Rapid Arctic warming accelerates permafrost thaw releasing aged organic matter (OM) to inland
aquatic ecosystems and ultimately, after transport via estuaries or deltas, to the Arctic Ocean
nearshore. Despite the importance of Arctic deltas, their functioning is still poorly studied. Here, we
examined seasonal fluctuations and spatial differences in the quantity and composition of OM in
the Lena Delta, measuring dissolved and particulate organic carbon (DOC and POC) concentrations,
carbon isotopes (δ13C and Δ14C), and total suspended matter (TSM). We compared deltaic POC to
the POC in the Lena River main stem over a ~1600 km transect, from Yakutsk to the Lena Delta. We
further examined and compared dynamics of DOC and POC in summer and winter across a ~140
km transect in the Lena Delta. TSM and POC concentrations decreased by 75 % during transit from
Yakutsk to the Lena Delta. 18 % of deltaic and 5 % of river main stem POC originated from Yedoma
deposits. Thus, despite lower concentrations of POC in the delta, amount of POC from Yedoma
deposits in deltaic waters were almost twice as large as in the main stem (0.07 ±0.02 and 0.04
±0.02 mg L-1, respectively). Deltaic POC was strongly depleted in 13C due to significant phytoplankton
contributions (~-68 ±6 %). Strong differences between winter and summer samples in DOC and POC
concentrations and their properties in the Lena Delta were also found. Combined analyses of DOC
and POC revealed that Pleistocene-aged Yedoma deposits were still actively degrading in winter
influencing the quantity and composition of OM of the Lena Delta and exported OC loads. Deltaic
processes control the type and amount of OM exported to the Arctic Ocean and require deeper
investigations as crucial processes for the riverine and oceans pathways in a warming Arctic