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[1] The export and D 14 C-age of dissolved organic carbon (DOC) was determined for the Yenisey, Lena, Ob', Mackenzie, and Yukon rivers for [2004][2005]. Concentrations of DOC elevate significantly with increasing discharge in these rivers, causing approximately 60% of the annual export to occur during a 2-month period following spring ice breakup. We present a total annual flux from the five rivers of $16 teragrams (Tg), and conservatively estimate that the total input of DOC to the Arctic Ocean is 25-36 Tg, which is $5-20% greater than previous fluxes. These fluxes are also $2.5Â greater than temperate rivers with similar watershed sizes and water discharge. D 14 C-DOC shows a clear relationship with hydrology. A small pool of DOC slightly depleted in D 14 C is exported with base flow. The large pool exported with spring thaw is enriched in D 14 C with respect to current-day atmospheric D 14 C-CO 2 values. A simple model predicts that $50% of DOC exported during the arctic spring thaw is 1-5 years old, $25% is 6-10 years in age, and 15% is 11-20 years old. The dominant spring melt period, a historically undersampled period, exports a large amount of young and presumably semilabile DOC to the Arctic Ocean.

forests. The three large Siberian rivers, Lena, Yenisei, and Ob, which also have the highest 58 proportion of forests within their watersheds, contribute about 90% of the total lignin discharge 59 to the Arctic Ocean. The composition of river DOC is also characterized by elevated levels of p-60 hydroxybenzenes, particularly during the low flow season, which indicates a larger contribution 61 from mosses and peat bogs. The lignin composition was strongly related to the average 14 C-age 62 of DOC supporting the abundance of young, boreal-vegetation-derived leachates during spring 63 flood, and older, soil-, peat-, and wetland-derived DOC during groundwater dominated low flow 64 conditions, particularly in the Ob and Yukon Rivers. We observed significant differences in 65 DOC concentration and composition between the rivers over the seasonal cycles with the 66 Mackenzie River being the most unique, the Lena River being similar to the Yenisei, and the 67 Yukon being most similar to the Ob. The observed relationship between the lignin phenol 68 composition and watershed characteristics suggests that DOC discharge from these rivers could 69 increase in a warmer climate under otherwise undisturbed conditions. 70 71 4

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.

[1] Quantification of sediment fluxes from rivers is fundamental to understanding landocean linkages in the Arctic. Numerous publications have focused on this subject over the past century, yet assessments of temporal trends are scarce and consensus on contemporary fluxes is lacking. Published estimates vary widely, but often provide little accessory information needed to interpret the differences. We present a pan-arctic synthesis of sediment flux from 19 arctic rivers, primarily focusing on contributions from the eight largest ones. For this synthesis, historical records and recent unpublished data were compiled from Russian, Canadian, and United States sources. Evaluation of these data revealed no long-term trends in sediment flux, but did show stepwise changes in the historical records of two of the rivers. In some cases, old values that do not reflect contemporary fluxes are still being reported, while in other cases, typographical errors have been propagated into the recent literature. Most of the discrepancy among published estimates, however, can be explained by differences in years of records examined and gauging stations used. Variations in sediment flux from year to year in arctic rivers are large, so estimates based on relatively few years can differ substantially. To determine best contemporary estimates of sediment flux for the eight largest arctic rivers, we used a combination of newly available data, historical records, and literature values. These estimates contribute to our understanding of carbon, nutrient, and contaminant transport to the Arctic Ocean and provide a baseline for detecting future anthropogenic or natural change in the Arctic.