Portal de Periódicos da CAPES

PRELIMINARY RESULTS AND INTERPRETATIONS OF MINERAL MAGNETIC ANALYSES OF A HOLOCENE SEDIMENT SUCCESSION FROM LAKE MAARDU NEAR TALLINN, ESTONIA

1993; Estonian Academy Publishers; Volume: 42; Issue: 2 Linguagem: Inglês

10.3176/geol.1993.2.04

1736-7913

Per Sandgren,

Geomagnetism and Paleomagnetism Studies

A core from Lake Maardu, 15 km east of Tallinn in the Republic of Estonia, has been analysed with respect to magnetic parameters.The 570 cm long core consists of gyttja resting on fine sand.Pollen, chemical composition, and magnetic analyses of this core have been carried out.The magnetic analyses include the determination of magnetic susceptibility and induced remanences in a forward field of 1 Tesla and a backfield of 0.1 Tesla.Radiocarbon dates and pollen diagrams as well as physical and chemical analyses, based on cores collected in 1988, were published by Saarse et al. (Caapce et al, 1990).A radiocarbon date from this paper of 9655470 МС years BP in the lowermost part of the gyttja dates the onset of the organic sedimentation.There is also stratigraphical evidence near the top of the period, between 1894 and 1939, when the lake was dried out.Despite the magnetically very weak sediments, a number of features can be identified in the magnetic records.These features are assumed to reflect changing conditions in the environment due to natural and anthropogenic effects.Two periods of human impact are indicated, the first one starting at around 280 cm and the second at 180 cm.Disturbances in the catchment due to human activity have caused increased erosion, seen in the magnetic records as alterations of the magnetic parameters.Both these periods are characterized by increasing magnetic concentrations, accompanied by a drop in the S-ratio as opposed to the periods with no human influence.In the lowermost part of the sediment succession variations in magnetic mineralogies are interpreted as the result of the Ancylus transgression, reflected as a period of lower S-ratios.A corresponding peak in the concentration parameters could be explained by changes in particle size distributions.