Fate of colloids during estuarine mixing in the Arctic

Abstract

Abstract. The estuarine behavior of organic carbon (OC) and trace elements (TE) was studied for the largest European sub-Arctic river, which is the Severnaya Dvina; this river has a deltaic estuary covered in ice during several hydrological seasons: summer (July 2010, 2012) and winter (March 2009) baseflow, and the November–December 2011 ice-free period. Colloidal forms of OC and TE were assessed for three pore size cutoffs (1, 10, and 50 kDa) using an in situ dialysis procedure. Conventionally dissolved (< 0.22 μm) fractions demonstrated clear conservative behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of freshwater with the White Sea; a significant (up to a factor of 10) concentration increase occurs with increases in salinity. Si and OC also displayed conservative behavior but with a pronounced decrease in concentration seawards. Rather conservative behavior, but with much smaller changes in concentration (variation within ±30%) over a full range of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE. Strong non-conservative behavior with coagulation/removal at low salinities (< 5‰) was exhibited by Fe, Al, Zr, Hf, and light REE. Finally, certain divalent metals exhibited non-conservative behavior with a concentration gain at low (~ 2–5‰, Ba, Mn) or intermediate (~ 10–15‰, Ba, Zn, Pb, Cd) salinities, which is most likely linked to TE desorption from suspended matter or sediment outflux. The most important result of this study is the elucidation of the behavior of the "truly" dissolved low molecular weight LMW< 1 kDa fraction containing Fe, OC, and a number of insoluble elements. The concentration of the LMW fraction either remains constant or increases its relative contribution to the overall dissolved (< 0.22 μm) pool as the salinity increases. Similarly, the relative proportion of colloidal (1 kDa–0.22 μm) pool for the OC and insoluble TE bound to ferric colloids systematically decreased seaward, with the largest decrease occurring at low (< 5‰) salinities. Overall, the observed decrease in the colloidal fraction may be related to the coagulation of organo-ferric colloids at the beginning of the mixing zone and therefore the replacement of the HMW1 kDa–0.22 μm portion by the LMW< 1 kDa fraction. These patterns are highly reproducible across different sampling seasons, suggesting significant enrichment of the mixing zone by the most labile (and potentially bioavailable) fraction of the OC, Fe and insoluble TE. The size fractionation of the colloidal material during estuarine mixing reflects a number of inorganic and biological processes, the relative contribution of which to element speciation varies depending on the hydrological stage and time of year. In particular, LMW< 1 kDa ligand production in the surface horizons of the mixing zone may be linked to heterotrophic mineralization of allochthonous DOM and/or photodestruction. Given the relatively low concentration of particulate versus dissolved load of most trace elements, desorption from the river suspended material was less pronounced than in other rivers in the world. As a result, the majority of dissolved components exhibited either conservative (OC and related elements such as divalent metals) or non-conservative, coagulation-controlled (Fe, Al, and insoluble TE associated with organo-ferric colloids) behavior. The climate warming at high latitudes is likely to intensify the production of LMW< 1 kDa organic ligands and the associated TE; therefore, the delivery of potentially bioavailable trace metal micronutrients from the land to the ocean may increase.

Organic carbon remobilized from thawing permafrost is resequestered by reactive iron on the Eurasian Arctic Shelf

“…These results are in accordance with previous studies suggesting that OC derived from erosion of thawing permafrost may have short residence times in the surface water and quickly settle to the sea floor Vonk et al, 2010aVonk et al, , 2010b. This data are in agreement with previous studies showing the important role of iron oxides in the transfer of dissolved OC (DOC) from the water column to the sediment [Barber et al, 2014;Gustafsson et al, 2000;Ingri et al, 2004;Pokrovsky and Schott, 2002;Pokrovsky et al, 2014;Riedel et al, 2013;Tipping, 1981]. Chlorophyll and particulate Fe have also been reported to be correlated in sub-Arctic water columns [Gelting et al, 2010].…”

“…Chlorophyll and particulate Fe have also been reported to be correlated in sub-Arctic water columns [Gelting et al, 2010]. This data are in agreement with previous studies showing the important role of iron oxides in the transfer of dissolved OC (DOC) from the water column to the sediment [Barber et al, 2014;Gustafsson et al, 2000;Ingri et al, 2004;Pokrovsky and Schott, 2002;Pokrovsky et al, 2014;Riedel et al, 2013;Tipping, 1981].…”

“…The highest concentrations were found off the Yenisey and Lena rivers' mouths, in the shelves of the Kara and Laptev Seas, where the OC-Fe represents 18 ± 6% and 14 ± 4% respectively, of the bulk sedimentary OC (Figure 1a and Table S2). These results are consistent with the proportion of OC linked to iron oxides coagulated in the estuary [Pokrovsky et al, 2014] and support the conceptual model in soils that the dominant form of iron shifts from iron oxides to precipitated organo-Fe complexes with increasing OC supply [Schwertmann et al, 1986]. In marine sediment ferric iron can not only work as an electron acceptor in the respiration of OC [Burdige, 1993] but also enhances the potential preservation of sedimentary OC by coating of iron phases by organic matter [Schwertmann, 1966].…”

Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats

“…REE behave almost conservative in organic-rich systems [31] and only a small percentage of the suspended particles and colloids are trapped in the estuary [31,49], while the remaining particles and colloids enter the open oceans. In marked contrast, in systems dominated by inorganic particles, REE behave non-conservatively [31,46,47,50] and >98% of the suspended material is trapped in the estuary [46,51]. Rousseau et al [50] showed that leached particulate REE from the Amazonas river contribute significantly to the dissolved REE budget and the Nd isotopic composition of the modern oceans, indicating that riverine REE input is an important source for the global modern oceans and its Nd isotope compositions.…”

Organic Matter and Mineral Interactions Modulate Flocculation Across Arctic River Mixing Zones

“…In some studies it was claimed that riverine DOM behaves conservatively, which means that DOM is a simple dilution and DOM concentration decreases linearly with increases in water salinity (Pokrovsky et al, ; Stein et al, ). A negligible increase in DOM concentrations at low salinity is considered to be an outlier of measured values and is not considered (Köhler et al, ).…”