Sediment gas studies related to methane and carbon dioxide in Lake
Tanganyika: Comparative core investigations on the Kalya Horst and
Slope
Student: Samuel Ogada Ochola
Mentors: Dr. Kiram E. Lezzar and Dr. Donald Adams
Introduction
The importance of sediment gases as part of the carbon and nitrogen budgets of the East African Rift lakes
has been addressed in only a few publications. The cycling of gases in the water column, and especially
sediments, are important ecosystem processes. With the exception of dissolved oxygen, information
concerning dissolved gases in the water column is fragmentary and scarce. Rudd (1980) reported on
methane oxidation in Lake Tanganyika. There seems to be a greater scarcity of published information for
gases in African lake sediments, with no information concerning concentrations of major sediment gases,
such as methane, carbon dioxide and nitrogen (Adams, 2001 in prep). According to Adams (2000),
greenhouse sediment gas studies have never been conducted in the East African lakes. Nonetheless, some
significant work has been carried out on methane oxidation in the water column in Lake Tanganyika (Rudd,
1980). The whole lake's water column methane oxidation rate varies seasonally from 3.8 to 5.8 mmol
CH4.m-2.d-1. The annual rate is tentatively estimated at 3.1 mmol CH4.m-2.Yr-1. This is equivalent to at
least 10% of annual primary productivity (Ruud, 1980).
Objectives
•
•
•
To collect initial sediment gas data
Correlate Kalya Horst and Slope methane and Carbon dioxide levels with depth.
Compare the methane data with OC content.
Methodology
A multi-corer aboard M/V Maman Benita was used to recover sediment multi-cores on Kalya horst and
slope. Two of these were sub-cored for sediment CH4 and CO2 studies. From the Kalya slope, gravity core
no. 4 was sub-cored from NP-01-MC1. From the Kalya Host, gravity core no. 2 was sub-cored from NP01-MC2. The internal diameter of the final core liners used for sediment glove bag processing was only
4.4cm while that of the multi-core liners was 10cm. The gravity sub-coring was done immediately after
retrieval to avoid loss of gases and were processed immediately on the ship. The sub-coring led to some
significant compaction and loss of a few millimetres of the core bottom. Core no. 4 was compacted by
11cm including bottom loss while core no. 2 was compacted by 2cm only.
A vacuum was created in the core liners by topping them up with lake water and capped without stirring up
the sediments. The multi-core was then opened and the inner core liner pulled out and capped at the
bottom. The cores were then kept upright and fitted with a rod (graduated at 1cm intervals) having a rubber
end for extruding sediments. A small diameter (4cm o.d) plastic liners was fitted to a core adapter syringe
sampling (CASS) system plastic slider, located at the top of the CASS system contained threaded holes to
hold a 25ml serum monovette plastic syringe and a glass scintillation vial (both pre-weighed). This slider
could be adjusted to either open or closed position. After sub-coring, the core was kept vertical and the
CASS system, (which replaced a top rubber stopper or piston), attached directly to the top of the liner. A
bottom piston was inserted and overlying water expelled with the slider in the open position. As the
sediment surface appeared the slider was closed and the entire CASS system and core placed horizontally
into a helium-filled glove bag for processing.
The helium-filled glove bag had oxygen levels periodically checked to avoid atmospheric contamination.
Levels below 0.35mg/l were acceptable above which more helium was pumped into the glove bag. A
second person applied pressure against the bottom piston, with a rod marked at 1cm intervals extending
outside the glove bag to force sediments into the monovette syringes. Syringes and vials were attached to
the slider, which was opened for sediment extrusion (1.5-2 cm core displacement, 10 to 20g wet
sediments), into the monovette. Sub-sampling was done along the core with intervals of 0.5 cm discarded
between samples. Half of the sediments were injected into scintillation vials for further analysis for percent
sediment water content. The remaining halves in the syringes were slid fast across the slider and capped
(with a silicon insert reinforced septum) for later injection of a helium headspace for pore water gas
extraction and later Gas Chromatograph (GC) processing. The whole core length was sampled ending with
19 syringes per core.
The syringes were then stored in a double-lined helium-filled plastic bag under refrigeration. The syringes
were kept in a cold water bath with temperatures below 22oC to cool (CH4 is temperature sensitive). An
exact volume of 5ml of helium headspace was added under water pumped with helium to avoid
contamination since nitrogen and oxygen measurements were desired. The syringes were then each put
into a helium-filled plastic bag and vibrated for 5 minutes horizontally to let all the sediment gases go into
helium. Syringes were vigorously shaken for 5 minutes vertically to create a headspace. 500µl of
headspace were removed with a helium-flushed side-port needle for GC gas analysis. Samples are injected
into the GC and transmitted into the Peak 2 chromatogram program on a laptop computer. Curves for the
different gas peaks were then traced and identified after certain retention times for both Flame Ionisation
Detector (FID) and Thermo-Conductivity Detector (TCD) for both IN/OUT positions.
The gas peaks were then picked out and integrated to isolate CH4 and CO2. All peak2 files were then
imported into MS-Excel based on individual retention times for more analysis and chart preparation. Areas
under peaks were normalised to % of the maximum value. Glass scintillation vials tare and wet weights
were recorded then caps removed and oven dried at 60oC. Dry weights were taken for duplicate analysis of
percent sediment (pore) water. Headspace equilibrium is based on the technique developed by McAuliffe
(1971) for analysis of hydrocarbons in water. Gas extraction with an inert headspace has been adapted to
sediment pore waters by Kaplan et al, (1979) for N2-gas production during denitrification, by Conrad et al,
(1985) for H2 and CH4 in sewage sludge and lake sediments. Naguib (1978) studied for CH4 and Fendinger
and Adams (1986) for Ar, O2, N2, CH4 and CO2 in lake sediments.
Observations and Results
Kalya Slope Core (NP 01 MC1)
The final gravity core length reduced to 47cm from the original 58cm after sub-coring. This can be
attributed to both sediment losses at the bottom or compaction (11cm). The first 23cm from
sediment/water interface were homogenous with a gas fraction at 10cm. Distinct laminae at 23cm.
Laminations at 26cm consist of alternating bands of light and brownish-yellow layers each half a cm of
diatom ooze flocculation to 54cm depth. The bottom 4cm is a black homogenous layer.
CH4 (based on norm. %) at the top of the core drops from 587 to 160mVSec at 5cm core depth. This
(Table 1 and Figure 1) represents 48.6% to 13.3% CH4 proportion. No CO2 is reported at this depth. CH4
increases steadily with depth between 6-12.5cm from 712-1209 mVSec representing 58.9 to 100%
proportions. No CO2 was reported here due to syringe contamination and GC error. Down from 13cm is a
sharp drop in CH4 activity to only 33.59mVSec or 2.78%. This contrasts with CO2, which starts to increase
from 31-49mVSec or 18-29% at 16cm. Down the core CH4 is kept steady at low values of 44-94mVSec or
3.7-7.83%. In contrast, CO2 levels down the last 25cm picks up above CH4 to 42-49mVSec with peaks of
167 and 169mVSec or 99.3 and 100% at 29cm and 23cm depths respectively.
Table 3 and Figure 3 shows the % pore water content. Pore water content in the Kalya Slope core at the
sediment H2O interface is 92.4% and drops to 88.7% at 5.8cm depth. The lowest % H2O is 81.8% at 8cm.
The interstitial water picks up to 90% at 10cm and remains stable through to 23cm between 90 and 92%
H2O. The content fluctuates down the core but generally decreases to 86% at 30cm depth.
Kalya Horst Core (NP 01 MC2)
The final sub-core length reduced to 54cm from the initial 56cm multicore length. Only 2cm was either
lost to compaction or bottom loss. Laminations start at 3.5cm from interface consisting of 2 white bands
For Table 1 and 2—CH4 and CO2 data—click on the link below:
Ochola_Table 1 & 2
Table 3
L. Tanganyika Sediment Interstitial Water Data
Nyanza Project 2001
Core # NP 01 02 MC2
Kalya Horst
Vial
No.
MidPoint
K105
Tare Weight
Wet
weight
Total wet
wt
Dry
Weight
15.528
25.665
10.14
16.204
0.75
Dry
Sed.
Pore
Water
Wt (g)
wt (g)
0.676
%
Water
Content
9.46
93.33
K76
3.25
15.534
19.750
4.22
15.836
0.301
3.91
92.85
K121
5.25
15.433
24.769
9.34
17.080
1.647
7.69
82.36
K72
7.25
15.509
22.841
7.33
16.179
0.670
6.66
90.86
K115
9.25
15.441
24.189
8.75
16.168
0.727
8.02
91.69
K67
11.25
15.493
20.917
5.42
15.964
0.471
4.95
91.31
K102
13.25
15.601
21.823
6.22
16.073
0.472
5.75
92.41
K61
15.25
15.513
22.034
6.52
16.083
0.570
5.95
91.26
K114
17.25
15.485
21.589
6.10
16.042
0.557
5.55
90.87
K122
19.25
15.437
21.441
6.00
15.979
0.542
5.46
90.97
K52
21.25
15.645
21.887
6.24
16.250
0.605
5.64
90.31
K116
23.25
15.511
24.626
9.12
16.458
0.947
8.17
89.61
K101
25.25
15.566
23.552
7.99
16.730
1.163
6.82
85.43
K68
27.25
15.513
24.178
8.66
16.868
1.355
7.31
84.37
K71
29.25
15.576
25.237
9.66
17.031
1.455
8.21
84.94
K120
31.25
15.481
23.124
7.64
16.455
0.974
6.67
87.26
K75
33.25
15.444
22.601
7.16
16.650
1.206
5.95
83.15
K123
35.25
15.542
22.932
7.39
16.890
1.348
6.04
81.76
K78
37.25
15.482
21.560
6.08
16.337
0.855
5.22
85.94
Fi gure 1: L. Tanganyika
Sediment CH4/CO2 Vs
Depth. Core NP 01 MC1.
(Kalya Slope)
0
25
Figure 2: L. Tanganyika
CH4/CO2 with Depth. Core
NP 01 02 MC2 (Kalya Horst)
Figure 3: L. Tangankyika Sediment Pore H2O
Content. Core No NP 01 MC2. (Kalya Horst)/ Core
NP 01 MC1 (Kalya Slope)
Water Content (% Ww)
Norm. %
50 75 100
0
Norm %
50
75
100
80
85
90
95
0
0
0
5
5
5
15
20
25
10
15
20
25
30
% CH4 (IN)
30
35
% CH4 (IN)
35
%CO2
% CO2
Depth in Core (cm )
Depth in Core (cm)
Depth in Core (cm)
10
10
15
20
25
40
30
35
% H2O Content
(Kalya Horst)
% H2O Content
(Kalya Slope)
40
separated by a brown layer. The 1cm thick band is composed of fine silt. The top 3cm is brown and
homogenous. At 4.5 to 21cm is black layer followed by distinct laminations at 21-22cm depth. These are
brown to olive grey. Further down are alternating bands of yellow diatomaceous ooze flocculation and
dark bands. At 37cm is a major change to black sediment facies. At 45-47 cm is a light band. Black
homogenous “clay” facies is dominant in the bottom 7cm.
CH4 (based on norm. %) in the Kalya horst (figure 2and table 2) core starts at 241mVSec or 15.7%
proportion at 3cm depth. This increases to 639-652mVSec at 5-7cm respectively representing 42%. No
CO2 recorded until 9cm depth, almost steady from 175, 25, 165, 141 and 189mVSec representing 32, 4, 30,
26 and 35% from 11-19cm depth. Below 11cm, CH4 level remains stable at 1079-1537mVSec as peak
values. The highest CH4 value is at 27cm depth. CO2 continues to increase down the core with the highest
100% level at 38cm of 539mVSec. The last sample in the core (Syringe 43/Vial 78 was discarded since
there was no headspace).
Table 3 shows the pore water content for the Kalya Horst core (NP 01 02 MC2) decreases steadily down
the core (Figure 3). The top has 93% H2O and goes down to 90% at 7cm depth. At 5cm though, H2O
content is lowest (82%). Up to 23cm depth the pore H2O is constant between 89-91% below, which a drop
to 85, and 84% at 29cm was recorded. A small increase (87%) is noted at 31cm but again maintains the
drop in water content to 81%.
Discussion
Methane and carbon dioxide flux are important in trying to understand carbon cycling in lakes in addition
to being palaeoclimatic record of the OM in the catchment at the time of deposition. These two gases also
constitute a large proportion of greenhouse gas emission. There are numerous publications on the
importance of CH4 in the cycling of carbon. As Adams (2001 pers. comm.) noted, internal cycling of gases
and their production and consumption are important ecosystem processes whereby carbon is returned to the
water column and eventually the atmosphere of fresh water lakes. Carbon cycling processes are mostly
inferred from temperate lakes that undergo seasonal temperature changes and water column mixing. As
Hecky (1978) pointed out, tropical aquatic systems might suffer extreme sensitivity to eutrofication
because of their endless summers and warm bottom waters.
From the preliminary data, the Kalya slope and horst have a significant variation in depth of
methanogenesis. Carbon dioxide production increases with depth in the horst sediments. CH4 production
in the horst sediments is inhibited for the first 10cm. This coincides with the zone of sulphate reduction.
Anaerobic oxidation of CH4 occurs between 13-18cm. The actual zone of methanogenesis goes on at 2230cm depth correlating with Guiles and Hartwell (2001, Nyanza Project) maximum OC content of 8.9%.
Kalya horst is isolated from terrestrial sediment input and most of the sedimentation is accounted for by
pelagic material settling down the lake bottom evidenced by the abundance of diatom ooze flocculation.
Anaerobic CH4 in the Kalya slope sediments goes on at a shallow depth (6-12.5cm). Moreover, as Hartwell
and Guiles (2001 Nyanza Project) noted from the same core, this is the depth of maximum OC content
(9%) in the sediments. This may be explained by the fact that at 10cm depth, there is a brown sediment
layer that is not yet reduced. The slope has terrestrial sediment input from rivers draining from Mahale
mountains. According to Ruud (1980), CH4 production rates in shallow lakes are directly related to rates of
particulate carbon input to sediments and are about 10-13% of primary productivity. In deeper lakes a
larger proportion of fixed carbon is mineralised before reaching the sediments (Adams, 2001 op. cit).
It is thought that the highest CH4 levels are directly proportional to OC content in the sediments. This
holds true for both the horst and slope. It is apparent that methanogenesis goes on at greater depths on the
horst as opposed to slope. It is possible that the brownish sediment facies have the highest CH4 formation.
From the ongoing, there seems to be more methanogenesis going on at depths between 60-120cm as
suggested by even higher OC content >11% at 63cm depth from graviy cores.
This is particularly true for Lake Tanganyika. As Adams (2001), the oxic-anoxic boundary, normally
found within sediments or at the sediment-water interface in oligotrophic lakes, would move upwards into
the water column, and can remain there continuously if thermal stratification persists. This boundary
would represent the zone of methanotrophy (oxidation of CH4 diffusing upwards from the sediments). As
lakes become more eutrophic, they tend to deposit greater amounts of organic materials in their bottom
deposits (Adams, 2001 op. cit). These substances fuel organic matter decomposition processes which
consume oxygen and produce reduced substances, such as methane (and carbon dioxide, as a
decomposition product), ammonium, sulphides and other elements (manganese and iron). The reduced
substances diffuse into the overlying water and further consume oxygen. Eventually the oxic-anoxic
interface moves upwards into the water column.
Conclusion
The cycling of gases in the water column, and especially the sediments, are important ecosystem processes.
It is becoming necessary to understand the biogeochemical gas cycles of all lakes of the world, but
especially in the tropical lakes where there is limited or sporadic information. These lakes are extremely
important resources for the local populations. Follow-up studies are needed to ascertain the spatial and
temporal OM deposition, CH4 and CO2 production rates on the lake. More interpretation will follow this
report to correlate the preliminary result with OC and core stratigraphy in addition to the actual gas
concentration. Of even much importance in future would be to measure lake surface gas emissions for
these two gases to get the total flux into the atmosphere as they contribute to global warming. Therefore,
processing of more gravity cores is needed to get a more complete record.
Acknowledgements
I am highly indebted to my geology mentor Dr. Kiram Lezzar for his advice and critique, much of which
was applied in this report. Richard Hartwell and Kathy Guiles for OC data. More thanks to Dr. Andy
Cohen for his ideas and concern as the project progressed. I am particularly grateful to Dr. Lezzar and Dr.
Donald Adams separately for their time and in carefully preparing me for this task. Special thanks to Dr.
James Scott for his new and exciting ideas and expertise on anaerobic bacteria that helped define the
discussion. I wish to particularly thank the US National Science Foundation Grant # ATM 9619458 (The
Nyanza Project), and the WWF for financial support of this project. Again, I thank Dr. Adams for
providing his GC, data, and laptop computer and for explaining gas data processing.
References
Adams D. D., 1994. Sediment Pore Water Sampling in Mudroch A and MacKnight, S.D, A Handbook of
techniques for Aquatic Sediment Sampling, 2nd ed., Lewis Publishers, London, pp 172-202.
Adams, D.D., 2001 in prep. Sediment Gas cycling in African lakes, with special reference to Lake
Victoria.
Adams, D.D., 2001 pers. comm. Nyanza Project 2001, Guest Researcher.
Hartwell, R.J., Guiles, K., 2001 Nyanza Project. Comparison of stratigraphic records between
contrasting depositional environments from the Kalya Ridge and Kavala Island Ridge, L. Tanganyika, East
Africa. This edition.
Naguib, M., 1978. A Rapid Method for the Quantitative Estimation of Dissolved Methane and its
Application in Ecological research. Arch. Hydrobiol. 82. 66.
Ruud, J.W.M., 1980. Methane Oxidation in Lake Tanganyika (East Africa). Limnol. Oceanogr., vol
25(5), 1980, 958-963.
List of Figures (saved in file: Gas data)
Figure 1: L. Tanganyika CH4/CO2 with Depth on Kalya Slope, NP 01MC1
Figure 2: L. Tanganyika CH4/CO2 with Depth on Kalya Horst, NP 01 MC2
Figure 3: Variation of % Pore Water Content on Kalya Slope and Horst with Depth.
List of Tables (saved in file: Pore water)
Table 1: Chromatogram Gas Data on Kalya Slope
Table 2: Chromatogram Gas Data on Kalya Horst
Table 3: Percent Pore Water Content with Depth on Kalya Slope and Horst