泰国湾百年来有机碳埋藏记录及环境响应

受热带季风气候和周边陆源输入的影响,低纬泰国湾海—陆相互作用强烈,是研究海洋沉积有机碳与陆源输入、海洋初级生产力等气候环境变化响应关系的理想区域.通过对泰国湾泥质区T43柱样中总有机碳(TOC)、总氮(TN)、稳定碳同位素(δ13C)以及粒度等指标的分析,基于210Pb建立的高分辨年代地层框架,重建了该区百年来有机碳的...     

苏北潮滩湿地不同生态带碳、氮、磷分布特征*

通过对比苏北潮滩湿地不同生态带的表层以及柱状沉积物中总有机碳、总氮、总磷和有机磷含量,并结合不同植被不同植株部位中碳、氮、磷的含量变化,分析了潮滩沉积物中碳、氮、磷的垂向和水平分布特征与规律,探讨了不同生态带以及潮滩植被对碳、氮、磷等生源要素的富集作用。对比分析结果表明:苏北潮滩湿地各生态带对不同的测量指标有着不同的富集作用,互花米草滩的总有机碳、总氮和有机磷含量要远大于其他几个生态带,光滩沉积物中总磷的含量最高; 粒度效应是控制互花米草前缘地带以及互花米草滩沉积物中有机碳和氮分布的一个重要因素,盐蒿和芦苇滩中有机碳和氮的分布更多的是受粒度之外的其他因素影响; 不同生态带表层沉积物中的C/N比值分布,大致可反映其有机物来源的差异,而不同生态带中柱状沉积物中的C/N比值相对接近,很难根据C/N比值大小来对不同生态带中的有机物来源进行判断,这可能是埋藏在柱状沉积物中的有机物更多的受到了早期成岩作用造成的。植被对潮滩湿地中碳、氮、磷的分布有着重要影响,3种物质在互花米草、盐蒿和芦苇中的含量差别不是很大,因此潮滩植被对沉积物中上述3种物质的贡献差别主要是由不同植被的生物量和其所处环境的沉积动力差异造成的。     

Sources of organic carbon in the Portuguese continental shelf sediments during the Holocene period

Organic C (OC) and total N (TN) concentrations, and stable isotope ratios (δ¹³C) in muddy deposit sediments of the Northern and Southern Portuguese continental shelf were used to identify sources of fine-sized organic matter (<63 μm) during the Holocene period. Sedimentary columns off the Guadiana (core CRIDA 05), Tagus (core MD 992332) and Douro (core KSGX 57) estuaries are characterised by elemental and isotopic values that reflect distinct sources of organic matter (OC/TN and δ¹³C ranging, respectively, from 8.5 to 21 and from −22.4‰ to −27‰). Intense supplies to the Guadiana continental shelf of fine terrigenous particles during the Younger-Dryas Event are closely linked with higher OC/TN values and lower δ¹³C ratios. During the postglacial transgression phase, an increasing contribution of marine supplies (up to 80%) occurred. Higher δ¹³C (up to −22.4‰) values and low OC/TN ratios (down to 8.5) are found as the sea level approaches the current one. The Upper Holocene records emphasize the return to enhanced terrestrial supplies except for the Little Climatic Optimum between the 11th and 15th centuries AD. This climatic event is especially obvious in the three cores as a return to marine production and a decrease in terrestrial sediment supply to the continental shelf. The return to a cooling event, the Little Ice Age, between the 15th and 19th centuries AD, is mirrored by decreased terrigenous supplies in core KSGX 57. Gradually increasing sedimentation in estuaries, as well as formation of coastal dune fields, have been hypothesized on the basis of increasing δ¹³C and decreasing OC, TN and OC/TN values.     

Solid-state C NMR analysis of size and density fractions of marine sediments: Insight into organic carbon sources and preservation mechanisms

Burial of organic carbon (OC) in ocean sediments acts as the ultimate long-term sink for both terrestrial and marine carbon, however, the mechanisms controlling the preservation of this carbon are poorly understood. To better understand these mechanisms, we applied solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, along with elemental, stable carbon isotopic (δ¹³C) and lignin phenol analyses, to size and density fractions of sediments influenced by either mixed terrestrial and marine OC inputs (Washington Coast slope) or dominantly marine inputs (Mexican Margin). Elemental, isotopic and lignin analyses all reveal that within the Washington Coast sediment, the OC mixes linearly between nitrogen-poor and ¹³C-depleted, lignin-rich OC in the large and light fractions and nitrogen-rich and ¹³C-enriched, lignin-poor OC in the small and dense fractions, suggesting that this sediment contains a two-component mixture of terrestrial vascular plant- and marine-derived OC. The integral areas of each of seven NMR spectral regions in the different samples trend linearly when plotted versus δ¹³C signature, with most R² values of 0.78 or greater, demonstrating that the NMR spectra of the two sources of carbon also mix linearly between the two endmembers. The terrestrial endmember in this sediment appears to be dominated by lignin and black carbon whereas the source of the marine endmember is less clear from the NMR spectra. In contrast, all of the analyses indicate that OC in the Mexican Margin sediment fractions is homogenous and derives almost exclusively from marine sources. It appears that selective preservation of (bio)chemically recalcitrant lignin and black carbon is the primary mechanism of preservation of terrestrial OC, whereas mineral-protection is the dominant mechanism preserving marine OC in the Washington coast sediment. There is little evidence showing that either preservation mechanism functions in the Mexican Margin sediments.     

Distribution and sources of carbon,nitrogen, phosphorus and biogenic silica in the sediments of Chilika lagoon

The present study investigated the spatial and vertical distribution of organic carbon (OC), total nitrogen (TN), total phosphorus (TP) and biogenic silica (BSi) in the sedimentary environments of Asia’s largest brackish water lagoon. Surface and core sediments were collected from various locations of the Chilika lagoon and were analysed for grain-size distribution and major elements in order to understand their distribution and sources. Sand is the dominant fraction followed by silt + clay. Primary production within the lagoon, terrestrial input from river discharge and anthropogenic activities in the vicinity of the lagoon control the distribution of OC, TN, TP and BSi in the surface as well as in the core sediments. Low C/N ratios in the surface sediments (3.49–3.41) and cores (4–11.86) suggest that phytoplankton and macroalgae may be major contributors of organic matter (OM) in the lagoon. BSi is mainly associated with the mud fraction. Core C5 from Balugaon region shows the highest concentration of OC ranging from 0.58–2.34%, especially in the upper 30 cm, due to direct discharge of large amounts of untreated sewage into the lagoon. The study highlights that Chilika is a dynamic ecosystem with a large contribution of OM by autochthonous sources with some input from anthropogenic sources as well.     

四川盆地海、陆相烃源岩有机质稳定碳同位素组成变化及其地球化学意义

采用质谱和色谱-同位素质谱技术,测定了500余个干酪根及饱和烃、芳烃组份、正构烷烃的δ13C值,以此揭示了四川盆地海、陆相烃源岩有机碳同位素组成随地质时代的变化特点及其在高成熟阶段的演化特征,并结合其他相关分析资料,应用碳同位素剖析了不同类型海、陆相烃源岩的有机质生源及沉积环境。研究结果表明,该盆地从震旦系灯影组到中三叠统雷口坡组的海相地层中,干酪根的碳同位素组成随层位变新呈逐渐变重的趋势,可能的原因之一是浮游植物的进化作用;而上三叠统须家河组至中侏罗统千佛崖组的陆相有机质碳同位素组成则有反向的年代变化,主要与生源构成和沉积环境性质有关。这些海、陆相烃源岩的有机源难以用干酪根δ13C值进行区分,而可用饱和烃与芳烃组份的δ13C及其CV值来区别。海洋和湖泊不同沉积相带烃源岩的干酪根具有明显不同的δ13C值,结合其他相关资料可识别其有机质来源。煤系地层中煤与泥岩在干酪根碳同位素组成上没有可区分性,而两者有不同的正构烷烃碳同位素分布曲线。从成熟晚期到高-过成熟阶段,海、陆相干酪根的碳同位素均变重1~2‰,煤系泥岩的正构烷烃碳同位素分布曲线由负向线型分布向平直型演变。这些碳同位素的变化特征为油气源对比和烃源识别提供了依据。     

Selected organic matter source indicators in the Orinoco,Nile and Changjiang deltas

Carbon isotopic composition, carbon to nitrogen ratios, carbon preference (n-C₂₃ to n-C₃₂), and microscopic examination of selected samples were compared and contrasted as organic matter source indicators. The Orinoco, Nile, and Changjiang deltas were sampled and analyzed. The δ ¹³C of organic matter increased with distance offshore at all three locations. Organic carbon content decreased seaward on the Orinoco and Changjiang deltas but increased on the Nile delta. C/N ratios decreased regularly with distance offshore on the Orinoco delta, ranged from 10 to 25, and correlated positively with δ ¹³C_OM. Estimations of the relative input of marine versus terrestrial organic matter to the sediments was dependent on the parameter analyzed. These discrepancies were primarily due to the definition of end-member compositions and the fraction of the total organic matter that is measured by a given parameter. Multiple-source indicators must be measured and integrated and end-members must be properly defined in order to obtain realistic input estimates.     

Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico

Suspended sediments (SS) from the Atchafalaya River (AR) and the Mississippi River and surficial sediment samples from seven shallow cross-shelf transects west of the AR in the northern Gulf of Mexico were examined using elemental (%OC, C/N), isotopic (δ¹³C, Δ¹⁴C), and terrigenous biomarker analyses. The organic matter (OM) delivered by the AR is isotopically enriched (∼−24.5‰) and relatively degraded, suggesting that soil-derived OM with a C4 signature is the predominant OM source for these SS. The shelf sediments display OC values that generally decrease seaward within each transect and westward, parallel to the coastline. A strong terrigenous C/N (29) signal is observed in sediments deposited close to the mouth of the river, but values along the remainder of the shelf fall within a narrow range (8-13), with no apparent offshore trends. Depleted stable carbon isotope (δ¹³C) values typical of C3 plant debris (−27‰) are found near the river mouth and become more enriched (−22 to −21‰) offshore. The spatial distribution of lignin in shelf sediments mirrors that of OC, with high lignin yields found inshore relative to that found offshore (water depth > 10 m).The isotopic and biomarker data indicate that at least two types of terrigenous OM are deposited within the study area. Relatively undegraded, C3 plant debris is deposited close to the mouth of the AR, whereas more degraded, isotopically enriched, soil-derived OM appears to be deposited along the remainder of the shelf. An important input from marine carbon is found at the stations offshore from the 10-m isobath. Quantification of the terrigenous component of sedimentary OM is complicated by the heterogeneous composition of the terrigenous end-member. A three-end-member mixing model is therefore required to more accurately evaluate the sources of OM deposited in the study area. The results of the mixing calculation indicate that terrigenous OM (soil-derived OM and vascular plant debris) accounts for ∼79% of the OM deposited as inshore sediments and 66% of OM deposited as offshore sediments. Importantly, the abundance of terrigenous OM is 40% higher in inshore sediments and nearly 85% higher in offshore sediments than indicated by a two-end-member mixing model. Such a result highlights the need to reevaluate the inputs and cycling of soil-derived OM in the coastal ocean.     

Raman spectroscopy and biomarker analysis reveal multiple carbon inputs to a Precambrian glacial sediment

Extractable biomarkers can help elucidate the environment and biota of ancient glaciations, although the method must be applied with care, as glacial sediments have a potential for incorporation of older detrital carbon. In Phanerozoic glacial sediments, the distinct elemental, molecular and isotopic compositions of the terrestrial and marine biomass allow discrimination between primary marine and redeposited terrestrial organic matter. However, as the Proterozoic biosphere was largely microbial and marine, biomarker and isotopic analyses are insufficient for distinguishing primary organic matter from secondary reworked organic matter. Here, we report the combined application of Raman spectroscopy and biomarker analysis to Precambrian glacial sediments, which, together, allows discrimination between mixed pools of organic carbon and provides a promising new approach for rapidly screening Precambrian sediments for immature organic matter amenable to biomarker analysis.     

Geochemical Characteristics of Amino Acids in Sediments of Lake Taihu, A Large, Shallow, Eutrophic Freshwater Lake of China

To examine the biogeochemistry of amino acids (AAs) in the sediment of Lake Taihu, surface sediments (0–3 cm) and deeper sediments (18–21 cm) were collected at 21 sites from different ecotype zones of the lake. AAs were extracted from the sediments, and the total hydrolyzable amino acids (THAA) were determined by high-performance liquid chromatography instrument. The THAA contents in Taihu sediment were much lower than that in marine sediments, ranging from 6.84 to 38.24 μmol g⁻¹ in surface sediments and from 2.91 to 18.75 μmol g⁻¹ in deeper sediments in Taihu, respectively. AAs were a major fraction of the organic matter (OM) and organic nitrogen in Taihu sediments. The AAs on average contributed 8.2% of organic carbon (OC) and 25.0% of total nitrogen (TN) from surface sediments, and 5.9% of OC and 20.5% of TN in deeper sediments, respectively. AA composition provided very useful information about the degradation of OM. Glycine (Gly) and lysine (Lys) were the predominant forms of AAs in the sediments, irrespective of lake regions, followed by alanine, glutamic acid, serine (Ser), and aspartic acid (Asp). The high concentrations of Gly, Lys, and Ser suggested that these forms of AAs were relatively refractory during OM degradation in sediments. The relationship between the Asp/Gly ratio and Ser + Thr [mol%] indicated that OM in surface sediment was relatively fresher than that in deeper sediments. The AAs-based degradation index (DI) gave a similar conclusion. The composition and DI of AAs in surface sediments are markedly different across different zones in Taihu. The percentages of AAs to organic carbon (AA-C%) and total nitrogen (AA-N%) were higher in phytoplankton-dominated zones than those in macrophyte-dominated zones. These results suggest that DI could provide useful information about the degradation of OM in shallow lakes such as Taihu.     

Isotopic compositions of tropical East African flora and their potential as source indicators of organic matter in coastal marine sediments

The C and N stable isotope compositions of some flora of East Africa from coastal Tanzania and Amboseli National Park (Kenya) are used to assess if they can be used as a terrestrial end member during the estimation of terrestrial fraction in coastal marine sediments. The results of C isotope composition of various tree leaves, which average −29.3 ± 1.4%, indicate that these tropical higher land plant species follow a Calvin-Benson or non-Kranz (C₃) type of metabolism. The results for grass species, which average −13.2 ± 2.4%, indicate that most of them follow a Hatch-Slack or Kranz (C₄) type of metabolism. However, some of the succulent plants from the Amboseli National Park have δ¹³C values that average −14.7%, an indication that they follow a CAM (Crassulacean Acid Metabolism) type of metabolism. The N isotope values are relatively higher than expected for the terrestrial organic material. The average δ¹⁵N values for both tree and grass samples are higher than 5% and fall within the range normally considered to be marine. The high enrichment in ¹⁵N may be related to the environmental conditions in which plants thrive. Plants growing in sandy, dry and overgrazed environments are expected to be enriched in ¹⁵N owing to full utilisation of all available N species, regardless of their isotopic compositions. Other processes which may cause an enrichment in ¹⁵N include adsorption by various types of clay minerals, supply of ¹⁵N-enriched nitrate through sea-spray, and local denitrification, especially in swampy and lake margins where the input of organic matter may be higher than the rate of decomposition.The stable isotopic composition of organic C and N for surficial organic matter for the coastal marine sediments averages −17.0 ± 0.9% and 5.4 ± 1.1%, respectively. These values indicate a substantial contribution of C₄ plants and sea grasses. However, contribution of C₄ relative to that of sea grasses can not be evaluated owing to the fact that there is no significant difference in the isotopic compositions between the two groups.In the savannah environment, where a contribution from the C₄ types of plants might be substantial, the δ¹³C value for a terrestrial end member needs to be established prior to evaluation of the terrestrially derived organic matter in the marine environment. Owing to a significant contribution of sea grasses to the total organic matter preserved in coastal marine sediments, the stable isotopes of organic C seem to have a limited applicability as source indicators in the East African coastal waters. Furthermore, the results indicate that N stable isotopes seem to have a limited applicability as source indicators in coastal waters of East Africa. However, more work needs to be conducted to determine the terrestrial and sea grass end member values for the coastal areas.     

The Diversity of Organic Matter in Marine Sediments and the Suspiciousness of Source Parameters: A Review

There are various types of Organic Matter (OM) in marine sediments, and it is of great significance to accurately estimate the source of them. The methods used in most researches include element ratio (C/N), stable isotopes (δ¹³C) and biomarkers (BIT), all of which are used for the whole sample as objects and calculated by end-member of terrestrial and marine source OM, respectively. However, OM is preserved in different ways, such as on its own recalcitrance or combining with inorganic minerals. Preservation differences make their respective parameter values change. After differential sedimentary by gravity or other factors,different preservation types of OM are enriched or dispersed, making some parameters distorted and even meaningless in some areas. Therefore, the parameters of OM measured by the whole sample are influenced by the source of OM and the way of deposition and preservation. Therefore, it is unreliable to use these parameters to study the source of OM without considering the way of preservation. Based on this, this paper put forward a method of particle separation to separate the sediment in different propertied and preservation types. According to the composition and characteristics of the OM at the specific grain level, the corresponding index and end-members value were used to calculate more accurate proportion of the OM. It is of profound significance to understand the production, aggregation, occurrence, migration and deposition of OM and its evolution in the entire carbon cycle.     

Sources of fine-sized organic matter in North Atlantic Heinrich Layers: δC and δN tracers

Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios (δ¹³C, δ¹⁵N) of fine (<50 μm) size fractions of deep-sea sediments from the central North Atlantic were employed to identify changes in sources of organic matter over the past 50 ka BP. Ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs (averages ± 1σ: OC/TN = 7.6 ± 0.8; δ¹³C = −22.8 ± 1.0‰; δ¹⁵N = 5.5 ± 0.6‰). δ¹³C, OC, and TN concentrations shift to higher values during the Holocene, indicating a gradual decrease of fine terrigenous supply to the North Atlantic. The unchanged δ¹⁵N record between last glacial and Holocene stages indicates that the central North Atlantic region remained oligotrophic at least during the past 50 ka BP, but additional studies are required to support this result in terms of nitrogen oceanic budget. During the phases of enhanced ice-rafted detrital supply corresponding to prominent Heinrich events (HL₁, HL₂, HL₄, and HL₅), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower δ¹³C (down to −28‰) and δ¹⁵N (down to 1.6‰) values and high OC:TN ratios (up to 14.7 ± 1.1) are found for HL₁, HL₂, and with lesser extent for HL₄. These values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter-bearing rock sources of the underlying geological basement. During HL₅, only the δ¹³C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL₅ to HL₁ (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant δ¹³C, increasing OC/TN and decreasing δ¹⁵N values.     

Organic matter and nutrients in an altered subtropical marsh system,Chiricahueto, NW Mexico

Seven sediment push-cores were extracted from Chiricahueto, a marsh affected by urban, industrial and agricultural wastes. Concentrations of total phosphorus (300-1,620 µg g^-1), organic carbon (4-39 mg g^-1) and total nitrogen (0.5-4.5 mg g^-1) in the sediments showed an exponential decrease with depth, related to the decomposition of organic matter (OM). Between 20 and 40% of OM initially deposited is degraded at the sediment-water interface under oxic conditions. Another fraction (40-60%) of non-refractory OM is decomposed within the sediments by oxidants other than oxygen. Likewise, the preservation of OM (<20%) was estimated as burial concentrations of C, N and P linked to organic compounds. The C/N ratios, '¹³C and '¹⁵N suggested that the major source of OM to the sediments derives from marine phytoplankton. The allochthonous sources of OM were overprinted by the high flux of marine autochthonous OM. However, an indirect terrestrial influence is recognised, in which high nutrient load derived from agricultural, domestic and industrial activities promoted high productivity.     

南海表层沉积物与沉降颗粒物中有机碳的δ13C对比研究及其古环境再造意义

对南海表层沉积物与沉降颗粒物有机碳的δ13C进行了对比研究,从古环境再造方面探讨了陆源和海源对南海表层沉积物的贡献。南海表层沉积物有机碳的δ13C比沉降颗粒物中有机碳的δ13C偏重表明,Suess效应对南海现代过程的影响不可忽视,两者的δ13C差值反映了Suess效应对南海现代过程的影响程度。利用两者的δ13C差值,对现代陆源和海源有机碳的δ13C进行修正后,计算得到,在南海陆架周围表层沉积物的有机碳中,陆源的比例为48%,海源的比例为52%;在远离陆架的表层沉积物的有机碳中,陆源的比例为14%,海源的比例为86%。     

Organic Facies Characteristics of Quaternary Sediments,Gulf of Izmit,Marmara Sea,Turkey

The classification of organic facies from samples obtained by submarine drilling between Hersek Burnu and Kaba Burun in the Gulf of Izmit was determined using organic geochemistry methods. The host sediment is composed of pebbles, sand, silt, and clay, some of which are rich in organic matter. Results indicate an early diagenesis stage according to total organic carbon (0.11-1.78%) and vitrinite reflectance (0.12-0.32%) values. Rock-Eval pyrolysis data display hydrogen indices (HI) of 3 to 97 mg HC/g TOC and oxygen indices (OI) of 29 to 245 mg HC/g CO₂. Microscopic studies show that the dominant organic matter is oxic and terrestrial. These data demonstrate that the organic facies were developed in this environment. The CD facies reflects a mixture of source materials including terrestrial plant detritus, and reworked and fine-grained amorphous organic matter. The D facies consists of highly decomposed, reworked organic matter. These types of organic facies generally appear to be weakly sorted in marine and lake sediments.     

Organic fades in Recent sediments of carbonate platforms: Southwestern Puerto Rico and Northern Belize

Our organic facies model of a platform environment is based on the analysis of (1) total organic carbon, (2) visual protokerogen types and hydrogen indices of the sediments, (3) sediment mineralogy and grain size, and (4) reef patterns, water circulation, and relief of the adjacent land.TOC values are lowest on reefs and outer platforms (0.2%); intermediate in sediments fringing carbonate islands (0.5–2.0%), on sea-grass meadows (0.5–0.7%), and in forereef lagoons (0.1–0.7%); highest in inner platform lagoons (0.5–5.0%) and mangrove swamps (2.7–3.5%). Paniculate organic matter comprises structured terrestrial, marine, and amorphous types. On average, terrestrial material accounts for 28% of OM in Puerto Rico and 12% in Belize. The mean hydrogen index of the sediment is 355 (mg hydrocarbons/g TOC) in Puerto Rico and 719 in Belize.This study indicates that TOC values are mainly related to platform subenvironment, while OM types are controlled by distance from the shoreline. Organic facies dominated by structural terrestrial OM occur closer to the shore, whereas toward the shelf margin marine and amorphous OM contents increase.     

The Effect of Organic Carbon-Iron Oxide Association on the Preservation of Sedimentary Organic Carbon in Marine Environments

Understanding the mechanisms responsible for long-term storage of organic carbon (OC) in marine environment is important for studying the marine carbon cycling and predicting how the global carbon cycle will respond to climate change. It is estimated that more than 20% of the OC in marine sediments is associated with iron oxides and thus these complexes are one of the most important factors in the long-term storage of OC. The OC-iron oxide (OC-Fe) association can be formed through either adsorption or co-precipitation, but the dominant mechanism of OC-Fe association in marine environments is co-precipitation. The combination of OC from different sources with iron oxides is selective. Iron oxides preferentially combine with marine OC in most marine environments, but in estuarine delta regions they prefer terrestrial OC. Due to large inputs of terrestrial materials, high primary production and frequent re-suspension, estuarine and marginal seas are suitable sites for OC-Fe association studies, which should be emphasized in the future.     

Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland)

We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ¹³C_TOC and δ¹⁵N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ¹³C_DIC were used to trace OM decomposition.The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH₄ and CO₂ in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO₃.The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes.     

The sequestration of terrestrial organic carbon in Arctic Ocean sediments: A comparison of methods and implications for regional carbon budgets

A variety of approaches have previously been developed to estimate the fraction of terrestrial or marine organic carbon present in aquatic sediments. The task of quantifying each component is especially important for the Arctic due to the regions’ sensitivity to global climate change and the potential for enhanced terrestrial organic carbon inputs with continued Arctic warming to alter carbon sequestration. Yet it is unclear how each approach compares in defining organic carbon sources in sediments as well as their impact on regional or pan-Arctic carbon budgets. Here, we investigated multiple methods: (1) two end-member mixing models utilizing bulk stable carbon isotopes; (2) the relationship between long-chain n-alkanes and organic carbon (ALKOC); (3) principal components analysis (PCA) combined with scaling of a large suite of lipid biomarkers; and (4) ratios of branched and isoprenoid glycerol dialkyl glycerol tetraether lipids (the BIT index) to calculate the fraction of terrestrial organic matter components preserved in Arctic marine sediments.Estimated terrestrial organic carbon content among approaches showed considerable variation for identical sediment samples. For a majority of the samples, the BIT index resulted in the lowest estimates for terrestrial organic carbon, corroborating recent suggestions that this proxy may represent a distinct fraction of terrestrial organic matter; i.e., peat or soil organic matter, as opposed to markers such as n-alkanes or long-chain fatty acids which measure higher plant wax inputs. Because of the patchy inputs of n-alkanes to this region from coastal erosion in the western Arctic, the ALKOC approach was not as effective as when applied to river-dominated margins found in the eastern Arctic. The difficulties in constraining a marine δ¹³C end-member limit the applicability of stable isotope mixing models in polar regions. Estimates of terrestrial organic carbon using the lipid-based PCA method and the bulk δ¹³C mixing model approach varied drastically at each site, suggesting that organic matter fractions such as amino acids or carbohydrates may affect bulk organic matter composition in a manner that is not captured in the lipid-based analysis. Overall, terrestrial organic matter inputs to the Chukchi and western Beaufort Seas using the average of the methods at each site ranged from 11% to 44%, indicating that land-derived organic matter plays a substantial role in carbon dynamics in the western Arctic Ocean.