δ13Corg Values Research Articles

Soil Inorganic Carbon Sequestration Following Afforestation Is Probably Induced by Pedogenic Carbonate Formation in Northwest China.

In arid and semiarid areas, the effects of afforestation on soil organic carbon (SOC) have received considerable attention. In these areas, in fact, soil inorganic carbon (SIC), rather than SOC, is the dominant form of carbon, with a reservoir approximately 2–10 times larger than that of SOC. A subtle fluctuation of SIC pool can strongly alter the regional carbon budget. However, few studies have focused on the variations in SIC, or have used stable soil carbon isotopes to analyze the reason for SIC variations following afforestation in degraded semiarid lands. In the Mu Us Desert, northwest China, we selected a shifting sand land (SL) and three nearby forestlands (Populus alba) with ages of 8 (P-8), 20 (P-20) and 30 (P-30) years, and measured SIC, SOC, soil organic and inorganic δ13C values (δ13C-SOC and δ13C-SIC) and other soil properties. The results showed that SIC stock at 0–100 cm in SL was 34.2 Mg ha-1, and it increased significantly to 42.5, 49.2, and 68.3 Mg ha-1 in P-8, P-20, and P-30 lands, respectively. Both δ13C-SIC and δ13C-SOC within the 0–100 cm soil layer in the three forestlands were more negative than those in SL, and gradually decreased with plantation age. Afforestation elevated soil fine particles only at a depth of 0–40 cm. The entire dataset (260 soil samples) exhibited a negative correlation between δ13C-SIC and SIC content (R2 = 0.71, P < 0.01), whereas it showed positive correlation between SOC content and SIC content (R2 = 0.52, P < 0.01) and between δ13C-SOC and δ13C-SIC (R2 = 0.63, P < 0.01). However, no correlation was observed between SIC content and soil fine particles. The results indicated that afforestation on shifting SL has a high potential to sequester SIC in degraded semiarid regions. The contribution of soil fine particle deposition by canopy to SIC sequestration is limited. The SIC sequestration following afforestation is very probably caused by pedogenic carbonate formation, which is closely related to SOC accumulation. Our findings suggest that SIC plays an important role in the carbon cycle in semiarid areas and that overlooking this carbon pool may substantially lead to underestimating carbon sequestration capacity following vegetation rehabilitation.

Controlling factors and dynamical formation models of lacustrine organic matter accumulation for the Jurassic Da'anzhai Member in the central Sichuan Basin, southwestern China

The lacustrine shale in the Jurassic Da'anzhai Member is considered as an excellent source rock in the central Sichuan Basin. However, geochemical studies of lacustrine organic matter (OM) accumulation factors and mechanisms are scarce. This study investigated the variations in total organic carbon (TOC), major elements, trace elements (TEs), and δ13C and δ18O values to reconstruct the paleoenvironmental conditions, biotic productivity, terrestrial inputs and other factors to improve the understanding of the controlling factors and dynamical formation models for lacustrine OM. The target shale was deposited in a fluctuating and complicated terrestrial open lake that exhibited oxic-suboxic oxygen levels, weak to moderate hydrodynamic conditions, a dry to humid climate, moderate weathering, and fresh to saline water conditions. The lake experienced moderate-high biotic productivity under a warm, humid climate and featured abundant flora and fauna. Compared to the Da13 shale, the Da1 shale featured higher salinity levels, a hotter and drier climate, greater weathering and more reduced conditions. The Da13 shale experienced stronger hydrodynamic forces and a more humid climate. Moreover, the TOC values of the Da13 correlate better with the indicators of redox conditions and exhibit little or no correlation with indicators of climate, weathering, salinity and biotic productivity, indicating that the lacustrine OM was mainly controlled by the redox conditions. Additionally, factors such as hydrodynamic conditions and terrestrial inputs exerted some degree of influence. Based on these factors and their relationships, two dynamical formation models are proposed: a high-salinity low-oxygen dynamical formation model (Model I) and a deep-water low-oxygen dynamical formation model (Model II). Model I stresses that the low oxygen levels were mainly caused by salinity, whereas Model II stresses that the low oxygen levels were mainly caused by deep depth. Both models can be classified as preservation models.

Chemostratigraphy of the Ediacaran basinal setting on the Yangtze platform, South China: Oceanographic and diagenetic aspects of the carbon isotopic depth gradient

AbstractThe Ediacaran Yangtze platform in South China, which represents depositional settings ranging from coastal to basinal, provides valuable information for understanding climate changes and animal evolution during the Ediacaran Period. Although the shallower settings have been investigated, research on the basinal sections has been limited. This has hampered efforts to establish stratigraphic correlations and understand the oceanographic setting of the Yangtze platform. In this paper, the chemostratigraphy of a basinal section at Fengtan, Hunan Province, is reported based on analyses of stable carbon isotope profiles in carbonates (δ13Ccarb), organic matter (δ13Corg), total organic carbon, 87Sr/86Sr ratios, and Mn, Rb, and Sr concentrations. The basinal section of the Doushantuo Formation, which is represented at Fengtan, provides data supporting regional correlations and oceanography. Three intervals in the Doushantuo Formation are correlated with the Three Gorges: (i) a negative δ13Ccarb anomaly with stable δ13Corg values and altered 87Sr/86Sr ratios in the lower section can be correlated to the boundary between Doushantuo Members 2 and 3 (Interval A); (ii) a relatively high δ13Ccarb anomaly with unaltered 87Sr/86Sr ratios (up to 0.7086) in the middle section corresponding to the lower part of Doushantuo Member 3 (Interval B); and (iii) a negative δ13Ccarb anomaly with lowered δ values in the upper section can be correlated to the long interval of negative δ13Ccarb (Interval C). The Gaskiers glaciation is likely represented in Interval A, and Interval C corresponds to the Shuram excursion reported for other Ediacaran localities. Our correlations confirm the depth gradient of δ13Ccarb in the Yangtze platform and imply that reductive conditions prevailed in the basinal section from the Early to Middle Ediacaran. Under such conditions, anaerobic degradation of organic carbon or methane perturbed the inorganic carbon isotopic compositions and was at least partly responsible for the depth gradient of δ13Ccarb.

Aquatic species dominate organic matter in Qinghai Lake during the Holocene: Evidence from eolian deposits around the lake

Total organic carbon (TOC) in lake sediments and its stable carbon isotope (δ13Corg) are widely applied to investigate paleoenvironmental changes even though their implications are complicated and multi-explanatory. Organic geochemistry studies of lake sediments from Qinghai Lake have been investigated, but some interpretations are controversial. In this study, TOC of one Holocene eolian section and δ13Corg of three eolian sections were studied around Qinghai Lake. Results indicate that the TOC content in eolian deposits was low during the Early Holocene, and gradually increased to high values during the Middle and Late Holocene. The variation in TOC content of eolian deposits is different from that in the lacustrine sequence from Qinghai Lake during the Holocene. The δ13Corg values in the eolian sections were relatively stable, with oscillation amplitudes of ~4‰ (ranging from -25.8‰ to -22.1‰), in contrast to ~10‰ variations in δ13Corg values (varying from -30‰ to -20‰) in lacustrine sediments. Through comparison of TOC and δ13Corg values between eolian deposits and lacustrine sediments, we can confirm indications that the organic matter in Qinghai Lake sediments during the Holocene was primarily a contribution of the aquatic species in the lake. This is significant for understanding the origin of organic matter in lake sediments on the northeastern Qinghai-Tibetan Plateau and for paleoenvironmental inferences using such proxies.

The role of land-marine teleconnections in the tropical proximal Permian-Triassic Marine Zone, Levant Basin, Israel: Insights from stable isotope pairing

Three Late Permian – early Middle Triassic successions (Avdat 1, Pleshet 1 and David 1 boreholes, Levant Basin, Israel), located in relatively proximal and distal order from land within a broad tropical mixed carbonate/siliciclastic open marine zone, were studied using carbonate and organic matter contents (organic and inorganic carbon) in order to demonstrate the degree of effect of the land-marine teleconnection on the isotopic signatures at the depositional environment. The δ13Ccarb profiles exhibit sequential negative/positive fluctuations, which are correlatable with the reported worldwide sequential negative-shift events, enabled further stratigraphic division of the successions to stages and sub-stages. The successions changed their relative siliciclastic content relative to the degree of influence of each terrestrial influx source (eastern or southern), an outcome of humid up to extreme aridization hinterland exchanges, actually recording the expansion or contraction of the paleo-ITCZ. The δ18O profiles exhibited a range of values (−5‰ to −7.5‰ on average) typical to the western NeoTethys and similar to the reported worldwide climate trends with three major warming periods: (I) Late Permian to the PTB; (II) Late Dienerian - most of the Smithian; (III) early-mid Anisian, and two relatively cool periods: Griesbachian-Dienerian and Late Smithian – Spathian, but each of the three periods exhibiting short respites with the opposite trend. The δ13Ccarb, δ18Ocarb and the δ13Corg profiles of the proximal position consistently differ in magnitude from the distal ones, assuming a high contribution and involvement of meteoric water rich in terrestrial OM derived from the nearby supercontinent and affecting also the original water δ18Oseawater value (calculated to about −3‰),which seemingly should be applied on the entire western Tethys seaway. During times of associations with maximum ITCZ contraction, the δ13Corg values of −31‰ to −33‰ in the distal succession exhibit the end member values of the regional marine OM, while values of −22.5‰ to −25‰ of the proximal succession are considered representing the regional terrestrial signature. Our data show good correlation between warming trends, the proximal/distal location of succession and the disparity of δ13Ccarb+org values that may explain the differences in reported worldwide values.

Organic geochemical investigations of the Dali Lake sediments in northern China: Implications for environment and climate changes of the last deglaciation in the East Asian summer monsoon margin

Millennial-scale environment and climate changes in the East Asian summer monsoon margin during the last deglaciation are reconstructed by systematic studies on the characteristic of sedimentary organic matter from Dali Lake in northern China. Concurrent increases in the TOC and TN concentrations indicate increases in terrestrial organic matter and nutrient inputs to the lake and a development of terrestrial vegetation and phytoplankton productivity related to increases in regional temperature and precipitation. C/N ratios reflect changes in the proportions of terrestrial and aquatic organic matter. Decreases in both δ13Corg and δ15N values indicate increases in the isotopically lighter, terrestrial carbon and nitrogen inputs to the lake, due to increases in surface runoffs; while a sharp decrease in the δ15N value implies a significant weakening in the biological activities of nitrifying and amonifying bacteria, due to abrupt decrease in the water temperature. The geochemical data indicate that regional temperature and precipitation exhibited increasing trends from 15,000 to 12,350calyrBP; temperature decreased abruptly at 12,350calyrBP and then maintained a low level from 12,350 to 11,400calyrBP, precipitation decreased to a relatively low level from 12,350 to 11,400calyrBP; and both temperature and precipitation returned to increase after 11,400calyrBP. The climate change in the Dali Lake region during the last deglaciation corresponds, within age uncertainties, to the Bølling-Allerød (BA) warm phase and Younger Dryas (YD) cold reversal occurring over northern high latitudes. However, the gradual and mild increasing trends of regional temperature and precipitation during the BA warm period contrasts with the general cooling trend in northern high latitude temperature, implying a dominant influence from increases in the Northern Hemisphere summer insolation; while the slight decreases in regional precipitation relative to the rapid and significant decreases in northern high latitude temperature during the YD cold period may have resulted from local moisture recycling or from an insensitive response of hydrology and ecology to the regional precipitation change.

A geochemical study of the Ediacaran discoidal fossil Aspidella preserved in limestones: Implications for its taphonomy and paleoecology.

The Ediacara biota features the rise of macroscopic complex life immediately before the Cambrian explosion. One of the most abundant and widely distributed elements of the Ediacara biota is the discoidal fossil Aspidella, which is interpreted as a subsurface holdfast possibly anchoring a frondose epibenthic organism. It is a morphologically simple fossil preserved mainly in siliciclastic rocks, which are unsuitable for comprehensive stable isotope geochemical analyses to decipher its taphonomy and paleoecology. In this regard, three-dimensionally preserved Aspidella fossils from upper Ediacaran limestones of the Khatyspyt Formation in the Olenek Uplift of northern Siberia offer a rare opportunity to leverage geochemistry for insights into their taphonomy and paleoecology. To take advantage of this opportunity, we analyzed δ13 Ccarb , δ18 Ocarb , δ13 Corg , δ34 Spyr , and iron speciation of the Khatyspyt Aspidella fossils and surrounding sediment matrix in order to investigate whether they hosted microbial symbionts, how they were fossilized, and the redox conditions of their ecological environments. Aspidella holdfasts and surrounding sediment matrix show indistinguishable δ13 Corg values, suggesting they did not host and derive significant amount of nutrients from microbial symbionts such as methanogens, methylotrophs, or sulfide-oxidizing bacteria. δ13 Ccarb , δ18 Ocarb , and δ34 Spyr data, along with petrographic observations, suggest that microbial sulfate reduction facilitated the preservation of Aspidella by promoting early authigenic calcite cementation in the holdfasts before matrix cementation and sediment compaction. Iron speciation data are equivocal, largely because of the low total iron concentrations. However, consideration of published sulfur isotope and biomarker data suggests that Aspidella likely lived in non-euxinic waters. It is possible that Aspidella was an opportunistic organism, colonizing the seafloor in large numbers when paleoenvironments were favorable. This study demonstrates that geochemical data of Ediacaran fossils preserved in limestones can offer important insights into the taphonomy and paleoecology of these enigmatic organisms living on the eve of the Cambrian explosion.

A multistratigraphic approach to pinpoint the Permian-Triassic boundary in continental deposits: The Zechstein–Lower Buntsandstein transition in Germany

The Central European Basin is very suitable for high-resolution multistratigraphy of Late Permian to Early Triassic continental deposits. Here the well exposed continuous transition of the lithostratigraphic Zechstein and Buntsandstein Groups of Central Germany was studied for isotope-chemostratigraphy (δ13Corg, δ13Ccarb, δ18Ocarb), major and trace element geochemistry, magnetostratigraphy, palynology, and conchostracan biostratigraphy. The analysed material was obtained from both classical key sections (abandoned Nelben clay pit, Caaschwitz quarries, Thale railway cut, abandoned Heinebach clay pit) and a recent drill core section (Caaschwitz 6/2012) spanning the Permian-Triassic boundary. The Zechstein–Buntsandstein transition of Central Germany consists of a complex sedimentary facies comprising sabkha, playa lake, aeolian, and fluvial deposits of predominantly red-coloured siliciclastics and intercalations of lacustrine oolitic limestones. The new data on δ13Corg range from −28.7 to −21.7 ‰ showing multiple excursions. Most prominent negative shifts correlate with intercalations of oolites and grey-coloured clayey siltstones, while higher δ13Corg values correspond to an onset of palaeosol overprint. The δ13Ccarb values range from −9.7 to −1.3 ‰ with largest variations recorded in dolomitic nodules from the Zechstein Group. In contrast to sedimentary facies shifts across the Zechstein-Buntsandstein boundary, major element values used as a proxy (CIA, CIA*, CIA-K) for weathering conditions indicate climatic stability. Trace element data used for a geochemical characterization of the Late Permian to Early Triassic transition in Central Germany indicate a decrease in Rb contents at the Zechstein-Buntsandstein boundary.New palynological data obtained from the Caaschwitz quarry section reveal occurrences of Late Permian palynomorphs in the Lower Fulda Formation, while Early Triassic elements were recorded in the upper part of the Upper Fulda Formation. The present study confirms an onset of a normal-polarized magnetozone in the Upper Fulda Formation of the Caaschwitz quarry section supporting an interregional correlation of this crucial stratigraphic interval with the normal magnetic polarity of the basal Early Triassic known from marine sections in other regions. Based on a synthesis of the multistratigraphic data, the Permian-Triassic boundary is proposed to be placed in the lower part of the Upper Fulda Formation, which is biostratigraphically confirmed by the first occurrence date of the Early Triassic Euestheria gutta-Palaeolimnadiopsis vilujensis conchostracan fauna. Rare records of conchostracans reported from the siliciclastic deposits of the lower to middle Zechstein Group may point to its potential for further biostratigraphic subdivision of the Late Permian continental deposits.

Carbon isotope analysis of whole-coal and vitrinite from intruded coals from the Illinois Basin: No isotopic evidence for thermogenic methane generation

Igneous intrusions into coals and organic-rich rocks may have contributed to global warming in the geologic past through the release of greenhouse gases. Evidence for a large release of thermogenic CH4 from the organics would include significant δ13Corg enrichment in the residual organic matter (OM). However, δ13Corg values of thermally altered OM in coals and shales adjacent to intrusions often show negative trends or, in some cases, ambiguous or positive trends. Previous studies have evaluated the δ13Corg of whole-coal samples rather than that of individual organic components, or macerals. As different macerals have different isotopic compositions, maceral-specific trends may be masked by variations in maceral composition of the bulk samples. This study evaluates the hypothesis that, if a large-scale release of 13C-depleted thermogenic CH4 resulted from intrusion of the coal, then it should have produced 13C-enriched coal residuum and, specifically, vitrinite (the most abundant component of the coal) adjacent to the intrusion. This study reports geochemical and petrographic data for whole-coal samples and vitrinite macerals (separated via density-gradient centrifugation, DGC) from a transect of thermally altered Springfield (No. 5) Coal (Pennsylvanian) in the Illinois Basin.Approaching the dike contact, mean vitrinite reflectance (Rr) increases from background levels of 0.55% up to ~4.8% and liptinites become indistinguishable from vitrinite. Isotropic and fine-grained circular mosaic coke is observed in samples within ~1.3m of the coal/intrusion contact. Approaching the intrusion, volatile matter (VM) decreases and fixed carbon (FC) and %C increase, whereas %H decreases. Total organic carbon (TOC) for whole coal decreases from 77% to 34%, whereas TOC for separated vitrinite fractions increases from ~66% to 93% toward the coal/intrusion contact. Density of DGC-separated vitrinites ranges from 1.27g/mL in the unaltered coal to 1.52g/mL in the sample adjacent to the coal/intrusion contact. Vitrinite density increases with increased Rr and %C, and decreases with increased %H and H/C.Despite these marked geochemical and petrographic changes, no significant changes in δ13Corg values of the whole coal (−25.3‰ to −24.9‰) or the separated vitrinites (−25.3‰ to −25.0‰) occur proximal to the intrusion. Changes in the isotopic signatures are not of a magnitude that would be expected if significant quantities of isotopically depleted thermogenic CH4 had been generated by the intrusive event. Moreover, no petrographic evidence supports the occurrence of condensed or immobilized thermal products due to rapid pyrolysis (such as 12C-rich pyrolytic carbon) close to the intrusion that could have moderated any changes in δ13Corg. Geochemical and petrographic results suggest that only minimal loss of CH4 was associated with the rapid heating of the Springfield (No. 5) Coal by the intrusion. These findings have significant implications for models linking past global warming to the intrusion of coals and carbonaceous shales.

Differential proxy responses to late Allerød and early Younger Dryas climatic change recorded in varved sediments of the Trzechowskie palaeolake in Northern Poland

High-resolution biological proxies (pollen, macrofossils, Cladocera and diatoms), geochemical data (μ-XRF element scans, TOC, C/N ratios, δ18Ocarb and δ13Corg values) and a robust chronology based on varve counting, AMS 14C dating and tephrochronology were applied to reconstruct lake system responses to rapid climatic and environmental changes of the Trzechowskie palaeolake (TRZ; Northern Poland) during the late Allerød – Younger Dryas (YD) transition. Palaeoecological and geochemical data at 5–15 years temporal resolution allowed tracing the dynamics of short-term shifts of the ecosystem triggered by abrupt climate change. The robust age control together with the high-resolution sampling allowed the detection of leads and lags between different proxies to the climate shift at the Allerød-Younger Dryas transition. Our results indicate (1) a water level decrease and an increase in wind activities during the late Allerød and the Allerød-YD transition, which caused intensified erosion in the catchment, (2) a two-decades delayed vegetation response in comparison to the lake depositional system. Comparison with the Lake Meerfelder Maar record revealed slightly different vegetation responses of the Trzechowskie palaeolake at the YD onset.

Sources and burial of organic carbon in the middle Okinawa Trough during late Quaternary paleoenvironmental change

The sediments from a piston core ECS12A recovered from the middle Okinawa Trough in the East China Sea were measured for total organic carbon (TOC), total nitrogen (TN), and other biogenic elements to provide constraints on the sources and burial rates of depositional organic matter (OM) and on the changes in primary productivity since 19 ka. The last glacial sediments (ca. 17–19ka) are characterized by low contents of biogenic elements and well-developed turbidite layers, suggesting low primary productivity but a high component of terrigenous sediment. With rising sea level and enhanced monsoons during the deglacial period, the proportion of marine OM gradually increased. The least negative δ13Corg values and the smallest grain size of sediments deposited ca. 10–14.5ka indicate high primary productivity and a sedimentary environment dominated by the marine component. The source and burial rates of OM in the Holocene sediments (ca. 5.4–10ka) were largely controlled by the intensification of the Kuroshio Current, which caused a slight decrease in primary productivity, but strengthened the oceanic circulation in the East China Sea. Overall, the source-to-sink process of OM in the Okinawa Trough is governed by complex interactions between sea level, climate and ocean circulation.

Subsoil 14C Dynamics in Different Types of Tropical and Subtropical Soils under Different Crop Management

AbstractRadiocarbon has been applied as a tracer to study carbon dynamics in different types of tropical soils, under paddy and non-paddy management on Java, Indonesia. The 14C concentrations were measured in samples of total organic carbon as well as in alkali-soluble humic acids, insoluble humin, and plant remains (roots, seeds, leaves) obtained from three sites with Andosols, Alisols, and Vertisols at different altitudes. In addition, the abundance and distribution of plant macrofossils in the soil column and organic δ13C values were determined. The results obtained so far are compared with those from a chronosequence of Cambisols in China. They indicate the input of fresh plant materials into the subsoil directly via roots and/or by soil cracks and bioturbation of aboveground litter. The total organic C and 14C concentrations show the usual decrease with increasing depth in paddy and non-paddy soils, reflecting the influence of direct input as well as the downward redistribution of organic material as particulates or dissolved organic carbon (DOC) depending on soil type (pedogenesis, plough-pan formation, drying cracks), crop type and management, and climatic factors. A disturbance of the Andosol around 0.4 m depth and a change in profile properties around 0.63 m in the Alisol limit the general conclusions.

Clay-mineral compositions of sediments in the Gaoping River-Sea system: Implications for weathering, sedimentary routing and carbon cycling

Small mountainous rivers (SMRs) play a crucial role in the global sediment and carbon cycle by rapid transfer of huge particulate and dissolved loads into the sea. Gaoping river-sea system is a good representative of SMRs in Oceania, having an extreme high denudation rate (15,000tkm−2yr−1) and a high burial efficiency of terrestrial particulate organic carbon (POC) in the Gaoping submarine canyon (GPSC). However, it is contentious whether silicate weathering or POC burial plays more important role in long-term carbon cycling in such a high active orogen. Clay minerals of fluvial sediments not only act as an important carrier of POC, but also encode valuable information of weathering regimes on land and dispersal routes in the seas. To uncover these informations, surface and core sediments are systematically collected from the Gaoping river-sea system for clay-mineral analysis. Comparative studies are carried out on the basis of new results and vast published data to better understand weathering processes and products in Oceanian islands and Southeast Asian continent. We find that clay-mineral compositions in the Gaoping River (GPR) and other Taiwan rivers are a legacy from provenance rocks in the alpine reach, characterized by predominant illite and chlorite and limited compositional change throughout the river courses. These sharply contradict the downstream increase in compositional complexity (rich in kaolinite and smectite) and chemical weathering intensity in Southeast Asian large rivers because of strong hydrolysis and active sediment recycling at the lowland reach with extensive floodplains. High consistency of clay-mineral compositions is also observed in the interior and exterior of GPSC. Careful examination shows that a slight offshore decrease in cumulative illite and chlorite content is accompanied by a slight increase in exotic clay-mineral inputs by ocean circulations. The combination of this finding with previously reported data on the offshore change in δ13Corg and Δ14Corg values suggests that petrogenic POC predominates in the river-dominated Gaoping continental margin with high preservation efficiency due to its tight bandage with clay minerals. Moreover, a tardy response of chemical weathering fluxes to high elevated physical erosion rates is remarkably examined in the Oceanian and global river gauging data. We therefore postulate that rapid transfer and high burial efficiency of terrestrial POC in the tectonically active high-standing islands and their adjacent continental margins play much more important role in the long-term carbon cycle than silicate weathering.

Lacustrine facies dependence of highly 13C-depleted organic matter during the global age of methanotrophy

Highly 13C-depleted organic matter reported from Neoarchean formations worldwide has led to the concept of a “Global Age of Methanotrophy” (GAM) in the Neoarchean. A temporal peak in the GAM is suggested by values as low as −61‰ that are reported from rocks deposited at ∼2.7Ga. Here we analyse previously reported values, report new field observations and isotope data, and re-evaluate the depositional settings of several units of this age. We find a statistically significant lowering of δ13Corg values in units of Neoarchean age compared to values reported from other Precambrian intervals, both older and younger, confirming the existence of the GAM. However, we also report a correlation between very low δ13Corg values and lacustrine units deposited during the Neoarchean. We hypothesize methanogenesis may have been promoted in some Neoarchean lakes due to local deficiencies of oxidants, specifically Fe3+ and SO4, relative to the Archean oceans. Lower availability of these oxidants could have limited higher energy yield metabolisms such as sulfate and iron reduction and provided an ecological niche for methanogens, ultimately resulting in the local burial of biomass highly depleted in 13C. We conclude that the exceptionally low δ13Corg values reported from formations deposited at ∼2.7Ga could represent the prevalence of closed basin depositional environments preserved in the limited outcrop available, rather than a peak in the global age of methanotrophy at this time.

Early Jurassic carbon-isotope excursion in the Qiangtang Basin (Tibet), the eastern Tethys: Implications for the Toarcian Oceanic anoxic event

The Early Jurassic was characterized by a global disturbance of the carbon cycle known as the Toarcian oceanic anoxic event (T-OAE). This event is recorded by a large negative carbon-isotope excursion (CIE) in marine carbonates and marine organic matter, as well as fossil wood. Here, we present new high-resolution carbonate (δ13Ccarb) and organic (δ13Corg) carbon-isotope records and total organic content from the Bilong Co section in the Qiangtang Basin, the eastern Tethys. This section is high in organic carbon, and black shale facies are also identified. The Bilong Co section shows heavy δ13Corg values compared to other reported marine organic carbon-isotope values for the T-OAE. This is explained by variable mixing of terrestrial and marine organic matter. Compared to European sections, the magnitude of the negative CIE in the Bilong Co section is slightly different, which could be explained by different local paleoenvironmental conditions and strata missing. Although these, the carbon-isotope profile from the Bilong Co area displays a similar trend to those in contemporaneous strata in the western Tethyan and Boreal realms of NW Europe, which is characterized by a distinct negative excursion in the Early Toarcian. Our new data from the Bilong Co section in N Tibet provide the first high-resolution record of the Early Toarcian Oceanic Anoxic Event in the eastern Tethys.

Mangrove degradation and response to anthropogenic disturbance in the Maowei Sea (SW China) since 1926 AD: Mangrove-derived OM and pollen

Mangrove forests, located at the interface between land and sea, have been impacted by an increase in intensive anthropogenic disturbance in developing countries or regions. In order to study the impact of human activity on mangrove forests, mangrove development was reconstructed over the last 130yr, using the contribution of mangrove-derived organic matter (OM) and mangrove pollen as proxies, from two sediment cores from the Maowei Sea (SW China). It is a semi-enclosed bay that receives a large amount of terrestrial material from the Qinjiang and Maoling rivers, with average sedimentation rate of 0.63–0.64cm/yr. The material accumulates mainly in the coast and its adjacent region, owing to weak water exchange through the channel. Sediment samples had C:N and δ13Corg values intermediate between mangrove leaves and flood plain sediments, indicating that OM sources could be apportioned as a mixture from only these two sources. Based on k-mean cluster analysis, mangrove development was divided into three stages since 1880 AD: (i) a flourishing period (1880–1926 AD), (ii) a phase of slow degradation (1926–1980 AD) and (iii) a time of rapid degradation (1980 AD to the present). The study indicates that anthropogenic activity, including reclamation of mangrove swamps for farmland and shrimp ponds, is the primary reason for mangrove degradation since 1926 AD, rather than climate change (temperature and precipitation).

Carbon and oxygen isotopic records from Lake Tuosu over the last 120 years in the Qaidam Basin, Northwestern China: The implications for paleoenvironmental reconstruction

Isotopic compositions of total organic carbon (TOC) and authigenic carbonate in lakes have been widely used to reconstruct paleoclimatic changes and the depositional environments of lake sediments. However, since these proxies are often controlled by multiple environmental factors, detailed examinations of modern environmental processes is necessary before further applying them into paleoclimatic studies, especially in arid/semi-arid northwestern China. Here we generate High-resolution multi-proxy sedimentary records from Lake Tuosu, a hydrologically closed, saline and alkaline lake located at the north margin of the Qaidam Basin, through analysis of carbon isotope of TOC, and δ18O and δ13C values of ostracods over the last 120years. Together with the meteorological data (precipitation and temperature), lake area record, and other tree-ring evidence, we examine how these sedimentary indices respond to changes in hydrologic balance and climate at interannual to decadal timescales. We found that sedimentary δ13Corg values resemble the variation of lake areas of Lake Tuosu over the last 40years, suggesting that δ13Corg values would be an ideal indicator of lake area/level fluctuations and thus effective moisture variations (precipitation vs. evaporation). However, ostracod δ18O, which was previously used as proxies of effective precipitation, is not well correlated with δ13Corg values in Lake Tuosu. Therefore, the changes of ostracod δ18O values cannot be straightforwardly explained as the effective precipitation. Instead, the isotopic composition of carbonate would be additionally controlled by other factors including isotopic compositions of input water and drainage pattern.

Distortion of carbon isotope excursion in bulk soil organic matter during the Paleocene-Eocene thermal maximum

The Paleocene-Eocene thermal maximum was a period of abrupt, transient global warming, fueled by a large release of 13C-depleted carbon and marked globally by a negative carbon isotope excursion. While the carbon isotope excursion is often identified in the carbon isotope ratios of bulk soil organic matter (δ13Corg), these records can be biased by factors associated with production, degradation, and sources of sedimentary carbon input. To better understand these factors, we compared δ13Corg values from Paleocene-Eocene thermal maximum rocks in the southeastern Bighorn Basin, Wyoming, with those derived from leaf wax n -alkanes (δ13C n -alk). While both δ13C n -alk and δ13Corg records indicate an abrupt, negative shift in δ13C values, the carbon isotope excursions observed in bulk organic matter are smaller in magnitude and shorter in duration than those in n -alkanes. To explore these discrepancies, we modeled predicted total plant tissue carbon isotope (δ13CTT) curves from the δ13C n -alk record using enrichment factors determined in modern C3 plants. Measured δ13Corg values are enriched in 13C relative to predicted δ13CTT, with greater enrichment during the Paleocene-Eocene thermal maximum than before or after. The greater 13C enrichment could reflect increased degradation of autochthonous organic matter, increased input of allochthonous fossil carbon enriched in 13C, or both. By comparing samples from organic-rich and organic-poor depositional environments, we infer that microbial degradation rates doubled during the Paleocene-Eocene thermal maximum, and we calculate that fossil carbon input increased ∼28%−63%. This approach to untangling the controls on the isotopic composition of bulk soil carbon is an important development that will inform not only future studies of global carbon cycle dynamics during the Paleocene-Eocene thermal maximum hyperthermal event, but also any study that seeks to correlate or estimate duration and magnitude of past events using soil organic carbon.

Mercury content in Atlantic sediments as a new indicator of the enlargement and reduction of Northern Hemisphere ice sheets

ABSTRACTThe total mercury content of Quaternary sediments over the last 1.1 Ma from IODP Site U1308 drilled in the North Atlantic Ocean has changed, varying with periodicities of 100 000 and 41 000 years. The Hg content of the sediment increased during glacial periods, associated with increases in ice‐rafted debris (IRD), and correlates positively with the total organic carbon (TOC) content and inversely with the absolute abundance of surface‐dwelling nannoplankton species. TOC/total nitrogen ratios and δ13Corg values of TOC indicate that the TOC deposited during glacial periods was marine and associated with increased Hg deposition. These observations suggest that marine Hg is incorporated into the organic matter produced by deep‐dwelling phytoplankton in the lower photic zone. Mercury‐ and IRD‐bearing icebergs flowed out to this area during glacial periods melt, increasing the Hg content of photic water, and depositing IRD on the ocean floor, where increased marine Hg during glacial periods was consumed by deep‐dwelling phytoplankton. Therefore, the change in the sediment Hg content from the North Atlantic Ocean is controlled by climatic change and can be used as a chemical indicator of Northern Hemisphere ice sheet variability.

The Toarcian Oceanic Anoxic Event (Early Jurassic) in the Neuquén Basin, Argentina: A Reassessment of Age and Carbon Isotope Stratigraphy

AbstractThe Toarcian oceanic anoxic event (T-OAE) is recorded by the presence of globally distributed marine organic carbon–rich black shales and a negative carbon isotope shift, with δ13Corg values as low as −33‰, interrupting an overarching positive excursion. Here we present new biostratigraphic data and high-resolution δ13Corg data from two Southern Hemisphere localities: Arroyo Serrucho in the north and Arroyo Lapa in the south of the Neuquen Basin, Argentina. Previous studies at these localities aimed to provide an accurate numerical age for the T-OAE and characterization of its carbon isotope stratigraphy. The new carbon isotope data and ammonite biostratigraphy presented here from Arroyo Serrucho show the T-OAE to be recorded lower in the section than supposed by previous authors, thus calling into question the published age of the T-OAE in this section. A newly investigated exposure at Arroyo Lapa North shows a complex carbon isotope record with at least three high-amplitude fluctuations in the h...