Dissolved and particulate organic carbon in Chesapeake Bay

We measured dissolved and particulate organic carbon (DOC and POC) in samples collected along 13 transects of the salinity gradient of Chesapeake Bay. Riverine DOC and POC end-members averaged 232±19 μM and 151±53 μM, respectively, and coastal DOC and POC end-members averaged 172±19 μM and 43±6 μM, respectively. Within the chlorophyll maximum, POC accumulated to concentrations 50–150 μM above those expected from conservative mixing and it was significantly correlated with chlorophylla, indicating phytoplankton origin. POC accumulated primarily in bottom waters in spring, and primarily in surface waters in summer. Net DOC accumulation (60–120 μM) was observed within and downstream of the chlorophyll maximum, primarily during spring and summer in both surface and bottom waters, and it also appeared to be derived from phytoplankton. In the turbidity maximum, there were also net decreases in chlorophylla (?3 μg l^?1 to ?22 μg l^?1) and POC concentrations (?2 μM to ?89 μM) and transient DOC increases (9–88 μM), primarily in summer. These occurred as freshwater plankton blooms mixed with turbid, low salinity seawater, and we attribute the observed POC and DOC changes to lysis and sedimentation of freshwater plankton. DOC accumulation in both regions of Chesapeake Bay was estimated to be greater than atmospheric or terrestrial organic carbon inputs and was equivalent to ≈10% of estuarine primary production.     

Application of pyrolysis–GC/MS for the characterisation of suspended particulate organic matter in the Mediterranean Sea: a comparison with the Black Sea

Suspended particulate organic matter (SPOM) both from the oligotrophic Mediterranean and from the more productive Black Sea has been analysed by pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) and by such conventional techniques as elemental analysis and fluorometry. The data achieved by the two approaches generally confirmed and complemented each other. The relative concentrations of pyrolysis products (termed markers) characteristic of chlorophyll (CHL), lipid, carbohydrate (CBH), and protein (PROT) components of SPOM have been determined. The vertical distribution of lipid markers was more uniform in the Mediterranean surface waters. Their relative abundance increased rapidly in the oxycline of the Black Sea, reaching their highest levels in the suboxic zone, where the protein composition of the SPOM changed significantly. In both seas, the relative concentrations of CHL markers increased consistently within the CHL maximum zone where the CBH markers were less abundant. No lignin markers from terrestrial vegetation were detected at significant levels in the pyrograms.     

The polymer-like organic material in the Orgueil meteorite

The insoluble organic material in the Orgueil (Cl) chondrite was analyzed by combined high vacuum pyrolysis-gas chromatography-mass spectrometry. Stepwise pyrolyses at 150, 300, 450 and 600°C of Orgueil meteorite powder which had been exhaustively extracted with solvents yielded a series of alkenes and alkanes to C₈, an extensive series of alkylbenzene isomers, thiophene, alkylthiophenes, and benzothiophene, together with the nitrogen- and oxygen-containing breakdown products, acetonitrile, acrylonitrile, benzonitrile, acetone and phenol. The Orgueil polymer fragmentation products are very similar both qualitatively and quantitatively to pyrolysis products of solvent-extracted Pueblito de Allende (C3) chondrite described in the literature.Changes in the relative abundances of polymer degradation products between 150 and 600°C imply the preferential loss of aliphatic and certain heteroatomic portions of the polymer at lower temperatures to leave highly condensed aromatic and heteroaromatic portions of the polymer which begin to fragment only at 450–600°C. The Orgueil polymer-like matter thus appears to be a complex mixture of polymerized materials having different thermal stabilities. Similarities between vacuum pyrolyzates of the Orgueil polymer and terrestrial kerogen suggest the possibility that meteorite organic matter may have been subjected on the meteorite parent bodies to diagenetic processes similar to those by which terrestrial kerogen is formed.     

Pyrolysis of modern wetland sediment: extracting climate records from fens in the Uinta Mountains and Fish Lake Plateau,Utah, USA

Rock‐Eval pyrolysis provides a quick, relatively inexpensive means of characterizing organic‐rich strata, and has been used for decades to understand global petroleum systems. Although designed to characterize ancient kerogens, pyrolysis is increasingly being used to understand Holocene systems as well. The ability of this technique to distinguish between types of preserved organic matter is useful in characterizing climatic evolution, particularly in systems sensitive to climatic fluctuation such as isolated fens and bogs. Cores collected from the Tokewanna and Garden Basin Cattail fens in central/eastern Utah exhibit variability of organic source, with the mixture of terrestrial and algal sources varying through time, as shown through the hydrogen index (HI) and oxygen index pyrolysis parameters. A sediment core was collected at each fen, and 176 samples were taken from the cores at 6‐cm intervals. Total organic carbon (TOC) for all samples ranges from 1.3 to 44.2%, with an average of 18.2% TOC. Samples range from 84 to 687 HI, equivalent to Type I (lacustrine algal) to Type III (terrestrial) organic material (OM). Variability in HI response represents mixing of the two OM sources, and the relative amount of aqueous organic input can be estimated through time based on age‐calibrated HI curves at the two sites. The balance of organic input serves as an accurate, high‐resolution proxy for climate, and calibration with palynological data near both sites confirms patterns shown by pyrolysis, showing the utility of this method in quickly, affordably and accurately characterizing Holocene sediments for use in understanding palaeoclimate.     

Turbidity Maximum Entrapment of Phytoplankton in the Chesapeake Bay

Estuarine turbidity maxima (ETM) play an important role in zooplankton and larval fish productivity in many estuaries. Yet in many of these systems, little is known about the food web that supports this secondary production. To see if phytoplankton have the potential to be a component of the ETM food web in the Chesapeake Bay estuary a series of cruises were carried out to determine the biomass distribution and floral composition of phytoplankton in and around the ETM during the winter and spring using fluorometry, high-performance liquid chromatography (HPLC), and microscopy. Two distinct phytoplankton communities were observed along the salinity gradient. In lower salinity waters, biomass was low and the community was composed mostly of diatoms, while in more saline waters biomass was high and the community was composed mostly of mixotrophic dinoflagellates, which were often concentrated in a thin layer below the pycnocline. Phytoplankton biomass was always low in the ETM, but high concentrations of phytoplankton pigment degradation products and cellular remains were often observed suggesting that this was an area of high phytoplankton mortality and/or an area where phytoplankton derived particulate organic matter was being trapped. These results, along with a box model analysis, suggest that under certain hydrodynamic conditions phytoplankton derived organic matter can be trapped in ETM and potentially play a role in fueling secondary production.     

Organic matter and deposition conditions of the Kashpir oil shales

The composition of organic matter was investigated in the oil shales and country rocks of the Kashpir deposit. The analysis of the aromatic fraction of bitumen showed the presence of isorenieratene derivatives, which indicates the accumulation of the sequence under anoxic conditions in the bottom waters of a paleobasin. Special attention was given to the composition of organosulfur compounds from the bitumen of rocks and products of kerogen pyrolysis. The concentrations of hydrocarbon structures occurring in the bitumen in a free state and in sulfur-bearing derivatives are comparable. The composition of the pyrolysis products of kerogen depends on the concentration of organic carbon in the rock: carbon-rich rock varieties contain kerogen whose pyrolysis yields relatively high concentrations of organosulfur compounds and low total contents of n-alkanes/n-alkenes-1.     

Distributions of uronic acids and O-methyl sugars in sinking and sedimentary particles in two coastal marine environments

Although recent research has indicated that bacteria may contribute an important fraction of biochemical residues in terrestrial and marine environments, it is difficult for geochemists to identify contributions from these ubiquitous and biochemically diverse organisms. Previous studies have suggested uronic acids and O-methyl sugars may be useful indicators of microbial abundance and activity, but have been limited primarily to analyses of a small number of isolated samples. We report here comparative distributions of O-methyl sugars, uronic acids, and aldoses in sediment trap material and sediments from Dabob Bay, WA and nearby Saanich Inlet, BC, where temporal and spatial trends may be used together with well-established patterns in other biochemicals to identify bacterial contributions against the background of other carbohydrate sources.O-methyl sugars and uronic acids were important contributors to the overall flux and burial of polysaccharide material in Dabob Bay and Saanich Inlet, composing ≤12 wt% of the total carbohydrate yields from sediment trap and sediment samples. O-methyl sugars accounted for an average of 5% of the carbohydrate yields from sediment trap materials and sediments, but were found rarely and only in low abundance in vascular plant tissues, phytoplankton, and kelp. In contrast, uronic acids were abundant products of sediment trap material and sediments, as well as vascular plant tissues, where in some cases they predominated among all carbohydrates. Uronic acid abundance in sediment trap material averaged 3% and ranged to >6% of total carbohydrate yields.The persistence of total minor sugar yields in water column collections from Dabob Bay throughout the seasonal cycle indicated they had a primary source that was not directly related to plankton bloom cycles nor pulsed inputs of vascular plant remains. Subsurface maxima in total minor sugar yields (and several individual components) within sediment cores from both sites indicate in situ sedimentary sources. Taken together, the observed environmental distributions strongly suggest that the minor sugar abundances in Dabob Bay and Saanich Inlet were controlled by in situ microbial production.     

Amino acid composition of suspended particles,sediment-trap material,and benthic sediment in the Potomac Estuary

Sediment trap deployments in estuaries provide a method for estimating the amount of organic material transported to the sediments from the euphotic zone. The amino acid composition of suspended particles, benthic sediment, and sediment-trap material collected at 2.4 m, 5.8 m, and 7.9 m depths in the Potomac Estuary was determined in stratified summer waters, and in well-mixed oxygenated waters (DO) in late fall. The total vertical flow, or flux, of material into the top traps ranged from 3 g m^?2 d^?1 in August to 4.9 g m^?2 d^?1 in October. The carbon and nitrogen fluxes increased in the deepest traps relative to the surface traps during both sampling periods, along with that of the total material flux (up to 47.3 g m^?2 d^?1 in the deepest trap), although the actual weight percent of organic carbon and organic nitrogen decreased with depth. Amino acid concentrations ranged from 129 mg g^?1 in surface water particulate material to 22 mg g^?1 in particulate material in 9-m-deep waters and in the benthic sediment. Amino acid concentrations from 2.4-mg-depth sediment traps averaged 104±29 mg g^?1 in stratified waters and 164±81 mg g^?1 in well-mixed waters. The deep trap samples averaed, 77.3±4.8 mg g^?1 amino acids in summer waters and 37±16 mg g^?1 in oxygenated fall waters. Amino acids comprised 13% to 39% of the organic carbon and 12% to 89% of the orgnaic nitrogen in these samples. Analysis of the flux results suggest that resuspension combined with lateral advection from adjacent slopes can account for up to 27% of the material in the deep traps when the estuary was well-mixed and unstratified. When the estuary was stratified in late summer, the amino acid carbon produced by primary productivity in the euphotic zone decreased by 85% (86% for total organic carbon) at the pycnocline at 6 m depth, leaving up to 15% of the vertical organic flux available for benthic sediment deposition.     

Sedimentary organic matter and source rock potential of the Paleocene Aaliji Formation in Qumar Oil Field,NE Iraq

The organic matter content of the Paleocene Aaliji Formation has been studied from the Qm-1 well in the Qumar Oil Field, NE Iraq. A palynofacies analysis revealed the obvious domination of amorphous organic matter (AOM) in the samples studied in addition to the different ratios of palynomorphs, phytoclasts and opaque organic material. The deposition of the various percentages of organic matter components and types of palynomorphs appear to have occurred in a proximal suboxic–anoxic basinal environment. The samples analysed showed relatively low percentages of total organic carbon, indicating a generally poor source rock. The thermal alteration index for the palynomorphs (dinoflagellates) observed and identified ranged between 2, 2+ and 3?, indicating an immature to early stage of maturity for the section studied. No clear differentiation between the stages of maturity within the section was identified. The reflectance measured for a few vitrinite particles at a depth of 2,900 m showed uncertainly of 0.46 % Ro, indicating a still thermally immature stage. The values obtained from pyrolysis analysis also supported the formation being in the early stages of maturity, i.e. an immature condition, with T _max values between 416 and 435 °C. The quality of the organic matter examined and analysed appeared to be mostly type III gas-prone kerogen, as discerned from the hydrogen index, oxygen index and other pyrolysis parameters. The parameters obtained and calculated from gas chromatography analysis performed on a sample at a depth of 2,900 m found marginally mature marine source organic matter.     

Generation of aliphatic acid anions and carbon dioxide by hydrous pyrolysis of crude oils

Two crude oils with relatively high (0.60 wt%) and low (0.18 wt%) oxygen contents were heated in the presence of water in gold-plated reactors at 300°C for 2348 h. The high-oxygen oil was also heated at 200°C for 5711 h. The compositions of aqueous organic acid anions of the oils and of the headspace gases were monitored inn order to investigate the distribution of organic acids that can be generated from liquid petroleum.The oil with higher oxygen content generated about five times as much organic anions as the other oil. The dominant organic anions produced were acetate, propionate and butyrate. Small amounts of formate, succinate, methyl succinate and oxalate were also produced. The dominant oxygen-containing product was CO₂, as has been observed in similar studies on the hydrous pyrolysis of kerogen. These results indicate that a significant portion (10–30%) of organic acid anions reported i be generated by thermal alteration of oils in reservoir rocks. The bulk of organic acid anions present in formation waters, however, is most likely generated by thermal alteration of kerogen in source rocks. Kerogen is more abundant than oil in sedimentary basins and the relative yields of organic acid anions reported from the hydrous pyrolysis of kerogen are much higher than the yields obtained for the two oils.     

The Bioavailability of Effluent-derived Organic Nitrogen along an Estuarine Salinity Gradient

Total maximum daily loads for nitrogen (N) are currently being established for the Chesapeake Bay watershed. While we know inorganic N is bioavailable in the environment and therefore its input contributes to cultural eutrophication, the bioavailability of organic N is unclear. Using bioassay experiments, we examined the impact of effluent-derived organic nitrogen (EON) from wastewater treatment plants on natural water samples collected along an estuarine/salinity gradient within the lower Chesapeake Bay watershed. All of the inorganic N and between 31% and 96% of the EON was removed during biotic bioassays within the first 2 days. Further, there was substantial abiotic reactivity of effluent N when it was added to natural water samples. Results demonstrate that organic and inorganic N in effluent is removed to support the growth of microbial communities. These are the first results aimed at assessing the reactivity of EON in natural waters along an estuarine/salinity gradient.     

Hypoxia and salinity in Virginia estuaries

Hypoxia, periods of reduced dissolved oxygen concentrations, has been observed not only in the Chesapeake Bay but also in the deeper waters of the Virginia estuaries that are tributaries to the Chesapeake Bay. When water temperature exceeded 20°C, minimum oxygen concentrations were observed to be <50% of saturation concentrations in 75%, 50% and 2% of the surveys in the estuaries of the Rappahannock, York and James rivers, respectively. The observation that hypoxia rarely occurred in the James River is surprising, given the fact that it receives the greatest amount of wastewater. Analysis of the oxygen budgets in these estuaries indicates that the variations in the frequency, duration, and severity of hypoxia are related to the net movement of bottom waters. This relationship has significant implications for the management of water quality and marine fisheries.     

Identification of important primary producers in a Chesapeake Bay tidal creek system using stable isotopes of carbon and sulfur

The use of multiple stable isotopes in the study of trophic relationships in temperate estuaries has usually been limited to euhaline systems, in which phytoplankton, benthic microalgae, andSpartina alterniflora are major sources of organic matter for consumers. Within large estuaries such as Chesapeake Bay, however, many species of consumers are found in the upper mesohaline to oligohaline portions. These lower salinity wetlands have a greater abundance of macrophytes that use C3 photosynthesis to fix carbon, in addition toS. alterniflora, which fixes carbon via the C4 photosynthetic pathway. In a broad survey of the biota and sediments of a brackish tidal creek tributary to Chesapeake Bay, combined δ¹³C and δ³⁴S measurements disclosed a balanced contribution to secondary production from phytoplankton, C3 macrophytes,Spartina sp., and benthic microalgae. Surface sediment δ¹³C suggested that the organic matter from C3 plants was derived both from allochthonous sources (terrestrial runoff) and from autochthonous production (marsh macrophytes). Unlike most estuarine systems studied to date, which are dominated by algae (phytoplankton and benthic microalgae) and C4 macrophytes, C3 plants are of greater importance in the diets of consumers in this low-salinity creek system.     

Dispersion and cycling of organic matter from the Sepik River outflow to the Papua New Guinea coast as determined from biomarkers

The Sepik River is a major contributor of water, sediment and associated organic loads to the coastal waters of northern New Guinea. With a narrow continental shelf and strong coastal currents, much of this exported material is available for long distance transport into the Bismarck Sea and beyond. CTD casts and associated instrument data showed that the river signature was visible in optical measurements of deep profiles from the Sepik Canyon. Discrete water samples were collected in clean Niskin bottles for organic analysis. Additional high volume samples for lipid classes were collected with Infiltrex samplers deployed on a floating mooring, along with two sediment trap arrays set at 100 and 260 m depth. The Infiltrex samplers were set at 55, 180, 200 and 220 m depth in an effort to target the surface layers and those of westward flowing water in the New Guinea Coastal Undercurrent (NGCUC). The samplers allowed analysis of lipid classes in both dissolved and particulate phases.Analyses of lignin phenols, hydrocarbons, fatty acids, sterols, and n-alcohols in the samples were used to estimate the cycling of the organic input from the river system to the coastal waters and possible entrainment in the NGCUC. The molecular biomarkers confirmed the Sepik River as a significant source of reduced carbon in the near coastal zone influenced by the plume. They indicated that ca. 90% of the organic carbon delivered to the coastal zone is dispersed or degraded, with only about 10% of terrestrial carbon being deposited on to the narrow shelf of coastal sediments. The dissolved lignin phenols provided evidence for desorption of organics from particles with increasing depth. However, this desorption process did not change the C/N ratio of the dissolved organic carbon (DOC) in the deep water of the Sepik Canyon. The hydrocarbons, sterols and fatty acids showed the change from lipid content dominated by phytoplankton in surface layers to zooplankton and bacterial biomarkers in deeper waters. The organic biomarkers provided evidence that some of the dissolved organic input from the Sepik River was injected into fast moving undercurrents. However, concentrations of terrestrial biomarkers were diluted and/or degraded to non-detectable levels within 100 km of the source.     

Hydropyrolysis of insoluble carbonaceous matter in the Murchison meteorite: new insights into its macromolecular structure

The major organic component of carbonaceous chondrites is a solvent-insoluble, high molecular weight macromolecular material that constitutes at least 70% of the total organic content in these meteorites. Analytical pyrolysis is often used to thermally decompose macromolecular organic matter in an inert atmosphere into lower molecular weight fragments that are more amenable to conventional organic analytical techniques. Hydropyrolysis refers to pyrolysis assisted by high hydrogen gas pressures and a dispersed catalytically-active molybdenum sulfide phase. Hydropyrolysis of meteorites has not been attempted previously although it is ideally suited to such studies due to its relatively high yields. Hydropyrolysis of the Murchison macromolecular material successfully releases significant amounts of high molecular weight PAH including phenanthrene, carbazole, fluoranthene, pyrene, chrysene, perylene, benzoperylene and coronene units with varying degrees of alklyation. Analysis of both the products and residue from hydropyrolysis reveals that the meteoritic organic network contains both labile (pyrolysable) and refractory (nonpyrolysable) fractions. Comparisons of hydropyrolysis yields of Murchison macromolecular materials with those from terrestrial coals indicate that the refractory component probably consists of a network dominated by at least five- or six-ring PAH units cross-linked together.     

Kerogen: chemical structure and formation conditions

The available data on the composition of the pyrolysis products of kerogen from the Mesozoic carbonaceous strata of the Russian Plate evidence that changes in the contents of total organic carbon (TOC) lead to a regular change of the mechanisms of organic-matter (OM) conservation in sediments. Each mechanism prevails for particular TOC contents. The initial increase in the TOC content of rocks is accounted for by the fact that the higher is the biologic productivity of the basin, the higher is the portion of nonmineralized organic matter. This is due mainly to the mechanism of selective accumulation of the most stable biochemical components such as algaenan. The appearance of H₂S first in the pore waters of sediment and then in the water column increases the degree of preservation of initial OM at the expense of its sulfurization. This process runs first in the lipid and then in the carbohydrate fractions of initial OM.     

The origin, deposition, and alteration of the organic material in Green River shale

Characteristics of the organic material in Green River Formation shale suggest that this material was derived from algae that grew in ancient Eocene lakes. The basin depressions for these nonmarine lakes were formed by the uplifting of the Rocky Mountain ranges. Large quantities of soluble salts flowed into these basins from the mountain streams, increasing the salinity of the lake waters until they became chemically stratified. In the upper, relatively fresh-water section of the lakes, abundant quantities of microscopic algae and other biota grew. Lesser amounts of terrestrial plant life remains were supplied in the form of wind- or water-borne spores and in the form of water-soluble organic materials entering the in-flowing waters. The lower section of the lake waters became highly reducing and stagnant because of lack of seasonal oxidative turnovers, thus providing ideal conditions for the accumulation and preservation of the organic debris from the lake biota.Precipitation of mineral carbonates and silicates from the highly saline waters provided most of the minerals that were co-deposited with the organic matter. Carbon dioxide, which evolved from the aquatic plant life, probably influenced mineral carbonate precipitation. During the life span of a few million of years the characteristics of the lake waters varied considerably and had significant effect upon the composition of both the inorganic and organic constituents of the Green River shale.Sixty to seventy-five core samples from each of three basins of the Green River Formation were analyzed for changes in both the organic and inorganic constituents. Considerable differences in the organic components of the soluble bitumen and of the insoluble kerogen were evident. Some of these differences appeared to be related to depth of burial and some to source material and the environment of the lake waters. Compositional differences relative to lithologic differences in the sediments of the three basins were found.     

Lignin pyrolysis products: Their structures and their significance as biomarkers

Pyrolysis in combination with gas chromatography and mass spectormetry was used to characterize softwood, hardwood and grass lignins as well as the corresponding synthetic dehydro-polymers. The method permitted differentiation of the three types of lignins. Softwood lignins yielded exclusively guaiacyl derivatives, coniferaldehyde and coniferyl alcohol being major compounds. Hardwood lignins gave rise to guaiacyl and syringyl derivatives, among which syringaldehyde, coniferyl alcohol and sinapyl alcohol were the most prominent. Grass lignins, represented by bamboo lignin, yielded p-vinylphenol as major compound. In addition, other guaiacyl and syringyl pyrolysis products were identified. The results indicate that guaiacyl and syringyl compounds are unique pyrolysis products of lignins and woods. Because of the relatively high resistance of lignins these pyrolysis products can be considered as characteristic biomarkers for terrestrial plant input.     

Hydrothermal pyrolysis of organic matter in Riphean mudstone

The catagenesis of organic matter (OM) was modeled by the hydrothermal pyrolysis of a source rock (Riphean mudstone from eastern Siberia). Isothermal experiments 72-h long were carried out in an aqueous environment in autoclaves at temperatures of 300, 310, 320,..., 370°. The pyrolysis products were analyzed for yield of extract, organic carbon, and parameters of Rock-Eval pyrolysis. The amount of the generated liquid hydrocarbon (HC) compounds increased to a temperature of 340°C and then decreased. The experimental trend of the hydrogen index (HI) dependence on the T _Max temperature generally coincided with that for natural OM maturation. The carbon isotopic composition of the insoluble (in organic solvents) OM remained practically unchanged in the course of the experiments. The carbon structure of the solid remnants of the experimental samples was ordered (after the experiments) with the origin of turbostratic graphite with a spacing of d ₀₀₂≈3.5 A°. We also conducted pyrolysis in a diamond anvil cell equipped with a digital camera in order to obtain additional qualitative and quantitative information on oil generation and emigration in the source rock and isolated kerogen. Chemical kinetic parameters of kerogen cracking were calculated for pyrolysis in an open system. The extrapolation of the high-temperature experimental results is discussed with reference to natural OM maturation.     

Predictive isotopic biogeochemistry: hydrocarbons from anoxic marine basins

Carbon isotopic compositions were determined for individual hydrocarbons in water column and sediment samples from the Cariaco Trench and Black Sea. In order to identify hydrocarbons derived from phytoplankton, the isotopic compositions expected for biomass of autotrophic organisms living in surface waters of both localities were calculated based on the concentrations of CO2(aq) and the isotopic compositions of dissolved inorganic carbon. These calculated values are compared to measured delta values for particulate organic carbon and for individual hydrocarbon compounds. Specifically, we find that lycopane is probably derived from phytoplankton and that diploptene is derived from the lipids of chemoautotrophs living above the oxic/anoxic boundary. Three acyclic isoprenoids that have been considered markers for methanogens, pentamethyleicosane and two hydrogenated squalenes, have different delta values and apparently do not derive from a common source. Based on the concentration profiles and isotopic compositions, the C31 and C33 n-alkanes and n-alkenes have a similar source, and both may have a planktonic origin. If so, previously assigned terrestrial origins of organic matter in some Black Sea sediments may be erroneous.