Description: 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.

Description: Oceanic emissions of the climate-relevant trace gases carbonyl sulfide (OCS) and carbon disulfide (CS2) are a major source to their atmospheric budget. Their current and future emission estimates are still uncertain due to incomplete process understanding and therefore inexact quantification across different biogeochemical regimes. Here we present the first concurrent measurements of both gases together with related fractions of the dissolved organic matter (DOM) pool, i.e., solid-phase extractable dissolved organic sulfur (DOSSPE, n=24, 0.16±0.04 µmol L−1), chromophoric (CDOM, n=76, 0.152±0.03), and fluorescent dissolved organic matter (FDOM, n=35), from the Peruvian upwelling region (Guayaquil, Ecuador to Antofagasta, Chile, October 2015). OCS was measured continuously with an equilibrator connected to an off-axis integrated cavity output spectrometer at the surface (29.8±19.8 pmol L−1) and at four profiles ranging down to 136 m. CS2 was measured at the surface (n=143, 17.8±9.0 pmol L−1) and below, ranging down to 1000 m (24 profiles). These observations were used to estimate in situ production rates and identify their drivers. We find different limiting factors of marine photoproduction: while OCS production is limited by the humic-like DOM fraction that can act as a photosensitizer, high CS2 production coincides with high DOSSPE concentration. Quantifying OCS photoproduction using a specific humic-like FDOM component as proxy, together with an updated parameterization for dark production, improves agreement with observations in a 1-D biogeochemical model. Our results will help to better predict oceanic concentrations and emissions of both gases on regional and, potentially, global scales.

Description: 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.

Description: The Arctic icescape is rapidly transforming from a thicker multiyear ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production. One critical challenge is to understand how productivity will change within the next decades. Recent studies have reported extensive phytoplankton blooms beneath ponded sea ice during summer, indicating that satellite-based Arctic annual primary production estimates may be significantly underestimated. Here we present a unique time-series of a phytoplankton spring bloom observed beneath snow-covered Arctic pack ice. The bloom, dominated by the haptophyte algae Phaeocystis pouchetii, caused near depletion of the surface nitrate inventory and a decline in dissolved inorganic carbon by 16 ± 6 g C m−2. Ocean circulation characteristics in the area indicated that the bloom developed in situ despite the snow-covered sea ice. Leads in the dynamic ice cover provided added sunlight necessary to initiate and sustain the bloom. Phytoplankton blooms beneath snow-covered ice might become more common and widespread in the future Arctic Ocean with frequent lead formation due to thinner and more dynamic sea ice despite projected increases in high-Arctic snowfall. This could alter productivity, marine food webs and carbon sequestration in the Arctic Ocean.

Description: A method is presented for the chemical characterization of natural organic matter (NOM). We combined reversed-phase chromatographic separation of NOM with high resolution inductively coupled plasma mass spectrometry. A desolvation technique was used to remove organic solvent derived from the preceding chromatographic separation. We applied our method to solid-phase extracted marine dissolved organic matter samples from South Atlantic and Antarctic surface waters. The method provided a direct and quantitative determination of dissolved organic phosphorus and sulfur in fractions of differing polarity and also allowed simultaneous speciation studies of trace elements. Dissolved organic carbon/phosphorus and carbon/sulfur ratios for the different chromatographic fractions of our two samples ranged between 341–3025 for C/P and 11–1225 for C/S. Differences in elemental distribution between the fractions were attributed to different biochemical environments of the samples. Sulfur was exclusively found in one hydrophilic fraction, while uranium showed a strong affinity to the hydrophobic fractions. Our method was designed to be easily adapted to other separation techniques. The elemental information will deliver valuable information for ultrahigh resolution molecular analyses.

Description: Continuing losses of multi-year sea ice (MYI) across the Arctic are causing first-year sea ice (FYI) to dominate the Arctic ice pack. Melting FYI provides a strong seasonal pulse of dissolved organic matter (DOM) into surface waters; however, the biological impact of this DOM input is unknown. Here we show that DOM additions cause important and contrasting changes in under-ice bacterioplankton abundance, production and species composition. Utilization of DOM was influenced by molecular size, with 10–100 kDa and 〉100 kDa DOM fractions promoting rapid growth of particular taxa, while uptake of sulfur and nitrogen-rich low molecular weight organic compounds shifted bacterial community composition. These results demonstrate the ecological impacts of DOM released from melting FYI, with wide-ranging consequences for the cycling of organic matter across regions of the Arctic Ocean transitioning from multi-year to seasonal sea ice as the climate continues to warm.