Organic Carbon Isotope Ratios Research Articles

High-resolution climate evolution derived from the sediment records of Erlongwan Maar Lake since 14 ka BP

Varve counts with AMS 14C, 137Cs and 210Pb dating of sediments (0–900 cm) from Erlongwan Maar Lake, NE China were used to establish a high-resolution chronology series for the late Quaternary. Dry density, total organic carbon (TOC) content, total nitrogen (TN) content, TOC/TN ratios and stable organic carbon isotope (δ 13Corg) ratios were continuously analyzed on this sediment profile. On the basis of lithological characters, sporo-pollen assemblages and geochemical analyses, we identified 6 climate stages within the last 14 ka BP. The time before the Holocene (14-11.4 ka BP) represents a higher-order oscillation climatic transitional period (I). The entire Holocene climate development (from 11.4 ka BP to present) exhibited an increasing temperature trend, although there were cold and warm alternations (II–VI). The periods included were: II (11.4-9.05 ka BP) warm-wet stage, III (9.05-7.4 ka BP) cold and warm fluctuation stage, IV (7.4-4.2 ka BP) smoothly warming climate stage, V (4.2-1.67 ka BP) climate optimum stage, and VI (from 1.67 ka BP to present) cool and drier stage. Each climate stage began with a warming event and ended with an abrupt cooling event. This climate change cycle had unequal time spaces that were progressively shorter over time. Several abrupt climate shifts occurred at about 9.4-9.05, 8.5-8.2, 7.8-7.4, 4.6-4.2, 3.7-3.25, 2-1.67 and 0.3-0.03 ka BP. Thus, it can be seen that the climate has been warming since 1920 AD, which indicates a new climate stage.

In situ iron isotope analyses of pyrite and organic carbon isotope ratios in the Fortescue Group: Metabolic variations of a Late Archean ecosystem

The biogeochemical cycle of the Late Archean ocean is important for understanding the relationships between biological activity and oxygenation of the atmosphere and ocean. Based on the detailed geological survey of the Fortescue Group in the Redmont area in Pilbara Craton, Western Australia, we carefully selected 44 samples for iron isotope analyses, which consist of sandstones, stromatolitic carbonate rocks, alternating mudstone/sandstone rocks, mudstones and cherts. Our in situ analyses of δ56Fe values of 210 pyrite grains in these samples show a large variation from −4.2‰ to +3.0‰. We also analyzed 128 and 40 carbon isotope compositions of organic (δ13Corg: −51.8 to −10.3‰) and inorganic (δ13Ccarb: −6.1 to 0.6‰) carbons, respectively.Microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio. Most pyrite grains with positive δ56Fe values show hexagonal, rectangular, and parallelogram shapes, which may replace former iron-oxide crystal systems: hematite, magnetite, and goethite, respectively. In contrast, more than half the pyrite grains with negative δ56Fe values show irregular forms. The correlation allows the possibility to solve the origin and the formation process of each grain of pyrite. The positive δ56Fe values suggest the partial oxidation of iron in an oxygen-limited environment. Some pyrites show very lower δ56Fe values below −2.2‰ suggesting a biological origin, probably due to microbial iron reduction.On the other hand, the pyrite is accompanied by isotopically very light organic carbon (δ13Corg: −51.8‰ to −40‰), which indicates aerobic or anaerobic methanotrophy. The coexistence of the low δ56Fe values and low δ13C values in the some rocks suggests anoxic oxidation of methane by iron-reduction (AOM/IR). The iron and carbon isotopes demonstrate the metabolic variations of microorganisms in a Late Archean shallow marine environment.

Influence of Spartina Colonization on the Supply and Accumulation of Organic Carbon in Tidal Salt Marshes of Northern Jiangsu Province, China

Abstract GAO, J.; BAI, F.; YANG, Y.; GAO S.; LIU, Z., and LI, J., 2012. Influence of Spartina colonization on the supply and accumulation of organic carbon in tidal salt marshes of northern Jiangsu Province, China. Two species of cordgrass (Spartina anglica Hubbard and Spartina alterniflora Loisel.) were artificially introduced to the intertidal zone of Jiangsu Province, China, in 1963 and 1982, respectively. However, as alien species, the impact of Spartina on the tidal flat ecosystem remains largely unknown. Suaeda salsa and Phragmites australis, two native plants in the intertidal zone of northern Jiangsu Province, China, are C3-type plants, whereas the Spartina species are C4-type plants. In this article, we investigate the vertical distribution of total organic carbon, total nitrogen, and organic carbon isotope ratios in different ecosystems associated with the northern Jiangsu tidal flats; analyze changes in the organic carbon sources in the tidal flat sediments after Spartina colonization; compare ...

Early Toarcian (Early Jurassic) oceanic anoxic event recorded in the shelf deposits in the northwestern Panthalassa: Evidence from the Nishinakayama Formation in the Toyora area, west Japan

The early Toarcian oceanic anoxic event (T-OAE) was marked by the worldwide burial of organic-rich black shales and a coincident pronounced negative carbon-isotope excursion of carbonates, organic matter, and fossil wood. This characteristic carbon-isotope perturbation and other geochemical records have been recognized in many sections in the European domain and in selected sections in the central and eastern Panthalassa, but not in the western Panthalassa. We present herein, for the first time, data of carbon isotope ratios of organic carbon, total organic carbon (TOC) contents, and sedimentary fabrics from the uppermost Pliensbachian to the lower Toarcian sequence in the Sakuraguchi-dani Valley, Toyora area, west Japan, that was deposited in the shelf basin of the northwestern Panthalassa. The carbon-isotope profile displays a similar trend to those in contemporaneous strata in the Tethys and in the eastern and central Panthalassic regions, which is characterized by a distinct negative excursion around the Tenuicostatum/Falciferum Zone boundary in the early Toarcian. This evidence suggests that the widely recognized negative carbon-isotope excursion represents a nature of the global carbon-cycle perturbation. Analyses of total organic carbon contents and sedimentary fabrics in the study section reveal that accumulation of organic carbon occurred and anoxic bottom-water was prevalent during the early Toarcian. The most organic-rich strata and bottom-water anoxia described in the study section, however, exist in the interval showing the carbon-isotope positive recovery after the abrupt negative shift, and this fact suggests that the accumulation of organic matters and development of marine anoxia were globally diachronous, even though the negative CIE was interpreted as a synchronous event. Our study also demonstrates that the early Toarcian anoxia in the northwestern Panthalassic basin was not a single and continuous anoxic event but was interrupted by episodic short-term oxygenation events.

Marine diatoms in polar and sub-polar environments and their application to Late Pleistocene paleoclimate reconstruction

Diatoms are one of the major phytoplankton groups in polar and sub-polar marine environments along with green algae and chrysophytes. Diatoms are composed of two components, a two-valve test made of amorphous silica and an organic cell encapsulated into the test. Mucilage covering the test and proteins embedded in the silica lattice of the test completes the organic pool of the diatoms. The preservation of these two components into deep-sea sediments allows for a large set of diatom-based proxies to infer past oceanographic and climatic changes in polar and sub-polar marine environments.Most diatom species in polar and sub-polar marine environments exhibit a narrow range of ecological preferences, especially in terms of sea-surface temperature and sea ice conditions. Preserved diatom assemblages in deep-sea sediments mirror the diatom assemblages in the phytoplankton. It is subsequently possible to extrapolate the relationships between diatom assemblages in surface sediments and modern parameters to down-core fossil assemblages to document past changes in sea-surface temperatures and sea ice conditions. Congruent analysis of biogenic silica and organic carbon and stable isotope ratios (O, Si in the silica matrix and C, N in the diatom-intrinsic organic matter) provides information on siliceous productivity, nutrient cycling and water mass circulation. Measurements of diatom biomarkers give complementary information on sea ice conditions and siliceous productivity.

Climatic changes since the last deglaciation inferred from a lacustrine sedimentary sequence in the eastern Nanling Mountains, south China

AbstractTwo sediment cores recovered from Dahu Swamp, which is located in eastern Nanling Mountains in south China, were selected for investigation of palaeoclimatic changes. Multi‐proxy records of the two cores including lithological variation, organic carbon isotope ratio, dry bulk density, organic matter content, magnetic susceptibility, humification degree, median grain size and geochemical proxies reveal that during the last deglaciation three drier phases correspond to the Oldest, Older and Younger Dryas cooling events, and the intercalated two wetter phases synchronise with the Bølling and Allerød warming events. The Holocene Optimum, which was resulted from a strengthening of the East Asian (EA) summer monsoon, occurred in the early and mid Holocene (ca. 10–6 cal. ka BP). In the mid and late Holocene (ca. 6–3 cal. ka BP), a prevailing dry climate suggested a weakening of the EA summer monsoon. The general trend of Holocene climatic changes in this study agrees with the 25° N summer solar insolation, suggesting that orbitally induced insolation may have played an important role in the Holocene climate in the study region. Copyright © 2010 John Wiley & Sons, Ltd.

Holocene paleolimnological changes in Lake Skallen Oike in the Syowa Station area of Antarctica inferred from organic components in a sediment core (Sk4C-02)

Antarctic climate changes influence environmental changes at both regional and local scales. Here we report Holocene paleolimnological changes in lake sediment core Sk4C-02 (length 378.0 cm) from Lake Skallen Oike in the Soya Kaigan region of East Antarctica inferred from analyses of sedimentary facies, a range of organic components, isotope ratios of organic carbon and nitrogen, and carbon-14 dating by Tandetron accelerator mass spectrometry. The sediment core was composed of clayish mud (378.0–152.5 cm) overlain by organic sediments (152.5 cm-surface). The age of the surface and the core bottom were 150 (AD1950-1640) and ca. 7,030 ± 73 calibrated years before present (cal BP), respectively, and the mean sedimentation rate was estimated to be 0.55 mm/year. Multi-proxy analyses revealed that the principal environmental change in the core is a transition from marine to lacustrine environments which occurred at a depth of 152.5 cm (ca. 3,590 cal BP). This was caused by relative sea level change brought about by ongoing retreat of glaciers during the mid-Holocene warming of Antarctica, and ongoing isostatic uplift which outpaced changes in global (eustatic) sea level. The mean isostatic uplift rate was calculated to be 2.8 mm/year. The coastal marine period (378.0–152.5 cm, ca. 7,030–3,590 cal BP) was characterized by low biological production with the predominance of diatoms. During the transition period from marine to freshwater conditions (152.5-approximately 135 cm, ca. 3,590–3,290 cal BP) the lake was stratified with marine water overlain by freshwater, with a chemocline and an anoxic (sulfidic) layer in the bottom of the photic zone. Green sulfur bacteria and Cryptophyta were the major photosynthetic organisms. The Cryptophyta appeared to be tolerant of the moderate salinity and stratified water conditions. The lacustrine period (approximately 135 cm-surface, ca. 3,290 cal BP-present) was characterized by high biological production by green algae (e.g. Comarium clepsydra and Oedegonium spp.) with some contributions from cyanobacteria and diatoms. Biological production during this period was 8.7 times higher than during the coastal marine period.

Sedimentary evidence of Late Holocene human activity in the Pearl River delta, China

AbstractThis study examines the sedimentary evidence of human activities during the last 4000 years in the Pearl River deltaic area. The analyses are focused on indentifying agricultural signatures present in the sedimentary record and establishing the timing of a change from a simple, rice‐based agriculture to a more advanced, diverse agriculture. The examination is based on modern sediment and plant samples and a sediment core collected from the deltaic area. The analyses include particle size and diatom analysis to determine the environmental conditions that were associated with the period of human activities. Organic carbon isotope ratios and major metal elements reveal an expansion in commercial crop production and metal smelting in the Pearl River delta area about 2000 years ago. The input of organic matter from introduced sugarcane, a C4 plant, elevates the bulk organic carbon isotope values in the estuarine sediments above that represented by other common agricultural crops in the study area, including rice, banana and lotus, which are all C3 plants. The increase in bulk organic isotopic value coincides with the rise in the concentration of copper, iron and lead in the sedimentary sequence, suggesting a wider use of metal tools. These results indicate that advanced agriculture started about 2000 years ago as an expansion in human population took place in the area. This record also provides sedimentary evidence that help ascertain the timing and type of human activities that are linked to subsequent land reclamation on the deltaic plain, resulting in rapid shoreline advancement in the last 2000 years. Copyright © 2010 John Wiley & Sons, Ltd.

A smooth negative shift of organic-carbon isotope ratios at an end-Permian mass extinction horizon in central pelagic Panthalassa

article i nfo The end-Permian mass extinction is associated with a global perturbation in the carbon isotopic composition of carbonates and organic matter, but such changes are not well documented from pelagic deep-sea sediments. One of the most continuous pelagic deep-sea Permian/Triassic boundary (PTB) sections, the Am-2 section, is exposed in the accretionary complexes of Japan. New data for organic carbon isotope ratios (δ 13 Corg) were obtained from 36 samples from section Am-2. The δ 13 Corg excursion curve exhibits a negative shift of 2.0‰ in the low-latitude, pelagic Panthalassic Ocean at the end of the Permian, which coincides with a radiolarian extinction. The δ 13 Corg values of pelagic, deep-sea Panthalassic sections and those of shallow- water sections from Panthalassic seamounts exhibit a smooth, negative shift that lacks temporary increases like those reported from Paleotethyan PTB sections. Absence of temporary δ 13

A smooth negative shift of organic carbon isotope ratios at an end-Permian mass extinction horizon in central pelagic Panthalassa

The end-Permian mass extinction is associated with a global perturbation in the carbon isotopic composition of carbonates and organic matter, but such changes are not well documented from pelagic deep-sea sediments. One of the most continuous pelagic deep-sea Permian/Triassic boundary (PTB) sections, the Am-2 section, is exposed in the accretionary complexes of Japan. New data for organic carbon isotope ratios ( δ 13C org) were obtained from 36 samples from section Am-2. The δ 13C org excursion curve exhibits a negative shift of 2.0‰ in the low-latitude, pelagic Panthalassic Ocean at the end of the Permian, which coincides with a radiolarian extinction. The δ 13C org values of pelagic, deep-sea Panthalassic sections and those of shallow-water sections from Panthalassic seamounts exhibit a smooth, negative shift that lacks temporary increases like those reported from Paleotethyan PTB sections. Absence of temporary δ 13C org increases at the PTB in Panthalassa may reflect less algal and bacterial blooming in pelagic Panthalassa compared to the shallow-water Paleotethys.

Distributions of polycyclic aromatic hydrocarbons in a sediment core from the north basin of Lake Biwa, Japan

We analyzed the vertical distributions of 19 polycyclic aromatic hydrocarbons (PAHs), total organic carbon (TOC), total nitrogen (TN) and stable carbon isotope ratio (δ 13C) in a 27 cm sediment core from the north basin of Lake Biwa in order to elucidate changes in PAH sources and events possibly affecting the lake over the last few decades. Both TOC and TN increased gradually from the deepest layer (TOC, 1.2 wt.%; TN, 0.14 wt.%) to the uppermost layer (TOC, 4.0 wt.%; TN, 0.45 wt.%). The TOC/TN ratio, a useful indicator for distinguishing between aquatic and terrestrial plant sources, increased remarkably at 11–13 cm (10.3–11.1 vs. 8.5–9.7 at other depths), indicating that large amounts of terrestrial material entered the lake when that layer was deposited. All PAHs except perylene gradually increased from the deepest layer, but not as clearly as TOC and TN. At 11–13 cm, all PAHs except perylene transiently decreased and perylene transiently increased. Since the PAH ratios used for the elucidation of sources did not change greatly throughout the core, we consider that the major source of PAHs in the lake has not changed during the last few decades. The brief decrease in PAHs at 11–13 cm could be the result of dilution by a large input of terrestrial material, which brought in perylene.

Humification degrees of a lacustrine sedimentary sequence as an indicator of past climatic changes in the last c. 49 000 years in South China

Zhong, W., Ma, Q. H., Xue, J. B., Zheng, Y. M., Cai, Y. & Ou, Y. J. 2009: Humification degrees of a lacustrine sedimentary sequence as an indicator of past climatic changes in the last c. 49 000 years in South China. Boreas, 10.1111/j.1502‐3885.2009.00119.x. ISSN 0300‐9483.An 860 cm long sediment core recovered from Dahu Swamp was used to test the potential of the degree of humification (expressed as the corrected absorbance values, HDcab) of lacustrine sediments as an indicator of past climatic changes in South China. The chronological sequence of this core was established using 10 AMS 14C dates and 10 conventional 14C dates and gives the bottom age of c. 48 800 cal. yr BP. Based on comparisons with multiproxy records, such as lithological variation, organic carbon isotope ratio, organic matter content and dry bulk density, we infer that the HDcab record is sensitive to local hydrological variations that are closely related to the East Asian (EA) summer monsoon precipitation. Higher HDcab values would indicate relatively drier conditions, suggesting a weakening of the EA summer monsoon and vice versa. Based on the HDcab, as well as other multiproxies, the climatic history in the last c. 49 000 years in the swamp area shows agreement with the Chinese stalagmite record and with the 25°N summer solar insolation, implying that orbitally induced insolation may have played a role in the last glacial period (LGP) climate in South China. Several millennial‐scale dry phases during the LGP show similarities with the stadials revealed by the ice core record in Greenland and from the Chinese cave stalagmites. Our study demonstrates that the HDcab of lacustrine sediments is a useful proxy of past climatic changes in South China.

Stable carbon and nitrogen isotopic signatures of organic matter sources in near-shore areas of Lake Victoria, East Africa

Stable isotope ratios of organic carbon and nitrogen (δ 13C and δ 15N) were measured in suspended particulate matter (SPM) and sediment in the near-shore areas of Tanzanian waters of Lake Victoria (LV) to identify the various sources of organic matter in different areas of the lake. Spatial variations in δ 13C, δ 15N, and C/N ratios in SPM and sediments in inshore areas were due to differences in the proportions and sources of autochthonous or allochthonous matter. Watershed characteristics, such as urbanization, and lake characteristics, such as algal blooms, also immensely influenced the stable isotope signal of the organic matter in sediments. Stable C and N ratios showed that inshore areas in bays without pronounced fluvial input were dominated by autochthonous planktonic organic matter; conversely, for bays receiving significant fluvial inflows, the composition of organic matter depended on the extent of preservation of shoreline vegetation. Signals resulting from input of anthropogenic organic waste as a result of urbanization were distinguishable from natural sources. This work demonstrates the potential to discriminate between input sources of organic material into lakes using stable isotope signals in sediment and suspended particulate matter.

Degradation and small-scale spatial homogenization of topsoils in intensively-grazed steppes of Northern China

Overgrazing has led to severe degradation and desertification of semi-arid grasslands in Northern China over the last decades. Despite the fact that vegetation is often heterogeneously distributed in semi-arid steppes, little attention has been drawn to the effect of grazing on the spatial distribution of soil properties. We determined the spatial pattern of soil organic carbon (SOC), total nitrogen (N tot), total sulphur (S tot), bulk density (BD), pH, Ah thickness, and carbon isotope ratios (δ 13C) at two continuously grazed (CG) and two ungrazed (UG79 = fenced and excluded from grazing in 1979) sites in Leymus chinensis and Stipa grandis dominated steppe ecosystems in Inner Mongolia, Northern China. Topsoils (0–4 cm) were sampled at each site using a large grid (120 m × 150 m) with 100 sampling points and a small plot (2 m × 2 m) with 40 points. Geostatistics were applied to elucidate the spatial distribution both at field (120 m × 150 m grid) and plant (2 m × 2 m plot) scale. Concentrations and stocks of SOC, N tot, S tot were significantly lower and BD significantly higher at both CG sites. At the field scale, semivariograms of these parameters showed a heterogeneous distribution at UG79 sites and a more homogeneous distribution at CG sites, whereas nugget to sill ratios indicated a high small-scale variability. At the plant scale, semivariances of all investigated parameters were one order of magnitude higher at UG79 sites than at CG sites. The heterogeneous pattern of topsoil properties at UG79 sites can be attributed to a mosaic of vegetation patches separated by bare soil. Ranges of autocorrelation were almost congruent with spatial expansions of grass tussocks and shrubs at both steppe types. At CG sites, consumption of biomass by sheep and hoof action removed vegetation patches and led to a homogenization of chemical and physical soil properties. We propose that the spatial distribution of topsoil properties at the plant scale (<2 m) could be used as an indicator for degradation in semi-arid grasslands. Our results further show that the maintenance of heterogeneous vegetation and associated topsoil structures is essential for the accumulation of SOM in semi-arid grassland ecosystems.

Coupling a High-Temperature Catalytic Oxidation Total Organic Carbon Analyzer to an Isotope Ratio Mass Spectrometer To Measure Natural-Abundance δ13C-Dissolved Organic Carbon in Marine and Freshwater Samples

The stable isotope composition of dissolved organic carbon (delta(13)C-DOC) provides powerful information toward understanding carbon sources and cycling, but analytical limitations have precluded its routine measurement in natural samples. Recent interfacing of wet oxidation-based dissolved organic carbon analyzers and isotope ratio mass spectrometers has simplified the measurement of delta(13)C-DOC in freshwaters, but the analysis of salty estuarine/marine samples still proves difficult. Here we describe the coupling of the more widespread high-temperature catalytic oxidation-based total organic carbon analyzer to an isotope ratio mass spectrometer (HTC-IRMS) through cryogenic trapping of analyte gases exiting the HTC analyzer for routine analysis of delta(13)C-DOC in aquatic and marine samples. Targeted elimination of major sources of background CO2 originating from the HTC analyzer allows for the routine measurement of samples over the natural range of DOC concentrations (from 40 microM to over 2000 microM), and salinities (<0.1-36 g/kg). Because consensus reference natural samples for delta(13)C-DOC do not exist, method validation was carried out with water-soluble stable isotope standards as well as previously measured natural samples (IAEA sucrose, Suwannee River Fulvic Acids, Deep Sargasso Sea consensus reference material, and St. Lawrence River water) and result in excellent delta(13)C-DOC accuracy (+/-0.2 per thousand) and precision (+/-0.3 per thousand).

Mississippian δ13C carb and conodont apatite δ18O records — Their relation to the Late Palaeozoic Glaciation

Carbon isotopes of whole rock carbonates and oxygen isotopes of conodont apatite from Late Devonian to Early Pennsylvanian sections in Europe and Laurentia were measured in order to reconstruct variations in the carbon cycle, marine palaeotemperature, and ice volume during the Mississippian. Conodont apatite δ 18O values show two major positive shifts of + 2‰ and + 1.5‰ V-SMOW in the late Tournaisian and Serpukhovian, respectively, that are interpreted to reflect climatic cooling and changes in ice volume. Carbon isotope ratios of inorganic and organic carbon show a major positive excursion with an amplitude of + 6.5‰ V-PDB in the Tournaisian and a positive shift of up to + 5‰ V-PDB in the Serpukhovian. The positive δ 13C excursions coincide with the deposition of organic carbon-rich black shales which indicate that organic carbon burial, lowering of atmospheric pCO 2, and climatic cooling may have occurred during these time intervals. However, while in the Tournaisian the positive shifts in δ 18O apatite and δ 13C coincide, in the Serpukhovian the positive shift in δ 18O apatite precedes the positive shift in δ 13C and raises the question as to whether changes in the global carbon cycle were the ultimate cause of the inferred climatic changes. The conodont apatite δ 18O values suggest that a first major cooling and potential glaciation event occurred in the Tournaisian with ice masses persisting into the Visean. The second glaciation event occurred in the Serpukhovian and culminated in the first glacial maximum of the Late Palaeozoic Glaciation.

Late glacial environment and climate development in northeastern China derived from geochemical and isotopic investigations of the varved sediment record from Lake Sihailongwan (Jilin Province)

Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) and stable organic carbon isotope (δ13Corg) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM. Phases of high TOC content, high TN content, depleted δ13Corg values and high δ15N values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point towards a reduced phytoplankton discrimination against 15N due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic (δ13Corg, δ15N) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450 to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial.

Assessment of nutrient sources and paleoproductivity during the past century in Longgan Lake, middle reaches of the Yangtze River, China

Total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were determined in combination with stable isotope ratios of carbon and nitrogen (δ13COrg, δ15N) in a 63 cm sediment core from Longgan Lake, located in the middle reaches of the Yangtze River, China. These geochemical and isotopic records provide a continuous history of lake productivity and trophic state of Longgan Lake since 1890. Variations of δ13COrg, TOC, TN and TP indicate that primary productivity of Longgan Lake increased continuously during the last century and that the trophic state of the lake shifted from oligotrophic to mestrotrophic conditions accordingly. Anthropogenic sources of organic carbon (OC), nitrogen (N) and phosphorus (P) were distinguished from their natural background in the sediments using mass accumulation rates. Element mass accumulation rates suggested increased human activities in the lake’s catchment since 1950s, were especially the utilization of artificial fertilizers amplified the anthropogenic input of N and P into the lake. In the course of the improved availability of dissolved nutrients also primary productivity of Longgan Lake increased, resulting in an increase of the Suess-effect corrected organic carbon isotope ratios. δ15N of bulk sediments show a marked shift towards lower values around 1950 that has been attributed to the input of nitrogen from chemical fertilizers characterized by relatively depleted isotopic signatures into the lake.

Carbon isotope fractionation during decomposition of organic matter in soils and paleosols: Implications for paleoecological interpretations of paleosols

Stable isotope ratios (reported as δ 13C) of organic carbon preserved in paleosols have potential utility as a valuable paleoecological indicator if the isotopic composition of paleobiomass is preserved in the paleosol organic carbon without fractionation. However, soil organic matter is in a constant state of decomposition in an open system, and frequently decomposes further during alteration after burial of the paleosol. During this decomposition, kinetic fractionation of 13C/ 12C in an open system results in concentration of 13C in the residual organic matter preserved in the soil and paleosol. This paper summarizes studies of modern soils which show that this 13C-enrichment follows a Rayleigh distillation process whereby the 13C/ 12C ratio is a function of the natural logarithm of the fraction of remaining soil organic carbon, leaving soil organic carbon 13C-enriched by up to ∼ 6‰ with respect to the original biomass. If the fraction of original biomass carbon remaining during these decomposition processes deviates significantly from 1, the soil organic carbon can be 13C-enriched by several per mil, and thus not a good paleoecological indicator of the δ 13C value of the original biomass. Therefore, effective paleoecological studies of δ 13C preserved in paleosol organic carbon must therefore demonstrate that the measured concentration of organic carbon in the paleosol is near its maximum concentration during soil formation, or that the paleosol organic matter does not include the 13C-enriched products of microbial decomposition. Suggestions are made for interpreting the effects of carbon isotope fractionation of organic matter, and for materials and methods most useful for paleoecological studies from bulk paleosol organic carbon.

Reconstructing marine redox conditions for the Early Cambrian Yangtze Platform: Evidence from biogenic sulphur and organic carbon isotopes

In order to constrain marine redox conditions for the sedimentary environments immediately preceding the rapid diversification of metazoans carbon and sulphur isotopes, as well as carbon, sulphur and iron abundances, were measured in shales, cherts, and carbonates from two Lower Cambrian (Nemakit-Daldynian to Atdabanian) sections belonging to a shallow shelf and deeper water basinal environment on the Yangtze Platform, South China. A largely anoxic and most probably sulphidic deeper water body for most of the Nemakit-Daldynian and lower Tommotian is supported by positive δ 34S values for sulphide, as well as high degree of pyritization (DOP) and total organic carbon (TOC) values. Additionally, an isotope fractionation between organic and carbonate carbon of > 32‰ suggests that sulphate reducing and sulphide oxidizing bacteria flourished and added to the biomass. Furthermore a positive correlation between δ 34S of sulphide and δ 13C of organic and possibly inorganic carbon is observed. In the lower Tommotian, low δ 13C values for organic and inorganic carbon together with lower δ 34S values for pyrite, if compared to underlying and overlying samples, have been measured. This is indicative of bacterial sulphate reduction taking place not only in the interstitial waters but also in an anoxic, possibly euxinic water column, and thus leading together with chemoautotrophic bacteria to recycling of organic matter, superimposed on the temporal evolution of the global carbon cycle. In the upper Tommotian and Atdabanian, decreasing DOP and TOC and high δ 34S values for pyrite and organic sulphur suggest that the water column became oxic and that bacterial sulphate reduction occurred primarily in the sediment, where sulphate limitation was soon established. Organic carbon and carbonate carbon isotope ratios increase, reflecting open marine conditions and oxic photosynthesis dominating carbon cycling in the shelf environment. In the basinal deeper water section, anoxic conditions were still present or re-established during the upper Tommotian. In the lower Atdabanian, δ 34S is variable, but mostly high in the deeper section, with DOP indicating an oscillation between oxic and dysoxic conditions.