Long chain alkenones preserved in Miocene lake sediments

Over the last few decades, the occurrence of long chain alkenones (LCAs) in Quaternary lacustrine settings has been reported, the distributions being remarkably different from those of open ocean settings. We report here the occurrence of LCAs with modern lake-type composition from Middle Miocene sedimentary strata in the Dahonggou Section, Qaidam Basin, northern Tibetan Plateau. This is the first report of LCAs with preserved lake compositions in pre-Quaternary lacustrine sediments. The discovery implies that the producers of modern lacustrine LCAs could originate from similar lake species living at least as far back as the Miocene epoch. It also sheds new light on the potential of “alkenones” as palaeotemperature indicators in lacustrine systems back to at least 10.5 Ma.     

江汉盆地潜江凹陷古近系盐湖沉积盐韵律及其古气候意义

盐湖沉积具有成盐多期性和长期性、沉积连续性、淡化-咸化周期性等特色,因而成为在恢复古气候环境、进行全球变化研究的重要研究对象之一。本文以我国东部独具特色的古近纪古盐湖盆地--江汉盆地潜江凹陷潜江组盐韵律为例,通过对王平1等3口井连续取心段的精细研究,在前人划分Ⅰ、Ⅱ、Ⅲ级盐韵律的基础上,首次划分出组成含盐层系基础韵律单元--Ⅳ级盐韵律,弄清了其沉积过程基本遵循从盐岩→（含泥）钙芒硝岩→含云泥岩（含泥云岩）→泥岩→白云岩→钙芒硝岩→盐岩的淡化-咸化序列和盐类矿物的析出顺序;解析了Ⅳ级盐韵律及其沉积组合记录与水体古盐度波动和短尺度（0.05ka -1.0ka）古气候干-湿变化之间的对应关系。根据典型暖相盐类矿物原生钙芒硝及其薄层在潜江组中广泛发育,可以推断江汉盆地流域在晚始新世-早渐新世潜江组沉积期间,所出现的干旱古气候背景属于暖旱型而非寒旱型。     

Hydroxy acids in Antarctic lake sediments and their geochemical significance

Hydroxy acids in sediments of Lakes Bonney, Fryxell, Joyce and Vanda, and unnamed ponds (B2, NF1, NF2 and L4) as well as in cyanobacterial mats from the McMurdo Sound region of southern Victoria Land in Antarctica have been studied to clarify their features and elucidate their source organisms. Normal and branched (iso and anteiso) 2-hydroxy acids were found in all the samples studied with the predominance of even- and odd-carbon numbers, respectively. The most dominant 2-hydroxy acids in the sediments were mainly short-chain components (<C₂₀). Normal and branched 3-hydroxy acids were detectewith the predominance of even- and odd-carbon numbers, respectively, in total concentrations between 0.48 and 53 μg/g of dry sediment. (ω-1)-Hydroxy acids were all long-chains (C₂₂, C₂₄, C₂₆, C₂₈ and C₃₀). 9,10-Dihydroxyhexadecanoic and/or 9,10-dihydroxyoctadecanoic acids were identified in all the sediments and a cyanobacterial mat. The composition of hydroxy acids differ considerably among the lakes and ponds, suggesting the difference of source organisms. These 2-, 3- and (ω-1)-hydroxy, and 9,10-dihydroxy acids may be derived from cyanobacteria and microalgae, in addition to non-photosynthetic microorganisms. Cyanobacteria and microalgae which are widely distributed in the world, may be important sources of hydroxy acids in the natural environments.     

Bacterial and algal hydrocarbons in sediments from a saline Antarctic lake, Ace Lake

Capillary gas chromatography-mass spectrometry (C-GC-MS) and Iatroscan thin layer chromatography-flame ionisation detection (TLC-FID) were used to study hydrocarbon distributions in a sediment core from Ace Lake, a saline, meromictic lake in the Vestfold Hills of Antarctica. Hydrocarbons were abundant in most core sections (up to 125/μg/g dry wt), particularly in near-surface samples, and the distributions were very complex. Major constituents were identified as phytane, 2,6,10,15,19-pentamethyleicosane, tetrahydrosqualene, a mixture of phytenes, cholesta-3,5-diene and fern-7-ene. Smaller amounts of sterenes and hopenes were also present. The predominance of the first 3 acyclic isoprenoids in sediments buried less than 30 cm is consistent with high populations of methanogenic bacteria known to be present.Phytenes were abundant in all core sections, and there was no relationship between their abundance and that of phytane which suggests that they were not derived from methanogenic bacteria. Phytadienes were minor constituents at all depths studied. An unusual feature of some distributions was the high concentrations of fern-7-ene which was the major hydrocarbon in the 20–25 cm core section. This alkene was only abundant in sediments which contained high concentrations of methanogen markers suggesting that it may also be indicative of anoxic depositional environments. A possible source might be from purple non-sulphur bacteria. High concentrations of straight-chain C₂₉ and C₃₄ alkenes were also found in these sediments but their origin has not been determined. Major changes in the hydrocarbon distributions with depth indicate that the depositional environment in the lake has altered dramatically since the lake was formed less than 8000 years ago. The present condition of permanent anoxic bottom waters probably developed only in the last 1000 years.     

太湖全新世沉积物的古盐度指标及其环境意义

沉积物中的Ｃａ／（Ｆｅ＋Ｃａ）比值和硼含量随水体含盐度的高低而相应增减，它们都是较好的古盐度指标。本文对太湖全新世的沉积物进行了古盐度指标的分析。并与现代湖泊和海洋沉积物作了对比，结果反映东太湖为陆相沉积环境，而西太湖则主要为海相沉积环境。此结论与古生物化石的结论一致，从而证明了太湖地区于全新世期间经历了大规模的海侵，并形成海湾－湖环境。同时也证明了这些古盐度指标是行之有效的。     

Distribution and isotopic composition of sulfur in lake sediments of northern Ontario

Sediments from unpolluted and highly polluted lakes in northern Ontario have been fractionated into acid volatile sulfide, HCl-soluble sulfur, elemental sulfur, pyrite sulfur, ester sulfate and carbon-bonded sulfur and the isotopic composition of each fraction determined. In general, reduced inorganic S constitutes 25–50% of the total S in the polluted surficial sediments, but is <20% in the unpolluted samples, with pyrite formation being a minor process of S diagenesis in lake sediment ecosystems. Organic S in the form of ester sulfate and carbon-bonded S predominates and both the C/S ratios and the isotopic data suggest that, in unpolluted lakes, plant detritus can be a major contributor of organic-S to the sediments. The depth profiles observed suggest that the more labile ester sulfate is diagenetically converted to the carbon-bonded form. For the polluted sediments from the Sudbury basin, the isotopic data suggest that (a) the elemental S is derived from the oxidation of acid volatile sulfide in the aerobic surficial sediments, and (b) the isotopically light reduced S species are incorporated into the organic material. S diagenesis in lake sediments generally results in the release of ³⁴S to the overlying water. The suggestion is made that sulfate concentrations over 5 mg/1 accompanied by an enrichment of surficial sediments with isotopically different S may signal significant inputs of pollutant S into the lake and its basin.     

Sterols as source indicators of organic materials in sediments

The sterol contents of plankton and sediment samples have been determined. Cholesterol, 22-dehydrocholesterol, brassicasterol, campesterol, 24-methylenecholesterol, stigmasterol, and β-sitosterol are the major sterols in these samples. Systematic variations have been observed in the compositions of the sterols from bay and Gulf of Mexico plankton and from sediments collected in the mouth of the Aransas River, Capano Bay, Aransas Bay, Corpus Christi Bay and the Gulf near Port Aransas. The concentration of cholesterol increases as the concentration of β-sitosterol decreases in the sample sequence of river inlet sediments-bay sediments-Gulf sediments-bay and Gulf plankton. Calculations based on either the cholesterol or the β-sitosterol contents of the sterol fractions of the fresh water sediments and the Gulf plankton and sediments suggest that 34% of the sterols in the Gulf sediments are derived from terrigenous sources.     

Late-glacial climatic oscillations as recorded in Swiss lake sediments

Regional pollen assemblage zones for the late-glacial period of the Swiss Plateau are introduced and defined. They include four major zones (Artemisia, Juniperus—Hippophaë, Betula, Pinus PAZ) with several subzones. Pollen and oxygen-isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000-9500 yr BP. The first oscillation, termed the Aegelsee fluctuation, occurs shortly before 12 000 yr BP and varve counts suggest its duration was ca. 100 yr. It is characterised by a short decrease in the oxygen isotopes as well as a short increase in NAP associated with a depression in birch pollen values. The second oscillation, which occurs in the δ¹⁸O record shortly before the deposition of the Laacher See Tephra (ca. 11 000 yr BP), is termed the Gerzensee fluctuation. It occurs during a pine-dominated phase and its vegetational effects cannot be determined palynologically. The most prominent regressive phase is the Younger Dryas biozone (ca. 10 700-10 000 yr BP) characterised by an increase in heliophilous NAP and low δ¹⁸O values. The Younger Dryas biozone can often be subdivided palynologically into two parts: a first part rich in grasses and juniper and a second part with higher Filipendula and birch values. During the Preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios. Comparison of the Swiss oxygen isotope profiles with the Greenland Dye 3 record suggests that not only the three major shifts in the δ¹⁸O curves but also the minor ones are closely comparable, suggesting some common climatic control.     

Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China

Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to ∼100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C₃₇/C₃₈ ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C_37:4 (the percentage of the C_37:4 alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C_37:4 index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C_37:4 values need further investigation.     

Distribution of n-alkanes in lacustrine sediments from subtropical lake in Brazil

The geochemical composition of lake bed sediments of a tropical reservoir in Brazil have been investigated. The C, N, P composition showed almost no variation between the different sampling points. All samples contained inorganic phosphorus (IP) ranging from 52 to 70%. The Redfield ratios show that the lake is without significant anthropogenic inputs. Most of the organic matter is composed by higher plants decomposition revealed in the total organic carbon (TOC):nitrogen (N) ratio ranging from 15.4 to 57.2. Also, TOC:organic phosphorus (OP) ranged from 265.3 to 933.6, suggesting that most of the organic matter has terrestrial origin from wood plants. The ratios suggest that most organic matter is influenced by the terrestrial characteristics of the watershed. Furthermore, the relative abundance of n-alkane homologues in the sediments was investigated. All samples have been analyzed for the n-alkanes C₈ to C₄₀. The sediments were dominated by n-alkanes C₂₅–C₃₈. It is concluded that n-alkane in sediments mainly come from terrestrial plants, however there is a contribution of submerged aquatic plants, especially in those sites in deeper areas of the lake. On the basis of P_aq index the n-alkanes in sediments comes from terrestrial plants, however there is a contribution from emerged/floating plants.The investigated lake seems to be considered as meso to eutrophic. Because of the relatively high primary productivity in the lakes, there is a substantial organic-matter flux to the sediments, which rapidly becomes anoxic. According to the pristine/phytane ratio the sediments represent an anoxic environment with values found between 0.38 and 1.72.     

Distribution of reduced inorganic sulfur compounds in lake sediments receiving acid mine drainage

The sediments of Lake Anna, Virginia, act as a major sink for incoming acid mine drainage (AMD) pollutants (Fe, SO₄²⁻, H⁺) due to bacterial sulfate reduction (SR). Acid-volatile sulfide (AVS), elemental S, and pyrite concentrations in the sediments of the polluted arm of the lake are significantly greater than those in unpolluted sections of the lake. Measurements of SR using ³⁵SSO₄²⁻ showed that AVS and S⁰ are the major short-term (48 h) products of SR in these sediments. Inorganic forms of S(AVS, S⁰, and FeS₂) made up from 60 to 100% of the total sediment S concentration. Pyrite concentrations in the sediment were high but decreased exponentially with distance from the AMD source, suggesting that the pyrite was deposited as stream detritus from the abandoned mines. Iron monosulfide and elemental S concentrations were highest at a station 1 km away from the AMD inflow, indicating formation in situ. There was no evidence for the formation of organic S species. The results suggest that in Fe- and S-rich locations such as those contaminated with acid mine drainage, the distribution of end products of SR may vary substantially from those reported for more moderate environments.     

晚第四纪湖泊沉积物盐类与碎屑矿物反相关关系的沉积学解释

湖泊沉积物中矿物组合是古气候环境研究的敏感性指标之一, 充分认识湖泊沉积物中各类矿物组合类型、沉积过程, 对正确解释矿物学指标有重要意义. 我国晚第四纪湖泊沉积记录中, 盐类矿物与碎屑矿物含量普遍存在反相关关系, 影响了矿物组合作为气候指标的解释和应用. 从地球科学的角度正确理解这种关系, 可以为湖泊动力学以及古气候环境定量研究提供重要的科学依据. 选择甘肃民勤盆地石羊河下游终端湖猪野泽不同位置5个全新世剖面, 在明确湖泊沉积物来源、搬运方式和沉积动力机制的基础上, 研究了以碳酸盐矿物为主的盐类矿物含量与粒度指标之间的关系. 结果表明:猪野泽各剖面砂层富含的中砂、细砂沉积物主要是来自于巴丹吉林沙漠和腾格里沙漠的风成砂; 水动力作用主导了湖相沉积层200 μm以下沉积物的沉积, 风力作用对20~70 μm粉砂组分沉积也有所贡献. 同时, 猪野泽湖相沉积层中高含量的碳酸盐主要来自于流水搬运, 湖相沉积层位200 μm以下的粉砂和极细砂是碳酸盐的主要富集区. 综上所述, 干旱区湖泊沉积物中盐类矿物含量与其沉积过程密切相关, 盐类矿物在全球变化研究中的应用要建立在充分研究其沉积动力机制的基础上.     

福建晋江龙湖中晚全新世沉积的环境意义

为研究福建沿海的全新世气候环境变化,对取自晋江龙湖的柱样进行了沉积相、粒度、磁化率、有机碳、有机氮等分析,结合加速器14〖KG-*5〗C测年结果,对龙湖的形成演化以及该地区中晚全新世的气候环境变化进行了研究。认为,龙湖形成于约5580 calaBP〖DK〗.〖KG-*5〗,之前为河流相沉积;约5580～850 calaBP〖DK〗.〖KG-*5〗,该区为深灰色粉沙和砂质粉沙的湖泊相沉积;约850～470 calaBP〖DK〗.〖KG-*5〗,龙湖水位下降,变为沼泽湿地;之后,湖泊水位再次上升,再次形成湖相沉积。各种气候环境指标分析结果显示,自龙湖形成以来,当地的气候呈阶段性相对干、湿变化,并呈变化幅度逐渐减小的趋势。这一方面反映了福建沿海中晚全新世气候的不稳定性,还反映了当地气候环境变化与太阳辐射的长周期变化有关。沼泽湿地的形成,反映当地降水的进一步减少,而湿地的形成年代,可与太阳活动、北大西洋冰筏、亚洲季风等突发事件对比,说明该沼泽层的形成与叠加在气候长期变化趋势下的突发气候事件有关。龙湖沉积记录也表明,气候变冷,福建沿海与台风有关的降水将减少;反之,全球变暖,福建沿海台风降水将增加。     

Sub-bottom temperature perturbations due to temperature variations at the boundary of inhomogeneous lake or oceanic sediments

For many years continental heat flow data were obtained from temperature gradients measured in boreholes and conductivity measurements made on representative cores. Because of the difficulty of obtaining deep holes in suitable regions, increasing use is being made of measurements made in the top few meters of the sediments at the bottom of continental lakes. For reliable heat flow results two conditions must be met. First, an accurate record of the temperature history at the top of the sediment should be available, preferably over a time span of 4 or 5 years; second, if the thermal properties are not constant with depth, then the depth-dependence should be known.Errors that may arise if either or both of these conditions cannot be met can be serious. Using various surface thermal histories we compare results from models assuming constant diffusivity, a diffusivity changing exponentially with depth and a layered medium, all being structures based upon observed variations near the sediment-water interface where conductivity may change by a factor of two over 2 or 3 m. In some situations, errors as large as several hundred percents may occur but in such cases it should be possible to deduce from the data that something has occurred to make the data unreliable; more realistically, there are occasions where a temperature-depth plot which is linear within normally acceptable limits of accuracy and with a gradient that is within the range of expectations may be as much as 100% in error.     

A history of fire and vegetation in northeastern Minnesota as recorded in lake sediments

The record of charcoal in lake sediments indicates that fire has always been an important ecological factor in the forest history of northeastern Minnesota. The annually laminated sediments of Lake of the Clouds permit precise dating of the charcoal peaks and record the changes in the influx of various pollen types. A detailed record of the past 1000 yr shows that the average frequency of fire is approximately 60–70 yr, with a range of about 20–100 yr. The amount of charcoal in sediments dating between 1000-500 y.a. is consistently higher than that for the last 500 yr, although the fire frequency for the two periods was not appreciably different. Pollen analysis shows no change or only short-term changes in the percentages of major pollen types following charcoal peaks.     

Metals in sediments of springs and cave streams as environmental indicators in karst areas

The content of heavy metals and arsenic in sediments of karst streams in southern Missouri was investigated for its potential use as an indicator of pollution. A three-step sequential extraction procedure was utilized for this purpose. The amount of trace elements bound to each extraction phase gives insight of its availability and geochemical dependence. These results were complemented with analyses of correlation and spatial variability. Although sediments collected in this study remained below EPAs critical value guidelines, concentration in the mobile phases and higher normalized Mn values successfully identified sites with concentrations higher than background levels. Correlation among elements was poor in most cases; among the trace metals only Cu and Zn correlated while Pb correlated with Ca, Al, Mn and Fe. Spatial variability analysis confirmed that natural variation among adjacent sediment samples is a common occurrence. The urban spring Ward Branch showed the highest levels of Cr, Zn, Pb and As.     

Periglacial phenomena and their significance as climatic and edaphic indicators

As formed by frost-action and the presence of intensely frozen ground periglacial phenomena, especially microrelief features, are good examples for geomorphic phenomena as being clearly dependent upon specific climatic conditions. Whereas climate controls and delimits the formation and occurrence of periglacial phenomena at a zonal and regional scale, non-climatic vegetational, topographical and edaphic factors are significant within the climatically defined boundary conditions at a smaller scale. It can be shown that each type of periglacial phenomenon is dependent on specific climatic and edaphic conditions. Data on these conditions are compiled in several tables. On the other hand, because of this dependence on specific environmental conditions periglacial phenomena are at the same time diagnostic indicators for such conditions. As to climate they are indicators for specific climatically defined types of frozen ground and for thermal conditions in terms of mean values, such as mean air temperatures, freezing and thawing indexes. Spatial associations of genetically different types of periglacial phenomena indicate the extent and regional subdivision of the periglacial environment and its climatic limits. The definition of the climate-diagnostic value of present periglacial phenomena is an essential perequisite for the palaeoclimatic interpretation phenomena are sensitive to climatic variations of small amplitude and as such they are also indicators of recent short-term climatic changes. As to edaphic conditions they indicate frost-action potential of the groud and various soil properties which have practical implications. As edaphic indicators periglacial phenomena are of significance for terrain evaluation programmes in the Arctic for practical purposes.     

Distribution and Variation of Ribonucleic Acid （RNA） and Protein and Its Hydrolysis Products in Lake Sediments

Protein and RNA in lake sediments tend to be decomposed progressively with time and sedimentation depth. Their concentrations tend to decrease starting from the sedimentation depth of 17 cm and that of 19 cm, respectively. However, the products of their decomposition —amino acids and nucleotides show different rules of variation. At the depth from 27 cm to 30 cm the amino acids are most abundant in the pore waters of lake sediments. Such variation tendency seems to be related to the extent to which microbes utilize amino acids and nucleotides. Due to polymerization in the geological processes and the adsorption of protein on minerals and organic polymers, below the sedimentation depth of 17 cm there is still a certain amount of protein in the sediments. With the time passing by, protein has been well preserved in various sediment layers, indicating that its decomposition is relatively limited. The peak values of protein content in the sediments of the two lakes are produced in the surface layers at the depth of 10 cm, implicating that the surface sediments are favorable to the release of protein. The contents of amino acids in the pore waters of lake sediments are closely related to the activities of microbes. Below the depth of 27 cm, the amino acids are significantly accumulated in Lake Aha sediments, probably indicating the weakening of microbial activities. The research project was financially supported jointly by the Ntional Natural Science Foundation of China (No. 40173038) and the Guizhou Provincial Foundation (No. 3090) and the Chinese Academy of Sciences Knowledge-Innovation Foundation (KZCX2-105).     

Pregnanes as molecular indicators for depositional environments of sediments and petroleum source rocks

Steroids with unconventional side chains have increasingly been applied as diagnostic markers for geological source and age assessments. However, one of the most distinctive characteristics, the abnormal abundance of pregnane and homopregnane in ancient sediments and petroleum, remains unresolved. Higher pregnane and homopregnane, as well as C₂₃–C₂₆ 20-n-alkylpregnanes, relative to the regular steranes were observed in samples collected from different petroleum basins in China. These included Precambrian marine carbonate-derived petroleum (NW Sichuan Basin), Lower Paleozoic marine marl derived crude oils (Tarim Basin), and Eocene hypersaline lacustrine carbonate source rocks and associated petroleum (Bohai Bay Basin). However, all of the samples have many common biomarker characteristics, such as pristane/phytane ratios < 1, low amounts of diasteranes and high C₂₉/C₃₀ hopane (∼0.6–1), C₃₅/C₃₄ hopane (mostly ⩾ 1) and dibenzothiophene/phenanthrene (DBT/PHEN, mostly 0.5–1) ratios revealing a contribution from anoxic carbonate/marl source rocks deposited in restricted, clastic-starved settings. We suggest that 5α,l4β,l7β-pregnane and homopregnane, as well as their higher C₂₃–C₂₆ homologues, are geological products derived from steroids bound to the kerogen by a sulfurized side chain. Carbon or carbonate minerals are considered to be natural catalysts for this cracking reaction via preferential cleavage of the bond between C-20 and C-22. Similar distributions occur in the short chain analogues of 4-methylsterane, triaromatic steroid and methyltriaromatic steroid hydrocarbons, providing circumstantial evidence for this proposal. The ratio of pregnane and homopregnane to the total regular steranes and the ratio of C₂₇ diasteranes to cholestanes can be sensitive indicators of sedimentary environments and facies. In general, high diasteranes and low pregnanes (with homologues) indicate an oxic water column or significant input of terrigenous organic matter in clay rich source rocks and some organic lean carbonate rocks. Low diasteranes with high pregnanes implies restricted, sulfur rich conditions, typical of anoxic carbonate source rocks. Furthermore, the two ratios may be useful to assess the variation of mineralogy and openness of source rock depositional settings.