Freshwater marshes are prevalent and important stores of carbon. They bury carbon in deeper soils, although reported rates of carbon accumulation are significantly higher over recent (decadal) versus longer (centennial and millennial) timescales. Intrinsic organic matter degradation, long-term climatic and ecological changes, and recent anthropogenic impacts on sediment fluxes and organic matter production may have a role in explaining this discrepancy, yet remain poorly understood for freshwater marshes. We collected a 4-m core from a riverine-influenced marsh in the watershed of Big Creek which drains into Lake Erie in southern Ontario, Canada, and conducted radiometric dating, elemental analyses, and programmed pyrolysis for organic matter characterization. Over the past 5,710 calibrated years, burial of organic (on average 26 ± 34 g C m-2 yr-1) and inorganic (22 ± 25 g C m-2 yr-1) carbon fractions has resulted in high rates of carbon accumulation. We found that elevated recent rates of organic carbon accumulation are driven by fractions that have low thermal stability and are predominantly from aquatic sources. This type of organic carbon is buried intermittently in deeper marsh sediments and corresponds to major hydro-fluvial events (e.g., Nipissing highstands), which coincide with regional marsh development. We deduce that lower fractions of labile carbon in deeper soils reflect long-term degradation, which underscores the notion that high recent rates of carbon accumulation are generally not sustained over centuries and millennia. Our research demonstrates the importance of identifying various carbon fractions in understanding carbon burial in freshwater marsh soils, and informing marsh conservation.

Peatland soils are of great interest for study and management because of their high carbon contents and known role in the global carbon cycle. However, carbon stocks have yet to be constrained in many wetland ecosystems. Relative to bogs, fens and saline coastal ecosystems, less is known about carbon stocks in freshwater marsh soils despite their global prevalence, and it is not well understood how disturbance of freshwater marshes may affect carbon-climate dynamics. To better understand the potential for freshwater marshes to be net carbon sinks, we review how freshwater marshes and associated soils are classified, and synthesize available data on short- and long-term rates of carbon accumulation in freshwater marsh soils in temperate North America. Although often described as mineral-based, our findings suggest that freshwater marshes are not restricted to mineral substrates, and that inconsistencies in classification may underestimate presumed carbon stocks. Organic carbon contents and bulk density measurements are highly variable, and can range between 1-45% and 0.04-1.5 g cm-3, respectively. Moreover, rates of carbon accumulation in freshwater marshes are often measured over recent time scales (50-100 years; on average 155 +/- 74 g C m-2 yr-1), while long-term rates (measured over centuries and millennia; on average 51 +/- 38 g C m-2 yr-1) are much more scarce. We suspect that short-term rates are markedly greater than long-term rates of carbon accumulation because they do not account for long-term carbon loss and may reflect large increases in sedimentation since European settlement in North America. However, we also suspect that long-term carbon storage in freshwater marsh soils is underestimated, and that freshwater marshes can have long-term rates of carbon accumulation similar to those reported for temperate peatlands. In this presentation, we will show that variability of rates of carbon accumulation, rates of sediment accretion, bulk density and organic carbon content in freshwater marshes needs to be better constrained in order to accurately quantify their regional and global carbon pools. We will discuss the importance for scientists to specify timeframes over which they are measuring rates of carbon accumulation so that the capacity for wetlands to be net carbon sinks can be correctly understood.