Die Sauerstoffisotope der biogenen Silikate des Holzmaares als mögliches Klimaarchiv: eine limnologisch-hydrochemische, sedimentologische und isotopengeochemische Prozessstudie

A two year case study at Lake Holzmaar (Eifel, Germany) provided new insights for the use of oxygen isotopes in diatom silica from lake sediments for palaeotemperature reconstructions. A complex framework of meteorological, hydrogeochemical, isotope geochemical and biological factors were identified as being responsible for the generation of $\delta^{18}O_{SIO2^{"}}$ signals in lacustrine sediments. The most important compartment for the conversion of temperature information into a sedimentary oxygen isotope signal is the epilimnion of the lake. Therein, the signal transformation from the atmosphere to the hydrosphere and from the hydrosphere into the biosphere takes place. Spatial and temporal variations of epilimnic water temperatures in Lake Holzmaar result in a characteristic seasonal pattern which is nearly linked to the corresponding air temperature variations. Oxygen isotope ratios of lake water are an essential factor of the whole system because they are the basis for fractionation of oxygen isotopes during valve formation. A striking seasonal pattern of $\delta^{18}O_{SIO2^{-}}$- variations was discovered in the epilimnion of Lake Holzmaar and, in a similar manner, in a neighbouring maar lake (Lake Meerfelder Maar). Presumably, this pattern is typical for Eifel maar lakes in detail and for lakes with similar hydrological settings in general. The different seasonal developments of epilimnic water temperatures and $\delta^{18}O_{SIO2^{"}}$- values led to a hysteresis relationship between these parameters causing an equivocal temperature-isotope relation. High resolution palaeoclimatic studies might be biased by this phenomenon, therefore, demanding a separation between spring and autumn diatom populations in the sediments. First results are reported on the recent planktonic diatom population of Lake Holzmaar, i.e. about timing and extent of growth in general and distinct blooms in detail. Laboratory experiments with diatoms under controlled temperature conditions demonstrate that the oxygen isotope fractionation between water and biogenic silica is temperature controlled in the range from 10 to 25°C but may additionally be influenced by a species specific dependency. Based on the obtained data, model calculations were used to simulate the temporal and spatial pattern of $\delta^{18}O_{SIO2^{-}}$ratios. The results predict isotope values for biogenic silica during the course of the year produced in the epilimnion of Lake Holzmaar. They show a rather high isotope shift of up to 4.5 %o, depicting a strong seasonal component of variation. These results emphasize the necessity for high resolution calibration studies on a seasonal basis to develop a reliable temperature-$\delta^{18}O_{SIO2}$ transfer function