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GEOCHEMICAL SAMPLE PROCESSING PROCEDURE
As soon as the piston core is retrieved on-deck, geochemical sample processing begins. Every
effort is made to minimize the time between core retrieval, sample processing, and storage at -l5
to -20C. Upon retrieval, the sediment core liner is labeled in 20-cm sections. The specification
for labeling the core with Section Numbers is shown in Table 1.
The labeled core is then cut into sections with a special core liner cutter developed by the Ocean
Drilling Program (ODP). This liner cutter eliminates plastic shavings that can contaminate the
sediment. A picture of a core being cut with this cutter is shown in Figure 1. The cutter is
clipped in place around the liner, and a special double-sided blade sinks into the liner about 3
mm. The cutter is then rotated around the liner, cutting a neat, shaving-free section end. Table 2
lists the specification for which three core sections are sampled. The section numbers selected
for geochemical processing vary with core recovery.
Table 1. Core Section Numbering Convention with Corresponding Depth Interval.
Section Number
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Depth Interval (cm)
0-20
20-40
40-60
60-80
80-100
100-120
120-140
140-160
160-180
180-200
200-220
220-240
240-260
260-280
280-300
300-320
320-340
340-360
360-380
380-400
400-420
420-440
440-460
460-480
480-500
500-520
520-540
540-560
560-580
580-600
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Figure 1. Core being cut into 20-cm sections using special core-liner cutter.
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Table 2. Core Sections Selected for Processing Based on Length of Core.
# of Core Sections Geochemical Section #s Archive Section #
1
01
none
2
01
02
none
3
01
02
03
none
4
01
02
04
03
5
02
03
05
04
6
03
04
06
05
7
04
05
07
06
8
05
06
08
07
9
05
07
09
08
10
05
08
10
09
11
06
09
11
10
12
06
10
12
11
13
06
10
13
12
14
06
10
14
13
15
06
10
15
14
16
06
10
16
15
17
06
11
17
16
18
06
11
18
17
19
10
15
19
18
20
10
15
20
19
21
10
15
21
20
22
11
16
22
21
23
11
16
23
22
24
11
16
24
23
25
11
20
25
24
26
11
20
26
25
27
11
20
27
26
28
15
21
28
27
29
15
21
29
28
30
15
21
30
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Each core section taken for processing is extruded and the top and bottom 1-cm of sediment is
discarded. The core material is then sampled for the various analytical requirements. An
example of three geochemistry sections and an archive section taken as a part of this program is
shown in Figure 2. This photograph shows 20-cm sections already cut from a core. They are
labeled “10”, “15”, and “20”. Referring to Table 2, these sections must have come from a core
containing 20 sections total length. This equates to a core that is 4.0 meters long. When such a
core is pulled from the core barrel, the liner containing the core material is immediately placed in
the graduated processing trough shown in Figure 1. The top of the core is adjusted in the trough
so that the top of the core lines up with the beginning of section one marked on the trough. The
bottom section is that length of core that is comprised of a full section. Almost always, this
means that the bottom section designated is longer than 20 cm, because the partial section below
it is not cut off prior to extruding. Through this procedure, the actual core recovery is almost
always slightly longer than core length logged for the core.
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TDI-Brooks
Figure 2. Sections taken for geochemical processing from a piston core.
Geochemical samples for total scanning fluorescence, C15+ hydrocarbons, and biomarkers are
extruded directly into labeled plastic ZipLok bags by inverting the bag inside-out and sampling
about one-third (6 cm) of the section length as shown in Figure 3. The bag is then carefully
reversed and sealed. This method eliminates contact with extraneous materials, thus minimizing
contamination. When necessary, clean stainless steel utensils are used to help in sampling.
Duplicate bags are filled from each section. One bag is for analysis and the other is for backup
(archive). The air is squeezed out of the bags, and they are sealed and laid flat in the freezer.
Figure 3. Extruding sediment for geochemical analysis.
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The final one-third (5 cm) of the section material is used to sample for headspace gases. This
material can be “popped” into a 500 mL can by connecting the liner to the groove in the top of
the can and forcing the plug of sediment into the can with a “thump” of the can onto the
processing table. An example of the final set of geochemical samples is shown in Figure 4. For
each core, the resulting set of labeled samples includes 3 pair of duplicate bags (6 bags total), 3
cans, and one whole-round section to be saved as an archive sample.
Figure 4. The set of geochemistry samples taken from a piston core.
After the 5-cm sediment section of sample for headspace analysis is placed into the 500 mL can,
165 mL of clean, degassed, poisoned (with 50g sodium azide per 5 L) seawater is added. The
headspace can is purged with nitrogen to remove ambient gases before sealing. The process of
purging the can’s headspace after water is added is shown in Figure 5. All samples are frozen at
-15° to -20°C.
Figure 5. Purging the headspace of a sample-filled can with nitrogen before sealing.
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A core log sheet is also completed for each site. The core log data are input directly into a
computer onboard ship. The core logs are bound and provided as a permanent record of the core
collection program. The core log consists of core description, core location, water depth,
sections sampled, depth of any oil-staining or gas pockets, presence of hydrogen sulfide, obvious
large fauna, sedimentary features, etc. Any unusual features are photographed and sampled. A
complete set of core logs for this program is included as a separate report or separate volume.
GC ANALYSIS OF CANS FOR HEADSPACE GASES
Headspace gas analysis refers to the determination of interstitial light hydrocarbon gases
including methane, ethane, propane, iso-butane, n-butane, neo-pentane, iso-pentane, and npentane (C1-C5). The headspace gases are sampled through the septum of the equilibrated
sample can with the needle of a gas-tight syringe or automated sample loop. One (1.0) mL of
gas is injected onto a packed GC column. The gases are separated and detected by a flame
ionization detector (GC/FID). External standards are used for quantitation. A calibration curve
is established by analyzing each of 5 calibration gas mixtures in replicate and fitting the data to a
straight line for each analyte using the least squares technique. Sample responses for each
analyte gas are then compared to the calibration curve to derive the concentrations of each gas in
each sample in ppmV headspace.
Analytes are identified based on their retention time. Figure 6 shows a chromatogram of
analysis of a sample containing substantial concentrations of thermogenic gas. In this figure, the
numbers associated with each peak are the retention times in minutes for that compound. For
example, the retention time for methane is 1.89 minutes and that for ethane is 3.77 minutes.
Figure 6. Chromatogram of thermogenic gases in a canned sample.
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The gas chromatograph’s ChemStation software automatically integrates analyte peaks based on
their retention times. The analyst visually confirms that each analyte peak is integrated correctly
by the software. Concentrations of analytes in samples are calculated based on the concentration
and response of external standard compounds.
If a sample contains greater than 500 ppmV of methane, the can is sent for carbon isotopic
analysis, otherwise, once analysis is complete, the headspace can is opened to measure the mass
of sediment and volume of headspace. From these measurements, the volume ratio of canned
sediment to headspace is calculated. This “mud/HS” ratio is estimated for unopened cans by
weighing the can and calculating the apparent ratio using the average density of all measured
samples. The concentrations of the light hydrocarbon gases in the can headspaces are then
multiplied by the mud/HS ratio of the corresponding can to yield the reported concentrations of
gas as volume gas at NTP per unit volume of sediment. They are reported as parts per million by
volume (ppmV).
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