Noble gas concentrations in lakes carry information on the water
temperature, salinity and air pressure prevailing during gas
exchange between the water and the atmosphere. During
sedimentation, part of the water at the lake bottom is
incorporated into the pore space of the sediment. The pore
water may therefore provide a natural archive of the past noble
gas concentrations in the lake, from which previously prevailing
climatic and environmental conditions can be reconstructed. As
a prerequisite to investigating the feasibility of employing noble
gases dissolved in sediment pore water as environmental
tracers, a suitable method of analysis first had to be developed.
This method, although based on procedures that are routinely
employed in sedimentology and noble gas analysis, is the first
such method that allows noble gas concentrations in sediment
pore water to be analysed reliably. In this new method, sediment
samples are taken from a sediment core and are transferred into
copper tubes, which are sealed gas-tight. To avoid
contamination or degassing, the sample is exposed neither to
the atmosphere nor to any other gas reservoir. The noble gases
are then extracted from the pore water under vacuum conditions
and are analysed by mass spectrometry. The analytical
uncertainties in the Ne, Ar, Kr, and Xe concentrations are <2%,
those in the isotope ratios 20Ne/22Ne and 40Ar/36Ar are
<0.1%. Similar uncertainties result for the He concentration and
the 3He/4He ratio, if the analysis is not affected by the
radiogenic He which may be released from the sediment grains
during extraction. Further, a model for the vertical advection and
diffusion of the noble gases in the pore water was developed.
The transport equations for the solid sed- iment, the pore water,
and the dissolved noble gases dissolved in the pore water were
derived using a realistic set of simplifying assumptions. The new
analytical method and the transport model were applied in dif-
ferent lakes to assess the feasability of using noble gases in
sediment pore water as environmental tracers. In Lake Zug
(Switzerland), the local vertical flux of terrigenic 4 He through
the sediment was determined from the vertical 4He profile in the
pore water to be 10^8 atoms/m^2/s, which is two orders of
magnitude smaller than the mean flux of terrigenic 4 He in the
area covered by Lake Zug. This leads to the hypothesis that the
4He input into Lake Zug is spatially highly inhomogeneous. In
Lake Issyk-Kul (Kyrgyzstan), a closed-basin saline lake, the
concentration profiles of Ne, Ar, Kr, and Xe in the pore water
indicate that during the mid-Holocene the salinity was more
than twice its present value, and that the lake level was several
hundred me- tres lower than it is at present. In Soppensee
(Switzerland), the noble gas concentrations in the pore water
show pronounced undersaturation, probably because the noble
gases escape from the pore water into the gas bubbles that form
in the sediment due to the supersaturation of the pore water
with biologically produced methane. This allowed the amount of
methane which was released in bubbles from the sediment to be
estimated from the extent of the noble gas depletion. The case
studies in these three lakes thus illustrate that dissolved noble
gases are suitable tracers for palaeoclimatic conditions and
physical transport processes in the sediment.