Total Organic Carbon Source Research Articles

Size-fractionated distribution of glycerol dialkyl glycerol tetraether core lipids in surface sediments of a large-river delta-front estuary

Glycerol dialkyl glycerol tetraether core lipids (GDGTs) are microbial biomarkers ubiquitously distributed in terrestrial and marine environments. Dispersal and fate of GDGTs in an estuary largely depends on sediment grain size, however, their size distribution patterns remain poorly understood. Here, surface sediments collected from the Changjiang Estuary were separated into <20, 20–32, 32–63, 63–125 and >125 μm fractions, and analyzed for GDGTs as well as total organic carbon (TOC), stable isotopic composition (δ13C) of TOC and lignin phenols, to investigate the size and spatial distributions of GDGTs and the particle size effects on GDGTs proxies in this large river delta-front estuary. The concentrations of isoprenoidal GDGTs (isoGDGTs) were higher in the finest fractions and in off-estuary sites. On the contrary, branched GDGTs (brGDGTs) were high not only in the finest fractions but in coarser fractions (>32 μm fractions), and thus at both near- and off-estuary sites. The branched and isoprenoid tetraether (BIT) index increased with increasing grain size, and decreased sharply from the estuary (~0.52) to the shelf (~0.16). BrGDGTs were positively correlated with crenarcheaol in both high and low BIT regions. The brGDGTIIIa/IIa ratios in all size fractions were <0.59, further indicating that the brGDGTs were mainly derived from terrestrial input with minimum in-situ production. Fractional TOC source assignments derived from the BIT index was significantly positively correlated with the fractions of terrestrial OC from a mixing model based on δ13C-TOC and lignin contents, indicating that BIT may track a broader pool of terrestrial OC than just soil OC. This work provides novel, yet preliminary insights into the size fractionated distribution characteristics of GDGTs and the applicability of BIT as a proxy for OC sources in estuarine sediments. More work is needed to further clarify the particle size effects on other GDGTs proxies in estuarine systems.

Spatial–temporal distribution of total organic carbon and its transportation in the Jiulong River Estuary

Spatial and temporal distributions of total organic carbon (TOC) in the Jiulong River Estuary (JRE) were determined using data collected during three cruises in summer 2010, autumn 2010, and spring 2011. The TOC concentration influencing factors were identified, and the export fluxes were calculated. TOC concentration ranges were 0.73–4.17 mg/L in summer, 0.90–5.32 mg/L in autumn, and 1.78–8.03 mg/L in spring, respectively. TOC concentrations of the surface water and nearshore area were higher than those of the bottom water and offshore area, respectively, and the maximum TOC content occurred in the JRE upper reaches. The TOC concentration decreased with increasing salinity and exhibited a significant positive correlation with petroleum and dissolved inorganic nitrogen (DIN), indicating the influence of terrestrial input. A weak relationship between TOC and chlorophyll-a indicated that phytoplankton was not the dominant source of TOC. TOC fluxes discharged into the JRE were 50.39 × 103 t/a in 2010 and 46.08 × 103 t/a in 2011, and those transported into the sea were 38.46 × 103 t/a in 2010 and 33.66 × 103 t/a in 2011, respectively, accounting for approximately 75% of the total estuary fluxes. This study elucidates the biogeochemical processes of estuarine organic carbon and provides a quantitative basis for the land–sea integration of carbon dioxide emission reduction and sink increase projects.

Potential organic matter management for industrial wastewater guidelines using advanced dissolved organic matter characterization tools

Since 2021, national standards of South Korea for industrial wastewater discharge to surface water have changed from chemical oxygen demand to total organic carbon for the organic matter. Conventional organic matter parameters (e.g., biochemical oxygen demand, chemical oxygen demand and total organic carbon) are limited means of understanding the behavior of dissolved organic matter in industrial wastewater treatment processes. Thus, the current study used advanced dissolved organic matter characterization tools (e.g., fluorescence excitation emission matrix and size exclusion chromatography-organic carbon detection) to scrutinize industrial wastewater characteristics from three full-scale industrial wastewater treatment plants (IWTPs). The tools were conducive to tracking industrial wastewater sources of total organic carbon, influencing the overall performance of IWTPs, and proposing alternative processes to lower total organic carbon concentration in the effluent. The results of this study suggest that the diagnosis of IWTPs based on dissolved organic matter characteristics could be a useful tool for providing more insight into total organic carbon management.

Biophilic elements in core sediments as records of coastal eutrophication in the Seto Inland Sea, Japan

This study presents vertical profiles of biophilic elements in core sediments from the Seto Inland Sea, Japan. We determined total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), biogenic silica (BSi), and phaeopigments in sediments from 80-cm-long cores collected from Hiuchi-Nada (Site A) and Harima-Nada (Site B). The vertical profiles are generally similar: all concentrations decrease downward from the top layer and show maximum values in the top layers for each parameter at both study areas. Among these biophilic elements, the correlation between BSi and TOC is particularly high, suggesting that diatoms are the main source of TOC in the sediments. The degradation rates at Site A in increasing order, as estimated by a steady-state diagenetic model (one-G model), are TOC ≈ BSi < TP < TN ≈ phaeopigments. However, these rates differ substantially from those at Site B, suggesting that the degradation rates might differ among sedimentation processes and environments. Using the one-G model and molecular composition (C:N) ratios, we estimate the origin (terrestrial and marine) for the amount of carbon before decomposition and followed its variation over the past approximately 200 years. We found that marine carbon is a major component and tends to be higher during the eutrophication period (1960–1990) at Site B, although not so much at Site A. At Site B the marine carbon content increases by about 25% during the eutrophication period, but at Site A no remarkable increase can be confirmed. These results are consistent with previous studies showing that the trend in primary production in the Seto Inland Sea since the eutrophication period has not been regionally homogeneous.

An increase in the slope of the concentration-discharge relation for total organic carbon in major rivers in New England, 1973 to 2019.

The mobilization and transport of organic carbon (OC) in rivers and delivery to the near-coastal ocean are important processes in the carbon cycle that are affected by both climate and anthropogenic activities. Riverine OC transport can affect carbon sequestration, contaminant transport, ocean acidification, the formation of toxic disinfection by-products, ocean temperature and phytoplankton productivity. There have been many studies reporting temporal trends in OC concentrations in comparatively small streams with minimal anthropogenic influences but there have been fewer studies on larger rivers and fewer still that have investigated changes in OC concentration-discharge (C-Q) relations. This study examined changes in C-Q relations for total organic carbon (TOC) from 1973 to 2019 in 8 rivers in New England, USA. TOC concentrations declined in all rivers, and in most rivers, and in most seasons, the slope of the C-Q relation increased between 1973 to 1995 and 1996 to 2019. The increase in C-Q slope between periods may be related to changes in the magnitude of TOC sources. The most likely sources to have changed are wastewater inputs, urban runoff, production through photosynthesis in aquatic systems, and runoff from agricultural and forestry practices. Changes in wetland abundance and changes in sulfate concentrations can be ruled out as drivers of the observed changes in C-Q.

Seasonal Variation in Drinking Water Disinfection By-products in Gharbia Governorate, Egypt

Chlorine is the most common disinfectant used in drinking water treatment because it is cheap and has an efficient germicidal ability. However, chlorine and organic matter reacting trihalomethanes (THMs) are suspected carcinogens. The major groups of disinfection by-products (DBPs) are THMs, haloacetic acids (HAAs), haloacetonitriles (HANs), and halogenated ketones (HKs). Exposure to these by-products increases the risk of cancers, abortion, low birth weight, and congenital disabilities. The wastewater contents of organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), as the source of total organic carbon (TOC) compounds, are oxidized by chlorine to produce DBPs. This study aimed to monitor the seasonal levels of THMs in Egypt compared with international standards using capillary gas chromatography. Results revealed that THMs mean values vary between 9.26 to 35.86 μg/l, while dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) mean values vary between 3.82 to 17.74 μg/l and 4.41 to 13.25 μg/l, respectively. The maximum THM and TCAA values were observed during the summer, probably due to high temperatures. While the maximum DCAA values were observed during the autumn due to the high levels of raw water TOC. In conclusion, continuous monitoring of THM and its species is highly recommended, taking into consideration how climate can influence THMs formation.

Recent trophic state changes of selected Florida lakes inferred from bulk sediment geochemical variables and biomarkers

Short sediment cores (80–100 cm) from three Florida (USA) lakes (Sheelar, Wauberg, and Apopka) that range in trophic status, were analyzed for total organic carbon (TOC), total nitrogen, stable isotopes of organic carbon (δ13CTOC) and biomarkers (n-alkanes and fatty acids), to identify the sources of organic matter in the lake deposits, and to link changes in primary productivity to anthropogenic activities during the last ~ 150 years. Relatively small (0.07 km2), ultra-oligotrophic Lake Sheelar is located in a state park, and sediment analyses indicate stable trophic status in the water body since at least the middle of the nineteenth century. Algal biomarkers are in low abundance throughout the core and terrestrial lipids and δ13CTOC values suggest that vascular plants were the primary source of TOC to the lake sediments during the period of record. In larger, eutrophic Lakes Wauberg (1.5 km2) and Apopka (125 km2), algal-derived biomarkers increase in recent sediments, whereas δ13CTOC values and concentrations of terrestrial biomarkers decrease toward the sediment surface. Increasing dominance through time of autochthonous carbon sources in the Lake Wauberg and Apopka sediment records coincides with specific anthropogenic activities in the respective watersheds. Submersed macrophytes in Lake Wauberg were replaced by algal communities in the mid-1980s, following expansion of residential development in the watershed. Biomarker data from the Lake Apopka core show there was an abrupt transition in the lake in the late 1940s, from domination by vascular plants to domination by algae, which has been documented by other paleolimnological studies that used alternative trophic state indicators. The trophic state shift in Lake Apopka corresponds to a period of increased nutrient input to the lake, associated with extensive farming along the north shore. Florida lakes have experienced different trophic state trajectories over the last ~ 150 years, driven by specific human activities in their respective watersheds.

Analysis of Total Organic Carbon Source Differences Between New and Old Cascade Reservoirs using Carbon and Nitrogen Isotopes

The ecological problems due to reservoir construction are causing unprecedented concern. To reveal the differences in organic carbon distribution characteristics and sediment sources of total organic carbon (TOC) between the old and new reservoirs, water samples, and sediment samples from reservoirs constructed in the three different periods of Miaowei, Gongguoqiao, and Dachaoshan were collected in November 2017. The temperature (T), dissolved oxygen (DO), TOC, redox potential (ORP), total nitrogen (TN), and total phosphorus (TP) of the water samples were measured. The isotopes 15N and 13C were used as indicators with IsoSource software to analyze the contributions of TOC sources and their source materials to the corresponding reservoir sediments, in order to explore the carbon cycle mechanism and evolution mode of reservoir. The results showed that the average concentrations of organic carbon in the waters of the Miaowei, Gongguoqiao, and Dachaoshan Reservoirs were 0.95 mg·L-1, 1.97 mg·L-1, and 4.64 mg·L-1, respectively. The range of organic carbon content in the corresponding sediments was 4.41-81.63 g·kg-1, 18.30-28.42 g·kg-1, and 9.16-14.46 g·kg-1, respectively. The cascade construction of the reservoirs resulted in a difference between the sediment sources of the new and old reservoirs and the surrounding recharge area, meaning that the TOC of the new and old reservoirs were significantly different. For the TOC of waterbodies, the difference between the thermodynamic state of water and dissolved oxygen indirectly affects the distribution trend of TOC. The sediments mainly reflect the influence of source elements, that is, the ability of the sedimentary environment to preserve organic matter was the main cause of the vertical distribution of DCS, MV, and GGQ sediments. In the evolution mode of cascade reservoir, the research shows that it can be preliminarily set as three stages. Firstly, due to the short age of MV, it is in the first stage and mainly accumulates the TOC from the upstream. GGQ is longer than the age of MV, and it is mainly used to decompose the upstream TOC, so it is defined as in the second stage. Finally, as an old reservoir, DCS mainly accumulates TOC sources around the reservoir, which can be regarded as the third stage.

Organic Carbon in the Surface Sediments from the Intensive Mariculture Zone of Sanggou Bay: Distribution, Seasonal Variations and Sources

Mariculture has rapidly grown worldwide, which might greatly influence the biogeochemical cycle of organic carbon in coastal seas. In this study, several geochemical parameters, including grain size composition, sedimentary total organic carbon (TOC), total nitrogen (TN), stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions, were analyzed for surface sediments collected from different mariculture zones of Sanggou Bay and in different seasons. We investigated the composition and distribution of organic matter in surface sediments and further evaluated the contribution of mariculture activities to TOC sources. The TOC and TN contents (mass percentage) in the bay were in the range of 0.14% to 1.45% and 0.03% to 0.20%, respectively. The spatial distribution indicated that sedimentary TOC and TN contents in shellfish monoculture and shellfish-kelp polyculture zones were higher than in other mariculture zones, which might be related to grain size composition and mariculture organisms. Seasonal variations of TOC contents were observed in different mariculture zones. The TOC/TN atomic ratio (C/N), δ13C and δ15N were in the ranges of 5.97 to 10.97, −21.76‰ to −13.14‰ and 2.13‰ to 8.08‰, respectively, implying that sedimentary organic matter in Sanggou Bay was the mixture of marine phytoplankton, terrestrial and maricultural sources. A simple mixing model based on δ13C was applied and the results indicated that the relative contributions of organic carbon sources in Sanggou Bay followed the order kelp (36.6%) > marine phytoplankton (28.7%) > shellfish bio-deposition (23.8%) > terrestrial input (10.9%). Surface sediments in Sanggou Bay were dominated by mariculture-derived organic carbon, which on average accounted for 60.4% of TOC.

Hydrocarbon potential and depositional environment of the Lower Cretaceous black mudstones and shales in the coastal Guangdong Province, China

Abstract The Mesozoic (especially the Cretaceous) sequences in the northern South China Sea (SCS) are important potential targets of oil and gas exploration; however, they are currently poorly understood. We use organic petrology and geochemistry to evaluate the hydrocarbon potential and study depositional environment of the Cretaceous black mudstones and shales from the Dayawan Section in the Guangdong Province, China. These mudstones and shales yield total organic carbon (TOC) contents of 0.63%–2.76% and most samples meet the requirement for higher TOC source rocks. Organic matters in the mudstones and shales were sourced mainly from higher plants as well as subordinate benthic macroalgae. Organic petrology and geochemical data suggested that the organic matters were dominated by Type-III and minor Type-II, and generally have low thermal maturity. Thus, we evaluated the black mudstones and shales have a potential for hydrocarbon generation (especially gas prone). Pr/Ph, trace elements, and mixture source of organic matter suggested that these black mudstones and shales formed in a coastal–lacustrine environment, characterized by weakly oxidizing−weakly reducing conditions and low-salinity. A marine transgression developing in the middle part of the section was recognized. Generally, the northern SCS, which exhibits the more marine influence, likely contains source rocks with higher TOC contents and better organic matter types. The Cretaceous source rocks with higher liquid hydrocarbon generation potential may occur locally in the Pearl River Mouth Basin.

Oceanographic and Climatic Change in the Bering Sea, Last Glacial Maximum to Holocene

AbstractPost‐glacial sea level rise led to a direct connection between the Arctic and Pacific Oceans via the Bering Strait. Consequently, the Bering Sea experienced changes in connectivity, size, and sediment sources that were among the most drastic of any ocean basin in the past 30,000 years. However, the sedimentary response to the interplay between climate change and sea level rise in high‐latitude settings such as Beringia remains poorly resolved. To ascertain changes in sediment delivery, productivity, and regional oceanography from the Last Glacial Maximum (LGM) to the Holocene, we analyzed sedimentological, geochemical, and isotopic characteristics of three sediment cores from the Bering Sea. Interpretations of productivity, terrestrial input, nutrient utilization, and circulation are based on organic carbon isotopes (δ13Corg), total organic carbon (TOC), bulk nitrogen isotopes, total organic nitrogen, carbon/nitrogen ratios, elemental X‐ray fluorescence data, grain size, and presence of laminated or dysoxic, green intervals. Principal component analysis of these data captures key climatic intervals. The LGM was characterized by low productivity across the region. In the Bering Sea, deglaciation began around 18–17 ka, with increasing terrestrial sediment and TOC input. Marine productivity increased during the Bølling‐Allerød when laminated sediments revealed dysoxic bottom waters where denitrification was extreme. The Younger Dryas manifested increased terrestrial input and decreased productivity, in contrast with the Pre‐Boreal, when productivity markedly rebounded. The Pre‐Boreal and Bølling‐Allerød were similarly productive, but changes in the source of TOC and a δ13Corg depletion suggest the influence of a gradually flooding Bering Shelf during the Pre‐Boreal and Holocene.

Spatial Distribution of Carbon, Nitrogen and Phosphorus within Surface Sediments in the Lower Lancang River: Pollution Assessment Related to Dams

Surface sediment is one of the main sources of nutrients in overlying water environments, and these can also indirectly reflect the degree of eutrophication. In this paper, the spatial distribution characteristics of total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) in the surface sediments of 11 sections in the lower Lancang River during flood season are analyzed, as well as the distribution of phosphorus in different forms. The main sources of TOC and its correlation with TN and TP are discussed and, finally, the pollution levels of the surface sediments are evaluated. The results show that the average content of TOC in the surface sediments of the lower Lancang River is 9003.75 mg/kg. The average TN content is 893.79 mg/kg, while the average TP content is 521.35 mg/kg. The TOC in the surface sediments of the lower Lancang River is derived mainly from algae and plankton in the river, and the TN and TP have similar sources. The total phosphorus in the surface sediments of the lower Lancang River is composed mainly of calcium-bound phosphorus (Ca-P). The evaluations of the organic pollution index and comprehensive pollution index show that the surface sediment pollution degree at the upper sections of the Nuozhadu and Jinghong Dams are more serious than those below the dams. Furthermore, the tributary sections are all slightly polluted, with the exception of the Mengyang River, which is considered moderately polluted.

Recent organic carbon sequestration in the shelf sediments of the Bohai Sea and Yellow Sea, China

This study provides an extensive depiction of regional scale sedimentary total organic carbon (TOC) sequestration in the Bohai Sea (BS) and Yellow Sea (YS), marginal system of the western Pacific Ocean. The spatial pattern of the sediment mass accumulation rate (MAR) in the BS and YS was summarized based on a 100-year timescale. The relatively higher MAR (3−7g/cm2/y) in the Yellow River estuary, compared to the adjacent areas, indicate a predominant impact of river discharge on the modern sedimentation process in the BS. Relatively stable sedimentary environment in the offshore mud deposits of the BS and YS was also identified based on the along-core sediment composition and radionuclides profiles. The corresponding spatial pattern of grain size and TOC content suggest a hydrodynamic constraint on the sedimentary TOC accumulation. Moreover, in spite of the various TOC sources revealed by the bulk organic matter (OM) proxies (C/N ratio and δ13C), the restriction of these bulk parameters with potential masking of the signature of terrigenous OM was also identified. The average burial flux of TOC (15.3g C/m2/y) was finally obtained with a total sequestration of 5.6×106t C/y, suggesting that the BS and YS serve as a significant repository of sedimentary TOC. The overall organic carbon storage capacity of the BS and YS is mainly sustained by the fluvial/land-based OM input, high phytoplankton primary productivity, convergent hydrodynamic and stable depositional settings.

Influence of total organic carbon deposition on the inventory of gas hydrate in the Indian continental margins

Total organic carbon (TOC) content of marine sediments represents residual carbon, originally derived from terrestrial and marine sources, which has survived seafloor and shallow subseafloor diagenesis. Ultimately, its preservation below the sulfate reduction zone in marine sediments drives methanogenesis. Within the gas hydrate stability zone (GHSZ), methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate. In this paper we examine the inventory and sources of TOC in sediments collected from four regions within the GHSZ along the Indian continental margins. The recovered sediments vary in age from Oligocene to recent. Mean TOC abundance is greatest in the Krishna–Godavari (K–G) Basin and decreases progressively to the Mahanadi basin, Andaman wedge, and Kerala–Konkan (K–K) Basin. This decrease in TOC is matched by a progressive increase in biogenic CaCO3 and increasing distance from terrestrial sources of organic matter and lithogenic materials. Organic carbon sources inferred from C/N and δ13CTOC range from terrestrial (K–G Basin) to mixed marine and terrestrial (Mahanadi Basin), to marine dominant (Andaman wedge and K–K Basin). In the K–G Basin, variation in the bulk δ13CTOC is consistent with changes in C3 and C4 vegetation driven by monsoon variability on glacial-interglacial timescales, whereas in the Mahanadi Basin a shift in the δ13CTOC likely reflects the onset of C4 plant deposition in the Late Miocene. A large shift the δ13CTOC in the K–K basin is consistent with a change from C3 to C4 dominated plants during the middle Miocene. We observe a close relationship between TOC content and gas hydrate saturation, but consider the role of sedimentation rates on the preservation of TOC in the zone of methanogenesis and advective flow of methane from depth. Although TOC contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data, seismic, borehole log, pressure core, and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions. Although TOC content may be a first order indicator for gas hydrate potential, the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur.

Predicting Total Organic Carbon Load with El Niño Southern Oscillation Phase Using Hybrid and Fuzzy Logic Approaches

Abstract. During drinking water treatment, chlorine reacts with total organic carbon (TOC) to form disinfection byproducts (DBP), some of which can be carcinogenic. Additional treatment required to remove TOC before chlorination significantly increases treatment cost. There are two main sources of TOC in a water supply reservoir: (1) the watershed draining to the reservoir, and (2) the internal loading within the reservoir. Of these two sources, the watershed TOC load can be significant, especially when the watershed has large wetland areas. TOC loads in the Southeast U.S. can be affected by El NiA±o Southern Oscillation (ENSO)-induced climate variability. Since ENSO can be forecasted a few months in advance, ENSO-based TOC load forecasts can be used to tailor additional treatment required for TOC removal (e.g., activated carbon) before chlorination, thus reducing treatment costs. The objectives of this study were to quantity the effect of ENSO on watershed TOC loads and develop data-driven modeling approaches for forecasting TOC loads. Four hybrid and four fuzzy logic models, with different model architectures, were developed for forecasting watershed TOC loads using temperature, precipitation, NiA±o 3.4, and Trans NiA±o Index (TNI). This study concludes that the hybrid models are suitable for estimating real-time TOC loads, while the fuzzy logic models are suitable for qualitative forecasting of TOC loads with one-month lead time. In addition, this study highlights the importance of incorporating ENSO information into data-driven models in ENSO-affected regions.

Total organic carbon isotopes: A novel proxy of lake level from Lake Qinghai in the Qinghai–Tibet Plateau, China

The isotopic compositions of total organic carbon (TOC) in lakes have been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments. However, the main factors that affect the carbon isotopes of TOC (source of organic material, water condition and others) may vary in different lake sediment records, which have limited the applicability of organic carbon isotopes in explaining biogeochemical and environmental changes in lakes. In this study, the organic carbon isotopic compositions of aquatic plants and surface sediments from Lake Qinghai and the living terrestrial plants and surface soils around the lake were systematically investigated to identify the sources of TOC in the sediments and the significance of the organic carbon isotopes of sedimentary TOC. We found that the aquatic plants in the deep water areas (>10m) were primarily dominated by Cladophora, but submerged plants (Potamogeton and Ruppia L.) are the dominant species in shallow water (<10m). The Cladophora have negative δ13Corg values (−33.6‰ to −28.6‰) that are caused by C3-like photosynthesis, but the submerged plants have enriched δ13Corg values (−17.8‰ to −15.4‰) that are caused by C4-like photosynthesis. In addition, the δ13Corg values of Cladophora become more negative with increasing water depth because of the slow photosynthetic rate caused by the weak light intensity at depth. The isotopic data indicate that the carbon isotopes of organic material in the surface sediments are primarily controlled by the types of aquatic plant and that the δ13Corg values of TOC can be used to indicate the variation of the water depth (lake level). The organic carbon isotopic data from the 1F core showed that the water was shallow (<10m) because of intense evaporation related to high temperatures even though precipitation sharply increased in the warm period during the early-mid Holocene. The lake level reached its maximum level at 3ka.

Variability of groundwater levels and total organic carbon in the riparian zone of a boreal catchment

[1] The riparian zone is a narrow corridor where hillslopes (and their associated hydrobiogeochemical processes) interface with the river system. As such, the riparian zone serves as the last piece of landscape with which water interacts as it transitions from being water flowing primarily through the landscape (i.e., shallow groundwater) to water flowing primarily on the landscape (i.e., stream water). This study investigates the spatiotemporal variability in riparian-zone soil water total organic carbon (TOC) and its relation to the shallow groundwater table using observations from the recently instrumented riparian observatory in the Krycklan catchment study area located in boreal northern Sweden. In general, there is a decrease in TOC concentration with depth down through the soil profile. The rate of this decrease was variable among the six monthly samplings used in this study. The spatial variability of soil water TOC in the riparian zone was connected to the spatial variability of the shallow groundwater levels. This demonstrated the importance of the temporal variation of flow pathways and the mixing of waters from different sources of TOC moving into and through the riparian zone. The coupled variation of the hydrologic and biogeochemical systems raised questions about the ability of simple lumped approaches to accurately predict how in-stream TOC concentrations will change with climate and/or land use. The integrated sampling approach in the riparian observatory covers both hydrologic and biogeochemical aspects of soil water TOC and provides a basis for development and testing of distributed, physically based transport models.

Decoupling of total organic carbon concentrations and humic substance fluorescence in a an extended temperate estuary

We present the first complete sections of humic substance fluorescence (HS fl)-values and total organic carbon (TOC) concentrations through the extended estuary of the Baltic Sea, Kattegat, and Skagerrak. Both HS fl-values and TOC concentrations correlated strongly with salinity, indicating terrestrial runoff as the main source for both components. However, their distributions do not result from a simple two end-member mixing and the two pools have different controls. In the Baltic Sea, surface water HS fl-values decreased from north to south, while TOC concentrations remained relatively constant, indicating an increasingly biologically-labile TOC pool from north to south. Both HS fl-values and TOC concentrations decreased strongly from the Baltic Sea to Kattegatt and Skagerrak. There were no major net sources or sinks for HS fl or TOC in Kattegat. However, the Jutland Current was an HS fl and TOC source for Skagerrak, probably derived from Danish rivers. Fraction of forested area in drainage basins of the Baltic Sea correlated positively with HS fl-values in freshwater end-members, but not with TOC concentrations. Land-runoff-volume relative to basin volume correlated positively with HS fl-values in surface waters, but not with TOC concentrations. In the Baltic Proper, deepwater oxygen concentrations correlated negatively with HS fl-values, indicating a significant source for HS fl (but not TOC) in suboxic and anoxic deepwaters. Photobleaching decreased HS fl-values in surface waters of the Baltic Proper, but had no effect on TOC concentrations. Despite terrestrial material being the main source for both TOC and HS, differences in control mechanisms result in a partial decoupling of their concentrations within the estuary.

Modeling estuarine-shelf exchanges in a deltaic estuary: Implications for coastal carbon budgets and hypoxia

The export of wetland-derived materials to the coastal ocean (i.e., the “Outwelling” hypothesis) has received considerable attention over the past several decades. While a number of studies have shown that estuaries export appreciable amounts of nutrients and carbon, few studies have attempted to estimate the importance of estuarine sources for the coastal carbon budgets in river-dominated coastal ecosystems. A novel tidal prism model was developed to examine estuarine-shelf exchanges in the Barataria estuary, a deltaic estuary located in the north-central Gulf of Mexico. This estuary has been the site of a massive wetland loss, and it has been hypothesized that carbon export from the eroding coastal wetlands supports the development of a large hypoxic zone in the coastal Gulf of Mexico. The model results show that the Barataria estuary receives nitrogen through the tidal passes and releases carbon to the coastal ocean. The mean calculated tidal water discharge of 6930 m 3 s −1 is equivalent to about 43% of the lower Mississippi River discharge. The annual total organic carbon (TOC) export is 109 million kg, or 57 gC m 2 yr −1 when prorated to the total water area of the estuary. This carbon export is equivalent to a loss of 0.5 m of wetland soil horizon over an area of 8.4 km 2, and accounts for about 34% of the observed annual wetland loss in the estuary between 1978 and 2000. Compared to the lower Mississippi River, the Barataria estuary appears to be a very small source of TOC for the northern Gulf of Mexico (2.7% of riverine TOC), and is unlikely to have a significant influence on the development of the Gulf's hypoxia.

Total Organic Carbon, an Important Tool in an Holistic Approach to Hydrocarbon Source Fingerprinting

The identification and allocation of multiple hydrocarbon sources in marine sediments is best achieved using an holistic approach. Total organic carbon (TOC) is one important tool that can constrain the contributions of specific sources and rule out incorrect source allocations in cases where inputs are dominated by fossil organic carbon. In a study of the benthic sediments from Prince William Sound (PWS) and the Gulf of Alaska (GOA), we find excellent agreement between measured TOC and TOC calculated from hydrocarbon fingerprint matches of polycyclic aromatic hydrocarbons (PAH) and chemical biomarkers. Confirmation by two such independent source indicators (TOC and fingerprint matches) provides evidence that source allocations determined by the fingerprint matches are robust and that the major TOC sources have been correctly identified. Fingerprint matches quantify the hydrocarbon contributions of various sources to the benthic sediments and the degree of hydrocarbon winnowing by waves and currents. TOC contents are then calculated using source allocation results from fingerprint matches and the TOCs of contributing sources. Comparisons of the actual sediment TOC values and those calculated from source allocation support our earlier published findings (5) that the natural petrogenic hydrocarbon background in sediments in this area comes from eroding Tertiary shales and associated oil seeps along the northern GOA coast and exclude thermally mature area coals from being important contributors to the PWS background due to their high TOC content.