Flux of Dissolved and Particulate Low-Temperature Pyrogenic Carbon from Two High-Latitude Rivers across the Spring Freshet Hydrograph

Allison N Myers-Pigg,Roman Teisserenc,Patrick Louchouarn

doi:10.3389/fmars.2017.00038

Abstract

A number of recent studies have documented that pyrogenic carbon (PyC) is an integral and significant proportion of DOM in worldwide rivers. This material originates from all fractions of the PyC continuum, from highly condensed PyC to more functionalized components that retain some structural identity of fuel molecules. Understanding the transfer of PyC to river systems is paramount for Arctic regions, given the projected increase in frequency and intensity of forest fires within these ecosystems. However, the environmental distribution and concentration of soluble and particulate PyC, parameters that govern the overall fate of PyC in aquatic systems, has so far been unstudied. Here, we analyze the concentration and phase distribution of the anhydrosugar biomarker levoglucosan, as a proxy for low-temperature PyC, in two high-latitude river systems: a small sub-Arctic Canadian river, the Great Whale River in northern Quebec, and the largest Arctic River, the Yenisei River in north-central Siberia. Low-temperature PyC, as estimated by levoglucosan concentrations, is exported predominantly in the dissolved phase. Peak export of low-temperature PyC occurs during the spring freshet period in both rivers. Seasonal variability of dissolved and particulate PyC export in each river elucidated that the export of PyC in the particulate and dissolved phases were temporally decoupled throughout the peak discharge events. While the present work confirms that levoglucosan is exported in particulate phase at a high enough level to enter sedimentary deposits and record historical wildfire signatures, as the phase distribution varies between rivers and during different flow regimes, spatial and temporal differences may affect the usage of levoglucosan as a PyC proxy in depositional settings.

Highlights

Vegetation fires affect carbon cycling in all major earth systems
The timing of peak export of low-temperature Py-DOC and Py-Particulate organic carbon (POC) were decoupled in the Yenisei River throughout the freshet period (Figure 3)
Free monomeric lignin phenol ratios in both phases, 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 pyrogenic carbon (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)

Summary

Introduction

Vegetation fires affect carbon cycling in all major earth systems. These fires release 1.6–2.8 petagrams of carbon (Pg C) to the atmosphere worldwide (Santín et al, 2016), mostly as gaseous CO2. At the highest range of the temperature continuum (>500◦C), PyC is characterized by increasing graphene-like structures in highly condensed turbostratic aromatics, marked by the increase in condensation levels of molecular markers such as BPCAs (Keiluweit et al, 2010; Schneider et al, 2011) This PyC-derived turbostratic carbon is relatively unordered (Nguyen et al, 2010) and is structurally different than graphite found in natural environments, which is derived from high temperature and pressure conditions (such as within metamorphic rock) and is likely more ordered and highly crystalline (Brandes et al, 2008). As the lability of organic matter decreases along the PyC continuum (Ascough et al, 2011), the reactivity potential of PyC in the environment decreases with higher combustion temperatures (Masiello, 2004)

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Results

Conclusion