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1.
Excess N from agriculture induces eutrophication in major river systems and hypoxia in coastal waters throughout the world. Much of this N is from headwaters far up the watersheds. In turn, much of the N in these headwaters is from ground-water discharge. Consequently, the concentrations and forms of N in groundwater are important factors affecting major aquatic ecosystems; despite this, few data exist for several species of N in groundwater and controls on speciation are ill-defined. Herein, we report N speciation for a spring and well that were selected to reflect agricultural impacts, and a spring and well that show little to no agricultural-N impact. Samples were characterized for NO 3−, NO 2−, N 2O, NH 4+, urea, particulate organic N(), and dissolved organic N(). These analytes were monitored in the agricultural spring for up to two years along with other analytes that we reported upon previously. For all samples, when oxidized N was present, the dominant species was NO 3− (88-98% of total fixed N pool) followed by (<4-12%) and only trace fractions of the other N analytes. In the non-agriculturally impacted well sample, which had no quantifiable NO 3− or dissolved O 2, comprised the dominant fraction (68%) followed by NH 4+ (32%), with only a trace balance comprised of other N analytes. Water drawn from the well, spring and a wetland situated in the agricultural watershed also were analyzed for dissolved N 2 and found to have a fugacity in excess of that of the atmosphere. H 2O 2 was analyzed in the agricultural spring to evaluate the O 2/H 2O 2 redox potential and compare it to other calculated potentials. The potential of the O 2/H 2O 2 couple was close in value to the NO 3−/NO 2− couple suggesting the important role of H 2O 2 as an O 2-reduction intermediate product and that O 2 and NO 3− are reduced concomitantly. The O 2/H 2O 2 and NO 3−/NO 2− couples also were close in value to a cluster of other inorganic N and Fe couples indicating near partial equilibrium among these species. Urea mineralization to NO 2− was found to approach equilibrium with the reduction of O 2 to H 2O 2. By modeling as amide functional groups, as justified by recent analytical work, similar thermodynamic calculations support that mineralization to NO 2− proceeds nearly to equilibrium with the reduction of O 2 to H 2O 2 as well. This near equilibration of redox couples for urea- and -oxidation with O 2-reduction places these two couples within the oxidized redox cluster that is shared among several other couples we have reported previously. In the monitored agricultural spring, [NO 3−] was lower in the summer than at other times, whereas [N 2O] was higher in the summer than at other times, perhaps reflecting a seasonal variation in the degree of denitrification reaction progress. No other N analytes were observed to vary seasonally in our study. In the well having no agricultural-N impact, C org/N org = 5.5, close to the typical value for natural aqueous systems of about 6.6. In the agricultural watershed C org/N org varied widely, from ∼1.2 to ?9. 相似文献
2.
The rates of organic carbon oxidation by O 2, NO 3?, MnO 2, Fe 2O 3 and SO 4? have been calculated for five pelagic Pacific and Atlantic sites using simple diffusion-reaction models. O 2 everywhere oxidizes > 90% of the raining C org; the fraction oxidized by the secondary oxidants decreases as the rain rate of organic C to the seafloor decreases. A large fraction of the C org escaping oxidation by O 2 is oxidized by the secondary oxidants. Hence while these oxidants play a small role in remineralization, they are important in regulating the burial of organic matter and the consequent removal from the oceans of reduced carbon and nutrients. 相似文献
3.
Based on a new mixing model of two end-members, the water column remineralization ratios of P/N/C org - O 2 = 1/13 ± 1/135 ± 18/170 ± 9 are obtained for the Hawaii Ocean Time-series (HOT) data set at station ALOHA. The traditional Redfield ratios of P/N/C org/–O 2 = 1/16/106/138 have standard deviations of more than 50%, when they are based on the average composition of phytoplankton. Apparently, the remineralization processes in the water column have smoothed out the observed large variability of plankton compositions. A new molar formula for the remineralized plankton may be written as 135H 280O 105N 13P or C 25(CH 2O) 101(CH 4) 9(NH 3) 13(H 3PO 4). Oxidation of this formula results inC 25(CH 2O) 101(CH 4) 9(NH 3) 13(H 3PO 4) + 170O 2 135CO 2 + 132H 2O + 13NO 3
- + H 2PO 4
- + 14H +.For comparison, remineralization using Redfield's formula gives:(CH 2O) 106(NH 3) 16(H 3PO 4) + 138O 2 106CO 2 + 122H 2O + 16NO 3
-+ H 2PO 4
- + 17H + 相似文献
4.
高氯酸盐(ClO4-)是一种有毒的无机阴离子,其环境污染问题引起了广泛关注,控制与修复高氯酸盐污染环境成为新的研究热点,其中微生物修复技术最具应用前景。国外已开展环境中高氯酸盐污染现状的调查工作,利用微生物修复技术去除高氯酸盐取得了一定的成果,但我国的相关研究较少。文章综述了近年来国内外高氯酸盐的污染现状及微生物修复高氯酸盐的最新研究进展。由于高氯酸盐的高水溶性和低吸附性,当前研究最多的是高氯酸盐水体污染,这些水体通过饮用水(源水)或食物链直接或间接地对人类健康造成影响。在微生物修复方面,从有机电子供体,无机电子供体(H2、Fe0等),电子受体(O2、NO3-、SO24-)及微生物的生长环境因素(pH值、温度、氧化还原电位、盐度)等方面总结了各因素对微生物修复技术去除高氯酸盐效果的影响,以期为我国开展微生物处理技术在高氯酸盐污染修复领域的实践应用提供参考。 相似文献
5.
The speciation and mobility of a variety of chemical species in groundwater are strongly influenced by redox and pH conditions. Dissolved O 2 (DO) and dissolved inorganic C (DIC) concentrations are significant controls of these conditions, respectively. It is not always clear what the major processes are that influence changes in the concentration of DO and DIC across a groundwater flowpath. The combined use of the stable isotope compositions of DO ( δ18O–DO) and DIC ( δ13C–DIC) has the potential to help investigators discriminate between sources and sinks of DO and DIC in groundwater systems. 相似文献
6.
We use the evolution of river sediment characteristics and sedimentary C org from the Himalayan range to the delta to study the transport of C org in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and C org content in order to evaluate the controls on C org loading. Contributions of C3 and C4 plants are estimated from C org stable isotopic composition (δ 13C org). We use the evolution of δ 13C org values from the Himalayan range to the delta in order to study the fate of C org during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar C org content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on C org loading. The general correlation between the total C org content and Al/Si ratio indicates that C org loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ 13C org values. In bed sediments, δ 13C org values are compatible with a dominant proportion of fossil C org derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ 13C org values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ 13C org values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ 13C org values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic C org coming from the Himalaya is oxidised and replaced by floodplain C org. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain. 相似文献
7.
As an indicator for terrestrial paleovegetation, the stable isotopic composition of total organic matter (δ 13C org) in loess sediments has been widely used for paleoclimatic reconstruction in western Europe, the Great Plains of North America and the Chinese Loess Plateau (CLP). However, little is known about the variation and paleoclimatic significance of the loess δ 13C org in arid Central Asia (ACA). We report δ 13C org data from an Axike (AXK) loess/paleosol profile from the eastern Ili Basin, eastern Central Asia. Along the profile, the δ 13C org values were more negative in the paleosol layers observed in the field and were confirmed by environmental magnetic proxies and a higher concentration of total organic carbon (TOC), consistent with results for western Europe and the northwestern CLP. Our results demonstrate that the loess δ 13C org in this region documents mainly the response of δ 13C of locally predominant C 3 plants to paleoclimatic variation, especially paleoprecipitation. Our results also suggest that the loess δ 13C org values in the area have the potential for quantitative paleoprecipitation reconstruction on the basis of detailed δ 13C org results from modern plants and surface soils in the future. 相似文献
8.
Attenuation characteristics of landfill leachate were examined for two uncontrolled landfills in Korea. The two landfills containing municipal wastes without appropriate bottom liner and leachate treatment system have different landfill age, waste volume, and most importantly different hydrogeologic settings. One landfill (Cheonan landfill) is situated in an open flat area while the other (Wonju landfill) is located in a valley. Variations of various parameters including dissolved organic carbon (DOC), dissolved oxygen (DO), alkalinity, pH, electrical conductivity (EC), redox potential (ORP), ammonia (NH 3), nitrate (NO 3−), sulfate (SO 42−), and chloride (Cl −) were examined along groundwater flow path. All these parameters were analyzed every month for a year. In the interior of the landfills, typical anaerobic conditions revealed by low DO and NO 3 concentrations, negative ORP values, high NH 3, alkalinity, and Cl − concentrations were observed. Generally, higher levels of contaminants (DOC, NH 3, and Cl −) were detected in the dry season while they were greatly lowered in the wet season. Significantly, large decrease of Cl - concentration in the wet season indicates that the dilution or mixing is one of dominant attenuation mechanisms of leachate. But detailed variation behaviors in the two landfills are different and they were largely dependent on permeability of surface and subsurface layers. The intermediately permeable surface of the landfills receives part of direct rainfall infiltration but most rainwater is lost to fast runoff. The practically impermeable surface of clayey silt (paddy field) at immediately adjacent to the Cheonan landfill boundary prevented direct rainwater infiltration and hence redox condition of the ground waters were largely affected by that of the upper landfill and the less permeable materials beneath the paddy fields prohibited dispersion of the landfill leachate into down gradient area. In the Wonju landfill, there are three different permeability divisions, the landfill region, the sandy open field and the paddy field. Roles of the landfill and paddy regions are very similar to those at the Cheonan. The very permeable sandy field receiving a large amount of rainwater infiltration plays a key role in controlling redox condition of the down gradient area and contaminant migration. This paper reports details of the attenuation and redox conditions of the landfill leachates at the two uncontrolled landfills. 相似文献
9.
The early Aptian abrupt carbon isotope excursion in marine carbonate and sedimentary organic matter reflects a major perturbation in the global carbon cycle. However, until now almost all the evidences of this event came from marine deposits. Here we present organic-carbon isotope (δ 13C org) data from the non-marine Jehol Group in western Liaoning, China. The lacustrine δ 13C org curve is marked by a relative long-lasting δ 13C org minimum followed by two stages of positive δ 13C org excursions that are well correlated with contemporaneous marine records. The carbon isotope correlation shows that the lacustrine strata of the Jehol Group were deposited at the same time of the early Aptian Oceanic Anoxic Event (OAE1a). The relative long-lasting δ 13C org minimum supports the hypothesis that volcanic CO 2 emission may have played the main role in triggering the negative δ 13C excursion and global warming at the onset of this event. In addition, the onset of δ 13C org minimum is concomitant with the radiation of the Jehol Biota, implying that the evolutionary radiation of the Jehol Biota may have been closely related with the increase in atmospheric CO 2 and temperature. 相似文献
10.
Aerobic mineralisation of C org in surface sedimentsof the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailedbox core C org concentration versus depth profiles and the modelling environment for early diageneticproblems MEDIA. The reactive fraction comprises 60–80% of the total C org reachingthe sediments and is largely oxidised within the surficial 10 cm. A non-reactive C orgfraction (G NR) dominates at depths >10 cm, and makes up20–40% of the total C org flux to the sediments. First-order rateconstants for decomposition of the reactive fraction calculated from theC org profiles range from 5.4 × 10 -3 to8.0 × 10 -3 y -1 to 8.0 × 10 -3 y -1. Total mineralization rates in thesurface sediment are between 1.7 and 2.6 mol C cm -2 y -1 and thus are typical for oligotrophic, deep-seaenvironments. The low fluxes and rapid remineralisation of C org are accompanied by 210Pb excess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments.Model results indicate a mismatch between the C org profiles and O 2 microprofileswhich were measured onboard ship. This can be attributed to a combination of decompression artefactsaffecting onboard measurement of the O 2 profiles or the leakage ofoxygen into the core during handling on deck. Furthermore, the used D b values, based on 210Pb, may not befully appropriate; calculations with higher D b values improve the O 2 fits. The surficial sediment 13C org values of -22 become less negative with increasing depth and decreasing C orgconcentrations. The major 13C change occurs in the top 3 to 4 cm and coincides with the interval weremost of the organic carbon oxidation takes place. This indicates that the reactive fractionof organic matter, commonly assumed to be marine, has a more negative 13C orgthan the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from thesediment data by means of literature algorithms yield low surface productivities(12–88 gC m -2 y -1), which are in good agreement with field measurements of primary productivity in otherstudies. Such values are, however, significantly lower than those indicated by recent productivitymaps of the area derived from satellite imagery (>100 gC m -2 y -1). 相似文献
11.
Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr −1 particulate terrestrial organic carbon with initial particle C org loading ∼0.7 mg m −2. C org loading is reduced to ∼0.3 mg m −2 in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1-4 cm yr −1. Carbon isotopic analyses (δ 13C, Δ 14C) of ΣCO 2 and C org demonstrate rapid (<100 yr) remineralization of both terrestrial (δ 13C <−28.6) and marine C org (δ 13C ∼−20.5) ranging in average age from modern (bomb) (Δ 14C ∼60) to ∼1000 yr (Δ 14C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine C org often preferentially dominates pore water ΣCO 2 relative to bulk C org in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO 2 reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of C org from particles in the topset-upper foreset zone (i.e. recycled marine C org leaves little residue). Preferential utilization of C org subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary C org composition can produce changes in 14C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ 14C of pore water ΣCO 2 averaged ∼ 300‰ greater than C org sediment between ∼1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C org compromises simple application of 14C for determination of sediment accumulation rates in diagenetically reactive deposits. 相似文献
12.
Biological carbon pumping (BCP) is a key process in which dissolved inorganic carbon in terrestrial aquatic ecosystems is utilized by aquatic autotrophs for photosynthesis and transformed into autochthonous organic matter (AOC). However, the mechanisms underlying BCP and the amount of generated AOC deposited effectively, are still poorly understood. Therefore, we conducted a systematic study combining modern hydrochemical monitoring and a sediment trap experiment in Fuxian Lake (Yunnan, SW China), the second-deepest plateau, oligotrophic freshwater lake in China. Temperature, pH, EC (electrical conductivity), DO (dissolved O2), [HCO3−], [Ca2+], SIc, partial CO2 (pCO2) pressure, and carbon isotopic compositions of HCO3− (δ13CDIC) in water from Fuxian Lake all displayed distinct seasonal and vertical variations. This was especially apparent in an inverse correlation between pCO2 and DO, indicating that variations of hydrochemistry in the lake water were mainly controlled by the metabolism of the aquatic phototrophs. Furthermore, the lowest C/N ratios and highest δ13Corg were recorded in the trap sediments. Analyses of the C/N ratio demonstrated that the proportions of AOC ranged from 30% to 100% of all OC, indicating that AOC was an important contributor to sedimentary organic matter (OC). It was calculated that the AOC flux in Fuxian Lake was 20.43 t C km−2 in 2017. Therefore, AOC produced by carbonate weathering and aquatic photosynthesis could potentially be a significant carbon sink and may have an important contribution to solving the lack of carbon sinks in the global carbon cycle. 相似文献
13.
During the Ordovician, huge biological revolutions and environmental changes happened in Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, but the cause of these events remains controversial. Herein, we conducted a paired carbon isotopic analysis of carbonate (δ 13C carb) and organic matter (δ 13C org) through the Ordovician in the Qiliao section on the Yangtze platform of South China. Our results showed that the δ 13C carb trend of the Qiliao section can be correlated with local and global curves. The δ 13C org trend seems is less clear than the δ 13C carb trend for stratigraphic correlations, but some δ 13C org positive excursions in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to the global carbon cycle during the Ordovician. Several known δ 13C carb and δ 13C org negative and positive excursions have been recognised in this study, including the early Floian Negative Inorganic Carbon (δ 13C carb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon (δ 13C carb) Excursion, the mid-Darriwilian Inorganic Carbon (δ 13C carb and δ 13C org) Excursion (MDICE), and the early Katian Guttenberg Inorganic Carbon (δ 13C carb and δ 13C org) Excursion (GICE). These positive excursions and a smooth decline trend of δ 13C org values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO 2 and then global cooling. 相似文献
14.
Isotope systematics are well defined for conventional sapropelic, Type I/II kerogens and their associated bacterial and thermogenic natural-gas products. These geochemical tools are used to estimate source type, maturity and depositional environment, and as a correlation technique. In many cases the natural gas signatures in near-surface samples and drill cuttings can be used to classify or predict a deeper lying source rock or reservoir.Corresponding interpretative schemes for coals, Type III kerogens and their associated hydrocarbons are progressing quickly. The shift in attention to humic sources is driven primarily by depletion of conventional oil and gas resources and the economic and societal requirements of coal and coal-bed methane.Carbon, hydrogen and nitrogen stable isotope variations can be large between different coals and humic kerogens. These differences can often be recognized in their bulk δ 13C org, δD org and δ 15N org values. Isotope signatures of coals can be diagnostic of several factors, including deposit age, type, geographic location, maturity and generation history. However, these characteristic isotopic variations are substantially better defined by the C-, H- and N-isotope ratios of the separate maceral groups, such as vitrinite, exinite and inertinite. This new application of stable isotopes, at the maceral and compound levels, have great potential to improve the interpretative precision over conventional whole coal or bulk techniques.Hydrocarbon gases, including coal gases, derived from coals and humic kerogens can be distinguished from Type I/II sources, based on their molecular rations, i.e., C1/( C2 + C3) and by comparing their stable isotope compositions, especially δ 13C CH4 and δD CH4. The δ 13C C2H6 can also be valuable, but ethane is generally present in small amount (<1 vol. %) and requires 相似文献
15.
This study has evaluated the main hydrogeochemical characteristics that affect the quality of shallow groundwater in the coastal area at Kimje in South Korea. In this area, the chemical composition of groundwater is mostly classified into Na +?Cl ? and Ca 2++Mg 2+?NO 3-+Cl ? types. These types are affected by three major factors: seawater intrusion, fertilizers and redox processes. The Na +?Cl ? type, which is generally localized in the coastal area, shows typical characteristics of groundwater affected not only by modern seawater but also by old seawater residing in the reclaimed land. In contrast, the Ca 2++Mg 2+?NO 3-+Cl ? type, which is usually found inland, is very contaminated by fertilizers. As groundwater flows from the inland area to the coast, the redox condition changes from an oxic condition to a suboxic/anoxic condition. In addition, the reclamation has caused a large amount of underlying organic matter. These effects provide optimum conditions for the occurrence of redox processes in the coastal area. 相似文献
16.
测定了新疆罗布泊地区湖相沉积物CK-2钻孔样品的总有机碳含量(TOC)及其同位素组成、碳酸盐含量和C/N比值等环境代用指标,以及石膏矿物的质谱-铀系年龄。测试结果表明,20~9kaB.P.期间沉积物δ 13C org.在-23.4‰~-16.1‰之间波动且阶段性明显,与TOC呈现良好的相关关系,整体变化趋势同南极Dome C冰芯中记录的全球大气CO 2浓度一致;C/N比值表明有机碳来源主要是陆生高等植物。因此大气CO 2浓度变化是影响20~9kaB.P.期间罗布泊湖相沉积物δ 13C org.值变化的主导因素,周围山体上C 3/C 4植物相对生物量的变化则是另一重要因素。依据δ 13C org.的变化序列将此时间段湖区古环境的演化分成6个阶段:20.0~14.1kaB.P.期间受到末次盛冰期的影响,气温偏低,湖水丰沛;14.1~13.3kaB.P.是一个气候不稳定期,冷暖波动较频繁,但以暖为趋势;13.3~12.8kaB.P.期间经历了一段冷期,于12.8kaB.P.结束了末次冰期,随后气候开始转暖至11.8kaB.P.;其后气温再次变冷并维持到10kaB.P.;最后从10kaB.P.进入全新世暖期。δ 13C org.序列明显向偏负方向变化,表明该地区变暖的趋势相当明显。罗布泊地区日益干旱化是全球气候变化的结果,尤其是受到全球CO 2浓度的不断升高所制约。 相似文献
17.
Sediment cores were collected from deep-water areas of Lake Chenghai, China in June 1997. The vertical profile of 137Cs activity gives reliable geochronological results. The results also indicate that sediment accumulation rates in deep-water areas of Lake Chenghai were relatively constant in recent decades, averaging 0.43 g cm − 2 y − 1, despite a variable organic carbon influx. 210Pb eq (= 226Ra) activity was relatively constant also, with an average value of 54.3 ± 3.2 Bq kg − 1. Vertical profiles of 210Pb ex (= 210Pb total − 226Ra) decreased exponentially, resulting in somewhat lower sediment accumulation rates (0.3 g cm − 2 y − 1). These lower rates are likely less reliable, as the relatively large fluctuations in 210Pb ex activities correlate closely to the organic carbon (C org) content of the sediments. For example, the vertical profile of 210Pb ex activity displays peaks at mass depths of 3.7-4.7 g cm − 2 (10-12 cm) and 10-11 g cm − 2(25-28 cm), similar to the maxima in the vertical profile of C org. This phenomenon must be related to the delivery of particulate organic matter (POM) from the water to the sediments, or to watershed soil erosion. Since the mean atomic ratios of H org / C org and C org / N org in Lake Chenghai sediments are 5.5 and 7.0, respectively, indicating that POM was predominantly derived from the remains of authigenic algae, this eliminates watershed erosion rates as a primary control on lake sedimentation rates as resolved by 210Pb ex. Sedimentation fluxes ( F(C org)) of particulate organic carbon since 1970 varied between 60 to 160 g m − 2 y − 1, and appeared to closely influence variations in 210Pb ex concentrations. For example, sedimentation fluxes of 210Pb ex (F( 210Pb ex)) showed maxima in the years 1972-1974 and 1986-1989, likely reflecting historical variations of lake biological productivity or carbon preservation. 相似文献
18.
The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245-0 ka) from the Niger and Congo deep-sea fans, however, reveal that C org/N tot ratios and isotopic signatures of bulk organic matter (δ 13C org) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased C org/N tot and δ 13C org (up to −17‰) were driven by generally nitrogen-poor but 13C-enriched terrigenous plant debris and SOM from C 4/C 3 vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C 3/C 4 vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower C org/N tot and δ 13C org (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and 12C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of C org/N tot and δ 13C org for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO 2. 相似文献
19.
Anthropogenic sources of carbon from landfill or waste leachate can promote reductive dissolution of in situ arsenic (As) and enhance the mobility of As in groundwater. Groundwater from residential-supply wells in a fractured crystalline-rock aquifer adjacent to a Superfund site in Raymond, New Hampshire, USA, showed evidence of locally enhanced As mobilization in relatively reducing (mixed oxic-anoxic to anoxic) conditions as determined by redox classification and other lines of evidence. Redox classification was determined from geochemical indicators based on threshold concentrations of dissolved oxygen (DO), nitrate (NO 3 – ), iron (Fe 2+), manganese (Mn 2+), and sulfate (SO 4 2– ). Redox conditions were evaluated also based on methane (CH 4), excess nitrogen gas (N 2) from denitrification, the oxidation state of dissolved As speciation (As(III) and As(V)), and several stable isotope ratios. Samples from the residential-supply wells primarily exhibit mixed redox conditions, as most have long open boreholes (typically 50–100?m) that receive water from multiple discrete fractures with contrasting groundwater chemistry and redox conditions. The methods employed in this study can be used at other sites to gauge redox conditions and the potential for As mobilization in complex fractured crystalline-rock aquifers where multiple lines of evidence are likely needed to understand As occurrence, mobility, and transport. 相似文献
20.
Groundwater samples were collected along a groundwater flow path in the Carrizo Sand aquifer in south Texas, USA. Field measurements that included pH, specific conductivity, temperature, dissolved oxygen (DO), oxidation–reduction potentials (Eh in mV), alkalinity, iron speciation, and H 2S concentrations were also conducted on site. The geochemistry (i.e., concentrations, shale-normalized patterns, and speciation) of dissolved rare element elements (REEs) in the Carrizo groundwaters are described as a function of distance along a flow path. Eh and other redox indicators (i.e., DO, Fe speciation, H 2S, U, and Re) indicate that redox conditions change along the flow path in the Carrizo Sand aquifer. Within the region of the aquifer proximal to the recharge zone, groundwaters exhibit both highly oxidizing and localized mildly reducing conditions. However, from roughly 10 km to the discharge zone, groundwaters are reducing and exhibit a progressive decrease in redox conditions. Dissolved REE geochemical behavior exhibits regular variations along the groundwater flow path in the Carrizo Sand aquifer. The changes in REE concentrations, shale-normalized patterns, and speciation indicate that REEs are not conservative tracers. With flow down-gradient, redox conditions, pH and solution composite, and adsorption modify groundwater REE concentrations, fractionation patterns, and speciation. 相似文献
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