Final Report Summary - SOURCE (Tracing Amazon soil organic carbon input from land to the ocean)
Project context and objectives
During the project, I studied the large tropical Amazon River system, investigating river-bed sediments and suspended particulate matter (SPM). Samples were collected in the Solimões-Amazon River main stream and its main tributaries (Negro, Madeira and Tapajós) in periods of high- and low-water discharge in 2005 and 2009. We determined variations in crenarchaeol and glycerol dialkyl glycerol tetraethers (GDGTs) concentrations as well as the Branched and Isoprenoid Tetraether (BIT) index. The results were subsequently compared with other commonly used proxies for terrestrial organic matter (OM) input such as C:N ratio and d13COC. The aims of this study were to trace the potential compositional alteration of soil OM along the transport pathway, and to quantify the relative contribution of soil-derived organic carbon (OC) to particulate OC and its discharge to the ocean.
Project results
Our results provide a qualitative and quantitative assessment of GDGT sources (soil vs. aquatic) and composition in the Amazon River and its tributaries. The GDGT-derived and bulk geochemical data indicate that riverine SPM and river-bed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and river-bed sediments did not predominantly originate from the High Andes' soils (>2500 m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower-montane forest areas in the Andes.
Our study also revealed seasonal and inter-annual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Óbidos, the last gauging station in the Amazon River, we estimated that 70-80 % of the POC pool in the river was derived from soil OC. However, care should be taken to use the BIT index since it showed non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.
During the project, I studied the large tropical Amazon River system, investigating river-bed sediments and suspended particulate matter (SPM). Samples were collected in the Solimões-Amazon River main stream and its main tributaries (Negro, Madeira and Tapajós) in periods of high- and low-water discharge in 2005 and 2009. We determined variations in crenarchaeol and glycerol dialkyl glycerol tetraethers (GDGTs) concentrations as well as the Branched and Isoprenoid Tetraether (BIT) index. The results were subsequently compared with other commonly used proxies for terrestrial organic matter (OM) input such as C:N ratio and d13COC. The aims of this study were to trace the potential compositional alteration of soil OM along the transport pathway, and to quantify the relative contribution of soil-derived organic carbon (OC) to particulate OC and its discharge to the ocean.
Project results
Our results provide a qualitative and quantitative assessment of GDGT sources (soil vs. aquatic) and composition in the Amazon River and its tributaries. The GDGT-derived and bulk geochemical data indicate that riverine SPM and river-bed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and river-bed sediments did not predominantly originate from the High Andes' soils (>2500 m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower-montane forest areas in the Andes.
Our study also revealed seasonal and inter-annual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Óbidos, the last gauging station in the Amazon River, we estimated that 70-80 % of the POC pool in the river was derived from soil OC. However, care should be taken to use the BIT index since it showed non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.