The impact of land-fast ice on the distribution of terrestrial dissolved organic matter in the Siberian Arctic shelf seas

Abstract

Abstract. Remobilization of soil carbon as a result of permafrost degradation in the drainage basin of the major Siberian rivers combined with higher precipitation in a warming climate potentially increase the flux of terrestrial derived dissolved organic matter (tDOM) into the Arctic Ocean. The Laptev (LS) and East Siberian Seas (ESS) receive enormous amounts of tDOM-rich river water, which undergoes at least one freeze-melt cycle in the Siberian Arctic shelf seas. To better understand how freezing and melting affect the tDOM dynamics in the LS and ESS, we sampled sea ice, river and seawater for their dissolved organic carbon (DOC) concentration and the colored fraction of dissolved organic matter. The sampling took place in different seasons over a period of 9 years (2010–2019). Our results suggest that the main factor regulating the tDOM distribution in the LS and ESS is the mixing of marine waters with freshwater sources carrying different tDOM concentrations. Of particular importance in this context are the 211 km3 of meltwater from land-fast ice from the LS, containing ~ 0.3 Tg DOC, which in spring mixes with 245 km3 of river water from the peak spring discharge of the Lena River, carrying ~ 2.4 Tg DOC into the LS. During the ice-free season, tDOM transport on the shelves takes place in the surface mixed layer, with the direction of transport depending on the prevailing wind direction. In winter, about 1.2 Tg of brine-related DOC, which was expelled from the growing land-fast ice in the LS, is transported in the near-surface water layer into the Transpolar Drift Stream that flows from the Siberian Shelf toward Greenland. The actual water depth in which the tDOM-rich brines are transported, depends mainly on the density stratification of the LS and ESS in the preceding summer and the amount of ice produced in winter. We suspect that climate change in the Arctic will fundamentally alter the dynamics of tDOM transport in the Arctic marginal seas, which will also have consequences for the Arctic carbon cycle.

Insights Into Water Mass Origins in the Central Arctic Ocean From In‐Situ Dissolved Organic Matter Fluorescence

“…LSHW temperatures are very close to freezing (for the most part within 0.3°C of freezing for their given salinity), which confirms the role that brine release during sea‐ice formation has on shaping the properties of these waters (Aagaard et al., 1981). Mixing with sea‐ice melt water is apparent at the very surface of the profiles where both DOM fluorescence and salinity is reduced (black profile in Figure 4) and is consistent with a recent study with extensive coverage of the Siberian shelf water (Hölemann et al., 2021).…”

Insights into Water Mass Origins in the Central Arctic Ocean from in-situ Dissolved Organic Matter Fluorescence.

“…LSHW temperatures are very close to freezing (for the most part within 0.3°C of freezing for their given salinity), which confirms the role that brine release during sea-ice formation has on shaping the properties of these waters (Aagaard et al, 1981). Mixing with sea-ice melt water is apparent at the very surface of the profiles where both DOM fluorescence and salinity is reduced (black profile in Figure 4) and is consistent with a recent study with extensive coverage of the Siberian shelf water (Hölemann et al, 2021).…”

Comment on bg-2020-462

“…In the following summer, when the land-fast ice melts, the tDOM-poor meltwater mixes with the ambient seawater and thus could dilute the tDOM concentration of the surface mixed layer (Mathis et al, 2005;Logvinova et al, 2016). On the inner shelf the melting starts close to the Lena Delta in June and progresses eastward while the seaward edge of the landfast-ice moves slowly southward (Selyuzhenok et al, 2015). In general, the ice in the southeastern LS retreats completely by the end of July.…”

“…northeastern LS shelf in March-April (Kwok and Morrison, 2011). At the continental margin residual eastward velocities can reach more than 20 cm s -1 (Janout et al, 2015). The tDOM-rich brine formed in the southeast LS in December could thus be transported over several hundred kilometres in a northeasterly direction until the end of June, when the land-fast ice melts.…”

“…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) (Juhls et al, 2020). The LS additionally receives freshwater from the outflow of the Kara Sea (KS), which transports river water from Ob and Yenisey through the Vilkitzky Strait into the northwestern LS (Janout et al, 2015).…”