Detailed temperature structure of the hot brines in the Atlantis II Deep area (Red Sea)

Marme Geology, 14 (1973) 1-14 © Elsevier Scientific Pubhshmg Company, Amsterdam - Printed m The Netherlands

DETAILED TEMPERATURE STRUCTURE OF THE HOT BRINES THE ATLANTIS II DEEP AREA (RED SEA)

IN

MARTIN SCHOELL and MARTIN HARTMANN Bundesanstalt pur Bodenforschung, Hannover (Germany) GeologTsches lnstltut der Umversltat Ktel, Ktel (Germany)

(Accepted for pubhcatlon September 29, 1972)

ABSTRACT Schoell, M and Hartmann, M, 1973 Detailed temperature structure of the hot bnnes m the Atlantis II Deep area (Red Sea) Mar Geol, 14 1-14 Semi-continuous temperature profiles m the Atlantis II Deep bnnes revealed a detailed picture of the spreading of the recently detected 59°C bnne This newly heated brine onglnates m the southwest basra of the Atlantis II Deep and is spreading to its other basins In some depressions m the basins, rehcts of the prevaous 56°C bnne were detected as a bottom layer In the north basra restncted inflow of the 59°C brine led to a maximum temperature of only 58 2°C Detaaled bathymetry of the Chain Deep area revealed the existence of three nearly or completely separate brine pools with dfffenng temperature profiles, maximum temperatures recorded are 46 2 and 52 4°C, respectively

INTRODUCTION First mvestlgattons of the hot brines m the AtlanUs II Deep area revealed a characterlStlcally layered temperature and sahnlty structure (Krause and Ztegenbem, 1966, Munns et a l , 1967, Ross, 1969, Pugh, 1969) The temperatures winch were measured 1966 m the Atlantis II Deep were 44 3°C for the upper layer and 56 5°C for the lower one A relnvestagat~on o f the hot brines in 1971 by rv "Chain" (Brewer et a l , 1971) showed a temperature increase m the upper and the lower layer o f 5 5°C and 2 7°C, respectively Because of tins new situation queslaons regarding the way of heat transfer into the basra and watinn the bnnes again came under chscusslon Dunng March and April 1971, immediately after the cruise o f rv "Chain" in February 1971, the German rv "Valdlvla" carried out an extensive samphng program m the Atlantis II Deep area During tins expedition twenty-eight hydrocasts were lowered, twenty o f them combined wtth semt-contmuous temperature/pressure measurements by means of a bathysonde The temperature measurements, winch are the subject o f tins paper, gave detailed mformaUon about the non.uniform temperature structure wltinn the Atlantls II Deep and the Chain Deep bnnes

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M SCHOELL AND M HARTMANN

MEASURING TECHNIQUES The bathysonde Is a commercial oceanograpinc Instrument (Howaldtswerke Deutsche Werft AG, Hamburg and IOel), winch records pressure, temperature, conducuvlty and sound velocity simultaneously The sensor data output is dtgltahzed and is transmitted to the sinp by means of a coaxial cable The sensor of the pressure gauge Is a bourdon tube and is coupled wath a potentlometer, winch is part of an a c -bridge that is automatically balanced The resolulaon of the pressure gauge ~s 1 dbar One hmltat~on of the pressure measurements m the hot bnnes ~s the temperature dependence of the bourdon tube, which leads to lower readings In the hot bnnes Since tins temperature-dependent drift of the pressure reading starts after a tm~e delay of about 5 mm, measurements wltinn tins time are considered not to be influenced by tins effect Tins assumptaon is supported by the good reproduclbdlty of the depth readings 0 e , for the depth of the top of the 59°C brine at different stataons) winch was -+ 3 dbar In spite of tins good relatwe accuracy there remain some difficulties concerning the calibration of the pressure gauge for the depth zones of the brines, so that it proved up to now Impossible to convert the pressure readings to meters Therefore, all pressure readmgs presented in tins paper are darect Instrument readings without any correctxon For comparison with prewous measurements the depths of the different layer tops are hsted in Table I TABLE I Depths of the tops ot the two bnne layers m the Atlantis II Deep Reference

Top of upper bnne

Top of lower bnne

Ross (1966) ("Chain" cruise, 1966) This paper ("Valdwla" crmse, 1971)

2009 m (TTP) 2010 m (IES) 2032 dbar (BS) 2015 m (NBES)

2042 m (TTP) 2048 m (IES) 2054 dbar (BS) 2044 m (NBES)

The values for the bathysonde have been calculated from Tables II and III The different systems of depth measurement are gwen m brackets TTP = telemeter temperature pmger, IES = reverted echosounder, BS = bathysonde, NBES = narrow-beam echo sounder m values are corrected after Matthews (1939), but no correctaons are made for sound velocity m the bnnes In tins paper no conclusions concerning the depth are drawn from the pressure measurements with respect to previous depth deterrrunatlons because of the above-mentioned difficulties of pressure-gauge cahbratlon The temperature-sensing element Is a platinum resistor winch Is part of an automaucally balanced wheatstone bndge Platinum resistance variations are compared with highprecision reference resistors Relatwe accuracy Is +--0 007°C, however, the resolution of

TEMPERATURE STRUCTURE OF THE HOT BRINES IN THE RED SEA

3

the &gltal reading Is only 0 02°C, 1 e , the temperature vanattons m&cated are slgmficant For the measurements m the hot bnnes the absolute accuracy is esUmated to be -+ 0 I°C RESULTS

Atlantzs H Deep The locataons of the bathysonde stataons are gwen m Fig 1 winch Is a slmphfied new bathymetrlc map (Preussag, 1971) of the Atlantts II Deep area Tins map Is based on echo-soundmg profiles run dunng the "Valdtvla" crmse The outer contour hne of the Aflantas II Deep area follows a depth of 1991 m I and coincides approximately with the beginning of the temperature anomaly Tins depth corresponds to a temperature zone of about 25°C (Hartmann, 1972) The contour hne at 2040 m roughly corresponds with the top of the 59°C bnne, whose echo reflex was found at 2044 m The depth of the top of the lower bnne (Table I) is of fundamental interest, inasmuch as a change of tins depth should reflect any input of new bnne rote the basra The &rect comparison of the echo-sounder recordings from rv "Meteor" (crmse 1964•65) and rv "Valchvla" (crtase 1971) revealed exactly the same depth for the top of the lower bnne It should be noted that both research vessels are eqmpped gqth the same echo-sounding systems (ELAC narrow-beam echo-sounder), so that systematic differences should be neghglble Tins statement of the unchanged depth of the upper boundary of the lower bnne contradicts the findings of Brewer et al (1971) who estabhshed a rise of the raterface of approxamately 6 m since 1966 According to the &stnbutlon of the 59°C brine, the Aflanlas II Deep Is obviously sub&wded rote four basins southwest basra, west basm, east basin, and north basra The 59°C bnne of the north and east basins have conneclaon voth the west and southwest basra only wltinn the area around station 94 The results of the temperature measurements are presented m Tables II and III for the lower and the upper bnne, respeclavely The sequence ofstataons m these Tables follows the sub&vision of the Atlantas II Deep Charactensttc temperature profiles for each basra are gwen m Fig 2 the stnlang chfferences can be seen Imme&ately In the southwest basra, a typical feature of the temperature structure of the lower bnne Is a temperature maximum at the top just beneath the interface with the upper bnne (Fig 2A, enlarged scale southwest basra) The transltaon zone from tins temperature maximum down to the zone of mean temperature Is characterized by a comparalavely strong osc~llataon of the temperature, mchcatmg actave convection currents in flus transltton zone Tins zone Is found only m the western parts of the Aflantts II Deep and there ~t ~s the most pronounced m the southwest basra The mare zone of the lower bnne shows a qmet temperature structure over a rode depth range For all stataons m the southwest All depths are given corrected after Matthews (1939)

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Fig i S1mphfied bathymetnc map of the Atlantis II Deep area (Pmussag, 197]) from the "Valdlvza" crulse ] 9 7 ] The 2000-m contour hne corresponds approxzmate]y to the uppermost scattering layer of the echo-sounder recordings at ] 991 m representing the upper boundary of the bnne/sea-water transltlon zone with a temperature of 25°C (Hartmann, ]972) The 2040-m ]me corresponds appro×]mately to the top of the 59°C b n n e and gives a clear chvzszon of the mdzcated basins The connection of the s o u t h w e s t basra to the east basra sttil remams questionable All depths are corrected after Matthews (1939), b u t no corrections are made for higher s o u n d velocities m the b n n e s

004 005 004

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2070-2127 2059-2130 2067-2105

2071-2130 2063-2155 2077-2185 2062-2174

pressure (dbar)

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59 23 59 21 59 17

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mean

003 005 0 05 004 006 0 06

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temperature (°C)

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pressure (dbar)

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22 16 74 76 75

mean

002 0 04 0 03 003 004

stand dev

temperature (°C)

Bottom layer (east basin)

The stations are hsted m the sequence of increasing dtstance from the Atlantis II Deep southwest basra Note the decreasing maximum-and mean temperatures m tlus sequence as well as the sub-division of a zone of lngher temperatures m stations 38, 6 1 , 1 0 1 , 1 0 5 (See also Fig 2 )

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58 18 58 12

59 22 59 24 59 25

23 31 24 35

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2054-2071 2053-2063 2055-2077 2055-2062

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mean

temperature (°C)

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pressure (dbar)

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Maximum temperatures, mean temperatures, and standard dexaatlons (68% confidence level) for the lower h n n e system of the Atlantis 11 Deep

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TABLE III Maxtmum temperatures, mean temperatures and standard devmlaons (68% confidence level) of the upper bnne layer of the AtlanUs II Deep Upper Zone

Lower zone temperature (° C)

temperature (° C)

Stauon

pressure (dbar)

mean

stand dev

pressure (dbar)

mean

stand dev

38 105 61 96 75 94 82 83 42 49 50 92 91 89

2030-2045 2032-2044 2035-2044 2032-2042 2035-2043 2031-2042 2032-2044 2033-2044 2014-2036 2032-2045 2036-2045 2032-2045 2032-2045 2030-2044

49 89 49 92 49 85 49 84 49 75 49 81 49 85 49 87 49 76 49 88 49 83 49 77 49 69 49 78

0 06 0 10 0 10 0 05 0 22 0 07 0 14 0 03 0 24 0 19 0 11 0 06 0 17 0 04

2045-2050 2044-2049 2045-2049 2043-2048 2044-2047 2045-2054 2044-2048 2045-2049 2036-2052 2045-2050 2048-2053 2046-2053 2046-2050 2045-2050

50 38 50 23 50 17 50 21 50 27 50 32 50 20 50 13 50 47 50 49 50 87 50 08 49 98 50 15

0 32 0 12 0 13 0 08 0 32 0 34 0 03 0 12 0 43 0 34 0 37 0 16 0 07 0 19

Note The stattons are hsted m the same sequence as m Table II All standard deviations calculated for the upper bnne exceed those of the lower bnne There are no regional differences either m mean or maxtmum temperatures

basin the " n e w " tugh temperatures rangmg between 59 19°C and 59 24°C were recorded down to the contact with the se&ment Ttus fact is charactensttc for the southwest basra and is a fundamental &stmctton m companson with the other basins Fig 2B shows a typical example of the temperature structure of the bnnes in the east basra There ~s neither a temperature maximum nor a m~xmg zone at the top of the lower bnne The mean temperatures recorded for the lower bnne range from 59 07 to 59 19°C These temperatures are slgmficantly lower than those recorded m the southwest basm Furthermore, the temperatures along the whole profde m the east basra show more scattenng than m the southwest basra, thts is clearly expressed by the standard devtattons (see Table II) The surpnsmg feature m the east basra Is the exastence of a further layer of colder bnne beneath the 59°C layer The temperatures o f flus layer decrease from the north (57 2°C at staUons 82 and 83) to the south (56 7°C at stations 42, 49 and 50) Between the colder b o t t o m bnne and the 59°C bnne there is a translUon zone at about 2100 dbar

Fig 2 A, B, C (pp 7-8) Representatwe temperature profiles from the different basins of the Atlantis II Deep from bathysonde lowermgs The temperature structure of the lower brine is given m enlarged scale For further explanaUon, see text

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C which is approximately the depth of the sill between the west and the east basin near station 94 The constant temperature m the vertical profile of this colder bnne layer mchcates that ~t ~s not generated by heat loss to the bottom, but that the 59°C bnne has spread above the colder bnne and trapped It m the separate depressions of the basins In the west basra two local morphological depressions have been found also to be filled with residues of the colder bottom bnne At stalaon 96 and stataon 74, bottom brine temperatures were recorded at 56 8°C and 57 2°C, respectwely A completely chfferent temperature structure of the lower bnne was recorded in the north basm (Fig 2C) the maxtmum temperatures at the top are 58 18°C (station 91) and 58 12°C (station 89) With increasing depth, the temperature decreases contanuously, the rmmmum temperature m this basra Is 53.80°C at a depth of 2188 dbar (stalaon 89) These generally low temperatures m the north basra are obwously caused by the strongly

TEMPERATURE STRUCTURE OF THE HOT BRINES IN THE RED SEA

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restncted posslbthty of exchange w~th the 59°C brine wtuch was recorded at stalaon 92. Between stataon 92 and 91 there is a sill which nearly approaches the depth of the 59°C bnne top, thus reducing the inflow of the 59°C bnne In contrast to the 59°C bnne, the upper 49°C bnne does not show comparable systematac or reglonal differences A new feature, however, Is a subchvmon of this brine layer an upper zone (2032-2044 dbar) shows nearly constant temperatures (mean value of all profiles 49.8°C), whereas m the lower zone of thas layer two steps of increased temperature were detected m nearly all profiles (Fig.2) The temperature records m this lower zone show more scattenng and range between 50 0°C and 51 2°C The transltmn zone from the 49 ° layer to the normal Red Sea deep water has a tluckness of approximately 4 0 - 5 0 dbar. A temperature of 25°C is reached at about 2000 dbar Within this transition zone the bathysonde profiles show shght steps, mchcatang a non. homogeneous increase of temperature Chain Deep area

Bathymetnc data have shown that the Chmn Deep area is not an entirely separate basra Taking the uppermost brine reflectaon of the echo-sounder which is the beginning of the temperature anomaly, there Is a narrow channel connectang the Atlantts II Deep and Chain Deep area The minimum depth of the channel (about 2010 m) gwes evadence that the 49°bnne layer of the Atlanlas II Deep has no connectton with the Cham Deep Further, the latest bathymetnc data revealed a subdlwslon of the Cham Deep into a northern and a southern part The sdl between the two brine pools seems to be less than 2000 m m depth The temperature profiles of the two Chaan Deeps are shown m Fig 3 together with a temperature profile of the Atlantis II Deep southwest basra

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Fig 3 Comparison of temperature prof'fles from the Atlantis II Deep southwest basin and the Chain Deeps (See also Table IV )

10

M SCHOELL AND M HARTMANN The temperature structure at the three stations m F~g.3 is nearly identical to a depth of

about 2000 dbar, m greater depths, however, there are differences m the temperature structure which confirm the b a t h y m e t n c results But despite the quantatatave differences, quahtatlve slmdan)aes are obwous for both the temperature structure and the c h l o n m t y stratxficatlon (see Table IV) TABLE IV Companson of hydrograpl~c results from the AflanUs II Deep and Chain Deep brme pools (see also Fig 3)

Atlantis II Deep (staUon 38) Chain Deep A (staUon 56) Chain Deep B (stanon 55) Dascovery Deep (stalaon 22)

Deepest bottle of hydrocast

Bathysonde

temperature CC)

chlonmty I (Oleo)

max temp (oc)

59 2

157 1

59 74

405

794

524

46 2

156 3

46 2

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155 9

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a The chlonmty data are slupboard-processed data from Mohr-Kundsen tltrattons (Grasshoff and Wenk, 1972) Relalave accuracy determined from double determmatlons Is +-0 1% which is for the given data approx -+0 1 to ± 0 2%0 C1 Corresponding to the depth range of the upper brine m the Atlantas II Deep there is also a zone of nearly constant temperature ( 3 8 - 4 0 ° C ) m the Chain Deep A The maxamum temperatures m both the Cham Deep and the Aflantas II Deep are reached m neady the same depth range The maximum chlonmty m the Chain Deep B ~s 156 °/oo It approaches the chlonmty o f the 59°C bnne m the Atlantis II Deep and is nearly identical to the chlonmty of the Discovery Deep Unfortunately, the maxamum c h l o n m t y m the Chain Deep A could not be determined since the deepest water sampler only reached the zone lmmecha. tely above the lower bnne, the chlonmty m this depth is 79 4 % o which is similar to the chlonmty of the 49°C brine layer m the Atlantis II Deep (mean value 81 8 °/oo) The chlonmtles thus mchcate a similar density structure of the Chain Deeps as compared to the A t l a n t a II Deep DISCUSSION OF THE RESULTS Startang point of the discussion of the presented results is the heating of the Atlantis II Deep bnnes w~tbJn the last six years The heaUng rate of the lower bnne was first stated

TEMPERATURE STRUCTURE OF THE HOT BRINES IN THE RED SEA

11

by Munns et al (1967) and determined for a Ume span of 20 months to +0.056°C per month Tins fact was confirmed by Brewer et al. (1971) with a mean heating rate (for a ume span of 51 months) of 0 053°C per month For the explanaUon of this tremendous heat transfer into the basra mainly two mechanisms winch are consistent with the thermal measurements have been drscussed (1) Convecuve heat transfer through the se&ment wltinn the whole area of the Atlantis II Deep, caused by movement of brine through the se&ment (Enckson and Simmons, 1969) (2) Locahzed rejection of ingh-sahnxty (possibly m sltu salt saturated) hot waters through vents (Turner, 1969, Pugh, 1969, Bxschoff, 1969). In view of the bathysonde measurements, heat transfer wRinn the whole area of the AtlanUs II Deep ~s not consistent with the temperature structure of the bnnes unmedlately above the sechments The described low-temperature layers m the west, east, and north basins excluswely reduce the area of active heat transfer from the bottom to the Atlanta II Deep southwest basra Tins reduction of the area of heat transfer consequently excludes the proposed convection theory by means of arguments winch have already been given by Turner (1969, p 168) Thus the temperatures of the lower bnne system winch are grapincally compiled m Fig 3 reflect the spreading of heated brine from locahzed sources as described by Turner (1969) and Pugh (1969) Accorchng to Tumer's model "reJected water uses In the form of narrow plumes, winch become &luted by m~xmg wRh their environment and then spread out across the whole layer, along the rater/ace at its top" The presented results gwe strong evidence that this process IS going on at present m the southwest basra of the AtlanUs II Deep The temperature maxuna at the top of the 59°C brine are most pronounced at staUons 38, 105, 61 and 101 and decrease to the north of the west basra Tins gives clear evidence that the present brine &scharge occurs m the southwest basra, where the former 56°C brine is entirely mixed w~th new brine of ingher temperature, resulting In the new 59°C bnne. The mechamsm of the sprea&ng of the bnnes along the interface with the 49°C bnne makes tins chscontmulty the most active transport and m~rang zone Tins is shown by the scattered temperatures m the zone just beneath the interface and the development of a new 50 2°C layer immediately above the interface A further charactensUc feature of Turner's model is that by means of this process the layer (1 e , the former 56°C brine) will be filled from the top w~th increasingly warm fluid Tins can be clearly seen m the temperature structure in the other basins of the Atlantis II Deep In the east basra there is a bottom layer wRh the temperature of the previous 56°C bnne and m the north basin tins mlxang process ~s still going on at the top of tins bnne layer The differences between the east and the north basra are obviously the result of bathymetry The north basra has only a narrow connection (north of station 92) w~th the 59°C brine The spreading picture of the "new" bnnes Is completed when the lateral temperature

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Fig 4 Maximum (1), mean (2) and rmmmum temperatures (3) of the lower brine system plotted versus increasing distance from the southwest basm, as It Is geographically shown m Fig 5 shifts are taken into account Both the maximum temperatures and the mean temperatures of the 59°C brine decrease continuously wath increasing distance from the southwest basin, which is as a picture of the present state shown in Fig 4 and 5 The results of the temperature measurements reveal a dynamic rather than a static situation in the Atlantis II Deep which confirms the statements of Brewer et al (1971) that the Atlantas II Deep is in an active phase of bnne discharge ACKNOWLEDGEMENTS

This mvesUgaUon was supported by the West German Mlmstry of EducaUon and Science, Bonn, and Preussag A G , Hannover We thank the following people A Preuss for the introduction to the bathysonde handhng on board rv "Valdivla", H Hofmann for the techmcal service at the bathysonde, H Rachter and C Lange for the used bathymetnc results and W Stahl for fruitful cooperation and discussions During all phases of mvesUgation the cooperaUon of H Backer is greatly acknowledged

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Fig 5 The present state of brine spreading m the Atlanlas II Deep as deduced from the bathysonde temperature proFdes REFERENCES Blschoff, J L , 1969 Red Sea geothermal bnne deposits their mineralogy, chemtstry and genesis In E T Degens and D A Ross (Editors), Hot Brmes and Recent Heavy-Metal Depostts m the R e d Sea Spnnger, New York, N Y , pp 3 6 8 4 0 1

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M SCHOELL AND M HARTMANN

Brewer, P ( , , Densmore, C D, Munns, R G and Stanley, R J , 1969 Hydrography of the Red Sea brines In E T Degens and D A Ross (Editors), Hot Brtnes and Recent Heavy-Metal Depostts in the Red Sea Spnnger, New York, N Y , pp 138-147 Brewer, P G , Wdson, T R S , Murray, J W, Munns, R G and Densmore, C D, 1971 Hydrographic observations on the Red Sea brines indicate a marked increase m temperature Nature, 231 37-38 Enckson, A J and Simmons, G , 1969 Thermal measurements in the Red Sea hot brine pools In E T Degens and D A Ross (Editors), Hot Brines and Recent Heavy-Metal Deposits m the Red Sea Spnnger, New York, N Y , pp 122-130 Grasshoff, K and Wenk, A., 1972 A modern version of the Mohr-Kundsen titration for chlorlmty m sea water (Unpubhshed manuscript ) Hartmann, M, 1972 Sound velocity data for the hot bnnes and corrected depth of the interface m the Atlantis II Deep Mar Geol, 12 M16-M20 Krause, G and Zlegenbem, J , 1969 Die Struktur des helssen salzrelchen ~llefenwassers lm zentralen Roten Meer Meteor Forsch l~rgebntsse, Relhe A, 1 5 3 - 5 8 Matthews, D J , 1939 Tables o f Veloctty o f Sound tn Pure Water and Sea Watet tn l~cho-soundtng and Sound ranging Hydrographic Department Admiralty, London Munns, R G , Stanley, R J and Densmore, C D , 1967 Hydrographic observations of the Red Sea bnnes Nature, 214 1215 Preussag, A G , 1971 Bathymetnc map of the Atlantis II Deep area (Unpubhshed) Pugh, D T , 1969 Temperature measurements m the bottom layers of the Red Sea bnnes In E T Degens and D A Ross (Editors), Hot Bnnes and Recent Heavy-Metal Depostts in the Red Sea Springer, New York, N Y , pp 158-163 Ross, D A , 1969 Temperature structure of the Red Sea bnnes In E T Degens and D A Ross (Editors), Hot Brines and Recent Heavy-Metal Depostts m the Red Sea Springer, New York, N Y , pp 148-152 Turner, J S , 1969 A physical interpretation of the observations of hot brine layers m the Red Sea In E T Degens and D A Ross (Editors), Hot Brines and Recent Heavy Metal Depostts m the Red Sea Springer, New York, N Y , pp 164-173