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Title: Sediment fluxes and carbon dynamics of a tropical river floodplain, Tana River, Kenya
Other Titles: Sediment fluxen en koolstof dynamiek in het overstromingsgebied van een tropische rivier (Tana River, Kenia)
Authors: Omengo, Fred Ochieng
Issue Date: 14-Oct-2016
Abstract: Recent research has highlighted the importance of rivers in the global carbon cycle. Rivers transport considerable amounts of sediment and carbon (C) to the ocean and act as a major conduit between the continental and oceanic component of C cycle. However, a large fraction of the C in the fluvial system does not reach the ocean, either due to mineralization during transport or storage in transitory stores within the fluvial network. Our understanding of the role of these transitory stores under the different climatic regions remains limited. This research focuses on the temporal significance of a tropical floodplain (as transitory C store) and its role in the river C and sediment fluxes. The study is based in the Tana River, Kenya a tropical lowland semi-arid floodplain. Our research area was in the Garissa-Garsen (GSA-GSN) reach of the floodplain ~ 380 km river length and ~1,000 km2. We have used a unique combination of advanced biogeochemical and geophysical techniques to characterize the interaction between the floodplain and the main river, quantifying the fluxes of C and sediments transported in the river and deposited within the Tana River floodplain and analysing the temporal significance of Tana River floodplains as a C sink.
During flood events, a significant amount of sediment is deposited within the GSA-GSN reach. Based on a combination of fallout radionuclide activities, we recorded a 50 yr mean sedimentation rates of 1.15-1.21 g cm-2 yr-1 and a 100 yr mean of 1.01 g cm-2 yr-1 (using 137Cs and 210Pbex, respectively). Event-based sediment deposition rates averaged between 2-15 mm vertical accretion, corresponding to an average of 0.58±0.42 g cm-2 (dry weight). Overbank flooding is common with a 73 yr flooding frequency of 1.05 flood yr-1 and an average of 1-2 Mt (15-30%) of the river sediment load being deposited in the GSA-GSN floodplain reach. Substantial amounts of organic carbon (OC) are deposited with this enormous amount of sediment. However, the deposited sediments are carbon poor at ~1.5% OC and the floodplain vegetation plays a big role in enrichment with additional OC inputs to levels above 3% OC at the (sub) surface layers. Sediment cores at floodplain locations therefore show relatively high OC concentrations (3-12% OC) in the (sub) surface layers, however, due to high mineralization rates, a sharp decrease of OC with depth is observed to less than 1% OC below ca. 60 cm depth.
Acknowledging that OC source is an important consideration in OC preservation, we investigated OC sources and storage with depth within the floodplain using d13C-OC as a proxy. Local disturbance by clearing of vegetation and subsistence agriculture caused homogenization of the d13C-OC depth profile, while fresh biomass inputs from the undisturbed floodplain locations appeared to accelerate mineralization of riverine sediment derived OC. Our data shows that despite the strong variability in 13C enrichment in deep layers at some sites, the OC in sediment deposits show no evidence of major shift in the dominant vegetation in the catchment. The post depositional dynamics of OC were dominated by a combination of isotope mixing, selective mineralization and kinetic fractionation upon OC mineralization making it difficult to constrain the principal mechanism responsible for the observed pattern in d13C-OC. Due to this intricate mechanism, it was difficult to properly model mineralization of below ground OC using the Introductory Carbon Balance Depth Explicit Model (ICBM-DE), a simplified two pool SOC mineralization model.
Finally when we made a budget of the fluxes both within the river and the floodplain, we found that during flooding, an intensive selective deposition in the different geomorphological featured within the floodplain occurred. This led to changes in grain size as different particle sizes were either remobilized or preferentially deposited in certain floodplain location. The sorting is also evidenced by high fluxes of fine grain size at both the upstream and downstream locations and at the same time deposition of fine materials within the floodplain. Remobilization of sediment mainly through bank incision and meander migrations was a major floodplain process. While about 6.7 Mt yr-1 sediment and 104 GgC yr-1 enters the floodplain at Garissa, 5.19 Mt yr-1 of sediment equivalent to 36 GgC yr-1 is mobilized and added to this pool annually through river incisions. Consequently, the majority of this sediment and C is lost, the main mechanisms include within river deposition (6.57 Mt yr-1 sediment or 66 GgC yr-1), overbank floodplain deposition (38 GgC yr-1, 2.45 Mt yr-1 or 27% of total sediment fluxes) and the rest is discharged as fluxes downstream of the floodplain in Garsen (34 GgC yr-1, 2.6 Mt yr-1, 24% of total sediment fluxes). Due to the intensive selective deposition and sorting of different grain sizes, the downstream riverine fluxes of suspended matter and particulate organic carbon pools are strongly influenced by these processes.
URI: 
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division Soil and Water Management
Division of Geography & Tourism

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