engleski

Geochronological and Palaeoenvironmental Reconstruction of the Loess-Palaeosol Sequence on the Island of Susak

The loess-palaeosol record on the Island of Susak in the North Adriatic Sea is a very detailed terrestrial archive of palaeoenvironmental changes. The island is made of up to 90 m thick Quaternary deposits which cover the Upper Cretaceous carbonate basement. The deposits consist of loess, reworked loess and sand, intercalated by numerous palaeosols and at least three tephra layers. A detailed geochronological study was performed (WACHA et al., in press ; submitted) on two major sections located on the southern part of the island. Infrared stimulated luminescence (IRSL) dating, as well as radiocarbon dating, showed that the whole loess-palaeosol sequence represents a very detailed Last Glacial record, spanning from Oxygen Isotope Stage (OIS) 5 to OIS2, with most of the deposits correlating to OIS3. A single aliquot regenerative dose (SAR) protocol on the polymineral fine-grained material and on coarse-grained feldspar samples, as well as the multiple aliquot additive dose (MAAD) protocol on the polymineral fine-grained material was used for dating. Ages were fading corrected. Age estimates were determined for the three tephra layers found on Susak and can be correlated to coeval tephras found in nearby volcanic regions. The origin of the tephras are very likely the Middle and South Italian volcanic provinces (Campanian Province and the Aeolian arc) (MIKULČIĆ PAVLAKOVIĆ et al., in press). Detailed mineralogical and geochemical investigations of the tephra layers, as well as the loess, sand and palaeosols, were done by MIKULČIĆ PAVLAKOVIĆ et al. (in press). In addition, high-resolution grain size, palaeomagnetic and rock magnetic properties of samples collected in a 2 cm resolution are under study. Such high-resolution investigations allow for palaeoenvironmental reconstructions. The preliminary results of the grain-size analysis show a generally higher amount in sand-sized particles than is usual in loess. This increased amount of coarse-grained material is the result of the vicinity to the source area which is the river Po and its tributaries in the North Adriatic area. The high resolution grain-size record shows an increase in the sedimentation rate during OIS3 which is in agreement with data from other loess regions in central Europe (FRECHEN, et al. 2003). The great thickness of the deposits is very likely the result of the vicinity to the source area, which is the river Po plain. During the Pleistocene glacio-eustatic marine regression, the sea level of the Adriatic Sea was well below the present day one and the Po plain extended to the South-east and was exposed to a strong wind activity resulting in material transport across the dry North Adriatic basin. The numerous palaeosols intercalated in the loess are the evidence of a climate that was milder than the climate which accompanied loess sedimentation in other nearby areas, for example in the Pannonian Basin. The mineralogical and geochemical (MIKULČIĆ PAVLAKOVIĆ et al., in press), as well as recent mineral magnetic studies of the deposits showed that the material comes from the Alpine region and the Po plain. The concentration dependent magnetic parameters (e.g. magnetic low field susceptibility, etc.) do not at all resemble the lithology. Grain size dependent magnetic parameters, however, (e.g. frequency dependent magnetic susceptibility, etc.) reveal the relative enhancement of pedogenetically controlled superparamagnetic particles and the formation of high-coercivity minerals in the pedohorizons. Thus, climatically governed relatively weak magneto-mineralogical alterations occur only in the macroscopically visible palaeosols. The current palaeomagnetic studies show a clear, well defined palaeomagnetic signal with dominant normal magnetization for the entire loess sequence. At the latest after alternating field demagnetization experiments some inverse polarities of natural remanent magnetization (NRM) change to stable normal palaeodirections. These results confirm the palaeomagnetic investigations published by BOGNAR et al. (2003). Further demagnetization experiments and more detailed rock magnetic investigations are still in progress. Based on the geochronology, grain-size analysis, the palaeomagnetic investigations and mineral magnetic studies we can conclude that the loess-palaeosol record on Susak is an extraordinary and valuable evidence of environmental and palaeoclimatic changes in the North Adriatic region during the Last Glacial-Interglacial cycle, as well as in nearby regions.

Susak; loess; palaeosol; tephra; geochronology; luminescence dating; grain size analysis; palaeomagnetism; OIS5-2

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