Watershed dissolved organic carbon transport: a modeling approach combining water travel times and reactivity continuum

<p>Although their contribution was neglected in the past, inland waters play a significant role in the carbon cycle and affect CO<sub>2</sub> global balance. Streams and rivers are now considered not only as pipelines but as active reactors able to collect and transform carbon from terrestrial ecosystems trough drainage, erosion, deposition and respiration. Quantifying the transfer of carbon from the terrestrial to the riverine ecosystems is thus of crucial importance to fully appreciate carbon cycle at the watershed, regional and global scales. Such transfer is largely controlled by the processes occurring in the critical zone where the carbon and water cycles are tightly coupled. Previous studies investigated how hydrological drivers can affect Dissolved Organic Carbon (DOC) concentration in streams highlighting an hysteretic and unsteady behavior for the DOC-discharge relationship. In this study, we focus on the drainage flux from hillslopes to stream and river networks during rainfall events combining a transport model for water and a model of carbon degradation in soil. Using high-frequency records of chloride and DOC in Plynlimon catchments (UK), we employ the recently developed StorAge Selection (SAS) theory to evaluate water travel time and its partition as evapotranspiration, discharge and storage. We combine this approach with the reactivity continuum  theory to model  carbon degradation along the flow paths using a gamma-distribution as probability density function of the quality. The developed model can thus predict not only the flux of DOC released from hillslopes but also its quality (i.e. lability). We also show how the variability of the DOC-discharge relationship can partially be explained by hydrological fluctuations.</p>

Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

Determination of blue carbon sequestration in seagrass sediments over climatic time scales (>100 years) relies on several assumptions, including no loss of particulate organic carbon (POC) after 1–2 years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates, and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition and mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49 and 51% after 100 years of burial for the surface and 20–22-cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO2, even after accounting for CO2 from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on δ13CO2, acidity and lack of sulfate reduction suggested methanogenesis. The results indicated the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

Blue carbon sequestration dynamics within tropical seagrass sediments: Long-term incubations for changes over climatic scales

Determination of blue carbon sequestration in seagrass sediments over climatic time scales relies on several assumptions, such as no loss of particulate organic carbon (POC) after one or two years, tight coupling between POC loss and CO2emissions, no dissolution of carbonates and removal of the stable black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition/mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49% and 51% after 100 years of burial and 20–22 cm horizons, respectively. However, the loss rate of POC was far greater than the release rate of CO2, both before and after accounting for CO2from anoxic particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2fixation. Instead, evidence based on δ13CO2, acidity and lack of sulphate reduction suggested methanogenesis. The results indicate the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

A reactivity continuum of particulate organic matter in a global ocean biogeochemical model

The marine biological carbon pump is dominated by the vertical transfer of Particulate Organic Carbon (POC) from the surface ocean to its interior. The efficiency of this transfer plays an important role in controlling the amount of atmospheric carbon that is sequestered in the ocean. Furthermore, the abundance and composition of POC is critical for the removal of numerous trace elements by scavenging, a number of which such as iron are essential for the growth of marine organisms, including phytoplankton. Observations and laboratory experiments have shown that POC is composed of numerous organic compounds that can have very different reactivities. Yet, this variable reactivity of POC has never been extensively considered, especially in modeling studies. Here, we introduced in the global ocean biogeochemical model NEMO-PISCES a description of the variable composition of POC based on the theoretical Reactivity Continuum Model proposed by (Boudreau and Ruddick, 1991). Our model experiments show that accounting for a variable lability of POC increases POC concentrations in the ocean’s interior by one to two orders of magnitude. This increase is mainly the consequence of a better preservation of small particles that sink slowly from the surface. Comparison with observations is significantly improved both in abundance and in size distribution. Furthermore, the amount of carbon that reaches the sediments is increased by more than a factor of two, which is in better agreement with global estimates of the sediment oxygen demand. The impact on the major macro-nutrients (nitrate and phosphate) remains modest. However, iron (Fe) distribution is strongly altered, especially in the upper mesopelagic zone as a result of more intense scavenging: Vertical gradients in Fe are milder in the upper ocean which appears to be closer to observations. Thus, our study shows that the variable lability of POC can play a critical role in the marine biogeochemical cycles which advocates for more dedicated in situ and laboratory experiments.