Chlorofluorocarbon CFC-11 and carbon tetrachloride removal in Saanich Inlet, an intermittently anoxic basin

Vertical distributions of the anthropogenic halocarbons CCl2F2 (CFC-12), CCl3F (CFC-11), and CCl4 (carbon tetrachloride) measured in Saanich Inlet, B.C., Canada are used to estimate the renewal rates of bottom water, and to study the non-conservative behavior of these compounds in marine waters. Dissolved CFC-11 and CCl4 were strongly depleted in the deep anoxic hydrogen sulphide-bearing water, with CFC-11 concentrations <30% of saturation levels, and CCl4 concentrations near blank (<1% of saturation) levels. In contrast, CFC-12 was near saturation throughout the water column. A short ( 10 day) incubation experiment using anoxic hydrogen sulphide-bearing water from Saanich Inlet showed rapid removal of CCl4 in non-poisoned samples. The removal was more rapid at 25°C than at 0°C or 40°C, supporting the hypothesis that the removal is biologically mediated. Removal rates of CCl4 in poisoned samples, and removal rates of CFC-11 and CFC-12 in both poisoned and non-poisoned samples were undetectable within the 10 day incubation period. Longer incubation experiments using anoxic hydrogen sulphide-bearing water indicated that the decay rate of CFC-12 in both poisoned and non-poisoned samples was not detectable within 2.5 years at 8°C and 4 months at 25°C, while CFC-11 in non-poisoned samples was reactive with in situ first-order removal coefficients of 0.5 yr−1 (8°C) and 1.2 yr−1 (25°C). Results from these incubation experiments and measurements in Saanich Inlet support previous studies and indicate that CFC-12 behaves as a conservative tracer even in strongly anoxic hydrogen sulphide-bearing waters. In oxygenated waters, the removal of CFC-11 and CFC-12 was undetectable within incubation periods of 2.5 years and 4 months at temperatures 8°C and 25°C, respectively. In contrast, CCl4 was reactive in oxygenated waters with a removal rate constant of 0.69 yr−1.