Permafrost degradation in the catchment of major Siberian rivers, combined with higher precipitation in a warming climate, could increase the flux of terrestrially derived dissolved organic matter (tDOM) into the Arctic Ocean (AO). Each year, ∼7.9 Tg of dissolved organic carbon (DOC) is discharged into the AO via the three largest rivers that flow into the Laptev Sea (LS) and East Siberian Sea (ESS). A significant proportion of this tDOM-rich river water undergoes at least one freeze–melt cycle in the land-fast ice that forms along the coast of the Laptev and East Siberian seas in winter.
To better understand how growth and melting of land-fast ice affect dissolved organic matter (DOM) dynamics in the LS and ESS, DOC concentrations and the optical properties of coloured dissolved organic matter (CDOM) in sea ice, river water and seawater were determined. The data set, covering different seasons over a 9-year period (2010–2019), was complemented by oceanographic measurements (T, S) and determination of the oxygen isotope composition of the seawater.
Although removal of tDOM cannot be ruled out, the study suggests that conservative mixing of high-tDOM river water and sea-ice meltwater with low-tDOM seawater is the major factor controlling the surface distribution of tDOM in the LS and ESS. A case study based on data from winter 2012 and spring 2014 reveals that the mixing of about 273 km3 of low-tDOM land-fast-ice meltwater with more than 200 km3 of high-tDOM Lena River water discharged during the spring freshet plays a dominant role in this respect.
The mixing of the two low-salinity surface water masses is possible because the meltwater and the river water of the spring freshet flow into the southeastern LS at the same time every year (May–July). In addition, budget calculations indicate that in the course of the growth of land-fast ice in the southeastern LS, ∼1.2 Tg DOC yr−1 (± 0.54 Tg) can be expelled from the growing ice in winter, together with brines.
These DOC-rich brines can then be transported across the shelves into the Arctic halocline and the Transpolar Drift Current flowing from the Siberian Shelf towards Greenland. The study of dissolved organic matter dynamics in the AO is important not only to decipher the Arctic carbon cycle but also because CDOM regulates physical processes such as radiative forcing in the upper ocean, which has important effects on sea surface temperature, water column stratification, biological productivity and UV penetration.
The mean annual air temperature in the Arctic continues to rise, resulting in a rapid decrease in summer sea-ice extent and volume, and a longer ice-free season. Furthermore, the terrestrial permafrost temperature is increasing, with major implications for the Arctic carbon cycle. Due to the accelerated degradation of terrestrial permafrost, an estimated 1035 ± 150 Pg of organic carbon stored in the upper 3 m of circumpolar permafrost soils can be either mineralised and mobilised as tDOM into the hydrosphere or released as gaseous emissions into the atmosphere.
The release of soil carbon into the hydrosphere in combination with an increasing freshwater discharge from Arctic rivers might thus increase the flux of tDOM into the ocean. Changes in the land–ocean fluxes of tDOM in the Arctic are of particular importance for the global carbon cycle since the rivers at high northern latitudes export significant quantities of tDOM to the AO.
Dissolved organic matter (DOM) is typically quantified via its carbon content (dissolved organic carbon; DOC), which contributes roughly half to the total mass of DOM. Currently, the annual riverine input of DOC into the AO is about 25–36 Tg C yr−1, with the six largest Arctic rivers discharging about 18–20 Tg C yr−1. The three major Siberian river systems (Ob, Yenisey and Lena) account for about 14 Tg C yr−1, with the Lena River alone discharging 6.8 Tg C yr−1 DOC into the Siberian Laptev Sea (LS).
The total riverine DOC flux to the LS is about 8.3 Tg C yr−1, with the Lena drainage basin (2.61×106 km2) accounting for about 70 % of the total area of the LS watershed. The LS additionally receives freshwater from the outflow of the Kara Sea (KS), which transports river water from Ob and Yenisey through the Vilkitsky Strait into the northwestern LS (Janout et al., 2015). The combined DOC flux of the two major rivers discharging to the East Siberian Sea (ESS; Kolyma and Indigirka) is ∼1.1 Tg DOC yr−1, which is ∼50 % of the total annual riverine DOC flux to the ESS (Manizza et al., 2009).
The terrigenous input (particulate and dissolved) of carbon by coastal erosion to the entire AO is estimated to be 15.4 Tg yr−1 (9.2–24.2 Tg yr−1 with 1 standard deviation; Terhaar et al., 2021). However, DOC concentrations measured in Guba Buor-Khaya bay east of the Lena Delta, a region known for the most rapid and extensive coastal erosion of the Siberian Arctic coastline, suggest that direct DOC input from coastal erosion must be small compared to the riverine DOC flux (Alling et al., 2010).
If the input of tDOM into the AO increases as a consequence of climate change, it is crucial to achieve a better understanding of tDOM transport dynamics and biogeochemical cycles. Previous studies in the Arctic marginal seas have reported a strong negative linear relationship between salinity and DOC, which implies that tDOM-rich river water mixes with DOM-poor marine waters from the AO without significant losses and gains along the salinity gradient (i.e. conservative mixing) (Kattner et al., 1999; Köhler et al., 2003; Amon, 2004; Amon and Meon, 2004; Shin and Tanaka, 2004; Gueguen et al., 2005; Matsuoka et al., 2012; Pugach and Pipko, 2013; Pavlov et al., 2016; Tanaka et al., 2016; Pugach et al., 2018).
In contrast, a number of studies indicate significant degradation of tDOM in the Arctic Ocean (Belanger et al., 2006; Alling et al., 2010;
