Chronostratigraphy and geomagnetics in loess-paleo soil sequences in Croatia

Loess-paleo soil sequences overlying early Pleistocene (?), red soils (aeolian red clays) Work on lines from the island of Susak and the northern Cape of Istria (Savudrija).

Susak Island

The loess-paleo soil sequences (LPSS) outcropping on the island of Susak (Croatia) are a unique archive of the climate development in the Adriatic area during the last ice age, and possibly also beyond. Up to 90 m thick Quaternary sediments were deposited which are now available for detailed paleoclimatic analysis. The work presented here was undertaken in close co-operation with colleagues from the S3 department, the University of Zagreb, and the Croatian Geological Survey in Zagreb. Despite the significant thickness of the profiles, no continuous sequence has been identified. The profiles reflect a very dynamic environmental development. The dust deposition and loess formation was repeatedly interrupted by soil formation, periods of enhanced sand deposition, and erosion by the activity of water and translocation. The measured grain sizes reveal a trend to higher values compared to typical loess. This interpretation is also supported by the geomagnetic signals. The values for the magnetic susceptibility of these sediments are much higher than from samples taken from the Pannonian Basin and the typical Eurasian loess. The cores for the enhanced susceptibility and grain size values  are almost certainly attributable to the proximal input of material from the Po river plain. The findings were published in Wacha et al. (2017. The calcareous basement at the floor of the loess-paleo soil sequence (LPSS) on the island of Susak is overlain by red clays. In addition, material similar to petrified Terra Rossa is also exposed in the cavities and karstified fissures of the Cretaceous limestones which form the basement. In addition to the overlying LPSS, the investigations presented here also looked at the age and genesis of the petrified TR. The petrified Terra Rossa represents eroded residues of various soils which form the former surface of the limestone basement before the formation of the red clay. Determining the age of the petrified Terra Rossa failed because of the extreme behaviour of the samples with respect to their geomagnetic properties. The material revealed extremely high susceptibility and magnetisation values. It was not possible to completely exclude the possibility of lightning strike magnetisation caused by the high magnetic fields associated with lightning. The observed magnetic parameters could, however, also be attributable to the presence of enormous quantities of extremely small super paramagnetic (SP) magnetite and maghemite particles. The magnetisation is extremely high, as well as being very unstable with respect to demagnetisation in magnetic alternating fields. A paper has been submitted comparing the properties of the petrified "Terra Rossa" with the overlying red clays, and is currently under review Durn et al. (2017).

Northern Cape of Istria (Savudrija)

A ~ 8m thick loess-paleo soil sequence (LPSS) is exposed on the northern cape of the Istrian peninsula near the village of Savudrija. In addition to numerous luminescence methods, analysis was also carried out using geomagnetic and paleomagnetic techniques. The aim was to date the regional aeolian loess and soil-generating activities, and to interpret the climatic development during the time period being investigated. Interpretation of the luminescence methods indicates that the LPSS in Savudrija was deposited during a period from 70 ka to 9 ka, and therefore the period covering the last ice age up to the Holocene. 

The variation in magnetic susceptibilities matches the model for magnetic enhancement in soil with much lower values in loess (magnetic enhancement). The magnetic susceptibility values partially correlate with the globally valid reference values, and confirm a time period of 65 to 20 ka for the LPSS, without taking into consideration the deepest soils. The age determination based on paleomagnetics (relative paleo intensities in the earth's magnetic field) corresponds roughly to the luminescence age. However, there are differences in detail. The extremely high susceptibility values of the red clay in the deepest part of the profile contradict an MIS 4 age for the deepest soil. More investigations are required here. The previous findings have been submitted for publication to Quaternary International.