Controls on the composition and lability of dissolved organic matter in Siberia's Kolyma River basin

High-latitude northern rivers export globally significant quantities of dissolved organic carbon (DOC) to the Arctic Ocean. Climate change, and its associated impacts on hydrology and potential mobilization of ancient organic matter from permafrost, is likely to modify the flux, composition, and thus biogeochemical cycling and fate of exported DOC in the Arctic. This study examined DOC concentration and the composition of dissolved organic matter (DOM) across the hydrograph in Siberia's Kolyma River, with a particular focus on the spring freshet period when the majority of the annual DOC load is exported. The composition of DOM within the Kolyma basin was characterized using absorbance-derived measurements (absorbance coefficienta330, specific UV absorbance (SUVA254), and spectral slope ratio SR) and fluorescence spectroscopy (fluorescence index and excitation-emission matrices (EEMs)), including parallel factor analyses of EEMs. Increased surface runoff during the spring freshet led to DOM optical properties indicative of terrestrial soil inputs with high humic-like fluorescence, SUVA254, and low SRand fluorescence index (FI). Under-ice waters, in contrast, displayed opposing trends in optical properties representing less aromatic, lower molecular weight DOM. We demonstrate that substantial losses of DOC can occur via biological (∼30% over 28 days) and photochemical pathways (>29% over 14 days), particularly in samples collected during the spring freshet. The emerging view is therefore that of a more dynamic and labile carbon pool than previously thought, where DOM composition plays a fundamental role in controlling the fate and removal of DOC at a pan-Arctic scale

Reconciling carbon-stock estimates for the Yedoma region

Permafrost soil organic carbon (C) in the Yedoma region comprises a large fraction of the total circumpolar permafrost C pool, yet estimates based on different approaches during the past decade have led to disagreement in the size and composition of the Yedoma region permafrost C pool. This research aims to reconcile different approaches and show that after accounting for thermokarst and fluvial erosion processes of this interglacial period, the Yedoma region C pool (456 ± 45 Pg C) is the sum of 172 ± 19 Pg Holocene-aged C and 284 ± 40 Pg Pleistocene-aged C. The size of the present-day Pleistocene-aged Yedoma C pool was originally estimated to be 450 Pg based on a mean deposit thickness of 25 m, 1×106 km2 areal extent, 2.6% total organic C content, 1.65times103 kg m−3dry bulk density, and 50% volumetric ice wedge content (Zimov et al. 2006). This estimate assumed that 17% of the Last Glacial Maximum yedoma C stock was lost to greenhouse gas production and emission when 50% of yedoma thawed beneath lakes during the Holocene. However, the regional scale yedoma C pool estimate of Zimov et al. (2006) did not include any Holocene C and assumed that all of the 450 Pg C was Pleistocene-aged. In subsequent global permafrost C syntheses, soil organic C content (SOCC, kg C m−2) data from the Northern Circumpolar Soil C Database (NCSCD) and Zimov et al. (2006) were used to estimate the soil organic C pool for the Yedoma region (450 Pg), assuming only Pleistocene-aged yedoma C from 3 to 25 m (407 Pg), and a mixture of C ages in the 0 to 3 m interval (43 Pg). A more recent synthesis of Yedoma-region C stocks based on extensive sampling by Strauss et al. (2013) took into account lower C bulk density values of yedoma, higher organic C concentrations of yedoma, a larger landscape fraction of thermokarst (70% of Yedoma region area), the larger C concentration of thermokarst, and remote-sensing based quantification of ice-wedge volumes. This synthesis produced lower mean- and median-based estimates of Yedoma-region C, 348+73 Pg and 211 +160/-153 Pg respectively. However, Strauss et al. (2013) focused on the remaining undisturbed yedoma and refrozen surface thermokarst deposits and thus did not include taberite deposits, which are the re-frozen remains of Yedoma previously thawed beneath thermokarst lakes and still present in large quantities on the landscape. In our study (Walter Anthony et al. 2014), we measured the dry bulk density directly on 89 yedoma and 311 thermokarst-basin samples, including taberites, collected in four yedoma subregions of the North Siberian Kolyma Lowlands. Multiplying the organic matter content of an individual sample by the same sample’s measured bulk density yielded an organic C bulk density data set for yedoma samples that was normally distributed. Combining our subregion-specific organic C bulk density results with those of Strauss et al. (2013) for other yedoma subregions extending to the far western extent of Siberian yedoma, we determined a mean organic C bulk density of yedoma for the total Yedoma region (26 ± 1.5 kg C m-3), which is similar to that previously suggested by Strauss et al. (2013) (27 kg C m-3 mean based approach; 16 kg C m-3 median based approach). Our estimate of the organic C pool size of undisturbed yedoma permafrost (129 ± 30 Pg Pleistocene C) in the 396,600 ± 39,700 km2 area that has not been degraded by thermokarst since the Last Glacial Maximum is based on this regional-mean C bulk density value. Our calculation also assumes an average yedoma deposit thickness of 25 m and 50% volumetric massive ice wedge content, as in previous estimates (Zimov et al. 2006, NCSCD; Table 1). Similar results found in the recent study of the Yedoma-region C inventory by Strauss et al.(2013) corroborate our estimate of the undisturbed yedoma C inventory. The size of the remaining yedoma C pool was estimated by Strauss et al. (2013) to be 112 Pg (vs. 129 Pg, this study) based on mean parameter values: organic C bulk density 27 kg C cm-3 (vs. 26.2 kg C cm-3 in this study), yedoma deposit thickness 19.4 m (vs. 25 m in this study), Yedoma volumetric ice wedge content 48% (vs. 50% in this study), and thermokarst extent (70% in both studies). The two studies took different approaches for estimating yedoma deposit thicknesses: Strauss et al. (2013) used 22 field sites from Siberia and Alaska with a mean thickness of 19.4 m; our calculations used a thickness value determined from Russian literature (25 ± 5m, references in Walter Anthony et al. 2014). The mean derived from our limited (n=17) field sites was 38 m in the Kolyma region. The two studies further differ slightly in calculating Yedoma-region area: Strauss et al. (2013), which focused on still frozen deposits vulnerable to future thaw, did not include thawed deposits in present-day lakes, but did include deposits in known smaller yedoma occurrences outside the core Yedoma region such as valleys of NW Canada, Chukotka, and the Taymyr Peninsula. Our study focused on the extent of core-yedoma deposits as well as organic-C stored in present-day Yedoma lake deposits. While differences in yedoma thickness and area values can impact upscaling calculations, efforts are underway by the Yedoma region synthesis IPA Action Group (Strauss and colleagues) to analyze more comprehensive data sets and better constrain the values. Based on our approach that includes a differentiation of thermokarst-lake facies, we estimate that 155 Pg Pleistocene-aged organic C is stored in thermokarst-lake basins and thermoerosional gullies in the Yedoma region of Beringia [155 Pg is the sum of 114 Pg in taberite deposits and 41 Pg in various lacustrine facies]. This 155 Pg Pleistocene-aged C represents the remains of yedoma that thawed and partially decomposed beneath and in thermokarst lakes and streams. Altogether we estimate a total Pleistocene-C pool size of 284 ± 40 Pg for the Beringian Yedoma region in the present day as the sum of Pleistocene C in undisturbed yedoma (129 Pg) and in thermokarst basins (155 Pg). Separately, Holocene-aged organic C assimilated and sequestered in deglacial thermokarst basins in the Yedoma-region is 159 ± 24 Pg. Our upscaling is based on the mean C stocks of individual permafrost exposures (Fig. 2e in Walter Anthony et al. 2014), which were normally distributed. To our knowledge, this is the first study to combine a geomorphologic classification of alas facies with C content, including the deeper lacustrine deposits, for the purpose of systematically upscaling to a regional alas C inventory. We did not measure the C content of Holocene terrestrial soils overlying undisturbed yedoma permafrost; however, applying values from the NCSCD in Siberia for Histels (44.3 kg C m-2, 9% of Yedoma region area), Orthels (26.0 kg C m-2, 17% of Yedoma region area) and Turbels (38.4 kg C m-2, 63% of Yedoma region area) to the extent of 1-m surface deposits overlying the area of undisturbed Yedoma permafrost (396,000 ± 39,600 km2), results in 12.9 ± 1.3 Pg of Holocene C. This calculation assumes that the 70/30 ratio of thermokarst to undisturbed yedoma applies across the Histel, Orthel and Turbel cover classes. Altogether, we estimate the Holocene and Pleistocene organic C pool size in the Yedoma region of Beringia as 456 ± 45 Pg (38% Holocene, 62% Pleistocene). Despite the differences in approaches and locations of study sites, similarities in the meanbased estimates of the Yedoma-region organic C pool size between Strauss et al. (2013) and this study corroborate our findings. Not accounting for diagenetically altered organic C from yedoma thawed in situ beneath lakes (taberites), Strauss et al. (2013) estimated 348 Pg C for the regional pool size. Without taberite C, our estimate would be similar (342 Pg C). For our study, focusing on the C balance shifts from the Pleistocene to the end of the Holocene, we show that taberite deposits are an important component and need to be included in the budget as these deposits are a large C pool that represents diagenetically-altered organic C from yedoma thawed in situ beneath lakes (Table 1b). Our estimate of yedoma-derived taberite deposits underlying thermokarst basins (114 Pg C), would bring the Yedoma-region C pool estimate by Strauss et al. (2013) up to 462 Pg C, which is similar to our estimate of 456 Pg C. In summary, the Yedoma-region organic C value (456 ± 45 Pg C, consisting of Pleistocene and Holocene C) determined by this study is similar to that calculated originally by Zimov et al. (2006) to represent only the Pleistocene yedoma C pool (450 Pg). Subsequently, the Pleistocene-aged yedoma C was considered to be 450 Pg C. Pleistocene-aged Yedoma carbon was considered to be >90% of the regional pool by the subsequent NCSCD syntheses for quantification of circumpolar permafrost carbon. The primary difference between the Yedoma-region C pool estimate presented here versus Zimov et al. (2006), which entered the NCSCD syntheses, is that in this study net C gains associated with a widespread thermokarst process are taken into account. The component of Pleistocene yedoma C was reduced in this study by 38% and a new Holocene-thermokarst C pool (159 Pg) was introduced. We lowered the Pleistocene-aged yedoma C pool based on larger, more recent data sets on yedoma’s dry bulk density by this study and Strauss et al. (2013) and based on our more recent map-based analysis showing a 20% larger areal extent of deep thermokarst activity in the Yedoma region. The major implications of this study pertain to the nature and fate of greenhouse gas emissions associated with permafrost thaw in the Yedoma region. Differentiation of the C pool in the Yedoma region (yedoma vs. thermokarst basins) is critical to understanding past and future C dynamics and climate feedbacks. Since a larger fraction of the yedoma landscape has already been degraded by thermokarst during the Holocene (70% instead of 50%), the size of the anaerobically-vulnerable yedoma C pool for the production of methane is 40% lower than that previously calculated. Second, there is concern that permafrost thaw will mobilize and release ’ancient’ organic C to the atmosphere. Assuming average radiocarbon ages of Pleistocene-yedoma and Holocene deposits of 30 kya and 6.5 kya respectively, accounting for the new Holocene-aged thermokarst C pool (159 Pg C) lowers the average age of the current Yedoma-region C pool by about one third. This result is important to global C-cycle modeling since C isotope signatures provide valuable constraints in models. Finally, given differences in permafrost soil organic matter origins for the Pleistocene-aged steppe-tundra yedoma C pool [accumulated under aerobic conditions; froze within decades to centuries after burial; and remained frozen for tens of thousands of years] and the lacustrine Holocene-aged C pool [accumulated predominately under anaerobic conditions and remained thawed for centuries to millennia prior to freezing after lakes drained], it is likely that organic matter degradability differs substantially between these two pools. This has implications for differences in their vulnerability to decomposition and greenhouse gas production under scenarios of permafrost thaw in the future. References: Strauss J., Schirrmeister L, Grosse G, Wetterich W, Ulrich M, Herzschuh U, Hubberten H-W. 2013. The deep permafrost carbon pool of the Yedoma region in Siberia and Alaska. Geophys. Res. Lett. 40, 6165–6170. Walter Anthony K M, Zimov SA, Grosse G, Jones MC, Anthony P, Chapin III FS, Finlay JC, Mack mC, Davydov S, Frenzel P, Frolking S. 2014. A shift of thermokarst lakes from carbon sources to sinks during the Holocene epoch. Nature, 511, 452-456, DOI:10.1038/nature13560. Zimov,SA, Schuur EAG, Chapin FS. 2006. Permafrost and the global carbon budget. Science 312: 1612–1613

Small-scale hydrological patterns in a Siberian permafrost ecosystem affected by drainage

Climate warming and associated accelerated permafrost thaw in the Arctic lead to a shift in landscape patterns, hydrologic conditions, and release of carbon. In this context, the lateral transport of carbon and shifts therein following thaw remain poorly understood. Crucial hydrologic factors affecting the lateral distribution of carbon include the depth of the saturated zone above the permafrost table with respect to changes in water table and thaw depth and the connectivity of water-saturated zones. Landscape conditions are expected to change in the future due to rising temperatures and polygonal or flat floodplain Arctic tundra areas in various states of degradation; hydrologic conditions will also change. This study is focused on an experimental site near Chersky, northeast Siberia, where a drainage ditch was constructed in 2004 to simulate landscape degradation features that result in drier soil conditions and channeled water flow. We compared water levels and thaw depths in the drained area (dry soil conditions) with those in an adjacent control area (wet soil conditions). We also identified the sources of water at the site via stable water isotope analysis. We found substantial spatiotemporal changes in the water conditions at the drained site: (i) lower water tables resulting in drier soil conditions, (ii) quicker water flow through drier areas, (iii) larger saturation zones in wetter areas, and (iv) a higher proportion of permafrost meltwater in the liquid phase towards the end of the growing season. These findings suggest decreased lateral connectivity throughout the drained area. Shifts in hydraulic connectivity in combination with a shift in vegetation abundance and water sources may impact carbon sources and sinks as well as transport pathways. Identifying lateral transport patterns in areas with degrading permafrost is therefore crucial.</p

Summer methane ebullition from a headwater catchment in Northeastern Siberia

Streams and rivers are active processors of terrestrial carbon and significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Recent studies suggest that ebullition may represent a sizable yet overlooked component of the total CH4 flux from these systems; however, there are no published CH4 ebullition estimates for streams or rivers in subarctic or arctic biomes, regions that store vast quantities of vulnerable, old organic carbon in permafrost soils. We quantified CH4 ebullition from headwater streams in a small arctic watershed in Northeastern Siberia. Ebullitive emissions were 0.64 mmol m-2 d-1, which is lower than the global average but approximately 2 times greater than the pan-arctic diffusive CH4 flux estimate reported in a recent synthesis of global freshwater CH4 emissions. The high CO2:CH4 of sediment bubbles (0.52) suggests that methane emissions may currently be constrained by resource competition between methanogens and microbes using more efficient metabolic strategies. Furthermore, the magnitude and frequency of ebullition events were greater as temperatures increased, suggesting that ebullition from streams could become a more prominent component of the regional CH4 flux in a warmer future

The effects of CO2, climate and land-use on terrestrial carbon balance, 1920-1992: An analysis with four process-based ecosystem models

The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term(1920–1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr−1, which is within the uncertainty of analysis based on CO2 and O2 budgets. Three of the four models indicated (in accordance with O2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Niño/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system

Variability in above- and belowground Carbon Stocks in a Siberian Larch Watershed

Permafrost soils store between 1330 and 1580Pg carbon (C), which is 3 times the amount of C in global vegetation, almost twice the amount of C in the atmosphere, and half of the global soil organic C pool. Despite the massive amount of C in permafrost, estimates of soil C storage in the high-latitude permafrost region are highly uncertain, primarily due to undersampling at all spatial scales; circumpolar soil C estimates lack sufficient continental spatial diversity, regional intensity, and replication at the field-site level. Siberian forests are particularly undersampled, yet the larch forests that dominate this region may store more than twice as much soil C as all other boreal forest types in the continuous permafrost zone combined. Here we present above- and belowground C stocks from 20 sites representing a gradient of stand age and structure in a larch watershed of the Kolyma River, near Chersky, Sakha Republic, Russia. We found that the majority of C stored in the top 1m of the watershed was stored belowground (92%), with 19% in the top 10cm of soil and 40% in the top 30cm. Carbon was more variable in surface soils (10cm; coefficient of variation (CV) = 0.35 between stands) than in the top 30cm (CV = 0.14) or soil profile to 1m (CV = 0.20). Combined active-layer and deep frozen deposits (surface – 15m) contained 205kgCm−2 (yedoma, non-ice wedge) and 331kgCm−2 (alas), which, even when accounting for landscape-level ice content, is an order of magnitude more C than that stored in the top meter of soil and 2 orders of magnitude more C than in aboveground biomass. Aboveground biomass was composed of primarily larch (53%) but also included understory vegetation (30%), woody debris (11%) and snag (6%) biomass. While aboveground biomass contained relatively little (8%) of the C stocks in the watershed, aboveground processes were linked to thaw depth and belowground C storage. Thaw depth was negatively related to stand age, and soil C density (top 10cm) was positively related to soil moisture and negatively related to moss and lichen cover. These results suggest that, as the climate warms, changes in stand age and structure may be as important as direct climate effects on belowground environmental conditions and permafrost C vulnerability

High-resolution bathymetry models for the Lena Delta and Kolyma Gulf coastal zones

Arctic river deltas and deltaic near-shore zones represent important land-ocean transition zones influencing sediment dynamics and nutrient fluxes from permafrost-affected terrestrial ecosystems into the coastal Arctic Ocean. To accurately model fluvial carbon and freshwater export from rapidly changing river catchments, as well assessing impacts of future change on the Arctic shelf and coastal ecosystems, we need to understand the sea floor characteristics and topographic variety of the coastal zones. To date, digital bathymetrical data from the poorly accessible, shallow and large areas of the eastern Siberian Arctic shelves are sparse. We have digitized bathymetrical information for nearly 75,000 locations from large-scale (1:25,000 – 1:500,000) current and historical nautical maps of the Lena Delta and the Kolyma Gulf region in Northeast Siberia. We present the first detailed and seamless digital models of coastal zone bathymetry for both delta/gulf regions in 50 m and 200 m spatial resolution. We validated the resulting bathymetry layers using a combination of our own water depth measurements and a collection of available depth measurements, which showed a strong correlation (r > 0.9). Our bathymetrical models will serve as an input for a high-resolution coupled hydrodynamic-ecosystem model to better quantify fluvial and coastal carbon fluxes to the Arctic Ocean but may be useful for a range of other studies related to Arctic delta and near-shore dynamics such as modelling of submarine permafrost, near-shore sea ice, or shelf sediment transport