Late Ordovician (Turinian–Chatfieldian) carbon isotope excursions and their stratigraphic and paleoceanographic significance

Five positive carbon isotope excursions are reported from Platteville–Decorah strata in the Upper Mississippi Valley. All occur in subtidal carbonate strata, and are recognized in the Mifflin, Grand Detour, Quimbys Mill, Spechts Ferry, and Guttenberg intervals. The positive carbon isotope excursions are developed in a Platteville–Decorah succession in which background δ¹³C values increase upward from about −2‰ at the base to about 0‰ Vienna Pee Dee belemnite (VPDB) at the top. A regional north–south δ¹³C gradient, with lighter values to the north and heavier values to the south is also noted. Peak excursion δ¹³C values of up to +2.75 are reported from the Quimbys Mill excursion, and up to +2.6 from the Guttenberg excursion, although there are considerable local changes in the magnitudes of these events. The Quimbys Mill, Spechts Ferry, and Guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities, and completely starve out in deeper, more offshore areas. Closely spaced chemostratigraphic profiles of these sculpted, pyrite-impregnated hardground surfaces show that they are associated with very abrupt centimeter-scale negative δ¹³C shifts of up to several per mil, possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events.     

Magnesian calcite and chamositic ooids forming shoals peripheral to Late Ordovician (Ashgill) muddy siliciclastic shores: southern Ontario

In southern Ontario, ooids are associated with two distinct facies associations in the Queenston Formation, the final stage of Late Ordovician (Ashgill) Taconic basin fill. One facies consists of thin ooid and bioclastic grainstones interbedded with mudrock, and lies near the base of the formation, and, in southwestern Ontario, also forms a local NW-thickening wedge near the middle of the formation. Ooids have radial-fibrous and radial-concentric fabrics (Type A), with chamosite, illite, and Fe-oxide laths at intercrystalline sites. Vertical lithologic and ooid abundance patterns indicate that thresholds to carbonate production were sensitive to changes in terrigenous sediment supply, sea level, circulation, accommodation space, and tectonism.

Ooids in the second facies association are admixed with abraded fragments of open-marine biota, or occur burrow fills, within a <30-cm-thick interval of mudrock near the top of the preserved Queenston succession, a few metres below the Ordovician–Silurian unconformity. Ooids have radial concentric and crosscutting patchy microcrystalline fabrics (Type B). This unit may represent a transgressive or stillstand deposit modified by bioturbation.

The extent of preserved fabric suggests that both ooid types were originally magnesian calcite, but Type A ooids underwent greater burial alteration. This is shown by crystalline mosaics that cross-cut relict primary fabrics; δ¹³C values (−1.82‰ to +0.67‰) and δ¹⁸O values (−4.46‰ to −10.57‰) more negative than marine calcite of similar age; Mn and Fe concentrations more elevated above expected marine values; and a luminescence similar to that of intergranular cements. Burial meteoric diagenesis was likely promoted by excellent permeability of the host sand. We interpret authigenic chamosite and Fe-oxide to reflect diagenesis of iron-bearing and clay detritus trapped during ooid growth. Type B ooids suffered less alteration: δ¹³C (+1.1‰ to +6.64‰) and δ¹⁸O (−3.04‰ to −4.81‰) values overlap the expected marine range, including ¹³C enrichment that occurs within the Hirnantian (latest Ordovician) excursion. Although Mn and Fe values are still higher than those of modern calcitic ooids, negligible luminescence suggests that recrystallization occurred in the presence of marine-derived pore fluids. Further burial alteration was inhibited due to low permeability of the host mud.

Type A ooid facies in the Queenston Formation forms an ancient analogue for lesser known Quaternary ooid shoals peripheral to tropical deltaic systems. The facies of Type B ooids, while more enigmatic, may preserve a geochemical herald of latest Ordovician climate change. The presence of minor chamosite in Type A ooids defines a possible distal facies of the well-known oolitic ironstones of similar age in the mid-continental USA.     

The Middle Caradoc Facies and Faunal Turnover in the Late Ordovician Baltoscandian palaeobasin

A Late Ordovician episode of remarkable biotic, climatic, sea level and facies changes, named here as the Middle Caradoc Facies and Faunal Turnover, took place in the Baltoscandian area. This paper presents an integrated overview of these changes in the critical middle Caradoc interval. Data are given on carbonate rock composition, distribution and grain-size composition of the siliciclastic material and the carbon isotopic composition of whole-rock carbonates in cores of Estonia and Sweden.

The Middle Caradoc Facies and Faunal Turnover can be described as a succession of related environmental changes. The turnover began with a positive excursion in carbonate δ¹³C and continued with sea level changes that led to a sedimentary hiatus on the shelf and a change from carbonate-dominated to siliciclastic-dominated sedimentation in the basin. The turnover ended with an extinction event and associated microfaunal crisis.

The middle Caradoc turnover in Baltoscandia is comparable to a similar succession of changes in North America. The turnover affected two palaeocontinents, and reflects a widespread, possibly global environmental change. Onset of glaciation on Gondwana and/or increased orogenic activity might have initiated the changes in ocean circulation and led to the initial carbon isotope excursion. The following sea level rise and faunal changes affected several different continents.     

Stable isotopic variations in modern herbivore tooth enamel, plants and water on the Tibetan Plateau: Implications for paleoclimate and paleoelevation reconstructions

Stable isotope analyses of fossil teeth and other authigenic minerals have been used to reconstruct the paleoenvironment and paleoelevation of the Tibetan Plateau. The accuracy of such reconstructions is limited by the lack of a comprehensive modern comparative database from the region. We analyzed the carbon and oxygen isotopic compositions (δ¹³C and δ¹⁸O values) of tooth enamel from modern herbivores, the δ¹³C values of grasses and the δ¹⁸O values of water samples collected from various elevations within the Tibetan Plateau to examine their relationships with modern environment/elevation. The δ¹³C values of enamel samples from horses, yaks and goats display a narrow range of variation, with a mean of − 10.7 ± 1.4‰ (n = 301), indicating that these modern herbivores were feeding predominantly on C3 plants, consistent with the current dominance of C3 vegetation in the region. Some of the samples have δ¹³C values between − 7.3 and − 10‰. Although these higher δ¹³C values could suggest consumption of some C4 plants by the animals, the lack of significant seasonal δ¹³C variations within individual teeth indicates that these higher enamel δ¹³C values are due to consumption of C3 plants experiencing water stress and/or some CAM plants rather than C4 plants. Our data show that the conservative “cut-off” δ¹³C value for a pure C3 diet within the Tibetan Plateau should be − 8‰ for modern herbivores and − 7‰ (or even − 6.5‰) for fossils if the region was as arid in the past as today. In contrast to the small intra-tooth δ¹³C variations within individual teeth, serial enamel samples display large intra-tooth δ¹⁸O variations, reflecting seasonal variations in the δ¹⁸O of meteoric water. The mean δ¹⁸O values of tooth enamel from yaks and horses show a strong correlation with water δ¹⁸O values, confirming that the δ¹⁸O of tooth enamel from obligate drinker generally tracks the δ¹⁸O of meteoric water. Unfortunately, elevation alone cannot explain most of the variance in the δ¹⁸O of precipitation and tooth enamel, suggesting that quantitative reconstruction of the paleoelevation of the Tibetan Plateau using re-constructed δ¹⁸O values of paleo-meteoric water from fossil enamel or other oxygen-bearing minerals is not warranted. For a given environment, horses have the lowest enamel–δ¹⁸O values while goats display the highest enamel–δ¹⁸O values among the species studied. The large inter-species δ¹⁸O variations are likely due to differences in physiology and diet/drinking behavior of the animals. This underscores the importance of species-specific studies when interpreting δ¹⁸O data of fossil mammalian teeth in a stratigraphic sequence as a record of paleoclimate changes.     

Timing and magnitude of early Aptian extreme warming: Unraveling primary δO variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean

In order to elucidate early Aptian marine paleotemperature evolution across the period of enhanced organic carbon (C_org)-burial [Oceanic Anoxic Event (OAE) 1a], stable isotope analyses were performed on pelagic limestones at Deep Sea Drilling Project Site 463, central Pacific Ocean. The δ¹⁸O data exhibit a distinct anomaly by ~ − 2‰ spanning the OAE 1a interval (i.e., a ~ 6 m-thick, phytoplanktonic C_org-rich unit constrained by magneto-, bio- and δ¹³C stratigraphy). Elucidation of paleotemperature significance of the δ¹⁸O shift is made by taking account of recent Sr/Ca evidence at the same section, which revealed that geochemical signals in carbonate-poor lithologies are relatively unaltered against burial diagenesis. By discriminating δ¹⁸O values from carbonate-poor samples (CaCO₃ contents = 5–30 wt.%), it appears that an abrupt rise in sea-surface temperatures (SSTs) by 8 °C (= − 1.7‰ shift in δ¹⁸O) occurred immediately before OAE 1a, whereas a cooling mode likely prevailed during the peak C_org-burial. In terms of its stratigraphic relationship as to the C_org-rich interval and to a pronounced negative δ¹³C excursion, as well as its timescale, the observed SST rise resembles those associated with the Paleocene–Eocene thermal maximum and, more strikingly, Jurassic Toarcian OAE. This observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism, which was likely initiated by the greenhouse effect via massive release of CH₄ or CO₂ from the isotopically-light carbon reservoir and terminated by a negative productivity feedback.     

Evidence that Early Carboniferous ostracods colonised coastal flood plain brackish water environments

A study of the stable isotope composition (δ¹⁸O, δ¹³C) of biogenic (ostracod, mollusc) and authigenic carbonates in the Ballagan Formation, Lower Carboniferous of Scotland, coupled with evidence from sedimentology and associated fossil fauna and flora, supports the argument that this formation was deposited in a coastal flood plain setting, in brackish (0.5 < 30‰ NaCl) and hypersaline (> 40‰ NaCl) waters, but in the absence of persistent normal marine conditions. The oxygen isotope data from the Ballagan Formation divide into three clusters: a diagenetic field defined by low δ¹⁸O (< − 11‰ VPDB); an intermediary field (δ¹⁸O − 11‰ to − 9‰) composed of a mixture of known primary and secondary (diagenetic) carbonates; and samples within the range of − 9‰ to − 4‰ which, as far as we can ascertain, are largely unaltered. No samples give typical Early Carboniferous δ¹⁸O marine values. Average marine carbonates from Europe have δ¹⁸O between − 4‰ to − 3‰. The Ballagan Formation carbonates were probably deposited in evaporated freshwater and/or brackish water. This conclusion is supported by the presence of evaporites (gypsum, anhydrite, halite pseudomorphs) and common desiccation-cracked mudstone surfaces throughout the Ballagan Formation, suggesting conditions of fluctuating salinity in ephemeral bodies of water. The stable isotope data support the notion that the ostracod assemblages of the Ballagan Formation were colonising brackish water and hypersaline ecologies on a coastal flood plain during the Early Carboniferous, a stage of development that may have encouraged their colonisation of fully non-marine (limnetic) environments during the later Carboniferous. The ostracods include cytherellacean and kloedenellacean species known from marginal marine sites elsewhere, but probably tolerant of brackish water, podocopid species such as ‘Bythocypris’ aequalis that may have been adapted for brackish water settings on coastal flood plains (ephemeral lakes and lagoons), and paraparchitacean-dominated assemblages that may signal harsh (hypersaline or desiccating) environments.     

天津沼泽湿地芦苇叶片的碳稳定同位素比值分布特征及其环境影响因素

天津芦苇(Phragmites australis)沼泽具有重要的生态功能。目前天津地区水体咸化、氮污染和水资源短缺问题严重, 显著影响了芦苇湿地的植物生理生态过程。植物稳定碳同位素组成(碳稳定同位素比值(δ¹³C))能够记录与植物生长过程相关联的环境变化信息, 反映植物对环境变化的生理生态适应特性。该研究调查了天津七里海、北大港和大黄堡湿地芦苇叶片的δ¹³C分布特征, 探讨了影响该地区叶片δ¹³C值变化的主要因素。研究表明: 1)天津芦苇湿地植物叶片δ¹³C的变化范围在-26.3‰ - -23.6 ‰之间, 平均值为-25.8‰; 2)芦苇叶片δ¹³C与底泥相对含水量呈显著负相关关系, 与底泥有效氮和全氮含量呈显著正相关关系, 而与底泥盐度和磷含量没有显著相关关系; 水分条件和底泥氮营养状况是影响叶片的δ¹³C值变化的主要因素; 3)淹水条件下, 芦苇叶片δ¹³C与叶片质量氮含量呈显著正相关关系, 与叶碳氮比呈显著负相关关系, 8月份七里海湿地干涸打破了此相关关系。当前环境压力下, 天津沼泽湿地干涸极大地改变了芦苇的氮、水平衡和植物对水、氮资源的利用策略, 而湿地干涸对该过程的影响要高于盐度和氮负荷增加。     

鼎湖山森林演替序列植物-土壤碳氮同位素特征

植物群落对水分利用和养分利用的优化策略, 土壤碳周转和氮循环过程对演替变化如何响应, 森林土壤有机碳积累机制等都是森林生态学需要解决的关键问题。然而, 这些生态学过程的变化在短时间内通过传统的研究手段难以被精确观测, 碳氮同位素(¹³C、¹⁵N)技术的应用或许能提供更多有价值的信息。该文通过对鼎湖山森林演替序列代表性群落——马尾松(Pinus massoniana)针叶林(PF)、针阔叶混交林(MF)和季风常绿阔叶林(BF)植物-土壤碳氮同位素自然丰度的测定, 分析了叶片稳定碳同位素比率(δ¹³C)和稳定氮同位素比率(δ¹⁵N)与其叶片元素含量的关系, 以及叶片-凋落物-土壤δ¹³C、δ¹⁵N在演替水平和垂直方向上的变化特征。结果显示: 1)主要优势树种叶片δ¹³C与其C:N极显著正相关(p < 0.01), 凋落物和各层土壤δ¹³C均表现为PF > MF > BF, 沿演替方向逐渐降低; 2)叶片δ¹⁵N与叶片N含量正相关(p = 0.05), 凋落物和表层土壤(0-10 cm) δ¹⁵N沿演替方向逐渐增大; 3)不同演替阶段土壤δ¹³C、δ¹⁵N均沿垂直剖面呈现增大的趋势。结果表明: 南亚热带地区植物群落的发展并不一定受水分利用和氮素利用的补偿制约; δ¹³C自然丰度法的应用有助于森林土壤有机碳积累机制, 尤其有助于成熟森林土壤“碳汇”机制的阐释; 植物-土壤δ¹⁵N值可作为评估土壤氮素有效性和生态系统“氮饱和”状态的潜在指标。     

Foliar stable carbon isotope ratios of Phragmites australis and the relevant environmental factors in marsh wetlands in Tianjin

Aims Phragmites australis marshes in Tianjin play an important role in ecosystem functioning. Wetlands of Tianjin municipality have been suffering from serious nitrogen loading, salinization and water shortage. The foliar stable carbon isotope ratio (δ¹³C) is a good parameter which records environmental change information associated with the plant growth process, and reflects physiological and ecological responses of plants to environment changes. The objective of this study is to investigate the effects of environment stress on the leaf δ¹³C of P. australis in marsh wetlands in Tianjin municipality.Methods This study was conducted in Qilihai, Beidagang, and Dahuangpu marsh wetlands. We investigated the foliar δ¹³C of P. australis and sediment properties, and evaluated the relationships between the foliar δ¹³C and sediment environmental factors. Important findings 1) Foliar δ¹³C ranged from -26.3‰ to -23.6‰, with an average value of -25.8‰. 2) Sediment water and nitrogen status were the important factors affecting reed foliar δ¹³C. Foliar δ¹³C was negatively correlated to sediment relative water content, and positively correlated to sediment total nitrogen and available nitrogen content. In contrast, foliar δ¹³C was not significantly correlated to sediment salinity and phosphorus content. 3) Leaf δ¹³C were significantly positively correlated with leaf nitrogen content, and negatively correlated with leaf carbon and nitrogen ratio across all site. However, these relationships were not detected due to the wetland drainage at Qilihai site in August. Wetland drainage changed the plant water and nitrogen balance, and further affected water and nitrogen utilization strategies of P. australis. Moreover, wetland drainage had stronger effects on these processes than nitrogen loading and salinization.     

13C and 15N isotopic signatures of plant-soil continuum along a successional gradient in Dinghushan Biosphere Reserve

Aims The optimal patterns of plant community for water use and nutrient utilization, the responses of soil carbon and nitrogen turnover processes to forest succession, and the mechanisms of soil organic carbon accumulation, are three critical issues in forest ecosystem study. It is difficult to accurately detect these ecological processes with conventional methodologies in the short term, yet the application of ¹³C and ¹⁵N natural abundance technique may yield important information about these processes.Methods This study was conducted in Dinghushan Biosphere Reserve. We investigated the natural isotopic abundance of both ¹³C and ¹⁵N of plant-soil continuum along a successional gradient from Pinus massoniana forest (PF) to coniferous and broad-leaved mixed forest (MF), and monsoon evergreen broad-leaved forest (BF). We also analyzed the correlations of foliar stable carbon isotope ratio (δ¹³C) and stable nitrogen isotope ratio (δ¹⁵N) with foliar elemental contents and the variations of soil δ¹³C and δ¹⁵N along soil profiles at different successional stages.Important findings A significant positive correlation between foliar δ¹³C and foliar C:N was observed. In both litter and soil, the δ¹³C values tended to decrease along the forest succession, with the order as PF > MF > BF. Foliar δ¹⁵N was positively correlated with foliar N content. The δ¹⁵N values of litter and upper soil (0-10 cm) increased with successional status. Both soil δ¹³C and δ¹⁵N values increased with increasing soil depth at all three forests. Our results imply that 1) trade-off between water use efficiency and nitrogen use efficiency did not necessarily exist in subtropical forests of China; 2) the application of isotopic technique could assist understanding of the mechanisms of soil carbon accumulation in subtropical forests, especially in old-grow forests; 3) the ¹⁵N natural abundance of plant-soil continuum could be a potential indicator of soil nitrogen availability and ecosystem nitrogen saturation status.     

Examination of an Oligocene lacustrine ecosystem using C and N stable isotopes

The Late Oligocene (25.8 Ma) Enspel Fossillagerstätte in Westerwald, Germany, contains a comprehensive fossil ecosystem preserved with specimens retaining morphological detail and a concentration of organic material. Stable carbon and nitrogen isotope analyses were used to examine the lacustrine ecosystem preserved in one stratigraphic horizon. These data suggest the presence of several trophic levels, including primary producers (diatoms and higher plants), primary consumers (e.g., tadpoles and insects), and secondary consumers (e.g., the fish species Paleorutilus enspelensis). Terrigenous and aquatic plants were associated with the lowest δ¹³C and δ¹⁵N values (mean plant = − 26.28‰ ± 0.45, 3.18‰ ± 1.04), primary consumers such as flies are one trophic level higher, and carnivores such as fish are yet another level higher. The δ¹⁵N values for P. enspelensis also showed enrichment in ¹⁵N with increasing body length, implying a shift in diet or feeding strategy with size. P. enspelensis and tadpole (Pelobates decheni) samples showed intraorganism fractionation between ‘muscle’ and ‘bone’ tissues. Stable carbon and nitrogen isotope data from the measurement of components (shale, leaves and seeds) common to a number of different stratigraphic horizons showed significant variation between horizons. A number of the features of the stable isotopic data are similar to those relationships seen in modern ecosystems and therefore suggest that stable isotope analyses can contribute to understanding ancient ecosystems.     

Variation in δC values for the seagrass Thalassia testudinum and its relations to mangrove carbon

Carbon isotope ratios (¹³C/¹²C) were measured for the leaves of the seagrass Thalassia testudinum Banks ex König and carbonates of shells collected at the seagrass beds from seven sites along the coast of southern Florida, U.S.A. The δ¹³C values of seagrass leaves ranged from −7.3 to −16.3‰ among different study sites, with a significantly lower mean value for seagrass leaves from those sites near mangrove forests (−12.8 ± 1.1‰) than those far from mangrove forests (−8.3 ± 0.9‰; P < 0.05). Furthermore, seagrass leaves from a shallow water area had significantly lower δ¹³C values than those found in a deep water area (P < 0.01). There was no significant variation in δ¹³C values between young and mature leaves (P = 0.59) or between the tip and base of a leaf blade (P = 0.46). Carbonates of shells also showed a significantly lower mean δ¹³C value in the mangrove areas (−2.3 ± 0.6‰) than in the non-mangrove areas (0.6 ± 0.3‰; P <0.025). In addition, the δ¹³C values of seagrass leaves were significantly correlated with those of shell carbonates (δ¹³C seagrass leaf = −9.1 + 1.3δ¹³C shell carbonate (R² = 0.83, P < 0.01)). These results indicated that the input of carbon dioxide from the mineralization of mangrove detritus caused the variation in carbon isotope ratios of seagrass leaves among different sites in this study.     

Tissue-specific variation of δC in mature canopy trees in a temperate forest in central Europe

Stable carbon isotope analysis has become a key tool in functional ecology, yet considerable natural variability often limits the interpretations. In this study we document the spatial, taxonomic, temporal and tissue-specific δ¹³C variability in 10 tree species of a temperate European forest. The Swiss Canopy Crane provided access to the three dimensional space within 55 trees 30–35 m high representing the genera Acer, Carpinus, Fagus, Prunus, Quercus, Tilia, Abies, Larix, Picea and Pinus. The results from six broad-leaved and four conifer species (seven deciduous, three evergreen) documented that the species effect was not significant in contrast to tissue-specific and spatial differences in the canopy. Year-to-year differences were not large but still significant.

Our analysis confirmed a significant difference between δ¹³C of foliage collected in the upper and lower canopy, but revealed no systematic differences with respect to azimuthal directions in tree crowns of the broad-leaved trees, as opposed to the conifers, which show clear differences between the sun-exposed and the shaded side. Tissue-specific differences were significant, despite surprisingly similar mean values for most tissue types. Such tissue effect was largely due to young branch xylem, which exhibited a systematic less negative deviation from the other tissue types. These findings were consistent across the species tested and provided some guidelines towards a representative sampling strategy for ¹³C analysis.     

商代前期中原地区多品种农作物种植制度的初探：以河南新郑望京楼遗址为例

农业经济的发展是华夏文明形成的最重要的前提条件之一,郑洛地区作为夏商王朝建都和统治的核心区域,其农业经济发展状况一直是学术界研究的重点问题。相关研究表明,多品种农作物种植制度在龙山时期中原地区的出现,在华夏文明的形成过程中可能起到了至关重要的作用。为了探索这一种植制度在商代前期中原地区的具体实践情况,本文选取河南新郑望京楼遗址夏商时期（二里头文化和二里岗文化时期）29例先民肢骨和23例先民肋骨进行C、N稳定同位素分析。结果表明,先民骨胶原的δ¹³C值变化范围为-18.1‰~7.0‰,平均值为-9.5‰±2.1‰（n=52）,δ¹⁵N值变化范围为7.3‰~10.5‰,平均值为8.9‰±0.7‰（n=52）,先民仍以C₄类食物（粟黍）为主,但是也包含少量C₃类（水稻、小麦或大豆）食物,证明中原地区自龙山时代出现的多品种农作物种植制度,在商代得以延续,但是粟作农业的主导地位,始终未发生明显变化,这可能与中原地区长期以来的旱作农业经济模式习惯有关。     

Carbon isotope fractionation in species of the torrenticolous families Podostemaceae and Hydrostachyaceae

Carbon isotope ratios of herbarium material from members of the fresh-water families Podostemaceae and Hydrostachyaceae (Rosidae) were analyzed. The levels of ¹³C were highly variable (Podostemaceae −12.8‰ to −38.55‰; Hydrostachyaceae −10.78‰ to −30.42‰), across as well as within species and across a wide geographic range.

We suggest that the high variance observed is due neither to a constant attribute of the species like the photosynthetic CO₂-carboxylase (in water plants with very high discrimination of the ¹³CO₂ probably Rubisco) nor to the constant structural peculiarities of these species. Rather, it is likely due to the ‘diffusional resistance’ for the CO₂-flux from the turbulent and/or fast flowing water, causing a very variable boundary layer on the plant surface.     

Sources and accumulation rates of organic carbon in an equatorial peat bog (Burundi, East Africa) during the Holocene: carbon isotope constraints

The relationship between organic carbon accumulation rates and ¹³C/¹²C ratios of total organic carbon (TOC) was investigated in an highland peat bog core (Ru-3) from Equatorial Africa. This core yielded a sequence spanning the last 14 kyr and was analysed with a 100–300 yr resolution for TOC-δ¹³C values. The Holocene section shows contrasted TOC accumulation regimes and TOC δ¹³C varying between −28.5 and −19.5‰ with a few very short ‘isotopic excursions' (dated at ca. 9.3, 7.5, 4.2 ka B.P.). The organic carbon accumulation rates range from 2 to 20 mg C cm⁻² yr⁻¹. They increase when TOC becomes more depleted in ¹³C, notably between 12 and 9.8 ka B.P., 8.5 and 7.8 ka B.P. and after 1.6 ka B.P. Periods of restricted carbon storage correspond to heavier TOC accumulation at 9.3, and between 7.5 and 1.6 ka B.P. At the study site, the δ-variations can be related to variable C4-plant inputs, and possibly, to changes in the fractionation between CO₂ and the organic carbon in C3 vascular plants. The Ru-3 record indicates restricted carbon storage during the periods of increased contribution from C4 plants and/or of decreased fractionation between CO₂ and organic carbon in C3 plants. Changes in TOC-δ¹³C values in core Ru-3 seem to match fluctuations of East Equatorial African lakes. High lake stands correspond to low δ¹³C intervals and vice versa. This points to indirect climatic forcing of δ¹³C changes in intertropical peats.     

哈民忙哈遗址人和动物骨骼的C、N稳定同位素分析

哈民忙哈遗址是迄今为止内蒙古乃至东北地区发现面积最大的一处史前聚落遗址,为重建当时的生产和生活情况、文化习俗甚至思想观念等都提供了弥足珍贵的实物资料。其中,重建该遗址先民的食物结构和生业经济,有助于揭示哈民忙哈遗址产生和兴盛的动因及科尔沁沙地史前文化的变迁过程。目前,多学科的研究成果已经初步显示哈民忙哈遗址先民的生业经济具有多样性,但各种生业模式的比重以及是否存在家畜饲养等问题还缺乏相应的了解。本文对该遗址87例人骨与18例动物骨骼进行了C、N稳定同位素分析,还原了人和动物的食物结构及生业经济面貌。结果显示,陆生野生动物的食物结构主要基于C₃植物类食物和少量的C₄农作物,而犬科动物则体现出家养动物的食谱特性：食物主要依赖C₄食物和人类食物残羹。C₄类粟黍农作物和以此饲喂的动物是该遗址先民的主要食物来源,其中粟黍农作物在食物结构中的地位尤其重要。在该遗址中,女性摄入相对较多的粟黍和相对较少的肉类,与男性在食物结构上存在显著差异。农耕、家畜饲养是先民最重要的经济行为,狩猎、渔猎和采集是生业的重要补充。     

黑河中游春玉米叶片水δD和δ18O的富集过程和影响因素

植物水的稳定同位素分馏过程是水在土壤-植物-大气连续体中循环的重要环节。以往研究由于叶片水¹⁸O同位素比值(δ¹⁸O _l,b)和氘(D)同位素比值(δD_l,b)(合称δ_l,b)实测数量少只能作为模型验证数据, 导致δ_l,b富集机制研究多集中于模型研究, 缺乏基于野外试验条件的δ_l,b富集的控制机制研究。叶片水δD_l,b和δ¹⁸O _l,b的富集程度(ΔD_l,b和Δ¹⁸O _l,b, 合称Δ_l,b)通常表示为δ_l,b与茎秆水D同位素比值(δD_x)和¹⁸O同位素比值(δ¹⁸O_x) (合称δ_x)之差, 即Δ_l,b = δ_l,b - δ_x。该研究以黑河中游沙漠绿洲春玉米(Zea mays)生态系统为研究对象, 重点采集和分析了季节和日尺度δ_l,b和δ_x数据, 配套开展了大气水汽δ¹⁸O和δD (合称δ_v)等辅助变量的原位连续观测, 探讨了季节和日尺度上的δ_l,b富集特征及其影响因素。结果表明: 叶片水δ_l,b和Δ_l,b的季节变化趋势不明显, 而受蒸腾作用影响表现出白天富集夜间贫化的单峰日变化特征。对于D来说, 无论季节尺度上还是日尺度上, 大气水汽δ_v和相对湿度是δD_l,b和ΔD_l,b的主要环境控制因素; 而对于¹⁸O来说, 无论季节尺度上还是日尺度上, 相对湿度是δ¹⁸O _l,b和Δ¹⁸O _l,b的主要环境控制因素。由于D和¹⁸O在热力学平衡分馏上有约8倍差异, 直接分析叶片水ΔD_l,b和Δ¹⁸O_l,b与影响因素的差异性, 有助于理解叶片水δD和δ¹⁸O富集过程以及对模型发展有一定的指导意义。     

Determination of δC values of valine in protein hydrolysate by gas chromatography–combustion isotope ratio mass spectrometry

A gas chromatographic–combustion isotope ratio mass spectrometric (GC–C-IRMS) method for the determination of [1-¹³C]valine enrichments in protein hydrolysates is described. Using a quick derivatization method, δ¹³C values of the N-methoxycarbonyl methyl ester of valine can be determined from baseline separated GC peaks. Evaluation studies with respect to precision, accuracy, linearity, reduction capacity of the CuO combustion furnace and isotope dilution as a result of derivatization, showed that our GC–C-IRMS system allows robust measurement of enrichments of [1-¹³C]valine in the range 0 to 1.5 MPE (S.D.±0.01 MPE, n=3). Therefore this method is suited to determine fractional synthetic rates (FSRs) of proteins as low as one-tenth of the FSR of human albumin, in studies using a primed, continuous (6 h) infusion with [1-¹³C]valine plasma enrichments of approximately 15 MPE and an hourly sampling schedule.     

Late Quaternary paleoproductivity changes off the Congo deduced from stable carbon isotopes of planktonic foraminifera

A detailed record (≈2-kyr intervals) of the difference in stable carbon isotopes (Δδ¹³C) between Globigerina bulloides and Globigerinoides ruber (pink) is used to reconstruct changes in upwelling intensity off the Congo River for the last 190,000 yr. Comparisons of the oxygen and carbon isotope data from this core with records from the Niger Fan and from pelagic cores in the eastern equatorial South Atlantic indicate that the Congo Fan isotope records do not contain a strong freshwater signal as is described off other major rivers. The temporal pattern of the Δδ¹³C signal correlates with the marine organic carbon record from the Congo Fan. Thus the planktonic Δδ¹³C record, reflecting past changes in upwelling intensity and nutrient content, corroborates the signal provided by sedimentary organic carbon, which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor.

The planktonic Δδ¹³C signal is characterized by a dominant 23-kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the Congo to precessional forcing. Minima in the Δδ¹³C record are aligned with periods of minimum boreal summer insolation over Central Africa reflecting an increase of upwelling and biological productivity off the Congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern South Atlantic. A strong response to changes in river discharge probably did not occur, indicating that fertilization by river-derived nutrients has played only a minor role with respect to Late Quaternary changes in the total amount of primary productivity off the Congo.