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PUBLICATIONS Global Biogeochemical Cycles RESEARCH ARTICLE 10.1002/2017GB005693 Key Points: • The large total annual flux of SBC in the ESAS indicates the importance of this Arctic shelf in the sequestration of SBC • SBC in the ESAS is composed essentially of Pleistocene ice complex deposits mobilized from thawing permafrost • Rising permafrost-derived SBC input is expected to increase the size of the refractory pool of organic carbon in the Arctic Ocean

Supporting Information: • Supporting Information S1 Correspondence to: J. A. Salvadó, [email protected]

Citation: Salvadó, J. A., Bröder, L., Andersson, A., Semiletov, I. P., & Gustafsson, Ö. (2017). Release of black carbon from thawing permafrost estimated by sequestration fluxes in the East Siberian Arctic Shelf recipient. Global Biogeochemical Cycles, 31, 1501–1515. https://doi.org/10.1002/ 2017GB005693 Received 17 APR 2017 Accepted 20 SEP 2017 Accepted article online 2 OCT 2017 Published online 20 OCT 2017

Release of Black Carbon From Thawing Permafrost Estimated by Sequestration Fluxes in the East Siberian Arctic Shelf Recipient Joan A. Salvadó1,2 , Lisa Bröder1,2 and Örjan Gustafsson1,2

, August Andersson1,2

, Igor P. Semiletov3,4,5,

1

Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden, 2Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden, 3Department of Geology and Minerals Prospecting, Tomsk Polytechnic University, Tomsk, Russia, 4Pacific Oceanological Institute, Russian Academy of Sciences Far Eastern Branch, Vladivostok, Russia, 5International Arctic Research Center, University Alaska Fairbanks, Fairbanks, AK, USA

Abstract Black carbon (BC) plays an important role in carbon burial in marine sediments globally. Yet the sequestration of BC in the Arctic Ocean is poorly understood. Here we assess the concentrations, fluxes, and sources of soot BC (SBC)—the most refractory component of BC—in sediments from the East Siberian Arctic Shelf (ESAS), the World’s largest shelf sea system. SBC concentrations in the contemporary shelf sediments range from 0.1 to 2.1 mg g 1 dw, corresponding to 2–12% of total organic carbon. The 210 Pb-derived fluxes of SBC (0.42–11 g m 2 yr 1) are higher or in the same range as fluxes reported for marine surface sediments closer to anthropogenic emissions. The total burial flux of SBC in the ESAS (~4,000 Gg yr 1) illustrates the great importance of this Arctic shelf in marine sequestration of SBC. The radiocarbon signal of the SBC shows more depleted yet also more uniform signatures ( 721 to 896‰; average of 774 ± 62‰) than of the non-SBC pool ( 304 to 728‰; average of 491 ± 163‰), suggesting that SBC is coming from an, on average, 5,900 ± 300 years older and more specific source than the non-SBC pool. We estimate that the atmospheric BC input to the ESAS is negligible (~0.6% of the SBC burial flux). Statistical source apportionment modeling suggests that the ESAS sedimentary SBC is remobilized by thawing of two permafrost carbon (PF/C) systems: surface soil permafrost (topsoil/PF; 25 ± 8%) and Pleistocene ice complex deposits (ICD/PF; 75 ± 8%). The SBC contribution to the total mobilized permafrost carbon (PF/C) increases with increasing distance from the coast (from 5 to 14%), indicating that the SBC is more recalcitrant than other forms of translocated PF/C. These results elucidate for the first time the key role of permafrost thaw in the transport of SBC to the Arctic Ocean. With ongoing global warming, these findings have implications for the biogeochemical carbon cycle, increasing the size of this refractory carbon pool in the Arctic Ocean.

1. Introduction Black carbon (BC) is refractory particulate matter produced during incomplete combustion of biomass and fossil fuels (Goldberg, 1985). BC describes a continuum of highly condensed structures, spanning from partly charred plant material through charcoal to soot with no general agreement on clear-cut boundaries (Schmidt et al., 2001). Soot BC (SBC) particles are formed from vapor-phase condensation reactions and are generally of submicron size (Gustafsson et al., 2001; Schmidt & Noack, 2000). Char BC result from incomplete combustion of solid fuels (e.g., trees and grass) and is much larger than SBC. Some charcoal can be broken down in size, but charred materials largely remain on the ground close to the place of production; however, SBC is subject to long-range transport processes, such as through the atmosphere and to the ocean away from major vegetation fire regions and heavily populated and industrialized areas. Further, SBC is more resistant to oxidation than char and charcoal, suggesting that SBC is also more recalcitrant (Elmquist et al., 2006, 2004; Nguyen et al., 2004).

©2017. American Geophysical Union. All Rights Reserved.

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Analysis of BC is challenging, in part due to BC not being a single chemical compound or a substance with unique chemical and/or physical characteristics. It does not seem possible to define BC with strict physical or chemical parameters; it is functionally and/or operationally defined. The conceptual view of BC varies between fields and even within a given field (e.g., atmospheric versus sediment versus soil fields). Several methods are currently used for the quantification of environmental BC. The atmospheric field is RELEASE OF BC FROM THAWING PERMAFROST

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traditionally dominated by commercially available optical (e.g., aethelometer) and thermal-optical (e.g., OC/EC) (Andersson et al., 2015; Winiger et al., 2016) techniques. In biogeochemical studies of soils and sediments, a technique determining SBC with chemothermal oxidation at 375°C in active airflow (CTO-375) (Gustafsson et al., 1997, 2001) is commonly applied. This method has been thoroughly evaluated with potentially interfering, nonpyrogenic organic matter, and for geochemical consistency of obtained field results (e.g., Elmquist et al., 2006; Gustafsson et al., 1997; Middelburg et al., 1999; Nguyen et al., 2004). The ubiquity of natural and anthropogenic combustion processes, and its refractory composition, make BC widely distributed throughout the biogeosphere, in compartments such as the atmosphere, ocean, icecaps, soils, and sediments. BC plays important roles in several Earth system processes. As an aerosol, BC affects Earth’s radiative balance through scattering and absorption of sunlight (Chung & Seinfeld, 2002; Jacobson, 2001; Ramanathan & Carmichael, 2008). In soils and sediments, BC may be a quantitatively important component of carbon removed from the more rapid biogeochemical cycling. An “ideal” complete combustion process would release all carbon as CO2, but formation of BC rapidly transfers biospheric carbon into the long-term geological carbon pool (Gustafsson & Gschwend, 1998; Kuhlbusch, 1998; Lohmann et al., 2009; Mitra et al., 2002; Seiler & Crutzen, 1980; Suman et al., 1997). The biogeochemical cycling of BC among the various compartments is therefore also relevant for predicting climate change (Bond et al., 2013). While many studies have focused on quantifying BC sources and transport of atmospheric BC (Andersson et al., 2015; Bond et al., 2013; Gustafsson et al., 2009; Kuhlbusch & Crutzen, 1995; Reddy et al., 2002; Srinivas & Sarin, 2014; Winiger et al., 2016, 2017), the global BC sinks and BC fluxes through various environmental compartments are still poorly understood (Druffel, 2004; Preston & Schmidt, 2006; Schmidt & Noack, 2000). It has been estimated, based on limited early data, that 90% of the marine deposition of BC occurs on continental shelves (Suman et al., 1997), yet the magnitude of this transfer to marine sediments is still controversial (Dickens et al., 2004; Elmquist et al., 2008; Flores-Cervantes et al., 2009; Gustafsson & Gschwend, 1998; Lohmann et al., 2009; Mitra et al., 2002, 2014). Previous studies have investigated dissolved BC in the marine system (Coppola & Druffel, 2016; Jaffe et al., 2013; Stubbins et al., 2015; Ziolkowski & Druffel, 2010), and other authors studied the fluxes of BC into marine sediments in only a few coastal margins, such as the Gulf of Maine (Gustafsson & Gschwend, 1998), the South Atlantic Ocean (Lohmann et al., 2009), the Northern European Shelf (Sanchez-Garcia et al., 2012), the East China Sea (Huang et al., 2016), and the Gulf of Thailand (Hu et al., 2016). However, little attention has been paid to the continental shelves of the Arctic Ocean, which is a potentially important region for studying marine margin BC. There are lots of circum-Arctic vegetation fires (Boike et al., 2016; Kozlov et al., 2008; Soja et al., 2004), and the Eurasian Arctic holds extensive shelf seas receiving massive terrestrial input. Although the atmospheric deposition of BC to the Arctic margin from anthropogenic activities in the Arctic might be low (Taketani et al., 2016), a few studies are indicating the importance of BC input to the Arctic Ocean carbon cycling. Fang et al. (2016) reported that the western Arctic Shelf was probably an effective location for burying particulate BC, with an average sinking rate of 73 ± 55 μg m 3 d 1. Elmquist et al. (2008) studied the BC export from Pan-Arctic Rivers, estimating that >200 Gg of SBC are discharged into the coastal seawater annually. Furthermore, Guo et al. (2004) reported that SBC made up 9 ± 4% of the total organic carbon (TOC) near river mouths along the Siberian Arctic coastline. That study also found a correlation of increasing Δ14C signatures of the total sedimentary OC with the SBC/TOC ratio, suggesting that thawing permafrost is releasing BC. However, this issue has remained unresolved, as the radiocarbon composition of the SBC was not measured in that or later studies. It is well known that peat lands and other land forms in boreal-Arctic systems are vulnerable to fires and associated carbon loss (Turetsky et al., 2015). There are extensive tundra and taiga fires in the northern Eurasia (Boike et al., 2016; Evangeliou et al., 2016; Kozlov et al., 2008; Soja et al., 2004). These boreal-Arctic fires are dominated by smoldering combustion, a flameless form of combustion that occurs more readily than flaming combustion. Smoldering fires can persist under low temperatures, high moisture content, and low-oxygen concentrations and as a result can burn for long periods (e.g., weeks and months) despite rain events or changes in fire weather (Huang & Rein, 2014; Rein et al., 2008, 2009; Rodionov et al., 2006). Smoldering fires have a severe impact on the local soil system, because the burning fuel is the organic portion of the soil itself. The prolonged heating from the slowly propagating fire can kill roots, seeds, and plant stems, and the affected layers of soil sustain large losses of biomass along with the formation of BC (Czimczik et al., 2003; Mack et al., 2011; Schuur et al., 2003). These natural peat and soil fires and the consequent formation of BC SALVADÓ ET AL.

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have occurred along Earth’s history and during the last glacial period (Verardo & Ruddiman, 1996). Smoldering fires produce both larger-sized char/charcoal and finer soot BC (SBC). A previous study found large amounts of BC in permafrost soils (Guggenberger et al., 2008). Nevertheless, it is still largely unknown to what extent this BC is released and transported to shelf sediments of the Arctic Ocean. Photodegradation by sunlight is the major identified mechanism for BC removal. Highly condensed structures are preferentially degraded by irradiation, and 28 days of exposure to sunlight of North Atlantic Deep Water caused a decrease of B6CA/B5CA from 0.32 to 0.23 (Stubbins et al., 2010, 2012). However, in the Arctic and at depth in the ocean photodegradation is unlikely, due to the low solar angle, scarcity of sunny days and that sunlight cannot penetrate far because of the high turbidity of Arctic shelves and rivers (Dittmar & Paeng, 2009). This study focuses on the sequestration fluxes of SBC on the East Siberian Arctic Shelf (ESAS), as it is both more resistant to oxidation and likely more prone to be fluvially remobilized and transported than marcoparticulate char and charcoal. Further, the ESAS is a particularly relevant region for investigating the sequestration of SBC and its possible release from thawing permafrost. This area is not only the largest continental shelf in the World, covering about one third of the Arctic Ocean (Stein & Macdonald, 2004), but it is also known to receive and sequester massive amounts of old permafrost carbon (PF/C) (Salvadó et al., 2015; Semiletov et al., 2011; Vonk et al., 2012). Recent observations indicate that this area is particularly susceptible to climate warming (Arndt et al., 2015; Zwiers, 2002), which may increase the export of leached-out surface soil permafrost (topsoil/PF) as well as the coastal erosion of ice complex deposits (ICD/PF or “Yedoma”) of thawing permafrost (Gunther et al., 2013; Vonk & Gustafsson, 2013; Vonk et al., 2012). Here we present the first assessment of SBC sequestration in Arctic shelf recipients. The objectives of this study are to (1) elucidate the contribution of SBC to the overall OC inventory in ESAS surface sediments, (2) quantitatively constrain the SBC sediment sequestration fluxes and compare these with previous studies in other ocean margin areas, and (3) resolve SBC source contributions to the ESAS by conducting statistical source apportionment of radiocarbon signatures of the SBC carbon pool.

2. Materials and Methods 2.1. Study Area and Sampling The ESAS is the largest shelf area of the world ocean (up to 800 km wide) with an average water depth of just 50 m (Stein & Macdonald, 2004). The ESAS consists of the Laptev Sea, the East Siberian Sea (ESS), and the Russian part of the Chuckchi Sea (Figure 1). Its coastline is draped by several thousand kilometers of actively eroding coastal cliffs, comprising ice-rich, fine-grained ice complex deposits (Gunther et al., 2013; Lantuit et al., 2013). This Pleistocene material (1 × 106 km2) dominating north-eastern Siberia (and parts of Alaska and north-western Canada) is quite distinct and more vulnerable (5–7 times higher retreat rates) than other Arctic permafrost bodies built of peat and mineral soil (Schirrmeister et al., 2011; Zimov et al., 2006). The Laptev Sea, between ~110°E and ~140°E, receives large amounts of freshwater (~745 km3 yr 1) mainly transported by the Lena River (Cooper et al., 2008; Semiletov et al., 2000) but most of the carbon inputs stem from coastal erosion of late Pleistocene ice complex deposit (Semiletov et al., 2011; Vonk et al., 2012). The ESS is made up of two different physical and biogeochemical regimes. The eastern ESS (E-ESS, east of ~160°E) is influenced by the Pacific inflow waters (Semiletov et al., 2005; Stein & Macdonald, 2004). In the western ESS (W-ESS), between ~140°E and ~160°E, river inputs and coastal erosion of thawing permafrost supply the major part of the organic carbon. Sediment sampling was conducted during two extensive expeditions on the ESAS. The International Siberian Shelf Study 2008 (ISSS-08) was performed on board the H/V Yakob Smirnitskiy (YS samples) and the smaller ship TB-0012 (TB samples) from August to September 2008 (Semiletov & Gustafsson, 2009). For the purpose of this particular SBC study, we studied sediment samples (n = 30), collected with a Van Veen grab sampler and a dual gravity corer (GEMAX, Kart Oy, Finland; modified at Stockholm University). The International Swedish-Russian-US campaign (SWE samples) was conducted on board the I/B Oden from July to August 2014 (SWERUS-C3). Sediment samples (n = 24) were collected using an Oktopus multicorer (8 Plexiglas tubes, 10 cm diameter), which was developed to collect samples of the seabed with an undisturbed sediment-water interface. The liners were made of polycarbonate and were 60 cm long with a 10 cm diameter. The multicorer was deployed with full weight (head weight about 500 kg) at a speed of 0.5 m/s near the seabed. Sediment cores were sectioned into 1 cm slices with an extruder (Kart Oy, Finland). The surface layers of the YS and TB

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grab samples were retrieved with stainless steel spatulas. Furthermore, in July 2006, permafrost samples (n = 6) were collected from the ICD bluffs along erosion profiles by the Lena and Kolyma Rivers (Figure 1). All samples were transferred into plastic bags and stored in the freezer ( 20°C). 2.2. SBC and TOC Analysis To analyze the SBC, we used the chemothermal oxidation method at 375°C (CTO-375) that has been extensively tested and described earlier (Gustafsson et al., 1997, 2001). The CTO-375 method detects carbon material of high thermal-oxidative resistance and is therefore well suited for quantification of soot-BC, whereas no CTO375-BC was detected for analyzed char-BC materials in previous studies (Elmquist et al., 2006). This method detects highly condensed refractory SBC and has been shown to be selective and reproducible (Elmquist et al., 2006; Hammes et al., 2007). The suitability of the method to natural matrices was previously investigated by testing both positive standards (BC-containing material such as diesel particles and charcoal) and negative standards (potentially interfering nonpyrogenic organic matter such as melanoidin, carbohydrates, proteins and other N-containing compounds, pollen, phytoplankton, wood, kerogen, and coal of increasing degree of maturity). Overall, a good method selectivity was observed with generally low interference/charring potential of the negative standards (Gustafsson et al., 1997). The undesired inclusion (i.e., false positives) of SBC due to charring was estimated to be from 0.3 to 3.7% depending on the aforementioned standards (Gustafsson et al., 2001). Figure 1. Spatial distributions of (a) SBC concentrations (% dw) and (b) SBC/TOC (%) of surface sediments from the ESAS. Surface sediment samples (black dots); samples with calculated depositional fluxes (black crosses); permafrost samples (yellow stars, two samples in each location). E-ESS, eastern East Siberian Sea and W-ESS, western East Siberian Sea. Maps were produced using natural-neighbor interpolation in ArcGIS Geostatistical Analyst with default parameters.

Similarly, in a BC analysis intercomparison (Currie et al., 2002), the CTO375 method measured a lower BC concentration for the analyzed reference material (NIST SRM-1649a) than other employed methods. This suggests that the CTO-375 method is less prone to charring (or incomplete oxidation of non-BC organic matter) than other methods but, besides oxidizing organic matter, also may remove some of the less condensed BC particles. The CTO375 SBC method returned consistent results between different laboratories that, for example, NIST SRM-1650 Diesel Particulate Matter (from a truck) contained 481 ± 5 mg/gdw (Gustafsson et al., 2001) or 457 ± 26 mg/gdw (Elmquist et al., 2006) (the rest being OC and inorganic components), while reports for the more recent NIST SRM-2975 Diesel Particulate Matter (from a forklift diesel truck) were reported at 682 ± 9 mg/gdw (Gustafsson et al., 2001), 661 ± 8 mg/gdw (Elmquist et al., 2006), and 630 ± 41 mg/gdw (Nguyen et al., 2004). Similarly, for environmental mixed material, there are reports for the CTO375-generated SBC for NIST SRM-1941b (marine sediment from Baltimore Harbor) of 5.8 ± 0.5 mg/gdw (Gustafsson et al., 2001); 5.7 ± 0.3 mg/gdw (Elmquist et al., 2006); and for NIST SRM-1944 (Sediment from New Jersey/New York Waterways) of 6.6 ± 1.6 mg/gdw (Gustafsson et al., 2001); 8.0 ± 0.2 mg/gdw (Reddy et al., 2002); and 8.6 ± 1.2 mg/gdw (Elmquist et al., 2006). Furthermore, Elmquist et al. (2006) performed also thorough testing of the method with the standard additions method to marine sediments (using three different NIST RMs)—the results gave good agreement with the direct CTO375 analysis of SBC in the same RM sediments. Briefly, dried sediments and permafrost samples were ground and ≤10 mg was weighed into Ag capsules (5 × 9 mm) and combusted at 375°C for 18 h under an active airflow (200 mL min 1). Subsequently, carbonates were removed by in situ microscale acidification with 1 M HCl and the samples were dried at 60°C. The residual carbon content, corresponding to SBC, was then analyzed in triplicates using a Carlo Erba NC2500 elemental analyzer connected via a split interface to a Finnigan MAT Delta Plus mass spectrometer at the Stable Isotope Laboratory of the Department of Geological Sciences at Stockholm University. Ag capsules without sediment samples (blanks) were also analyzed using the same method described above and showed and average of 0.3 μg of SBC. SBC results were then corrected with these blanks (S/N was

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about 20). The total organic carbon (TOC) content was measured in the same way as the SBC pool without the 375°C CTO treatment. A subset of these samples (n = 8), after the same acidification and drying steps, were analyzed also for its radiocarbon content (Δ14C). To harvest enough carbon for the soot-BC radiocarbon measurements, six-ten identical runs, depending on BC concentration, of 10 mg subportions of sediments were combusted (375°C, 18 h). The combusted sediment portions were pooled together and analyzed at the U.S.-NSF National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility at Woods Hole Oceanographic Institution. Uncertainties of Δ14C were ±0.002 (fraction modern error). 2.3. Quantification of 210Pb-Based Sedimentation Rates To estimate the annual organic carbon accumulation rates in the sediment cores from the Laptev Sea (SWE-14, SWE-23, SWE-24, and YS-6), we used 210Pb dating. 210Pb measurements were carried out by gamma spectroscopy at the Department of Geosciences of the Swedish Museum of Natural History, Stockholm, on an EG&G ORTEC® coaxial low-energy photon spectrometer (Gammadata AB, Uppsala, Sweden) containing a High-Purity Germanium detector. Prior to analysis, the samples (~10 g dry weight) were placed in prewashed HDPE containers and left standing for 3 weeks to obtain secular equilibrium. Decays of 210Pb (46.52 keV) and 226 Ra (185.99 keV) were counted for 20–100 h depending on their activity. The results were blank corrected by measuring decay rates for an empty container. An externally calibrated standard (pitchblende, Stackebo, Sweden) was added to three samples after analysis, and then the measurements were repeated to correct for the relative efficiency of the detector. The measured standard deviations for each sample were on average 8% for 210Pb and 11% for 226Ra. Excess 210Pb (210Pbxs), that is, the unsupported fraction of 210Pb, was determined by difference in activity of 210Pb and 226Ra (Cutshall et al., 1983). Assuming a constant atmospheric input, linear sedimentation rates were determined from the slope of the linear fit to the natural logarithm of 210Pbxs over the sediment core depth (Appleby & Oldfield, 1992). For the sediment cores from the East Siberian Sea (YS-22, YS-26, YS-36, YS-37, YS-90, YS-93, YS-98, and YS-120), we used the fluxes of organic carbon previously calculated by Vonk et al. (2012). 2.4. Estimation of Atmospheric SBC Flux in the ESAS Atmospheric depositional fluxes of aerosol SBC, including both dry and wet deposition, represent a potential source of SBC to continental shelf sediments (Fang et al., 2015, 2016; Huang et al., 2016; Sanchez-Garcia et al., 2012). Thus, to evaluate the importance of the atmospheric input to the sediment sequestration of SBC and to better understand the regional SBC source-to-sink processes, we estimated the atmospheric SBC depositional rate to the ESAS. The magnitude of SBC atmospheric flux is the sum of dry and wet deposition across the air-sea interface. The dry SBC deposition was calculated by Fdry = C · Vdry, where Fdry is the flux of atmospheric dry deposition, C is the atmospheric SBC concentration, and Vdry is the dry-deposition velocity (0.001 m s 1) (Srinivas & Sarin, 2014). In addition, the wet deposition can be taken for the Arctic as 85–90% of the total atmospheric SBC flux (Wang et al., 2011). We could not find studies about atmospheric SBC concentrations in the marine system of the ESAS. However, Winiger et al. (2017) recently reported 2 years of observations (2012–2014) at Tiksi (East Siberian Arctic) and established a strong seasonality in BC concentrations (8 ng m 3 to 302 ng m 3), with an overall average of 47 ± 67 ng m 3. Other observed BC concentrations in Abisko (northern Sweden) showed an annual average for the year 2012 of 27 ng m 3 (Winiger et al., 2016). Annual mean concentrations of BC in Alert (Nunavut, Canada) were ~50 ng m 3 (aethalometer data adjusted to BC) for the period of 1997–2007 (Gong et al., 2010) and in the Zeppelin Observatory (Svalbard, Norway) records showed 39 ng m 3 BC (aethalometer data) for the period of 1998–2007 (Eleftheriadis et al., 2009). The atmospheric BC concentrations in those studies present some differences. If we take the highest annual atmospheric mean concentration of BC reported in the Arctic (~50 ng m 3) (Gong et al., 2010) to calculate the dry deposition, which is very similar to the average BC concentration in Tiksi (Winiger et al., 2017), and multiply this deposition by 10, as the dry BC deposition flux represents only the 10–15% of the total atmospheric BC flux (Wang et al., 2011), we can estimate a total atmospheric BC flux in the ESAS on the order of 0.016 g m 2 yr 1.This is only ~0.6% of the SBC sediment burial flux calculated in this study (see section 3). 2.5. Statistical Source Apportionment A mass balance isotope model was used to delineate the main sources of SBC to the ESAS. Flux estimates of this study show that terrestrial influx dominates the marine SBC pool, with atmospheric deposition playing

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only a negligible role. The source apportionment was therefore focused on estimating the relative contributions from the two major terrestrial pools: ICD/PF and topsoil/PF, using Δ14C as a key source marker. The ICD/PF end-member is constrained as 940 ± 45‰, whereas topsoil is given by 232 ± 132‰ (Vonk et al., 2012). To account for the variability of the end-members and the Δ14C measurement uncertainty (estimated as 50‰), a Markov chain Monte Carlo (MCMC) approach was used (Andersson et al., 2015; Bosch et al., 2015). The topsoil end-member was described as a normal distribution with mean and standard deviation defined by literature values. The ICD/PF end-member was described using a shifted exponential distribution (starting at 1000‰), with a decay rate defined as the inverse of the standard deviation (1/45‰ 1). An exponential distribution (rather than a normal distribution) described more accurately the empirical distribution of the end-member data. The mixed distribution is then an exponentially mixed Gaussian distribution (see supporting information for detailed description). The MCMC calculations were made using in-house written Matlab (ver. 2014b) scripts, using 100,000 iterations, a burn-in of 10,000 and a data thinning of 10, and the stochastic perturbation was adjusted to obtain an acceptance rate of ~0.23.

3. Results and Discussion 3.1. Occurrence and Distribution of SBC Soot Black Carbon (SBC) was detected in all surface sediment samples from the ESAS. Concentrations ranged from 0.01 to 0.21% dry weight (dw), with an average value of 0.08 ± 0.04% dw (Tables 1 and 2 and Figure 1a). The highest values were found near the shore and off the Lena River mouth and showed a decreasing concentration trend with increasing distance from the coast. Near the coast, concentrations of SBC were higher than 0.12% dw, while on the outer shelf, levels decreased to less than 0.04% dw. There were no significant differences between the Laptev Sea (0.09 ± 0.06% dw), the W-ESS (0.06 ± 0.03% dw), and the E-ESS (0.07 ± 0.03% dw). We hypothesize that these findings could be related to the preferentially direct land-based input of SBC through fluvial discharge and coastal erosion of thawing permafrost. Elmquist et al. (2008) reported, with a much smaller number of observations, that the highest values of SBC were eastward, in regions with greatest coverage of continuous permafrost. We also analyzed the SBC in permafrost samples along erosion profiles by the Lena and Kolyma Rivers and obtained SBC concentrations between 0.02 and 0.15% dw soil (average of 0.07 ± 0.04% dw soil) (Figure 1 and Table 2). These permafrost values were thus in the same range as the ones we observed in shelf sediments. If we compare the SBC concentrations on the ESAS with other ocean margin studies using the same CTO-375 technique, our values were broadly similar as reports from surface sediments in Arctic coasts (0.02–0.25% dw) (Elmquist et al., 2008; Guo et al., 2004), African and South American coasts (0.04–0.17% dw) (Lohmann et al., 2009), the Iberian margin of the Atlantic Ocean (0.05–0.16% dw) (Middelburg et al., 1999), and the East China Sea (0.03–0.15% dw) (Huang et al., 2016), but in the lower range of SBC concentrations in the Gulf of Maine (0.01–0.69%) (Gustafsson & Gschwend, 1998), the Black Sea (0.2–0.8% dw) (Middelburg et al., 1999), and the Swedish continental shelf (0.04–1.77% dw) (Sanchez-Garcia et al., 2012, 2010) (Table 2). SBC may represent a significant fraction of the sedimentary organic carbon as its recalcitrant character makes it more resistant to degradation than other organic matter. The organic carbon content in ESAS sediments varied between 0.4 and 2.1% of the sedimentary dry weight (average of 1.2 ± 0.4%) (Table 1). The SBC/TOC ratio ranged from 2.2 to 11.8% with an average value of 6.0 ± 2.2% (Table 1 and Figure 1b). The SBC fraction also presented higher contributions in nearshore samples, particularly off the Kolyma and Lena Rivers, showing decreasing concentrations with increasing distance from the coast. While SBC/TOC was higher than 8% for nearshore samples, it decreased to less than 4% on the outer-shelf (Figure 1b). On the other hand, we do not see differences between the three physical and biogeochemical regimes of the study area, the Laptev Sea (6.4 ± 2.2%), the W-ESS (5.3 ± 1.9%), and the E-ESS (6.2 ± 2.3%). SBC/TOC ratios are the result of two independent processes, the fluxes of SBC, and the fluxes of TOC (both terrestrial and marine) to the sediments. Considering SBC is a refractory subfraction of soil organic matter, our off-shelf decreasing SBC concentrations may be interpreted as a result of dilution with marine organic matter during transport and/or hydrodynamic sorting along the water- and sediment-dispersal system. Our estimates of the SBC/TOC ratios were similar to other estimates in the literature generated with the same CTO-375 method (Table 2). These SBC/TOC values were in the same range as earlier studies in near-coastal

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Table 1 Data of Bulk Organic Carbon and Soot Black Carbon in Surface Sediments From the East Siberian Arctic Shel Sample

Region

Sampling year

Latitude (°N)

Longitude (°E)

SBC (% dw)

SBC/TOC (%)

TOC (%)

SWE-6 SWE-13 SWE-14 SWE-23 SWE-24 SWE-26 SWE-28 SWE-29 SWE-38 YS-4 YS-6 YS-9 YS-13 YS-14 YS-19 Tb-19 Tb-24 Tb-43 Tb-51 SWE-39 SWE-40 SWE-43 SWE-44 SWE-46 SWE-48 SWE-49 SWE-50 SWE-55 YS-22 YS-23 YS-26 YS-28 YS-30 YS-120 SWE-52 SWE-57 SWE-59 SWE-60 SWE-61 SWE-66 YS-31 YS-34B YS-36 YS-37 YS-38 YS-39 YS-40 YS-86 YS-88 YS-90 YS-93 YS-95 YS-98 YS-102

Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea Laptev Sea W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS W-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS E-ESS

2014 2014 2014 2014 2014 2014 2014 2014 2014 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2014 2014 2014 2014 2014 2014 2014 2014 2014 2008 2008 2008 2008 2008 2008 2014 2014 2014 2014 2014 2014 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008

77.15 76.78 76.89 76.17 75.60 76.47 77.34 77.75 78.48 75.99 74.72 73.37 71.97 71.63 73.03 72.09 71.76 72.89 72.46 77.68 77.68 76.78 76.27 76.40 76.62 76.53 75.76 74.84 72.88 72.79 72.46 72.65 71.36 73.29 75.00 74.42 74.43 73.52 74.11 75.84 71.59 69.71 69.82 70.13 70.70 71.22 71.48 75.30 75.10 74.67 74.42 74.42 75.55 76.56

127.35 125.83 127.80 129.33 129.56 132.04 135.01 136.54 137.27 129.98 130.02 130.00 131.70 130.05 133.46 132.78 131.17 131.93 131.66 141.37 144.69 147.79 146.03 149.88 153.37 156.92 158.53 159.33 140.63 142.67 150.60 154.19 152.15 155.17 161.02 163.69 168.49 169.46 170.90 174.41 161.69 162.69 166.00 168.01 169.13 169.37 170.55 174.40 172.19 172.39 166.00 161.34 160.75 160.07

0.04 0.06 0.03 0.04 0.06 0.07 0.06 0.05 0.04 0.11 0.11 0.12 0.13 0.21 0.10 0.04 0.16 0.13 0.21 0.01 0.01 0.04 0.06 0.05 0.04 0.04 0.04 0.06 0.13 0.09 0.09 0.08 0.11 0.08 0.03 0.05 0.04 0.04 0.05 0.03 0.06 0.15 0.07 0.08 0.09 0.12 0.11 0.08 0.08 0.06 0.06 0.09 0.07 0.07

5.51 4.68 2.91 2.22 5.82 5.56 4.21 5.11 4.95 7.56 6.83 8.51 6.69 10.30 5.56 8.16 8.42 8.80 9.61 2.89 3.54 3.90 5.39 4.32 2.81 3.18 3.58 5.74 8.22 7.36 7.75 7.08 6.92 6.90 3.71 3.23 2.36 4.55 2.62 4.08 8.69 11.80 6.97 6.50 7.22 8.00 7.07 6.59 6.40 6.30 5.44 7.22 7.33 7.00

0.8 1.3 0.9 1.6 1.1 1.2 1.4 1.1 0.9 0.5 0.4 1.1 1.2 1.1 1.4 1.3 1.2 0.8 1.0 1.6 1.7 0.9 1.8 0.8 1.4 1.6 1.5 2.0 2.1 1.8 1.5 1.2 1.1 1.1 1.6 0.7 1.3 1.1 1.2 1.2 1.6 1.5 1.2 1.3 1.0 1.1 1.2 1.0 1.0 1.2 0.4 1.9 1.5 2.1

14

Δ C-TOC (‰) 364 314 333 284 441 417 422 437 465 423 543 504 557 622 549 517 495 457 447 479 460 345 368 515 326 716 741 672 682 600 546 296 466 308 239 448 624 554 547 489 462 427 425 391 447 332 398 511 525 476

14

Δ C-SBC (‰) 738 721 722 896 802 749 742 823 -

Arctic sediments (0.9–16.9%) (Elmquist et al., 2008; Guo et al., 2004), the African coast (3–12%) (Lohmann et al., 2009), and the Gulf of Maine (3.1–7.3%) (Gustafsson & Gschwend, 1998) but lower than ratios found for sediments from the Swedish continental shelf (2–44%) (Sanchez-Garcia et al., 2012), the East China Sea (12–65%) (Huang et al., 2016), the South American coast (8–34%) (Lohmann et al., 2009), the Iberian

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Table 2 Soot Black Carbon (SBC) Concentrations and Sediment Fluxes Measured in This Study and in Other Marine Systems Using the Same CTO-375 Method a

Region

n

East Siberian Arctic Shelf Arctic Permafrost Soils Arctic Atmospheric Flux Pan-Arctic River Mouths Siberian Arctic Coastline East China Sea Swedish Continental Shelf South American Coast African Coast Iberian Margin (Atlantic) Black Sea Gulf of Maine, USA

54 6 10 13 52 120 14 9 5 9 6

a

2

SBC (% dw)

SBC/TOC (%)

Flux (g/m yr)

Reference

0.01–0.21 0.02–0.15 0.02–0.15 0.01–0.25 0.03–0.15 0.06–1.77 0.04–0.17 0.08–0.16 0.05–0.16 0.2–0.8 0.01–0.07

2.2–11.8 0.3–9.4 3–9.7 0.9–16.9 12–65 2–44 8–34 3–12 20–31 13–25 3.1–7.3

0.42–11 (2.6) 0.016 0.1–34 (3.5) 2.7–81 (11) 0.01–0.03 (0.01) 0.01–0.08 (0.03) 0.86–1.9 (1.3)

This study This study This study Elmquist et al. (2008) Guo et al. (2004) Huang et al. (2016) Sanchez-Garcia et al. (2012) Lohmann et al. (2009) Lohmann et al. (2009) Middelburg et al. (1999) Middelburg et al. (1999) Gustafsson and Gschwend (1998)

Number of samples.

Margin (20–31%) (Middelburg et al., 1999), and the Black Sea (13–25%) (Middelburg et al., 1999) (Table 2). The lower fraction of SBC compared to other shelf areas around the world could be explained by the higher inputs of nonpyrogenic terrigenous organic carbon to the ESAS. 3.2. Burial Fluxes of SBC It is advantageous to compare fluxes of SBC rather than concentrations, because it avoids matrix dilution effects. To our knowledge, these are the first depositional SBC fluxes reported for marine sediments of the Arctic. The SBC burial fluxes for surface sediments of the ESAS were calculated using SBC concentrations and mass accumulation rates for each sediment core. We assume that sediment recycling (i.e., resuspension of sediments followed by redeposition) should not reset the 210Pb “clock.” Terrestrial-C sequestration flux in this shelf system is on the same order as bottom-up estimates from river and coastal erosion (e.g., Vonk et al., 2012). Thus, we suspect that this process may not change the overall picture. The SBC fluxes varied between 0.42 and 11 g m 2 yr 1 in the ESAS (Tables 1 and 2 and Figure 2). These data showed an offshore-decreasing trend. The highest SBC fluxes were calculated for nearshore locations, particularly for sample YS-22 (11 g m 2 yr 1) (Figures 2 and 3). The SBC fluxes in the different regimes were Laptev Sea (1.3 ± 0.9 g m 2 yr 1), the W-ESS (5.5 ± 4.5 g m 2 yr 1), and the E-ESS (2 ± 1 g m 2 yr 1). These small dissimilarities may be related to the biogeochemical characteristics of the shelf regimes. While in the W-ESS, coastal erosion of late Pleistocene ice complex deposits is delivering large amounts of organic carbon to the sediments (Vonk et al., 2012), in the Laptev Sea, and E-ESS most organic carbon is coming from the Lena and Kolyma Rivers (Karlsson et al., 2015; Salvadó et al., 2016) and marine primary production (Semiletov et al., 2005).

Figure 2. Distribution of SBC flux (g/m2·yr) on the ESAS, based on the sites with calculated depositional fluxes (see Figure 1). E-ESS, eastern East Siberian Sea and W-ESS, western East Siberian Sea. The map was produced using naturalneighbor interpolation in ArcGIS Geostatistical Analyst with default parameters.

SALVADÓ ET AL.

Comparing these fluxes with previous studies in shelf sediments using the same CTO-375 method, the ESAS SBC fluxes are higher or in the same range as fluxes reported for marine areas closer to anthropogenic emissions than the ESAS (Table 2). For example, SBC fluxes in the Gulf of Maine ranged from 0.86 to 1.9 g m 2 yr 1 (Gustafsson & Gschwend, 1998), from 0.01 to 0.08 in African and South American margins (Lohmann et al., 2009) and a wide range from 0.1 to 34 g m 2 yr 1 in the East China Sea (Huang et al., 2016). Further, these Siberian-Arctic shelf SBC fluxes were slightly larger than a previously calculated average flux for the world’s continental margins (0.65 g m 2 yr 1) (Mitra et al., 2014). This highlights the importance of the ESAS in the sequestration of SBC. On the other hand, SBC fluxes in the ESAS are lower than those on the continental shelf sediments of the North Sea and Baltic Sea (2.7–81 g m 2 yr 1), where the SBC influx derives


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largely from long-range atmospheric transport of SBC from densely populated and industrialized parts of Northern Europe (SanchezGarcia et al., 2012). Based on the calculated SBC burial fluxes and the accumulation area of the Laptev Shelf (498,000 km2) and the East Siberian Shelf (987,000 km2) we can estimate the total annual burial flux of sequestered SBC in the ESAS to be on the order of ~4,000 Gg yr 1. This SBC burial flux integrated over the extensive ESAS system, draining large areas of tundra and taiga, may be compared with SBC sequestration for other marginal sea areas. For instance, a SBC burial flux of 400–800 Gg yr 1 was estimated for the New England continental shelf (Gustafsson & Gschwend, 1998), 480–700 Gg yr 1 for the South Atlantic shelves and basins (Lohmann et al., 2009), Figure 3. Derived SBC sediment fluxes (g/m2·yr) versus distance from the coast. ~1,100 Gg yr 1 for the Northern European shelves (Sanchez-Garcia et al., 2012), 700 Gg yr 1 for the world ocean (Dickens et al., 2004), 1 and 18,000 Gg yr for the World’s continental shelves (Mitra et al., 2014) (Table 3). Despite the discrepancies between some of these existing estimates, which demonstrate the need to interpret them with caution, our estimated area-integrated SBC burial in the extensive ESAS is between 4 and 10 times higher than for other reported shelf areas and corresponds to about 22% of the estimated SBC sequestration in the World continental shelves. These findings stress the important role of the ESAS for the sedimentary sequestration of SBC. To better understand the global SBC cycle, it is useful to compare the sequestration of SBC in the ESAS with other large-scale SBC fluxes across the biogeosphere, such as riverine export and atmospheric emissions (Table 3). Our annual marine sediment flux of SBC in the ESAS is much higher than previous studies in river systems. For instance, a SBC discharge of 200 Gg yr 1 was estimated for the seven largest Arctic rivers (Elmquist et al., 2008), 500 Gg yr 1 in the Mississippi River (Mitra et al., 2002), and 24 Gg yr 1 in the much smaller Chesapeake Bay Rivers (Mannino & Harvey, 2004). Dissolved BC flux of 2,800 Gg yr 1 was estimated for the major Pan-Arctic Rivers (Stubbins et al., 2015), and 27,000 Gg yr 1 for the Global Rivers (Jaffe et al., 2013). Estimates of global atmospheric BC emissions span over nearly 2 order of magnitudes from 5,000–20,000 Gg yr 1 of anthropogenic and natural emissions in the entire world (Bond et al., 2004) to 4,700–270,000 Gg yr 1 of only natural emissions (Kuhlbusch & Crutzen, 1995) (Table 3). Key aspects to consider in this interpretation are the large uncertainties in the emission models and differences in employed BC analytical techniques. Nevertheless, these estimated SBC fluxes across different interfaces in the biogeosphere underscores the importance of the ESAS (~4,000 Gg yr 1) in the sequestration of SBC. Next, Table 3 Comparison of SBC Sediment Fluxes From This Study With Other Marine Areas, Export Fluxes From Rivers, and Atmospheric Emission Fluxes a

SBC interface

n

ESAS World Continental Shelves Northern European Shelf New England Shelf South Atlantic Shelves + Basin World ocean

54 126 15 23 7

F (Gg/yr)

Reference

~ 4,000 18,000 ~1,100 400–800 480–700 700

This study Mitra et al. (2014) Sanchez-Garcia et al. (2012) Gustafsson and Gschwend (1998) Lohmann et al. (2009) Dickens et al. (2004)

Marine sediment fluxes

b

Major Pan-Arctic Rivers b Global flux Pan-Arctic Rivers Chesapeake Bay Rivers Mississippi River

World (only natural emissions) World (atmospheric and natural emissions) a

Number of samples.

SALVADÓ ET AL.

River export fluxes b 6 2,800 b 174 27,000 10 200 24 500 Atmospheric emission fluxes 4,700–270,000 4,000–20,000

Stubbins et al. (2015) Jaffe et al. (2013) Elmquist et al. (2008) Mannino and Harvey (2004) Mitra et al. (2002) Kuhlbusch and Crutzen (1995) Bond et al. (2004)

b

Only dissolved black carbon.


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environmental factors that affect the transport of SBC to the ESAS, such as coastal erosion of thawing permafrost, will be considered in contrast to previous SBC studies. 3.3. Contrasting Δ14C Values of SBC Versus Non-SBC Fractions Radiocarbon measurements (Δ14C) for both surface-sediment SBC and bulk organic carbon were performed to evaluate the age and origin of the SBC fraction. The Δ14C signatures of sedimentary SBC ranged from 721 to 896‰ (average of 774 ± 62‰), whereas Δ14C for the non-SBC fraction varied between 304 and 728‰ (average of 491 ± 163‰) (Table 1 and Figure 4). It is notable that the radiocarbon signatures of the sequestered SBC were not correlated with the trends of the non-SBC fraction or the bulk organic carbon. This also lends credence to the integrity of the CTO375-based technique to isolate the SBC fraction without substantial charring of OC (Elmquist et al., Figure 4. Longitudinal distribution of radiocarbon contents in the SBC, the 2006; Gustafsson et al., 2001; Hammes et al., 2007). The Δ14C signatures non-SBC and the bulk sedimentary organic carbon (SOC). of SBC exhibited relatively uniform values, whereas the bulk organic carbon and the non-SBC fraction depicted a much higher isotopic variability (Figure 4). The 14C ages of non-SBC comprised a broader range of values, varying from ancient (more than 10,000 14C yr) to relatively modern ages (~3,000 14C yr), with younger ages in the Laptev Sea and the E-ESS and older ages in the W-ESS. In contrast, radiocarbon ages of SBC depicted similar values in all samples (more than 10,000 14C yr), implying that SBC is coming from an older and more specific source, plausibly either from atmospheric input of fossil fuel combustion or from mobilization of thawing old permafrost. The contribution of atmospheric SBC in the surface sediments of the ESAS can be quantified by comparing the SBC fluxes in the sediments with the estimated atmospheric BC flux in the Arctic (0.016 g m 2 yr 1; see section 2 and Table 2). The estimated atmospheric BC flux in the ESAS is ~24 Gg yr 1 and represents only ~0.6% of the total SBC flux that was constrained for the surface sediments of the ESAS. Thus, we can consider that the atmospheric input of SBC in the ESAS is negligible and there must be other sources of SBC. Previous studies also implied the predominance of other transport mechanisms over atmospheric deposition to shelf sediments (Elmquist et al., 2008; Huang et al., 2016; Lohmann et al., 2009; Mitra et al., 2002; Suman et al., 1997), but here it is reported for the first time that the atmospheric input of SBC indeed is negligible (~0.6% of the total burial flux of SBC). The large fluxes of old SBC in the ESAS thus more likely reflect release from old permafrost in the drainage basin and along the shoreline. It is known that there are extensive smoldering fires in northern Eurasia peatlands (Boike et al., 2016; Evangeliou et al., 2016; Kozlov et al., 2008; Soja et al., 2004). These smoldering fires are not violent fires but rather low-temperature long-term pyrolysis. Once ignited, they are particularly difficult to extinguish despite extensive rains or firefighting attempts and can linger for long periods of time (weeks up to years) and spread over extensive areas and deep into the soil (Page et al., 2002). These fires represent a large contribution to biomass consumption and a significant source of BC to the blackened tundra soils (Czimczik et al., 2003; Mack et al., 2011; Rodionov et al., 2006). To explain the radiocarbon offsets between SBC and non-SBC, we must consider when the SBC was produced and the mechanisms that transport SBC and organic carbon from their land-based sources to the shelf sediments. Before humans started to combust globally significant amounts of fossil fuels, terrestrial biomass burning produced all SBC, which thus had the radiocarbon signature of the terrestrial biosphere. If this SBC was transported directly from production to the shelf sediments, its radiocarbon age would not be substantially different from the concurrently deposited non-SBC pool. However, we see a substantial age offset between SBC and non-SBC in the surface sediments (5,900 ± 3,000 year). Previous studies in sediments from other ocean basins have also reported radiocarbon offsets (Coppola et al., 2014; Masiello & Druffel, 1998). These age differences can be explained if SBC is stored in at least one intermediate carbon pool between terrestrial production and deposition in marine sediments. In this study, we found high SBC concentrations in old permafrost samples along erosion profiles of the Lena and Kolyma riverbanks (Table 2), similar to those observed in ESAS shelf sediments. Other studies also found large amounts of BC in other late Pleistocene

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reservoirs (Verardo & Ruddiman, 1996) and permafrost soils (Guggenberger et al., 2008). Therefore, it seems that “holding pool” candidates in the Siberian Arctic are topsoil permafrost (topsoil/PF) and Pleistocene ice complex deposits (ICD/PF), which could readily release and transport SBC to the ESAS through aquatic conduits and the “boundless carbon cycle” (Vonk & Gustafsson, 2013). This release could be accelerated by regional warming of the East Siberian Arctic and the associated increase of fluvial discharge and coastal erosion of old permafrost pools. 3.4. Source Apportionment of SBC and Carbon Cycle Implications To constrain the contribution of thawing topsoil/PF and ICD/PF to the sequestered SBC, we applied an end-member mixing model, solved Figure 5. Map of the contribution to SBC from the mobilized permafrost carbon with a Markov chain Monte Carlo model (Andersson et al., 2015; 14 (PF/C) of the ESAS, based on the sites with Δ C data. Pie charts represent the Bosch et al., 2015). The end-members were characterized by their radiorelative contributions of the source pools (ICD/PF-SBC, red; topsoil/PF-SBC, carbon fingerprints based on an extensive compilation of data from the orange) to the sedimentary SBC. E-ESS, eastern East Siberian Sea and W-ESS, western East Siberian Sea. Map performed using natural-neighbor interpolation study area (Karlsson et al., 2011; Vonk, Sanchez-Garcia, et al., 2010; Vonk et al., 2012). It is important to note that topsoil/PF and ICD/PF are terresin ArcGIS Geostatistical Analyst with default parameters. trial end-members with partly similar transport or mobilization routes, coastal erosion, and/or river transport but from different pools of SBC. While topsoil/PF represents material from the annual thaw layer of permafrost, which is remobilized through hydraulic conduits and exported fluvially to the coastal ocean, ICD/PF (or “Yedoma”) consists of Pleistocene freeze-locked deposits that are introduced largely by landscape collapse processes such as thermokarst and riverbed/coastal erosion (Gunther et al., 2013; Lantuit et al., 2013; Sanchez-Garcia et al., 2014; Vonk & Gustafsson, 2013). The modeling yielded relative contributions to the SBC of 10–32% from topsoil/PF and 68–90% from ICD/PF (Figure 5). Hence, sedimentary SBC on the extensive ESAS originates predominantly from ICD/PF (75 ± 8%). These dual-isotopebased source apportionment results are broadly in agreement with an early study that deduced that SBC was being released from long-term storage in continental soils, especially through permafrost thawing and riverbank and coastal erosions (Guo et al., 2004). Moreover, it has been suggested that the SBC emission fluxes could have been higher during the late Pleistocene epoch (Verardo & Ruddiman, 1996). The mobilization of thawing permafrost SBC (PF/SBC) may explain the high SBC concentrations found in sediments of the Arctic Ocean. Permafrost is, hence, not only a crucial factor in the storage of organic carbon but also of SBC. We can characterize the contribution of SBC to the mobilized permafrost carbon (PF/C) of the ESAS by using the source apportionment data of the SBC presented above and the bulk sedimentary organic carbon from previous studies (Salvadó et al., 2015; Vonk et al., 2012). If we consider that the mobilized PF/SBC represents ~100% of the SBC on the ESAS (since the atmospheric SBC input is negligible), between 5 and 14% (10 ± 3%) of the total PF/C in the ESAS is constituted by SBC (Figure 5). We observe differences among the three physical and biogeochemical regimes of the study area; the SBC/PF represents 7 ± 3% of the PF/C in the Laptev Sea, 9 ± 0.4% in the W-ESS, and 11 ± 2% in the E-ESS. Further, the SBC contribution to the sedimentary translocated PF/C increases with increasing distance from the coast (Figure 5). This increasing contribution, the opposite pattern to that observed for SBC concentrations and SBC/TOC ratios, may be caused by preferential degradation of non-SBC PF/C (i.e., PF/OC) during cross-shelf transport. Photodegradation by sunlight is the major identified mechanism for BC removal (Stubbins et al., 2010, 2012). Marques et al. (2017) reported that 20–40% of dissolved BC was lost enroute to the ocean in the Paraíba do Sul River (Brasil). However, at depth in the ocean, BC behaves almost conservatively, suggesting minimal loss when light is not present (Dittmar & Paeng, 2009). We hypothesize that the loss of SBC, the more recalcitrant form of BC, from thawing permafrost to ESAS sediments may be minimal due to low solar angle, scarcity of sunny days in the region and that sunlight cannot penetrate far into the water because of the high turbidity of Lena and Kolyma Rivers and ESAS seawater. Therefore, if SBC can reach areas of no light before being photodegraded, it may get reburied in marine sediments. Sequestration of PF/SBC in Arctic shelf sediments on the order of 10 ± 3% of the mobilized total PF/C has implications for the carbon cycle in the Arctic land-ocean interface. These findings may also help to SALVADÓ ET AL.


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explain why some of the PF/C is less accessible to degradation and thus prone to be selectively preserved (Bröder et al., 2016; Dutta et al., 2006; Vonk, Sanchez-Garcia, et al., 2010; Vonk, van Dongen, et al., 2010). Degradation of thawing permafrost will have different consequences for PF/C and PF/SBC, because SBC is a more refractory component of soil organic matter. With ongoing global warming, rising permafrost-derived SBC input from coastal erosion and river discharge may thus increase the size of the refractory pool of carbon that is mobilized and translocated to the Arctic Ocean.

4. Conclusions This study provides an assessment of the concentrations, the sediment sequestration fluxes, and the sources of SBC on the extensive ESAS, the World’s largest shelf sea system. The results demonstrate the key role of thawed-out permafrost in the transport of SBC to the Arctic Ocean. The SBC represents a significant fraction of the total sedimentary organic carbon, exhibiting higher contributions in nearshore samples and decreasing concentrations with increasing distance from the coast, due to dilution with marine organic matter and/or hydrodynamic sorting during sediment transport. The SBC fluxes also show a similar off-shelf decreasing pattern yet are higher or in the same range as fluxes reported for marine areas closer to anthropogenic emissions. Further, the area-integrated annual sequestration flux in the ESAS shows the importance of this shelf area for the marine sequestration of SBC. Radiocarbon signatures of the bulk organic carbon and the non-SBC fraction show a high isotopic variability in the analyzed sediment samples; however, SBC exhibits relatively more uniform and systematically more depleted Δ14C values, inferring that SBC is coming from an older and more specific source. We estimate that the atmospheric input of SBC to the ESAS is negligible compared to this sediment sequestration flux. Consequently, the substantial age offsets between SBC and non-SBC fractions suggest that SBC in ESAS sediments is coming from the mobilization of a thawing old permafrost reservoir. Statistical modeling indicates that sedimentary SBC is mainly stemming from Pleistocene freeze-locked deposits (ICD/PF or “Yedoma”) that are mobilized largely by landscape collapse processes such as thermokarst and riverbed/coastal erosion.

Acknowledgments We would like to thank the scientific crew and personnel of the International Siberian Shelf Study 2008 (ISSS-08) and the SWERUS-C3 expedition 2014. The ISSS-08 and SWERUS-C3 campaigns were supported by the Knut and Alice Wallenberg Foundation, the Headquarters of the Russian Academy of Sciences (RAS), the Far Eastern Branch of the RAS, the Swedish Research Council (VR contracts 621-2007-4631 and 621-2013-5297), the Swedish Polar Research Secretariat, the Russian Foundation of Basic Research (08-0513572, 08-05-00191-a, and 07-0500050a), the Nordic Council of Ministers Cryosphere-Climate-Carbon Initiative (project Defrost, contract 23001), the European Research Council (ERC-AdG project CC-TOP #695331 to Ö. Gustafsson), the U.S. National Science Foundation (OPP ARC 0909546), and partial funding of a PhD student position from the Climate Research School of the Bolin Centre for Climate Research at Stockholm University. I.P. Semiletov also thanks the Russian Government for support (megagrant #2013–220–04– 157 under contract 14.Z50.31.0012). J.A. Salvadó acknowledges EU financial support as a Marie Curie grant (FP7PEOPLE-2012-IEF; project 328049). All data are available in tables and figures associated with the paper.

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The SBC contribution to the translocated total PF/C increases with increasing distance from the coast, the opposite trend to what is observed for SBC concentrations, suggesting that non-SBC PF/C is preferentially degraded during cross-shelf transport. Overall, these findings indicate the important role of thawing permafrost in the release and transport of SBC to the Arctic Ocean. Additional effort should aim to also measure SBC fluxes in other Arctic shelves and in the interior basins.

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