GASES IN BOTTOM SEDIMENTS
F. C. W. OLSON AND BOB WILDER
U.S. Navy Mine Defense Laboratory
Panama City, Florida
ABSTRACT
An activated carbon chromatographic column was used in Fisher-Gulf
Gas Partitioner to analyze one ml gas samples from shallow water
sediments. Small bubbles rising from undisturbed hard bottoms were
pure 0" large bubbles pure CO". Methane and H"S were found only
in mud-muck bottoms with copious evolution of gas.
Although it is common knowledge that gases do occur in bottom
sediments, very little information on this subject can be found in the
literature. Perhaps one reason for this is the difficulty of analyzing the
small gas samples which are usually obtained. The availability in our
laboratory of a Fisher-Gulf Gas Partitioner gave us an opportunity
to make some studies of a preliminary nature on the composition of
gases found in our local sediments.
The chromatographic
column supplied by the manufacturer as a
standard item uses tri-m-cresyl phosphate as the liquid coating on
inert granules. We found this to be unsatisfactory, particularly when
H:!S was present. A column 7 ft long, of 0.25 inch OD copper tubing
packed with a mixture of 32 to 60 mesh activated charcoal proved
to have suitable characteristics, when helium was used as a carrier
gas. It was tested with pure N:!, O:!, CO:!, CH4, H:!S and H:!O vapor
and various mixtures of these gases as well as air. Water vapor and
H:!S were not retained in the column. The inability to detect water
vapor was no real loss and no effect from H"S was preferred to the
troublesome effect of H:!S in the tri-m-cresyl phosphate column.
The retention times of N", 0" and air were so close that it was not
possible to separate them, at least with the flow rates, temperatures
and sensitivity settings appropriate to the detection of CO:! and CH4•
Optimum settings for a 1 ml sample were: temperature,
140°C;
flow rate, 40 ml/minute;
sensitivity, 20. Under these .conditions,
the retention time of N:!, O:! or air was 1 minute, CH4 was 2 minutes
and CO:! was 6.5 to 9 minutes. Although the retention time of CO:!
was not consistent, the shape of the curve was distinctive enough so
that the peak could be identified with little difficulty.
Field Tests: Gas samples were taken at several locations in shallowwater zones of St. Andrew Bay. Sand bottom, grass (mixed Thalassia
and Diplanthera) bottom and mud flats were sampled by water dis-
208
Bulletin of Marine Science of the Gulf and Caribbean
[11(2)
placement in a 4 oz. glass jar placed over an inverted funnel of about
18 inch diameter and 20 inch height. In some tests, the funnel was
left undisturbed for 10 to 30 minutes; in others, the bottom under the
funnel was raked and otherwise thoroughly disturbed to release gases
trapped in 'the sediment. A third procedure was to collect bubbles
individually as they were seen to rise from the bottom. These varied
from about 1 to 4 mm in diameter. In a few instances, large quantities
of bubbles were seen to rise after the bottom had been disturbed by
walking on it. These, too, were collected individually until a sufficiently large sample (2 to 5 ml) was obtained.
Typical results of these field tests are given in Table 1. Where O~ is
listed, its presence is inferred since O~ is expected to be liberated by
photosynthesis. A 1 ml sample was used in the gas analyzer.
TABLE
TYPICAL
Beltom Type and Method
of Collection
Grass bottom, undisturbed for
30 min., funnel
Disturbed mud bottom, funnel
Disturbed grass bottom, funnel
Disturbed sand bottom, funnel
Bog bottom, Lake Poltergeist, -,copious evolution, no funnel
required
Soft mud bottom with free
evolution of gas bubbles
collected individually
Large bubbles individually
collected from slightly
disturbed grass bottom
*A dystrophic
1
ANALYSES OF GASES IN SHALLOW-WATER SEDIMENTS
OF ST. ANDREW BAY, BAY COUNTY, FLORIDA
salt-water
Quantity
of Gases
Collected
Per Cent Composition by Weight
10 ml
100% 0"
75 ml
23 ml
52 ml
Unlimited
lagoon at 30c9'44"N
4% 0,,96% CO,
13.2% 0",86.8% CO,
20.6% 0" 79.6% CO,
0.4% 0" 1% CH" 98.6% CO,
plus obvious but indeterminate
amount of H,S
5.5% 0" 4.4% CH" 90.1 % CO,
and 85c44'30"W.
The field observations and analyses permit some fairly definite
conclusions which, while neither profound nor particularly novel.
are consistent with what one might expect. Bubbles arising from
grass flats were nearly always small. When collected with the funnel.
analysis showed the gas to contain no CH{ or CO~. This leads to
the inference that it is pure O~. When the bottom was disturbed, large
bubbles were always seen to rise. When collected individually, the
1961]
Olson & Wilder: Bottom Sediment Gases
209
composition was 100% CO~. When collected with the funnel, up to
20% oxygen was found, but the only other gas was CO~. It is apparent then, that the major constituent, if not the only one, of gases
within the sediment is CO~ and this accumulates as large bubbles.
Methane was found only in soft mud or muck bottoms exhibiting
free evolution of gas.