this brine is heated by high-temperature hydrothermal activity. Seismic maps from the 1970s suggest, however, that the seafloor under and around the deep is basaltic, leading to the question of how this brine formed. In this project we aim to (a) determine the seafloor geology of the mid-ocean rift in the Jeddah transect in detail, looking especially for the presence of volcanics and evaporites and (b) locate the hydrothermal plumes and smokers associated with the venting to determine the chemistry of the hydrothermal fluids. Combining this information will potentially allow us, for the first time, to place constraints on the direction and distance over which hydrothermal recharge occurs.

Scientific Goals

The highly saline brine pools of the Red Sea rift all seem to accumulate in basins related to active ocean opening. In some cases (such as the Atlantis II Deep) these brine pools show elevated temperatures and are underlain by metal-rich sediments indicating a hydrothermal input to the pool. This exotic combination of hydrothermalism and oceanography provides the opportunity for unique testing of models for hydrothermal systems. The following fundamental questions will be investigated:

What is the distribution and geometry of basalt and evaporite in the rift axis?
This is the fundamental information necessary to answer the other questions. A combination of bathymetry, side-scan and sediment-penetrating echosounding from an AUV will be used to answer this question, seafloor sampling will provide ground-truthing. Seismic refraction data will provide information on the subsurface structure and on the extent of basalt/ evaporite layers as well as on the geometry of the rift axis at depth.

Where and how is hydrothermal venting occurring into the Atlantis II Deep?
The evidence (from steadily rising temperatures and noble gases for venting of high-temperature (>70°C) hydrothermal fluids in this Deep is overwhelming. Where (centre or margin of the Deep) this venting is occurring is however unknown but is of great importance. Any venting (whether this fluid is highly saline or of “normal” (± seawater) chlorinity) should produce a buoyant hydrothermal plume due to temperature (and possibly also salinity) contrast. The particular acoustic properties of the brine surface mean that the influence of the plume on this surface should be mappable with multibeam sonar by an AUV. This will provide the first opportunity to directly sample and hence determine the chemical nature of the hydrothermal fluids, providing information for the next question;

What is the origin of the Atlantis II brines?
There seem to be two possibilities: they are produced passively by dissolution of evaporites exposed on the rift walls and then heated by mixing with hydrothermal fluid or they are produced during infiltration of seawater into the seafloor during hydrothermal recharge. Determining the salinity of the venting fluids could answer this question – if they show seawater-like salinities they are probably derived by bare-rock infiltration outside the deep, if they are brines then infiltration probably occurs through the evaporite layers. Both options, combined with knowledge of seafloor geology from (1) will allow the distance and direction of recharge to be constrained.