Marine Chemistry, 31 (1990) 35-62 35 Elsevier Science Publishers B.V., Amsterdam Some chemical characteristics of the brines in Bannock and Tyro Basins: salinity, sulphur compounds, Ca 2+, F-, pH, At, PO4 3- , SiO2, NH3 Davide Bregant, Giulio Catalano, Giuseppe Civitarese and Anna Luchetta C.N.R., Istituto Talassografico di Trieste, (Italy) ABSTRACT Bregant, D., Catalano, G., Civitarese, G. and Luchetta, A., 1990. Some chemical characteristics of the brines in Bannock and Tyro Basins: salinity, sulphur compounds, Ca2+, F-, pH, At, P034 - , Si02, NH3. Mar. Chem., 31: 35-62. Results of the chemical investigationon the Bannock and Tyro Basins are reported. Both basins were found to be hypersaline ( ~ 10 times higher than salinityof normal seawater) and anoxic. In all investigated basins a region of transition,a few meters thick, was identifiedat depths > 3327 dbar. It ischaracterized by a sharp gradient of salinity, and allconcentrations of analysed species increase strongly except for dissolved oxygen and nitrate,which immediately drop to zero.This region appears as a sharp boundary that prevents mixing. As a result,in the presence of organic matter, an anoxic condition developed with the complete depletion of dissolvedoxygen. At the same time, hydrogen sulphide and ammonium accumulated within the brine. Between the Bannock and the Tyro brines differencesoccur in the measured concentrations of H2S, SO~-, Ca 2+ and NH3. There are some differ- ences also within the Bannock area sub-basins. The Libeccio sub-basin, in the Bannock area,contains a double-layered brine:the upper layeris ~ 140 dbar thick and the lower layer is ~ 300 dbar thick. A second interfacebetween upper and lower brines develops at a depth of ~ 3500 dbar. Nearly allof the measured concentrations vs. depth show the double layer,with the exception of ammonium, the concentration of which remains nearly constant throughout the anoxic column. Profilesof the other species analyzed show remarkable differenceson passing from the upper to the lower brine. Hydrogen sulphide, sulphate and fluorideconcentrations appear constant and then increase at the second interface. The calcium concentration isalsoconstant in the upper brine, but decreases at the second interface.Total alkalinityand phosphate concentrations show a maximum peak just below the first interface.However, afterpassing through the second interfaceallthe chemical parameters exhibit an almost constant behaviour down to the bottom. Hypersaline conditions are attributed to the dissolution of Messinian evaporite,and anoxia is sug- gested to originate from the oxidation of organic matter present in sediments and from the absence of bottom water circulationin such a deep and enclosed environment. The chemical conditions can be summarized as follows:in the Libeccio Basin the values forthe species analysed have the ranges: 39-321 psu for 'salinity', 8.2-6.5 for pH, 2.7-4.0 mM for totalalkalinity, 0.2- 0 mM for dissolved oxygen, 0-1669 pM for hydrogen sulphide, 0-198/zM for thiol,31-99 mM for sul- phate, 11-21 mM for calcium, 7-100/zM for fluoride, 0.2-3080/LM for ammonium, 5.8-0/tM for nitrite, 0.2-12 pM for phosphate and 8-130 #M for silicate. 0304-4203/90/$03.50 © 1990 -- Elsevier Science Publishers B.V.