5.1. Fluvial Transport of POCnon-fossil: Capacity and Supply
 Our results demonstrate that Corg and Fnf do not decrease at high Qw (Figure 2) and thus that POCnon-fossil is not diluted at the peak of large flood events (Figure 1b). This leads to a positive correlation between POCnon-fossil concentration and Qw/Qmean (Figure 3) which is analogous to that commonly observed between Qw/Qmean and SSC in mountain rivers [Hovius et al., 2000; Fuller et al., 2003; Hicks et al., 2004a; Kao and Milliman, 2008; Hovius et al., 2011]. For clastic sediment, SSC increase with Qw is often attributed to variability in: i) the capacity of the river to transport sediment as suspended load; and ii) the supply of suspendable sediment (sand, silt and clay) to the river channel. In mountain rivers a third factor may also be important, namely the production of suspended sediment by pebble abrasion at high levels of bed shear stress and associated bed load transport [Attal and Lavé, 2009]. We hypothesize that these factors also control POCnon-fossil transport and examine their potential roles herein.
 The capacity of a river to entrain and transport fine sediment increases with water flow velocity and turbulence [Garcia and Parker, 1991]. Given the restricted channel geometry in bedrock rivers [Turowski et al., 2008], capacity is likely to increase with Qw. Turbulent mixing, typical of mountain river channels with large scale bed roughness, may also increase the entrainment rate and transport capacity of the flow [Jackson, 1976]. POCnon-fossil should be less dense than the accompanying mineral sediment load, even when waterlogged [Buxton, 2010], causing its propensity for entrainment and transport to increase rapidly with Qw [Hamm et al., 2011]. However, in five of the catchments we observe negative values for c-POCnon-fossil, the linear intercept between POCnon-fossil concentration and Qw (Table 2). The physical meaning of a negative intercept implies either a threshold for motion for POCnon-fossil, which may be the case for coarse woody debris (CWD) [West et al., 2011; Wohl, 2011] but seems unlikely for fine POCnon-fossil [Hamm et al., 2011], or a limit on the transport of POCnon-fossil in river channels imposed by its supply. River channels in Taiwan are characterized by a lack of vegetation due to frequent flooding preventing colonization by plants [Hartshorn et al., 2002] and therefore the supply of POCnon-fossil must originate from forested hillslopes.
 The rate at which geomorphic processes erode the landscape are known to depend on the steepness of the topography on which they act [Roering et al., 2001], and high rates of physical erosion by landsliding and overland flow are therefore expected to occur in Taiwan. Overland flow preferentially mobilizes loose material and POCnon-fossil from surface soils [Gomi et al., 2008]. Bedrock landslides can remove entire tracts of mountain forest and soil, harvesting the whole biomass and mixing it with POCfossil [Hilton et al., 2008b; West et al., 2011; Hilton et al., 2011b]. The influence of supply on POCnon-fossil transport can be examined using the hysteresis of POCnon-fossil and Qw during individual flood events, as documented by Hilton et al. [2008a]. That study demonstrated that after several hours of sustained rainfall, enhanced POCnon-fossil concentrations were observed across a range in Qwwhen compared to dry intervals. Rainfall activates geomorphic processes of overland flow and landsliding and leads to efficient hillslope-channel coupling and the supply of POCnon-fossil. In addition, at high flood stage the river has capacity to transport CWD [West et al., 2011] the mechanical attrition of which may also enhance POCnon-fossil concentrations in the river suspended load [cf. Attal and Lavé, 2009]. In contrast, during periods without substantial rainfall, supply from hillslopes is minimal and POCnon-fossil is likely to be sourced from channels, where bed sediments are typically dominated by POCfossil [Hilton et al., 2010]. Thus, POCnon-fossil concentrations are lower for similar hydraulic conditions [Hilton et al., 2008a].
 Organic carbon measurements on samples collected during the flood caused by Typhoon Haitang in the Peinan River are consistent with these observations [Eglinton, 2008]. Measured precipitation on 19 July 2005 totaled 110 mm in Taitung (Figure 1b) near to the gauging station (22.76°N, 121.15°E, data from the Central Weather Bureau, Taiwan, http://www.cwb.gov.tw/). On that day, the sample collected 14 h prior the peak of the flood, on the steep rising limb, had a POCnon-fossil concentration of 160 ± 40 mg L−1 with Fnf = 0.39 ± 0.09. 32 h after the flood peak (09:40 21 July 2005), POCnon-fossil concentration had dropped by 75% to 40 ± 15 mg L−1 (Fnf = 0.24 ± 0.09) despite only a slight decrease (∼10%) in Qw/Qmean from 16 to 14. The marked drop in POCnon-fossilconcentration was co-incident with the cessation of heavy precipitation over the catchment (Figure 1b). These results demonstrate that while landsliding and overland flow are moderated by slope angle [Dietrich et al., 2003], their temporal occurrence is stochastic [Benda and Dunne, 1997; Hovius et al., 2000]. As a result, the fluvial transport of fine POCnon-fossil may vary at a given transport capacity (Qw) due to the specific timing and location of POCnon-fossil supply to the river. This explanation is also consistent with the observed variability in POCnon-fossil concentration for individual catchments (Figure 3) and confirms the importance of POCnon-fossil supply during rainfall [Hilton et al., 2008a], when erosion processes efficiently couple forested hillslopes to the river channel.
 The relative importance of the POCnon-fossil supply processes identified here (overland flow, bedrock landslides, mechanical attrition) remains an avenue for future research. However, the observed lack of Fnf decrease with increasing Qw provides some insight (Figure 2). As established, bedrock landslides are ubiquitous in Taiwan [e.g., Lin et al., 2008] and known to be crucial for delivering clastic sediment to river networks at the peak of floods [Hovius et al., 2000; Fuller et al., 2003; Dadson et al., 2005; Hilton et al., 2008a]. However, erosion of POC by this process can decrease Fnf (decrease POCnon-fossil:POCfossil ratio) at times of high sediment delivery. As the surface area of a bedrock landslide increases (i.e., its POCnon-fossil erosion) it is known that its volume (i.e., sediment and POCfossil erosion) increases as a power law with an exponent >1.2 [Guzzetti et al., 2009; Larsen et al., 2010], implying large landslides can dig deeper and reduce Fnf [Hilton et al., 2008b]. Therefore, the observation of elevated Fnf during high flow (Figures 1b and 2) implies supply of POCnon-fossil by a process other than deep bedrock landslides. Mobilization of surface materials by overland flow, and mechanical attrition of CWD do not contribute POCfossil. One or both of these processes must contribute significantly to POCnon-fossilfluxes in floods. These considerations support conclusions from the Western Southern Alps, New Zealand. There, decadal estimates of landslide-driven POCnon-fossil yield were lower than estimates of fluvial export, requiring additional processes of POCnon-fossil supply from the mountain hillslopes [Hilton et al., 2011b].
5.2. Enhancement of POCnon-fossil Transport
 Rainfall-driven changes in erosional supply underlie a strong climatic control on the mobilization and transport of POCnon-fossil (Figure 3), which should have a similar expression in each catchment. However, it is clear that the positive relationship between POCnon-fossil concentration and Qw/Qmean is not constant for Taiwanese Rivers. This is articulated in the range in gradients of the linear best fit to the data (m-POCnon-fossil), from 0.27 ± 0.08 to 6.43 ± 0.78 (Table 2). m-POCnon-fossil can be viewed as an enhancement factor, with a steeper gradient reflecting increased loading of POCnon-fossil across a range of hydrological conditions. As established previously (Section 5.1), supply is likely to be the main control on the variability in POCnon-fossil concentration, rather than transport capacity in these rivers. Thus, enhancement should relate primarily to the efficiency of erosion processes delivering POCnon-fossil from hillslopes to channels.
 The Taiwanese rivers have a positive trend between m-POCnon-fossil and the area of the catchment with steep slopes above typical thresholds for mass wasting and erosion processes (>35°) (Figure 5). Between the Linpien River (Figure 3c) and the Liwu River (Figure 3a) the trend is nonlinear (n = 8). Such a trend is consistent with the mechanics of the geomorphic processes responsible for POCnon-fossil supply [Gomi et al., 2008; West et al., 2011; Hilton et al., 2011b]. Landsliding and overland flow processes are both stochastic and their rates of occurrence are a nonlinear, threshold functions of slope and runoff [Benda and Dunne, 1997; Roering et al., 1999; Hovius et al., 2000; Dietrich et al., 2003]. Steepening the topography of a catchment should increase the rate of POCnon-fossil supply, but only once hydrological thresholds are surpassed. This explains both the increase in POCnon-fossil with Qw (Figure 3) and enhanced rate of POCnon-fossil supply when steep slopes contribute more importantly to the catchment hypsometry (Figure 5).
Figure 5. The gradient of the linear relationship between POCnon-fossil and Qw/Qmean (Figure 3) for catchments which returned a significant fit (m-POCnon-fossil, Table 2) plotted against the proportion of catchment area with slope angles >35°. Shading of each point reflects the suspended sediment yield (Table 1). A nonlinear fit is shown to 8 of the catchments excluding the Peinan River.
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 The Peinan River, in the southwest of Taiwan, has an m-POCnon-fossil of 6.43 ± 0.78 and lies significantly off the trend in the data set (Figure 5). To explain the higher loads of POCnon-fossil in this catchment, we note that it also has had a very high suspended sediment yield for the study period, over the last four decades [Dadson et al., 2003] and when compared to its mountain headwaters in the Wulu and Yenping catchments (Table 1 and Figure 4b). This may relate to active tectonic deformation of Pleistocene-Recent sediments in the Longitudinal Valley [Ho, 1986]. While the Wulu and Yenping mountain tributaries are located upstream (Figure 4b), the Peinan trunk river has cut into these recently uplifted, poorly consolidated sediments which contain POCnon-fossil [Shyu et al., 2006; Ramsey et al., 2007]. Supply of clastic sediment and POCnon-fossil from these deposits provides a mechanism to enhance fluvial POCnon-fossil concentration across all Qw (Figure 3b) and increase both the SSY and POCnon-fossil yield. Cannibalism of young, uplifted foreland deposits may be an important mechanism by which POCnon-fossilis re-mobilized in larger fluvial systems exiting active mountain belts [Bouchez et al., 2010; Galy and Eglinton, 2011].
5.3. Export of POCnon-fossil From Subtropical Mountain Forest
 The climatic (Figures 1b and 3) and geomorphic factors (Figure 5) that influence transport of POCnon-fossil in Taiwan's mountain rivers also affect their clastic load [Dietrich et al., 2003; Dadson et al., 2003; Hicks et al., 2004a; Galewsky et al., 2006; Kao and Milliman, 2008]. As a result, a strong positive relationship exists between POCnon-fossil yield and suspended sediment yield over two orders of magnitude in this mountain belt (Figure 4a). The data show no evidence for dilution of POCnon-fossil yields at very high physical erosion rates. The average rate of POCnon-fossil transfer of 21 ± 10 tC km−2 yr−1 represents an export of 0.12 ± 0.08% yr−1 of the total organic carbon stock in vegetation and soil, of 11 ± 5 × 103 tC km−2 and 7 ± 2 × 103 tC km−2, respectively [Chang et al., 2006; West et al., 2011]. These export rates are high when compared to rates of geomorphic disturbance in mountain forest. In the western Southern Alps, New Zealand, bedrock landslides disturb forested surfaces at a rate 0.03% yr−1 [Hilton et al., 2011b] and in Central America, disturbance rates are 10 times lower [Restrepo and Alvarez, 2006]. However, the POCnon-fossilexport rates here are likely to include important input from non-bedrock landslide inputs (overland flow, mechanical attrition of CWD) as previously discussed.
 The fluvial POCnon-fossil export from the mountain forest has important implications for carbon cycling at the regional scale. In the absence of other output fluxes (e.g., respiration), it sets a bound on the amount of time available for organic matter to age in the landscape (τnon-fossil, yr). At a depletion-rate of 0.12 ± 0.08% yr−1, physical erosion sets a timescale of 830 ± 530 yr for the aging of the organic carbon stock in vegetation and soil. Across Taiwan, the maximum τnon-fossil imposed by physical erosion is ∼15,000 yr in the Hsiukuluan River (Figure 6). Given the dominant role of respiration to carbon loss in terrestrial ecosystems, the estimates of τnon-fossil are not directly comparable to estimates of residence time in vegetation and soil. These account for all input and output fluxes and recognize different pools of carbon which turnover at different rates [Trumbore, 1993]. However, the limit on biomass aging set by POCnon-fossilexport is consistent with the range of conventional radiocarbon ages of surface soils (A-E Horizons) in Taiwan, which reach a maximum of 4169 yr [Hilton et al., 2008a] with the majority falling between 340 and 1540 yr (Figure 6).
Figure 6. The time available for POCnon-fossil aging imposed by physical erosion (τnon-fossil, yr) as a function of physical erosion rate (mm yr−1) calculated from suspended sediment yields for catchments in Taiwan (circles). Triangles indicate the measured 14C-age of surface soils (A-E horizons) in the Central Range [Hilton et al., 2008a].
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 The data from Taiwan suggest that suspended sediment yields of ∼3000–4000 t km−2 yr−1 (physical erosion rates of ∼1–2 mm yr−1 with sediment density of 2.5 t m−3) can limit τnon-fossil to ∼8000 yr (Figure 6). Thus, it appears that even modest rates of physical erosion can reduce or even eliminate the potential for very long timescales (>10,000 yr) available for pools of organic matter in soils to age, regardless of their respiration rate [Trumbore, 1993; Torn et al., 1997]. POCnon-fossil export thus plays an important role in montane ecosystem turnover, likely to promote young sections of forest where net productivity is most efficient [Restrepo et al., 2009] and inhibit ecosystem retrogression [Wardle et al., 2004; Peltzer et al., 2010]. Physical erosion rates of 1–2 mm yr−1 are exceeded in many mountain belts [Galy and France-Lanord, 2001; Dadson et al., 2003; Hicks et al., 2004b; Gabet et al., 2008; Milliman and Farnsworth, 2011] suggesting that erosion may limit τnon-fossil in mountain forest at the global scale. At very high erosion rates of >10 mm yr−1, the physical processes impose a timescale for aging (Figure 6) which encroaches on the centennial rates of turnover in vegetation and components of soil organic carbon [Trumbore, 1993; Torn et al., 1997]. Clearly, the findings here demonstrate that the impact of rapid geomorphic process rates on nutrient and carbon cycling in mountain forests warrants further assessment.
5.4. Wider Implications for the Carbon Cycle
 The erosion and export of POCnon-fossil by mountain rivers represents a lateral flux of recently fixed atmospheric CO2 and its fate is important for our understanding of the global carbon cycle [Berner, 1982; Hayes et al., 1999]. If this material is buried in sedimentary deposits while the POCnon-fossil is replaced by new primary productivity on land, then this transfer represents a net sink of atmospheric CO2. Efficient burial of POCnon-fossil offshore Taiwan may be driven by the very high suspended sediment loads of the mountain rivers which deliver ∼380 × 106 t yr−1 to the ocean [Dadson et al., 2003], causing rapid accumulation rates in depocenters, a first order control on organic carbon burial efficiency [Canfield, 1994; Galy et al., 2007]. Hyperpycnal river plumes, arising when SSC>40 g L−1 at the river mouth [Mulder and Syvitski, 1995], can trigger turbidity currents which are also thought to play an important role by rapidly delivering POCnon-fossil carried by floodwaters (Figure 1b) to deep marine sediments [Dadson et al., 2005; Kao et al., 2006; Nakajima, 2006; Saller et al., 2006; Hilton et al., 2008a]. While the fate of POCnon-fossil remains to be fully assessed, it seems likely that a large proportion of the 0.5 ± 0.2 × 106 tC yr−1 of POCnon-fossil delivered to the oceans from Taiwan is buried.
 The significance of the transfer of POCnon-fossilfrom Taiwan to the ocean is evident from comparison to a well-studied source-to-sink region from the Himalayan mountain belt to Bay of Bengal. There, an estimated 3.7 × 106 tC yr−1 of POCnon-fossilis delivered by the Ganga-Brahmaputra rivers and sequestered from a continental source region ∼50 times larger than Taiwan [Galy et al., 2007]. The conservative estimate of POCnon-fossil flux from the small mountain island represents ∼15% of this value and ∼1% of the estimated total terrestrial organic carbon burial in the oceans [Schlünz and Schneider, 2000]. Evidently, mountain islands are important not only for the erosion and transfer of POCfossil [Blair et al., 2003; Leithold et al., 2006; Kao et al., 2008; Hilton et al., 2011a], but also in the transfer of carbon recently fixed from atmospheric CO2.
 Our data suggest that, for a constant set of geomorphic conditions, the fluvial transfer of POCnon-fossil from mountain catchments is driven by climate (Figure 3) through the activation of erosion and transport processes during heavy rainfall (Figure 1b). A move to a wetter, stormier climate over mountain forest should enhance the erosional export of POCnon-fossil. In settings with strong coupling between depositional sinks and terrestrial inputs [e.g., Leithold and Hope, 1999; Kao et al., 2006] this offers a feedback in the Earth System, whereby climate modifies rates of carbon sequestration through erosion and burial of POCnon-fossil [e.g., Hilton et al., 2008a]. In addition, the data from Taiwan suggest that this carbon transfer is moderated by the catchment geomorphology (Figures 4a and 5). Rapid rates of plate convergence and the uplift of competent metamorphic rocks set prime conditions for the rapid erosion and fluvial export of POCnon-fossil concomitant with large amounts of clastic sediment [Galy et al., 2007; Hilton et al., 2008a, 2008b]. On orogenic timescales, this implies a tectonic forcing of the carbon cycle which may lead to net changes in the size of the organic carbon reservoir and influence atmospheric greenhouse-gas concentrations [Derry and France-Lanord, 1996; France-Lanord and Derry, 1997; Hayes et al., 1999] via a carbon transfer that is sensitive to climatic conditions [cf. West et al., 2005].