Lighter δ13C Values Research Articles

Composition and sources of suspended particles in the Pacific Arctic region

During the 6th (2014) and 7th (2016) Chinese Arctic Expedition (CHINARE), samples of suspended particulate matter (SPM) were collected from both surface (depth: <1.0 m) and subsurface (depth: approximately between 10 and 150 m) waters over the northern shelf of the Bering Sea and in the western Arctic Ocean. To investigate the distribution and sources of organic matter in both the surface water and the vertical profile, the concentration and stable carbon isotopic composition of SPM, particulate organic carbon (POC), and particulate nitrogen (only in surface water samples) were determined, and some particle samples were selected for examination using scanning electron microscopy. Results showed apparent geographical partitioning and temporal variation in both the concentration and the composition of SPM. Higher SPM concentrations were observed in nearshore, shelf break, and sea ice edge areas; the distribution of POC concentration displayed a similar pattern, with higher values found from the northern part of the Bering Shelf to southern parts of the Chukchi Shelf. In surface water, SPM mainly comprised clay and detrital minerals with higher POC contents, lighter δ13C values, and higher POC/PN ratios, indicating organic matter predominantly derived from terrestrial sources in areas south of St. Lawrence Island and north of 73°N. The downward trend of heavier δ13C values, together with reduction in clay and detrital minerals, suggests that vertical transport of SPM is hindered by stratification, resulting in transport of terrestrial materials toward northern basin areas. In the Chukchi Slope and Canada Basin areas, extremely light δ13C values (as low as −33.41‰ PDB) were mainly observed at depths of 20–60 m, where the Polar Mixed Layer (PML) intersects with the Upper Halocline Layer (UHL). Under the condition of low sea ice extent in 2016, the POC-δ13C values were heavier in the PML than in the UHL in the Chukchi Slope and Canada Basin areas. These findings provide insights into the sources, transport, and fate of organic matter in the Pacific Arctic region, which have important implications for understanding the biogeochemical cycles and ecosystem dynamics in this remote and rapidly changing environment.

Oil origin and migration direction in Wushi sag, Beibuwan Basin, South China Sea

The Wushi sag is one of the most important petroleum-bearing units located in the Beibuwan Basin, South China Sea. Insufficient understanding of oil origin and migration direction, and lack of characterizing oil differential accumulation distribution patterns in Wushi sag greatly restrict further hydrocarbon exploration. In this study, oil-source rock genetic relationships and migration directions were analyzed in detail by combining geochemical analysis of 45 source rock samples, 23 crude oil samples, 12 oil sandstone samples, and basin modeling results. The geochemical data show there is distinguishable heterogeneity in Liushagang Formation (Fm) source rock in Wushi sag, which is divided into two types. The first type (including the Member (Mbr) 23 and Mbr 3 of Liushagang Fm source rocks) was deposited in a weak reducing to suboxic and fresh to brackish water environment with a predominance of aquatic algal plankton input. The second type (including the Mbr 1 and Mbr 21 of Liushagang Fm source rocks) was deposited in a suboxic and fresh to brackish water environment with mixed inputs of aquatic algal plankton and terrestrial higher plants. Three groups of crude oils (A, B, and C) are identified using their physical properties and biomarker data. The group A oil is characterized by a higher abundance of oleanane, lower abundance of C30 4-methylsterane, higher values of Pr/Ph, and lighter δ13C values, and is mainly derived from the Mbr 21 of Liushagang Fm source rock. Group B and group C oils are characterized by a lower abundance of oleanane and higher abundance of C30 4-methylsterane, but present some differences in Pr/Ph and δ13C values. Both groups of oils are sourced from the Mbr 23 of Liushagang Fm source rock, and their differences in geochemical characteristics indicate the lateral heterogeneity of source rock. Oil migration directions are determined by combining maturity parameters, pyrrolic nitrogen compounds, and also basin modeling results. The preferential oil migration directions in Wushi sag are from the central to the northeast and south of the sag. The accumulation distribution of different groups of oils is mainly controlled by the vertical and lateral heterogeneity of Mbr 2 of Liushagang Fm source rock.

Jurassic – Cretaceous boundary record in Carpathian sedimentary sequences

An integrated study of micro- and nannoplankton distribution, O and C isotopes and palaeomagnetic record of nine Carpathian sections provided a key interpretation tool to characterize the Jurassic–Cretaceous (J/K) boundary transition. Biostratigraphy supported by saccocomids, radiolarians, calpionellids, dinoflagellates and nannofossils helped to distinguish biostratigraphic zones which could have been correlated both with stable isotopes and with magnetostratigraphy. The Alpina Event (lorica size change) accompanied by Nannoconus wintereri and N. steinmannii minor FO's designates the J/K boundary.Sedimentary model in N Mediterranean Tethys margin consists of elevated blocks with slow (1–2 mm/ka) Rosso Ammonitico carbonate sedimentation separated by dysoxic depressions. Tectonic instability has been indicated by Upper Jurassic fluxoturbidites.Late early Tithonian (M21n to M20r) saccocomid abundance increases with positive δ18O isotope ratios, similar excursion between M20n and M19r accompanied late Tithonian crassicollarian boom. The δ13C variation (>0.2‰) confronted with microplankton quantity variations: positive excursion occurred both during early Tithonian Saccocoma sp., and during late Tithonian calpionellid and nannoconid blooms. Higher up in the Berriasian “Maiolica” sequence, lighter δ13C values prevailed.During late Tithonian (Praetintinnopsella-to earliest Crassicollaria zones; M20n1n to M19r), Saccocoma microfacies has been substituted by a calpionellid one. Reorganization of Tethyan oceanic currents during opening of the Hispanic corridor started planktonic carbonate particle „rain“ resulting in the Maiolica facies dominance. Sedimentary rate raised ten times. Calpionella alpina lorica size change event was synchronneous with FO's of Nannoconus wintereri and N. steinmannii minor and with negative δ18O trend of latest Tithonian (Colomi Subzone) warming. Expressiveness of eccentricity and obliquity cycles indicates climatic optimum conditions.

Reconstruction of 7500 years of coastal environmental change impacting seagrass ecosystem dynamics in Oyster Harbour (SW Australia)

Seagrass ecosystems, which have important functions such as coastal protection and blue carbon sequestration, are threatened by anthropogenic pressure including climate change. Long-term data series from seagrass sedimentary archives (mats) can be used to understand natural cycles of environmental change and answer key questions related to contemporary management. A 7500 yr sediment record from Posidonia australis meadows in Oyster Harbour (Albany, SW Australia) was subjected to multiproxy reconstruction by means of pigment analysis (UHPLC), analytical pyrolysis (Py-GC–MS), carbonate content, δ13C and δ15N stable isotope ratios, organic C (Corg) content, Corg/N ratio and glomalin-related soil proteins (GRSP). The study revealed a brackish lagoon (7500–7000 cal yr BP) that was transformed in an open marine environment (7000–4100 cal yr BP) due to Holocene transgression. Earliest evidence of seagrass establishment was detected around 4500 cal yr BP, and meadow extension accelerated between 4100 and 3700 cal yr BP. The meadow environment was surprisingly resistant against environmental perturbations, as the mat, composed of P. australis seagrass fibres embedded within a siliciclastic mineral matrix containing biogenic carbonates, continued to develop steadily until 190 cal yr BP (1830 CE). Then, shifts in several proxies (pigments, GRSP) showed evidence of terrestrial runoff-triggered eutrophication/turbidity (likely driven by forest clearance and agricultural activities after European settlement), but the seagrass showed resilience (no decline of the proportion of seagrass-derived Corg). By contrast, since ~1930 CE seagrass retreat is evident in the biogeochemical record: lighter δ13C values, lower lignin abundance and shifts in pigment abundance and types, affecting the balance between seagrass inputs and alternative sources, as was observed in previous studies of the area. The findings show that pigment proxies are useful early indicators of shifts in seagrass ecosystem condition, while lignocellulose and other pyrolysis products are useful proxies of more profound ecosystem alterations that influence seagrass abundance. The record indicates that the climax seagrass ecosystem condition, which prevailed for several millennia, had been impacted over the last century. Management of seagrass and coastal ecosystems should aim to avoid crossing ecological thresholds and diminish local impacts aggravating those of global change.

Carbon and nutrient accumulation in mangrove sediments affected by multiple environmental changes

In order to assess the impact of anthropogenic activities on carbon and nutrient accumulation, total organic carbon (TOC), nitrogen (TN), and phosphorus (TP) accumulation rates were examined in a 210Pb-dated mangrove sediment core from Sepetiba Bay, Brazil, a coastal region impacted by multiple environmental changes during the previous century. A 50-cm length sediment core was collected from a mangrove forest in Sepetiba Bay. Sediment subsamples were analyzed to measure TOC, TN, δ13C, and δ15N using an elemental analyzer attached to an isotope ratio mass spectrometer Thermo Finnigan Model Delta Plus XP, whereas colorimetric analysis were used to measure TP. For 210Pbex analyses, gamma-ray measurements were performed in a semiplanar intrinsic germanium high purity coaxial detector, coupled to a multichannel analyzer, whereas the sediment accumulation rate (SAR) was calculated according to the constant initial concentration (CIC) method. Also, carbon and nutrient fluxes were calculated using SAR and TOC, TN, and TP contents, whereas statistical differences were evaluated by ANOVA + Tukey HSD analysis with previous data normalization. The calculated sedimentation rate (~ 8.1 mm year−1) since the early 1900s was up to threefold higher than the global mean determined for mangrove forests (~ 2.8 mm year−1) and the regional sea level rise (~ 3.2 mm year−1). Significantly higher TOC, TN, and TP fluxes, up to nearly 1000, 90, and 15 g m−2 year−1, respectively, were observed after the water diversion from a nearby drainage basin in the 1950s and an increase in sewage effluent input, which increased in the early 1990s. After this period, lighter δ13C values (~ − 25‰) indicate an increased importance of the terrestrial organic matter source, while lower TOC:TN ratios (~ 11) and heavier δ15N values (~ + 9‰) suggest an increased influence of anthropogenic fertilization on inorganic nitrogen accumulation. The significantly higher accumulation rates during the last decades evidenced the role of mangrove sediments as sinks for anthropogenically enhanced inputs of carbon and nutrients. Also, studies on carbon and nutrient accumulation evidenced the need for further research in eutrophic coastal areas.

Geochemical characteristics and genetic origin of crude oil in the Fushan sag, Beibuwan Basin, South China Sea

Abstract The Fushan sag is an important petroleum-bearing unit in the Beibuwan Basin, South China Sea. However, genetic relationship between oil and source rocks has not been well understood. In this work, 43 mudstone samples and 22 crude oil samples in the sag were collected for molecular geochemical and isotopic analyses to investigate this relationship. The geochemical data shows that the mudstones in Liushagang formation have poor to good oil generation potential with TOC ranging from 0.45 to 2.19 wt% and the kerogen is mainly type Ⅱ. Two types of crude oils were identified in this study. The first type of Group A oils including the subgroup A1 and subgroup A2 has the characteristics of low maturity with low saturated hydrocarbon content and lighter δ13C values in the Huachang uplift. The source rocks for the Group A oils were deposited under suboxic to oxic and freshwater environment containing more terrestrial higher plant and less angiosperm. The subgroup A1 oils in Els11 reservoirs are separated from subgroup A2 in the Els12 reservoirs by relatively lower maturity. The subgroup A1 and subgroup A2 oils are closely related to the Els1 to Els21 and Els12 to Els21 source rocks, respectively. While the second type of oils including Group B and Group C oils has higher level of maturity with higher saturated hydrocarbon content and heavier δ13C values. The source rocks of Group B oils were deposited in a freshwater environment with lower salinity, and more angiosperms and terrestrial higher plants input than those of Group C oils. Two subgroups are divided in the Group B oils, of which subgroup B1 oils have higher maturity and their source rocks were deposited in lower salinity with more terrestrial higher plant input than that of subgroup B2 oils. The subgroup B1 oils occurring in the Els31 reservoirs from the Huachang uplift are likely related to Els31 source rocks, while subgroup B2 oils in the Els22 reservoirs from the southern slope area of the Bailian subsag is linked with mixed source of Els22 and Els31 mudstones. The Group C oils are distributed in Els22 and Els31 reservoirs in the Huachang uplift and their oil source are from Els32 mudstones. Two petroleum plays are likely divided by the mfs in Els2 based on the oil-source correlation in the sag. Group B and Group C oils belong to the lower play, and Group A oils only occur in the upper play. The distribution and accumulation of different groups of oils are mainly controlled by the special two-layered tectonostratigraphic framework and the transport ability of fault zone in the sag. This study clarifies the types of oils, as well as the oil-source correlation and the spatial distribution of different groups of oils. The results could benefit the further petroleum exploration and resource evaluation in the sag.

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

The hydrothermal caves linked to active faulting can potentially harbour subterranean atmospheres with a distinctive gaseous composition with deep endogenous gases, such as carbon dioxide (CO2) and methane (CH4). In this study, we provide insight into the sourcing, mixing, and biogeochemical processes involved in the dynamic of deep endogenous gas formation in an exceptionally dynamic hypogenic karst system (Vapour Cave, southern Spain) associated with active faulting. The cave environment is characterized by a prevailing combination of rising warm air with large CO2outgassing (&gt;1%) and highly diluted CH4with an endogenous origin. Theδ13CCO2data, which ranges from −4.5 to −7.5‰, point to a mantle-rooted CO2that is likely generated by the thermal decarbonation of underlying marine carbonates, combined with degassing from CO2-rich groundwater. A pooled analysis ofδ13CCO2data from exterior, cave, and soil indicates that the upwelling of geogenic CO2has a clear influence on soil air, which further suggests a potential for the release of CO2along fractured carbonates. CH4molar fractions and theirδD andδ13C values (ranging from −77 to −48‰ and from −52 to −30‰, respectively) suggest that the methane reaching Vapour Cave is the remnant of a larger source of CH4, which was likely generated by microbial reduction of carbonates. This CH4has been affected by a postgenetic microbial oxidation, such that the gas samples have changed in both molecular and isotopic composition after formation and during migration through the cave environment. Yet, in the deepest cave locations (i.e., 30 m below the surface), measured concentration values of deep endogenous CH4are higher than in atmospheric with lighterδ13C values with respect to those found in the local atmosphere, which indicates that Vapour Cave may occasionally act as a net source of CH4to the open atmosphere.

The origin and accumulation of multi-phase reservoirs in the east Tabei uplift, Tarim Basin, China

Abstract A giant deep-strata oil-gas field with complex fluid characteristics was recently discovered in the Hade-Yuke area of the east Tabei uplift (Tarim Basin, NW China). The deep fluids show a lateral co-existence of several gas, condensate and oil phase reservoirs characterized through the integration of complimentary geochemical (e.g., bulk composition, biomarker and stable carbon isotope) and geological data. Typical features of the gas and condensate reservoirs, which were mainly distributed in the eastern parts of the study site, included high GOR (gas-to-oil ratio), hydrocarbon gases with relatively heavy δ13C values and crude oils with high wax content. In contrast oil reservoirs, which mostly occurred in more westerly locations, had lower GOR values, lighter δ13C values and lower wax content, indicating gradual change of fluid characters. The petroleum accumulations derive from two hydrocarbon charge events: an early oil charge in the Late Hercynian and a later gas charge in the Late Himalayan which migrated from east to west, leading to a gradual variation in fluid phase and characteristics. Alteration of paleo oil reservoirs by this secondary gas charge was therefore primarily responsible for the complex fluid character and multi-phase reservoirs presently in place. Secondary condensates were formed from the retrograde condensation of paleo reservoirs. Our re-construction of the hydrocarbon accumulation process encourages future exploration endeavours to target commercially viable gas accumulations in the deep east strata and oil reservoirs in the west.

Stable Isotope (δ13C, δ15N, δ18O, δD) Composition and Nutrient Concentration of Red Sea Primary Producers

Measurements of isotopic composition of marine primary producers are a valuable tool to follow and trace the source and cycling of organic matter in the marine systems, as well to describe the physiological status of aquatic photosynthetic organisms. Although stable isotope data abounds in the literature, relatively limited information regarding the isotopic signatures of marine primary producers is available for the Red Sea. Here we present data on carbon concentration (and nitrogen when possible) of phytoplankton, macroalgae, seagrasses, mangroves and salt-marsh plants, and examine how their isotopic signatures differed among plant types across a north-south gradient in the Red Sea. We also tested the potential use of deuterium, δD, to distinguish among primary producers whose carbon isotopic values may overlap. Our findings showed a clear differentiation of carbon and nitrogen content between the different groups of primary producers, as well as between species. Seagrasses and mangroves had on average larger carbon (30 and 49 % of C, respectively) and nitrogen content (1.8 % N) than other groups. In terms of stable carbon isotopes, seagrasses and macroalgae tended to be heavier (-7.3 ‰ and -13.3 ‰, respectively) than halophytes, mangroves, and phytoplankton, which showed statistically similar and lighter δ13C values (between -24 ‰ and -26 ‰). There was a tendency for the nitrogen isotopic composition of seagrass and macroalgae to become lighter from the southern to the northern Red Sea, in parallel to a decline in nitrogen concentration in the tissues, indicative of a higher dependence of nitrogen fixation as a source of nitrogen toward the more oligotrophic northern Red Sea. Our results showed an overlap in the δ13C and δ15N values between macroalgae and seagrasses; however, their δD values were significantly different (seagrasses -56.6 ± 2.8 ‰ and macroalgae -95.7 ± 3.4 ‰). This remarkable difference offers a promising alternative for ecological studies where a similar range of isotopic values could mask different potential sources.

Impacts of Altered Hydrology on the Sources of Particulate Organic Carbon on the Diet ofCrassostrea virginicain the Northern Everglades, Florida, USA

ABSTRACT Stable isotope composition of the eastern oyster Crassostrea virginica was characterized from three estuaries in the Ten Thousand Islands (Florida). Freshwater in flow from watershed management is affecting this region and has deeply modified the salinity gradient. Stable carbon and nitrogen isotopes were used to trace specific sources of organic matter and the influence of these sources of carbon on the diet of the eastern oyster in the Greater Everglades. Oysters, as well as particulate organic matter (POM) and benthicmicroalgae (BMA) were sampled in three bays at three different stations from upstream to downstream during both wet and dry seasons. Significant salinity differences were observed in all estuaries between summer (wet) and winter (dry) seasons and were linked to freshwater inputs into the bays. Temperature followed a typical seasonal trend. Oysters were enriched in δ15N compared with POM and BMA, and similarly or slightly enriched in δ13C for both seasons. Lighter δ13C values in th...

14 CO 2 analysis of soil gas: Evaluation of sample size limits and sampling devices

Abstract Radiocarbon (14C) analysis of CO2 respired from soils or sediments is a valuable tool to identify different carbon sources. The collection and processing of the CO2, however, is challenging and prone to contamination. We thus continuously improve our handling procedures and present a refined method for the collection of even small amounts of CO2 in molecular sieve cartridges (MSCs) for accelerator mass spectrometry 14C analysis. Using a modified vacuum rig and an improved desorption procedure, we were able to increase the CO2 recovery from the MSC (95%) as well as the sample throughput compared to our previous study. By processing series of different sample size, we show that our MSCs can be used for CO2 samples of as small as 50 µg C. The contamination by exogenous carbon determined in these laboratory tests, was less than 2.0 µg C from fossil and less than 3.0 µg C from modern sources. Additionally, we tested two sampling devices for the collection of CO2 samples released from soils or sediments, including a respiration chamber and a depth sampler, which are connected to the MSC. We obtained a very promising, low process blank for the entire CO2 sampling and purification procedure of ∼0.004 F14C (equal to 44,000 yrs BP) and ∼0.003 F14C (equal to 47,000 yrs BP). In contrast to previous studies, we observed no isotopic fractionation towards lighter δ13C values during the passive sampling with the depth samplers.

Sedimentology and Geochemistry of the 2930 Ma Red Lake–Wallace Lake Carbonate Platform, Western Superior Province, Canada

AbstractChemical sedimentary rocks in the Red Lake–Wallace Lake area of the Canadian Shield form the Earth's oldest known carbonate platform and, as such, provide a unique opportunity to explore the floor of a 2930 Myr old warm, shallow sea. Peritidal depositional features dominate the platform top, ranging from colloform crusts, teepee structures, and evaporate pseudomorphs in the supratidal; pseudomorph crystal fans and laterally linked domal stromatolites, associated with stromatactis‐like structures, sheet cracks and liminoid fenestral fabrics extending from the lower supratidal through the intertidal; and herring‐bone cross‐stratification, isolated domal stromatolites and herring‐bone calcite cement in tidal channels. These lithofacies indicate a low energy, restricted evaporitic environment. Limited subtidal platform top deposits are characterized by laterally linked domal stromatolites and pseudomorph crystal fans on a larger scale than those found in the intertidal areas. A transitional lithofacies to deeper water were deposited further offshore. It consists of ribbon rock (mixed laminae and beds of carbonate, slate and iron oxide sediments), slump structures composed of intraclastic carbonate lithoclasts in a marl matrix and carbonate‐associated iron formation. Basinal deposits consist of chert and chert‐oxide facies iron formation. Peritidal lithofacies are composed of ferroan dolomite, whereas deep subtidal to upper slope lithofacies are composed of calcite. The dolomites have O and Sr isotopic ratios which were probably reset during dolomitization, whereas only O is altered in the limestones. The δ13C values for all the carbonate samples were 0 ± 1·1‰V‐PDB, with samples formed in deeper water having lighter δ13C values, suggesting the deeper open ocean had a lighter δ13C budget than water in the platform interior. Post Archean Australian Shale normalized rare earth element patterns for the carbonate samples have positive La and Eu anomalies, suprachondritic Y/Ho ratios and slight heavy rare earth element enrichment in most samples. Basin lithofacies are characterized by heavy rare earth element enrichment and positive Eu anomalies. One sample had a significant negative Ce anomaly. These data probably indicate restricted circulation in the supra and intertidal areas and possibly the development of spatially limited areas where oxygen production could move the redox boundary out into the water column.

Characterization of Eocene lacustrine source rocks and their oils in the Beibuwan Basin, offshore South China Sea

Two hundred twenty five rock samples and thirty seven oil samples from the Beibuwan Basin, South China Sea, were analyzed with geochemical and organic petrological techniques to evaluate the Eocene lacustrine source rocks, and investigate controls on their properties and the distribution of different oil families in the basin. Two types of organic facies are recognized in the Liushagang (LS) Formation. The first organic facies is algal-dominated, and mainly occurs in the organic-rich, laminated mudstones of the middle member of the LS (LS-2) that were deposited in an anoxic, stratified, medium–deep lake environment. It is geochemically identified by its high abundance of C30 4-methylsteranes and heavy δ13C values in the range of -22.4 to -27.5‰. The organic matter in this organic facies comprises type I and II1 kerogens, with its macerals dominated by fluorescent amorphous organic matter (AOM) and exinites, indicating a highly oil-prone character. The second organic facies is of terrestrial algal origin, and is mainly identified in the non-laminated mudstones of the upper (LS-1) and lower (LS-3) members of the Liushagang Formation that were deposited in shallow, dysoxic, weakly stratified, fresh-water environments. Source rocks of the second organic facies mainly contain type II1–II2 kerogens with mixed macerals of AOM, internites and vitrinites. It is geochemically differentiated from the algal-dominated organic facies by its relatively low abundance of C30 4-methylsteranes and lighter δ13C values in the range of -27.20 to -28.67‰. Three oil groups are identified by their biomarkers and stable carbon isotopes. The first two groups (A and B) are probably end-members of two major oil families (A and B) that correspond to the algal-dominated organic facies and algal-terrestrial organic facies, respectively. Most of the discovered oils belong to Group A oils that are characterized by a high abundance of C30 4-methylsteranes and heavy δ13C values, and show a good correlation with the algal-dominated organic facies in LS-2. Group B oils are found only within the LS-1 and LS-3 reservoirs, and they are recognized by their relatively low content of C30 4-methylsteranes and lighter δ13C values, showing a close relation to the algal-terrestrial source facies within the LS-1 and LS-3members, respectively. Group C oils display intermediate biomarker features and stable carbon isotope values, and are interpreted to be a mixture of Group A and B oils. The oil-source correlation reveals a strong control of organic facies on the geographic distribution of oil groups or oil fields in the basin.

An ocean-floor carbonaceous sedimentary sequence in the 3.2-Ga Dixon Island Formation, coastal Pilbara terrane, Western Australia

The Dixon Island Formation of the Pilbara terrane, Western Australia, extends from Cleaverville Beach to the Dixon Island coast, and is the only example worldwide of a coastal outcrop of a 3.2–3.1Ga low-grade greenstone belt. The Dixon Island Formation was situated in an immature island arc setting and comprises siliceous, carbonaceous deep-water sediments that contain evidence for hydrothermal and microbial activity. The extensive outcrop along the coastline makes it possible to examine in detail the characteristics of Mesoarchean sedimentation in a hydrothermal environment. This study focuses on a continuously exposed carbonaceous, black chert succession in the central part of the northern coastline outcrop on Dixon Island. At this site, a 20-m-thick, carbonaceous, black chert sequence conformably overlies basement rocks of highly altered komatiite–rhyolite tuffs. The black chert sequence formed well-bedded black chert with carbonaceous peloidal matter and fragmented grains, and the sequence is homogeneous and finely laminated. In this sequence, evidence of low-temperature hydrothermal fluid, sediments and alteration stractures are well preserved in the lowermost section, indicating that high levels of hydrothermal activity occurred on the ocean floor during deposition. In particular, swarms of black chert veins provide evidence of post-volcanic hydrothermal activity that released organic matter and silica to the ocean. The carbonaceous peloidal textures and δ13Corg values of sediments located just above the basement, which hosts the vein swarms, suggest that the veins were the conduits for hydrothermal fluid which contained organic-rich silica material and that flowed onto the seafloor to form the homogeneous carbonaceous cherts along with hydrothermal-related sediment. The δ13Corg values of organic matter in the black cherts range from −42‰ to −22‰ (average=−31.9‰; n=313). Lighter δ13C values (−35‰ to −42‰) characterize carbonaceous laminated black chert located ∼5m above the basement, where biogenic structures (e.g. biomat bed, microfossil structures) are found. The lighter δ13C values might be related to methanotrophic micro-organic activity within the sediments during hydrothermal activity. In summary, we reconstructed the sedimentary environment upon a Mesoarchean hydrothermal ocean floor that was a site of microbial activity and local methanogenesis.

The Little Ice Age: evidence from a sediment record in Gullmar Fjord, Swedish west coast

Abstract. We discuss the climatic and environmental changes during the last millennium in NE Europe based on a ca. 8-m long high-resolved and well-dated marine sediment record from the deepest basin of Gullmar Fjord (SW Sweden). According to the 210Pb- and 14C-datings, the record includes the period of the late Holocene characterised by anomalously cold summers and well-known as the Little Ice Age (LIA). Using benthic foraminiferal stratigraphy, lithology, bulk sediment geochemistry and stable carbon isotopes we reconstruct various phases of the cold period, identify its timing in the study area and discuss the land–sea interactions occurring during that time. The onset of the LIA is indicated by an increase in cold-water foraminiferal species Adercotryma glomerata at ~ 1350 AD The first phase of the LIA was characterised by a stormy climate and higher productivity, which is indicated by a foraminiferal unit of Nonionella iridea and Cassidulina laevigata. Maximum abundances of N. iridea probably mirror a short and abrupt warming event at ~ 1600 AD. It is likely that due to land use changes in the second part of the LIA there was an increased input of terrestrial organic matter to the fjord, which is indicated by lighter δ13C values and an increase of detritivorous and omnivorous species such as Textularia earlandi and Eggerelloides scaber. The climate deterioration during the climax of the LIA (1675–1704 AD), as suggested by the increase of agglutinated species, presence of Hyalinea balthica, and a decline of N. iridea may have driven the decline in primary productivity during this time period.

Organic carbon and nitrogen composition in the sediment of the Kara Sea, Arctic Ocean during the Last Glacial Maximum to Holocene times

[1] A sharp decrease in Corg/Ntot ratios and apparently heavier stable isotope ratios of Corg and Ntot (δ13Corg and δ15Ntot) after deglaciation were observed in sediment cores St-25 and St-34 collected from the Saint Anna Trough in the Kara Sea, exhibiting a change of sediment origin from terrestrial to marine during the LGM and Holocene times. Organic nitrogen (Norg) contributed about 40% to total nitrogen (Ntot) during the Holocene, but the Norg fraction was hardly observed in the lower LGM sediments (below 120 cm depth in St-25 and beneath 170 cm in St-34). This may be due to a permanent cover of sea ice in the area during the LGM. In addition, the δ13C values for OClabile and OCresi were remarkably different between LGM and Holocene sediment. The unexpected lighter δ13C values for OCresi for the LGM period might be due to methanotrophic bacteria in the sediment of Kara Sea.

Fluid venting activity on the Costa Rica margin: new results from authigenic carbonates

Carbonate precipitates on mounds and along tectonic scarps off the Costa Rica margin are manifestations of subduction-induced dewatering. The long-term dewatering history is recorded in mineralogical, petrological and isotope signals of carbonates recovered from these sites. The carbonates are strongly depleted in δ13C (−11 to −53‰ PDB) and enriched in δ18O (+4 to +8‰ PDB). Thermogenic methane and biogenic methane were identified as sources of the carbon. Chemoherm carbonates and seepage-associated carbonates formed in a focused flow regime have lighter δ13C values, while others formed in a more diffusive flow regime have slightly enriched C isotope values. Three fluid components were inferred based on the calculation of equilibrium δ18O: clay dehydration water, gas hydrate water and seawater. Calculated equilibrium δ18O values of carbonates from different down-core depths as well as from different precipitation stages show that the δ18O of the precipitating fluid is progressively depleted with time. Dolostones showing a methane-C source and a well constrained O-isotope signature are thought to have formed at depth in the sediment and subsequently became exhumed. Glauconitic sandstones cemented by methane-derived carbonate provide evidence that fluid and solid material have been expelled by the mud volcano.

Radiocarbon and δ13C Values Related to Ground‐Water Recharge and Mixing

AbstractGround‐water levels in the Upper Floridan aquifer beneath the southeastern coast of South Carolina have undergone pumpage‐induced declines approaching 20 ft below sea level at the southern end of Hilton Head Island. This scenario suggests the potential exists for the inducement of recharge to the Upper Floridan aquifer across the island, which could affect the quality of water being pumped by wells. However, low radiocarbon concentrations in ground‐water samples (0.5 to 1.4 ± 0.1 PMC) indicate that most of the water is relict ground water reflecting prepumpage ground‐water flow conditions in the Upper Floridan aquifer. The isotopic data indicate long residence times and water‐chemistry evolution more characteristic of ground‐water recharge occurring farther inland prior to the commencement of pumpage in the late 1800s.Radiocarbon concentrations (as Percent Modern Carbon) and stable carbon isotope ratios (as δ13C in dissolved inorganic carbon) determined during this study and reported in other studies on and around Hilton Head Island varied in a systematic manner. Heavier δ13C values (–2.8 to –1.6 per mil) in ground water beneath southern Hilton Head Island reflect ground‐water discharge from prepumpage flowpaths originating over 100 miles away, hence a depletion in radiocarbon concentration with corrected ground‐water ages no younger than 16,000 yrs BP. In contrast, lighter δ13C values (–13.9 to –8.67 per mil) beneath the northern part of the island indicate recent recharge as a result of water‐level declines, and recharge in areas off the island that have not changed as a result of pumpage (evidenced by enrichment in radiocarbon with corrected ground‐water ages no older than 4,000 yrs BP). This suggests that the δ13C composition of ground water in the Upper Floridan aquifer is a useful indicator of mixing between ground waters from different sources, and can be used to delineate recharge‐discharge patterns. This approach may be applicable to other aquifers of highly evolved ground‐water chemistry in regional carbonate aquifer systems that may be receiving recent recharge. Moreover, this approach could prove useful in delineating the contribution of recent water being captured by pumped wells as part of wellhead protection programs designed to assess aquifer vulnerability from surficial contaminant sources.

Anomalous 13C enrichment in modern marine organic carbon

Marine organic carbon is heavier isotopically (13C enriched) than most land-plant or terrestrial organic C1. Accordingly, δ13C values of organic C in modern marine sediments are routinely interpreted in terms of the relative proportions of marine and terrestrial sources of the preserved organic matter2,3. When independent geochemical techniques are used to evaluate the source of organic matter in Cretaceous or older rocks, those rocks containing mostly marine organic C are found typically to have lighter (more-negative) δ13C values than rocks containing mostly terrestrial organic C. Here we conclude that marine photosynthesis in mid-Cretaceous and earlier oceans generally resulted in a greater fractionation of C isotopes and produced organic C having lighter δ13C values. Modern marine photosynthesis may be occurring under unusual geological conditions (higher oceanic primary production rates, lower ) that limit dissolved CO2 availability and minimize carbon isotope fractionation4.