Verlag des Forschungszentrums Jülich
JUEL-3523 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]18OSiO2- 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 neatly 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]d18OH2O-
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 watertemperatures and [delta]18OH2O-
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]18OSiO2-
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 ‰, 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]18OSiO2 transfer function.
Lücke, Andreas
Die Sauerstoffisotope der biogenen Silikate des Holzmaares als mögliches Klimaarchiv - Eine limnologisch-hydrochemische, sedimentologische und isotopengeochemische Prozeßstudie
136 S., 1998
Neuerscheinungen
Schriften des Forschungszentrums Jülich
Ihre Ansprechperson
Heike Lexis
+49 2461 61-5367
zb-publikation@fz-juelich.de