The evolution of a tropical rainforest/grassland mosaic in southeastern Brazil since 28,000 C yr BP based on carbon isotopes and pollen records

The lack of paleoecological records from the montane Atlantic Rainforest of coastal Brazil, a hotspot of biological diversity, has been a major obstacle to our understanding of the vegetational changes since the last glacial cycle. We present carbon isotope and pollen records to assess the impact of the glaciation on the native vegetation of the Serra do Mar rainforest in São Paulo, Brazil. From ca. 28,000 to  22,000 ¹⁴C yr BP, a subtropical forest with conifer trees is indicative of cool and humid conditions. In agreement carbon isotopic data on soil organic matter suggest the presence of C₃ plants and perhaps C₄ plants from  28,000 to  19,000 ¹⁴C yr BP. The significant increase in the sedimentation rate and algal spores from  19,450 to  19,000 ¹⁴C yr BP indicates increasing humidity, associated to an erosion process between  19,000 and  15,600 ¹⁴C yr BP. From  15,600 ¹⁴C yr BP to present there is a substantial increase in arboreal elements and herbs, indicating more humid and warmer climate. From  19,000 to  1000 ¹⁴C yr BP, δ¹³C values indicated the predominance of C₃ plants. These results are in agreement with studies in speleothems of caves, which suggest humid conditions during the last glacial maximum.     

The dynamics of organic matter in soil size and density fractions traced by stable carbon isotopes

1IntroductionSoilorganicmatter (SOM)isakeysourceofnutrientsforplantgrowth ,itisessentialforthemaintenanceofsoilstructureanditcontributestotheabilityofsoiltoretainnutrientsandwater.AnimprovedunderstandingofSOMdynamicsiscentraltothedevelopmentofmoreenvironmentallysoundandsustainablepracticesofagriculturalmanagement (Collinsetal.,2 0 0 0 ) .Avarietyofcon ceptualmethodshavebeenusedtodescribetheprocessesofSOMaccumulationandturnover (Jen kinsonandRayner,1 977;Duxburyetal.1 989;Partonetal.,1 99…     

The Variation of Soil Organic Matter in a Forest-Cultivation Sequence Traced by Stable Carbon Isotopes

On the basis of different photosynthetic pathways.there is an obvious difference in δ^13C values between C3 and C4 plants,In terms of this characteristic,we analyzed the organic carbon content (forestlands:1.81%-16.00%;farmland:0.45%-2.22%) and δ^13C values(forestlands:-23.86‰--27.12‰;farmland:-19.66‰--23.26‰)of three profile-soil samples either in farmland or in forestland near the Maolan Karst virgin forest,where there were developed plant C3 plants previously and now are C4 plants.Results showed that the deforestation has accelerated the decomposition rate of soil organic matter and reduced the proportion of active components in soil organic matter and thus soil fertility.     

Stable carbon isotopic composition of soil organic matter in the karst areas of Southwest China

This study dealt with the distribution characteristics of soil organic carbon （SOC） and the variation of stable carbon isotopic composition （δ^13C values） with depth in six soil profiles, including two soil types and three vegetation forms in the karst areas of Southwest China. The δ^13C values of plant-dominant species, leaf litter and soils were measured using the sealed-tube high-temperature combustion method. Soil organic carbon contents of the limestone soil profiles are all above 11.4 g/kg, with the highest value of 71.1 g/kg in the surface soil. However, the contents vary between 2.9 g/kg and 46.0 g/kg in three yellow soil profiles. The difference between the maximum and minimum δ^13C values of soil organic matter （SOM） changes from 2.2‰ to 2.9‰ for the three yellow soil profiles. But it changes from 0.8‰ to 1.6‰ for the limestone soil profiles. The contrast research indicated that there existed significant difference in vertical pattems of organic carbon and δ^13C values of SOM between yellow soil and limestone soil. This difference may reflect site-specific factors, such as soil type, vegetation form, soil pH value, and clay content, etc., which control the contents of different organic components comprising SOM and soil carbon turnover rates in the profiles. The vertical variation patterns of stable carbon isotope in SOM have a distinct regional character in the karst areas.     

Variation characteristics of CO2 in a newly-excavated soil profile,Chinese Loess Plateau: Excavation-induced ancient soil organic carbon decomposition

Soils of the Chinese Loess Plateau(CLP)contain substantial amounts of soil inorganic carbon(SIC),as well as recent and ancient soil organic carbon(SOC).With the advent of the Anthropocene,human perturbation,including excavation,has increased soil CO₂ emission from the huge loess carbon pool.This study aims to determine the potential of loess CO₂ emission induced by excavation.Soil CO₂ were continuously monitored for seven years on a newly-excavated profile in the central CLP and the stable C isotope compositions of soil CO₂ and SOC were used to identify their sources.The results showed that the soil CO₂ concentrations ranged from 830μL·L^-1 to 11190μL·L^-1 with an annually reducing trend after excavation,indicating that the human excavation can induce CO₂ production in loess profile.Theδ¹³ C of CO₂ ranged from–21.27‰to–19.22‰(mean:–20.11‰),with positive deviation from top to bottom.The range of δ¹³CSOC was–24.0‰to–21.1‰with an average of–23.1‰.Theδ¹³ C-CO₂ in this study has a positive relationship with the reversed CO₂ concentration,and it is calculated that 80.22%of the soil CO₂ in this profile is from the microbial decomposition of SOC and 19.78%from the degasification during carbonate precipitation.We conclude that the human excavation can significantly enhance the decomposition of the ancient OC in loess during the first two years after perturbation,producing and releasing soil CO₂ to atmosphere.     

Biomarker geochemistry of marine organic matter in the Hushan and Chaohu areas,Lower Yangtze region

Marine strata are widely exposed in the Hushan and Chaohu areas, Lower Yangtze region. As biomarker geochemistry of the strata has not been well documented, this paper deals with the biomarker composition of representative samples collected from the Silurian, Carboniferous and Triassic systems and their geological implications, thus providing clues to marine organic matter. On the basis of experimental results, it is shown that abundant biomarkers (e.g. n-alkanes, isoprenoids, terpanes and steranes) were detected. As organic matter in the strata is highly to over mature in general based on petrologic microobservation, some biomarkers (mainly n-alkanes) except terpanes and steranes cannot reflect the source, depositional environment and maturity of organic matter. Thus, primarily based on analyses of the terpanes and steranes, it is suggested that organic matter in the Silurian and Carboniferous strata is derived mainly from lower organisms, while higher plants are predominant in the Triassic organic matter. This further indicates that the depositional environment may have transformed from the marine to continental facies in the Late Triassic. These results provide new evidence for the study of regional depositional evolution, and have enriched the study of biological composition of organic matter. In addition, the biomarker geochemistry of organic matter at high to over maturation stage is addressed.     

Molecular and bulk isotopic analyses of organic matter in marls of the Mulhouse Basin (Tertiary, Alsace, France)

Contents of 13C in kerogens and carbonates in 21 samples from a core of the MAX borehole, Mulhouse Evaporite Basin, range from -27.3 to -23.5 and -3.7 to -1.8% vs PDB, respectively. Organic nitrogen in the same samples is enriched in 15N relative to atmospheric N2 by 12.2-15.7%. Hydrogen indices and delta values for kerogens vary systematically with facies, averaging 493 mg HC/g Corg and -25.7% in the most saline facies (dominated by inputs from aquatic sources) and 267 mg HC/g Corg and -23.7% in the least saline facies (50/50 aquatic/terrigenous). Values of delta were measured for individual aliphatic hydrocarbons from three samples representing three different organic facies. For all samples, terrigenous inputs were unusually rich in 13C, the estimated delta value for bulk terrigenous debris, apparently derived partly from CAM plants, being -22.5%. In the most saline facies, isotopic evidence indicates the mixing of 13C-depleted products of photosynthetic bacteria with 13C-enriched products of halotolerant eukaryotic algae. At lower salinities, a change in the producer community is marked by a decrease in the 13C content of algal lipids. The content of 13C in algal lipids increases in the least saline facies, due either to succession of different organisms or to decreased concentrations of dissolved CO2.     

37 ka以来日本海沉积物有机质碳和氮稳定同位素变化及其古海洋学意义

海洋沉积物有机质碳氮稳定同位素(δ¹³C、δ¹⁵N)广泛用于有机质来源示踪、古生产力和古海洋环境重建。日本海沉积物δ¹³C和δ¹⁵N值一个显著特征是在末次冰盛期(LGM)同步负偏,但是对这一现象产生的原因以及他们的演化过程的认识仍然存在明显不足。在本研究中,我们详细调查了37 ka以来日本海中部LV53-23-1岩心沉积物δ¹³C和δ¹⁵N演化历史。结果显示,沉积物δ¹³C和δ¹⁵N分别介于-26.3‰至-22.5‰和1.6‰至6.1‰,低值出现在LGM(26.5~17 ka)暗色层状泥发育时期,指示较强的陆源输入贡献。在Heinrich冰阶1时期(17~14.5 ka),δ¹³C和δ¹⁵N快速正偏,表明日本海海洋环境发生了明显的转换,对应于对马海峡淹没及对马暖流入侵。14.5 ka之后,沉积物δ¹⁵N值恢复到5‰,与开阔大洋海水硝酸盐的δ¹⁵N值近似。我们采用二端员混合模型粗略地估算了有机质来源的相对贡献。LGM时期陆源有机质贡献介于65%至80%,14.5 ka以后海源有机质贡献介于60%至80%。除了增加的陆源有机质贡献以外,LGM时期沉积物δ¹⁵N亏损还涉及如下过程:(1)较高的含Fe沙尘供给提高日本海表层海洋生物固氮效率;(2)缺氧环境盛行减弱成岩作用对沉积物δ¹⁵N影响。37 ka以来,日本海沉积物δ¹³C和δ¹⁵N变化与有机质来源、营养盐的供给、表层生产力和沉积物氧化还原条件相关,实际受海平面和全球气候制约。     

大巴山前缘五峰组-龙马溪组干酪根碳同位素特征与有机质类型

大巴山前缘五峰组-龙马溪组有机质类型组成存在较大争议。在万源曹家、城口周溪、巫溪田坝、巫溪白鹿和巴东两河口等地区采集代表性样品进行有机碳含量、成熟度和干酪根碳同位素分析。五峰组-龙马溪组δ¹³C组成分布为-31.1‰~-28.2‰,田坝地区δ¹³C最轻,两河口地区δ¹³C最重。大巴山前缘干酪根类型是碳同位素组成的主要控制因素,有机质成熟度和有机碳含量影响较小。当有机质处于高成熟演化阶段,有机碳含量越高,δ¹³C组成偏轻;有机质处于过成熟演化阶段,有机碳含量越高,δ¹³C组成偏重,变化幅度小于1.2‰。按照干酪根碳同位素组成划分干酪根类型原则,大巴山前缘五峰组-龙马溪组有机质类型为I型(腐泥型)和Ⅱ₁型(腐殖腐泥型)。     

Precise indices based on n-alkane distribution for quantifying sources of sedimentary organic matter in coastal systems

Precise indices based on n-alkane signatures were developed in order to determine the sources and composition of sedimentary organic matter (SOM) in coastal systems. The Arcachon Bay (France), a well-studied temperate lagoon, was used as an example of a complex coastal system sheltering a wide diversity of OM sources. Three main groups of sources were well discriminated from their n-alkane signatures: seagrass (Zostera sp.) produced mainly n-C₁₇, n-C₁₉, n-C₂₁, n-C₂₃ and n-C₂₅ alkanes, algae (Rhodophyta, Chlorophyta) produced n-C₁₅ and n-C₁₇ and the terrigenous input [Quercus sp., Spartina sp. and river suspended particulate OM (SPOM)] was characterized by n-C₂₅, n-C₂₇, n-C₂₉, n-C₃₁ and n-C₃₃. From the above and literature n-alkane fingerprints, we developed a set of indices (n-alkane ratios) to quantify the contribution of these three major sources of the SOM. At the Arcachon Bay scale, they indicated that SOM was composed mainly of seagrass (ca. 53 ± 19%) and terrestrial (ca. 41 ± 17%) material, followed by algae (ca. 6 ± 9%). Moreover, the new n-alkane indices exhibited more relevant spatial patterns than classical ones – the TAR (C₂₇ + C₂₉ + C₃₁/C₁₅ + C₁₇ + C₁₉; terrestrial to aquatic ratio) and the P_aq (C₂₃ + C₂₅/C₂₃ + C₂₅ + C₂₉ + C₃₁; aquatic plant %) – with a greater contribution from marine sources in the central part of the lagoon where a high density of Zostera seagrass was observed. Therefore, the development of precise indices adapted to the local diversity of OM sources is needed when using n-alkanes for quantifying the source composition of SOM in complex coastal systems.     

Local-scale spatial variability of soil organic carbon and its stock in the hilly area of the Loess Plateau, China

Soil organic carbon (SOC) is one of the key components for assessing soil quality. Meanwhile, the changes in the stocks SOC may have large potential impact on global climate. It is increasingly important to estimate the SOC stock precisely and to investigate its variability. In this study, Yangjuangou watershed was selected to investigate the SOC distribution under different land uses. We found that SOC concentration decreased with increasing soil depth under all land uses and was significantly different across the vertical soil profile (P < 0.01). However, considering effect of land use on SOC, it is only significant (P < 0.01) in the topsoil (0-5 cm) layer. This indicated that land use has a large effect on the stocks of SOC in the surface soil. The stratification ratio of SOC > 1.2 may mean that soil quality is improving. The order of the SOC density (0-30 cm) under different land uses is forestland > orchard land > grassland > immature forestland > terraced cropland. The SOC stock is found to be as large as 2.67 × 10³ t (0-30 cm) in this watershed. Considering time effect of restoration, the slope cropland just abandoned is more efficient for SOC accumulation than trees planted in the semi-arid hilly loess area.     

The impact of climate changes during the Holocene on vegetation in northern French Guiana

The impact of climatic changes that occurred during the last glacial maximum and the Holocene on vegetation changes in the Amazon Basin and the Guiana Shield are still widely debated. The aim of our study was to investigate whether major changes in vegetation (i.e. transitions between rainforests and C₄ savannas) occurred in northern French Guiana during the Holocene. We measured variations in the δ¹³C of soil organic matter at eight sites now occupied by forest or savannah. The forest sites were selected to cover two regions (forest refugia and peneplains) which are thought to have experienced different intensities of disturbance during the latest Pleistocene and the Holocene. We found that none of the forest sites underwent major disturbances during the Holocene, i.e. they were not replaced by C₄ savannahs or C₄ forest savannahs for long periods. Our results thus suggest that tropical rainforests in northern French Guiana were resilient to drier climatic conditions during the Holocene. Nevertheless, geographical and vertical variations in the ¹³C of SOM were compatible with minor changes in vegetation, variations in soil processes or in soil physical properties.     

Multi‐proxy constraints on the significance of covariant δ13C values in carbonate and organic carbon during the early Mississippian

This study investigates the covariation between carbonate and organic δ¹³C values in a proximal to distal transect of four outcrops in the Madison Limestone in the Western United States Rockies, combined with δ³⁴S values of carbonate associated sulphate, the concentration of acid‐insoluble material and measurements of total organic carbon. These new geochemical datasets not only allow for an evaluation of carbon isotope covariance during one of the largest perturbations to the global carbon cycle over the past 550 Myr, but also constrain the cause of the excursion in carbonate δ¹³C values. The results support the hypothesis that a period of anoxia did not play a role in generating the positive carbonate δ¹³C values, but rather favour interpretations by previous workers that the proliferation of land plants destabilized the Carboniferous carbon cycle, setting the stage for a significant change in the carbonate δ¹³C values of contemporaneous marine carbonates. These results also demonstrate that one of the largest perturbations to the global carbon cycle did not produce synchronous variations in carbonate and organic δ¹³C values, emphasizing the importance of local depositional controls on carbon isotope covariance in the geological record in both modern and ancient environments.     

The Role of Organic Matter in the Formation of Siderite from Xuanlong Area,Hebei Province

Based on the analysis of siderite distribution,occurrence,chemical compositionk,structureal characteristics,carbon-oxygen isotopic characteristics and relationship between siderite and hematite,this paper presents a systematic study of siderite in the region studied.suggesting that the siderite in the Xuanlong area genetically resulted from organically reduced primary hematite during the diagenesis.The ferric and ferrous relations directly depend on organic contents.In the presence of organic matter ferrous iron can be converted to ferric iron through or ganic reduction.The above conclusion has also been proved by organic geochemistry.data.     

The Role of Organic Matter During Metal Enrichment in Permian Kupferschiefer of the Rudna Mine,Southwest Poland

The bulk composition of organic matter and saturated and aromatic hydrocarbons extracted from 16 samples collected from two Kuperschiefer profiles in the Rudna mine,Southwest Poland has been analyzed to study the role of organic matter during base metal enrichment in the Kupferschiefer shale.The results indicated that the extract yields and saturated hydrocarbon yields decreased with increasing base metal contents.GC and GC/MS analyses indicated that n -alkanes and alkylated aromatic compounds were depleted and may have served as hydrogen donators for thermochemical sulfate reduction.The enrichment of base metal is closely connected with the destruction of hydrocarbons.     

Palynofacies and organic geochemistry studies of organic matter from a wetland system of southern Brazil influenced by different hydrological regimes in the Quaternary

The main goal of this study was to quantitatively and qualitatively characterize the sedimentary organic matter (OM) and demonstrate the usefulness of geochemistry and palynofacies analysis for obtaining paleoenvironmental data for the Holocene in southernmost Brazil. The results indicate that during the time interval from 10,586 cal yr BP to the present, the study area housed a wetland characterized by different hydrologic regimes. The basal peaty deposits correspond to a phase influenced mainly by the groundwater table, whereas the upper deposits composed of silty organic mud indicate fluvial influence related to river overflow events. In a similar manner, the TOC (total organic carbon) and TS (total sulfur) contents are higher in the basal portion of the profile, decreasing toward the top. These findings could be related to granulometry alterations that are linked to hydrologic regimes or anthropogenic interference in the landscape dynamics. Anomalous TS content observed in one of the samples might be due to an external source and perhaps related to the presence of thermal springs in the region. These types of areas have potential as a modern reference that can be applied in the reconstruction of past analogous environments such as coal deposits associated with fluvial paleoenvironments.     

模拟土样有机碳和矿物质对TCE吸附贡献的实验研究

为研究均一介质条件下有机碳含量及矿物质对TCE吸附行为的影响,简化了土壤环境的复杂性和异质性,以固定矿物质(高岭土∶石英砂=3∶7)作为土壤基体,添加不同质量分数的腐殖质(foc=0.16%~2.29%)配制成模拟土样,进行矿物质和模拟土样对TCE的吸附批实验。实验结果显示,TCE的吸附等温线呈非线性。随有机碳含量增加,表现为线性吸附增强,矿物质对TCE的吸附贡献随有机碳含量增加而减小。foc>0.82%时,矿物质对吸附作用的贡献率<5%;foc>1%时,可以基本忽略矿物质对吸附的影响。此外,TCE初始浓度也会影响有机碳和矿物质的吸附能力。用Freundlich模型分别拟合TCE吸附等温线的低浓度段和高浓度段(以Ce=500μg/L为浓度高低的分界),Freundlich指数n值呈现由大到小的趋势;TCE初始浓度越高,有机碳的吸附贡献率相对上升,而矿物质的贡献率则下降。TCE初始浓度在50~500μg/L之间、foc由0.16%增大到1%时,矿物质的吸附贡献率范围由28%~16%缩小到3%~1%之间,此时TCE初始浓度对有机碳和矿物质的吸附贡献率基本没有影响。     

夏季丰水期河流输入对太湖有色可溶性有机物的贡献

结合2008年夏季丰水期对太湖上游直湖港、大浦河、长兜港3条河流有色可溶性有机物(CDOM)吸收和三维荧光的测定分析,探讨了夏季丰水期时河流输入对太湖中CDOM的贡献.3条河流中直湖港CDOM浓度最高,大浦河次之,长兜港最低,反映了太湖北部外源河流污染物输入大于西南部.3条河流内CDOM吸收α(355)的均值为(4.7...