Dissolved inorganic carbon isotopes of a typical alpine river on the Tibetan Plateau revealing carbon sources,wetland effect and river recharge

The Nyangqu River, the largest right bank tributary of the Yarlung Zangbo River in the Qinghai–Tibet Plateau, was representative of an alpine riverine carbon cycle experiencing climate change. In this study, dissolved inorganic carbon (DIC) spatial and seasonal variations, as well as their carbon isotopic compositions (δ¹³C_DIC) in river water and groundwater were systematically investigated to provide constraints on DIC sources, recharge and cycling. Significant changes in the δ¹³C_DIC values (from −2.9‰ to −23.4‰) of the water samples were considered to be the result of different contributions of two dominant DIC origins: soil CO₂ dissolution and carbonate weathering. Three types of rock weathering (dissolution of carbonate minerals by H₂CO₃ and H₂SO₄, and silicate dissolution by H₂CO₃) were found to control the DIC input into the riverine system. In DIC cycling, groundwater played a significant role in delivering DIC to the surface water, and DIC supply from tributaries to the main stream increased from the dry season to the wet season. Notably, the depleted δ¹³C_DIC ‘peak’ around the 88.9° longitude, especially in the September groundwater samples, indicated the presence of ‘special’ DIC, which was attributed to the oxidation of methane from the Jiangsa wetland located nearby. This wetland could provide large amounts of soil organic matter available for bacterial degradation, producing ¹³C-depleted methane. Our study provided insights regarding the role of wetlands in riverine carbon cycles and highlighted the contribution of groundwater to alpine riverine DIC cycles.     

Origin and chemical and isotopic evolution of dissolved inorganic carbon (DIC) in groundwater of the Okavango Delta,Botswana

The origin and the chemical and isotopic evolution of dissolved inorganic carbon (DIC) in groundwater of the Okavango Delta in semi-arid Botswana were investigated using DIC and major ion concentrations and stable oxygen, hydrogen and carbon isotopes (δD, δ¹⁸O and δ¹³C_DIC). The δD and δ¹⁸O indicated that groundwater was recharged by evaporated river water and unevaporated rain. The river water and shallow (<10 m) groundwater are Ca–Na–HCO₃ type and the deep (≥10 m) groundwater is Na–K–HCO₃ to HCO₃–Cl–SO₄ to Cl–SO₄–HCO₃. Compared to river water, the mean DIC concentrations were 2 times higher in shallow groundwater, 7 times higher in deep groundwater and 24 times higher in island groundwater. The δ¹³C_DIC indicate that DIC production in groundwater is from organic matter oxidation and in island groundwater from organic matter oxidation and dissolution of sodium carbonate salts. The ionic and isotopic evolution of the groundwater relative to evaporated river water indicates two independent pools of DIC.     

Contrasting carbon export dynamics of human impacted and pristine tropical catchments in response to a short‐lived discharge event

Utilising newly available instrumentation, the carbon balance in two small tropical catchments was measured during two discharge events at high temporal resolution. Catchments share similar climatic conditions, but differ in land use with one draining a pristine rainforest catchment, the other a fully cleared and cultivated catchment. The necessity of high resolution sampling in small catchments was illustrated in each catchment, where significant chemical changes occurred in the space of a few hours or less. Dissolved and particulate carbon transport dominated carbon export from the rainforest catchment during high flow, but was surpassed by degassing of CO₂ less than 4 h after the discharge peak. In contrast, particulate organic carbon dominated export from the cleared catchment, in all flow conditions with CO₂ evasion accounting for 5–23% of total carbon flux. Stable isotopes of dissolved inorganic carbon (DIC) in the ephemeral rainforest catchment decreased quickly from ~1.5 ‰ to ~ ?16 ‰ in 5 h from the flood beginning. A two‐point mixing model revealed that in the initial pulse, over 90% of the DIC was of rainwater origin, decreasing to below 30% in low flow. In the cultivated catchment, δ¹³C_DIC values varied significantly less (?11.0 to ?12.2 ‰) but revealed a complex interaction between surface runoff and groundwater sources, with groundwater DIC becoming proportionally more important in high flow, due to activation of macropores downstream. This work adds to an increasing body of work that recognises the importance of rapid, short‐lived hydrological events in low‐order catchments to global carbon dynamics. Copyright © 2013 John Wiley & Sons, Ltd.     

Controls on fluvial carbon efflux from eroding peatland catchments

Global peatlands store an unparalleled proportion of total global organic carbon but it is vulnerable to erosion into fluvial systems. Fluvial networks are being recognized as areas of carbon transformation, with eroded particulate organic carbon processed to dissolved organic carbon and CO₂. Existing studies indicate biodegradation and photodegradation as key processes controlling the transformation of organic carbon in fluvial systems, with initial concentrations of dissolved organic carbon (DOC) identified as a control on the rate of carbon mineralization. This study manipulates temperature and incident light intensity to investigate carbon mineralization rates in laboratory simulations of peatland sediment transport into fluvial systems. By directly measuring gaseous CO₂ emissions from sampled stream water, the relationship of temperature and light intensity with carbon efflux is identified. In simulations where sediment (as particulate organic matter, POM) is absent, temperature is consistently the dominant factor influencing carbon efflux rates. This influence is independent of the initial DOC concentration of the water sample. In simulations where POM was added, representing a peatland river receiving eroded terrestrial sediment, initial DOC concentration predicts 79% of the variation in total gaseous carbon efflux whereas temperature and light intensity predict 12% and 3%, respectively. When sampled stream water's mineralization rates in the presence of added POM are analysed independently, removing DOC as a model variable, the dominant variable affecting CO₂ efflux is opposite for each sample. This study presents novel data suggesting peatland erosion introduces further complexity to dynamic stream systems where rates of carbon transformation processes and the influence of specific environmental variables are interdependent. Anthropogenic climate change is identified as a leading risk factor perpetuating peatland erosion; therefore, understanding the fate of terrestrial sediment in rivers and further quantifying the benefits of protecting peatland soils will be of increasing importance to carbon budgeting and ecosystem function studies.     

Isotope distribution of dissolved carbonate species in southeastern coastal aquifers of Sicily (Italy)

Concentrations of major ions and the δ¹³C composition of dissolved inorganic carbon in groundwater and submarine groundwater discharges in the area between Siracusa and Ragusa provinces, southeastern Sicily, representing coastal carbonate aquifers, are presented and discussed. Most of groundwater analysed belongs to calcium bicarbonate type, in agreement with the geological nature of carbonate host rocks. Carbonate groundwater acquires, besides the dissolution of carbonate minerals, dissolved carbon (and the relative isotopic composition) from the atmosphere and from soil biological activity. In fact, δ¹³C values and total dissolved inorganic carbon contents show that both these sources contribute to carbon dissolved species in the waters studied. Finally, mixing with seawater in the second main factor of groundwater mineralization Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluating the impact of soil redistribution on the in situ mineralization of soil organic carbon

Soil erosion, transport and deposition by water drastically affect the distribution of soil organic carbon (SOC) within a landscape. Moreover, soil redistribution may have a large impact on the exchange of carbon (C) between the pedosphere and the atmosphere. One of the large information gaps within this research domain, concerns the fate of SOC after erosion by water. According to different (mainly laboratory) studies, soil redistribution leads to aggregate breakdown, thereby exposing the contained SOC to mineralization. Our study aims to quantify the extent to which such increased mineralization occurs in a real field situation. Carbon dioxide (CO₂)‐efflux was measured in the field after an important erosion event for a continuous period of 112 days. The specific situation on the field ensured that almost none of eroded SOC was exported from the field. Measurements of CO₂‐efflux were done in areas with sediment deposition, as well as in comparable areas without sedimentation. Comparison of these measurements allowed the net effect of soil deposition on CO₂‐efflux to be assessed. Field data were complemented by measurements on incubated, undisturbed soil core samples, in order to disentangle the contribution of environmental factors (moisture, temperature) from any erosional effect on CO₂‐efflux. Results of these measurements on the field showed that CO₂‐efflux was regulated by a complex interplay of different factors (mostly soil porosity, soil moisture and soil temperature). In combination with the incubation measurements, it could be concluded that the processes of erosion and transport indeed led to an increased mineralization of SOC, as a result of aggregate breakdown and exposure of previously encapsulated SOC. This effect was, however, much smaller than observed in previous laboratory studies. Moreover, it was only important in the first weeks, immediately after the erosion event. The calculated net erosional effect on CO₂‐efflux represented a mere 1·6% of total SOC, originally present in the soil. Copyright © 2010 John Wiley & Sons, Ltd.