Suzanne A.G. Leroy

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.

Development of vegetation and continental aridity in northwestern Africa during the Late Pliocene: the pollen record of ODP site 658

Abstract A 200 m long marine pollen record from ODP Site 658 (21°N, 19°W) reveals cyclic fluctuations in vegetation and continental climate in northwestern Africa from 3.7 to 1.7 Ma. These cycles parallel oxygen isotope stages. Prior to 3.5 Ma, the distribution of tropical forests and mangrove swamps reached Cape Blanc, 5°N of the present distribution. Between 3.5 and 2.6 Ma, forests occurred at this latitude during irregular intervals and nearly disappeared afterwards. Likewise, a Saharan paleoriver flowed continuously until isotope Stage 134 (3.35 Ma). When river discharge ceased, wind transport of pollen grains prevailed over fluvial transport. Pollen indicators of trade winds gradually increased between 3.3 and 2.5 Ma. A strong aridification of the climate of northwestern Africa occurred during isotope Stage 130 (3.26 Ma). Afterwards, humid conditions reestablished followed by another aridification around 2.7 Ma. Repetitive latitudinal shifts of vegetation zones ranging from wooded savanna to desert flora dominated for the first time between between 2.6 and 2.4 Ma as a response to the glacial stages 104, 100 and 98. Although climatic conditions, recorded in the Pliocene, were not as dry as those of the middle and Late Pleistocene, latitudinal vegetation shifts near the end of the Pliocene resembled those of the interglacial-glacial cycles of the Brunhes chron.

Near East Desertification: Evidence from the Dead Sea

Christoph Heim, Norbert R. Nowaczyk, Jo¨rg F.W. NegendankGeoForschungsZentrum, Bereich 3.3 Sedimente und Beckenbildung,Telegrafenberg A26, D-14473 Potsdam, GermanySuzanne A.G. LeroyCentre for Palaeoecology, School of Geosciences,Queen’s University of Belfast, Northern Ireland, UKZvi Ben-AvrahamDead Sea Research Center, Tel Aviv University, Ramat Aviv, 69978, Israel

The high sensitivity of the palynological record in the Vico maar lacustrine sequence (Latium, Italy) highlights the climatic gradient through Europe for the last 90 ka

The palynological study of the Vice maar lacustrine sequence (Italy) is realized from the correlation of three drillholes. The time span covers a period from the end of Oxygen Isotope Substage 5b till the beginning of Stage 1 (from ca. 90 to ca. 10 ka BP). At the base of the pollen diagram (pollen zone 0), a Younger Dryas-like fluctuation is observed. After pollen zone 1, which is a well developed forest period (Substage 5a), the forests of pollen zones 3 and 5 correspond to extensive climate improvements. These last two zones are correlated to the Ognon complex of the Grande Pile (France) and to interstadial periods 20 and 19 defined in the GRIP ice core (Greenland) (72.6-66.2 ka) before the Stage 5/4 transition. These two interstadial periods are global events found in the Greenland and Antarctica ice sheets, as well as in North Atlantic marine cores. In other pollen records from north of the Alps, changes have been recorded but they are less pronounced than in Vico. Pollen zone 7 (Stage 3) shows ca. 7 warm/humid fluctuations, that might have a link to the Dansgaard-Oeschger events (Greenland). During zone 9, the Lateglacial interstadial is recorded as well as the Younger Dryas event. The localization of Vice near plant refugia and far from the ice cap enables its pollen diagram to record abrupt and intense response to climate changes, possibly through a link to the Atlantic Ocean via westerly winds

The European Modern Pollen Database (EMPD) project

Modern pollen samples provide an invaluable research tool for helping to interpret the quaternary fossil pollen record, allowing investigation of the relationship between pollen as the proxy and the environmental parameters such as vegetation, land-use, and climate that the pollen proxy represents. The European Modern Pollen Database (EMPD) is a new initiative within the European Pollen Database (EPD) to establish a publicly accessible repository of modern (surface sample) pollen data. This new database will complement the EPD, which at present holds only fossil sedimentary pollen data. The EMPD is freely available online to the scientific community and currently has information on almost 5,000 pollen samples from throughout the Euro-Siberian and Mediterranean regions, contributed by over 40 individuals and research groups. Here we describe how the EMPD was constructed, the various tables and their fields, problems and errors, quality controls, and continuing efforts to improve the available data.

TOWARDS THE LOWERING OF THE PLIOCENE/PLEISTOCENE BOUNDARY TO THE GAUSS-MATUYAMA REVERSAL

Abstract The present Pliocene/Pleistocene boundary is at the top of the Olduvai normal subchron. Many data now favour the lowering of the Pliocene/Pleistocene boundary to the Gauss-Matuyama reversal. This affords an opportunity to give to the boundary a more global significance in keeping with recent evidence.

Latest Pliocene pollen and leaf floras from Bernasso palaeolake (Escandorgue Massif, Hérault, France)

Abstract Bernasso palaeocanyon (Herault, France) is in the Escandorgue volcanic massif, south of the Massif Central. The first glacial-interglacial cycles of the Plio-Pleistocene period are recorded in the infilling of this canyon and are rich in macroflora (mainly leaves) and microflora (mainly pollen). Pollen zones I and III represent a steppe environment, in contrast to pollen zone II, a forested one. This study concentrates on pollen zone II for which new pollen spectra and abundant leaves are described owing to new outcrops and a 8 m long drillhole (BN I) through the deposit. Pollen zone II-a poorly illustrates an open oak forest that could correspond to a pioneering phase after a steppe period (pollen zone I). No sampling of the sediment between those yielding pollen zones II-a and II-b was possible. Therefore little is known of the intermediate vegetational phases. Pollen zones II-b to II-d show a diversified mixed temperate forest. Pollen zones II-a to II-d are probably steps in vegetation successions of an entire interglacial period such as the Nogaret interglacial, defined in a neighbouring diatomite deposit of similar age. The macrofloral study is based on 800 samples. Most abundant specimens are Carpinus suborientalis, Parrotia persica, Acer and Carya. The Bernasso macroflora reflects a mesothermic forest similar to the present vegetation south of the Caspian sea. The difference between the dominant genera in the macroflora and microflora leads to an improvement in the interpretation of pollen percentages. There is an under-representation of pollen from Carpinus orientalis, Parrotia persica and Acer sp. and over-representation of Carya minor, Zelkova, Pinus and Tsuga. The Hyrcanian region and to a lesser degree the Euxinian region, serve as models for the Pliocene flora. Compared to present-day Bernasso, the Late Pliocene climate was milder and had more precipitation throughout the year with a reduced decrease during the summer leading to reduced seasonal contrast. The estimated duration for pollen zone II, part of an odd oxygen isotope stage driven by an obliquity cycle (41 ka), is less than 10 ka. The age estimates (based on K/Ar dates, palaeomagnetism and cyclopalynostratigraphy) indicate that Bernasso floras developed during the time interval between ca. 2.16 and 1.96 Ma. Therefore, Bernasso pollen zones II-b to II-d could correlate to part of an oxygen isotope stage between stage 81 and stage 73.

Marine palynology of the ODP site 658 (N-W Africa) and its contribution to the stratigraphy of Late Pliocene

Abstract A relatively high resolution pollen analysis (a sample each ca 5 ka) has been made on ODP site 658, off Cap Blanc, N-W Africa. In the same cores detailed palaeomagnetism, biostratigraphy, sedimentology and stable isotopes provided a chronology based on the cyclic forcing by the astronomical parameters. The period previous to 3.5 Ma corresponds to a warmer and wetter climate from northwestern Europe as far south as equatorial West Africa. Long term variation shows a first step towards a drier climate between 3.5 and 3.2 Ma. At 3.2 Ma, the pollen record establishes the aridification of northwestern Africa probably in connection with enhanced Trade winds. This phase may correspond to a preliminary stage of the Northern Hemisphere glaciation. Spectral analyses show a positive correlation between low deep sea temperatures in the Atlantic Ocean and aridity in northwestern Africa. After a period of re-established humidity, a second and stronger step to aridification started at ca. 2.6 Ma. Before that date, there is a northern emplacement of savannah and tropical forest that probably shifted southwards afterwards. Already from 2.8 Ma, a reduction of savannah vegetation took place, probably as a result of the development of a desert in West Africa. However, mean percentages of steppe elements of North Africa are five times lower during the Pliocene than during the Late Pleistocene indicating that arid periods were still less prolonged and/or less severe during the Late Pliocene. Spectral analyses suggest that forcing of the climate by obliquity started after or ca. 2.6 Ma.

Pollen Record of the Saint-macaire Maar (herault, Southern France) - a Lower Pleistocene Glacial Phase in the Languedoc Coastal-plain

The age of the Saint-Macaire maar sediment, in the northwest Mediterranean coastal plain, is between 1.4 and about 0.7 Ma. The pollen diagrams indicate an open xeric vegetation (steppe). The moderate percentages of thermophilous trees support the hypothesis of vegetation refugia along the north Mediterranean shoreline as far back as Early Pleistocene.

Palynological evidence of Azolla nilotica Dec. in recent Holocene of the eastern Nile Delta and palaeoenvironment

Megaspores, microspores and massulae of the free-floating fern, Azolla nilotica, were found in Late Holocene sediments obtained by coring in the eastern Nile Delta. Nowadays, the nearest station for this fern is southern Sudan. The determination of the species is based on spiny projections on the megaspore body and on the verrucate microspores. Palynological studies reveal that the habitat of the fern consisted of extensive papyrus marshes, now disappeared. Several causes for the disappearance of the fern from the Nile Delta are proposed amongst which the most probable is human influence which has completely modified the vegetation and the hydrology.