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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (21 followers)
Geophysical Research Letters     Full-text available via subscription   (41 followers)
Global Biogeochemical Cycles     Full-text available via subscription   (3 followers)
Journal of Advances in Modeling Earth Systems     Open Access   (2 followers)
Journal of Geophysical Research : Atmospheres     Partially Free   (18 followers)
Journal of Geophysical Research : Biogeosciences     Full-text available via subscription   (5 followers)
Journal of Geophysical Research : Earth Surface     Partially Free   (22 followers)
Journal of Geophysical Research : Oceans     Partially Free   (14 followers)
Journal of Geophysical Research : Planets     Full-text available via subscription   (12 followers)
Journal of Geophysical Research : Solid Earth     Full-text available via subscription   (21 followers)
Journal of Geophysical Research : Space Physics     Full-text available via subscription   (13 followers)
Paleoceanography     Full-text available via subscription   (4 followers)
Radio Science     Full-text available via subscription   (3 followers)
Reviews of Geophysics     Full-text available via subscription   (17 followers)
Space Weather     Full-text available via subscription   (3 followers)
Tectonics     Full-text available via subscription   (7 followers)
Water Resources Research     Full-text available via subscription   (82 followers)
Global Biogeochemical Cycles    [5 followers]  Follow    
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     ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.4]   [H-I: 109]
  • Linking variability in soil solution dissolved organic carbon to climate,
           soil type and vegetation type
    • 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 80 cm and analyzed it with the aim, firstly, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil and vegetation types at global scale and, secondly, 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.
  • Intra‐annual variability of organic carbon concentrations in running
           waters: drivers along a climatic gradient
    • 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. Unravelling 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 1400 km 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 inter‐annual trends.
  • Natural and anthropogenic variations in atmospheric mercury deposition
           during the Holocene near Quelccaya Ice Cap, Peru
    • Abstract: Mercury (Hg) is a toxic metal that is transported globally through the atmosphere. The emission of Hg from mineral reservoirs and subsequent recycling in surface reservoirs (i.e., soil/biomass, ocean, and atmosphere) are fundamental to the modern global Hg cycle, yet past emissions from anthropogenic and natural sources are not fully constrained. We use a sediment core from Yanacocha, a headwater lake in southeastern Peru, to study the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. From 12.3 to 3.5 ka, Hg fluxes in the record are relatively constant (mean ± 1σ: 1.4 ± 0.6 µg m‐2 a‐1, n = 189). Past Hg deposition does not correlate with changes in regional temperature and precipitation, inferred from nearby paleoclimate records, or with most large volcanic events that occurred regionally, in the Andean Central Volcanic Zone (~300‐400 km from Yanacocha), and globally. In B.C. 1450 (3.4 ka), Hg fluxes abruptly increased and reached the Holocene‐maximum flux (6.7 µg m‐2 a‐1) in B.C. 1200, concurrent with a ~100‐year peak in Fe and chalcophile metals (As, Ag, Tl) and the presence of framboidal pyrite. Continuously elevated Hg fluxes from B.C. 1200‐500 suggest a protracted mining‐dust source near Yanacocha that is identical in timing to documented pre‐Incan cinnabar mining in central Peru. During Incan and Colonial time (A.D. 1450‐1650), Hg deposition remains elevated relative to background levels but lower relative to other Hg records from sediment cores in central Peru, indicating a limited spatial extent of preindustrial Hg emissions. Hg fluxes from A.D. 1980 to 2011 (4.0 ± 1.0 µg m‐2 a‐1, n = 5) are 3.0 ± 1.5 times greater than pre‐anthropogenic fluxes and are similar to modern fluxes documented in remote lakes around the world.
  • Representative regional sampling of carbon dioxide and methane
           concentrations in hemiboreal headwater streams reveal underestimates in
           less systematic approaches
    • Abstract: Boreal headwater streams have been identified as hotspots for water‐air exchange of greenhouse gases (GHG´s). Despite these findings GHG concentrations and fluxes in headwaters are to a great extent unexplored at large (regional/national) scales. This study was the first to systematically determine the concentrations of CO2 and CH4 in hemiboreal (southern boreal and boreonemoral) headwater streams. The use of a headspace sampling method focusing on GHG´s 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 supersaturation level of CO2 and CH4. All streams were supersaturated relative to the atmosphere in CO2 (median concentration, 1.9 (±1.1) mg C L‐1) and the majority in CH4 (median concentration, 7.1 (±54.0) µg C L‐1 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 is something that predictive models need to consider. The large and representative data set allowed for comparison between a headspace and an alkalinity‐based method for determining CO2 in these headwaters. 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 alkalinities low enough (
    • Abstract: In the tropical Andes, fires play an important cultural and ecological role. However, there have been very few systematic studies aimed at understanding the biomass burning dynamics in the area. This paper seeks to advance on our understanding of burning regimes in this region, with the first detailed and comprehensive assessment of fire occurrence and the derived gross biomass burning emissions of an area of the Peruvian tropical Andes. We selected for analysis an area of 2.8 million hectares at altitudes over 2000 m. We analyzed fire occurrence over a 12‐year period with three types of satellite data: active fire pixels from the MODerate Resolution Imaging Spectroradiometer (MODIS) MCD14ML product, burned area scars from the MODIS MCD45 product, and higher resolution Landsat 5 TM imagery. Fire dynamics showed a large intra‐ and inter‐annual variability, with most fires occurring May‐October (the period coinciding with the dry season), and year 2005 having the largest number of fires and burned area. Total area burned decreased with increasing rainfall until a given rainfall threshold beyond which no relationship was found. The estimated fire return interval (FRI) for the area is 37 years for grasslands, which is within the range reported for grasslands and 65 years for forests, which is remarkably shorter than other reported FRI in tropical moist forests. The greatest contribution (60‐70%, depending of the data source) to biomass burning emissions came from burned montane cloud forests (4.5 million Mg CO2 over the study period), despite accounting for only 7.4‐10% of the total burned area. Gross aboveground biomass emissions (7.55 ± 2.14 Tg CO2; 0.43 ± 0.04 Tg CO; 24,012 ± 2,685 Mg CH4 for the study area) were larger than previously reported for the Tropical Andes.
  • The contribution of aeolian sand and dust to iron fertilization of
           phytoplankton blooms in southwestern Ross Sea, Antarctica
    • Abstract: Iron (Fe) limitation during the austral summer is a persistent characteristic of primary production in the Ross Sea, Antarctica. Recent observations reveal low dissolved Fe (dFe) concentrations in the Ross Sea polynya after the dFe winter reserve has been consumed in association with high rates of primary production, suggesting significant new sources of dFe are required to sustain the phytoplankton bloom during this period. The accumulation of locally‐derived aeolian sand and dust (ASD) on sea ice is a potential source. To constrain aeolian Fe inputs from melting sea ice, we determined ASD mass accumulation rates as well as the total and soluble Fe content on first year sea ice in McMurdo Sound (Latitude 77.5ºS; Longitude 165ºE), southwestern (SW) Ross Sea. The mean ASD mass accumulation rate was ~1.5 g m‐2 yr‐1, total Fe content of this ASD was 4 ± 1 wt % and the percentage of soluble Fe was 11 ± 1 %. Assuming our results are representative of the 7400 km2 McMurdo Sound region, we use our mean estimate of the bulk aeolian dFe flux of 122.1 µmol m‐2 yr‐1 to calculate that aeolian Fe potentially supports between 9.0 x 109 and 4.1 x 1011 mol C yr‐1 (0.1‐4.9 Tg C yr‐1) of new primary production in McMurdo Sound. This equates to ~15 % of new primary production in the SW Ross Sea, suggesting that locally‐derived aeolian dFe is a minor component of seasonal Fe supply. Consequently, this study highlights the need to quantify other sources of dFe to the Ross Sea region, which can potentially sustain phytoplankton blooms during the austral summer. In comparison to other estimates in the Antarctic region, McMurdo Sound has very high ASD and represents an upper limit of dFe that can be contributed to the ocean from melting sea ice in the Ross Sea.
  • Issue Information
    • Abstract: No abstract is available for this article.
  • Ecological processes dominate the 13C land disequilibrium in a Rocky
           Mountain subalpine forest
    • Abstract: Fossil fuel combustion has increased atmospheric CO2 by ≈ 115 µmol mol‐1 since 1750, and decreased its carbon isotope composition (δ13C) by 1.7‐2 ‰ (the 13C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ13C of CO2 released by terrestrial ecosystems is expected to differ from the δ13C of CO2 assimilated by land plants during photosynthesis. This isotopic difference between land‐atmosphere respiration (δR) and photosynthetic assimilation (δA) fluxes gives rise to the 13C land disequilibrium (D). Contemporary understanding suggests that over annual and longer time scales, D is determined primarily by the Suess effect, and thus D is generally positive (δR > δA). A seven‐year record of biosphere‐atmosphere carbon exchange was used to evaluate the seasonality of δA and δR, and the 13C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ13C of net land‐atmosphere exchange during day and night, and combined with tower‐based flux observations to assess δA and δR. The disequilibrium varied seasonally, and when flux‐weighted was opposite in sign than expected from the Suess effect (D = ‐0.75 ± 0.21 ‰ or ‐0.88 ± 0.10 ‰ depending on method). Seasonality in D appeared to be driven by photosynthetic discrimination (Δcanopy) responding to environmental factors. Possible explanations for negative D include: 1) changes in Δcanopy over decades as CO2 and temperature have risen, and/or 2) post‐photosynthetic fractionation processes leading to sequestration of isotopically‐enriched carbon in long‐lived pools like wood and soil.
  • Drivers of pCO2 variability in two contrasting coral reef lagoons: The
           influence of submarine groundwater discharge
    • Abstract: The impact of groundwater on pCO2 variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of pCO2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high pCO2 (5,501 µatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and pCO2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO2 (549 µatm) than Heron Island (471 µatm), however, pCO2 exhibited a greater diel range in Heron Island (778 µatm) than in Rarotonga (507 µatm). SGD flux rates were quantified using a radon (222Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free‐CO2 m−2 d−1 from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free‐CO2 m−2 d−1. Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO2 m−2 d−1 in Rarotonga and Heron Island, respectively), with SGD‐derived free‐CO2 most likely contributing to the outgassing of CO2. Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO3) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.
  • Air‐sea CO2 fluxes in the California Current: Impacts of model
           resolution and coastal topography
    • Abstract: The present study uses a suite of coupled physical‐biogeochemical model simulations at 1/3°, 1/10°, and 1/30° to assess the impact of horizontal resolution on air‐sea CO2 fluxes in the California Current System (CCS), a relevant issue for downscaling between coarser resolution global climate models and higher resolution regional models. The results demonstrate that horizontal resolution is important to reproduce the sharp transition between near‐shore outgassing and offshore absorption, as well as to resolve the regions of enhanced near‐shore outgassing in the lee of capes. The width of the outgassing region is overestimated when horizontal resolution is not eddy‐resolving (i.e., 1/3°), but becomes more dependent on shelf topography for eddy‐resolving simulations (i.e., 1/10° and 1/30°). Enhanced near‐shore outgassing is associated with local increases in wind‐driven upwelling in the lee of capes (i.e., expansion fans), meaning that sufficient horizontal resolution is needed both in the ocean circulation model and in the wind field forcing the model. From a global carbon budget perspective, the model indicates that biological production generates sufficient absorption within a few hundred kilometers of the coast to offset near‐shore outgassing, which is consistent with the notion that mid‐latitude eastern boundary current upwelling systems act both as a sink and source for atmospheric CO2. Based on the 1/30° solution, the CCS between 35‐45N and out to 600 km offshore is as a net carbon sink of ca. 6 TgC yr−1, with the 1/10° solution underestimating this value by less than 10% and the 1/3° solution by a factor of three.
  • Calcium carbonate dissolution in the upper 1000 m of the eastern
           North Atlantic
    • Abstract: Recent analyses suggest that considerable CaCO3 dissolution may occur in the upper water column of the ocean (< 1500 m). This study uses the distribution of particulate calcium from high‐resolution suspended matter sampling along the CLIVAR/CO2 Repeat Hydrography A16N transect in 2003 to estimate CaCO3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400–1000 m) in the eastern tropical North Atlantic. The CaCO3 dissolution rate in this region is estimated to be 0.9 mmol CaCO3 m‐2 d‐1, indicating this region is a hotspot for upper water column CaCO3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO3 m‐2 d‐1) and are comparable to previous estimates of CaCO3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface‐ocean CaCO3 standing stocks suggests that biologically‐mediated CaCO3 dissolution may be occurring in the top 1000 m of the Atlantic.
  • The viscosity effect on marine particle flux – a climate relevant
           feedback mechanism
    • Abstract: Oceanic uptake and long‐term storage of atmospheric carbon dioxide (CO2) are strongly driven by the marine ‘biological pump’, i.e. sinking of biotically fixed inorganic carbon and nutrients from the surface into the deep ocean [Sarmiento and Bender, 1994; Volk and Hoffert, 1985]. Sinking velocity of marine particles depends on seawater viscosity, which is strongly controlled by temperature [Sharqawy et al., 2010]. Consequently, marine particle flux is accelerated as ocean temperatures increase under global warming [Bach et al., 2012]. Here we show that this previously overlooked 'viscosity effect' could have profound impacts on marine biogeochemical cycling and carbon uptake over the next centuries to millennia. In our global‐warming simulation, the viscosity effect accelerates particle sinking by up to 25%, thereby effectively reducing the portion of organic matter that is respired in the surface ocean. Accordingly, the biological carbon pump's efficiency increases, enhancing the sequestration of atmospheric CO2 into the ocean. This effect becomes particularly important on longer timescales when warming reaches the ocean interior. At the end of our simulation (4000 AD) oceanic carbon uptake is 17% higher, atmospheric CO2 concentration is 180 ppm lower, and the increase in global average surface temperature is 8% weaker when considering the viscosity effect. Consequently, the viscosity effect could act as a long‐term negative feedback mechanism in the global climate system.
  • The fate of terrigenous dissolved organic carbon in a
           river‐influenced ocean margin
    • Abstract: The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air‐sea CO2 exchange in ocean margins. However, the extreme heterogeneity of river‐influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations, and thereby obscures the role of tDOC in biogeochemical cycles. Here, a lignin based optical proxy for tDOC and a shelf‐wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi‐Atchafalaya River System (M‐ARS) to the Louisiana shelf. The mass balance revealed that ~40% of the tDOC discharged by the M‐ARS during March 2009‐2010 was mineralized to CO2 on the Louisiana shelf, with 2/3 of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates several‐fold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for ~94% of the tDOC mineralization on an annual basis, and suggest that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms ocean margins are major sinks of the tDOC discharged by rivers, and indicate tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2.
  • Soil organic carbon sequestration in upland soils of northern China under
           variable fertilizer management and climate change scenarios
    • Abstract: We determined the historical change in soil organic carbon (SOC) stocks from long‐term inorganic fertilizer and/or organic manure trials (maize and wheat dominated rotations) that represent major soil types and climatic conditions of northern China. Soil carbon (RothC, Rothamsted, UK) and general circulation models (BCCR, Bjerknes Centre for Climate Research, Norway, and IPSL, Institute Pierre Simon Laplace, France) were validated using these field trial data sets. We then applied these models to predict future change in SOC stocks to 2100 using two net primary production (NPP) carbon input scenarios (i.e., current NPP or 1% yr‐1 NPP increase). Here we show that the conversion rate of plant residues to SOC was higher in single‐cropping sites than in double‐cropping sites. The prediction of future SOC sequestration potential indicated that these soils will be a net source of carbon dioxide (CO2) under no fertilizer inputs. Even when inorganic nutrients were applied the additional carbon input from increased plant residues could not meet the depletion of SOC in parts of northern China. Manure or straw application could however improve the SOC sequestration potential at all study sites. The SOC sequestration potential in northern China was estimated to be −4.3 to 18.2 t C ha‐1 by 2100. The effect of projected climate change on the annual rate of SOC change did not differ significantly between climate scenarios. The average annual rate of SOC change under current and increased NPP scenarios (when using the IPSL and BCCR models at 850 ppm CO2) was ca. 0.136 t C ha‐1 yr‐1 in northern China. These findings highlight the need to maintain, and where possible increase, organic carbon inputs into these farming systems which are rapidly becoming inorganic fertilizer intensive.
  • A Growing Oceanic Carbon Uptake: Results from an inversion study of
           surface pCO2 data
    • Abstract: Concerted community efforts have been devoted to producing an authoritative climatology of air‐sea CO 2 fluxes [Takahashi et al., 2009], but identifying decadal trends in CO 2 fluxes has proven to be more challenging. The available surface pCO 2 estimates are too sparse to separate long‐term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo [MCMC] sampling as a novel technique for estimating the historical pCO 2 at the ocean surface. The result is a plausible history of surface pCO 2 based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO 2 data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO 2, with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr − 1 of anthropogenic carbon and an increase in the global annual uptake over the 30‐year time period of 0.4 ± 0.1 PgC yr − 1 decade − 1. The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling.
  • Potential future dynamics of carbon fluxes and pools in New England
           forests and their climatic sensitivities: a model‐based study
    • 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 the dynamic ecosystem model LPJ‐GUESS 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 279 gC/m2/yr and New England forests sequestered CO2 by 149 gC/m2/yr in the baseline period (1971–2000). Under nine future CCSs, NPP and RH were modeled to increase by an average rate of 0.85 and 0.56 gC/m2/yr2 during 1971–2099. The asymmetric increase in modeled NPP and RH resulted in New England forests sequestering atmospheric CO2 at a net rate of 0.29 gC/m2/yr2 with increases in both vegetation and soil C. In addition, changes in C fluxes and pools varied spatially with generally larger changes (except for NEE) occurring in southern compared to northern New England. Model 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 elevation of CO2 levels, climate warming has the potential to change New England forests from C sinks to sources in the late 21st century.
  • The role of soil processes in δ18O terrestrial climate proxies
    • Abstract: A paleoclimate interpretation of a terrestrial hydrologic proxy such as the δ18O of tree cellulose or speleothem calcite may be biased or mis‐interpreted if the isotopic composition of the soil water from which the proxy originated undergoes isotopic exhange or fractionation. In this study, we use a global isotope‐enabled land surface model (IsoLSM) to investigate how the δ18O of precipitation may be altered in a soil column due to evaporation and vertical moisture transport. In order to assess how precipitation and evaporation contribute to the soil water isotopic variability, we compare seasonal and interannual changes in simulated xylem water δ18O within a control simulation and in a suite of sensitivity experiments where the effect of precipitation δ18O, water vapor δ18O, and soil water evaporation are independently removed. The simulations, carried out for the period 1979 to 2004, reveal that in semi‐arid regions, such as the southwest United States, the seasonal cycle in xylem water δ18O is strongly affected by evaporative loss during the dry season and evaporation can also constitute as much as 50% of the interannual δ18O variance. Additional simulations, including soil water tagging experiments, indicate that upward fluxes of soil water occur during drier periods. For soil water δ18O profiles that are isotopically more depleted in 18O at depth, this imparts a low isotopic signature to xylem water δ18O during such dry intervals. Hence, without taking into account vertical moisture transport in the soils, low δ18O years could be misinterpreted as wet conditions (due to decreased evaporative enrichment) when instead drier conditions are equally as likely.
  • Incorporating microbial ecology concepts into global soil mineralization
           models to improve predictions of carbon and nitrogen fluxes
    • Abstract: Global models of soil carbon (C) and nitrogen (N) fluxes become increasingly needed to describe climate change impacts, yet they typically have limited ability to reflect microbial activities that may affect global‐scale soil dynamics. Benefiting from recent advances in microbial knowledge, we evaluated critical assumptions on microbial processes to be applied in global models. We conducted a sensitivity analysis of soil respiration rates (Cmin) and N mineralization rates (Nmin) for different model structures and parameters regarding microbial processes, and validated them with laboratory incubation data of diverse soils. Predicted Cmin was sensitive to microbial biomass, and the model fit to observed Cmin improved when using site‐specific microbial biomass. Cmin was less affected by the approach of microbial substrate consumption (i.e. linear, multiplicative, or Michaelis–Menten kinetics). The sensitivity of Cmin to increasing soil N fertility was idiosyncratic and depended on the assumed mechanism of microbial C:N stoichiometry effects: a C overflow mechanism upon N limitation (with decreased microbial growth efficiency) led to the best model fit. Altogether, inclusion of microbial processes reduced prediction errors by 26% (for Cmin) and 7% (for Nmin) in our validation dataset. Our study identified two important aspects to incorporate into global models: site‐specific microbial biomass and microbial C:N stoichiometry effects. The former requires better understandings of spatial patterns of microbial biomass and its drivers, while the latter urges for further conceptual progress on C–N interactions. With such advancements, we envision improved predictions of global C and N fluxes for a current and projected climate.
  • Factors influencing export of dissolved inorganic nitrogen by major
           rivers: a new, seasonal, spatially explicit, global model
    • Abstract: Substantial effort has focused on understanding spatial variation in dissolved inorganic nitrogen (DIN) export to the coastal zone and specific basins have been studied in depth. Much less is known, however, about seasonal patterns and controls of coastal DIN delivery across large spatial scales. Understanding seasonal patterns of DIN export is critical to efforts to predict impacts of coastal eutrophication, such as algal blooms and hypoxic areas, which are often seasonal phenomena. Here we describe, test, and apply a global model that predicts seasonal DIN export to coastal regions for >6,000 rivers using the Nutrient Export from Watersheds (NEWS2) model. NEWS2‐DIN‐S used spatially explicit, seasonal N inputs and was calibrated with measured DIN yield (kg N km‐2 season‐1) for 77 rivers, distributed globally. Of the characteristics considered, DIN‐transport efficiency was positively related to runoff and negatively related to temperature (r2 = 0.34‐0.60, depending on season p 
  • CO2 and CH4 emissions from streams in a lake‐rich landscape:
           Patterns, controls and regional significance
    • Abstract: Aquatic ecosystems are important components of landscape carbon budgets. In lake‐rich landscapes, both lakes and streams may be important sources of carbon gases (CO2 and CH4) to the atmosphere, but the processes that control gas concentrations and emissions in these interconnected landscapes have not been adequately addressed. We use multiple datasets that vary in their spatial and temporal extent during 2001‐2012 to investigate the carbon gas source strength of streams in a lake‐rich landscape and to determine the contribution of lakes, metabolism and groundwater to stream CO2 and CH4. We show that streams emit roughly the same mass of CO2 (23.4 Gg C yr‐1; 0.49 mol CO2 m‐2 day‐1) as lakes at a regional scale (27 Gg C yr‐1), and that stream CH4 emissions (189 Mg C yr‐1; 8.46 mmol CH4 m‐2 day‐1) are an important component of the regional greenhouse gas balance. Gas transfer velocity variability (range = 0.34 to 13.5 m day‐1) contributed to the variability of gas flux in this landscape. Groundwater inputs and in‐stream metabolism control stream gas supersaturation at the landscape scale, while carbon cycling in lakes and deep groundwaters do not control downstream gas emissions. Our results indicate the need to consider connectivity of all aquatic ecosystems (lakes, streams, wetlands and groundwater) in lake‐rich landscapes and their connections with the terrestrial environment in order to understand the full nature of the carbon cycle.
  • Introducing a terrestrial carbon pool in warm desert bedrock mountains,
           Southwestern USA
    • Abstract: Growth of the Phoenix metropolitan area led to road cut or house platform exposures of the internal bedrock material of surrounding semi‐arid mountain ranges. Similar exposures in the surrounding Sonoran and Mojave Deserts reveal the presence of sedimentary calcium carbonate infilling the pre‐existing fracture matrix of the bedrock. Field surveys at 31 sites with bedrock fractures filled with carbonate, referred to as BFFC in the following text, reveal an average of 0.079 ± 0.036 mTC/m2 stored in the upper 2 m of analyzed bedrock exposures. Back‐scattered electron microscopy images indicate the presence of carbonate at the micron scale, not included in this estimate of carbon storage. Pilot radiocarbon and Sr isotope analyses suggests that one of the surveyed BFFC veins was flushed into the bedrock from a non‐bedrock source during the wetter last glacial period in the late Pleistocene.
  • Global Assessment of Ocean Carbon Export by Combining Satellite
           Observations and Food‐Web Models
    • Abstract: The export of organic carbon from the surface ocean by sinking particles is an important, yet highly uncertain, component of the global carbon cycle. Here, we introduce a mechanistic assessment of the global ocean carbon export using satellite observations, including determinations of net primary production (NPP) and the slope of the particle size spectrum, to drive a food‐web model that estimates the production of sinking zooplankton feces and algal aggregates comprising the sinking particle flux at the base of the euphotic zone. The synthesis of observations and models reveals fundamentally different and ecologically consistent regional‐scale patterns in export and export efficiency not found in previous global carbon export assessments. The model reproduces regional‐scale particle export field observations and predicts a climatological mean global carbon export from the euphotic zone of ~6 Pg C y‐1. Global export estimates show small variation (typically 
  • Evaluating soil biogeochemistry parameterizations in Earth system models
           with observations
    • Abstract: Soils contain large reservoirs of terrestrial carbon (C), yet soil C dynamics simulated in Earth systems models show little agreement with each other or with observational datasets. This uncertainty underscores the need to develop a framework to more thoroughly evaluate model parameterizations, structures, and projections. Towards this end we used an analytical solution to calculate approximate equilibrium soil C pools for the Community Land Model version 4 (CLM4cn) and DAYCENT soil biogeochemistry models. Neither model generated sufficient soil C pools when forced with litterfall inputs from CLM4cn; however, global totals and spatial correlations of soil C pools for both models improved when calculated with litterfall inputs derived from observational data. DAYCENT required additional modifications to simulate soil C pools in deeper soils (0‐100 cm). Our best simulations produced global soil C pools totaling 746 and 978 Pg C for CLM4cn and DAYCENT parameterizations, respectively; compared to observational estimates of 1259 Pg C. In spite of their differences in complexity and equilibrium soil C pools, predictions of soil C losses with warming temperatures through 2100 were strikingly similar for both models. Ultimately, CLM4cn and DAYCENT come from the same class of models that represent the turnover of soil C as a first‐order decay process. While this approach may have utility in calculating steady state soil C pools, the applicability of this model configuration in transient simulations remains poorly evaluated.
  • Natural variability and anthropogenic change in equatorial Pacific surface
           ocean pCO2 and pH
    • Abstract: The equatorial Pacific is a dynamic region that plays an important role in the global carbon cycle. This region is the largest oceanic source of carbon dioxide (CO2) to the atmosphere, which varies interannually dependent on the El Niño‐Southern Oscillation (ENSO) and other climatic and oceanic drivers. We present high‐resolution observations of surface ocean CO2 partial pressure (pCO2) at four fixed locations in the Niño 3.4 area with datasets encompassing 10 ENSO warm and cold events from 1997 to 2011. The mooring observations confirm that ENSO controls much of the interannual variability in surface seawater pCO2 with values ranging from 315 to 578 µatm. The mooring time series also capture the temporal variability necessary to make the first estimates of long‐term pH trends in the equatorial Pacific, which suggest the combination of ocean acidification and decadal variability create conditions for high rates of pH change since the beginning of the mooring record. Anthropogenic CO2 increases play a dominant role in significant observed seawater pCO2 trends of +2.3 to +3.3 µatm yr‐1 and pH trends of ‐0.0018 to ‐0.0026 yr‐1 across the full time series in this region. However, increased upwelling driven by increased trade winds, a shallower thermocline, and increased frequency of La Niña events also contribute an average of 40% of the observed trends since 1998. These trends are higher than previous estimates based on underway observations and suggest that the equatorial Pacific is contributing a greater amount of CO2 to the atmospheric CO2 inventory over the last decade.
  • Constraining surface carbon fluxes using in situ measurements of carbonyl
           sulfide and carbon dioxide
    • Abstract: Understanding the processes that control the terrestrial exchange of carbon is critical for assessing atmospheric CO2 budgets. Carbonyl sulfide (COS) is taken up by vegetation during photosynthesis following a pathway that mirrors CO2, but has a small or non‐existent emission component. This tracer could thus provide a means to separate photosynthetic and respiration fluxes. We present field measurements of COS and CO2 mixing ratios made during the summer of 2012 in a forest, senescent grassland and riparian ecosystem using a laser absorption spectrometer installed in a mobile trailer. Measurements of leaf fluxes with a branch‐bag gas‐exchange system were made across species from 10 genera of trees, and soil fluxes were measured with a flow‐through chamber across a number of soil types. These data provide an extensive characterization of surface COS fluxes and show: (1) the existence of a narrow normalized daytime uptake ratio of COS to CO2 across vascular plant species of 1.7, providing critical information for the application of COS to estimate photosynthetic CO2 fluxes and (2) a temperature‐dependent normalized uptake ratio of COS to CO2 from soils. Significant nighttime uptake of COS was observed in broad‐leafed species and revealed active stomatal opening hours prior to sunrise. Joint measurements of ambient near‐surface COS and CO2 concentrations are used here alongside the flux measurements to partition the relative influence that leaf and soil fluxes and entrainment of air from above have on the surface carbon budget. The time‐dependent co‐variation of COS and CO2 in the ambient surface air sheds light on the dominant processes influencing the surface carbon budget in ways that cannot be done with measurements of only CO2. The results provide a number of critical constraints on the processes that control surface COS exchange, which can be used to diagnose the robustness of global models that are beginningto use COS to constrain terrestrial carbon exchange.
  • Do Trichodesmium spp. populations in the North Atlantic export most of the
           nitrogen they fix'
    • Abstract: A new observational synthesis of diazotrophic biomass and nitrogen fixation provides the opportunity for systematic quantitative evaluation of these aspects in biogeochemical models. One such model of the Atlantic Ocean is scrutinized, and the simulated biomass is found to be an order of magnitude too low. Initial attempts to increase biomass levels through decreasing grazing and other loss terms caused an unrealistic buildup of nitrate in the upper ocean. Two key changes to the model structure facilitated a closer match to the observed biomass and nitrogen fixation rates: addition of a pathway for export of diazotrophically‐fixed organic material, and uptake of inorganic nitrogen by the diazotroph population. These changes, along with a few other revisions to existing model parameterizations, facilitate more accurate simulation of basin‐scale distributions of diazotrophic biomass, as well as mesoscale variations contained therein. The resulting solutions suggest that Trichodesmium spp. populations of the North Atlantic export the vast majority of the nitrogen they fix, a finding that awaits assessment through direct observation.
  • Observational constraints on the distribution, seasonality, and
           environmental predictors of North American boreal methane emissions
    • Abstract: Wetlands comprise the single largest global source of atmospheric methane, but current flux estimates disagree in both magnitude and distribution at the continental scale. This study uses atmospheric methane observations over North America from 2007–2008 and a geostatistical inversion to improve understanding of Canadian methane fluxes and associated biogeochemical models. The results bridge an existing gap between traditional top‐down, inversion studies, which typically emphasize total emission budgets, and biogeochemical models, which usually emphasize environmental processes. The conclusions of this study are threefold. First, the most complete process‐based methane models do not always describe available atmospheric methane observations better than simple models. In this study, a relatively simple model of wetland distribution, soil moisture, and soil temperature outperformed more complex model formulations. Second, we find that wetland methane fluxes have a broader spatial distribution across western Canada and into the northern US than represented in existing flux models. Finally, we calculate total methane budgets for Canada and for the Hudson Bay Lowlands, a large wetland region (50–60°N, 75–96°W). Over these lowlands, we find total methane fluxes of 1.8 ± 0.24 TgC yr− 1, a number in the mid‐range of previous estimates. Our total Canadian methane budget of 16.0 ± 1.2 TgC yr− 1 is larger than existing inventories, primarily due to high anthropogenic emissions in Alberta. However, methane observations are sparse in western Canada, and additional measurements over Alberta will constrain anthropogenic sources in that province with greater confidence.
  • New insights into sea ice nitrogen biogeochemical dynamics from the
           nitrogen isotopes
    • Abstract: We report nitrogen (N) isotopic measurements of nitrate (NO3‐), total dissolved nitrogen (TDN), and particulate nitrogen (PN) from Antarctic pack ice in the Bellingshausen Sea during the early spring and in the Weddell Sea during the late spring. The normalized concentrations of total fixed N within the ice are ~2‐fold higher than in the underlying seawater, indicating that sea ice continues to exchange fixed N with seawater. Partial nitrate assimilation into immobile organic matter, with low‐δ15N, leads to the subsequent loss of high‐δ15N nitrate during brine convection. The combined effect is to lower the δ15N of total fixed sea ice N relative to the winter‐supplied nitrate. In this regard, the effect of incomplete nitrate consumption in sea ice is similar to that in the summertime surface ocean. However, the degree of nitrate consumption is greater in ice, leading to a higher δ15N for the organic N produced (3.9‰) than in the open Antarctic Zone (0.6‰). Relative to previous suggestions of very high‐δ15N export from sea ice (up to 41‰), this study indicates that it would be more difficult than previously thought for sea ice to explain the high δ15N of ice age Antarctic sediments. The partitioning of N isotopes between PN and DON + NH4+ (dissolved organic nitrogen + ammonium) suggests an imbalance between DON + NH4+ production and consumption processes in early spring, which was subsequently attenuated by the development of an efficient regeneration loop in late spring. Despite the 15N enrichment associated with nitrate assimilation, the δ15N of sea ice nitrate is frequently lower than that of seawater providing direct evidence of the products of nitrification within sea ice, with the mass and isotopic balances suggesting that nitrification supplies a substantial fraction (up to ~70%) of NO3‐ assimilated within the ice.
  • The counterintuitive effect of summer‐to‐fall mixed layer
           deepening on eukaryotic new production in the Sargasso Sea
    • Abstract: The Sargasso Sea is characterized by strong summertime stratification that is thought to drive oligotrophy, but export production is surprisingly similar to that of high‐latitude regions with ample major nutrient supply. Here, we use the summer‐to‐fall progression in the northwestern Sargasso Sea to investigate the relationship between upper ocean stratification and phytoplankton nitrogen (N) uptake. Euphotic zone particles collected in July, October, and December were sorted by flow cytometry, and the 15 N/14 N of separated prokaryotic and eukaryotic phytoplankton was analyzed. The 15 N/14 N of Prochlorococcus and Synechococcus was always low, indicating uniform reliance on recycled N. In July and in two fall profiles, the 15 N/14 N of eukaryotic phytoplankton was high, reflecting consumption of subsurface nitrate. In three other fall profiles, eukaryotic 15 N/14 N was similar to prokaryote 15 N/14 N, suggesting a shift toward more complete reliance on recycled N. The progressive deepening of the mixed layer from summer to fall, although reducing the surface‐to‐deep density contrast, increases the density difference of the euphotic zone as a whole from underlying nutrient‐rich waters, which may play a role in the observed decline in euphotic zone nitrate supply into the fall. The apparent summertime nitrate supply to the euphotic zone, when the mixed layer is shallowest, may help to explain the surprisingly high export production of the subtropical and tropical ocean.
  • Estimating Impacts of Lichens and Bryophytes on Global Biogeochemical
    • Abstract: Lichens and bryophytes may significantly affect global biogeochemical cycles by fixation of nitrogen and biotic enhancement of surface weathering rates. Most of the studies suggesting these effects, however, are either conceptual or rely on upscaling of regional estimates to obtain global numbers. Here, we use a different method, based on estimates of net carbon uptake, to quantify the impacts of lichens and bryophytes on biogeochemical cycles at the global scale. We focus on three processes, namelynitrogen fixation, phosphorus uptake and chemical weathering. Our estimates have the form of potential rates, which means that we quantify the amount of nitrogen and phosphorus needed by the organisms to build up biomass, also accounting for resorption and leaching of nutrients. Subsequently, we use potential phosphorus uptake on bare ground to estimate chemical weathering by the organisms, assuming that they release weathering agents to obtain phosphorus. The predicted requirement for nitrogen ranges from 3.5 to 34 Tg yr − 1 and for phosphorus it ranges from 0.46 to 4.6 Tg yr − 1. Estimates of chemical weathering are between 0.058 and 1.1 km 3 yr − 1 of rock. These values seem to have a realistic order of magnitude and they support the notion that lichens and bryophytes have the potential to play an important role for biogeochemical cycles.
  • Climate Warming Shifts Carbon Allocation from Stemwood to Roots in
           Calcium‐Depleted Spruce Forests
    • Abstract: Increased greening of northern forests, measured by the Normalized Difference Vegetation Index (NDVI), has been presented as evidence that a warmer climate has increased both net primary productivity (NPP) and the carbon sink in boreal forests. However, higher production and greener canopies may accompany changes in carbon allocation that favor foliage or fine roots over less decomposable woody biomass. Furthermore, tree core data throughout mid‐ and northern latitudes have revealed a divergence problem (DP), a weakening in tree ring responses to warming over the past half century that is receiving increasing attention, but remains poorly understood. Often, the same sites exhibit trend inconsistency phenomenon (TIP), namely positive, or no trends in growing season NDVI where negative trends in tree ring indexes are observed. Here we studied growth of two Norway spruce (Picea abies) stands in western Russia that exhibited both the DP and TIP but were subject to soil acidification and calcium depletion of differing timing and severity. Our results link the decline in radial growth starting in 1980 to a shift in carbon allocation from wood to roots driven by a combination of two factors: (a) soil acidification that depleted calcium and impaired root function and (b) earlier onset of the growing season that further taxed the root system. The latter change in phenology appears to act as a trigger at both sites to push trees into nutrient limitation as the demand for Ca increased with the longer growing season, thereby causing the shift in carbon allocation. © 2013 American Geophysical Union. All rights reserved.
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