ALBERT

Description: Here, we present the implementation of the freshwater carbon (C) cycle in the Dynamic In-stream Chemistry module (CARBON-DISC), which is part of the Integrated Model to Assess the Global Environment-Dynamic Global Nutrient Model (IMAGE-DGNM). A coupled hydrology-biogeochemistry approach with 0.5 by 0.5-degree resolution accounts for the spatial and temporal variability in dynamic conditions in the aquatic continuum using independent global databases. This process-based model resolves the concentrations, transformations and transfer fluxes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and terrestrial and autochthonous particulate organic carbon (POC) from headwaters to river mouth with a time step of 1 month for the period 1950–2000. This is a major step forward in basin scale modelling of the C processing in freshwater systems, since simulated results can be validated at every location and point in time, and the model can be applied for retrodiction and to analyse future scenarios. Validation of the model with long-term measurement data shows a fair agreement, considering that this is a global model. To analyse the performance of the full production-respiration DISC module, two other schemes are presented, including an abiotic system excluding any in-stream processing of DOC and allochthonous production, and an extended abiotic system including heterotrophic respiration, but excluding production. Furthermore, a sensitivity analysis shows that many parameters, such as temperature, solar radiation, organic sediment mineralization rate and C inputs, including particulate organic carbon from terrestrial vegetation and dissolved inorganic carbon from groundwater, strongly affect atmosphere-freshwater exchange of CO2.

Description: Cable bacteria are multicellular, filamentous microorganisms that are capable of transporting electrons over centimeter-scale distances. Although recently discovered, these bacteria appear to be widely present in the seafloor, and when active they exert a strong imprint on the local geochemistry. In particular, their electrogenic metabolism induces unusually strong pH excursions in aquatic sediments, which induces considerable mineral dissolution, and subsequent mineral reprecipitation. However, at present, it is unknown whether and how cable bacteria play an active or direct role in the mineral reprecipitation process. To this end we present an explorative study of the formation of sedimentary minerals in and near filamentous cable bacteria using a combined approach of electron microscopy and spectroscopic techniques. Our observations reveal the formation of polyphosphate granules within the cells and two different types of biomineral formation directly associated with multicellular filaments of these cable bacteria: (i) the attachment and incorporation of clay particles in a coating surrounding the bacteria and (ii) encrustation of the cell envelope by iron minerals. These findings suggest a complex interaction between cable bacteria and the surrounding sediment matrix, and a substantial imprint of the electrogenic metabolism on mineral diagenesis and sedimentary biogeochemical cycling. In particular, the encrustation process leaves many open questions for further research. For example, we hypothesize that the complete encrustation of filaments might create a diffusion barrier and negatively impact the metabolism of the cable bacteria.

Description: Cenozoic stable carbon (δ13C) and oxygen (δ18O) isotope ratios of deep-sea foraminiferal calcite co-vary with the 405 kyr eccentricity cycle, suggesting a link between orbital forcing, the climate system, and the carbon cycle. Variations in δ18O are partly forced by ice-volume changes that have mostly occurred since the Oligocene. The cyclic δ13C–δ18O co-variation is found in both ice-free and glaciated climate states, however. Consequently, there should be a mechanism that forces the δ13C cycles independently of ice dynamics. In search of this mechanism, we simulate the response of several key components of the carbon cycle to orbital forcing in the Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir model (LOSCAR). We force the model by changing the burial of organic carbon in the ocean with various astronomical solutions and noise and study the response of the main carbon cycle tracers. Consistent with previous work, the simulations reveal that low-frequency oscillations in the forcing are preferentially amplified relative to higher frequencies. However, while oceanic δ13C mainly varies with a 405 kyr period in the model, the dynamics of dissolved inorganic carbon in the oceans and of atmospheric CO2 are dominated by the 2.4 Myr cycle of eccentricity. This implies that the total ocean and atmosphere carbon inventory is strongly influenced by carbon cycle variability that exceeds the timescale of the 405 kyr period (such as silicate weathering). To test the applicability of the model results, we assemble a long (∼22 Myr) δ13C and δ18O composite record spanning the Eocene to Miocene (34–12 Ma) and perform spectral analysis to assess the presence of the 2.4 Myr cycle. We find that, while the 2.4 Myr cycle appears to be overshadowed by long-term changes in the composite record, it is present as an amplitude modulator of the 405 and 100 kyr eccentricity cycles.

Description: Dissolved carbon leaching in and from soils plays an important role in C transport along the terrestrial-aquatic continuum. However, a global overview and analysis of dissolved carbon in soil solutions, covering a wide range of vegetation types and climates, is lacking. We compiled a global database on annual average dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in soil solutions, including potential governing factors, with 762 entries from 351 different sites covering a range of climate zones, land cover types and soil classes. Using this database we develop regression models to calculate topsoil concentrations, and concentrations vs. depth in the subsoil at the global scale. For DIC, the lack of a proportional globally distributed cover inhibits analysis on a global scale. For DOC, annual average concentrations range from 1.7 to 88.3 (median = 25.27) mg C/L for topsoils and from 0.42 to 372.1 (median = 5.50) mg C/L for subsoils (excluding lab incubations). Highest topsoil values occur in forests of cooler, humid zones. In topsoils, multiple regression showed that precipitation is the most significant factor. Our global topsoil DOC model (R2 = 0.36) uses precipitation, soil class, climate zone and land cover type as model factors. Our global subsoil model describes DOC concentrations vs. depth for different USDA soil classes (overall R2 = 0.45). Highest subsoil concentrations are calculated for Histosols.