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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.

-The 13C : l*C of suspended particulate organic C (POC), dissolved organic C (DOC), and dissolved inorganic C (DIC) were measured during 1982-l 984 at 11 main-channel and 7 tributary stations over an 1,800 km reach of the Amazon River between Vargem Grande and Obidos, Brazil. The measured 613C range vs. marine carbonate (PDB) was -32 to -269~ for suspended POC, -30 to -289m for DOC, and -26 to -129~ for DIC. The 613C of the fine particulate organic C (FPOC) decreased downriver from Vargem Grande, with values lowest during the fallingwater portion of the runoff cycle; these trends were the result primarily of input of IC-depleted FPOC from tributaries draining the lowland regions of the Amazon basin and floodplain soils. The 613C of the FPOC at Obidos implies that at least 35% of the POC exported by the Amazon River is derived from the lowland portion of the Amazon basin. The 613C of DIC decreased downriver with the lowest values measured during falling water; these trends were due primarily to within-river respiration and tributary input. The 613C of the DIC suggests that -40% of the organic matter being respired in the river is C, plant material derived from floodplain grasses.Within its basin, the discharge of chemically diverse tributaries is blended by the Amazon River, resulting in main-channel concentrations of chemical species and sediment that are close to the world average for rivers (Stallard and Edmond 1983). In the main channel of the river, we found that -20% of the total C was particulate organic

Distributions of oxygen, argon, nitrogen, and radon in the upper ocean of the subarctic Pacific distinguish the fluxes controlling the oxygen mass balance during the summers of 1987 and 1988. The difference between the net O2 flux (in mmol m−2 d−1) to the atmosphere via gas exchange (32) and the integrated decrease with time (−14) is balanced by biological production (13‐17), air injection by bubble entrainment (5), and O2 flux to the thermocline −(0‐4). Nitrogen/argon and oxygen/argon ratios reveal that ˜15% of the oxygen supersaturation in summer is produced by air injection and ˜40% by biological production, with the rest induced by surface water warming. Our estimate of biologically induced oxygen production when translated stoichiometrically to nitrogen uptake agrees to within error estimates with both the particulate and dissolved nitrogen mass balances for the upper ocean determined in the SUPER program during the same time period. The oxygen mass balance requires a net carbon production in the euphotic zone of ˜140 mg C m−2 d−1 (PQ=1.5), which is 20–30% of the level of 14C primary production determined by SUPER investigators.

The 13C/12C of atmospheric methane (CH4) was measured at Point Barrow (71°N, 156°W), Olympic Peninsula (48°N, 126°W), Mauna Loa (19°N, 155°W), and Cape Grim (41°S, 144°E) between 1987 and 1989. The global average δ13CPDB from these measurements (n = 208) was −47.20 ± 0.13%o. The lowest mean annual δ13C value of‐47.61 ± 0.14‰ was measured at Point Barrow with values increasing to ‐47.03 ± 0.14‰ at Cape Grim. The seasonal cycle in the δ13C of CH4 was greatest at Point Barrow, with an amplitude of 0.5‰, and varied inversely with concentration. The isotopic fractionation during CH4 oxidation is calculated to be 0.993 ± 0.002 based on the measured CH4 concentration and δ13C values. The 14C content of atmospheric CH4, measured at monthly intervals at the Olympic Peninsula site between 1987 and 1989, is increasing at 1.4 ± 0.5 pM yr−1, primarily owing to 14CH4 release from nuclear reactors. The global average 14C content of 122 pM for CH4 implies a fossil methane source strength that is 16% of the total source. The global mean δ13C of −47.2‰, when coupled with the 14C results, implies that ∼11% of the total CH4 release rate is derived from biomass burning. These results indicate for a total CH4 source of ∼550 Tg yr−1 that natural gas release accounts for ∼90 Tg yr−1 and biomass burning yields ∼60 Tg yr−1. Preliminary analyses of the δ13C data using a three‐dimensional chemical tracer model indicate that the observed meridional gradients in the annual average δ13C and concentration of CH4 are most closely matched with a CH4 source scenario in which 11% of the CH4 is derived from biomass burning.