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1.
The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate. 相似文献
2.
The paired chemical reactions, Ca 2+ + 2HCO 3 ? ? CaCO 3 + CO 2 + H 2O, overestimate the ratio of CO 2 flux to CaCO 3 flux during the precipitation or dissolution of CaCO 3 in seawater. This ratio, which has been termed ??, is about 0.6 in surface seawater at 25°C and at equilibrium with contemporary atmospheric CO 2 and increases towards 1.0 as seawater cools and pCO 2 increases. These conclusions are based on field observations, laboratory experiments, and equilibrium calculations for the seawater carbonate system. Yet global geochemical modeling indicates that small departures of ?? from 1.0 would cause dramatic, rapid, and unrealistic change in atmospheric CO 2. ?? can be meaningfully calculated for a water sample whether or not it is in equilibrium with the atmosphere. The analysis presented here demonstrates that the atmospheric CO 2 balance can be maintained constant with respect to seawater CaCO 3 reactions if one considers the difference between CaCO 3 precipitation and burial and differing values for ?? (both <1.0) in regions of precipitation and dissolution within the ocean. 相似文献
3.
In the present investigation, an age model of carbonate‐rich cores from a seamount top in the Central Indian Basin (CIB) was constructed using both isotopic ( 230Th excess, AMS 14C, oxygen isotopes) and biostratigraphic methods. The chronologies using the two methods are in good agreement, yielding a record of the late Middle Pleistocene to the Pleistocene–Holocene transition (550 to 11.5 ka). The first appearance datum (FAD) of the radiolarian Buccinosphaera invaginata (180 ka) and coccolith Emiliania huxleyi (268 ka) and the last appearance datum (LAD) of the radiolarian Stylatractus universus (425 ka) were used. A monsoon‐induced productivity increase was inferred from carbonate, organic carbon and δ 13C records in response to the Mid‐Brunhes Climatic Shift (MBCS), consistent with an increased global productivity. While the coccolith diversity increased, a decrease in coccolith productivity was found during the MBCS. At nearly the same time period, earlier records from the equatorial Indian Ocean, western Indian Ocean and eastern Africa have shown an increased productivity in response to the influence of westerlies and increased monsoon. The influence of easterlies from Australia and the intensification of aridity are evidenced by increased kaolinite content and clay‐sized sediments in response to the MBCS. An increased abundance of Globorotalia menardii and other resistant species beginning from marine isotope stage (MIS) 11 and the proliferation of coccolith Gephyrocapsa spp. indicate increased dissolution, which is consistent with the widespread global carbonate dissolution during this period. The relatively high carbonate dissolution during the transition period of MIS 3/2 and glacial to interglacial periods (MIS 6, 7 and 8) may be due to the enhanced flow of corrosive Antarctic Bottom Water (AABW) into the CIB. 相似文献
4.
About two hydrological years of continuous data of discharge, temperature, electrical conductivity and pH have been recorded
at the Glarey spring in the Tsanfleuron glaciated karst area in the Swiss Alps, to understand how glaciated karst aquifer
systems respond hydrochemically to diurnal and seasonal recharge variations, and how calcite dissolution by glacial meltwater
contributes to the atmospheric CO 2 sink. A thermodynamic model was used to link the continuous data to monthly water quality data allowing the calculation of
CO 2 partial pressures and calcite saturation indexes. The results show diurnal and seasonal hydrochemical variations controlled
chiefly by air temperature, the latter influencing karst aquifer recharge by ice and snowmelt. Karst process-related atmospheric
CO 2 sinks were more than four times higher in the melting season than those in the freezing season. This finding has implication
for understanding the atmospheric CO 2 sink in glaciated carbonate rock terrains: the carbon sink will increase with increasing runoff caused by global warming,
i.e., carbonate weathering provides a negative feedback for anthropogenic CO 2 release. However, this is a transient regulation effect that is most efficient when glacial meltwater production is highest,
which in turn depends on the future climatic evolution. 相似文献
5.
Carbonate rock outcrops cover 9%–16% of the continental area and are the principal source of the dissolved inorganic carbon (DIC) transferred by rivers to the oceans, a consequence their dissolution. Current estimations suggest that the flux falls between 0.1–0.6 PgC/a. Taking the intermediate value (0.3 PgC/a), it is equal to 18% of current estimates of the terrestrial vegetation net carbon sink and 38% of the soil carbon sink. In China, the carbon flux from carbonate rock dissolution is estimated to be 0.016 PgC/a, which accounts for 21%, 87.5%–150% and 2.3 times of the forest, shrub and grassland net carbon sinks respectively, as well as 23%–40% of the soil carbon sink flux. Carbonate dissolution is sensitive to environmental and climatic changes, the rate being closely correlated with precipitation, temperature, also with soil and vegetation cover. HCO3- in the water is affected by hydrophyte photosynthesis, resulting in part of the HCO3? being converted into DOC and POC, which may enhance the potential of carbon sequestration by carbonate rock dissolution. The possible turnover time of this carbon is roughly equal to that of the sea water cycle (2000a). The uptake of atmospheric/soil CO2 by carbonate rock dissolution thus plays an important role in the global carbon cycle, being one of the most important sinks. A major research need is to better evaluate the net effect of this sink in comparison to an oceanic source from carbonate mineral precipitation. 相似文献
6.
To accurately predict future CO 2 levels in the atmosphere, which is crucial in predicting global climate change, the sources and sinks of the atmospheric
CO 2 and their change over time must be determined. In this paper, some typical cases are examined using published and unpublished
data. Firstly, the sensitivity of carbonate rock weathering (including the effects by both dissolution and reprecipitation
of carbonate) to the change of soil CO 2 and runoff will be discussed, and then the net amount of CO 2 removed from the atmosphere in the carbonate rock areas of mainland China and the world will be determined by the hydrochem-discharge
and carbonate-rock-tablet methods, to obtain an estimate of the contribution of carbonate rock weathering to the atmospheric
CO 2 sink. These contributions are about 0.018 billion metric tons of carbon/a and 0.11 billion metric tons of carbon/a for China
and the world, respectively. Further, by the DBL (Diffusion Boundary Layer)-model calculation, the potential CO 2 sink by carbonate rock dissolution is estimated to be 0.41 billion metric tons of carbon/a for the world. Therefore, the
potential CO 2 source by carbonate reprecipitation is 0.3 billion metric tons of carbon/a.
Received: 12 May 1999 · Accepted: 16 August 1999 相似文献
7.
It is widely accepted that chemical weathering of Ca–silicate rocks could potentially control long-term climate change by providing feedback interaction with atmospheric CO 2 drawdown by means of precipitation of carbonate, and that in contrast weathering of carbonate rocks has not an equivalent impact because all of the CO 2 consumed in the weathering process is returned to the atmosphere by the comparatively rapid precipitation of carbonates in the oceans. Here, it is shown that the rapid kinetics of carbonate dissolution and the importance of small amounts of carbonate minerals in controlling the dissolved inorganic C (DIC) of silicate watersheds, coupled with aquatic photosynthetic uptake of the weathering-related DIC and burial of some of the resulting organic C, suggest that the atmospheric CO 2 sink from carbonate weathering may previously have been underestimated by a factor of about 3, amounting to 0.477 Pg C/a. This indicates that the contribution of silicate weathering to the atmospheric CO 2 sink may be only 6%, while the other 94% is by carbonate weathering. Therefore, the atmospheric CO 2 sink by carbonate weathering might be significant in controlling both the short-term and long-term climate changes. This questions the traditional point of view that only chemical weathering of Ca–silicate rocks potentially controls long-term climate change. 相似文献
8.
Epochs of changing atmospheric CO 2 and seawater CO 2–carbonic acid system chemistry and acidification have occurred during the Phanerozoic at various time scales. On the longer geologic time scale, as sea level rose and fell and continental free board decreased and increased, respectively, the riverine fluxes of Ca, Mg, DIC, and total alkalinity to the coastal ocean varied and helped regulate the C chemistry of seawater, but nevertheless there were major epochs of ocean acidification ( OA). On the shorter glacial–interglacial time scale from the Last Glacial Maximum (LGM) to late preindustrial time, riverine fluxes of DIC, total alkalinity, and N and P nutrients increased and along with rising sea level, atmospheric PCO 2 and temperature led, among other changes, to a slightly deceasing pH of coastal and open ocean waters, and to increasing net ecosystem calcification and decreasing net heterotrophy in coastal ocean waters. From late preindustrial time to the present and projected into the 21st century, human activities, such as fossil fuel and land-use emissions of CO 2 to the atmosphere, increasing application of N and P nutrient subsidies and combustion N to the landscape, and sewage discharges of C, N, P have led, and will continue to lead, to significant modifications of coastal ocean waters. The changes include a rapid decline in pH and carbonate saturation state (modern problem of ocean acidification), a shift toward dissolution of carbonate substrates exceeding production, potentially leading to the “demise” of the coral reefs, reversal of the direction of the sea-to-air flux of CO 2 and enhanced biological production and burial of organic C, a small sink of anthropogenic CO 2, accompanied by a continuous trend toward increasing autotrophy in coastal waters. 相似文献
9.
The draw down of CO 2 from the atmosphere during mineral weathering plays a major role in the global budget of this greenhouse gas. Silicate minerals remove twice the CO 2 of carbonate minerals per mole of calcium in runoff during weathering. Bedrock weathering chemistry was investigated in the White River watershed of northeastern USA to investigate whether there are seasonal differences in carbonate and silicate weathering chemistry. Geographic Information Systems analyses of bedrock geology were combined with major element concentrations in river waters to gain an understanding of the consistency of mineral weathering during three seasons. The percent of carbonate mineralogy comprising the bedrock in tributaries of the White River varied from less than 5% to 45% by area. A mass balance calculation using major element concentrations in waters was applied to estimate the seasonal relationships between bedrock geology and bicarbonate flux. In all tributaries and the main stem of the White River the highest calculated percent of bicarbonate from carbonate mineral weathering was measured in the late fall. The results suggest that carbonate and silicate bedrock weathering processes are seasonally controlled. Thus single season sampling could not accurately represent an entire year's geochemical budget. In the White River, water samples obtained solely during the summer would consistently underestimate the total yearly source of bicarbonate from carbonate bedrock weathering. The same sample set would also provide data that would lead to an underestimation of the yearly atmospheric CO 2 draw down by bedrock weathering in the watershed. For example at four of the seven locations studied there was an almost two-fold difference between summer and spring calculated atmospheric CO 2 consumption rates. 相似文献
10.
Far from equilibrium enstatite dissolution rates both open to atmospheric CO 2 and CO 2 purged were measured as a function of solution pH from 8 to 13 in batch reactors at room temperature. Congruent dissolution was observed after an initial period of incongruent dissolution with preferential Si release from the enstatite. Steady-state dissolution rates in open to atmospheric CO 2 conditions decrease with increase in solution pH from 8 to 12 similar to the behavior reported by other investigators. Judging from the pH 13 dissolution rate, rates increase with pH above pH 12. This is thought to occur because of the increase in overall negative surface charges on enstatite as Mg surface sites become negative above pH 12.4, the pH of zero surface charge of MgO.Steady-state dissolution rates of enstatite increase above pH 10 when CO 2 was purged by performing the experiments in a N 2 atmosphere. This suggests inhibition of dissolution rates above pH 10 when experiments were open to the atmosphere. The dissolved carbonate in these solutions becomes dominantly CO 32− above pH 10.33. It is argued that CO 32− forms a >Mg 2-CO 3 complex at positively charged Mg surface sites on enstatite, resulting in stabilization of the surface Si-O bonds. Therefore, removal of solution carbonate results in an increase in dissolution rates of enstatite above pH 10. The log rate of CO 2-purged enstatite dissolution in moles per cm 2 per s as a function of increasing pH above pH 10 is equal to 0.35. This is consistent with the model of silicate mineral dissolution in the absence of surface carbonation in alkaline solutions proposed earlier in the literature. 相似文献
11.
The goal of this study was to highlight the occurrence of an additional proton-promoted weathering pathway of carbonate rocks in agricultural areas where N-fertilizers are extensively spread, and to estimate its consequences on riverine alkalinity and uptake of CO 2 by weathering. We surveyed 25 small streams in the calcareous molassic Gascogne area located in the Garonne river basin (south-western France) that drain cultivated or forested catchments for their major element compositions during different hydrologic periods. Among these catchments, the Hay and the Montoussé, two experimental catchments, were monitored on a weekly basis. Studies in the literature from other small carbonate catchments in Europe were dissected in the same way. In areas of intensive agriculture, the molar ratio (Ca + Mg)/HCO 3 in surface waters is significantly higher (0.7 on average) than in areas of low anthropogenic pressure (0.5). This corresponds to a decrease in riverine alkalinity, which can reach 80% during storm events. This relative loss of alkalinity correlates well with the content in surface waters. In cultivated areas, the contribution of atmospheric/soil CO 2 to the total riverine alkalinity (CO 2 ATM-SOIL/HCO 3) is less than 50% (expected value for carbonate basins), and it decreases when the nitrate concentration increases. This loss of alkalinity can be attributed to the substitution of carbonic acid (natural weathering pathway) by protons produced by nitrification of N-fertilizers (anthropogenic weathering pathway) occurring in soils during carbonate dissolution. As a consequence of these processes, the alkalinity over the last 30 years shows a decreasing trend in the Save river (one of the main Garonne river tributaries, draining an agricultural catchment), while the nitrate and calcium plus magnesium contents are increasing.We estimated that the contribution of atmospheric/soil CO 2 to riverine alkalinity decreased by about 7-17% on average for all the studied catchments. Using these values, the deficit of CO 2 uptake can be estimated as up to 0.22-0.53 and 12-29 Tg 1 yr −1 CO 2 on a country scale (France) and a global scale, respectively. These losses represent up to 5.7-13.4% and only 1.6-3.8% of the total CO 2 flux naturally consumed by carbonate dissolution, for France and on a global scale, respectively. Nevertheless, this loss of alkalinity relative to the Ca + Mg content relates to carbonate weathering by protons from N-fertilizers nitrification, which is a net source of CO 2 for the atmosphere. This anthropogenic CO 2 source is not negligible since it could reach 6-15% of CO 2 uptake by natural silicate weathering and could consequently partly counterbalance this natural CO 2 sink. 相似文献
12.
Activities to provide energy for an expanding population are increasingly disrupting and changing the concentration of atmospheric gases that increase global temperature. Increased CO 2 and temperature have a clear effect on growth and production of rice as they are key factors in photosynthesis. Rice yields could be increased with increased levels of CO 2, however, the rise of CO 2 may be accompanied by an increase in global temperature. The effect of doubling CO 2 levels on rice production was predicted using rice crop models. They showed different effects of climate change in different countries. A simulation of the Southeast Asian region indicated that a doubling of CO 2 increases yield, whereas an increase in temperature decreases yield.Enhanced UV-B radiation resulting for stratographic ozone depletion has been demonstrated to significantly reduce plant height, leaf area and dry weight of two rice cultivars under glasshouse conditions. Data are still insufficient, however, for conclusive results on the effect of UV-B radiation on rice growth under field conditions.Rice production itself has a significant effect on global warming and atmospheric chemistry through methane emission from flooded ricefields. Water regime, soil properties and the rice plant are major factors controlling the flux of methane in ricefields. Global and regional estimates of methane emission rates are still highly uncertain and tentative. Integration of mechanistic modeling of methane fluxes with geographic information systems of factors controlling these processes are required to improve estimates and predictions. 相似文献
13.
The most suitable candidates for subsurface storage of CO 2 are depleted gas fields. Their ability to retain CO 2 can however be influenced by the effect which impurities in the CO 2 stream (e.g. H 2S and SO 2) have on the mineralogy of reservoir and seal. In order to investigate the effects of SO 2 we carried out laboratory experiments on reservoir and cap rock core samples from gas fields in the northeast of the Netherlands. The rock samples were contained in reactor vessels for 30 days in contact with CO 2 and 100 ppm SO 2 under in-situ conditions (300 bar, 100 °C). The vessels also contained brine with the same composition as in the actual reservoir. Furthermore equilibrium modeling was carried out using PHREEQC software in order to model the experiments on caprock samples.After the experiments the permeability of the reservoir samples had increased by a factor of 1.2–2.2 as a result of dissolution of primary reservoir minerals. Analysis of the associated brine samples before and after the experiments showed that concentrations of K, Si and Al had increased, indicative of silicate mineral dissolution.In the caprock samples, composed of carbonate and anhydrite minerals, permeability changed by a factor of 0.79–23. The increase in permeability is proportional to the amount of carbonate in the caprock. With higher carbonate content in comparison with anhydrite the permeability increase is higher due to the additional carbonate dissolution. This dependency of permeability variations was verified by the modeling study. Hence, caprock with a higher anhydrite content in comparison with carbonate minerals has a lower risk of leakage after co-injection of 100 ppmv SO 2 with CO 2. 相似文献
14.
Release of CO 2 from surface ocean water owing to precipitation of CaCO 3 and the imbalance between biological production of organic matter and its respiration, and their net removal from surface
water to sedimentary storage was studied by means of a quotient θ = (CO 2 flux to the atmosphere)/(CaCO 3 precipitated). θ depends not only on water temperature and atmospheric CO 2 concentration but also on the CaCO 3 and organic carbon masses formed. In CO 2 generation by CaCO 3 precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric
CO 2 concentration (195–375 ppmv), and increasing CaCO 3 precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic
carbon counteracts the CO 2 production by carbonate precipitation and it results in lower CO 2 emissions from the surface layer. When atmospheric CO 2 increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO 2 transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric
CO 2. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon
from land, the calculated CO 2 flux to the atmosphere is a function of the CaCO 3 and C org net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO 2 flux near the Last Glacial Maximum is 17 to 7×10 12 mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time
it is 38–42×10 12 mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic
emissions of CO 2 to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total
alkalinity owing to the CaCO 3 storage and consequent emission of CO 2. These results are in agreement with the conclusions of a number of other investigators. As the CO 2 uptake in mineral weathering is a major flux in the global carbon cycle, the CO 2 weathering pathway that originates in the CO 2 produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large
imbalances of the atmosphere and land organic reservoir at 10 2–10 4-year time scales. 相似文献
15.
A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO 2 consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ34S values of 16 to 20‰ CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS 2. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO 2 fluxes from silicate weathering range from ~ 0.012 × 10 6 to 0.039 × 10 6 mol/km 2/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO 2 consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO 2 consumption rates in particular, based on major ion abundances, should be treated with skepticism. 相似文献
16.
We evaluate whether the global weathering budget is near steady state for the pre-anthropogenic modern environment by assessing the magnitude of acidity-generating volcanic exhalations. The weathering rate induced by volcanic acid fluxes, of which the CO 2 flux is the most important, can be expressed as an average release rate of dissolved silica, based on a model feldspar-weathering scheme, and the ratio of carbonate-to-silicate rock weathering. The theoretically predicted flux of silica from chemical weathering is slightly smaller than the estimated global riverine silica flux. After adjustment for carbonate weathering, the riverine dissolved bicarbonate flux is larger than the volcanic carbon degassing rate by a factor of about three. There are substantial uncertainties associated with the calculated and observed flux values, but the modern system may either not be in steady state, or additional, “unknown” carbon sources may exist. The closure errors in the predicted budgets and observed riverine fluxes suggest that continental weathering rates might have had an impact on atmospheric CO 2 levels at a time scale of 10 3-10 4 years, and that enhanced weathering rates during glacial periods might have been a factor in the reduced glacial atmospheric CO 2 levels. Recent anthropogenic emissions of carbon and sulfur have a much larger acid-generating capacity than the natural fluxes. Estimated potential weathering budgets to neutralize these fluxes are far in excess of observed values. A theoretical scenario for a return to steady state at the current anthropogenic acidity emissions (disregarding the temporary buffering action of the ocean reservoir) requires either significantly lower pH values in continental surface waters as a result of storage of strong acids, and/or higher temperatures as a result of enhanced atmospheric CO 2 levels in order to create weathering rates that can neutralize the total flux of anthropogenic and natural background acidity. 相似文献
17.
Rising atmospheric pCO 2 and ocean acidification originating from human activities could result in increased dissolution of metastable carbonate minerals
in shallow-water marine sediments. In the present study, in situ dissolution of carbonate sedimentary particles in Devil’s
Hole, Bermuda, was observed during summer when thermally driven density stratification restricted mixing between the bottom
water and the surface mixed layer and microbial decomposition of organic matter in the subthermocline layer produced pCO 2 levels similar to or higher than those levels anticipated by the end of the 21st century. Trends in both seawater chemistry
and the composition of sediments in Devil’s Hole indicate that Mg-calcite minerals are subject to selective dissolution under
conditions of elevated pCO 2. The derived rates of dissolution based on observed changes in excess alkalinity and estimates of vertical eddy diffusion
ranged from 0.2 mmol to 0.8 mmol CaCO 3 m −2 h −1. On a yearly basis, this range corresponds to 175–701 g CaCO 3 m −2 year −1; the latter rate is close to 50% of the estimate of the current average global coral reef calcification rate of about 1,500 g
CaCO 3 m −2 year −1. Considering a reduction in marine calcification of 40% by the year 2100, or 90% by 2300, as a result of surface ocean acidification,
the combination of high rates of carbonate dissolution and reduced rates of calcification implies that coral reefs and other
carbonate sediment environments within the 21st and following centuries could be subject to a net loss in carbonate material
as a result of increasing pCO 2 arising from burning of fossil fuels. 相似文献
18.
In a mid-continental North American grassland, solute concentrations in shallow, limestone-hosted groundwater and adjacent surface water cycle annually and have increased steadily over the 15-year study period, 1991-2005, inclusive. Modeled groundwater CO 2, verified by measurements of recent samples, increased from 10 −2.05 atm to 10 −1.94 atm, about a 20% increase, from 1991 to 2005. The measured groundwater alkalinity and alkaline-earth element concentrations also increased over that time period. We propose that carbonate minerals dissolve in response to lowered pH that occurs during an annual carbonate-mineral saturation cycle. The cycle starts with low saturation during late summer and autumn when dissolved CO 2 is high. As dissolved CO 2 decreases in the spring and early summer, carbonates become oversaturated, but oversaturation does not exceed the threshold for precipitation. We propose that groundwater is a CO 2 sink through weathering of limestone: soil-generated CO 2 is transformed to alkalinity through dissolution of calcite or dolomite. The annual cycle and long-term increase in shallow groundwater CO 2 is similar to, but greater than, atmospheric CO 2. 相似文献
19.
We conducted CO 2–water–rock interaction experiments to elucidate the dissolution characteristics and geochemical trapping potential of three different altered andesitic to rhyolitic tuffaceous rocks (Tsugawa, Ushikiri and Daijima tuffaceous rock) relative to fresh mid-ocean ridge basalt. The experiments were performed under 1 MPa CO 2 pressure to reproduce the water–rock–CO 2 interactions in CO 2 storage situations. Basalt showed high acid neutralization potential and rapid dissolution of silicate minerals. Two of the tuffaceous rocks (Ushikiri and Daijima) showed relatively high solubility trapping potential, mainly due to the dissolution of carbonate minerals in the andesitic Ushikiri tuffaceous rock and the ion-exchange reaction with zeolite minerals in the rhyolitic Daijima tuffaceous rock. The mineral trapping potential of the Ushikiri tuffaceous rock was found to be relatively high, due to the rapid dissolution of Mg- and Ca-bearing silicate minerals. Our experimental results suggest that regions of porous and andesitic tuffaceous rock hold global promise as CO 2 storage sites. 相似文献
20.
Physiochemical controls on the carbonate geochemistry of large river systems are important regulators of carbon exchange between terrestrial and marine reservoirs on human time scales. Although many studies have focused on large-scale river carbon fluxes, there are few investigations of mechanistic aspects of carbonate mass balance and transport at the catchment scale. We determined elemental and carbonate geochemistry and mass balances for net carbonate dissolution fluxes from the forested, mid-latitude Huron River watershed, established on carbonate-rich unconfined glacial drift aquifers. Shallow groundwaters are near equilibrium with respect to calcite at pCO 2 values up to 25 times atmospheric values. Surface waters are largely groundwater fed and exhibit chemical evolution due to CO 2 degassing, carbonate precipitation in lakes and wetlands, and anthropogenic introduction of road salts (NaCl and CaCl 2). Because the source groundwater Mg 2+/HCO 3 ? ratio is fairly constant, this parameter permits mass balances to be made between carbonate dissolution and back precipitation after groundwater discharge. Typically, precipitation does not occur until IAP/K calcite values exceed 10 times supersaturation. Stream chemistry changes little thereafter even though streams remain highly supersaturated for calcite. Our data taken together with historical United States Geological Survey (USGS) data show that alkalinity losses to carbonate precipitation are most significant during periods of lowest discharge. Thus, on an annual basis, the large carbon flux from carbonate dissolution in soil zones is only decreased by a relatively small amount by the back precipitation of calcium carbonate. 相似文献
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