Michel Servant

Holocene precipitation and atmospheric changes inferred from river paleowetlands in the Bolivian Andes

Abstract In the southern tropical Andes, previous studies performed on lacustrine basins display large water-level fluctuations due to changes in the precipitation minus evaporation balance during the Holocene. Significant changes in groundwater levels were also inferred from river paleowetland deposits in northern Chile and the northern Bolivian Altiplano but the discrepancies which appeared in the data from paleolakes and paleowetlands are still not understood. In this paper we present stratigraphy and diatom analyses in new paleowetland records from non-glacial valleys. These data, compared with previous results, show that non-stormy precipitation (recorded by fine and/or organic sedimentation) dominated continuously from ∼11 200 to ∼1500 cal yr BP and that convective rainfall (recorded by strong erosion) occurred only episodically in the northern Altiplano. Convective rainfall was similar to the type of precipitation which nowadays occurs during the rainy season (austral summer) when the tropical easterlies from the Atlantic reach the Bolivian Andes. Non-stormy precipitation is interpreted as a result of an intensification of cold-air incursions from southern polar regions, in good agreement with data from Brazil. The maximum intensity in cold-air incursions occurred during the Early Holocene and coincided with a climatic optimum in Antarctica and a considerably reduced El Nino Southern Oscillation (ENSO). During the Middle and Late Holocene, cold-air incursions decreased and the ENSO frequency increased. We suggest that the interactions between the southern high and low latitudes, by means of cold-air incursions and associated changes in the west wind flow, have been the main mechanisms involved in climatic changes at the latitudes of Bolivia.

Lake levels in the southern Bolivian Altiplano (19°–21°S.) during the Late Glacial based on diatom studies

This study is focused on the endorheic Uyuni-Coipasa Basin located in the southern Bolivian Altiplano. Stratigraphical and fossil diatom studies based on a detailed radiocarbon chronology revealed six phases in water-level changes and paleosalinity variations. At 15,430 f 80 yr B.P,, lacustrine conditions settled in the southern Bolivian Altiplano. A saline lake, characterized by benthic meso-metasaline species, reached N +4 m altitude above the present bottom of the basin. After 15,430 f 80 yr B.P.. the level rapidly rose to N +27 m, as suggested by a tychoplanktonic mesosaline flora. Between -14,500 years and -13,000 years, finely laminated sediments at N 4-32 m contained successively a dominance of epiphytic mesosaline to hypcrsnlinc spccics nnd tychoplnnktonic oligosnlinc di~toi~~s, intlicritinp; wcak flrtctiinlions in water-level and salinity. At 13,000 years, strong changes in the diatom flora occurred: epiphytic oligo-hypersaline diatoms were replaced by planktonic meso-polysaline species. They indicate a deep salt lake (the lake level reached - +lo0 m). After -12,000 years, the lake level abruptly dropped, as suggested by fluviatile sediments with a benthic meso- polysaline diztom flora. The main lake was replaced by shallow saline ponds. A wet pulse occurred at -1 1,400 years, characterized by IOW water level.(-

Paléolimnologie et paléoclimatologie de l'Ouest-Cameroun au cours des 5 000 dernières années, à partir de l'étude des diatomées du lac Ossa

7 m) and high salinity. This lacustrine phase remained until 10,400 yr B.P. These data indicate changes in Precipitation minus Evaporation (P-E). Our regional interpretations are based on a comparison with the available data on the northern (Lake Titicaca) and southern (Lipez area) Bolivian Altiplano and on the northern Chilean Altiplano (Atacama Desert).

Le dernier maximum glaciaire (21 000-17 000 14C ans B.P.) dans les Andes tropicales de Bolivie d'après l'étude des diatomées

In Lake Ossa, the relative abundance of pH, water level and Saharan aerosol markers suggests that this lake was characterized by an alternating reinforcement and attenuation of pluriannual variability. The transfer function diatom/mean pluriannual bathymetry applied on fossil diatom flora of OW4 core indicates limited pluriannual bathymetric variations during the last 5 500 yr B.P. The water budget remained stable. The strong short-term variability of the climate after 2 700 yr B.P. explains the forest modifications.

Palaeoclimates and Their Consequences on Forest Composition

Abstract A diatom study, carried out on a core recovered in the Southern Altiplano (Coipasa salt lake 19 °S, 68 °W) currently almost completely dry, shows that during the last glacial maximum the Coipasa salar was entirely occupied by a large shallow lake. Available data for the northern Altiplano (Lake Titicaca, 16 °S,69 °W) indicate a water level 17 m lower than today. This opposition is explained by decreased tropical precipitations whose effects registered by Lake Titicaca were obliterated in the Coipasa salar by increased winter precipitation.

Mise en évidence de quatre phases d'ouverture de la forêt dense dans le sud-est de l'Amazonie au cours des 60 000 dernières années. Première comparaison avec d'autres régions tropicales

The analysis of most plant populations in natural forests clearly retraces the mechanisms of sylvigenesis, based on the occurrence of small-scale incidents — in particular treefall gaps — more or less regularly distributed in time and space (van der Meer et al. chapter 24). However, a number of ‘anomalies’ detected in the population structure or in the distribution of some species cannot be adequately explained by the internal dynamic processes which occur on the century time scale. Plants react at different speeds to perturbations, and major events, even very ancient ones such as the perturbations recorded in the sediments, have probably also left a durable mark in the present organisation of the vegetation. We can therefore hypothesise that relatively ancient events, which would have occurred on a much larger scale than treefall gaps, would have caused large modifications of the forest ecosystems and left long-lasting tell-tale signs.