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The concentration and I80 : I60 ratio of dissolved oxygen were measured for 23 rivers and lakes of the Amazon Basin during 1988, 1990, and 199 1. With only two exceptions, the rivers and lakes had dissolved oxygen concentrations that were at 20-90% of atmospheric saturation levels. The aI80 of the dissolved oxygen ranged from 15 to 30?& (vs. SMOW). The 6180 for the lakes were the lowest at 15-23o/oo. &I80 <24.2'?& (the atmospheric equilibrium value) are the result of photosynthetic oxygen input. The 6180 of the rivers, in contrast, ranged from 24 to 30?&. aI80 > 24.2%0 resulted from respiration. Despite this clear difference between the 6180 for rivers and lakes, these water bodies had similar levels of oxygen undersaturation. The aI80 and dissolved oxygen concentrations are used to determine the ratio of community respiration (R) to gross photosynthesis (P) rates. R : P varied between -1 and 1.5 for lakes and between 1.5 and 4 for rivers. For all rivers and lakes, the measured 6180 indicated the presence of photosynthetically produced oxygen, with the highest proportion occurring in lakes. The 6180 of dissolved oxygen is a unique tracer of photosynthetic oxygen and provides, through a determination of R : P, a means of quantifying the heterotrophic state of freshwaters.

The end-Triassic mass extinction is one of the five most catastrophic in Phanerozoic Earth history. Here we report carbon isotope evidence of a pronounced productivity collapse at the boundary, coincident with a sudden extinction among marine plankton, from stratigraphic sections on the Queen Charlotte Islands, British Columbia, Canada. This signal is similar to (though smaller than) the carbon isotope excursions associated with the Permian-Triassic and Cretaceous-Tertiary events.

We present experimental results that show that the kinetic isotopic fractionation during gas exchange is 0.9972 ± 0.0002 for oxygen, 0.9992 ± 0.0002 for methane, 0.9987 ± 0.0001 for nitrogen and 0.982 ± 0.002 for hydrogen, and that the equilibrium fractionation between water and gas phases is 1.037 for hydrogen. We show that the isotopic fractionation during gas transfer for these gases is not equal to the square root of their reduced mass in water, as would be predicted by an extension of the kinetic theory of ideal gases to dissolved gases. The use of isotopes as tracers of biogeochemical gases requires knowledge of the fractionation factor for air‐water gas transfer; there have been few direct measurements of these factors.