Late Glacial sediments of the Stará Jímka paleolake and the first finding of Laacher See Tephra in the Czech Republic
Abstrakt
The Late Glacial sedimentary sequence of the Stará Jímka paleolake (Bohemian Forest, Czech Republic) is characterized by the prevalence of detrital minerals with an elevated proportion of organic matter and abundant diatom valves in relatively warm periods. The mineral debris originated mainly from local migmatites and mica-schists (or glacial and deluvial deposits derived from the same material).
Low background contents of CaO (< 0.20 wt%) and Na2O (< 0.50 wt%) reflect chemical weathering. Correlation analysis of trace elements shows that the mineral debris concentrates Rb, Cs, Ga, Cr, Zr, Hf, light rare earth elements, Th, U, Pb, Ba, Sr and weakly Zn, whereas Mo, Co, Se, As, Ni, Cu, Br and weakly Au are allocated predominantly to sulphides and organic matter.
The sequence contains a continuous horizon with the Laacher See Tephra (LST), causing contrasting maxima of Na, Ca, Cl and Nb in bulk composition. While the phonolitic tephra corresponds to the lower part of the middle LST unit it also contains phosphorus-rich relicts of older sediments with small euhedral (air-transported) volcanic crystals.
Such sediments may have occurred in depressions at an older volcano prior to the major Laacher See eruption (~ 12,880 cal. BP).