东海内陆架有机碳的源—汇过程及其沉积记录

大陆边缘海是不同来源、不同性质有机碳沉积和埋藏的主要场所,在全球碳的生物地球化学循环过程中具有重要地位.东海内陆架接收大量陆源有机碳,并且具有较高的海洋生产力,是研究沉积有机碳来源、输运和埋藏的理想场所,已取得大量研究成果.在对相关文献进行系统整理的基础上,以沉积学的视角对前人研究成果进行了梳理,旨在为后续相关研究提供...     

Influence of the Yangtze River and grain size on the spatial variations of heavy metals and organic carbon in the East China Sea continental shelf sediments

Bulk heavy metal (Fe, Mn, Zn, Cu, Pb, Cd), Al, organic carbon and carbonate concentrations, grain sizes, and δC¹³ of the organic carbon distributions were studied in sediments collected throughout the East China Sea continental shelf and the Yangtze River Delta. The results demonstrated that terrigenous sediments from the Yangtze River is a dominating factor controlling the spatial variations of heavy metals and organic carbon concentrations on the East China Sea continental shelf. In addition, grain size and recent anthropogenic influences are also major factors modifying the spatial and vertical variations of heavy metals.Large spatial variations with a band type distribution of heavy metals, grain size, organic carbon and carbonate were observed. Higher concentrations of heavy metal and light δC¹³ of the organic carbon were found primarily in the Deltaic and inner shelf sediments. The band type distribution generally followed the coastline with little variations in the north–south direction. Away from the Delta and inner shelf (west–east direction), most heavy metal concentrations decreased rapidly with the exception of Cd where high concentrations of Cd were also found in the carbonate-rich shelf break sediments. Coarse-grained relict sediments and biogenic carbonate are two primary diluting agents for the fine-grained aluminosilicate sediments from the Yangtze River with high concentrations of heavy metals.Unusually high concentrations of Cu, Pb, and Cd showed both spatially and vertically that more pollution prevention measures are needed in the Yangtze River drainage basin in order to prevent further heavy metal pollution of the East China Sea inner continental shelf.     

Authigenic carbonate burial in the Late Devonian–Early Mississippian Bakken Formation (Williston Basin,USA)

Late Devonian (Famennian) marine successions globally are typified by organic-rich black shales deposited in anoxic and euxinic waters and the cessation of shelf carbonate sedimentation. This global ‘carbonate crisis’, known as the Hangenberg Event, coincides with a major extinction of reef-building metazoans and perturbations to the global carbon cycle, evidenced by positive carbon-isotope excursions of up to 4‰. It has been suggested that authigenic carbonate, formed as cements in sedimentary pore spaces during early burial diagenesis, is a significant mass fraction of the total global carbon burial flux, particularly during periods of low oxygen concentration. Because some authigenic carbonate could have originated from remineralization of organic carbon in sediments, it is possible for this reservoir to be isotopically depleted and thereby drive changes in the carbon isotopic composition of seawater. This study presents bulk isotopic and elemental analyses from fine-grained siliciclastics of the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA) to assess the volume and isotopic composition of carbonates in these sediments. Carbonate in the Bakken black shales occurs primarily as microscopic disseminated dolomite rhombs and calcite cements that, together, comprise a significant mass-fraction (ca 9%). The elemental composition of the shales is indicative of a dynamic anoxic to sulphidic palaeoenvironment, likely supported by a fluctuating chemocline. Despite forming in an environment favourable to remineralization of organic matter and the precipitation of isotopically depleted authigenic carbonates, the majority of carbon isotope measurements of disseminated carbonate fall between −3‰ and +3‰, with systematically more depleted carbonates in the deeper-water portions of the basin. Thus, although there is evidence for a significant total mass-fraction of carbonate with contribution from remineralized organic matter, Bakken authigenic carbonates suggest that Famennian black shales are unlikely to be sufficiently ¹³C-depleted relative to water column dissolved inorganic carbon to serve as a major lever on seawater isotopic composition.     

From land to sea: provenance,composition, and preservation of organic matter in a marine sediment record from the North-East Greenland shelf spanning the Younger Dryas–Holocene

The organic matter content of marine sediments is often used to infer past changes in ocean conditions. However, the organic carbon pool preserved in coastal sediments is a complex mixture derived from different sources and may not reflect in situ processes. In this study, we combine taxonomic identification of reworked palynomorphs with pyrolysis organic geochemistry and reflected-light organic petrographic microscopy to investigate the provenance, composition and preservation of organic matter in a marine sediment core retrieved from the NE Greenland shelf. Our study reveals continuous yet variable input of land-derived organic carbon to the marine environment throughout the late Younger Dryas–Holocene, with the highest input of inert carbon in the late Younger Dryas. Although the sediments contain some recent marine palynomorphs, there is no other evidence of fresh marine organic carbon. In contrast, our results indicate that these shelf sediments represent a significant sink of recycled organic carbon. The results of pyrolysis geochemistry revealed that ~90% of the total organic carbon in the sediments is inert. The organic petrography analyses revealed that >70–84% of the organic carbon in the sediment core is terrigenous. Reworked dinoflagellate cysts showed a continuous provenance of Cretaceous land-derived material, most likely from the nearby Clavering Island. Our study points to the importance of constraining the organic matter origin, composition and preservation in marine sediments to achieve more accurate palaeoenvironmental reconstructions based on organic proxies.     

陆架边缘海沉积物有机碳矿化及其对海洋碳循环的影响

边缘海沉积物是海洋重要的碳储库,其内部的碳循环主要是由有机质矿化分解过程来驱动的。有机碳进入边缘海沉积物后,矿化分解为溶解无机碳(DIC)进入沉积物孔隙水并扩散到上层水柱,参与海洋系统碳循环;同时还有部分DIC与钙镁等离子结合形成自生碳酸盐,保存于沉积物碳库。从生物地球化学角度探讨有机质埋藏机制和效率,在此基础上重点综述沉积物硫酸盐还原、产甲烷和甲烷厌氧氧化过程的耦合机制,以及有机质矿化对自生碳酸盐形成的影响等方面的研究进展,以期加深对陆架边缘海沉积物在全球碳循环收支平衡中的作用及其气候环境效应的认识。     

Organic Carbon Isotope Geochemistry of the Neoproterozoic Doushantuo Formation, South China

The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.     

Sedimentary Organic Carbon Mineralization and Its Contribution to the Marine Carbon Cycle in the Marginal Seas

Continental margin sediments are important ocean carbon repository, and the internal carbon cycle is mainly driven by the mineralization processes of sedimentary organic matter. Most organic carbon is transformed to Dissolved Inorganic Carbon (DIC) by mineralization processes after being delivered to continental margin sediments, and DIC from pore water diffuses into the upper water column and participates in the ocean carbon cycle. At the same time, some DIC combines ions such as Ca²⁺ and Mg²⁺ and precipitates as authigenic carbonate minerals so that carbon is stored in the deposits. Based on the biogeochemical study of the mechanism and efficiency of organic matter burial, we discussed the interaction among sulfate reduction, methanogenesis and anaerobic oxidation of methane, and the effect of organic mineralization on the formation of authigenic carbonate. By reviewing the above-mentioned aspects, we can obtain a better understanding of the role of continental margin sediments in the global carbon cycling budgets as well as its climate and environmental effects.     

Matrix protected organic matter in a river dominated margin: A possible mechanism to sequester terrestrial organic matter?

The provenance of organic matter in surface sediments from the northern Gulf of Mexico was investigated by analyzing the compositions of lipid biomarkers (n-alkanes, fatty acids, sterols) liberated after a series of chemical treatments designed to remove different organo-mineral matrix associations (i.e. freely extractable, base-hydrolyzable, unhydrolyzable). Bulk analyses of the organic matter (carbon content, carbon:nitrogen ratios, stable and radiocarbon isotopic analyses) were also performed on the intact sediments and their non-hydrolyzable, demineralized residue. We found recognizable lipids from distinct sources, including terrestrial vascular plants, bacteria and marine algae and zooplankton, within each of the isolated fractions. Based on the lipid signatures and bulk compositions, the organic matter within the unhydrolyzable fractions appeared to be the most diagenetically altered, was the oldest in age, and had the highest abundance of terrigenous lipids. In contrast, the base-hydrolyzable fraction was the most diagentically unaltered, had the youngest ages and was most enriched in N and marine lipids. Our results indicate that fresh, autochthonous organic matter is the most important contributor to base-hydrolyzable lipids, whereas highly altered allochthonous sources appear to be predominant source of unhydrolyzable lipids in the surface sediments from the Atchafalaya River shelf. Overall, the lipid biomarker signatures of intact sediments were biased towards the autochthonous source because many of the organic compounds indicative of degraded, terrigenous sources were protected from extraction and saponification by organo-mineral matrices. It is only after these protective matrices were removed by treatment with HCl and HF that these compounds became evident.     

The persistence of memory: the fate of ancient sedimentary organic carbon in a modern sedimentary system

The cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales. The burial of organic carbon occurs predominantly at sea in association with clay-sized particles derived from the erosion of uplifted continental rocks. It follows that the history of the fine-grained particles on land may bear on the nature of the organic carbon buried. In this study, the evolution of clay-associated organic matter was followed from bedrock source to the seabed in the Eel River sedimentary system of northern California using natural abundance ¹³C and ¹⁴C tracers. Approximately half of the fine-grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted Mesozoic-Tertiary Franciscan Complex of the watershed. The short residence time of friable soils on steep hill slopes, coupled with rapid sediment accumulation rates on the shelf-slope, act to preserve the ancient organic carbon. A comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope. The bimodal mixture of ancient and modern C in soils and sediments may be characteristic of many short, mountainous rivers. If the Eel River chemistry is typical of such rivers, more than 40 Tg of ancient organic C may be delivered to the world’s oceans each year. A flux of that magnitude would have a significant influence on marine and global C-cycles.     

Factors controlling the burial of organic carbon in laminated and bioturbated sediments off NW Mexico: implications for hydrocarbon preservation

Factors controlling the burial of organic carbon (OC) in Late Quaternary sediments on the NW Mexican continental margin are assessed using a suite of box and piston cores strategically located on the shelf-slope rise with respect to the intense oxygen minimum in this region. An OC concentration maximum occurs on the mid-slope, below the core of an intense water-column O₂ minimum, due to current winnowing on the outer shelf, the preferential accumulation of organic matter in fine-grained deposits, and the offshore decrease in the settling flux of organic detritus. The organic matter at all water depths is overwhelmingly marine. Hydrogen indices (HI) are higher on the slope (>300 mg HC/g TOC) than on the shelf (<300 mg HC/g TOC), where current winnowing has promoted organic matter degradation, but there is no difference in HI in slope sediments accumulating under well oxygenated and O₂-deficient conditions. The degree of winnowing appears to be the primary factor affecting the preservational quality of organic matter deposited on this margin.Rates of accumulation of OC and opal are all higher in the interglacial intervals when compared with the glacial deposits over the last 140,000 yr. However, matrix-corrected HI values in the mid- and lower-slope cores are invariant and are similar to values in the laminated intervals from the oxygen-minimum site. Thus, cyclic changes in organic carbon accumulation on this margin have been controlled by production variations rather than differential preservation. HI values in Late Quaternary sediments from several continental margins, including NW Mexico, and euxinic basins correspond to type II kerogen, irrespective of bottom water O₂ concentrations. Therefore, the preservation of oil-prone kerogen in productive margin settings does not appear to be restricted to sediments deposited under conditions of low bottom water O₂ concentrations as envisioned in models of petroleum source-rock deposition.     

自然和人类活动作用下月湖沉积物有机质来源及其演变

潟湖海湾可以保存环境演化和人类活动的良好记录,碳氮元素含量及同位素特征可用来有效地反演这些演化。对位于山东半岛的月湖柱状样和沿岸表层样总有机碳、总氮、稳定碳氮同位素的分析表明,陆源物质对月湖沉积物和有机质的贡献很小,其沉积物中有机质的主要来源是维管植物大叶藻和各种海洋藻类。在2002年前大约140 a的沉积历史中,大叶藻对沉积物有机质的贡献比例呈上升趋势,其中在约1930年和1980年有两次明显的上升,前者可能因为种群的快速繁育,后者则可能是1979年口门筑坝后由于湾内环境恶化,大叶藻开始大量死亡的表现。月湖海洋藻类的有机碳埋藏通量在沉积记录中保持相对稳定,而大叶藻则增加了一倍,但两者都有一定程度的波动。研究结果表明自然环境演变（如沙嘴生长、口门缩小）和人类活动（如筑坝、修筑虾池）对月湖生态环境及有机碳埋藏的影响在沉积物中有清晰的记录。     

The contrasting geochemical behaviours of iodine and bromine in recent sediments from the Namibian shelf

The vertical distribution of iodine, bromine and organic carbon has been examined in sediment cores from a range of environments on the Namibian shelf. The relationship between Br and C org. is linear, and that between I and C org. is variable, for all surface sediments; I/C org. ratios show a decrease of about one order of magnitude between the outer shelf oxidising sediments ($\text{250 × 10}{}^{\mathrm{?4}}$) and the organic-rich inner shelf sediments ($\text{20 × 10}{}^{\mathrm{?4}}$). The contrasting behaviour of the halogens in surface sediments is explained by differences in the amount of halogen absorbed by living organisms within the euphotic zone and on seston on the seabed. It is suggested that sorption by seston occurs only in oxidising sediment where free O₂ is available. Hence, iodine is sorbed by seston in the outer shelf environment, but is not sorbed by the reducing sediments of the inner shelf. Here the iodine in the sediment represents only that taken up by plankton. On the outer shelf, 50–80% of the total iodine in the organic matter is sorbed by seston. The principal site of Br uptake is not known.The distribution of C org. in subsurface sediments broadly reflects that found at the surface, although there is a slight decrease with depth in the outer shelf cores due to dilution by terrigenous materials. In the inner shelf cores, there is no change in the relationship of iodine and bromine to organic carbon at depth. Those from the mid shelf, and especially the outer shelf, on the other hand, show decreases in both I/C org. and Br/C org. ratios, reaching values at about 70 cm depth that are similar to those in surface reduced sediments from the inner shelf. Over this depth interval I/C org. ratios decrease by a factor of five while Br/C org. ratios show a two fold decrease. These changes in the ratios at depth imply that diagenesis within the reducing cores is negligible compared with that of oxidised sediments. The implications of diagenesis with regard to halogen recycling in sediments are briefly discussed.     

Geochemistry of surface sediments in the southwestern East/Japan Sea

We analyzed 77 surface sediment samples collected in the southwestern East/Japan Sea from the Korea Strait through the Ulleung Basin and the Korea Plateau for grain size, calcium carbonate, organic carbon, and major (Na, Mg, Al, Fe, K, Ca, and Ti) and trace elements (P, Mn, Sr, Li, Sc, V, Cr, Co, Ni, Zn, Cu, and Pb).The chemical composition of the surface sediments was found to be highly variable spatially. Cluster analysis of surface sediment chemical compositions indicated five major geochemical sedimentary environments: basin, lower slope, coast and upper slope, inner shelf, and outer shelf. Continental-shelf sediments were rich in shell fragments and had relict and coarse-grained characteristics. Recent fine-grained sediments were only distributed in coastal, slope, and basin areas. Concentrations of Al, K, Ca, Ti, Cr, and Sc were highest in the coastal and upper slope areas and decreased with water depth. Elemental ratios using major and trace elements indicated that coastal and upper slope detrital sediments were mixtures of sediments derived from the Changjiang (Yangtze) and Nakdong Rivers. Although the concentrations of organic carbon, P, Mn, V, Co, Ni, Cu, and Pb increased with water depth, their distribution patterns indicated authigenic (V, Cu, and Pb) and diagenetic (Fe, P, Mn, Co, and Ni) origins. The distribution pattern with water depth suggested that the chemical composition of surface sediment was determined by sedimentologic and geochemical processes, such as the supply of detrital and biogenic materials, and authigenic and post-depositional diagenetic processes in sediments.     

Carbon Isotopic Evolution of the Late Ediacaran Gaojiashan Biota on the Northern Yangtze Platform,South China

Metazoan fossils in the Gaojiashan Biota are famous for being well preserved and may provide new insights into the early evolution and skeletonization of Metazoans. We are studying the isotopic compositions of organic and carbonate carbon from a sequence of sedimentary rocks at the Gaojiashan section, northern Yangtze Platform, Shaanxi Province of China. Organic carbon isotope values display a range between –30.8‰ and –24.7‰ with clear stratigraphic variations. Carbonate carbon isotope data vary between 0.1‰ and +6‰. Positive δ13C values from sediments with Gaojiashan biota reflect temporal variations in carbon turnover, i.e. an increasing in photosynthetic carbon fixation followed by an increasing subsequent fractional organic carbon burial, and that related to bio-radiation such as increasing algae, bacteria, and original creatures productivity in biomass. These secular variations are interpreted to reflect perturbations of the regional carbon cycle, specifically changes in the fractional burial of organic carbon, and discuss the relationship between Gaojiashan biota and paleoenvrionmental variation.     

The radiocarbon age of organic carbon in marine surface sediments

Long-term carbon cycling and climate change are strongly dependent on organic carbon (OC) burial in marine sediments. Radiocarbon (¹⁴C) has been widely used to constrain the sources, sinks, and processing of sedimentary OC. To elucidate the dominant controls on the radiocarbon content of total organic carbon (¹⁴C_TOC) accumulating in surface sediments we construct a box model that predicts ¹⁴C_TOC in the sediment mixed layer (measured as fraction modern, Fm). Our model defines three distinct OC pools (“degradable,” “semi-labile,” and “refractory”) and assumes that ¹⁴C_TOC flux to sediments is exclusively derived from surface ocean primary productivity, and hence follows a “generic” surface ocean dissolved inorganic carbon (DIC) bomb curve. Model predictions are compared to a set of 75 surface sediment samples, which span a wide geographic range and reflect diverse water column and depositional conditions, and for which sedimentation rate and mixed-layer depth are well characterized. Our model overestimates the Fm value for a majority (65%) of these sites, especially at shallow water depths and for sites characterized by depleted δ¹³C_TOC values. The model is most sensitive to sedimentation rate and mixed-layer depth. Therefore, slight changes to these parameters can lead to a match between modeled and measured Fm values at many sites. Yet, in some cases, we find that measured Fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed-layer depth. These results point to sources of pre-aged OC to surface sediments and implicate soil-derived terrestrial OC, reworked marine OC, and/or anthropogenic carbon as important components of the organic matter present in surface sediments. This approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales.     

Grain size and geochemistry of surface sediments in northwestern continental shelf of the South China Sea

Eighty-six surface sediments collected from the northwestern continental shelf of the South China Sea (SCS) were analyzed for grain size distribution, calcium carbonate, organic carbon, and major and trace element compositions to investigate sediment provenance and factors controlling their geochemical composition. Sediments from the eastern continental shelf of Hainan Island have higher sand and lower clay content, while the samples from the nearshore Hainan Island have higher contents of gravel and clay. Calcium carbonate contents in samples show a positive correlation with water depth in northwestern shelf of SCS, suggesting that it is related to biological factors. However, the nearshore sediments have higher contents of organic carbon compared to those of the outer shelf, possibly suggesting that the terrigenous organic matter usually deposited in nearshore environments such as bays and estuaries. Compared with the upper continental crust, the samples have relatively lower contents of SiO₂ and Al₂O₃, higher than those of the Pearl and Red river sediments. The low contents of K₂O and Na₂O in sediments from the northwestern continental shelf are consistent with intense chemical weathering in the river basin due to the seasonally hot and humid climate regime. The sediments mainly consist of three components, including the gravel fraction composed of calcareous debris, the sand fraction composed of quartz, and the silt and clay fractions mainly composed of clay minerals. The content of each component depends on grain size, sediment source, biogenesis, and hydrodynamic conditions, which finally controls the chemical composition of the sediments. The distributions of Co/Al₂O₃, Cr/Al₂O₃, and Zr/Sc ratios for sediments in the northwestern continental shelf suggest that source rocks are mainly composed of felsic rocks rather than mafic rocks. There is a distinct difference in sediment source between eastern and western shelf sediments; the eastern shelf sediments are characterized by high Zr/Sc ratios mainly derived from the Pearl River, while the western shelf sediments have relatively low values of Zr/Sc indicating a main contribution possibly sourced from the Red River Basin. Terrigenous materials from Hainan Island usually influence the geochemistry of sediments deposited in the nearshore area.     

Pore water fluoride in Peru continental margin sediments: Uptake from seawater

Fluoride analyses display downward decreasing pore water gradients in Peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters, presumably by incorporation into carbonate fluorapatite. The profiles can be modeled as first-order F-removal with rate constants of ~3 yr^?1 and asymptotic F-concentrations deep in the cores of 35–45 μM, almost one-half the seawater value. The integrated flux of fluoride from seawater into organic-rich shelf sediments in coastal-upwelling zones (phosphatic muds) yields a contemporaneous global F-burial of 0.54 × 10¹⁰ mol-F yr^?1, about one-fifth the burial in other sinks (mostly carbonates and opal). The associated burial flux of phosphorus in shelf phosphorites is about 1.6 × 10¹⁰ mol-P yr^?1, comparable to P-burial in the deep sea with organic matter (~1.4 × 10¹⁰ mol yr^?1) and biogenic carbonates (~1.4 × 10¹⁰ mol yr^?1). Thus phosphorite formation on the Peru shelf is a significant contemporaneous process.     

南海表层沉积物与沉降颗粒物中有机碳的δ13C对比研究及其古环境再造意义

对南海表层沉积物与沉降颗粒物有机碳的δ13C进行了对比研究,从古环境再造方面探讨了陆源和海源对南海表层沉积物的贡献。南海表层沉积物有机碳的δ13C比沉降颗粒物中有机碳的δ13C偏重表明,Suess效应对南海现代过程的影响不可忽视,两者的δ13C差值反映了Suess效应对南海现代过程的影响程度。利用两者的δ13C差值,对现代陆源和海源有机碳的δ13C进行修正后,计算得到,在南海陆架周围表层沉积物的有机碳中,陆源的比例为48%,海源的比例为52%;在远离陆架的表层沉积物的有机碳中,陆源的比例为14%,海源的比例为86%。     

长江三角洲晚新生代沉积物有机碳、总氮和碳酸盐组成及古环境意义

长江三角洲地区埋深达320m的PD钻孔揭示了上新世以来河湖相、滨海相和三角洲相的沉积环境特征。钻孔沉积物中有机碳、总氮和碳酸盐组成变化较大,但具有一定的规律性。上新世东部地区气候较暖湿,古湖泊发育,化学风化强烈,有机质保存率较低;早更新世气候波动较大,相对温湿气候下高等植被发育;中、晚更新世长江流域气候逐渐变冷干,晚第四纪气候变化较显著,流域植被类型发生明显变化;全新世气候转暖,有机质保存率高。有机碳、总氮和碳酸盐组成揭示的古气候变化与孢粉、沉积地层等资料相当吻合,与相邻内陆地区的第四纪古环境研究认识也具有明显的可比性。本研究也揭示,运用有机碳、总氮和碳酸盐组成重建我国东部地区第四纪古环境和季风演化史具有相当的难度,必须充分考虑沉积地层不连续性、粒度、有机质来源、埋藏成岩环境以及陆海相互作用等多因素的复杂影响,同时需要结合有机碳同位素等分析来深化古环境研究。     

Sources of Terrestrial Organic Carbon in the Mississippi Plume Region: Evidence for the Importance of Coastal Marsh Inputs

High sedimentation rates along river-dominated margins make these systems important repositories for organic carbon derived from both allochthonous and autochthonous sources. Using elemental carbon/nitrogen ratios, molecular biomarker (lignin phenol), and stable carbon isotopic (bulk and compound-specific) analyses, this study examined the sources of organic carbon to the Louisiana shelf within one of the primary dispersive pathways of the Mississippi River. Surface sediment samples were collected from stations across the inner, mid, and outer Louisiana shelf, within the Mississippi River plume region, during two cruises in the spring and fall of 2000. Lignin biomarker data showed spatial patterns in terrestrial source plant materials within the river plume, such that sediments near the mouth of the Mississippi River were comparatively less degraded and richer in C₄ plant carbon than those found at mid-depth regions of the shelf. A molecular and stable isotope-based mixing model defining riverine, marsh, and marine organic carbon suggested that the highest organic carbon inputs to the shelf in spring were from marine sources (55?C61% marine organic carbon), while riverine organic carbon was the highest (63%) in fall, likely due to lower inputs of marine organic carbon at this time compared with the spring season. This model also indicated that marsh inputs, ranging from 19 to 34% and 3?C15% of the organic carbon in spring and fall, respectively, were significantly more important sources of organic carbon on the inner Louisiana shelf than previously suggested. Finally, we propose that the decomposition of terrestrial-derived organic carbon (from the river and local wetlands sources) in mobile muds may serve as a largely unexplored additional source of oxygen-consuming organic carbon in hypoxic bottom waters of the Louisiana shelf.