Geochemical evidence for holocene palaeodischarge variations in lacustrine records from the Westeifel Volcanic Field, Germany: Schalkenmehrener Maar and Meerfelder Maar

Various chemical elements (Fe, Mn, U, Mo) can be transported in both dissolved and solid form and then deposited into lakes. Geochemical profiles of these elements in lacustrine records;re therefore potential palaeoclimatic proxies since there should be a positive correlation between discharge from the catchment and dissolved element input. Flux rates for the dissolved input of uranium Ul(t) and the total allochthonous minerogenic deposit were calculated for an annually laminated sediment record from the Lake Schalkenmehrener Maar (SMM). Calculated UI(t) values were critically examined with the help of monitoring data (hydrochemistry, soil chemistry) from the modem Meerfelder Maar (MFM) catchment. The calculated Ul(t)-profile and the variation of varve thickness show a corresponding pattern. The palaeoproductivity of Lake SMM with respect to diatomaceous matter was clearly controlled by the discharge from the catchment. The deposition rate of allochthonous minerogenic matter, which received significant aeolian contributions from a remote source during the early Holocene, has been found to correlate with the Ul(t) values. The deposit of dust on the lake surface is undoubtedly influenced by rainfall events during which fine particles are washed out from the atmosphere. A higher discharge of groundwater from the lake's catchment is indicated at around 10 200 (D1), 8500 (D2), 7150 (D3), 6000 (D4) and between about 4050 and 3300 varve years BP (D5), Significantly lower discharge was obtained for periods around 11 000, 9500, 5000 and after about 3050 varve years BP. There are several indications of a lowering of the SMM lake level between 8100 and 7300 varve years BP. A drier climate can, therefore, be suggested as likely far this period. In addition to three early Holocene periods, which display an increase in element input into Lake MFM (thus supporting the positive discharge indications D1, D2, D3 from the SMM sediment profile), a general increase of discharge since the Neolithic period due to anthropogenic forest clearance can be inferred from the MFM sediment record.