The nature of the physiology and thermal regulation of the nonavian dinosaurs is the subject of debate. Previously, arguments have been made for both endothermic and ectothermic metabolisms on the basis of differing methodologies. We used clumped isotope thermometry to determine body temperatures from the fossilized teeth of large Jurassic sauropods. Our data indicate body temperatures of 36° to 38°C, which are similar to those of most modern mammals. This temperature range is 4° to 7°C lower than predicted by a model that showed scaling of dinosaur body temperature with mass, which could indicate that sauropods had mechanisms to prevent excessively high body temperatures being reached because of their gigantic size.

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Microbial nitrite reduction plays an important role in the nitrogen cycle, producing the first gaseous product in the denitrification pathway. The role of nitrite reduction in the environment can be assessed using stable isotope measurements of nitrite. Here, we present estimates for nitrogen (N) and oxygen (O) isotope fractionation during nitrite reduction catalyzed by copper‐containing nitrite reductase (Cu‐NIR) and cytochrome cd1‐containing nitrite reductase (Fe‐NIR). A Rayleigh fractionation model was used to calculate the N and O isotope effects, 15ε and 18ε respectively, from time‐course measurements of nitrite concentration and isotopic composition in batch culture experiments. For three strains of denitrifier carrying the Cu‐NIR, 15ε = 22 ± 2‰ and 18ε = 2 ± 2‰ (95% confidence interval). For three strains of denitrifier carrying the Fe‐NIR, 15ε = 8 ± 2 and 18ε = 6 ± 2‰ (95% confidence interval). These isotope effects for nitrite reduction are significantly different from each other. Furthermore, 15ε and 18ε do not show a 1 : 1 relationship, as has been assumed. The difference between the isotope effects for these two families of enzymes is likely due to a mechanical difference in how the enzymes bind nitrite. The Cu‐NIR binds to both O atoms and the Fe‐NIR only binds to the N, allowing either NO bond to be cleaved and imparting a larger isotope effect for O than for the Cu‐NIR. Utilizing these new N isotope effects for nitrite reduction in oxygen minimum zone N cycle models results in higher rates of nitrite oxidation than previously modeled.

Abstract. Nitrite (NO2-) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (NO3-), as well as water column N loss processes denitrification and anammox. In models of the marine N cycle, NO2- is often not considered as a separate state variable, since NO3- occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, NO2- represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understand the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Improving N cycle models by including NO2- is important in order to better quantify N cycling rates in ODZs, particularly N loss rates. Here we present the expansion of a global 3-D inverse N cycle model to include NO2- as a reactive intermediate as well as the processes that produce and consume NO2- in marine ODZs. NO2- accumulation in ODZs is accurately represented by the model involving NO3- reduction, NO2- reduction, NO2- oxidation, and anammox. We model both 14N and 15N and use a compilation of oceanographic measurements of NO3- and NO2- concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and NO2- oxidation, and we find that the larger (more negative) inverse isotope effects for NO2- oxidation, along with relatively high rates of NO2-, oxidation give a better simulation of NO3- and NO2- concentrations and isotopes in marine ODZs.

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