On carbon transport and fate in the East Siberian Arctic land–shelf–atmosphere system

A notable characteristic of the ESAS is large gradients of hydrological and biogeochemical parameters that correspond to geographically critical contrasts in the Arctic system where the Pacific and local shelf waters interact over the shelf. That is reflected in the sedimentation regime (Semiletov et al 2000, 2005, van Dongen et al 2008, Gustafsson et al 2011).

The distribution of δ¹³C-OC in the surface bottom sediments was used to distinguish between the two biogeochemical provinces, western and eastern, in the ESAS (Semiletov et al 2005). This approach was later confirmed with more detailed considerations using biomarkers, isotopes, and C/N ratios (Vetrov et al 2008, Vonk et al 2010). The western geochemical province extends from Vilkitsky Strait (∼120E) in the west to Cape Chukochiy (roughly 172E) in the east, where it is approximately bounded by the isolines of OC and nitrogen isotope ratios: δ¹³C_org =− 24.5‰ and δ¹⁵N_org = 7‰. The western province is strongly influenced by particulate material (PM) transport induced by coastal erosion and fresh water (FW) transport by Lena runoff. PM concentrations in the western province are as much as ten times higher than in the eastern geochemical province, which extends from Cape Chukochiy to the Bering Strait and is highly affected by Pacific inflow (Semiletov et al 2005).

Recent studies indicate a connection among the atmospheric circulation and the hydrological and geochemical characteristics of the water masses in this region (Semiletov et al 2005, Anderson et al 2009, 2011, Porcelli et al 2009). Multi-year variability of the carbonate system and dissolved CH₄ dynamics and efflux of CO₂ into the atmosphere confirm the significance of changing atmospheric circulation patterns (Pipko et al 2005, 2008, 2011, Shakhova and Semiletov 2007, 2008). An important effect on distribution of Atlantic and Pacific waters in the upper Arctic Ocean could be exerted by a change in direction of water transport associated with a shift from the cyclonic (Zn) mode to the anticyclonic (Az) mode of atmospheric circulation over the Arctic basin. Under Az mode conditions the trans-Arctic current (which transports FW northward from the Asian shelf) is intensified and shifted toward Siberia (Proshutinsky and Johnson 1997). The switch of atmospheric circulation regimes (from Az to Zn mode) causes a significant inter-annual shift in the position of the trans-Arctic current, and its correlation (0.78) with hydrological conditions (anomalies of temperature and salinity in the East Siberian Sea) is quite high (Shpaikher and Yankina 1969). The river runoff also determines development of the low salinity Siberian coastal current (SCC) that follows the coasts from west to east (Weingartner et al 1999). Due to wind forcing the SCC is highly variable. Under cyclonic atmospheric conditions the SCC exists as a narrow jet along the coast (for example, in summer 2003), while under anticyclonic atmospheric circulation it was less confined and was not observed in the easternmost part of the ESAS (Weingartner et al 1999, Savelieva et al 2008, Pipko et al 2011). Thus, the variability in position of the frontal zone (FZ) between the 'Pacific waters' and 'local shelf waters' could be considered an indicator of regional climate change as reflected in alternations of the Zn and Az circulation regimes (Semiletov et al 2005). Further comprehensive studies are required to establish process models with different time-scale resolutions, from intra-seasonal to inter-annual to decadal.

The sediment OM is generally a mixture between marine and terrigenous material that is characterized by a range of C/N ratios between 6 and 15; typically the lower values characterize marine OM, and the higher values characterize the terrigenous material (Stein and Macdonald 2004). The distribution of δ¹⁵N_org also shows an eastward change from its 'terrestrial' value of less than 6 to the 'marine' value of more than 8 (Walsh et al 1989). The C/N ratio is negatively correlated with δ¹³N_org (r =− 0.73) and δ¹⁵N_org (r =− 0.77). The spatial pattern of δ¹³C_org distribution in the bottom sediment was used to quantitatively estimate the contribution of terrestrial OM (CTOM) to the East Siberian Sea sediment west and east of the geochemical FZ. Following Walsh et al (1989), the amount of OC derived from the terrestrial end-member (δ¹³C_ter) can be calculated from the data as

$$\begin{eqnarray*} &&\mathrm{CTOM}~(\% )=({\delta }^{13}{\mathrm{C}}_{\mathrm{o}}-{\delta }^{13}{\mathrm{C}}_{\mathrm{mar}})100/({\delta }^{13}{\mathrm{C}}_{\mathrm{ter}}-{\delta }^{13}{\mathrm{C}}_{\mathrm{mar}}), \end{eqnarray*}$$

where 'o', 'mar', and 'ter' refer to the δ¹³C_org values of observed, marine and terrestrial sediment, respectively. Given that the two end members are δ¹³C_ter of  − 27‰, typical of higher plants, and δ¹³C_mar of  − 21‰, typical of phytoplankton (Walsh et al 1989, Naidu et al 2000), the CTOM values for the western and eastern geochemical provinces (bounded by CTOM = 70%) become 86% and 52% of OC_ter, respectively (Semiletov et al 2005). Calculations indicate that there is a significant amount of OC_ter stored within sediments, especially in the nearshore zone most strongly influenced by coastal erosion; between the Dmitry Laptev Strait and the Kolyma River mouth, the OC is almost all of terrestrial origin (from 81% to 100%, mean CTOM = 93%). In contrast, almost half of the sediments underneath transformed Pacific-origin water (the eastern province) are marine in origin. To illustrate the anomalous export of terrestrial OM into the ESAS we show the integrated plume of PM and terrestrial OM in POC for the period 2003–7 (figure 4) from data obtained east of the Lena Delta.

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These observations support the hypothesis of a dominant role for coastal erosion (Semiletov 1999a, 1999b) in ESAS sedimentation and in the dynamics of the OC and carbonate system. It can be seen that the PM and the terrestrial OM/POC plume from the Lena is limited to the narrow nearshore zone where most of the Lena River burden settles. The pattern also shows a negligible direct influence of Lena transport of PM into the East Siberian Sea as was earlier suggested (Semiletov et al 2005).

General differences between the oceanographic and geochemical regimes of the western and eastern provinces in the East Siberian Sea are reflected in the size fraction distribution of the surface sediments. The mean psammite (sand) fraction more than doubled from the western province to the eastern province, while the aleurite (silt) fraction was nearly halved (Semiletov et al 2005). The percentage contribution of the pelite (clay) fraction (particle size  =   <0.01 mm) is almost the same in the western and eastern provinces; however, the pelite fraction is almost completely terrestrial in origin in the western province, but exhibits a mix of marine and terrestrial origins in the eastern province. The highest OC concentration (up to 1.9%) exists in the pelite fraction. The dominant presence of terrestrial eroded material in ESAS sedimentation is illustrated by the distribution of surface sediments in the ESAS (figure 5); >95% of the entire ESAS area is covered by clayey silt, the main product of coastal erosion (Dudarev et al 2006b).

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It is necessary to quantitatively assess the fraction of terrestrial OC (OC_terr) contributed by riverine runoff and the fraction from coastal erosion; in order to provide a benchmark of the composition, provenance, and extent of degradation of the OC_terr, it is very important to distinguish between these two sources. The molecular and radiocarbon composition of the OC_terr released by the GSARs indicate clear patterns relatable to vegetation and climate variations across the 4000 km continent-scale climosequence of the Siberian Arctic (Guo et al 2004, van Dongen et al 2008, Vonk et al 2010, Gustafsson et al 2011). Black carbon ranged from 3 to 10% of total OC; the highest fractions in the permafrost-covered East Siberian region showed a ¹⁴C (radiocarbon) mass balance, suggesting about 20% from biomass burning and the balance from fossil fuel combustion or relict black carbon in uplifted source rocks (Elmquist et al 2008). Among the lipid biomarkers, there is a large contribution of C₂₃ and C₂₅ n-alkanes relative to other homologs, suggesting a substantial contribution from sphagnum, with a decreasing trend eastward. Increasing concentrations of both high-molecular-weight (HMW) n-alkanoic acids and beta-sitosterol relative to HMW n-alkanes toward East Siberia is consistent with increasing permafrost and a shorter annual thaw period (Vonk et al 2010, Gustafsson et al 2011).

Compound-specific radiocarbon analysis yielded similar modern ¹⁴C fraction values for long-chained n-alkanes and n-alkanoic acids, which increased toward the east. The ¹⁴C signal of the bulk OC exhibited an opposite spatial trend. We suggest that these gradients reflect decoupled hydraulic conduits for fresh and aged OC_terr. Since this decoupling is related to the extent of permafrost, there may be repercussions for OC_terr remobilization under a warming climate (van Dongen et al 2008, Vonk et al 2010, Gustafsson et al 2011). If the climate in the ESAS region becomes more like the current climate in the West Siberian region, these results would predict a greater degree (compared to that in western Siberia) of decomposition of the old terrestrial OM released by coastal erosion (figure 2) and the Lena and other eastern GSARs, and thus greater remineralization and release as CO₂.

Rivers transport large quantities of both dissolved and particulate matter and therefore they are carriers of a wide variety of chemical signals to the sea margin (Semiletov 1999a, 1999b, Guo et al 2004, Semiletov et al 2007, Pipko et al 2008, 2010, 2011, van Dongen et al 2008, Anderson et al 2009, 2011, Vonk et al 2010, Charkin et al 2011, Karlsson et al 2011). The degradation of particulate OM (POM) causes the formation of an anomalously high partial pressure of CO₂ (pCO₂) and nutrient concentrations in the nearshore zone, where the river signal overlaps with the biogeochemical signal from highly eroded coastal ice complexes enriched by old terrestrial organics (Semiletov 1999a, 1999b, Semiletov et al 2007, 2011). The POM signal of coastal erosion leads to a pCO₂ increase in bottom water near the highly eroded ice complexes by up to ∼4000 µatm, while pCO₂ values in the surface water strongly impacted by river runoff were usually observed to be ∼1000 µatm or less (figure 6).

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This is consistent with levels of pCO₂ measured in carbon plumes from the GSARs along the northern sea route (figure 7); pCO₂usually appears in surface water at levels <500 µatm (Semiletov et al 2007, 2011).

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Note that rates of oxidation of POM and OM from surface sediments must be quite high to produce a homogeneous distribution of anomalously high pCO₂ values ranging between 2000 and 3000 µatm in the completely mixed water column (figure 6(a), Stations 13–15); the highest values were found in the bottom water of the Buor-Khaya Gulf (figure 6(b), Stations 123–125) where the water column is strongly stratified (Charkin et al 2011). High rates of eroded OM oxidation were confirmed by direct measurements of CO₂ release (measured in September 2006 and 2008 using the Li-Cor chamber technique) in the air above the bluff of the highly eroded ice complex at the northern cape of Muostakh Island in the Laptev Sea; CO₂ efflux to the atmosphere usually ranged from tens up to hundreds of mM per square meter per day.

Recycling of limiting nutrients from old terrestrial OM could influence the element stoichiometry (and 'new production') (Codispoti and Richards 1968, Cauwet and Sidorov 1996, Jones et al 1998, Kattner et al 1999). Our data show that the decay of POM and OM at the sediment surface results in low in situ pH values (as low as 7.24, total scale) and a naturally acidified marine environment. The ESAS exhibits the lowest values of calcium carbonate saturation ever reported for the open marine environment. In some areas of the Laptev Sea (such as at Stations 123–125, figure 6(b)) acidification lowers the saturation state of calcite and aragonite in the surface waters to 0.4 and 0.2, and in the bottom water to 0.2 and 0.1, respectively; these values are even lower than those measured in the East Siberian Sea (Anderson et al 2011). In contrast to eroded POM, dissolved organic carbon (DOC) was believed to be bio-unavailable (Dittmar and Kattner 2003). However, the DOC concentrations measured in samples from the ISSS 2008 cruise showed a strong non-conservative pattern in areas with FW residence times of several years (Alling et al 2010). The average DOC losses during mixing along the ESAS shelf were estimated to be 30–50%, suggesting an additional and unaccounted source of CO₂ to the atmosphere.

Permafrost degradation takes place during climate warming and marine transgression. At present, the mean annual onshore permafrost temperature ranges from  − 11 °C in the coastal zone (Grigoriev et al 2006, Rachold et al 2007), to  − 12 to  − 15 °C in the northern ESAS (Novosibirsky Islands, 73 °N–76 °N), giving a mean latitudinal gradient of about 1.5 °C/100 km (Romanovskii et al 2005). The bed of the high-latitude, shallow ESAS has been alternately exposed to the air and inundated with seawater during glacial and interglacial periods, respectively. Because of its shallowness and location, the ESAS experiences unique climatological transitions. During previous glacial periods, when sea level was more than 100 m lower than at present, the coastline extended up to 1000 km further northward, exposing the continental shelf above the sea surface and, thus, increasing the area of the Siberian coastal accumulative plain by a factor of five (Romanovskii et al 2005). During the last glaciations, the exposed ESAS permafrost was 4–6 °C colder than now (Romanovskii et al 2000); therefore, the mean annual permafrost temperature ranged from  − 15 °C in the southern part of the ESAS to  − 19 to  − 21 °C at its northern periphery. Extensive permafrost formation occurred because freezing extended hundreds of meters deep. Upwardly migrating CH₄ became trapped below the impermeable permafrost and was transformed into permafrost-related Arctic hydrates (Soloviev et al 1987, Kvenvolden 1988). Because they are permafrost-controlled, these hydrates destabilize when sub-sea permafrost experiences warming (Makogon et al 2007). The transition from the last glacial period to the current warm Holocene was accompanied by a sea level rise that inundated the previously exposed shelf area 5–12 kyr ago (Fleming et al 1998). Shallow shelf water annual mean temperatures ranged from  − 1.8 °C to slightly above 0 °C; therefore, inundation resulted in a warming at the seafloor boundary by up to 13–20 °C (versus ∼12–17 °C suggested by Soloviev et al (1987) relative to the temperatures under which permafrost originated in cold epochs when sediments were not covered with water. Over time, ocean heat together with geothermal heat flux-drive submerged permafrost to a new equilibrium state and cause it to thaw. This process first occurs in the areas of fault zones and river estuaries where geothermal heat flux and annual water temperatures are the greatest (Romanovskii et al 2005, Nicolsky and Shakhova 2010, Charkin et al 2011). Thus, as permafrost degradation takes place, thaw bulbs, called taliks, form in limited areas (Soloviev et al 1987, Kvenvolden 1988). Thaw bulbs that penetrate through permafrost are called open taliks (Romanovskii et al 2005, 2000). Taliks could be considered, first, to be an environment favorable for modern methanogenesis from thawed old carbon and, second, to form gas migration pathways by which CH₄ can be released from seabed deposits to the water column and further to the atmosphere (Shakhova et al 2010b, 2010c). The discontinuous presence of taliks is manifested by extremely high spatial variability in concentrations of dissolved CH₄, which correlate with spikes in mixing ratios of CH₄ in the atmosphere like those shown in figure 8.

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Additional destabilization of seabed CH₄ deposits could be caused by sediment settlement and adjustment caused by partial hydrate destabilization, seismic and tectonic events, and formation of drainage features in permafrost, where they are interpreted as providing possible migration pathways (Hyndman and Dallimore 2001, Wright et al 2008). Recently detected warmer water temperatures near the seabed may also impact the stability of the ESAS shelf's submarine permafrost (Holemann et al 2011, Shakhova and Semiletov 2007, 2008).

Our measurements of CH₄ taken in 1994–9 and 2003–10 over the ESAS demonstrate that the system is in a destabilization period (Semiletov 1999a, Shakhova et al 2010a, 2010b, 2010c). Moreover, unlike most other sub-sea locations where CH₄ oxidizes within the water column, a substantial amount of CH₄ released at the seafloor in the shallow ESAS is delivered to the atmosphere (Shakhova et al 2010b, 2010c). Among the factors controlling the fluxes of CH₄ to the atmosphere over the ESAS, we discovered seasonal features such as ice coverage, wind speeds and wave heights (frequency of storm events); depth of the water column, which affects transition time from the bottom to the surface and the amount of CH₄ removed by oxidation as well as water mixing during deep fall convection; and fraction of CH₄ transported by bubbles relative to that transported through diffusion, because bubble-mediated CH₄ transport is far more efficient than diffusion (Shakhova et al 2010b, 2010c). We suggest that fluxes from the ESAS could contribute to rising atmospheric CH₄ because the ESAS serves as a source of CH₄, annually releasing ∼8 Tg CH₄ to the atmosphere. A number of flux components remain to be quantified and added to the existing evaluation (Shakhova et al 2010c).