Bedding characteristics of Holocene sediments from salt lakes of the northern Great Plains, Western Canada

The northern Great Plains region of western Canada contains many saline and hypersaline lakes. These lakes exhibit great diversity in geochemical and sedimentological characteristics which results in a wide range of bedding features and lamination types. Because of the high brine salinities and supersaturation with respect to many carbonate and sulfate evaporitic minerals, chemical laminae and beds are the most common stratification types observed. Simple monomineralic carbonate or sulfate layers as well as beds composed of complex mixtures of aragonite, magnesite, hydromagnesite, mirabilite, gypsum, epsomite,and/or bloedite occur frequently in Holocene sequences from these saline lakes. In addition, biolaminae, including microbialite bedding and accretionary tufa and travertine deposits, are present. Due to the dominance of chemical sedimentary processes operating in these lakes, physical laminae are uncommon. Other observed bedding features and sedimentary structures consist of distinctive pedogenic-cryogenic dry zones, salt karst structures, and clastic dykes and diapirs. Although paleoenvironmental investigations of these well-bedded sequences have just recently begun, several basins provide examples of the nature of paleolimnological information that can be derived from the salt lakes of the northern Great Plains. The chemical and biological laminae preserved in the Holocene sequence of Waldsea Lake provide evidence for significant fluctuations in brine chemistry and chemocline depth in this meromictic basin. Freefight Lake, another hypersaline meromictic lake, contains a relatively thick sequence of rapidly deposited, deep-water salts underlain by finely laminated carbonates, sulfates, and microbial mat sediments. These very thin, undisturbed laminae, combined with exceedingly high rates of offshore evaporite mineral accumulation, provide an excellent opportunity for high resolution geochemical and hydrologic reconstructions in a part of the region distinguished by a paucity of other sources of paleoenvironmental information. Chappice Lake, a shallow, hypersaline brine pool, contains a wealth of paleoenvironmental information. Although the basin probably never experienced the deep-water conditions that earmark Waldsea and Freefight lakes, nonetheless, finely laminated and well-bedded sequences abound in the Holocene record of Chappice Lake. The endogenic magnesium and calcium carbonates and sulfates comprising these laminae can be used to interpret the history of brine chemistry fluctuations which may then help to understand past changes in the hydrologic budget and groundwater inflow.     

Lithostratigraphy and recent sedimentation history of Little Manitou Lake,Saskatchewan, Canada

Little Manitou Lake is a topographically closed, hypersaline lake that occupies a long, linear glacial meltwater channel in the northern Great Plains of western Canada. Most of the modern and late Holocene sediment in the lake has been generated from within the basin itself, either by endogenic inorganic precipitation or by other authigenic processes. These endogenic and authigenic precipitates, composed of mainly very soluble sulfate salts and sparingly soluble carbonates, provide an explicit record of the past chemical and hydrological fluctuations that have occurred in the lake. Although detailed chronostratigraphy is incomplete, preliminary¹⁴C dating indicates an age of about 2000 years for the oldest sediment recovered from the basin.Five subsurface sedimentary facies are identified in offshore cores. From the base these are: (i) structureless, gray clay, (ii) gypsiferous mud, (iii) structureless, organic-rich mud, (iv) finely laminated aragonitic mud, and (v) Na and Mg sulfate salts. The lithostratigraphy and variation in the mineralogical composition of the sediment indicate that Little Manitou Lake experienced significant water level changes and compositional fluctuations during the past several millennia. The basal clays indicate a relatively deep, freshwater lake existed about 2000 years ago, but was soon followed by a period of low water/playa sedimentation and a negative hydrological budget in the basin. Water levels gradually increased after about 1500 years ago in response to a cooler and wetter climate. This resulted in development of a meromictic, saline to hypersaline lake characterized by periodic carbonate (aragonite) whitings. Water levels again decreased about 1000 years ago, resulting in a breakdown of meromixis and initiation of subaqueous evaporitic salt precipitation. Although the brine in Little Manitou Lake has fluctuated between Na-SO₄ and Mg-Na-SO₄ -Cl types during the past 1000 years, water levels and overall salinities have remained relatively constant.Palliser Triangle Global Change Contribution No. 16.     

Rates of sediment deposition in a hypersaline lake in the northern Great Plains,western Canada

Remarkably high rates of sedimentation (up to nearly 60 kg m⁻² yr⁻¹) are documented for offshore areas of Freefight Lake, a deep, hypersaline, meromictic lake in the northern Great Plains of Canada. Most material in the offshore sediment traps consists of soluble and suparingly soluble endogenic salts. Deposition of allogenic sediment is minor. The observed high sedimentation rates are in response to several mechanism: (i) freeseout precipitation of salts from the mixolimnion; (ii) redistribution of shallow water endogenic precipitates by turbidity flows and interflow processes; and (iii) precipitation of salts from within the monimolimnion Because the bottom water mass of the lake is essentially isothermal and isochemical, large scale temperature induced remobilization of these salts at or near the sediment-water interface is unlikely. Similarly, the morphology of the basin suggests that resuspension of bottom sediment by waves in the offshore areas of the basin is negligible. Although the sedimentation rates observed in Freefight Lake are extraordinarily high and considerably greater than other modern lacustrine sedimentation rates documented in the literature to date, the observed rates are entirely compatible with suggested accumulation rates of ancient evaporite sequences. Palliser Tringle Global Change Contribution Number 6.     

Occurrence and significance of a cold-water carbonate pseudomorph in microbialites from a saline lake

Micron-scale pseudomorphs of calcite after ikaite were discovered in microbialites from Manito Lake, a large hypersaline lake in the Great Plains of western Canada. Although environmental conditions in the lakes of this region (seasonally cold temperatures, high salinities and elevated productivity) suggest that ikaite should be common, this is the first documentation of lacustrine ikaite in Canada and the Great Plains region of North America. The calcite pseudomorphs form a porous dendritic fabric, comprise the interiors of massive shoreline microbialite mounds and pinnacles, and are encased in centimeter-scale, laminated dolomite-aragonite rinds. Because of the limited thermodynamic stability of ikaite at or near freezing temperatures, the psuedomorphs of this mineral provide evidence of an extended cold episode during the first millennium AD. The Manito deposits also show high δ¹³C values, indicating elevated productivity and δ¹⁸O signatures are consistent with precipitation in a cold, somewhat fresher lake than present.     

Diatom,pollen, and geochemical evidence for the palaeosalinity of Medicine Lake,S. Dakota,during the late-Wisconsin and early Holocene

Medicine Lake is a highly saline meromictic lake in eastern South Dakota. A lithostratigraphic and biostratigraphic study of the late-glacial and early post-glacial sediments shows that it was a relatively deep dilute freshwater lake during the period of the Picea pollen zone. When spruce forest changed to a Betula and then to a Quercus/Ulmus woodland in the early post-glacial period, the lake water became more concentrated but remained fresh. However, during the subsequent rapid transition to prairie in the early Holocene, when Gramineae, Ambrosia, and Artemisia dominated dry-land vegetation, the freshwater diatom flora was progressively replaced by a saline assemblage characterized by Cyclotella quillensis, Chaetoceros, and eventually Cyclotella caspia. The lake became permanently saline at about 9000 yr BP. A comparison of the fossil diatom assemblages with surface-sediment samples from a range of lakes in the Dakotas indicates that the change involved an increase in conductivity from about 500 S cm^–1 in the late-glacial period to > 10 000 S cm^–1 in the early Holocene. This rapid change is also marked by an abrupt increase in sulphate concentration in the sediment, the first appearance of bands of gypsum crystals, and the beginning of a well-laminated core sequence that continues through the remaining sediment record. Conditions of high salinity have prevailed to the present day.Contribution 299, Limnological Research Center.     

Holocene evolution of the crater lake at Malha,Northwest Sudan

A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 ¹⁴C years BP.Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lake's Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso- to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool.The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early- to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lake's history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point.     

白洋淀地区晚冰期以来的气候和环境演变

华北平原晚冰期以来气候环境演变研究对该地区社会发展、灾害风险评估和科学应对未来全球增温背景下极端降水和洪涝事件具有重要意义。本文以华北平原中部白洋淀地区高阳剖面（BG-2019）为研究对象,通过高精度AMS¹⁴C、OSL定年技术和高分辨率孢粉组合、粒度组分分析,恢复和重建了白洋淀地区晚冰期以来（距今13710 a—今）区域植被演替和气候环境变化历史。结果显示：BG-2019剖面在距今10270~13710 a和距今4630~5400 a发育湖相沉积,距今3470~3700 a发育沼泽相沉积;距今7130~8000 a发育河流—入湖三角洲相沉积,距今3700~4630 a和距今3230~3470 a发育河流相沉积;距今8000~10270 a和距今5400~7130 a存在明显的沉积间断/地层缺失;表明采样剖面所在位置缺乏连续的湖相地层。晚冰期白洋淀为局地小湖沼;中全新世湖沼较发育、范围广,但也不是连续广袤的湖相沉积;晚全新世湖泊范围收缩。晚冰期和全新世白洋淀流域植被景观存在显著差异;晚冰期气候寒冷干燥,平原发育以蒿属、藜亚科、禾本科和菊科等为主的草地,周围山地森林覆盖度低;中全新世气候温暖湿润,平原大部仍发育以蒿属、藜亚科和禾本科为主的草地,湖区水蕨和水生植物繁盛,周围山地生长松属、栎属为主的针阔混交林,森林覆盖度增高;晚全新世气候温和偏干,平原仍是以蒿属、藜亚科和禾本科等为主的草地,西部山地生长以松属为主的针阔混交林,森林覆盖度较高。     

Holocene paleohydrology of a hypersaline lake in southeastern Alberta

Analyses of pollen, plant macrofossils, sediment mineralogy, geochemistry, and lithology of cores from Chappice Lake, southeastern Alberta, provide an outline of paleohydrological changes spanning the last 7300 radiocarbon years. Situated near the northern margin of the Great Plains, Chappice Lake is currently a small (1.5 km²), shallow (<1 m), hypersaline lake. Results of this study suggest that the lake has experienced significant changes in water level and chemistry during the Holocene.From 7300 to 6000 BP the lake oscillated between relatively high stands and desiccation. From 6000 to 4400 BP it was smaller than present and ponded highly saline water. Although extreme water level variations of the preceding period had ceased, pronounced seasonal fluctuations persisted. Between 4400 and 2600 BP, lake level was more stable but gradually rising. Carbonates were a major component of the sediments deposited during this interval. A large, relatively fresh lake existed from 2600 to 1000 BP. Illite was the dominant mineral deposited during this period, but since then has been a minor constituent in a mineral suite dominated by detrital silicates. A series of low-water, high-salinity stands occurred between 1000 and 600 BP, although these low stands were not as pronounced as low-water intervals in the middle Holocene. Relatively high water levels were sustained from 600 BP until the late 1800s. The lake declined significantly in the last one hundred years, notably during the historically documented droughts of the late 1800s, 1920s, 1930s, and 1980s.The timing of paleohydrological events at Chappice Lake corresponds closely with well documented Holocene climatic intervals, such as the Hypsithermal, Neoglaciation, Medieval Warm Period, and Little Ice Age. In addition, historic lake-level fluctuations can be related directly to climate. As a result, the Chappice Lake sedimentary succession offers a rare opportunity to obtain a high-resolution, surrogate record of Holocene climate on the northern Great Plains, and to observe the response of lake chemistry and biota to significant environmental change.Geological Survey of Canada Contribution No. 45191, Palliser Triangle Global Change Contribution No. 2This publication is the first of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor for this series.     

Early-Holocene geochemical evolution of saline Medicine Lake,South Dakota

Medicine Lake is a highly saline, meromictic, magnesium sulfate, closed-basin lake in northeastern South Dakota. The geochemical, mineralogical, and magnetic stratigraphies of sediments deposited from about 10.8 to 4.5 ka B.P. document the evolution of the saline brine in response to climatic change in the early to mid-Holocene. During the spruce occupation of the Medicine Lake catchment (10.8–10.0 ka B.P.), dark-grey massive basal sediments with low total-sulfur and carbonate content, upwardly increasing organic-carbon content, and high magnetic susceptibility were deposited in a deep freshwater lake. As the vegetation in the area changed from spruce to birch to oak and elm and finally to prairie between 10.0 and 9.2 ka B.P., and as the lake became shallow and salinity increased from <2 to >10%, light-and dark-grey calcareous and organic-carbon-rich banded sediments with low total-sulfur content and low magnetic susceptibility were deposited. Previous studies have shown that during the forest/prairie transition the lake then changed abruptly from fresh to saline as it lost a substantial portion of its volume. During the early prairie period (9.2–5.5 ka B.P.), alternating sections of aragonite-rich laminae and grey massive sediments with high total-sulfur content and multiple gypsum layers were deposited in a meromictic environment under conditions of fluctuating lake levels and salinity. Continued aridity during the mid-Holocene (5.5–4.5 ka B.P.) probably maintained the lake at relatively low levels and high salinity as dark-grey generally massive sediments with moderate total-sulfur, carbonate, and organic-carbon content and no measurable magnetic susceptibility were deposited.     

Mineralogy and lithostratigraphy of Harris Lake,southwestern Saskatchewan,Canada

Harris Lake, a small, groundwater fed lake in the Cypress Hills area of Saskatchewan, is one of the few lacustrine basins in the Great Plains that contains a complete, uninterrupted record of Holocene sedimentation. The lithostratigraphy and variation in the mineralogical composition of the sediments in this basin provide insight into the paleolimnology and paleohydrology of the lake and surrounding watershed. Although there is no evidence that the basin was dry for extended periods during the Holocene, the lake did experience numerous short-lived episodes of high salinity, as well as significant changes in solute composition during the early to mid-Holocene. An abrupt change, from a lake dominated by detrital sediments to one characterized almost entirely by endogenic deposition, occurred about 4000 years ago in response to the combined influence of forestation of the watershed and diversion of major fluvial and detrital influx by landsliding. These adjustments to the Harris Lake drainage basin were likely the result of the onset of cooler and wetter climatic conditions after 4500 B.P. During the late Holocene, slope failure continued to sporadically provide fresh clastic material to the otherwise endogenic-sediment dominated lake.     

Paleochemistry and paleohydrology of Ceylon Lake,a salt-dominated playa basin in the northern Great Plains,Canada

In the western part of the Canadian Prairies, there are thousands of small, closed-basin saline lakes. Most of these lakes are ephemeral, filling with water during the spring and drying completely by late summer. Ceylon Lake, located in southern Saskatchewan, is typical of many of these shallow ephemeral lacustrine basins. The stratigraphic sequence recovered from this salt playa can be subdivided into six distinct facies types: (a) icelaid gravelly clay loam diamicton; (b) fluvial massive bedded to laminated sand; (c) lacustrine laminated calcareous clay and silt; (d) lacustrine laminated gypsiferous clay and silt; (e) lacustrine black, anoxic, nonlaminated, organic-rich mud; and (f) lacustrine salt. The crystalline salt facies, which can be up to 9 meters thick, is comprised mainly of sodium and sodium + magnesium sulfates, with smaller and more variable proportions of other sulfates, halides, carbonates, and insoluble clastic detritus.Although a variety of postdepositional processes have significantly altered the nature and stratigraphic relationships in the basin, the sediment fill does record, in a general way, the fluctuating depositional, hydrological, and geochemical conditions that existed in the basin since deglaciation. The Ceylon Lake basin originated about 15 000 years ago as meltwater from the retreating glacial ice cut a major spillway system in the drift and bedrock. The initial (early Holocene) phases of lacustrine sedimentation in Ceylon Lake occurred in a relatively deep freshwater lake. By about 6000 years B.P., the lake had become much shallower with numerous episodes of complete drying and subaerial exposure. The most recent 5000 years of deposition in the basin have been dominated by evaporite sedimentation. The composition of the soluble salts deposited during this time indicates some degree of cyclic sedimentation superimposed on an overall gradual shift from a sodium dominated brine to one of mixed sodium and magnesium.     

The Holocene history of West Basin Lake,Victoria, Australia; chemical changes based on fossil biota and sediment mineralogy

Analyses of diatoms, ostracods, pollen and sediment mineralogy from a 524 cm core from a stratified, hypersaline crater lake, West Basin, Victoria, has revealed clear shifts in the lake's water balance and chemistry and the region's climate over the last 10 000 years. Diatom and ostracod analyses reveal lake water salinity changes which are consistent with the conditions suitable for the precipitation of the carbonate and other minerals identified using x-ray diffraction analysis. The fluctuations in lake water balance deduced from diatom and ostracod inferred lake salinity suggest that the lake began to fill at the beginning of the Holocene and was saline and shallow. Toward the mid-Holocene the water levels rose and yet the lake remained largely saline. The late Holocene is marked by a return to more shallow but fluctuating, water conditions. Through the whole period, the regional dryland vegetation was dominated by open sclerophyll woodland. Both the lacustrine and regional environments interpreted here are consistent with those from Holocene records elsewhere in the region.This is the fourth in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

Reconstruction of paleoenvironments of the Great Salt Lake Basin during the late Cenozoic

This study summarizes the results of micropaleontological, sedimentological, and isotope geochemical analyses of cuttings from five deep wells drilled in the Great Salt Lake (Utah, USA). Spanning the last 5.0 million yrs, our environmental history of the Great Salt Lake distinguishes four intervals based on paleobiological and sedimentological characteristics, using a previously developed tephrochronology for age control. For most of its history, the Great Salt Lake Basin has been occupied by a mixture of marsh, shallow lacustrine and sand flat conditions. In contrast, open lake conditions, typical of the Bonneville cycles and the modern Great Salt Lake apparently have only dominated the basin for the past 0.6-0.8 Ma. The two main structural basins in the study area (the North and South Basins) experienced different lacustrine histories. Large but frequently saline lakes occupied the North Basin after about 0.6 Ma. In the South Basin, ephemeral, saline lacustrine conditions started at 2.1 Ma and developed to full lacustrine conditions at 0.3 Ma. Our paleoenvironmental interpretations are broadly consistent with the aquatic palynological records from the same wells, as well as with the prior core- and outcrop-based lines of evidence. However, the differences in lake history between the North and South Basin have not been previously recognized.     

A record of early to middle Holocene environmental change inferred from lake deposits beneath a sabkha sequence in the Central Sahara (Seggedim, NE Niger)

Seggedim is a remote site in NE Niger where previous studies demonstrated the presence of Holocene paleolake deposits beneath an existing sabkha. In 2005, we retrieved a 15-m core that contains a high-resolution record of early to middle Holocene environmental change. Mineralogical, chemical and petrographic data were used to infer paleoenvironmental conditions recorded by successive lake stages. An exceptionally stable lake regime existed from 10.6 to 7.3 cal ka BP. This regime, reflecting moist conditions, was characterized by deposition of continuously laminated diatomite with high organic matter content, formed under meromictic conditions, with very low detrital input. A lake regime shift occurred at 7.3 cal ka BP, interpreted as the beginning of a transition to arid conditions. This shift was initially marked by a change to predominantly clastic sedimentation, followed by increased sand input during later stages, culminating in the onset of the sabkha stage. Because of the presence of reworked organic material in the deposits, reliable radiocarbon dating of the succession of lake stages could not be achieved.     

Climate and human impact on a meromictic lake during the last 6,000 years (Montcortès Lake,Central Pyrenees,Spain)

Sedimentological, mineralogical and compositional analyses performed on short gravity cores and long Kullenberg cores from meromictic Montcortès Lake (Pre-Pyrenean Range, NE Spain) reveal large depositional changes during the last 6,000 cal years. The limnological characteristics of this karstic lake, including its meromictic nature, relatively high surface area/depth ratio (surface area ~0.1 km²; z _max = 30 m), and steep margins, facilitated deposition and preservation of finely laminated facies, punctuated by clastic layers corresponding to turbidite events. The robust age model is based on 17 AMS ¹⁴C dates. Slope instability caused large gravitational deposits during the middle Holocene, prior to 6 ka BP, and in the late Holocene, prior to 1,600 and 1,000 cal yr BP). Relatively shallower lake conditions prevailed during the middle Holocene (6,000–3,500 cal years BP). Afterwards, deeper environments dominated, with deposition of varves containing preserved calcite laminae. Increased carbonate production and lower clastic input occurred during the Iberian-Roman Period, the Little Ice Age, and the twentieth century. Although modulated by climate variability, changes in sediment delivery to the lake reflect modifications of agricultural practices and population pressure in the watershed. Two episodes of higher clastic input to the lake have been identified: 1) 690–1460 AD, coinciding with an increase in farming activity in the area and the Medieval Climate Anomaly, and 2) 1770–1950 AD, including the last phase of the Little Ice Age and the maximum human occupation in late nineteenth and early twentieth centuries.     

The Holocene development of Kushu Lake on Rebun Island in Hokkaido, Japan

Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation.     

毛乌素沙漠东南缘滴哨沟湾剖面DGS1层段粒度特征及其指示的全新世气候变化

分析滴哨沟湾剖面全新统DGS1段的粒度数据,认为流动沙丘砂、泥炭、砂质古土壤、古固定-半固定沙丘砂到湖沼相和次生黄土的颗粒总体上变细,分选变差。结合腹足类动物化石和湖沼相地层中石英砂粒表面电镜扫描,认为DGS1粒度结果实际上是全新世以来多次冬夏季风的交互演替过程的气候-地质记录。据此,将DGS1粒度信号记录的全新世气候分为4个阶段——全新世早期好转期、全新世鼎盛期、大暖期向寒冷期转变波动期和降温不稳定的沙漠化频繁变化时期。     

Resolving the chronology of recent lake sediments: an example from Devils Lake,North Dakota

Devils Lake is a closed, saline lake in North Dakota; it is typical of lake basins in the Great Plains Region, where windy conditions and fluctuating water-levels disturb sediment and confound chronostratigraphy. Pollen analysis and ²¹⁰Pb dating of two cores collected from bathymetrically contrasting embayments demonstrate (1) how certain agriculture-related pollen types differ in their value as chronostratigraphic markers, (2) how pollen and ²¹⁰Pb stratigraphies can be reconciled to determine the approximate depth of sediment mixing, and (3) the importance of coring-site selection, especially in lakes with unstable sedimentary conditions.     

Recent evolution of Lake Arreo,northern Spain: influences of land use change and climate

We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes that affect the lake catchment. Chronology is based on “floating” discontinuous varve counts and ¹³⁷Cs and ¹⁴C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans.