Dissolved Inorganic Carbon Production Research Articles

Escalating Carbon Export from High-Elevation Rivers in a Warming Climate.

High-elevation mountains have experienced disproportionately rapid warming, yet the effect of warming on the lateral export of terrestrial carbon to rivers remains poorly explored and understood in these regions. Here, we present a long-term data set of dissolved inorganic carbon (DIC) and a more detailed, short-term data set of DIC, δ13CDIC, and organic carbon from two major rivers of the Qinghai-Tibetan Plateau, the Jinsha River (JSR) and the Yalong River (YLR). In the higher-elevation JSR with ∼51% continuous permafrost coverage, warming (>3 °C) and increasing precipitation coincided with substantially increased DIC concentrations by 35% and fluxes by 110%. In the lower-elevation YLR with ∼14% continuous permafrost, such increases did not occur despite a comparable extent of warming. Riverine concentrations of dissolved and particulate organic carbon increased with discharge (mobilization) in both rivers. In the JSR, DIC concentrations transitioned from dilution (decreasing concentration with discharge) in earlier, colder years to chemostasis (relatively constant concentration) in later, warmer years. This changing pattern, together with lighter δ13CDIC under high discharge, suggests that permafrost thawing boosts DIC production and export via enhancing soil respiration and weathering. These findings reveal the predominant role of warming in altering carbon lateral export by escalating concentrations and fluxes and modifying export patterns.

Production and fluxes of inorganic carbon and alkalinity in a subarctic subterranean estuary

In this study, we focus on the biogeochemical processes that produce both dissolved inorganic carbon (DIC) and total alkalinity (TA) along a subarctic subterranean estuary (STE) located in the Gulf of St. Lawrence (Magdalen Island, Qc, Canada) in order to evaluate the DIC and TA fluxes as well as the buffering capacity of the exported groundwater to coastal waters. DIC and TA do not behave conservatively during mixing along the groundwater flow path and this implies the occurrence of internal redox reactions that control both their production and consumption. In addition, we show that the origin and composition of the organic carbon within the system alter the carbonate parameters by generating low pH conditions (5.9 - 7.2) and contributing to non-carbonate alkalinity (NCA) that accounts for more than 30% of TA. Whereas iron cycling plays a key role in the production of DIC in the fresh and low-salinity groundwaters, the precipitation of sulfide minerals neutralize the acidity produced by the metabolically produced CO2, in the saline groundwater where sulfate is available. The STE pCO2, computed from the DIC-pHNBS pair ranged from a few ppm to 16000 ppm that results in a CO2 evasion rate of up to 310 mol m−2d−1 to the atmosphere. Based on Darcy flow and the mean concentrations of DIC and carbonate alkalinity (Ac = TA - NCA) in the discharge zone, fluxes derived from submarine groundwater discharge were estimated at 1.43 and 0.70 mol m−2d−1 for DIC and Ac, respectively. Despite a major part of the metabolic CO2 being lost along the groundwater flow path, the SGD-derived DIC flux was still greater than the Ac flux, implying that groundwater discharge reduces the buffering capacity of the receiving coastal waters. This site-specific scale study demonstrates the importance of diagenetic reactions and organic matter remineralization processes on carbonate system parameters in STE. Our results highlight that subarctic STEs could be hot spots of CO2 evasion and a source of acidification to coastal waters that should be considered in carbon budgets.

Amplified Production and Export of Dissolved Inorganic Carbon During Hot and Wet Subtropical Monsoon

AbstractUnderstanding the origins and processes of riverine dissolved inorganic carbon (DIC) is crucial for predicting the global carbon cycle with projected, more frequent climate extremes yet our knowledge has remained fragmented. Here we ask: How and how much do DIC production and export vary across space (shallow vs. deep, uphill vs. depression) and time (daily, seasonal, and annual)? How do the relative contributions of biogenic (soil respiration) and geogenic (carbonate weathering) sources differ under different temperature and hydrological conditions? We answer these questions using a catchment‐scale reactive transport model constrained by stream flow, stable water isotopes, stream DIC, and carbon isotope data from a headwater karstic catchment in southwest China in a subtropical monsoon climate. Results show climate seasonality regulates the timing of DIC production and export. In hot‐wet seasons, high temperature accelerates soil respiration and carbonate weathering (up to a factor of three) via elevating soil CO2 and carbonate solubility, whereas high discharge enhances export by two orders of magnitude compared to cold‐dry seasons. Carbonate weathering is driven more by soil CO2 than water flow. At the annual scale, 92.9% and 7.1% of DIC was produced in shallow and deep zone, respectively, whereas 64.5% and 35.5% of DIC was exported from shallow and deep zone, respectively. These results highlight the uniqueness of subtropical karst areas as synchronous reactors and transporters of DIC during the hot‐wet monsoon, contrasting the asynchronous production and export in other climate regions. A future hotter and wetter climate with more intensive storms in the region may further intensify DIC production and export, accentuating the potential of subtropical karst regions as global hot spots for carbon cycling.

The benthic-pelagic coupling affects the surface water carbonate system above groundwater-charged coastal sediments

Submarine groundwater discharge (SGD) can be a significant source of dissolved nutrients, inorganic and organic carbon, and trace metals in the ocean and therefore can be a driver for the benthic-pelagic coupling. However, the influence of hypoxic or anoxic SGD on the carbonate system of coastal seawater is still poorly understood. In the present study, the production of dissolved inorganic carbon (DIC) and alkalinity (AT) in coastal sediments has been investigated under the impact of oxygen-deficient SGD and was estimated based on the offset between the measured data and the conservative mixing of the end members. Production of ATand DIC was primarily caused by denitrification and sulphate reduction. The ATand DIC concentrations in SGD decreased by approximately 32% and 37% mainly due to mixing with seawater counterbalanced by reoxidation and CO2release into the atmosphere. Total SGD-ATand SGD-DIC fluxes ranged from 0.1 to 0.2mol m-2d-1and from 0.2 to 0.3mol m-2d-1, respectively. These fluxes are probably the reason why the seawater in the Bay of Puck is enriched in ATand DIC compared to the open waters of the Baltic Sea. Additionally, SGD had low pH and was undersaturated with respect to the forms of the aragonite and calcite minerals of CaCO3. The seawater of the Bay of Puck also turned out to be undersaturated in summer (Inner Bay) and fall (Outer Bay). We hypoth​e​size that SGD can potentially contribute to ocean acidification and affect the functioning of the calcifying invertebrates.

Dissolved Organic Matter Composition Determines Its Susceptibility to Complete and Partial Photooxidation within Lakes.

Dissolved organic matter (DOM) plays an important role in carbon cycling within inland surface waters. Under sunlight irradiation, DOM undergoes complete photooxidation to produce carbon dioxide (CO2) and partial photooxidation that alters the molecular composition of DOM. However, a mechanistic understanding of the relationship between DOM composition and its susceptibility to partial and complete photooxidation in surface waters is currently lacking. This work combines light exposure experiments with high-resolution mass spectrometry to investigate DOM photooxidation using two DOM isolates and DOM from 16 lakes that vary in trophic status and size. High ratios of oxygen consumption to dissolved inorganic carbon (DIC) production demonstrate that all samples undergo extensive partial photooxidation. At the molecular level, more oxidized, aromatic DOM formulas are associated with oxygen consumption and DIC production. Bulk level measurements indicate that DOM becomes less aromatic and lower in apparent molecular weight following partial photooxidation, and there is molecular level evidence of oxygen addition and loss of CO2 in all samples. However, formulas most susceptible to photooxidation vary depending on the initial DOM composition. Collectively, this work provides insights into the relationship between DOM composition and photooxidation, which has important implications for carbon cycling in diverse surface waters.

Inorganic carbon outwelling from a Mediterranean seagrass meadow using radium isotopes

Seagrass meadows are ‘blue carbon’ ecosystems widely recognised for their potential role in climate change mitigation. Previous studies have focused mainly on carbon storage within meadows and sediments. However, little is known about contribution of outwelling (i.e., lateral transport) to seagrass carbon budgets. Here, radium isotopes (223Ra and 224Ra) were used to assess dissolved inorganic carbon (DIC) and total alkalinity (TA) outwelling from a Mediterranean Posidonia oceanica meadow during early autumn. DIC outwelling was 114 ± 61 mmol m−2 day−1 and exceeded above-meadow CO2 outgassing (3 ± 1 mmol m−2 day−1). Production of DIC was uncoupled from TA and fuelled by net heterotrophy and aerobic processes within the meadow. The small export of TA (5 ± 6 mmol m−2 day−1) implied that ∼90% of outwelled DIC may return to the atmosphere as CO2 in offshore waters. Combining these fluxes with above-meadow outgassing suggested a total carbon loss that exceeded long term burial in sediments. Overall, the meadow acted as a carbon source to the atmosphere during the early autumn season. Further studies quantifying outwelling at multiple spatial and temporal scales are required to better resolve seagrass carbon budgets and their contribution to carbon sequestration.

Anaerobic carbon oxidation in sediment of two Brazilian mangrove forests: the influence of tree roots and crab burrows

This study evaluated the effects of biogenic structures (tree roots and crab burrows) on sediment carbon (C), sulfur (S), and iron (Fe) biogeochemistry during the wet season in the Olaria mangrove forests near the city of Cananéia, São Paulo state, Brazil and the Nobrega mangrove forest approximately 2 km from the city. Anaerobic C oxidation pathways were assessed from sediment profiles and anaerobic incubations and related to the abundance of biogenic structures in the form of pneumatophores and crab burrows. Porewater depth profiles of dissolved inorganic carbon (DIC) and SO42- were less steep in the presence than absence of biogenic structures. While Fe(II) appeared unaffected by biogenic structures, Fe(III) levels were significantly higher in the upper 4 cm of the sediment in the presence than absence of vegetation and bioturbation. Surprisingly, the concentration of Fe(III) in this layer was 2-6 times higher in the Nobrega forest (6-13 µmol cm-3) than in the Olaria forest (1.5-6.5 µmol cm-3). Accordingly, depth integrated sulfate reduction (SR) tended to be highest at Olaria, while iron reduction (FeR) was highest at Nobrega. SR accounted for 54-83% of DIC production, with no diference between forested sites, while FeR accounted for 8-24% of DIC production, with a 2-3 times higher contribution in the Nobrega versus the Olaria forest. The results suggest that mangrove roots and crab burrows in mangrove sediments only promote FeR at the expense of SR in the Nobrega forest. It appears that anthropogenic discharge from Cananéia city may have overridden the impact of biogenic structures on sediment redox conditions at Olaria, thereby diminishing the role of FeR without strong stimulation of overall C oxidation rates.

Decoupling of Estuarine Hypoxia and Acidification as Revealed by Historical Water Quality Data.

Hypoxia and acidification are commonly coupled in eutrophic aquatic environments because aerobic respiration is usually dominant in bottom waters and can lower dissolved oxygen (DO) and pH simultaneously. However, the degree of coupling, which can be weakened by non-aerobic respiration and CaCO3 cycling, has not been adequately assessed. In this study, we applied a box model to 20 years of water quality monitoring data to explore the relationship between hypoxia and acidification along the mainstem of Chesapeake Bay. In the early summer, dissolved inorganic carbon (DIC) production in mid-bay bottom waters was dominated by aerobic respiration, contributing to DO and pH declines. In contrast, late-summer DIC production was higher than that expected from aerobic respiration, suggesting potential buffering processes, such as calcium carbonate dissolution, which would elevate pH in hypoxic waters. These findings are consistent with contrasting seasonal relationships between riverine nitrogen (N) loads and hypoxic and acidified volumes. The N loads were associated with increased hypoxic and acidified volumes in June, but only increased hypoxic volumes in August, when acidified volume declines instead. Our study reveals that the magnitude of this decoupling varies interannually with watershed nutrient inputs, which has implications for the management of co-stressors in estuarine systems.

Seasonal and Spatial Production Patterns of Dissolved Inorganic Carbon and Total Alkalinity in a Shallow Beach Aquifer

Dissolved inorganic carbon (DIC) and total alkalinity (AlkT) fluxes to the nearshore ocean can directly impact the rates of primary production, coral reef formation, coastal ocean acidification, and continental shelf ecology. Current understanding of the transformations that DIC and AlkT undergo as they move from land to sea are limited, leading to difficulties in estimating future DIC and AlkT export that may be altered under a changing climate. While much research has focused on carbon fluxes in carbon-rich mangroves and coastal wetlands, DIC and AlkT transformations and distributions in sandy beach aquifers, which are comparatively carbon-poor, have not been studied as extensively. We monitored DIC and AlkT concentrations in a sandy beach system over six sampling events spanning two years. Substantial changes to DIC and AlkT occurred along subsurface flowpaths due to aerobic respiration and anoxic reactions, resulting in an additional mean flux to the ocean of 191 and 134 mmol/d per meter length of shoreline, respectively. The chemical alterations occurred within the saltwater-freshwater mixing zone beneath the beach surface. Both aerobic and anaerobic reactions actively contributed to DIC and AlkT production within the system, as indicated by DIC: AlkT and dDIC:dAlkT ratios relative to the theoretical dilution line. The work indicates that beach aquifers support active transformation of inorganic carbon and highlights a potentially important and overlooked source of DIC and AlkT to coastal systems.

Hydrology influences carbon flux through metabolic pathways in the hypolimnion of a Mediterranean reservoir

Global change is modifying meteorological and hydrological factors that influence the thermal regime of water bodies. These modifications can lead to longer stratification periods with enlarged hypolimnetic anoxic periods, which can promote heterotrophic anaerobic processes and alter reservoir carbon cycling. Here, we quantified aerobic and anaerobic heterotrophic processes (aerobic respiration, denitrification, iron and manganese reduction, sulfate reduction, and methanogenesis) on dissolved inorganic carbon (DIC) production in the hypolimnion of a Mediterranean reservoir (El Gergal, Spain) under two contrasting hydrological conditions: a wet year with heavy direct rainfall and frequent water inputs from upstream reservoirs, and a dry year with scarce rainfall and negligible water inputs. During the wet year, water inputs and rainfall induced low water column thermal stability and earlier turnover. By contrast, thermal stratification was longer and more stable during the dry year. During wet conditions, we observed lower DIC accumulation in the hypolimnion, mainly due to weaker sulfate reduction and methanogenesis. By contrast, longer stratification during the dry year promoted higher hypolimnetic DIC accumulation, resulting from enhanced methanogenesis and sulfate reduction, thus increasing methane emissions and impairing reservoir water quality. Aerobic respiration, denitrification and metal reduction produced a similar amount of DIC in the hypolimnion during the two studied years. All in all, biological and geochemical (calcite dissolution) processes explained most of hypolimnetic DIC accumulation during stratification regardless of the hydrological conditions, but there is still ~ 30% of hypolimnetic DIC production that cannot be explained by the processes contemplated in this study and the assumptions made.

Inter‐laboratory differences in the apparent quantum yield for the photochemical production of dissolved inorganic carbon in inland waters and implications for photochemical rate modeling

AbstractSolar radiation initiates photochemical oxidation of dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) in inland waters, contributing to their carbon dioxide emissions to the atmosphere. Models can determine photochemical DIC production over large spatiotemporal scales and assess its role in aquatic C cycling. The apparent quantum yield (AQY) spectrum for photochemical DIC production, defined as mol DIC produced per mol chromophoric dissolved organic matter‐absorbed photons, is a critical model parameter. In previous studies, the principle for the determination of AQY spectra is the same but methodological specifics differ, and the extent to which these differences influence AQY spectra and simulated aquatic DIC photoproduction is unclear. Here, four laboratories determined AQY spectra from water samples of eight inland waters that are situated in Alaska, Finland, and Sweden and span a nearly 10‐fold range in DOM absorption coefficients. All AQY values fell within the range previously reported for inland waters. The inter‐laboratory coefficient of variation (CV) for wavelength‐integrated AQY spectra (300–450 nm) averaged 38% ± 3% SE, and the inter‐water CV averaged 63% ± 1%. The inter‐laboratory CV for simulated photochemical DIC production (conducted for the five Swedish lakes) averaged 49% ± 12%, and the inter‐water CV averaged 77% ± 10%. This uncertainty is not surprising given the complexities and methodological choices involved in determining DIC AQY spectra and needs to be considered when applying photochemical rate modeling. Thus, we also highlight current methodological limitations and suggest future improvements for DIC AQY determination to reduce inter‐laboratory uncertainty.

Carbon‑sulfur coupling in a seasonally hypoxic, high-sulfate reservoir in SW China: Evidence from stable C[sbnd]S isotopes and sulfate-reducing bacteria

Anthropogenic input of sulfate (SO42−) in reservoirs may enhance bacterial sulfate reduction (BSR) under seasonally hypoxic conditions in the water column. However, factors that control BSR and its coupling to organic carbon (OC) mineralization in seasonally hypoxic reservoirs remain unclear. The present study elucidates the coupling processes by analyzing the concentrations and isotopic composition of dissolved inorganic carbon (DIC) and sulfur (SO42−, sulfide) species, and the microbial community in water of the Aha reservoir, SW China, which has high SO42− concentration due to the inputs from acid mine drainage about twenty years ago. The water column at two sites in July and October revealed significant thermal stratification. In the hypoxic bottom water, the δ13C-DIC decreased while the δ34S-SO42− increased, implying organic carbon mineralization due to BSR. The magnitude of S isotope fractionation (Δ34S, obtained from δ34Ssulfate-δ34Ssulfide) during the process of BSR fell in the range of 3.4‰ to 27.0‰ in July and 21.6‰ to 31.8‰ in October, suggesting a change in the community of sulfate-reducing bacteria (SRB). The relatively low water column stability in October compared to that in July weakened the difference of water chemistry and ultimately affected the SRB diversity. The production of DIC (ΔDIC) scaled a strong positive relationship with the Δ34S in July (p < 0.01), indicating that high OC availability favored the survival of incomplete oxidizers of SRB. However, in October, Δ13C-DIC was correlated with the Δ34S in the bottom hypoxic water (p < 0.01), implying that newly degraded OC depleted in 13C could favor the dominance of complete oxidizers of SRB which caused greater S isotope fractionation. Moreover, the sulfide supplied by BSR might stimulate the reductive dissolution of Fe and Mn oxides (Fe(O)OH and MnO2). The present study helps to understand the coupling of C and S in seasonally hypoxic reservoirs characterized by high SO42− concentration.

Evaluating the Arabian Sea as a regional source of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;: seasonal variability and drivers

Abstract. The Arabian Sea (AS) was confirmed to be a net emitter of CO2 to the atmosphere during the international Joint Global Ocean Flux Study program of the 1990s, but since then few in situ data have been collected, leaving data-based methods to calculate air–sea exchange with fewer and potentially out-of-date data. Additionally, coarse-resolution models underestimate CO2 flux compared to other approaches. To address these shortcomings, we employ a high-resolution (1/24∘) regional model to quantify the seasonal cycle of air–sea CO2 exchange in the AS by focusing on two main contributing factors, pCO2 and winds. We compare the model to available in situ pCO2 data and find that uncertainties in dissolved inorganic carbon (DIC) and total alkalinity (TA) lead to the greatest discrepancies. Nevertheless, the model is more successful than neural network approaches in replicating the large variability in summertime pCO2 because it captures the AS's intense monsoon dynamics. In the seasonal pCO2 cycle, temperature plays the major role in determining surface pCO2 except where DIC delivery is important in summer upwelling areas. Since seasonal temperature forcing is relatively uniform, pCO2 differences between the AS's subregions are mostly caused by geographic DIC gradients. We find that primary productivity during both summer and winter monsoon blooms, but also generally, is insufficient to offset the physical delivery of DIC to the surface, resulting in limited biological control of CO2 release. The most intense air–sea CO2 exchange occurs during the summer monsoon when outgassing rates reach ∼ 6 molCm-2yr-1 in the upwelling regions of Oman and Somalia, but the entire AS contributes CO2 to the atmosphere. Despite a regional spring maximum of pCO2 driven by surface heating, CO2 exchange rates peak in summer due to winds, which account for ∼ 90 % of the summer CO2 flux variability vs. 6 % for pCO2. In comparison with other estimates, we find that the AS emits ∼ 160 Tg C yr−1, slightly higher than previously reported. Altogether, there is 2× variability in annual flux magnitude across methodologies considered. Future attempts to reduce the variability in estimates will likely require more in situ carbon data. Since summer monsoon winds are critical in determining flux both directly and indirectly through temperature, DIC, TA, mixing, and primary production effects on pCO2, studies looking to predict CO2 emissions in the AS with ongoing climate change will need to correctly resolve their timing, strength, and upwelling dynamics.

Seasonal Mixing and Biological Controls of the Carbonate System in a River-Dominated Continental Shelf Subject to Eutrophication and Hypoxia in the Northern Gulf of Mexico

Large rivers export a large amount of dissolved inorganic carbon (DIC) and nutrients to continental shelves; and subsequent river-to-sea mixing, eutrophication, and seasonal hypoxia (dissolved oxygen &amp;lt; 2 mg⋅L–1) can further modify DIC and nutrient distributions and fluxes. However, quantitative studies of seasonal carbonate variations on shelves are still insufficient. We collected total alkalinity (TA), DIC, and NO3– data from nine cruises conducted between 2006 and 2010 on the northern Gulf of Mexico continental shelf, an area strongly influenced by the Mississippi and Atchafalaya Rivers. We applied a three-end-member model (based on salinity and potential alkalinity) to our data to remove the contribution of physical mixing to DIC and nitrate distribution patterns and to derive the net in situ removal of DIC and nitrate (ΔDIC and ΔNO3–, respectively). Systematic analyses demonstrated that the seasonal net DIC removal in the near-surface water was strong during summer and weak in winter. The peak in net DIC production in the near-bottom, subsurface waters of the inner and middle sections of the shelf occurred between July and September; it was coupled, but with a time lag, to the peak in the net DIC removal that occurred in the near-surface waters in June. A similar 2-month delay (i.e., January vs. November) could also be observed between their minima. A detailed examination of the relationship between ΔDIC and ΔNO3– demonstrates that net biological activity was the dominant factor of DIC removal and addition. Other effects, such as air–sea CO2 gas exchange, wetland exports, CaCO3 precipitation, and a regional variation of the Redfield ratio, were relatively minor. We suggest that the delayed coupling between eutrophic surface and hypoxic bottom waters reported here may also be seen in the carbon and nutrient cycles of other nutrient-rich, river-dominated ocean margins worldwide.

Coupled effects of hydrology and temperature on temporal dynamics of dissolved carbon in the Min River, Tibetan Plateau

Studying the temporal dynamics and fluxes of riverine dissolved carbon is crucial in understanding the regional and global carbon cycles under various climatic conditions. Here, we studied the behaviors and fluxes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) responding to various hydrologic conditions in the Min River, originated from Tibetan Plateau. The DIC concentrations decreased with increasing runoff, partially reflecting dilution effect, which may be ascribed to the shortened fluid transit time and then the reduced contact time with rocks. Nevertheless, DOC concentrations were positively correlated with runoff, which can be attributed to that a large amount of soil organic carbon flowed into the river as a result of the strong flushing effect. The negative relationship between δ13CDIC and runoff could be explained by soil CO2 influx and organic matter degradation during the high flow season. ΔDIC (the production of DIC with changing hydrologic conditions) had a strong positive correlation with water temperature due to the accelerated DIC production rates by high temperature, which always co-varied with intense precipitation in Asian monsoonal regions. The mean DIC/DOC ratio in the Min River was 15.09, and the DOC and DIC fluxes were 1.1 and 15.2 t C km−2 yr−1, respectively, for the studied year. And the DOC and DIC fluxes varied dramatically with runoff changes, suggesting that hydrologic conditions were critical factors for the variations in dissolved carbon export. This study shows that carbon dynamics of rivers draining from the Tibetan Plateau are greatly affected by short-term climatic variabilities, which has implications for understanding global carbon cycle under future climate change.

Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

Mangroves and salt marshes are among the most productive ecosystems in the global coastal ocean. Mangroves store more carbon (739 Mg CORG ha−1) than salt marshes (334 Mg CORG ha−1), but the latter sequester proportionally more (24%) net primary production (NPP) than mangroves (12%). Mangroves exhibit greater rates of gross primary production (GPP), aboveground net primary production (AGNPP) and plant respiration (RC), with higher PGPP/RC ratios, but salt marshes exhibit greater rates of below-ground NPP (BGNPP). Mangroves have greater rates of subsurface DIC production and, unlike salt marshes, exhibit active microbial decomposition to a soil depth of 1 m. Salt marshes release more CH4 from soil and creek waters and export more dissolved CH4, but mangroves release more CO2 from tidal waters and export greater amounts of particulate organic carbon (POC), dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC), to adjacent waters. Both ecosystems contribute only a small proportion of GPP, RE (ecosystem respiration) and NEP (net ecosystem production) to the global coastal ocean due to their small global area, but contribute 72% of air–sea CO2 exchange of the world’s wetlands and estuaries and contribute 34% of DIC export and 17% of DOC + POC export to the world’s coastal ocean. Thus, both wetland ecosystems contribute disproportionately to carbon flow of the global coastal ocean.

Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates

Abstract. The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CH2O in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH4) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modelled using reaction-transport equation vertical profiles of the concentration and isotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Furthermore, we modelled solute porewater profiles reported in the literature for four other seasonally anoxic lake basins. A total of 17 independent porewater datasets are analyzed. CH4 and DIC production rates associated with methanogenesis at the five seasonally anoxic sites collectively show that the fermenting OM has a mean (± SD) carbon oxidation state (COS) value of -1.4±0.3. This value is much lower than the value of zero expected from carbohydrate fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM in hypolimnetic sediments and propose to include the COS in the formulation of OM fermentation in models applied to lake sediments to better quantify sediment CH4 outflux. This study highlights the potential of mass balancing the products of OM mineralization to characterize labile substrates undergoing fermentation in sediments.

Summertime Oxygen Depletion and Acidification in Bohai Sea, China

Increasing attention has recently been drawn to oxygen depletion and ocean acidification in coastal seas and their impacts on the marine ecosystem. Intensive organic matter degradation combined with weak water exchange is considered to be mainly responsible for bottom water oxygen depletion and acidification. Hypoxia and acidification in the bottom water of Bohai Sea have been frequently observed in summer seasons over the past decade. In this study, monthly surveys were conducted along an inshore-offshore transect in the north Bohai Sea from May to August in 2017 and 2018. The physical structure of the water column in the study area was characterized by a well-mixed surface layer and an essentially homogeneous deep layer that were separated by a strong pycnocline over the period from mid-Jun to late August in both years. Dissolved oxygen (DO), pH and dissolved inorganic carbon (DIC) in the water column were also split into two layers by the pycnocline and varied little in each layer. No significant interannual variations were observed for the temporal progressions of DO, pH and DIC in the bottom water. Average DO concentration and pH in the bottom water linearly decreased with time from May to August while DIC presented an opposite trend. DO became increasingly unsaturated from mid-June to late August when DO decreased to 96 µmol L -1 and pH to 7.74. The molar ratio of net oxygen consumption to DIC production was 1.42, similar to the Redfield ratio (1.30). Pelagic respiration (> 60%) linked to degradation of fresh organic matter prevailed over sediment respiration (< 40%) in contributing to the oxygen depletion and acidification observed in this shallow water body. The duration and intensity of oxygen depletion and acidification in the study area were strongly affected by stormy weather events such as typhoons.

Control factors of DIC in the Y3 seamount waters of the Western Pacific Ocean

An investigation was carried out in the Y3 seamount area of the Western Pacific Ocean in December 2014, and the distribution of dissolved inorganic carbon (DIC) and its relationship with environmental factors in this area were explored. The results show that DIC concentration was higher in the adjacent waters of the Y3 seamount area, and the uplift of DIC isolines at the stations was close to the seamount. Meanwhile, interaction between the North Equatorial Current (NEC) and the Y3 seamount aff ected the DIC distribution, i.e., the upwelling in the same direction of the NEC was obvious, resulting in a decreasing trend of average concentration of DIC in the 200 m water column from the top to the two sides in this direction but in the cross direction. The DIC concentration increased with the water depth increase, and its distribution was aff ected by various environmental factors. In the surface water, high temperature was a decisive factor for the decrease of the DIC concentration, but the photosynthesis of phytoplankton showing only a weak influence. In the North Pacific Tropic Water (NPTW), DIC production rate from organic matter decomposition was higher than that of DIC consumption by phytoplankton photosynthesis, leading to a continual increase of DIC. In the North Pacific Intermediate Water (NPIW), organic matter decomposition played a leading role in the increase of DIC. In the deep water, decomposition of organic matter weakened, and the dissolution of CaCO3 controlled the carbonate system, and DIC had the smallest variation range.

Impact of methane occurrence on iron speciation in the sediments of the Gdansk Basin (Southern Baltic Sea)

Due to changing climate conditions, such inland seas as the Baltic are expected to become more eutrophicated and less saline (causing lower availability of sulphates). This may promote methanogenesis as the main process of organic matter (OM) degradation in marine sediments. Presence of methane, in turn, may affect biogeochemical cycling of many elements, including iron. Thus, in the present study we attempted to investigate the influence of CH4 on the Fe forms in marine sediments. Sediment cores were collected from three physico-chemically different stations within the Southern Baltic, taking into account such parameters as water depth, OM content, bottom zone oxygenation and distance from the Vistula River (main source of anthropogenic material). At two sampling stations methane was present in shallow sediments (P1, MET2) and at one station (W6) any traces of this gas were not determined. Iron species (FeCARB, FeOX1, FeOX2, FeMAG, FePRS, FeT, FeFeS2, FeFeS) in sediments were investigated using the sequential extraction. Pore water was analysed to obtain vertical profiles of hydrogen sulphide, sulphate and dissolved inorganic carbon (DIC). Additionally, such parameters as water and OM content, CH4 and Eh in sediments were determined. Results showed that methanogenesis affects the biogeochemical iron cycling in sediments of the Southern Baltic, leading to the increase of carbonates containing ferrous iron (as a result of DIC production during intensive OSR, AOM and methanogenesis) and decrease of ferric iron compounds used in the AOM. Moreover, in the areas of lower salinity, pyrite formation is limited by an insufficient amount of hydrogen sulphide, leading to the situation when a significant part of Fe is accumulated in the form of monosulphides. In turn, in the areas of higher salinity, where oxygen deficiencies occur more often, more hydrogen sulphide is present in pore water. Pyrite formation is then limited by iron, not by sulphur.