Organic Carbon Isotopic Composition Research Articles

Carbon Isotope Records Indicative of Paleoceanographical Events at the Latest Permian Dalong Formation at Shangsi, Northeast Sichuan, China

Paired organic and carbonate carbon isotope compositions of Late Permian Wujiaping and Dalong formations at Shangsi, Northeast Sichuan were analyzed by MAT 251. An abrupt negative excursion in the two isotope records was observed in the middle part of Dalong Formation, in association with a drop in the carbon isotope difference of the two records and an increase of total organic carbon (TOC) content. The negative drop of the paired carbon isotope records is suggestive of the input of 12 C-enriched CO 2 . The molecular ratios of pristane to phytane and dibenzothiophene to phenanthrene indicate the anoxic condition in this interval. The enhanced TOC content is indicative of the elevated preservation of organic matter due to the anoxic condition. These isotopic and organic geochemical data probably infer the occurrence of the upwelling in this interval. The additional contribution of volcanism activity observed in South China cannot be excluded to the input of 12 C-enriched CO 2 and the negative shifts in carbon isotope composition of bulk organic matter and carbonate.

A 130-ka reconstruction of precipitation on the Chinese Loess Plateau from organic carbon isotopes

We study the relationship between organic carbon isotopes (δ 13C TOC) in soil and mean annual precipitation (MAP) in the East Asian monsoon area by combining the results of carbon isotope studies of plants, C 4/C 3 ratios, and precipitation in the modern ecosystem of the Chinese Loess Plateau. Based on this relationship, we estimate the amount of paleoprecipitation that has occurred in the southeastern part of the Loess Plateau since the last interglacial stage using the carbon isotopic compositions of the soil organic matter (SOM). The results show that the annual precipitation in that region was high during the interglacial stages, with a maximum value of 850 mm, and that it decreased during the last glacial stage to a minimum value of 350 mm. We suggest that the use of organic carbon isotopic composition in paleosol sediments is a reliable method for reconstructing paleoprecipitation due to the clear relationships between soil δ 13C TOC, C 4/C 3 abundance, and precipitation in modern ecosystems.

C and N elemental and stable isotopic signatures in sedimentary organic matter from Lake Pamvotis (Greece)

Organic carbon (Corg) and total nitrogen (N) concentrations in combination with stable organic carbon (δ13Corg) and nitrogen (δ15N) isotopic composition were used to investigate the organic matter sources and influence of anthropogenic activities on recent sediments of eutrophic Lake Pamvotis (Greece) and oligotrophic Lake Bohinj (Slovenia). Corg and N concentrations increase upcore showing a progressive nutrient enrichment in both lakes. The C/N ratios vary with depth and indicate that allochthonous and autochthonous sources contributed to sedimentary organic matter. Nevertheless, an excursion to relatively low surface sediment C/N ratios suggests that recently the latter predominates. Finally, δ13Corg and δ15N records showed changes in carbon and nitrogen cycles related predominantly to local anthropogenic activities in Lake Pamvotis and to anthropogenic activities (i.e. woodcutting) and global atmospheric changes in Lake Bohinj.

Carbon geochemistry of serpentinites in the Lost City Hydrothermal System (30°N, MAR)

The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and δ 13C TC values ranging from −28.7‰ to +2.3‰. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (δ 13C TOC: −28.9‰ to −21.5‰) and variations in δ 13C TC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C 15 to C 30. Longer-chain hydrocarbons (up to C 40) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C 16 to n-C 20 alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important—as yet unidentified—reservoir for dissolved organic carbon (DOC) from seawater.

Paleoceanographic implications of nitrogen and organic carbon isotopic excursions in mid-Pleistocene sapropels from the Tyrrhenian and Levantine Basins, Mediterranean Sea

Comparison of the total nitrogen and organic carbon isotopic compositions of sediment sequences from the Tyrrhenian and Levantine Basins that contain three same-age sapropels reveals evidence of both similarities and differences in mid-Pleistocene sapropel-forming conditions in the western and eastern Mediterranean Sea. Total δ 15N values systematically decrease at both locations from 5‰ in background sediments to − 1‰ in the sapropels. The lower δ 15N values signify that nitrogen fixation was important to the increased marine productivity implied by the higher organic carbon concentrations, which is consistent with widespread salinity stratification of surface waters when sapropels accumulated. Organic δ 13C values systematically increase from − 25‰ in the background sediments to − 22‰ in the sapropels at the Tyrrhenian site, which confirms that increased marine productivity accompanied sapropel deposition. In contrast, δ 13C values decrease in the sapropel layers in the Levantine Basin even though organic carbon concentrations are higher in these sapropels. The different types of δ 13C excursions associated with increased organic matter production at the two locations indicates a difference in the dissolved carbon pool from a combination of greater fluvial delivery of continental carbon and stronger recycling of organic matter in the Levantine Sea than in the Tyrrhenian Sea. This difference implies that the shifts to wetter climate during times of sapropel deposition were stronger in the eastern than in the western Mediterranean.

Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation

Recent geochemical data from Oman, Newfoundland, and the western United States suggest that long-term oxidation of Ediacaran oceans resulted in progressive depletion of a large dissolved organic carbon (DOC) reservoir and potentially triggered the radiation of acanthomorphic acritarchs, algae, macroscopic Ediacara organisms, and, subsequently, motile bilaterian animals. However, the hypothesized coupling between ocean oxidation and evolution is contingent on the reliability of continuous geochemical and paleontological data in individual sections and of intercontinental correlations. Here we report high-resolution geochemical data from the fossil-rich Doushantuo Formation (635-551 Ma) in South China that confirm trends from other broadly equivalent sections and highlight key features that have not been observed in most sections or have received little attention. First, samples from the lower Doushantuo Formation are characterized by remarkably stable delta(13)C(org) (carbon isotope composition of organic carbon) values but variable delta(34)S(CAS) (sulfur isotope composition of carbonate-associated sulfate) values, which are consistent with a large isotopically buffered DOC reservoir and relatively low sulfate concentrations. Second, there are three profound negative delta(13)C(carb) (carbon isotope composition of carbonate) excursions in the Ediacaran Period. The negative delta(13)C(carb) excursions in the middle and upper Doushantuo Formation record pulsed oxidation of the deep oceanic DOC reservoir. The oxidation events appear to be coupled with eukaryote diversity in the Doushantuo basin. Comparison with other early Ediacaran basins suggests spatial heterogeneity of eukaryote distribution and redox conditions. We hypothesize that the distribution of early Ediacaran eukaryotes likely tracked redox conditions and that only after approximately 551 Ma (when Ediacaran oceans were pervasively oxidized) did evolution of oxygen-requiring taxa reach global distribution.

Pretreatment technique for siderite removal for organic carbon isotope and C:N ratio analysis in geological samples

A method for the removal of siderite from geological samples to determine organic carbon isotope compositions using elemental analysis isotope ratio mass spectrometry is presented which includes calculations for % organic carbon in samples that contain diagenetic carbonate. The proposed method employs in situ acidification of geological samples with 6 N HCl and silver capsule sample holders and was tested on modern peach leaf samples (NIST 1547) and ancient lacustrine samples from Valles Caldera, New Mexico. The in situ acidification technique eliminates potential errors associated with the removal of soluble organic material using standard acid decanting techniques and allows for removal of the less soluble siderite, which is not efficiently removed using vapor acidification techniques.

Photosynthetic Pathway of Steppe Vegetation and the Stable Carbon Isotope Composition of Organic Matter in the Russian Chernozem

This study was conducted to determine the stable carbon isotope composition of soil organic carbon (C) under different land uses in the Russian Chernozem, one of the typical soils in grasslands. Three sites were sampled: a native grassland field (not cultivated for at least 300 years), an adjacent 50‐year continuous fallow field in the V. V. Alekhin Central Chernozem Biosphere State Reserve in the Kursk region of Russia, and a continuously cropped field in the Experimental Station of the Kursk Institute of Agronomy and Soil Erosion Control. All sampled soils were classified as Haplic Chernozem. The stable C isotope composition of organic C (δ13C values vary from −24.7‰ to −25.9‰) is determined by the C3‐type plants dominating in the region. Agricultural crops grown in the region are predominantly C3‐type with the exception of corn. Corn cropping slightly altered the stable C isotope signature of soil organic C. Presence of carbonates in the soil can interfere with the stable C isotope signature of soil organic C at depth. It can result in high variability of the stable C isotope signature of organic C where carbonates are present, and therefore caution should be applied when using these values for determination of the proportion of C3/C4‐type plants contributing to soil organic‐matter formation.

Paleoceanography of the middle Eocene Arctic Ocean based on geochemical measurements of biogenic matter

Geochemical analyses of biogenic opal, total organic carbon (TOC), total sulfur (TS), and stable sulfur isotopic composition were conducted on the middle Eocene section of the ACEX cores obtained by IODP Expedition 302. The analyzed %TS contents were high in all the examined intervals, indicating sufficient sea water was present in the deep layer of the paleo Arctic basin in contrast with the low salinity surface waters determined by freshwater siliceous microfossils. From the high %TS in Lithological Unit 1/6, the extent of sea water supply from outside of the Arctic basin appeared to have increased after 45 Ma. The euxinic condition of the bottom water is suggested by the TOC-TS diagram. The anoxic environment was brought about by the estuarine type circulation pattern and salinity stratification. The light sulfur isotope composition ($ delta^{34} $S) indicates the microbial sulfate reduction in an open system. The trend of $ delta^{34} $S shift toward lower values with time is an indication of the history that the water exchange between the Arctic Ocean and the outside basin became progressively extensive. The high %TS, the plotted pattern of TOC vs TS, and the extremely low sulfur isotopic values are unique and not comparable with the modern analogous such as the Black Sea. These results illustrate that the middle Eocene Arctic Ocean retained characteristically unique environments on the geochemical aspect.

Climatic dependency of soil organic carbon isotopic composition along the S–N Transect from 34°N to 52°N in central-east Asia

We explored the relationships between surface-soil (1–20 cm) organic carbon isotopic signatures and associated climatic factors in central-east Asia in an attempt to develop transfer functions that can be used to retrieve the paleoclimatic information stored in the thick eolian–paleosol sequences within the area. Our analysis shows that the negative correlation between the surface-soil organic δ 13C values and the mean annual precipitation is robust ( R 2 = 0.453; n = 196; p < 0.05) and the negative correlation with the growing-season (April–September) precipitation is more significant ( R 2 = 0.4966; n = 196; p < 0.05). Our study further shows that the positive correlation between the surface-soil organic δ 13C values and mean growing-season aridity is most significant ( R 2 = 0.5805; n = 196; p < 0.05). We have smoothed both the organic δ 13C values and the mean growing-season aridity values using a 3-point moving-window average-filter method in an attempt to remove some of random errors and found that the positive correlation between the two is further increased ( R 2 = 0.7784; n = 192; p < 0.05). These robust linear relationships demonstrate their value in reconstructing paleoclimate changes in the study area. The documented climatic dependency of the surface-soil carbon isotopic composition in the study area might have resulted both from the humidity-related isotopic enrichment processes of the dominant C 3 plants (stomatal conductance and photosynthetic discrimination) and from the aridity-related abundance of C 4 plants (mainly Chenopodiaceae species) along the S–N bioclimatic gradient.

Isotopic composition of organic carbon and elemental carbon in PM2.5 in Hangzhou, China

In order to identify the sources of organic carbon (OC) and elemental carbon (EC) in airborne PM2.5 in Hangzhou, OC and EC were oxidized to CO2 offline in two thermal steps, and their carbon isotope compositions were measured by mass spectrometer. Results showed that the average δ13COC values were −52.8‰, −48.1‰, −50.0‰ and −52.5‰ in spring, summer, autumn, and winter, respectively, and the annual average value was −50.9‰ (−85.0‰ to −35.6‰), whereas δ13CEC values were −26.4‰, −26.9‰, −26.7‰ and −25.9‰ in the four seasons, and its annual value was −26.5‰ (−30.5‰ to −23.8‰). Large differences were found between isotope compositions of the two types of carbon. δ13COC values were much smaller than those of δ13CEC, and varied in a large range, and the ratios in both spring and winter were the smallest. δ13CEC varied in a small range, and its mean values in four seasons were nearly the same. This suggests that the carbon isotope compositions of OC and EC in PM2.5 can provide useful information in distinguishing their sources.

Organic Carbon Isotopic Evolution during the Ediacaran‐Cambrian Transition Interval in Eastern Guizhou, South China: Paleoenvironmental and Stratigraphic Implications

Abstract: Secular variations of carbon isotopic composition of organic carbon can be used in the study of global environmental variation, the carbon cycle, stratigraphic delimitation, and biological evolution, etc. Organic carbon isotopic analysis of the Nangao and Zhalagou sections in eastern Guizhou reveals a negative excursion near the Precambrian‐Cambrian boundary that correlates with a distinct carbonate carbon isotopic negative excursion at this boundary globally. Our results also demonstrate that several alternating positive and negative shifts occur in the Meishucunian, and an obvious negative anomaly appears at the boundary between the Meishucunian and Qiongzhusian. The isotope values are stable in the middle and lower parts but became more positive in the upper part of the Qiongzhusian. Evolution of organic carbon isotopes from the two sections in the deepwater facies can be well correlated with that of the carbonate carbon isotopes from the section in the shallow water facies. Integrated with other stratigraphic tools, we can precisely establish a lower Cambrian stratigraphic framework from shallow shelf to deep basin of the Yangtze Platform.

Principal stages in evolution of Precambrian organic world: Communication 1. Archean and Early Proterozoic

Expounded in this work are the results of critical consideration of published and original data on biologic nature and appearance chronology of different groups of Archean and Lower Proterozoic (3.5–1.65 Ga) paleontological remains known from geological record. Conclusions are substantiated by morphological analysis of structurally preserved microfossils, their facies distribution, and by inferable genesis and principal evolutionary trends of Archean stromatolites. A special attention is paid to variations of organic and carbonate carbon isotope composition in sedimentary successions with paleontological remains and to recent information about discovered, most ancient biomarkers of large groups of organic world. As a result of this approach, a detailed model of Precambrian organic world evolution is suggested.

Trends of soil organic matter turnover in the salt marsh of the Yangtze River estuary

Characteristics and tidal flat trends of soil organic matter (SOM) turnover were studied for the Chongmingdongtan Salt Marsh in the Yangtze River estuary, based on analyses of stable carbon isotope composition (δ13C), grain sizes and contents of particulate organic carbon (POC), total nitrogen (TN) and inorganic carbon (TIC) for three cores excavated from high tidal flat, middle tidal flat and bare flat. Results demonstrate that correlations between soil POC contents and δ13C values of the salt marsh cores were similar to those between soil organic carbon (SOC) contents and δ13C values of the upper soil layers of mountainous soil profiles with different altitudes. SOM of salt marsh was generally younger than 100 years, and originated mainly from topsoil erosions in catchments of the Yangtze River. Correlations of TN content with C/N ratio, POC content with TIC content and POC content with δ13C values for the cores suggest that turnover degrees of SOM from the salt marsh are overall low, and trends of SOM turnover are clear from the bare flat to the high tidal flat. Bare flat samples show characteristics of original sediments, with minor SOM turnover. Turnover processes of SOM have occurred and are discernable in the high and middle tidal flats, and the mixing degrees of SOM compartments with different turnover rates increase with evolution of the muddy tidal flat. The exclusive strata structure of alternate muddy laminae and silty laminae originated from dynamic depositional processes on muddy tidal flat was a great obstacle to vertical migration of dissolved materials, and SOM turnover was then constrained. The muddy tidal flat processes exerted direct influences on sequestration and turnover of SOM in the salt marsh, and had great constraints on the spatial and temporal characteristics of SOM turnover of the Chongmingdongtan Salt Marsh in the Yangtze River estuary.

Contemporaneous massive subaerial volcanism and late cretaceous Oceanic Anoxic Event 2

Oceanic Anoxic Events (OAEs) are geological time intervals characterized by extremely high burial rates of organic carbon that led to deposition of organic-rich “black shales” in the global ocean. It has been proposed that oceanic anoxic events are ultimately caused by massive volcanism associated with formation of large igneous provinces (LIPs) because of chronological similarities, but no general consensus has developed yet. To investigate the possibility of LIP volcanism instigating OAEs, we measured stable isotopic compositions of bulk organic carbon ( δ 13C org) and lead (Pb) isotopic compositions in the silicate sediment fraction across the Bonarelli black shale (Italy), a type stratigraphic section for the end-Cenomanian OAE (OAE-2; 94 Ma). Ultra-high-resolution δ 13C org records determined every 1.5 mm capture a 3‰ sharp negative shift at the base of the Bonarelli. At the same stratigraphic level, Pb isotopic compositions in the silicate sediment fraction exhibit significant shifts toward characteristic values of volcanic rocks from contemporaneous LIPs (Caribbean and Madagascar flood basalts). These data suggest a rapid, substantial increase in the relative supply of silicate minerals from the two LIPs. Massive subaerial volcanism associated with LIP formation provides a simple explanation for these two isotopic geochemical signals via release of a huge amount of carbon dioxide (∼ 10 5 Gt CO 2) and particulate materials into the atmosphere, which resulted in a rapid negative shift of δ 13C in sea water and changes in Pb isotopic compositions in the silicate sediment fraction, respectively. We interpret that massive volcanism triggered significant climatic changes, inducing biotic crises and oceanic anoxia.

Content and isotopic composition of organic carbon within a flood layer in the Po River prodelta (Adriatic Sea)

Thirty-three surface sediment samples from cross-shelf transects on the northern Adriatic shelf were collected in December 2000, soon after a 100-yr flood of the Po River, in order to determine the distribution of organic carbon (OC) along the main sediment dispersal system. To evaluate the temporal variability, stations were re-occupied eight times at seasonal intervals until June 2003. Downcore sediment profiles from two sites characterized by high flood deposit thicknesses were also examined to assess the OC variability within the flood layer. In December 2000, the highest contents of OC (up to 1.24 wt%) were measured in front of the main distributary mouths (Pila, Tolle and Gnocca-Goro) where the greatest thicknesses of the flood deposit were recorded. However, the influence of the Po di Gnocca-Goro sediment supply on the OC surface distribution declined after ∼1.5 years from the fall-2000 river flood, probably because these mouths are less active when the water discharge is lower. The δ 13C of organic matter was used to trace the dispersal of fluvial OC on the continental shelf. The δ 13C values ranged from −25.9‰ to −23.1‰. The fraction of fluvially derived organic particles decreased with increasing water depth according to a radial dispersal pattern around the Po River delta. This pattern persisted in all cruises. δ 13C values increased progressively until April 2002, suggesting an increasing marine contribution to the OC content but decreased again following a second minor flood event in November 2002. The molar C/N ratio was on average 10.0±1.6, with slightly lower values in southern and central areas. Assuming contributions from three OC end-members (terrestrial, riverine and marine), a mixing model based on δ 13C and the ratio of N to C (statistically more robust than C/N; Goñi, M.A., Teixeir, M.J., Perkley, D.W., 2003. Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA). Estuarine, Coastal and Shelf Science 57, 1023–1048) was applied in order to quantitatively assess the OC sources for Po shelf sediments. δ 13C is significantly and positively correlated with the marine OC fraction. The terrestrial fraction is inversely correlated with N/C, while the riverine fraction is positively correlated with N/C. The terrestrial OC source was the most abundant end-member (>70%) showing only little temporal variability regardless of the Po River water discharge. Temporal and spatial changes in OC composition suggest that: (a) the Po River prodelta is always dominated from terrestrial OC input; (b) the Po della Pila supplies most terrestrial OC, whilst other tributaries (e.g., Po di Gnocca-Goro) are secondary sources. However, these mouths are as important as the Po della Pila in affecting the riverine OC signature; (c) offshore, biological primary production raises the marine OC contribution. At two sites on the Po River prodelta, the 2000-flood deposit shows slight but consistent compositional changes of organic matter (N/C and δ 13C) which can help to recognize other flood events in the sedimentary record. The OC budget for the 2000-flood deposit accounts for a terrestrial+riverine OC supply of 68–162 Gg (10 9 g) against an OC deposition of 106–162 Gg (excluding the marine fraction), which implies a rapid and efficient sedimentation of the flood material, and scarce or negligible export out the study area. Flood events may thus enhance terrestrial carbon burial, whereas marine carbon arrives more slowly and may be largely mineralized at the sediment–water interface.

Paleo-environmental evolution as revealed by analysis of organic carbon and nitrogen: A case of coastal Taipei Basin in Northern Taiwan

In an attempt to discriminate different sources of organic matter and reconstruct the paleo-environment of the Taipei Basin, northern Taiwan over the past 250,000 years, organic carbon and nitrogen content (C/N ratios) and isotope composition (δ13C and δ15N) are measured for sediment samples collected from core Wu-ku drilled at the depocenter in western Taipei Basin. The high δ13C values (up to -17.0‰) and high C/N ratio (up to 14.9 and 13.0, respectively) indicate that this area underwent a period of relative aridity during MIS 2 and mid MIS 6. Another two periods of high and variable δ15N values (up to 15.5 and 13.5‰, respectively) in MIS 1 and MIS 7 represent different nutrient condition in the basin. In MIS 1, high δ15N variability along with increasing TOC may reflect an unstable trend of aquatic productivity related to the dynamic mixing ratios of seawater and fresh water. The high δ15N values observed in the sediments of MIS 7 could be explained by the employment of a 15N-enriched nitrate from denitrification as the main source of nitrogen for primary producers.

High resolution carbon isotopic analysis across the Ordovician-Silurian boundary of the Wangjiawan section, South China

High resolution organic carbon isotopic compositions (δ13Corg) were analyzed for the samples from Wangjiawan section, Yichang, south China. Three negative shifts have been recognized in the uppermost Ordovician. The results indicate that the shifts were related to environmental changes during the development and the decline of the Gondwana ice sheet. Carbonate deposited in the ice period records, inorganic carbon isotope (δ13Ccarb) curve showing a different trend from δ13Corg : δ13Ccarb shows a sharp and negative shift while δ13Corg shifts positive. The δ13Ccarb curve is a local trend in the restricted Yangtze basin. The in-situ oxidation of organic matter enhanced by glacier induced oceanic circulation in narrow and shallow Yangtze basin, and decreased δ13Ccarb in the carbonate bed.

How depositional conditions control input, composition, and degradation of organic matter in sediments from the Chilean coastal upwelling region

In a comprehensive study, we compared depositional conditions, organic matter (OM) composition, and organic carbon turnover in sediments from two different depositional systems along the Chilean continental margin: at ∼23° S off Antofagasta and at ∼36° S off Concepción. Both sites lie within the Chilean coastal upwelling system and have an extended oxygen minimum zone in the water column. However, the northern site (23° S) borders the Atacama Desert, while the southern site (36° S) has a humid hinterland. Eight surface sediment cores (up to 30 cm long) from water depths of 126–1350 m were investigated for excess 210Pb ( 210Pb xs) activity, total organic and total inorganic carbon concentrations (TOC and TIC, respectively), C/N-ratios, organic carbon isotopic compositions (δ 13C), chlorin concentrations, Chlorin Indices (CI), and sulfate reduction rates (SRR). Sediment accumulation rates obtained from 210Pb-analysis were similar in both regions (0.04–0.15 cm yr −1 at 23° S, 0.10–0.19 cm yr −1 at 36° S), although total 210Pb xs fluxes indicated that the vertical particle flux was higher at 36° S than at 23° S. We propose that sediment focusing in isolated deposition centers led to high sediment accumulation rates at 23° S. Furthermore, there were no indications for sediment mixing at 23° S, while bioturbation was intense at 36° S. δ 13C-values (−24.5‰ to −20.1‰ vs. VPDB) and C/N-ratios (molar, 8.6–12.8) were characteristic of a predominantly marine origin of the sedimentary OM in both investigated areas. The extent of OM alteration in the water column was partly reflected in the surface sediments as chlorin concentrations decreased and C/N-ratios and CI increased with increasing water depth of the sampling site. SRR were lower at 23° S (areal SRR 0.12–0.60 mmol m −2 d −1) than at 36° S (areal SRR 0.82–1.18 mmol m −2 d −1), which was partly due to the greater water depth of most of the sediments investigated in the northern region and consistent with a lower quality of the sedimentary OM at 23° S. Reaction rate constants for TOC degradation that were obtained from measured SRR ( k SRR; 0.0004–0.0022 yr −1) showed a good correspondence to k TOC that were derived from the depth profiles of TOC (0.0003–0.0014 yr −1). Both, k SRR and k TOC, reflect differences in OM composition. At 36° S they were related to the degradation state of bulk OM (represented by C/N-ratios), whereas near 23° S they were related to the freshness of a small fraction of labile OM (represented by CI). Our study shows that although rates of organic carbon accumulation were similar in both investigated sites, the extent and kinetics of organic carbon degradation were closely linked to differing depositional conditions.

Interannual variation in stable carbon and nitrogen isotope biogeochemistry of the Mattaponi River, Virginia

Seasonal and interannual variation of the stable carbon (C) and nitrogen (N) isotope composition of suspended particulate organic matter (POM) was measured in the brackish and tidal freshwater regions of the Mattaponi River, a tributary of the York River, Virginia, and a pristine end member on a continuum of anthropogenic modification within Chesapeake Bay. A principal components analysis indicated that seasonal variation was related to physical mixing and river discharge. Freshwater POM had high C : N (.12), depleted particulate organic carbon isotopic composition (d13CPOC, 226% to 230%), and depleted particulate nitrogen isotopic composition (d15NPN, 2–10%) compared to brackish water POM, which had lower C : N and enriched d13CPOC (224% to 227%) and d15NPN (7–15%). During high discharge events, the d13CPOC was enriched, the d15NPN depleted, and the C : N high relative to low discharge periods, indicating a large contribution from terrestrial-derived material. Within tidal freshwater, POM was comprised of humic-rich sediment, vascular plant matter, and phytoplankton produced in situ. Nonconservative mixing behavior was observed. Endogenously produced phytoplankton increased POC concentrations in tidal freshwater and oligohaline portions during base flows. Where estuarine and riverine POM mixed, the isotopic composition of the POM was homogenized, blurring source-specific characters observed upriver and thereby emphasizing the need to characterize the freshwater end member of estuaries carefully in order to identify POM sources. In estuaries, identifying the origin of particulate organic matter (POM) is difficult because POM is received from multiple sources, including riparian vegetation, adjacent marsh vegetation, submerged and emergent aquatic vegetation and associated epiphytes, and phytoplankton produced in situ. Early research using the stable isotope composition of estuarine POM to identify its dominant origins and fates led investigators to conclude that estuarine phytoplankton and terrestrial material were the major contributors to estuarine organic matter (OM; dissolved and particulate fractions) and that these sources