>Dear Listmembers,
>Has anyone measuring the carbon isotope values of freshwater aquatic
>grasses observed values in the range of -40 to -43VPDB? If you have
>observed such values, what is the explanation for such low values
In one system I work in we have measured aquatic mosses and algae
with values of -40 permil, so not restricted to grasses.
There are a fair number of papers on this. In moving waters, one
mechanism that leads to such depletion is as Andy Johnston pointed
out due to diffusion of CO2 in the water, and in essence the
thickness of the boundary layer around the plant, which in turn can
be related to water velocity. My understanding is:
High flow = small boundary layer = fast diffusion and high 13C
discrimination due to large CO2 pool available relative to plant
Slow flow = larger boundary layer = slow diffusion and less 13C
discrimination due to plant CO2 requirements coming closer to
available CO2 pool.
This seems to be a major control and probably more influential than
variation in 13C(DIC).
Check out the work of Keeley and Sandquist, and  Jacque Finlay for
more on this.

I have a few questions of my own....
I am collecting samples for bulk DOM analyses and as it requires time
to extract sufficient  DOM for stable and radiocarbon measurements,
I will be poisoning the samples with saturated HgCl2 solution. Does
anyone have any comments on the amount of HgCl2 soln to add?  One
paper I read suggested 1ml per 100ml of solution, but another paper
suggested 1ml per litre.  Some preliminary sampling shows the HgCl2
to ppt out with the DOM (post rotary evaporation and freeze-drying)
which clearly dilutes the DOM and also makes the powder quite
corrosive, so I would prefer to keep the HgCl2 concentration as low
as possible.

And leading on from that, if anyone out there has experience on
analysing such DOM samples (possibly containing HgCl2, but likely
very acidic anyway) by CF-IRMS can they let me know how they do it.
The one time I tried it, the samples (once wrapped in sufficient tin
to stop dissolving the capsule that I could place it in the carousel)
reacted with the glass furnace tube, and ruined it. I was now going
to return to closed tube combustion, but wondered if there was a
sneakier way of neutralising the sample instead, that would also give
me easier access to C:N data.

Dr. Susan Waldron,
NERC Advanced Research Fellow
Scottish Universities Research and Reactor Centre,
Scottish Enterprise Technology Park,
East Kilbride G75 0QF,

Tel: (01355) 270142
Fax: (01355) 229898
email: [log in to unmask]