On Fri, 13 Mar 1998, Margaret B. Lansing wrote:

> Dear List Members,
>
> I am having trouble removing carbonates from Lake Michigan coastal
> sediments/sediment trap materials.
> The sample material is silty clay, with low
> organic content (<2%), and around 2% carbonate content.

 This brings up a potential problem that concens me as we make the change
from sealed tube combustion to continuous flow measurement of 13C and 15N
of particulate organic matter. My concern is the clay. Traditionaly, we
acid treat the samples just as you do Margaret, but in a sealed tube
combustion, the samples are evacuated while heating to ~90C to drive off
primarily water that is strongly bound to any clay in the sediment
samples. Any adsorbed CO2 is incidentaly removed.
 Has anyone out there established whether or not CF measurements of
organic carbon from clay-containing sediments are affected by CO2 adsorbed
onto the clay?

David Wilbur              206-685-9288
Stable Isotope Lab        206-685-2366
School of Oceanography    206-685-3351 fax
Box 357940                [log in to unmask]
Univ. of Washington
Seattle, WA 98195-7940 USA
http://www.ocean.washington.edu/research/sil/index.html