Lake-level Data Research Articles

The sedimentary and palynological records of Serpent River Bog, and revised early Holocene lake-level changes in the Lake Huron and Georgian Bay region

Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve 14C dates on peat, wood and plant macrofossils combined with previously published 14C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which had been previously identified as seismo-stratigraphic sequence boundaries.

A multi-source satellite data approach for modelling Lake Turkana water level: calibration and validation using satellite altimetry data

Abstract. Lake Turkana is one of the largest desert lakes in the world and is characterized by high degrees of inter- and intra-annual fluctuations. The hydrology and water balance of this lake have not been well understood due to its remote location and unavailability of reliable ground truth datasets. Managing surface water resources is a great challenge in areas where in-situ data are either limited or unavailable. In this study, multi-source satellite-driven data such as satellite-based rainfall estimates, modelled runoff, evapotranspiration, and a digital elevation dataset were used to model Lake Turkana water levels from 1998 to 2009. Due to the unavailability of reliable lake level data, an approach is presented to calibrate and validate the water balance model of Lake Turkana using a composite lake level product of TOPEX/Poseidon, Jason-1, and ENVISAT satellite altimetry data. Model validation results showed that the satellite-driven water balance model can satisfactorily capture the patterns and seasonal variations of the Lake Turkana water level fluctuations with a Pearson's correlation coefficient of 0.90 and a Nash-Sutcliffe Coefficient of Efficiency (NSCE) of 0.80 during the validation period (2004–2009). Model error estimates were within 10% of the natural variability of the lake. Our analysis indicated that fluctuations in Lake Turkana water levels are mainly driven by lake inflows and over-the-lake evaporation. Over-the-lake rainfall contributes only up to 30% of lake evaporative demand. During the modelling time period, Lake Turkana showed seasonal variations of 1–2 m. The lake level fluctuated in the range up to 4 m between the years 1998–2009. This study demonstrated the usefulness of satellite altimetry data to calibrate and validate the satellite-driven hydrological model for Lake Turkana without using any in-situ data. Furthermore, for Lake Turkana, we identified and outlined opportunities and challenges of using a calibrated satellite-driven water balance model for (i) quantitative assessment of the impact of basin developmental activities on lake levels and for (ii) forecasting lake level changes and their impact on fisheries. From this study, we suggest that globally available satellite altimetry data provide a unique opportunity for calibration and validation of hydrologic models in ungauged basins.

Hydrologic connectivity of oxbow lakes along the lower Guadalupe River, Texas: The influence of geomorphic and climatic controls on the “flood pulse concept”

Summary Hydrologic connectivity between rivers and floodplains is fundamental to the ecological integrity of meandering river corridors. This study integrates two important concepts in floodplain hydrology; the flood pulse concept and the oxbow lake cycle. High temporal resolution (0.5–3.0 h) lake level data from vented pressure transducers and river discharge data (0.25 h resolution) were utilized to develop standardized indices of hydrologic connectivity for two oxbow lakes along the lower Guadalupe River, Texas over a 3.4 yr period. The lakes were influenced by hydrologic conditions representative of the northwestern Gulf region, notably a severe drought and a wet season associated with La Nina and El Nino, respectively. Because the data were obtained over the same period it enables a direct comparison of surficial hydrologic connectivity within the evolutionary geomorphic framework of the oxbow lake cycle. A recently (13 yr old) formed oxbow lake was connected 5.9 times per yr and an older partially infilled oxbow lake was connected 3.8 times per yr during the study period. Each oxbow lake was connected by discharge pulses below bankfull discharge, and the signature of hydrologic connectivity varied significantly. The flow duration for connectivity was 18% and 3% for the recently formed and older oxbow lake, respectively. Bivariate relations between discharge and lake stage varied significantly for the two oxbow lakes. The recently formed oxbow lake exhibited highly linear relations between discharge and lake levels, and lake drainage closely mirrored flood wave recession. Older oxbow lake connectivity was complex, as discharge and lake level relations were nonlinear and exhibited pronounced negative hysteresis, which increased with sequential flood pulses. While the older oxbow lake was more sensitive to short term low flow conditions, both lakes were completely desiccated during an extended La Nina influenced drought. This indicates a substantial impact to alluvial aquifers, which were unable to supply groundwater to oxbow lakes during extended dry periods. The “flood pulse concept” applied with only river discharge data is overly simplistic and does not adequately characterize hydrologic connectivity for distinct types of oxbow lakes along meandering river floodplains. With limited lake level data, however, great potential exists for development of effective “integrated floodplain management” plans that more appropriately considers variable hydrologic connectivity inherent within the oxbow lake cycle.

Hydrological Dynamics and Human Impact on Ecosystems of Lake Tana, Northwestern Ethiopia

Lake Tana, biggest lake in Ethiopia, is very important water resource for community living and depending on the lake’s resources. However, the recent development activities at the catchment areas have negatively affected the water of the lake, and the climate of the region. This study delineated how land cover modification, climate change, population increase and development activities within the catchment have influenced the hydrological dynamics of Lake Tana. The land cover modification of southern part of catchment of Lake Tana in the years 1973, 1986, 1995 and 2008 were examined using land sat images. For the last 30 years, temperature, rainfall and lake level data were evaluated to identify change in climate and lake level. The annual means of temperature, rainfall, and lake level with their decadal variation and trends were analyzed with the support of linear regression model to assess trend change. Population change for the last 25 years and development activities were also assessed. The results have revealed that within the last 35 years more than 6.2% of lake’s area was converted to other land covers. Parallel to this period within the catchment there was declining of rainfall and lake level and rise in temperature. There was also increase of population and intensification of use of lake’s water in recent times. Thus, it is implied that change in Lake Tana water was caused by human factors and climate changes. Key Words: Lake Tana; Hydrological Dynamics; Climate Change; Population Increase; Land Modification; Development Activities

Model, proxy and isotopic perspectives on the East African Humid Period

Both North and East Africa experienced more humid conditions during the early and mid-Holocene epoch (11,000–5000 yr BP; 11–5 ka) relative to today. The North African Humid Period has been a major focus of paleoclimatic study, and represents a response of the hydrological cycle to the increase in boreal summer insolation and associated ocean, atmosphere and land surface feedbacks. Meanwhile, the mechanisms that caused the coeval East African Humid Period are poorly understood. Here, we use results from isotope-enabled coupled climate modeling experiments to investigate the cause of the East African Humid Period. The modeling results are interpreted alongside proxy records of both water balance and the isotopic composition of rainfall. Our simulations show that the orbitally-induced increase in dry season precipitation and the subsequent reduction in precipitation seasonality can explain the East African Humid Period, and this scenario agrees well with regional lake level and pollen paleoclimate data. Changes in zonal moisture flux from both the Atlantic and Indian Ocean account for the simulated increase in precipitation from June through November. Isotopic paleoclimate data and simulated changes in moisture source demonstrate that the western East African Rift Valley in particular experienced more humid conditions due to the influx of Atlantic moisture and enhanced convergence along the Congo Air Boundary. Our study demonstrates that zonal changes in moisture advection are an important determinant of climate variability in the East African region.

Orbital, ice sheet, and possible solar controls on Holocene moisture trends in the North Atlantic drainage basin

Holocene patterns of hydroclimatic change in the North Atlantic region are poorly understood despite the importance to climate of the region9s freshwater flux to the ocean. Here we reconstruct the history of moisture balance in the North Atlantic drainage by compiling lake-level records from 95 lakes in Europe and North America. We show that recent millennia were significantly wetter than much of the Holocene in both regions. Only 15% of lakes studied in the North Atlantic basin were low in the past millennium. In contrast, 50% of lakes were low at 10 cal kyr B.P. Low moisture levels in the early Holocene likely resulted from high summer and fall insolation and the influence of the continental ice sheets; water levels rose in many areas as these influences diminished. A principal components analysis of the lake-level data, however, also indicates that Holocene atmospheric circulation changes, possibly driven by solar activity, generated additional moisture variation, including low water levels at 5–2 cal kyr B.P. in eastern North America and Scandinavia.

SWAT-Based Streamflow and Embayment Modeling of Karst-Affected Chapel Branch Watershed, South Carolina

SWAT is a GIS-based basin-scale model widely used for the characterization of hydrology and water quality of large, complex watersheds; however, SWAT has not been fully tested in watersheds with karst geomorphology and downstream reservoir-like embayment. In this study, SWAT was applied to test its ability to predict monthly streamflow dynamics for a 1,555 ha karst watershed, Chapel Branch Creek, which drains to a large embayment and is comprised of highly diverse land uses. SWAT was able to accurately simulate the monthly streamflow at a cave spring (CS) outlet draining mostly agricultural and forested lands and a golf course plus an unknown groundwater discharging area, only after adding known monthly subsurface inputs as a point source at that location. Monthly streamflows at two other locations, both with multiple land uses, were overpredicted when lower lake levels were prevalent as a result of surface water flow to groundwater (losing streams). The model underpredicted the flows during rising lake levels, likely due to high conductivity and also a deep percolation coefficient representing flow lost to shallow and deep groundwater. At the main watershed outlet, a wide section performing as a reservoir embayment (R-E), the model was able to more accurately simulate the measured mean monthly outflows. The R-E storage was estimated by using a daily water balance approach with upstream inflows, rainfall, and PET as inputs and using parameters obtained by bathymetric survey, LiDAR, and downstream lake level data. Results demonstrated the substantial influence of the karst features in the water balance, with conduit and diffuse flow as an explanation for the missing upstream flows appearing via subsurface conveyance to the downstream cave spring, thus providing a more accurate simulation at the embayment outlet. Results also highlighted the influences of downstream lake levels and karst voids/conduits on the watershed hydrologic balance. Simulation performance of hydrology could be improved with more accurate DEMs obtained from LiDAR for karst feature identification and related modification of SWAT parameters. This SWAT modeling effort may have implications on nutrient and sediment loading estimates for TMDL development and implementation in karst watersheds with large downstream embayments that have significant changes in water level due to adjoining lakes.

A model of surface water hydrology of the Great Lakes, North America during the past 16,000 years

A model of glacial isostatic adjustment (GIA) is combined with GIS methods to predict the surface water history of the Great Lakes region of North America during the past 16,000years. The extent of lakes continually changed as the Laurentide ice sheet margin fluctuated and GIA affected the elevations of ice-free outlets. Given only a modern digital elevation model, an earth viscoelastic structure and an ice sheet history, the model adequately represents the Great Lakes history that was developed by glacial geologists during more than a century of field studies. The model predicts the location and elevations of the lowest ice-free outlets and the extent and bathymetry of the proglacial and post-glacial lakes. Results indicate that the Lake Algonquin shoreline, formed about 13,000years ago, plunges below the present level of Lake Michigan rather than becoming subhorizontal as was once thought leading early geologists to conclude the region was stable and not experiencing GIA. Rather our study indicates that the southern region of the Great Lakes is subsiding, a conclusion reinforced by modern GPS and lake level data. GIA also contributed to many avulsions of river channels during the past 10,000years as post-glacial tilting promoted stream capture.

Quantitative estimates of temperature and precipitation changes over the last millennium from pollen and lake-level data at Lake Joux, Swiss Jura Mountains

This paper presents quantitative climate estimates for the last millennium, using a multi-proxy approach with pollen and lake-level data from Lake Joux (Swiss Jura Mountains). The climate reconstruction, based on the Modern Analogue Technique, indicates warmer and drier conditions during the Medieval Warm Period (MWP). MWP was preceded by a short-lived cold humid event around AD 1060, and followed by a rapid return around AD 1400 to cooler and wetter conditions which generally characterize the Little Ice Age (LIA). Around AD 1450 (solar Spörer minimum), the LIA attained a temperature minimum and a summer precipitation maximum. The solar Maunder minimum around AD 1690 corresponded at Joux to rather mild temperatures but maximal annual precipitation. These results generally agree with other records from neighbouring Alpine regions. However, there are differences in the timing of the LIA temperature minimum depending on the proxy and/or the method used for the reconstruction. As a working hypothesis, the hydrological signal associated with the MWP and LIA oscillations at Lake Joux may have been mainly driven by a shift around AD 1400 from positive to negative NAO modes in response to variations in solar irradiance possibly coupled with changes in the Atlantic meridional overturning circulation.

Changes in the Coastline and Water Level of the Akşehir and Eber Lakes Between 1975 and 2009

The Aksehir and Eber Lakes, relatively shallow, small freshwater lakes with an area of 361 km2 and 150 km2 and average depth of 7 m and 2 m (1998), respectively in southwestern Turkey, have experienced a severe decline in water levels in recent decades. This study aimed to investigate coastline and water level changes of lakes and identify the causes for the decline in lake levels. Nine Landsat images from different times, monthly temperature, precipitation, discharge, lake level records and population data were used to analyze these changes. From 1975 to 2009, the water surface areas of the Aksehir and Eber Lakes decreased from 356,929 to 126,482 km2 and from 119,882 to 85,663 km2, a loss of 64.5% and 28.4% over the 34-year period, respectively. From 1975 to 2004, the Aksehir Lake level declined by 2.67 m from 956.02 m to 953.35 and the Eber Lake level declined by 2.03 m from 966.75 m to 964.72 m from 1975 to 2004 based on ground lake level data (in situ). The results of the temperature and precipitation analysis showed that although the annual mean climatic factors vary substantially, they show small increasing trend over the record periods. Annual discharge records on the Akarcay River and its tributaries decreased over the basin during the same period. Irrigation systems, three dams and seven pounds built in recent decades for agricultural irrigation and domestic use, made the major impact on lowering the lake levels because they derive water from the river for human use upstream of the lakes’ catchments. Population growth, rising water consumption for agricultural and domestic purposes and building dams has led to lake levels declining. The change of lake levels might depend more on anthropogenic factors than on climatic factors.

Late-Holocene climatic variability south of the Alps as recorded by lake-level fluctuations at Lake Ledro, Trentino, Italy

A lake-level record for the late Holocene at Lake Ledro (Trentino, northeastern Italy) is presented. It is based on the sediment and pollen analysis of a 1.75 m high stratigraphic section observed on the southern shore (site Ledro I) and a 3.2 m long sediment core taken from a littoral mire on the southeastern shore (site Ledro II). The chronology is derived from 15 radiocarbon dates and pollen stratigraphy. The late-Holocene composite record established from these two sediment sequences gives evidence of centennial-scale fluctuations with highstands at c. 3400, 2600, 1700, 1200 and 400 cal. BP, in agreement with various palaeohydrological records established in central and northern Italy, as well as north of theAlps. In addition, high lake-level conditions at c. 2000 cal. BP may be the equivalent of stronger river discharge observed at the same time in Central Italy’s rivers. In agreement with the lake-level record of Accesa (Tuscany), the Ledro record also suggests a relatively complex palaeohydrological pattern for the period around 4000 cal. BP. On a millennial scale, sediment hiatuses observed in the lower part of the Ledro I sediment sequence indicate that, except for a highstand occurring just after 7500 cal. BP, lower lake levels generally prevailed rather before c. 4000 cal. BP than afterwards. Finally, the lake-level data obtained at Lake Ledro indicate that the relative continuity of settlements in humid areas of northern Italy during the Bronze Age (in contrast to their general abandonment north of the Alps between c. 3450 and 3150 cal. BP), does not reflect different regional patterns of climatic and palaeohydrological conditions. In contrast, the rise in lake level dated to c. 3400 cal. BP at Ledro appears to coincide with a worldwide climate reversal, observed in both the hemispheres, while palaeoenvironmental and archaeological data collected at Lake Ledro may suggest, as a working hypothesis, a relative emancipation of protohistoric societies from climatic conditions.

Quantitative reconstruction of climatic variations during the Bronze and early Iron ages based on pollen and lake-level data in the NW Alps, France

Vegetation and lake-level data from the archaeological site of Tresserve, on the eastern shore of Lake Le Bourget (Savoie, France), are used to provide quantitative estimates of climatic variables over the period 4000–2300 cal BP in the northern French Pre-Alps, and to examine the possible impact of climatic changes on societies of the Bronze and early Iron Ages. The results obtained indicate that phases of higher lake level at 3500–3100 and 2750–2350 cal BP coincided with major climate reversals in the North Atlantic area. In west-central Europe, they were marked by cooler and wetter conditions. These two successive events may have affected ancient agricultural communities in west-central Europe by provoking harvest failures, more particularly due to increasing precipitation during the growing season. However, archaeological data in the region of Franche-Comté (Jura Mountains, eastern France) show a general expansion of population density from the middle Bronze Age to the early Iron Age. This suggests a relative emancipation of proto-historic societies from climatic conditions, probably in relation to the spread of new modes of social and economic organisation.

Variations in Lake Levels during the Holocene in North America: An Indicator of Changes in Atmospheric Circulation Patterns

Fluctuations in the extent of closed lakes provide a detailed record of regional and continental variations in mean annual water budget. The temporal sequence of hydrological fluctuations during the Holocene in North America has been reconstructed using information from the Oxford Lake-Level Data Bank. This data base includes 67 basins from the Americas north of the equator. Maps of lake status, an index of relative depth, are presented for the period 10,000 to 0 yr BP. The early Holocene was characterised by increasingly arid conditions, which led to widespread low lake levels in the mid-latitudes by 9,000 yr BP. By 6,000 yr BP this zone of low lakes extended from 32oto 51oN. Many of the features of the present day lake-level pattern, particularly high lake levels north of 46oN and along the eastern seaboard, were established by 3.000 yr BP. Four distinctive regional patterns of lake behaviour through time are apparent. Histograms of lake status from 20,000 to 0 yr BP are presented for each of these regions. They illustrate the temporal patterns of lake-level fluctuations on a time scale of 103 — 104yr. Changes in lake status over North America are interpreted as indicating displacements in major features of the general circulation, specifically the zonal Westerlies and the Equatorial Trough, as reflected by changes in air mass trajectories and hence the position of air mass boundaries over the continent.

Multiproxy late Holocene peat records from Ireland: towards a regional palaeoclimate curve

AbstractTwo new peat‐based climate records from Ireland covering the late Holocene are presented. The sequences are dated by a strong chronological framework formed by AMS radiocarbon dates and SCPs. Three proxy indicators (testate amoebae, macrofossils and humification) have been determined allowing the limitations and strengths of each to be identified and utilised to provide a bog surface wetness (BSW) record for both sites. Age–depth models take into account the potential for accumulation rates to vary with bog vegetation. The records from each site have been used to derive a combined BSW record that displays changes to a wetter/cooler climate from ca. AD 30 (1920 BP), ca. AD 310 (1640 BP), ca. AD 805 (1145 BP), ca. AD 1040 (910 BP) and ca. AD 1300 (650 BP). Changes follow closely those identified in a northern Britain composite BSW record and largely correspond with lake‐level data in central France suggesting the main changes in water balance were coherent over a large region. Correspondence with increases in IRD and slower Iceland‐Scotland Overflow Water (ISOW) suggests that these changes were related to oceanic forcing influencing the track of dominant westerly air flow over Ireland. Copyright © 2007 John Wiley & Sons, Ltd.

Early-Holocene climatic oscillations recorded by lake-level fluctuations in west-central Europe and in central Italy

On the basis of lake-level data, the period of the Preboreal oscillation (PBO) at 11 300–11 150 cal yr BP as defined by the GRIP oxygen-isotope record appears to correspond to wetter climatic conditions in west-central Europe and to a marked drying in north-central Italy. Additional short-lived phases of higher lake level have been identified before the PBO at ca 11 450–11 400 and 11 350 cal yr BP in west-central Europe, and at ca 11 500 cal yr BP in Italy. Such multiple climatic oscillations around the PBO have been observed in various records in northwestern Europe. On the basis of various proxies, a map of changes in moisture over western Europe during the PBO indicates a mid-latitude zone between 58 and 43°N characterised by wetter conditions, while a drier climate developed in southern and northern Europe. This wet middle zone shows a larger extension and suggests a more meandering, weaker Atlantic Westerly Jet during the PBO than during the 8.2 kyr event. A comparison of lake-level records in west-central Europe with (1) outbursts from North American and north-European proglacial lakes, and (2) variations in solar activity as reflected by 14C and the 10Be records supports the hypothesis that the PBO was a response to successive meltwater pulses at 11 300, 11 250, 11 200 and 11 170 cal yr BP and to a sudden decrease in solar activity at 11 250 cal yr BP. This study points to the necessity of developing integrated multi-proxy approaches to construct more robust regional event stratigraphies, and of better documenting palaeohydrological changes in the Mediterranean area.

Lake level and trophic state variables among a population of shallow Florida lakes and within individual lakes

Monthly total phosphorus, total nitrogen, and chlorophyll concentrations, Secchi depth, and lake water level data for 84 Florida lakes were used to examine relations between trophic state variables and water level fluctuation. Lake size averaged 566 ha (range 4.0 to 5609 ha), with the period of record for individual lakes averaging 57 months (range 7 to 175 months). Lake level fluctuation for individual lakes averaged 1.3 m (range 0.1 to 3.5 m). The lakes also ranged from oligotrophic to hypereutrophic, with average chlorophyll values for individual lakes ranging from 1 to 97 µg·L–1. No overall relation between trophic state variables and lake level fluctuation could be found among the population of lakes. However, individual lakes showed direct, inverse, or no significant relations between lake trophic state variables and water level fluctuation, regardless of the magnitude of water level fluctuation. These data suggest that predicting how water level fluctuations will impact trophic state variables among a population of lakes will be difficult, if not impossible, and that any accurate predictions will have to be made after first examining several mechanisms within individual lake systems.

Quantitative reconstruction of mid-Holocene climatic variations in the northern Alpine foreland based on Lake Morat (Swiss Plateau) and Lake Annecy (French Pre-Alps) data

Vegetation and lake-level data from the sites of Lake Morat (Switzerland) and Lake Annecy (eastern France) are used to provide quantitative estimates of climatic variables over the period 6600-5500 cal.yr BP in the nor-them Subalpine zone, and to test the method of climate reconstruction based on modern pollen analogues constrained by lake-level data. The results obtained from both sites suggest: (1) that phases of higher lake level coincided with a cooling and a shortening of the growing season and an increase in annual precipitation, and (2) that during climatic reversals summer was more affected than winter. They also provide evidence of a sensitive method of reconstruction of climatic variables capable of capturing minor climatic oscillations such as reflected by weakly pronounced lake-level fluctuations. However, comparison of results obtained at Lake Morat and Lake Annecy, with each other and with results from other sites in central Europe, also clearly indicates that either anthropogenic disturbances of the vegetation cover or an overrepresentation of taxa due to particular site exposition can bias the reconstruction of variations in climatic parameters. In addition to pluri-centennial climatic oscillations, both the Lake Morat and Lake Annecy records show a general trend toward temperature cooling over the period considered. This may be related to a reduction in summer insolation induced by orbital factors. Finally, the quantitative estimates of climatic parameters reconstructed at Lake Morat and Lake Annecy help to increase our understanding of the possible impact of climatic oscillations on ancient agricultural communities.

Environmental and climatic changes in the Jura mountains (eastern France) during the Lateglacial–Holocene transition: a multi-proxy record from Lake Lautrey

This paper presents a multi-proxy reconstruction of the climatic and environmental changes during the Last Glacial–Interglacial transition as recorded by a sediment sequence from Lake Lautrey (Jura, eastern France). This reconstruction is based on analysis of pollen, chironomid, organic matter, oxygen-isotope, mineralogical, magnetic susceptibility and inferred lake-level data at a high temporal resolution. The chronology is derived using AMS radiocarbon dates, the position of the Laacher See Tephra (LST), and of correlation between the Lautrey and GRIP oxygen-isotope records. This data set reveals a detailed sequence of environmental changes in the Jura mountains from Greenland Stadial 2a to the early Holocene. Biotic and abiotic indicators allow the recognition of major abrupt changes associated with the GS-2a/GI-1e, GI-1a/GS-1 and GS-1/Preboreal transitions, and other minor fluctuations related to the cold events GI-1d, GI-1b and the Preboreal oscillation (PBO). They also suggest additional cooling spells at ca 14,550 and 14,350 cal yr BP (Intra-Bølling Cold Periods), at ca 13,500 and ca 12,700 cal yr BP just before the GS-1 onset, and at ca 11,350 cal yr BP just before the PBO, as well as an intriguing brief warming episode within GS-1 at ca 12,080 cal yr BP. Summer temperature increased by ca 5 °C at the start of GI-1e, and by 1.5–3 °C at the Holocene onset, while it decreased by ca 3–4 °C at the beginning of GS-1. Major changes in local hydrology and in seasonality appear to be also associated with the GS-2a/GI-1e, GI-1a/GS-1 and GS-1/Preboreal transitions. Pollen and abiotic indicators suggest a greater sensitivity of the vegetation cover to climatic oscillations during the first part of the Lateglacial Interstadial than during the second part (after ca 13,700 cal yr BP), when a closed forest had been restored in this area. By contrast, the restoration of forest cover took less than 300 yr after the end of GS-1. At the beginning of GI-1e and GS-1, no lag occurs (within the sampling resolution of 20–50 yr) in the responses of aquatic (chironomids) and terrestrial (pollen) ecosystems, while, at the onset of the Holocene, the response of the vegetation appears slightly delayed in comparison with that of the chironomid community. Finally, the recognition of two successive tephra layers, which were deposited just before the LST at ca 12,950 cal yr BP and which originated from Le Puy de la Nugère (Massif Central, France), provides an additional tephrochronological tool for correlation between Lateglacial European sequences.

Some mechanisms of mid-Holocene climate change in Europe, inferred from comparing PMIP models to data

We propose a new approach for comparing mid-Holocene climates from 18 PMIP simulations with climate reconstructions of winter and growing season temperatures and the annual water budget inferred from European pollen and lake-level data. A cluster analysis is used to extract patterns of multivariate climate response from the reconstructions; these are then compared to the patterns simulated by models. According to paleodata, summers during mid-Holocene were warmer-than-present in the north, and cooler-than-present in the south, while winters were colder-than-present in the southwest but milder-than-present in the northeast. Whereas warmer summers and colder winters may easily be explained as a direct response to the amplified seasonal cycle of insolation during the mid-Holocene, the other recorded responses are less straightforward to explain. We have identified, from the models that correctly simulate the recorded climate change, two important atmospheric and hydrological processes that can compensate for direct insolation effects. First, a stronger-than-present airflow from southwestern Europe that veers to the north over Eastern Europe, in winter, can consistently explain the reconstructed changes in this season's temperatures and water budget. Second, the increased winter soil moisture allows a shift of the partitioning of net radiative energy towards latent rather than sensible heat fluxes, thereby decreasing surface temperature during the following summer season. Our approach therefore solves one of the recurring problems in model-data comparisons that arises when a model simulates the correct response but in the wrong location (as a consequence, for instance, of model resolution and topography).

Contrasting patterns of hydrological changes in Europe in response to Holocene climate cooling phases

The quantitative reconstruction of climatic parameters from pollen and lake-level data obtained at Saint-Jorioz, Lake Annecy (eastern France), gives evidence for cooler and wetter conditions during the 8.2 ka cold event. A comparison of these regional data with other hydrological records reconstructed in Europe for the same period suggests, as a working hypothesis, that mid-latitudes between ca 50° and 43° underwent wetter conditions in response to the cooling, whereas northern and southern Europe were marked by drier climate, in the latter case leading to an interruption of the sapropel 1 formation in the Mediterranean. A similar hydrological tri-partition of Europe can be observed during other Holocene cooling phases associated with North Atlantic IRD events. Data indicate, that the middle zone characterised by wetter climate conditions could have had a more extended latitudinal amplitude during phases of climate cooling weaker than the 8.2 ka event. The differences in expansion of the wet mid-European zone depending on Holocene climate cooling phases could reflect variations in the strength of the Atlantic Westerly Jet in relation with the thermal gradient between high and low latitudes.