Geomorphology and hydrology of Nee Soon 
43 
based on a mass balance approach (Nguyen, 2017). These calculations suggest physical 
erosion accounts for only about 24% of total denudation, compared with 76% for 
chemical weathering and loss from the soils. 
Low denudation rates are not unexpected in the Nee Soon catchment, given the 
gently sloping terrain (steepest stream slope is ~5°) and dominance of forest vegetation. 
The unusually high contribution of chemical weathering (43-84%) is indicative of the 
warm temperatures and high rainfall of this tropical locale (NO 1.39017°, E103.80893°). 
We believe that the physical erosion rate (5.6 ± 0.5 Mg km 2 yr 1 ) is slow compared 
with other studied granite-derived soil environments (e.g. in Riebe et al., 2001, 2004). 
However, because chemical weathering is high (accounting for c. 76% of the total 
denudation), biochemical processes likely play an important role in soil formation. 
Our field observations suggest that tree uprooting, which causes redistribution 
downslope of the soil in the root mat, as well as bioturbation by abundant termite and 
ant activity extending into the B horizon, are important for soil formation/alteration. 
Moreover, volumetric strain calculations suggest that rock deformation and soil 
formation has been intense over a period of hundreds to thousands of years (Nguyen, 
2017). 
Dating of swamp sediments by the use of 137 Cs and 210 Pb(ex) suggests that a 
period of accelerated erosion may have occurred during and/or since the 1950s due to 
disturbance from the construction and maintenance of a water pipeline in the catchment 
and other peripheral activities at the lower catchment. Again, accelerated erosion 
related to these disturbances is not substantial today. Accelerated erosion associated 
with forest conversion to agriculture in the upper parts of the catchment also cannot be 
ascertained with these methods. 
Sedimentation rates 
We collected cores from the middle and lower swampy areas with a 6.36 cm gouge 
auger to determine sediment deposition and to support a variety of analyses (description, 
total organic carbon (TOC), bulk density, texture, elemental and oxides concentrations, 
radioisotope dating). Core depths vary from 1 to 1.95 m depending on the ability 
to penetrate subsurface material and recover an intact core. For the lower reach, we 
focus primarily on one core, but use others to provide additional material for analysis 
and to examine spatial patterns of deposition. Ages of various layers were determined 
from a variety of isotope techniques ( 14 C, 210 Pb and 137 Cs) on pollen, charcoal, and 
sediments. Thus, our interpretations reference a composite core, constructed from data 
from several cores (Fig. 4). 
Within the total length of the composite core for the lower swamp, we demarcate 
several distinct layers having highly variable deposition rates (Table 2; see also Fig. 4). 
The ages of these layers extend back through the Holocene (0-11,700 BP) to before 
the Late Glacial Maximum (10,000-13,000 BP). However, the contemporary periods 
of maximum disturbance related to forest conversion to agriculture (from about 1850) 
are missing due to channel incision (layer 2), as mentioned above. 
Only three dates can be determined for the middle swamp core. Radiocarbon 
