Carbon transport by the Lena River from its headwaters to the Arctic Ocean, with emphasis on fluvial input of terrestrial particulate organic carbon vs. carbon transport by coastal erosion

Abstract

The Lena River integrates biogeochemical signals from its vast drainage basin, and the integrated signal reaches far out over the Arctic Ocean. Transformation of riverine organic carbon (OC) into mineral carbon, and mineral carbon into the organic form in the Lena River watershed, can be considered to be quasi-steady-state processes. An increase in Lena discharge exerts opposite effects on total organic (TOC) and total inorganic (TCO₂) carbon: TOC concentration increases, while TCO₂ concentration decreases. Significant inter-annual variability in mean values of TCO₂, TOC, and their sum (total carbon, TC) has been found. This variability is determined by changes in land hydrology which cause differences in the Lena River discharge. There is a negative correlation in the Lena River between TC in September and its mean discharge in August; a time shift of about one month is required for water to travel from Yakutsk to the Laptev Sea. Total carbon entering the sea with the Lena discharge is estimated to be almost 10 Tg C yr⁻¹. The annual Lena River discharge of particulate organic carbon (POC) can be as high as 0.38 Tg (moderate to high estimate). If we instead accept Lisytsin's (1994) statement that 85–95 % of total particulate matter (PM) (and POC) precipitates on the marginal "filter", then only about 0.03–0.04 Tg of Lena River POC reaches the Laptev Sea. The Lena's POC export would then be two orders of magnitude less than the annual input of eroded terrestrial carbon onto the shelf of the Laptev and East Siberian seas, which is estimated to be about 4 Tg. Observations support the hypothesis of a dominant role for coastal erosion (Semiletov, 1999a, b) in East Siberian Arctic Shelf (ESAS) sedimentation and the dynamics of the carbon/carbonate system. The Lena River is characterized by relatively high concentrations of the primary greenhouse gases, dissolved carbon dioxide (CO₂) and methane (CH₄). During all seasons the river is supersaturated in CO₂ compared to the atmosphere, by up to 1.5–2 fold in summer, and 4–5 fold in winter. This results in a significant CO₂ supersaturation in the adjacent coastal sea. Localized areas of dissolved CH₄ along the Lena River and in the Lena delta channels may reach 100 nM, but the CH₄ concentration decreases to 5–20 nM towards the sea, which suggests that riverborne export of CH₄ plays but a minor role in determining the ESAS CH₄ budget in coastal waters. Instead, the seabed appears to be the source that provides most of the CH₄ to the Arctic Ocean.