Oppenheimer: 



we call "Marken". We restrict the term, "Marken" or mark- 

 ings, absolutely to inorganic things but it is possible that a 

 crustacean moving over the surface leaves a trail or marking. 



There is a reaction which might be interesting to the understand- 

 ing of these burrowing animals. That is when you have an an- 

 aerobic sediment and sulphate reducing bacteria active, you get 

 a lot of hydrogen sulphide produced and this creates an acid sed- 

 iment. When the organism comes into there and burrows a hole, 

 it allows the oxygenated water to penetrate. Then the anaerobic 

 acid sediment is converted to an aerobic state and thus the pH 

 rises. It is very possible under certain conditions in which there 

 are carbonates dissolved in a more acid anaerobic part of the 

 sediment that you w^ill get calcium carbonate precipitated due to 

 the change in pH around the side of the burrow. 



Chronic: I was going to ask if that could happen to iron as well. 



Oppenheimer: Well, the iron is changed from the reduced to the oxidized state. 



Chronic: 



Oppenheimer: 



Odum: 



Is it apt to be precipitated as a limonitic band? 



It is already precipitated as the iron sulphide and it is converted 

 to a hydroxide so you just get a change of state of the precipitate; 

 instead of black it is red. I do not know if there is any mobility 

 of the iron although iron hydroxide in the ferrous state is sup- 

 posed to be mobile at minus 200 millivolts in the anaerobic state 

 so there might be somie migration there too. 



Do you have any suggestion as to what graptolytes are ? 



Hantzschel: No. 



Odum: 



Hantzschel: 



Odum: 



They are not tracks I presume ? 



They are preserved fossils which are out of discussion today. 

 There is a very fine investigation of these by the Polish paleont- 

 ologists. 



But it could be some secretion from something couldn't it? 



Schuckmann: I think that studies of thin sections and so on indicate that that 

 could not be the case. 



Bradley: May I comment on Dr. Oppenheimer's observation? I can't say 



what the pH was in the mud that I studied but I had the opposite 

 experience measuring the pH and eH in the tidal flats on the 

 Maine coast where the sediment is reducing and there are sul- 

 phate reducing bacteria and in none of those did we find that the 



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