Inorganic Carbon Pool Research Articles

The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event

Abstract The Paleoproterozoic Francevillian succession of Gabon has figured prominently in concepts about Earth’s early oxygenation and genesis of a large positive excursion in carbon-isotope values, the Lomagundi-Jatuli event (LJE). Here we present a detailed study of a 139-m-long core of Francevillian rocks marked by carbonate δ13C (δ13Ccarb) values of 5‰–9‰ that decline upsection to near 0‰, a trend inferred by many workers as a fingerprint of the LJE and its termination. However, we show that the shift in δ13Ccarb values coincides with a facies change: shallow-marine facies are marked by the strongly positive values, whereas deeper-marine facies (below storm wave base) are at ∼0‰. The most circumspect interpretation of such facies dependence of δ13Ccarb is that shallow-marine settings record the isotope effects of local physical and biochemical processes driving the ambient dissolved inorganic carbon (DIC) pool to heavier values, and the lighter values (∼0‰) in deeper-water facies track the DIC of the open-marine realm where δ13C was largely unaffected by fractionations occurring in shallow-water settings. Further, a transgressing redoxcline created conditions for precipitation of Mn-bearing minerals and chemotrophic microbial biota, including methane cycling communities evident by organic δ13C (δ13Corg) values of −47‰ and Δδcarb-org values as high as 46‰. Thus, the Francevillian C-isotope profile reflects basin-specific conditions and is not a priori an indicator of global C-cycle disturbances nor of the termination of the LJE.

Combining Inorganic and Organic Carbon Stable Isotope Signatures in the Schwalbenberg Loess-Palaeosol-Sequence Near Remagen (Middle Rhine Valley, Germany)

Western Central European Loess-Palaeosol-Sequences (LPS) provide valuable terrestrial records of palaeoenvironmental conditions, which formed in response to variability in the North Atlantic climate systems. Over the last full glacial cycle (~130 ka), climate oscillations within these systems are best documented in deep sea and ice cores; the responses of terrestrial systems are not yet fully understood. A better understanding of metabolism governing input and output variables of organic- and inorganic C pools is, however, crucial for investigating landscape-atmospheric feedback processes and in particularly, for understanding the formation of calcareous LPS as environmental archives. Here we quantify the contributions of primary carbonates (PC) and secondary carbonates (SC) to the overall inorganic carbon pool down a LPS at the Schwalbenberg site, based on the natural abundance ratio of stable carbon isotopes (δ13C). This facilitates detailed insights into the carbonate metabolism and hence, loessification and percolation processes. PC accumulate predominantly in cold phases during periods of reduced biological activity and become leached during wetter and warmer periods contemporary with higher rates of SC re-precipitation and total organic carbon (TOC) increases due to enhanced biomass production. We find that mineral dust input is most significant during stadials, as well as towards the end of warmer interstadials, characterised by gradual cooling back to stadial conditions. Pedogenesis in the Schwalbenberg LPS kept pace with surface accumulation of mineral dust. This indicates that palaeosols are of accretionary nature, which gives raise to the idea of incorporation of former topsoils in preserved subsoil horizons. Our study decodes fundamental aspects of the link between atmospheric dust circulation and terrestrial records in western Central Europe. In addition, interdependencies between factors governing the regional moisture budged and LPS can be reconstructed in a more holistic way than before.

Temporal Variations of Inorganic Carbon Pools and Air-Sea CO 2 Fluxes in the North Pacific Subtropical Gyre Evaluated from 30 Years of Sustained Field Observations

Temporal Variations of Inorganic Carbon Pools and Air-Sea CO 2 Fluxes in the North Pacific Subtropical Gyre Evaluated from 30 Years of Sustained Field Observations

Thermal stability of soil carbon pools: Inferences on soil nature and evolution

The quantification of soil carbon pools is a pressing topic both for the agriculture productivity and to evaluate the Greenhouse Gases (GHG) sequestration potential, therefore a rapid and precise analytical protocol for carbon speciation is needed. Temperature-dependent differentiation of soil carbon in compliance with the DIN (German Institute for Standardization) 19539 standard has been applied for the first time on 24 agricultural soil samples from the Po River Plain (Italy), with the aim of investigate their thermal behavior in the 50−900 °C interval. The results invariably show the existence of three soil carbon pools having different thermal stabilities, namely, thermally labile organic carbon (TOC400), residual oxidizable carbon (ROC) and total inorganic carbon (TIC900), in the intervals of 300−400 °C, 510−600 °C and 700−900 °C, respectively. Significant relationships have been observed between the above mentioned organic and inorganic carbon pools and the associated isotopic composition: 1) inverse correlation between TOC400/ROC and δ13C links thermal stability and soil organic matter (SOM) composition; 2) direct correlation between carbonate breakdown temperature and δ13C denotes the mineralogical association of the inorganic pool. The results give clues regarding the nature and evolution of soil carbon pools.

Sequestration of inorganic carbon in groundwater of arid saline-alkali area

It has been reported that the saline/alkaline groundwater aquifer is a potentially large active inorganic carbon pool. This paper reviews the formation mechanism and the carbon storage path of inorganic carbon in groundwater of arid saline-alkali area. The geochemical carbon sequestration processes (CO2-water-rock interaction and mineral capture) known as the “capture-rereaction” model, are considered to be the most effective means of long-term CO2 storage, since the saline aquifers are covered with thick saline-alkali soils, once CO2 is involved in rock weathering or mineralization, it will become a part of the geological structure that can hardly be released again. This review provides an important reference for the carbon sequestration mechanism of arid underground saline-alkali aquifers, and is helpful for the estimation of regional carbon balance and the mitigation of Greenhouse Effect.

Regional Characteristics of the Temporal Variability in the Global Particulate Inorganic Carbon Inventory

AbstractCoccolithophores are a biogeochemically important calcifying group of phytoplankton that exert significant influence on the global carbon cycle. They can modulate the air‐sea flux of CO2 through the processes of photosynthesis and calcification and, as one of the primary contributors to the oceanic particulate inorganic carbon (PIC) pool, promote the export of organic carbon to depth. Here we present the first interannually resolved, global analysis of PIC standing stock. Average, global PIC standing stock in the top 100 m is estimated to be 27.04 ± 4.33 Tg PIC, with turnover times of ~7 days, which suggests PIC is likely removed by active processes such as grazing or rapid sinking, mediated through biogenic packaging (i.e., fecal pellets). We find that the Southern Hemisphere plays a significant role in the variability in PIC inventories and that interannual variability in PIC standing stock is driven primarily by variability in the midlatitude oceanic gyres and regions within the Great Calcite Belt of the Southern Ocean. Our results provide a framework against which future changes in global PIC standing stocks may be assessed.

Carbon fluxes and stocks in a carbonate-rich chenier plain

Coastal wetlands play an important role in the global carbon cycle because they have high ecosystem productivity and carbon sequestration capability. Previous research focused on the carbon dynamics of organic-rich ecosystems (e.g., salt marshes, mangroves), while little attention has paid to the carbon cycling of carbonate-rich ecosystems such as chenier plains, sandy or shelly beach ridges that are parts of strand plains. Here we examined the carbon stocks and fluxes of the poorly studied chenier plain in the Yellow River Delta, China. The inorganic and organic carbon pools in the top 1 m sediment were 444 ± 92 Mg C ha−1 and 89 ± 7 Mg C ha−1, respectively. The average CO2 net sequestration was −177 ± 51 Mg C ha−1, indicating that the chenier plain had a net CO2 evasion during the entire soil formation process. With plant growth, however, the chenier plain provided a significant carbon sink (395 g C m−2 a−1) on a per-unit area basis. The annual gross ecosystem productivity (GEP) and ecosystem respiration (Re) were 1067 and 672 g C m−2 a−1, respectively. The carbon sink strength of the chenier plain was comparable to that of organic-rich salt marshes on a per-unit area basis. The biomass carbon pool was 5.0 ± 1.4 Mg C ha−1 in the chenier plain. Moreover, the sediment inorganic carbon content in mudflats was significantly lower than that of vegetated habitats. Our results showed that plant grown in a chenier plain could significantly enhance carbon sequestration by increasing organic carbon storage. Inorganic carbon storage should be considered in blue carbon inventories because inorganic carbon dominates the carbon pool and is important in the carbon cycling in a chenier plain. Our findings can help us better understand the carbon cycling of carbonate-rich coastal ecosystems and can inform chenier plain conservation and restoration efforts.

Coupled anaerobic and aerobic microbial processes for Mn-carbonate precipitation: A realistic model of inorganic carbon pool formation

Abstract Mn carbonate is the main MnII mineral phase that precipitates in suboxic to anoxic environments. The coupled processes of MnIV oxide bioreduction and organic oxidation serve as dominant factors leading to Mn carbonate precipitation. This study examined the simultaneous respiration of oxygen and birnessite by a facultatively anaerobic bacterium, the Dietzia strain DQ12-45-1b (45-1b), and discussed the possible mechanism of rhodochrosite precipitation under general oxic environments. Compared to anaerobic experiments, the more rapid growth of 45-1b under aerobic conditions caused faster oxidation of acetate (1.0 × 103 μM h−1) and accumulation of HCO3− (5.5 × 102 μM h−1) within 72 h, which was coupled to a dramatic increase in pH from 7.0 to more than 9.2. By virtue of the higher biomass and bioactivity in the aerobic condition, the bioreduction of MnIV was accelerated and it caused a higher accumulating rate of soluble reduced Mn (4.0 μΜ h−1) than that in the anaerobic condition (2.0 μΜ h−1). Those rates indicated that an anaerobic-aerobic sub-interface was present in the aerobic system, in which anaerobic and aerobic respiration co-occurred to give rise to sufficient Mn(II) and alkalinity, thus, increased the supersaturation index (SI) for rhodochrosite. The mineral intermediates and products were identified by time-course XRD, SEM, and Raman spectra. Manganite (MnOOH) was found as the transient intermediate, which suggested the stepwise one-electron transfer mechanism of birnessite reduction. The dialysis tube, lysed cells, dead cells and two-compartment experiments suggested that the living 45-1b not only carried out a direct extracellular electron transfer for birnessite reduction but also provided necessary nucleation sites for rhodochrosite precipitation. Furthermore, both the isotope experiments and Raman analysis showed that the carbon source in rhodochrosite was mainly 13C isotope-labeled acetate, which corresponded well with the geological isotopic records. Finally, a conceptual model of Mn carbonate precipitation at oxic-suboxic/anoxic interfaces that could be possibly present in soil and sedimentary environments was proposed based on three prerequisites: (i) sufficient Mn(II) produced on an aerobic-anaerobic sub-interface, (ii) adequate alkalinity, and (iii) nucleation sites provided by cell surfaces. This model highlights the role of aerobic respiration in Mn(IV) reduction and Mn-carbonate formation, and may suggest a realistic way for inorganic carbon storage.

Kinetic effects during the experimental transition of aragonite to calcite in aqueous solution: Insights from clumped and oxygen isotope signatures

Abstract Paleoenvironmental reconstructions based on the clumped isotopes (Δ47) and traditional isotope (δ13C or δ18O) techniques are often problematic for carbonates that have undergone diagenetic alteration. One of the most common types of diagenesis is the transition between polymorphs, such as the replacement of aragonite by calcite. The isotope fractionation during such transitions in aqueous solutions remains unclear. We conducted a series of aragonite-to-calcite transition experiments in aqueous solutions of varying salinity and experiment durations at 25 °C and 90 °C to examine the variations of δ13C, δ18O, and Δ47 values for carbonates at varying degrees of the transition. The results confirm the retarding effect of Mg2+ and the catalytic effect of Na+ and Ca2+ on the transition, which are consistent with previous findings. Compared with the results from the transitions at 25 °C, the 90 °C experiments show a greater transition to calcite and a more depleted oxygen isotope composition. However, both clumped and carbon isotope values show no significant variation in the various experiments, suggesting that they are unaffected by mineralogical transition. Based on published equilibrium calibrations for δ18O and Δ47, the results demonstrate that kinetic effects in isotope systems are controlled primarily by the rate of polymorph transition, but there is a significant kinetic difference between the clumped isotope bond (13C 18O) reordering and the 18O exchange within the dissolved inorganic carbon pool. This kinetic difference results in partial equilibration in δ18O with no significant reordering in Δ47 toward their equilibriums, which can be accounted for by the difference of equilibration rates between the oxygen isotopes bound to 12C and those bound to 13C. This study provides a clear observation of the response of isotope systems to carbonate polymorph transitions and sheds light on their reliability as paleoenvironmental proxies.

Evidence for carbon cycling in a large freshwater lake in the Balkans over the last 0.5 million years using the isotopic composition of bulk organic matter

In the DEEP core from the Lake Ohrid ICDP drilling project, the carbon isotope composition of bulk organic matter (δ13CTOC) over the last 516 ka shows a negative correlation with total organic carbon (TOC) and total inorganic carbon (TIC). This relationship is marked by periods of lower δ13CTOC values corresponding to higher TIC and TOC. Along with TOC/TN, the correlation between δ13CTOC and δ13CTIC suggests that most of the organic matter in the core is from aquatic primary production within the lake. The combination of TOC, TIC, and δ13CTOC is able to disentangle long-term glacial/interglacial cycles and, to a lesser extent, millennial scale climate variability. Over the longer term, δ13CTOC shows modest variability, indicating that the δ13C of the dissolved inorganic carbon (DIC) pool is stabilised by the supply of karst spring water characterised by δ13CDIC influenced by the bedrock δ13C value, and the long residence time of the lake water and well mixed upper water column promoting equilibration with atmospheric CO2. However, comparison between arboreal pollen (AP%), TIC and TOC data indicates that the δ13CTOC signal is modulated by the leaching of soil CO2 through runoff and spring discharge, changes in primary productivity, and recycling of organic matter within the lake, all affecting δ13CDIC. Exceptionally low δ13CTOC during some interglacial periods (e.g. MIS7 and MIS9) possibly indicate rapid intensification of organic matter recycling and/or increasing stratification and enhanced methanogenesis, even if the latter process is not supported by the sedimentological data.

Biochar increased field soil inorganic carbon content five years after application

Soil inorganic carbon (SIC) plays an important role in terrestrial ecosystem carbon cycling, especially in arid and semi-arid areas. Biochar has becoming a great of interest as a mean for carbon sequestration, resulting from its high content of carbon and long-term persistence in soil. Moreover, there is abundance of information about the effect of biochar on the turnover of soil organic carbon; however, up to date, no study has been done on the impact of biochar on soil inorganic carbon pool despite biochar contains inorganic carbon. This study was conducted in a long-term field condition to investigate the effect of different biochar application rates (0, 30, 60 and 90 t ha−1) on soil inorganic carbon content and composition in the soil profile (0–100 cm). Using stable δ13C isotope signatures, the SIC was distinguished into lithogenic and pedogenic inorganic carbon. The results showed that long-term biochar application, at 30, 60 and 90 t ha−1, increased soil total inorganic carbon by 18.8, 42.4 and 62.3% and native soil inorganic carbon by 7.8, 20.2 and 28.3%, respectively, in the 0–20 cm soil layer. Biochar application at these rates also increased total inorganic carbon in the 20–40 cm soil layer by 13.4, 22.8 and 30.5%, respectively, but did not influence the total inorganic carbon content below the 40 cm soil depth. Moreover, as biochar application rate increased, δ13C of native soil inorganic carbon decreased, which indicated that pedogenic inorganic carbon was formed. Biochar application rates were positively related to the pedogenic inorganic carbon contents, however, it did not influence the lithogenic inorganic carbon contents. This is the first study about the effect of biochar application on native inorganic carbon content and its composition, which strengthens the understanding about the role of biochar in soil C sequestration.

Effects of nitrogen fertilizer application on carbon dioxide emissions from soils with different inorganic carbon contents.

The application of nitrogen (N) fertilizer results in decreases of soil pH, but its effects on CO2 emission from soils with different inorganic carbon contents remain unclear. An closed-jar incubation experiment was conducted to examine the effects of N fertilizer and nitrification inhibitor (DCD) on soil pH and CO2 emissions from three soil types with different contents of soil inorganic carbon (SIC), including paddy soil (PS), lime concretion black soil (CS), and eum-orthic anthrosols (AS). There were three treatments for each soil type, including control (N0), 0.2 g·kg-1 N fertilizer (N0.2), and its combination with DCD (N0.2+DCD). Soil pH, contents of mineral N (NH4+, NO3-), and CO2 emissions were measured. The results showed that N fertilizer addition significantly reduced soil pH and increased soil CO2 cumulative emissions in each type of soil. Compared to control, the CO2 cumulative emissions after 49 days incubation from the three soil types were enhanced by 39.4%, 23.4%, and 71.8% for PS, CS, AS soils, respectively. The soil pH of N0.2+DCD for three soil types were significantly higher than N0.2 after 49 days incubation, indicating that DCD application inhibited soil nitrification process. There were no significant differences in the mean CO2 cumulative emissions of PS and CS soils between N0.2 and N0.2+DCD treatments; however, N0.2+DCD treatment significantly reduced cumulative CO2 emissions from AS soil by 12.5%. Soil inorganic carbon can effectively buffer soil acidification caused by N fertilizer addition. The CO2 emission in calcareous soil following N addition is not only derived from the mineralization of soil organic carbon, but also from the dissolution of inorganic carbon. Given the large differences in soil inorganic carbon content in different regions of China, the problem of soil acidification and soil inorganic carbon pool consumption caused by long-term large amount of N fertilizer inputs deserve more attention.

A coastal coccolithophore maintains pH homeostasis and switches carbon sources in response to ocean acidification

Ocean acidification will potentially inhibit calcification by marine organisms; however, the response of the most prolific ocean calcifiers, coccolithophores, to this perturbation remains under characterized. Here we report novel chemical constraints on the response of the widespread coccolithophore species Ochrosphaera neapolitana (O. neapolitana) to changing-CO2 conditions. We cultured this algae under three pCO2-controlled seawater pH conditions (8.05, 8.22, and 8.33). Boron isotopes within the algae’s extracellular calcite plates show that this species maintains a constant pH at the calcification site, regardless of CO2-induced changes in pH of the surrounding seawater. Carbon and oxygen isotopes in the algae’s calcite plates and carbon isotopes in the algae’s organic matter suggest that O. neapolitana utilize carbon from a single internal dissolved inorganic carbon (DIC) pool for both calcification and photosynthesis, and that a greater proportion of dissolved CO2 relative to HCO3− enters the internal DIC pool under acidified conditions. These two observations may explain how O. neapolitana continues calcifying and photosynthesizing at a constant rate under different atmospheric-pCO2 conditions.

Investigating the environmental interpretation of oxygen and carbon isotope data from whole and fragmented bivalve shells

Sclerochronological data from whole bivalve shells have been used extensively to derive palaeoenvironmental information. However, little is known about the relevance of shell fragments more commonly preserved in the sediment record. Here, we investigate the oxygen and carbon isotope composition of Dreissena carinata fragments from a core recovered from Lake Dojran (FYRO Macedonia/Greece) to identify their relevance and efficacy as a proxy in palaeoenvironmental studies. We use a modern Dreissena shell to calibrate the relationship between the bivalve and its contemporary environment, which suggests their isotope composition is primarily a function of temperature and water balance. The range of fragment isotope data from the core overlaps with that of unbroken fossil shells, suggesting the fragments broadly record lakewater conditions across the time of deposition. A comparison of the isotope composition of shell fragments and endogenic carbonate shows an offset between the two sets of data, which is likely due to temperature differences between surface and bottom waters, the timing of carbonate precipitation, and productivity-controlled stratification of the dissolved inorganic carbon pool. Shell fragment isotope data seem to reflect the signal of environmental change recorded in other proxy data from the same core and may potentially be used (like endogenic carbonate) to provide information on past changes in lake level.

Carbonic anhydrase activity in seaweeds: overview and recommendations for measuring activity with an electrometric method, using Macrocystis pyrifera as a model species

Carbonic anhydrase (CA) plays an important physiological role in all biological systems by accelerating the interconversion of CO2 and HCO3−. In algae, CA is essential for photosynthesis: external CA (CAext) dehydrates HCO3−, enhancing the supply of CO2 to the cell surface, and internal CA (CAint) interconverts HCO3− and CO2 to maintain the inorganic carbon (Ci) pool and supply CO2 to RuBisCO. We first conducted a literature review comparing the conditions in which CA extraction and measurement have been carried out, using the commonly used Wilbur–Anderson method. We found that the assay has been widely modified since its introduction in 1948, mostly without being optimized for the species tested. Based on the review, an optimized protocol for measuring CA in Macrocystis pyrifera was developed, which showed that the assay conditions can strongly affect CA activity. Tris–HCl buffer gave the highest levels of CA activity, but phosphate buffer reduced activity significantly. Buffers containing polyvinylpyrrolidone (PVP) and dithiothreitol (DTT) stabilized CA. Using the optimized assay, CAext and CAint activities were readily measured in Macrocystis with higher precision compared to the non-optimized method. The CAint activity was 2 × higher than CAext, which is attributed to the Ci uptake mechanisms of Macrocystis. This study suggests that the CA assay needs to be optimized for each species prior to experimental work to obtain both accurate and precise results.

Carbonic anhydrase, coral calcification and a new model of stable isotope vital effects

The stable isotope compositions of biogenic carbonates have been used for paleoceanographic and paleoclimatic reconstructions for decades, and produced some of the most iconic records in the field. However, we still lack a fully mechanistic understanding of the stable isotope proxies, especially the biological overprint on the environmental signals termed “vital effects”. A ubiquitous feature of stable isotope vital effects in marine calcifying organisms is a strong correlation between δ18O and δ13C in a range of values that are depleted from inorganic calcite/aragonite. Two mechanisms have been proposed to explain this correlation, one based on kinetic isotope effects during CO2(aq)-HCO3− inter-conversion, the other based on equilibrium isotope exchange during pH dependent speciation of the dissolved inorganic carbon (DIC) pool. Neither mechanism explains all the stable isotope features observed in biogenic carbonates. Here we present a fully kinetic model of biomineralization and its isotope effects using deep-sea corals as a test organism. A key component of our model is the consideration of the enzyme carbonic anhydrase in catalyzing the CO2(aq)-HCO3− inter-conversion reactions in the extracellular calcifying fluid (ECF). We find that the amount of carbonic anhydrase not only modulates the carbonate chemistry of the calcifying fluid, but also helps explain the slope of the δ18O-δ13C correlation. Differences in CA activity in the biomineralization process can possibly explain the observed range of δ18O-δ13C slopes in different calcifying organisms. A mechanistic understanding of stable isotope vital effects with numerical models can help us develop better paleoceanographic tracers.

Thermal separation coupled with elemental and isotopic analysis: A method for soil carbon characterisation

A new analytical approach (Thermally Based Separation, TBS) for the characterisation of inorganic (IC) and organic (OC) carbon pools in alluvial soils has been tested. The method does not involve any chemical pre-treatment of samples and relies on different thermal stabilities of IC and OC pools, which can be accurately determined with an EA-IRMS analytical system. Elemental (C, g/kg) and isotopic (δ13C, ‰) carbon data were used to check the reliability of the TBS method by calculating the mass balances using the measured IC and OC fractions and total carbon (TC). TBS was applied to 7 soil samples from the Padanian alluvial plain (NE Italy) that were collected from different depths and characterised by textural/geochemical heterogeneity. The method allowed for a mean carbon elemental recovery of 99% (SD = 2%), and mean isotopic deviation (Δ13C) between theoretical (δ13COCXOC + δ13CICXIC)/(XOC + XIC) and measured δ13CTC of 0.2‰ (SD = 0.3‰). A comparison of TBS with other conventional methods for carbon separation provided insights into the possible effects of sample acidification on the soil organic pools. The results suggest a higher robustness of TBS compared to conventional methods in the determination of organic and inorganic carbon pools in soils. This is because TBS bypasses any possible fractionation derived from the hydrolysis of soil organic matter and therefore accurately determines the carbon isotopic composition of the OC and IC fractions. TBS appears to be more robust than conventional methods and is independent from the IC/OC ratio of the samples, from the unpredictable complexity of soil organic matter and from the arbitrary application of the acidification routine. On this basis, TBS represents a very promising approach for a correct and complete characterisation of soil carbon pools.

Inorganic carbon sequestration capacity and soil carbon assimilation pathway of deserts in arid region

The inorganic carbon sequestration capacity of deserts in an arid region is now still questioned for their accuracy. The key question is whether the result of CO2 absorption from aboveground flux observation can be reflected in the soil carbon pool or not. In the arid region of northwest China, desert is divided into three types based on the landscape and vegetation characteristics, namely gravel desert (Gobi), sand desert, and silt desert. By measuring and analyzing the soil particle composition, total salt content, organic carbon and inorganic carbon content, the changes in soil organic carbon (SOC) and soil inorganic carbon (SIC) density and their storage in different type deserts were studied, and then they were compared with that in the alpine ecosystem of southwest China. The results showed that the average SIC densities in 0–30 cm surface soil layer and 1 m soil depth in different type deserts were 2.8 and 10.1 kg C m−2, respectively. Comparatively, the SIC density of silt desert was the highest, and the value over 12 kg C m−2 in 1 m soil depth. In our study, the average total carbon densities in 0–30 cm desert soil layer was 3.5 kg C m−2, and in 1 m soil depth was 12.1 kg C m−2. According to the value of 100 g C m−2 a−1 to calculate, the carbon accumulation in 0–30 cm surface soil layer and 1 m soil depth need 35 years and 121 years, respectively. If the value is less than 100 g C m−2 a−1, a longer period is required, showed that it will take a long time for the formation of desert inorganic carbon pool. In terms of material content, the average bulk density in 1 m soil depth was 1.52 g cm−3, and the total carbon accumulated in desert soil accounted for 0.8% of soil mass in 121 years, of which the inorganic carbon accounted for 0.7% of soil mass. For the soil absorption and inorganic sequestration to CO2, we proposed the concept of soil carbon assimilation. Carbon sequestration in desert ecosystems is plant carbon assimilation and soil carbon assimilation, which is a combination of organic and inorganic processes. The SIC density and storage of desert soil are five times that of SOC. The pathway of soil carbon assimilation can be divided into three stages: the reaction stage between CO2 and H2O, the reaction stage between CO2 or weak carbonic acid and soil solution cation, and the stage that is the dissolved and precipitated carbonates which attached to soil particles and downward deposition. The soil carbon assimilation capacity was enhanced by increasing the soil organic carbon, salt content, water content, and soil silt and clay content. To recovering and conservation of desert vegetation, the desert is reclaimed into farmland under the carrying capacity of water resources, the SOC density was not only increased, the soil carbon assimilation capacity, SIC density and storage are also enhanced greatly. Under conditions of future climatic warming, soil inorganic carbon sequestration capacity will be increased and carbon sequestration capacity of the desert ecosystem will be enhanced with increasing precipitation in the arid region. The development of irrigated agriculture in the arid desert region will be beneficial to a collaborative improvement in production and ecological functions in the arid region.

Influence of Glacier Melting and River Discharges on the Nutrient Distribution and DIC Recycling in the Southern Chilean Patagonia

AbstractThe Chilean Patagonia constitutes one of the most important and extensive fjord systems worldwide, therefore can be used as a natural laboratory to elucidate the pathway of both organic and inorganic matter in the receiving environment. In this study we use data collected during an intensive oceanographic cruise along the Magellan Strait into the Almirantazgo Fjord in southern Patagonia to evaluate how different sources of dissolved inorganic carbon (DIC) and recycling may impact particulate organic carbon (POC) δ13C and influence the nutrients and carbonate system spatial distribution. The carbonate system presented large spatial heterogeneity. The lowest total alkalinity and DIC were associated to freshwater dilution observed near melting glaciers. The δ13CDIC analysis suggests that most DIC in the upper 50 m depth was not derived from terrestrial organic matter remineralization. 13C‐depleted riverine and ice‐melting DIC influence the DIC pool along the study area, but due to that DIC concentration from rivers and glaciers is relatively low, atmospheric carbon contribution or biological processes seem to be more relevant. Intense undersaturation of CO2 was observed in high chlorophyll waters. Respired DIC coming from the bottom waters seems to be almost insignificant for the inorganic carbon pool and therefore do not impact significantly the stable carbon isotopic composition of dissolved organic carbon and POC in the upper 50 m depth. Considering the combined effect of cold and low alkalinity waters due to ice melting, our results highlight the importance of these processes in determining corrosive waters for CaCO3 and local acidification processes associated to calving glacier in fjord ecosystems.

The spatial and interannual dynamics of the surface water carbonate system and air–sea CO<sub>2</sub> fluxes in the outer shelf and slope of the Eurasian Arctic Ocean

Abstract. The Arctic is undergoing dramatic changes which cover the entire range of natural processes, from extreme increases in the temperatures of air, soil, and water, to changes in the cryosphere, the biodiversity of Arctic waters, and land vegetation. Small changes in the largest marine carbon pool, the dissolved inorganic carbon pool, can have a profound impact on the carbon dioxide (CO2) flux between the ocean and the atmosphere, and the feedback of this flux to climate. Knowledge of relevant processes in the Arctic seas improves the evaluation and projection of carbon cycle dynamics under current conditions of rapid climate change. Investigation of the CO2 system in the outer shelf and continental slope waters of the Eurasian Arctic seas (the Barents, Kara, Laptev, and East Siberian seas) during 2006, 2007, and 2009 revealed a general trend in the surface water partial pressure of CO2 (pCO2) distribution, which manifested as an increase in pCO2 values eastward. The existence of this trend was defined by different oceanographic and biogeochemical regimes in the western and eastern parts of the study area; the trend is likely increasing due to a combination of factors determined by contemporary change in the Arctic climate, each change in turn evoking a series of synergistic effects. A high-resolution in situ investigation of the carbonate system parameters of the four Arctic seas was carried out in the warm season of 2007; this year was characterized by the next-to-lowest historic sea-ice extent in the Arctic Ocean, on satellite record, to that date. The study showed the different responses of the seawater carbonate system to the environment changes in the western vs. the eastern Eurasian Arctic seas. The large, open, highly productive water area in the northern Barents Sea enhances atmospheric CO2 uptake. In contrast, the uptake of CO2 was strongly weakened in the outer shelf and slope waters of the East Siberian Arctic seas under the 2007 environmental conditions. The surface seawater appears in equilibrium or slightly supersaturated by CO2 relative to atmosphere because of the increasing influence of river runoff and its input of terrestrial organic matter that mineralizes, in combination with the high surface water temperature during sea-ice-free conditions. This investigation shows the importance of processes that vary on small scales, both in time and space, for estimating the air–sea exchange of CO2. It stresses the need for high-resolution coverage of ocean observations as well as time series. Furthermore, time series must include multi-year studies in the dynamic regions of the Arctic Ocean during these times of environmental change.