Bernd Zolitschka

Rapid environmental changes in southern Europe during the last glacial period

Oxygen-isotope records from Greenland ice cores, indicate numerous rapid climate fluctuations during the last glacial period. North Atlantic marine sediment cores show comparable variability in sea surface temperature and the deposition of ice-rafted debris. In contrast, very few continental records of this time period provide the temporal resolution and environmental sensitivity necessary to reveal the extent and effects of these environmental fluctuations on the continents. Here we present high-resolution geochemical, physical and pollen data from lake sediments in Italy and from a Mediterranean sediment core, linked by a common tephrochronology. Our lacustrine sequence extends to the past 102,000 years. Many of its features correlate well with the Greenland ice-core records, demonstrating that the closely coupled ocean–atmosphere system of the Northern Hemisphere during the last glacial extended its influence at least as far as the central Mediterranean region. Numerous vegetation changes were rapid, frequently occurring in less than 200 years, showing that the terrestrial biosphere participated fully in last-glacial climate variability. Earlier than 65,000 years ago, our record shows more climate fluctuations than are apparent in the Greenland ice cores. Together, the multi-proxy data from the continental and marine records reveal differences in the seasonal character of climate during successive interstadials, and provide a step towards determining the underlying mechanisms of the centennial–millennial-scale variability.

Human and climatic impact on the environment as derived from colluvial, fluvial and lacustrine archives: examples from the Bronze Age to the Migration period, Germany

Investigation of colluvial, fluvial and lacustrine sediment archives from 12 sites in Germany for the last ca 5000 years demonstrates that there is no synchronous development of the cultural landscape. This can only be explained, if climate is not the dominating control mechanism. However, to a certain degree there is a climatic influence, like during the slight climatic deteriorations immediately following the Holocene climatic optimum, or the stronger ones during the 1st millennium BC and during the Little Ice Age. As shown by all the different archives, human impact strongly increased and became the dominant factor which often made the climatic influence difficult to detect or even invisible. There are indications that human activities may have been triggered by climatic changes. Although the pattern of habitation was in general dependent on the environment offered by nature, also political and socio-economic factors have been involved, resulting in varying occupation patterns in prehistoric and medieval times.

A 14,000 year sediment yield record from western germany based on annually laminated lake sediments

The reconstruction of sediment yield data for the catchment of Lake Holzmaar (205.8 ha) in the Eifel mountains (western Germany) is based on high resolution geochemical and physical analyses of an absolutely dated sediment record. This technique provides flux rates of allochthonous minerogenic deposition for the last 13,840 years. The Pleistocene/Holocene transition is characterized by instable soils and vegetation caused by adjustment of the catchment to interglacial climatic conditions. These instabilities are the reason for elevated mean sediment yields in the catchment of 16 t km−2 yr−1 until 9800 cal. BP. During most of the Holocene erosive processes are insignificant and sediment yields are very low (1.5 t km−2 yr−1). Since humans cleared the forests for agriculture and early industrial purposes the natural environment has been disturbed and sediment yields increased to mean values of 14 t km−2 yr−1 during the last 2750 years. Comparisons of reconstructions from the Eifel area with similar data from southern Sweden and with measured data from lakes, reservoirs and monitoring sites suggest that quantitative reconstructions based on Lake Holzmaar sediments may be regarded as reliable despite uncertainties related to the methods of reconstruction applied.

Ams Radiocarbon Dating of Annually Laminated Sediments From Lake Holzmaar, Germany

AMS radiocarbon ages have been determined on terrestrial macrofossils selected from the annually laminated sediments of lake Holzmaar (Germany). The radiocarbon chronology of this lake covers the last 12.6 ka. Comparison of the radiocarbon dated varve chronology with tree ring data shows that an additional 878 years have to be added to the varve chronology. The corrected C-14 varve chronology of Holzmaar reaches back to ca. 13.8 ka cal. BP and compares favourably with the results from Soppensee (Switzerland) (Hajdas et al., 1993). The corrected ages for the onset and the end of the Younger Dryas biozone are 11,940 cal. BP and 11,490 cal. BP, respectively. The ash layer of the Laacher See volcanic eruption is dated at 12,201 +/- 224 cal. BP and the Ulmener Tephra layer is dated at 10,904 cal. BP.

Annually dated late Weichselian continental paleoclimate record from the Eifel, Germany

A record extending back to 23 220 calendar yr B.P. was obtained from varved sediments from Lake Holzmaar, Germany, by applying multiple dating methods and interlake comparison with Meerfelder Maar. Variations in the Weichselian inorganic carbon content resulting from intensified eolian activities coincide with proxy parameters of dust accumulation in Greenland. The timing for the end of the last glacial maximum at Holzmaar agrees with the Greenland Ice Sheet Project (GISP2) data but suggests dating discrepancies with the Greenland Ice Core Project data (GRIP). Assuming that periods of maximum dust deposition in the Eifel and Greenland coincide with ice advances in northeastern Germany, the Holzmaar record provides a means of dating three late Weichselian terminal moraines.

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

Annually-laminated clastic sediments preserve a high resolution proxy record of paleoclimate, provided that allochthonous sedimentation represents a response to meteorological forcing of watershed sediment transfer. Here, we demonstrate this linkage, and illustrate a calibration process using the most recent 40 years of a varve record from Lake C2 (82°50′ N; 78°00′ W), three years of field measurements, and meteorological data for 1951–92 from nearby AES weather station Alert. Field measurements were used to correlate proxies of the energy available for snowmelt (e.g. air temperature) and daily suspended sediment discharge (SSQ). Our calibration was extended through use of weather data from Alert. Both mean daily air temperature at Echo, and daily SSQ, were well correlated with air temperature at 600 m above Alert, as obtained from the 1200 Z (0800 LST) rawinsonde sounding. Accordingly, we used pooled 1990 and 1992 Alert 600 m data to predict the lagged daily sediment discharge into Lake C2 (adj. r2=0.43). Daily values were summed each year in order to produce an annual series of predicted sediment transfer to the lake. The original varve chronology was based on eight sediment cores recovered from the deep basin of the lake (>80 m). Although low-frequency fluctuations of the varve and predicted SSQ series agree, slight tuning of the varve record optimizes the correlation between them. Adjustments were based on examination of weather data for specific years, reexamination of sediment core thin sections, and by aligning fluctuations in the two series which closely matched. Although the original chronology is reasonably well correlated with 600 m temperatures at Alert (for JJA mean, r=0.41, significant at 0.01), the adjusted chronology is both better correlated and contains a more precise climate signal (r=0.54 for July mean, significant at 0.01). This is the first calibrated varve record produced from Arctic lake sediments, and demonstrates that varves from Lake C2 contain a paleoclimatic record. We believe the post-facto manipulations required to produce the adjusted varve chronology are reasonable given the uncertainties inherent in varve counting, and the lack of any independent corroborating chronostratigraphic markers.

Sedimentology, dating and palaeoclimatic interpretation of A 76.3 ka record from Lago Grande di Monticchio, southern Italy

Abstract Detailed sedimentological and microstratigraphical investigations of lacustrine sediments from Lago Grande di Monticchio (southern Italy) provide a time scale based on rates of sedimentation obtained from annual laminations. Although not annually laminated throughout, a time scale covering the last 76.3 ka is available by interpolation of sedimentation rates. Results of this chronology agree with radiocarbon dates, and with Ar Ar dates on tephra layers from the same lacustrine sediment sequence. Occurrence of varves in combination with physical and geochemical properties are related to variations in the trophic state of the lake and to changing runoff, both controlled by palaeoclimatic conditions. Water depth was relatively shallow with dominating bioturbation and only few varves preserved for most of the Pleniglacial. A marked increase in precipitation and related runoff characterises the early Last Glacial Maximum when water depth reached a maximum and clastic varves were deposited. Subsequently water depth was shallower again. A drastic change towards nutrient rich conditions occurred at the Pleistocene/Holocene transition. During the Holocene organic varves were deposited. Two periods with higher organic productivity were recognised prior to the Last Glacial Maximum around 50 ka and 73 ka. These intervals are interpreted as the result of interstadial conditions. The oldes of the two events shows laminations comparable to those at the Glacial/Holocene transition. The periods with increased minerogenic influx may be characterised as stadial conditions, lasting from 14–40 ka and from 50–73 ka.

VARVED LAKE SEDIMENTS

Varved or annually laminated lake sediments offer an incremental dating technique that also provides high-resolution climatic and environmental data and information about related forcing mechanisms. The first definition of varves by De Geer (1912) was restricted to rhythmically deposited proglacial clays. A century later the meaning was extended to include all annually laminated sediments deposited on continents and in the ocean. Varves are formed under favorable basin configurations due to the pronounced seasonal climatic forcing that controls all processes in the lake’s water column and in the catchment area. In addition to their formation, the preservation of varves is important, as bioturbation often destroys all seasonal laminas. Varved lake sediments are composed of laminations with contrasting color and composition as clastic, biogenic, and evaporitic lacustrine sediments, depending on the overall climatic conditions. To establish a varve chronology, the annual character of laminations needs to be verified and possible sources of error have to be determined. The latter can best be achieved by applying the multiple dating approach. If the time frame is eventually assured, such annually laminated sediment archives provide a wealth of information and can be exploited by interdisciplinary multiproxy investigations.

Palaeoclimate, chronology and vegetation history of the Weichselian Lateglacial: comparative analysis of data from three cores at Lago Grande di Monticchio, southern Italy

Abstract Palynological and chronological data are presented for the Lateglacial from three cores taken at different locations in the basin of Lago Grande di Monticchio. Plant macrofossil data from one core provide complementary evidence of the palaeovegetation. Quantitative palaeoclimate reconstructions are made from the pollen data using pollen-climate response surfaces. The chronological framework for Late-glacial events as well as the palaeoclimate are compared with other sources of evidence of Lateglacial palaeoenvironments. Surface conditions in the North Atlantic appear to be more directly linked to climatic conditions in Italy than is the temperature in Greenland. Nonetheless, the palynological record exhibits high-frequency variability that reflects high-frequency climatic changes paralleling those seen in the ice core records from Greenland.

Taking varves to bits: Scanning electron microscopy in the study of laminated sediments and varves

Conventional high resolution studies of varved sediments are able to identify clastic and biogenic laminae, but are often unable to resolve the nature of fine-scale lamination contained therein. This intra-annual signal provides us with the highest potential resolution from the sedimentary record and can be resolved using scanning electron microscopy (SEM). Six case studies from lacustrine and marine settings are presented to illustrate the combination of clastic and biogenic fabric types typically found in laminated sediments. Clastic laminae fabrics include those which originate through grain settling and those which are ‘event deposits’. The correct identification of event deposits is essential if varves are to be used chronologically. SEM-based biogenic laminae fabric studies have identified seasonal faunal successions where individual laminae may be less than 100 μ thick and most recently, deep chlorophyll maxima (DCM) summer diatom floras, providing an insight into seasonal scale processes. High resolution lamina fabric studies can provide a basis for generating records of seasonal and inter-annual variability, thus contributing to our understanding of lacustrine and marine processes and palaeoenvironmental interpretation.