The impact of the freeze–melt cycle of land-fast ice on the distribution of dissolved organic matter in the Laptev and East Siberian seas (Siberian Arctic)

Abstract

Abstract. 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, we determined DOC concentrations and the optical properties of coloured dissolved organic matter (CDOM) in sea ice, river water and seawater. 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, our 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 (containing ∼ 0.3 Tg DOC) with more than 200 km3 of high-tDOM Lena River water discharged during the spring freshet (∼ 2.8 Tg DOC yr−1) 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.

Using Remotely Sensed Sea Surface Salinity and Colored Detrital Matter to Characterize Freshened Surface Layers in the Kara and Laptev Seas during the Ice-Free Season

“…We expect that the changes in the SSS-CDM relationship over time are primarily related to sampling limitations (i.e., as the data availability varied from year to year due to cloud coverage). The overall agreement between CDM and SSS observed by satellite in low-salinity waters was in line with the negative linear correlation observed by in situ profiles between CDOM absorption and salinity close to the river mouth by [38,40]. Using a CDOM product would have been a viable alternative option for this study, as it is a good descriptor for tracing freshwater.…”

“…Local ice melt water should have a low CDM signature; therefore, this would allow us to distinguish between sea ice melt water and river water, as detailed in [73] where they used SSS and Ocean Color data to discriminate between water masses. However, the analysis of isotope characteristics of freshened surface layers in the Kara Sea has revealed that its volume is composed of river water directly mixed with saline water [40,[74][75][76]. During late summer and autumn, the large river discharge determines the freshwater balance, while the contribution of sea ice melt is negligible [48].…”

“…Negative linear correlations between the SSS and CDM, K490, or CDOM have frequently been reported for numerous river estuaries, as the river water is rich in dissolved and suspended matter and is fresh, having a diluting effect as a result of entrainment with saline seawater. Although physical, chemical, and biological processes across the fluvial-marine transition zone (e.g., mixing with more than one endmember, influence of landfast sea ice melt, photochemical oxidation, or microbial utilization) can cause deviations from the linear conservative mixing [38][39][40], the negative linear correlation between SSS and optical parameters allows us to infer one variable by using the other as a proxy.…”

Serpentine (Floating) Ice Channels and their Interaction with Riverbed Permafrost in the Lena River Delta, Russia

“…The interruption of flow through channels in the northern part of the Lena River Delta (Tumatskaya Channel) effectively turns off the winter under-ice freshwater supply to northern coastal waters. This may explain the observed high salinity (>20) and low turbidity of upper water beneath the landfast ice in winter compared to the outflow region east of the delta (Wegner et al, 2017;Hölemann et al, 2021). Furthermore, blocked channels probably play an important role for ice jams in the early stage of the freshet in spring (Beltaos et al, 2012;Kozlov and Kuleshov, 2019).…”

Dissolved organic matter characterization in soils and streams in a small coastal low-Arctic catchment

“…All DOC samples have been filtered through glass fiber filters with 0.7 µm nominal pore size. The DOC concentration and δ 13 C DOC were measured with an Aurora 1030 DOC analyzer (OI Analytical, USA) connected via a Conflow V to an isotope ratio mass spectrometer (IRMS, Delta V, Thermo Scientific, Germany) at the University of Hamburg (Germany) (stream water incubation samples), the Alfred Wegener Institute Helmholtz Centre for Polar Research (Bremerhaven, Germany) (bulk porewater samples) following Hölemann et al (2021) and at North Carolina State University (Raleigh, North Carolina, USA) (stream water bulk samples and porewater incubation samples). DOC concentrations were used to quantify the total amount of OC in a dissolved state within the watershed systems, and δ 13 C DOC was used to derive the origin and relative degradation state of OM.…”