Seasonal and spatial variations in riverine DOC exports in permafrost-dominated Arctic river basins

Abstract

• Seasonality of riverine DOC exports is strongly associated with hydrological regimes. • The extent and degradation of permafrost regulate riverine DOC mobilization. • Water temperature is an important factor influencing riverine DOC exports. Climate warming is accelerating the release of voluminous organic carbon from thawing permafrost into the Arctic Ocean via riverine transport. However, the seasonal variations in riverine dissolved organic carbon ( DOC ) exports in Arctic river basins with different areal extents of permafrost and how changes in water temperature ( T w ) impact seasonal DOC exports are not fully understood. In this study, the concentrations, ages and seasonality of riverine DOC in the estuaries of six major Arctic rivers (Ob, Yenisei, Lena, Kolyma, Yukon and Mackenzie) were analysed using Arctic Great Rivers Observatory (ArcticGRO) datasets from 2003 to 2019. The results showed that DOC concentrations generally increased with the increases in the streamflow, but always dropped to the minimum with the oldest Δ 14 C-DOC ages (as old as 1650 years BP) in the freezing period (November–April), when the streamflow originates predominantly from groundwater. During the flood pulse period (May or June), a rapid increase in riverine DOC concentration with younger organic carbon (Δ 14 C values from −61 to 152‰) was observed, likely associated with snowmelt-dominated runoff regimes (lower δ 18 O-H 2 O of approximately −20.4 ± 1.6‰). During the ice-free period (June–September), DOC concentrations decreased due to the enhanced dilution of streamflow from precipitation. In the Lena and Kolyma River basins with large areal extents of continuous permafrost, over 70% of DOC flux exported during the ice-free period originated from DOC sources from ∼410 years and ∼230 years BP to the present, respectively; this suggests that greater permafrost extents restrict the release of older DOC into rivers. However, riverine DOC exports likely respond positively to changes in T w during the ice-free period. In addition, such a positive response is likely to be enhanced in basins with larger percentages of sporadic permafrost, thicker active layers, more precipitation and less soil organic carbon. Ultimately, under a warming climate, riverine DOC exports are expected to rise with increasing river water temperatures.