| Abstract: | Variations in the natural abundance of stable carbon isotopes have been examined in an assorted spectrum of marine samples from the South Indian Ocean. All major sites of carbon isotope fractionation were investigated. δ¹³ values for dissolved in organic carbon were lighter than those reported for the South Pacific. Organic carbon oxidation approximated from δ¹³ changes agrees with published values of the loss of organic matter in near surface waters. Under special conditions, δ¹³ of ΣCO₂ was used to calculate upwelling at 65°S. Carbon isotope fractionation during photosynthetic fixation of inorganic carbon by marine phytoplankton exhibits a minimum at approximately 16°C., increasing at higher and lower temperatures. Below 15°C. the fractionation factor is approximately 0.6°/οο/°C. Species of higher marine organisms reflect the isotopic composition of their food source. Metabolic intermediates and purified extracted compounds exhibit temperature dependences on isotopic composition different from the whole organism; lipids having the largest dependence, sugars and amino acids the smallest. Evidence also suggests isotope fractionation occurs during metabolic polymerization of sugars and amino acids. Isotopic analyses of particulate organic matter indicates that this material is not biologically refractory. Significant fractionation and gross quantitative changes which occur at the benthic sea-sediment interface seemingly indicate high microbial activity. The use of the δ¹³ composition of sedimentary organic carbon as a paleotemperature indicator appears valid under open ocean conditions in areas free from terrigeneous influence. |
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