| Summary: | The variations of the stable isotopes of hydrogen (protium and deuterium) were determined in the various water masses of the Gulf of Mexico. Surface waters of the central Gulf were found to have an average δD = 5.6%, relative to SMOW. The deep water masses, Subtropical Underwater, Antarctic Intermediate Water and Gulf Bottom Water, were found to be characterized by deuterium contents of 5.5, -0.5 and 0.9%, respectively. These values were found to be comparable with the δD values of these water masses in their source regions. The δD values for the fresh water contribution to the Gulf via the Mississippi and Coatzacoalcos Rivers were found to be -42.3%, and -19.3%, respectively. The value for the net fresh water addition to the Gulf was determined to be ~-33%. Using these values it is shown that the isotopic composition of near shore waters can be explained by a simple mixing model of fresh water from runoff and sea water modified by processes of advection. In addition, it is possible to follow the various water masses of the Gulf by their deuterium contents. Using an evaporation-precipitation model of the Gulf, the annual evaporation and precipitation were found to be 172 and 77 cm/y respectively. The hydrogen isotope fractionation during the thermal cracking of n-octadecane was studied in relation to previous studies of this system using carbon isotopes. The deuterium depletion in methane generated during pyrolysis was found to be -200, -139 and -86% at 630, 530 and 430°C respectively. It was shown that inverse isotope effects were probably responsible for the reversed temperature dependence observed. This and previous studies on the carbon isotope fractionation of this system suggest that the multiplicity of bond types available for breaking and formation and the isotopic fractionation associated with the cracking of decomposition products of the original hydrocarbon are primary factors in determining the isotopic composition of the methane produced. Samples of total plankton, plankton lipid and Euphausia lipid showed a δD(%.) range of -18%, to -178%. The total plankton ranged from -18% to -130% while the lipids ranged from -116% to -178%. The fractionation from total plankton to lipid was approximately -40 to -50%. These values were found to be comparable with those of other workers.
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