Bjørg Stabell

Freshwater Diatom Influx in Intertropical Atlantic: Relationships with Continental Records from Africa

Diatom assemblages from modern West African rivers and from lacustrine sediments subjected to deflation represent the present-day sources of continental diatoms to the sea. Diatom productivity in large rivers is high, especially for the genus Melosira. Windblown diatoms derive mainly from the central and northern Sahara (summer dust plume) where saline chloride-water assemblages are widespread, or from the southern edge of the Sahara (winter dust plume) where Melosira-rich assemblages of dilute water predominate. Freshwater diatom peaks in Atlantic cores may reflect (1) phases of increased river influx, correlated with humid episodes on the continent or (2) phases of enhanced deflation and wind transport during arid episodes (the single hypothesis for fine sediments from mid-ocean sites). Genus Melosira dominates the freshwater assemblages of many modern and fossil marine samples, whatever the transport agent is. Therefore, it is not an accurate paleoclimate indicator by itself, but associated taxa may provide information on the environmental and geographical origin of the displaced diatoms. This tentative approach shows that freshwater diatoms in Atlantic cores may be a good tool for reconstructing paleoclimates and for establishing continent-ocean correlations if species analyses are made and if the continental distribution of the taxa encountered is considered.

PARALIA THYBERGII SP. NOV.—ANOTHER FOSSIL PARALIA

A diatom species belonging to the genus Paralia dominates assemblages in Oligocene deposits from the northern North Sea. The species resembles P. crenulata (Grun.) Glezer in the light microscope. SEM studies, however, show that the species differ in mantle structure. The new species is given the name Paralia thybergii and described on the basis of light and scanning electron microscopy.

A PRE-INDUSTRIAL ACIDIFICATION EVENT SHOWN BY DIATOM ANALYSIS OF A RECENTLY ACIDIFIED LAKE IN SOUTHERN NORWAY

The pre-iridustrial pH history of a lake in an area of southern Norway which presently has heavy acid precipitation is elucidated on the basis of an 800-year record of its diatom assemblage. The pH history is interpreted in terms of the pH-tolerance of the recorded assemblages. An acidification event appears to have started around AD 1350, and to have lasted for a maximum of 300 years, as indicated by the marked increase in the relative abundance of Asterionella ralfsii W. Sm. This event coincided with the abandonment of farms in the catchment area and re-establishment of coniferous forest following the Black Death, as shown by pollen analysis. Even in the unlikely case that A. ralfsii be considered acidophilus, the pre-industrial event is evident. A. ralfsii disappeared about AD 1700 after a consistent decline from about AD 1600. This is consistent with the re-establishment of farms and deforestation of the area. The lake supported a rich fish population until about 1950, when the fish were eradicated by...

Morphology, valve ultrastructure and stratigraphical variability of Discostella taxa in a tropical crater lake, northern Tanzania

Morphological and ultrastructural variability of Discostella Houk et Klee (Bacillariophyceae) valves from a small crater lake in northern Tanzania have been examined under scanning electron microscope (SEM) and light microscope (LM). Three types of Discostella are described; Discostella woltereckii, as well as a small variety of D. woltereckii denoted Discostella woltereckii var. minor Öberg, Risberg et Stabell and an unidentified species, Discostella sp. The diatom record during the last ~100 years is dominated by D. woltereckii and D. woltereckii var. minor. The lithology and diatom stratigraphy of a 36 cm sediment surface core show a period of low lake level with extensive aquatic vegetation from around AD 1600. Sometime around AD 1900 there is a substantial rise in the lake water level indicated by an increase in euplanktonic Aulacoseira ambigua and facultative planktonic Discostella spp.

Deflation and Humidity During the Last 700 Ka in NW Africa from the Marine Record

Freshwater diatoms from a core off Cape Blanc have been used to reconstruct atmospheric circulation and climate on the African continent. Maxima in freshwater diatoms occurred mainly in the late phase of interglacial stages. From the freshwater diatom record it can be inferred that intertropical aridity and increased wind strength was initiated just prior to global icegrowth.