δC¹³ Ratios of several soil and cave fill samples

Doug Love

1. Jay Gregg and I are trying to answer two different questions, so we split the presentation. Two of the samples are from my study area, and may contain ice-age material. I am looking for clear differences between recently disturbed soil and possibly ancient soil in these samples:

1. Type section of the Sunnyside Loam from in front of the new USDA HQ.

formerly farmed, now a waste area.

2. Bog soil from a permanent wetlands in nearby Indian Creek

3. Ruined Tobacco Land, now forested, near Hamilton Cemetery, Greenbelt

4. Rich humus from between roots of the Witch Tree, Greenbelt

5. recently worked garden soil, 3-D Plateau Place, Greenbelt

6. black soil from a possibly glaciated slot valley above C&O Canal

7. from the floor of Snyder's Landing Cave #2, possibly old glacial soil filtered down through sinkholes.

2. Preparation

Dr. A. J. Kaufman of the Geology Dept. showed us how to prepare samples to be tested for % Carbon and δC¹³ ratios. His lab techs Chrissy France and Nick Collins were also very helpful.

a. Samples were dried in a drying oven for 24 hours.

b. a few grams of each sample were crushed with mortar and pestle, after visible plant and animal debris were removed.

c. Crushed samples were put in small sealed containers, and the mortar and pestle were washed and dried to prevent contamination of the next sample.

d. Weighing of the samples was done in a clean lab, inside a vibration-proof booth, on a marble table.

i. Less than a gram of each sample was placed in pre-weighed tin cups, which were then weighed again when filled to determine sample weight.

ii. A small screwdriver was used as a sample shovel.

iii. The filled tin cups were then crimped and placed in a plastic holder to be tested.

iv. Two cups of each sample and 10 control samples of pure urea were processed.

v. An attempt was made to use .2 to .4 gram of low-carbon soil and .5 to 1 gram of high-carbon soil.

e. Carbonates had to be acidified, cleaned, dried and reweighed.

i. Sample 7 reacted strongly.

ii. Sample 1 may have reacted a little, so it was also decarbonated.

f. Finally, after Nick Collins fixes the machine, Chrissy France ran the samples.

3. Results

a. Results were given to us as permil vs. the PDB standard, so no conversion was necessary.

b. saturated values for samples 4, 5 and 7 were discarded.

c. differences between sample 1 and decarbonated sample 1b were minor, so it was concluded that samples 1-6 contained little or no carbonate.

d. Decarbonated sample 7b was compared with samples 1-6, and was found to be 5.73 ‰ higher than the others. This was considered within the 3-8‰ reduction suggested by Deleens et al in Sundquist and Visser, but far below the 15-25‰ reduction suggested by OLeary in the same paper. I concluded that there was no forest cover here in glacial times, as all of the photosynthetic activity was in C4 plants.

4. Other benefits of doing this study

a. We were exposed to sample preparation and analysis techniques

b. We gained a good practical knowledge of what a microgram is

References:

Sundquist, E. T., and K Visser, USGS, Woods Hole, MA, The Geologic History of the Carbon Cycle, Ch. 8 of Treatise on Geochemistry, pp 425-472

O'Leary, MH (1988) Carbon Isotopes in Photosynthesis. BioScience 38, 328-335

Deleens E., Ferhi A., and Queiroz O., (1983) Carbon isotope fractionation by plants using the C4 pathway Physiol. Veg. 21, 897-905