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The high resolution OIS 3 loess record from the Sand Pit on Susak Island, Croatia

The loess-palaeosol record from the island of Susak provides an excellent archive of palaeoenvironmental changes of the Northern Adriatic area. More than 30 m thick loess deposits which cover the Cretaceous carbonate basement have been the matter of interest for scientists for a long time. The loess on Susak is intercalated by numerous palaeosols, at least three tephra layers and several horizons of carbonate concretions. Recently the deposits on Susak have been dated using infrared stimulated luminescence (IRSL) and radiocarbon dating methods (Wacha et al., 2011a, b ; Mikulčić Pavlaković et al., 2011) providing a very detailed chronological framework. The results showed that the deposits on Susak cover a timeframe from oxygen isotope stage (OIS) 5 (and older) to OIS2 with the OIS3 record predominating. This study is extended with a multidisciplinary research approach, which among others includes high-resolution grain- size, palaeomagnetic and rock magnetic analysis. Such high-resolution investigations allow for detailed palaeoenvironmental reconstructions. The Sand Pit section is situated on the south- eastern part of the Susak bay. There an approximately 30 m thick loess-palaeosol sequence is exposed consisting of a thick brown palaeosol in the lower part, on top of which there is loess intercalated with several thin brown palaeosols, a sand horizon and a tephra layer in the upper part of the section. This section was sampled for high- resolution grain size (every 2 cm) and palaeomagnetic (every 10 cm) analyses. The grain-size distribution shows a generally higher amount in sand-sized particles than is usual in loess. It also reveals the changes in the dynamics of the sedimentation. The great thickness and the increased amount of coarse- grained material could be a result of the vicinity to the source area which was the river Po plane with its tributaries in the North Adriatic area. The high resolution grain-size record shows a higher sedimentation rate during OIS3. 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 palaeosols intercalated in the loess could be the evidence of a climate that was milder than the climate which accompanied loess accumulation in other nearby areas, for example in the Pannonian Basin. The concentration dependent magnetic parameters (e.g. magnetic low field susceptibility, etc.) in accordance to the grain size distribution curves show a clear difference between 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 study shows a clear, well defined palaeomagnetic signal with dominant normal magnetization for the entire loess sequence and thus correlates to the Brunhes palaeomagnetic epoch in accordance with the palaeomagnetic investigations published by Bognar et al. (2003). After alternating field demagnetization experiments, two samples in the uppermost loess section show bipolar palaeodirections ; one less stable inverse and one normal direction stable at higher AF-fields. This behaviour could imply to short lasting excursions of the Earth magnetic Field (like the Mono Lake or Laschamp) but without further evidence this cannot be proven. Fortunately it was possible to determine the relative palaeointensity (RPI) data for the uppermost loess part of the section by normalising the remanence intensity signal with susceptibility values in order to get the remanence independent from concentration variations of the magnetic minerals. By comparison with the GLOPIS RPI- stack (Laj et al., 2004) it is possible to date the two excursions (minimum in local intensity) as Mono Lake (~32 ka) and as Laschamp (~41 ka) excursions respectively. These results are in accordance to those from infrared stimulated luminescence (IRSL) and radiocarbon dating (Wacha et al., 2011a, b).

Susak ; grain size analysis ; environmental magnetism ; palaeomagnetism

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