Patterns and implications of plant-soil δC and δN values in African savanna ecosystems

Southern African savannas are mixed plant communities where C₃ trees co-exist with C₄ grasses. Here foliar δ¹⁵N and δ¹³C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ¹⁵N increased as aridity rose for both C₃ and C₄ plants for both seasons, although the magnitude of the increase was different for C₃ and C₄ plants and for two seasons. Soil δ¹⁵N also significantly increased with aridity. Foliar δ¹³C increased with aridity for C₃ plants in the wet season but not in the dry season, whereas in C₄ plants the relationship was more complex and non-linear. The consistently higher foliar δ¹⁵N for C₃ plants suggests that C₄ plants may be a superior competitor for nitrogen. The different foliar δ¹³C relationships with rainfall may indicate that the C₃ plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree-grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.     

Controls of δS and δO in dissolved sulphate: Learning from a detailed survey in the Llobregat River (Spain)

The S and O isotopic composition of dissolved SO₄, used as a tracer for SO₄ sources, was applied to the water of the Llobregat River system (NE Spain). The survey was carried out at 30 sites where surface water was sampled on a monthly basis over a period of 2a. The concentration of dissolved SO₄ varied from 20 to 1575 mg L⁻¹. Sulphur isotopic compositions clustered in two populations: one – 93% of the samples – had positive values with a mode of +9‰; the other had negative values and a mode of −5‰. Data for δ¹⁸O_SO4 showed a mean value of +11‰, with no bi-modal distribution, though lower values of δ¹⁸O corresponded to samples with negative δ³⁴S. These values can not be explained solely by the contribution of bedrock SO₄ sources: that is, sulphide oxidation and the weathering of outcrops of sulphates, though numerous chemical sediments exist in the basin. Even in a river with a high concentration of natural sources of dissolved SO₄, such as the Llobregat River, the δ³⁴S values suggest that dissolved SO₄ is controlled by a complex mix of both natural and anthropogenic sources. The main anthropogenic sources in this basin are fertilizers, sewage, potash mine effluent and power plant emissions. Detailed river water sampling, together with the chemical and isotopic characterisation of the main anthropogenic inputs, allowed determination of the influence of redox processes, as well as identification of the contribution of natural and anthropogenic SO₄ sources and detection of spatial variations and seasonal changes among these sources. For instance, in the Llobregat River the input of fertilisers is well marked seasonally. Minimum values of δ³⁴S are reported during fertilization periods – from January to March – indicating a higher contribution of this source. The dual isotope approach, δ³⁴S and δ¹⁸O, is useful to better constrain the sources of SO₄. Moreover, in small-scale studies, where the inputs are well known and limited, the mixing models can be enhanced and the contribution of the different sources can be quantified to some extent.     

K–Ar dating and δO–δD tracing of illitization within and outside the Shea Creek uranium prospect,Athabasca Basin,Canada

Isotope analyses (K–Ar, δ¹⁸O and δD) were performed on illite from both the sandstone cover and the underlying basement, close to and distant from Shea Creek, an unconformity-type U deposit (Athabasca Basin, Canada); the illite had previously been characterized crystallographically. In the barren areas away from deposit, illite is mainly of the cis-vacant 1M polytype occurring as relatively coarse-grained lath-shaped particles, while it occurs as fine-grained particles of the trans-vacant 1M type next to and in the U mineralized strata. The tectonic-induced hydrothermal system that favored illite crystallization was multi-episodic 1453 ± 2, 1330 ± 20 and probably about 1235 Ma ago. These illite-forming episodes appear to have occurred contemporaneously to those favoring the concentration of the associated U oxides, which were dated independently by the U–Pb method in the Shea Creek deposits and elsewhere in the Athabasca Basin.     

A multi-isotope (δD, δO,Sr/Sr,and δB) approach for identifying saltwater intrusion and resolving groundwater evolution along the Western Caprock Escarpment of the Southern High Plains,New Mexico

Declining water levels in arid and semi-arid regions increase an aquifer’s vulnerability to natural and anthropogenic influences. A multi-isotope (δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ¹¹B) approach was used to resolve the geochemical evolution of groundwater in a declining aquifer in a semi-arid region of the southwestern USA as groundwater composition reacts to source-water mixing, cross-formational flow including saltwater intrusion, water–rock interaction, and likely agricultural recharge. Sub-aquifers or local flow systems are present in the Southern High Plains aquifer along the Western Caprock Escarpment in New Mexico, and the study site’s local flow system contains a Na–Cl, high dissolved-solids groundwater that flows from the escarpment until it mixes with a high quality regional aquifer or regional flow system. The local flow system contains water that is similar in composition to the underlying, upper Dockum Group aquifer. Saltwater found in the upper Dockum Group aquifer likely originates in the adjacent Pecos River Basin and crosses beneath or possibly through the hydrologic divide of the Western Caprock Escarpment. Strontium concentrations of 0.9–31 mg/L and a ⁸⁷Sr/⁸⁶Sr range of 0.70845–0.70906 were sufficient to estimate source-water fractions, mixing patterns, and contributions from chemical weathering through mass balance inverse calculations. Boron concentrations (59–1740 mg/L) and δ¹¹B values (+6.0–+46.0‰) were used to confirm source-water mixing, further evaluate water–rock interaction, and examine the influence of possible agricultural recharge. Alteration of B concentrations and δ¹¹B values in an area of likely agricultural recharge indicated the loss of B and decrease in δ¹¹B values likely from plant uptake, adsorption, and weathering contributions in the soil/vadose zone prior to recharge. The effectiveness of ⁸⁷Sr/⁸⁶Sr and δ¹¹B for resolving the geochemical influences in groundwater in the Southern High Plains along the Western Caprock Escarpment allowed for the reinterpretation of the isotopic composition of water that has been shown to be highly variable in the Southern High Plains. This study shows the utility of a multi-isotope approach for resolving the geochemical evolution of groundwater in an aquifer that has a complex relationship with underlying aquifers and the applicability of these isotopes as indicators of the alteration of source waters from natural or anthropogenic influences.     

Stable isotope records (δO and δC) of Lower-Middle Jurassic belemnites from the Western Balkan mountains (Bulgaria): Palaeoenvironmental application

Stable O and C isotope data of 110 Upper Pliensbachian-Lower Bajocian belemnites have been obtained and used to attempt a reconstruction of palaeotemperature and its variation in two epicontinental depositional environments from the Western Balkan mountains (Bulgaria). The samples were collected from 3 sections with high-resolution ammonite subdivision. Initially taphonomic, cathodoluminescence and geochemical analyses were used for evidence of diagenetic alteration. Non-luminescent parts of the belemnite rostra have been sampled for isotope analyses and 76 samples, having δ¹⁸O < −4‰ (PDB), δ¹³C > −0.5‰ (PDB), Fe < 250 ppm, Mn < 50 ppm, Sr > 950 ppm and Sr/Mn ratio > 80 were used for palaeotemperature interpretations. The O and C isotope data generally exhibit little stratigraphical variability with minor fluctuations, however, there are prominent positive C isotope excursions and coeval negative O isotope shifts detected in the Lower Toarcian Tenuicostatum, Falciferum and Bifrons Zones. The O isotope data, interpreted in terms of palaeotemperature, revealed relatively high seawater temperatures during the Toarcian, Aalenian and Early Bajocian, with detectable temperature rises during the Early Toarcian (maximum value of 29.6 °C). Both C isotope maxima and O isotope minima are used as evidence of the Early Toarcian anoxic event reported from many localities of the same age and in similar facies in Western Europe. In the study the latter is recognized as 3 episodes, which are closely related with the seawater temperature maxima. This isotope record pattern is considered as a consequence of a global Tethyan transgression during the Early Toarcian.     

低δ18O岩浆岩的成因

具有显著低于正常幔源岩浆δ¹⁸O值的岩浆岩是地球上比较罕见的一种岩石,其形成需要特殊的岩浆过程。虽然岩浆分异过程基本上不改变氧同位素组成,但水岩反应可以明显改变岩石的氧同位素组成,高温水岩反应会导致岩石的氧同位素组成显著降低,这得到了理论计算的支持。对黄石高原流纹岩和冰岛玄武岩这两个典型的低δ¹⁸O岩浆实例的长期研究发现, 在地表遭受了高温热液蚀变的低δ¹⁸O岩石部分熔融或者被岩浆同化是形成低δ¹⁸O岩浆的重要机制,这通常通过裂谷构造带破火山口的垮塌来实现。塞舌尔和碾子山代表了结晶于低δ¹⁸O岩浆的花岗岩实例,其形成需要源区有亏损¹⁸O的地壳物质参与。大别-苏鲁造山带超高压变质岩亏损¹⁸O的幅度和这些亏损¹⁸O岩石的分布面积都非常令人瞩目。对这些超高压变质岩的原岩进行氧同位素研究,能够为其这些低δ¹⁸O岩石的成因提供有力的制约。目前已经在北淮阳花岗岩中报道了具有新元古代年龄和低δ¹⁸O 值的锆石。研究显示,这些花岗岩可能经历了两次高温水岩反应,第一次导致了低¹⁸O岩浆的形成和低δ¹⁸O值锆石的结晶,第二次则进一步降低了其它矿物的δ¹⁸O值,导致了氧同位素不平衡。对这些低δ¹⁸O花岗岩的深入研究,特别是进行原位微区分析,不仅有助于我们深入认识低δ¹⁸O岩浆的形成机理,而且有助于进一步理解水岩反应中元素和同位素的地球化学行为。     

Micro-Fourier Transform Infrared (FT-IR) and δD value investigation of hydrothermal vein quartz: Interpretation of fluid inclusion δD values in hydrothermal systems

Some recent studies have suggested that the hydrogen isotopic composition (δD) of hydrothermal fluids, released in vacuo by thermal decrepitation of quartz, are not always accurately revealed. We report the results of a step-heating δD value study of vein quartz, hosted by Lower Palaeozoic rocks in SW England, which was analyzed by micro-FT-IR for hydrogen speciation, before and after fluid extraction at temperatures between 750 and 1500 °C. The δD values of individual aliquots of released water vary between −3‰ and −208‰, with the lowest values generally corresponding to the highest temperature fractions and samples of relatively low yield. The data show significant departures from geologically reasonable δD. Micro-FT-IR analyses show that a variety of OH species are present within the vein quartz, with significant intra and inter sample variation. Typically a broad absorption due to molecular water, in the region 3400 cm⁻¹ is observed, along with bands attributed to Li-OH and Al-OH. On heating, the broad absorption due to molecular water is reduced, accompanied by a measurable loss of Li-OH species. The latter becomes more pronounced in the higher temperature fractions (>750 °C). These data support earlier studies which indicated that contributions from the contrasting OH reservoirs in quartz can significantly influence the reported δD values. These new data also suggest that the incorporation of OH released from Li-OH sites in the quartz may be the most important factor in the generation of the anomalous values for these samples.     

Effect of plant life form on relationship between δD values of leaf wax n-alkanes and altitude along Mount Taibai, China

We present δD_wax values from different forms of plants and soils, and δD_sw values from soil water along the northern slope of Mount Taibai, China. The results show a highly negative linear correlation of the δD_wax values for soils with altitude (R² 0.74) and we observed the same correlation for δD_sw values of soil water with altitude (R² 0.68). The δD_wax of living plants behaves like the soil, but does not exhibit a significant linear correlation with altitude (R² 0.11). The δD_wax values of woody plants and grasses also show a similar trend with respect to altitude with significant and no linear correlation, respectively (R² 0.50 for woody plants and 0.17 for grass), which suggest that the “altitude effect” can not be well documented for the δD_wax values of living plants, which may be due to differences in plant type and/or evapotranspiration controlled by the plant microclimate. The ε_wax-sw values of woody plants, grasses and soil show minor fluctuations with altitude. However, the ε_wax-sw and δD_wax values of woody plants are roughly 51‰ and 50‰ more positive, respectively, than those of grasses, suggesting that an “altitude effect” could be documented in the δD_wax of woody plants and grasses, with each responding independently to changes in precipitation along the altitude transect. Additionally, the ε_wax-sw values of soil are relatively constant with altitude, suggesting that the altitudinal change in the proportions between woody plant and grass input to soils will likely change the relationship between the δD_wax values of soil n-alkanes and altitude.     

Ferromanganese nodules of freshwater reservoirs of Ol’khon Island (Baikal) and the Kulunda Plain (West Siberia)

Morphology and mineralogical and geochemical compositions of freshwater ferromanganese nodules of Ol’khon Island (Sasa Formation) and Kulunda Plain (Lake Porozhnee) were studied. The study has shown rhythmic structures of the nodules, formed by macro- and microlayers with mineralized microflora. The layers are composed of either crystalline Mn mineral phases and finely dispersed Fe phases (Lake Porozhnee) or, on the contrary, crystalline goethite and X-ray amorphous Mn phases (Ol’khon Island). Separation of Mn and Fe mineral phases in the nodules proceeded during their formation and diagenesis. The freshwater nodules show both high (Lake Porozhnee) and low (Ol’khon Island) Mn/Fe ratios. The predominance of Fe phase in the Ol’khon nodules accounts for their high contents of REE, including Ce. The Porozhnee nodules grew, most likely, more rapidly, which is reflected in their low REE contents and Ce anomaly. The examined chemical and mineral compositions, textures, and structures of the nodules testify to the low-temperature hydrothermal source of their ore substance.     

Compound- and enzyme-specific phosphodiester hydrolysis mechanisms revealed by δO of dissolved inorganic phosphate: Implications for marine P cycling

We have studied the oxygen isotope signature of inorganic phosphate (P_i) generated by hydrolysis of nucleic acid phosphodiester (P-diester) compounds by cell-free enzymes (Deoxyribonuclease 1, Phosphodiesterase 1, Alkaline phosphatase) and microbial cultures at natural isotopic abundances. We demonstrate that the diesterase-catalyzed hydrolytic step leads to incorporation of at least one water O into released P_i for a total of two O atoms from water incorporated into P_i released from P-diesters. In the presence of Phosphodiesterase 1, ¹⁶O is preferentially incorporated into nucleotides released from DNA; whereas ¹⁸O is preferentially incorporated into nucleotides released from RNA. A strong consistency between predicted O-isotope regeneration signatures based on results of cell-free enzyme experiments and measured isotopic signatures from independent experiments with E. coli cultures was observed and confirms proposed models for phosphoester hydrolysis. Results from these studies made at natural ¹⁸O abundance levels provide a new tool, enzyme-specific O-isotope fractionation, for investigations of organophosphate metabolism and phosphorus cycling pathways in natural aquatic systems.     

Passage of Tropical Storm Allison (2001) over southeast Texas recorded in δO values of Ostracoda

Freshwater Ostracoda collected in ephemeral pond-waters derived from Tropical Storm Allison (2001, Texas) recorded the unusually low oxygen-isotope values of that storm. Therefore, the potential clearly exists, in locations where tropical cyclones make landfall, to obtain a long-term record of tropical cyclone activity from fossil ostracode calcite.     

Effect of precipitation regime on δD values of soil n-alkanes from elevation gradients - Implications for the study of paleo-elevation

We measured δD values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in δD value with elevation in the wetter region greatly exceeded that in drier region by, −1.9 ± 0.1‰/100 m and −1.4 ± 1.0‰/100 m respectively. The apparent fractionation between leaf wax and precipitation ε_wax-p values in the wetter region (ca. −164‰) were more negative than those in drier region (ca. −125‰ above 3200 m).We also measured δD values in leaves of six common living trees (values from −287‰ to −193‰) from Gongga Shan, ranging from about 2900-4200 m. The abundance-weighted average values of the n-alkanes (δD_wax) show a strong reverse correlation with sample source elevation (R² 0.78 for soils from Gongga Shan; R² 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane δD_wax faithfully records the precipitation δD and that the isotopic altitude effect of precipitation controls δD_wax altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane δD values on elevation for the eastern Plateau, but a complex relationship between n-alkane δD values and elevation for the northwestern Plateau. The δD_wax values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment δD_wax values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of δD values with elevation.     

Calcium isotope constraints on the uptake and sources of Ca in a base-poor forest: A new concept of combining stable (δCa) and radiogenic (εCa) signals

Plant-available reserves of major base cations, Ca²⁺ and Mg²⁺, decreased markedly in soils over the past century, thus posing a potential threat to forest ecosystem health. Trees are thought to obtain dissolved Ca²⁺ ions mainly from an easily accessible soil-water reservoir also termed the ‘exchangeable cation pool’. The status of Ca reserves in this soil pool is sensitive to anthropogenic perturbations such as soil acidification induced by acid rain and/or excessive timber harvesting. Here we show that in a base-poor forest of the northeastern USA (i.e. Wachusett Mountain, Massachusetts) the ‘exchangeable Ca pool’ of deeper mineral soils has a unique isotope signature that is significantly enriched in the radiogenic ⁴⁰Ca, due to the dissolution of K-rich silicate minerals such as biotite. Using a simple isotope mass balance, and assuming that the input of Ca from biotite has a ε_Ca signature of ∼16, the results of our calculation indicate that the weathering of biotite may supply a sizeable fraction, up to 25%, of Ca²⁺ ions into the ‘exchangeable cation pool’ of deeper mineral soils. Importantly, samples of local vegetation (i.e. woody tissues of red oak) show no detectable excess of the radiogenic ⁴⁰Ca, and based on our model the upper limit of a possible biotite-derived Ca contribution in vegetation is estimated at ∼5%. We also found no evidence of the radiogenic ⁴⁰Ca signal in the samples of forest floor and the uppermost organic-rich soils (0-15 cm depth), which in turn suggest that over the long-term development of the forest and its organic matter accumulation, the vegetation growth must have also relied primarily on the non-radiogenic Ca sources. Based on our experimental data, such sources may include (i) wet atmospheric deposition, (ii) the organically-complexed Ca in topsoil horizons, and (iii) chemical weathering and/or fungal-mediated dissolution of apatite and Ca-rich plagioclase. Hence, our stable and radiogenic Ca isotope data indicate that the studied base-poor forest is able to bypass the ‘exchangeable cation pool’ of deeper (i.e. below 15 cm) mineral soils, and still manages to meet its nutritional requirements with respect to Ca. Another important implication of this study is that the organically-complexed Ca in the topsoil horizon (0-15 cm depth) has to be tightly bound to the ion exchange sites, otherwise the large radiogenic ⁴⁰Ca signatures present in the ‘exchangeable cation pool’ of deep mineral soils would be swamped by the downward gravitational flux of non-radiogenic Ca from the decaying organic matter and litterfall. Hence, the limited mobility of the organically-complexed Ca in soils and its tight biological cycling could explain the lack of a significant impact of vegetation on the Ca isotope systematics observed in large rivers.     

δ31Corg perturbations preserved by the interglacial Datangpo Formation in South China with implications for stratigraphic correlation and carbon cycle

Palaeoclimatic and palaeoenvironmental reconstructions of the Cryogenian Period have attracted attention in relation to the debated “Snowball Earth ” hypothesis and the early evolution of metazoan life. The carbon cycle and redox conditions of the Sturtian-Marinoan non-glacial interval have been subjected to much controversy in the past decades because of the lack of a high-resolution stratigraphic correlation scheme. As one of the typical Sturtian-Marinoan interglacial deposits, the Datangpo Fo...     

Middle Holocene Bison diet and mobility in the eastern Great Plains (USA) based on δC, δO, and Sr/Sr analyses of tooth enamel carbonate

During the Holocene, bison (Bison bison) were key components of the Great Plains landscape. This study utilizes serial stable isotope analyses (tooth enamel carbonate) of 29 individuals from five middle Holocene (∼ 7–8.5 ka) archaeological sites to address seasonal variability in movement patterns and grazing behavior of bison populations in the eastern Great Plains. Stable carbon isotopes (δ¹³C) indicate a bison diet that is similar to the C3/C4 composition of modern tallgrass prairies, while ⁸⁷Sr/⁸⁶Sr values generally indicate very little seasonal movement (< 50 km) and relatively limited inter-annual movement (< 500 km) over the course of 4–5 yr. Analyses of variability in serial stable oxygen isotope samples (δ¹⁸O) further substantiate a model of localized bison herds that adhered to upland areas of the eastern Plains and prairie–forest border.     

Influence of oxygen fugacity on the solubility of nitrogen, carbon, and hydrogen in FeO-Na2O-SiO2-Al2O3 melts in equilibrium with metallic iron at 1.5 GPa and 1400°C

It is assumed in the theories of Earth formation that the composition of gases extracted by primary planetary magmas is formed by the large-scale melting of the early mantle, which occurred in the presence of a metallic Fe phase. The molten Fe metal and silicate materials underwent gravitational migration, which affected the fractionation of siderophile elements. Volatile compounds had to form simultaneously in the zones of large-scale melting of the early Earth; their compositions were controlled by interaction with silicate and metallic melts. This process remains poorly understood.     

The application of δ13C and C/N ratios as indicators of organic carbon sources and paleoenvironmental change of the mangrove ecosystem from Ba Lat Estuary, Red River, Vietnam

In this study, two sediment cores (~70 cm) were collected from separate mangrove forests straddling the Ba Lat Estuary, Red River of northern Vietnam, to examine the origins of sedimentary organic carbon (SOC) and reconstruct the paleoenvironment. In addition, mangrove leaves and particulate organic matter were collected and measured for δ¹³C to trace the origins of SOC. The cores were analyzed by high-resolution sections for δ¹³C, TOC, C/N ratios, sediment grain size, water content, and porosity, with values of δ¹³C, TOC, and C/N ratios ranging from −28.19 to −22.5‰, 2.14–30.94 mg/g, and 10.29–18.32, respectively. The δ¹³C and TOC relationship indicated that there were some small residual effects of diagenetic processes on TOC and δ¹³C values in mangrove sediments. However, the shifts of δ¹³C and C/N ratios from the bottom to the surface sediment of the cores explained the change in organic matter sources, with values of C/N > 12 and δ¹³C < −25‰, and C/N < 12 and δ¹³C > −25‰ indicated terrestrial (e.g., mangrove litter) and marine phytoplankton sources, respectively. The covarying δ¹³C, C/N ratios, and sediment grain sizes during the past 100 years in sediment cores showed that the paleoenvironment may be reconstructed into three environments (subtidal, tidal flat, and intertidal mangrove). General trends in δ¹³C and C/N followed a gradual increase in the C/N ratio and a concomitant decrease in δ¹³C from the subtidal, through to tidal flat, and to the intertidal mangrove. δ¹³C and C/N ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem.     

Organic carbon isotope ratios (δ13C) of Arctic Amerasian Continental shelf sediments

Organic matter origins are inferred from carbon isotope ratios ('¹³C) in recent continental shelf sediments and major rivers from 465 locations from the north Bering-Chukchi-East Siberian-Beaufort Sea, Arctic Amerasia. Generally, there is a cross-shelf increase in '¹³C, which is due to progressive increased contribution seaward of marine-derived organic carbon to surface sediments. This conclusion is supported by the correlations between sediment '¹³C, OC/N, and '¹⁵N. The sources of total organic carbon (TOC) to the Amerasian margin sediments are primarily from marine water-column phytoplankton and terrigenous C₃ plants constituted of tundra taiga and angiosperms. In contrast to more temperate regions, the source of TOC from terrigenous C₄ and CAM plants to the study area is probably insignificant because these plants do not exist in the northern high latitudes. The input of carbon to the northern Alaskan shelf sediments from nearshore kelp community (Laminaria solidungula) is generally insignificant as indicated by the absence of high sediment '¹³C values (-16.5 to -13.6‰) which are typical of the macrophytes. Our study suggests that the isotopic composition of sediment TOC has potential application in reconstructing temporal changes in delivery and accumulation of organic matter resulting from glacial-interglacial changes in sea level and environments. Furthermore, recycling and advection of the extensive deposits of terrestrially derived organic matter from land, or the wide Amerasian margin, could be a mechanism for elevating total CO₂ and pCO₂ in the Arctic Basin halocline.     

桂林地区不同类型岩溶地下水中δ13CDIC、δ18O的特征及意义

对桂林地区地下河水、岩溶大泉中的δ13CDIC、δ18O及Ca2+、Sr2+含量进行了测试分析。结果表明,地下河水的δ13CDIC值范围为–15.99‰～–12.29‰,平均值为(–14.03±1.15)‰;δ18O值范围为–6.63‰～–5.78‰,平均值为(–6.24±0.24)‰。岩溶大泉的δ13CDIC值范围为–15.26‰～–9.22‰,平均值为(–12.05±1.57)‰;δ18O值范围为–6.97‰～–3.19‰,平均值为(–5.68±0.97)‰。岩溶大泉的δ13CDIC值、δ18O值比地下河水的分别偏重1.98‰和0.56‰。通过分析发现,水的循环方式引起了不同类型地下水的同位素差异,地下河以管道流的形式进行循环,循环速度快,水岩作用时间短,碳酸盐岩碳的贡献相对较少;同时,出口处水所经历的蒸发作用时间也短,水的δ18O值偏轻。岩溶大泉以裂隙流的形式进行循环,循环速度慢,水岩作用时间长,碳酸盐岩碳的贡献相对较多;泉口处水所经历的蒸发作用时间也长,水的δ18O值偏重。Sr/Ca值与δ13CDIC值具有正相关关系,而与δ18O值的相关性差。这意味着δ13CDIC值与Sr/Ca值一样,可以在一定程度上反映出地下水的径流条件。     

Adsorption and stabilization of organic carbon by mineral surfaces in soils

Soils are the largest carbon reservoir in the terrestrial system. Soils contain about three times more carbon than vegetation and twice as much as that present in the atmosphere. Soil organic matter （SOM） is very complex in composition and structure, formed of heterogeneous substances and generally associated with minerals in soils. SOM is classified as labile and stable fractions on the basis of residence time, determined not only by the chemical composition of SOM, but also by types of protection or bonds within soils. The stable carbon fraction is protected either physically or chemically. To understand the process of SOM stabilization, physicochemical properties of organic-mineral complexes were determined by Fourier transformed infrared （FTIR） with attenuated total reflectance （ATR） and diffuse reflectance （DRIFT）, atomic force microscopy （AFM）, and nuclear magnetic spectroscopy （NMR）. Humic acids and carboxylic acids with relatively short carbon chains were used as sorbates, and goethite, kaolinite, and montmorillonite as adsorbents. Humic acid was fractionated during adsorption on the minerals, which was highly influenced by the characteristics of minerals. For instance, long-chain aliphatic carbon was likely to be adsorbed onto the surface of kaolinite and montmorillonite, while goethite surface attracted carboxylic functional groups of humic acid.