Low Lake Levels Research Articles

A late glacial Antarctic climate teleconnection and variable Holocene seasonality at Lake Pupuke, Auckland, New Zealand

We present the first continuous paleolimnological reconstruction from the North Island of New Zealand (37°S) that spans the last 48.2 cal kyr. A tephra- and radiocarbon-based chronology was developed to infer the timing of marked paleolimnological changes in Lake Pupuke, Auckland, New Zealand, identified using sedimentology, magnetic susceptibility, grain size and geochemistry (carbon, nitrogen and sulphur concentrations and fluxes, carbon and nitrogen stable isotopes). Variable erosional influx, biomass and benthic REDOX conditions are linked to changing effective precipitation and seasonality within three inferred broad intervals of climatic change: (1) the Last Glacial Coldest Phase (LGCP) of reduced effective precipitation and cooler temperatures, from 28.8 to 18.0 cal kyr BP, (2) the Last Glacial Interglacial Transition (LGIT) of increasing effective precipitation and warmer conditions, from 18.0 to 10.2 cal kyr BP, and (3) a Holocene interval of high effective precipitation, beginning with a warm period of limited seasonality from 10.2 cal kyr BP and followed by increasing seasonality from 7.6 cal kyr BP. The LGCP and LGIT also contain millennial-scale climate events, including the coldest inferred glacial conditions during the LGCP from 27.8 to 26.0 and 22.0–19.0 cal kyr BP, and a climate reversal in the LGIT associated with lower lake level, from 14.5 to 13.8 cal kyr BP, coeval with the Antarctic Cold Reversal. The onset of seasonal thermal stratification occurred at 5.7 cal kyr BP and was linked to natural eutrophication of Lake Pupuke, which produced enhanced organic sedimentation.

Late Holocene paleoenvironmental changes in Northeast Brazil recorded by organic matter in lacustrine sediments of Lake Boqueirão

Northeast Brazil represents a key area in terms of Holocene environmental changes in South America due to its distinct semi-arid climate at near equatorial latitudes as well as to a wetter Early and Middle Holocene paleoclimate in comparison with other South America regions, which also experienced a reduced monsoon at that time. We studied the lacustrine sediments of Lake Boqueirão, which is located at 5°S latitude on the Brazilian Atlantic coast; it is directly influenced by displacement of the Intertropical Convergence Zone (ITCZ). A short (1m) core “Boqc0701” was collected at a 7.5m water depth during a seismic survey. Organic matter (OM) geochemical [Rock‐Eval Total Organic Carbon (TOC), Hydrogen Index (HI), Oxygen Index (OI)] and petrographical studies allowed us to distinguish five sedimentary intervals during the last 3kyr. Phase E (3000 to 2050calyr BP) presents the largest TOC, HI and OI fluctuations in the entire core. The most probable cause of these variations is a great instability in the lake level during this period. Phase D (2050 to 1830calyr BP) is marked by an increase in sedimentation rate and a higher contribution from a well-preserved OM with an algal origin; this was interpreted as a higher and more stable lake level. During phase C (1830 to 1550calyrs BP), low HI and high OI indicate the input of more oxidized, degraded and detrital OM, reflecting a lower lake level. In phase B (1550 to 1470calyr BP), the OM quality and quantity (HI and TOC) indicate an algal contribution. During phase A (1470 to 570calyr BP), the high stability of the proxies may be due to a deep environment where OM fluctuations are strongly buffered. These changes in lake level can be compared with other tropical South America high resolution records. A correspondence seems to exist between phases of low ENSO, cool North Atlantic Ocean and Boqueirão wet phases. We did not encounter this same pattern during the last 1050years, during which the 1050–570calyrs BP interval was generally wet (although probable fluctuations are buffered in our proxies) while the North Atlantic was warm (Mann et al., 2009). A prolonged dryness occurred in Lake Boqueirão from 570calyrs BP until recent decades. This phase corresponds to the Little Ice Age characterized by a cooler North Atlantic Ocean (Mann et al., 2009). These data indicate that the teleconnection pattern between the tropical Atlantic, ENSO and Northeast Brazil rainfalls has changed in the past.

Multiple early Holocene climate oscillations at Silver Lake, New Jersey and their possible linkage with outburst floods

Episodic discharges from the Laurentide Ice Sheet are considered to be one of the major causes of climate oscillations during the glacial to interglacial transition. These outburst floods disrupted thermohaline circulation and might have caused cooling events in the North Atlantic region and beyond, including the Younger Dryas, Preboreal Oscillation, and 8.2-ka events. However, few terrestrial sedimentary records show regional climate changes directly linked to outburst floods during the early Holocene. Here we present lithologic and isotopic data from Silver Lake in northern New Jersey to document temperature and moisture changes during the early Holocene and to discuss possible linkage with outburst flooding events. The lithologic record shows multiple intervals with coupled decreases in carbonate and increases in organic matter during the early Holocene, suggesting lower lake levels and drier climate conditions. Simultaneous with these changes were decreases in carbonate δ18O values, likely reflecting a decrease in air temperature during these dry periods. The apparent correlation of these sedimentary events at Silver Lake with documented outburst floods from the melting ice sheet suggests a possible causal connection, implying that the climate in the Mid‐Atlantic region was extremely sensitive to perturbations of ocean circulation.

Lithostratigraphic and geochronological framework for the paleoenvironmental reconstruction of the last ∼36 ka cal BP from a sediment record from Lake Iznik (NW Turkey)

New sediment cores were recovered from two sites in the central part of Lake Iznik with the overall aim of reconstructing past environmental conditions of the Marmara region. The composite profile presented here, IZN09/LC2&LC3, encompasses the late Pleistocene to Holocene transition (c. 36 ka cal BP) which is the longest lacustrine record in this region obtained to date. A lithostratigraphical and geochronological framework builds the basis to establish a composite section for first inferences on the paleo Lake Iznik. The recovered sedimentary record was divided into five stratigraphic units which can be correlated between the different coring locations. The proposed age-depth model is based on eleven 14C dates (eight radiocarbon-dated levels) and two tephra layers, supported by three OSL ages. The modeled age distribution of the Ca/Ti ratio and magnetic susceptibility express variations in the carbonate accumulation in balance with clastic sediment input.Starting from the end of MIS 3 with high clastic input, Lake Iznik passed through a low lake-level during the Last Glacial Maximum (LGM), delineated by a sublitoral sedimentary facies, and reduced sedimentation rates. After c. 18 ka cal BP, the onset of primary carbonate deposition might be linked to meltwater inflow into the lake as well as onset of lake productivity. From this time onward, there is a gradual increase in carbonate accumulation punctuated by the occurrence of an iron-sulfidic layer which coincides with the Younger Dryas event. At the early Holocene, the lake presents a minimum level as reflected by the maximum carbonate production, followed by a lake level rise at c. 9 ka cal BP when it reached a level similar to the modern situation.

Carbonate horizons, paleosols, and lake flooding cycles: Beds I and II of Olduvai Gorge, Tanzania

This study documents the petrology and stable isotope geochemistry of carbonates from six horizons from Beds I and II of Olduvai Gorge, Tanzania. The studied succession, immediately below and above Tuff IF, consists of interbedded waxy and earthy claystones with discrete carbonate horizons and thin sandstones. The succession was deposited in response to repeated flooding and withdrawal of a saline-alkaline lake. The carbonates and their overlying disconformities are important because they help define the surfaces on which hominin activity took place and allow very high-resolution correlation of geographically separated levels of hominin exploitation.The range of different carbonates includes unambiguous land-surface and pedogenic features including calcified rootmat horizons, rhizocretions, and micritic nodules, together with less determinate sparry calcite nodules. Stellate nodules are interpreted as pseudomorphs after sulfate-roses. The carbonate nodules are synsedimentary features, truncated by fluvial and other erosional surfaces. The isotopic composition of the carbonates is variable with δ18O ranging from −7.0‰ to −4.3‰, and δ13C from −8.5‰ to −1.6‰. A covariant increase in δ13C and δ18O repeats in each carbonate horizon and in individual nodules (inner to outer layers): it reflects the evolution of synsedimentary groundwaters. At times of low lake level, the carbonates started to precipitate from meteoric waters with low isotopic values and continued to form as lake levels rose and the waters became increasingly saline. Some of the samples have a last-stage cement of strontium rich dolomite, which supports late-stage flooding by the saline-alkaline lake. Previous studies of carbonate horizons from Olduvai have interpreted carbon isotope values in terms of changes in C3 and C4 plants that colonized the land surface. This study demonstrates that in some instances the isotope values from carbonates deposited in these lake marginal settings reflect changes in hydrology rather than vegetation.

Climatic and environmental changes reflected by lake-level fluctuations at Gerzensee from 14,850 to 13,050 yr BP

High-resolution sediment analysis at Gerzensee, Swiss Plateau, focused on the reconstruction of lake-level changes during the last glaciation Greenland Stadial 2a (GS-2a) and a large portion of Lateglacial including the Gerzensee oscillation. The chronology is derived from a comparison of the oxygen-isotope stratigraphy established at Gerzensee with that of the NGRIP ice core. On a multi-millennial scale differences between the lake-level and oxygen-isotope records established from cores GEJ–GEK reflect a complex interplay among climate, vegetation and lake level during retreat of the ice sheet. But on a multi-centennial scale both the lake-level and oxygen-isotope records show a general agreement, i.e. cool periods such as Greenland Stadial GS-2a and Greenland Interstadials-1d and -1b, which coincided with more positive water budgets and, conversely, warm periods (Greenland Interstadials-1e and -1c) with more negative water budget. This is in agreement with the regional pattern of palaeohydrological changes reconstructed from previous studies. Despite possible reduced annual precipitation during GS-2a, the maintenance of relatively high lake-level conditions at Gerzensee at this time could have resulted from reduced evaporation and stronger runoff on bare slopes, whereas during the Lateglacial Interstadial the phases of lake-level lowering may have been associated with summer dryness linked to increasing summer insolation.

A diatom record of recent environmental change in Lake Duluti, northern Tanzania

Lake Duluti is a small, topographically closed crater lake located on the flanks of Mt Meru, northern Tanzania. Analyses of diatoms in three short sediment cores and four modern samples from Lake Duluti were used to infer past environmental changes. 210Pb and 137Cs activity profiles combined with AMS 14C dates provide the chronological framework. Weak agreement between the 210Pb and 14C records, together with dating uncertainty, precludes construction of precise age models. The modern diatom flora, from plankton and three periphytic habitats, is dominated by Aulacoseira ambigua (Grunow) Simonsen, Gomphonema parvulum (Kutzing) Grunow and Nitzschia amphibia Grunow. All three cores display similar stratigraphic succession, but the relative ratio of habitats represented by the diatoms varies substantially between cores. Diatoms indicate that the oldest part of the record is characterized by relatively low lake level and swampy vegetation. In the late nineteenth or early twentieth century there was a rapid lake level rise and the swamp turned into an open-water lake. High lake levels have prevailed since that time.

Late Holocene change in climate and atmospheric circulation inferred from geochemical records at Kepler Lake, south-central Alaska

Climate records during the last millennium are essential in placing recent anthropogenic-induced climate change into the context of natural climatic variability. However, detailed records are still sparse in Alaska, and these records would help elucidate climate patterns and possible forcing mechanisms. Here we present a multiple-proxy sedimentary record from Kepler Lake in south-central Alaska to reconstruct climatic and environmental changes over the last 800 years. Two short cores (85 and 101 cm long) from this groundwater-fed marl lake provide a detailed stable isotope and sediment lithological record with chronology based on four AMS 14C dates on terrestrial macrofossils and 210Pb analysis. The δ18O values of inorganic calcite (CaCO3) range from −17.0 to −15.7 ‰, with the highest values during the period of 1450–1850 AD, coeval with the well-documented Little Ice Age (LIA) cold interval in Alaska. The high δ18O values during the cold LIA are interpreted as reflecting shifts in atmospheric circulation. A weakening of the wintertime Aleutian low pressure system residing over the Gulf of Alaska during the LIA would have resulted in 18O-enriched winter precipitation as well as a colder and possibly drier winter climate in south-central Alaska. Also, elevated calcite contents of >80 % during the LIA reflect a lowering of lake level and/or enhanced seasonality (warmer summer and colder winter), as calcite precipitation in freshwater lakes is primarily a function of peak summer temperature and water depth. This interpretation is also supported by high δ13C values, likely reflecting high aquatic productivity or increased residence times of the lake water during lower lake levels. The lower lake levels and warmer summers would have increased evaporative enrichment in 18O, also contributing to the high δ18O values during the LIA. Our results indicate that changes in atmospheric circulation were an important component of climate change during the last millennium, exerting strong influence on regional climate in Alaska and the Arctic.

A sedimentary and geochemical record of water-level changes from Rantin Lake, Yukon, Canada

A multi-proxy analysis of two sediment cores from Rantin Lake are used to reconstruct past lake-level changes and to make inferences about millennial-scale variations in precipitation/evaporation (P/E) balance in the southern Yukon, Canada between 10,900 and 3,100 cal yr BP. Analyses of calcium carbonate and organic matter concentration, magnetic susceptibility, titanium content, dry bulk density, and macrofossils are used to reconstruct water-level changes. The development of sand layers and deformed sediments at the deep-water core site (i.e. Core A-06) prior to ~10,900 cal yr BP suggest that lake level was lower at this time. Fine-grained organic sediment deposited from 10,600 to 9,500 cal yr BP indicates a rise in lake level. The formation of an unconformity at the shallow cores site (Core C-06) and the deposition of shallow-water calcium carbonate-rich facies at the Core A-06 site between ~9,500 and ~8,500 cal yr BP suggest lower lake levels at this time. Shallow-water facies gradually transition into a sand layer that likely represents shoreline reworking during an extreme lowstand that occurred at ~8,400 cal yr BP. Following this low water level, fine-grained organic-rich sediment formed by ~8,200 cal yr BP, suggesting deeper water conditions at core site A-06. Calcium carbonate concentrations are relatively low in sediment deposited from ~6,300 to 3,100 cal yr BP in Core A-06, indicating that lake level was comparatively higher during the middle and late Holocene. In general, results from this study suggest that the early Holocene was characterized by high P/E from ~10,500 to 9,500 cal yr BP, low P/E from ~9,500 to 8,400 cal yr BP, and return to higher P/E from ~8,200 to 3,100 cal yr BP.

Desalinate or divert? Coastal non-market values as a decision tool for an integrated water management policy: The case of the Jordan River basin

This paper deals with a cost effective analysis of two options to increase the water supply in Israel. The first policy is to divert 300 Million Cubic Meters (MCM) of water from the Sea of Galilee (SOG) to the central part of Israel. This policy is the existing one. The second policy is to replace this diversion with desalinated water plants that will be built on the Mediterranean Coast (MC). These two options carry both market and non-market consequences. The first policy has a negative effect on the SOG itself due to the lower lake level. It also carries some negative consequences on the Jordan River (JR) and the Dead Sea (DS) which are located downstream. The second policy involves water production at a higher cost and has negative external effects of scarce coastal land usage and high energy consumption. A Payment Card (PC) Contingent Valuation (CV) survey was performed at the four sites (the SOG, the DS, the JR and the MS). We show that when one takes these non-use values into account, the preferred solution will shift from the usage of the SOG to the desalination policy.

Holocene hydrological and climatic change on the northern Mongolian Plateau based on multi-proxy records from Lake Gun Nuur

A multi-proxy study including analyses of δ13Corg for the lake sediment core GN-02 and grain size, TOC, CaCO3 content, δ13Ccarb and δ18Ocarb of bulk carbonate, and the mineralogy of the parallel core GN-04 from Gun Nuur was performed to reconstruct the Holocene hydrology and climate on the northern Mongolian Plateau. The chronology was established using 40 14C dates of bulk organic matter in addition to nine previously published radiocarbon dates for core GN-02, and further five 14C dates for the new core GN-04. A lake reservoir effect of 1060 14Cyears was determined as the intercept of the high-resolution GN-02 age-depth model at the modern sediment surface. The size of the reservoir effect is supported by the age of the core-top sample (1200±40 14Cyears) and the determined difference between a wood-derived radiocarbon age from the GN-02 core base and the age-model inferred age for bulk organic matter at the same stratigraphic level (1000 14Cyears).Low lake level and prevailing aeolian sediment deposition at Gun Nuur under dry conditions were recorded during the earliest Holocene (>10,800–10,300cal a BP). Gun Nuur expanded under significantly wetter conditions between 10,300 and 7000cal a BP. Unstable climate conditions existed in the mid Holocene (7000–2500cal a BP) and three periods of low lake-levels and significantly drier conditions were recorded between 7000–5700, 4100–3600 and 3000–2500cal a BP. Intermediate lake levels were inferred for the intervening periods. Around 2500cal a BP, the climate change and wetter conditions were established again. As a consequence, the lake level of Gun Nuur rose again due to higher effective moisture and the relatively wet present conditions were achieved ca. 1600cal a BP. Our results suggest that the initial Holocene climate change on the northern Mongolian Plateau was not accompanied by a rapid increase in precipitation as on the Tibetan Plateau. The establishment of wetter conditions in northern Mongolia lagged behind the early Holocene moisture increase on the Tibetan Plateau by ca. 1000years. Subsiding dry air in the north of the Tibetan Plateau resulted from the strengthened summer monsoon on the Tibetan Plateau during the period of maximum summer insolation and probably inhibited a significant precipitation increase in Mongolia. The significant moisture increase in the Gun Nuur region at ca. 10.3calka BP is probably not related to the northward shift of the present summer monsoon boundary or the moisture delivery from the northern Atlantic through the westerlies. Instead, water from melting snow, ice and frozen ground and the generation of precipitation from the local recycling of moisture are discussed as possible moisture source for the early onset of wetter conditions on the Mongolian Plateau.

Environmental changes during the past 50000 years recorded by Lake Wulagaigaobi sediment, Dongwuzhumuqinqi, Inner Mongolia

中国北方季风边缘区的湖泊对环境的反应很敏感,乌拉盖高毕是位于内蒙古高原东北部的一个已干涸的内陆封闭湖泊.研究剖面总厚度为405 cm,根据取得的9个AMS¹⁴C及光释光测年数据,建立了51.9 ka BP以来的年代序列.结合岩性的变化、粒度组分分析、粒度参数(标准偏差、偏度、峰度等)及Rb/Sr比值特征进行了详细的分析,恢复了乌拉盖高毕湖泊沉积记录的5万年来的环境演化过程.将0~405 cm的剖面划分为4个较大的气候阶段:51.9-44.1 ka BP(405~343 cm),气候为干冷期,各环境指标显示为滨湖沉积,湖泊水位较低;44.1-28.5 ka BP(343~130 cm),气候为温湿期,湖泊逐渐扩张,湖泊水位升高;28.5-11.38 ka BP(130~35 cm),气候转冷,湖面萎缩,并在盛冰期出现砂楔,该阶段后期湖泊干涸;11.38 ka BP至今(35~0 cm),气温回升.;Lakes located in the monsoon marginal zone in North China are sensitive to climate change. Lake Wulagaigaobi is an enclosed lake in Inner Mongolia of China, which is in the semiarid and sub-humid ecotone on the northeast margin of the Inner Mongolia Plateau. In this paper, a sediment profile of 405 cm deep was chose. The measured data of the sediment in AMS¹⁴C and OSL age revealed that the sediment profile was deposited since 51.9 ka BP. Combined with chronological data and the analyses of sedimentary characteristics, including grain-size distribution, grain-size frequency, grain-size parameters, and trace elements, the evolutionary process and features of the lake environment were reconstructed during the past 50000 years. Results showed that there were four climate periods:51.9-44.1 ka BP (405-343 cm), cold and dry period, a lower lake-level; 44.1-28.5 ka BP (343-130 cm), warm and wet period, the lake-level fluctuated; 28.5-11.38 ka BP (130-35 cm), cold and dry period; 11.38 ka BP-today (35-0 cm), warm and wet period.

Exploiting seasonal habitat use of the common carp, Cyprinus carpio, in a lacustrine system for management and eradication

The control of invasive species is a major challenge to the preservation of native ecosystems, both terrestrial and aquatic. A population of the invasive common carp, Cyprinus carpio, was detected in Lakes Sorell and Crescent, Tasmania, Australia, in the mid-1990s. Early detection allowed for the containment of their population to this lacustrine system. Radio-telemetry was subsequently used over a 5-year period to understand seasonal distribution of carp in relation to environmental parameters to guide efficient eradication strategies. The present study found that fish displayed increased mobility during spring–summer periods, moving into shallow habitat rich in macrophytes, particularly during years of high lake levels. During years of low lake levels, this pattern was altered with frequent use of a rocky ‘secondary’ habitat in Lake Sorell. During winter, carp congregated in deeper habitat in Lake Sorell, whereas no habitat-specific winter aggregations were found in Lake Crescent. The increased knowledge of spatio-temporal preferences of carp provided a basis for deploying barriers to facilitate the capture of fish and sabotage spawning events. The results have led to the eradication of carp from Lake Crescent and a significant reduction of their population in Lake Sorell.

Response of the Truckee River to lowering base level at Pyramid Lake, Nevada, based on historical air photos and LiDAR data

The Truckee River in western Nevada has severely incised in response to a net 20 m of lake-level lowering of Pyramid Lake over the past 120 yr, leaving a suite of cut terraces that extend ~15 km upstream from the lake where incision was arrested by bedrock in the channel. Channel planform changes and terrace development were mapped over a 70 yr period using rectified aerial imagery and light detection and ranging (LiDAR) data in ArcGIS. Discrete point elevations were extracted from the LiDAR data for channel remnants on each of the different-aged terraces and combined with channel distance measurements derived from the appropriate aerial imagery to derive true channel gradients for each of the photo years. These same point elevation measurements were also resolved to a common valley distance (CVD), as is often done when studying prehistorical terrace sequences. Comparison of the true channel distance (TCD) profiles to the CVD profiles demonstrates that the TCD profiles are longer and less steep than the CVD profiles. These differences exist because resolving terrace elevations to a CVD does not account for decreasing sinuosities and straighter channel planforms through time. As base level fell to its historical low elevation, the channel became steeper, straighter, and smoother, which increased the rate of sediment transport and delivery to Pyramid Lake. Over the past 120 yr, ~60,000,000 m 3 of sediment have been removed from the bed and banks of the lower river and redeposited in the lake, temporarily increasing the sedimentation rate. Larger trenches through deltaic and lacustrine deposits occupied by rivers and streams flowing into Pyramid Lake and other closed basins may reflect similar rapid responses to lowering base levels at times in the past. Therefore, lowering lake levels at Pyramid Lake and elsewhere during relatively dry times may be at least partly responsible for transient increases in sedimentation rates in lacustrine archives.

A 15,000 year record of vegetation and climate change from a treeline lake in the Rocky Mountains, Wyoming, USA

Future climate projections predict warming at high elevations that will impact treeline species, but complex topographic relief in mountains complicates ecologic response, and we have a limited number of long-term studies examining vegetation change related to climate. In this study, pollen and conifer stomata were analyzed from a 2.3 m sediment core extending to 15,330 cal. yr BP recovered from a treeline lake in the Rocky Mountains of Wyoming. Both pollen and stomata record a sequence of vegetation and climate change similar in most respects to other regional studies, with sagebrush steppe and lowered treeline during the Late Pleistocene, rapid upward movement of treeline beginning about 11,500 cal. yr BP, treeline above modern between ~9000 and 6000 cal. yr BP, and then moving downslope ~5000 cal. yr BP, reaching modern limits by ~3000 cal. yr BP. Between 6000 and 5000 cal. yr BP sediments become increasingly organic and sedimentation rates increase. We interpret this as evidence for lower lake levels during an extended dry period with warmer summer temperatures and treeline advance. The complex topography of the Rocky Mountains makes it challenging to identify regional patterns associated with short term climatic variability, but our results contribute to gaining a better understanding of past ecologic responses at high elevation sites.

Past environmental and climatic changes during the last 7200 cal yr BP in Adamawa plateau (Northern-Cameroun) based on fossil diatoms and sedimentary carbon isotopic records from Lake Mbalang

Abstract. Past limnological conditions of Lake Mbalang (7°19′ N, 13°44′ E, altitude: 1130 m) and vegetation type were reconstructed from diatoms and sedimentary stable carbon isotope records (δ13C) since 7200 cal yr BP. The data showed that before 3600 cal yr BP, the water column was dominantly stable except around 5000–5300 cal yr BP where diatoms evidenced a mixed upper water layer and δ13C data suggest more forested vegetation in the landscape. These stable conditions can be explained by a strong monsoon flux and relatively northern position of the ITCZ that entailed high or low rainfall well distributed over the year, allowing the development of mountainous forest taxa. The decreasing trend of the monsoon flux towards the mid-Holocene was affected by several abrupt centennial to millennial-scale weakening at 6700, 5800–6000, 5000–5300, 4500 and 3600 cal yr BP. However, their impact on the vegetation is not visible, probably because rainfall distribution was favourable to forest maintenance or extension. After 3600 cal yr BP, the water column became very mixed as a result of more intense NE trade winds (Harmattan) that led at ~3000 cal yr BP to the establishment of savannah in the vegetation landscape. At that time, rainfall was probably reduced following the southward shift of the ITCZ, and the distribution of yearly rainfall was not favourable anymore to forest development. A strong seasonality with a marked dry season was established, conditions that maintained the savannah vegetation until today. Diatom data suggest the lake did not dry up during the last 7200cal yr BP; however, a low lake level observed at 2400–2100 cal yr BP is contemporaneous to a climatic event evidenced in several areas of tropical Africa and could correspond to the southernmost position of the ITCZ. Other low lake levels are observed at 1800 and 1400 cal yr BP, after which the lake rose to its present level.

Holocene history of environment and human impact on two islands in the Ostholstein lakeland area, Northern Germany

Holocene sediment cores from contemporary terrestrialised kettle holes on two islands in the Ostholstein lakeland area, Northern Germany, were investigated by palaeoenvironmental methods. Records from such isolated locations, such as these island mires, are an important source of information on small-scale vegetation changes. Pollen, non-pollen palynomorphs and macrofossils were used to reveal anthropogenic impact as well as lake level fluctuations. Changes in the peat decomposition and the ratio of organic to minerogenic content in the sediment facilitated correlation with past ground water table levels and accordingly with former lake levels. The palynological surveys indicate that human impact on the islands expanded in prehistoric times during phases of low lake levels or periods of stable hydrological conditions. The first distinct low level is recorded between ca. 8210 and 7740 cal. yr b.p. Another major low level period was detected for the time span from approximately 5290 to 4600 cal. yr b.p. After a subsequent period with a wetter climate, indicated by rising and fluctuating lake levels culminating at ca. 2420 to 2180 cal. yr b.p., a third lowering occurred until about 660 cal. yr b.p. This period was interrupted by minor fluctuations in lake levels from ca. 1770 to 1630 cal. yr b.p. and from ca. 1370 to 1010 cal. yr b.p.

Cementation characteristics and age of beachrocks in a fresh-water environment, Lake İznik, NW Turkey

This paper focuses on the cement characteristics and optical luminescence age of late Pleistocene–Holocene beachrock, exposed on the southeast shore of freshwater Lake İznik in northwest Turkey, based on field observations, various spectroscopic analyses and optical dating results. The studied beachrock is a poorly-sorted coastal conglomerate composed mostly of gravels derived from surrounding volcanic rocks and marbles as well as quartz and carbonate grains. We identified different types of cements; dominated by micritic envelopes, bladed isopachous aragonite rims, void fills, radial aggregates and meniscus bridges, implying no single origin. Cementation characteristics indicate that marine-like cement micro-fabrics may occur in freshwater lake environments where the lake-water chemistry favors carbonate precipitation. OSL data revealed the existence of older beachrock dating back to 20.2 ka, together with younger Holocene-age beachrock of between 5.6 and 2.4 ka. This suggests that younger beachrocks have been superimposed on older cemented carbonates through the removal of carbonates by wave motion (splash and spray) during lower lake level conditions.

Analysis of Shoreline Changes (1959–2004) in Point Pelee National Park, Canada

Abstract Shoreline change rates of Point Pelee National Park, Ontario are estimated for the period 1959–2004 using orthoair photos. The eastern side of the park had varying recession regardless of Lake Erie water-level changes, especially in Northeast Beach, where it averaged −3.14 m/y. The western side of the park generally exhibited accretion and had an inverse relationship with Lake Erie water levels. Longer duration of ice coverage during winter, favourable topography, and sedimentary conditions led to the observed difference in shoreline response. Future climate change patterns are expected to result in lower lake levels and less winter ice coverage, both of which will likely result in a net loss to the park's land habitat.

Late Pleistocene lake and glaciation evolution on the northeastern Qinghai–Tibetan Plateau: a review

Late Pleistocene paleoclimatic history on northeastern Qinghai–Tibetan Plateau (QTP) has been reconstructed mainly from lake sediments; however, data regarding dry–wet climate changes reported in this region are still not clear and controversial. Based on shoreline features and highstand lacustrine sediments around lakes on the QTP, high lake level histories in this paper were summarized and compared with paleoclimatic records from lake sediments, ice core and glaciation evolution surrounding mountains on the NE QTP during late Pleistocene. The results indicate that periods of high lake level occurred at MIS 5, MIS 3 and early-middle Holocene and most likely corresponding to warm and wet climate periods, while periods of low lake level existed in intervening intervals, corresponding to cold and dry climate periods, which most likely coincide with glacial advances surrounding high mountains. With an exception, no wide glacial advance in study area was found during MIS 3, possibly suggesting that effective moisture is lower than that in the other region of NE QTP in this period.