Sources Of Sedimentary Organic Matter Research Articles

Spatial differentiation of sediment organic matter isotopic composition and inferred sources in a temperate forest lake catchment

Freshwater sediments are important carbon reservoirs, but the extent to which different components of soil or aquatic organic matter (OM) are deposited in these sediments is not well defined. Bulk sediment carbon (δ13C) and nitrogen (δ15N) stable isotope ratios, as well as radiocarbon, are valuable tracers for sediment OM sources, but there are few studies comparing the isotopic composition of soil and sediment OM at the catchment scale. We analyzed spatial variation in δ13C, δ15N, 14C, and C:N ratios in OM from soils, stream and lake sediments, and aquatic plants and algae, in a temperate forest lake catchment in southern Quebec, and used a Bayesian model to estimate source mixtures for sediment OM. Sediments at the stream mouths entering the lake were characterized by high C:N ratios, high fraction modern carbon (Fm), and low δ13C, indicating preferential deposition of plant-derived OM. In contrast, sediments sampled further upstream during a period of low streamflow indicated a larger proportion of microbial OM based on low C:N ratios and high δ15N. In lake sediments we observed zonation of OM isotopic composition by water depth. Shallow sediments (0–1 m water depth) were characterized by high amounts of plant-derived OM, while intermediate-depth sediments (1–3 m) were characterized by high δ13C, indicating an increased input of OM from aquatic plants. Deep lake sediments (> 4 m) were characterized by low δ13C and Fm values, which likely reflect greater input of phytoplankton OM. Stream sediments downstream of the lake exhibited high δ15N and low Fm values, implying a greater input of aged microbial biomass from soils. Our results indicate catchment-scale spatial differentiation in the source of OM in sediments, with zones of preferential deposition of terrestrial plant, aquatic plant, phytoplankton, and soil microbial biomass.

Assessing spatiotemporal sources of biogenic and anthropogenic sedimentary organic matter from the mainstream Haihe River, China: Using n-alkanes as indicators

Sedimentary organic matter (SOM) plays an important role in the transportation and transformation of various pollutants and the carbon cycle in aquatic and terrestrial ecosystems, especially for seagoing rivers. However, few studies have focused on the sources and factors of SOM in rivers under the significant pressure of high urbanization and industrialization. In this study, we adopted the molecular markers of n-alkanes and their proxies in the mainstream Haihe River to reveal the spatiotemporal distributions and biogenic and anthropogenic sources of SOM. The concentrations of Σ29n-alkanes, Σbiogenicn-alkanes, and Σanthropogenicn-alkanes were 4985.6 (127.5–26,296.6), 1872.1 (38.1–9216.9), and 3113.5 (89.4–18,259.7) ng·g−1 dw (dry weight), respectively. Hybrid sources of n-alkanes were found in this study. The composition distribution and proxies of n-alkanes showed that woody and herbaceous plants are the main sources of biogenic SOM, while incomplete fossil fuel burning and heavy oil emissions served as the main contributors to anthropogenic SOM in the mainstream Haihe River, especially through industrial activities. The average chain length of biogenic n-alkanes (ACLbio) was verified to quantify the relative contributions of biogenic sources of SOM and proxies; the average chain length of anthropogenic n-alkanes (ACLanthro), and the ratio of unit short‑carbon to unit long‑carbon anthropogenic n-alkanes (L/H) were verified to quantify the relative contributions of anthropogenic sources of SOM in the river system. Impacts from sedimentary geochemistry (such as total organic carbon (TOC) and grain size of sediments) on n-alkanes were explored, and the correlations of Σ29n-alkanes with TOC and grain size of the river sediment indicated that terrestrial organic matter was the main source of SOM, while emissions from incomplete combustion and intensive manufacturer activities should also not be neglected.

Isotopic and traits-based trophic diversity of Canadian Beaufort Sea benthic communities in relation to food supply

In the Arctic, climate-driven alterations to the quality and quantity of organic matter reaching the seafloor will likely affect benthic food web function. We used biomass-weighted diversity measures based on trophic traits (those related to feeding behaviours, e.g., mobility, size, food source), stable isotope ratios (δ15N and δ13C), and taxonomy to assess linkages between benthic food web structure and indicators of food supply in the shelf and slope ecosystems of the Canadian Beaufort Sea and Amundsen Gulf (down to 500 m). Benthic communities across both regions used a similar diversity of sedimentary organic matter sources according to stable isotopes, despite known differences between regions in organic matter input and benthic–pelagic coupling. Shelf-edge and upper-slope communities exhibited relatively high trophic trait diversity, which likely reflected the exploitation of pulsed food inputs associated with dynamic processes at the shelf break that affect the production and advection of organic matter to the benthos. Pairwise relationships between trophic traits and sedimentary proxies of benthic food supply were not significant at the regional scale. However, cluster analyses supported the notion that trophic trait composition was influenced by proximity to the shelf edge, where food supply is influenced by episodic events that may not be reflected by in situ sedimentary proxies. Our findings add to growing evidence that evaluating trophic traits composition may provide more information regarding functional responses to changes in benthic food supply than either isotopic or taxonomic diversity indices alone.

Biogenic and anthropogenic sources of sedimentary organic matter in marine coastal areas: A multi-proxy approach based on bulk and molecular markers

The investigation of organic matter sources is an especially challenging issue for marine coastal areas influenced by both natural and anthropogenic inputs. This study focuses on the provenance of sedimentary organic matter deposited in the Gulf of Gdańsk (southern Baltic) and Oslofjord/Drammensfjord (southern Norway) based on the analysis of multiple specific proxies in recent sediments (0–5 cm layer) sampled at 12 sites in 2014. Bulk markers (total organic carbon/total nitrogen [TOC/TN] and stable organic carbon isotopes [δ13CTOC]) and various molecular geochemical markers of organic matter origin (n-alkanes, branched alkanes, unresolved complex mixture [UCM], polycyclic aromatic hydrocarbons [PAHs], hopanes, steranes, and polar steroids including sterols and stanols) were used to comprehensively assess the origin of sedimentary organic matter in the studied basins. The results of this study indicate that the combined use of TOC/TN and δ13CTOC may lead to ambiguous outcomes, and anthropogenic input may affect the sedimentary TOC/TN ratio. The analysis of molecular proxies allowed the identification of locations most impacted by anthropogenic organic matter derived from petrogenic sources and/or sewage effluents and the classification of the studied sites based on the contribution of marine/terrestrial biogenic matter. This study demonstrates that key issues, such as the forms of sedimentary chemical markers, post-depositional changes, and specific diagenetic products, although neglected by some researchers, must be considered for the reliable assessment of organic matter sources. To the best of our knowledge, this is the first such extensive study on surface sediments from the Gulf of Gdańsk and Oslofjord/Drammensfjord in terms of the use of both bulk and molecular markers to investigate organic matter sources.

Apportioning sedimentary organic matter sources and its degradation state: Inferences based on aliphatic hydrocarbons, amino acids and δ15N

The sources and state of sedimentary organic matter (SOM) in fresh water aquatic systems are important to understand the carbon cycling in terrestrial environments. The composition of organic matter in the lake sediments demonstrates the physical and chemical condition of the lake ecosystems. However, the systematic and structured investigations focussed on to understand the source and fate of organic matters within eutrophic lakes is still far from clear. The present study is focusing on the implications of amino acids (AA), aliphatic hydrocarbons and bulk geochemical (C/N, δ15N) proxies to understand the distribution, sources and state of sedimentary organic matter in Ahansar Lake from Kashmir valley, India. The relatively low C/N ratios along with high AA contents indicate enhanced aquatic productivity in the lake system. Likewise, the dominance of the mid-chain monomethyl alkanes (MMAs), highly branched isoprenoids (HBIs), botryococcenes, steroids and triterpenoids suggest OM sourced from periphyton remains. Furthermore, the presence of C27, C28 and C29 diagenetically altered steroids also reflects a major algal contribution. The spatial variability of Paq demonstrates their applicability as a proxy for the contribution of aquatic vegetation. The ratio of individual amino acids (oxic/anoxic ratio) and low Pr/Ph (pristane/phytane) values indicate anoxic nature of the current depositional environment. This also leads to significant organic matter preservation as revealed by amino acid indices (e.g., degradation index – DI and reactivity index - RI). These data collectively demonstrate the systematic investigation and comprehensive understanding of source of sedimentary organic matters and respective depositional condition via multiple indicators. Overall, understanding the OM molecular composition and its spatial heterogeneity in a lake system is important to better constrain the fate of organic carbon, and assess the pollution risks as well as adopt relevant management strategies.

Tracing organic matter sources in the estuarine sediments of Vanga, Kenya, and provenance implications

Mangrove ecosystems can potentially and indirectly modulate the effects of sea level rise due to their ability to trap and retain sediments. In order to gain a better understanding of the potential sediment sources and dynamics in the mangroves of Vanga, Kenya, inter-seasonal riverine, estuarine and coastal/marine sediments together with agricultural soil and mangrove tissues were studied. Elemental ratios (C/N) and carbon (δ13C) stable isotope composition, were analysed to disentangle sources of sediment organic matter (SOM) in the Vanga, Estuary. The organic carbon (C) content, C/N ratios and δ13C values ranged between 0.1 and 2.4%, 9.9 to 35.5 and −24.8 to −19.9‰ respectively. These contents were also as expected relatively higher in mangrove tissues than in all the sediment samples. δ13C values of mangrove sediments were higher than those of the mangrove tissues, which was attributed to rapid decomposition, dilution by freshwater and mineral organic matter (OM) discharged by the Umba River as well as outwashing by ebb tides. On average, C/N ratios were also observed to be greater than 10 in all samples collected during both the dry and wet season. This has been shown to reflect a pronounced contribution of allochthonous organic matter. Using a combination of δ13C values and C/N ratio, the dominance of riverine POM was also evident from a Bayesian Stable Isotope Mixing Model in R-program (SIMMR) that indicated OM source mixing but with a dominant (∼60%) contribution of riverine OM. The observed and modeled variability of SOM in the Vanga estuary points to the hypothesized influence of the Umba River on Vanga Estuary sediments but with evidence of potential material exchange and influence of tidal fluxes within the estuary.

A multiproxy study of intertidal surface sediments from two macrotidal estuarine systems (Canche, Authie) in northern France: Insights into environmental processes

Estuaries are considered one of the most dynamic ecosystems in the world, ensuring high production and transfer of large quantities of organic matter (OM) from continents to the ocean. Despite the large influence of estuaries in the functioning of coastal ecosystems, our understanding of OM dynamics in these buffering coastal environments remains fragmented. In this frame, our study was carried out to describe and understand the main drivers of OM distribution in two estuaries (Canche, Authie) of the eastern English Channel, based on descriptive and quantitative approaches. For this purpose, in February and March 2018, surface sediments were sampled at low tide along cross-shore transects in the intertidal zone of each estuary. Analysis of sediment grain size, porosity, proportion of aggregated particles, total carbonate content and organic geochemical characteristics (total OM content, atomic TOC/TN ratio, δ13C value) of surface sediments allowed identifying four biosedimentary facies: sand flat, mud flat, low and high salt marshes. In parallel, the elemental and isotopic characterization of the suspended particulate OM inputs by the rivers, the surface biofilms and the halophytic vegetation was performed to trace the three major sources contributing to sedimentary OM in both estuaries. Spatial distribution of OM in surface sediments (ranging between 0.6 and 26.9% dry weight) evidenced higher contents along marsh platforms than in sand and mud flats. and the OM pool was mainly constituted of a mixing of C3 terrestrial vascular plants, marine and riverine phytoplankton. The comparison between both estuaries highlighted that the geomorphology impacted by natural and anthropic factors plays a preponderant role in the distribution and the sources of sedimentary OM by influencing vegetation communities and hydrodynamic processes. In the context of sea level rise, an improved understanding of the OM dynamics within each biosedimentary facies is crucial to better constrain and assess the role of macrotidal estuaries in carbon cycle.

Fourier transform infrared spectroscopy as a tracer of organic matter sources in lake sediments

The source of sedimentary organic matter in lakes can help to elucidate climate and catchment variation and processes that reflect lake development. Common techniques for tracing sediment organic matter sources, such as the stable isotopes and elemental concentrations of C and N, can be too imprecise to identify the specific provenance of organic matter. By contrast, organic geochemical techniques such as gas or liquid chromatography and nuclear magnetic resonance provide detailed organic molecular characterisation but are both expensive and time consuming. Fourier Transform Infrared (FTIR) spectroscopy is a rapid, non-destructive, and well-established method for determining the constituents of lake sediments. However, the potential for identifying the sources of organic matter in lake sediments has not been fully explored. In this study, we assess the extent to which FTIR can be used to identify varying organic matter sources through analysis of modern autotrophs from Blue Lake, North Stradbroke Island, Australia. We investigated spectral processing techniques to identify the approach that could most accurately classify autotroph samples. Three autotroph groups (i.e algae, aquatic macrophytes, and terrestrial) were correctly classified 90% of the time. Processed spectra then became the basis of a model that used multivariate random forests to estimate sediment organic matter composition source from a sediment record from Blue Lake that spans the last 7500 years. FTIR-based estimates suggested that throughout the history of the lake, algae contributed the highest amount of organic matter to the sediment samples. These results allow a refinement of a previous study of C:N and δ13C from the same core and suggests that alterations in C:N and, particularly, δ13C reflect chemical changes in algae through time. This study demonstrates that FTIR spectroscopy is a promising tool to elucidate sources of sediment organic matter in lake sediments.

Sources of sedimentary organic matter and assessment of heavy-metal levels in estuarine sediments after Fundão dam breach

Mining tailing is one of many anthropogenic stressors affecting estuarine systems around the world. On the November 5, 2015, the Fundão dam breached, releasing 52 million m3 of iron mining residues affecting Doce River until its estuary, spreading a large plume along the Brazilian coast. The present study characterizes the impact of the mining tailing in these environments and the changes in sedimentary organic matter (SOM) sourced at Doce River estuary (DRE) and adjacent estuaries over time. Three sediment cores and surface sediment samples were collected along the coast of Espírito Santo State. Elemental (C, N) and stable isotope (δ15N and δ13C) fingerprint methods were used to characterize sedimentary organic matter sources. The indexes of sedimentary organic matter stability (recalcitrance index) were quantified by FTIRS technique. Besides, WDXRF analyzes were carried out to statistically determine metal baseline levels and assess the present and past pollution degrees of sediments using pollution indexes (PLI). Results indicate that Doce River estuary significantly provides Fundão dam's tailings (77 % ± 9 %) to the coastal region, with a PLI categorized as moderately polluted to highly polluted (PLI = 3.1), even two years after the dam breach. Estuaries north from Doce River Estuary presented a more significant influence of marine organic sedimentation sources with lower PLI (unpolluted). In contrast, estuaries south from DRE presented greater terrestrial influence with higher PLI values (unpolluted to moderate polluted). Further, changes in SOM stability were noticed mainly at Doce River Estuary, where the recalcitrance index did not show the expected natural pattern. Finally, the findings indicate that Fundão dam failure had a significant impact on Doce River Estuary sediment quality at different moments after Fundão breach, and this ecosystem is expected to continue to suffer from this indefinitely. However, the direct effect of tailings deposition was not observed in the sampled estuaries, PLI shows that ecosystems in the south already suffer from other pollution sources, which increases the environmental risks in a possible tailing deposition scenario.

Organic Matter Compositions and Loadings in River Sediments From Humid Tropical Volcanic Luzon Island of the Philippines

AbstractTropical rivers deliver ∼60% of particulate organic carbon to the world ocean. However, compositions and loadings of sedimentary organic matter (OM) from tropical small mountainous rivers are largely unknown. Here, we provide an initial constrain on sources of sedimentary OM from 28 fluvial systems across Luzon in the Philippines by measuring total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C) and radiocarbon activity of TOC (expressed as fraction modern—Fm), as well as grain size and mineral surface area (SA) of sediments. Results indicate that sediments in Luzon rivers contain both contemporary and 14C‐depleted OM (Fm: 0.71–1.06, mean 0.97 ± 0.07) with a wide range of δ13C values (−28.3‰ to −17.7‰, −24.9 ± 2.2‰). This is attributed to the OM sources originated from modern surface soil and 14C‐depleted subsoil and deep soil vegetated by C3 and C4 plants, with mean fraction of C3 plant at 80% ± 11%. Minor input from bedrock may also contribute to the 14C‐depleted OM in sediments, accounting for 6% ± 6%. Sediments in most rivers are featured by low OC loadings (OC/SA ratio < 0.4 mg C m−2), owing either to a less OM input or intensive OM degradation. The estimated yields of particulate OC from Luzon vary between 3.2 and 3.7 t km−2 yr−1, which is higher than most tropical large rivers.

Linking soils and streams: Chemical composition and sources of eroded organic matter during rainfall events in a Loess hilly-gully region of China

Ascertaining the chemical compositions and sources of eroded organic matter (OM) is of significant importance for understanding the flux and fate of laterally transported OM at the catchment scale. Limited information, however, is available for the sources and dynamics of the eroded OM in bulk and soluble phases in relation to land-use types and rainfall events from an erosion prone landscape. In this study, we collected the top-soils from different land-uses and gully bank in uplands over seven rainfall events in 2016 to address this issue at two contrasting watersheds (with vs. without soil conservation measures) in the Loess hilly-gully region of China. Chemical compositions of eroded OM in bulk sediments and stream waters at the stream channels during rainfall processes were determined by spectroscopic analysis and source partition was inferred with the stable isotopic technique. Our results indicated that OM from forested soils had higher aromaticity, hydrophobic fraction, humification, molecular weight, and proportion of humic-like substance compared with other land-use types. The calculation with isotope mixing model indicated that the eroded bulk OM sources in sediments mainly originated from gully bank, orchard, and forests at the watershed coupled with soil conservation measures. Further, gully banks were the main contributor of eroded OM during rainfall events only at the watershed without conservation practices. Moreover, the principal component analysis suggested that the dissolved organic matter in stream waters were primarily derived from microbial OM sources (i.e. biological production of OM) in deep soils during rainfall events at both contrasting watersheds. Our findings highlight the effects of soil conservation measures and rainfall events on the nature and reactivity of eroded OM in various forms issuing from watersheds.

Tracing the sources of sedimentary organic matter in Nanyue small watershed based on 13C, 15N and C/N

Losses of organic matter in agricultural watersheds result in eutrophication and land degra-dation, which not only threaten water quality and food security, but also lead to environmental problems such as the greenhouse gases emission. We used 13C, 15N and C/N as fingerprint markers to trace the sources of sedimentary organic matter at the outlet in the Nanyue small watershed. We analyzed the spatial distribution in watershed sedimentary organic matter and soils of typical land use types, including forest, paddy field, and vegetable fields. The Bayesian stable isotope mixing model was used to quantitatively estimate the contribution of different sources. The results showed that there was significant spatial variation of δ13C. The δ13C of sediment organic matter (-22.6‰±0.53‰) and forest soil (-23.13‰±1.71‰) was significantly higher than that of paddy soil (-25.24‰±1.4‰). The differences of δ15N among the sources were not significant, with sediment having the maximum (4.37±0.83)‰ and forest soil having the minimum (2.38±1.97)‰. Forest soil had the highest C/N of 16.66±7.18, while paddy soil had the lowest C/N of 11.95±0.92. The results of the Bayesian stable isotope mixture model showed that the contribution rates of forest land, paddy fields and vegetable fields to the organic matter deposited at the outlet in the watershed were 19.6%, 15.7%, and 64.7%, respectively. Paddy filed and vegetable field had a combined contribution rate of 80.4%. It was concluded that, soils of agricultural land were the main sources of organic matter deposited in the Nanyue small watershed, and that nutrient loss in the watershed would be effectively controlled by optimizing farmland management.

Influence of sedimentary organic matter sources on the distribution characteristics and preservation status of organic carbon, nitrogen, phosphorus, and biogenic silica in the Daya Bay, northern South China Sea

Surface sediment samples were collected from Daya Bay in October 2018, and analyzed for total organic carbon (OC), total nitrogen (TN) and their stable isotopes (δ13C and δ15N), total phosphorus (TP), biogenic silica (BSi), sediment textures and specific surface area (SSA). The primary objective was to evaluate the influence of mariculture/aquaculture on the distribution characteristics of organic matter (OM), and preservation status of OC, TN, TP, and BSi in sediments. The average δ13C and δ15N values, and OC/TN ratios were −21.27‰, 6.74‰, and 8.90, respectively. Monte Carlo simulation results revealed that mariculture/aquaculture biodeposits accounted for >40% of the buried OM at sites where the breeding rafts and cages are located, whereas marine OM increased gradually to the open sea. Terrestrial OM was generally low accounting for 17% by average. The contents and distribution characteristics of biogenic elements were more influenced by mariculture/aquaculture and primary productivity than sediment textures. Lower OC/SSA (0.3–1.2 mg OC/m2), TN/SSA (~0.05–0.18 mg TN/m2), and TP/SSA (0.02–0.04 mg TP/m2) loadings indicated that increased sequestration of labile OM in a coastal bay could contribute to significant degradation of recalcitrant OM in sediments with significant loss of P relative to OC. Nitrogen contamination in surface sediments was due to increased injection of aquaculture biodeposits, and may pose a detrimental effect on the ecological sustainability of the bay. Higher BSi/SSA loadings (0.9–1.7 mg BSi/m2) revealed that BSi was more preserved, and that BSi-based proxy could be used for paleo-productivity studies. However, such preservation may induce adverse dissolved silicate limitation in a bay perturbed by eutrophication. Fine-grained sediments (clay and silt) accounted for >77% of the sediment texture types with higher SSA, and while controlling the contents of biogenic elements under given depositional conditions were not the main determining factors of OC, TN, TP, and BSi preservation.

The Stable Carbon Isotopic Compositions of n-Alkanes in Sediments of the Bohai and North Yellow Seas: Implications for Sources of Sedimentary Organic Matter

Stable carbon isotopic compositions of n-alkanes in surface sediments of the Bohai and North Yellow Seas were investigated to elucidate sources of sedimentary organic matter in these seas. The long-chain n-alkanes in surface sediments are predominantly long-chain C27, C29, and C31 types, with obvious odd carbon predominance. The δ13C values of long-chain n-C27, n-C29, and n-C31 alkanes are −30.8%±0.59‰, −31.9%±0.6‰, and −32.1%±1.09‰, respectively, within the range of n-alkanes of C3 terrestrial higher plants. This suggests that sedimentary n-alkanes are derived mainly from terrestrial higher plants. Compound-specific carbon isotopic analysis of long-chain n-alkanes indicates that C3 terrestrial higher plants predominate (64%–79%), with angiosperms being the main contributors. The n-alkane δ13C values indicate that mid-chain n-alkanes in sediments are derived mainly from aquatic emergent macrophytes, with significant petroleum pollution and bacterial degradation sources for short-chain n-alkanes.

Historical reconstructions of sedimentary organic matter sources and phytoplankton evolution in the Jiaozhou Bay based on sterols and carbon isotope

Bulk organic matter proxies including total organic carbon (TOC), total nitrogen (TN), C/N ratio and carbon stable isotopic composition (δ13C) combined with sterols in a sediment core were studied to reconstruct both organic matter (OM) sources and phytoplankton evolutions of the Jiaozhou Bay (JZB) during the past ~ 80 years. The OM source allocations were calculated based on δ13C and sterol. The results showed that the marine OM (MOM) input was the dominant OM sources, with the marine organic carbon (OCM) proportion of 54.2–78.4% and marine sterol proportion of 63.9–72.7%. The Terrestrial OM (TOM) contribution increased especially since the 1960s, mainly attributed to the increased sewage discharge and usage of fertilizer. Elevated marine primary productivity since the 1980s was mainly attributed to the increased nutrient inputs. Evolution of diatom compared with dinoflagellate in the JZB was closely related to the anthropogenic forcing and climate change.

Sources of sedimentary organic matter from the cage fish farm in Lake Poyang, China

Sources of sedimentary organic matter from the cage fish farm in Lake Poyang, China

Distribution, sources, and decomposition of soil organic matter along a salinity gradient in estuarine wetlands characterized by C:N ratio, δ13 C-δ15 N, and lignin biomarker.

Despite increasing recognition of the critical role of coastal wetlands in mitigating climate change, sea-level rise, and salinity increase, soil organic carbon (SOC) sequestration mechanisms in estuarine wetlands remain poorly understood. Here, we present new results on the source, decomposition, and storage of SOC in estuarine wetlands with four vegetation types, including single Phragmites australis (P, habitat I), a mixture of P. australis and Suaeda salsa (P+S, habitat II), single S. salsa (S, habitat III), and tidal flat (TF, habitat IV) across a salinity gradient. Values of δ13 C increased with depth in aerobic soil layers (0-40cm) but slightly decreased in anaerobic soil layers (40-100cm). The δ15 N was significantly enriched in soil organic matter at all depths than in the living plant tissues, indicating a preferential decomposition of 14 N-enriched organic components. Thus, the kinetic isotope fractionation during microbial degradation and the preferential substrate utilization are the dominant mechanisms in regulating isotopic compositions in aerobic and anaerobic conditions, respectively. Stable isotopic (δ13 C and δ15 N), elemental (C and N), and lignin composition (inherited (Ad/Al)s and C/V) were not completely consistent in reflecting the differences in SOC decomposition or accumulation among four vegetation types, possibly due to differences in litter inputs, root distributions, substrate quality, water-table level, salinity, and microbial community composition/activity. Organic C contents and storage decreased from upstream to downstream, likely due to primarily changes in autochthonous sources (e.g., decreased onsite plant biomass input) and allochthonous materials (e.g., decreased fluvially transported upland river inputs, and increased tidally induced marine algae and phytoplankton). Our results revealed that multiple indicators are essential to unravel the degree of SOC decomposition and accumulation, and a combination of C:N ratios, δ13 C, δ15 N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river-estuary-ocean continuum.

Impact of Terrestrial Input on Deep-Sea Benthic Archaeal Community Structure in South China Sea Sediments.

Archaea are widespread in marine sediments and play important roles in the cycling of sedimentary organic carbon. However, factors controlling the distribution of archaea in marine sediments are not well understood. Here we investigated benthic archaeal communities over glacial-interglacial cycles in the northern South China Sea and evaluated their responses to sediment organic matter sources and inter-species interactions. Archaea in sediments deposited during the interglacial period Marine Isotope Stage (MIS) 1 (Holocene) were significantly different from those in sediments deposited in MIS 2 and MIS 3 of the Last Glacial Period when terrestrial input to the South China Sea was enhanced based on analysis of the long-chain n-alkane C31. The absolute archaeal 16S rRNA gene abundance in subsurface sediments was highest in MIS 2, coincident with high sedimentation rates and high concentrations of total organic carbon. Soil Crenarchaeotic Group (SCG; Nitrososphaerales) species, the most abundant ammonia-oxidizing archaea in soils, increased dramatically during MIS 2, likely reflecting transport of terrestrial archaea during glacial periods with high sedimentation rates. Co-occurrence network analyses indicated significant association of SCG archaea with benthic deep-sea microbes such as Bathyarchaeota and Thermoprofundales in MIS 2 and MIS 3, suggesting potential interactions among these archaeal groups. Meanwhile, Thermoprofundales abundance was positively correlated with total organic carbon (TOC), along with n-alkane C31 and sedimentation rate, indicating that Thermoprofundales may be particularly important in processing of organic carbon in deep-sea sediments. Collectively, these results demonstrate that the composition of heterotrophic benthic archaea in the South China Sea may be influenced by terrestrial organic input in tune with glacial-interglacial cycles, suggesting a plausible link between global climate change and microbial population dynamics in deep-sea marine sediments.

Changes in terrestrial ecosystems across the Cretaceous-Paleogene boundary in western Canada inferred from plant wax lipid distributions and isotopic measurements

Changes in terrestrial environments across the Cretaceous-Paleogene boundary, including plant ecology and carbon and water-cycling, remain poorly defined. Fluvial sediments spanning the Cretaceous-Paleogene (K-Pg) boundary of southern Saskatchewan, Canada contain well preserved plant wax n-alkanes that provide a means of reconstructing changes to plant ecology and carbon and water cycling during this mass extinction event. We measured n-alkane carbon (δ13C) and hydrogen (δ2H) isotope ratios in two sedimentary sections and applied established fractionation factors to estimate the isotopic compositions of precipitation and bulk sedimentary organic matter sources. We also analyzed the distribution of n-alkanes as an indicator of the relative abundance of aquatic and terrestrial plants. We find a consistent shift towards a greater relative abundance of longer-chain n-alkanes across the boundary, implying a persistent increase in the relative abundance of terrestrial plants in the sedimentary basin. This is consistent with an increase in birch and elm palynomorphs immediately above the boundary. We hypothesize the extinction of all large herbivores at the boundary may have facilitated this transition to a terrestrial angiosperm dominated flora immediately after the boundary. We also find that the region was characterized by isotopically light precipitation, with δ2H values between −95‰ to −160‰, but do not observe evidence for major millennial-scale changes in regional precipitation isotopic composition spanning the boundary. n-Alkanes derived from both aquatic and terrestrial plants at one site display an upward trend in δ13C values of approximately 2‰ across the K-Pg boundary. This suggests millennial-scale local or global carbon-cycle variability altering either plant carbon isotope fractionation or the carbon isotope composition of dissolved inorganic carbon and atmospheric CO2. Overall our results suggest that carbon and water cycle changes associated with the K-Pg impact in terrestrial environments in western Canada were short-lived, but ecological shifts in plant communities were longer-lasting.

Carbon and nitrogen isotopes to distinguish sources of sedimentary organic matter in a Caribbean estuary

ABSTRACT Carbon and nitrogen stable isotope compositions (δ13C and δ15N) of organic matter (OM) and total organic carbon to total nitrogen ratio (Corg/TN) in a sediment core collected in Sagua estuary (Cuba), were investigated to elucidate the origin of the Sedimentary OM (SOM) and changes in its main sources, over the last 100 years. Results showed almost constant values in the elemental and isotope composition of SOM from 1908 to 1970 with an abrupt change after 1970. From 1970 to 2005, δ13C increased from -21.2 up to -19.3 ‰, while δ15N declined from 1.5 to values close to 0 ‰. The output of the mass-balance model for the identification of OM sources indicated that δ13C and Corg/TN values are generally influenced by marine Particulate OM (POM) sources. Between 1900 and 1970, the main OM source in sediments was marine POM (>85 %), with freshwater POM contributing ca. 15%. Since 1970, the establishment of the Alacranes Dam determined drastic environmental changes influencing the OM sources in the area. Mixing models pointed to seagrasses (79 %) as the main contributors to SOM in the first period, while since 1973 onward, the contribution of human-derived sources such as fertilizers and urban discharges became greater. This information can provide baseline data for the environmental management of the Sagua watershed.