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1.
The sources and distributions of terrigenous organic matter (OM) were investigated in a small tropical estuary in the Hainan Island, South China. Plants, suspended particulate matter (SPM), and surface sediments samples in the estuary and coast were collected. Bulk properties [organic carbon (OC%), total nitrogen (TN%), stable carbon isotope (δ13C) and grain size] and lignin phenol concentrations were measured. OC% of mangrove plants was (43.4 ± 2.1)%, which is similar to the values reported for mangrove plants in other regions. OC% of sediment samples ranged from 0.07% to 1.42%, and they were related to the sediment texture. Lignin phenols in the sediment ranged from 5.16 mg/100 mg OC in the uppermost station to 0.51mg/100mg OC in the coast. The molar ratio of organic carbon to total nitrogen (C/N) (～7) and δ 13 C (～-31.1×10-3 ) of riverine SPM revealed that the major OM sources of riverine SPM were aquatic OM (phytoplankton and/or bacteria). Moreover, the lower lignin concentration (Λ8) and higher (Ad/Al)v of lignin phenols suggest that terrestrial OM in riverine SPM were mainly from soil. Furthermore, C/N ratio, δ13C and lignin phenols reveal that mangrove plants were the predominant OM sources of mangrove surface sediment. Based on the δ13C and lignin phenols, it can be concluded that the major OM sources in estuarine and coastal surface sediments were marine phytoplankton, riverine SPM and mangrove surface sediment. In addition, the higher (Ad/Al)v of lignin phenols in those coastal sediments indicate that seagrass might be a potential OM source in coastal sediments, however, the lower (Ad/Al)v in the estuarine sediments in turn suggests that seagrass could not be transported to the mangrove fringed region. A three-end-member model which is based on lignin concentrations and δ13C was applied to evaluate the contribution of mangroves to the organic matter preserved in the surface sediments. Around the mangrove fringed region, mangrove could contribute more than 50% to the sedimentary OM, and this value is much higher than riverine OM. Nevertheless, mangrove OM could not be efficiently transported to the coastal region. Our study suggests that mangrove forest is an important OM source in this small estuary.  相似文献   

2.
《Marine Chemistry》2007,103(1-2):172-184
Here we report on temporal changes in the concentration and composition of lignin phenols in high molecular weight (< 0.2 μm, > 1 kDa) dissolved organic matter (HMW DOM) collected from the lower Mississippi and Pearl Rivers (MR and PR) (USA). Monthly water samples were collected at a station in the lower reach in each river from August 2001 to August 2003. Significantly higher concentrations of lignin and Λ8 values (mg lignin phenols in 100 mg organic carbon) in the Pearl River than in the Mississippi River, reflected sporadic inputs of terrestrial DOM during rainstorm events from wetlands and forest soils. Larger seasonal variations in lignin concentration and composition in the Pearl River, compared to the Mississippi River, were attributed to shifts in organic matter sources from topsoil inputs during rainstorm events to groundwater inputs and in situ production during base flow in this small river. Conversely, lower Λ8 and vanillic acid to vanillin ratios [(Ad/Al)v] in the HMW DOM of the lower Mississippi River may be a result of a lower export rate of lignin from agricultural soils due to lower carbon storage in the expansive agricultural systems of the Mississippi River watershed, as well as dilution of phytoplankton DOM inputs. Large seasonal changes in lignin concentration and Λ8 (linked at times with river discharge), and minimal variability in the composition of lignin phenols, likely represented an integrated signal of soil-derived vascular inputs from the upstream drainage basin. If we are to better understand the controls of organic matter delivery to the coastal zone from both small and large rivers, sampling strategies need to be adjusted to account for the different scales of hydrologic response time and in situ processing associated with different residence times.  相似文献   

3.
《Marine Chemistry》2005,93(1):53-73
The provenance of organic matter in sediments from the Mackenzie River and Beaufort Shelf was investigated using the stable carbon and radiocarbon isotopic compositions of bulk organic matter and the stable carbon isotopic compositions of individual organic compounds, including lignin-derived phenols and lipid-derived fatty acids. Most river suspended sediments and shelf surface sediments contained organic carbon characterized by highly depleted Δ14C values that were consistent with average radiocarbon ages exceeding 7000 years. The stable carbon isotopic signatures of lignin phenols were uniformly depleted (−25≥δ13C≥−32‰), indicating the predominant contributions of C3 vascular plant sources. The isotopic compositions of C14 and C16 fatty acids exhibited important contrasts between the river (−36‰ to −40‰) and shelf (−25‰ to −29‰) sediments that were consistent with contributions from freshwater algae and/or vascular plants in the former and marine phytoplankton in the latter. Using 14C isotopic mass balance, the abundances of modern and ancient organic matter were quantitatively constrained. The fate of organic matter in the Beaufort Shelf was explored by normalizing these abundances to the specific surface area of sediments. Ancient organic carbon, which may include old pre-aged soil material as well as fossil bitumen or kerogen, accounted for the majority (∼70%) of the particulate organic matter exported by the Mackenzie River and deposited in surface sediments of the Beaufort Shelf. Modern organic carbon accounted for ∼30% in both river and shelf sediments, with significant contributions from vascular plant-derived materials in both river and shelf samples and from marine algae in the shelf sediments. Respiration (and/or leaching) of particle-bound marine organic matter dominates the carbon metabolism in the Mackenzie Delta/Beaufort Shelf region. However, land-derived pools, including modern carbon derived from vascular plants as well as ancient carbon also appeared to undergo a degree of post-depositional degradation prior to burial in the shelf. These novel source apportionments are reflected in an updated carbon budget for the study area.  相似文献   

4.
Sedimentary material from coastal and nearshore areas in the Mississippi Delta region are comprised of different organic carbon sources with diverse ages that require isotopic and elemental records for resolving the various sources of plant residues. Carbon isotopic ((13)C, (14)C) values were used to differentiate contributions from plants using the C3, C4, and/or CAM (crassulacean acid metabolism) carbon fixation pathways., and iodine concentrations indicated that wetland plant residues are a significant source of organic carbon in a sediment core from the Mississippi River delta region collected at a 60 m water depth. This sediment core had been extensively described in Oktay et al. [Oktay, S.D., Santschi, P.H., Moran, J.E., Sharma, P., 2000. The (129)Iodine Bomb Pulse Recorded in Mississippi River delta Sediments: Results from Isotopes of I, Pu, Cs, Pb, and C. Geochim. Cosmochim. Acta 64 (6), 989-996.] and significantly, includes unique features that had not previously been seen in the marine environment. These special features include a plutonium isotopic close-in fallout record that indicates a purely terrestrial source for these sediment particles and the elements associated with it, and a distinct iodine isotopic peak (as well as peaks for plutonium and cesium isotopes) that indicate little bioturbation in this core. Our carbon isotopic and iodine data can thus be compared to published records of changes in drainage basin land use, river hydrology, and hydrodynamic sorting of suspended particles to elucidate if these changes are reflected in nearshore sediments. This comparison suggests a significant contribution for organic carbon (OC) from C4 plants to these sediments during the 1950's to early 1960's. Relative older carbon isotopes, and episodically high iodine concentrations (up to 34 ppm) were observed during this time period that (1) indicate sediment deposition that is coincident with the times of major hydrological changes induced from dam and levee building in both the upper and lower reaches of the Mississippi River drainage basin, and (2) suggest episodic organic carbon deposition from wetland plant residues.  相似文献   

5.
Late-Quaternary sections (about 1 Ma) from the Congo deep-sea fan (ODP Leg 175, site 1075) were used to reconstruct the terrigenous organic matter supply to the easternmost equatorial Atlantic Ocean. Variations in quantity and quality of the riverine organic matter reflect the interaction between the paleoclimatic development within the continental catchment area and the paleoceanographic conditions in the Congo river plume. To characterize the delivery of organic matter from terrigenous and marine sources, we used elemental and bulk carbon isotopic analyses, Rock-Eval pyrolysis, lignin chemistry, and organic petrology. High-amplitude fluctuations occurring about every 15-25 ka reveal a mainly precessional control on organic sedimentation. Results from Rock-Eval pyrolysis indicate a mixed kerogen type III/II, as would be anticipated in front of a major river. Fluctuations in Tmax from Rock-Eval pyrolysis demonstrate pronounced cyclic changes in the delivery of low- and high-mature organic matter. Contribution of the low-mature organic fraction was strongest during warm climates supporting enhanced marine production offshore of the Congo. Organic petrological observations confirm the existence of abundant terrigenous plant tissues, both non-oxidized (vitrinite) and oxidized (inertinite). Charcoal-like organic matter (fusinite) is attributed to periods of increased bush fires in the continental hinterland, and implies more arid climatic conditions. Results from ratios of specific phenolic lignin components suggest that terrigenous organic matter in Late-Quaternary sections of site 1075 mainly derives from non-woody angiosperm tissue, i.e., grasses and leaves. Correlation between the amount of specific lignin phenols and the bulk '13Corg signature fosters the conclusion that an appreciable amount of the terrigenous organic fraction derives from C4 plant matter. This may cause an underestimation of the terrigenous proportion of bulk organic matter when assessments are based on bulk carbon isotopic signatures alone.  相似文献   

6.
《Marine Chemistry》2001,73(3-4):253-271
The influence of mangrove-fringed tropical estuaries on coastal carbon budgets has been widely recognised. However, a quantitative differentiation between riverine and mangrove-derived inputs to the dissolved (DOM) and microparticulate organic matter (POM) pool of these environments has been hitherto not possible. Based on lignin-derived phenols and stable carbon isotopes a chemical signature for mangrove, terrestrial and marine-derived organic matter was established for a mangrove estuary in North Brazil. A mixing model was applied to calculate the contribution of each of the three sources to the DOM and POM pool in the estuary throughout 18 tidal cycles in the course of one year. Best source assignment for POM was reached with the yield of lignin phenols and δ13C as paired indicators, while the origin of DOM was best identified by the yield of lignin phenols and the acid to aldehyde ratio of vanillyl phenols. Although only about 6% of the fluvial catchment area is covered by mangroves, their contribution to the estuarine DOM and POM pool generally exceeded several times the terrigenic input from the hinterland. This outwelling of mangrove-derived organic matter was enhanced during the rainy season. DOM and POM were exported from the mangrove to the estuary in similar proportions. Most mangrove-POM was rapidly removed from the water column, while mangrove-DOM behaved conservatively. In contrast, terrestrial DOM was almost entirely removed in the outer part of the estuary, which was accompanied by a concomitant increase in terrestrial POM. This seems to be the result of a geochemical barrier zone for this type of DOM in the estuary. Generally, a high proportion of mangrove-DOM was present in the outer part of the estuary, even at high tide. This indicates DOM outwelling from mangroves in adjacent bays or estuaries and points to similar driving forces controlling this process on a regional scale. Mangroves probably play a more important role than rivers for marine carbon budgets along the North Brazilian coast south of the Amazon estuary.  相似文献   

7.
In order to characterize the sources and fate of organic matter (OM) in the Pichavaram estuarine-mangrove ecosystem (east coast of India), stable isotope (δ13C and δ15N) ratios and molecular lignin analyses were conducted in plant litter, benthic algae, sediment, particulate matter and in a variety of benthic invertebrate species. The δ13C signature of plant litter ranges from −29.75‰ to −27.64‰ suggesting that mangrove trees follow the C3 photosynthetic pathway. Sedimentary δ13C signature (−28.92‰ to −25.34‰) demonstrates the greater influence of plant litter organic matter on sedimentary organic matter. Suspended particulate organic pool was influenced by terrestrial source and also seems to be influenced by the marine phytoplankton. Enriched signature of δ15N in surface sediments (4.66–8.01‰; avg. 6.69‰) suggesting the influence of anthropogenic nitrogen from agricultural fields and human settlements. Spatial chemical variability in availability of nitrogen and plant associated microbial interactions demonstrate variability in δ15N signature in mangrove plant litter. Two (lower and higher) trophic levels of invertebrates were identified with and observed >4‰ gradient in δ13C signal between these two trophic groups. The observed δ13C values suggest that the lower level invertebrates feed on phytoplankton and higher level organisms have a mixed source of diet, phytoplankton, sediment and particulate organic matter. Lignin phenol analyses explain that the benthic surface layer was almost free of lignin. The ratio between syringyl phenols to vanillyl phenols (S/V) is 1.14–1.32 (avg. 1.23) and cinnamyl phenols to vanillyl phenols (C/V) is 0.17–0.31 (avg. 0.24), demonstrate non-woody angiosperm tissues was the major sources of lignin to this ecosystem, while aldehyde to acid ratios (Ad/Al) describe diagenetic nature of sediment and is moderately to less degraded. A two-end-member mixing model indicate that the terrigenous OM was dominant in the estuarine zones, while in the mangrove zone terrigenous supply accounts for 60% and marine input accounts for 40%.  相似文献   

8.
对长江口泥质区24Z孔的沉积物柱状样进行了粒度和元素分析,根据沉积物的粒度特征可将岩芯从下到上分为3个阶段:A阶段(1931—1964年)粒度参数的波动较小,各层位平均粒度Φ在5.58~7.65;B阶段(1964—1983年)粒度参数波动范围比A阶段更小,整个阶段没有出现明显峰值,Φ在6.51~7.37;C阶段(1983—2003年)粒度参数波动范围明显增大,出现多个峰值,Φ在5.59~7.46。元素Zr和Rb质量比(mZr/mRb)的大小实际上反映了粗粒级矿物与黏土粒级矿物的相对含量的高低,24Z孔的沉积物中mZr/mRb波动范围在1.02~3.59,出现了多个明显的峰值。利用24Z孔沉积物平均粒度和mZr/mRb识别出多个突变层位,结合长江中下游洪水水文记录,发现突变层位年代与20世纪80年代后的水文洪水年份有着较好的对应,这可能与长江主泓的变化有关。对比突变层位与正常层位的粒度特征,突变层位沉积物的粒度频率曲线呈负偏态且峰高而尖,正常层位粒度频率曲线呈正偏态且峰低而宽,同时,概率累积曲线也指示突变层位沉积物在沉积过程中受到了更强的水动力作用。研究长江口泥质区南部24Z孔的沉积物粒度特征和对洪水事件的沉积响应,可以更好地了解长江洪水所携带的粗颗粒物质在长江口泥质区的分布范围,有助于重建长江流域古洪水从而更好的认识长江流域洪水发生的规律。  相似文献   

9.
三峡工程运行对洞庭湖与荆江三口关系的影响分析   总被引：1，自引：0，他引：1  
三峡水库于2003年6月1日正式开始蓄水,其防洪、发电、航运等综合效益日益明显。根据水文泥沙观测资料,采用BP(Back Propagation)神经网络、变差系数法、双累积曲线法等方法分析三峡工程的运行对洞庭湖与荆江三口关系的影响,结果表明:(1)从年际和年代际尺度上看,长江上游降水量减少和三峡工程建设等人类活动是影响洞庭湖水沙变异的主要因素;(2)长江中下游荆江三口分流分沙锐减,并呈现三口口门趋于淤积,藕池河和虎渡河逐渐走向衰亡;(3)从洞庭湖泥沙沉积量过程线来看,2003—2010年洞庭湖的累计泥沙淤积量仅为2301×104t,比多年平均值减少95.6%;(4)运用BP神经网络对洞庭湖出湖水量和沙量进行模拟,结果显示模拟精度满足洞庭湖出湖水沙预测的需要。  相似文献   

10.
Continental shelves play a major role as transition zone during transport of multiply-sourced organic matter into the deep sea. In order to obtain a comprehensive understanding of the origin and fractionation processes of organic matter at the NW Iberian margin, 40 surface sediment samples were analyzed for a structurally diverse range of lipid biomarkers, lignin phenols, grain size distribution, organic carbon content (TOC), its stable carbon isotopic composition (δ13CTOC), and the organic carbon to nitrogen ratio (TOC/TN). The biomarker inventory reflected a heterogeneous mixture of organic matter from various marine and terrestrial sources. Soil- and vascular plant-derived continental organic matter, indicated by lignin phenols and plant-derived triterpenoids, was primarily associated with the silt fraction and transported by river run-off. The spatial distribution patterns of higher plant-derived waxes, long-chain n-alkanes, n-alcohols, and n-fatty acids suggested distinct different transport mechanisms and/or sources. The branched tetraether index, a molecular proxy expressing the relative abundance of branched dialkyl tetraethers vs. crenarchaeol and considered to signal soil-derived organic matter, was not as sensitive as the other molecular indicators in detecting continental organic matter. Hydrodynamic sorting processes on the shelf resulted in a separation of different types of terrestrial organic matter; grass and leaf fragments and soil organic matter were preferentially transported offshore and deposited in areas of lower hydrodynamic energy. Algal lipid biomarker distributions indicated a complex community of marine plankton contributing to organic matter. Spatial and seasonal patterns of phytoplankton growth primarily controlled the distribution of algal organic matter components. The interplay of all of these processes controls production, distribution, and deposition of organic matter and results in three distinct provinces at the Galicia–Minho shelf: (I) fresh marine organic matter dominated the inner shelf region; (II) high inputs of terrestrial organic matter and high TOC content characterized the mid-shelf deposited mudbelt; (III) lower concentrations of relatively degraded organic matter with increased proportions of refractory terrestrial components dominated the outer shelf and continental slope.  相似文献   

11.
Lignin phenols were measured in the sediments of Sepitiba Bay, Rio de Janeiro, Brazil and in bedload sediments and suspended sediments of the four major fluvial inputs to the bay; São Francisco and Guandu Channels and the Guarda and Cação Rivers. Fluvial suspended lignin yields (Σ8 3.5–14.6 mgC 10 g dw−1) vary little between the wet and dry seasons and are poorly correlated with fluvial chlorophyll concentrations (0.8–50.2 μgC L−1). Despite current land use practices that favor grassland agriculture or industrial uses, fluvial lignin compositions are dominated by a degraded leaf-sourced material. The exception is the Guarda River, which has a slight influence from grasses. The Lignin Phenol Vegetation Index, coupled with acid/aldehyde and 3.5 Db/V ratios, indicate that degraded leaf-derived phenols are also the primary preserved lignin component in the bay. The presence of fringe Typha sp. and Spartina sp. grass beds surrounding portions of the Bay are not reflected in the lignin signature. Instead, lignin entering the bay appears to reflect the erosion of soils containing a degraded signature from the former Atlantic rain forest that once dominated the watershed, instead of containing a significant signature derived from current agricultural uses. A three-component mixing model using the LPVI, atomic N:C ratios, and stable carbon isotopes (which range between –26.8 and –21.8‰) supports the hypothesis that fluvial inputs to the bay are dominated by planktonic matter (78% of the input), with lignin dominated by leaf (14% of the input) over grass (6%). Sediments are composed of a roughly 50–50 mixture of autochthonous material and terrigenous material, with lignin being primarily sourced from leaf.  相似文献   

12.
为了解胶州湾水体和表层沉积物营养环境状况及其主要影响因素,于2019年8月在胶州湾30个站位点采集了海水和表层沉积物样品,并于2021年5月在胶州湾沿岸采集了18个站位点的水样,对水体溶解无机态营养盐浓度和组成以及表层沉积物中总有机碳、总氮、总磷及生物硅含量和碳、氮稳定同位素（δ13C、δ15N）进行了分析。结果表明,胶州湾内水体和沿岸水体中溶解无机氮、溶解无机磷和溶解硅酸盐浓度空间分布相近,高值均位于湾东北部,主要受到河流输入和沿岸污水排放的影响,低值主要出现在湾中部和湾口处。结合近30年来的历史数据分析发现,胶州湾夏季营养盐浓度在1990?2008年期间呈持续上升的趋势,政府实施的污染物总量控制措施以及河流径流量下降使得2006年以来营养盐浓度呈现下降的趋势,该变化在空间上主要体现为大沽河氮、磷输入量的减少及其对应的湾西部营养盐高值的消失。胶州湾氮、磷营养盐输入的不平衡使得“磷限制”在2000年后逐渐加剧。胶州湾表层沉积物中总有机碳、总氮、总磷含量高值均集中于东北部和东部沿岸,结合生物硅和水体营养盐含量分析显示,这主要是河流与排污输入及其带来的高初级生产力造成的,沉积物生源要素与水体营养盐在空间分布上存在较好的耦合关系。沉积物粒度较粗对有机质保存的不利影响以及湾口较强的水动力作用共同导致了湾西部、中部以及湾口的生源要素含量较低。δ13C以及二端元混合模型显示,胶州湾表层沉积物有机质来源总体以海源为主,平均占比为64%,东部沿岸受陆源输入影响较明显。δ15N的空间分布显示,胶州湾表层沉积物中氮元素受到了海水养殖与污水排放的共同影响。水体和沉积物营养环境现状共同表明,对东北部河流和沿岸污水排放的控制是后期胶州湾污染治理的关键。  相似文献   

13.
This study investigates species community patterns and sediment relationships of benthic macrofauna off the Rhone river delta. Along a WE transect, changes in diversity, density, biomass and trophic structure were coupled with the mean extension of the river dilution plume. Species number and diversity were at a minimum off the river mouth, below the area of the minimum surface salinity. Density decreased by a factor 2 and biomass by a factor 5 from the fluvial to the marine system. These features are due to high overload of terrestrial organic matter in the river prodelta as evidenced by the carbon isotopic signature of surface sediment and by pigment content. On the basis of a non-metric MDS analysis and of Dufrêne and Legendre method (1997), groups of stations and characteristic species associated were identified. These species, mainly small capitellids, spionids, lumbrinerids and sternaspids, correspond to a successional dynamic in response to changes in sedimentation conditions, mainly in organic matter quality of the surface sediment. The succession observed in space was similar to described in macrotidal estuaries and off other deltaic systems all over the world and to that observed in time following the Rhone river severe flood events. Results suggest that organic matter quality is an important factor with regard to benthic macrofauna successions and recovery mechanisms following disturbances. The differences observed between the Rhone deltaic system and the general model of relations between shelf processes and discharge of large rivers are attributed to a more regular supply of organic material from terrestrial origin on the Rhone continental shelf.  相似文献   

14.
The Galicia-Minho Shelf features two large mud patches, the Douro and the Galicia Mud patches. These are recent sediment bodies that have accumulated under a combination of conditions including: (1) abundant supplies of sediment; (2) morphological barriers that act as sediment traps; and (3) hydrographic conditions that favour the accumulation of fine sediment in these sinks. This paper describes the mechanisms controlling the deposition of the fine-grained sediment depositions and the processes that result in resuspension processes on the Galicia-Minho Shelf.Fine-grained sediments are provided from discharges from the river basins on the southern sector of the shelf, mainly the Douro and Minho rivers. Sediments are exported from river estuaries onto the shelf during episodic flood events. In contrast, most of the sediments originating from the Galician hinterland fail to contribute significantly to sedimentation on the shelf, because they are retained in the Galician Rías, which function as sediment traps.Sediments deposited on the shelf are frequently remobilized, particularly during southwesterly storms that coincide with downwelling conditions. Once in suspension the fine-grained sediments are transported northwards by the poleward flowing bottom currents and are eventually deposited on the Douro and Galicia Mud patches after a series of resuspension events. The locations of the two mud patches are strongly influenced by the shelf morphology.Fines already deposited on the mud patches are occasionally reintroduced into the system by large storm events. Some material from the Douro Mud patch and adjacent areas is re-deposited in the Galicia Mud patch. It is probable that sediments re-suspended from the Galicia Mud patch are carried off the shelf when storm events coincide with downwelling conditions.  相似文献   

15.
Hydrodynamic processes sort and redistribute organic matter (OM) and minerals on continental margins. Density fractionations were conducted on sediments from diverse margins (Mexico margin, Gulf of Mexico, Mississippi River delta, Eel River margin) to investigate the nature, provenance and age of OM among density fractions. Mass, elemental (C and N), lignin, and surface area distributions, as well as stable carbon and radiocarbon isotopic compositions were measured. The lowest density fractions (< 1.6 g cm− 3) contained the highest organic carbon (OC) (up to 45%) and lignin concentrations (up to 8 mg g− 1) due to abundant woody debris, whereas high density fractions (> 2.5 g cm− 3) were OC-poor (%OC < 0.5) mineral material. Most sediment mass was found in the mesodensity fractions (1.6 to 2.5 g cm− 3) that contained the highest proportion of OC (up to ~ 75%) for each sediment. Stable carbon isotope compositions (δ13C − 25.5‰ to − 22.9‰) show terrigenous OC as a significant component of density isolates from the river-dominated sediments (Gulf of Mexico, Mississippi River, and Eel margin), whereas the Mexico margin, least influenced by riverine input, was dominated by autochthonous marine OC (δ13C ~ − 21.5‰). Radiocarbon compositions of density fractions indicate significant pre-aged OC (Δ14C as low as − 900‰) in river-influenced sediments but not on the Mexico margin (Δ14C > − 200‰). Ratios of vanillic acid to vanillin (Ad/Al)v among lignin oxidation products increase with increasing particle density suggesting variable lignin sources or selective degradation of lignin among the different density fractions.  相似文献   

16.
Sedimentary, isotopic and bulk geochemical proxies measured in sediment samples of five gravity cores collected in the distal part of the Ogooue turbidite system (around 4000 m-depth) were used to develop a conceptual model to describe the accumulation of terrigenous organic matter (OM) during the last 200,000 yrs BP in the eastern part of the Gulf of Guinea. This model takes into account the influence of the different depositional processes (turbiditic vs hemipelagic sedimentation), geomorphological features and sea-level variations.Total organic carbon (TOC) and the stable organic carbon isotopes of the OM (δ13C) variability follow the highstand/lowstand (interglacial/glacial) cyclicity with a very low accumulation rate of terrigenous OM during periods of high sea-level and higher accumulation rate during period of low sea-level. A sea-level of 80–120 m below present day seems to favor the transfer of terrigenous sediments to the deep offshore environment through the turbidite system and thanks to the connection of the canyons heads with the river system presently located at the shelf edge at −120 m water depth.In this system, terrigenous OM matter delivered by the river accumulate in the sediments via two main processes. Indeed, a part of the terrigenous OM settles in combination with the finest particles forming hemipelagites, while another part, formed of very well preserved land plant debris, is transported and deposited far offshore with turbidity currents. The proportion of terrigenous OM accumulated due to turbidity currents is important as it can represent more than 70% of the carbon accumulated during sea-level lowstand. Moreover, terrigenous OM seems to preferentially accumulate in the levees and the lobes of the system notably due to the higher frequency of organic-rich turbidites.This study demonstrates that gravity flows, influenced by the sea-level variations, can significantly affect the terrigenous OM budget of the deep offshore Atlantic margins and that channel-levee complexes as well as turbidite lobes can be regarded as good sink for terrestrial organic carbon. These processes should be taken into consideration in the context of source rocks exploration but also for the estimation of the general carbon accumulation in ocean sediment.  相似文献   

17.
通过南海南沙群岛海区NS97-13柱样磁化率各向异性系统测试、统计计算和测年等分析,给出了沉积物的磁化率椭球几何形态,重建了研究海区的古水流方向、沉积物的沉积形式。研究结果表明:(1)压扁型椭球(E>1)的择优方向位于磁性线理-磁性叶理直线的上方,夹角β的范围为0°<β≤33.14°,拉长型椭球(E<1)的择优方向位于磁性线理-磁性叶理直线的下方,夹角α的范围为33.14°<α<56.86°,指示磁性颗粒沉降时受动力作用的大小及其受力方向不同;(2)由磁化率各向异性参数(磁性线理-各向异性度,磁性叶理-各向异性度)确定的沉积物有改造沉积、再沉积和稳定沉积三种形式,前两种形式掺杂在稳定沉积层中的不同部位,构成部分混合沉积;(3)最大磁化率和中间磁化率的轴向分布表征研究海区距今近62 ka以来古水流方向主要为南北和北东-南西向,总磁化率记录了Heinrich层中的冷事件H1—H6。  相似文献   

18.
In the Russian sector of the Gdansk Basin (Baltic Sea), high organic matter influx fuels microbial processes resulting in the formation of reduced sediments with elevated methane concentrations. Investigated areas of geoacoustic anomalies (～245 km2) were found to contain three distinct geomorphologic structures (pockmarks), with a total area of ～1 km2. Methane anomalies recorded in the water above one of these pockmarks were traced as high as 10 m above the bottom. In pockmark sediments, sulfate reduction and anaerobic oxidation of methane (AOM) occurred at high rates of 33 and 50 µmol dm?3 day?1, respectively. Integrated over 0–180 cm sediment depths, AOM exceeded methanogenesis almost tenfold. High AOM rates resulted from methane influx from deeper sediment layers. The δ13C signature of methane carbon (?78.1 to ?71.1‰) indicates the biogenic origin of pockmark methane. In pockmark sediments, up to 70% of reduced sulfur compounds was possibly produced via AOM.  相似文献   

19.
《Marine Chemistry》2001,73(1):65-81
Radiocarbon (Δ14C) abundance and stable carbon isotope (δ13C) compositions were measured for total lipid, total hydrolyzable amino acids (THAA), total carbohydrates (TCHO), and acid-insoluble organic fractions separated from phytoplankton, zooplankton, and sediment cores collected from two abyssal sites, one in the northeast (NE) Pacific Ocean and one in the Southern Ocean. These results are compared with those obtained for a separate core from the NE Pacific and reported by Wang et al. [Geochim. Cosmochim. Acta 62 (1998) 1365.]. An uncharacterized acid-insoluble fraction dominated (43–57%) the sediment TOC pool at both sites. Sediment collected from the NE Pacific exhibited higher TOC, TN. and higher Δ14C values and contained both labile (THAA and TCHO) and refractory (lipid and acid-insoluble) fractions. In contrast, sediment from the Southern Ocean had much lower TOC and Δ14C values, which indicated that organic matter was extremely refractory. Sedimentation rates calculated from 14C ages of TOC for both sites indicate that the differences in the organic composition and Δ14C signatures of organic matter in the sediments likely resulted from the differences in production and deposition of organic matter to the sediment, and the diagenetic stages of sedimentary organic matter in the two oceans.Unlike Δ14C, stable carbon isotopic (δ13C) compositions of TOC in the sediments of the two oceans had similar values, which reflect not only the organic matter input from marine-derived sources but also the nature of degraded, predominately uncharacterized organic fraction in these sediments. The differences in δ13C values among the compound classes in sediments at both sites can be attributed to the carbon isotopic signatures in original sources during photosynthesis and associated environmental factors. Preferential degradation of organic matter and heterotrophic effects may also play important roles for the observed δ13C variations and these effects need to be further investigated.  相似文献   

20.
The Northwest Atlantic margin is characterized by high biological productivity in shelf and slope surface waters. In addition to carbon supply to underlying sediments, the persistent, intermediate depth nepheloid layers emanating from the continental shelves, and bottom nepheloid layers maintained by strong bottom currents associated with the southward flowing Deep Western Boundary Current (DWBC), provide conduits for export of organic carbon over the margin and/or to the interior ocean. As a part of a project to understand dynamics of particulate organic carbon (POC) cycling in this region, we examined the bulk and molecular properties of time-series sediment trap samples obtained at 968 m, 1976 m, and 2938 m depths from a bottom-tethered mooring on the New England slope (water depth, 2988 m). Frequent occurrences of higher fluxes in deep relative to shallower sediment traps and low Δ14C values of sinking POC together provide strong evidence for significant lateral transport of aged organic matter over the margin. Comparison of biogeochemical properties such as aluminum concentration and flux, and iron concentration between samples intercepted at different depths shows that particles collected by the deepest trap had more complex sources than the shallower ones. These data also suggest that at least two modes of lateral transport exist over the New England margin. Based on radiocarbon mass balance, about 30% (±10%) of sinking POC in all sediment traps is estimated to be derived from lateral transport of resuspended sediment. A strong correlation between Δ14C values and aluminum concentrations suggests that the aged organic matter is associated with lithogenic particles. Our results suggest that lateral transport of organic matter, particularly that resulting from sediment resuspension, should be considered in addition to vertical supply of organic matter derived from primary production, in order to understand carbon cycling and export over continental margins.  相似文献   

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