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1.
Two sediment cores collected in Chaleur Trough, Gulf of St. Lawrence are compared in terms of organic geochemistry, sediment texture and foraminifera. One of the cores was collected from a depression and contained methane in the range of 5,970–14,230 ppm, in comparison to 42–106 ppm in the core of open environment. The sediments of the methanerich core are finer, higher in organic carbon, extractable organic matter, plant pigments and contained more diverse foraminiferal assemblages. The results of these geological investigations tend to suggest a relatively fast rate of sedimentation in the depression. This results in the preservation of organic compounds and development of anaerobic subsurface conditions, consequently giving rise to high concentrations of methane through fermentative processes. No other gaseous hydrocarbons were detected hence the possibility of seepage of petroleum gases appears to be remote.  相似文献   

2.
The La Luna Formation (Maraca section), Maracaibo Basin, was studied by means of V and Ni analysis of the bitumen, total organic carbon (TOC), total sulfur (St), major elements (Si, Al, Fe, Mg, Mn, Ca, Ti, Na, K, P), trace elements (V, Ni, Co, Cr, Cu, and Zn), and electron microprobe analysis (EPMA) of the whole rock, and St, major elements (Si, Al, Fe, Mg, Mn, Ca, Ti, Na, K, P), trace elements (V, Ni, Co, Cr, Cu, Zn, Mo, Ba, U, Th) and rare earth elements (La, Ce, Nd, Sn, Eu, Th, Yb, Lu) of the carbonate-free fraction. The results are discussed based on the organic and inorganic association of trace elements and their use as paleoenvironmental indicators of sedimentation. An association between V and organic matter is suggested by means of correlation between V and Ni vs. TOC, the use of EPMA (whole rock) and V and Ni concentrations (carbonate-free fraction), whereas Ni is found in the organic matter and the sulfide phase. Fe is present as massive and framboidal pyrite, whereas Zn precipitates into a separate phase (sphalerite), and Ni, Cu and, in some cases, Zn, can be found as sulfides associated with pyrite. Concentrations of V and Ni (bitumen), TOC, St, V, Ni, Cr, Cu and Zn (whole rock), U, Th, Mo (carbonate-free fraction) are indicative of changes in the dysoxic sedimentation conditions in the chert layers (TOC, St, V, Ni, Cu and low Zn and V/Cr <4) to euxinic anoxic conditions in the argillaceous limestone (TOC, St, V, Ni, Cu and high Zn and V/Cr >4). In the sequence corresponding to the argillaceous limestone, variations in the concentrations of TOC, St, V, Ni, Zn, Cu and Cr (whole rock) can be observed, also suggesting variable sedimentation conditions. The following is proposed: (i) sedimentation intervals under euxinic conditions associated with high contribution and/or preservation of organic matter, allowing a high concentration level of V and Ni in the organic phase and the accumulation of Cu, Zn and Ni (in a smaller proportion) in the sulfide phase; (ii) sedimentation intervals under anoxic conditions and in the presence of relatively lower H2S, which allowed lower concentrations of V and Ni in the organic phase and higher concentrations of Cu, Zn, and Ni in the sulfide phase. Rare earth elements (REE) concentrations exhibit a marked increase in Ce, Nd, Sm, Eu, Y and Lu for the QM-3 interval, relative to Post-Archean Average Shale (PAAS). REE enrichment in shales has been related to the presence of phosphate minerals such as monazite or apatite. However, these minerals were not detected through XRD or EPMA in the whole rock or in the carbonate-free fraction. The association of REE with organic matter is suggested due to the absence of phosphate minerals, although assessment of these elements require further analysis.  相似文献   

3.
The Mn contents and sedimentation rates of two cores from different areas of the Peru Basin have been determined. The southern core is associated with Mn nodules of conventional slow accretion rates (~ mm/106 yr) while the northern one accompanies nodules with very high growth rates (~102 mm/106 yr). The depletion of Mn observed within the top 40 cm of the cores is interpreted as resulting from a diagenetic remobilization of Mn+2 due to the oxidative characteristics of the sedimentary column. In both cores, the calculated flux of Mn provided by the sediment is higher than the total Mn recovered in the overlying nodules. This indicates that the Mn content of the Peru Basin nodules can be supplied by diagenetic processes from the sediment, not only for the slowly growing nodules but also for the extremely fast growing concretions found in the north of the basin.  相似文献   

4.
Lignin oxidation products and stable carbon isotope distributions are used to investigate the sources, transport, and chemical stability of land-derived organic matter in dated cores of modern sediment from the southern Washington State continental shelf and slope. There is no evidence for significant chemical alteration of lignin compounds in these sediments for time periods of up to 400 yr. Gymnosperm woods and nonwoody angiosperm tissues account for most of the land-derived organic matter in the deposits. These land plant remains have an average δ13C of approximately ?25.5% and are concentrated in a narrow band of silty sediment which extends northward from the Columbia River mouth along the mid-shelf. Marine organic matter having an approximate δ13C of ?21.5%, strongly predominates in most other shelf and slope environments. Net fluxes of land-derived organic matter into the surface 5 cm of the cores vary directly with sediment accumulation rates. Net fluxes of marine organic material into the surface sediments are highest in environments which favor the preservation of organic matter, but correspond to less than 1% of the primary productivity in the overlying waters.  相似文献   

5.
Analysis of 3-m sediment cores revealed that profiles of carbon (C), sulfur (S), and iron (Fe) varied with relative distance from marine and terrestrial sediment sources in Tomales Bay California. Despite relatively high sedimentation rates throughout the bay (historically 3–30 mm yr−1), sulfate reduction of deposited organic matter led to free-sulfide accumulation in sediments only at the location farthest from terrestrial runoff, the source of reactive iron. Acid-volatile sulfide concentrations in all sediments (<10 μmol g−1) were low relative to concentrations of chromiumreducible sulfide (up to 400 μmol g−1 farthest from the reactive iron source). A calculated index of iron availability, used to describe sediment resistance to build-up of free sulfide, was lowest at this location. Recent, upward shifts in reactive Fe concentration and in the relative contribution of terrestrial orgnic carbon (measured as a shift in δ13C of bulk sediment organic matter) in all cores indicated that erosion and transport of sediments from the watershed surrounding Tomales Bay increased after European settlement in the 1850s.  相似文献   

6.
In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,210Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr?1 (0.25 to 0.60 cm yr?1) based on the constant flux model, 0.40±0.15 cm yr?1 (0.15 to 0.58 cm yr?1) based on the constant activity model, and 0.44±0.11 cm yr?1 (0.30 to 0.59 cm yr?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess210Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.  相似文献   

7.
陆相断陷咸化湖盆有机质差异富集因素探讨   总被引：1，自引：0，他引：1  
我国中、新生代陆相断陷咸化湖盆广泛发育含盐泥页岩,蕴藏着大量页岩油资源。页岩油“甜点”发育与有机质富集密切相关,但含盐泥页岩有机质差异富集主控因素研究相对较少,且存在争议。以渤海湾盆地东濮凹陷为例,针对古近系沙三段泥页岩取芯,开展总有机碳（TOC）、扫描电镜、能谱分析以及主量、微量和稀土元素分析,探讨泥页岩的有机质丰度与古气候、古盐度、沉积速率、古生产力和氧化还原条件的关系。结果表明:沙三段泥页岩的有机质富集主要受控于古生产力、古盐度和沉积速率。古生产力越大,有机质富集程度越高;随古盐度和沉积速率升高,有机质富集程度先增大后减小。氧化还原条件对有机质富集的影响不大,这与泥页岩整体发育于缺氧的还原条件有关。只有在高的古生产力、适当的古盐度和适当的沉积速率背景下,方才利于沉积有机质富集。泥页岩的有机质差异富集是非常规油气沉积学研究的重要内容之一,揭示有机质差异富集主控因素,对我国陆相页岩油勘探开发具有重要意义。  相似文献   

8.
Chesapeake Bay sediments were examined for biogeochemical evidence of eutrophication trends using two mesohaline sediment cores. Measurements of 210Pb geochronology and sediment profiles of organic carbon, nitrogen, organic phosphorus, inorganic phosphorus, and biogenis silica (BSi) were used used to develop temporal concentration trends. Recent sediments have 2–3 times as much organic carbon and nitrogen as sediments from 80 to 100 yr ago, but the increases result from both changes in organic matter deposition and time-dependent changes in organic matter decomposition rates. Despite increases in phosphorus loading, no major changes in phosphorus concentration were noted throughout most of the century; anthropogenic phosphorus deposition, though not evident in sulfidic mid-bay sediments, must occur in more oxidizing sediment environments in both the northern and southern bays. Temporal trends in BSi concentrations are much less evident and the lack of substantial increases in this century suggest that BSi inputs may be capped by late spring-summer Si limitation.  相似文献   

9.
Persistent inorganic constitutents preserved in sediments of aquatic ecosystems record temporal variability of biogeochemical functioning and anthropogenic impacts.210Pb and137Cs dating techniques were used to study the past variations of heavy metals (Pb, Cu, and Zn) and accumulation rates of sediments for Tivoli South Bay, in the Hudson River National Estuarine Research Reserve ecosystem. South Bay, a tidal freshwater embayment of the Hudson, may play an important role in the sediment dynamics of this important river. The measured sedimentation rate range of 0.59 to 2.92 cm yr−1 suggests that rapid accumulation occurred during the time period represented by the length of the cores (approximately the past 50 yr). Direct measurements of sediment exchange with the Hudson River reveal high variability in the sediment flux from one tidal cycle to the next. Net exchange does not seem to be adequate to explain sediment accumulation rates in the bay as measured by210Pb and137Cs. The difference may be supplied from upland streams or the Hudson River during storm events. Concentrations of the metals Pb, Cu and Zn were found to be well correlated with each other within individual cores at five of six sites tested. This suggests a common proximate source for the three metals at a specific site. The evidence is consistent with mixing in some environmental compartment before delivery to the bay. While metals self-correlate within individual cores, absolute concentrations, depth distribution patterns, and ratios of the metals to each other vary among the cores collected at different locations within the bay. Organic matter, Fe content, and particle size distribution of sediments do not account for the intercore variations in metal concentration. It is likely that cores collected from different sites may have derived metals from different sources, such as watershed streams and tidal exchange with the Hudson River.  相似文献   

10.
Enriched concentrations of vanadium and nickel have been noted in a variety of naturally occurring organic substances including crude oils, asphalts, and organic matter in some sedimentary rocks. Vanadium and nickel concentrations in bitumens extracted from a variety of organic sedimentary rock types of different geological ages and geographical areas range from less than 0.2 to 4760 ppm and less than 7 to 1240 ppm, respectively. Vanadium concentrations showed a polymodal frequency distribution, while nickel concentrations showed a near-normal frequency distribution. The concentrations of these two metals showed no significant correlations with bitumen content, organic carbon content, or proportionality between bitumen and organic contents. Enriched vanadium and nickel concentrations greater than 100 ppm are only observed in bitumens that are associated with Type II and Type I kerogens. Conversely, bitumens associated with Type III kerogens contained vanadium and nickel concentrations less than 100 ppm.The high stability of vanadium and nickel in crude oils, asphalts, and bitumens suggest that they occur in tetrapyrrole complexes. These complexes may occur as free molecules or assimulated subunits in macromolecules because of their availability in anaerobic systems, small atomic radii, and favorable electron configurations. The potential for an organic sediment to be enriched in these two metals depends upon the amount of tetrapyrroles preserved in its organic matter. Tetrapyrrole preservation preferentially decreases in organic matter as exposure time to aerobic conditions increases. The potential for vanadium and nickel enrichment is therefore the highest in organic matter derived from algae that encountered anaerobic conditions early in their depositional history. Metallation of tetrapyrrole complexes appears to occur within sediments prior to their lithification, and interstitial waters are the most likely source for enriched concentrations of vanadium and nickel. The amount of metal enrichment in a sediment depends upon the diffusion of metal cations from its overlying water body, and this source is only effective as long as the sediment system remains open. The complete metallation of tetrapyrroles is most favorable under conditions of slow sedimentation rates, which would allow open sediment systems to be maintained for long durations.  相似文献   

11.
Macrotidal salt marshes play an important role in sedimentary processes in estuaries and can act as a sink for fine sediments and contaminants. This study examines sedimentation rates and the history of heavy metal accumulation in the Allen Creek salt marsh in the Bay of Fundy, Canada. Pb-210 and Cs-137 measurements and accelerated mass spectrometer (AMS) dating indicate a sedimentation rate of about 1.1 cm/year, which is consistent with independent observations. Elevated normalized concentrations of As in the upper section of the deposit may reflect an increase in organic matter content while a consistent decrease in Mn toward the surface of the section may be due to a decrease in natural supply. A peak in metal concentrations in the early to mid twentieth century is attributed to inputs from local foundries.  相似文献   

12.
Radiocarbon-dated sediment cores from the Champlain Valley (northeastern USA) contain stratigraphic and micropaleontologic evidence for multiple, high-magnitude, freshwater discharges from North American proglacial lakes to the North Atlantic. Of particular interest are two large, closely spaced outflows that entered the North Atlantic Ocean via the St. Lawrence estuary about 13,200–12,900 cal yr BP, near the beginning of the Younger Dryas cold event. We estimate from varve chronology, sedimentation rates and proglacial lake volumes that the duration of the first outflow was less than 1 yr and its discharge was approximately 0.1 Sv (1 Sverdrup = 106 m3 s−1). The second outflow lasted about a century with a sustained discharge sufficient to keep the Champlain Sea relatively fresh for its duration. According to climate models, both outflows may have had sufficient discharge, duration and timing to affect meridional ocean circulation and climate. In this report we compare the proglacial lake discharge record in the Champlain and St. Lawrence valleys to paleoclimate records from Greenland Ice cores and Cariaco Basin and discuss the two-step nature of the inception of the Younger Dryas.  相似文献   

13.
Sulfate reduction and sulfur-iron geochemistry were studied in 5-6 m deep gravity cores of Holocene mud from Aarhus Bay (Denmark). A goal was to understand whether sulfate is generated by re-oxidation of sulfide throughout the sulfate and methane zones, which might explain the abundance of active sulfate reducers deep below the main sulfate zone. Sulfate penetrated down to 130 cm where methane started to build up and where the concentration of free sulfide peaked at 5.5 mM. Below this sulfate-methane transition, sulfide diffused downwards to a sulfidization front at 520 cm depth, below which dissolved iron, Fe2+, accumulated in the pore water. Sulfate reduction rates measured by 35S-tracer incubations in the sulfate zone were high due to high concentrations of reactive organic matter. Within the sulfate-methane transition, sulfate reduction was distinctly stimulated by the anaerobic oxidation of methane. In the methane zone below, sulfate remained at positive “background” concentrations of <0.5 mM down to the sulfidization front. Sulfate reduction decreased steeply to rates which at 300-500 cm depth were 0.2-1 pmol SO42− cm−3 d−1, i.e., 4-5 orders of magnitude lower than rates measured near the sediment surface. The turn-over time of sulfate increased from 3 years at 12 cm depth to 100-1000 years down in the methane zone. Sulfate reduction in the methane zone accounted for only 0.1% of sulfate reduction in the entire sediment column and was apparently limited by the low pore water concentration of sulfate and the low availability of organic substrates. Amendment of the sediment with both sulfate and organic substrates immediately caused a 10- to 40-fold higher, “potential sulfate reduction” which showed that a physiologically intact community of sulfate reducing bacteria was present. The “background” sulfate concentration appears to be generated from the reaction of downwards diffusing sulfide with deeply buried Fe(III) species, such as poorly-reactive iron oxides or iron bound in reactive silicates. The oxidation of sulfide to sulfate in the sulfidic sediment may involve the formation of elemental sulfur and thiosulfate and their further disproportionation to sulfide and sulfate. The net reaction of sulfide and Fe(III) to form pyrite requires an additional oxidant, irrespective of the formation of sulfate. This could be CO2 which is reduced with H2 to methane. The methane subsequently diffuses upwards to become re-oxidized at the sulfate-methane transition and thereby removes excess reducing power and enables the formation of excess sulfate. We show here how the combination of these well-established sulfur-iron-carbon reactions may lead to the deep formation of sulfate and drive a cryptic sulfur cycle. The iron-rich post-glacial sediments underlying Holocene marine mud stimulate the strong sub-surface sulfide reoxidation observed in Aarhus Bay and are a result of the glacial to interglacial history of the Baltic Sea area. Yet, processes similar to the ones described here probably occur widespread in marine sediments, in particular along the ocean margins.  相似文献   

14.
Data on abundance and isotopic composition of porewater and sedimentary sulfur species are reported for relatively uncontaminated and highly contaminated fine-grained anoxic sediments of St. Andrew Bay, Florida. A strong contrast in amount and composition of sedimentary organic matter at the two sites allows a comparative study of the historical effects of increased organic loading on sulfur cycling and sulfur isotopic fractionation. In the contaminated sediments, an increase in organic loading caused increased sedimentary carbon/sulfur ratios and resulted in higher rates of bacterial sulfate reduction, but a lower efficiency of sulfide oxidation. These differences are well reflected in the isotopic composition of dissolved sulfate, sulfide, and sedimentary pyrite. Concentration and isotopic profiles of dissolved sulfate, organic carbon, and total sulfur suggest that the anaerobic decomposition of organic matter is most active in the upper 8cm but proceeds at very slow rates below this depth. The rapid formation of more than 90% of pyrite in the uppermost 2 cm which corresponds to about 3 years of sediment deposition allows the use of pyrite isotopic composition for tracing changing diagenetic conditions. Sediment profiles of the sulfur isotopic composition of pyrite reflect present-day higher rates of bacterial sulfate reduction and lower rates of sulfide oxidation, and record a profound change in the diagenetic cycling of sulfur in the contaminated sediments coincident with urban and industrial development of the St. Andrew Bay area.  相似文献   

15.
Distinctive planktonic foraminiferal assemblages which characterize particular late Quaternary sapropel layers in deep basin sediments from the eastern Mediterranean Sea have been identified using cluster analysis. Three distinct clusters allow for identification and intercore correlation of the nine sapropels deposited during the last 250,000 yr. Cluster 1, representing sapropel layers S1 and S9, exhibits low abundances of Neogloboquadrina dutertrei and high abundances of Globigerinoides ruber; Cluster 2, which groups S3, S5, and S7, contains high abundances of G. ruber, N. dutertrei, and Globigerina bulloides, and Cluster 3, which includes samples from S4, S6, and S8, is marked by extremely abundant N. dutertrei and G. bulloides, and rare G. ruber. Analysis of sedimentation rates in 14 cores reveals the following approximate ages for the sapropel layers: S2 = 52,000 yr B.P.; S3 = 81,000–78,000 yr B.P.; S4 = 100,000–98,000 yr B.P.; and S5 = 125,000–116,000 yr B.P. As previously suggested, sedimentation rates on the Mediterranean Ridge were determined to be relatively constant during the last 127,000 yr. In contrast, basin sedimentation rates have fluctuated markedly from lower rates during interglacial stage 5 to higher rates during the last glacial episode. These glacial/interglacial differences are most pronounced in the northern Ionian Basin, because of increased terrigenous sediment deposition during glacial episodes. Unusually high biogenic sedimentation rates occurred in an arc south of Crete during the deposition of sapropel S5, probably due to higher productivity in this region.  相似文献   

16.
运用γ谱仪,对采自福建省兴化湾外近海海域的D37和FJ3-12孔进行了 210Pb 、226Ra和 137Cs 活度的测试,得到这3种核素活度随深度变化特征。分别运用 210Pb 法的CFCS模式(恒定通量恒定沉积模式,Constant Flux and Constant Sedimentation)和CRS模式(恒定供给速率模式,Constant Rate of Supply)以及 137Cs 时标法计算平均沉积速率,发现3种方式的计算结果存在一定差异。在排除粒度变化对核素剖面的影响后,对比不同取样深度的结果发现,指数衰减剖面不完整度(取样深度未达 210Pb 的本底值区)对运用CFCS和CRS模式计算平均沉积速率均有不利影响,其中对CRS模式的影响随剖面不完整性愈强而愈显著。为此提出了相关校正方法作为参考,即先根据CFCS模式估算平均沉积速率和相应的210Pbex累积量,再通过CRS法建立钻孔年龄框架,由此可计算得出D37和FJ3-12孔的平均沉积速率分别为2.76cm/yr和4.53cm/yr。  相似文献   

17.
The radial-growth patterns of white spruce were studied on a number of trees growing in subarctic dunes along the eastern coast of Hudson Bay to calculate the rates of accumulation, erosion, and migration of cold-climate sand dunes. The average rate of sand accumulation in sheltered dunes (forest sites) was 2.5 to 3.3 cm/yr, which is two to three times lower than in highly exposed dunes with a rate of sedimentation of 7.65 cm/yr. The average erosion rate was 1.4–1.7 cm/yr, about two times lower than the accumulation rate. The migration rate of sheltered dunes was 18 to 30 cm/yr, three to five times lower than for an exposed dune which advanced at a speed of 74 cm/yr. This migration rate established for highly exposed dunes in the Subarctic with tree-ring methods is about 10 times lower than that established for a barchan in the Sahara with other methods.  相似文献   

18.
福建罗源湾潮滩沉积过程对人类活动和台风事件的响应   总被引：1，自引：0，他引：1  
王爱军  叶翔 《沉积学报》2013,31(4):639-645
潮滩地区是人类活动和台风事件影响最为显著的地区之一,利用潮滩沉积记录可以提取影响区域重大人类活动和台风事件的历史变化。罗源湾潮滩中上部盐沼和光滩柱状沉积物粒度分析结果显示,罗源湾潮滩沉积物以细颗粒为主,盐沼内部和潮滩中上部光滩柱状沉积物平均粒径分别为5.41~45.00 μm和5.4~68.82 μm,但在不同深度出现多次沉积物变粗的现象;利用210Pb测试计算得到的沉积速率分别为1.96 cm/a和1.23 cm/a;近年来的围填海活动导致潮间带地区沉积速率显著增大,互花米草的引种也进一步导致潮间带上部沉积速率增大。台风的强烈影响使得潮间带沉积出现间断,在沉积间断界面出现沉积物粒度变粗的现象,由此推断了近170年来显著影响罗源湾地区的台风事件沉积层的位置,并由此计算了不同时段的沉积速率。根据柱状沉积物粒度分析结果,不仅可以反演近百年来的对罗源湾产生重要影响的台风事件,也可以反演20世纪50年代以来罗源湾经历的几次较大规模的人类活动。  相似文献   

19.
The role of acetate in the biogeochemical cycling of organic matter in contemporary marine anoxic sediments of Skan Bay, Alaska was investigated with inhibition and quasi in situ turnover experiments. The turnover time for acetate oxidation in the upper 30 cm of the sediment column is ca. 1 hr. A molybdate inhibition experiment indicated that sulfate reducing bacteria were responsible for more than 95% of acetate oxidation. However, measured acetate oxidation rates exceeded sulfate reduction rates indicating that acetate oxidation rates are overestimated. Values for acetate concentration calculated from sulfate reduction rates (0.3–3.4 μM) were considerably lower than directly measured acetate concentrations (3.1–10.8 μM). Much of the chemically measured acetate may be microbially unavailable, perhaps in the form of a soluble or colloidal complex. A sorption experiment indicates that 10% to 40% of added acetate associates with Skan Bay sediment particles. Production of methane from acetate was detected only at 2 m depth.  相似文献   

20.
Abstract. The occurrence, lithology, and stratigraphic setting of buried manganese deposits and associated host sediments in cores obtained on Legs 123–210 of the Ocean Drilling Program (ODP) are examined in order to establish the formative environment and conditions of preservation. Fossil manganese nodule and crusts are found to have formed or deposited throughout the period from 100 Ma to the present, with an additional example of formation near 137 Ma, suggesting that the deep-sea environment has been oxic and suitable for the formation of manganese nodules and crusts since the Cretaceous. Many manganese nodules and crusts occur on horizons corresponding to hiatuses in sedimentation or periods of slow sedimentation, consistent with the environment in which modern nodules form (sedimentation rate less than 10 m/m.y.). Sediments overlying the fossil nodules and crusts are oozes or biogenic sediments with sedimentation rates of 1–18 m/m.y. Low total organic carbon (<0.1 wt%) in the overlying sediments and high sulfate content (>25 mM) in interstitial water around the manganese horizon suggest that no strong reduction occurred within the overlying sediments. Coverage by biogenic sediments containing only small amounts of organic matter is therefore considered important for the preservation of manganese nodules and crusts. Manganese carbonate occurs sporadically as nodules, concretions or thin layers in various host sediments, including clay, calcareous ooze and siliceous ooze with sedimentation rates of 6–125 m/m.y. Hiatuses are rare around the host sediments of manganese carbonate. Higher total organic carbon (0.2–1.8 wt%) in the host sediments and lower sulfate content (0–25 mM) in interstitial water around the manganese carbonate horizon suggest that reduction in association with decomposition of organic matter would have proceeded in the host sediments.  相似文献   

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