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From: [log in to unmask] href="mailto:[log in to unmask]">Forizs Istvan

To: [log in to unmask] href="mailto:[log in to unmask]">[log in to unmask]

Sent: Monday, May 02, 2005 3:20 PM

Subject: [ISOGEOCHEM] about Lake Balaton

Dear Pier,

thank you very much for your comment!
Really, I forgot to mention that this lake is very shallow. Its average depth is only 3.5 meters, and its maximum depth is 12 meters! And as a conseqence it is very sensitive for environmental change or fluctuation (amount of precipitation, amount of nitrate-phosphate loaded to the lake, ....).

We have some problem with the water temperature time series as well. The temperature was measured at 7 o'clock in the morning, which (we think) is not characteristic for the daily average temperature. So there is another place at the Lake Balaton, where the temperature is measured at 7 o'clock and at 12 o'clock as well. Using the correlation between the these two temperature data we calculated the temperature at 12 o'clock at Balatonszeplak, and we used this calculated temperature for calculating the theoretical d18O(aragonite). I have a feeling that even this calculated temperature is not good for characterizing the daily average temperature of the mud, where the Unio Pictorum lived. Keeping in mind that the water depth at the place where the Unio was picked up in September 2004 was about 40-50 cm in the years of 2002 and 2003, I think that the solar radiation could reach the bottom easily and warmed it up during daytimes, so the average daily temperature of the bottom was higher than the calculated temperature for the water (12 o'clock).
 We have tested this idea. From the difference between the calculated and the measured d18O(aragonite) values we have calculated the temperature difference needed to meet the two d18O(aragonite) values. But this temperature difference turned out to be too big (8-16 degrees Celsius). So big temperature difference is unlikely. There must be other reason as well, and we think it must be traced along the d18O values.
At Balatonszeplak there is no natural inflow. As I know at this moment there is no artificial inflow iether. But I have an idea, that when the water level of the lake is very low (like in 2002-2003), shallow groundwater from the vicinity can flow and discharge to the lake in very little amount, and this can affect the isotopic composition of pore water of the mud, where Unio Pictorum lives.

If you have any comment, I am glad to read it.

Istvan

Pier de Groot wrote:
Dear Istvan,

I already guessed it could be the lake Balaton you was studying. There is a very important factor you did not mention about this lake, which I am certain you know but many others probably do not. This lake has a very large surface compared to depth. Size of surface roughly is 80 x 10 km, but the maximum depth is only about 80 meters if I remember right. Many parts of the lake are very shallow (one can walk into the water at the south coast at least for one or two kms without loosing bottom under ones feet) and will not provide enough space for isotopically different layers of water at those parts. Also, this large surface causes huge evaporation compared to total water in the lake reservoir compared with many other lakes. Water temperature of most of the lake is quite high during summer periods, and changes in temperature may be rather fast because of the large surface to total water volume. The strong influence of wet or dry years confirm this too I think. In short, this is not a common type of lake.

Just a note to raise the consiousness of the special character of the lake, fact you know better than I do, I am certain. Lake Balaton simply represents a rare type in the world. Probably for you it is so common knowledge that you may overlook this fact.... Comparing with other lakes will be difficult for the reasons given above I think.
Say hallo to Attila Demeny from me.

Regards,
Pier.
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Dr. FÓRIZS István ......... Istvan FORIZS Ph.D.
Magyar Tudományos Akadémia, Geokémiai Kutatóintézet
Institute for Geochemical Research, Hungarian Academy of Sciences
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