High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian reservoir

Abstract

<p><strong>Abstract.</strong> Reservoir sediments sequester significant amounts of organic carbon (OC), but at the same time, high amounts of methane (CH<sub>4</sub>) can be produced during the degradation of sediment OC. Hydropower is expanding in the Amazon basin, but the potential effects of river damming on the biogeochemistry of the Amazon river system can at present not be gauged due to a lack of studies. Here we present results from the first investigation of OC burial and CH<sub>4</sub> concentrations in the sediments of an Amazonian reservoir. We performed sub-bottom profiling, sediment coring and sediment pore water analysis in the Curuá-Una reservoir (Amazon, Brazil) during rising and falling water periods. A mean sediment accumulation rate of 0.6 cm yr<sup>−1</sup> and a mean OC burial rate of 91 g C m<sup>−2</sup> yr<sup>−1</sup> were found, which is the highest OC burial rate on record for low-latitude reservoirs, probably resulting from high OC deposition onto the sediment compensating for high OC mineralization at 28–30 °C water temperature. Elevated OC burial was found near the dam, and close to major river inflow areas. C : N ratios between 10.3 and 17 (mean ± SD: 12.9 ± 2.1) indicate that both land-derived and aquatic OC accumulate in CUN sediments. About 29 % of the sediment pore water samples had dissolved CH<sub>4</sub> close to saturation concentration, a higher share than other hydroelectric reservoirs, indicating a high potential for CH<sub>4</sub> ebullition, particularly in river inflow areas.</p>

Hotspots of Diffusive CO2 and CH4 Emission From Tropical Reservoirs Shift Through Time

“…Instead, we dealt with ebullition in CDU and FUN in a separate study, which revealed that CH 4 ebullition alone made up for 60% and 99% of CO 2 -equivalent C emission in CDU and FUN, respectively (Linkhorst et al, submitted). High potential for CH 4 ebullition from sediments of CUN has also been reported before (Quadra et al, 2020). Accordingly, any assessment of total reservoir CH 4 emission, which, again, we did not intended to do here, needs to include measurements of CH 4 ebullition, at high spatial resolution and during different hydrological seasons (Grinham et al, 2018;Linkhorst et al, 2020).…”

“…CO 2 and CH 4 emission from reservoirs can be highly variable in space due to the heterogeneity of flooded terrestrial habitats, hydrological gradients (e.g., across the longitudinal axis from river to dam), and variability in internal primary production (DelSontro et al, 2016;Linkhorst et al, 2020;Pacheco et al, 2015;Paranaíba et al, 2018;Teodoru et al, 2012). Moreover, the complex bathymetry and hydrodynamics of reservoirs imply that sedimentation is heterogeneous in space, which also contributes to the spatial variability in emission rates since both CO 2 and CH 4 are produced during organic matter degradation in sediments (Delsontro et al, 2011;Mendonça et al, 2014;Quadra et al, 2020;Sobek et al, 2012). Hence, accurate quantification of greenhouse gas (GHG) emission from reservoirs requires well-resolved spatial coverage to reduce the uncertainties behind the spatial estimates (Deemer et al, 2016;Paranaíba et al, 2018;Teodoru et al, 2012).…”