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Publisher: AGU   (Total: 17 journals)   [Sort by number of followers]

Showing 1 - 17 of 17 Journals sorted alphabetically
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 100, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 12, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 109)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 24)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 48)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 45)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 96)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 42)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 113)
Paleoceanography     Full-text available via subscription   (Followers: 7, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 36, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 33, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 16, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 13, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 76, SJR: 2.661, h-index: 144)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [12 followers]  Follow
    
   Full-text available via subscription Subscription journal  (Not entitled to full-text)
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Mixed layer carbon cycling at the Kuroshio Extension Observatory
    • Authors: Andrea J. Fassbender; Christopher L. Sabine, Meghan F. Cronin, Adrienne J. Sutton
      Abstract: Seven years of data from the NOAA Kuroshio Extension Observatory (KEO) surface mooring, located in the North Pacific Ocean carbon sink region, were used to evaluate drivers of mixed layer carbon cycling. A time-dependent mass balance approach relying on two carbon tracers was used to diagnostically evaluate how surface ocean processes influence mixed layer carbon concentrations over the annual cycle. Results indicate that the annual physical carbon input is predominantly balanced by biological carbon uptake during the intense spring bloom. Net annual gas exchange that adds carbon to the mixed layer and the opposing influence of net precipitation that dilutes carbon concentrations, make up smaller contributions to the annual mixed layer carbon budget. Decomposing the biological term into annual net community production (aNCP) and calcium carbonate production (aCaCO3) yields 7 ± 3 mol C m-2 yr-1 aNCP and 0.5 ± 0.3 mol C m-2 yr-1 aCaCO3, giving an annually integrated particulate inorganic carbon to particulate organic carbon production ratio of 0.07 ± 0.05, as a lower limit. Although we find that vertical physical processes dominate carbon input to the mixed layer at KEO, it remains unclear how horizontal features, such as eddies, influence carbon production and export by altering nutrient supply as well as the depth of winter ventilation. Further research evaluating linkages between Kuroshio Extension jet instabilities, eddy activity, and nutrient supply mechanisms is needed to adequately characterize the drivers and sensitivities of carbon cycling near KEO.
      PubDate: 2017-01-13T22:10:24.855091-05:
      DOI: 10.1002/2016GB005547
       
  • Declining spatial efficiency of global cropland nitrogen allocation
    • Authors: Nathaniel D. Mueller; Luis Lassaletta, Bryan Runck, Gilles Billen, Josette Garnier, James S. Gerber
      Abstract: Efficiently allocating nitrogen (N) across space maximizes crop productivity for a given amount of N input and reduces N losses to the environment. Here we quantify changes in the global spatial efficiency of cropland N use by calculating historical tradeoff frontiers relating N inputs to possible N yield assuming efficient allocation. Time-series cropland N budgets from 1961 to 2009 characterize the evolution of N input-yield response functions across 12 regions and are the basis for constructing tradeoff frontiers. Improvements in agronomic technology have substantially increased cropping system yield potentials and expanded N-driven crop production possibilities. However, we find these gains are compromised by the declining spatial efficiency of N use across regions. Since the start of the Green Revolution, N inputs and yields have moved farther from the optimal frontier over time; in recent years (1994–2009), global N surplus has grown to a value that is 69% greater than what is possible with efficient N allocation between regions. To reflect regional pollution and agricultural development goals, we construct scenarios that restrict reallocation, finding that these changes only slightly decrease potential gains in nitrogen use efficiency. Our results are inherently conservative due to the regional unit of analysis, meaning a larger potential exists than is quantified here for cross-scale policies to promote spatially efficient N use.
      PubDate: 2017-01-12T20:30:29.905431-05:
      DOI: 10.1002/2016GB005515
       
  • Thorium distributions in high and low dust regions and the significance
           for iron supply
    • Authors: Christopher T. Hayes; Jeffrey Rosen, David McGee, Edward A. Boyle
      Abstract: Thorium and uranium isotopes (232Th, 230Th, 238U, and 234U) were investigated to refine their use for estimating mineral dust deposition and Fe delivery to the ocean. U concentrations and isotope ratios were consistent with conservative behavior and can safely be described using published U-salinity relationships and global average seawater isotopic composition. Near Barbados, waters affected by the Amazon outflow contained elevated 232Th. This signals one region where the thorium-dust method is inaccurate because of a confounding continental input. Dissolved 232Th fluxes in this region suggest that Amazonian Fe supply to the adjacent open ocean is much larger than local atmospheric deposition. The colloidal content of dissolved Th south of Bermuda was found to be quite small (2-6%), similar to that found north of Hawaii, despite the order of magnitude higher dust deposition in the Atlantic. This finding supports the assumption that dissolved 232Th and 230Th are scavenged at the same rate despite their different sources and also sheds light on the increase of dissolved 232Th fluxes with integrated depth. Outside the region influenced by Amazon River waters, dissolved 232Th fluxes are compared with Bermudan aerosol Fe deposition to estimate that fractional Th solubility is around 20% in this region. Finally, new dissolved and soluble Fe, Mn and Cr data from the subtropical North Pacific support the idea that Fe concentrations in the remote ocean are highly buffered, whereas 232Th has a larger dynamic range between high and low dust regions.
      PubDate: 2017-01-11T15:05:22.293763-05:
      DOI: 10.1002/2016GB005511
       
  • Upwelling and Primary Production during the U.S. GEOTRACES East Pacific
           Zonal Transect
    • Authors: David Kadko
      Abstract: The 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling region (PCU). In this work the flux of nitrate into the euphotic zone is derived for stations within the PCU using a previously developed method whereby dilution of the water column 7Be inventory by upwelled 7Be-free water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles are used to constrain vertical diffusivity within the upper thermocline. These transport terms are applied to nitrate profiles to estimate net community production between 79oW and 104oW along the EPZT, which includes the zone of active upwelling to the edge of the oligotrophic gyre. With a simple, one-dimensional model the calculated upwelling rates were inversely related to mixed layer temperature and ranged from 0-3.0 m/d. Results using a depth-dependent upwelling rate with a component of horizontal advection are also described. Vertical diffusivities near the base of the euphotic zone were in the range 1.7 - 4.5 x 10-4 m2/s. These values are compared to those generated by analysis of temperature profiles. Net community production averaged 15 mmol C/m2/d for stations between 84oW and 104oW, and was 134 mmol C/m2/d for the furthest inshore station at 79oW which displayed the lowest SST and greatest rate of upwelling.
      PubDate: 2017-01-11T14:50:48.01206-05:0
      DOI: 10.1002/2016GB005554
       
  • Mechanisms responsible for high N2O emissions from sub-Arctic permafrost
           peatlands studied via stable isotope techniques
    • Authors: J. Gil; T. Perez, K. Boering, P. J. Martikainen, C. Biasi
      Abstract: Recent field studies have shown that there are habitats in the sub-Arctic tundra emitting N2O at exceptionally high rates. In this study stable isotope techniques were applied to characterize the processes responsible for these high N2O emissions which have been found from bare peat surfaces in permafrost peatlands. The results include the first data on the nitrogen and oxygen isotopic composition of N2O emitted from Arctic tundra. The emission-weighted average δ15Nbulk value for N2O of -13.0 ± 2.0‰ (mean ± s.d; n = 8) from the bare peat surfaces falls within the range of the emission-weighted average values from other natural ecosystems, but is distinct from those for managed/agricultural ecosystems. This implies that if in the future, a smaller rate in the overall decreasing trend of δ15NbulkN2O tropospheric isotopic composition is found, it cannot only be attributed to agricultural N2O emission reductions from mitigation actions but also from soils in natural ecosystems that may be emitting more N2O to the atmosphere due to warmer conditions. The SP values from emitted N2O range from -30‰ to 58‰, indicating a temporal shift of microbial production and consumption of N2O during the sampling period. Soil emission SP data suggest that the N2O emission in sub-Arctic tundra are more likely to be produced by nitrifier denitrification in the relatively drier study year, but due to variable published SP values for N2O production processes in soils, this interpretation has to be taken with caution. According to SP values at depth, denitrification was the main N2O production pathway. To better address the usefulness of SP in partitioning microbial mechanisms in soils further studies in soils mesocosms are required.
      PubDate: 2017-01-05T21:30:22.165009-05:
      DOI: 10.1002/2015GB005370
       
  • Influence of biological carbon export on ocean carbon uptake over the
           annual cycle across the North Pacific Ocean
    • Authors: Hilary I. Palevsky; Paul D. Quay
      Abstract: We evaluate the influences of biological carbon export, physical circulation, and temperature-driven solubility changes on air-sea CO2 flux across the North Pacific basin (35°N–50°N, 142°E–125°W) throughout the full annual cycle by constructing mixed layer budgets for dissolved inorganic carbon (DIC) and pCO2, determined on fifteen container ship transects between Hong Kong and Long Beach, CA from 2008 to 2012. Annual air-sea CO2 flux is greatest in the western North Pacific and decreases eastward across the basin (2.7 ± 0.9 mol C m-2 yr-1 west of 170°E, as compared to 2.1 ± 0.3 mol C m-2 yr-1 east of 160°W). East of 160°W, DIC removal by annual net community production (NCP) more than fully offsets the DIC increase due to air-sea CO2 flux. However, in the region west of 170°E influenced by deep winter mixing, annual NCP only offsets ~20% of the DIC increase due to air-sea CO2 flux, requiring significant DIC removal by geostrophic advection. Temperature-driven solubility changes have no net influence on pCO2 and account for
      PubDate: 2017-01-05T20:30:21.474642-05:
      DOI: 10.1002/2016GB005527
       
  • Organic matter remineralization in marine sediments: a Pan-Arctic
           synthesis
    • Authors: Solveig Bourgeois; Philippe Archambault, Ursula Witte
      Abstract: Climate change in the Arctic is ongoing and causes drastic modification on the ecosystem functioning. In soft-bottom environments, organic matter remineralization is considered an important ecosystem function. Here we provide a large scale assessment of the current knowledge on the benthic organic matter remineralization and its potential response to climate change. Sediment oxygen demand (SOD) values (n = 1154), measured throughout the Arctic, were gathered from 30 publications and 16 databases and nutrient flux values, available in a far lesser extent (n 
      PubDate: 2017-01-05T20:20:26.706277-05:
      DOI: 10.1002/2016GB005378
       
  • Particle flux in the oceans: Challenging the steady state assumption
    • Authors: Sarah L. C. Giering; Richard Sanders, Adrian P. Martin, Stephanie A. Henson, Jennifer S. Riley, Chris M. Marsay, David Johns
      Abstract: Atmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption; that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (
      PubDate: 2017-01-02T19:45:45.559175-05:
      DOI: 10.1002/2016GB005424
       
  • Issue Information
    • Pages: 1755 - 1755
      Abstract: No abstract is available for this article.
      PubDate: 2017-01-11T19:07:31.336167-05:
      DOI: 10.1002/gbc.20342
       
  • Ocean productivity from space: Commentary
    • Authors: Mati Kahru
      Abstract: Ocean color measurements from satellites have been used to estimate global oceanic productivity for about 30 years but the approach still has many problems. A combination of more sophisticated satellite products with improved models has the potential of higher accuracy but in reality the improvement in accuracy during last two decades has been minimal. Persistent cloud cover over the oceans and low sun elevation over polar areas severely limit the potential of operational satellite ocean color measurements. A combination of remote measurements from both satellites and sub-orbital platforms as well as from a large number of autonomous devices in the ocean can overcome these limitations in the future.
      PubDate: 2016-12-28T16:30:21.450377-05:
      DOI: 10.1002/2016GB005582
       
  • Increasing net ecosystem biomass production of Canada's boreal and
           temperate forests despite decline in dry climates
    • Authors: Robbie A. Hember; Werner A. Kurz, Nicholas C. Coops
      Abstract: Repeated measurements of tree biomass at field plots describe recovery from disturbances, sampling artifacts, and potential effects of environmental change on forest ecosystems. Challenges in differentiating between intrinsic and extrinsic sources of variation, both in theory and in practice, continues to confound claims of an anthropogenic carbon sink in forest biomass. Here, we analyzed observations at 10,307 plots across southern ecozones of Canada to investigate temporal trends in stand-level biomass growth (G), biomass loss due to mortality (M), and net ecosystem biomass production (NEBP) of intact stands. Net extrinsic forcing (Fex) was expressed by the collective dependence of biomass fluxes on climate anomalies, nitrogen deposition (N), and atmospheric carbon dioxide concentration (C). Inferences drawn directly from linear mixed effects model coefficients only reflect the static behaviour of the model specifically at field plot locations. We, therefore, defined a dynamic landscape-scale net extrinsic forcing (Fex′), which additionally accounted for potential negative feedback responses to anthropogenic growth enhancement. Simulations were performed over 1501-2012 to estimate Fex′. Overall, Fex′ was positive, suggesting that environmental changes drove a 90 percent increase in NEBP. The increase in NEBP was confined to wet regions, while the biomass sink in dry regions decreased, suggesting that large expanses of northern forests, historically located near the boundary between wet and dry climates, may be at high risk of decline under continued increases in evaporative demand. These results have important implications for the greenhouse gas balance of Canada's forest sector.
      PubDate: 2016-12-26T01:50:23.531446-05:
      DOI: 10.1002/2016GB005459
       
  • Avoidable impacts of ocean warming on marine primary production: Insights
           from the CESM ensembles
    • Authors: Kristen M. Krumhardt; Nicole S. Lovenduski, Matthew C. Long, Keith Lindsay
      Abstract: As anthropogenic emissions and warming continue to alter Earth's environment, it is essential to highlight future impacts that can be avoided through mitigation. Here, we use two ensembles of the Community Earth System Model (CESM) run under the business-as-usual scenario, RCP 8.5, and the mitigation scenario, RCP 4.5, to identify avoidable impacts of anthropogenic warming on marine net primary production (NPP). We emphasize the use of ensembles so as to distinguish long-term, anthropogenic trends in marine productivity from internal variability. 20th century globally-integrated marine NPP is 55.7 ± 1 Pg C, with much of the variability attributable to certain regions (e.g., the equatorial Pacific). CESM projections indicate that global marine NPP will drop by ∼4% by 2080 if we follow RCP 8.5, but only by 2% under RCP 4.5. The response to warming on a global scale includes compensating regional effects; NPP increases in polar and eastern equatorial Pacific waters, but decreases in the Atlantic, western Pacific, and Indian Ocean. The two main phytoplankton groups simulated in CESM show distinct responses: diatoms decrease their NPP, while small phytoplankton NPP increases over the mid-21st century. Trends in NPP from mid-21st century to 2080 are significantly different between the two emission scenarios mainly in the Atlantic Ocean basin and therefore impacts here are “avoidable” if we follow RCP 4.5, rather than RCP 8.5. In contrast, changes in NPP on a global scale and in most areas of the Pacific and Indian basins and the Southern Ocean are not distinguishable between forcing scenarios.
      PubDate: 2016-12-16T22:30:29.122296-05:
      DOI: 10.1002/2016GB005528
       
  • Interactions between land use change and carbon cycle feedbacks
    • Authors: Natalie M. Mahowald; James T. Randerson, Keith Lindsay, Ernesto Munoz, Scott C. Doney, Peter Lawrence, Sarah Schlunegger, Daniel S. Ward, David Lawrence, Forrest Hoffman
      Abstract: Using the Community Earth System Model, we explore the role of human land use and land cover change (LULCC) in modifying the terrestrial carbon budget in simulations forced by Representative Concentration Pathway 8.5, extended to year 2300. Overall, conversion of land (e.g. from forest to croplands via deforestation) results in a model-estimated, cumulative carbon loss of 490 PgC between 1850 and 2300, larger than the 230 PgC loss of carbon caused by climate change over this same interval. The LULCC carbon loss is a combination of a direct loss at the time of conversion and an indirect loss from the reduction of potential terrestrial carbon sinks. Approximately 40% of the carbon loss associated with LULCC in the simulations arises from direct human modification of the land surface; the remaining 60% is an indirect consequence of the loss of potential natural carbon sinks. Because of the multi-century carbon cycle legacy of current land use decisions, a globally averaged amplification factor of 2.6 must be applied to 2015 land use carbon losses to adjust for indirect effects. This estimate is 30% higher when considering the carbon cycle evolution after 2100. Most of the terrestrial uptake of anthropogenic carbon in the model occurs from the influence of rising atmospheric CO2 on photosynthesis in trees, and thus model-projected carbon feedbacks are especially sensitive to deforestation.
      PubDate: 2016-12-16T11:00:26.595185-05:
      DOI: 10.1002/2016GB005374
       
  • Anthropogenic CO2 Accumulation and Uptake Rates in the Pacific Ocean based
           on Changes in the 13C/12C of Dissolved Inorganic Carbon
    • Authors: P. Quay; R. Sonnerup, D. Munro, C. Sweeney
      Abstract: The anthropogenic CO2 accumulation rate for the Pacific Ocean was estimated from the decrease in δ13C of the dissolved inorganic carbon measured on six WOCE cruises during the 1990s and repeated during CLIVAR in the 2000s. A mean depth-integrated anthropogenic δ13C change of -83 ± 20 ‰ m decade-1 was estimated for the basin using the multiple linear regression approach. The largest anthropogenic δ13C decreases occurred between 40°S and 60°S, whereas the smallest decreases occurred in the Southern Ocean and subpolar North Pacific. A mean anthropogenic CO2 accumulation rate of 0.41 ± 0.13 mol C m-2 yr-1 (0.82 ± 0.26 Pg C yr-1) was determined based on observed δ13C changes and is in agreement with previous observation- and model-based estimates. The mean DIC13 inventory change of -178 ± 43 ‰ mol m-2 decade-1 was primarily the result of air-sea CO2 exchange acting on the measured air-sea δ13C disequilibrium of ~ -1.2 ± 0.1 ‰. Regional differences between the DIC13 inventory change and air-sea 13CO2 flux yielded net anthropogenic CO2 uptake rates (independent of ∆pCO2) that ranged from ~0 to 1 mol m-2 yr-1 and basin-wide mean of 1.2 ± 1.5 Pg C yr-1. High rates of surface ocean DIC increase and δ13C decrease observed in the Drake Passage (53°-60°S) support above average anthropogenic CO2 accumulation since 2005. Observed δ13C changes in the Pacific Ocean indicate that ocean transport significantly impacted the anthropogenic CO2 distribution and illustrate the utility of δ13C as a tracer to unravel the processes controlling the present and future accumulation of anthropogenic CO2 in the ocean.
      PubDate: 2016-12-14T12:35:30.984635-05:
      DOI: 10.1002/2016GB005460
       
  • What proportion of riverine nutrients reaches the open ocean?
    • Authors: Jonathan Sharples; Jack J. Middelburg, Katja Fennel, Timothy D. Jickells
      Abstract: Globally rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported, and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time.
      PubDate: 2016-12-09T06:25:41.431956-05:
      DOI: 10.1002/2016GB005483
       
  • Evaluating the drought response of CMIP5 models using global gross primary
           productivity, leaf area, precipitation and soil moisture data
    • Authors: Y. Huang; S. Gerber, T. Huang, J. W. Lichstein
      Abstract: Realistic representation of vegetation's response to drought is important for understanding terrestrial carbon cycling. We evaluated 9 Earth System Models from the historical experiment of the Coupled Model Intercomparison Project Phase 5 (CMIP5) for the response of gross primary productivity (GPP) and leaf area index (LAI) to hydrological anomalies. Hydrological anomalies were characterized by the standardized precipitation index (SPI) and surface soil moisture anomalies (SMA). GPP and LAI in models were on average more responsive to SPI than in observations revealed through several indicators. First, we find higher mean correlations between global annual anomalies of GPP and SPI in models than observations. Second, the maximum correlation between GPP and SPI across 1-24 month timescales is higher in models than observations. And finally we found stronger excursions of GPP to extreme dry or wet events. Similar to GPP, LAI responded more to SPI in models than observations. The over-response of models is smaller if evaluated based on SMA instead of SPI. LAI responses to SMA are inconsistent among models, showing both higher and lower LAI when soil moisture is reduced. The timescale of maximum correlation is shorter in models than the observation for GPP and the markedly different response timescales among models for LAI indicate gaps in understanding how variability of water availability affects foliar cover. The discrepancy of responses derived from SPI and SMA among models, and between models and observations calls for improvement in understanding the dynamics of plant-available water in addition to how vegetation responds to these anomalies.
      PubDate: 2016-12-08T17:52:25.796067-05:
      DOI: 10.1002/2016GB005480
       
  • Two Centuries of Nitrogen Dynamics: Legacy Sources and Sinks in the
           Mississippi and Susquehanna River Basins
    • Authors: K. J. Van Meter; N. B. Basu, P. Van Cappellen
      Abstract: Global flows of reactive nitrogen (N) have increased significantly over the last century in response to agricultural intensification and elevated levels of atmospheric deposition. Despite widespread implementation conservation measures, N concentrations in surface waters are often remaining steady or continuing to increase. Although such lack of response has been attributed to time lags associated with legacy N stores in subsurface reservoirs, it is unclear what the magnitudes of such stores are and how they are partitioned between shallow soil and deeper groundwater reservoirs. Here we have synthesized data to develop a 214-year (1800 – 2014) trajectory of N inputs to the land surface of the continental U.S. We have concurrently developed a parsimonious, process-based model, ELEMeNT that pairs this input trajectory with a travel time-based approach to simulate transport and retention along subsurface pathways. Using the model, we have reconstructed historic nitrate yields at the outlets of two major U.S. watersheds, the Mississippi River Basin (MRB) and Susquehanna River Basin (SRB). Our results show significant N loading above baseline levels in both watersheds before the widespread use of commercial N fertilizers, largely due to the conversion of forest and grassland to row-crop agriculture. Model results also allow us to quantify the magnitudes of legacy N in soil and groundwater pools, and to highlight the dominance of soil legacies in MRB and groundwater legacies in SRB. Approximately 55% and 18% of the current annual N loads in the MRB and SRB were found to be older than 10 years of age.
      PubDate: 2016-12-01T18:21:03.807771-05:
      DOI: 10.1002/2016GB005498
       
  • Dissolved organic matter composition of Arctic rivers: linking permafrost
           and parent material to riverine carbon
    • Authors: Jonathan A. O'Donnell; George R. Aiken, David K. Swanson, Santosh Panda, Kenna D. Butler, Andrew P. Baltensperger
      Abstract: Recent climate change in the Arctic is driving permafrost thaw, which has important implications for regional hydrology and global carbon dynamics. Permafrost is an important control on groundwater dynamics and the amount and chemical composition of dissolved organic matter (DOM) transported by high-latitude rivers. The consequences of permafrost thaw for riverine DOM dynamics will likely vary across space and time, due in part to spatial variation in ecosystem properties in Arctic watersheds. Here, we examined watershed controls on DOM composition in 69 rivers draining heterogeneous landscapes across a broad region of Arctic Alaska. We characterized DOM using bulk dissolved organic carbon (DOC) concentration, optical properties, and chemical fractionation, and classified watersheds based on permafrost characteristics (mapping of parent material and ground-ice content, modeling of thermal state) and ecotypes. Parent material and ground-ice content significantly affected the amount and composition of DOM. DOC concentrations were higher in watersheds underlain by fine-grained loess compared to watersheds underlain by coarse-grained sand or shallow bedrock. DOC concentration was also higher in rivers draining ice-rich landscapes compared to rivers draining ice-poor landscapes. Similarly, specific ultraviolet absorbance (SUVA254, an index of DOM aromaticity) values were highest in watersheds underlain by fine-grained deposits or ice-rich permafrost. We also observed differences in hydrophobic organic acids, hydrophilic compounds, and DOM fluorescence across watersheds. Both DOC concentration and SUVA254 were negatively correlated with watershed active layer thickness, as determined by high-resolution permafrost modeling. Together, these findings highlight how spatial variations in permafrost physical and thermal properties can influence riverine DOM.
      PubDate: 2016-11-24T13:50:23.023529-05:
      DOI: 10.1002/2016GB005482
       
  • Modelled Chl:C ratio and derived estimates of phytoplankton carbon biomass
           and its contribution to total particulate organic carbon in the global
           surface ocean
    • Authors: Lionel Arteaga; Markus Pahlow, Andreas Oschlies
      Abstract: Chlorophyll (Chl) is a distinctive component of autotrophic organisms, often used as an indicator of phytoplankton biomass in the ocean. However, assessment of phytoplankton biomass from Chl relies on the accurate estimation of the Chl:C ratio. Here we present global patterns of Chl:C ratios in the surface ocean obtained from a phytoplankton growth model that accounts for the optimal acclimation of phytoplankton to ambient nutrient, light, and temperature conditions. The model agrees largely with observed/expected global patterns of Chl:C. Combining our Chl:C estimates with satellite Chl and particulate organic carbon (POC), we infer phytoplankton C concentration in the surface ocean and its contribution to the total POC pool. Our results suggest that the portion of POC corresponding to living phytoplankton is higher in subtropical latitudes and less productive regions (∼ 30–70%), and decreases to ∼ 10–30% towards high latitudes and productive regions. An important caveat of our model is the lack of iron limiting effects on phytoplankton physiology. Comparison of our predicted phytoplankton biomass with an independent estimate of total POC reveals a positive correlation between nitrate concentrations and non-photosynthetic POC in the surface ocean. This correlation disappears when a constant Chl:C is applied. Our analysis is not constrained by assumptions of constant Chl:C or phytoplankton:POC ratio, providing a novel independent analysis of phytoplankton biomass in the surface ocean. These results highlight the importance of accounting for the variability in Chl:C, and its application in distinguishing the autotrophic and heterotrophic components in the assemblage of the marine plankton ecosystem.
      PubDate: 2016-11-18T01:15:31.056892-05:
      DOI: 10.1002/2016GB005458
       
  • Two black carbon pools transported by the Changjiang and Huanghe Rivers in
           China
    • Authors: Xuchen Wang; Caili Xu, Ellen M. Druffel, Yuejun Xue, Yuanzhi Qi
      Abstract: Major rivers play important roles in transporting large amounts of terrestrial organic matter from land to the ocean each year, and the organic matter carried by rivers contains a significant fraction of black carbon (BC). A recent study estimated that 0.027 Gt of BC is transported in the dissolved phase by rivers each year, which accounts for ~ 10% of the global flux of dissolved organic carbon (DOC) [Jaffé et al., 2013]. The relative sources of this large amount of riverine dissolved black carbon (DBC) from biomass burning (young, modern 14C) and fossil fuel (old, 14C-free) combustion are not known. We present radiocarbon measurements of BC in both dissolved and particulate phases transported by the Changjiang and Huanghe Rivers, the two largest rivers in China, during 2015. We show that two, distinct BC pools (young and old) were carried by the rivers. The DBC pool was much younger than the particulate BC (PBC) pool. Mass balance calculations indicate that most (78 - 85%) of the DBC in the Changjiang and Huanghe Rivers was derived from biomass burning, and only 15 – 22% was from fossil fuel combustion. In contrast, PBC from biomass burning and fossil fuel combustion were approximately equal in these two rivers. Export of PBC and DBC by the rivers are decoupled, and fluxes of PBC were 4.1 and 6.7 times higher than DBC in the Changjiang and Huanghe Rivers, respectively. The 14C age differences of the two BC pools suggest that BC derived from biomass burning and fossil fuel combustion are mobilized in different phases and on different time scales in these rivers.
      PubDate: 2016-11-16T18:12:56.976585-05:
      DOI: 10.1002/2016GB005509
       
  • Fire and deforestation dynamics in Amazonia (1973-2014)
    • Authors: M. J. E. Marle; R. D. Field, G. R. Werf, I. A. Estrada Wagt, R. A. Houghton, L. V. Rizzo, P. Artaxo, K. Tsigaridis
      Abstract: Consistent long-term estimates of fire emissions are important to understand the changing role of fire in the global carbon cycle and to assess the relative importance of humans and climate in shaping fire regimes. However, there is limited information on fire emissions from before the satellite-era. We show that in the Amazon region, including the Arc of Deforestation and Bolivia, visibility observations derived from weather stations could explain 61% of the variability in satellite-based estimates of bottom-up fire emissions since 1997, and 42% of the variability in satellite-based estimates of total column carbon monoxide concentrations since 2001. This enabled us to reconstruct the fire history of this region since 1973 when visibility information became available. Our estimates indicate that until 1987 relatively few fires occurred in this region and that fire emissions increased rapidly over the 1990s. We found that this pattern agreed reasonably well with forest loss datasets, indicating that although natural fires may occur here deforestation and degradation were the main cause of fires. Compared to fire emissions estimates based on Food and Agricultural Organization's Global Forest and Resources Assessment (FAO-FRA) data, our results were substantially lower up to the 1990s, after which they were more in line. Our visibility-based fire emissions dataset can help constrain dynamic global vegetation models and atmospheric models with a better representation of the anthropogenic fire regime.AGU Index terms: 0305 Aerosols and particles, 0315 Biosphere/atmosphere interactions, 0428 Carbon cycling, 0480 Remote sensing, 1632 Land cover change
      PubDate: 2016-11-16T16:45:26.858339-05:
      DOI: 10.1002/2016GB005445
       
  • The CAFE Model: A Net Production Model for Global Ocean Phytoplankton
    • Authors: Greg M. Silsbe; Michael J. Beherenfeld, Kimberly H. Halsey, Allen J. Milligan, Toby K. Westberry
      Abstract: The Carbon, Absorption, and Fluorescence Euphotic-resolving (CAFE) net primary production model is an adaptable framework for advancing global ocean productivity assessments by exploiting state-of-the-art satellite ocean color analyses and addressing key physiological and ecological attributes of phytoplankton. Here, we present the first implementation of the CAFE model that incorporates inherent optical properties derived from ocean color measurements into a mechanistic and accurate model of phytoplankton growth rates (μ) and net phytoplankton production (NPP). The CAFE model calculates NPP as the product of energy absorption (QPAR), and the efficiency (φμ) by which absorbed energy is converted into carbon biomass (CPhyto), while μ is calculated as NPP normalized to CPhyto. The CAFE model performance is evaluated alongside 21 other NPP models against a spatially robust and globally representative set of direct NPP measurements. This analysis demonstrates that the CAFE model explains the greatest amount of variance and has the lowest model bias relative to other NPP models analyzed with this dataset. Global oceanic NPP from the CAFE model (52 Pg C m-2 yr-1) and mean division rates (0.34 d-1) are derived from climatological satellite data (2002-2014). This manuscript discusses and validates individual CAFE model parameters (e.g. QPAR, φμ), provides detailed sensitivity analyses, and compares the CAFE model results and parameterization to other widely cited models.
      PubDate: 2016-11-03T13:08:30.775007-05:
      DOI: 10.1002/2016GB005521
       
 
 
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