A compact history of peat in the Cumberland Marshes river delta A compact history of peat in the Cumberland Marshes river delta A compact history of peat in the Cumberland Marshes river delta A compact history of peat in the Cumberland Marshes river delta
S. van Asselen* and E. Stouthamer
*Corresponding author:
*Corresponding author: *Corresponding author:
*Corresponding author: s.vanasselen@geo.uu.nl. Department of Physical Geography,
Faculty of Geosciences, Utrecht University, P.O. Box 80.115, 3508 TC Utrecht, The Netherlands.
Tel.:+31(0)30 2532779. Website: www.geo.uu.nl/fg/palaeogeography.
• Peat is most compressible of all natural soils.
• Many Holocene deltas comprise thick peat layers.
• Compaction of peat leads to substantial amounts of subsidence.
• Subsidence leads to:
- relative sea level rise (land inundation, coastal wetland loss) - damage to construction works
- creation of accommodation space for fluvial deposition
BACKGROUND BACKGROUND BACKGROUND BACKGROUND
• What are the amount and rate of peat compaction that occurred in our study area over short timescales?
• Which factors influence peat compaction?
• How does peat compaction affect fluvial deposition patterns?
• …and ultimately: How does peat formation and compaction influence avulsion, and hence delta evolution?
QUESTIONS QUESTIONS QUESTIONS QUESTIONS
FIELDWORK FIELDWORK FIELDWORK
FIELDWORK – – – – METHOD & RESULTS METHOD & RESULTS METHOD & RESULTS METHOD & RESULTS
CONCLUSIONS CONCLUSIONS CONCLUSIONS CONCLUSIONS
A device was developed that allows un- disturbed sampling of uncompacted peat.
CROSS SECTIONS CROSS SECTIONS CROSS SECTIONS CROSS SECTIONS
STUDY AREA STUDY AREA STUDY AREA STUDY AREA
The Cumberland Marshes (Canada).
A former peatland is invaded by
rivers following an avulsion in the 1870s.
At each study site cross sections are constructed
perpendicular to the main flow
direction.
NEW PEAT CORING DEVICE NEW PEAT CORING DEVICE NEW PEAT CORING DEVICE NEW PEAT CORING DEVICE
DRY BULK DENSITY DRY BULK DENSITY DRY BULK DENSITY DRY BULK DENSITY
Calculating the uncompacted dry bulk
density of compacted peat.
SUBSAMPLING PEAT CORES SUBSAMPLING PEAT CORES SUBSAMPLING PEAT CORES SUBSAMPLING PEAT CORES
Peat cores were
sampled at a 5 cm
interval to determine dry bulk density and
organic matter content.
0.0 0.2 0.4 0.6 0.8 -0.5
0.0 0.5 1.0 1.5 2.0
146 145
dry density [g/cm 3 ]
0 20 40 60 80 100 -0.5
0.0 0.5 1.0 1.5 2.0
om [%]
d e p th [ -]
0 20 40 60 80 100
0.0 0.2 0.4 0.6
0.8 compacted peat - 145
uncompacted peat samples compacted peat - 146
om [%]
d ry b u lk d e n si ty [ g /c m 3 ]
* om 0228 .
exp 0 84
.
0 −
ρ =
76 . 2 0
R =
DECOMPACTION DECOMPACTION DECOMPACTION DECOMPACTION
0 20 40 60 80 100 0
2 4 6 8 10
146 145
om [%]
d e c o m p a c te d h e ig h t [c m ]
Calculating decompaction height per 5 cm.
1. In our study area peat layers are compacted up to 53% of their original thickness in decades time (shortly after start of loading!).
2. Most important factors influencing peat compaction are: a) thickness of peat layer, b) thickness of overburden, c) peat type (especially organic matter content) and d) time since start of loading.
3. Peat compaction leads to relatively thick natural levees and crevasse-splay deposits.
4. Peat compaction enhances vertical aggradation which results in channel belt geometries with low width/depth ratios.
5. Based on our study we suggest that on short timescales peat compaction fixes river channels and hence inhibits avulsion.
pvc liners sharp cutting edge
bolt holes stainless steel tube
ventilation tube hollow rod
vacuum pump
undisturbed sample in middle liner
bolt holes
screw for
extension rods
80 mm
106 mm
liner run up
1 1
5 compacted peat sample
1 1
h calculating height of uncompacted peat sample
h=(DBD
comp
/DBD
uncomp
)*5
145 147 148 146
James Bond river
N S
Transect site 2
100 cm 10 m Peat
Floodplain deposits (caly, silty clay) Natural levee/crevasse deposits (fine) Natural levee / crevasse deposits (coarse) Substratum (fine-grained fluvial deposits)
End of coring
105Coring number
Legend
Water
14C dated peat sample (no results yet)
0.0 2.5 5.0 7.5 10.0
40 60 80 100
JB M R NAC
M uskeg South M uskeg M id
