The role of biogeochemical cycling for the formation and preservation of varved sediments in Soppensee (Switzerland)

We analyzed seasonally aggregated observations of temperature, conductivity, dissolved oxygen and dissolved inorganic carbon from Soppensee (District of Lucerne, Switzerland) for the yrs 1980 to 1993. Holomictic Soppensee is characterized by a strong summer stratification with a thin epilimnion separated from an anoxic hypolimnion by a strong pycnocline formed by thermal and chemical gradients. A vertical one-dimensional model was developed to simulate the observed seasonal cycles of carbon and oxygen. The processes of net community production, mineralization of organic matter, precipitation and dissolution of calcite, gas exchange, in- and outflow, sedimentation and vertical eddy diffusion are included. According to the model, the annual net community production is estimated to about 110 g C m^-2 yr^-1 and the annual net primary production to about 330 to 440 g C m^-2 yr^-1, which is a typical value for eutrophic lakes. A mass balance of the carbon cycle indicates that most of the inflow comes from groundwater which is super-saturated with respect to atmospheric CO₂. Therefore the surface waters exhibit a large capacity for calcite precipitation. The results of the model are used to constrain the conditions that favor the formation of varved sediments in Soppensee during thousands of yrs. Model calculations show that the deep waters would still turn anoxic even if the sedimentation rate of organic matter were decreased to 25%. Several physical factors such as biogenic stabilization of the deep waters due to calcite dissolution and low input of wind energy are responsible for the long term anoxia in Soppensee.     

The climatic signal in varved sediments from Lake C2, northern Ellesmere Island, Canada

Annually-laminated clastic sediments preserve a high resolution proxy record of paleoclimate, provided that allochthonous sedimentation represents a response to meteorological forcing of watershed sediment transfer. Here, we demonstrate this linkage, and illustrate a calibration process using the most recent 40 years of a varve record from Lake C2 (82°50 N; 78°00 W), three years of field measurements, and meteorological data for 1951–92 from nearby AES weather station Alert. Field measurements were used to correlate proxies of the energy available for snowmelt (e.g. air temperature) and daily suspended sediment discharge (SSQ). Our calibration was extended through use of weather data from Alert. Both mean daily air temperature at Echo, and daily SSQ, were well correlated with air temperature at 600 m above Alert, as obtained from the 1200 Z (0800 LST) rawinsonde sounding. Accordingly, we used pooled 1990 and 1992 Alert 600 m data to predict the lagged daily sediment discharge into Lake C2 (adj. r ²=0.43). Daily values were summed each year in order to produce an annual series of predicted sediment transfer to the lake. The original varve chronology was based on eight sediment cores recovered from the deep basin of the lake (>80 m). Although low-frequency fluctuations of the varve and predicted SSQ series agree, slight tuning of the varve record optimizes the correlation between them. Adjustments were based on examination of weather data for specific years, reexamination of sediment core thin sections, and by aligning fluctuations in the two series which closely matched. Although the original chronology is reasonably well correlated with 600 m temperatures at Alert (for JJA mean, r=0.41, significant at 0.01), the adjusted chronology is both better correlated and contains a more precise climate signal (r=0.54 for July mean, significant at 0.01). This is the first calibrated varve record produced from Arctic lake sediments, and demonstrates that varves from Lake C2 contain a paleoclimatic record. We believe the post-facto manipulations required to produce the adjusted varve chronology are reasonable given the uncertainties inherent in varve counting, and the lack of any independent corroborating chronostratigraphic markers.This is the ninth in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.     

Sediment indicators of meromixis: comparison of laminations, diatoms, and sediment chemistry in Brownie Lake, Minneapolis, USA

Meromixis has several powerful effects on lakes, yet there is no single definitive sediment indicator of meromixis. In this study three sediment indicators of meromixis were compared in Brownie Lake, Minneapolis, Minnesota, a small eutrophic lake that became meromictic around 1925. The results show that in Brownie the onset of laminations and changes in the iron to manganese ratio most likely occurred before the development of permanently anoxic bottom water and that changes in the diatom assemblage occurred later, most likely only when meromixis was well developed.This is the 11th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

The response of sedimentation in three varved lacustrine sequences to air temperature,precipitation and human impact

Human activity and climatic forcing have influenced sedimentation in three of Finland's deepest lakes during the last centuries. High-resolution sediment sequences of Lake Päijänne, Lake Pääjärvi and Lake Pyhäjärvi represent records of the last 440 years, 839 years and 633 years, respectively. The accumulation rates of dry matter, organic carbon and biogenic silica refer to changes in human activity in the catchments. However, they also reveal the importance of climatic forcing on lake sediment deposition. A significant correlation was found between instrumentally measured records of temperature (163 years) and precipitation (148 years), and varve thickness. Warm winter months indicating a short ice-cover period have the strongest control on varve thickness. This shows that wind-driven resuspension of littoral material is the forcing mechanism of climate on lake sediments. The long-term human-induced erosion pulses observed may even have magnified the climatic signals in some cases. Nevertheless, increased anthropogenic field erosion in the catchment, and the associated leaching of bioavailable nutrients, hampers the observation of climate signals, especially during the last 50 years, in the lakes studied.This is the second paper a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Palaeolimnology Symposium held 19-April, 1993 at the Australian National University, Canberra, Australia. Dr A. F. Lotter and Dr. M. Sturm served as guest editors for these papers.     

Tertiary and Quaternary vegetation history of the Great Salt Lake, Utah, USA

Pollen analysis of 5 wells drilled to bedrock in the Great Salt Lake, Utah, USA provide a record vegetation change over the last ca 13.5 Ma. Over 440 pollen samples have been counted. The longest record presented is for the mid-lake Bridge Well. Close-interval (3–10 Ka) sampling is presented for the upper Indian Cove well. Chronologic control is provided by identification volcanic tephra and by K/Ar, Ar/Ar, and fission-track dates. Ash determinations are based on electron microprobe analyses of iron, calcium, and other elements compared to Neogene ash data at the University of Utah. Sedimentation begins 38 Ma, with good pollen preservation is sediments younger than 13.5 Ma, and no obvious gaps in sedimentation after 6 Ma.The upland vegetation is desert from the late Miocene onward, with Sarcobatus and Ephedra pollen dominance during the late Miocene (<5 Ma). Chenopodiaceae-Amaranthus, and Artemisia gain dominance during the Pliocene (5–2.5 Ma) and early Pleistocene. Pinus and Artemisia sharply increase in the late Pleistocene (0.75 Ma). The pollen of 'Tertiary exotics' (elm, hickory, Ostrya-Carpinus) is rare, but persists into the Pleistocene. Major vegetation – climatic events occur 3.7 and 2.5 Ma. Increased pollen concentration and sedimentation rate after 310 Ka are attributed to the diversion of the Bear River into the Bonneville Basin.Glacial-interglacial cycles appear as alternations of Chenopodiaceae-Amaranthus (interglacial) vs Artemisia (glacial) during the Pliocene and early Pleistocene, and of Cupressaceae, Sarcobatus, and Gramineae (interglacial) vs Picea, Abies, and Pseudotsuga (glacial) during the late Pleistocene. Pluvial cycles are separated by interpluvial peaks in percentages of wetground and aquatic types. Six interpluvials are indicated during the last 759 Ka, with pluvial cycles of ca 100 Ka cyclicity back to 1.5 Ma.     

A late Holocene pollen record of deforestation and environmental change from the Lake Tauanui catchment, Northland, New Zealand

Late Holocene pollen and sediment records from the Lake Tauanui catchment, northern New Zealand, indicate that the lake formed about 5500 years ago following a series of volcanic events in the Tauanui Volcanic Centre. These volcanic events initiated a volcanosere resulting in a mixed conifer-hardwood forest. Dacrydium cupressinum was the dominant tree. Agathis australis was always present. Changes similar to those registered in other Northland pollen diagrams are apparent. At ca 4000 yr B.P., when the climate became cooler and drier than before, a fire occurred in the catchment area causing erosion of the surrounding slopes and some destruction of forest. Fluctuations in abundance of many forest species, including Ascarina lucida, A. australis and D. cupressinum, from ca 3500 yr B.P. indicate repeated disturbance, increasingly so after 1600 yr B.P. Summer droughts and increased frequency of cyclonic winds are suggested as the cause. Major anthropogenic deforestation events defined by palynology occurred across many parts of the New Zealand landscape at ca 700 yr B.P. At Lake Tauanui anthropogenic forest disturbance, radiocarbon dated to ca 1000 yr B.P., is indicated by significant decline in all tree and shrub elements with concomitant increase in pteridophytes, especially Pteridium esculentum. Charcoal concentration increases steadily from the onset of disturbance, and in the final phase after the arrival of Europeans, major clearance of vegetation is indicated. Herbs increase markedly in this period, in diversity and abundance.     

Relationship between surface pollen assemblages and vegetation in Luonan Basin,Eastern Qinling Mountains,Central China

The catchment of South Luohe River in Central China is an important region for investigating modern pollen-environment relationship, because it is located in the transitional zone between south and north China, an environment which is sensitive to climate changes. In this study, 40 surface samples under ten vegetation types were collected to reveal the relationship between pollen assemblages and vegetation. The results show that the surface pollen assemblages reflect the vegetation quite well. In forest topsoils, the average of arboreal pollen content is greater than 40%, and the Selaginella sinensis spore is high. As to sparse forest grassland and shrub community, the average arboreal pollen is 13.2% and 16.6% respectively, and the shrub pollen is relatively higher than that of grassland samples. The grassland and farmland are characterized by low percentage of tree and shrub pollen (<10% and <1%), and high percentage of herbs (>80%). Pinus, Quercus and some other arboreal pollen can indicate the regional vegetation because of their dispersal ability. Quercus pollen is under-representative and so is Pinus. Artemisia pollen is significantly over-represented, has poor correlation with the plant coverage, and may reflect human disturbance. Gramineae can indicate plant quite well, but with low representation. High content of Chenopodiaceae probably suggests human impact. Predominant Selaginella sinensis can be used as an indicator of forest environment. Cluster analysis and principal components analysis of pollen assemblages can distinguish forest and non-forest vegetation well. The former method is better at separating pine and mixed forests, while the latter is more stable and could better differentiate farmland and other non-forest area. The first axis of PCA mainly reflects the humidity.     

A 7000-Year Record of Environmental Change,Bear Lake,Rocky Mountain National Park,USA

Few long-term records of the fire history of Rocky Mountain National Park exist. Data from a lake sediment core was used to reconstruct changes in vegetation and fire frequencies over the last 7000 cal yr. Bear Lake is a high-elevation lake surrounded by subalpine vegetation in Rocky Mountain National Park, Colorado. Pollen data indicate that a warm and dry climate prevailed between ca. 7000 and 5000 cal yr BP. Temperatures increased until shortly before ca. 3500 cal yr BP when evidence for a marked decline is seen. Cooler-than-present conditions were maintained until ca. 1700 cal yr BP, when conditions transitioned to more like those of the present-day climate. Based on macroscopic charcoal analyses, fire frequency had varied between two and five episodes per 1000 years. The largest peak in charcoal was at ca. 590 cal yr BP. The fire return interval has varied with climate over time; however, we calculate a fire return interval of 325 years over the past 7000 years. Given these results, fire activity is likely to increase under current Intergovernmental Panel on Climate Change climate projections of an increase in annual temperatures.     

基于氢氧同位素与水化学的潮白河流域地下水水循环特征

为了研究变化环境下潮白河流域地下水水循环规律,通过现场调查,结合环境同位素及水化学应用,对潮白河流域浅层和深层地下水采样,测定其氢、氧环境同位素及水化学成分,通过分析其变化特征判明地下水的补给来源以及各含水层的相互联系。降水和地下水中的环境同位素δD和δ¹⁸O组成分析表明,降水是山前地下水的主要补给源,山区浅层地下水受蒸发影响非常强烈。水化学研究结果表明,山区地下水水质以 Ca²⁺和 HCO^-₃为主,属Ca²⁺-Mg²⁺-HCO^-₃型地下水。山前地下水类型为Ca²⁺-Mg²⁺-HCO^-₃、 Na⁺-K⁺-HCO^-₃、Mg²⁺-Ca²⁺-HCO^-₃和 Ca²⁺-Mg²⁺-Cl^--SO^2-₄。平原区地下水为Mg²⁺, Na⁺和HCO^-₃。滨海冲积海积平原为Ca²⁺-Mg²⁺-HCO^-₃型和Ca²⁺-Mg²⁺-Cl^--SO^2-₄型地下水。水化学分析证实了越流补给的存在。Ca²⁺ 和 HCO^-₃离子均呈山区高、山前和平原低、而滨海增高的趋势。沿潮白河流向地下水类型变化为:Ca²⁺-Mg²⁺-HCO^-₃ Na⁺＝K⁺-HCO^-₃ Ca²⁺-Mg²⁺-HCO^-₃。     

The glacilacustrine sedimentary environment of Bowser Lake in the northern Coast Mountains of British Columbia, Canada

Bowser Lake, a fiord lake in the northern Coast Mountains of British Columbia, contains a thick Holocene fill consisting mainly of silt and clay varves. These sediments were carried into the lake by proglacial Bowser River which drains a high-energy, heavily glacierized basin. Sedimentation in the lake is controlled by seasonal snow and ice melt, by autumn rainstorms, and by rare, but very large jökulhlaups from glacier-dammed lakes in the upper Bowser River basin which complicate environmental inferences from the sedimentary record. Sediment is dispersed through the deep western part of the lake by energetic turbidity currents. The turbidity currents apparently do not overtop a sill that separates the western basin from much shallower areas to the east. Large amounts of silt and clay are deposited from suspension in the eastern part of the lake, but sediment accumulation rates there are much lower than to the west. Several strong acoustic reflectors punctuate the varved fill in the western basin; these may be thick or relatively coarse beds deposited during jökulhlaups or exceptionally large storms. The contemporary sediment yield to Bowser Lake, estimated from sediments in the lake, is about 360 t km^-2a^-1. This is a relatively high value, but it is less than yields insome other, similar montane basins with extensive snow and ice cover.The most likely explanation for the difference is that large amounts of sediment have been, and continue to be, stored on the Bowser delta andin small proglacial lakes.     

Variability in diatom and chrysophyte assemblages and inferred pH: paleolimnological studies of Big Moose Lake,New York,USA

We measured variability in the composition of diatom and chrysophyte assemblages, and the pH inferred from these assemblages, in sediment samples from Big Moose Lake, in the Adirondack Mountains of New York. Replicate samples were analyzed from (1) a single sediment core interval, (2) 12 different intervals from each of 3 separate cores, and (3) 10 widely spaced surface sediment samples (0–1 cm). The variability associated with sample preparation (subsampling, processing, and counting) was relatively small compared to between-core and within-lake variability. The relative abundances of the dominant diatom taxa varied to a greater extent than those of the chrysophyte scale assemblages. Standard deviations of pH inferences for multiple counts from the same sediment interval from diatom, chrysophyte, and diatom plus chrysophyte inference equations were 0.04 (n=8), 0.06 (n=32), and 0.06 (n=8) of a pH unit, respectively. Stratigraphic analysis of diatoms and chrysophytes from three widely spaced pelagic sediment cores provided a similar record of lake acidification trends, although with slight differences in temporal rates of change. Average standard deviations of pH inferences from diatom, chrysophyte and diatom plus chrysophyte inference equations for eight sediment intervals representing similar time periods but in different cores were 0.10, 0.20, and 0.09 pH unit, respectively. Our data support the assumption that a single sediment core can provide an accurate representation of historical change in a lake. The major sources of diatom variability in the surface sediments (i.e., top 1.0 cm) were (1) differences in diatom assemblage contributions from benthic and littoral sources, and (2) the rapid change in assemblage composition with sediment depth, which is characteristic of recently acidified lakes. Because scaled chrysophytes are exclusively planktonic, their spatial distribution in lake sediments is less variable than the diatom assemblages. Standard deviations of pH inferences for 10 widely spaced surface sediment samples from diatom, chrysophyte and diatom plus chrysophyte inference equations were 0.21, 0.09, and 0.16 of a pH unit, respectively.     

Relationship between pollen assemblages and organic geochemical proxies and the response to climate change in the Zhuye Lake sediments

This paper examines the relationship among organic geochemical proxies （TOC, C/N ratio and ~13C） and pollen assemblages in Zhuye Lake sediments since the Late Glacial. Results show that the reaction extent of organic geochemical proxies and pollen as- semblages to environment changes are different. Organic geochemical proxies are sensitive to overall environmental change, while pollen assemblages indicate detailed information of environmental change. For the entire sedimentary section （except the sand layer fi＇om the bottom of the section）, when values of TOC, C/N ratio and total pollen concentrations are high, 813C values are low, and vice versa. The different responses of organic geochemical proxies and pollen records in Zhuye Lake are mainly due to their different sensitivity and diverse influencing factors in different environmental conditions.     

Relationship between pollen assemblages and organic geochemical proxies and the response to climate change in the Zhuye Lake sediments

This paper examines the relationship among organic geochemical proxies (TOC, C/N ratio and δ¹³C) and pollen assemblages in Zhuye Lake sediments since the Late Glacial. Results show that the reaction extent of organic geochemical proxies and pollen assemblages to environment changes are different. Organic geochemical proxies are sensitive to overall environmental change, while pollen assemblages indicate detailed information of environmental change. For the entire sedimentary section (except the sand layer from the bottom of the section), when values of TOC, C/N ratio and total pollen concentrations are high, δ¹³C values are low, and vice versa. The different responses of organic geochemical proxies and pollen records in Zhuye Lake are mainly due to their different sensitivity and diverse influencing factors in different environmental conditions.     

Big lake records preserved in a little lake’s sediment: an example from Silver Lake,Michigan, USA

We reconstruct postglacial lake-level history within the Lake Michigan basin using soil stratigraphy, ground-penetrating radar (GPR), sedimentology and ¹⁴C data from the Silver Lake basin, which lies adjacent to Lake Michigan. Stratigraphy in nine vibracores recovered from the floor of Silver Lake appears to reflect fluctuation of water levels in the Lake Michigan basin. Aeolian activity within the study area from 3,000 years (cal yr. B.P.) to the present was inferred from analysis of buried soils, an aerial photograph sequence, and GPR. Sediments in and around Silver Lake appear to contain a paleoenvironmental record that spans the entire post-glacial history of the Lake Michigan basin. We suggest that (1) a pre-Nipissing rather than a Nipissing barrier separated Silver Lake basin from the Lake Michigan basin, (2) that the Nipissing transgression elevated the water table in the Silver Lake basin about 6,500 cal yr. B.P., resulting in reestablishment of a lake within the basin, and (3) that recent dune migration into Silver Lake is associated with levels of Lake Michigan. This is the fourth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.     

Observations on the diagenetic behaviour of arsenic in a saline lake: Pyramid Lake,Nevada

We present, to our knowledge, the first dissolved arsenic (As) data from the pore waters of a closed-basin, saline lake, Pyramid Lake, USA. The As concentrations first decrease across the sediment-water interface to values below lake water concentrations and then rapidly increase to values as high as 5.85mol L^–1 at a depth of 15 cm. This maximum value is reached at the top of the sulfate reduction zone. The profile suggests that As is remobilized in the suboxic and anoxic zones of the sediments and is reprecipitated near the sediment-water interface under oxic conditions.     

Paleoecology and its application to fire and vegetation management in Kootenay National Park, British Columbia

High-resolution analysis of macroscopic charcoal and pollen ratios were used to reconstruct a 10,000 yr history of fire and vegetation change around Dog Lake, now in the Montane Spruce biogeoclimatic zone of southeastern British Columbia. Lake sediment charcoal records suggest that fire was more frequent in the early Holocene from 10,000 to 8200 calendar yrs BP, when climate was warmer and drier than today and forest fuels were limited. Fire frequency increased and reached its maximum during the early to mid-Holocene from 8200 to 4000 calendar yrs BP, corresponding to the dry and warm Hypsithermal period in the Rocky Mountains. During the Hypsithermal period forests around Dog Lake were dominated by Pseudotsuga/Larix,Pinus and open meadows of Poaceae that were subject to frequent fire. From 4000 calendar yrs BP to present, fires became less frequent with the onset of cooler and wetter Neoglacial climate and an increase in wet-closed Picea and Abies forests in the valley. Changes in fire frequency are supported by dry-open/wet-closed pollen ratio data indicating that forest type and disturbance regimes vary with changing climate. The fire frequency and forest cover reconstructions from Dog Lake are a first attempt at defining a range of natural variability for Montane Spruce forests in southeastern British Columbia. Fire and vegetation management in Kootenay National Park can now use this century to millennial-scale range of variability to define the context of current forest conditions and potential changes under global warming scenarios.     

新疆博斯腾湖流域风沙环境特征

通过风资料的统计和计算,对博斯腾湖流域风沙活动强度环境特征进行了系统的研究。结果表明:博斯腾湖流域年平均风速较小,一般1.6～3.0 m·s^-1;有效起沙风作用时间存在明显的区域差异,其中巴音布鲁克起沙风天数占全年总天数的55.59%,焉耆、轮台、巴音郭楞及和静分别为18.75%、12.72%、11.48%、11.23%,其余各站均低于10%;随着风速等级的增加,起沙风出现的频率相应减少,基本上都集中在6.1～10.0 m·s^-1;研究区方向变率指数(合成输沙势/输沙势,RDP/DP)0.49~0.68,属于中比率,风况特征属于纯双峰风况或锐双峰风况。输沙势属于低风能环境,且存在较大的区域差异。     

Relationship between C:N ratios of lake sediments, organic matter sources, and historical deforestation in Lake Pleasant, Massachusetts, USA

The C:N ratios of lake sediments may reflect proportions of terrestrial and algal carbon contributing to accumulation of sediment. This possibility was tested in Lake Pleasant, Massachussetts, USA which underwent watershed deforestation in about 1780 A.D. The C:N profile of a 70-cm sediment core clearly reflected deforestation through a rise in C:N ratio caused by an increase in watershed contributions to sedimentary carbon. Spatial variability of C:N in modern surficial sediments is small compared to the change caused by deforestation.     

洞庭湖植被对降水的响应

We analyzed the Normalized Difference Vegetation Index (NDVI) from satellite images and precipitation data from meteorological stations from 1998 to 2007 in the Dongting Lake wetland watershed to better understand the eco-hydrological effect of atmospheric precipitation and its relationship with vegetation. First,we analyzed its general spatio-temporal distribution using its mean,standard deviation and linear trend. Then,we used the Empirical Orthogonal Functions (EOF) method to decompose the NDVI and precipitation data into spatial and temporal modes. We selected four leading modes based on North and Scree test rules and analyzed the synchronous seasonal and inter-annual variability between the vegetation index and precipitation,distinguishing time-lagged correlations between EOF modes with the correlative degree analysis method. According to our detailed analyses,the vegetation index and precipitation exhibit a prominent correlation in spatial distribution and seasonal variation. At the 90% confidence level,the time lag is around 110 to 140 days,which matches well with the seasonal variation.     

Analysis of vegetation response to rainfall with satellite images in Dongting Lake

We analyzed the Normalized Difference Vegetation Index (NDVI) from satellite images and precipitation data from meteorological stations from 1998 to 2007 in the Dongting Lake wetland watershed to better understand the eco-hydrological effect of atmospheric precipitation and its relationship with vegetation. First, we analyzed its general spatio-temporal distribution using its mean, standard deviation and linear trend. Then, we used the Empirical Orthogonal Functions (EOF) method to decompose the NDVI and precipitation data into spatial and temporal modes. We selected four leading modes based on North and Scree test rules and analyzed the synchronous seasonal and inter-annual variability between the vegetation index and precipitation, distinguishing time-lagged correlations between EOF modes with the correlative degree analysis method. According to our detailed analyses, the vegetation index and precipitation exhibit a prominent correlation in spatial distribution and seasonal variation. At the 90% confidence level, the time lag is around 110 to 140 days, which matches well with the seasonal variation.