Salt and water budget calculations for the Late Miocene Lake Pannon

Abstract

The water body which once covered a large area of the Pannonian Basin in the Late Miocene is commonly referred to as
'Lake Pannon'. However, up until now it has never been proved that it had any direct connection with the sea. Although the
high level of faunal endemism suggests a long-lived lake (GEARY et al. 2000, MAGYAR 2009), some of its described features
are difficult to explain if the environment was nothing other than lacustrine. Besides the occurrence of marine microfossils
and some (albeit debated) seismic stratigraphic results, the most important sign of a possible marine connection is the fact
that the salinity of the water body appears to have remained permanently within a narrow interval (8–15‰).
Given the above, the modelling of the water and salt budget of the ancient water body could contribute to providing
an answer as to whether the environment was lacustrine or marine. In the current study, such a model is introduced after
assembling the necessary parameters: (i) extent of the lake surface and the discharge area,(ii) run-off factor, (iii)
precipitation and salinity of the lake and (iv) the inflowing water. The results of the calculations show that the known
salinity could have remained steady and without a marine connection if the lake had an outflow of a few 10 m3
/s. However,
the variability of the level of Lake Pannon would have caused large fluctuations in the discharge of an outflowing river;
given this fact, the effects of a temporary cessation and an increase of the outflow on the salinity were also tested. Based
on the model, a period with no outflow would cause the salinity to exceed 15‰ after about 100 kyr. However, the presence
of a major outflowing river (with a discharge of a few 1000 m3
/s, like the modern Danube) would reduce the salinity to
below a level of 8‰ within several hundred years. Therefore the outflow of Lake Pannon probably did not occur in a river,
but through subsurface water. This possibility is also supported by some modern analogues.
The uncertainty of the numerical results was assessed using the Monte Carlo simulation; the uncertainty proved to be
small enough to show that there had be no weakening or anything to affect the palaeogeographical implications. Overall,
the model of the water and salt budget confirms that the known brackish salinity of Lake Pannon could have been stable
even if the water body remained continuously lacustrine with no seawater inflow.