Levoglucosan concentrations in the dissolved and particulate phase increase with increasing discharge (Figure 2). Particulate organic carbon (POC) in the Yenisei River was 0.08 ± 0.02 kg OC kg⁻¹ throughout the sampling period. Monomeric lignin phenol ratios (syringyl/vanillyl: S/V) are 0.44 ± 0.21 in the Yenisei River (Table 1), and the ratios of levoglucosan to its isomer mannosan (L/M) are 4.2 ± 1.2 (Table 1).

The timing of peak export of low-temperature Py-DOC and Py-POC were decoupled in the Yenisei River throughout the freshet period (Figure 3). Low-temperature Py-DOC peaks during the rising limb of the hydrograph, while low-temperature Py-POC peaks during the recession limb for the Yenisei River (Figure 3). LogK_d values, the measure of environmental phase distribution, for levoglucosan were nearly one order of magnitude lower during peak flow in the Yenisei River (Figure S2) than during base flow, indicating a higher proportion of total PyC in the dissolved phase during high flow regimes. LogK_d values of levoglucosan are within the same range as estimated logK_d values for higher temperature PyC markers (BPCA) in rivers (Table 2 and references therein).

POC in the Great Whale River was 0.17 ± 0.08 kg OC kg⁻¹ throughout the sampling period. Monomeric lignin phenol ratios S/V are 0.29 ± 0.19 and L/M ratios are 3.2 ± 3.7 (Table 1). In the Great Whale River, the flux of levoglucosan and low-temperature Py-DOC increased with increasing discharge, while the flux of low-temperature Py-POC decreased with increasing discharge (Figures 2, 3). LogK_d values decreased with increasing discharge (Figure S2).

Free monomeric lignin phenol ratios in both phases (syringyl/vanillyl: S/V), coupled with levoglucosan and its isomer, mannosan (L/M), are within the range of values reported for gymnosperm source inputs in charcoals, aerosols, and soils, suggesting that low-temperature PyC in the Yenisei River and the Great Whale River in both phases are influenced predominantly by gymnosperm fire fuels (Table 1, source ratios from Kuo et al., 2011a). These ratios are consistent with previously reported source reconstructions for Py-DOC in Arctic Rivers (Myers-Pigg et al., 2015).

Absolute concentrations of levoglucosan in the dissolved phase are on the lower end of the range previously reported in river DOM. For the Yenisei River, concentrations in this study are 0.06 ± 0.04 μg L⁻¹ vs. 0.8 ± 0.5 μg L⁻¹ from 2004 to 2006 (Myers-Pigg et al., 2015). Differences in the observed concentrations may be dependent on interannual variability in the mechanisms that govern export of PyC, such as hydrological flow regimes. However, further research must be done to ascertain the varying importance of the mechanisms that control PyC export to and through aquatic systems. Concentrations of levoglucosan in the particulate phase of the studied rivers are also low (Figure 2; Table 2, 0.73 ± 1.1 μg gdw⁻¹) compared to concentrations of levoglucosan in surface sediments from a river plume (1.3–6.9 μg gdw⁻¹) of a small, mountainous stream (Hunsinger et al., 2008). It should be noted that erosion, resuspension, and deposition of PyC along transport is particularly high in mountainous systems (Cotrufo et al., 2016). This may affect the total amount of PyC transported in the particulate phase to estuarine systems compared to the relatively larger river systems in this study, which also have relatively smaller proportions of total carbon exported in the particulate phase.

In watersheds that contain permafrost coverage (such as the watersheds in this study), the permafrost can act as a boundary layer, causing rapid translocation of water to river systems during spring thaw events, which influences SOM transport to rivers. The depth of the soil active layer, in which organic matter has the ability to exchange with the surface terrestrial ecosystem, though generally quite shallow in these regions, tends to vary based on location within a basin. In a subwatershed of the Yenisei River, storage of PyC is largest in areas with permafrost and shallow active layers, whereas mineral soils with thick active layer or lacking permafrost stored less PyC, suggesting increased mobility of PyC from the latter (Guggenberger et al., 2008). The same study showed that most of the export of PyC occurs during the period of snowmelt with dominance of surface flow pointing to a strong relationship between PyC concentrations and hydrological regime. This may help to explain the observed lack of relationship between fire frequency and riverine export of dissolved black carbon (DBC as determined by BPCAs; Ding et al., 2013, 2014), and the strong reported relationships between DBC and hydrological cycling (Jaffe et al., 2013), particularly in Arctic systems (Stubbins et al., 2015).

Here, low-temperature Py-DOC peaks during the rising limb of the hydrograph in the Yenisei River (Figure 3A). This early peak of low-temperature Py-DOC is not mirrored in bulk DOC concentrations, which increase linearly with discharge (R² = 0.82, p < 0.001; graph not shown). This apparently “early” release of Py-DOC has not been previously observed, as Py-DOC from both high and low-temperature portions of the continuum have been previously correlated to discharge and DOC in these and other systems (Jaffe et al., 2013; Myers-Pigg et al., 2015; Stubbins et al., 2015). However, here we present the first dataset with targeted high resolution sampling during spring peak flow. Hence, the decoupling of Py-DOC and DOC during the rising limb may be a phenomenon not previously captured. During the rest of the hydrograph (low flow, peak flow, and the falling limb) low-temperature Py-DOC is correlated to bulk DOC (R² = 0.63, p < 0.001), as has been previously observed. Therefore, further high-resolution sampling is needed to understand Py-DOC dynamics during the early part of the spring freshet period in other years to determine if the periodic decoupling of Py-DOC and DOC is systematic or only an isolated case. This decoupling of low-temperature Py-DOC and DOC in the rising limb of the hydrograph may be linked to the mobilization of a different source of Py-DOC during this period than during other flow regimes.

Although very little research has been done on the transport of particulate PyC from river systems, it has been suggested that transport in this phase may be of importance in recently fire-affected ecosystems (Wagner et al., 2015), where increased surface run off (Moody et al., 2013), soil erosion (Shakesby and Doerr, 2006), and increases in TSS (Ryan et al., 2011) have been observed. In the study regions, particulate carbon export is often a small proportion of the total carbon export; POC export accounts for ~15% of the total OC flux from the pan-Arctic per year (Holmes et al., 2012; McClelland et al., 2016). Around 23% of the total POC export occurs during the spring freshet period in both the Great Whale River and Yenisei River (Hudon et al., 1996; McClelland et al., 2016), making high-resolution sampling important to capture temporal variability in the Py-POC flux. As previous work estimates that Py-POC export in Arctic Rivers is between 3 and 9% of total POC fluxes (Elmquist et al., 2008), transport of Py-POC may be an important portion of the total POC in Arctic Rivers. Here, we find that low-temperature Py-POC comprises 4.5 ± 2.8% of the low-temperature PyC flux from the Yenisei River (Figure 3; Table 3). These relatively lower levels of export of low-temperature Py-POC in our systems has implications for the usage of levoglucosan to track fire activity in depositional records, as low-temperature PyC determined from levoglucosan is exported predominantly in the dissolved phase. In the Yenisei River, low-temperature Py-POC export peaks during the falling limb of the freshet hydrograph (Figure 3C), which may reflect varying sources of PyC between phases or variations in export mechanisms between the two phases. For example, the composition and bioavailability of OM exported from soils in high-latitude systems is a function of active layer depth and mineral soil association (Kawahigashi et al., 2006). Terrestrial organic carbon (as determined by n-alkanoic acids) in the particulate phase has been found to associate with two distinct pools of soil carbon released during the spring freshet period; a young pool dominated by humics and an old pool with a mineral soil component (Vonk et al., 2010). This observed difference in mobilization of terrestrial OC may also influence the mobilization of PyC in high-latitude systems, which in turn, affects availability for transformations during transport, as a large portion of labile materials, if associated with a mineral layer, may be protected from biodegradation during transport (Hedges et al., 1997; Kaiser and Guggenberger, 2000; Vonk et al., 2010). During the freshet period, low-temperature PyC is proportionally exported more in the dissolved phase than during low-flow (Figure 3; Figure S2). This may be due to hydrophilic DOM, such as low-temperature PyC (chemical functionality in Figure 1), bypassing association with soils within the active layer, and directly entering the aquatic system (Kawahigashi et al., 2006) during high-flow regimes. Because high-flow regimes release the largest proportion of yearly DOM and POM during a relatively short time period (Amon et al., 2012; Holmes et al., 2012; McClelland et al., 2016), these flow regimes are similarly expected to contribute substantially to the overall flux of PyC from high-latitude rivers. Additionally, since PyC is predominantly released in the dissolved phase, this may allow highly functionalized and hydrophilic portions of PyC to be more accessible to in-situ microbial degradation and lead to increased riverine respiration rates during this period (Masiello and Louchouarn, 2013).

In the Great Whale River, physical protection of PyC may regulate its release from soils. Only around 10% of the total DOC is released in this river system during the ice break-up period (Hudon et al., 1996), as opposed to almost half of the DOC released from the major Arctic Rivers during the spring freshet period (Holmes et al., 2012). This observation suggests that the timing and release of organic matter from small southern boreal watersheds are driven by different controls than those operating in the major Arctic Rivers. The flux of organic matter may be normalized in these latter systems by the size of the watersheds, whereas soil heterogeneities in smaller southern boreal watersheds may show a strong influence of soil processes. For example, in southern boreal forests, accumulation of large, long-lived stocks of PyC have been observed in mineral horizons with little to no soil profile development (Miesel et al., 2015). The opposite has been reported in boreal forests containing mineral soils underlying thick active layers (Guggenberger et al., 2008). The Great Whale River watershed is itself highly heterogeneous and contains a variety of soil types including discontinuous permafrost (Bhiry et al., 2011). However, and in conjunction with high fire return intervals, the export of PyC from this watershed may be more regulated by soil processes than hydrological flow regimes, as highlighted by the lack of a clear relationship of PyC export with hydrography (Table 3; Figure 3).

The similarity between phase distribution calculated for high-temperature PyC components (BPCA and PAHs) and low-temperature PyC (levoglucosan) suggest that low- and high-temperature PyC may be exported similarly from river systems (Table 2). However, as physical weathering has been shown to be an important loss process for both high- and low-temperature charcoals, although on very different time scales (Naisse et al., 2015), understanding the ages of these exported carbon pools is necessary before final conclusions on their similarities can be determined. Additionally, given the fact that bulk DOC and POC in Arctic River systems have vastly different ages (e.g., Goñi et al., 2005; Amon et al., 2012), we must understand the sources and ages of dissolved and particulate PyC in order to fully understand how export of Py-DOC and Py-POC might be linked both temporally and throughout the PyC continuum. For example, high-temperature PyC (as determined by the CTO-375 method; PyC range covered by the method in Figure 1) has been found to contain two distinct sources between the particulate and dissolved phases in river systems (Wang et al., 2016), and much of this high-temperature PyC can be derived from fossil fuels (up to 33% of the DBC and around 50% of the PBC). In contrast, the sources of low-temperature PyC (derived from biomass burning) using biomarker source ratios appear to be similar in our studied rivers (S/V and L/M; suggesting gymnosperm sources in both phases). Therefore, it may become important to characterize potential variability of PyC generation during biomass burning and its effects on PyC mobilization. Isotopic approaches may help identify some of this variability (e.g., isotopic fractionation of levoglucosan occurs during atmospheric processes; Sang et al., 2016).

The apparent order of magnitude increase in the ratio of Py-DOC to Py-POC for high-temperature PyC from rivers to oceans (Table 2; calculated from Ziolkowski and Druffel, 2010; Coppola et al., 2014; Wagner et al., 2015) suggests that Py-POC plays an increasingly important role in PyC cycling through transport from riverine to oceanic systems. This may be due to the potential degradation of dissolved PyC during transit (Stubbins et al., 2012; Masiello and Louchouarn, 2013; Ward et al., 2014; Myers-Pigg et al., 2015) and/or the importance sorption of Py-DOC to Py-POC in marine systems (Coppola et al., 2014). Sorption processes and transfer to sediments are considered as one of the major loss processes for Py-DOC in the world's ocean (Coppola et al., 2014). Py-DOC exported from Arctic Rivers could sorb to sinking particulates once it enters the Arctic Ocean. This has implications on how much Py-DOC may survive transport and end up in regions of North Atlantic Deep Water formation (Stubbins et al., 2015), vs. what is sorbed to sinking particles and buried on the vast Arctic continental shelves. Measures of Py-POC in Arctic Ocean sediments and sinking particles (using CTO-375 method) suggest that Py-POC quickly sinks and is stored within coastal margins (Fang et al., 2016). To understand loss terms and PyC cycling, the relative proportion of all component of the PyC continuum in the dissolved and particulate phases during transfer to and within ocean systems must be further explored.

The percentage of the total PyC flux from these rivers in the particulate phase varies dramatically throughout the hydrograph. During low-flow stages (<2 × 10⁴ m³ s ⁻¹), low-temperature Py-POC was 3.1 ± 2.3% of the total PyC flux in the Yenisei River, while during high flow low-temperature Py-POC was 5.1 ± 2.8% of this flux. This is much less than a calculated distribution of Py-POC in worldwide flux estimates (~16% of total PyC flux from rivers; Santín et al., 2016). This is also lower than previous estimates of Py-POC fluxes from the pan-Arctic (~8% of total PyC flux) using pan-Arctic Py-POC flux estimates calculated using BC/POC ratios from Elmquist et al. (2008) and pan-Arctic POC fluxes from McClelland et al. (2016) and pan-Arctic Py-DOC fluxes from Stubbins et al. (2015). However, as our estimates measure in-situ phase distribution based on biomarker composition within rivers, they can represent a conservative estimate of the amount of Py-POC exported from the studied Arctic Rivers. Additionally, previous Py-POC flux estimates from Arctic Rivers rely on measurements of the most refractory PyC (using the CTO-375 method; see Figure 1 for range of detection within the PyC continuum) in estuarine sediments to estimate in-situ Py-POC fluxes (see Elmquist et al. (2008) for further explanation of previous Py-POC flux calculations). As refractory PyC analytical methods (such as CTO-375 method) may overestimate PyC contribution in environmental samples (Hammes et al., 2007), higher temperature PyC can be derived from both fossil fuel combustion and biomass burning, and the most refractory portion of the PyC continuum measured by these techniques may be more particle associated than other forms of PyC, the previously reported flux estimates may overestimate the actual distribution of PyC transported from Arctic Rivers. In environments where POC export is a proportionally important part of the TOC flux, the relative influence of Py-POC on the total export of PyC from the region may be of higher significance. For example, in the Great Whale River, total POC export accounts 18.9% of the total OC export (Hudon et al., 1996), slightly higher than the average Arctic River system (~15%; Holmes et al., 2012; McClelland et al., 2016). In this river system, Py-POC was 8.9 ± 17.5% of the total PyC flux (Figure 3; Table 3). The export of low-temperature PyC varied with the hydrograph (Figure 3), implying that the export of PyC in the particulate phase varies throughout the year. This is important to consider when looking at PyC in deposited sedimentary records and estimating the impact of fire on past climate, as the records of these signatures may vary with hydrographic regimes and particulate matter loadings.

Considering the changing fire regimes in the Arctic (Kelly et al., 2015), the intrinsic tie of Py-DOC with discharge in the region (Myers-Pigg et al., 2015; Stubbins et al., 2015), and changing hydrological regimes (Wrona et al., 2016), the transport of PyC from Arctic terrestrial to aquatic systems is likely to increase. As the Arctic Ocean continental margins have recently been suggested as an efficient location for PyC burial (Fang et al., 2016), understanding the phase distribution of PyC during transport through rivers and estuaries is relevant for understanding overall PyC fluxes, the potential degradation of PyC during transit, and losses of PyC from the active carbon cycle through burial into sediments.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.