Carbon Burial In Sediments Research Articles

Estimates of carbon sequestration potential in an expanding Arctic fjord (Hornsund, Svalbard) affected by dark plumes of glacial meltwater

Abstract. In polar regions, glaciers are retreating onto land, gradually widening ice-free coastal waters, which are known to act as new sinks of atmospheric carbon. However, the increasing delivery of inorganic suspended particulate matter (iSPM) with meltwater might significantly impact their capacity to contribute to carbon sequestration. Here, we present an analysis of satellite, meteorological, and SPM data as well as results of a coupled physical–biogeochemical model (1D GOTM-ECOSMO-E2E-Polar) with a newly implemented iSPM group to show the impact of iSPM on the ecosystem dynamics in a warming polar fjord (Hornsund, European Arctic) with numerous shallow-grounded marine-terminating glaciers. Our results indicate that with a longer melt season (9 d per decade, 1979–2022), the loss of sea ice cover (44 d per decade, 1982–2021) and the formation of new marine habitats after the retreat of marine-terminating glaciers (around 100 km2 in 1976–2022, a 38 % increase in the total area), glacial meltwater has transported increasing loads of iSPM from land (3.7 g m−3 per decade, reconstructed for 1979–2022). The simulated light limitation induced by the iSPM input delayed and decreased the peaks in phytoplankton, zooplankton, and macrobenthos. The newly ice-free areas still markedly contributed to plankton primary and secondary production and carbon burial in sediments (5.1, 2.0, and 0.9 Gg C yr−1, respectively, on average for 2005–2009 in the iSPM scenario). However, these values would have been 5.0, 2.1, and 0.1 Gg C yr−1 higher, respectively, without the iSPM input. Since carbon burial was the least affected by iSPM (a decrease of around 16 %, in comparison to 50 % for plankton primary and secondary production), the impact of marine ice loss and enhanced land–ocean connectivity should be investigated further in the context of carbon fluxes in expanding polar fjords.

Recycling and deposition of inorganic carbon from calcium carbonate encrustations of charophytes

AbstractMany aquatic primary producers can use bicarbonates as a carbon source for photosynthesis. Charophytes of the two genera: Chara and Nitellopsis are quite efficient in this process. Some species of these macroalgae produce carbonate encrustations, mainly calcium carbonate, constituting up to 86% of the summer maximum dry weight of the standing crop. In this study, we analyzed the fate of inorganic carbon accumulated this way in Chara spp. and Nitellopsis obtusa from six Polish lakes located in two regions (warmer W Poland and cooler NE Poland). Our study distinguished two groups of charophyte species that differed in the way of CaCO3 release from their summer standing crops. On average, the corticate Chara rudis and C. tomentosa belonging to the first group were less efficient in depositing CaCO3 from summer to autumn than the less corticate C. contraria and ecorticate N. obtusa of the second group. The latter two species were more efficient in inorganic carbon burial in sediments. On the contrary, dissolution of encrustation was more typical of the first species group and was facilitated by decreasing the pH and saturation index of calcite in lake water. The final output of CaCO3 loss mainly resulted from combined species‐specific features, lake water properties and overwintering patterns. Our study revealed that inorganic carbon cycling through charophytes involves burial and dissolution and is more complex than previously thought.

Methane Emissions in Seagrass Meadows as a Small Offset to Carbon Sequestration

AbstractSeagrass meadows are effective carbon sinks due to high primary production and sequestration in sediments. However, methane (CH4) emissions can partially counteract their carbon sink capacity. Here, we measured diffusive sediment‐water and sea‐air CO2 and CH4 fluxes in a coastal embayment dominated by Posidonia oceanica in the Mediterranean Sea. High‐resolution timeseries observations revealed large spatial and temporal variability in CH4 concentrations (2–36 nM). Lower sea‐air CH4 emissions were observed in an area with dense seagrass meadows compared to patchy seagrass. A 6%−40% decrease of CH4 concentration in the surface water around noon indicates that photosynthesis likely limits CH4 fluxes. Sediments were the major CH4 source as implied from radon (a natural porewater tracer) observations and evidence for methanogenesis in deeper sediments. CH4 sediment‐water fluxes (0.1 ± 0.1–0.4 ± 0.1 μmol m−2 d−1) were higher than average sea‐air CH4 emissions (0.12 ± 0.10 μmol m−2 d−1), suggesting that dilution and CH4 oxidation in the water column could reduce net CH4 fluxes into the atmosphere. Overall, relatively low sea‐air CH4 fluxes likely represent the net emissions from subtidal seagrass habitat not influenced by allochthonous CH4 sources. The local CH4 emissions in P. oceanica can offset less than 1% of the carbon burial in sediments (142 ± 69 g CO2eq m−2 yr−1). Combining our results with earlier observations in other seagrass meadows worldwide reveals that global CH4 emissions only offset a small fraction (<2%) of carbon sequestration in sediments from seagrass meadows.

Prevalence of Autotrophy in Non-humic African Lakes

Heterotrophic respiration of organic matter (OM) is thought to dominate over aquatic primary production (PP) in most freshwater lake ecosystems. This paradigm implies that lateral transport of OM from the terrestrial biosphere subsidize the major fraction of aquatic respiration and that many lakes are a net source of carbon dioxide (CO2) to atmosphere. Nevertheless, African lakes were absent of the datasets upon which this paradigm was built. Here, we report a comprehensive and methodologically consistent data set of pelagic PP and community respiration (CR) obtained over the last decade in contrasting non-humic African lakes including 5 of the East African Great lakes (Tanganyika, Kivu, Edward, Albert, Victoria) and smaller shallow lakes located in Eastern Africa. Also, we determined the partial pressure of CO2 in surface waters and examined the sources and dynamics of organic and inorganic carbon by means of stable isotope tools across a wide range of physical and chemical conditions and productivity status. Our observations revealed that the threshold value at which the equivalence between PP and CR is met is substantially lower in Africa (10 mmol C m−3 d−1) than at higher latitude (25 mmol C m−3 d−1), suggesting that non-humic African lakes tend to be more autotrophic than expected from empirical relationships derived from data collected in boreal and temperate regions. Integrated at the regional scale, we estimate that PP is about 20 times higher than the organic carbon burial in sediments. It implies that a large fraction (< 90%) of PP is effectively recycled in the warm water column of non-humic African lakes.

Global carbon budget of reservoirs is overturned by the quantification of drawdown areas

Reservoir drawdown areas—where sediment is exposed to the atmosphere due to water-level fluctuations—are hotspots for carbon dioxide (CO2) emissions. However, the global extent of drawdown areas is unknown, precluding an accurate assessment of the carbon budget of reservoirs. Here we show, on the basis of satellite observations of 6,794 reservoirs between 1985 and 2015, that 15% of the global reservoir area was dry. Exposure of drawdown areas was most pronounced in reservoirs close to the tropics and shows a complex dependence on climatic (precipitation, temperature) and anthropogenic (water use) drivers. We re-assessed the global carbon emissions from reservoirs by apportioning CO2 and methane emissions to water surfaces and drawdown areas using published areal emission rates. The new estimate assigns 26.2 (15–40) (95% confidence interval) TgCO2-C yr−1 to drawdown areas, and increases current global CO2 emissions from reservoirs by 53% (60.3 (43.2–79.5) TgCO2-C yr−1). Taking into account drawdown areas, the ratio between carbon emissions and carbon burial in sediments is 2.02 (1.04–4.26). This suggests that reservoirs emit more carbon than they bury, challenging the current understanding that reservoirs are net carbon sinks. Thus, consideration of drawdown areas overturns our conception of the role of reservoirs in the carbon cycle.

The mangrove CO2 pump: Tidally driven pore‐water exchange

AbstractAtmospheric carbon dioxide (CO2) is fixed by mangrove vegetation and stored in its biomass and sediments. Part of the sediment carbon can be exported to coastal waters via tidally driven pore‐water exchange. Here, we quantify pore water‐derived dissolved CO2 export using in situ, high‐resolution observations of 222Rn and CO2 over a spring‐neap tidal cycle in a mangrove‐fringed estuary (Coffs Creek, Australia). 222Rn‐derived pore‐water exchange rates were 11.5–34.9 cm d−1 (23.0 ± 6.7) over 30 tidal cycles. Pore‐water exchange released CO2 from intertidal sediment at rates of 61–213 (136 ± 43) mmol m−2 d−1. This is equivalent to ~ 94% of the total CO2 input into the estuary and approximately two times of the water‐atmosphere CO2 emission. These observations reveal that tidal pumping is a major regulator of both mangrove pore‐water exchange and associated dissolved CO2 export to the ocean. Combining our estimates with literature data, a first‐order global pore water‐derived dissolved CO2 export from mangroves was estimated to be 83 ± 50 Tg C yr−1. This is higher than an earlier estimates of global mangrove CO2 emissions to the atmosphere (34.1 ± 5.4 Tg C yr−1) and carbon burial in sediments (18.4–34.4 Tg C yr−1), implying that pore water‐derived CO2 escapes to the atmosphere within and beyond mangrove waters. Overall, CO2‐rich pore water seems to be a widespread, important pathway of CO2 into mangrove‐dominated estuaries and should be considered in mangrove carbon assessments in the context of global climate change and blue carbon.

Mangroves as a Source of Greenhouse Gases to the Atmosphere and Alkalinity and Dissolved Carbon to the Coastal Ocean: A Case Study From the Everglades National Park, Florida

AbstractMost research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial in sediments. However, the few studies that have quantified lateral exchange of carbon and alkalinity indicate that the dissolved carbon and alkalinity export may be several‐fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon, and greenhouse gas fluxes of the mangrove‐dominated Shark River estuary located in the Everglades National Park in Florida, USA. Spatial surveys and 29‐hr time series were conducted to assess total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) dynamics. Dissolved carbon and greenhouse gas concentrations were coupled to porewater input, which was examined using radon‐222. Shark River was a source of CO2 (92 mmol/m2/day), CH4 (56 μmol/m2/day), and N2O (2 μmol/m2/day) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m2/day, DOC = 39 mmol/m2/day, normalized to mangrove area) was several‐fold higher than previously reported carbon burial rates in the study area (~28 mmol/m2/day). The majority of the DIC was exported as TAlk (97 mmol/m2/day), which remains dissolved in the ocean for millennia and, therefore, represents a long‐term sink for atmospheric carbon. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon, and greenhouse gas fluxes should be considered in future blue carbon budgets.

Enhanced Organic Carbon Burial in Sediments of Oxygen Minimum Zones Upon Ocean Deoxygenation

Oxygen minimum zones (OMZs) in the ocean are expanding. This expansion is attributed to global warming and may continue over the next 10 to 100 kyrs due to multiple climate CO$_\text{2}$-driven factors. The expansion of oxygen-deficient waters has the potential to enhance organic carbon burial in marine sediments, thereby providing a negative feedback on global warming. Here, we study the response of dissolved oxygen in the ocean to increased phosphorus and iron inputs due to CO$_\text{2}$-driven enhanced weathering and increased dust emissions, respectively. We use an ocean biogeochemical model coupled to a general ocean circulation model (the Hamburg Oceanic Carbon Cycle model, HAMOCC 2.0) to assess the impact of such regional deoxygenation on organic carbon burial in the modern ocean on time scales of up to 200~kyrs. We find that an increase in input of phosphorus and iron leads to an expansion of the area of the OMZ impinging on continental margin sediments and a significant decline in bottom water oxygen in the open ocean relative to pre-industrial conditions. The associated increase in organic carbon burial could contribute to the drawdown of $\sim$1600~Gt of carbon, which is equivalent to the total amount of CO$_\text{2}$ in the atmosphere predicted for the year 2100 in a business as usual scenario, on time scales of up to 50 kyrs. The corresponding areal extent of sediments overlain by bottom waters with little or no oxygen as estimated by the model is not very different from the minimum area estimated for two major oceanic anoxic events in Earth's past. Such events were associated with major perturbations of the oceanic carbon cycle, including high rates of organic carbon burial. We conclude that organic carbon burial in low oxygen areas in the ocean could contribute to removal of anthropogenic CO$_\text{2}$ from the atmosphere on long time scales.

Non-native mangroves support carbon storage, sediment carbon burial, and accretion of coastal ecosystems.

Mangrove forests play an important role in climate change adaptation and mitigation by maintaining coastline elevations relative to sea level rise, protecting coastal infrastructure from storm damage, and storing substantial quantities of carbon (C) in live and detrital pools. Determining the efficacy of mangroves in achieving climate goals can be complicated by difficulty in quantifying C inputs (i.e., differentiating newer inputs from younger trees from older residual C pools), and mitigation assessments rarely consider potential offsets to CO2 storage by methane (CH4 ) production in mangrove sediments. The establishment of non-native Rhizophora mangle along Hawaiian coastlines over the last century offers an opportunity to examine the role mangroves play in climate mitigation and adaptation both globally and locally as novel ecosystems. We quantified total ecosystem C storage, sedimentation, accretion, sediment organic C burial and CH4 emissions from ~70year old R. mangle stands and adjacent uninvaded mudflats. Ecosystem C stocks of mangrove stands exceeded mudflats by 434±33MgC/ha, and mangrove establishment increased average coastal accretion by 460%. Sediment organic C burial increased 10-fold (to 4.5MgCha-1 year-1 ), double the global mean for old growth mangrove forests, suggesting that C accumulation from younger trees may occur faster than previously thought, with implications for mangrove restoration. Simulations indicate that increased CH4 emissions from sediments offset ecosystem CO2 storage by only 2%-4%, equivalent to 30-60MgCO2 -eq/ha over mangrove lifetime (100year sustained global warming potential). Results highlight the importance of mangroves as novel systems that can rapidly accumulate C, have a net positive atmospheric greenhouse gas removal effect, and support shoreline accretion rates that outpace current sea level rise. Sequestration potential of novel mangrove forests should be taken into account when considering their removal or management, especially in the context of climate mitigation goals.

Sources and dry deposition of carbonaceous aerosols over the coastal East China Sea: Implications for anthropogenic pollutant pathways and deposition

75 paired TSP and PM2.5 samples were collected over four seasons on Huaniao Island (HNI), an island that lies downwind of continental pollutants emitted from mainland China to the East China Sea (ECS). These samples were analyzed for organic carbon (OC) and elemental carbon (EC), with a special focus on char-EC (char) and soot-EC (soot), to understand their sources, and the scale and extent of pollution and dry deposition over the coastal ECS. The results showed that char concentrations in PM2.5 and TSP averaged from 0.13 to 1.01 and 0.31–1.44 μg m−3; while for soot, they were from 0.03 to 0.21 and 0.16–0.56 μg m−3, respectively. 69.0% of the char and 36.4% of the soot were present in PM2.5. The char showed apparent seasonal variations, with highest concentrations in winter and lowest in summer; while soot displayed maximum concentrations in fall and minimum in summer. The char/soot ratios in PM2.5 averaged from 3.29 to 17.22; while for TSP, they were from 1.20 to 7.07. Both of the ratios in PM2.5 and TSP were highest in winter and lowest in fall. Comparisons of seasonal variations in OC/EC and char/soot ratios confirmed that char/soot may be a more effective indicator of carbonaceous aerosol source identification than OC/EC. Annual average atmospheric dry deposition fluxes of OC and EC into ECS were estimated to be 229 and 107 μg m−2 d−1, respectively, and their deposition fluxes significantly increased during episodes. It was estimated that the loadings of OC + EC and EC accounted for 1.3% and 4.1% of the total organic carbon and EC in ECS surface sediments, respectively, implying a relatively small contribution of OC and EC dry deposition to organic carbon burial. This finding also indicates a possibly more important contribution of wet deposition to organic carbon burial in sediments of ECS, and this factor should be considered for future study.

Carbon sinks/sources in the Yellow and East China Seas—Air-sea interface exchange, dissolution in seawater, and burial in sediments

The sinks/sources of carbon in the Yellow Sea (YS) and East China Sea (ECS), which are important continental shelf seas in China, could exert a great influence on coastal ecosystem dynamics and the regional climate change process. The CO2 exchange process across the seawater-air interface, dissolved and particulate carbon in seawater, and carbon burial in sediments were studied to understand the sinks/sources of carbon in the continental shelf seas of China. The YS and the ECS generally have different patterns of seasonal air-sea CO2 exchange. In the YS, regions west of 124°E can absorb CO2 from the atmosphere during spring and winter, and release CO2 to the atmosphere during summer and autumn. The entire YS is considered as a CO2 source throughout the year with respect to the atmosphere, but there are still uncertainties regarding the exact air-sea CO2 exchange flux. Surface temperature and phytoplankton production were the key controlling factors of the air-sea CO2 exchange flux in the offshore region and nearshore region of the YS, respectively. The ECS can absorb CO2 during spring, summer, and winter and release CO2 to the atmosphere during autumn. The annual average exchange rate in the ECS was −4.2±3.2 mmol m−2 d−1 and it served as an obvious sink for atmospheric CO2 with an air-sea exchange flux of 13.7×106 t. The controlling factors of the air-sea CO2 exchange in the ECS varied significantly seasonally. Storage of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the YS and the ECS were 425×106 t and 1364×106 t, and 28.2×106 t and 54.1×106 t, respectively. Long-term observation showed that the DOC content in the YS had a decreasing trend, indicating that the “practical carbon sink” in the YS was decreasing. The total amount of particulate organic carbon (POC) stored in the YS and ECS was 10.6×106 t, which was comparable to the air-sea CO2 flux in these two continental shelf seas. The amounts of carbon sequestered by phytoplankton in the YS and the ECS were 60.42×106 t and 153.41×106 t, respectively. Artificial breeding of macroalgae could effectively enhance blue carbon sequestration, which could fix 0.36×106–0.45×106 t of carbon annually. Organic carbon (OC) buried in the sediments of the YS was estimated to be 4.75×106 t, and OC of marine origin was 3.03×106 t, accounting for 5.0% of the TOC fixed by phytoplankton primary production. In the ECS, the corresponding depositional flux of OC in the sediment was estimated to be 7.4×106 t yr−1 , and the marine-origin OC was 5.5×106 t, accounting for 5.4% of the phytoplankton primary production. Due to the relatively high average depositional flux of OC in the sediment, the YS and ECS have considerable potential to store a vast amount of “blue carbon.”

CO2 emission and organic carbon burial in the Xinanjiang Reservoir

In order to understand the effect of river impoundment on carbon dynamics, a large reservoir in a subtropical area, the Xinanjiang Reservoir, was investigated in detail. CO2 emissions from the water–air interface was studied, as was organic carbon burial in sediment. The results show a significant seasonal difference in CO2 emissions. River impoundment led to the enhancement of aquatic photosynthesis, generating large amounts of authigenic organic carbon that was then buried in sediment.

Macronutrient processing by temperate lakes: A dynamic model for long-term, large-scale application

We developed a model of the biogeochemical and sedimentation behaviour of carbon (C), nitrogen (N) and phosphorus (P) in lakes, designed to be used in long-term (decades to centuries) and large-scale (104–105km2) macronutrient modelling, with a focus on human-induced changes. The model represents settling of inflow suspended particulate matter, production and settling of phytoplankton, decomposition of organic matter in surface sediment, denitrification, and DOM flocculation and decomposition. The model uses 19 parameters, 13 of which are fixed a priori. The remaining 6 were obtained by fitting data from 109 temperate lakes, together with other information from the literature, which between them characterised the stoichiometric incorporation of N and P into phytoplankton via photosynthesis, whole-lake retention of N and P, N removal by denitrification, and the sediment burial of C, N and P. To run the model over the long periods of time necessary to simulate sediment accumulation and properties, simple assumptions were made about increases in inflow concentrations and loads of dissolved N and P and of catchment-derived particulate matter (CPM) during the 20th century. Agreement between observations and calculations is only approximate, but the model is able to capture wide trends in the lakewater and sediment variables, while also making reasonable predictions of net primary production. Modelled results suggest that allochthonous sources of carbon (CPM and dissolved organic matter) contribute more to sediment carbon than the production and settling of algal biomass, but the relative contribution due to algal biomass has increased over time. Simulations for 8 UK lakes with sediment records suggest that during the 20th century average carbon fixation increased 6-fold and carbon burial in sediments by 70%, while the delivery of suspended sediment from the catchments increased by 40% and sediment burial rates of N and P by 131% and 185% respectively.

Ecosystem-based carbon footprinting of marine engineering projects

Dredging projects use large amounts of fuel, which leads to substantial carbon dioxide (CO2) emissions. Until now, reducing the carbon footprint of dredging projects has mainly involved investigating the possibilities of dredging schemes and vessels that are more fuel efficient. A reduction in the order of 10–20% may be within reach, most of it is a win–win situation since fuel reduction will also reduce costs. A further reduction in carbon dioxide emissions may be possible on a project-by-project basis but will involve a trade-off between dredging costs and carbon dioxide emissions. Marine engineering projects also have an impact on primary production and formation of organic carbon and on sedimentation processes and burial of organic carbon in sediments. Both impacts influence carbon sequestration and can be a substantial or even overriding factor in the ‘carbon footprint’ of a project. The carbon footprint of a marine engineering project is often larger and more complex than previously anticipated. However, a more comprehensive carbon footprint also shows that a significant reduction in carbon dioxide emissions is possible when the designs and dredging and maintenance schemes stimulate sequestration of organic carbon in the wider environment.

Detecting sinks and sources of CO2 and CH4 by ferrybox-based measurements in the Baltic Sea: Three case studies

A fully automated measurement system for recording of the surface water CO2 partial pressure, pCO2, was deployed on VOS (voluntary observation ship) “Finnpartner/Finnmaid” in 2003. Since 2009, an amendment of the system also allows for the continuous detection of the surface water partial pressure of methane, pCH4. The ship commutes regularly at 2–3day intervals between the Gulf of Finland (Helsinki) and the Mecklenburg Bight (Lübeck) in the southwest of the Baltic Sea.The pCO2 data in the central Gotland Sea showed a pronounced seasonality that was mainly controlled by the biological production and decomposition of organic matter in combination with stratification/mixing of the water column. CO2 consumption in spring/summer caused pCO2 that were by up to 300μatm below the atmospheric level. In contrast, the pCO2 exceeded the atmospheric values during autumn/winter when deep mixing transports CO2-enriched water to the surface. To identify the central Baltic Sea as a sink or source for atmospheric CO2, an air–sea CO2 gas exchange balance was established for three selected years (2005, 2008 and 2009). During each year the surface water acted as a net sink for atmospheric CO2 with uptake rates ranging between 0.60 and 0.89molm−2yr−1. The rates correspond approximately to the enhanced carbon burial in sediments during the last century and suggest a link between eutrophication and CO2 uptake.The data of the surface methane concentration are used to focus on situations were extraordinarily high methane concentrations were observed. Temporary methane peaks were observed south of the Island of Gotland, which could clearly be attributed to frequent upwelling events.Between spring 2012 and 2013, Finnmaid went at a few occasions to St. Petersburg in the east of the Gulf of Finland. Methane concentration of up to 130nmolL−1 where found close to the River Neva mouth but decreased rapidly to the west. The plume of methane-enriched waters was observed farthest to the west during the winter period. This was attributed to air–sea gas exchange that was most effective during summer but inhibited during winter because of the ice coverage.

Modern carbon burial in Lake Qinghai, China

The quantification of carbon burial in lake sediments, and carbon fluxes derived from different origins are crucial to understand modern lacustrine carbon budgets, and to assess the role of lakes in the global carbon cycle. In this study, we estimated carbon burial in the sediment of Lake Qinghai, the largest inland lake in China, and the carbon fluxes derived from different origins. We find that: (1) The organic carbon burial rate in lake sediment is approximately 7.23gm−2a−1, which is comparable to rates documented in many large lakes worldwide. We determined that the flux of riverine particulate organic carbon (POC) is approximately 10 times higher than that of dissolved organic carbon (DOC). Organic matter in lake sediments is primarily derived from POC in lake water, of which approximately 80% is of terrestrial origin. (2) The inorganic carbon burial rate in lake sediment is slightly higher than that of organic carbon. The flux of riverine dissolved inorganic carbon (DIC) is approximately 20 times that of DOC, and more than 70% of the riverine DIC is drawn directly and/or indirectly from atmospheric CO2. (3) Both DIC and DOC are concentrated in lake water, suggesting that the lake serves as a sink for both organic and inorganic carbon over long term timescales. (4) Our analysis suggests that the carbon burial rates in Lake Qinghai would be much higher in warmer climatic periods than in cold ones, implying a growing role in the global carbon cycle under a continued global warming scenario.

Sedimentation and Organic Carbon Burial in the Yangtze River and Hudson River Estuaries: Implications for the Global Carbon Budget

One of the most important challenges in global climate change research is balancing the carbon budget within the global carbon cycle. Carbon burial in sediments at the land–ocean interface has been difficult to quantify and model because it represents non-steady-state boundary conditions that are also affected by human activities. In this study, we document carbon burial rates in the Yangtze River (1.6–4.9 × 1012 gC year−1) and Hudson River (1.8–3.6 × 1010 gC year−1) estuaries and integrate our results with carbon burial rates determined by others in the world’s 25 largest river-estuarine systems (6–11 × 1013 gC year−1). Our results indicate that carbon burial in estuaries, bays, coves, lagoons, mud flats, marshes, mangroves, and other highly productive or protected low-energy areas at the land–ocean interface along the entirety of the world’s coastlines may serve as an unrecognized sink within the global carbon budget.

Silicon as a permanent-carbon sedimentation tracer

A procedure to quantify permanent carbon (C) sedimentation rates was required to compare these rates to methane (CH4) and carbon dioxide (CO2) water–air emission rates measured during reservoir C flux studies. Therefore, a new method to estimate C burial rates using silicon (Si) as a tracer was devised and applied. Burial rates in 8 tropical reservoirs were measured. Ages of these 8 reservoirs varied between 3.7 and 49 years. Each reservoir was surveyed 3 times during 1 year. Median burial rate was 78 (min 12, max 516; n = 66) mg C m−2 d−1. Trapped C (Ct) rates were also measured; the resulting median was 845 mg C m−2 d−1 (min 179, max 19 064; n = 40). Burial efficiency (comparison between C burial rate and Ct rate) was ~10%. Carbon burial efficiency of the 8 reservoirs showed strong dependence on bottom water temperature, efficiency being halved for each 3.4 °C increase in annual average temperature of reservoir bottom water. This finding strongly supported the adequacy of the Si-tracer method for rate measurements of carbon burial in sediments. Simultaneous with our new Si-tracer method we conducted traditional lead 210 isotope (210Pb) dating. The resulting median was 133 (min 11, max 441; n = 15) mg C m−2 d−1. Compared to the Si-tracer median, the 210Pb-dating technique resulted in a higher C median burial rate because the sampling sites that lacked sediment (and therefore contributed a null burial rate) were, in retrospect, erroneously disregarded.

Modeling phosphorus cycling and carbon burial during Cretaceous Oceanic Anoxic Events

In the greenhouse world of the Cretaceous, there were episodes of oxygen depletion in the deep ocean associated with enhanced organic carbon burial in sediments (black shale formation). In this study, we use a box model of the oceanic phosphorus (P), organic carbon (orgC) and oxygen cycles to explore the hypothesis that variations in marine P availability control deep ocean oxygen depletion (anoxia) and the formation of black shales under Cretaceous ocean conditions. We find that, for the Cretaceous ocean, with large continental shelves, slow oceanic overturning and high sea surface temperatures, Oceanic Anoxic Events (OAEs) can be triggered by enhanced P supply from land, and that the system is particularly sensitive to oceanic mixing. In our baseline scenario, the deep sea becomes completely anoxic, while the shelves attain only partial anoxia. Sedimentary burial differs between the shelves and open ocean: while organic carbon burial is enhanced in both regions, deep sea reactive P burial decreases dramatically under anoxia, but not on the shelves, where oxygen depletion is not complete. Furthermore, our model results imply that OAEs can be sustained by P recycling from sediments under low-oxygen conditions. Ultimately, however, the feedbacks which result in the accumulation of atmospheric oxygen terminate the anoxic event. Atmospheric oxygen is modulated by land processes such as forest fires and oxidative weathering, which limit the rise of atmospheric O 2. Our model findings are corroborated by P burial data from the geological record for OAE2 (approximately 94 Myr BP). Through a sensitivity analysis we identify two necessary criteria for OAEs: low mixing of surface and deep waters (poor ocean ventilation) and enhanced sedimentary P recycling under low oxygen conditions. When these criteria are met, ocean anoxia is a robust result to a mild increase of continental supply of phosphorus, under a wide range of environmental conditions.

Burial of organic carbon and carbonate on inner shelf of the northern South China Sea during the postglacial period

Two vibrocores from the inner shelf off Hong Kong are investigated to compare the contents of organic and inorganic carbon in postglacial sediments. The compositions of organic elements and carbonate are highly variable in the core sediments, but overall drop within the compositional ranges of modern seabed sediments in the Zhujiang estuarine and its shelf area. The Holocene sediments in the inner shelf have never been subject to subaerial exposure and the organic matter and carbonate can be preserved well. The burial of carbon in river-dominated shelf environments is highly dependent on the river flux with time. Nevertheless, it is difficult to establish a simple relationship between carbon burial in sediments in relation to climatic changes of basin-wide scale due to complex controls of production, transport and deposition of organic matter and carbonate. Our study suggests that the organic carbon to nitrogen ratio can not reliably identify the sources of depositional organic matters because of selective decomposition of organic matter components during humification and sedimentation. Caution is therefore needed in using organic elemental compositions as indicators of organic matter sources and paleoenvironmental changes in the East Asian continental shelves where intense river-sea interaction and variable carbon flux in geologic record occur.