Terrestrial Organic Carbon Research Articles

Bound to rock

Geochemistry Organic matter binds to phyllosilicates, a process which affects both its transport and chemical stability. How does that affect the fate of terrestrial organic carbon that enters the ocean? Blattmann et al. show that organic carbon derived from soils is stripped from mineral surfaces upon discharge and dispersal into the ocean, whereas organic matter derived from ancient rocks is preserved there. Their results show that preservation of continentally derived organic matter in marine sediments is controlled largely by phyllosilicate mineralogy. Science , this issue p. [742][1] [1]: /lookup/doi/10.1126/science.aax5345

PAES and PAHs in the surface sediments of the East China Sea: Occurrence, distribution and influence factors

A total of 29 sediment samples were collected from the East China Sea (ECS), with the Yangtze River estuary and the Zhejiang costal area. These sediment samples were analyzed for 6 phthalate esters (PAEs) and 16 polycyclic aromatic hydrocarbons (PAHs): the ΣPAEs and ΣPAHs concentrations ranged between 1649.5 and 8451.5 ng g−1 (mean = 3446.3 ng g−1) and 57.5–364.5 ng g−1 (mean = 166.2 ng g−1), respectively. Overall, the PAEs and PAHs concentrations gradually decreasing in the offshore and southward directions: their compositions and distributions suggest they could have mainly derived from the Yangtze River. In particular, their distribution was influenced by the sources’ proximity, hydrodynamics, and sediment geochemistry (i.e., TOC content and grain size). A classical two-end member model was utilized to estimate the fraction of terrestrial organic carbon in the sediments of the ECS. When the sediment was dominated by terrestrial-derived organic matter (OM), the concentrations of PAEs and PAHs were significantly correlated to the TOC content and gran size of the sediments. In contrast, the poor correlation of TOC content and grain size with PAEs in those sediments dominated by marine-derived OM, implied that the distribution of PAEs in the ECS was mainly related to land-based inputs, (especially to that of the Yangtze River). Regardless of the origin of most of the OM contained in the sediments, we observed positive correlations between the TOC content, and grain size of those containing PAHs. These results suggest that the distribution of PAHs in the ECS was not only related to the Yangtze River input, but also to the geochemical characteristics of the sediments.

Terrestrial Vegetation Drives Methane Production in the Sediments of two German Reservoirs

Inland waters and reservoirs in particular are significant sources of methane to the atmosphere. However, little information is available on the extent to which organic carbon from terrestrial vegetation or from internal photosynthesis fuels the methane production. This limits our ability to constrain methane emissions efficiently. We studied the isotopic composition (13C, 14C) of pelagic and sedimentary carbon sources in two small German reservoirs. The methane was enriched by radiocarbon with isotopic ranges (∆14C 5‰ to 31‰) near to fresh terrestrial organic carbon (OC, 17‰ to 26‰). In contrast, potential source OC produced by internal photosynthesis was characterized by negative ∆14C values (−30‰ and −25‰) as derived from signatures of inorganic carbon in the reservoirs. The particulate OC in stream supplies (terrestrial OC) was also 14C depleted in almost all cases, but highly variable in ∆14C (−131‰ to 42‰). Although the import of terrestrial OC was lower than the amount of OC produced by reservoir-internal photosynthesis, we conclude that the methane production was predominantly fuelled by catchment vegetation. The utilized terrestrial OC was of contemporary origin, fixed within years to decades before sampling and supplemented with reservoir-internal or aged terrestrial OC. Our results indicate that terrestrial biomass is an important driver of methane production in reservoirs receiving significant imports of terrestrial OC.

Geochemical fractionation, potential bioavailability and ecological risk of phosphorus in surface sediments of the Cross River estuary system and adjacent shelf, South East Nigeria (West Africa)

Abstract Phosphorus (P) as an essential macronutrient, plays a crucial role in the regulation of biological productivity and biogeochemical cycling of other biogenic elements in aquatic environments. Different geochemical fractions of sedimentary P were separated and quantified using the sequential extraction (SEDEX) method to evaluate the abundance, sources, distribution, bioavailability and ecological risk of P in surface sediments of the Cross River estuary system (CRES) and adjacent shelf. Sediment grain size composition and bulk organic carbon were also determined. Total P (TP) ranged from 9.21–18.2 μmol/g (average, 13.30 ± 2.08 μmol/g), and comprised mostly of inorganic P (IP) (IP/TP = ~87%). Generally, authigenic P (Ca-P) was the dominant IP fraction (Ca-P/IP = ~37 ± 11.1%), representing an important long-term P reservoir. The range of organic carbon (OC) to organic P (OP) ratio (OC/OP) indicated the predominance of terrestrial OC, and that leachable OP (Le-OP) may be the major OP form in surface sediments of the study area. The partitioning and distribution of different sedimentary P fractions were controlled by the biogeo- and physicochemical conditions favorable for P transformation, sediment grain size composition and sources of sedimentary organic matter. The ratio (Fe/P) of the extractable Fe in citrate-dithionite-bicarbonate solution (FeCDB) to ferric oxide-bound P(Fe-P) revealed the presence of reactive Fe- matrices having strong sorption capacity for P. The contents of potential bioavailable P (BAP) (~34–60% of TP) indicated that P amount in the overlying water column may increase substantially if released from surface sediments. The Fe-P was the major component of BAP, and the relationship between BAP and extractable FeCDB highlighted the significant impact of Fe(III) (oxyhdr)oxides on P mobility and bioavailability. The estimated P enrichment index (PEI) indicated that surface sediment is less impacted by anthropogenic P and poses less ecological risk with respect to sedimentary TP load in the study area.

Land plant evolution decreased, rather than increased, weathering rates

Abstract The repeated evolution of trees is widely thought to have enhanced the capacity of silicate weathering via the impact of deep rooting. However, land plants are also responsible for wetland assembly and organic carbon burial. The total burial output of carbon via both organic and inorganic deposition must balance input to the exogenic system from volcanic outgassing on million-year time scales. Increased partitioning of carbon burial toward organic carbon and away from inorganic carbon reduces the marine carbonate burial flux, necessitating a lowered total flux of alkalinity to the oceans to maintain mass balance in the Earth’s surface carbon cycle. This flux includes the nutrient delivery from the terrestrial vegetation implicated as a driver of marine evolution, extinction, and environmental change including anoxia and black shale formation. Here, the burial of terrestrial organic carbon, first substantially in the Devonian and continuing through to the present, is argued to require a reduction in silicate weathering rates when compared to earlier times, given the independence of volcanic outgassing from weathering on short time scales. Land plants still may cause reductions in steady-state atmospheric CO2 levels, but via increasing the silicate weathering feedback strength, not silicate weathering rates. The mass-balance constraints on the long-term carbon cycle provide a mechanism for linking how land plant evolution simultaneously increased nutrient recycling and weathering efficiency of the Earth’s surface.

Comparison of sedimentary organic carbon loading in the Yap Trench and other marine environments

Knowledge about organic carbon loadings (ratio of sedimentary organic carbon (SOC) content to specific surface area (SSA)) and the fate of organic carbon (OC) is critical to understand the marine carbon cycle. We investigated the variations in the patterns of OC loadings and the preservation capacities of sedimentary OC in the Yap Trench and other marine environments. The average OC loading in sediment cores from various marine environments decreases with increasing water depth at a rate of ~0.06 mg OC/(m2·km) (R2 =0.23, P<0.01). Distinct low OC loadings (0.09±0.04 mg OC/m2) were observed in the Yap Trench, with the lowest values as ~0.02 mg OC/m2. A further comparative analysis indicated that OC/SSA=0.2 mg OC/m2 is a good indicator to distinguish between oxic deep-sea regions and suboxic energetic deltaic areas. Regression analysis between OC loading and bulk carbon isotope compositions indicates that marine OC (δ13C ~−20.4‰ to −18.6‰) dominates the lost OC within the Yap Trench and does not differ from that of the abyssal zone. In contrast, terrestrial OC with δ13C values of approximately −27.4‰ to −20.5‰ was the major source of remineralized OC in the sublittoral zone. The ratios of OC loadings in the bottom layer relative to those in the top layers of sediment cores indicate that the preservation capacities of hadal trenches are much lower than those of other environments, and only approximately 30% of the SOC deposited in hadal trenches is finally buried. The value is equivalent to 0.066% of the primary production-derived OC and much lower than the global ocean average (~0.3%). Overall, the hadal zone exhibits the lowest OC loading and preservation capacity of SOC of the different marine environments investigated, despite the occurrence of a notable funneling effect.

Mineralogical control on the fate of continentally derived organic matter in the ocean

First-order relationships between organic matter content and mineral surface area have been widely reported and are implicated in stabilization and long-term preservation of organic matter. However, the nature and stability of organomineral interactions and their connection with mineralogical composition have remained uncertain. In this study, we find that continentally derived organic matter of pedogenic origin is stripped from smectite mineral surfaces upon discharge, dispersal, and sedimentation in distal ocean settings. In contrast, organic matter sourced from ancient rocks that is tightly associated with mica and chlorite endures in the marine realm. These results imply that the persistence of continentally derived organic matter in ocean sediments is controlled to a first order by phyllosilicate mineralogy.

A comparative study of total organic carbon-δ13C signatures in the Triassic–Jurassic transitional beds of the Central European Basin and western Tethys shelf seas

Stratigraphic studies are an integral component in understanding the chronology of events that led to the end-Triassic mass extinction, by resolving causal relationships between environmental upheavals and biotic response. Successful correlation of Triassic–Jurassic (Tr–J) successions is complicated by the disappearance of macro-fossils that are otherwise central components in stratigraphic studies. This problem is exacerbated in multiple Tr–J sections situated in Europe, where the so-called “Event Beds” – assumed to demarcate the extinction interval – are virtually devoid of fossils. An alternative stratigraphic approach entails the reconstruction of carbon isotope records, where stratigraphic fluctuations in carbon isotope composition are considered to track changes in the global biogeochemical carbon cycle. The predominance of carbonate-lean sections has prompted the reconstruction of total organic carbon (TOC)-based carbon isotope records. However, bulk rock derived TOC is the diagenetically stabilized remnant of organic components that accumulated on the sea floor, and which can originate from multiple sources. In this study, we assess long-term TOC-based carbon isotope trends at two sites: Bonenburg (Central European Basin) and Kuhjoch (the Tr–J Global Stratotype Section and Point; western Tethys shelf seas). We focus on the TOC 13C-enrichment of the Event Beds with the aim of deciphering stratigraphic fluctuations in relation to their main driver (the exogenic carbon pool versus organic matter source changes). By studying the systematic co-variance of several sedimentary parameters (TOC, total nitrogen [TN], and the palynomorph composition), we infer that the TOC composition is possibly characterized by insignificant organic matter source changes in terms of the marine and terrestrial organic carbon contributions. By contrast, a clay mineralogical shift to more K-depleted minerals as well as the elevated occurrence of wood fragments in the Event Beds suggest a terrestrial organic matter source shift from immature substrates to substrates predominated by “pre-aged” or “fossil organic matter” under a changing continental weathering regime. This outcome urges for reservations when interpreting TOC-based carbon isotope records in terms of global C-cycle perturbations, especially when coinciding with lithological and mineralogical changes. On a more positive note, the shift towards positive carbon isotope values appears to be a recurring feature, possibly testifying to a globally significant climate-controlled weathering regime shift.

Terrestrial organic carbon storage modes based on relationship between soil and lake carbon, China

Terrestrial ecosystems have received considerable attention as a significant sink for organic carbon at regional to global scales. Previous studies were focused on assessment and quantification of carbon sinks for one ecotype, and few have worked on the interconnection of terrestrial sinks. In this paper, we synthesized the data from China's second national soil survey and direct measurements from 54 lakes. Meanwhile, we investigated the controlling factors of carbon accumulation dynamics in soils and lakes. Results showed varied spatial distribution of soil and lake organic carbon in different regions, and three storage modes were found. The storage mode of watershed collection was observed in the region of the Qinghai-Tibetan Plateau, while the northeast China and Yunnan-Guizhou Plateau revealed another storage mode of autochthonous deposition, and the mode of human activities affection was represented by the East Plain and Mongolia-Xinjiang Plateau. The spatial difference throughout China was regulated by various climate patterns, geological conditions and anthropogenic interference. Our results provide insights into carbon storage modes in various regions, and also inform strategies for enhancing global carbon sequestration and future mitigation policies towards global climate change.

Spatiotemporal variation of organic geochemical properties since the mid-Miocene in the deep South China Sea (IODP Expedition 349)

We performed an extensive survey of total organic carbon (OC) and stable OC composition of in the deep South China Sea sediments from the International Ocean Discovery Program (IODP) Expedition 349 Sites U1431, U1433 and Ocean Drilling Program (ODP) Leg 184 Site 1148. Our results reveal dramatic spatial and temporal variations in organic geochemical characteristics among the three Sites. Before ~8 Ma, OC deposited at Site U1431 may be predominantly terrigenous, whereas OC deposited at Sites U1433 and 1148 is mainly of marine origin. After ~8 Ma, OC accumulation at the three Sites progressively increases, mainly due to enhancement of marine primary productivity and input of terrestrial organic matter; the three sites receive mainly marine OC, whereas Sites U1433 and 1148 receive relatively more terrestrial OC compared with Site 1431. Additionally, methane concentrations vary from ~20 to ~100,000 ppmv at Site U1433 and from ~10 to ~600 ppmv at Site 1148 with higher geothermal gradients (78 and 84 °C/km, respectively). In contrast, methane concentrations are <5 ppmv at Site U1431 with lower geothermal gradient (14.8 °C/km). Such high methane concentrations and ratios of methane to ethane at Site U1433 suggest that the gaseous hydrocarbons in the southwestern sub-basin (deep water depth, >4000 km) are primarily sourced from microbial production. Our findings suggest that the sources, trajectories, and fates of OC buried in the deep South China Sea since the mid-Miocene are spatially and temporally variable, controlled by sedimentological processes in the sub-basins, and regional tectonic evolution and long-term paleoenvironmental changes.

Molecular isotopic insights into hydrodynamic controls on fluvial suspended particulate organic matter transport

Abstract Hydrodynamic sorting has been shown to strongly influence the composition and age of organic carbon (OC) during sediment transport and burial in the marine environment, yet sorting effects on terrestrial OC (OCterr) in fluvial systems remain poorly understood. We conducted size fractionation of suspended particle samples from the lower Yellow River, China, and examined variations in mass distribution and carbon isotopic (δ13C and Δ14C) composition of bulk OC and specific biomarkers among grain size fractions in order to investigate the influence of hydrodynamic sorting and selective transport on organic matter export. In general, the 16–32 μm and 32–63 μm fractions contributed the most of sediment mass while the majority of the OC resided in the 16–32 μm fraction. Over 80% of OC and n-fatty acids (FAs) were concentrated in 32 μm), characterized by relatively low SA, OC%, and Δ14COC values, but higher Δ14C values of C26+28+30 FAs, are inferred to reflect a combination of bedrock-derived detrital sediment and fresh vascular-plant material (e.g., plant fragments). In contrast, finer particles (

Spatial and seasonal variability of sedimentary organic matter and its origin in an algal-blooming eutrophic lake

Abstract Knowledge of the origin of sedimentary organic matter (SOM) in lakes contributes to our understanding of the carbon cycle and provides valuable guidance for lake ecosystem management. To elucidate the changes in the spatial and seasonal origins of SOM in a eutrophic lake, we collected sediment at 31 sampling points in Lake Hachiro, northern Japan, in June and November of 2016 and April and June of 2017, and measured total organic carbon (TOC) and fatty acid biomarkers. We found that TOC and the origin of SOM showed both spatial and seasonal variations. TOC was relatively high in the eastern part of the lake and may reflect the hydrological conditions therein. The average TOC of sediment collected from the 31 sampling points increased from June to November 2016, concomitant with an increase in cyanobacterial fatty acids. A cyanobacterial bloom was observed in the water column from July to October of that year, and the input of cyanobacterial carbon to the sediment may explain the TOC increase during this period. We also detected an increase in TOC from November 2016 to April 2017, which coincided with increases in fatty acids of terrestrial and diatom origin. In this regard, we suspect that meltwater inflow during this period may have borne large quantities of terrestrial organic matter, and diatoms are known to predominate in Lake Hachiro during the spring. In contrast, from April to June 2017, we recorded decreases in TOC and all fatty acid biomarkers, with the exception of those derived from bacteria, indicating that decomposition processes prevailed during this period. Quantitative estimates indicated that, despite the spatial and seasonal variations, terrestrial organic carbon was the predominant carbon source in Lake Hachiro, even when the waters were eutrophic and algal blooms were present.

Quantifying terrestrial carbon in freshwater food webs using amino acid isotope analysis: Case study with an endemic cavefish

Abstract Flow of terrestrial carbon though aquatic ecosystems (allochthony) is an important but underestimated component of the global carbon cycle. A lack of clear consensus about the importance of allochthonous (terrestrial) organic carbon is sometimes attributed to uncertainties associated with conventional ‘bulk’ isotope data, the most widely used ecological tracer. Amino acid‐specific isotope analysis is an emerging research method promising to address existing limitations of bulk C and N isotope analyses. We tested the efficacy of amino acid δ13C data as a generalizable measure of allochthony by analysing an aggregated dataset (n = 168) of primary and secondary data of carbon sources from disparate geographical locations across the globe. We found the δ13C fingerprints amino acids to be consistently distinct between allochthonous (terrestrial) and autochthonous (aquatic) carbon sources. We also found that our approach is most effective when we use only essential amino acid tracers (i.e. isoleucine, leucine, phenylalanine, threonine and valine). Predictive trends in δ13C fingerprints appear to be largely compatible across studies and/or laboratories. As a case study, we used this approach to quantify the contribution of terrestrial carbon to an endemic cavefish, Cryptotora thamicola, and found that its biomass was comprised largely of autochthonous carbon (~75%).

Plant detritus is selectively consumed by estuarine copepods and can augment their survival

Particulate material comprising the detrital remains of terrestrial plants and macrophytes is a substantial source of organic matter to estuaries and therefore has the potential to support the energy demands of the pelagic aquatic food web. Despite the prevalence of macrophytic or terrestrial particulate organic carbon (tPOC), phytoplankton are nutritionally superior and are thought to be the primary food resource for zooplankton. However, estuarine phytoplankton primary productivity abundances can wax and wane, and often production cannot meet heterotrophic energy needs. In this study, we examined how tPOC (detritus of macrophytes and grasses) may affect survival of a calanoid copepod (Eurytemora affinis) common in the San Francisco Estuary (SFE), an estuary with relatively low phytoplankton primary productivity. Using chemical biomarkers and a targeted DNA metagenomic methodology, we show that E. affinis consumed tPOC (dominated by Schoenoplectus sp., or tule) even when phytoplankton were abundant and tPOC was scarce. Furthermore, we found that a mixed diet of phytoplankton and terrestrial material (1:3 carbon ratio) enhanced the survival of E. affinis over a diet of phytoplankton alone. These data show that tPOC can be a vital supplementary food source for zooplankton, perhaps extending survival during low phytoplankton periods, and may help explain elevated zooplankton abundances in tidal wetlands and other detrital-dominated regions.

Linking Direct Measurements of Turbidity Currents to Submarine Canyon-Floor Deposits

Submarine canyons are conduits for episodic and powerful sediment density flows (commonly called turbidity currents) that move globally significant amounts of terrestrial sediment and organic carbon into the deep sea, forming some of the largest sedimentary deposits on Earth. The only record available for most turbidity currents is the deposit they leave behind. Therefore, to understand turbidity current processes, we need to determine the degree to which these flows are represented by their deposits. However, linking flows and deposits is a major long-standing scientific challenge. There are few detailed measurements from submarine turbidity currents in action, and even fewer direct measurements that can be compared to resulting seabed deposits. Recently, an extensive array of moorings along Monterey Canyon, offshore California, took measurements and samples during sediment density flow events, providing the most comprehensive dataset to date of turbidity current flows and their deposits. Here, we use sediment trap samples, velocity measurements, and seafloor cores to document how sand is transported through a submarine canyon, and how the transported sediment is represented in seafloor deposits. Sediment trap samples from events contain primarily fine to medium-grained sand with sharp bases, normal grading, and muddy tops. Sediment captured from the water column during the flow shows normal grading, which is broadly consistent with the initial peak and waning of flow velocities measured at a single height within the flow, and may be enhanced by collapsing flows. Flow events contain coarser sand concentrated toward the seafloor and larger grain sizes on the seafloor or in the dense near-bed layer, possibly representative of stratified flows. Although flow velocity varies, sand grain sizes in sediment traps are similar over distances of 50 km down-canyon, suggesting that grain size is an unfaithful record of down-canyon changes in maximum flow speeds. Sand transported within flow events and sampled in sediment traps is similar to sand sampled from the seafloor shortly after the events, but traps do not contain pebbles and gravel common in seabed deposits. Seabed deposits thus appear to faithfully record the sand component that is transported in the water column during sub-annual turbidity currents.

Effects of Sediment Delivery Changes on Carbon Burial Within the Modern and Abandoned Delta Lobes of the Yellow River, China

Globally, deltaic wetlands sequester large volumes of terrestrial and marine-derived organic carbon. Diminishing sediment supply via river diversions, dams, and/or sea level rise threatens this stored carbon by enhancing erosion, thereby potentially releasing CO2 back to the atmosphere during remineralization of organic matter. The Yellow River delta, located in the Bohai Sea, China, has undergone intense anthropogenic manipulation since the 1950s including rerouting of the river mouth to expand the delta for oil exploitation. The goal of this study is to identify the impacts of river course diversions on sources and rates of carbon burial in the modern and abandoned delta lobes of the Yellow River delta. In 2016, we collected four cores total in abandoned and modern deltaic lobes and measured vertical accretion, total carbon, total nitrogen, δ13C, and n-alkanes. The highest average mass accretion rate of 12,470.1 g m−2 year−1 is observed in the abandoned delta, although it no longer receives direct river sediment input. The modern and abandoned deltas are currently outpacing sea level rise, but vertical accretion rates are influenced by sediment trapping practices in the modern delta and redistribution of eroded sediments in the abandoned. Average carbon burial rates across both delta sites vary between 7.2 and 14.9 g OC m−2 year−1. Sediment-associated carbon at both sites is dominantly sourced from the Loess Plateau. To conserve wetlands across the Yellow River delta, sediment management practices that periodically reintroduce sediment-laden river water to former river courses, such as in the Mississippi delta, are suggested.

Assessment of land use/land cover changes induced by Jizan Dam, Saudi Arabia, and their effect on soil organic carbon

Land use change has been influenced by human activity and has altered an enormous part of the natural landscapes of developing countries. Meanwhile, as the largest pool of terrestrial organic carbon, soils interact strongly with atmospheric composition, climate, and land cover change. The objectives of this study were to assess the land cover and existing land use changes in Jizan Basin, Jizan Province, Saudi Arabia, as an aftermath when Jizan Dam started working, and, subsequently, to assess soil organic carbon distribution in different land uses in the study area. Three cloud-free Landsat MSS, ETM+, and OLI images covering the study area were employed for analysis. Images were acquired in years 1972, 2000, and 2017, respectively. All images were manipulated using ERDAS Imagine 10 software to achieve the aims of this study. The final output maps have been produced using Arc GIS 10.1. Different methods of data transformation and analysis were applied including normalized difference vegetation index (NDVI). A supervised classification approach was applied to categorize the images in different land cover classes. Five land use/land cover classes were identified: vegetation, bare soil (sand), bare soil (rocks), urban area, and water bodies. The results show that the vegetation of Jizan Basin increased between 1972 and 2017. The area increased 4% of its total area between 1972 and 2000 with an average annual increment of 0.45%. This annual rate increased to 1.05% during the period of 2000–2017. The major change has been found in the bare area (sand) cover which decreased during the period of 2000–2017 to about 42 km2 (9%). The results prevail that there is a significant increase in vegetated land in Jizan Basin which could be connected to Jizan Dam construction. It is important to mention that in conducting the present study, actual clear land cover changes took place as the result of agricultural land development. These changes in land cover influenced soil organic carbon sequestration.

Distribution and Sources of Organic Matter in Surface Sediments of the Northern Bering and Chukchi Seas by Using Bulk and Tetraether Proxies

The organic matter (OM) preserved in Arctic Ocean sediments is of great importance to the global carbon budget. However, works that apply multiple proxies to determine the distribution and concentration of organic carbon (OC) in the surface sediments of the northern Bering and Chukchi Seas remain limited. Here a multiproxy approach based on bulk OM parameters and the branched vs. isoprenoid tetraether (BIT) index was used to investigate the distribution and sources of OM in the surface sediments of the northern Bering and Chukchi Seas. Binary and ternary mixing models were applied to trace the contribution of different OC sources to the total OC in the study area. The δ13C values of the sediments provided by the binary model showed that the proportion of terrestrial OC fell in the range of 27.4%−79.8% (46.2% on average). The BIT index returned the lowest fraction (4.8%−27.3%, 12.0% on average). The ternary mixing model was employed to determine the plant-, soil-, and marine-derived fractions of the total OM. The ternary model showed that 11.5%±6.3%, 31.4%±9.5%, and 57.1%±12.4% of OM in the sediment of the study area was derived from soil, plants, and marine sources, respectively. The differences in OM composition between the west and east sides of the Chukchi Sea were controlled by OM inputs from key water masses (i.e., Anadyr Water and Alaska Coastal Water), river discharge, and the nutrient supply from the Pacific inflow that supports marine productivity.

Quantifying the carbon source of pedogenic calcite veins in weathered limestone: implications for the terrestrial carbon cycle

The continent is the second largest carbon sink on Earth’s surface. With the diversification of vascular land plants in the late Paleozoic, terrestrial organic carbon burial is represented by massive coal formation, while the development of soil profiles would account for both organic and inorganic carbon burial. As compared with soil organic carbon, inorganic carbon burial, collectively known as the soil carbonate, would have a greater impact on the long-term carbon cycle. Soil carbonate would have multiple carbon sources, including dissolution of host calcareous rocks, dissolved inorganic carbon from freshwater, and oxidation of organic matter, but the host calcareous rock dissolution would not cause atmospheric CO2 drawdown. Thus, to evaluate the potential effect of soil carbonate formation on the atmospheric pCO2 level, different carbon sources of soil carbonate should be quantitatively differentiated. In this study, we analyzed the carbon and magnesium isotopes of pedogenic calcite veins developed in a heavily weathered outcrop, consisting of limestone of the early Paleogene Guanzhuang Group in North China. Based on the C and Mg isotope data, we developed a numerical model to quantify the carbon source of calcite veins. The modeling results indicate that 4–37 wt% of carbon in these calcite veins was derived from atmospheric CO2. The low contribution from atmospheric CO2 might be attributed to the host limestone that might have diluted the atmospheric CO2 sink. Nevertheless, taking this value into consideration, it is estimated that soil carbonate formation would lower 1 ppm atmospheric CO2 within 2000 years, i.e., soil carbonate alone would sequester all atmospheric CO2 within 1 million years. Finally, our study suggests the C–Mg isotope system might be a better tool in quantifying the carbon source of soil carbonate.

Sources and burial fluxes of sedimentary organic carbon in the northern Bering Sea and the northern Chukchi Sea in response to global warming

The Arctic and subarctic seas are the major CO2 sink areas on earth. In this study, the vertical variation characteristics of organic carbon, total nitrogen and their ratio (Corg/Nt), stable isotopes δ13C and δ15N, and BIT (branched and isoprenoid tetraether) index of GDGTs (glycerol dialkyl glycerol tetraethers) in combination with 210Pb-dating were used to analyze the changes in the marine and terrestrial sources of organic carbon in the northern Bering Sea (site 1), western Beaufort Sea slope (site 2) and northern Chukchi Sea (site 3). Organic carbon burial fluxes (OCBF) in the context of global warming were also explored at sites 1 and 3. The results showed that organic matter in these sediments were a mixed input of marine and terrestrial sources, and the BIT index and δ13C of site 2 suggested that the terrestrial soil organic matter was dominant. Based on a combination of 210Pb dating and Corg, the sedimentary OCBF at site 1 was 2.29–3.65 mg cm−2 y−1, and at site 3 was 0.00–0.41 mg cm−2 y−1. The temperature anomalies and sea ice changes in the Arctic in the past 100 years were compared with the burial fluxes of the terrestrial organic carbon. At site 1, the results indicated that fast melting of seasonal sea ice led to earlier arrival of ice algae bloom, enhanced zooplankton feeding and reduced carbon burial from 1947 to 2010, and the sudden increase in carbon burial after 2010 was attributed to an increase in primary productivity and terrestrial organic matter input due to an accelerated melting of sea ice. There was a smaller change in marine organic carbon content in site 3, but OCBF increased after a pre-1965 decrease, mainly controlled by terrestrial organic matter input associated with temperature rising and sea ice melting during recent decades.