Hot brine pools found in Red Sea

Press, Volume CXIV, Issue 33422, 2 January 1974, Page 11

Hot brine pools found in Red Sea

(By Dr JOHN GR1BBIN)

In the mid-1960s oceanographers discovered the first of what proved to be a chain of deep brine pools running along the centre of the Red Sea. These deeps contain hot water which is very rich in metallic salts compared with the surrounding sea and, indeed, with the oceans of the world.

As well as the natural interest aroused by this peculiar phenomenon, the discoveries came just at the time when the geophysical concepts of plate tectonics—according to which the continents are continually in motion, carried on plates of the Earth’s crust which drift over the molten rocks beneath—were gaining wide acceptance.

The ridge which runs down the centre of the Red Sea marks a boundary between two continents now moving apart, and any information about that ridge has a bearing on modem geophysical theories.

But the greatest excitement lies in the discovery that the brines of the Red Sea pools have in them hundreds of times the concentration of solid material usually found dissolved in seawater, and that the pools’ sedimentary deposits are rich in heavy metals, including gold, silver, copper, lead and zinc. If these are a feature of all enclosed basins associated with rift valleys, such as the Red Sea, then they could provide a valuable addition to the natural resources available to man.

But progress toward understanding the occurrence of these brines and sediments required enough of them to be discovered and investigated. In 1966 only three were known about — hardly sufficient to form a basis for generalisation. New pools found Last year, however, the situation was completely transformed: a German team investigating the area from! the RV Valdivia found 13 new brine pools and several other areas where sediments seem to have been laid down by pools which are n~ longer! active. This brought the total number of known deeps to 171 and for the first time it was practicable to make com-! parative investigations of individual pools in order to see which properties were char-i acteristic of all features like the rift valley and ridge of the Red Sea, and which werei peculiar to individual pools. , From a geological point of view it is now clear that the: southern and central parts of . the Red Sea are rather differ-, ent from each other.

For example, in the region btween the Nereus Deep and the Suakin Deep the gently sloping sea floor at a depth of 500 m to 600 m falls away steeply to a central trough about 1 km deep, and the brine pools are very steepsided depressions, more than 2 km deep overall, lying inside this trough. Further south, however, the *m floor is made up of long flat tarraces and this explains why brine pools art not found there — it is simply that there are no narrow and deep holes in which the brine can

remain undisturbed by currents.

In the steep-sided deeps the brines form layers of different salt concentrations, with the most concentrated layer at the bottom. Transition zone The higher layers become gradually less in concentration until, at the top of the pool but still 1 km. below the surface of the sea, there is a transition zone which blends into normal Red Sea waters.

But where do the heavy metals in the pools come from?

There are two particularly favoured ideas.

The first suggests that the Red Sea dried out at some time in the fairly recent I geological past and has since 'been resubmerged. If that happened, then the water in the deeper holes of the trench system would have evaporated slowly, leaving salt deposits which are now dissolving into the sea-water presently covering the area. The second idea is that deposits of metals and other substances might have been formed in this region of geological activity by the tectonic processes that are now slowly widening the Red Sea into an embryonic ocean. Certainly, it is this regional activity which accounts for the high temperature of the pools, and J makes the Atlantis II Deep, perhaps others, active | today. Salinity increases Typically, the water in the deeps shows a salinity {(including salts of many different metals) which increases from the 40 or so .parts per thousand char- ! acteristic of sea-water to imore than 250 parts per thousand — over an increase I in depth of only a couple of I hundred metres. At the same time, the I temperature of the brine increases from about 22deg. Centigrade to about 44deg. Centigrade. These figures were obtained for the DisIcovery Deep by a team from Britain’s National Institute of Oceanography. I In the case of the Atlantis 111 Deep, which has been investigated by many teams i on several research ships, there has been an increase in the temperature of the brine from 55.9de». Centigrade in 1965 to 60.1 deg. Centigrade in 1972. Convection currents have been measured in this deep;

as the water in the bottom layer is heated by the processes occurring in the Earth’s crust beneath it the lowest layer rises through cooler layers. Because of this activity, the Atlantis II Deep is now spilling over into others nearby; the metal-rich brine is naturally denser than seawater, ana sinks downwards unless it is heated. So in this case hot brine rises from the deep, meets ordinary seawater and is cooled until it falls back to the local sea floor.

Must be connected As well as this kind of activity linking brine pools over the sea floor, some of the pools have such similar concentrations of metals and nearly identical temperatures that it seems they must be connected, even though the geography of the sea floor prevents the kind of overflow seen in the Atlantis II Deep. So they must be connected by tunnels below the sea floor.

All this evidence suggests strong links between the pools and tectonic activity. But what of the other possibilities? One of the most recent investigations of Red Sea brine pools was carried out by a team from the Applied Geochemistry Research Group of Imperial College, London. The expedition, aboard the tug "Nereus,” formed a joint project with the Kingdom of Saudi Arabia and was typical of the international collaboration which plays such an important part in large-scale studies of this kind. Different activity Dr J. S, Tooms, Dr R. Holms and Dr A. Horowitz have now reported the results of a detailed investigation of the Oceanographer Deep, which seems to be quite different in origin and activity from some of the other deeps, for example Atlantis II Deep. The Oceanographer Deep lies slightly outside the central valley of the Red Sea, at 26deg 16.6 min N, 30deg 01 min E. Just to the east and west of the deep sea floor is at a depth of 960 m, while the bottom of the deep itself is at 1528 m. The width of the depression is only 0.6 nautical mile at the 1350 m contour and 0.3 nautical mile at the 1450 m contour, and its overall lenght seems to be less than two nautical miles. As well as obtaining samples of brine from the deep, the “Nereus” expedition took a core sample from the underlying sediments. The amount of metals such as iron and copper found In the brine was more than in normal sea-water but less than in active pools such as the Atlantis II Deep; for example, normal sea-water

contains 0.02 milligrams of iron in each kilogram of water, Atlantis II Deep contains 0.08 g/kg (80 mg/kg) and the Oceanographer Deep contains 0.2 mg/kg. According to the Imperial College team the core from the bottom sediments is "highly organic” and “gave off an extremely strong odour” of hydrogen sulphide. Distribution of metallic elements in the core is very like that of similar cores from the Norwegian fiords. All this evidence suggests that the Oceanographer Deep the most northerly of the brine pools yet found in the Red Sea, is indeed an effect of the region have dried out and been resubmerged. The deposits of salts that result —evaporites—are like some found recently in the Mediterranean, which indicate that that sea has repeatedly dried out and been resubmerged. For the geophysicists, this discovery of a second type of Red Sea brine pool will help to determine the kinds of geological activity going on there. From the point of view of discovering workable deposits of ores the evidence is again encouraging because the differences between the Oceanographer Deep and some of the more southerly pools indicates that the latter have not been formed by the evaporite process and so must be associated with metal-rich regions produced by tectonic activity. We can expect that geophysicists may soon be able to predict where in the world similar tectonic activity in the distant past may nave produced ore deposits we can mine today, or, at least tomorrow. From “Spectrum.”