Ruth Drescher-Schneider

Holocene seasonality changes in the central Mediterranean region reconstructed from the pollen sequences of Lake Accesa (Italy) and Tenaghi Philippon (Greece)

This study presents pollen-based climate reconstructions of Holocene temperature and precipitation seasonality for two high-resolution pollen sequences from the central (Lake Accesa, central Italy) and eastern Mediterranean (Tenaghi Philippon, Greece) regions. The quantitative climate reconstruction uses multiple methods to provide an improved assessment of the uncertainties involved in palaeoclimate reconstructions. The multimethod approach comprises Partial Least Squares regression, Weighted Average Partial Least Squares regression, the Modern Analogues Technique, and the Non-Metric-Multidimensional Scaling/Generalized Additive Model method. We find two distinct climate intervals during the Holocene. The first is a moist period from 9500 to 7800 cal. BP characterised by wet winters and dry summers, resulting in a strongly seasonal hydrological contrast (stronger than today) that is interrupted by a short-lived event around 8200 cal. BP. This event is characterised by wet winters and summers at Accesa whereas at Tenaghi Philippon the signal is stronger, reversing the established seasonal pattern, with dry winters and wet summers. The second interval represents a later aridification phase, with a reduced seasonal contrast and lower overall precipitation, lasting from 7800 to 5000 cal. BP. Present-day Mediterranean conditions were established between 2500 and 2000 cal. BP. Many studies show the Holocene to have a complex pattern of climatic change across the Mediterranean regions. Our results confirm the traditional understanding of an evolution from wetter (early Holocene) to drier climatic conditions (late Holocene), but highlight the role of changing seasonality during this time. Our data yield new insights into the aspect of seasonality changes, and explain the apparent discrepancies between the previously available climate information based on pollen, lake-levels and isotopes by invoking changes in precipitation seasonality.

Patterns of variation in late-glacial pollen stratigraphy along a northwest-southeast transect through Switzerland—A numerical analysis

Palynological data from seven late-glacial sites on a transect from the Swiss Plateau, through the North and Central Alps to the South Alps are analysed numerically. Constrained classification procedures were used to derive local pollen-assemblage zones for each site. Detrended correspondence analysis was used to compare pollen-stratigraphical similarities and differences within and between sites and to estimate the amount of palynological change at each site. Canonical correspondence analysis was used to display graphically patterns of variation between local zones within the sites and between the seven sites, and to partition the total palynological variance into within-site temporal variation and between-site spatial variation. The numerical results suggest that no two sites have identical late-glacial pollen stratigraphies, that the South Alps site has a very different pollen stratigraphy compared with the other sites, that the major spatial patterns of variation correspond to site location, and thus that there was considerable variation in late-glacial pollen rain along the transect, just as there is today. The total palynological variance is partitioned into 73% temporal variance, 13.2% spatial variance and 13.8% unexplained variance. The Younger Dryas ‘event’ is poorly reflected in the South Alps but is well reflected as a palynological revertence towards pre-Allerod assemblages at mid- and high-elevation sites in the North and Central Alps. It is also well reflected as changes in pollen composition at one site on the Swiss Plateau, but is poorly reflected at a nearby site on the Swiss Plateau.

A contribution towards the reconstruction of Eemian vegetation and climate in central Europe: first results of pollen and oxygen-isotope investigations from Mondsee, Austria

In this paper, the results of pollen analysis and oxygen-isotope investigations of two new cores from Mondsee are discussed. The climatic progression from the end of the penultimate glaciation to the end of the Eemian interglacial is compared with reconstructions from Bispingen and Gräbern, northern Germany. The rise in temperature, between thePinus phase and the climate optimum in theCorylus phase, appears to have occurred in two steps. Evidence for climatic deterioration is first recorded during thePicea-Pinus phase, i.e. after theCarpinus phase. These reconstructions are in agreement with those based on the Gräbern pollen data, but contrast with recent reconstructions based on the Bispingen pollen profile and the GRIP ice core from Greenland.

Reconsidering the current stratigraphy of the Alpine Lateglacial

The sedimentary and morphological evidence for Lateglacial glacier fluctuations in the Lienz area provides a strong case against the currently used pentapartite stratigraphic subdivision of the Alpine Lateglacial (ALG; c. 19–11.7 ka) i.e. the timespan between the Würmian Pleniglacial (= Alpine Last Glacial Maximum; AlpLGM) and the beginning of the Holocene. The results of comprehensive geological mapping (including the detection of mass movements) supported by geochronological data and pollen analysis revealed that the ALGrecord of the Schobergruppe mountains and the Lienz Dolomites can be subdivided into four unconformity-bounded (allostratgraphic) units which are linked to three climatostratigraphically-defined phases of glacier activity. Delta deposits and till of local glaciers document the phase of ice-decay after the AlpLGM. Between this period and the Bølling/Allerød (B/A) interstadial only one glacier stabilisation with massive end moraines, correlated with the Gschnitz stadial, is evident. Multiple end moraines prove the presence of very active glacier tongues during the Younger Dryas aged Egesen stadial. The 10Be exposure dating of an end moraine, previously attributed to the Daun stadial (pre-B/A interstadial) based on ∆ELA values, provided an age of 12.8 ± 0.6 ka indicating it is of Younger Dryas age. This case highlights the pitfalls of the commonly used ∆ELA-based stratigraphic ALG subdivision and the subsequent derivation of palaeoclimatic implications. ∆ELAs are still considered as a useful tool for correlation on the local scale e.g. in one mountain group with a quite comparable topography and lithology and taking into account the limitations, especially the impact of debris cover. However, our results show that a stratigraphic correlation across the whole Alpine chain via ∆ELAs is not a successful approach potentially leading to bias and, eventually, to circular arguments. Eine Neubetrachtung der aktuellen Stratigrafie des Alpinen Spätglazials: Implikationen aus den sedimentären und morphologischen Belegen des Gebietes um Lienz (Tirol/Österreich) Kurzfassung: Die morphologischen und sedimentären Belege aus dem Raum Lienz liefern starke Argumente gegen die bisher angewandte, fünfgliedrige Stratigraphie des Alpinen Spätglazials (ALG; ca 19–11.7 ka), dem Zeitraum nach dem Würm-Hochglazial (= Alpines Letztes Glaziales Maximum; AlpLGM). Die Resultate einer flächendeckenden geologischen Kartierung (inklusive Erfassung von Massenbewegungen) unterstützt durch geochronologische Methoden sowie Pollenanalysen zeigen, dass sich die ALG-Abfolgen in der Schobergruppe und den Lienzer Dolomiten jeweils in vier „unconformity-bounded units“ im Sinne der Allostratigraphie untergliedern lassen. Diese belegen drei klimato-stratgraphisch korrelierbare Phasen. Deltasedimente und Grundbzw. Seitenmoränen von Lokalgletschern dokumentieren die Eiszerfallsphase unmittelbar nach dem AlpLGM. Nach der Eiszerfallsphase und vor dem Bölling/Alleröd (B/A)-Interstadial gibt es nur eine markante Phase der Gletscherstabilisierung, die mit dem Gschnitz-Stadial korreliert wird. Multiple Endmoränenwälle belegen aktive Gletscherzungen in der Jüngeren Dryas während des Egesen Stadials. Eine zuvor dem Daun-Stadial (prä-B/A-Interstadial) aufgrund von Schneegrenzdepressionswerten (∆ELA) zugeordneter Gletscherstand wurde mit 10Be auf 12.8 ± 0.6 ka datiert und entspricht dem Egesen-Maximum. Damit kann gezeigt werden, dass die bisherige Praxis ∆ELA-Werte zur stratigrafischen Korrelation über größere Räume zu benutzen nicht nur untauglich ist, sondern letztlich zu Fehlschlüssen hinsichtlich Paläoklima führt. ∆ELA-Werte werden nach wie vor als ein nützliches Werkzeug für Korrelationen im lokalen Maßstab betrachtet, so beispielsweise innerhalb einer Gebirgsgruppe mit vergleichsweise ähnlicher Topographie und Lithologie sowie unter Berücksichtigung von Einschränkungen wie z.B. dem Einfluss einer ehemaligen Schuttbedeckung. Jedenfalls zeigen unsere Resultate, dass eine stratigrafische Korrelation mittels ΔELA-Werten quer über die Alpen kein erfolgreicher Ansatz ist, der zu einer Verzerrung der Resultate und schließlich zu Zirkelschlüssen führt.

Millennial scale variability of denudation rates for the last 15 kyr inferred from the detrital 10Be record of Lake Stappitz in the Hohe Tauern massif, Austrian Alps

Reconstructing paleo-denudation rates over Holocene timescales in an Alpine catchment provides a unique opportunity to isolate the climatic forcing of denudation from other tectonic or anthropogenic effects. Cosmogenic 10Be on two sediment cores from Lake Stappitz (Austrian Alps) were measured yielding a 15-kyr-long catchment-averaged denudation record of the upstream Seebach Valley. The persistence of a lake at the outlet of the valley fixed the baselevel, and the high mean elevation minimizes anthropogenic impacts. The 10Be record indicates a decrease in the proportion of paraglacial sediments from 15 to 7 kyr cal. BP after which the 10Be concentrations are considered to reflect hillslope erosion and thus can be converted to denudation rates. These ones significantly fluctuated over this time period: lower hillslope erosion rates of ca. 0.4 mm/year dated between 5 and 7 kyr cal. BP correlate with a stable climate, sparse flooding events and elevated temperatures that favoured the widespread growth of stabilizing soils and vegetation. Higher hillslope erosion rates of ca. 0.8 mm/year over the last ~4 kyr correlate with a variable, cooler climate where frequent flooding events enhance denudation of less protected hillslopes. Overall, our results suggest a tight coupling of climate and hillslope erosion in alpine landscapes as it has been observed in other parts of the Alps.