Local formation of varved sediments in a karstic collapse depression of Lake Banyoles (NE Spain)

Abstract

Banyoles is the largest and deepest lake of karstic-tectonic origin in the Iberian Peninsula. The lake comprises several circular sub-basins characterized by different oxygenation conditions at their hypolimnions.The multiproxy analysis of a > 5 m long sediment core combined with high resolution seismic stratigraphy (3.5 kHz pinger and multi-frequency Chirp surveys), allow a precise reconstruction of the evolution of a karstic depression (named B3) until present times.Local meromictic conditions in this sub-basin have been conducive to deposition and preservation of ca. 85 cm of varved sediments since the late 19th century. The onset of these conditions is likely related to lake waters eutrophication caused by increasing farming activities in the watershed. Increasing clastic input and organic productivity during the second half of the 20th century have also been recorded within the laminated sediments, revealing an intensification of human impact and warmer water temperatures