Isotopic Composition Of Organic Matter Research Articles

Organic Matter in the Ocean: Indicators of Biogeochemical Processes

The article presents informative organogeochemical indicators used in oceanology to characterize biogeochemical processes. A classification based on data on suspended particulate matter, plankton, benthos, bottom sediments, and sea, river, and pore water has been developed. Clusters have been identified that include the elemental and isotopic composition of organic matter (OM), the group composition of lipids, aliphatic and alicyclic hydrocarbons, higher alcohols, macromolecular acids and ketones, isoprenoids, lignin and its structural monomers, and PAHs. The most informative indices are considered, which make it possible to assess the genesis of the original OM, the degree of its transformation, sources, and the ratio of aquatic and terrigenous components. These indices reflect the influx of riverine and aeolian material and permafrost thawing and make it possible to reconstruct the temperature of river waters and coastal upwellings. They significantly complement biooceanological, lithological, and geochemical studies and expand the biogeochemical methodology for studying the carbon cycle.

Assessment of kerogen types and source rock potential of Lower Jurassic successions in the Mandawa Basin, SE Tanzania

Variability in organic matter types and quality of source rock intervals across evolving rift-basins has been the focus of previous studies, which have shown that due to dynamic sedimentary regimes and water depths such source intervals occur sporadically and restricted to specific stratigraphic horizons. Such heterogeneous settings require detailed geochemical analysis to document variations in source quality. This study presents a comprehensive geochemical analysis of source rock intervals in Lower Jurassic successions (Mbuo, Nondwa and Mihambia Formations) in the Mandawa Basin, on-shore Tanzania, in order to constrain organic matter types and quality. Organic matter richness was measured by Total Organic Carbon (TOC) analysis, while quality and thermal maturity of organic matter was assessed by programmed pyrolysis, complemented by organic carbon isotope analysis (δ13Corg) of sediment samples collected from outcrops, cores, and cuttings samples. TOC and programmed pyrolysis datasets from existing literature were synthesized in this study to better understand organic matter quality and thermal maturity evolution on regional scale. These data were examined in relation to paleogeography to gain insight into the factors regulating the distribution of organic matter quality within the Mandawa Basin.Average TOC contents in Lower Jurassic sediments fluctuate from 0.31 to 4.83 wt% with an average of 1.4 wt%, whereas, kerogen yield (S2) and hydrogen index (HI) remain generally low, with values ranging from 0.01 to 8.6 (mg HC/g rock) with an average (0.92 mg HC/g rock) as well as from 2 to 233 (mg HC/g TOC) with an average of 46 (mg HC/g TOC), respectively. Organic matter is composed mainly of type III (gas prone) to IV (inert) kerogens with lesser admixture of type II-III kerogen (oil-gas prone), which have been deeply buried and undergone a wider range of thermal diagenesis, from early to overmature stage. The δ13Corg isotopic composition of organic matter is consistent with kerogen typing, indicating terrestrial organic matter with a heavier signature during sea level low stands. Upon marine incursions δ13Corg shifts to isotopically more depleted marine organic matter. This implies that the investigated source intervals have poor to fair source potential. Only a few intervals of limited thickness in the Upper Mbuo, Nondwa, and Mihambia Formations of the Mita Gamma-1 and East Lika-1 wells, as well as in the Lower Mbuo Formation in the Mbuo-1 well, have been locally enriched. The enrichment is attributed to periodic marine ingressions that have persisted since the Pliensbachian period and suggests the presence of oil generation potential.A comparison of TOC and programmed pyrolysis borehole data from this study to those compiled from literature in a paleogeographical context reveals high spatial variability. The southeastern and northwestern parts of the study area have the highest source potential, indicating high organic matter productivity in the dysoxic to anoxic water column, which promoted the preservation of organic matter. In contrast, the eastern and western parts generally show low source potential, indicating degraded organic matter and significant terrestrial influxes from emergent land that were deposited in shallow, mixed oxygenated water with low to moderate organic matter productivity.

Tooth enamel nitrogen isotope composition records trophic position: a tool for reconstructing food webs

Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs; however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotope composition of organic matter preserved in mammalian tooth enamel (δ15Nenamel) records diet and trophic position. The δ15Nenamel of modern African mammals shows a 3.7‰ increase between herbivores and carnivores as expected from trophic enrichment, and there is a strong positive correlation between δ15Nenamel and δ15Nbone-collagen values from the same individuals. Additionally, δ15Nenamel values of Late Pleistocene fossil teeth preserve diet and trophic level information, despite complete diagenetic loss of collagen in the same specimens. We demonstrate that δ15Nenamel represents a powerful geochemical proxy for diet that is applicable to fossils and can help delineate major dietary transitions in ancient vertebrate lineages.

Reconstructing the Paleo‐Ecological Diet of Snow Petrels (Pagodroma nivea) From Modern Samples and Fossil Deposits: Implications for Southern Ocean Paleoenvironmental Reconstructions

AbstractSnow petrels (Pagodroma nivea), which are endemic to the Antarctic region, produce proventricular stomach oil from ingested food for feeding purposes but also spit the oil in the immediate surrounds of the nests, where it forms encrustations over time (Antarctic mumiyo). These deposits provide a unique opportunity to understand the paleo‐ecological diet of snow petrels and because the seabirds forage in the ocean, they potentially provide an archive of past marine environmental conditions in the Southern Ocean. For validating methods for reconstructions we use compositional data obtained on modern stomach oils and DNA data from fecal samples of snow petrels. We find that the distribution of carboxylic acid compounds in modern stomach oils and in the fossil deposits are consistent with variable contributions of fish and krill, which are the main constituents of modern snow petrel diet, and allows inference of past changes in snow petrel diet from the fossil record. Analyses of mumiyo deposits from six regions in East Antarctica reveal systematic differences in the isotopic composition of organic matter (δ13C and δ15N) and carboxylic acid patterns. This may suggest regional and/or temporal variability in the composition of snow petrels diet, likely differing in response to the prevailing environmental conditions in the foraging range of the birds, such as sea‐ice variability, polynya activity and primary productivity. Our study provides confidence for using these approaches for broader scale paleo‐studies in the future and for an assessment of the temporal changes and regional variability in snow petrel diet.

Overprinting of δ13Corg signatures in the sedimentary record linked to hydrocarbon migration: Evidence from Paleozoic drill cores from the Ordos Basin, China

Carbon isotopic compositions of organic matter (δ13Corg) provide critical information about changes in the global carbon cycle through Earth's history. However, several biotic and abiotic processes occurring after sediment deposition may overprint the original geochemical signals, complicating their paleoenvironmental interpretation. Here, we report multiple large and rapid δ13Corg swings (ranging from −30.8‰ to −23.3‰) from three Paleozoic drill cores in the Ordos Basin, Northwest China, which may reflect different organic contributions related to lithology. As a potentially important organic matter source, migrated hydrocarbons may substantially alter the original δ13Corg and total organic carbon contents (TOC), and their oxidation may contribute to forming 13C- and 18O-depelted carbonate cements in siliciclastic-dominated strata at depth. Cross-plots of δ13Corg and TOC from samples of the three drill cores all show similar hyperbolic trends that are well simulated using Rayleigh fractionation and a two-component mixing model, thus providing potential criteria for identifying the reservoir and the related hydrocarbon source in oil exploration. Our data call for a critical reassessment of the δ13Corg records of sedimentary rocks if hydrocarbon redistribution or mixing of organic sources is suspected to have occurred during the burial history.

Recent global warming induces the coupling of dissimilar long-term sedimentary signatures in two adjacent volcanic lakes (Azores Archipelago, Portugal)

Paleoclimatological information derived from the study of lacustrine sedimentary records is not only biased by taphonomical processes but also by potential differences in the expression of climate variability in the sediments due to site-specific factors. Using a multiproxy approach (the elemental and isotopic compositions of organic matter, diatom assemblages, and marker pigments of algae and cyanobacteria), we study the different environmental signatures recorded since the Little Ice Age (LIA) in the sediments of two volcanic lakes located within the same caldera on São Miguel Island (Azores Archipelago). Lake Santiago is a crater lake whose eutrophic status in the last stage of the LIA was linked to external nutrient inputs associated with this humid period. Its post-LIA evolution was forced by changes in the thermal structure of the water, which determined its degree of mixing and therefore nutrient availability through recycling from the hypolimnion. In contrast, the decadal to centennial limnological evolution of Lake Azul, a caldera lake 2.5 km from Lake Santiago, shows geochemical and micropaleontological signatures disconnected from climate variability until 1980/1990 CE due to its greater exposure to the fallout of tephra after a catastrophic eruption in c. 1290 CE. Only after 1980/1990 CE did a global warming scenario induce a common ecological restructuring of both lakes, involving the replacement of turbulence-loving algal taxa by species adapted to strengthening water column stratification. Nevertheless, this shift was relatively gradual in Lake Azul but more sudden in Lake Santiago, indicating that the local site-specific components still had an effect on the expression of climate change in the sediments. Despite the short history of anthropogenic pressure (compared to their continental counterparts) and the large atmospheric patterns operating over the Azores Archipelago, the sedimentary records of these two adjacent oceanic volcanic lakes reacted quite differently to climate changes.

Iron-mediated anaerobic ammonium oxidation recorded in the early Archean ferruginous ocean.

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ15 N values found in well-preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much-needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ15 N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non-quantitative oxidation of ammonium via the Feammox pathway, a metabolic co-cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ15 N signatures in Archean sediments.

Eccentricity paced paleoenvironment evolution and microbial community structure in the Gulf of Mexico during the outgoing Early Eocene Climate Optimum

Orbital-driven climate fluctuations and associated variations in the carbon cycle over short- and long-term time scales can be recorded in sedimentary archives. Bulk geochemical, biomarker, and stable isotope signatures in sediments deposited at the end of the Early Eocene Climatic Optimum (EECO) recovered from the Chicxulub impact crater in the Gulf of Mexico show a strong relationship with Milankovitch cycles, which play a critical role in controlling climatic and environmental oscillations. Our study represents the first highly spatially-resolved biomarker and bulk geochemical record from the EECO. The bulk δ13Ckerogen data records the Milankovitch eccentricity-paced variability of continental weathering throughout the studied interval. Biomarkers (and indices) indicative of redox conditions [e.g., pristane (Pr)/phytane (Ph) ratios], water column stratification and/or salinity conditions (e.g., Gammacerane Index), photic zone euxinia (e.g., isorenieratane, chlorobactene, and okenane) and those that can differentiate between algal communities such as dinoflagellates (dinosteranes), marine pelagophytes (24-n-propylcholestane), chlorophyte algae (24-iso-propylcholestane), and prasinophytes (C28/C29 sterane ratios) show changes controlled by orbital eccentricity frequencies. In particular, eccentricity maxima were marked by more reducing/salinity stratified water conditions, photic zone euxinic episodes, and higher (relative) abundances of prasinophytes. In contrast, eccentricity minima were marked by more oxic water conditions and an increase in cyanobacterial markers. The δ13C offset observed between phytane and C17–C19n-alkanes may represent shifts between a predominance of autotrophic vs. heterotrophic communities controlled by orbital eccentricity. The direct response of molecular and isotopic composition of organic matter to orbitally controlled climate change in the early Eocene could be proven here for the first time and may be more prevalent in Paleogene sediments worldwide.

A Dramatic Marine Environment Change in the Beibu Gulf of the South China Sea around 3.2 kyr BP

Abstract The profile geochemical characteristics of a seafloor sediment core, collected from the Beibu Gulf of the South China Sea, have been investigated in terms of the sediment grain size, the content of major and trace elements, the total organic carbon (TOC) and organic nitrogen (TN), and the carbon isotopic composition of organic matter (δ13Corg) and AMS 14C dating. These results show that the core sediments were deposited since 11.3 kyr BP and the profile could be clearly divided into the lower and upper sediment section based on geochemical characteristics. The lower section covers the depth of 86 cm to 200 cm and was deposited from 3.2 to 11.3 kyr BP, while the upper section is at the depth of 4 to 86 cm and represents sediments from 0.2 to 3.2 kyr BP. The upper section sediments are charcteristized by a much finer grain size and a clearly positive deviation of the δ13Corg value. Obviously, the overall profile geochemical characteristics indicate that the upper section was deposited under a significantly deeper seawater depth associated with a relatively weaker hydrodynamic and a better preservation condition for organic matter, while the low section was deposited under a relatively shallow seawater depth with the relatively stronger hydrodynamic condition and a relatively poor preservation condition for organic matter. Finally, it is suggested that the transition between the lower section and the upper section occurring around 3.2 kyr BP was mostly likely due to a dramatic marine environmental change caused by the rapid seafloor subsidence associated with the further expansion of the Qiongzhou Strait. And further multidisciplinary studies are needed to better reveal and understand different aspects of the significance of this marine environment change in the Beibu Gulf of the South China Sea.

Spatial Characterization of Stable Isotope Composition of Organic Carbon from Farmland Soils in Chongqing

Soil was sampled from 182 profiles in typical farmlands of Chongqing and analyzed for the stable carbon isotope composition of organic matter (δ13CSOC). The results showed that the values of δ13CSOC for each soil profile were gradually increasing with increasing soil depth, and the mean values were (-23.63±1.53)‰, (-22.43±1.59)‰, and (-21.42±1.90)‰ for surface, middle, and bottom layers, respectively. The δ13CSOC values in the northeastern region of Chongqing tended to be more negative, whereas those in central Chongqing were less negative. Paddy fields showed the most negative values of δ13CSOC, followed by rice-upland rotating fields and upland fields, with the average being (-25.32±0.93)‰, (-23.17±1.37)‰, and (-24.75±1.28)‰ for the surface layers, respectively. For different soil types, the δ13C values in the surface layers were in the order of paddy soil<alluvial soil<purplish soil<calcareous soil<yellow soil. According to the regression-tree analysis, the crop types predominantly influenced the variation in δ13CSOC in surface soils, and soil types mainly affected that in the middle- and bottom-layer soils. Other factors, such as soil properties (TN, SOC, and pH) and meteorological conditions (precipitation and air temperature) played only minor roles in the variation of δ13CSOC. In short, the stable isotope composition of organic carbon in the surface soils was primarily controlled by the input carbon source, whereas that in the deeper layers was closely linked with carbon cycling processes within the soils.

Multiproxy records of temperature, precipitation and vegetation on the central Chinese Loess Plateau over the past 200,000 years

The strength of the East Asian Summer Monsoon (EASM) and associated moisture availability have been linked to the spatial distribution of occurrence of C3 and C4 vegetation on the Chinese Loess Plateau (CLP). Variations in the stable carbon isotopic composition of organic matter in loess-paleosol sequences from several locations on the CLP indicate that vegetation has shifted from mainly C3 plants during cool, dry glacials to more C4 plants during warm, wet interglacials. Although increased temperatures generally lead to an expansion of C4 vegetation, increased humidity has an opposite effect, as does increasing pCO2, leaving the exact driver(s) of vegetation change, and thus EASM strength, elusive. Here we reconstruct continuous, directly comparable records of air temperature, monsoon precipitation, and vegetation type over the past 200,000 years based on branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids, plant leaf waxes and their isotopic stable carbon and hydrogen isotopic composition (δ13Cwax and δ2Hwax, respectively) preserved at Lingtai on the central CLP. BrGDGT-based temperatures vary between 12 and 21 °C over glacial-interglacial cycles, and consistently lead changes in loess proxies (grain size, magnetic susceptibility), as also observed elsewhere on the CLP. Variations in δ13Cwax are only minor (<2‰) and indicate that C3 plants have continuously dominated at Lingtai, in contrast to a nearby section where C4 plants flourished during interglacials. This difference can be explained by the site elevation (∼1300 m above sea level), resulting in air temperatures too low for widespread C4 vegetation occurrence. Instead, variations in the average chain length of the leaf waxes coinciding with large shifts in δ2Hwax (∼40‰) during glacial-interglacial transitions suggest that changes in rainfall source and seasonality may have pushed C3 woody vegetation present during glacials to more non-woody C3 vegetation during interglacials, as a result of differences in respective water-use efficiency. Our multiproxy records thus indicate that subtle changes in the C3 species composition can be used to reveal variations in EASM precipitation dynamics in elevated areas where temperature exerts a first order control on the vegetation type.

Ecology and climate sensitivity of a groundwater-fed lake on subtropical North Stradbroke Island (Minjerribah), Queensland, Australia over the last 7500 years

Lake sediments are important archives of past climate variability and lake responses to climate. In order to accurately infer past climates, it is necessary to understand, and account for, the ecological processes that affect the record of indicators preserved in lake sediment. This is particularly the case with respect to the concentration of carbon and nitrogen (TOC, TN, and calculated C:N), and the stable isotope composition of organic matter preserved in lake sediments. These are common, yet ambiguous, tracers of environmental change. Ideally, palaeoenvironmental reconstructions using the concentration and isotope composition of organic matter should be grounded in a detailed understanding of the sources of the organic material. This study documents the history and evolution of Blue Lake, an environmentally and culturally important oligotrophic, groundwater window lake on North Stradbroke Island, Queensland, Australia. We utilise organic matter δ13C, TOC, TN, and C:N from a 2.4 m sediment core with a basal age of 7.5 cal kyr BP, to investigate changing organic matter sources as a measure of the climate sensitivity of Blue Lake. This interpretation is supported by data from contemporary algae, aquatic and terrestrial plants, and catchment soils. We show that lake nutrient dynamics drove an increase in algal biomass at 4.2 cal kyr BP. This change coincides with a widely documented intensification of the El Niño-Southern Oscillation, which we infer to have influenced lake nutrient concentrations by reducing groundwater throughflow. Climatic changes resulted in marked changes in lake primary productivity, despite relatively little turnover of the lake diatom flora and catchment vegetation. This suggests that south-east Queensland dune lakes are sensitive to climate changes and helps to refine past and future palaeoclimate research using sediments from these lakes. It also indicates that increased nutrient concentrations in Blue Lake may result from projected changes in 21st Century climate.

Signatures of natural to anthropogenic transition in lake sediments from the Central Himalaya using stable isotopes

Stable isotopic compositions of carbon (C) and nitrogen (N) in lake sediments have the potential to decipher natural and anthropogenic influences in a region, which has undergone significant cultural changes in the recent past. In this study, using carbon (δ13C) and nitrogen (δ15N) isotopic compositions of organic matter along with C isotopic ratio of black carbon (δ13CBC) in sediments collected from a lake in the Lesser Himalaya, an attempt has been made to decipher the biogeochemical changes in the lake under natural and anthropogenic stresses along with fire history of the region. The sediment core, dated using 210Pb and 137Cs, went back to 1949 AD, where isotopic compositions and elemental ratios (TOC/TN) suggested that the lake biogeochemistry was largely controlled by in-lake primary productivity. A noticeable change in isotopic compositions and elemental ratios was observed in the early 1970s, possibly due to increased input of soil organic matter from the catchment, which coincided with the urbanization and other human interventions in the region. At the same time, a consistent increase in differences between the measured and Suess effect corrected δ13C of organic matter from the early 1970s to 2016 AD indicated towards the increased utilization of fossil fuel-induced atmospheric CO2 by the lake biota. Also, a sudden shift in δ13CBC in the early 1970s showed an increase in fossil fuel-induced black C in the region during that time. Overall, a comparison of the current study with other lakes in the region revealed the effect of land use change and population growth on the lake biogeochemistry, which was clearly recorded in elemental contents and stable isotopic composition of different components of lake sediments. However, the response of each lake depended on the location (i.e., distance from the city) and size of the catchment. The larger lakes in the region appeared to have suffered the anthropogenic stress through both atmospheric and terrestrial routes, whereas the smaller and secluded lake, as in the present study, was largely influenced via atmospheric factors.

Biomarker and Carbon Isotopic Evidence of Marine Incursions in the Himalayan Foreland Basin During Its Overfilled Stage

AbstractThe final stages of evolution of the Himalayan foreland basin (HFB) are preserved in the Siwalik Group of rocks deposited by meandering and/or braided rivers in the western and central regions of HFB. However, the time‐equivalent deposits in the eastern part of the foreland provide contradictory evidence of both terrestrial and marine environments. To address the ambiguity, molecular level characterization and stable isotopic composition of organic matter (OM) have been employed in the late Miocene‐Pliocene sequence of the eastern HFB. The n‐alkane distribution, carbon isotopic (δ13C) signature of n‐alkanes and distribution of hopane and sterane isomers suggest OM contributions from both marine and terrestrial sources during the late Miocene period. Increase in short‐chain n‐alkane abundance and gammacerane index, low pristane/phytane ratio, higher δ13C values, higher regular sterane/17α‐hopane, C31R/C30‐hopane and C27/C29 steranes ratios and presence of C30 sterane provides substantial evidence of stratified anoxic conditions and marine influences at specific stratigraphic intervals. During the late Miocene period, mixing of marine OM sources with terrestrial sources argue for marginal marine depositional conditions amid fluvial‐dominating environments. The entry of marine waters in the eastern HFB through the pre‐existing cratonic troughs possibly resulted from eustatic or relative sea‐level rise. No further evidence of marine incursions is observed in the younger Pliocene sediments. The higher detrital influx from the rising Himalayas, the onset of Northern Hemisphere Glaciation and evolution of physical barriers such as Shillong Massifs and Barind Tracts altogether led to the cessation of marine incursions into the HFB.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

Here we present the carbon isotopic composition of dissolved inorganic carbon (DIC) and the sulfur isotopic composition of sulfate, along with changes in sulfate concentrations, of the pore fluid collected from a series of sediment cores located along a depth transect on the Iberian Margin. We use these data to explore the coupling of microbial sulfate reduction (MSR) to organic carbon oxidation in the uppermost (up to nine meters) sediment. We argue that the combined use of the carbon and sulfur isotopic composition, of DIC and sulfate respectively, in sedimentary pore fluids, viewed through a δ13CDIC vs. δ34SSO4 cross plot, reveals significant insight into the nature of carbon-sulfur coupling in marine sedimentary pore fluids on continental margins. Our data show systemic changes in the carbon and sulfur isotopic composition of DIC and sulfate (respectively) where, at all sites, the carbon isotopic composition of the DIC decreases before the sulfur isotopic composition of sulfate increases. We compare our results to global data and show that this behavior persists over a range of sediment types, locations and water depths. We use a reactive-transport model to show how changes in the amount of DIC in seawater, the carbon isotopic composition of organic matter, the amount of organic carbon oxidation by early diagenetic reactions, and the presence and source of methane influence the carbon and sulfur isotopic composition of sedimentary pore fluids and the shape of the δ13CDIC vs. δ34SSO4 cross plot. The δ13C of the DIC released during sulfate reduction and sulfate-driven anaerobic oxidation of methane is a major control on the minimum δ13CDIC value in the δ13CDIC vs. δ34SSO4 cross plot, with the δ13C of the organic carbon being important during both MSR and combined sulfate reduction, sulfate-driven AOM and methanogenesis.

Hybrid event beds generated by erosional bulking of modern hyperpycnal flows on the Choshui River delta front, Taiwan Strait

AbstractHyperpycnal flows are important agents for transporting detrital sediments from rivers to oceans. Previous studies often assumed that the deposits of flood‐controlled delta fronts would be dominated by graded sand with common hummocky cross‐stratification. This study documents, for the first time, hybrid event beds and plumite deposits inside the succession of a modern delta front. This delta in the Taiwan Strait is fed by the Choshui River, which is considered a highly efficient sediment transport system with individual floods with extremely high suspended sediment concentrations. The deltaic event beds recorded in the core were interpreted as hyperpycnal flow‐generated hybrid event beds and turbidites triggered by hypopycnal flows. Accelerator Mass Spectrometry 14C dating, grain‐size analysis and measurements of stable isotopic composition of organic matter were conducted to delineate the depositional process of the recognized beds. The internal subdivisions of the hybrid event beds were differentiated mainly based on sedimentary textures, including cohesive mud content, sand content and sorting. The disorganized portion (division H3) appears internally chaotic and contains large rafted substrate clasts but also displays an upward increase in dispersed cohesive mud from 35% (division H3a) to 50% (division H3b). In contrast, massive H1 divisions are characterized by much lower cohesive mud of ca 8%. The vertical arrangement between depositional facies allows the discrimination of three hybrid event bed types. The stable carbon isotopic composition of the organic matter reveals that the cohesive mud in each division of the hybrid event beds was sourced from marine substrate, rather than supplied by the original hyperpycnal flows. Therefore, the hybrid event beds are generated by energetic hyperpycnal flows, which can delaminate the muddy sea‐floor and incorporate large quantities of substrate fragments. The bulking of erosional hyperpycnal turbulent flow forces flow transformation and generates a more cohesive flow with a turbulent, dense head and trailing bipartite debris flow. The occurrence of hybrid event beds in modern shallow delta front settings is also an important novelty, implying a high density of hyperpycnal flows at the mouth of sediment‐laden rivers as well as the crucial impact of substrate entrainment on the development of hybrid event beds.

Anthropocene climate warming enhances autochthonous carbon cycling in an upland Arctic lake, Disko Island, West Greenland

Abstract. The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial–aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the low–high Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and lakes received varying amounts of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in high-latitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤1.5 mg L−1). Sedimentary carbon/nitrogen (C/N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210Pb-dated sediment core to determine how carbon cycling in a lake–catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA∼1860 CE), which accelerated after ca. 1950, led to melt of glacier ice and permafrost, releasing nutrients and DOC to the lake and stimulating pronounced aquatic algal production, as shown by a &gt;10-fold increase in β-carotene, indicative of a major regime shift. We also demonstrate that recent increases in catchment terrestrial vegetation cover contributed to the autochthonous response. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, when compared with extensively vegetated low-Arctic systems.

Carbon Isotope Chemostratigraphy Across the Permian-Triassic Boundary at Chaotian, China: Implications for the Global Methane Cycle in the Aftermath of the Extinction

During the end-Permian extinction, a substantial amount of methane (CH4) was likely released into the ocean-atmosphere system associated with the Siberian Traps volcanism, although fluctuations in the global CH4cycle in the aftermath of the extinction remain poorly understood. The carbon (C) isotopic composition of carbonate (δ13Ccarb) across the Permian-Triassic boundary (P-TB) was analyzed at Chaotian, South China. The δ13Ccarbvalues decrease from ca. +1 to –2‰ across the P-TB, possibly caused by a collapse of primary productivity associated with the shallow-marine extinction. The frequent intercalation of felsic tuff layers around the P-TB suggests that a volcanogenic carbon dioxide (CO2) input to the surface oceans may also have contributed to the δ13Ccarbdecline. The magnitude of the δ13Ccarbdecrease (∼3‰) is substantially smaller than the magnitude of a decrease in C isotopic composition of organic matter (δ13Corg) in the same P-TB interval (∼7‰). This apparent δ13Ccarb-δ13Corgdecoupling could be explained by proliferation of methanogen (“methanogenic burst”) in the sediments. A global δ13C compilation shows a large variation in marine δ13Corgrecords, implying that the “methanogenic burst” according to the Siberian Traps volcanism may have contributed, at least in part, to the δ13Corgvariability and to the elevated CH4levels in the atmosphere. The present and previous observations allow us to infer that the global CH4cycle may have fluctuated substantially in the aftermath of the extinction.

An eco-parametric method to derive sedimentation rates for coastal saltmarshes

The saltmarsh plant Spartina alterniflora was introduced to the Jiangsu coasts, China and serves as an ecological engineer to reduce near-bed shear stress, trap fine-grained sediments and protect the coast from wave-induced erosion. The saltmarshes thus could record the Spartina colonization-driven changes within the sedimentary layers. Based on these ecological and sedimentological changes in sediments, we present a new eco-parametric method to estimate the sedimentation rate for the newly-formed wetlands in the Yancheng Wetland Nature Reserve for Rare Birds, Jiangsu. Sediment cores and satellite imagery were used to identify the thickness of accumulated sediment layers and the time since the Spartina colonization. We defined the original ground on which Spartina alterniflora initially colonized using pigment concentrations, grain size and stable carbon isotopic compositions of organic matter (δ13C) in sediments. We also determined the time mark of the Spartina colonization by examining the Landsat images over 1982–2018 to discriminate the Spartina alterniflora from other native plants and geomorphological features. These two datasets yielded a sedimentation rate of 3.3 cm/yr for Core A and of 9.6 cm/yr for Core B, the latter evidenced by an increase of ~ 0.51 m in the bed level from 2008 to 2014. Combining the 210Pb dating method, we further estimated the sedimentation rate for the layers beneath the original ground, which was comparable to that of the bare flats in the Jiangsu coast. Even though this new method is only applicable to newly-formed saltmarshes, it helps identify the recent sedimentation events as well as reveal the environmental changes and the evolution of saltmarsh-bare flat systems due to the interplay between vegetation, hydrodynamics and sediment dynamics. It thus could be an efficient and cost-effective tool for an improved understanding of the response of coastal wetlands to a changing climate/environment.

Stable Carbon Isotopic Composition of Organic Matter of the Late Pleistocene and Holocene Soils of the Baikal Region

The results of studying stable carbon isotopes of the organic matter of Late Pleistocene and Holocene soils of the Baikal region are presented. The δ13С values in soils of different ages varies from –30.00 to –21.36‰, which indicates the formation of their organic matter from C3 plants at least in the past 35 ka. Fractionation of isotopes during the organic matter mineralization leads to an increase in δ13С values with the depth. At the same time, in soils of steppe and forest-steppe landscapes, an increase in δ13С is about 3‰, whereas in forest soils it can reach 4–5‰, which is most likely associated with a slow transformation of organic residues in the upper horizons of forest soils. In the Late Glacial and Early Holocene soils, an elevated content of organic carbon is noted, in which the stable carbon isotopic composition is lighter than that in the recent soils. This may be explained by the influence of permafrost slowing down the transformation of organic matter. In the sequence starting with rather dry soils of the Ol’khon region and the Selenga Mountains to the forest-steppe soils of the Upper Angara region with its more humid conditions and, further, to the taiga soils in the south of Central Siberia, there is a marked decline of δ13С values in the organic matter. Thus, we deal with a bright example of the δ13С dependence on the soil water supply, i.e., on precipitation during the growing season. Reconstruction of precipitation based on δ13С values attests to a gradual aridization of the climate starting from the Late Glacial period with a maximum drying in the Middle Holocene, while in the Late Holocene the level of humidization increases. A tendency towards a gradual decrease in humidity was also observed during MIS-3. It is probable that the most humid climate was during the formation of the Early Karga soils. In the range of 36–31 ka BP, climatic humidity decreased and became comparable with the modern one. At the end of MIS-3, climatic humidity was below the current level.