Contrasting composition of terrigenous organic matter in the dissolved, particulate and sedimentary organic carbon pools on the outer East Siberian Arctic Shelf
Fluvial discharge and coastal erosion of the permafrost-dominated East Siberian Arctic delivers large quantities of terrigenous organic carbon (Terr-OC) to marine waters. The composition and fate of the remobilized Terr-OC needs to be better constrained as it impacts the potential for a climate–carbon feedback. In the present study, the bulk isotope (δ13C and Δ14C) and macromolecular (lignin-derived phenols) composition of the cross-shelf exported organic carbon (OC) in different marine pools is evaluated. For this purpose, as part of the SWERUS-C3 expedition (July–September 2014), sediment organic carbon (SOC) as well as water column (from surface and near-bottom seawater) dissolved organic carbon (DOC) and particulate organic carbon (POC) samples were collected along the outer shelves of the Kara Sea, Laptev Sea and East Siberian Sea. The results show 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−1) were 1 order of magnitude higher than POC (20–360 µg L−1), with higher concentrations towards the Lena River plume. The δ13C 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 Δ14C 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. Further, 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 δ13C signatures under the sea-ice extent. Lignin phenols exhibited higher OC-normalized concentrations in the SOC (0.10–2.34 mg g−1 OC) and DOC (0.08–2.40 mg g−1 OC) than in the POC (0.03–1.14 mg g−1 OC). The good relationship between lignin and Δ14C signatures in the DOC suggests that a significant fraction of the outer-shelf DOC comes from “young” Terr-OC. By contrast, the slightly negative correlation between lignin phenols and Δ14C signatures in POC, with higher lignin concentrations in older POC from near-bottom waters, may reflect the off-shelf transport of OC from remobilized permafrost in the nepheloid layer. Syringyl ∕ vanillyl and cinnamyl ∕ vannillyl phenol ratios presented distinct clustering between DOC, POC and SOC, implying that those pools may be carrying different Terr-OC of partially different origin. Moreover, 3,5-dihydroxybenzoic acid to vanillyl phenol ratios and p-coumaric acid to ferulic acid ratios, used as a diagenetic indicators, enhanced in POC and SOC, suggesting more degradation within these pools. Overall, the key contrast between enhanced lignin yields both in the youngest DOC and the oldest POC samples reflects a significant decoupling of terrestrial OC sources and pathways.
The sources of atmospheric black carbon at a European gateway to the ArcticDownload
Black carbon (BC) aerosols from incomplete combustion of biomass and fossil fuel contribute to Arctic climate warming. Models—seeking to advise mitigation policy—are challenged in reproducing observations of seasonally varying BC concentrations in the Arctic air. Here we compare year-round observations of BC and its d13C/D14C-diagnosed sources in Arctic Scandinavia, with tailored simulations from an atmospheric transport model. The model predictions for this European gateway to the Arctic are greatly improved when the emission inventory of anthropogenic sources is amended by satellite-derived estimates of BC emis- sions from fires. Both BC concentrations (R2=0.89, P<0.05) and source contributions (R2=0.77, P<0.05) are accurately mimicked and linked to predominantly European emis- sions. This improved model skill allows for more accurate assessment of sources and effects of BC in the Arctic, and a more credible scientific underpinning of policy efforts aimed at efficiently reducing BC emissions reaching the European Arctic.
Fate of terrigenous organic matter across the Laptev Sea from the mouth of the Lena River to the deep sea of the Arctic interior
Ongoing global warming in high latitudes may cause an increasing supply of permafrost-derived organic carbon through both river discharge and coastal erosion to the Arctic shelves. Mobilized permafrost carbon can be either buried in sediments, transported to the deep sea or degraded to CO2 and outgassed, potentially constituting a positive feedback to climate change. This study aims to assess the fate of terrigenous organic carbon (TerrOC) in the Arctic marine environment by exploring how it changes in concentration, composition and degradation status across the wide Laptev Sea shelf. We analyzed a suite of terrestrial biomarkers as well as source-diagnostic bulk carbon isotopes (δ13C, Δ14C) in surface sediments from a Laptev Sea transect spanning more than 800 km from the Lena River mouth (< 10 m water depth) across the shelf to the slope and rise (2000–3000 m water depth). These data provide a broad view on different TerrOC pools and their behavior during cross-shelf transport. The concentrations of lignin phenols, cutin acids and high-molecular-weight (HMW) wax lipids (tracers of vascular plants) decrease by 89–99 % along the transect. Molecular-based degradation proxies for TerrOC (e.g., the carbon preference index of HMW lipids, the HMW acids ∕ alkanes ratio and the acid ∕ aldehyde ratio of lignin phenols) display a trend to more degraded TerrOC with increasing distance from the coast. We infer that the degree of degradation of permafrost-derived TerrOC is a function of the time spent under oxic conditions during protracted cross-shelf transport. Future work should therefore seek to constrain cross-shelf transport times in order to compute a TerrOC degradation rate and thereby help to quantify potential carbon–climate feedbacks.
Isotope-Based Source Apportionment of EC Aerosol Particles during Winter High-Pollution Events at the Zeppelin Observatory, Svalbard
Black carbon (BC) aerosol particles contribute
to climate warming of the Arctic, yet both the sources and the
source-related effects are currently poorly constrained.
Bottom-up emission inventory (EI) approaches are challenged
for BC in general and the Arctic in particular. For example,
estimates from three different EI models on the fractional
contribution to BC from biomass burning (north of 60° N)
vary between 11% and 68%, each acknowledging large
uncertainties. Here we present the first dual-carbon isotope-
based (Δ14C and δ13C) source apportionment of elemental
carbon (EC), the mass-based correspondent to optically
defined BC, in the Arctic atmosphere. It targeted 14 high-
loading and high-pollution events during January through
March of 2009 at the Zeppelin Observatory (79° N; Svalbard,
Norway), with these representing one-third of the total sampling period that was yet responsible for three-quarters of the total EC loading. The top-down source-diagnostic 14C fingerprint constrained that 52 ± 15% (n = 12) of the EC stemmed from biomass burning. Including also two samples with 95% and 98% biomass contribution yield 57 ± 21% of EC from biomass burning. Significant variability in the stable carbon isotope signature indicated temporally shifting emissions between different fossil sources, likely including liquid fossil and gas flaring. Improved source constraints of Arctic BC both aids better understanding of effects and guides policy actions to mitigate emissions.
Source Apportionment of Polycyclic Aromatic Hydrocarbons in Central European Soils with Compound-Specific Triple Isotopes (delta C-13, Delta C-14, and delta H-2)
Radiocarbon-based source apportionment of elemental carbon aerosols at two South Asian receptor observatories over a full annual cycle
Regionally-Varying Combustion Sources of the January 2013 Severe Haze Events over Eastern China
Thick haze plagued northeastern China in January 2013, strongly affecting both regional climate and human respiratory health. Here, we present dual carbon isotope constrained (Delta C-14 and delta C-13) source apportionment for combustion-derived black carbon aerosol (BC) for three key hotspot regions (megacities): North China Plain (NCP, Beijing), the Yangtze River Delta (YRD, Shanghai), and the Pearl River Delta (PRD, Guangzhou) for January 2013. BC, here quantified as elemental carbon (EC), is one of the most health-detrimental components of PM2.5 and a strong climate warming agent. The results show that these severe haze events were equally affected (similar to 30%) by biomass combustion in all three regions, whereas the sources of the dominant fossil fuel component was dramatically different between north and south. In the NCP region, coal combustion accounted for 66% (46-74%, 95% C.I.) of the EC, whereas, in the YRD and PRD regions, liquid fossil fuel combustion (e.g., traffic) stood for 46% (18-66%) and 58% (38-68%), respectively. Taken together, these findings suggest the need for a regionally-specific description of BC sources in climate models and regionally-tailored mitigation to combat severe air pollution events in East Asia.
Water-soluble organic carbon aerosols during a full New Delhi winter: Isotope-based source apportionment and optical properties
Water-soluble organic carbon (WSOC) is a major constituent (similar to 20-80%) of the total organic carbon aerosol over the Indian subcontinent during the dry winter season. Due to its multiple primary and secondary formation pathways, the sources of WSOC are poorly characterized. In this study, we present radiocarbon constraints on the biomass versus fossil sources of WSOC in PM2.5 for the 2010/2011 winter period for the megacity Delhi, situated in the northern part of the heavily polluted Indo-Gangetic Plain. The fossil fuel contribution to Delhi WSOC (21 +/- 4%) is similar to that recently found at two South Asian background sites. In contrast, the stable carbon isotopic composition of Delhi WSOC is less enriched in C-13 relative to that at the two receptor sites. Although potentially influenced also by source variability, this indicates that near-source WSOC is less affected by atmospheric aging. In addition, the light absorptive properties of Delhi WSOC were studied. The mass absorption cross section at 365 nm (MAC(365)) was 1.1-2.7m(2)/g with an Absorption angstrom ngstrom Exponent ranging between 3.1 and 9.3. Using a simplistic model the relative absorptive forcing of the WSOC compared to elemental carbon in 2010/2011 wintertime Delhi was estimated to range between 3 and 11%. Taken together, this near-source study shows that WSOC in urban Delhi comes mainly (79%) from biomass burning/biogenic sources. Furthermore, it is less influenced by photochemical aging compared to WSOC at South Asian regional receptor sites and contributes with a relatively small direct absorptive forcing effect.
Sources and light absorption of water-soluble organic carbon aerosols in the outflow from northern China
High loadings of anthropogenic carbonaceous aerosols in Chinese air influence the air quality for over one billion people and impact the regional climate. A large fraction (17-80 %) of this aerosol carbon is water-soluble, promoting cloud formation and thus climate cooling. Recent findings, however, suggest that water-soluble carbonaceous aerosols also absorb sunlight, bringing additional direct and indirect climate warming effects, yet the extent and nature of light absorption by this water-soluble "brown carbon" and its relation to sources is poorly understood. Here, we combine source estimates constrained by dual carbon isotopes with light-absorption measurements of water-soluble organic carbon (WSOC) for a March 2011 campaign at the Korea Climate Observatory at Gosan (KCOG), a receptor station in SE Yellow Sea for the outflow from northern China. The mass absorption cross section at 365 nm (MAC(365)) of WSOC for air masses from N. China were in general higher (0.8-1.1 m(2) g(-1)), than from other source regions (0.3-0.8 m(2) g(-1)). However, this effect corresponds to only 2-10% of the radiative forcing caused by light absorption by elemental carbon. Radiocarbon constraints show that the WSOC in Chinese outflow had significantly higher fraction fossil sources (30-50 %) compared to previous findings in S. Asia, N. America and Europe. Stable carbon (delta C-13) measurements were consistent with aging during long-range air mass transport for this large fraction of carbonaceous aerosols.