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1.
Reported herein are the results of eight soil CO2 efflux surveys performed from 2006 to 2011 at Timanfaya Volcanic Field (TVF), Lanzarote Island with the aim of evaluating the long- and short-term temporal variations of the diffuse CO2 emission. Soil CO2 efflux values ranged from non-detectable up to 34.2 g m−2 d−1, with the highest values measured in September 2008. Conditional sequential Gaussian simulations (sGs) were applied to construct soil CO2 efflux distribution maps and to estimate the total CO2 output from the studied area at the TVF. Soil CO2 efflux maps showed a high spatial and temporal variability. Total CO2 emission rates ranged between 41 and 518 t d−1, February 2011 (winter) being the season when maximum diffuse CO2 emission rates were observed. To investigate the influence of external variables on the soil CO2 efflux, a geochemical station (LZT01) was installed at TVF to measure continuously the soil CO2 efflux between July 2010 and March 2012 Since external factors such as barometric pressure, rainfall, soil water content, soil and air temperatures, and wind speed influence strongly the observed soil CO2 effluxes, multiple regression analysis was applied to the time series recorded by the automatic geochemical station LZT01 to remove the contribution of these external factors. The influence of meteorological variables on soil CO2 efflux oscillations accounts for 13% of total variance, with barometric pressure, rainfall and/or soil water content having the most influence in the control of the soil CO2 efflux. These observations along with the results from the eight soil gas surveys performed at TVF indicate that the short and long-term trends in the diffuse CO2 degassing are mainly controlled by environmental factors.  相似文献   

2.
Irragric anthrosols form as a result of prolonged deposition of fine sediments from irrigation water. Ancient irragric soils centuries to millennia old occur in several world regions, especially in arid environments of Asia and the Americas. This article presents evidence for an ancient irragric anthrosol in the North American Southwest, along the Snaketown Canal System in the Middle Gila River Valley, Arizona. This pedostratigraphic unit was formed as a result of a millennium of irrigation by Hohokam farmers from A.D. 450 to 1450. The irragric soil consists of a mantle of silty‐to‐loamy textures with minimal soil formation overlying a natural argillic horizon on a Pleistocene stream terrace. A soil mapped independently by the United States Department of Agriculture‐Natural Resources Conservation Service with these horizons corresponds closely with the canal system. Soil within the canal system tends to be lower in salt, sodium, and pH compared with external soils. This suggests that the irragric process improved soil for crop production through long‐term leaching and additions of fresh sediments with the irrigation water. This anthropogenic process of canal sedimentation has had a long‐lasting impact on the sedimentary record and soils in this arid environment.  相似文献   

3.
Assessing the global C budget requires a better understanding of the effect of environmental factors on soil CO2 efflux from both experiments and theoretical research, especially in different desertified lands in the Qinghai–Tibet Plateau. Based on the enclosed chamber method, soil CO2 efflux in four different desertified lands and one control [alpine meadow (AM)] were measured in June, August and September, 2008, respectively. Soil CO2 efflux rates at the top, the middle, the bottom of a slope and the flat in front of the slope were obtained at Maduo County. The results showed that average daily soil CO2 efflux rates were 3.72, 2.65, 2.68, 0.59 and 0.37 g m−2 day−1 in the AM, lightly (LDL), moderately (MDL), severely (SDL) and very severely desertified lands (VSDL) during the growing season, respectively. Soil CO2 efflux decreased with the change of desertification. The response of soil CO2 efflux to environmental factors was adequately described by the linear model; models accounted for 76, 65, 72, 59 and 71% of the variability on soil CO2 efflux in the AM, LDL, MDL, SDL and VSDL, respectively. Any environmental factor, however, was insufficient to explain the soil CO2 efflux; the common influence could perfectly reflect soil CO2 efflux response to the desertification change.  相似文献   

4.
A study area in an arid region of southern New Mexico is in basin-and-range topography and includes both a river valley and a closed basin. Holocene soils occur in valley fills and low terraces between Pleistocene fans, in and near drainageways on the fan-piedmont, on ridges, and in dunes. Holocene soils suggest the character of initial development in soils that are much older and more complex, and record the beginnings of various soil horizons. Noncalcareous brown or reddish brown B horizons have formed in low-carbonate parent materials of stable sites. Incipient development of the argillic horizon and the Haplargids occurs at stable sites in very gravelly materials that are about 1–2000 yr old. The cambic horizon and Camborthids occur in adjacent low-gravel materials of the same age. The argillic horizon occurs continuously in soils of earliest Holocene, particularly in very gravelly materials. Where soils have been truncated, as in areas affected by landscape dissection, argillic and cambic horizons are usually absent and the soils are Torripsamments, Torriorthents, or Torrifluvents depending on content of sand, gravel, and organic carbon. In high-carbonate parent materials, noncalcareous, reddish brown B horizons have not formed at any time in the Holocene. Most of these soils are Torriorthents or Torrifluvents although an incipient calcic horizon has formed in some of the oldest Holocene soils; the latter are Calciorthids. Horizons of carbonate accumulation are the best and most common pedogenic indicators of soil age. Stage I carbonate horizons are a major feature of pedogenesis in the Holocene. Because of additions of carbonate from the atmosphere, carbonate horizons are morphologically similar whether they have formed in high or low-carbonate alluvium. The carbonate accumulations are illuvial.Some Holocene deposits apparently resulted from changes in climate. Others, such as the youthful deposits of coppice dunes, apparently were caused by man's introduction of cattle and subsequent overgrazing and seed dispersal.  相似文献   

5.
Soil CO2 efflux from an ecosystem responds to the active layer thawing depth (H) significantly. A Li-8100 system was used to monitor the CO2 exchange from a wet meadow ecosystem during a freeze–thaw cycle of the active layer in a permafrost region on the Qinghai-Tibet Plateau. An exponential regression equation ( $ F_{\text{soil\, flux}} = 1.84e^{0.023H} + 5.06\,R^{2} = 0.96 $ ) has been established on the basis of observed soil CO2 efflux versus the thawed soil thickness. Using this equation, the total soil CO2 efflux during an annual freeze–thaw cycle has been calculated to be approximately 8.18 × 1010 mg C. The results suggest that freeze–thaw cycles in the active layer play an important role in soil CO2 emissions and that thawed soil thickness is the major factor controlling CO2 fluxes from the wet meadow ecosystem in permafrost regions on the Qinghai-Tibet Plateau. It can be concluded that with active layer thickening due to permafrost degradation, massive amounts of soil carbon would be emitted as greenhouse gases, and the permafrost region would become a carbon source with a positive feedback effect on climate warming. Hence, more attention should be paid to the influences of the active layer changes on soil carbon emission from these permafrost regions.  相似文献   

6.
A study of the soil profiles in the central Transvaal showed that in most areas a thick top layer of transported soils is present. Since this transported horizon has moved for considerable distances and because mixing took place in the process, the use of soil geochemistry in the search for hidden ore bodies tends to yield erratic results. In the present investigation an attempt was made to use volatile elements such as Hg, As, S and C as CO2 as geochemical indicator elements and to establish whether epigenetic haloes of these elements are imprinted on the transported soils. The geochemical behaviour of these elements are compared with elements such as Ni, Co, Cu and Fe, which would tend to migrate with the transported soil.The geochemical study was done on the Platreef in the Bushveld Complex, north of Potgietersrus. The Platreef consists of a stratified horizon which contains appreciable amounts of chalcopyrite, pyrrhotite, pentlandite and platinum-group elements. Four traverses across the ore body were investigated and the results indicate that in cases where the soil cover is relatively thin, approximately 1 to 2 metres, all the elements studied can be used to locate the ore body. Those traverses where the soil cover is thicker and where transported soils are definitely present, only mercury yields significant anomalies, while siderophile elements give erratic and poor results. The mercury anomalies are usually not displaced and even tend to delineate mineralized sub-zones of the Platreef.Arsenic, sulphur and carbon measured as total CO2, also give erratic results in the cases where a thick soil cover is present. The fact that arsenic tends to be fixed relatively easily in soils, either in laterite particles or in surface limestone, apparently is the prime reason for its erratic behaviour. The results indicate that both S and C would yield unsatisfactory results if these elements are measured as total S and C in the soils. The use of gas species may be more successful.A surprising but consistent result, was the tendency for mercury to be concentrated in the soils above faults, which intersect the ore body at depth. The behaviour of mercury in soils taken along the strike of a fault indicated that a logarithmic relationship exists between the mercury concentration in the soil and the depth of the ore body.  相似文献   

7.
Between 1996 and 2001 an experimental set up in a chaparral community near San Diego, CA, examined various plant and ecosystem responses to CO2 concentrations ranging from 250 to 750 μl l− 1. These experiments indicated a significant increase in soil C sequestration as CO2 rose above the ambient levels. In 2003, two years after the cessation of the CO2 treatments, we returned to this site to examine soil C dynamics with a particular emphasis on stability of specific pools of C. We found that in as little as two years, C content in the surface soils (0–15 cm) of previously CO2 enriched plots had dropped to levels below those of the ambient and pretreatment soils. In contrast, C retained in response to CO2 enrichment was more durable in the deeper soil layers (> 25 cm deep) where both organic and inorganic C were on average 26% and 55% greater, respectively, than C content of ambient plots. Using stable isotope tracers, we found that treatment C represented 25% of total soil C and contributed to 55% of soil CO2 efflux, suggesting that most of treatment C is readily accessible to decomposers. We also found that, C present before CO2 fumigation was decomposed at a faster rate in the plots that were exposed to elevated CO2 than in those exposed to ambient CO2 levels. To our knowledge, this is the first report that allows for a detail accounting of soil C after ceasing CO2 treatments. Our study provides a unique insight to how stable the accrued soil C is as CO2 increases in the atmosphere.  相似文献   

8.
Field experiments on the CO2 flux of alpine meadow soil in the Qilian Mountain were conducted along the elevation gradient during the growing season of 2004 and 2005. The soil CO2 flux was measured using the Li-6400-09 soil respiration chamber attached to the Li-6400 portable photosynthesis system. The effects of water and heat and roots on the soil CO2 flux were statistically analyzed. The results show that soil CO2 flux along the elevation gradient gradually decreases. The soil CO2 flux was low at night, with lowest value occurring between 0200 and 0600 hours, started to rise rapidly during 0700–0830 hours, and then descend during 1600–1830 hours. The peak CO2 efflux appears during 1100–1600 hours. The diurnal average of soil CO2 efflux was between 0.56 ± 0.32 and 2.53 ± 0.76 μmol m−2 s−1. Seasonally, soil CO2 fluxes are relatively high in summer and autumn and low in spring and winter. The soil CO2 efflux, from the highest to the lowest in the ranking order, occurred in July and August (4.736 μmol m−2 s−1), June and September, and May and October, respectively. The soil CO2 efflux during the growing season is positively correlated with soil temperature, root biomass and soil water content.  相似文献   

9.
Rare earth element (REE) distributions and Pb isotope compositions were explored in soils varying in age from ca. 0.4 to ?300 ka, developed on moraines in the Wind River Mountains, Wyoming. Soil extracts (0.6 M HCl) were used to examine the soil labile pool while the major element distribution in soil profiles was used to determine the extent of weathering at different soil depths. The results show that the chondrite-normalized REE patterns of the deepest bulk soil within each profile reflects the composition of the moraine till, except for the oldest soil. Up to ca. 12 ka, the soil extract fraction is enriched in light REE, indicating early release of light REE to the soil labile pool while that of the two oldest soils are relatively enriched in heavy REE. In the soil extracts the La/Sm ratio normalized to the deepest soil (LaD/SmD) decreases systematically with soil age. Similarly, the Eu-anomaly in the deepest soil from each profile (EuD/EuD*) decreases slightly with soil age in the three young soils; however, EuD/EuD* increases with soil age in the older soils. The systematic trends of these two ratios indicate the depletion of light REE in young soils and the enrichment of Eu and heavy REE in the older soils. Based on the Pb isotope ratios, the relative contribution of Pb to the soil labile pool via mineral weathering of U- or Th-rich phases was assessed for the different stages of weathering. The whole-soil profile 208Pb/204Pb ratio was found to decrease with soil age and with LaD/SmD, whereas it increased with the EuD/EuD* ratio. In each horizon, Pb isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) ratio generally decrease with soil age. In order to overcome possible effects from parent material heterogeneity, the amount of radiogenic Pb as compared to the whole-soil composition was calculated and this was found to decrease systematically with soil age.  相似文献   

10.
The terrestrial carbon cycle and the role of atmospheric CO2 concentrations in controlling global temperatures can be inferred from the study of ancient soils (paleosols). Soil-formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric pCO2 from ancient terrestrial records. In the case of goethite, optimum sampling strategies for reconstructing pCO2 focus on the portion of the soil profile that displays steep gradients in both soil CO2 concentration and δ13C values of soil CO2 such that a keeling plot can be developed for a given soil and atmospheric pCO2 can be calculated from it. We report data from a Carboniferous paleosol that depart from the expected linear trends. The results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil CO2, over a range of timescales, and can record these variations in the carbon isotope composition and mole fraction of Fe(CO3)OH in solid solution with goethite. We explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired CO2 or in the residence time of carbon in soils. The implications of this result are overestimation of paleoatmospheric pCO2 from pedogenic goethite.  相似文献   

11.
《Applied Geochemistry》2004,19(8):1217-1232
Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO2 (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m−2 h−1 at pH 2.78 in the batch experiments to 0.009 μmol Si m−2 h−1 at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H+ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO2 demonstrated that dissolution rates were independent of CO2 within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO2 have been elevated over the last decade. This release of magmatic CO2 has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO2 may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO2 can be excluded.  相似文献   

12.
Coastal wetlands, among the most productive ecosystems, are important global reservoirs of carbon (C). Accelerated sea level rise (SLR) and saltwater intrusion in coastal wetlands increase salinity and inundation depth, causing uncertain effects on plant and soil processes that drive C storage. We exposed peat-soil monoliths with sawgrass (Cladium jamaicense) plants from a brackish marsh to continuous treatments of salinity (elevated (~?20 ppt) vs. ambient (~?10 ppt)) and inundation levels (submerged (water above soil surface) vs. exposed (water level 4 cm below soil surface)) for 18 months. We quantified changes in soil biogeochemistry, plant productivity, and whole-ecosystem C flux (gross ecosystem productivity, GEP; ecosystem respiration, ER). Elevated salinity had no effect on soil CO2 and CH4 efflux, but it reduced ER and GEP by 42 and 72%, respectively. Control monoliths exposed to ambient salinity had greater net ecosystem productivity (NEP), storing up to nine times more C than plants and soils exposed to elevated salinity. Submersion suppressed soil CO2 efflux but had no effect on NEP. Decreased plant productivity and soil organic C inputs with saltwater intrusion are likely mechanisms of net declines in soil C storage, which may affect the ability of coastal peat marshes to adapt to rising seas.  相似文献   

13.
This study reports the first field test of a multi-channel, auto-dilution, steady-state, soil–CO2 flux monitoring system being developed to help understand the pathways by which fugitive CO2 from a geologic sequestration site migrates to the surface. The test was conducted from late August through mid-October 2008 at the Zero Emissions Research and Technology project site located in Bozeman, MT. Twenty steady-state and five non-steady-state flux chambers were installed in a 10 × 15 m area, one boundary of which was directly above a shallow (2-m depth) horizontal injection well located 0.5 m below the water table. A total flux of 52 kg CO2 day−1 was injected into the well for 13 days and the efflux from the soil was monitored by the chambers before, during, and for 33 days after the injection. The results showed a rapid increase in soil efflux once injection started, with maximal values reached within 3–7 days in most chambers. Efflux returned to background levels within a similar time period after injection ceased. A radial efflux pattern was observed to at least 2 m from the injection well, and evidence for movement of the CO2 plume during the injection, presumably due to groundwater flow, was seen. The steady-state chambers yielded very stable data, but threefold to fivefold higher fluxes than the non-steady-state chambers. The higher fluxes were attributed to vacuum induced in the steady-state chambers by narrow vent tubes. High winds resulted in significant decreases in measured soil CO2 efflux, presumably by enhancing efflux from soil outside the chambers.  相似文献   

14.
Biological soil crusts (BSCs) are an important cover in arid desert landscapes, and have a profound effect on the CO2 exchange in the desert system. Although a large number of studies have focused on the CO2 flux at the soil–air interface, relatively few studies have examined the soil CO2 concentration in individual layers of the soil profile. In this study, the spatiotemporal dynamics of CO2 concentration throughout the soil profile under two typical BSCs (algae crusts and moss crusts) and its driving factors were examined in a revegetated sandy area of the Tengger Desert from Mar 2010 to Oct 2012. Our results showed that the mean values of the vertical soil CO2 concentrations under algal crusts and moss crusts were 600–1,200 μmol/mol at the 0–40 cm soil profiles and increased linearly with soil depth. Daily CO2 concentrations showed a single-peak curve and often had a 1–2 h time delay after the maximum soil temperature. During the rainy season, the mean soil CO2 concentration profile was 1,200–2,000 μmol/mol, which was 2–5 times higher as compared to the dry season (400–800 μmol/mol). Annually, soil moisture content was the key limiting factor of the soil CO2 concentration, but at the daily time scale, soil temperature was the main limiting factor. Combined with infiltration depth of crusted soils, we predicted that precipitation of 10–15 mm was the most effective driving factor in arid desert regions.  相似文献   

15.
Accurate measurements of soil CO2 concentrations (pCO2) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO2, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO2 distribution can predictably vary with topographic position. We measured soil pCO2 at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO2 (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO2 generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO2 because it controls soil depth, texture, porosity, and moisture, which govern soil CO2 diffusive fluxes. The highest pCO2 values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO2 diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO2 because they are shallower (≤0.6 m) and drier, resulting in high CO2 diffusion through soil profiles. Aspect was a minor predictor of soil pCO2: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO2 than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO2 was similar across the watershed, the amplitude of the pCO2 peak was higher in the deep soils due to more variable moisture content. The high pCO2 observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO2 alone. CO2 is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO2 dynamics would be improved by including landscape controls on soil pCO2.  相似文献   

16.
 The properties of soils on previously dated sand dunes from Robe to Naracoorte in South Australia were examined. In these areas younger sand dunes are composed of fresh sand, but older sand dunes are composed of calcarenited sand. The soils on the sand dunes developed successionally by the age of sand dunes. The soil properties of these sand dunes differ depending on the ages of the sand dunes. The properties of sand particles in soils are as follows: (1) On the sand dunes of 4300 years B.P., A/C profile developed (Rendzina). On the sand dunes older than 125 000 years B.P. and on the plateau of Tertiary limestone, soil profiles of A1/AB/B/C on the sand dunes of 83 000 years B.P. and A1/A3/B1/B2/C (Terra rossa) are well developed. (2) Within the sand of A/C horizons of the sand dunes with the age of 4300 year B.P., the calcite grain content is about 64%, and the quartz content is about 35%. Within the B horizons of soils on the dunes from 83 000 years B.P. to 347 000 years B.P., the calcite grain content is only 1–2%; however, the quartz grain content is about 92%. In the B2 horizons of soils on the dune of 690 000 years B.P. and on the Tertiary plateau, there are some calcite grains but the quartz grain content is about 96%. (3) The average size of quartz grains in the soils on the sand dunes from 4300 B.P. to 347 000 years B.P. is generally smaller, but the average size of quartz on the sand dunes of 690 000 year B.P. becomes larger and the grains are well rounded. On the Tertiary limestone plateau, the average quartz size becomes again smaller, and the grains are more rounded. (4) Fet in B2 horizon of the soil profiles increases clearly corresponding to the age. Iron activity expressed by Feo/Fed also shows a close relation to the chronological sequence. The B horizon of the soil profiles shows a drastic decrease of Feo/Fed according to the age. Iron crystalinity, (Fed-Feo)/Fet, has a tendency for a positive relation with increasing age. Received: 1 June 1995 · Accepted: 4 December 1995  相似文献   

17.
Chloroform is a common groundwater pollutant but also a natural compound in forest ecosystems. Leaching of natural chloroform from forest soil to groundwater was followed by regular analysis of soil air and groundwater from multilevel wells at four different sites in Denmark for a period of up to 4 a. Significant seasonal variation in chloroform was observed in soil air 0.5 m below surface ranging at one site from 120 ppb by volume in summer to 20 ppb during winter. With depth, the seasonal variation diminished gradually, ranging from 30 ppb in summer to 20 ppb during winter, near the groundwater table. Chloroform in the shallowest groundwater ranged from 0.5–1.5 μg L−1 at one site to 2–5 μg L−1 at another site showing no clear correlation with season. Comparing changes in chloroform in soil air versus depth with on-site recorded meteorological data indicated that a clear relationship appears between rain events and leaching of chloroform. Chloroform in top soil air co-varied with CO2 given a delay of 3–4 weeks providing evidence for its biological origin. This was confirmed during laboratory incubation experiments which further located the fermentation layer as the most chloroform producing soil horizon. Sorption of chloroform to soils, examined using 14C–CHCl3, correlated with organic matter content, being high in the upper organic rich soils and low in the deeper more minerogenic soils. The marked decrease in chloroform in soil with depth may in part be due to microbial degradation which was shown to occur at all depths by laboratory tests using 14C–CHCl3.  相似文献   

18.
The role of CO2 in karst has been of interest for decades, and emphasized by IGCP 379, International Geoscience Programme, UNESCO started in 1995. There are still open questions about the dynamics of carbon in karst systems, particularly the flux of carbon between the surface and subsurface and between different components in the karst subsurface. This research report focuses on the variations of hydrochemistry and PCO2 (partial pressures of carbon dioxide) in subtropical karst groundwater, using high-resolution auto-monitoring hydrochemical data (15-min intervals). The aim of this study was to understand how hydrochemistry and PCO2 in karst systems respond to recharge over different time scales and what the controlling factors are. An auto-monitoring hydrochemistry station was installed about 300 m upstream from the exit in the active stream channel of Xueyu Cave, a typical subtropical karst cave. Four years of high-resolution continuous pH, specific conductivity (Spc), temperature and water-level data were collected. A thermodynamic model was used to link the continuous data to monthly water quality data, allowing the calculation of CO2 partial pressures and calcite saturation (SIc) levels on a continuous basis. Seasonal, diurnal and storm-scale variations were captured in the hydrochemistry and calculated PCO2 records, indicating that the cave stream is a dynamic and variable system. Seasonal features (higher specific conductivity and lower pH in summer; lower specific conductivity and higher pH in winter) tend to covary with temperature which influences the production of CO2 in soils, thus being the driving force for the variations (the soil CO2 effect). Due to the buffer effect of a thick vadose zone and large void cave space, diurnal variations are not obvious compared with epikarst springs in SW China. Storm-scale fluctuations due to storm events occur during the summer rainy season. Piston flow effects, dilution and soil CO2 effects determine the variations in different storm events. At the beginning of the rains, the piston effect drives the variations, characterized by increase in Spc, SIc and pH in the cave stream and decrease in PCO2. With heavy rainfall, decrease in Spc shows control by the dilution effect, while decrease in SIc and pH and increase in PCO2 indicates the greater influence of soil CO2. These results imply that the soil and cave voids are important factors influencing the hydrochemical evolution of karst groundwater. Future works need to use such high-resolution technology widely for tracing the PCO2 and hydrochemical variations in different karst aquifers.  相似文献   

19.
Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The δ13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. δ13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco2 value of about 1 × PAL, but the scatter in the measured δ13C values of organic matter permits a calculated maximum Pco2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments.The average δ13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ13C value for all profiles of −5.4‰. Thus, the value of Δ13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco2 and organic matter δ13C values were the same for both paleosol types. Furthermore, the atmospheric Pco2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco2 (permitted by one standard deviation of the organic matter δ13C value) of ∼5 × PAL.If, however, measured average δ13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric Pco2 using these 13C-enriched organic values would underestimate the actual atmospheric Pco2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2.The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the δ13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco2.  相似文献   

20.
Alkaline soils occupy approximately 5 % of the Earth’s land surface (7 million km), and this may increase due to the global trend towards increasing desertification, yet the extent to which these soils modulate carbon dynamics on regional and global scales is inadequately studied and poorly understood. Railroad Valley (RRV) playa (Nevada, USA) is a semi-arid playa with highly alkaline soils (pH > 10) and no vegetation. The extreme, alkaline environment and absence of vascular vegetation make RRV an ideal site to investigate the role of physiochemical processes of soil-atmosphere CO2 exchange. Both field and laboratory investigations were conducted. This work shows how the atmospheric CO2 mixing ratio decreases at nighttime at RRV playa to a value well below the average global background CO2 concentration. Laboratory investigations using soil samples collected at RRV playa confirmed that CO2 uptake by RRV playa soils occurs when temperatures are decreased. Both field and laboratory studies suggest that the alkaline RRV soil acts as a CO2 reservoir during colder periods, such as at nighttime. These results highlight the importance of investigating carbon dynamics in previously understudied environments. Given how little information is available on the CO2 flux in desert and semi-arid alkaline ecosystems lacking vegetation, our findings draw attention to these environments as becoming increasingly important for carbon fluxes on regional and global scales.  相似文献   

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