Sources and preservation of organic matter in Plum Island salt marsh sediments (MA, USA): long-chain n-alkanes and stable carbon isotope compositions

Elemental (TOC, TN, C/N) and stable carbon isotopic (δ¹³C) compositions and n-alkane (nC_16–38) concentrations were measured for Spartina alterniflora, a C₄ marsh grass, Typha latifolia, a C₃ marsh grass, and three sediment cores collected from middle and upper estuarine sites from the Plum Island salt marshes. Our results indicated that the organic matter preserved in the sediments was highly affected by the marsh plants that dominated the sampling sites. δ¹³C values of organic matter preserved in the upper fresh water site sediment were more negative (−23.0±0.3‰) as affected by the C₃ plants than the values of organic matter preserved in the sediments of middle (−18.9±0.8‰) and mud flat sites (−19.4±0.1‰) as influenced mainly by the C₄ marsh plants. The distribution of n-alkanes measured in all sediments showed similar patterns as those determined in the marsh grasses S. alterniflora and T. latifolia, and nC₂₁ to nC₃₃ long-chain n-alkanes were the major compounds determined in all sediment samples. The strong odd-to-even carbon numbered n-alkane predominance was found in all three sediments and nC₂₉ was the most abundant homologue in all samples measured. Both δ¹³C compositions of organic matter and n-alkane distributions in these sediments indicate that the marsh plants could contribute significant amount of organic matter preserved in Plum Island salt marsh sediments. This suggests that salt marshes play an important role in the cycling of nutrients and organic carbon in the estuary and adjacent coastal waters.     

Transport and fate of Danube Delta terrestrial organic matter in the Northwest Black Sea mixing zone

Within the framework of the European project EROS 21, a biogeochemical study of particles transported from the Danube Delta to the Northwestern Black Sea whose carbon cycle is dominated by riverine inputs was carried out in spring off the Sulina branch of the Danube Delta. The distribution of particulate organic carbon (POC), chlorophyll a (Chl a), C/N, and δ¹³C evidenced an omnipresent contribution of terrestrial organic matter throughout the study area together with a dilution of these inputs by freshwater and marine organisms. Four lipid series, n-alkanoic acids, n-alkanes, n-alkanols, and sterols were analyzed by gas chromatography and gas chromatography/mass spectrometry. Several signature compounds were selected to delineate dispersion of terrestrial organic carbon: (1) long-chain n-alkanoic acids in the range C₂₄–C₃₄, long-chain n-alkanes in the range C₂₅–C₃₅, long-chain n-alkanols in the range C₂₂–C₃₀, 24-ethylcholesta-5,22-dien-3β-ol (29Δ^5,22) and 24-ethylcholesterol (29Δ⁵) for vascular plant-derived material and (2) coprostanol (27Δ^0,5β) for faecal contamination associated with sewage effluents. A marked decrease was observed between the concentrations of different vascular plant markers characterizing the two end members: riverine at salinity 0.3 and marine at salinity 15.5. The decrease observed for marine/riverine end members (expressed as a function of organic carbon) varied in a large range, from 4% for n-alkanes to 18.6%, 20.4% and 24% for n-fatty acids, n-alkanols and sterols, respectively. These values reflect a combination of various processes: size-selective particle sedimentation, resuspension of different particle pools of different sizes and ages, and/or selective biological utilization. The multi-marker approach also suggested the liberation in the mixing zone of terrestrial moieties, tightly trapped in macromolecular structures of the riverine material. The greatest decrease for marine/riverine end members was observed for coprostanol (0.9%), underlining the efficiency of the mixing zone as a sink for sewage-derived carbon.     

Contrasting chemical and isotopic compositions of organic matter in Changjiang (Yangtze River) estuarine and East China Sea shelf sediments

To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0–2 cm) were 5–10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C₂₆-C₃₃ long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC₁₅ to nC₂₂ compounds. Long-chain (>C₂₀) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (<C₂₀) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk δ ¹³C_TOCTOC. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.     

Fluxes,source and transport of organic matter in the western Sea of Okhotsk: Stable carbon isotopic ratios of n-alkanes and total organic carbon

Settling particles and surface sediments collected from the western region of the Sea of Okhotsk were analyzed for total organic carbon (TOC), long-chain n-alkanes and their stable carbon isotope ratio (δ¹³C) to investigate sources and transport of total and terrestrial organic matter in the western region of the sea. The δ¹³C measurements of TOC in time-series sediment traps indicate lateral transport of resuspended organic matter from the northwestern continental shelf to the area off Sakhalin via the dense shelf water (DSW) flow at intermediate depth. The n-alkanes in the surface sediments showed strong odd carbon number predominance with relatively lighter δ¹³C values (from −33‰ to −30‰). They fall within the typical values of C3-angiosperms, which is the main vegetation in east Russia, including the Amur River basin. On the other hand, the molecular distributions and δ¹³C values of n-alkanes in the settling particles clearly showed two different sources: terrestrial plant and petroleum in the Sea of Okhotsk. We reconstructed seasonal change in the fluxes of terrestrial n-alkanes in settling particles using the mixing model proposed by Lichtfouse and Eglinton [1995. ¹³C and ¹⁴C evidence of a soil by fossil fuel and reconstruction of the composition of the pollutant. Organic Geochemistry 23, 969–973]. Results of the terrestrial n-alkane fluxes indicate that there are two transport pathways of terrestrial plant n-alkanes to sediments off Sakhalin, the Sea of Okhotsk. One is lateral transport of resuspended particles with lithogenic material from the northwestern continental shelf by the DSW flow. Another is the vertical transport of terrestrial plant n-alkanes, which is independent of transport of lithogenic material. The latter may include dry/wet deposition of aerosol particles derived from terrestrial higher plants possibly associated with forest fires in Siberia.     

The sources and accumulation rate of sedimentary organic matter in the Pearl River Estuary and adjacent coastal area, Southern China

In this study, the contents, sources and accumulation rate of sedimentary organic matter (OM) in the Pearl River Estuary (PRE) and adjacent coastal area were investigated. The stable carbon isotopic composition (δ¹³C) is a reliable geochemical proxy and was used to indicate the OM origin here. Nevertheless, the organic carbon and nitrogen molar ratios (TOC/TN) and the stable nitrogen isotopic composition (δ¹⁵N) were affected by diagenesis and could be the supplementary indicators. The sources of OM were estimated based on the two end-member model. The results showed that in the estuary, sedimentary OM originated from terrestrial and aquatic mixing origins, whereas, OM in coastal sediments was dominantly algae-derived. The accumulation rate of sedimentary OM was analyzed based on ²¹⁰Pb dating. Due to the sampling sites and the distinct hydraulic environments, the accumulation rates of TOC, aquatic and terrestrial OC were obviously higher in the estuary than in coastal area. TOC accumulation rates were 18–27 mg cm⁻² y⁻¹ in the estuary, and 0.84–3.6 mg cm⁻² y⁻¹ in coastal area. Aquatic OC accumulation rates were 7.9–11.3, 0.8–1.3, and 2.6–3.1 mg cm⁻² y⁻¹, and terrestrial OC accumulation rates were 9.7–16.3, 0.02–0.14, 0.16–0.42 mg cm⁻² y⁻¹ in cores 2, 5, 6, respectively. It could be seen from the high accumulation rate of organic matter in the estuary that, when nutrients increased in the river, phytoplankton biomass and productivity would also have increased. As a result, phytoplankton sinking and organic matter sedimentation usually increased with primary productivity, resulting in the observed accumulation rate of aquatic OC in the estuary. Furthermore, terrestrial OC accumulation rates in the estuary and coastal area showed an increasing trend with the age.     

Sources and distribution of organic matter in a river-dominated estuary (Winyah Bay, SC, USA)

The sources and distribution of organic matter (OM) in surface waters and sediments from Winyah Bay (South Carolina, USA) were investigated using a variety of analytical techniques, including elemental, stable isotope and organic biomarker analyses. Several locations along the estuary salinity gradient were sampled during four different periods of contrasting river discharge and tidal range. The dissolved organic carbon (DOC) concentrations of surface waters ranged from 7 mg l⁻¹ in the lower bay stations closest to the ocean to 20 mg l⁻¹ in the river and upper bay samples. There was a general linear relationship between DOC concentrations and salinity in three of the four sampling periods. In contrast, particulate organic carbon (POC) concentrations were significantly lower (0.1–3 mg l⁻¹) and showed no relationship with salinity. The high molecular weight dissolved OM (HMW DOM) isolated from selected water samples collected along the bay displayed atomic carbon:nitrogen ratios ([C/N]a) and stable carbon isotopic compositions of organic carbon (δ¹³C_OC) that ranged from 10 to 30 and from −28 to −25‰, respectively. Combined, such compositions indicate that in most HMW DOM samples, the majority of the OM originates from terrigenous sources, with smaller contributions from riverine and estuarine phytoplankton. In contrast, the [C/N]a ratios of particulate OM (POM) samples varied significantly among the collection periods, ranging from low values of 5 to high values of >20. Overall, the trends in [C/N]a ratios indicated that algal sources of POM were most important during the early and late summer, whereas terrigenous sources dominated in the winter and early spring.In Winyah Bay bottom sediments, the concentrations of the mineral-associated OM were positively correlated with sediment surface area. The [C/N]a ratios and δ¹³C_OC compositions of the bulk sedimentary OM ranged from 5 to 45 and from −28 to −23‰, respectively. These compositions were consistent with predominant contributions of terrigenous sources and lesser (but significant) inputs of freshwater, estuarine and marine phytoplankton. The highest terrigenous contents were found in sediments from the river and upper bay sites, with smaller contributions to the lower parts of the estuary. The yields of lignin-derived CuO oxidation products from Winyah Bay sediments indicated that the terrigenous OM in these samples was composed of variable mixtures of relatively fresh vascular plant detritus and moderately altered soil OM. Based on the lignin phenol compositions, most of this material appeared to be derived from angiosperm and gymnosperm vascular plant sources similar to those found in the upland coastal forests in this region. A few samples displayed lignin compositions that suggested a more significant contribution from marsh C₃ grasses. However, there was no evidence of inputs of Spartina alterniflora (a C₄ grass) remains from the salt marshes that surround the lower sections of Winyah Bay.     

Terrestrial n-alkane signatures in the middle Okinawa Trough during the post-glacial transgression: control by sea level and paleovegetation confounded by offshore transport

We report records of land plant-derived long-chain n-alkanes since 37 ka b.p. from a sediment core in the middle Okinawa Trough, East China Sea. The data show the content and carbon preference index (CPI; degree of freshness) of n-alkanes generally decreasing as sea level rose, which could be explained by coastline retreat resulting in an increased transport route of n-alkanes toward the study site. The n-alkane CPI returned to higher values during the Holocene highstand, however, suggesting sea-level rise was not the only cause for the decline of freshness of n-alkanes. Paleovegetation changes in terms of C₃ vs. C₄ contributions inferred from n-alkane δ¹³C are overall consistent with published marine and terrestrial records during two distinct intervals: from 37.0 to 15.2 ka b.p., and from 7.6 ka b.p. to the present. However, n-alkane δ¹³C excursions from 15.2 to 7.6 ka b.p. are difficult to reconcile with terrestrial signatures. This disagreement, along with other possible causes for the decline of freshness of n-alkanes, and the higher-energy sedimentary environment inferred from increased mean grain sizes and silt/clay ratios during this time period, is consistent with existing knowledge of offshore transport of materials previously stored on the extensive continental shelf during the post-glacial transgression. We therefore suggest that n-alkane records from the Okinawa Trough should be used only cautiously to infer deglacial vegetation and sea-level changes.     

Controls on carbon cycling in two contrasting temperate zone estuaries: The Tyne and Tweed, UK

In order to evaluate the respiration–photosynthesis dynamics in two contrasting North Sea estuaries, pH, temperature, alkalinity, chlorophyll-a (chl-a), and isotopic ratios of dissolved inorganic carbon (δ¹³C_DIC) and dissolved oxygen (δ¹⁸O_DO) were measured in the Tyne (July 2003) and Tweed (July 2003 and December 2003) estuaries. Using a concentration-dependent isotope mixing line, δ¹³C_DIC values in the Tweed (July 2003) demonstrated mostly conservative behaviour across the estuary, reflecting mixing between riverine and marine sources, although some samples were slightly more ¹³C-enriched than predicted δ¹³C_DIC values. Low pCO₂ (less than 2 times atmospheric pressure) and ¹⁸O-depleted δ¹⁸O_DO signatures below equilibrium with the atmosphere provided further evidence for net autotrophy in the Tweed estuary in summer 2003. Conversely, in the Tyne during the summer and in the Tweed during the winter higher pCO₂ (up to 6.5 and 14.4 times atmospheric partial pressure in the Tweed and Tyne, respectively), slightly ¹³C-depleted δ¹³C_DIC and ¹⁸O-enriched δ¹⁸O_DO values indicated heterotrophy as the dominant process. The relatively large releases of CO₂ observed during these two estuarine surveys can be attributed to significant oxidation of terrigenous organic matter (OM). This study therefore demonstrates the usefulness of combined δ¹⁸O_DO and δ¹³C_DIC isotopes in examining the relationship between respiration–photosynthesis dynamics and the fate of terrestrially derived OM during estuarine mixing.     

Oceanographic controls on the carbon isotopic compositions of sinking particles from the Cariaco Basin

This study examined the relationship between carbon isotopic composition of sinking organic matter (OM) and the biological, physical and chemical properties of the surface ocean in the Cariaco Basin. The ¹³C/¹²C ratio of OM (δ¹³C_org) in sinking particles was determined on sediment trap samples from four depths collected from 1996 to 1999 as part of the CArbon Retention In A Colored Ocean time series. Water column properties, including temperature, productivity, chlorophyll and concentration of dissolved CO₂, were concurrently measured on monthly cruises. The δ¹³C_org varied from a high of –17.7‰ to a low of –22.6‰ during the study period. The variation of the δ¹³C_org throughout seasonal cycles was directly proportional to the strength of upwelling and was negatively correlated with temperature (r²=0.64). During the 1996–1997 upwelling event, the strongest during the study period, the δ¹³C_org increased by 4.4‰ whereas during the 1998–1999 upwelling event, the weakest during the study period, the δ¹³C_org only increased by 3.3‰. Contrary to most previous studies, we observed a negative relationship (r²=0.53) between [CO_2 aq] and the estimated isotopic fractionation factor (ε_p). However, there was no correlation between ε_p and the calculated growth rates indicating that there was non-diffusive uptake of carbon into phytoplankton cells. It thus appears that [CO_2 aq] does not control the δ¹³C_org in the water column of the study site. The best explanation for the isotopic enrichment observed is a carbon concentrating mechanism (CCM) in phytoplankton. The existence of a CCM in phytoplankton has major implications for the interpretation of the δ¹³C_org in the Cariaco Basin.     

The trophic ecology of key megafaunal species at the Pakistan Margin: Evidence from stable isotopes and lipid biomarkers

The Arabian Sea is subject to intense seasonality resulting from biannual monsoons, which lead to associated large particulate fluxes and an abundance of organic carbon, a potential food source at the seafloor for benthic detritivores. We used the stable isotopes of carbon and nitrogen alongside lipid analyses to examine potential food sources (particulate and sedimentary organic matter, POM and SOM respectively) in order to determine trophic linkages for the twelve most abundant megafaunal species (Pontocaris sp., Solenocera sp., Munidopsis aff. scobina, Actinoscyphia sp., Actinauge sp., Echinoptilum sp., Pennatula aff. grandis, Astropecten sp. Amphiura sp. Ophiura euryplax, Phormosoma placenta and Hyalinoecia sp.) at the Pakistan Margin between 140 and 1400 m water depth. This transect spans a steep gradient in oxygen concentrations and POM flux. Ranges of δ¹³C and δ¹⁵N values were narrow in POM and SOM (4‰ and 2‰ for δ¹³C and δ¹⁵N, respectively) with little evidence of temporal variability. Labile lipid compounds in SOM originating from phytoplankton did exhibit seasonal change in their concentrations at the shallowest sites, 140 and 300 m. Benthic megafauna had broad ranges in δ¹³C and δ¹⁵N (>10‰ and >8‰ for δ¹³C and δ¹⁵N, respectively) suggesting they occupy several trophic levels and utilize a variety of food sources. There is evidence for feeding niche separation between and within trophic groups. Lipid biomarkers in animal tissues indicate a mixture of food sources originating from both phytoplankton (C_20:5(n-3) and C_22:6(n-3)) and invertebrate prey (C_20:1 and C_22:1). Biomarkers originating from phytodetritus are conserved through trophic transfer to the predator/scavengers. Six species (Pontocaris sp., Solenocera sp., Actinoscyphia sp., Echinoptilum sp., Amphiura sp. and Hyalinoecia sp.) showed a significant biochemical response to the seasonal supply of food and probably adapt their trophic strategy to low food availability. Biotransformation of assimilated lipids by megafauna is evident from polyunsaturated fatty acid distributions, for example, Echinoptilum sp. converts C_20:5(n-3) to C_24:6(n-3).     

Sources and distribution of allochthonous organic matter in surface sediment from the Seomjin River to the southern inner shelf of Korea

The spatial distributions of δ¹³C, δ¹⁵N, and n-alkanes were investigated to determine the source and transportation of allochthonous organic matter from the mouth of the Seomjin River to the southern inner shelf break of Korea. Total organic carbon (%) ranged from 0.3% to 1.6% (average = 0.80%, n = 81), and the C/N ratio varied from 2.4 to 12.4 (average = 6.76, n = 81). The δ¹³C values ranged from ?25.86 to ?20.26‰ (average = ?21.47‰, n = 81), and δ¹⁵N values ranged from 4.37‰ to 8.57‰ (average = 6.72‰, n = 81). The contribution of the terrestrial fraction of organic matter to the total ranged from 4.4% to 97.7% (average = 24.4%, n = 81), suggesting higher amounts around the catchment area and lower amounts in the offshore area. The concentration of total n-alkanes (nC₂₅ ? nC₃₅) was higher at the boundary between the outer bay and inner shelf break (BOBIS). Average chain length and the carbon preference index both indicated that major leaf wax n-alkanes accounted for the observed distribution of terrestrial organic matter, and were dominant in the inner shelf break (around BOBIS) and outer shelf break. Based on the spatial distribution of the total n-alkanes and the sum of nC₂₇, nC₂₉, and nC₃₁, the terrestrial organic matter distribution was considered to be controlled by local oceanographic conditions, especially at the center of the BOBIS. In addition to enabling the distribution and source of terrestrial organic matter to be identified, the n-alkanes indicated that minor anthropogenic allochthonous organic materials were superimposed on the total organic materials in the central part of Yeosu Bay and the catchment area. The n-alkane indices revealed weathered petroleum contamination, with contamination levels being relatively low at the present time.     

长江口及邻近海域现代沉积物中正构烷烃分子组合特征及其对有机碳运移分布的指示

报道了长江口及邻近海域现代沉积物中正构烷烃的浓度及分布特征,通过因子分析法对正构烷烃来源进行了探讨.结果表明,调查站位正构烷烃主要可归纳为3种类型:陆源输入优势型(单峰群)、陆源和海洋内生混合类型(双峰群)和石油类污染类型(单峰型,不具奇偶优势).长江口邻近站位正构烷烃色谱指标的突变,是长江河口区2种不同水团造成沉积物差异的客观反映.除P4外,研究站位总正构烷烃含量(∑n-Alk)与有机碳总量(TOC)相关性良好,且长江口东南-浙江沿岸软泥区正构烷烃的陆源高等植物组分(TER-Alk)、海洋内生组分(PL-1)、奇偶碳优势指数(CPI)等指标与运移距离呈线形关系.在因子分析显示不同来源的4种正购烷烃中,以陆源烷烃输入比重最大(51.5%),在陆源烷烃中又以东海河流物质贡献最大(49.1%);根据因子负荷差异,推测东海北部沉积有机质可能多数来源于苏北沿岸及老黄河口水下三角洲,冲绳海槽区则可能大部分来源于长江及东海内陆架物质,并探讨了其运移机理.     

Stable carbon isotope distribution of particulate organic matter in the ocean: a model study

The stable carbon isotopic composition of particulate organic matter in the ocean, δ¹³C_POC, shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ¹³C_POC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ¹³C_POC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ¹³C_POC are much greater than in δ¹³C_DIC, suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ¹³C_POC. Understanding the factors that control isotope variability is a prerequisite when applying δ¹³C_POC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ¹³C_POC distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO₂ in seawater and the organic material produced by algae, _P, is a function of the ambient CO₂ concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, _P, depends on the CO₂ concentration in seawater and on the (instantaneous) growth rates, μ_i, of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ¹³C_POC distribution. The interhemispheric asymmetry of δ¹³C_POC can mostly be attributed to the interhemispheric asymmetry of CO₂ concentration in the water. However, the strong seasonal variations of δ¹³C_POC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.     

Composition of the organic matter of the water,suspended matter,and bottom sediments in Nha Trang Bay in Vietnam in the South China Sea

Research has been conducted in Nha Trang Bay (Southern Vietnam, the South China Sea) at the section from the estuary of the Cai River to the marine part of the bay, as well as in the area of coral reefs. The objects of the studies are the river and sea waters, the suspended matter, and the bottom sediments. Data on the dissolved organic carbon and the total nitrogen in the water are obtained. The organic carbon content is estimated in the suspended matter; the organic carbon and the molecular and group composition of the n-alkanes are determined in the bottom sediments. The molecular and group composition of the n-alkanes in the bottom sediments of the landfill have made it possible to identify three types of organic matter (OM): marine, mixed, and of mainly terrigenous origin. All the types of OM are closely related to the specificity of the sedimentation and the hydrodynamics of the waters in this water area.     

Cycling of dissolved and particulate nitrogen and carbon in the Framvaren Fjord, Norway: stable isotopic variations

The reaction pathways of nitrogen and carbon in the Framvaren Fjord (Norway) were studied through stable isotope analysis (δ¹⁵N and δ¹³C) of dissolved inorganic and particulate organic matter (POM). The variations in the isotopic compositions of the various C and N pools within the water column were use to evaluate the historical deposition of material to the sediments. The high δ¹⁵N-NH₄⁺ at the O₂/H₂S interface, as a consequence of microbial uptake between 19 and 25 m, results in extremely depleted δ¹⁵N-particulate nitrogen (PN) of approximately 1‰ within the particulate maximum at approximately 19 m. The carbon isotopic distribution of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) within the interface suggests that the distinct microbial flora (Chromatium sp. and Chlorobium sp.) fractionate inorganic carbon to different degrees. The extremely light δ¹³C-POC within the interface (−31‰) appears to be a result of carbon uptake by Chromatium sp. while δ¹³C-POC of −12‰ is more indicative of Chlorobium sp. Nitrogen isotopic mass balance calculations suggested that approximately 75% of the material sinking to the sediments was derived from the dense particulate maximum between 19 and 25 m. The sediment distribution of nitrogen isotopes varied from 2‰ at the surface to approximately 6‰ at 30 cm. The nitrogen isotopic variations with depth may be an indicator of the depth or position of the O₂/H₂S interface in the fjord. Low sediment δ¹⁵N indicated that the interface was within the photic zone of the water column, while more enriched values suggested that the interface was lower in the water column potentially allowing for less fractionation during biological incorporation of dissolved inorganic nitrogen. Results indicate that the dense layers of photo-autotrophic bacteria in the upper water column impart unique carbon and nitrogen isotopic signals that help follow processes within the water column and deposition to the sediments.     

Higher plant n-alkane average chain length as an indicator of petrogenic hydrocarbon contamination in marine sediments

The n-alkane average chain length (ACL) is the weight-averaged number of carbon atoms of the higher plant C₂₅–C₃₃ n-alkanes. The abundance of individual n-alkanes from higher plant sources generally increases with increasing carbon number in coastal marine sediments around Taiwan, but this trend is reversed for petrogenic hydrocarbons. The ACL would potentially be lowered if petrogenic hydrocarbons were added to sediments containing biogenic hydrocarbons alone. To test this idea, a marine environment off southwestern Taiwan known to contain both biogenic and petrogenic hydrocarbons and two nearby rivers were selected for investigating possible difference in ACL values between their sediments. The average CPI of C₂₅–C₃₃ n-alkanes was 4.08 ± 2.04 (range 1.90–8.96, n = 15) for the river sediments and 1.70 ± 0.16 (range 1.43–1.97, n = 15) for the marine sediments. The ACL of C₂₅–C₃₃ n-alkanes for river sediments ranged from 29.2 to 30.5 (average 29.9 ± 0.4), and for marine sediments from 28.4 to 29.3 (average 28.9 ± 0.3). The ACL difference between marine and river sediments was significant (Student's t test at 99% confidence) although it appeared small. It is suggested that the ACL can be an additional indicator for detection of petrogenic hydrocarbons in coastal marine sediments.     

Basin-Scale Geographic Patterns of Bacterioplankton Biomass and Production in the Subarctic Pacific, July–September 1997

Bacterial biomass and production rate were measured in the surface (0–100 m) and mesopelagic layers (100–1,000 m) in the subarctic Pacific and the Bering Sea between July–September, 1997. Depth profiles were determined at stations occupied in oceanic domains including the subarctic gyres (western, Bering Sea, and Gulf of Alaska) and a boundary region south of the gyres. In the surface layer (0–100 m), both bacterial biomass and production were generally high in the western and Bering Sea gyres, with the tendency of decrease toward east. This geographic pattern was consistent with the dominant regime of phytoplankton biomass at the time of our survey. A significant portion of variation in bacterial production was explained by the concentration of chlorophyll a (r ² = 0.340, n = 60, P < 0.001) and, to the greater extent, by the concentration of semilabile total organic carbon (SL-TOC = TOC at a given depth—TOC at 1,000 m, r ² = 0.488, n = 59, P < 0.0001). Temperature significantly improved the regression model: temperature and chlorophyll jointly explained 60% of variation in bacterial production. These results support the hypothesis that bacteiral growth is largely regulated by the combination of temperature and the supply of dissolved organic carbon in subarctic surface waters. In the mesopelagic layer (100–1,000 m), the geographic pattern of bacterial production was strikingly different from the surface phytoplankton distribution: the production was high in the boundary region where the phytoplankton biomass was lowest. Bacterial growth appeared to be largely controlled by the supply of organic carbon, as indicated by the strong dependency of bacterial production on SL-TOC (r ² = 0.753, n = 75, P < 0.0001). The spatial uncoupling between surface phytoplankton and mesopelagic bacterial production suggests that the supply rate of labile dissolved organic carbon in the mesopelagic zone does not simply reflect the magnitude of the particulate organic carbon flux in the subarctic Pacific.     

Carbon and nitrogen isotope composition of particulate organic matter in relation to mucilage formation in the northern Adriatic Sea

Carbon and nitrogen stable isotope ratios of particulate organic matter (POM) were studied approximately weekly during spring and summer 2003 and 2004 in the Gulf of Trieste (northern Adriatic Sea) in order to track the temporal variations and differences between two years. In parallel, particulate organic carbon (POC) and particulate nitrogen (PN), phytoplankton biomass (chlorophyll a), and N and P nutrients were monitored. All studied parameters, especially N and P nutrients and chlorophyll a, showed higher concentrations and larger variability in spring 2004. As a consequence the macroaggregates were produced in late spring 2004. The C and N isotope composition of POM was not directly linked to phytoplankton biomass dynamics. The δ¹³C_POC values covaried with temperature. In 2004, δ¹³C_POC variations followed the δ¹⁵N_PN values as well as the δ¹³C_DIC values which were probably more dependent on the photosynthetic use of ¹²C. Variations in δ¹⁵N_POM values were most probably the consequence of variations in N nutrient sources used in phytoplankton assimilation. The significant correlation between δ¹⁵N_PN values and nitrate concentrations in 2004 implies intense nitrate assimilation in the presence of higher nitrate concentration. This suggests nitrate as the key nutrient in the »new primary production«, later producing macroaggregates with a mean δ¹³C and δ¹⁵N values of − 19‰ and 5‰, respectively. A low fractionation factor ε, < 1‰, lower than that reported in other marine and lacustrine systems, was found probably to be a consequence of distinct phytoplankton species, i.e. several classes of autotrophic nanoflagellates, and specific growth conditions present in the Gulf of Trieste. The tentative use of C isotope composition of POM revealed a higher contribution of allochthonous organic matter in 2004 compared to 2003 due to higher riverine inflow.     

Organic geochemical and palynological evidence for terrigenous origin of the organic matter in Aegean Sea sapropel S1

Organic geochemistry and micropaleontology are used to determine the origin of sapropel S1 in the Aegean Sea. Low-molecular-weight (C₁₅, C₁₇ and C₁₉) n-alkane data show that net primary productivity (NPP) increased from ∼14,000 to 10,000 yr BP at the glacial interglacial transition, but the onset of S1 at 9600 yr BP marks a sharp decline in NPP, which remained low until ∼8200 yr BP. The start of sapropel deposition is marked by increased total organic carbon (TOC) and pollen-spore concentrations, together with increased high-molecular-weight (C₂₇, C₂₉, C₃₁ and C₃₃) n-alkanes. Pollen assemblages show large influx of tree pollen from central-northern European forests. Increases in high-molecular-weight n-alkanes suggest greater influx of fresh vascular plant material at the start of S1, although the amount is small compared to other insoluble organic matter. Palynological studies showed that most of this insoluble organic matter are flocks of dark-brown amorphous kerogen, typical of terrigenous humic compounds. From ∼8200 yr BP to the top of S1 at ∼6400 yr BP, there is a decline in high-molecular-weight n-alkanes and terrigenous kerogen, and an increase in low-molecular-weight n-alkanes, suggesting that NPP recovered during the later deposition of S1 in the Aegean Sea. The increase in low-molecular-weight n-alkanes coincides with the recovery of coccolithophores and dinoflagellates, suggesting that these phytoplankton are primarily responsible for the low-molecular-weight n-alkane variations. These data from the Aegean Sea support the model for sapropel deposition resulting from increased influx of TOC during times of stagnant bottom water, but disagree with Mediterranean models prescribing a large increase in marine productivity.     

Differential transport and degradation of bulk organic carbon and specific terrestrial biomarkers in the surface waters of a sub-arctic brackish bay mixing zone

Detailed organic geochemical analyses were performed on surface water particulate samples of the lower Kalix River and northern Bothnian Bay collected during the spring flood of 2005. Both bulk geochemical and molecular biomarker analyses indicated a predominance of terrestrially-derived particulate organic matter (POM), both of higher plant and Sphagnum origin in the low salinity zone (LSZ) of the Kalix River estuary, with an increasing contribution of marine-derived POM in the offshore Bothnian Bay basin.Two-dimensional box modeling of the mixed surface layer in the LSZ indicated that 65% of the particulate organic carbon (POC) and between 73 and 93% of the terrestrial biomarker classes analyzed (high molecular weight n-alkanes, n-alkanoic acids and n-alkanols as well as sitosterol) were degraded in the course of their weeklong transit through the inner LSZ during the spring flood. This corresponds to field-based degradation rate constants for the biomarkers of 0.5 and 2.5 day^− 1, which are similar to results reported from mesocosm experiments for related compounds. The degradation rate constant for terrestrial POC of 0.38 day^− 1 was about 20 times larger than for DOC and suggests that POC mineralization stands for 44% of the total mineralization, which is much larger than previously considered.This sub-arctic river-export regime has a geochemistry resembling that of neighboring western Russian Arctic Rivers, suggesting that a large part of the OM coastally exported from northernmost Eurasian soils may be degraded within the vicinity of the river mouths and putatively be released as carbon dioxide. The 65% degradation of terrestrial POC in the coastal surface water of this sub-arctic recipient is substantially larger than a global-average of 35% used in recent budget estimates of the fate of terrestrially-exported POC on the pan-arctic shelves. Considering ongoing and predicted changes in the Arctic Region due to global warming a more efficient degradation of river-exported terrestrial POC may have far-reaching consequences for the large-scale biogeochemical cycling of carbon in the pan-arctic region and beyond.