MAGNETIC SUSCEPTIBILITY DISTRIBUTION IN LAKE SEDIMENTS INFERRED FROM SHORT CORES COLLECTED IN THE DANUBE DELTA AND RAZIM – SINOIE LAGOONAL COMPLEX. II. RESULTS FROM THE LAGOONAL COMPLEX

Sorin - Corneliu Rădan, Geological Institute of Romania, Bucharest, Romania
Silviu Rădan, Irina Catianis, GeoEcoMar, Bucharest, Romania

Abstract

The article represents the second part of a composite paper regarding the enviromagnetic results obtained for a number of short sediment cores, collected by GeoEcoMar in the cruises carried out during the last five years in the very important wetlands of the Southeastern Romania, i.e. in the Danube Delta and Razim/Razelm – Sinoie Lagoonal Complex. Thus, while in the first part, the deltaic lakes were in our attention, the present paper is focused on the magnetic susceptibility (MS; k) vertical distribution recorded for 31 sediment cores taken out from the lakes of the lagoonal complex, during 2009-2011 interval. In some cases, the correlation of the enviromagnetic parameter MS with the contents of the main lithological components (i.e., SIL – Mineral/SILiciclastic fraction; TOM – Total Organic Matter; CAR – CARbonates) is analysed and discussed. By using a transparent Hydro-Bios corer, sediment columns up to ca 80 cm depth below the water/sediment interface were sampled from the aquatic ecosystems of the lagoonal complex. The sedimentary environments investigated were located in the Razim/Razelm (23 cores), Goloviţa (4), Zmeica (2) and Sinoie (2) Lakes. The sediment cores were sliced in order to study the vertical distribution of the magnetic susceptibility, associated with the detailed lithological composition. The k magnetic parameter, as well as the SIL, TOM and CAR contents were determined for each sediment slice acquired from the respective sampling intervals. The analysis of the magnetic susceptibility variations along the sediment columns has relieved the trends of the lithological vertical variations in the investigated cores. The magneto-lithological models show the coincidence of the sedimentation zones characterized by high k values with those outlined as dominantly siliciclastic. As an example, in the Razim Lake, the (detrital) siliciclastic material is mainly provided by the three important channels, i.e. Dunavăţ, Mustaca and Dranov. Therefore, the sediment core R 09-21, taken out from the Dranov Channel mouth, has been remarked by a high magnetic susceptibility regime (264.96´10-6 - 1107.59´10-6 SI). The highest k values (calibrated to the classes Vc and Vd) were recorded in the sampling interval 34-42 cm, associated with a sandy intercalation, deposited as a result of a strong flood. Such data suggest the existence of important changes of the hydrological regime in the lakes, during the time (other data are added in the paper). A correlation of the enviromagnetic variations at the scale of the entire Lagoonal Complex, accompanied by absolute age determinations, could contribute to the knowledge of the lacustrine system evolution during the last hundreds or thousands of years. The results confirm the importance of the magnetic susceptibility use in the environmental magnetism. This magnetic parameter provides information on the sediment quality, on the evolution of the sedimentogenetic processes, as well as on the changes caused by the natural or the anthropogenic pressures. The variety of the aquatic ecosystems and the diversity of the sediment deposits in the different sedimentation environments are well characterized by specific magnetic susceptibility fingerprints. There is also the case of the MS fingerprinting recovered from the sediment columns of the cores taken out from the main lakes of the Razim – Sinoie Lagoonal Complex.

Keywords: enviromagnetics, magnetic susceptibility, lake sediment, lithological composition, sediment core, Razim (Razelm) – Sinoie Lagoonal Complex