Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

Tropical rivers emit large amounts of carbon dioxide (CO2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and well-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km2) of the Nyong watershed (Cameroon 27 800 km2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO2 m−2 yr−1) compared to stream 1 (286 ± 228 gC-CO2 m−2 yr−1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr−1) and wetland (4.0 ± 1.5 MgC yr−1) to the stream represented 3 %–5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr−1) and 73 % (33.0 ± 12.4 MgC yr−1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO2 degassing from the entire river network was 652 ± 161 GgC-CO2 yr−1 (23.4 ± 5.8 MgC CO2 km−2 yr−1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO2 m−2 yr−1, which implied that only ∼ 10 % of the CO2 degassing at the water–air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr−1 (10.3 ± 5.8 MgC km−2 yr−1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the river–wetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds.

Geochemical Modeling of Scale Formation due to Cooling and CO2-degassing in San Kamphaeng Geothermal Field, Northern Thailand

Scaling in a geothermal piping system can cause serious problems by reducing flow rates and energy efficiency. In this work, scaling potential of San Kamphaeng (SK) geothermal energy, Northern Thailand was assessed based on geochemical model simulation using physical and chemical properties of hot spring water. Water samples from surface seepage and groundwater wells, analyzed by ICP-OES and ion chromatograph methods for chemical constituents, were dominated by Ca-HCO3 facies having partial pressure of carbon dioxide of 10–2.67 to 10–1.75 atm which is higher than ambient atmospheric CO2 content. Surface seepage samples have lower temperature (60.9°C) than deep groundwater (83.1°C) and reservoir (127.1°C, based on silica geothermometry). Geochemical characteristics of the hot spring water indicated significant difference in chemical properties between surface seepage and deep, hot groundwater as a result of mineral precipitation along the flow paths and inside well casing. Scales were mainly composed of carbonates, silica, Fe-Mn oxides. Geochemical simulations based on multiple chemical reaction equilibria in PHREEQC were performed to confirm scale formation from cooling and CO2-degassing processes. Simulation results showed total cumulative scaling potential (maximum possible precipitation) from 267-m deep well was estimated as 582.2 mg/L, but only 50.4% of scaling potential actually took place at SK hot springs. In addition, maximum possible carbon dioxide outflux to atmosphere from degassing process in SK geothermal field, estimated from the degassing process, was 6,960 ton/year indicating a continuous source of greenhouse gas that may contribute to climate change. Keywords: Degassing, Geochemical modeling, PHREEQC, San Kamphaeng Hot Springs, Scaling

Aquifer (Hydrogeological) Study of Southern Areas of Prayagraj, India

The physical and chemical characteristics of spring and well water samples were studied for two years to assess the origin of groundwater and determine the factors driving the geochemical composition. The ionic speciation and mineral dissolution/precipitation were calculated. Water wells, characterising groundwater circulation at shallow depths are moderate to high mineralised waters of Na-HCO3 type. In contrast to the shallow environment, the CO2-rich, deeper water is of the Ca-HCO3-SO4 type and undergoes significant changes in the baseline chemistry along flow lines with increasing residence time. The main factors controlling the groundwater composition and its seasonal variations are the geology, because of the presence of carbonate formations, the elevation and the rate of karst development. In both groups, the carbonate chemistry was a diagnostic approach. The super-saturation with respect to calcite indicates CO2 degassing, occurring either inside the aquifer in open conduits or at the outlet in reservoirs. Interaction between groundwater and surrounding rocks is believed to be the main process responsible for the observed chemical characteristics of groundwater in the study area. Mathematical equations were also derived involving the hydro geological variables for better prediction of the aquifer.

Geochemical evidence of volcanic plumbing system processes from fumarolic gases and diffuse CO2 degassing of Taal volcano, Philippines, prior to the January 2020 eruption

<p>Significant temporal variations in the chemical and isotopic composition of Taal fumarolic gas as well as in diffuse CO<sub>2</sub> emission from Taal Main Crater Lake (TMLC) have been observed across the ~12 years of geochemical monitoring (Arpa et al., 2013; Hernández et a., 2017), with significant high CO<sub>2 </sub>degassing rates, typical of plume degassing volcanoes, measured in 2011 and 2017. In addition to these CO<sub>2</sub> surveys at the TCML, soil CO<sub>2</sub> efflux continuous monitoring was implemented at Taal volcano since 2016 and a clear increasing trend of the soil CO<sub>2</sub> efflux in 2017 was also observed. Increasing trends on the fumarolic CO<sub>2</sub>/St, He/CO<sub>2</sub>, CO/CO<sub>2</sub> and CO<sub>2</sub>/CH<sub>4</sub> ratios were recorded during the period 2010-2011 whereas increasing SO<sub>2</sub>/H<sub>2</sub>S, H<sub>2</sub>/CO<sub>2</sub> ratios were recorded during the period 2017-2018. A decreasing on the CO<sub>2</sub>/CH<sub>4</sub> and CO<sub>2</sub>/St ratios was observed for 2017-2018. These changes are attributed to an increased contribution of magmatic fluids to the hydrothermal system in both periods. Observed changes in H<sub>2</sub> and CO contents suggest increases in temperature and pressure in the upper parts of the hydrothermal system of Taal volcano. The <sup>3</sup>He/<sup>4</sup>He ratios corrected (Rc/Ra), and δ<sup>13</sup>C of fumarolic gases also increased during the periods 2010-2011 and 2017-2018 before the eruption onset. During this study, diffuse CO<sub>2</sub> emission values measured at TMCL showed a wide range of values from >0.5 g m<sup>−2</sup> d<sup>−1</sup> up to 84,902 g m<sup>−2</sup> d<sup>−1</sup>. The observed relatively high and anomalous diffuse CO<sub>2</sub> emission rate across the ~12 years reached values of 4,670 ± 159 t d<sup>-1 </sup>on March 24, 2011, and 3,858 ± 584 t d<sup>-1</sup> on November 11, 2017. The average value of the soil CO<sub>2</sub> efflux data measured by the geochemical station showed oscillations around background values until 14 March, 2017. Since then at 22:00 hours, a sharp increase of soil CO<sub>2</sub> efflux from ~0.1 up to 1.1 kg m<sup>-2</sup> d<sup>-1</sup> was measured in 9 hours and continued to show a sustained increase in time up to 2.9 kg m<sup>-2</sup> d<sup>-1</sup> in 2 November, that represents the main long-term variation of the soil CO<sub>2</sub> emission time series. All the above variations might be produced by two episodes of magmatic intrusion which favored degassing of a gas-rich magma at depth. During the 2010-2011 the magmatic intrusion of volatile-rich magma might have occurred from the mid-crustal storage region at shallower depths producing important changes in pressure and temperature conditions, whereas a new injection of more degassed magma into the deepest zone of the hydrothermal system occurring in 2017-2018 might have favored the accumulation of gases in the subsurface, promoting conditions leading to a phreatic eruption. These geochemical observations are most simply explained by magma recharge to the system, and represent the earliest warning precursor signals to the January 2020 eruptive activity.</p><p>Arpa, M.C., et al., 2013. Bull. Volcanol. 75, 747. https://doi.org/10.1007/s00445-013-0747-9.</p><p>Hernández, P.A., et al.,  2017. Geol. Soc. Lond. Spec. Publ. 437:131–152. https://doi.org/10.1144/SP437.17.</p>

A full-waveform inversion workflow for estimationing ambient seismic source distributions from Rayleigh-wave multicomponent crosscorrelations

<p>Estimation of ambient seismic source distributions (e.g. location and strength) is important for studies of seismic source mechanisms and subsurface structures. It is current state of the art to estimate the source distribution by applying full-waveform inversion (FWI) to seismic crosscorrelations. We previously theoretically demonstrated the advantage of Rayleigh-wave multicomponent crosscorrelations in the FWI estimation process. In this presentation, we utilize the crosscorrelations from real ambient seismic data acquired in Hartoušov, Czech Republic, where the seismic sources are CO2 degassing areas at Earth’s surface (i.e. fumaroles or mofettes).  We develop a complete workflow from the raw data to the FWI estimation. We demonstrate that the multicomponent crosscorrelations can better constrain the source distribution than vertical-component crosscorrelations in both elastic media and anelastic media, even when we use an elastic forward model in the inversion process. Our inversion results indicate a strong seismic source near strong CO2 gas flux areas.</p>

Variability of Carbonate Isotope Signatures in a Hydrothermally Influenced System: Insights from the Pastos Grandes Caldera (Bolivia)

Laguna Pastos Grandes (Bolivia), nesting in a volcanic caldera, is a large, palustrine-to-lacustrine system fed by meteoric and hydrothermal calco–carbonic fluids. These different fluid inputs favor a complex mosaic of depositional environments, including hydrothermal springs, pools, and an ephemeral lake, producing abundant present-day carbonates developing over a Holocene carbonate crust dated by U–Th. Present-day carbonates (muds, concretions, and microbialites) recorded a large range of isotope variations, reaching 13.9‰ in δ13C and 11.1‰ in δ18O. Sedimentological and geochemical data indicated that the main processes influencing the isotope record were: (i) rapid CO2 degassing and temperature decreases along hydrothermal discharges; (ii) strong evaporation favored by the arid high-altitude Andean climate, locally enhanced by capillary water rise within microbial mats or by wind-induced spray falling on vadose concretions. Unlike past or present perennial lake systems in Central Andes, the short residence time of brine waters in the ephemeral central lake prevents enrichment of lacustrine carbonates in 13C and 18O. The very low fraction modern F14C in these present-day carbonates demonstrates that incorporation of fossil magmatic carbon related to the volcanic context also prevents any radiocarbon dating. The use of isotopes for the interpretation of ancient continental series should always be accompanied by a thorough characterization of the environmental setting.