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1.	Alling, Vanja, et al. (author) Non-conservative behavior of dissolved organic carbon across the Laptev and East Siberian Seas 2010 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 24, s. GB4033- Journal article (peer-reviewed)abstract Climate change is expected to have a strong effect on the Eastern Siberian Arctic Shelf (ESAS) region, which includes 40% of the Arctic shelves and comprises the Laptev and East Siberian seas. The largest organic carbon pool, the dissolved organic carbon (DOC), may change significantly due to changes in both riverine inputs and transformation rates; however, the present DOC inventories and transformation patterns are poorly understood. Using samples from the International Siberian Shelf Study 2008, this study examines for the first time DOC removal in Arctic shelf waters with residence times that range from months to years. Removals of up to 10%–20% were found in the Lena River estuary, consistent with earlier studies in this area, where surface waters were shown to have a residence time of approximately 2 months. In contrast, the DOC concentrations showed a strong nonconservative pattern in areas with freshwater residence times of several years. The average losses of DOC were estimated to be 30%–50% during mixing along the shelf, corresponding to a first-order removal rate constant of 0.3 yr−1. These data provide the first observational evidence for losses of DOC in the Arctic shelf seas, and the calculated DOC deficit reflects DOC losses that are higher than recent model estimates for the region. Overall, a large proportion of riverine DOC is removed from the surface waters across the Arctic shelves. Such significant losses must be included in models of the carbon cycle for the Arctic Ocean, especially since the breakdown of terrestrial DOC to CO2 in Arctic shelf seas may constitute a positive feedback mechanism for Arctic climate warming. These data also provide a baseline for considering the effects of future changes in carbon fluxes, as the vast northern carbon-rich permafrost areas draining into the Arctic are affected by global warming.
2.	Ask, Jenny, et al. (author) Net ecosystem production in clear-water and brown-water lakes 2012 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 26, s. GB1017- Journal article (peer-reviewed)abstract We studied 15 lakes in northern Sweden with respect to primary production and respiration in benthic and pelagic habitats. The lakes were characterized by different concentrations of colored dissolved organic carbon (DOC) of terrestrial origin, forming a gradient ranging from clear-water to brown-water lakes. Primary production decreased and respiration increased on a whole-lake scale along the gradient of increasing DOC. Thus, the lakes became more net heterotrophic, i.e., had lower net ecosystem production (NEP = gross primary production - community respiration), with increasing terrestrial DOC and this change coincided with increasing partial pressure of carbon dioxide (pCO(2)) in the surface waters. The single most important process for the increasing net heterotrophy along the DOC gradient was pelagic respiration of terrestrial organic carbon. In spite of high metabolic activity in the benthic habitat, benthic primary production and benthic respiration decreased simultaneously with increasing DOC, showing that the benthic habitat was in metabolic balance throughout the gradient. Therefore, the net heterotrophic states of the lakes depended on the terrestrial DOC export to lakes and the concomitant respiration of terrestrial organic carbon in the pelagic habitat.
3.	Björkman, Mats P., 1978, et al. (author) Winter carbon dioxide effluxes from Arctic ecosystems: An overview and comparison of methodologies 2010 In: Global Biogeochemical Cycles. ; 24, s. GB3010- Journal article (peer-reviewed)abstract The winter CO2 efflux from subnivean environments is an important component of annual C budgets in arctic ecosystems and consequently makes prediction and estimations of winter processes as well as incorporations of these processes into existing models important. Several methods have been used for estimating winter CO2 effluxes, involving different assumptions about the snow pack, all aiming to quantify CO2 production. Here, four different methods are compared and discussed: (1) measurements with a chamber on the snow surface, Fsnow; (2) chamber measurements directly on the soil, Fsoil, after snow removal; (3) diffusion measurements, F2-point, within the snow pack; and (4) a trace gas technique, FSF6, with multiple gas sampling within the snow pack. According to measurements collected from shallow and deep snow cover in High-Arctic Svalbard and Sub-Arctic Sweden during the winter of 2007-2008, the four methods differ by up to two orders of magnitude in their estimates of total winter emissions. The highest mean winter CO2 effluxes, 7.7-216.8 mg CO2 m-2 h-1, were observed using Fsoil and lowest values, 0.8-12.6 mg CO2 m-2 h-1, using FSF6. The Fsnow and F2-point methods were both within the lower range, 2.1-15.1 mg CO2 m-2 h-1 and 6.8-11.2 mg CO2 m-2 h-1, respectively. These differences are considered to be a result of contrasting methods, but also because the assumptions within the methods are not the same when quantifying CO2 production and effluxes to the atmosphere. Since snow can act as a barrier to CO2, Fsoil is assumed to measure soil production, whereas FSF6, Fsnow and F2-point are considered better approaches for quantifying exchange processes between the soil, snow, and the atmosphere. This study indicates that estimates of winter CO2 emissions may vary more as a result of the method used than due to the actual variation in soil CO2 production or release. This is a major concern, especially when CO2 efflux data are used in climate models or in carbon budget calculations, thus highlighting the need for further development and validation of accurate and appropriate techniques.
4.	Campeau, Audrey, et al. (author) Regional contribution of CO2 and CH4 fluxes from the fluvial network in a lowland boreal landscape of Quebec 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28:1, s. 57-69 Journal article (peer-reviewed)abstract Boreal rivers and streams are known as hot spots of CO2 emissions, yet their contribution to CH4 emissions has traditionally been assumed to be negligible, due to the spatially fragmented data and lack of regional studies addressing both gases simultaneously. Here we explore the regional patterns in river CO2 and CH4 concentrations (pCO(2) and pCH(4)), gas exchange coefficient (k), and the resulting emissions in a lowland boreal region of Northern Quebec. Rivers and streams were systematically supersaturated in both gases, with both pCO(2) and pCH(4) declining along the river continuum. The k was on average low and increased with stream order, consistent with the hydrology of this flat landscape. The smallest streams (order 1), which represent <20% of the total river surface, contributed over 35% of the total fluvial greenhouse gas (GHG) emissions. The end of winter and the spring thaw periods, which are rarely included in annual emission budgets, contributed on average 21% of the annual GHG emissions. As a whole, the fluvial network acted as significant source of both CO2 and CH4, releasing on average 1.5 tons of C (CO2 eq) yr(-1)km(-2) of landscape, of which CH4 emissions contributed approximately 34%. We estimate that fluvial CH4 emissions represent 41% of the regional aquatic (lakes, reservoirs, and rivers) CH4 emissions, despite the relatively small riverine surface (4.3% of the total aquatic surface). We conclude that these fluvial networks in boreal lowlands play a disproportionately large role as hot spots for CO2 and more unexpectedly for CH4 emissions. Key Points pCO(2) and pCH(4) decrease, whereas the k600 increases with increasing stream order Small streams and spring thaw period play a large role in regional C balance Rivers are significant sources of CO2 and unexpectedly large sources of CH4
5.	Daniau, A. -L, et al. (author) predictability of biomass burning in response to climate changes 2012 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 26, s. GB4007- Journal article (peer-reviewed)abstract Climate is an important control on biomass burning, but the sensitivity of fire to changes in temperature and moisture balance has not been quantified. We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo-fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.
6.	Giesler, Reiner, et al. (author) Spatiotemporal variations of pCO(2) and delta C-13-DIC in subarctic streams in northern Sweden 2013 In: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 27:1, s. 176-186 Journal article (peer-reviewed)abstract Current predictions of climate-related changes in high-latitude environments suggest major effects on the C export in streams and rivers. To what extent this will also affect the stream water CO2 concentrations is poorly understood. In this study we examined the spatiotemporal variation in partial pressure of CO2 (pCO(2)) and in stable isotopic composition of dissolved inorganic carbon (delta C-13-DIC) in subarctic streams in northern Sweden. The selected watersheds are characterized by large variations in high-latitude boreal forest and tundra and differences in bedrock. We found that all streams generally were supersaturated in pCO(2) with an average concentration of 850 mu atm. The variability in pCO(2) across streams was poorly related to vegetation cover, and carbonaceous bedrock influence was manifested in high DIC concentrations but not reflected in either stream pCO(2) or delta C-13-DIC. Stream water pCO(2) values were highest during winter base flow when we also observed the lowest delta C-13-DIC values, and this pattern is interpreted as a high contribution from CO2 from soil respiration. Summer base flow delta C-13-DIC values probably are more affected by in situ stream processes such as aquatic production/respiration and degassing. A challenge for further studies will be to disentangle the origin of stream water CO2 and quantify their relative importance.
7.	Gustafsson, Erik, et al. (author) External total alkalinity loads versus internal generation : The influence of nonriverine alkalinity sources in the Baltic Sea 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28:11, s. 1358-1370 Journal article (peer-reviewed)abstract In this study we first present updated riverine total alkalinity (TA) loads to the various Baltic Sea sub-basins, based on monthly measurements in 82 of the major rivers that represent 85% of the total runoff. Simulations in the coupled physical-biogeochemical BALTSEM (BAltic sea Long-Term large Scale Eutrophication Model) model show that these river loads together with North Sea water inflows are not sufficient to reproduce observed TA concentrations in the system, demonstrating the large influence from internal sources. Budget calculations indicate that the required internal TA generation must be similar to river loads in magnitude. The nonriverine source in the system amounts to about 2.4mmolm(-2) d(-1) on average. We argue here that the majority of this source is related to denitrification together with unresolved sediment processes such as burial of reduced sulfur and/or silicate weathering. This hypothesis is supported by studies on sediment processes on a global scale and also by data from sediment cores in the Baltic Sea. In a model simulation with all internal TA sources and sinks switched on, the net absorption of atmospheric CO2 increased by 0.78mol C m(-2) yr(-1) compared to a simulation where TA was treated as a passive tracer. Our results clearly illustrate how pelagic TA sources together with anaerobic mineralization in coastal sediments generate a significant carbon sink along the aquatic continuum, mitigating CO2 evasions from coastal and estuarine systems.
8.	Harrison, John A., et al. (author) Global importance, patterns, and controls of dissolved silica retention in lakes and reservoirs 2012 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 26:2, s. n/a-n/a Journal article (peer-reviewed)abstract Lentic water bodies (lakes and reservoirs) offer favorable conditions for silica (SiO2) burial in sediments. Recent global estimates suggest that (1) lentic SiO2 trapping is a globally important SiO2 flux, and (2) through reservoir construction, humans have dramatically altered river dissolved SiO2 (DSi) transport and coastal DSi delivery. However, regional to global scale patterns and controls of DSi removal in lentic systems are poorly constrained. Here we use 27 published lake and reservoir DSi budgets to develop insights into patterns and controls of lentic DSi retention and to develop a new, spatially explicit, global model of lentic DSi removal called SiRReLa (Silica Retention in Reservoirs and Lakes). In our analysis, lentic DSi removal (kg SiO2 yr−1) was significantly and positively related to DSi loading (P < 0.0001; r2 = 0.98), and DSi removal efficiency was significantly and positively related to water residence time (P < 0.0001; r2 = 0.68). In addition, DSi settling rates were, on average, 6.5-fold higher in eutrophic systems than in non-eutrophic systems (median settling velocities: 11.1 and 1.7 m yr−1 for eutrophic and non-eutrophic systems, respectively; P < 0.01). SiRReLa, which incorporates these insights, performed quite well in predicting both total DSi removal (kg SiO2 yr−1; Nash Sutcliffe Efficiency (N.S.E) = 0.88) and DSi removal efficiency (% Si removed; N.S.E. = 0.75), with no detectable bias in the model. Global application of SiRReLa confirms that lentic systems are important sinks for DSi, removing 89.1 Tg DSi yr−1 from watersheds globally, roughly 19–38% of all DSi inputs to surface waters. Small lakes and reservoirs (<50 km2) were critical in the analysis, retaining 81% (72 Tg DSi yr−1) of the globally retained total. Furthermore, although reservoirs occupy just 6% of the global lentic surface area, they retained approximately 35% of the total DSi removed by lentic systems. Regional hot spots for lentic DSi removal were identified and imply that lentic systems can remove the vast majority of DSi across a large fraction of Earth's land surface. Finally, a sensitivity analysis indicates that future improvements in DSi trapping and transport models should focus on improving estimates of DSi input to surface waters.
9.	Hugelius, Gustaf (author) Spatial upscaling using thematic maps : an analysis of uncertainties in permafrost soil carbon estimates 2012 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 26, s. GB2026- Journal article (peer-reviewed)abstract Studies of periglacial regions confirm their importance in the global carbon (C) cycle, but estimates of ecosystem C storage or green-house gas fluxes from these remote areas are generally poorly constrained and quantitative estimates of upscaling uncertainties are lacking. In this study, a regional database describing soil organic carbon (SOC) storage in periglacial terrain (European Russian Arctic) was used to evaluate spatial upscaling from point measurements using thematic maps. The selection of classes for upscaling and the need for replication in soil sampling were statistically evaluated. Upscaling using a land cover classification and a soil map estimated SOC storage to 48.5 and 47.0 kg C m(-2), respectively with 95% confidence intervals (CI) within +/- 8%. When corrected for spatial errors in the LCC upscaling proxy, SOC was estimated to 46.5 kg C m(-2) with a 95% CI reflecting propagated variance from both natural variability and spatial errors of +/- 11%. Artificially decreasing the size of the database used for upscaling showed that relatively stable results could be achieved with lower replication in some upscaling classes. Decreased spatial resolution for upscaling from 30 m to 1 km had little impact on SOC estimates in this region, but classification accuracy was dramatically reduced and land cover classes show different, sometimes nonlinear, responses to scale. The methods and recommendations presented here can provide guidelines for any future study where point observations of a variable are upscaled using remotely sensed thematic maps or classifications and potential applications for circum-arctic studies are discussed. For future upscaling studies at large geographic scales, a priori determination of sample sizes and tests to insure unimodal and statistically independent samples are recommended. If these prerequisites are not fulfilled, classes may be merged or subdivided prior to upscaling.
10.	Khalili, Maria, et al. (author) Nitrogen and carbon interactions between boreal soils and lakes 2010 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 24 Journal article (peer-reviewed)abstract In this study, we used a large data set on nitrogen (N) and carbon (C) from Swedish boreal soils and lake waters to investigate N and C interactions between soils and lake waters. To link thousands of soils sites with hundreds of lake sites distributed all over Sweden, we gridded the data and found a significant relation between gridded C:N ratios of the organic soil layer and the ones of lake waters. We also found evidence of N deposition having depressed the C:N ratios of lake waters more than the ones of organic soil layers. In lake waters N strongly increased toward southern Sweden, mainly in the form of nitrate-nitrogen (NO(3)(-)-N) which we primarily attribute to an increased NO(3)(-)-N input from the boreal soils into the lakes. In contrast to N we found a much weaker direct relationship for C between soils and lake waters over Sweden. Instead, lake C was strongly related to lake morphometry and catchment characteristics. Our results indicate that large-scale variations in soil C content are not directly linked to C concentrations in lake waters, whereas soil N seems to leach in small amounts from the soils directly into the lakes in form of NO(3)(-)-N. Such differences in N and C interactions between soils and lake waters give important insights into the global biogeochemical cycling of N and C.
11.	Koehler, Birgit, 1980-, et al. (author) Sunlight-induced carbon dioxide emissions from inland waters 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28:7, s. 696-711 Journal article (peer-reviewed)abstract The emissions of carbon dioxide (CO2) from inland waters are substantial on a global scale. Yet, the fundamental question remains open which proportion of these CO2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55 to 69°N in Sweden. The sunlight-induced production of dissolved inorganic carbon (DIC) averaged 3.8 ± 0.04 g C m-2 yr-1, which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 ± 1 kt C yr-1 was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO2 emissions from Swedish lakes. With a median depth of 3.2 m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which correspond to the absorbed photons. Therefore, the slope of the regression line represents the wavelength- and depth-integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr-1 under overcast and clear sky, respectively. Consequently, these directly sunlight-induced CO2 emissions contribute up to about one tenth to the global CO2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.
12.	Laudon, Hjalmar, et al. (author) Linking variability in soil solution dissolved organic carbon to climate, soil type, and vegetation type 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28, s. 497-509 Journal article (peer-reviewed)abstract Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux, i.e., the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation, and climate conditions. We compiled a database on DOC in soil solution down to 80cm and analyzed it with the aim, first, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil, and vegetation types at global scale and second, to identify potential determinants of the site-to-site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil, and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R2=0.67-0.99) in DOC concentrations in mineral European forest soils correlates with NH4+, C/N, Al, and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.
13.	Majdi, Hooshang (author) Measuring and modeling the spectrum of fine-root turnover times in three forests using isotopes, minirhizotrons, and the Radix model 2010 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 24 Journal article (peer-reviewed)abstract Fine root (<2 mm) cycling rates are important for understanding plant ecology and carbon fluxes in forests, but they are difficult to determine and remain uncertain. This paper synthesizes minirhizotron and isotopic data and a root model and concludes that (1) fine roots have a spectrum of turnover times ranging from months to many years and (2) the mean age of live root biomass (A) and the mean age of roots when they die (i.e., their turnover time (tau)) are not equal. We estimated A and tau of fine roots in three forests using the root model Radix. For short-lived roots, we constrained tau with existing minirhizotron data; for long-lived roots, we used new radiocarbon measurements of roots sampled by diameter size class and root branch order. Long-lived root pools had site mean tau of 8-13 y and 5-9 y when sampled by diameter and branch order, respectively. Mean turnover times across sites were in general not significantly different as a function of branch-order, size class, or depth. Our modeling results indicate that similar to 20% of fine root biomass has turnover times of about a year, and similar to 80% has decadal turnover times. This partitioning is reflected in our predicted mean ages of similar to 9 y and turnover times of similar to 3 y. We estimate that fine root mortality contributes between 38 and 104 g C m(-2) y(-1) to soil in these forests. These estimates are 20 to 80% of previous estimates in these and similar forests, in part because we explicitly account for the large portion of fine-root biomass with decadal cycling rates. Our work shows that both fast and slow cycling roots must be modeled jointly to account for the heterogeneous nature of fine-root dynamics.
14.	Qureshi, Asif, et al. (author) Quantifying uncertainties in the global mass balance of mercury 2011 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 25, s. GB4012- Journal article (peer-reviewed)abstract We quantify uncertainties in the global mass balance of mercury and identify research priorities to reduce these uncertainties. This is accomplished by developing a new spatially resolved global multimedia model (WorM(3)) that quantitatively describes the fate of mercury at a process level, and conducting an uncertainty analysis on its unit-world variant which computes similar global estimates. In our modeling approach, all mass transfer processes and reactions in ocean water, soil and vegetation, are represented as pseudo-first order; reactions in air are represented using the ratios of observed concentrations of mercury species. We use Monte Carlo analysis to estimate uncertainties in the unit-world modeled global mass balance of mercury and quantitatively identify the input parameters which contribute most to these uncertainties. A key finding is that uncertainties in input parameters that describe the rates of reduction and oxidation reactions in surface ocean contribute more than uncertainties in anthropogenic emissions to the total uncertainties in atmospheric concentration and depositional fluxes of mercury. More research should therefore be targeted toward understanding of these oceanic processes.
15.	Sanchez-Garcia, Laura, et al. (author) Inventories and behavior of particulate organic carbon in the Laptev and East Siberian seas 2011 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 25, s. GB2007- Journal article (peer-reviewed)abstract Fluvial and erosional release processes in permafrost-dominated Eurasian Arctic cause transport of large amounts of particulate organic carbon (POC) to coastal waters. The marine fate of this terrestrial POC (terr-POC), water column degradation, burial in shelf sediments, or export to depth, impacts the potential for climate-carbon feedback. As part of the International Siberian Shelf Study (ISSS-08; August-September 2008), the POC distribution, inventory, and fate in the water column of the extensive yet poorly studied Eurasian Arctic Shelf seas were investigated. The POC concentration spanned 1-152 mu M, with highest values in the SE Laptev Sea. The POC inventory was constrained for the Laptev (1.32 +/- 0.09 Tg) and East Siberian seas (2.85 +/- 0.20 Tg). A hydraulic residence time of 3.5 +/- 2 years for these Siberian shelf seas yielded a combined annual terr-POC removal flux of 3.9 +/- 1.4 Tg yr(-1). Accounting for sediment burial and shelf-break exchange, the terr-POC water column degradation was similar to 2.5 +/- 1.6 Tg yr(-1), corresponding to a first-order terr-POC degradation rate constant of 1.4 +/- 0.9 yr(-1), which is 5-10 times faster than reported for terr-DOC degradation in the Arctic Ocean. This terr-POC degradation flux thus contributes substantially to the dissolved inorganic carbon excess of 10 Tg C observed during ISSS-08 for these waters. This evaluation suggests that extensive decay of terr-POC occurs already in the water column and contributes to outgassing of CO(2). This process should be considered as a geographically dislocated carbon-climate coupling where thawing of vulnerable permafrost carbon on land is eventually adding CO(2) above the ocean.
16.	Sánchez-García, Laura, et al. (author) The sequestration sink of soot black carbon in the Northern European Shelf sediments 2012 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 26 Journal article (peer-reviewed)abstract To test the hypothesis that ocean margin sediments are a key final repository in the large-scale biogeospheric cycling of soot black carbon (soot-BC), an extensive survey was conducted along the ∼2,000 km stretch of the Swedish Continental Shelf (SCS). The soot-BC content in the 120 spatially distributed SCS sediments was 0.180.13 0.26% dw (median with interquartile ranges), corresponding to ∼5% of total organic carbon. Using side-scan sonar constraints to estimate the areal fraction of postglacial clay sediments that are accumulation bottoms (15% of SCS), the soot-BC inventory in the SCS mixed surface sediment was estimated at ∼4,000 Gg. Combining this with radiochronological constraints on sediment mass accumulation fluxes, the soot-BC sink on the SCS was ∼300 Gg/yr, which yielded an area-extrapolated estimate for the Northern European Shelf (NES) of ∼1,100 Gg/yr. This sediment soot-BC sink is ∼50 times larger than the river discharge fluxes of soot-BC to these coastal waters, however, of similar magnitude as estimates of atmospheric soot-BC emission from the upwind European continent. While large uncertainties remain regarding the large-scale to global BC cycle, this study combines with two previous investigations to suggest that continental shelf sediments are a major final repository of atmospheric soot-BC. Future progress on the soot-BC cycle and how it interacts with the full carbon cycle is likely to benefit from 14C determinations of the sedimentary soot-BC and similar extensive studies of coastal sediment in complementary regimes such as off heavily soot-BC-producing areas in S and E Asia and on the large pan-Arctic shelf. Copyright 2012 by the American Geophysical Union.
17.	Serrano, O., et al. (author) Millennial scale impact on the marine biogeochemical cycle of mercury from early mining on the Iberian Peninsula 2013 In: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 27:1, s. 21-30 Journal article (peer-reviewed)abstract The high-resolution mercury record of a Posidonia oceanica mat in the northwest Mediterranean provides an unprecedented testimony of changes in environmental mercury (Hg) loading to the coastal marine environment over the past 4315 yr BP. The period reconstructed made it possible to establish tentative preanthropogenic background Hg levels for the area (6.8 +/- 1.5 ng g(-1) in bulk sediments). A small, but significant, anthropogenic Hg increase was identifiable by similar to 2500 yr BP, in agreement with the beginning of intense mining in Spain. Changes in the record suggest four major periods of anthropogenic Hg pollution inputs to the Mediterranean: first, during the Roman Empire (2100-1800 yr BP); second, in the Late Middle Ages (970-650 yr BP); third, in the modern historical era (530-380 yr BP); and fourth, in the industrial period (last 250 years), with Hg concentrations two-, four-, five-, and tenfold higher than background concentrations, respectively. Hg from anthropogenic sources has dominated during the last millennium (increase from similar to 12 to similar to 100 ng g(-1)), which can be related to the widespread historical exploitation of ore resources on the Iberian Peninsula. The chronology of Hg concentrations in the mat archive, together with other Hg pollution records from the Iberian Peninsula, suggests regional-scale Hg transport and deposition and shows earlier marine Hg pollution than elsewhere in Europe. Moreover, the mat also records a higher number of historic contamination phases, in comparison with other natural archives, probably due to the fact that the bioaccumulating capacity of P. oceanica magnify environmental changes in Hg concentrations. In this study, we demonstrate the uniqueness of P. oceanica meadows as a long-term archive recording trends in Hg abundance in the marine coastal environment, as well as its potential role in the Mediterranean as a long-term Hg sink.
18.	Wallin, Marcus B., et al. (author) Spatiotemporal variability of the gas transfer coefficient (KCO2) in boreal streams : Implications for large scale estimates of CO2 evasion 2011 In: Global Biogeochemical Cycles. - : John Wiley & Sons. - 0886-6236 .- 1944-9224. ; 25:3, s. GB3025- Journal article (peer-reviewed)abstract Boreal streams represent potentially important conduits for the exchange of carbon dioxide (CO(2)) between terrestrial ecosystems and the atmosphere. The gas transfer coefficient of CO(2) (K(CO2)) is a key variable in estimating this source strength, but the scarcity of measured values in lotic systems creates a risk of incorrect flux estimates even when stream gas concentrations are well known. This study used 114 independent measurements of K(CO2) from 14 stream reaches in a boreal headwater system to determine and predict spatiotemporal variability in K(CO2). The K(CO2) values ranged from 0.001 to 0.207 min(-1) across the 14 sites. Median K(CO2) for a specific site was positively correlated with the slope of the stream reach, with higher gas transfer coefficients occurring in steeper stream sections. Combining slope with a width/depth index of the stream reach explained 83% of the spatial variability in K(CO2). Temporal variability was more difficult to predict and was strongly site specific. Variation in K(CO2), rather than pCO(2), was the main determinant of stream CO(2) evasion. Applying published generalized gas transfer velocities produced an error of up to 100% in median instantaneous evasion rates compared to the use of actual measured K(CO2) values from our field study. Using the significant relationship to local slope, the median K(CO2) was predicted for 300,000 km of watercourses (ranging in stream order 1-4) in the forested landscape of boreal/nemoral Sweden. The range in modeled stream order specific median K(CO2) was 0.017-0.028 min(-1) and there was a clear gradient of increasing K(CO2) with lower stream order. We conclude that accurate regional scale estimates of CO(2) evasion fluxes from running waters are possible, but require a good understanding of gas exchange at the water surface.
19.	Wallin, Marcus, et al. (author) Representative regional sampling of carbon dioxide and methane concentrations in hemiboreal headwater streams reveal underestimates in less systematic approaches 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28, s. 465-479 Journal article (peer-reviewed)abstract Boreal headwater streams have been identified as hot spots for evasion of greenhouse gases (GHGs). This study was the first to systematically determine the concentrations of CO2 and CH4 in hemiboreal headwater streams. The use of a headspace sampling method focusing on GHGs in combination with a statistically representative selection of more than 200 streams across two regions in Sweden was the basis for defining the base flow concentrations of CO2 and CH4. All streams were supersaturated relative to the atmosphere in CO2 and the majority in CH4 for the 82% of streams in which CH4 was detected. The spatial variability in both CO2 and CH4 was high but positively related to total organic carbon, mean annual temperature, and proportion of peatland in the catchment. There were, however, regional differences in the spatial controls, which are something that predictive models need to consider. The data set allowed for comparison between a headspace and an alkalinity-based method for determining CO2. More than 50% of the streams contained no alkalinity which made the alkalinity-based determination of CO2 impossible. In addition, half of the streams with alkalinity had alkalinities low enough (<0.07meqL(-1)) to make the CO2 determination very uncertain. The streams with low pH and no alkalinity contained median CO2 concentrations that were 45% higher than the streams containing alkalinity. Therefore, large-scale generalizations about CO2 in such headwaters will be significantly underestimated if (1) headwaters are underrepresented and (2) the headwaters are sampled but CO2 is calculated from their alkalinity.Key Points 207 statistically selected headwaters were directly sampled for CO2 and CH4 All streams were supersaturated in CO2 and CH4 but with large spatial variation Indirect methods will create bias in large scale data sets of headwater CO2
20.	Winterdahl, Mattias, et al. (author) Intra-annual variability of organic carbon concentrations in running waters : Drivers along a climatic gradient 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28:4, s. 451-464 Journal article (peer-reviewed)abstract Trends in surface water dissolved organic carbon (DOC) concentrations have received considerable scientific interest during recent decades. However, intra-annual DOC variability is often orders of magnitude larger than long-term trends. Unraveling the controls on intra-annual DOC dynamics holds the key to a better understanding of long-term changes and their ecological significance. We quantified and characterized intra-annual DOC variability and compared it with long-term DOC trends in 136 streams and rivers, varying in size and geographical characteristics, across a 1400km latitudinal gradient during 2000-2010. Discharge, temperature, and month of the year were the most significant predictors of intra-annual DOC variability in a majority of the running waters. Relationships between DOC, discharge, and temperature were, however, different along a mean annual temperature (MAT) gradient. Running waters with low MAT generally displayed positive DOC-discharge correlations whereas the relationships in sites with higher MAT were more variable. This reflected contrasting relationships between temperature and discharge with discharge positively correlated with temperature in cold areas, while it was negatively correlated with temperature in catchments with higher MAT. Sites where flow, temperature, and month were poorly related to intra-annual DOC dynamics were large catchments or sites with extensive upstream lake cover. DOC trends were generally much smaller than intra-annual DOC variability and did not show any north-south gradient. Our findings suggest that DOC in running waters could respond to a changing climate in ways not predictable, or even discernible, from extrapolation of recent interannual trends. Key Points Large-scale characterization of intra-annual DOC dynamics in running waters Discharge, temperature and month are significant predictors of DOC variability Shifting patterns in DOC dynamics along a 1400 km climatic gradient
21.	Winton, V.H.L., et al. (author) The contribution of aeolian sand and dust to iron fertilization of phytoplankton blooms in southwestern Ross Sea, Antarctica 2014 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. Journal article (peer-reviewed)
22.	Young, Giles H. F., et al. (author) Age trends in tree ring growth and isotopic archives : A case study of Pinus sylvestris L. from northwestern Norway 2011 In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 25, s. GB2020- Journal article (peer-reviewed)abstract Measurements of tree ring width and relative density have contributed significantly to many of the large-scale reconstructions of past climatic change, but to extract the climate signal it is first necessary to remove any nonclimatic age-related trends. This detrending can limit the lower-frequency climate information that may be extracted from the archive (the segment length curse). This paper uses a data set of ring widths, maximum latewood density and stable carbon and oxygen isotopes from 28 annually resolved series of known-age Pinus sylvestris L. trees in northwestern Norway to test whether stable isotopes in tree rings require an equivalent statistical detrending. Results indicate that stable oxygen and carbon isotope ratios from tree rings whose cambial age exceeds c.50 years exhibit no significant age trends and thus may be used to reconstruct environmental variability and physiological processes at this site without the potential loss of low-frequency information associated with detrending.
23.	Calleja, Maria Ll., et al. (author) Prevalence of strong vertical CO2 and O-2 variability in the top meters of the ocean 2013 In: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236. ; 27:3, s. 941-949 Journal article (peer-reviewed)abstract The gradient in the partial pressure of carbon dioxide (pCO(2)) across the air-sea boundary layer is the main driving force for the air-sea CO2 flux. Global data bases for surface seawater pCO(2) are actually based on pCO(2) measurements from several meters below the sea surface, assuming a homogeneous distribution between the diffusive boundary layer and the upper top meters of the ocean. Compiling vertical profiles of pCO(2), temperature, and dissolved oxygen in the upper 5-8 m of the ocean from different biogeographical areas, we detected a mean difference between the boundary layer and 5 m pCO(2) of 131 mu atm. Temperature gradients accounted for only 11% of this pCO(2) gradient in the top meters of the ocean; thus, pointing to a heterogeneous biological activity underneath the air-sea boundary layer as the main factor controlling the top meters pCO(2) variability. Observations of pCO(2) just beneath the air-sea boundary layer should be further investigated in order to estimate possible biases in calculating global air-sea CO2 fluxes.
24.	Karlsson, Anders, et al. (author) Distribution, transport, and production of volatile halocarbons in the upper waters of the ice-covered high Arctic Ocean 2013 In: Global Biogeochemical Cycles. - 0886-6236. ; 27:4, s. 1246-1261 Journal article (peer-reviewed)abstract Volatile halogenated compounds (CHBr3, CH2Br2, CHBr2Cl, and CH2ClI) were measured in the water column and in sea ice brine across the Arctic Ocean, from Barrow, Alaska, to Svalbard, during the Beringia 2005 expedition (August–September) with RV/IB Oden. High concentrations of brominated compounds (up to 42 pmol kg−1 of bromoform) were found under multiyear ice in the surface waters over the Makarov Basin and the Lomonosov Ridge, near the North Pole. Even higher concentrations (bromoform up to 160 pmol kg−1) were found in sea ice brine. We propose that the high load of riverine dissolved organic matter that is transported in the Transpolar Drift is a main factor responsible for the high concentration of brominated volatile compounds found in sea ice brine and upper waters and that cycles of freezing and thawing during the transport enhance the transfer of halocarbons to the seawater. The iodinated compound (CH2ClI) showed a completely different distribution with highest concentrations in water of Pacific origin in the mixed layer and upper halocline of the northern Canada Basin and over the Alpha Ridge. In the southern Canada Basin, low concentrations of halocarbons were found in upper waters. Higher concentrations in water of Pacific origin, especially on the continental shelf, indicate production in the shelf regions, likely in the Chukchi Sea and the East Siberian Sea.
25.	Metzl, Nicolas, et al. (author) Recent acceleration of the sea surface fCO2 growth rate in the North Atlantic subpolar gyre (1993–2008) revealed by winter observations 2010 In: GLOBAL BIOGEOCHEMICAL CYCLES. - 0886-6236. ; 24 Journal article (peer-reviewed)abstract Recent studies based on ocean and atmospheric carbon dioxide (CO2) observations, suggesting that the ocean carbon uptake has been reduced, may help explain the increase in the fraction of anthropogenic CO2 emissions that remain in the atmosphere. Is it a response to climate change or a signal of ocean natural variability or both? Regional process analyses are needed to follow the ocean carbon uptake and to enable better attributions of the observed changes. Here, we describe the evolution of the surface ocean CO2 fugacity (fCO2 oc) over the period 1993–2008 in the North Atlantic subpolar gyre (NASPG). This analysis is based primarily on observations of dissolved inorganic carbon (DIC) and total alkalinity (TA) conducted at different seasons in the NASPG between Iceland and Canada. The fCO2 oc trends based on DIC and TA data are also compared with direct fCO2 measurements obtained between 2003 and 2007 in the same region. During winters 1993–2003, the fCO2 oc growth rate was 3.7 (±0.6) matm yr−1, higher than in the atmosphere, 1.8 (±0.1) matm yr−1. This translates to a reduction of the ocean carbon uptake primarily explained by sea surface warming, up to 0.24 (±0.04) °C yr−1. This warming is a consequence of advection of warm water northward from the North Atlantic into the Irminger basin, which occurred as the North Atlantic Oscillation (NAO) index moved into a negative phase in winter 1995/1996. In winter 2001–2008, the fCO2 oc rise was particularly fast, between 5.8 (±1.1) and 7.2 (±1.3) matm yr−1 depending on the region, more than twice the atmospheric growth rate of 2.1 (±0.2) matm yr−1, and in the winter of 2007–2008 the area was supersaturated with CO2. As opposed to the 1990s, this appears to be almost entirely due to changes in seawater carbonate chemistry, the combination of increasing DIC and decreasing of TA. The rapid fCO2 oc increase was not only driven by regional uptake of anthropogenic CO2 but was also likely controlled by a recent increase in convective processes‐vertical mixing in the NASPG and cannot be directly associated with NAO variability. The fCO2 oc increase observed in 2001–2008 leads to a significant drop in pH of −0.069 (±0.007) decade−1.
26.	Tang, Guoping, et al. (author) Potential future dynamics of carbon fluxes and pools in New England forests and their climatic sensitivities: A model-based study 2014 In: Global Biogeochemical Cycles. - 0886-6236. ; 28:3, s. 286-299 Journal article (peer-reviewed)abstract Projections of terrestrial carbon (C) dynamics must account for interannual variation in ecosystem C exchange associated with climate change, increasing atmospheric CO2 concentration, and species dynamics. We used a dynamic ecosystem model to (i) project the potential dynamics of C in New England forests under nine climate change scenarios (CCSs) for the 21st century and (ii) examine the sensitivity of potential C dynamics to changes in climate and atmospheric CO2 concentration. Our results indicated that forest net primary productivity (NPP) and soil heterotrophic respiration (RH) averaged 428 and 279gC/m(2)/yr and New England forests sequestered CO2 by 149gC/m(2)/yr in the baseline period (1971-2000). Under the nine future CCSs, NPP and RH were modeled to increase by an average rate of 0.85 and 0.56gC/m(2)/yr(2) during 1971-2099. The asymmetric increase in NPP and RH resulted in New England forests sequestering atmospheric CO2 at a net rate of 0.29gC/m(2)/yr(2) with increases in vegetation and soil C. Simulations also indicated that climate warming alone decreases NPP, resulting in a net efflux of C from forests. In contrast, increasing precipitation by itself stimulates CO2 sequestration by forests. At the individual cell level, however, changes in temperature or precipitation can either positively or negatively affect consequent C dynamics. Elevation of CO2 levels was found to be the biggest driver for modeled future enhancement of C sequestration. Without the elevation of CO2 levels, climate warming has the potential to change New England forests from C sinks to sources in the late 21st century. Key Points Carbon sequestration in New England forests Complexity of climatic sensitivities of carbon dynamics Future potential carbon dynamics
27.	Ulfsbo, Adam, 1985, et al. (author) Late summer net community production in the central Arctic Ocean using multiple approaches 2014 In: Global Biogeochemical Cycles. - 0886-6236. ; 28:10, s. 1129-1148 Journal article (peer-reviewed)abstract Large-scale patterns of net community production (NCP) were estimated during the late summer cruise ARK-XXVI/3 (TransArc, August/September 2011) to the central Arctic Ocean. Several approaches were used based on the following: (i) continuous measurements of surface water oxygen to argon ratios (O2/Ar), (ii) underway measurements of surface partial pressure of carbon dioxide (pCO2), (iii) discrete samples of dissolved inorganic carbon, and (iv) dissolved inorganic nitrogen and phosphate. The NCP estimates agreed well within the uncertainties associated with each approach. The highest late summer NCP (up to 6mol C m−2) was observed in the marginal sea ice zone region. Low values (<1mol C m−2) were found in the sea ice-covered deep basins with a strong spatial variability. Lowest values were found in the Amundsen Basin and moderate values in the Nansen and Makarov Basins with slightly higher estimates over the Mendeleev Ridge. Our findings support a coupling of NCP to sea ice coverage and nutrient supply and thus stress a potential change in spatial and temporal distribution of NCP in a future Arctic Ocean. To follow the evolution of NCP in space and time, it is suggested to apply one or several of these approaches in shipboard investigations with a time interval of 3 to 5 years.
28.	Yuan, Wenping, et al. (author) Redefinition and global estimation of basal ecosystem respiration rate 2011 In: Global Biogeochemical Cycles. - 0886-6236. ; 25 Journal article (peer-reviewed)abstract Basal ecosystem respiration rate (BR), the ecosystem respiration rate at a given temperature, is a common and important parameter in empirical models for quantifying ecosystem respiration (ER) globally. Numerous studies have indicated that BR varies in space. However, many empirical ER models still use a global constant BR largely due to the lack of a functional description for BR. In this study, we redefined BR to be ecosystem respiration rate at the mean annual temperature. To test the validity of this concept, we conducted a synthesis analysis using 276 site-years of eddy covariance data, from 79 research sites located at latitudes ranging from similar to 3 degrees S to similar to 70 degrees N. Results showed that mean annual ER rate closely matches ER rate at mean annual temperature. Incorporation of site-specific BR into global ER model substantially improved simulated ER compared to an invariant BR at all sites. These results confirm that ER at the mean annual temperature can be considered as BR in empirical models. A strong correlation was found between the mean annual ER and mean annual gross primary production (GPP). Consequently, GPP, which is typically more accurately modeled, can be used to estimate BR. A light use efficiency GPP model (i.e., EC-LUE) was applied to estimate global GPP, BR and ER with input data from MERRA (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate resolution Imaging Spectroradiometer). The global ER was 103 Pg C yr (-1), with the highest respiration rate over tropical forests and the lowest value in dry and high-latitude areas.
29.	Ziehn, T, et al. (author) Investigating spatial differentiation of model parameters in a carbon cycle data assimilation system 2011 In: Global Biogeochemical Cycles. - 0886-6236. ; 25, s. 2021-2021 Journal article (peer-reviewed)abstract Better estimates of the net exchange of CO(2) between the atmosphere and the terrestrial biosphere are urgently needed to improve predictions of future CO(2) levels in the atmosphere. The carbon cycle data assimilation system (CCDAS) offers the capability of inversion, while it is at the same time based on a process model that can be used independent of observational data. CCDAS allows the assimilation of atmospheric CO(2) concentrations into the terrestrial biosphere model BETHY, constraining its process parameters via an adjoint approach. Here, we investigate the effect of spatial differentiation of a universal carbon balance parameter of BETHY on posterior net CO(2) fluxes and their uncertainties. The parameter, beta, determines the characteristics of the slowly decomposing soil carbon pool and represents processes that are difficult to model explicitly. Two cases are studied with an assimilation period of 1979 to 2003. In the base case, there is a separate beta for each plant functional type (PFT). In the regionalization case, beta is differentiated not only by PFT, but also according to each of 11 large continental regions as used by the TransCom project. We find that the choice of spatial differentiation has a profound impact not only on the posterior (optimized) fluxes and their uncertainties, but even more so on the spatial covariance of the uncertainties. Differences are most pronounced in tropical regions, where observations are sparse. While regionalization leads to an improved fit to the observations by about 20% compared to the base case, we notice large spatial variations in the posterior net CO(2) flux on a grid cell level. The results illustrate the need for universal process formulations in global-scale atmospheric CO(2) inversion studies, at least as long as the observational network is too sparse to resolve spatial fluctuations at the regional scale.
30.	Ziehn, T., et al. (author) On the capability of Monte Carlo and adjoint inversion techniques to derive posterior parameter uncertainties in terrestrial ecosystem models 2012 In: Global Biogeochemical Cycles. - 0886-6236. ; 26, s. 3025-3025 Journal article (peer-reviewed)abstract Terrestrial ecosystem models (TEMs) contain the coupling of many biogeochemical processes with a large number of parameters involved. In many cases those parameters are highly uncertain. In order to reduce those uncertainties, parameter estimation methods can be applied, which allow the model to be constrained against observations. We compare the performance and results of two such parameter estimation techniques - the Metropolis algorithm (MA) which is a Markov Chain Monte Carlo (MCMC) method and the adjoint approach as it is used in the Carbon Cycle Data Assimilation System (CCDAS). Both techniques are applied here to derive the posterior probability density function (PDF) for 19 parameters of the Biosphere Energy Transfer and Hydrology (BETHY) scheme. We also use the MA to sample the posterior parameter distribution from the adjoint inversion. This allows us to assess if the commonly made assumption in variational data assimilation, that everything is normally distributed, holds. The comparison of the posterior parameter PDF derived by both methods shows that in most cases an approximation of the PDF by a normal distribution as used by the adjoint approach is a valid assumption. The results also indicate that the global minimum has been identified by both methods for the given set up. However, the adjoint approach outperforms the MA by several orders of magnitude in terms of computational time. Both methods show good agreement in the PDF of estimated net carbon fluxes for the decades of the 1980s and 1990s.

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