Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes, or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters. Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average pCO2 of 3100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (> 500 km2). Because of their large proportion in global river networks (> 96% of the total number of streams), headwaters contribute large—but still poorly quantified—amounts of CO2 to the atmosphere. Conservative estimates imply that globally 36% (i.e., 0.93 Pg C yr−1) of total CO2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO2 sources, concentrations, and fluxes. To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multiscale geophysical imaging techniques.

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Light stable isotope analyses of hydrogen (2H/1H) and oxygen (18O/16O) of water are used in many terrestrial and marine aquatic studies. The advantage of using stable isotope ratios is that water molecules serve as ubiquitous and already present natural tracers. Within recent years, these analyses have been revolutionized by the development of new isotope ratio laser spectroscopy (IRIS) systems that are cheaper, more robust, and mobile compared with traditional isotope ratio mass spectrometry (IRMS). Although easier to operate, laser systems also need thorough calibration with international reference materials, and raw data need correction for analytical effects (i.e., memory and drift). This study presents modifications to the hardware for liquid water injection, an optimized sequence layout and a simple post‐run correction procedure. These protocols will maximize precision, accuracy, and sample throughput via an efficient memory correction. The number of injections per unknown sample can be reduced to 4 or less. This procedure meets the demands of faster throughput with reduced costs per analysis. Procedures presented here are based on real analyses. They were also verified by an international proficiency test and traditional IRMS techniques.

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Oxygen isotope ratios of well-preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the Middle and Late Devonian. By assuming an oxygen isotopic composition of 1‰ V-SMOW for Devonian seawater, the oxygen isotope values of Eifelian and early Givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 C. Late Givetian and Frasnian palaeotemperatures calculated from d 18 O values of conodont apatite are close to 25 C in the early Frasnian and increase to 32 C in the latest Frasnian and early Famennian. Oxygen isotope ratios of late Givetian and Frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite d 18 O values and give unrealistically warm temperatures ranging from 30 to 40 C. Diagenetic recrystallization of shell calcite, different habitats of conodonts and brachiopods, as well as non-equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the 18 O depletion of brachiopod calcite. Moreover, the 18 O depletion of brachiopod calcite with respect to equilibrium d 18 O values calculated from conodont apatite is too large to be explained by a change in seawater pH that might have influenced the oxygen isotopic composition of brachiopod calcite. The realistic palaeotemperatures derived from d 18 O apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of Palaeozoic oceans more faithfully than brachiopod calcite, and do not support the hypothesis that the 18 O/ 16 O ratio of Devonian seawater was significantly different from that of the modern ocean.