Past Lake-level Changes Research Articles

Reconstruction of Holocene lake-level changes in Lake Xinias, central Greece

A study of past lake-level changes in Lake Xinias covering the last 40 000 years was presented in an earlier paper. In that paper, changes during the Holocene were only briefly dealt with, because of poor sedimentary representation. In this paper a supplementary transect of cores is described, in which Holocene lake-level changes are better recorded. The correlation of the cores was based on pollen and lithostratigraphic analyses, and a curve was constructed showing recorded lake-level changes. The curve from Lake Xinias was compared with an earlier compilation of lake-level changes from the Balkans, mainly from Greece. The compilation shows rising lake levels during the earlier part of the Holocene, and successively lowered levels during the later part. Most lake levels were high between about 8000 and 5500 cal. yr BP. The reconstruction from Lake Xinias indcates similar changes during the early and late Holocene. However, it differs by indicating a lowering in lake level, culminating around 7000 cal. yr BP, interrupting and dividing the period of mid-Holocene high lake-level stand. Further studies are required to show if this lowering in Lake Xinias was regionally representative and caused by climate change, or was caused by tectonic disturbance locally affecting lake level.

Vegetation history and lake-level changes from the Younger Dryas to the present in Eastern Pyrenees (France): pollen, plant macrofossils and lithostratigraphy from Lake Racou (2000 m a.s.l.)

A multi-proxy palaeoecological investigation including pollen, plant macrofossil, radiocarbon and sedimentological analyses, was performed on a small mountain lake in the Eastern Pyrenees. This has allowed the reconstruction of: (1) the vegetation history of the area based on five pollen diagrams and eight AMS14C dates and (2) the past lake-level changes, based on plant macrofossil, lithological and pollen analysis of two stratigraphical transects correlated by pollen analysis. The palaeolake may have appeared before the Younger Dryas; the lake-level was low and the vegetation dominated by cold steppic grasslands. The lake-level rose to its highest level during the Holocene in the Middle Atlantic (at ca. 5060±45 b.p.). Postglacial forests (Quercetum mixtum and Abieto-Fagetum) developed progressively in the lower part of the valley, while dense Pinus uncinata forests rapidly invaded the surroundings of the mire and remained the dominant local vegetation until present. The observed lowering of the lake levels during the Late Atlantic and the Subboreal (from 5060 ± B.P. to 3590±40 b.p.) was related to the overgrowth of the mire. The first obvious indications of anthropogenic disturbances of the vegetation are recorded at the Atlantic/Subboreal boundary as a reduction in the forest component, which has accelerated during the last two millennia.

Controls on loss-on-ignition variation in cores from two shallow lakes in the northeastern United States

Loss-on-ignition analysis of Quaternary lake sediments provides an inexpensive and easy way to investigate past environmental changes. The mass loss on ignition at 550 °C (LOI) from lake sediment cores may vary because of temporal changes in: (1) sediment composition controlled by factors such as productivity, inorganic inputs, and decomposition; and (2) the patterns of sediment accumulation controlled by factors such as basin morphology and water level. Climatic changes can alter both. Here, modern surface samples and transects of sediment cores, collected across small (<10 ha), shallow (<4 m) lakes in the northeastern United States, show that LOI varies little (2–5%) across the deep portions of these small lakes at a given time. Large changes in LOI occur only at the transition into the littoral (shallow) zone. LOI variations in sediment cores that exceed 2–5%, therefore, appear to represent meaningful environmental changes. However, because of the many possible controls, changes in the LOI of a single core are often hard to interpret. Multiple cores increase the interpretability. At lakes studied here, similar LOI trends among several cores confirm that some LOI changes resulted from basin-wide shifts in sediment composition. Differences among cores, however, developed during the early- and mid-Holocene and indicate that the edge of the littoral zone moved towards the centers of the lakes during two periods of low lake levels, at ca. 11 000–8000 and ca. 5400–3000 cal yr B.P. The basin-wide balance of sediment sources controlled the LOI from deep-water sediments, but sedimentation patterns, which changed as lake levels changed, were also important. LOI differences among cores may therefore help identify past lake-level changes in other lakes.

Postglacial climate in the St. Lawrence lowlands, southern Québec: pollen and lake-level evidence

Pollen and lake-level data are used to reconstruct past climate changes in the St. Lawrence lowlands, southern Québec. Past lake-level changes are assessed from sedimentological, pollen and macrofossil records from a single shallow-water core from Lac Hertel, which lies in the central part of the studied area. Three low lake-level phases are recognised: prior to 8000, 7600–6600 and 4800–3400 cal. BP. The modern analogue method is applied to pollen data from seven well-dated sites from the St. Lawrence lowlands and adjacent mountain areas, constrained and unconstrained by lake-level changes. The reconstructed climate changes are congruent with the pattern of climate changes known from eastern North America: a dry and cold late-glacial episode due to the presence of pro-glacial lakes and seas; a rapid warming between 12 500 and 11 000 cal. BP possibly caused by increasing summer insolation; a dry period from 10 000 to 6500 cal. BP; a brief cooling between 9000 and 8000 cal. BP, possibly related to a summer cooling of Arctic airmasses; a temperature maximum around 8000 cal. BP; and finally, a progressive decrease in summer temperature and an increase in (winter?) precipitation over the 4500 last years. These results show that it is possible to reveal seasonal patterns in climate by combining pollen and lake-level data.

Orbital forcing signal in sediments of two Amazonianlakes

Paleolimnological data from two ancient lakes at 0° latitudein Amazonia indicate that past lake level changes reflect precessional (19,000to 22,000 years) variations in insolation over the last 170,000 years. Waterlevel in Lakes Pata and Verde, Brazil, is determined by the ratio ofprecipitation:leakage. Times of low lake level are indicated in sediments byoxidized clays, evidence of algal blooms and high K+ concentrations.Peak K+ values are attributed to biogenic concentration when thelake was reduced to a shallow, productive pool. Low lake phases correlate withmaximum insolation during the dry season (June–July–August). Thusthe last glacial maximum 18,000 to 22,000 yr BP was a wet time in northernAmazonia, and the driest period of the last 170,000 years was from 35,000 to27,000 yr BP. These results from near the equator do not imply thatprecipitation changed synchronously across Amazonia, because geographiclocations throughout the vast watershed were undoubtedly influenced by localmoisture sources.

Geochemical and physical characteristics of river and lake sediments at Naivasha, Kenya

Prior studies on Lake Naivasha relevant to understanding sediment dynamics include a bathymetric map, a paleolimnological study of fossil invertebrate assemblages in lake sediment, an overview of lake level fluctuations throughout the 20th century, and identification of a dynamic assemblage of macrophyte zones that has responded both to these changes in lake level and to more recent, alien species. Sediment samples collected from the rivers systems and the lake were examined physically and chemically. River sediment characteristics reflect geology and geomorphological processes in the catchment, whereas lake sediment stratigraphy has responded to past lake level changes. Such changes have caused significant changes in aquatic vegetation assemblages. Present day sediment dynamics in the lake are governed by the presence of river point sources in the north and wave-induced re-suspension, such that sediments introduced by rivers are transported in easterly and southerly directions, and are eventually deposited in the eastern, central and southern parts of the lake. Sedimentary deposition is also occurring in northern areas that once were protected by papyrus swamp vegetation but now only have a narrow fringe, highlighting the important role of swamp vegetation in filtering out suspended particulates and thereby controlling water quality in the lake. Geochemical analyses of river and lake sediments indicate that they represent fairly undisturbed background conditions. Higher-than-expected concentrations of cadmium, iron, nickel and zinc found in both river and lake sediment are likely to derive from volcanic rocks and/or lateritic soils found in the lake catchment.

Reconstruction and paleoclimatic interpretation of Holocene lake-level changes in Lac de Saint-Léger, Haute-Provence, southeast France

Abstract Lac de Saint-Leger is located in the transitional region between Central Europe and the Mediterranean. The reconstruction of the past lake-level changes was based on plant macrofossil and mollusc analyses of two stratigraphic transects correlated by pollen analysis. Past changes in lake level were quantitatively reconstructed from changes in the recorded level of overgrowth. In the beginning of the Holocene the lake level was high; however, a progressive lowering occurred from about 7500 B.P. Between about 4500 B.P. and 4000 B.P. there was a moderate rise in lake level. Subsequently the lowering was continued and culminated between about 3300 B.P. and 2500 B.P. From the reconstruction there then occurred a progressive rise in lake level until the present. The lake-level changes recorded in Lac de Saint-Leger are compared with a compilation of lake-level data from southern France. The major long-term changes can be supposed to be regionally significant and related to climatic changes. To obtain a basis for paleoclimatic interpretation of the changes in lake level, the reference pollen diagram from Lac de Saint-Leger was used for a quantitative reconstruction of paleoclimate. During the middle and later part of the Holocene there is a reasonable consistency between the recorded pattern of lake-level changes and the paleoclimatic reconstruction. During the earlier part of the Holocene there are some disagreements, which may be explained by the shortage in adequate modern pollen analogues. The paleoclimatic reconstruction indicates that the major changes in lake level were mostly related to changes in precipitation.

Reconstruction of Holocene lake-level changes in Lake Kalvsj�n, southern Sweden, with a contribution to the local palaeohydrology at the Elm Decline

Past lake-level changes in Lake Kalvsjon are reconstructed and compared with the changes recorded in nearby Lake Bysjon. The two major lowerings of lake level noted in Lake Bysjon are also recorded in Lake Kalvsjon, the older lowering taking place between ca. 9500–9200 B.P. and the younger occurring between ca. 6500–3000 B.P. A distinct decrease in the frequency ofUlmus, corresponding to the classical Elm Decline, is recorded in the pollen diagram from Lake Kalvsjon. The high rapidity of the decline strongly suggests that a pathogenic attack was primarily responsible. However, both human interference and palaeohydrological change may have interacted by disturbing the surrounding forests and increasing the susceptibility of elm to pathogenic attack. In the Lake Kalvsjon area, the disturbance resulting from palaeohydrological change is assumed to have been more influential in pre-disposing the forest to an outbreak of elm disease than any human interference.

The plant-macrofossil record of past lake-level changes

Plant-macrofossil analysis is one of the most useful biostratigraphical methods for the reconstruction of former lake-level changes. The distribution of submerged, floating-leaved and emergent lake-shore vegetation is mainly dependant on water depth, but water chemistry and nutrient status must also be taken into account when interpreting water-level changes. Lake-level studies should be based on the investigation of several littoral cores along a transect perpendicular to the lake-shore. Multiple cores are essential for separating genuine lake-level changes from other processes influencing the plant-macrofossil record. Physical analyses of sediment stratigraphy provide important additional information to the plant-fossil record, because natural infilling processes and erosion from the catchment must be distinguished from climatic events causing a change in the water level. Here we review several important concepts, including suitability of lakes for lake-level study, the degree of detail required in the analysis, and macrofossil records of lake-level changes, and illustrate those concepts by examples from southern Sweden and Minnesota. We discuss how to reconcile alternative hypotheses for the stratigraphic changes seen in the macrofossil assemblages.

Groundwater control of closed-basin lake levels under steady-state conditions

Groundwater exerts both local and regional controls on closed-basin lake-level changes. Locally, the equifer surrounding a lake can modify how lake-level changes result from a shift in surficial water balance. “Lake pumping” is defined as the net steady-state removal of lake water via surficial processes, namely evaporation minus direct precipitation and minus any input from overland runoff that reaches the lake. A simple analytic groundwater model of a circular lake next to an infinitely long river shows that the sensitivity of the lake level to a change in lake pumping is proportional to the distance from the lake to the river. Further, lakes with large surface areas are more sensitive than small lakes to a shift in lake pumping. Regionally, lake levels change from a shift in the surrounding water-table elevation. A review of a simple analytic groundwater model of a water table between two infinitely long canals demonstrates that a shift in groundwater recharge changes the water-table elevation most near the middle of the interfluve. Three types of lake-level sensitivities (two local, one regional) are defined as the partial derivative of lake level with respect to: (1) lake size (radius); (2) lake pumping rate; and (3) regional groundwater recharge. All three sensitivities may be further categorized as “positional sensitivities” because their values are a function of the lake's position in the watershed. Specifically, lakes far from rivers are more susceptible to lake-level change than are lakes close to rivers. Both simple models further demonstrate that lake-level sensitivity to the above factors is inversely proportional to the permeability of the surficial aquifer. When compared to a system of a more realistic geometric arrangement of lakes and rivers, the simple models underestimate lake-level change in response to both local and regional factors. Knowledge of the sensitivities of lakes can be important in identifying those lakes on the modern landscape that are susceptible to lake-level change, and in interpreting the causes of past lake-level changes as revealed by geologic investigations.