G. Landmann

Unusual distributions of long-chain alkenones and tetrahymanol from the highly alkaline Lake Van, Turkey

Long-chain C37 to C40 alkenones with di-, tri-, and tetra-unsaturation are very abundant in sediment trap material and Holocene to Late Pleistocene core samples from the Earths largest soda lake, Lake Van (Turkey). Thus, the known distribution range of these typical biomarkers for haptophyte microalgae is extended to highly alkaline environments. The observed unsaturation patterns differ strikingly from those found in open marine haptophytes and sediments by an enhanced relative abundance of the tetra-unsaturated compounds, especially the C37:4 methyl ketone. Their preponderance is suggested to be a facies marker pattern for lacustrine and marginal marine areas of sedimentation. Using published U37K calibrations, no reliable absolute temperatures were obtained for the Lake Van samples. Accordingly, marine sea surface temperature determinations based on long-chain alkenones should be applied with caution when a contribution of these compounds from coastal or nonmarine sources can not be excluded. The presence of tetrahymanol and gammacer-3-one in the Lake Van materials is attributed to organic matter contributions of ciliates. The relative abundance of long-chain alkenones and of tetrahymanol/gammacer-3-one is considered to reflect changes in the environmental conditions, in particular in the hydrological setting. We suggest that times of pronounced stagnation are recognised by very high tetrahymanol/gammacer-3-one concentrations together with drastically increased stanol/stenol ratios, and intervals of enhanced convection or of high freshwater input are characterised by high alkenone contributions.

Dating Late Glacial abrupt climate changes in the 14,570 yr long continuous varve record of Lake Van, Turkey

In summer 1990, during the third international expedition to Lake Van, eastern Anatolia, 10 sediment cores were retrieved from depths up to 446 m. As reported earlier, the sediments of the lake are finely laminated. The seven cores, recovered up to 30 km apart in the main lake basin, presented sediment sequences which correlate well with respect to ash layers and prominent colour changes, but also lamina for lamina. Here we report on the detailed evaluation of this record, which is varved continuously back to 14,570 yr B.P. (calendar years before 1950 AD). It is independent of 14C calibration, i.e. it is not a floating record, and it is the only detailed varve chronology known from the semi-arid Mediterranean region. Important Late Glacial events, such as the termination of the Oldest and Younger Dryas are clearly recorded in the sediments. Chronozones were defined on the basis of changes of the deposition rate and of chemical composition caused by environmental changes. Analysis of the annual deposition rates revealed abrupt changes within only a few years, declining for example by approximately 30% in the transition period between the Oldest Dryas and the Bolling. In most cases, alterations observed in the sedimentation rates are reflected in changes of the geochemical parameters, such as organic and inorganic carbon, opal, and the major elements Si, Ca, Mg, Al. Our results and palynological studies, performed on material recovered in an earlier expedition, are used to reconstruct palaeoenvironmental conditions. In this study, the termination of the Younger Dryas is dated to 10,920±132 yr B.P. This is younger than the recently published Greenland ice core dates but in accordance with, for example, the central European dendrochronology. We suspect, that higher sediment deposition rates during the cold periods are due to rapid melting and intense wash out of soil, which was fairly loose because of sparse vegetation. This would lead to higher river discharges. Based on the observed increase of the deposition rate in the record, melting of glaciers can only be detected after the termination of the Younger Dryas.

Climatically induced lake level changes at Lake Van, Turkey, during the Pleistocene/Holocene Transition

Sediment core K10 from Lake Van (eastern Turkey) provides a continuous varve record back to 14,570 calendar years B.P. (before present, 1950), the longest unbroken and non-floating lake varve sequence yet described. The underlying sediment is unvarved and hard. Changes in the aragonite/calcite ratio, the presence of protodolomite and magnesite in certain profile sections, the annual record of the sedimentation rate, the water content of the sediment, the concentrations of organic carbon and opal, and the texture of the sediments from this core provide a record of the lake level history. The new chronology enabled us to redate the old pollen profile [van Zeist and Woldring, 1978a, b] and to establish an accurate timescale for the reconstructed lake level change. Carbon 14 dates show that the highest lake terrace corresponds to high lake level at around 19,000 years B.P. during the Last Glacial, >70 m above its present level. Before 15,000 years B.P. the lake must have been completely dry, marking a reduction of lake level by 500 m in maximum 4000 years. Beginning at 14,600 years B.P. and ending at 12,040 years B.P., the lake level recovered by 250 m to fall again during the next 1400 years. By 10,600 years B.P. the lake began to rise and reached, following another regression between 9000 and 8100 years B.P., the Holocene highstand by about 7500 years B.P., dropping to todays level at about 3000 years B.P.

Geochemical fingerprints by activation analysis of tephra layers in Lake Van sediments, Turkey

We discuss geochemical and sedimentological characteristics of 12 tephra layers, intercalated within the finely laminated sediments of Lake Van. Within the about 15kyr long sediment record studied, volcanic activity concentrated in the periods 2.6-7.2 and 11.9-12.9kyr B.P. Concentrations of 25 elements provide the geochemical fingerprint of each tephra layer and allow comparison to literature values of potential source volcanoes such as Mts. Nemrut and Süphan. The youngest two tephra layers (and probably also the other three ashes from the 2.6-7.2kyr B.P. eruptions) originate from the Nemrut volcano. The source of the older tephra (11.9-12.9kyr B.P.), however, remains unidentified.

On the origin of red sea submarine canyons

nachhaltig beeinfluBte ([5], gemessen 37~ 122~ Mit Ausnahme des Chich6n beschreiben die Kurven bereits die Abklingphase der stratosph/irischen Belastung, in der der Abbau den Antransport von Aerosolen und deren Neubildung aus der Gasphase fiberwiegt. Im Falle der Chich6n-Eruption ist die Abklingphase noch nicht erreicht, d.h. die im Aquatorbereich die Erde umkreisende Aerosol-Wolke ist noch so konzentriert, dab weiterhin mehr Aerosol polw/irts wandert als in mittleren Breiten aussedimentiert. Das entspricht auch den Lidar-Beobachtungen aus Hawaii und Japan, wo im Oktober erheblich h6here Rfickstreuverhfiltnisse gemessen wurden als in Garmisch-Partenkirchen [6]. Unter Zuhilfenahme von Modellvorstellungen lassen sich aus den LidarDaten Riickschlfisse auf die Beeinflussung des stratosph/irischen Strahlungshaushalts ziehen. Mit der Annahme, dab zumindest geniigend lange nach einem Vulkanausbruch in der Stratosph/ire das vieldiskutierte 75% H2SOr HzO-Aerosol vorliegt [5, 7, 8], k6nnen aus den Rfickstreuwerten optische Dicken abgeleitet werden. Tabelle I zeigt die Zunahme der optischen Dicke ab April 1982, wobei dieser erste Weft noch nicht v o n d e r Chich6nEruption gest6rt ist. Die Werte sind Untergrund-korrigiert mit dem mittleten Wert des Jahres 1978, das mit seiner geringen vulkanischen Aktivit/it als ungest6rtes Referenzjahr gelten darf. Pollack et al. [7] haben gezeigt, dab eine Zunahme der optischen Dicke um 0,1 zu einer Abnahme der Erdoberfl/ichen-Temperatur um I~ fiihrt, vorausgesetzt die St6rung h/ilt lange genug an, um das System Atmosphfire-Ozean zu beeinflussen. Eine statistisch signifikante Abkiihlung um etwa 0,3 ~ im Jahr nach grol3en Eruptionen wurde von Mass und Schneider ermittelt [9]. Wegen der Gr613e der gegenw~irtig gemessenen St6rung und weft das Aerosol-Reservoir der/iquatorialen Stratosph/ire noch nicht ersch6pft ist, kann man erwarten, dab die Stratosph/ire auf Jahre erheblich belastet sein wird und daher mit einer Abk/ihlung um einige Zehntel Grad gerechnet werden mul3.