The chemistry of the anoxic waters in the Cariaco Trench

Jia Zhong Zhang, Frank J. Millero

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75 Scopus citations


The Cariaco Trench has been widely used as a natural laboratory for the study of anaerobic processes since the deep waters were found to be anoxic in 1954. In the summer of 1990 on a cruise of the R.V. Thomas Washington, we had the opportunity to measure a number of chemical parameters (salinity, H2S, O2, pH, TCO2, NH4+, PO43-, SiO2, SO32- and S2O32-) in the eastern and western basins of the Cariaco Trench. The oxic-anoxic interface was located at 330 m in both basins. The concentrations of H2S, NH4+, PO43- and SiO2 in the deep waters of the Cariaco Trench have increased with time. The linear extrapolations of H2S and NH4+ with time indicate that the trench was oxic around 1915. This turnover may be related to earthquakes that occurred in 1990 and 1929. The ratio of C/N/P/S in the anoxic waters was found to be close to 106:16:1:53, the ratio predicted from the oxidation of phytoplankton by sulfate. Micromolar concentrations of SO32- and S2O32- were found below the interface in the intermediate waters. These intermediates appear to be due to the oxidation of H2S by the sinking of oxygenated surface waters. The rates of oxidation of H2S with oxygen also were measured in the Cariaco Trench waters at 25°C. Measurements were made on surface waters, deep waters and mixtures of deep and surface waters. Oxidation rates were found to be 10 times faster in the deep waters than for surface seawater with added H2S due to Fe2+ in the deep waters. The intermediates SO32- and S2O32- formed during the oxidation of H2S were also determined. A simple kinetic model was used to fit the results.

Original languageEnglish (US)
Pages (from-to)1023-1041
Number of pages19
JournalDeep-Sea Research Part I
Issue number5
StatePublished - May 1993

ASJC Scopus subject areas

  • Oceanography
  • Aquatic Science


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