STATE OF THE SEDIMENTARY ENVIRONMENTS OF THE MATITA - MERHEI DEPRESSION (DANUBE DELTA, ROMANIA) AS INFERRED FROM MAGNETIC SUSCEPTIBILITY DATA

摘要

The magnetic susceptibility technique is used to characterise sedimentary environments of the lacustrine ecosystems in the Matita - Merhei Depression (M-M.D). The area is located in the northeastern part of the fluvial delta plain of the Danube Delta, between the Chilia and Sulina distributaries. The lakes of this depression (e.g., L Merhei, L. Babina, L Matita, L. Trei Ozere and L. Bogdaproste) are protected from the direct fluvial influx; therefore they are not significantly affected by the Danube supplies. A confined sedimentary environment, induced by a relatively reduced exchange of water, is characteristic for these lakes. The sediments are mainly organic-mineral and carbonate-mineral muds. The important participation of the organic matter, as well as of the carbonate minerals - which are defined by a diamagnetic behaviour - is clearly reflected by the (k) magnetic susceptibility (MS) values. A MS scale with 5 classes is used to calibrate the bottom sediments. There is a good connection between the MS values and the lithological types of sediments (eg., muds, muds rich in vegetal/organic detritus, silty muds, fine sandy muds), as well as a significant correlation with mineralogical and geochemical data. The magnetic susceptibility is a sensitive sedimentological indicator. The comparative analysis presented in this paper is based upon MS measurements performed on hundreds of grab samples collected during 10 scientific cruises, organized in the period 1978-1999. The magnetic susceptibility fingerprints recovered from the lake sediments of the Matita - Merhei Depression are expressed by low and very low (even negative) k values, mainly calibrated to the class I, with k <10x10~(-6) SIu., and to the class II of the MS scale, with k values placed within the interval 10x10~(-6) SIu. + 75x10~(-6) SIu. As regards the geo-ecological framework, this depression is located at the end of a long way of access of the Danube waters, where the natural conditions did not change too much after the vast hydrotechnical works carried out in the 1980s. The magnetic susceptibility characterisation of the bottom sediments of M-M.D lakes during a period lasting more than two decades confirms that the environmental and geo-ecological conditions of this aquatic domain of the Danube Delta were nearly preserved. For this reason, the natural and/or anthropogenic pressure over the submerged sedimentary environments in this area can be considered not important, providing slight alterations only.