Subaqueous geothermal activity revealed by lacustrine sediments of the acidic Nakadake crater lake, Aso Volcano, Japan

Lacustrine sediments were sampled from the inaccessible acidic (pH = 0.43) Nakadake crater lake of Aso Volcano, Japan by a simple method. The sediments contain an extremely high content (74 wt.%) of sulfur, which exits as elemental sulfur, gypsum and anhydrite. The abundant elemental sulfur is likely formed by the reaction of SO₂ and H₂S gases and by the SO₂ disproportionation reaction in magmatic hydrothermal system below the crater lake. Based on the sulfur content of sediments and measurements of elevation change of the crater bottom, the sulfur accumulation rate at the Nakadake crater lake was calculated as 250 tonne/day, which is comparable with the SO₂ emission rate (200–600 tonne/day) from the Nakadake crater. The sediments include a small amount (9%) of clear glass shards that are apparently not altered in spite of the high reactivity of hyperacid lake water. This finding suggests that the clear glass shards are fragments of recently emitted magmas from fumaroles on the bottom of the crater lake and the magma emissions continuously occur even in quiescent periods.     

湖泊沉积物碳酸盐含量的XRD半定量分析

湖泊沉积中的碳酸盐对于气候环境具有很好的指示意义.利用化学分析方法(气量法)和X衍射(XRD)方法,对青藏高原可可西里库赛湖KS-2006孔沉积物中的碳酸盐含量进行了半定量分析.结果表明:虽然KS-2006孔沉积物中的矿物种类达9种之多,且XRD的测量误差(±5%)高于气量法的测量误差(±0.8%);但XRD法获得的KS-2006孔沉积物碳酸盐总含量的平均值(18.9%)以及波动范围(7%~39%),同气量法所获得的碳酸盐含量的平均值(19.4%)和波动范围(5.0%~30.7%)基本一致,而且二者具有很好的正相关关系(相关系数R²为0.3940),随深度的波动趋势一致,说明XRD方法在半定量分析湖泊沉积物碳酸盐矿物种类及其含量方面,具有很高的可靠性.     

湖泊沉积物中微量金属二次迁移过程中微生物作用的实验研究

蓄积在湖泊沉积物中污染物质某些情况下可以成为威胁上覆水体水质安全的二次污染源．根据贵州省阿哈湖季节性缺氧的特性,通过控制氧化还原状况,设计了对该湖沉积物-水柱的原样／抑菌条件的培养实验．实验发现,微生物活动对界面附近氧化还原反应具有控制作用;改变水体的含氧状况可以显著影响上覆水体水质,包括表观性状和水体中污染物含量．聚类分析结果表明,早期成岩过程中Mn,Ga,SO4 2-,Cu,Cr,Pb和Co,Ni,Fe,Sc,V,Rb两大类分别具有相似的地球化学行为．根据实验结果,计算了厌氧培养过程中,微量元素的最大释放量,发现铁、锰在厌氧过程中可以大量向水体释放．     

Weak chemical weathering during the Little Ice Age recorded by lake sediments

Low magnetic susceptibility, low Sr content and hence high Rb/Sr ratio in the lake sediment sequence indicate a weak chemical weathering process under arid and cold climate of the Little Ice Age in a single closed lake watershed. According to different geochemical behavior between rubidium and strontium in earth surface processes, variation of Rb/Sr ratios in the lake sediment sequence can be used as an effective geochemical proxy with definite climatic significance of chemical weathering in watershed. Unlike chemical weathering process in tropic zone and modern temperate-humid climate, concordant changes in both Sr content and magnetic susceptibility with δ¹⁸O values of Dunde ice core suggest that the weak chemical weathering was controlled by air temperature during the Little Ice Age maximum. After the Little Ice Age, chemical weathering intensity was controlled also gradually by precipitation with increasing in temperature.     

阳澄湖近代沉积物的古湖泊学研究

对阳澄湖两上短柱状样进行了地上代以及各种环境指标分析。结果表明,湖底沉积物由被长期沉积间断所分割的３个岩性层组成：下部层Ｃ属再沉积的更新世晚硬土层;中部层Ｂ为全新世沉积;上部层Ａ为近１００年以来的表层沉积。对层Ａ的详细分析显示,本世纪以来,特别是７０－８０年代以来,金属元素,营养元素和色素的沉积通量明显提高,说明湖泊重金属污染和富营养化程度的不断加重。     

Environmental changes reflected by the lake sediments of the South Hongshan Lake, Northwest Tibet

The 1.07-m long lake core with 1 cm interval cutting, which was obtained by drilling in the South Hongshan Lake of Northwest Tibet, was dated by the 210Pb and 137Cs methods, and a 150-year consecutive lake sedimentary sequence (1840─1997) with 1.4 year resolution was obtained. Some environmental proxies, such as the total organic carbon (TOC), total nitrogen (TN), ratio of TOC to TN (TOC/TN), trace chemistry elements (TCE), CaCO3, grain size, richness of ostracoda etc. showed that they are of well coincidence. These results implied that the environmental background varied from the cold-wet period in the late 19th century, to the warm-wet period from the end of the 19th century to the 1920s and to the warm-dry period since the 1920s. There were sub-variations since the 1920s: the cold-dry/warm-wet fluctuation from 1922 to 1960, the intensively warm-dry period since 1960 with a short cold-wet period in the mid-1970s to the end of the 1980s. The humid period from the mid-1970s to the end of the 1970s and the dry period beginning from the early 1980s were well documented by climatic data of the nearby weather station records while the grain size was well correlated to the annual precipitation. Compared with the records from Guliya ice core in the same area, the TOC proxy in the lake core indicating warm/cold conditions well corresponded to the ( 18O records representing temperature variations in the ice core. However, the proxies with dry/wet significance in the lake core were different from the variations of snow accumulation reflected by the ice core. It can be concluded that the chosen environmental proxies have clear environmental significance and the lake sediments can reflect climatic and environmental changes at high-resolution.     

Testing the potential of luminescence dating of high-alpine lake sediments

The potential of luminescence dating of high-alpine lacustrine sediments is tested on samples taken from three high-altitude moraine-bounded lakes in the Swiss Alps. Independent age control is provided by radiocarbon ages and detailed palynological data in all cases. All samples show good luminescence characteristics (no thermal transfer, good dose recovery and recycling), but two samples show rather low quartz signals. Radioactive disequilibria in the uranium decay chain observed in one sample are accounted for by using a dynamic modelling approach. Because all cores had largely dried out and water content had not been measured after sampling, we developed a modern limnological approach to retrospectively establish palaeo-water content. Applying average water content values from a dataset of modern sediments with similar characteristics to the samples investigated here we obtained ages that match the independent age control. Whereas the low-signal quartz separates consistently underestimate these ages, the polymineral samples more accurately match them, do not suffer from anomalous fading and are thus considered reliable. This study demonstrates for the first time that applying luminescence dating to high-alpine, ice-proximal lacustrine sediments is a promising avenue to obtain a chronology for such depositional environments.     

Adsorption of metal ions on bed sediments

Abstract

The adsorption of lead and zinc ions on bed sediments of the River Kali in western Uttar Pradesh, India, has been studied. The role of the coarser sediment fraction (210–250 μm) in controlling metal pollution has been elucidated and compared to those of the clay and silt fractions. The parameters controlling metal uptake, viz., solution pH, sediment dose, contact time, and particle size have been evaluated. The optimum contact time needed to reach equilibrium is of the order of 45 min for both the metal ions. The extent of adsorption increases with an increase of pH. Furthermore, the adsorption of the metal ions increases with increasing adsorbent doses and decreases with adsorbent particle size. The two geochemical phases of iron and manganese oxide act as the active support material for the adsorption of the metal ions.     

Impregnation of organic lake sediments for palaeomagnetic measurements

A procedure for impregnating high water-content sediments for palaeomagnetic measurements is described and applied to an organic lake sediment core. Comparison of results obtained by a standard sub-sampling procedure, and by the impregnation method, demonstrates a significant improvement in the scatter of the palaeomagnetic directions using the latter method.     

On the possibility of obtaining relative paleointensities from lake sediments

Relative paleointensities are obtained from a 6-m sediment core from Lake St. Croix, Minnesota, spanning the time range from 445 to 1740 years B.P. To normalize the natural remanent magnetization (NRM) for variations in the magnetic content, a laboratory-induced remanence is chosen, whose alternating field (AF) demagnetization curves most closely resemble the NRM demagnetization curves. By plotting the ratio of the NRM to the normalizing remanence versus AF demagnetizing field, H_AF, for samples of the same sediment horizon, as well as for samples from different horizons, estimates are obtained for expected uncertainties in the relative paleointensities. For the Lake St. Croix sediments the anhysteretic remanence (ARM) demagnetization curves are very similar to those of the NRM's, and ARM is therefore used as the normalization parameter. Because the sediment exhibits homogeneous remanence properties throughout, and H_AF = 100Oe is the optimum “cleaning” field for the entire core, NRM₁₀₀/ARM₁₀₀ is evaluated to represent the fluctuations of the relative paleointensity. Our relative paleointensity data exhibit the same general features as obtained from archeomagnetic studies. The intensity increases as one goes back in time with a peak near 800 years B.P., representing an increase in the intensity of up to 60%. Apparent periodicities in the intensity of 300–400 years are observed.     

Environmental records of carbon in recent lake sediments

Based on careful sampling and accurate analysis of recent sediments in Lake Chenghai, this paper discusses the staggered positive/negative correlation between organic carbon (OC) concentration and inorganic carbon (IC) concentration. The result indicates that temperature change, and its induced relative changes are the main factors affecting the relationship between IC concentration, δ¹³C of carbonate and OC concentration. When temperature and its induced photosynthesis strength change control autochthonous calcite precipitation, OC concentration is positively correlated with IC concentration and δ¹³C of carbonate. When temperature and its induced physical/chemical changes dominate, OC concentration displays negative correlation with IC concentration and δ¹³C of carbonate. IC concentration and δ¹³C of carbonate in sediments of Lake Chenghai are good proxies for climatic warm/cold changes.     

浅钻岩芯揭示的固城湖4000年来环境演化

对固城湖6.2m深的现代沉积物柱状岩芯进行了放射性同位素、粒度、有机质含量、有机质σ~(13)C和孢粉分析。沉积记录巾各项环境指标的变化表明,4ka以来固城湖经历了高湖面—低湖面—高湖面的显著变化。气候变化是影响湖泊环境演化的重要自然因素,但2.5ka以来人类活动对湖泊演化的影响显得更为明显。特别是2.5ka和1.1ka两次人类对湖泊水系的改造是引起湖泊环境两次突变的主要因素。     

底泥释磷及其对杭州西湖富营养化的影响

杭州西湖是一个小型浅水湖泊,底泥由上部藻骸腐泥和下部泥炭层构成,其显著特点是有机碳含量特别高,氮含量也相当高,而磷的富集程度相对较低。通过实验室和现场模拟研究,考察了pH值、温度、溶解氧、氧化还原电位及上覆水种类等环境因素对底泥释磷量和释磷速率的影响。上覆水pH值在6.5—7.0范围内底泥释磷量最低;在较高或较低pH值时,释磷量倍增。升高水温或降低上覆水溶解氧浓度均能加速释磷。实验室模拟西湖底泥最大释磷量为0.368μgP/g。夏季现场模拟平均释磷速率为1.02mg P/m~2·d;由此估算底泥释磷量达1.346tP/a,相当于年平均外部入湖磷负荷的36.4％。底泥释磷对西湖富营养化起着不容忽视的影响。     

Methane levels in shallow subtropical lake sediments: Dependence on the trophic status of the lake and allochthonous input

The vertical sediment profiles (10 cm) of the margins of three shallow subtropical lakes (Rio Grande, Brazil) with different trophic states and surrounding areas were evaluated to identify the effects of the allochthonous input on the methane concentration in the sediment. Sediment cores were collected to quantify the organic matter content (OM) and total carbon (TC), total nitrogen (TN), total phosphorous (TP) and methane (CH₄) concentrations.The three lakes were distinguished according to the trophic status and classified as oligotrophic, dystrophic and eutrophic. The natural characteristics of the dystrophic and eutrophic lakes have been changed due to the allochthonous input of leaves and twigs (Eucalyptus sp.) and the excreta of birds, respectively. In the eutrophic lake, the allochthonous input contributed to high autochthonous production. The highest values of OM, TC, TN and TP were found in the superficial sediments of the dystrophic and eutrophic lakes. The accumulation of allochthonous organic matter in the littoral zone promoted changes in the vertical sediment profiles and contributed to increases in the CH₄ concentrations in the sediment.     

2.5ka来新疆吉力湖湖泊沉积记录的气候环境变化

选择位于西风区的吉力湖为研究对象,利用~(210)Pb、~(137)Cs和AMS~(14)C测年手段,建立了吉力湖沉积岩芯2.5ka以来的时间序列。对湖泊沉积物的粒度、总有机碳、总氮以及有机碳同位素等多环境指标的综合分析,重建了吉力湖2.5ka以来的气候环境演化特征。2.5ka以来吉力湖气候环境变化大致经历了5个阶段:500BC-50BC,暖干;50BC-650AD,冷湿,气候不稳定,冷暖波动较大;650-1350AD,暖干,对应于中世纪暖期(MWP),1350-1850AD,冷干,对应于小冰期(LIA);1850AD以来,温干,其中20世纪20年代后,尤其是70年代以来,气候变暖。吉力湖多环境指标记录的气候环境变化与其它地质记录以及历史文献记录均具有较好的一致性。     

Dynamics of phosphorus in sediments of a naturally acidic lake

The mechanisms which controls the fixation and/or release of P in sediment of an extremely acidic lake（pH = 2.0 to 3.0） and its response to the influence of eutrophic urban waste water were investigated.The results,in the chemical composition,in the mineralogy of the sediment and in the material as obtained from sediment traps,show that the lake sediments are mainly volcanic material reflecting volcanic features of the basin.The sedimentation rate calculated for the lake(2.5×10^-2 mg m^-2 day^-1) was higher than that observed in other similar glacial lakes in both Andean Patagonia and also elsewhere in the world.The Total Phosphorus concentration in sediments was higher than figures reported by other authors for mining acid lakes,and the main fraction of P was found associated with organic matter.There was no control by Fe or Al on P,because both are in solution at pH < 3.0.It was concluded that changes in the natural input of nutrients（derivatives of Copahue volcano fluid,the discharge of sewage,or basin run-off） are responsible for a high concentration of SRP and N-NH₄⁺ in the lake.Laboratory experiments showed that sediments have no ability to retain phosphorus and a continuous release of P from the sediments into the water column was observed.The assays where the pH was artificially increased showed that the P still remains in solution until at least pH 7.0.It was concluded that changes in the natural input of nutrients due to:1) the volcanic fluids,2) the increase in sewage charges,or 3) surface runoff upstream,maintain a high trophic state with high concentrations of dissolved P and N-NH₄⁺,although the threshold of neutral pH in the lake is exceeded.This study will enable a better understanding about of the mechanism of release/fixation of phosphorus in acidic sediments in order to assist in making decisions regarding the conservation and management of this natural environment.     

Identification of greigite in lake sediments and its magnetic significance

Detailed rock magnetic investigations and mineralogical analysis were conducted on lacustrine sediments recovered from Zoigê Basin, eastern Tibetan Plateau. Greigite, with fine and homogenous grain size, is found as a main magnetic carrier, which is suspected to be of biochemical origin. In contrast to some reports, greigite within the sediments is not oxidized even after exposure to air for a few years, which could be due to the fact that greigite is sealed by silicate.     

Structure of hakone caldera as revealed by drilling

Hakone caldera, now 8 by 12 km in diameter, was formed by collapse of a center of a volcano probably 2700 m high. The collapse took place at two separate periods each of which was followed by periods of deep denudation. The central part of the caldera has been covered by a thick pile of lavas of post-caldera cones and domes. For the purpose of finding thermal spring, drilling to depths of a few hundred to one thousand meters was carried out at various points within the caldera except for its central region. The study of the drill cores revealed that the average amount of subsidence at points 2 and 3 km away from the base of the present caldera wall is 600 m and 1200 m respectively, and probably more than 1800 m in the middle of the caldera. Within the caldera, the pre-caldera lavas and pyroclastic rocks are either lacking or much thinner than would be expected. It is concluded therefore that the present topographic depression of the caldera owed its origin to both subsidence and denudation. It is inferred that the subsidence took place along a complicated system of concentric faults combined with tilting of individual fault blocks toward the middle of the caldera. The magma reservoir into which the fault blocks sank probably had a shape of a cupola with a diameter comparable to or a little smaller than the diameter of the caldera.