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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 55, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 25)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 80, SJR: 2.189, h-index: 121)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [5 followers]  Follow
    
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Climate extremes dominate seasonal and interannual variations in carbon
           export from the Mississippi River Basin
    • Abstract: Knowledge about the annual and seasonal patterns of organic and inorganic carbon (C) exports from the major rivers of the world to the coastal ocean are essential for our understanding and potential management of the global C budget so as to limit anthropogenic modification of global climate. Unfortunately our predictive understanding of what controls the timing, magnitude and quality of carbon export is still rudimentary. Here we use a process‐based coupled hydrologic/ecosystem biogeochemistry model (the Dynamic Land Ecosystem Model, DLEM) to examine how climate variability and extreme events, changing land use, and atmospheric chemistry have affected the annual and seasonal patterns of C exports from the Mississippi River basin to the Gulf of Mexico. Our process‐based simulations estimate that the average annual exports of dissolved organic C (DOC), particulate organic C (POC), and dissolved inorganic C (DIC) in the 2000s was 2.6 ± 0.4 Tg C yr−1, 3.4 ± 0.3 Tg C yr−1 and 18.8 ± 3.4 Tg C yr−1, respectively. Although land‐use change was the most important agent of change in C export over the past century, climate variability and extreme events (such as flooding and drought) were primarily responsible for seasonal and interannual variations in C export from the basin. The maximum seasonal export of DIC occurred in summer while for maximum DOC and POC occurred in winter. Relative to the 10‐year average (2001–2010), our modeling analysis indicates that the years of maximal and minimal C export co‐occurred with wet and dry years (2008: 32% above average and 2006: 32% below average). Given IPCC‐predicted changes in climate variability and the severity of rain events and droughts of wet and dry years for the remainder of the 21st Century, our modeling results suggest major changes in the riverine link between the terrestrial and oceanic realms, which are likely to have a major impact on carbon delivery to the coastal ocean.
      PubDate: 2015-08-06T12:38:46.634104-05:
      DOI: 10.1002/2014GB005068
       
  • The multiple fates of sinking particles in the North Atlantic Ocean
    • Authors: James R. Collins; Bethanie R. Edwards, Kimberlee Thamatrakoln, Justin E. Ossolinski, Giacomo R. DiTullio, Kay D. Bidle, Scott C. Doney, Benjamin A. S. Van Mooy
      Abstract: The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here, we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle‐attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle‐attached microbial communities each spanned more than an order of magnitude. Rates of substrate‐specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 d‐1. A comparison of these substrate‐specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m‐2 d‐1, on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth‐integrated mesopelagic respiration and carbon inputs.
      PubDate: 2015-08-04T01:03:40.41616-05:0
      DOI: 10.1002/2014GB005037
       
  • Small phytoplankton drive high summertime carbon and nutrient export in
           the Gulf of California and Eastern Tropical North Pacific
    • Abstract: Summertime carbon, nitrogen and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, PC and PN concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol·dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. Highest C fixation rates were associated with diatom dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton and diazotroph dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including
      PubDate: 2015-08-02T23:05:00.189407-05:
      DOI: 10.1002/2015GB005134
       
  • Plant nutrients do not covary with soil nutrients under changing climatic
           conditions
    • Abstract: Nitrogen (N) and phosphorus (P) play vital roles in plant growth and development. Yet, how climate regimes and soil fertility influence plant N and P stoichiometry is not well understood, especially in the belowground plant parts. Here we investigated plant above‐ and belowground N and P concentrations ([N] and [P]) and their stoichiometry in three dominant genera along a 2200‐km long climatic gradient in northern China. Results showed that temperature explained more variation of [N] and [P] in C4 plants, whereas precipitation exerted a stronger influence on [N] and [P] in C3 plants. Both plant above‐ and belowground [N] and [P] increased with decreasing precipitation and increasing temperature yet were negatively correlated with soil [N] and [P]. Plant N:P ratios were unrelated with all climate and soil variables. Plant above‐ and belowground [N] followed an allometric scaling relationship but the allocation of [P] was isometric. These results imply that internal processes stabilize plant N:P ratios and hence tissue N:P ratios may not be an effective parameters for predicting plant nutrient limitation. Our results also imply that past positive relationships between plant and nutrient stocks may be challenged under changing climatic conditions. While any modeling would need to be able to replicate currently observed relationships, it is conceivable that some relationships, such as those between temperature or rainfall and carbon:nutrient ratios, should be different under changing climatic conditions.
      PubDate: 2015-07-30T05:51:57.005544-05:
      DOI: 10.1002/2015GB005089
       
  • Phosphorus budget in the water‐agro‐food system at nested
           scales in two contrasted regions of the world (ASEAN‐8 and
           EU‐27)
    • Abstract: Phosphorus (P) plays a strategic role in agricultural production as well as in the occurrence of freshwater and marine eutrophication episodes throughout the world. Moreover, the scarcity and uneven distribution of minable P resources is raising concerns about the sustainability of long‐term exploitation. In this paper we analyze the P cycle in anthropic systems with an original multi‐scale approach (world region, country, and large basin scales) in two contrasting world regions representative of different trajectories in socioeconomic development for the 1961–2009 period: Europe (EU‐27)/France and the Seine River Basin, and Asia (ASEAN‐8)/Vietnam and the Red River Basin. Our approach highlights different trends in the agricultural and food production systems of the two regions. Whereas crop production increased until the 1980s in Europe and France and has stabilized thereafter, in ASEAN‐8 and Vietnam it began to increase in the 1980s and it is still rising today. These trends are related to the increasing use of fertilizers, although in European countries the amount of fertilizers sharply decreased after the 1980s. On average, the total P delivered from rivers to the sea is three times higher for ASEAN‐8 (300 kgP km‐2 yr‐1) than for EU‐27 countries (100 kgP km‐2 yr‐1) and is twice as high in the Red River (200 kgP km‐2 yr‐1) than in the Seine River (110 kgP km‐2 yr‐1), with agricultural losses to water in ASEAN‐8 three times higher than in EU‐27. Based on the P flux budgets, this study discusses early warnings and management options according to the particularities of the two world regions, newly integrating the perspective of surface water quality with agricultural issues (fertilizers, crop production, and surplus), food/feed exchanges, and diet, defining the so‐called water‐agro‐food system.
      PubDate: 2015-07-30T05:51:42.093853-05:
      DOI: 10.1002/2015GB005147
       
  • Sources of new nitrogen in the Indian Ocean
    • Abstract: Quantifying the different sources of nitrogen (N) within the N cycle is crucial to gain insights in oceanic phytoplankton production. To understand the controls of primary productivity and the associated capture of CO2 through photosynthesis in the southeastern Indian Ocean we compiled the physical and biogeochemical data from 4 voyages conducted in 2010, 2011, 2012 and 2013. Overall higher NH4+ assimilation rates (~530 μmol m−2 h−1) relative to NO3− assimilation rates (~375 μmol m−2 h−1) suggest that the assimilation dynamics of C are primarily regulated by microbial regeneration in our region. N2 fixation rates did not decline when other source of dissolved inorganic nitrogen (DIN) were available, although the assimilation of N2 is a highly energetic process. Our data showed that the diazotrophic community assimilated ~2 nmol N L−1 h−1 at relative elevated NH4+ assimilation rates ~12 nmol L−1 h−1 and NO3− assimilation rates ~6 nmol L−1 h−1. The small diffusive deep‐water NO3− fluxes could not support the measured NO3− assimilation rates and consequently point towards another source of dissolved inorganic NO3−. Highest NO2− values coincided consistently with shallow lower dissolved O2layers (100–200 m; 100‐180μmol L−1). These results suggest that nitrification above the pycnocline could be a significant component of the N cycle in the eastern Indian Ocean. In our analysis we provide a conceptual understanding how NO3− in the photic zone could be derived from new N through N2‐fixation. We conclude with the hypothesis that N injected through N2 fixation can be recycled within the photic zone as NH4+, and sequentially oxidized to NO2− and NO3− in shallow lower DO layers.
      PubDate: 2015-07-28T15:18:20.12004-05:0
      DOI: 10.1002/2015GB005194
       
  • Influence of ENSO and the NAO on Terrestrial Carbon Uptake in the
           Texas‐northern Mexico Region
    • Authors: Nicholas C. Parazoo; Elizabeth Barnes, John Worden, Anna. B. Harper, Kevin W Bowman, Christian Frankenberg, Sebastian Wolf, Marcy Litvak, Trevor F. Keenan
      Abstract: Climate extremes such as drought and heat waves can cause substantial reductions in terrestrial carbon uptake. Advancing projections of the carbon uptake response to future climate extremes depends on (1) identifying mechanistic links between the carbon cycle and atmospheric drivers, (2) detecting and attributing uptake changes, and (3) evaluating models of land response and atmospheric forcing. Here, we combine model simulations, remote sensing products, and ground observations to investigate the impact of climate variability on carbon uptake in the Texas‐northern Mexico region. Specifically, we (1) examine the relationship between drought, carbon uptake, and variability of ENSO and the NAO using JULES biosphere simulations from 1950–2012, (2) quantify changes in carbon uptake during record drought conditions in 2011, and (3) evaluate JULES carbon uptake and soil moisture in 2011 using observations from remote sensing and a network of flux towers in the region. Long term simulations reveal systematic decreases in regional‐scale carbon uptake during negative phases of ENSO and NAO, including amplified reductions of gross primary production (GPP) (−0.42 ± 0.18 Pg C yr‐1) and net ecosystem production (NEP) (−0.14 ± 0.11 Pg C yr‐1) during strong La Niña years. The 2011 mega‐drought caused some of the largest declines of GPP (−0.50 Pg C yr‐1) and NEP (−0.23 Pg C yr‐1) in our simulations. In 2011, consistent declines were found in observations, including high correlation of GPP and surface soil moisture (r = 0.82 ± 0.23, p = 0.012) in remote sensing based products. These results suggest a large‐scale response of carbon uptake to ENSO and NAO, and highlight a need to improve model predictions of ENSO and NAO in order to improve predictions of future impacts on the carbon cycle and the associated feedbacks to climate change.
      PubDate: 2015-07-28T15:17:02.797609-05:
      DOI: 10.1002/2015GB005125
       
  • A thank you to our GBC Reviewers
    • PubDate: 2015-07-27T00:39:32.325531-05:
      DOI: 10.1002/2015GB005233
       
  • Criteria for rejection of papers without review
    • PubDate: 2015-07-25T16:26:25.340715-05:
      DOI: 10.1002/2015GB005234
       
  • The mutual importance of anthropogenically and climate induced changes in
           global vegetation cover for future land carbon emissions in the
           MPI‐ESM CMIP5 simulations
    • Authors: R. Schneck; C. H. Reick, J. Pongratz, V. Gayler
      Abstract: Based on the MPI‐ESM simulations for the Coupled Model Intercomparison Project Phase 5 (CMIP5) and on simulations with the submodel Cbalone we disentangle the influence of natural and anthropogenic vegetation changes on land carbon emissions for the years 1850 till 2300. According to our simulations, climate induced changes in distribution and productivity of natural vegetation strongly mitigates future carbon emissions from anthropogenic landuse and landcover change (LULCC). Depending on the assumed scenario, the accumulated carbon emissions until the year 2100 are reduced between 22 and 49% and until 2300 between 45 and 261%. The carbon storage due to climate‐induced vegetation change is generally stronger in combination with LULCC. This is because in our simulations anthropogenic vegetation changes support more productive Plant Functional Types (PFTs). After stopping LULCC in the year 2100 the refilling of depleted C pools on formerly transformed land takes (dependent on the scenario) timescales of centuries.
      PubDate: 2015-07-24T14:08:12.197437-05:
      DOI: 10.1002/2014GB004959
       
  • Recent Amazon Climate as background for possible ongoing and future
           changes of Amazon humid forests
    • Abstract: Recent analyses of Amazon runoff and gridded precipitation data suggest an intensification of the hydrological cycle over the past few decades in the following sense: wet‐season precipitation and peak river runoff (since ∼ 1980) as well as annual‐mean precipitation (since ∼ 1990) have increased while dry‐season precipitation and minimum runoff have slightly decreased. There has also been an increase in the frequency of anomalously severe floods and droughts. Here we extend and expand these analyses to characterize recent climate state and change, as a background for possible ongoing and future changes of these forests. The contrasting recent changes in wet and dry season precipitation have continued and are generally consistent with changes in catchment‐level peak and minimum river runoff as well as a positive trend of water vapour inflow into the basin. Consistent with the river records the increased vapour inflow is concentrated to the wet season. Temperature has been rising by 0.7∘C since 1980 with more pronounced warming during dry months. Suggestions for the cause of the observed changes of the hydrological cycle come from patterns in tropical sea surface temperatures (SST's). Tropical and North Atlantic SST's have increased rapidly and steadily since 1990, while Pacific SST's have shifted from a negative Pacific Decadal Oscillation (PDO) phase (approximately pre 1990) with warm eastern Pacific temperatures to a positive phase with cold eastern Pacific temperatures. These SST conditions have been shown to be associated with an increase in precipitation over most of the Amazon except the south and south‐west. If ongoing changes continue we expect these to be generally beneficial for forests in those regions where there is an increase in precipitation with the exception of floodplain forests. An increase in flood‐pulse height and duration could lead to increased mortality at higher levels of the floodplain and, over the long term, to a lateral shift of the zonally stratified floodplain forest communities. Negative effects on forests are mainly expected in the south‐west and south, which have become slightly drier and hotter, consistent with tree mortality trends observed at the RAINFOR forest plot census network.
      PubDate: 2015-07-22T09:05:53.202115-05:
      DOI: 10.1002/2014GB005080
       
  • Understanding large‐extent controls of soil organic carbon storage
           in relation to soil depth and soil‐landscape systems
    • Abstract: In this work we aimed at developing a conceptual framework in which we improve our understanding of the controlling factors for soil organic carbon (SOC) over vast areas at different depths. We postulated that variability in SOC may be better explained by modelling SOC within soil‐landscape systems (SLSs). The study was performed in mainland France and explanatory SOC models were developed for the sampled topsoil (0–30 cm) and subsoil (>30 cm), using both directed and undirected data mining techniques. With this study we demonstrated that there is a shift in controlling factors both in space and depth which were mainly related to 1) typical SLSs characteristics and 2) human induced changes to SLSs. The controlling factors in relation to depth alter from predominantly biotic to more abiotic with increasing depth. Especially, water availability, soil texture and physical protection control deeper stored SOC. In SLSs with similar SOC levels, different combinations of physical protection, the input of organic matter and climatic conditions largely determined the SOC level. The SLSs approach provided the means to partition the data into datasets that were having homogenous conditions with respect to this combination of controlling factors. This information may provide important information on the carbon storage and sequestration potential of a soil.
      PubDate: 2015-07-21T16:28:21.636805-05:
      DOI: 10.1002/2015GB005178
       
  • Relevance of methodological choices for accounting of land use change
           carbon fluxes
    • Authors: Eberhard Hansis; Steven J. Davis, Julia Pongratz
      Abstract: Accounting for carbon fluxes from land use and land cover change (LULCC) generally requires choosing from multiple options of how to attribute the fluxes to regions and to LULCC activities. Applying a newly‐developed and spatially‐explicit bookkeeping model BLUE (bookkeeping of land use emissions) we quantify LULCC fluxes and attribute them to land‐use activities and countries by a range of different accounting methods. We present results with respect to a Kyoto Protocol‐like “commitment” accounting period, using land use emissions of 2008‐12 as example scenario. We assess the effect of accounting methods that vary (1) the temporal evolution of carbon stocks, (2) the state of the carbon stocks at the beginning of the period, (3) the temporal attribution of carbon fluxes during the period, and (4) treatment of LULCC fluxes that occurred prior to the beginning of the period. We show that the methodological choices result in grossly different estimates of carbon fluxes for the different attribution definitions.
      PubDate: 2015-07-21T16:02:13.785754-05:
      DOI: 10.1002/2014GB004997
       
  • Decoupling of net community and export production on submesoscales in the
           Sargasso Sea
    • Authors: M. L. Estapa; D. A. Siegel, K. O. Buesseler, R. H. R. Stanley, M. W. Lomas, N. B. Nelson
      Abstract: Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux‐regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed‐layer biogeochemical parameters and proxies for NCP and EP during ten, high spatial‐resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common, but rarely did the same parameters co‐vary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in > 5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect‐averaged EP determinations were slightly smaller than similar NCP values (Type‐II regression slope of 0.93, std = 0.32); but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations.
      PubDate: 2015-07-18T16:01:30.024777-05:
      DOI: 10.1002/2014GB004913
       
  • Global oceanic emission of ammonia: constraints from seawater and
           atmospheric observations
    • Authors: F. Paulot; D. J. Jacob, M. T. Johnson, T. G. Bell, A. R. Baker, W. C. Keene, I. D. Lima, S. C. Doney, C. A. Stock
      Abstract: Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia (NHx(sw)), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here, we compare [NHx(sw)] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [NHx(sw)] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NHx within observed ranges. The resulting global ocean emissions is 2.5 TgN a−1, much lower than current literature values(7–23 TgN a−1), including the widely used GEIA inventory (8 TgN a−1). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NHx over most of the ocean in the Northern hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a−1, comparable in magnitude to other natural sources from open fires and soils.
      PubDate: 2015-07-17T20:01:20.713827-05:
      DOI: 10.1002/2015GB005106
       
  • Sea‐air CO2 exchange in the western Arctic coastal ocean
    • Abstract: The biogeochemical seascape of the western Arctic coastal ocean is in rapid transition. Changes in sea ice cover will be accompanied by alterations in sea‐air carbon dioxide (CO2) exchange, of which the latter has been difficult to constrain owing to sparse temporal and spatial datasets. Previous assessments of sea‐air CO2 flux have targeted specific sub‐regional areas of the western Arctic coastal ocean. Here a holistic approach is taken to determine the net sea‐air CO2 flux over this broad region. We compiled and analyzed an extensive dataset of nearly 600,000 surface seawater CO2 partial pressure (pCO2) measurements spanning 2003 through 2014. Using space‐time co‐located, reconstructed atmospheric pCO2 values coupled with the seawater pCO2 dataset, monthly climatologies of sea‐air pCO2 differences (∆pCO2) were created on a 0.2° latitude x 0.5° longitude grid. Sea‐air CO2 fluxes were computed using the ∆pCO2 grid and gas transfer rates calculated from a climatology of wind speed second moments. Fluxes were calculated with and without the presence of sea ice, treating sea ice as an imperfect barrier to gas exchange. This allowed for carbon uptake by the western Arctic coastal ocean to be assessed under existing and reduced sea ice cover conditions, in which carbon uptake increased 30% over the current 10.9 ± 5.7 Tg C (1 Tg = 1012 g) yr−1 of sea ice adjusted exchange in the region. This assessment extends beyond previous sub‐regional estimates in the region in an all‐inclusive manner, and points to key unresolved aspects that must be targeted by future research.
      PubDate: 2015-07-16T01:10:26.0701-05:00
      DOI: 10.1002/2015GB005153
       
  • Ocean nutrient pathways associated with the passage of a storm
    • Authors: Anna Rumyantseva; Natasha Lucas, Tom Rippeth, Adrian Martin, Stuart C. Painter, Timothy J. Boyd, Stephanie Henson
      Abstract: Storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open North Atlantic Ocean. In this study we use an interdisciplinary dataset collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and subsequent shear‐spiking at the pycnocline due to interactions of storm generated inertial currents with wind. The post‐storm increase in surface layer nitrate (by ~20 mmol m−2) was predominantly driven by the first pathway: nutrient intrusion during the storm. Alignment of post‐storm inertial currents and surface wind stress caused shear instabilities at the ocean pycnocline, forming the second pathway for nutrient transport into the euphotic zone. During the alignment period, pulses of high turbulent nitrate flux through the pycnocline (up to 1 mmol m−2 day−1; approximately 25 times higher than the background flux) were detected. However, the impact of the post‐storm supply was an order of magnitude lower than during the storm due to the short duration of the pulses. Cumulatively, the storm passage was equivalent to 2.5‐5 % of the nitrate supplied by winter convection and had a significant effect compared to previously reported (sub)‐mesoscale dynamics in the region. As storms occur frequently, they can form an important component in local nutrient budgets.
      PubDate: 2015-07-16T01:09:58.494909-05:
      DOI: 10.1002/2015GB005097
       
  • Soluble iron inputs to the Southern Ocean through recent andesitic to
           rhyolitic volcanic ash eruptions from the Patagonian Andes
    • Abstract: Patagonia, due to its geographic position and the dominance of westerly winds, is a key area that contributes to the supply of nutrients to the Southern Ocean, both through mineral dust as well as the periodic deposits of volcanic ash. Here we evaluate the characteristics of Fe dissolved (into soluble and colloidal species) from volcanic ash for three recent southern Andes volcanic eruptions having contrasting features and chemical compositions. Contact between cloud waters (wet deposition) and end‐members of andesitic (Hudson volcano) and rhyolitic (Chaitén volcano) materials was simulated. Results indicate higher Fe release and faster liberation rates in the andesitic material. Fe release during particle‐seawater interaction (dry deposition), have higher rates in rhyolitic type ashes. Rhyolitic ashes under acidic conditions release Fe in higher amounts and at a slower rate, while in those samples containing mostly glass shards, Fe release was lower and faster. The 2011 Puyehue eruption was observed by a dust monitoring station. Puyehue‐type eruptions can contribute soluble Fe to the ocean via dry or wet deposition, nearly reaching the limit required for phytoplankton growth. In contrast, the input of Fe after processing by an acidic eruption plume could raise the amount of dissolved Fe in surface ocean waters several times, above the threshold required to initiate phytoplankton blooms. A single eruption like the Puyehue one represents more than half of the yearly Fe‐flux contributed by dust.
      PubDate: 2015-07-14T10:58:03.974088-05:
      DOI: 10.1002/2015GB005177
       
  • Short‐term variability in euphotic zone biogeochemistry and primary
           productivity at Station ALOHA: A case study of summer 2012
    • Abstract: Time‐series observations are critical to understanding the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time‐series (HOT) program has maintained near‐monthly sampling at Station ALOHA (22° 45′N, 158° 00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and identified ecosystem variability over seasonal to interannual time‐scales. To further extend the temporal resolution of these near‐monthly time‐series observations, an extensive field campaign was conducted during July‐September 2012 at Station ALOHA with near‐daily sampling of upper water‐column biogeochemistry, phytoplankton abundance, and activity. The resulting dataset provides biogeochemical measurements at high temporal resolution and documented two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea‐surface salinity minimum feature which was prominent in the upper water‐column (0–50 m) for a period of approximately 30 days. The shipboard observations during July‐September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea‐surface salinity minimum feature in the NPSG.
      PubDate: 2015-07-14T10:31:10.947074-05:
      DOI: 10.1002/2015GB005141
       
  • Response of the Amazon carbon balance to the 2010 drought derived with
           CarbonTracker South America
    • Abstract: Two major droughts in the past decade had large impacts on carbon exchange in the Amazon. Recent analysis of vertical profile measurements of atmospheric CO2 and CO by Gatti et al. [2014] suggests that the 2010 drought turned the normally close‐to‐neutral annual Amazon carbon balance into a substantial source of nearly 0.5 PgC/yr, revealing a strong drought response. In this study, we revisit this hypothesis and interpret not only the same CO2/CO vertical profile measurements, but also additional constraints on carbon exchange such as satellite observations of CO, burned area, and fire hotspots. The results from our CarbonTracker South America data assimilation system suggest that carbon uptake by vegetation was indeed reduced in 2010, but that the magnitude of the decrease strongly depends on the estimated 2010 and 2011 biomass burning emissions. We have used fire products based on burned area (GFED4), satellite‐observed CO columns (IASI), fire radiative power (GFASv1) and fire hotspots (FINNv1), and found an increase in biomass burning emissions in 2010 compared to 2011 of 0.16 to 0.24 PgC/yr. We derived a decrease of biospheric uptake ranging from 0.08 to 0.26 PgC/yr, with the range determined from a set of alternative inversions using different biomass burning estimates. Our numerical analysis of the 2010 Amazon drought results in a total reduction of carbon uptake of 0.24 to 0.50 PgC/yr and turns the balance from carbon sink to source. Our findings support the suggestion that the hydrological cycle will be an important driver of future changes in Amazonian carbon exchange.
      PubDate: 2015-07-02T14:44:12.074049-05:
      DOI: 10.1002/2014GB005082
       
  • Combined Impact of Catchment Size, Land Cover and Precipitation on
           Streamflow and Total Dissolved Nitrogen: A Global Comparative Analysis
    • Authors: Erika L. Gallo; Tom Meixner, Hadi Aoubid, Kathleen A. Lohse, Paul D. Brooks
      Abstract: Nitrogen (N) loading is a global stressor to fresh and salt water systems with cascading effects on ecosystem processes [Galloway et al., 2003; Galloway et al., 2004]. However, it is unclear if generalized global response patterns exist between discharge and N sourcing and retention with respect to land cover and precipitation. Using data compiled from 78 catchments from across the world, we identified how discharge and total dissolved nitrogen (TDN) vary with precipitation and land cover; and how TDN yields deviate from a generalized global response pattern. Area‐weighted discharge regressions indicate that catchment size and the absence of vegetation largely control hydrologic responses. TDN concentrations and yields varied significantly (p
      PubDate: 2015-07-02T14:43:33.096705-05:
      DOI: 10.1002/2015GB005154
       
  • Water mass mixing is the dominant control on the zinc distribution in the
           North Atlantic Ocean
    • Authors: Saeed Roshan; Jingfeng Wu
      Abstract: Dissolved zinc (dZn) concentration was determined in the North Atlantic during the US GEOTRACES 2010&2011 cruise (GOETRACES GA03). A relatively poor linear correlation (R2=0.756) was observed between dZn and silicic acid (Si) the slope of which was 0.0577 nM:µmol/kg. We attribute the relatively poor dZn‐Si correlation to the following processes: a. differential regeneration of zinc relative to silicic acid; b. mixing of multiple water masses that have different Zn/Si; c. zinc sources such as sedimentary or hydrothermal. To quantitatively distinguish these possibilities, we use the results of Optimum Multi‐Parameter Water Mass Analysis by Jenkins et al. [2015] to model the zinc distribution below 500m. We hypothesized two scenarios: conservative mixing and regenerative mixing. The first scenario (conservative) could be modeled to results in a correlation with observations with a R2=0.846. In the second scenario, we took a Si‐related regeneration into account which could model the observations with a R2=0.867. Through this regenerative mixing scenario, we estimated a Zn/Si=0.0548 nM:µmol/kg that may be more realistic than linear regression slope due to accounting for process b. However, this did not improve the model substantially (R2=0.867 vs.0.846) which may indicate the insignificant effect of remineralization on the zinc distribution in this region. The relative weakness in the model‐observation correlation (R2~0.85 for both scenarios) imply that processes a and c may be plausible. Furthermore, dZn in the upper 500m exhibited a very poor correlation with apparent oxygen utilization (AOU) suggesting a minimal role for the organic matter‐associated remineralization process.
      PubDate: 2015-07-01T12:37:23.505278-05:
      DOI: 10.1002/2014GB005026
       
  • Transports and budgets of anthropogenic CO2 in the tropical North Atlantic
           in 1992–93 and 2010–11
    • Abstract: The meridional transport of anthropogenic CO2 (Cant) in the tropical North Atlantic (TNA) is investigated using data from transoceanic sections along 7.5°N and 24.5 °N, carried out in the early 1990s and 2010s. The net Cant transport across both sections is northward. At 7.5°N, this transport increased from 315 ± 47 kmol s−1 in 1993 to 493 ± 51 kmol s−1 in 2010; similarly, across 24.5°N it grew from 530 ± 46 kmol s−1 in 1992 to 662 ± 49 kmol s−1 in 2011. These changes result from modifications in the intermediate and deep circulation patterns, as well as from Cant increase within the thermocline waters. In deep waters, lateral advection causes a net Cant input of 112 ± 60 kmol s−1 (234 ± 65 kmol s−1) in 1992–93 (2010–11); within these deep waters, the storage rate of Cant is not statistically different from the net Cant input, 139 ± 21 kmol s−1 (188 ± 21 kmol s−1) in 1992–93 (2010–11). The Cant increase in deep waters is due to the large injection of Cant across the 24.5°N by the Deep Western Boundary Current and the northward recirculation of North Atlantic Deep Water along 7.5°N. In contrast, a large net Cant output in the upper layer is caused by the Florida Current. Despite this net Cant output, the Cant accumulates at a rate of 215 ± 24 kmol s−1 (291 ± 24 kmol s−1) referenced to year 1993 (2010). From the two Cant budgets, we infer a Cant air‐sea flux of 0.23 ± 0.02 Pg yr−1in the TNA, much larger than previous estimates.
      PubDate: 2015-06-22T09:12:46.349955-05:
      DOI: 10.1002/2014GB005075
       
  • A new look at ocean carbon remineralization for estimating
           deep‐water sequestration
    • Authors: Lionel Guidi; Louis Legendre, Gabriel Reygondeau, Julia Uitz, Lars Stemmann, Stephanie A. Henson
      Abstract: The “biological carbon pump” causes carbon sequestration in deep waters by downward transfer of organic matter, mostly as particles. This mechanism depends to a great extent on the uptake of CO2 by marine plankton in surface waters, and subsequent sinking of particulate organic carbon (POC) through the water column. Most of the sinking POC is remineralized during its downward transit and modest changes in remineralization have substantial feedbacks on atmospheric CO2 concentrations, but little is known about global variability in remineralization. Here we assess this variability based on modern underwater particle imaging combined with field POC flux data, and discuss the potential sources of variations. We show a significant relationship between remineralization and the size structure of the phytoplankton assemblage. We obtain the first regionalized estimates of remineralization in biogeochemical provinces, where these estimates range between ‐50 and +100% of the commonly used globally uniform remineralization value. We apply the regionalized values to satellite‐derived estimates of upper ocean POC export to calculate regionalized and ocean‐wide deep carbon fluxes and sequestration. The resulting value of global organic carbon sequestration at 2000 m is 0.33 Pg C y‐1, and 0.72 Pg C y‐1 at the depth of the top of the permanent pycnocline, which is up to 3 times higher than the value resulting from the commonly‐used approach based on uniform remineralization and constant sequestration depth. These results stress that variable remineralization length scale and sequestration depth should be used to model ocean carbon sequestration and feedbacks on the atmosphere.
      PubDate: 2015-06-18T19:25:21.63808-05:0
      DOI: 10.1002/2014GB005063
       
  • Why are biotic iron pools uniform across high‐ and low‐iron
           pelagic ecosystems'
    • Authors: P.W. Boyd; R.F. Strzepek, M.J. Ellwood, D.A. Hutchins, S.D. Nodder, B.S. Twining, S.W. Wilhelm
      Abstract: Dissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here, we compare mixed‐layer biotic iron inventories from a low‐iron (~0.06 nmol L−1) subantarctic (FeCycle study) and a seasonally high‐iron (~0.6 nmol L−1) subtropical (FeCycle II study) site. Both studies were quasi‐Lagrangian, and had multi‐day occupation, common sampling protocols, and indirect estimates of biotic iron (from a limited range of available published biovolume/carbon/iron quotas). Biotic iron pools were comparable (~100± 30 pmol L−1) for low‐ and high‐iron waters, despite a ten‐fold difference in dissolved iron concentrations. Consistency in biotic iron inventories (~80±24 pmol L−1, largely estimated using a limited range of available quotas) was also conspicuous for three Southern Ocean polar sites. Insights into the extent to which uniformity in biotic iron inventories was driven by the need to apply common iron quotas obtained from laboratory cultures were provided from FeCycle II. The observed two‐ to three‐fold range of iron quotas during the evolution of FeCycle II subtropical bloom was much less than reported from laboratory monocultures. Furthermore, the iron recycling efficiency varied by fourfold during FeCycle II, increasing as stocks of new iron were depleted, suggesting that quotas and iron recycling efficiencies together set biotic iron pools. Hence, site‐specific differences in iron recycling efficiencies (which provide 20‐50% and 90% of total iron supply in high and low iron waters, respectively) helps offset the differences in new iron inputs between low‐ and high‐iron sites. Future parameterisation of iron in biogeochemical models must focus on the drivers of biotic iron inventories, including the differing iron requirements of the resident biota, and the subsequent fate (retention/export/recycling) of the biotic iron.
      PubDate: 2015-06-17T08:03:43.288577-05:
      DOI: 10.1002/2014GB005014
       
  • Origin and fluxes of nitrous oxide along a latitudinal transect in western
           North Pacific: Controls and regional significance
    • Authors: Florian Breider; Chisato Yoshikawa, Hitomi Abe, Sakae Toyoda, Naohiro Yoshida
      Abstract: Nitrous oxide (N2O) is an atmospheric trace gas playing an important role in both radiative forcing and stratospheric ozone depletion. The oceans are the second most important natural source of N2O. The magnitude of the flux of this source is poorly constrained. Moreover the relative importance of the microbial processes leading to the formation or the consumption of N2O in oceans remains unclear. We present here fluxes, isotope and isotopomer signatures of N2O measured at three stations located along a latitudinal transect in subtropical and subarctic Western North Pacific. These results indicate that about 30% to 55% of the oceanic flux of N2O to the atmosphere originates from the deep euphotic and shallow aphotic zones. The sea‐to‐air fluxes of N2O calculated using an isotope mass balance model indicate that the emission rate of N2O in subarctic waters is about two times higher than in oligotrophic subtropical waters suggesting that nutrient‐rich water coming from the western subarctic gyre stimulates the N2O production. Moreover, isotopomer analysis has revealed that in shallow water N2O originates from nitrification and nitrifier‐denitrification processes and its distribution in the water column is partly controlled by the incident solar radiation. The results of this study contribute to better constrain the global N2O budget and provide important information to better predict the future evolution of the oceanic emissions of N2O.
      PubDate: 2015-06-17T08:03:26.97157-05:0
      DOI: 10.1002/2014GB004977
       
  • Biological and physical controls on N2, O2 and CO2 distributions in
           contrasting Southern Ocean surface waters
    • Abstract: We present measurements of pCO2, O2 concentration, biological oxygen saturation (ΔO2/Ar) and N2 saturation (ΔN2) in Southern Ocean surface waters during austral summer, 2010–2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chla) concentrations in regions of frontal mixing and sea‐ice melt. pCO2 and ΔO2 /Ar exhibited large spatial gradients (range 90 to 450 µatm and −10 to 60%, respectively) and co‐varied strongly with Chla. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time‐scales of O2 and CO2 air‐sea equilibration. We measured significant oceanic CO2 uptake, with a mean air‐sea flux (~ −10 mmol m−2 d−1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean ΔN2 of +2.5 %), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean ΔO2phys = 2.1 %). Box model calculations were able to reproduce much of the spatial variability of ΔN2 and ΔO2phys along the cruise track, demonstrating significant effects of air‐sea exchange processes (e.g. atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment‐corrected surface ΔO2 /Ar data, ranged from ~ −40 to > 300 mmol O2 m−2 d−1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea‐ice melt, reflecting physical controls on surface water light fields and nutrient availability.
      PubDate: 2015-06-16T06:18:50.485864-05:
      DOI: 10.1002/2014GB004975
       
  • Flux and budget of black carbon (BC) in the continental shelf seas
           adjacent to Chinese high BC emission source regions
    • Authors: Yin Fang; Yingjun Chen, Chongguo Tian, Tian Lin, Limin Hu, Guopei Huang, Jianhui Tang, Jun Li, Gan Zhang
      Abstract: This study conducted the first comprehensive investigation of sedimentary black carbon (BC) concentration, flux and budget in the continental shelves of “Bohai Sea (BS) and Yellow Sea (YS)”, based on measurements of BC in 191 surface sediments, 36 riverine water and 2 seawater samples, as well as the reported dataset of the atmospheric samples from seven coastal cities in the Bohai Rim. BC concentrations in these matrices were measured using the method of thermal/optical reflectance. The spatial distribution of the BC concentration in surface sediments was largely influenced by the regional hydrodynamic conditions, with high values mainly occurring in the central mud areas where fine‐grained particles (median diameters >6 Φ (i.e.,
      PubDate: 2015-06-10T10:42:21.623058-05:
      DOI: 10.1002/2014GB004985
       
  • Multi‐decadal trends of oxygen and their controlling factors in the
           western North Pacific
    • Authors: Daisuke Sasano; Yusuke Takatani, Naohiro Kosugi, Toshiya Nakano, Takashi Midorikawa, Masao Ishii
      Abstract: The rate of change of dissolved oxygen (O2) concentrations was analyzed over 1987–2011 for the high‐frequency repeat section along 165°E in the western North Pacific. Significant trends towards decreasing O2 were detected in the northern subtropical to subtropical‐subarctic transition zones over a broad range of isopycnal horizons. On 25.3σθ between 25°N‐30°N in North Pacific Subtropical Mode Water, the rate of O2 decrease reached −0.45 ± 0.16 µmol kg−1 yr−1. It is largely attributed to a deepening of isopycnal horizons and to a reduction in oxygen solubility associated with ocean warming. In North Pacific Intermediate Water, the rate of O2 decrease was elevated (−0.44 ± 0.14 µmol kg−1 yr−1 on 26.8σθ) and was associated with net increases in apparent oxygen utilization in the source waters. On 27.3σθ in the subtropical Oxygen Minimum Layer (OML) between 32.5°N‐35°N, the rate of O2 decrease was significant (−0.22 ± 0.05 µmol kg−1 yr−1). It was likely due to the increases in westward transport of low‐oxygen water. These various drivers controlling changes in O2 along the 165°E section are the same as those acting along 137°E (analyzed previously), and also account for the differences in the rate of O2 decrease between these sections. Additionally, in the tropical OML near 26.8σθ between 5°N‐10°N, significant trends toward increasing O2 were detected in both sections (+0.36 ± 0.04 µmol kg−1 yr−1 in the 165°E section). These results demonstrate that warming and circulation changes are causing multi‐decadal changes in dissolved O2 over wide expanses of the western North Pacific.
      PubDate: 2015-06-08T21:58:52.516376-05:
      DOI: 10.1002/2014GB005065
       
  • WATER MASS AGE AND AGEING DRIVING CHROMOPHORIC DISSOLVED ORGANIC MATTER IN
           THE DARK GLOBAL OCEAN
    • Abstract: The omnipresence of chromophoric dissolved organic matter (CDOM) in the open ocean enables its use as a tracer for biochemical processes throughout the global overturning circulation. We made an inventory of CDOM optical properties, ideal water age (τ) and apparent oxygen utilization (AOU) along the Atlantic, Indian and Pacific Ocean waters sampled during the Malaspina 2010 expedition. A water mass analysis was applied to obtain intrinsic, hereinafter archetypal, values of τ, AOU, oxygen utilisation rate (OUR), and CDOM absorption coefficients, spectral slopes and quantum yield for each one of the 22 water types intercepted during this circumnavigation. Archetypal values of AOU and OUR have been used to trace the differential influence of water mass ageing and ageing rates, respectively, on CDOM variables. Whereas the absorption coefficient at 325nm (a325) and the fluorescence quantum yield at 340 nm (Φ340) increased, the spectral slope over the wavelength range 275–295 nm (S275–295) and the ratio of spectral slopes over the ranges 275 –295 nm and 350–400 nm (SR) decreased significantly with water mass ageing (AOU). Combination of the slope of the linear regression between archetypal AOU and a325 with the estimated global OUR allowed us to obtain a CDOM turnover time of 634 ± 120 years, which exceeds the flushing time of the dark ocean (>200 m) by 46%. This positive relationship supports the assumption of in situ production and accumulation of CDOM as a by‐product of microbial metabolism as water masses turn older. Furthermore, our data evidence that global‐scale CDOM quantity (a325) is more dependant on ageing (AOU), whereas CDOM quality (S275–295, SR, Φ340) is more dependent on ageing rate (OUR).
      PubDate: 2015-06-02T20:46:12.123557-05:
      DOI: 10.1002/2014GB005048
       
  • On the influence of “non‐Redfield” dissolved organic
           nutrient dynamics on the spatial distribution of N2 fixation and the size
           of the marine fixed nitrogen inventory
    • Authors: Christopher J. Somes; Andreas Oschlies
      Abstract: Dissolved organic nitrogen (DON) and phosphorus (DOP) represent the most abundant form of their respective nutrient pool in the surface layer of the oligotrophic oceans and play an important role in nutrient cycling and productivity. Since DOP is generally more labile than DON, it provides additional P that may stimulate growth of N2‐fixing diazotrophs that supply fixed nitrogen to balance denitrification in the ocean. In this study, we introduce semi‐recalcitrant components of DON and DOP as state variables in an existing global ocean–atmosphere‐sea ice‐biogeochemistry model of intermediate complexity to assess their impact on the spatial distribution of N2‐fixation and the size of the marine fixed nitrogen inventory. Large‐scale surface datasets of global DON and Atlantic Ocean DOP are used to constrain the model. Our simulations suggest that both preferential DOP remineralization and phytoplankton DOP uptake are important “non‐Redfield” processes (i.e., deviate from molar N:P=16) that need to be accounted for to explain the observed patterns of DOP. Additional non‐Redfield DOP sensitivity experiments testing DOM production rate uncertainties that best reproduce the observed spatial patterns of DON and DOP stimulate additional N2‐fixation that increases the size of the global marine fixed nitrogen inventory by 4.7±1.7% compared to the simulation assuming Redfield DOM stoichiometry that underestimates the observed nitrogen inventory. The extra 8 Tg yr−1 of N2‐fixation stimulated in the Atlantic Ocean is mainly responsible for this increase due to its large spatial separation from water column denitrification, which buffers any potential nitrogen surplus in the Pacific Ocean. Our study suggests that the marine fixed nitrogen budget is sensitive to non‐Redfield DOP dynamics because access to the relatively labile DOP pool expands the ecological niche for N2‐fixing diazotrophs.
      PubDate: 2015-05-25T05:59:17.465865-05:
      DOI: 10.1002/2014GB005050
       
  • Century‐scale patterns and trends of global pyrogenic carbon
           emissions and fire impacts on the terrestrial carbon balance
    • Abstract: Fires have consumed a large amount of terrestrial organic carbon, and significantly influenced terrestrial ecosystems and the physical climate system over the past century. Although biomass burning has been widely investigated at a global level in recent decades via satellite observations, less work has been conducted to examine the century‐scale changes in global fire regimes and fire impacts on the terrestrial carbon balance. In this study, we investigated global pyrogenic carbon emissions and fire impacts on the terrestrial carbon fluxes from 1901 to 2010 by using a process‐based land ecosystem model. Our results show a significant declining trend in global pyrogenic carbon emissions between the early 20th century and the mid‐1980s, but a significant upward trend between the mid‐1980s and the 2000s as a result of more frequent fires in ecosystems with high carbon storage, such as peatlands and tropical forests. Over the past 110 years, average pyrogenic carbon emissions were estimated to be 2.43 Pg C year‐1 (1 Pg = 1015 g), and global average combustion rate (defined as carbon emissions per unit area burned) was 537.85 g C m‐2 burned area. Due to the impacts of fires, the net primary productivity and carbon sink of global terrestrial ecosystems were reduced by 4.14 Pg C year‐1 and 0.57 Pg C year‐1, respectively. Our study suggests that special attention should be paid to fire activities in the peatlands and tropical forests in the future. Practical management strategies, such as minimizing forest logging and reducing the rate of cropland expansion in the humid regions, are in need to reduce fire risk and mitigate fire‐induced greenhouse gases emissions.
       
  • Silicon pools in human impacted soils of temperate zones
    • Abstract: Besides well‐known effects of climate and parent material on silicate weathering the role of land use change as a driver in the global silicon cycle is not well known. Changes in vegetation cover have altered reservoirs of silicon and carbon in plants and soils. This has potential consequences for plant‐Si availability, agricultural yields and coastal eutrophication, as Si is a beneficial element for many crop plants and an essential nutrient for diatom growth. We here examined the role of sustained and intensive land use and human disturbance on silicon (Si) pool distribution in soils with similar climatological and bulk mineralogical characteristics. We show that land use impacts both biogenic and non‐biogenic Si pools. While biogenic Si strongly decreases along the land use change gradient (from forest to croplands), pedogenic silica fractions (e.g. pedogenic clays) increase in top soils with a long duration of cultivation and soil disturbance. Our results suggest that non‐biogenic Si pools might compensate for the loss of reactive biogenic silicon in temperate zones.
       
  • Increased influence of nitrogen limitation on CO2 emissions from future
           land use and land‐use change
    • Abstract: In the latest projections of future greenhouse gas emissions for the Intergovernmental Panel on Climate Change (IPCC), few Earth System Models included the effect of nitrogen limitation, a key process limiting forest regrowth. Few included forest management (wood harvest). We estimate the impacts of nitrogen limitation on the CO2 emissions from land use and land‐use change (LULUC), including wood harvest, for the period 1900‐2100. We use a land‐surface model that includes a fully coupled carbon and nitrogen cycle, and accounts for forest regrowth processes following agricultural abandonment and wood harvest. Future projections are based on the four Representation Concentration Pathways used in the IPCC Fifth Assessment Report, and we account for uncertainty in future climate for each scenario based on ensembles of climate model outputs. Results show that excluding nitrogen limitation will underestimate global LULUC emissions by 34‐52 PgC (20‐30%) during the 20th century (range across three different historical LULUC reconstructions) and by 128‐187 PgC (90‐150%) during the 21st century (range across the four IPCC scenarios). The full range for estimated LULUC emissions during the 21st century including climate model uncertainty is 91 to 227 PgC (with nitrogen limitation included). The underestimation increases with time because: (1) Projected annual wood harvest rates from forests summed over the 21st century are 380‐1080% higher compared to those of the 20th century, resulting in more regrowing secondary forests, (2) Nitrogen limitation reduces the CO2 fertilization effect on net primary production of regrowing secondary forests following wood harvest and agricultural abandonment, and (3) Nitrogen limitation effect is aggravated by the gradual loss of soil nitrogen from LULUC disturbance. Our study implies that: (1) Nitrogen limitation of CO2 uptake is substantial and sensitive to nitrogen inputs, (2) If LULUC emissions are larger than previously estimated in studies without nitrogen limitation, then meeting the same climate mitigation target would require an equivalent additional reduction of fossil fuel emissions, (3) The effectiveness of land‐based mitigation strategies will critically depend on the interactions between nutrient limitations and secondary forests resulting from LULUC, and (4) It is important for terrestrial biosphere models to consider nitrogen constraint in estimates of the strength of future land carbon uptake.
       
  • On the Southern Ocean CO2 uptake and the role of the biological carbon
           pump in the 21st century
    • Abstract: We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming‐ and freshening‐driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement ona dominance of either the SAM or the warming signal south of 44 ∘ S. In the southernmost zone, i.e., south of 58∘ S, they concur on an increase of biological export production, while between 44 and 58∘ S the models lack consensus on the sign of change in export. Yet, in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO 2 (aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44∘ S all models show a predominance of the warming signal and a nutrient‐driven reduction of export production. As a consequence, the share of the regions south of 44∘ S to the total uptake of the Southern Ocean south of 30∘ S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm‐REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate‐driven changes of natural CO2 exchange offsetting that trend only to a limited degree (∼10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30∘S.
       
  • Issue Information
    • Abstract: Cover: In Somes and Oschlies [doi 10.1002/2014GB005050], comparison of surface (0–50m) (a) map and (b) zonally averaged DON observations [Letscher et al., 2013] with annual semirecalcitrant DON from the model experiments (c) Redfield DOM (RedDOM), (d) preferential DOP remineralization (pref_DOP_remin), (e) preferential DOP recycling and phytoplankton DOP uptake (nonRedDOP), (f ) non‐Redfield DOP with low DOM production (low_nonRedDOP), (g) non‐Redfield DOP with high DOM production (high_nonRedDOP), and (h) fast recycling non‐Redfield DOP (fast_nonRedDOP). Note that the zonally averaged model results in (b) are taken only from locations where observations exist. See pp. 973–993.
       
  • Strong dependence of CO2 emissions from anthropogenic land cover change on
           initial land cover and soil carbon parametrization
    • Abstract: The quantification of sources and sinks of carbon from land use and land cover changes (LULCC) is uncertain. We investigated how the parametrization of LULCC and of organic matter decomposition, as well as initial land cover affect the historical and future carbon fluxes in an Earth System Model (ESM). Using the land component of the Max‐Planck‐Institute ESM, we found that the historical (1750–2010) LULCC flux varied up to 25% depending on the fraction of biomass which enters the atmosphere directly due to burning or is used in short‐lived products. We found an uncertainty in the decadal LULCC fluxes of the recent past due to the parametrization of decomposition and direct emissions of 0.6 Pg C yr−1, which is three times larger than the un‐certainty previously attributed to model and method in general. Pre‐industrial natural land cover had a larger effect on decadal LULCC fluxes than the aforementioned parameter sensitivity (1.0 Pg C yr−1). Re‐gional differences between reconstructed and dynamically‐computed land cover, in particular at low‐latitudes, led to differences in historical LULCC emissions of 84–114 Pg C, globally. This effect is larger than the effects of forest regrowth, shifting cultivation or climate feedbacks and comparable to the effect of differences among studies in the terminology of LULCC. In general, we find that the practice of calibrating the net land carbon balance to provide realistic boundary conditions for the climate component of an ESM hampers the applicability of the land component outside its primary field of application.
       
  • The imbalance of new and export production in the Western Antarctic
           Peninsula, a potentially “leaky” ecosystem
    • Abstract: To quantify the balance between new production and vertical nitrogen export of sinking particles, we measured nitrate uptake, net nitrate drawdown, ΔO2 /Ar‐based net community production, sediment trap flux, and 234Th export at a coastal site near Palmer Station, Antarctica during the phytoplankton growing season from October 2012 to March 2013. We also measured nitrate uptake and 234Th export throughout the northern western Antarctic Peninsula (WAP) region on a cruise in January 2013. We used a non‐steady state 234Th equation with temporally‐varying upwelling rates and an irradiance‐based phytoplankton production model to correct our export and new production estimates in the complex coastal site near Palmer Station. Results unequivocally showed that nitrate uptake and net community production were significantly greater than the sinking particle export on region‐wide spatial scales and season‐long temporal scales. At our coastal site, new production (105±17.4 mg N m−2 d−1, mean±st.err.) was 5.3 times greater than vertical nitrogen export (20.4±2.4 mg N m−2 d‐1). On the January cruise in the northern WAP, new production (47.9±14.4 mg N m−2 d‐1) was 2.4 times greater than export (19.9±1.4 mg N m−2 d−1). Much of this imbalance can be attributed to diffusive losses of particulate nitrogen from the surface ocean due to diapycnal mixing, indicative of a “leaky” WAP ecosystem. If these diffusive losses are common in other systems where new production exceeds export, it may be necessary to revise current estimates of the ocean's biological pump.
       
  • Seasonal variations, origin and fate of settling diatoms in the Southern
           Ocean tracked by silicon isotope records in deep sediment traps
    • Abstract: The Southern Ocean plays a pivotal role in the control of atmospheric CO2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace limiting nutrient and dissolved silicon (DSi) as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here, we use δ30Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ and Subantarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ30Si composition reveal a range of insights, including that i) the sinking rate of BSi exceeds 200 m d−1 in summer in the AZ, yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, ii) occasional vertical mixing events affect the δ30Si composition of exported BSi in both the SAZ and AZ, iii) the δ30Si signature of diatoms is well conserved through the water column despite strong BSi and POC attenuation at depth, and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.
       
  • An observational assessment of the influence of mesoscale and submesoscale
           heterogeneity on ocean biogeochemical reactions
    • Abstract: Numerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100 km ‐1 km). The causes and consequences of physical processes at these scales (‘eddy advection’) influencing biogeochemistry have received much attention. Less studied, the non‐linear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed ‘eddy reactions’. Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modelling work has previously suggested that the neglected ‘eddy reactions’ may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates, and not taken as a justification for ignoring eddy reactions. Compared to modelling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions.
       
  • Is atmospheric phosphorus pollution altering global alpine lake
           stoichiometry'
    • Abstract: Anthropogenic activities have significantly altered atmospheric chemistry and changed the global mobility of key macronutrients. Here, we show that contemporary global patterns in nitrogen (N) and phosphorus (P) emissions drive large hemispheric variation in precipitation chemistry. These global patterns of nutrient emission and deposition (N:P) are in turn closely reflected in the water chemistry of naturally oligotrophic lakes (r2=0.81, p
       
 
 
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