A possible varving mechanism for meromictic lakes

A variety of mechanisms have been proposed to explain how light and dark couplets are formed in the profundal sediments of meromictic lakes but none of these explanations acknowledges the significance of the sudden and massive mortality of mixolimnetic anaerobic bacteria. Data collected from sedimentation chambers suspended below the chemocline in a small meromictic lake indicate that a massive mortality of photosynthetic bacteria was induced by a sudden intrusion of partially oxygenated water into the anaerobic lower mixolimnion. The resulting deposition of anaerobic bacteria (0.45 mg dry wt cm^?2 day^?1) contributed 60% of the annual sediment influx during a 14-day period. It is postulated that the sudden sedimentation of the anaerobic bacteria inhabiting the lower mixolimnion of some meromictic lakes contributed to the formation of the dark-colored laminae in the sediments of these lakes.     

沉积物中细菌叶绿素的环境指示意义

沉积物中的光合细菌与细菌叶绿素（BChl）包含着丰富的古湖泊和古海洋信息。对光合细菌的生存环境,BChl的分布、保存、演化规律等方面进行了阐述,并探讨了湖泊和海洋沉积物中BChl的环境指示意义。BChl作为厌氧光合细菌的主要色素,不但可以指示水体中厌氧光合细菌的演变,而且能够反映化变层深度变化和水体环境特征,揭示季节性和年际尺度的气候变化。同时,根据沉积物中BChl含量可以估算古气候环境下湖泊和海洋的厌氧初级生产力。     

Sulfur isotopic evidence for chemocline upward excursions during the end-Permian mass extinction

The latest Permian was a time of major change in ocean chemistry, accompanying the greatest mass extinction of the Phanerozoic. To examine the nature of these changes, samples from two well-studied marine sections that span the Permian-Triassic boundary have been analyzed: the Meishan and Shangsi sections located in Southern China. Isotopic analysis of the carbonate-associated sulfate in these samples provides a detailed record of several isotopic shifts in δ³⁴S_CAS approaching and across the PTB, ranging from +30 to −15‰ (VCDT), with repeated asynchronous fluctuations at the two locations. We interpret the patterns of isotopic shifts, in conjunction with other data, to indicate a shallow unstable chemocline overlying euxinic deep-water which periodically upwelled into the photic zone. These chemocline upward excursion events introduced sulfide to the photic zone stimulating a bloom of phototrophic sulfur oxidizing bacteria. We hypothesize that elemental sulfur globules produced by these organisms and ³⁴S-depleted pyrite produced in the euxinic water column were deposited in the sediment; later oxidation led to incorporation as CAS. This created the large changes to the δ³⁴S_CAS observed in the latest Permian at these locations.     

Historical change of phytoplankton in a eutrophic lake in Japan as determined by analysis of photosynthetic pigments in a lakebed sediment core

No pre-1977 quantitative data exist that describe phytoplankton biomass in the water column of Lake Suwa, a Japanese eutrophic lake, although several studies have been conducted since. To assess recent environmental changes with time in the lake, phytoplankton composition was determined from analysis of photosynthetic pigments in the sediment core. Specifically, the total chlorophyll a (TCh-a) concentration in the water column before 1977 was estimated for comparison with that of the water column and sediment deposited since 1977. The depth profile of zeaxanthin, a potential indicator of cyanobacteria activity, substantially increased from 1960 to 1973 (from 9.53 to 76.7?nmol?g^?1). Additionally, the concentration of fucoxanthin, diadinoxanthin and diatoxanthin, originating from diatoms and dinoflagellates, increased slightly during this period (from 1.64 to 4.90, 1.86 to 3.54, and 4.00 to 3.26?nmol?g^?1). Cyanobacteria have been the major algal species in the lake since 1949; however, the ratio of fucoxanthin versus total algal carotenoids has gradually increased since 1981 indicating diatoms have increased relative to cyanobacteria since 1979, and that organic matter inflow and nutrient concentrations have resulted in changes in the phytoplankton composition. Since 1977, the chlorophyll a concentration in water in the lake had a very similar pattern as the TCh-a concentration in the sediment. The concentration in the water column also corresponds to those in the sediment even before 1977. These results indicate that it might be possible to predict the phytoplankton biomass of the water column based on the TCh-a in the water and sediment in this lake.     

Palaeohydrological significance of late Quaternary strontium isotope ratios in a tropical lake

Ostracods preserved in late Quaternary sediments of Wallywash Great Pond, a fresh coastal lake in SW Jamaica, record temporal variations in the strontium-isotope composition of lake water. Oxygen-isotope and Sr/Ca ratios in ostracods reveal temporal variations in the lake's hydrology, related to effective precipitation, and in its salinity related to varying marine-saline groundwater input from changes in relative sea level. Evaluation of isotopic and trace-element data indicates that the stratigraphic variations in ⁸⁷Sr/⁸⁶Sr ratios during the late Quaternary are best explained by climatically-controlled hydrological changes. During wetter periods, the lake's Sr budget was dominated by springwater input with relatively low ⁸⁷Sr/⁸⁶Sr ratio, whereas during drier times reduced springflow, possibly coupled with input of more-radiogenic Sr from other sources, such as sea-spray aerosols and perhaps Saharan dust, led to an increase in the Sr-isotope ratio of the lake water. Despite proximity of the lake to the sea and evidence for slight intrusion of marine saline groundwater in the past, however, the extent of marine input appears to have had limited influence on the lake's Sr-isotope ratios. Whereas the ⁸⁷Sr/⁸⁶Sr ratios cannot be used as a palaeosalinity proxy in this particular lake, they do provide valuable information about the mechanisms underlying hydrological change.     

松辽盆地东缘岳王城剖面嫩江组二段生物地层及其对湖侵事件的响应

松辽盆地嫩江组二段下部沉积期发生了大规模的湖侵事件,在整个盆地范围形成一套黑色泥页岩夹油页岩。然而,前人对嫩江组二段的地层及相关地质事件的研究主要基于井下岩心,地表露头不多见。通过调查,在松辽盆地东缘农安县青山口第二松花江沿岸发现一新的剖面——岳王城剖面,出露连续的嫩江组一段顶部—二段下部地层。通过对该套地层进行厘米级的描述和详细的分层,将该剖面地层划分为24层,累计厚度31.54m。第1层为嫩江组一段顶部,为青灰色砂质泥岩,含少量介形类化石;第2~24层为嫩江组二段下部,主体为暗色泥页岩夹油页岩、泥灰岩等,含丰富的介形类、叶肢介、鱼类,少量有孔虫及其他生物。此外,在嫩江组底部还发现一层火山灰和一层磷块岩。对其中的介形类化石进行初步研究,识别出11属18种。通过介形虫古生态和沉积学研究,并结合其他手段,初步探讨了该沉积期的湖泊环境,认为嫩二段下部沉积期经历了一次大规模湖侵,从一段顶部的滨浅湖过渡为二段下部的深湖环境,之后湖平面在波动中逐渐下降。     

Diagenetic cycling of arsenic in the sediments of eutrophic Baldeggersee,Switzerland

Sedimentation and benthic release of As was determined in Baldeggersee, a eutrophic lake in central Switzerland. Sediment traps recorded As sedimentation during 1994, including a flood event in spring. Diagenetic processes were studied using porewater profiles at the sediment–water interface and in deeper sediment strata deposited in the mesotrophic lake (before 1885). Sediment cores were used to calculate the accumulation and to construct the balance of sedimentation and remobilisation. The results showed that the lake sediment acts as an efficient sink for As. Only 22% of the particulate As flux reaching the sediment surface was remobilised at the sediment–water interface. The As accumulation in the recent varved section of the eutrophic lake was 40 mg As m⁻² a⁻¹. Iron reduction in older sediment caused a remobilisation of 1.2 mg As m⁻² a⁻¹. This upward flux from the deeper sediment was quantitatively immobilised in the recent sulfidic sediments. The flood event in spring contributed about 34% of the yearly sediment load and led to distinct peak profiles of dissolved As in the porewater. This evidence for rapid remobilisation disappeared within months.     

Black Sea chemocline oscillations during the Holocene: molecular and isotopic studies of marginal sediments

We measured δ¹³C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked difference in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites.     

Evidence for climatic variability and its impact on human development during the Neolithic from Loughmeenaghan,County Sligo,Ireland

High‐resolution pollen and geochemical analyses conducted on a sediment profile from a small lake in County Sligo, Ireland, revealed that human development during the Neolithic was influenced by pronounced climatic oscillations. The primeval woodland around the lake experienced a considerable transformation coinciding with the elm decline at 3810 BC. The subsequent increase in summer temperatures and decrease in precipitation favoured wheat cultivation in the lake's catchment area, which was practised for approximately 140 years. A shift towards pastoral farming took place with the establishment of exceptionally dry conditions between 3650 and 3560 BC, when lake level and influx of allochthonous material were notably low. The onset of cool and wet conditions at the transition from the Early to Middle Neolithic possibly caused the initial decline of human activity in the area. Periods of particularly high precipitation during the Middle and early Late Neolithic contributed to the abandonment of the area by the first farmers. Comparison of the proxy record from the study site with other palynological and archaeological records from Ireland suggests that climatic variability on the decadal to centennial scale represented a primary control on the nature and duration of farming practices during the Neolithic. Copyright © 2012 John Wiley & Sons, Ltd.     

Organofacies and paleoenvironment of the Oligocene Maikop series of Angeharan (eastern Azerbaijan)

The Maikop Formation, deposited in eastern Azerbaijan during Oligocene and Early Miocene times, contains prolific source rocks with primarily Type II organic matter. Paleontological analyses of dinoflagellate cysts revealed a Lower to Upper Oligocene age for the investigated succession near Angeharan. A major contribution of aquatic organisms (diatoms, green algae, dinoflagellates, chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids and δ¹³C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulfur bacteria. Higher inputs of terrigenous organic matter occurred during deposition of the Upper Oligocene units of the Maikop Formation from Angeharan mountains. The terpenoid hydrocarbon composition argues for angiosperm dominated vegetation in the Shamakhy–Gobustan area.High primary bioproductivity resulted in a stratified water column and the accumulation of organic matter rich sediments in the Lower Oligocene units of the Maikop Formation. Organic carbon accumulation during this period occurred in a permanently (salinity-) stratified, mesohaline environment with free H₂S in the water column. This is indicated by low pristane/phytane ratios of all sediments (varying from 0.37–0.69), lower methylated-(trimethyltridecyl)chromans ratio in the lower units and their higher contents of aryl isoprenoids and highly branched isoprenoid thiophenes. Subsequently, the depositional environment changed to normal marine conditions with oxygen deficient bottom water. The retreat of the chemocline towards the sediment–water interface and enhanced oxic respiration of OM during deposition of the Upper Oligocene Maikop sediments is proposed.Parallel depth trends in δ¹³C of total OM, n-alkanes, isoprenoids and steranes argue for changes in the regional carbon cycle, associated with the changing environmental conditions. Increased remineralisation of OM in a more oxygenated water column is suggested to result in low TOC and hydrocarbon contents, as well as ¹⁵N enriched total nitrogen of the Upper Oligocene units.     

6600 years of earthquake record in the Julian Alps (Lake Bohinj,Slovenia)

Sequences of lake sediments often form long and continuous records that may be sensitive recorders of seismic shaking. A multi‐proxy analysis of Lake Bohinj sediments associated with a well‐constrained chronology was conducted to reconstruct Holocene seismic activity in the Julian Alps (Slovenia). A seismic reflection survey and sedimentological analyses identified 29 homogenite‐type deposits related to mass‐wasting deposits. The most recent homogenites can be linked to historical regional earthquakes (i.e. 1348 ad , 1511 ad and 1690 ad ) with strong epicentral intensity [greater than ‘damaging’ (VIII ) on the Medvedev–Sponheuer–Karnik scale]. The correlation between the historical earthquake data set and the homogenites identified in a core isolated from local stream inputs, allows interpretation of all similar deposits as earthquake related. This work extends the earthquake chronicle of the last 6600 years in this area with a total of 29 events recorded. The early Holocene sedimentary record is disturbed by a seismic event (6617 ± 94 cal yr bp ) that reworked previously deposited sediment and led to a thick sediment deposit identified in the seismic survey. The period between 3500 cal yr bp and 2000 cal yr bp is characterized by a major destabilization in the watershed by human activities that led to increases in erosion and sedimentation rates. This change increased the lake's sensitivity to recording an earthquake (earthquake‐sensitivity threshold index) with the occurrence of 72 turbidite‐type deposits over this period. The high turbidite frequency identified could be the consequence of this change in lake earthquake sensitivity and thus these turbidites could be triggered by earthquake shaking, as other origins are discarded. This study illustrates why it is not acceptable to propose a return period for seismic activity recorded in lake sediment if the sedimentation rate varies significantly.     

Sediment accumulation and its effects on a Mississippi River oxbow lake

Recent sediment accumulation rates were measured in Moon Lake, a large (10.1 km²) Mississippi River oxbow lake in northwestern Mississippi. Moon Lake, which receives channeled inflow from an intensively cultivated soybean, rice, and cotton watershed (166 km²) and limited overland flow from surrounding lands, exhibited depositional patterns that were associated with (1) points of inflow, (2) flow patterns, and (3) lake morphology. From 1954 to 1965, 70 percent of the lake bottom experienced accumulation rates greater than 2 cm/yr. Accumulation rates exceeded 4 cm/yr in areas of delta formation. Changes in cropping systems during the 1960s, from cotton to soybeans and rice which require less cultivation, resulted in significantly (a = 0.01) less sediment accumulation during the period 1965- 1982 when 86 percent of the lake averaged less than 2 cm/yr sediment deposition. If current sediment accumulation rates continue, open water habitat in the lake will be reduced by only 3 to 7 percent during the next 50 years.Contribution of the Sedimentation Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Oxford, MS 38655     

食盐析出实验对盐湖盆地沉积研究的启示

将物理模拟实验结果与地震、岩芯资料相结合,对东濮凹陷盐岩沉积的古地理位置、成因机制提出新的认识。研究认为:洼陷带的边缘、中央隆起带等浅水部位是盐岩沉积较早的部位,可以沉积厚度大、粒度小的单盐层,而靠近湖盆中心,矿物结晶时间充分,沉积的盐岩粒度大,单盐层厚度可大可小,甚至可以缺失,盐岩沉积的古地理位置受古盐度影响明显;盐湖盆地绝大多部分地区,湖水有一定深度时便已形成湖底盐层,湖水变浅时盐层已具备一定厚度,保证了其下部泥页岩的还原环境,盐岩沉积后基本保持了原有的构造形态,其上可以再次接受泥页岩沉积;湖盆边缘的渗滤带在湖水退去以后仍可以沉积盐岩,该处盐层最薄、粒度细小,下部地层常见氧化环境标志。     

Recent environmental changes in Laguna Mar Chiquita (central Argentina): a sedimentary model for a highly variable saline lake

Abstract Laguna Mar Chiquita, a highly variable closed saline lake located in the Pampean plains of central Argentina, is presently the largest saline lake in South America (≈ 6000 km²). Recent variations in its hydrological budget have produced dry and wet intervals that resulted in distinctive lake level fluctuations. Results of a multiproxy study of a set of sedimentary cores indicate that the system has clearly recorded these hydrological variations from the end of the Little Ice Age (≈ ad 1770) to the present. Sedimentological and geochemical data combined with a robust chronology based on ²¹⁰Pb profiles and historical data provide the framework for a sedimentary model of a lacustrine basin with highly variable water depth and salinity. Lake level drops and concurrent increases in salinity promoted the development of gypsum–calcite–halite layers and a marked decrease in primary productivity. The deposits of these dry stages are evaporite‐bearing sediments with a low organic matter content. Conversely, highstands are recorded as diatomaceous organic matter‐rich muds. Average bulk sediment accumulation rose from 0·22 g cm^?2 year^?1 in lowstands to 0·32 g cm^?2 year^?1 during highstands. These results show that Laguna Mar Chiquita is a good sensor of high‐ and low‐frequency changes in the recent hydrological budget and, therefore, document climatic changes at middle latitudes in south‐eastern South America. Dry conditions were mostly dominant until the last quarter of the twentieth century, when a humid interval without precedent during the last 240 years of the lake's recorded history started. Thus, it is an ideal system to model sedimentary and geochemical response to environmental changes in a saline lacustrine basin.     

Iron in a post-glacial lake sediment core; a Mössbauer effect study

The distribution of iron in a 6-m core of post-glacial sediment from an oligotrophic lake (Connistonwater, England) was determined, principally by Mossbauer spectroscopy on dried samples. The immediate post-glacial deposits contain 4.8 wt. % of iron, with a Fe²⁺; Fe³⁺ ratio ～- 4. The iron there is predominantly in the form of chlorite, but there are small amounts in hematite and illite. The distribution of iron is different, and very variable in the recent sediments (~ < 13,000 BP), which contain 25–35 per cent organic matter and 5.2 wt. % of iron relative to the inorganic fraction. Typically half the iron is present there as chlorite, and the rest is ferric, mostly in the form of an amorphous gel which is also present in undried samples. To explain the observed ferrous:ferric profile, it is proposed that the latter includes iron which was once mobile, having been leached from the Fe²⁺-bearing clays under reducing conditions in the soils of the drainage basin, or in the sediment itself. It was subsequently precipitated as ferric hydroxide on contact with the oxic lake water. In contrast, the ferrous iron in the sediments is immobile iron, which remained locked in the chlorite phase of the clay particles as they were carried from soil to sediment intact.All the sediments are rather inhomogeneous. Chlorite, and especially hematite, are mechanically concentrated in pink varves in the immediate post-glacial deposits. In the partly inorganic sediments, the concentration of ferrous iron (chlorite) is approximately uniform, but the ferric content may differ by a factor of five between regions only a few millimeters apart.     

The timing of regional Lateglacial events and post-glacial sedimentation rates from Lake Superior

We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand.The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035±170 cal. BP (calibrated years before 1950, ca 7950–8250 ¹⁴C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.     

Evidence for gammacerane as an indicator of water column stratification

A new route for the formation of gammacerane from tetrahymanol is proposed; in addition to dehydration and hydrogenation, sulphurisation and early C-S cleavage are shown to be important in the pathway of formation, especially in marine sediments. Evidence is twofold. First, relatively large amounts of the gammacerane skeleton are sequestered in S-rich macromolecular aggregates formed by natural sulphurisation of functionalised lipids. Selective cleavage of polysulphide linkages with MeLi/MeI led to formation of 3-methylthiogammacerane, indicating that the gammacerane skeleton is primarily bound via sulphur at position 3, consistent with the idea that tetrahymanol (or the corresponding ketone) is the precursor for gammacerane. Second, upon mild artificial maturation of two sediments using hydrous pyrolysis, gammacerane is released from S-rich macromolecular aggregates by cleavage of the relatively weak C-S bonds. The stable carbon isotopic compositions of gammacerane and lipids derived from primary producers and green sulphur bacteria in both the Miocene Gessoso-solfifera and Upper Jurassic Allgau Formations indicate that gammacerane is derived from bacterivorous ciliates which were partially feeding on green sulphur bacteria. This demonstrates that anaerobic ciliates living at or below the chemocline are important sources for gammacerane, consistent with the fact that ciliates only biosynthesize tetrahymanol if their diet is deprived of sterols. This leads to the conclusion that gammacerane is an indicator for water column stratification, which solves two current enigmas in gammacerane geochemistry. Firstly, it explains why gammacerane is often found in sediments deposited under hypersaline conditions but is not necessarily restricted to this type of deposits. Secondly, it explains why lacustrine deposits may contain abundant gammacerane since most lakes in the temperate climatic zones are stratified during summer.     

A late Devensian glaciolacustrine fan-delta at Rhosesmor,Clwyd, North Wales

Glacigenic sediments exposed at Rhosesmor, Clwyd, North Wales, were deposited in a small proglacial lake ponded by the stagnating margin of the Late Devensian Irish Sea icesheet. Three major fades assemblages are identified and show a prograding sequence of topset, foreset, and bottomset deposits associated with two Gilbert-style deltas stacked one above the other. The lower delta is associated with a lake level at c. 180 m OD. A subsequent ice-marginal readvance raised the lake to c. 192 m OD, drowned the lower delta and caused the growth of a second delta by an iceward shift of facies. It is estimated that the life of the lake was of the order of 90 years during which average sediment in-fill amounted to some 4±10⁴ m³ per annum.     

The Lastglacial and Holocene seismostratigraphy and sediment distribution of Lake Bolshoye Shchuchye,Polar Ural Mountains,Arctic Russia

Seismostratigraphical studies of the 11.8‐km²‐large and ~140‐m‐deep Lake Bolshoye Shchuchye, Polar Ural Mountains, reveal up to 160‐m‐thick acoustically laminated sediments in the lake basin. Using a dense grid of seismic lines, the spatial and temporal distributions of the sedimentary history have been reconstructed. Three regional seismic horizons have been identified and correlated with the well‐dated 24‐m‐long sediment core retrieved from the lake. Isopach maps constructed from the seismic data show four phases of sedimentation. A contour map of the deepest regional seismic reflector represents the earliest hemipelagic sedimentation in the lake. Three contour maps represent time intervals covering the last 23 cal. ka based on the well‐dated core stratigraphy from the lake. The detailed time constraints on the upper stratigraphical units in the lake allow calculation of the lake's development in terms of sediment fluxes and the denudation rates from the Last Glacial Maximum (LGM) to the present. The sedimentation in Lake Bolshoye Shchuchye has been dominated by hemipelagic processes during at least the last 24 cal. ka BP only locally interrupted by delta progradation and slope processes. A major shift in the sediment accumulation at c. 18.7 cal. ka BP is interpreted to mark the end of the local glacial maximum, greatly reduced denudation and the onset of the deglaciation period; this also demonstrates how fast the glaciers melted and possibly disappeared at the end of the LGM. The denudation rate during the Holocene is only a fifth of the LGM rate. The age of the oldest stratified sediments in Lake Bolshoye Shchuchye is not well constrained, but estimated as c. 50–60 ka.