Particulate organic carbon and nitrogen export from major Arctic rivers

Abstract

Northern rivers connect a land area of approximately 20.5 million km 2 to the Arctic Ocean and surrounding seas. These rivers account for~10% of global river discharge and transport massive quantities of dissolved and particulate materials that reflect watershed sources and impact biogeochemical cycling in the ocean. In this paper, multiyear data sets from a coordinated sampling program are used to characterize particulate organic carbon (POC) and particulate nitrogen (PN) export from the six largest rivers within the pan-Arctic watershed (Yenisey, Lena, Ob', Mackenzie, Yukon, Kolyma). Together, these rivers export an average of 3055 × 10 9 g of POC and 368 × 10 9 g of PN each year. Scaled up to the pan-Arctic watershed as a whole, fluvial export estimates increase to 5767 × 10 9 g and 695 × 10 9 g of POC and PN per year, respectively.POC export is substantially lower than dissolved organic carbon export by these rivers, whereas PN export is roughly equal to dissolved nitrogen export. Seasonal patterns in concentrations and source/composition indicators (C:N, δ 13 C, Δ 14 C, δ 15 N) are broadly similar among rivers, but distinct regional differences are also evident. For example, average radiocarbon ages of POC range from~2000 (Ob') to~5500 (Mackenzie) years before present. Rapid changes within the Arctic system as a consequence of global warming make it challenging to establish a contemporary baseline of fluvial export, but the results presented in this paper capture variability and quantify average conditions for nearly a decade at the beginning of the 21st century.

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

“…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.…”

“…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).…”

“…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.…”

“…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 highlatitude 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).…”

Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition

“…The Arctic Ocean off northern Siberia receives large quantities of dissolved and particulate terrestrial organic carbon via continental runoff and coastal erosion (Alling et al, 2010;Dittmar and Kattner, 2003;McClelland et al, 2016;Sánchez-García et al, 2011;Semiletov et al, 2013;Vonk et al, 2012). The land-derived material that does not settle in the coastal zone further travels across the continental margin reaching out to the outer-shelf region resuspended within the ben- thic nepheloid layer or in suspension within the surface river plume (Fichot et al, 2013;Sánchez-García et al, 2011;Wegner et al, 2003).…”