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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: 31, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 137, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 16, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 5, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 139)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 30)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 55)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 54)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 123)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 51)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 133)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 38, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 37, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 19, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 16, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 83, SJR: 2.661, h-index: 144)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [16 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Quantifying the limitation to world cereal production due to soil
           phosphorus status
    • Abstract: Phosphorus (P) is an essential element for plant growth. Low P availability in soils is likely to limit crop yields in many parts of the world, but this effect has never been quantified at the global scale by process‐based models. Here we attempt to estimate P limitation in 3 major cereals worldwide for the year 2000 by combining information on soil P distribution in croplands and a generic crop model, while accounting for the nature of soil‐plant P transport. As a global average, the diffusion limited soil P supply meets the crop's P demand corresponding to the climatic yield potential, due to the legacy soil P in highly fertilized areas. However, when focusing on the spatial distribution of P supply vs. demand, we found strong limitation in regions like North and South America, Africa and Eastern Europe. Averaged over grid‐cells where P supply is lower than demand, the global yield gap due to soil P is estimated at 22, 55 and 26 % in winter wheat, maize and rice. Assuming that a fraction (20%) of the annual P applied in fertilizers is directly available to the plant, the global P yield gap lowers by only 5 – 10 % underlying the importance of the existing soil P supply in sustaining crop yields. The study offers a base for exploring P limitation in crops worldwide, but with certain limitations remaining. These could be better accounted for by describing the agricultural P cycle with a fully coupled and mechanistic soil‐crop model.
       
  • Issue Information
    • Abstract: No abstract is available for this article.
       
  • Trends and variability of global fire emissions due to historical
           anthropogenic activities
    • Abstract: Globally, fires are a major source of carbon from the terrestrial biosphere to the atmosphere, occurring on a seasonal cycle and with substantial interannual variability. To understand past trends and variability in sources and sinks of terrestrial carbon, we need quantitative estimates of global fire distributions. Here we introduce an updated version of the Fire Including Natural and Agricultural Lands model, version 2 (FINAL.2), modified to include multi‐day burning and enhanced fire spread rate in forest crowns. We demonstrate that the improved model reproduces the interannual variability and spatial distribution of fire emissions reported in present day remotely sensed inventories. We use FINAL.2 to simulate historical (post‐1700) fires and attribute past fire trends and variability to individual drivers: land use and land cover change, population growth, and lightning variability. Global fire emissions of carbon increase by about 10% between 1700 and 1900, reaching a maximum of 3.4 PgC yr‐1 in the 1910s, followed by a decrease to about 5% below year 1700 levels by 2010. The decrease in emissions from the 1910s to the present day is driven mainly by land use change, with a smaller contribution from increased fire suppression due to increased human population, and is largest in Sub‐Saharan Africa and South Asia. Interannual variability of global fire emissions is similar in the present day as in the early historical period, but present day wildfires would be more variable in the absence of land use change.
       
  • Insights into the biogeochemical cycling of iron, nitrate and phosphate
           across a 5300 km South Pacific zonal section (153°E‐150°W)
    • Abstract: Iron, phosphate and nitrate are essential nutrients for phytoplankton growth and hence their supply into the surface ocean controls oceanic primary production. Here, we present a GEOTRACES zonal section (GP13; 30‐33oS, 153oE‐150oW) extending eastwards from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L‐1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L‐1 (170oW‐150oW). The supply of dissolved iron into the upper ocean (
       
  • Characterizing CDOM Spectral Variability Across Diverse Regions and
           Spectral Ranges
    • Abstract: Satellite remote sensing of colored dissolved organic matter (CDOM) has focused on CDOM absorption (aCDOM) at a reference wavelength, as its magnitude provides insight into the underwater light field and large‐scale biogeochemical processes. CDOM spectral slope, SCDOM, has been treated as a constant or semi‐constant parameter in satellite retrievals of aCDOM despite significant regional and temporal variability. SCDOM and other optical metrics provide insights into CDOM composition, processing, food web dynamics, and carbon cycling. To date, much of this work relies on fluorescence techniques or aCDOM in spectral ranges unavailable to current and planned satellite sensors (e.g.
       
  • Estimating the Cross‐Shelf Export of Riverine Materials, Part I: General
           Relationships from an Idealized Numerical Model
    • Abstract: Rivers deliver large amounts of terrestrially derived materials (such as nutrients, sediments, and pollutants) to the coastal ocean, but a global quantification of the fate of this delivery is lacking. Nutrients can accumulate on shelves, potentially driving high levels of primary production with negative consequences like hypoxia, or be exported across the shelf to the open ocean where impacts are minimized. Global biogeochemical models cannot resolve the relatively small‐scale processes governing river plume dynamics and cross‐shelf export; instead, river inputs are often parameterized assuming an “all or nothing” approach. Recently, Sharples et al. [GBC, doi:10.1002/2016GB005483, 2017] proposed the SP number – a dimensionless number relating the estimated size of a plume as a function of latitude to the local shelf width – as a simple estimator of cross‐shelf export. We extend their work, which is solely based on theoretical and empirical scaling arguments, and address some of its limitations using a numerical model of an idealized river plume. In a large number of simulations, we test whether the SP number can accurately describe export in unforced cases, and with tidal and wind forcing imposed. Our numerical experiments confirm that the SP number can be used to estimate export and enable refinement of the quantitative relationships proposed by Sharples et al. We show that, in general, external forcing has only a weak influence compared to latitude and derive empirical relationships from the results of the numerical experiments that can be used to estimate riverine freshwater export to the open ocean.
       
  • Estimating the Cross‐Shelf Export of Riverine Materials, Part II:
           Estimates of Global Freshwater and Nutrient Export
    • Abstract: Rivers deliver large amounts of fresh water, nutrients, and other terrestrially derived materials to the coastal ocean. Where inputs accumulate on the shelf, harmful effects such as hypoxia and eutrophication can result. In contrast, where export to the open ocean is efficient riverine inputs contribute to global biogeochemical budgets. Assessing the fate of riverine inputs is difficult on a global scale. Global ocean models are generally too coarse to resolve the relatively small‐scale features of river plumes. High‐resolution regional models have been developed for individual river plume systems, but it is impractical to apply this approach globally to all rivers. Recently, generalized parameterizations have been proposed to estimate the export of riverine fresh water to the open ocean [Sharples et al. GBC, doi:10.1002/2016GB005483, 2017; Izett and Fennel submitted]. Here the relationships of Izett and Fennel [submitted] are used to derive global estimates of open‐ocean export of fresh water and dissolved inorganic silicate, dissolved organic carbon, and dissolved organic and inorganic phosphorus and nitrogen. We estimate that only 15–53% of riverine fresh water reaches the open ocean directly in river plumes; nutrient export is even less efficient because of processing on continental shelves. Due to geographic differences in riverine nutrient delivery, dissolved silicate is most efficiently exported to the open ocean (7‐56.7%), while dissolved inorganic nitrogen is the least efficiently exported (2.8–44.3%). These results are consistent with previous estimates and provide a simple way to parameterize export to the open ocean in global models.
       
  • Influence of vegetation growth on the enhanced seasonality of atmospheric
           CO2
    • Abstract: The amplitude of seasonal fluctuations in concentrations of atmospheric CO2 has increased over recent decades. Model‐based studies suggest that this increase could be the result of enhanced vegetation growth during the growing season and ecosystem respiration in the non‐growing season. Here, we investigated seasonal changes in vegetation growth derived from satellite‐based observations of the normalized difference vegetation index (NDVI) from 1982 to 2013. We found notable agreement between these observations and interannual variations of seasonality of global atmospheric CO2, suggesting that terrestrial vegetation growth is the dominant driver of the seasonality of the concentration of atmospheric CO2. Specifically, we found that the trend in seasonality of global vegetation growth was not continuous from 1982 to 2013, and that it increased substantially after 2001. In response, the trend of seasonality in the concentration of atmospheric CO2 stalled from 1982 to 2000, but increased from 2001 onwards. This 2001 change in the growth seasonality trend was largely a result of decreased NDVI during spring and winter. CMIP5 models were unable to reproduce this observed seasonality. Our results showed the dominant role played by vegetation growth in determining atmospheric CO2 seasonality, highlighting the need to improve representation of vegetation growth in current terrestrial models to adequately indicate seasonal changes in the concentration of atmospheric CO2.
       
  • Large differences in global and regional total soil carbon stock estimates
           based on SoilGrids, HWSD and NCSCD: Intercomparison and evaluation based
           on field data from USA, England, Wales and France
    • Abstract: Soils are the major component of the terrestrial ecosystem and the largest organic carbon reservoir on Earth. However, they are a non‐renewable natural resource and especially reactive to human disturbance and climate change. Despite its importance, soil carbon dynamics is an important source of uncertainty for future climate predictions and there is a growing need for more precise information to better understand the mechanisms controlling soil carbon dynamics and better constrain Earth system models.The aim of our work is to compare soil organic carbon stocks given by different global and regional databases that already exist. We calculated global and regional soil carbon stocks at 1m depth given by three existing databases (SoilGrids, the Harmonized World Soil Database, and the Northern Circumpolar Soil Carbon Database). We observed that total stocks predicted by each product differ greatly: it's estimated to be around 3400 Pg by SoilGrids and is about 2500 Pg according to HWSD. This difference is marked in particular for boreal regions where differences can be related to high disparities in soil organic carbon concentration. Differences in other regions are more limited and may be related to differences in bulk density estimates. Finally, evaluation of the three datasets vs ground truth data shows that i) there is a significant difference in spatial patterns between ground truth data and compared datasets and that ii) datasets underestimate by more than 40% the soil organic carbon stock compared to field data.
       
  • Marine N2O emissions from nitrification and denitrification constrained by
           modern observations and projected in multi‐millennial global warming
           simulations
    • Abstract: Nitrous oxide (N2O) is a potent greenhouse gas (GHG) and ozone destructing agent, yet, global estimates of N2O emissions are uncertain. Marine N2O stems from nitrification and denitrification processes which depend on organic matter cycling and dissolved oxygen (O2). We introduce N2O as an obligate intermediate product of denitrification and as an O2‐dependent byproduct from nitrification in the Bern3D ocean model. A large model ensemble is used to probabilistically constrain modern and to project marine N2O production for a low (RCP2.6) and high GHG (RCP8.5) scenario extended to AD 10,000. Water‐column N2O and surface ocean partial pressure N2O data serve as constraints in this Bayesian framework. The constrained median for modern N2O production is 4.5 (±1σ‐range: 3.0 to 6.1) Tg N yr−1, where 4.5% stems from denitrification. Modeled denitrification is 65.1 (40.9 to 91.6) Tg N yr−1, well within current estimates. For high GHG forcing, N2O production decreases by 7.7% over this century due to decreasing organic matter export and remineralization. Thereafter, production increases slowly by 21% due to wide‐spread deoxygenation and high remineralization. Deoxygenation peaks in two millennia and the global O2 inventory is reduced by a factor of two compared to today. Net denitrification is responsible for 7.8% of the long‐term increase in N2O production. On millennial timescales, marine N2O emissions constitute a small, positive feedback to climate change. Our simulations reveal tight coupling between the marine carbon cycle, O2, N2O, and climate.
       
  • In situ tropical peatland fire emission factors and their variability, as
           determined by field measurements in Peninsula Malaysia.
    • Abstract: Fires in tropical peatlands account for >25% of estimated total greenhouse gas emissions from deforestation and degradation. Despite significant global and regional impacts, our understanding of specific gaseous fire emission factors (EFs) from tropical peat burning is limited to a handful of studies. Furthermore, there is substantial variability in EFs between sampled fires and/or studies. For example, methane EFs vary by 91% between studies.Here we present new fire EFs for the tropical peatland ecosystem; the first EFs measured for Malaysian peatlands, and only the second comprehensive study of EFs in this crucial environment. During August 2015 (under El Niño conditions) and July 2016, we embarked on field campaigns to measure gaseous emissions at multiple peatland fires burning on deforested land in Southeast Pahang (2015) and oil palm plantations in North Selangor (2016), Peninsula Malaysia. Gaseous emissions were measured using open‐path Fourier transform infrared spectroscopy. The IR spectra were used to retrieve mole fractions of twelve different gases present within the smoke (including carbon dioxide and methane), and these measurements used to calculate EFs. Peat samples were taken at each burn site for physicochemical analysis and to explore possible relationships between specific physicochemical properties and fire EFs. Here we present the first evidence to indicate that substrate bulk density affects methane fire EFs reported here. This novel explanation of inter‐plume, within‐biome variability should be considered by those undertaking greenhouse gas accounting and haze forecasting in this region, and is of importance to peatland management, particularly with respect to artificial compaction.
       
  • Vertical distributions of coccolithophores, PIC, POC, biogenic silica and
           chlorophyll a throughout the global ocean
    • Abstract: Coccolithophores are a critical component of global biogeochemistry, export fluxes and seawater optical properties. We derive globally‐significant relationships to estimate integrated coccolithophore and coccolith concentrations as well as integrated concentrations of particulate inorganic carbon (PIC) from their respective surface concentration. We also examine surface versus integral relationships for other biogeochemical variables contributed by all phytoplankton (e.g. chlorophyll a, particulate organic carbon (POC)) or diatoms (biogenic silica; BSi). Integrals are calculated using both 100m integrals and euphotic zone integrals (depth of 1% surface photosynthetically available radiation). Surface concentrations are parameterized in either volumetric units (e.g. m‐3) or values integrated over the top optical depth. Various relationships between surface concentrations and integrated values demonstrate that when surface concentrations are above a specific threshold, the vertical distribution of the property is biased to the surface layer, and when surface concentrations are below a specific threshold, the vertical distribution of the properties are biased to subsurface maxima. Results also show a highly predictable decrease in explained‐variance as vertical distributions become more vertically heterogeneous. These relationships have fundamental utility for extrapolating surface ocean color remote sensing measurements to 100 m depth or to the base of the euphotic zone, well beyond the depths of detection for passive ocean color remote sensors. Greatest integrated concentrations of PIC, coccoliths and coccolithophores are found when there is moderate stratification at the base of the euphotic zone.
       
  • Limited carbonate dissolution by boring microflora at two volcanically
           acidified temperate sites: Ischia (Italy, Mediterranean Sea) and Faial
           (Azores, NE Atlantic Ocean)
    • Abstract: In situ effects of ocean acidification on carbonate dissolution by microboring flora, also called biogenic dissolution, have only been studied once in tropical environments. Naturally acidified seawaters due to CO2 vents offer a perfect setting to study these effects in temperate systems. Three sites were selected at Ischia (Italy, Mediterranean Sea) with one experiencing ambient pH and the two others a mean pHT of 7.2 and 7.5. At Faial (Azores, NE Atlantic), one site with ambient pH and one acidified site with a mean pHT of 7.4 were selected. Experiments were carried out during 1.5 months and 6 months in Azores and Ischia, respectively, to determine the effects of OA on microboring communities in various carbonate substrates. Low pH influenced negatively boring microflora development by limiting their depth of penetration and abundance in substrates. Biogenic dissolution was thus reduced by a factor 3 to 7 depending on sites and substrate types. At sites with ambient pH in Faial, biogenic dissolution contributed up to 23% to the total weight loss, while it contributed less than 1% to the total weight loss of substrates at the acidified sites. Most of the dissolution at these sites was due to chemical dissolution (often Ω ≤ 1). Such conditions maintained microboring communities at a pioneer stage with a limited depth of penetration in substrates. Our results, together with previous findings which showed an increase of biogenic dissolution at pH >7.7, suggest that there is a pH tipping point below which microborer development and thus, carbonate biogenic dissolution, is strongly limited.
       
  • Long‐term drainage reduces CO2 uptake and CH4 emissions in a
           Siberian permafrost ecosystem
    • Abstract: Permafrost landscapes in northern high latitudes with their massive organic carbon stocks are an important, poorly known, component of the global carbon cycle. However, in light of future Arctic warming, the sustainability of these carbon pools is uncertain. To a large part, this is due to a limited understanding of the carbon cycle processes because of sparse observations in Arctic permafrost ecosystems. Here, we present an eddy covariance dataset covering more than three years of continuous CO2 and CH4 flux observations within a moist tussock tundra ecosystem near Chersky in North‐eastern Siberia. Through parallel observations of a disturbed (drained) area and a control area nearby, we aim to evaluate the long‐term effects of a persistently lowered water table on the net vertical carbon exchange budgets and the dominating biogeochemical mechanisms. Persistently drier soils trigger systematic shifts in the tundra ecosystem carbon cycle patterns. Both, uptake rates of CO2 and emissions of CH4 decreased. Year‐round measurements emphasize the importance of the non‐growing season – in particular the “zero‐curtain” period in the fall – to the annual budget. Approximately 60% of the CO2 uptake in the growing‐season is lost during the cold‐seasons, while CH4 emissions during the non‐growing season account for 30% of the annual budget. Year‐to‐year variability in temperature conditions during the late growing season were identified as the primary control of the interannual variability observed in the CO2 and CH4 fluxes.
       
  • Biological and climate controls on North Atlantic marine carbon dynamics
           over the last millennium: Insights from an absolutely‐dated shell based
           record from the North Icelandic Shelf
    • Abstract: Given the rapid increase in atmospheric carbon dioxide concentrations (pCO2) over the industrial era there is a pressing need to construct long term records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon. Here we reconstruct the historical biological and climatic controls on the carbon isotopic (δ13C‐shell) composition of the North Icelandic shelf waters over the last millennium derived from the shells of the long‐lived marine bivalve mollusc Arctica islandica. Variability in the annually resolved δ13C‐shell record is dominated by multi‐decadal variability with a negative trend (‐0.003±0.002‰yr‐1) over the industrial era (1800‐2000 CE). This trend is consistent with the marine Suess effect brought about by the sequestration of isotopically light carbon (δ13C of CO2) derived from the burning of fossil fuels. Comparison of the δ13C‐shell record with contemporary proxy archives, over the last millennium, and instrumental data over the 20th century, highlight that both biological (primary production) and physical environmental factors such as relative shifts in the proportion of Subpolar Mode Waters and Arctic Intermediate Waters entrained onto the North Icelandic shelf, atmospheric circulation patterns associated with the winter North Atlantic Oscillation, and subpolar gyre sea surface temperatures and salinity, are the likely mechanisms that contribute to natural variations in seawater δ13C variability on the North Icelandic shelf. Contrasting δ13C fractionation processes associated with these biological and physical mechanisms likely cause the attenuated local marine Suess effect signal at this locality.
       
 
 
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