Dear List Members,

I am having trouble removing carbonates from Lake Michigan coastal
sediments/sediment trap materials.

Our usual procedure in preparation for measuring del C-13, del N-15, %OC &
%ON on lake sediment and trap samples:

Freeze dry, grind w/mortar and pestle,acidify with 1N HCl, shake on a shaker
table for 24 hrs, dry at 60 C, and regrind w/mortar and pestle.

For these coastal southern Lake Michigan samples this procedure has failed
to drive off all the carbonates. (We can tell by the much heavier than
expected del C-13 values). The sample material is silty clay, with low
organic content (<2%), and around 2% carbonate content. We do not know what
type of carbonates these are but suspect they are comprised in part of
recently eroded bluff materials from the western shore of Lake Michigan near
Milwaukee.

As a second attempt I reground the material, acidified it again with 1N HCl,
and then used a Tekmar sonic probe disrupter (six minutes per sample on
pulse setting), shook on a shaker table again for 24 hrs, dried in a 60 C
oven, and reground.  This seemed to eliminate the carbonates from all but a
stubborn few of the samples.

We are puzzled by this problem and would appreciate any input you may have
regarding either our prep procedures or the composition of these materials.

Thank you, Margaret
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Margaret B. Lansing
U.S. Dept. of Commerce
National Oceanic & Atmospheric Administration
Great Lakes Environmental Research Laboratory
2205 Commonwealth Blvd.
Ann Arbor, MI  48105
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313-741-2289 (lab)
313-741-2210 (office)
313-741-2055 (fax)
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