Groundwater data improve modelling of headwater stream CO<sub>2</sub> outgassing with a stable DIC isotope approach

Abstract

Abstract. A large portion of terrestrially derived carbon outgasses as carbon dioxide (CO 2 ) from streams and rivers to the atmosphere. Particularly, the amount of CO 2 outgassing from small headwater streams is highly uncertain. Conservative estimates suggest that they contribute 36 % (i.e. 0.93 petagrams (Pg) C yr −1 ) of total CO 2 outgassing from all fluvial ecosystems on the globe. In this study, stream pCO 2 , dissolved inorganic carbon (DIC), and δ 13 C DIC data were used to determine CO 2 outgassing from an acidic headwater stream in the Uhlířská catchment (Czech Republic). This stream drains a catchment with silicate bedrock. The applied stable isotope model is based on the principle that the 13 C / 12 C ratio of its sources and the intensity of CO 2 outgassing control the isotope ratio of DIC in stream water. It avoids the use of the gas transfer velocity parameter (k), which is highly variable and mostly difficult to constrain. Model results indicate that CO 2 outgassing contributed more than 80 % to the annual stream inorganic carbon loss in the Uhlířská catchment. This translated to a CO 2 outgassing rate from the stream of 34.9 kg C m −2 yr −1 when normalised to the stream surface area. Large temporal variations with maximum values shortly before spring snowmelt and in summer emphasise the need for investigations at higher temporal resolution. We improved the model uncertainty by incorporating groundwater data to better constrain the isotope compositions of initial DIC. Due to the large global abundance of acidic, humic-rich headwaters, we underline the importance of this integral approach for global applications.

Fluvial CO2 and CH4 patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

“…Our second approach to estimate broad scale emissions used a mass balance approach with a Miller–Tans plot to estimate the isotopic composition of the source of the dissolved fluvial CO 2 , in this case indicating the source of δ 13 C‐CO 2 entering low order streams (Strahler order 1 and 2) from groundwater and/or soil water. Identifying the groundwater source δ 13 C‐CO 2 improves the ability to model CO 2 emissions (Marx et al, ) and our approach (back‐calculating the source) has the advantage of determining the broad‐scale groundwater and soil water δ 13 C‐CO 2 actually entering the stream. In this method, the slope of a model II regression (major axis method) of δ 13 ‐CO 2 × CO 2 against CO 2 (more details in Supporting Information) can be used to provide an estimate of the isotopic source signature (Campeau et al, ; Campeau, Wallin, et al, ; Horgby et al, ; Miller & Tans, ).…”

Dominance of in situ produced particulate organic carbon in a subtropical reservoir inferred from carbon stable isotopes

“…This was done as outlined in Marx et al (2018) by using pH and HCO 3 - with the following Eq. 63 . where HCO 3 − is the concentration of bicarbonate, H + is 10 −pH , K 1 is the temperature-dependent first dissociation constant for the dissociation of H 2 CO 3 (mol L −1 ), and K H is the Henry’s law constant in mol L −1 atm −1 .…”

Using δ13C to reveal the importance of different water transport pathways in two nested karst basins, Southwest China

“…C 3 plants have δ 13 C values that range from -22‰ to -30‰, with an average approximate value of -27‰ (Marx et al, 2018;van Geldern et al, 2015). Along with photosynthesis, the main soil CO 2 transport pathways include rivers and streams.…”

“…Accordingly, the objective of this study was to determine the importance of different transport pathways in surface and groundwater in a karst river basin (comprised of nested karst watersheds) using δ 13 C DIC to trace C sources. However, the present researches generally focused on relationship between surface water and groundwater or C sources of groundwater in karst watersheds (Zeng et al, 2015;Deirmendjian and Abril 2018;Marx et al 2018).…”