Sediment mobilisation in Lake Alaotra catchment, MadagascarVao Fenotiana Razanamahandry, Liesa Brosens, Marjolein Dewaele, Benjamin Campforts, Liesbet Jacobs, Tantely Razafimbelo, Tovonarivo Rafolisy, Nils Broothaerts, Gert Verstraeten, Gerard Govers, Steven Bouillon. KU Leuven, Leuven, Belgium/University of Antananarivo, Antananarivo, Madagascar.
Project MaLESA
1.Initiation of Lavaka 2.Carbon and Sedimenttransport
3.Sedimentary archives and Environmental reconstruction
LAVAKA (Malagasy word) = “gullies”, is a part of erosion features that occurs
in many regions of Madagascar
High erosion rate in Madagascar: human driven?
Global Rainfall Erosivity Map (Source: European Soil Data Centre, 2017)
Background and research question
Lavaka-prone regions in Madagascar
Source:The Geological Society of America, 2010
Lake Alaotra: largest lake in Madagascar
Background and research question
Tracing sediment and organic carbon transfer in lake Alaotra regionsources, mobilisation and deposition.
Proxies: Organic carbon (OC) content and d13C of OC
Hillslopes
River
Reservoirs/lakesediments
APPROACHES
Isotope fractionation of Soil organic carbon gives an information about past vegetation
above ground
Organic carbon fluxes and d13C of plants and soils:
APPROACHES
soil profiles (grassland and forest hillslopes)
lake sediment cores marshes core floodplain cores riverine and lacustrine water
sampling ( regular sampling)
Study area: Lake Alaotra catchment and sampling points
%OC
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Depth (cm)
0
50
100
150
200
PPL V
PPL B
PPL C
PPL M
PPL UM
PPL T
%OC
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Depth (cm)
0
50
100
150
200
PNL V
PNL B
PNL C
PNL M
PNL UM
PNL T
%OC
0 1 2 3 4 5 6
Depth (cm)
0
50
100
150
200
F1 V
F1 B
F1 C
F1 M
F1 UM
F1 T
%OC
0 1 2 3 4 5 6
Depth (cm)
0
50
100
150
200
F2 V
F2 B
F2 C
F2 M
F2 UM
F2 T
Grassland soil : OC content extremely low and decreases with depth
Forest soil profiles: OC% decreases with depth, OC % forest soil > OC % of grassland (OC stock of forest is two times higher)
93Ton/ha
OC
181Ton/ha
OC
Figure 1 : Organic carbon content of soil profiles : 2 grassland profiles (a and b) and 2 forest soil profiles (c and d)
(a) (b) (c) (d)
RESULTS
9
13C
-26 -24 -22 -20 -18 -16 -14
Depth (cm)
0
50
100
150
200
PNL V
PNL B
PNL C
PNL M
PNL UM
PNL T
13C
-26 -24 -22 -20 -18 -16 -14
Depth (cm)
0
50
100
150
200
PPL V
PPL B
PPL C
PPL M
PPL UM
PPL T
13C
-28 -26 -24 -22 -20
Depth (cm)
0
50
100
150
200
F1 V
F1 B
F1 C
F1 M
F1 UM
F1 T
13C
-28 -26 -24 -22 -20
Depth (cm)
0
50
100
150
200
F2 V
F2 B
F2 C
F2 M
F2 UM
F2 T
d13C of Grassland soil profiles decreases with depth and is between C3
and C4 signatured13C of Forest soil profiles increases with depth and is consistent with
long-term C3 cover
Figure 2 : d13C of Organic carbon of soil profiles : 2 grassland soil profiles (a and b) and 2 forest soil profiles (c and d)
(a) (b) (c) (d)
RESULTS
OC(%)
0 10 20 30 40
Depth
(cm
)
0
4
8
12
16
20
24
28
32
36
40
44
48
13C(‰)
-20 -18 -16 -14
0
4
8
12
16
20
24
28
32
36
40
44
48
Figure 3: Characteristics of organic carbon of lake sediment core in the south of the lake “T2”(a) Organic carbon content plotted against core depth,(b) δ13C of organic carbon plotted against core depth.
(b)(a)
RESULTS
OC of lake sediment core “T2” higher than soil OC and δ13C ranges between -20 to -14 ‰
OC (%)
0 5 10 15 20 25
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
13C (‰)
-22 -20 -18 -16 -14
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Figure 4: Characteristics of organic carbon of lake sediment core in the south of the lake “Alaotra 1”(a) Organic carbon content plotted against core depth,(b) δ13C of organic carbon plotted against core depth.
RESULTS
OC of lake sediment core “Alaotra 1” are higher than soil and δ13C ranges between -22 to -14 ‰
OC (%)
0 5 10 15 20 25
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
13C (‰)
-22 -20 -18 -16 -14
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Figure 5: Characteristics of organic carbon of lake sediment core in the south of the lake “Alaotra 2”(a) Organic carbon content plotted against core depth,(b) δ13C of organic carbon plotted against core depth.
(b)(a)
OC of lake sediment core “Alaotra 2” are higher than soil and δ13C ranges between -22 to -14 ‰
RESULTS
Sediment core
1) Marshes vegetation and Peat
Internal primary productionSuspended organic material
2) Fluvial input and hillslope erosion
OC : 0.4-1.8%δ13C : - 24 to -18‰
OC %: 10% -60%δ13C : - 24 to -14‰
OC: 5-35% δ13C :-22 to-14‰
POC : 10-33 %δ13C : - 29 to -24‰POC/chl a: 5373) High Primary production
Figure 6: Characteristic of organic carbon from the 3 potential sources of OC in the lake sediment core of Lake Alaotra.
3 potential sources of OC in lake sediment core of Lake Alaotra
RESULTS
Application of isotopic mixing model (MixSIAR) on Lake Sediment core “T2” Marshes are the primary sources of organic carbon
(a) (b) (c) (d) (e)
Figure 7: Estimated proportion of organic carbon from potential sources in lake sediment core “T2” by using d13C of organic carbon,
(a) Organic carbon content plotted against core depth and age (cal a BP),(b) δ13C plotted against core depth,(c) Proportion of Soil-derived organic carbon plotted against core depth,(d) Proportion of Marshes-derived organic carbon plotted against core depth,(e) Proportion of internal primary
production-derived organic carbon in lake sediment core plotted against core depth.
OC (%)
0 10 20 30 40
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
13C (‰)
-20 -18 -16 -14 -12
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
Application of isotopic mixing model (MixSIAR) on lake sediment core “Alaotra 1”Marshes are the primary sources of organic carbon
OC (%)
0 5 10 15 20 25
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
13C (‰)
-22 -20 -18 -16 -14
Dep
th (
cm
)0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Figure 8: Estimated proportion of organic carbon from potential sources in lake sediment core by “Alaotra 1 ” by using d13C of organic carbon,(a) Organic carbon content plotted against core depth,(b) δ13C plotted against core depth(c) Proportion of Soil-derived, organic carbon plotted against core depth,(d) Proportion of Marshes-derived organic carbon plotted against core depth,(e) Proportion of internal
primary production-derived organic carbon in lake sediment core plotted against core depth.
(a) (b) (c) (d) (e)
OC (%)
0 5 10 15 20
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
13C (‰)
-22 -20 -18 -16 -14
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
De
pth
(cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
154
Age (cal a BP)
363
573
858
1176
1495
1813
1964
2044
2124
5417
10087
15691
18960
Application of isotopic mixing model (MixSIAR) on lake sediment core “Alaotra2” Marshes are the primary sources of organic carbon
OC (%)
0 5 10 15 20 25
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
13C (‰)
-22 -20 -18 -16 -14
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0.0 0.5 1.0
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Figure 9: Estimated proportion of organic carbon from potential sources in lake sediment core “Alaotra 2 ” by using d13C of organic carbon,(a) Organic carbon content plotted against core depth,(b) δ13C plotted against core depth,(c) Proportion of Soil-derived organic carbon plotted against core depth,(d) Proportion of Marshes-derived organic carbon plotted against core depth,(e) Proportion of internal
primary production-derived organic carbon in lake sediment core plotted against core depth.
(a) (b) (c) (d) (e)
Floodplain are likely a key sink of soil-derived sediment
%OC
0.01 0.1 1 10
13C
-30
-28
-26
-24
-22
-20
-18
-16
-14
-12
Grassland Soil
Forest Soil
Floodplain AND1
Floodplain MAR1
Floodplain AND2
Figure 10: δ13C plotted against organic carbon content of grassland and forest soil profiles and floodplain cores.
• Soil in the grassland and forest hillslope has a lower OC content (0-2% for grassland soil)
• d13C of Grassland soil profiles: indicates a shift of C3 to C4 vegetation
• Lake sediment core has a high organic carbon content.
• Majority of lake sediment OC is not soil-derived, but originates from surrounding marshes.
• Floodplains are likely a key sink for soil-derived sediments.
d13C and OC tracers give an information on environmental change : carbon content change and marshes vegetation,
not Insufficient to understand the entire sediment and carbon transfer in the Malagasy landscape
Consider another proxies : pollen or charcoal in lake sediment core .
CONCLUSIONS
