THE CARBONATE FACTORY IN KARSTIC ENVIRONMENTS: SOURCES AND PROCESSES IN QUATERNARY IBERIAN LAKES
Clastic carbonate-deposition dominated in glaciated mountains (Enol Lake, northern Spain) during glacial times and it was greatly controlled by surface drainage and geomorphology,. Increased endogenic carbonate productivity occurred during Holocene warmer periods. Clastic carbonates are also dominant in some small karstic lakes (Taravilla Lake, Iberian Range) where Late Holocene sediment infill is composed of fining upward sequences deposited during flood events. In most lakes, endogenic carbonate production occur in two settings: i) littoral platforms dominated by Chara meadows and ii) epilimnetic zone. Continuous preservation of varves since Mid Holocene only occurs in the deepest lake (Montcortès Lake, up to 30 m) where calcite laminae textures (massive, fining upward and coarsening upward) reflect changes in the seasonality of calcite precipitation. However, varves have been formed and preserved in most of the lakes during short periods associated to increased water depth and more frequent anoxic conditions. Reworking of littoral carbonates is common during shallow lake stages.
Lakes with very active karstic processes, particularly subaqueous spring discharge, are prone to intense re-sedimentation processes. In Banyoles lake (NE Spain), increased spring discharge after intense rain periods are responsible for large remobilization and re-suspension of the sediments accumulated in the deepest areas, leading to the deposition of thick homogeinites.
The Quaternary Spanish lake record underlines the large variability of facies, carbonate sources and depositional subenvironments in lake sequences. A correct interpretation of carbonate sources and depositional history is key to use lake sequences as archives of past global changes.