Final Report Summary - ACTIVE PERMAFROST (Activation of old carbon from thawing permafrost in Arctic Siberia) During the ARCTIC PERMAFROST project, Dr. Salvadó studied the sequestration of OC by reactive iron (OC-Fe). The OC-Fe ranged between 0.5 and 22% on the Eurasian Arctic Shelf, with higher values in the Kara Sea (KS) (18 ± 6%) and the Laptev Sea (LS) (14 ± 4%). The D14C/d13C signatures of the OC-Fe were substantially older and more terrestrial than the bulk sediment OC in the LS but younger and more dominated by marine plankton sources in the East Siberian Sea (ESS). Statistical source apportionment modeling revealed that reactive iron phases resequestered 15 ± 5% of thawing PF/C in the LS and 6.4 ± 5% in the ESS, derived from both coastal erosion of ice complex deposit and thawing topsoil. This Fe-associated trap of PF/C constitutes a reduction of the degradation/outgassing and thus also an attenuation of the PF/C-climate feedback. These findings thus have implications for the carbon cycle in the warming Arctic. Further studies should seek to elucidate structural composition of the remobilized PF/C fraction that gets associated with reactive iron phases as well as the nature of these interactions.Furthermore, he studied the bulk isotope (d13C and D14C) and macromolecular (lignin-derived phenols) composition of the cross-shelf exported organic carbon (OC) in different marine pools. These results showed that the Lena River and the DOC may have a preferential role in the transport of Terr-OC to the outer shelf. DOC concentrations (740-3600 μg/L) were one order of magnitude higher than POC (20-360 μg/L), with higher concentrations towards to the Lena River plume. The d13C signatures in the three carbon pools varied from -23.9±1.9‰ in the SOC, -26.1±1.2‰ in the DOC and -27.1±1.9‰ in the POC. The D14C values ranged between -395±83‰ (SOC), -226±92‰ (DOC) and -113±122‰ (POC). These stable and radiocarbon isotopes were also different between the Laptev Sea and the East Siberian Sea. Both DOC and POC showed a depleted and younger trend off the Lena River plume. Moreover, the Pacific inflow and the sea ice coverage, which works as a barrier preventing the input of “young” DOC and POC, seem to have a strong influence in these carbon pools, presenting older and more enriched d13C signatures under the sea ice extent. The high abundance of Terr-OC in the outer ESAS, particularly in the dissolved and sedimentary carbon pools, is a clear indicator of the magnitude of shelf to basin transport. Taken together, the results suggest that DOC, POC and SOC are composed of partially different Terr-OC. While DOC is strongly affected by buoyant freshwater plumes transporting young Terr-OC from topsoil and/or recently produced vascular pant material, near-bottom POC and SOC carries off-shelf old OC released from thawing permafrost.Dr. Salvadó also analysed polybrominated diphenyl ethers (PBDEs) and organochlorine pesticides (OCPs) in the water masses of the Arctic Ocean. 14PBDE concentrations in the Polar Mixed Layer (PML; a surface water mass) ranged from 0.3 to 11.2 pg·L−1, with higher concentrations in the pan-Arctic shelf seas and lower levels in the interior basin. BDE-209 was the dominant congener in most of the pan-Arctic areas except for the ones close to North America, where penta-BDE and tetra-BDE congeners predominate. In deep-water masses, 14PBDE concentrations were up to 1 order of magnitude higher than in the PML. Whereas BDE-209 decreased with depth, the less-brominated congeners, particularly BDE-47 and BDE-99, increased down through the water column. Likewise, concentrations of BDE-71 -a congener not present in any PBDE commercial mixture- increased with depth, which potentially was the result of debromination of BDE-209. The inventories in the three water masses of the Central Arctic Basin (PML, intermediate Atlantic Water Layer, and the Arctic Deep Water Layer) were 158 ± 77 kg, 6320 ± 235 kg and 30800 ± 3100 kg, respectively. The total load of PBDEs in the entire Arctic Ocean showed that only a minor fraction of PBDEs emissions are transported to the Arctic Ocean. These findings represent the first PBDE data in the deep-water compartments of an ocean.Finally, during the ARCTIC PERMAFROST project Dr. Salvadó assessed the concentrations, fluxes and sources of soot black carbon (SBC) -the most refractory component of black carbon- in the East Siberian Arctic Shelf (ESAS). SBC concentrations in the contemporary shelf sediments ranged from 0.01 to 0.21% dw, corresponding to 2-12% of total organic carbon. The 210Pb-derived fluxes of SBC (0.42-11 g·m-2·yr-1) were much higher or in the same range as fluxes reported for marine areas closer to anthropogenic emissions. The total burial flux of SBC in the ESAS (~4,000 Gg/yr) shed light on the great importance of this Arctic shelf in the sequestration of SBC. Radiocarbon isotopes of the SBC showed more uniform and depleted signatures (-721 to -896‰; average of -774±62‰) than of the non-SBC pool (-304 to -728‰; average of 491±163‰), suggesting that SBC was coming from an older and more specific source. He estimated that the atmospheric BC input to the ESAS is negligible (~0.6% of the SBC burial flux). Furthermore, statistical source apportionment modeling revealed that sedimentary SBC was composed by leached-out surface soil permafrost (topsoil/PF; 25±8%) and Pleistocene ice complex deposits (ICD/PF; 75±8%) mobilized from thawing permafrost. The SBC contribution in the mobilized permafrost carbon (PF/C) enhanced with increasing distance from the coast (from 5 to 14%), the opposite pattern than the observed for SBC concentrations in the same sediment samples, indicating that PF/C is being degraded across the shelf transport. These results elucidate for the first time the key role of climate warming and the consequent thawing permafrost in the transport of SBC to the Arctic Ocean. With ongoing global warming, these findings may have implications for the biogeochemical carbon cycle, increasing the size of this refractory pool of organic carbon in the Arctic Ocean. Further studies are needed to constrain the export of pyrogenic terrestrial organic carbon versus non-pyrogenic organic carbon to the Arctic Ocean and the biogeochemical consequences of ongoing climate and environmental change in the northern region.
