Arctic rivers are known to export large quantities of carbon by discharge of dissolved and particulate organic matter, and in a warming and progressively moister Arctic, these exports may increase resulting in a reduction of continental carbon stocks in the region. In particular, mobilization of fossil carbon from terrestrial reservoirs, stored predominantly in Yedoma deposits, will result in a net carbon loss. Therefore, the radiocarbon (14C) contents of carbon exported via rivers are of great interest to understand the on-going processes. Recent work has shown that both particulate and dissolved organic matter exported by the Lena, one of the great rivers draining Siberian permafrost regions into the Laptev Sea, consists of a complex mixture of material derived from multiple sources (e.g., Winterfeld et al., 2015, Dubinenkov et al., 2014). Organic matter derived from the different sources likely differs in its reactivity once released from the frozen deposits into the river waters. For example, it has been shown that ancient carbon is very rapidly respired, leading to predominantly modern 14C signatures of dissolved organic carbon (DOC) in Arctic river waters discharged to the ocean (Mann et al., 2015). Arctic rivers are characterized by highly variable discharge rates with a pronounced maximum during the spring freshet associated with highest concentrations of DOC and particulate organic carbon (POC). Most studies investigating the isotopic composition and quality of carbon exported by Arctic rivers, however, rely on samples taken in summer during base flow, which is due to the logistical challenges associated with sampling in the remote Siberian permafrost regions. Here we present a record of δ13C and ∆14C of DOC and POC collected between late May during the freshet and late August 2014 in the Lena Delta, and compare them with δ13C and ∆14C of DOC and POC sampled in central Siberia. The latter represent the hinterland of the large rivers, while the Lena Delta data are considered to contain an integrated signal of the watershed. The central Siberian POC is generally younger than the Lena Delta POC in spring. Throughout spring and summer, POC becomes progressively older in central Siberia, while an initial trend towards older values in the spring samples from the Lena Delta is reversed in summer, associated with a shift towards more depleted δ13C values. We interpret these aging trends as reflecting progressive thawing throughout the ice-free season, resulting in mobilization of progressively older carbon from deeper thawed layers. The summer reversal indicates admixture of fresh organic matter. We furthermore analysed the biomarker composition of Lena Delta particulate organic matter collected in spring and summer. From spring to summer, we observe trends in abundance of individual leaf-wax derived biomarkers indicating higher abundance of algal biomass in the summer particles. Trends in biomarkers associated with soil microbes suggest a shift in sources through the ice-free season. Our data illustrate that considering the seasonal evolution of carbon discharge from Arctic rivers will be required to understand the underlying mechanisms and to predict future changes. Dubinenkov, I., R. Flerus, P. Schmitt-Kopplin, G. Kattner, B.P. Koch (2014): Origin-specific molecular signatures of dissolved organic matter in the Lena Delta. Biogeochemistry, doi: 10.1007/s10533-014-0049-0 Mann, P.J., T.I. Eglinton, C.P. McIntyre, N. Zimov, A. Davydova, J.E. Vonk, R.M. Holmes, R.G.M. Spencer (2015): Utilization of ancient permafrost carbon in headwaters of Arctic fluvial networks. Nature communications, doi: 10.1038/ncomms8856 Winterfeld, M., T. Laepple, G. Mollenhauer (2015): Characterization of particulate organic matter in the Lena River delta and adjacent nearshore zone, NE Siberia – Part I: Radiocarbon inventories. Biogeosciences, doi: 10.5194/bg-12-3769-2015