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The Orca basin is an anoxic, brine-filled intraslope depression (2,250 m deep) located on the Louisiana continental slope in the Gulf of Mexico. The Dead Sea, also brine filled, is only 40 m deep. Both pH (6.83) and ^SgrCO2 (5.04 mM) in the Orca brine are higher than in the Dead Sea brine (pH = 5.9-6.5, ^SgrCO2 = 2.53 to 2.59 mM). These comparisons and our laboratory experiments on carbonate dissociation constant indicate that Orca basin brine and Dead Sea brine have quite different carbonate chemistries. Carbonate interactions in the Dead Sea brine are strongly influenced by its bulk ion composition, especially the magnesium and calcium enrichments, which are 2.8 and 3.7 times, respectively, relative to normal seawater. These enrichments cause a dec ease in the second dissociation constant of carbonic acid. The Orca basin is depleted in magnesium and calcium and has a carbonate system that resembles a NaCl-saturated seawater.
Sass and Ben-Yaakov have attributed the low pH in the Dead Sea to ion pairing of Mg CO3o. Carbonate interactions in the Orca basin can be explained by the increased NaCl effect on the dissociation constants, with pH being largely controlled by an input of biogenic CO2 from the sediments. This addition of CO2 is significant and accounts for the higher ^SgrCO2 levels that were found. These differences in the carbonate system between these two hypersaline bodies may result from differences in their origin.
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