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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)   [Sort by number of followers]   [Restore default list]

Showing 1 - 17 of 17 Journals sorted alphabetically
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 23, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 56, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 7, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 3, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 26)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 9)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 26)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 18)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 27)
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: 5, 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: 4, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 66, SJR: 2.189, h-index: 121)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [7 followers]  Follow
    
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • When Can Ocean Acidification Impacts be Detected from Decadal Alkalinity
           Measurements?
    • Abstract: We use a large initial condition suite of simulations (30 runs) with an Earth system model to assess the detectability of biogeochemical impacts of ocean acidification (OA) on the marine alkalinity distribution from decadally‐repeated hydrographic measurements such as those produced by the Global Ship‐Based Hydrographic Investigations Program (GO‐SHIP). Detection of these impacts is complicated by alkalinity changes from variability and long term trends in freshwater and organic matter cycling and ocean circulation. In our ensemble simulation, variability in freshwater cycling generates large changes in alkalinity that obscure the changes of interest and prevent the attribution of observed alkalinity redistribution to OA. These complications from freshwater cycling can be mostly avoided through salinity normalization of alkalinity. With the salinity‐normalized alkalinity, modeled OA impacts are broadly detectable in the surface of the subtropical gyres by 2030. Discrepancies between this finding and the finding of an earlier analysis suggest these estimates are strongly sensitive to the patterns of calcium carbonate export simulated by the model. OA impacts are detectable later in the subpolar and equatorial regions due to slower responses of alkalinity to OA in these regions and greater seasonal equatorial alkalinity variability. OA impacts are detectable later at depth despite lower variability due to smaller rates of change and consistent measurement uncertainty.
      PubDate: 2016-04-18T19:05:38.338592-05:
      DOI: 10.1002/2015GB005308
       
  • Potential use of the N2/Ar ratio as a constraint on the oceanic fixed
           nitrogen loss
    • Authors: M. Shigemitsu; N. Gruber, A. Oka, Y. Yamanaka
      Abstract: Using a global ocean biogeochemical model, we investigate the suitability of the N2/Ar supersaturation ratio (ΔN2/Ar) as a tracer of marine nitrogen fixation and denitrification, i.e., the main biological processes that add or remove fixed nitrogen from the ocean. In a series of factorial simulations, we demonstrate that in regions away from the oxygen minimum zones (OMZs) the ΔN2/Ar characteristics are mostly determined by benthic denitrification occurring in the deep ocean with minor contributions from benthic and water‐column denitrification at shallower depths. In the OMZs, the subsurface maxima of ΔN2/Ar are mainly determined by water‐column denitrification. In contrast, nitrogen fixation has little impact on ΔN2/Ar owing to the rapid loss of the N2 supersaturation signal through air‐sea exchange. We thus conclude that ΔN2/Ar can act as a powerful constraint on water column and benthic denitrification occurring in intermediate‐to‐deep waters, but it cannot be used to estimate nitrogen fixation. The comparison of the currently very limited observations of the ΔN2/Ar with our model results shows an acceptable level of agreement, suggesting that the model's prescribed rates and distribution of benthic and water‐column denitrification (i.e., 140 and 52 TgN yr‐1, respectively) are reasonable and confirms the results derived from other constraints.
      PubDate: 2016-04-07T20:42:21.238923-05:
      DOI: 10.1002/2015GB005297
       
  • Nitrogen trace gas fluxes from a semi‐arid subtropical savanna under
           woody legume encroachment
    • Authors: Fiona M Soper; Thomas W Boutton, Peter M Groffman, Jed P Sparks
      Abstract: Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and climate. These systems are experiencing widespread encroachment by woody plants, frequently associated with large increases in soil N, with no consensus on implications for trace gas emissions. We investigated the impact of encroachment by N‐fixing tree Prosopis glandulosa on total reactive N gas flux (Nt = NO + N2O + NOy + NH3) from south Texas savanna soils over two years. Contrary to expectations, upland Prosopis groves did not have greater Nt fluxes than adjacent unencroached grasslands. However, abiotic conditions (temperature, rainfall, and topography) were strong drivers. Emissions from moist, low‐lying Prosopis playas were up to three times higher than from Prosopis uplands. Though NO dominated emissions, NH3 and NOy (non‐NO oxidized N) comprised 12‐16% of the total summer N flux (up to 7.9 µg N m‐2 h‐1). Flux responses to soil wetting were temperature‐dependent for NO, NH3 and NOy: a 15 mm rainfall event increased flux 3‐22 fold after 24 hours in summer, but had no effect in winter. Repeated soil wetting reduced N flux responses, indicating substrate depletion as a likely control. Rapid (
      PubDate: 2016-03-30T18:55:39.513151-05:
      DOI: 10.1002/2015GB005298
       
  • Temperature influence on phytoplankton community growth rates
    • Authors: Elliot Sherman; J. Keith Moore, Francois Primeau, David Tanouye
      Abstract: A large database of field estimates of phytoplankton community growth rates in natural populations was compiled and analyzed to determine the apparent temperature effect on phytoplankton community growth rate. We conducted an ordinary least squares regression to optimize the parameters in two commonly used growth‐temperature relations (Arrhenius and Q10 models). Both equations fit the observational data equally with the optimized parameter values. The optimum apparent Q10 value was 1.47 ± 0.08 [95% C.I.]. Microzooplankton grazing rates closely matched the temperature trends for phytoplankton growth. This likely reflects a dynamic adjustment of biomass and grazing rates by the microzooplankton to match their available food source, illustrating tight coupling of phytoplankton growth and microzooplankton grazing rates. The field‐measured temperature effect and growth rates were compared with estimates from the satellite Carbon‐based Productivity Model (CbPM) and three Earth System Models (ESMs), with model output extracted at the same month and sampling locations as the observations. The optimized, apparent Q10 value calculated for the CbPM was 1.51, with overestimation of growth rates. The apparent Q10 value in the Community Earth System Model (CESM V1.0) was 1.65, with modest underestimation of growth rates. The GFDL‐ESM2M and GFDL‐ESM2G models produced apparent Q10 values of 1.52 and 1.39 respectively. Models with an apparent Q10 that is significantly greater than ~1.5 will overestimate the phytoplankton community growth response to the ongoing climate warming, and will have spatial biases in estimated growth rates for the current era.
      PubDate: 2016-03-18T01:06:47.331976-05:
      DOI: 10.1002/2015GB005272
       
  • Estimated storage of amorphous silica (ASi) in soils of the
           circum‐Arctic tundra region
    • Authors: H Alfredsson; W Clymans, G Hugelius, P Kuhry, DJ Conley
      Abstract: We investigated the vertical distribution, storage, landscape partitioning and spatial variability of soil amorphous silica (ASi) at four different sites underlain by continuous permafrost, and representative of mountainous and lowland tundra, in the circum‐Arctic region. Based on a larger set of data, we present the first estimate of the ASi soil reservoir (0 – 1 m depth) in circum‐Arctic tundra terrain. At all sites, the vertical distribution of ASi concentrations followed the pattern of either 1) declining concentrations with depth (most common) or 2) increasing/maximum concentrations with depth. Our results suggest that a set of processes, including biological control, solifluction and other slope processes, cryoturbation and formation of inorganic precipitates, influence vertical distributions of ASi in permafrost terrain, with the capacity to retain stored ASi on millennial time scales. At the four study sites, areal ASi storage (0 – 1 m) is generally higher in graminoid tundra compared to wetlands. Our circum‐Arctic upscaling estimates, based on both vegetation and soil classification separately, suggest a storage amounting to 219 ± 28 to 274 ± 33 Tmol Si, respectively, of which at least 30% is stored in permafrost. This estimate would account for about 3 % of the global soil ASi storage while occupying an equal portion of the global land area. This result does not support the hypothesis that the circum‐Arctic tundra soil ASi reservoir contains relatively higher amounts of ASi than other biomes globally as demonstrated for carbon. Nevertheless, climate warming has the potential to significantly alter ASi storage and terrestrial Si cycling in the Arctic.
      PubDate: 2016-02-25T22:44:47.081449-05:
      DOI: 10.1002/2015GB005344
       
  • The long‐term impact of magnesium in seawater on foraminiferal
           mineralogy: Mechanism and consequences
    • Abstract: Foraminifera are unicellular protists, primarily known for their calcium carbonate shells that provide an extensive fossil record. This record, ranging from Cambrian to Present shows both major shifts and gradual changes in the relative occurrence of taxa producing different polymorphs of carbonate. Here we present evidence for coupling between shifts in calcite‐ versus aragonite‐producing species and periods with respectively low and high seawater Mg/Ca throughout the Phanerozoic. During periods when seawater Mg/Ca is
      PubDate: 2016-02-22T05:52:47.786477-05:
      DOI: 10.1002/2015GB005241
       
  • Linking nutrient loading & oxygen in the coastal ocean: a new global
           scale model
    • Authors: Daniel C. Reed; John A. Harrison
      Abstract: Recent decades have witnessed an exponential spread of low‐oxygen regions in the coastal ocean due at least in‐part to enhanced terrestrial nutrient inputs. Oxygen deprivation is a major stressor on marine ecosystems, there is a great need to quantitatively link shifts in nutrient loading with changes in oxygen concentrations. To this end, we have developed and here describe, evaluate, and apply the Coastal Ocean Oxygen Linked to Benthic Exchange And Nutrient Supply model (COOLBEANS), a first‐of‐its‐kind, spatially‐explicit (with 152 coastal segments), global model of coastal oxygen and nutrient dynamics. In COOLBEANS, benthic oxygen demand (BOD) is calculated using empirical models for aerobic respiration, iron reduction, and sulfate reduction, while oxygen supply is represented by a simple parameterization of exchange between surface and bottom waters. A nutrient cycling component translates shifts in riverine nutrient inputs into changes in organic matter delivery to sediments and, ultimately, oxygen uptake. Modeled BOD reproduces observations reasonably well (Nash‐Sutcliffe efficiency = 0.71) and estimates of exchange between surface and bottom waters correlate with stratification. The model examines sensitivity of bottom‐water oxygen to changes in nutrient inputs and vertical exchange between surface and bottom waters, highlighting the importance of this vertical exchange in defining the susceptibility of a system to oxygen depletion. These sensitivities along with estimated maximum hypoxic areas that are supported by present day nutrient loads are consistent with existing hypoxic regions. Sensitivities are put into context by applying historic changes in nitrogen loading observed in the Gulf of Mexico to the global coastal ocean, demonstrating that such loads would drive many systems anoxic or even sulfidic.
      PubDate: 2016-02-19T21:06:35.68807-05:0
      DOI: 10.1002/2015GB005303
       
  • Decadal variability in the oxygen inventory of North Atlantic Subtropical
           Underwater captured by sustained, long‐term oceanographic
           time‐series observations
    • Abstract: Historical observations of potential temperature (θ), salinity (S), and dissolved oxygen concentrations (O2) in the tropical and subtropical North Atlantic (0–500 m; 0–40°N, 10–90°W) were examined to understand decadal‐scale changes in O2 in Subtropical Underwater (STUW). STUW is observed at four of the longest, sustained ocean biogeochemical and ecological time‐series stations, namely the CARIACO Ocean Time‐Series Program (10.5°N, 64.7°W), the Bermuda Atlantic Time‐series Study (BATS; 31.7°N, 64.2°W), Hydrostation “S” (32.1°N, 64.4°W), and the European Station for Time‐series in the Ocean, Canary Islands (ESTOC; 29.2°N, 15.5°W). Observations over similar time periods at CARIACO (1996–2013), BATS (1988–2011), and Hydrostation “S” (1980 – 2013) show that STUW O2 has decreased approximately 0.71, 0.28, and 0.37 µmol kg‐1 yr‐1, respectively. No apparent change in STUW O2 was observed at ESTOC over the course of the time series (1994–2013). Ship observation data for the tropical and subtropical North Atlantic archived at NOAA NODC show that, between 1980 and 2013, STUW O2 (upper ~300 m) declined 0.58 µmol kg‐1 yr‐1 in the southeastern Caribbean Sea (10–15°N, 60–70°W), and 0.68 µmol kg‐1 yr‐1 in the western subtropical North Atlantic (30–35°N, 60–65°W). A declining O2 trend was not observed in the eastern subtropical North Atlantic (25–30°N, 15–20°W) over the same period. Most of the observed O2 loss seems to result from shifts in ventilation associated with decreased wind‐driven mixing and a slowing down in STUW formation rates, rather than changes in diffusive air‐sea O2 gas exchange or changes in the biological oceanography of the North Atlantic. Variability of STUW O2 showed a significant relationship with the wintertime (January – March) Atlantic Multidecadal Oscillation index (AMO, R2 = 0.32). During negative wintertime AMO years trade winds are typically stronger between 10°N and 30°N. These conditions stimulate the formation and ventilation of STUW. The decreasing trend in STUW O2 in the three decades spanning 1980 through 2013 reflects the shift from a strongly negative wintertime AMO between the mid‐1980's and mid‐1990's to a positive wintertime AMO observed between the mid‐1990's and 2013. These changes in STUW O2 were captured by the CARIACO, BATS, and Hydrostation “S” time series stations. Sustained positive AMO conditions could lead to further de‐oxygenation in tropical and sub‐tropical North Atlantic upper waters.
      PubDate: 2016-02-17T22:01:17.235748-05:
      DOI: 10.1002/2015GB005183
       
  • Saharan versus local influence on atmospheric aerosol deposition in the
           Southern Iberian Peninsula: Significance for N and P inputs
    • Abstract: A novel methodology was used to evaluate the contribution of Saharan dust to the atmospheric deposition of particulate material (PM), total phosphorus (TP), and total nitrogen (TN) in the south‐eastern Iberian Peninsula. Dry and wet aerosol depositions were measured weekly during two one‐year periods at one site and simultaneously during spring‐summer of the same years at two other sites (inter‐site distance of ~ 40 km). Statistical relationships among depositions at the different sites permitted differentiation of Saharan dust inputs from locally derived dust. PM and TP depositions were synchronous among the three study sites; the synchrony was elevated during periods of Saharan intrusions (evaluated by air mass retro‐trajectories analyses), but no temporal correlation was observed during periods without Saharan intrusions. According to ANOVA results, PM and TP depositions were both significantly affected by Saharan intrusions. During weeks with Saharan intrusions, PM deposition increased around 85% above background levels, with no differences among the three sites, while TP deposition increased by 1.1 µmol TP m‐2 d‐1, i.e., 29% to 81% above background levels depending on the site. There were no correlations or differences in TN deposition among sites or as a function of Saharan intrusion periods. The annual contribution of PM and TP from Saharan dust was 75 kg ha‐1 and 0.07 kg P ha‐1, respectively, which can be considered a genuine input for the ecosystems in this area. This novel approach is likely to be valid in any area in the world under atmospheric deposition of long‐range transported material.
      PubDate: 2016-02-10T22:33:24.011476-05:
      DOI: 10.1002/2015GB005254
       
  • Multiple sources of soluble atmospheric iron to Antarctic waters
    • Authors: V.H.L. Winton; R. Edwards, B. Delmonte, A. Ellis, P.S. Andersson, A. Bowie, N.A.N. Bertler, P. Neff, A. Tuohy
      Abstract: The Ross Sea, Antarctica, is a highly productive region of the Southern Ocean. Significant new sources of iron (Fe) are required to sustain phytoplankton blooms in the austral summer. Atmospheric deposition is one potential source. The fractional solubility of Fe is an important variable determining Fe availability for biological uptake. To constrain aerosol Fe inputs to the Ross Sea region, fractional solubility of Fe was analyzed in a snow pit from Roosevelt Island, eastern Ross Sea. In addition, aluminum, dust, and refractory black carbon (rBC) concentrations were analyzed, to determine the contribution of mineral dust and combustion sources to the supply of aerosol Fe. We estimate exceptionally high dissolved Fe (dFe) flux of 1.2 x 10‐6 g m‐2 y‐1 and total dissolvable Fe (TDFe) flux of 140 x 10‐6 g m‐2 y‐1 for 2011/2012. Deposition of dust, Fe, Al, and rBC occurs primarily during spring‐summer. The observed background fractional Fe solubility of ~0.7 % is consistent with a mineral dust source. Radiogenic isotopic ratios and particle size distribution of dust indicates that the site is influenced by local and remote sources. In 2011/2012 summer, relatively high dFe concentrations paralleled both mineral dust and rBC deposition. Around half of the annual aerosol Fe deposition occurred in the austral summer phytoplankton growth season; however the fractional Fe solubility was low. Our results suggest that the seasonality of dFe deposition can vary, and should be considered on longer glacial‐interglacial timescales.
      PubDate: 2016-02-06T13:03:13.112625-05:
      DOI: 10.1002/2015GB005265
       
  • Contribution of regional sources to atmospheric methane over the Amazon
           Basin in 2010 and 2011
    • Authors: Chris Wilson; Manuel Gloor, Luciana V. Gatti, John B. Miller, Sarah A. Monks, Joey McNorton, A. Anthony Bloom, Luana Basso, Martyn P. Chipperfield
      Abstract: We present an assessment of methane (CH4) atmospheric concentrations over the Amazon Basin for 2010 and 2011 using a 3‐D atmospheric chemical transport model, two wetland emission models and new observations made during bi‐weekly flights made over four locations within the Basin. We attempt to constrain Basin‐wide CH4 emissions using the observations, and, since 2010 was an unusually dry year, we assess the effect of this drought on Amazonian methane emissions. We find that South American emissions contribute up to 150 ppb to concentrations at the sites, mainly originating from within the Basin. Our atmospheric model simulations agree reasonably well with measurements at three of the locations (0.28 ≤ r2 ≤ 0.63, mean bias ≤ 9.5 ppb). Attempts to improve the simulated background CH4 concentration through analysis of simulated and observed sulphur hexafluoride concentrations do not improve the model performance, however. Through minimisation of seasonal biases between the simulated and observed atmospheric concentrations, we scale our prior emission inventories to derive total Basin‐wide methane emissions of 36.5‐41.1 Tg(CH4)/yr in 2010 and 31.6‐38.8 Tg(CH4)/yr in 2011. These totals suggest that the Amazon contributes significantly (up to 7%) to global CH4 emissions. Our analysis indicates that factors other than precipitation, such as temperature variations or tree mortality, may have affected microbial emission rates. However, given the uncertainty of our emission estimates, we cannot say definitively whether the non‐combustion emissions from the region were different in 2010 and 2011, despite contrasting meteorological conditions between the two years.
      PubDate: 2016-02-06T12:41:56.048753-05:
      DOI: 10.1002/2015GB005300
       
  • A Mass Budget for Mercury and Methylmercury in the Arctic Ocean
    • Abstract: Elevated biological concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, are observed throughout the Arctic Ocean but major sources and degradation pathways in seawater are not well understood. We develop a mass budget for mercury species in the Arctic Ocean based on available data since 2004 and discuss implications and uncertainties. Our calculations show that high total Hg in Arctic seawater relative to other basins reflect large freshwater inputs and sea ice cover that inhibits losses through evasion. We find that most net MeHg production (20 Mg a‐1) occurs in the subsurface ocean (20‐200 m). There, it is converted to dimethylmercury (Me2Hg: 17 Mg a‐1), which diffuses to the polar mixed layer and evades to the atmosphere (14 Mg a‐1). Me2Hg has a short atmospheric lifetime and rapidly degrades back to MeHg. We postulate that most evaded Me2Hg is re‐deposited as MeHg and that atmospheric deposition is the largest net MeHg source (8 Mg a‐1) to the biologically productive surface ocean. MeHg concentrations in Arctic Ocean seawater are elevated compared to lower latitudes. Riverine MeHg inputs accounts for an approximately 15% of inputs to the surface ocean (2.5 Mg a‐1) but greater importance in the future is likely given increasing freshwater discharges and permafrost melt. This may offset potential declines driven by increasing evasion from ice‐free surface waters. Geochemical model simulations illustrate that for the most biologically relevant regions of the ocean, regulatory actions that decrease Hg inputs have the capacity to rapidly affect aquatic Hg concentrations.
      PubDate: 2016-02-03T17:18:26.409506-05:
      DOI: 10.1002/2015GB005280
       
  • Issue Information
    • Pages: 399 - 399
      Abstract: No abstract is available for this article.
      PubDate: 2016-04-17T20:40:53.147355-05:
      DOI: 10.1002/gbc.20333
       
  • Nitrogen deposition to lakes in national parks of the western Great Lakes
           region: Isotopic signatures, watershed retention, and algal shifts
    • Authors: William O. Hobbs; Brenda Moraska Lafrancois, Robert Stottlemyer, David Toczydlowski, Daniel R. Engstrom, Mark B. Edlund, James E. Almendinger, Kristin E. Strock, David VanderMeulen, Joan E. Elias, Jasmine E. Saros
      First page: 514
      Abstract: Atmospheric deposition is a primary source of reactive nitrogen (Nr) to undisturbed watersheds of the Great Lakes region of the US, raising concerns over whether enhanced delivery over recent decades has affected lake ecosystems. The National Atmospheric Deposition Program (NADP) has been measuring Nr deposition in this region for over 35 years. Here we explore the relationships among NADP‐measured Nr‐deposition, nitrogen stable isotopes (δ15Ν) in lake sediments, and the response of algal communities in 28 lakes situated in national parks of the western Great Lakes region of the US. We find that 36% of the lakes preserve a sediment δ15Ν record that is statistically correlated with some form of Nr‐deposition (total dissolved inorganic N, nitrate, or ammonium). Furthermore, measured long‐term (since 1982) nitrogen biogeochemistry and inferred critical nitrogen loads suggest that watershed nitrogen retention and climate strongly affect whether sediment δ15N is related to Nr‐deposition in lake sediment records. Measurements of algal change over the last ~ 150 years suggest that Nr‐deposition, in‐lake nutrient cycling, and watershed inputs are important factors affecting diatom community composition, in addition to direct climatic effects on lake physical limnology. The findings suggest that bulk sediment δ15N does reflect Nr‐deposition in some instances. In addition, this study highlights the interactive effects of Nr‐deposition and climate variability.
      PubDate: 2016-03-19T03:40:52.91913-05:0
      DOI: 10.1002/2015GB005228
       
  • Longitudinal distributions of dicarboxylic acids, ω‐oxoacids,
           pyruvic acid, α‐dicarbonyls and fatty acids in the marine
           aerosols from the central Pacific including equatorial upwelling
    • Authors: Mir Md. Mozammal Hoque; Kimitaka Kawamura
      First page: 534
      Abstract: Remote marine aerosol samples (total suspended particles, TSP) were collected during a cruise in the central Pacific from Japan to Mexico (1°59’N‐35°N and 171°54’E‐90°58’W). The aerosol samples were analyzed for dicarboxylic acids (C2‐C11), ω‐oxoacids, pyruvic acid, α‐dicarbonyls and fatty acids as well as organic and elemental carbon (OC/EC), water‐soluble organic carbon (WSOC) and total nitrogen (WSTN). During the study, diacids were the most abundant compound class followed by fatty acids, ω‐oxoacids and α‐dicarbonyls. Molecular compositions of diacids showed a predominance of oxalic (C2) acid followed by malonic (C3) and succinic (C4) acids. Oxalic acid comprises 74% of total diacids. This result suggests that photochemical production of oxalic acid is significant over the central Pacific. Spatial distributions of diacids, ω‐oxoacids, pyruvic acid, α‐dicarbonyls and fatty acids together with TC and WSTN showed higher abundances in the eastern equatorial Pacific where the upwelling of high nutrient waters followed by high biological productivity is common, indicating that their in‐situ production is important in the warmer central Pacific through photochemical oxidation from their gaseous and particulate precursors. This study demonstrates that there is a strong linkage in biogeochemical cycles of carbon in the sea‐air interface via ocean upwelling, phytoplankton productivity, sea‐to‐air emissions of organic matter and formation of secondary organic aerosols in the eastern equatorial Pacific.
      PubDate: 2016-03-29T19:45:59.454919-05:
      DOI: 10.1002/2015GB005346
       
  • Seasonal trends of Amazonian rainforest phenology, net primary
           productivity, and carbon allocation.
    • Abstract: The seasonality of solar irradiance and precipitation may regulate seasonal variations in tropical forests carbon cycling. Controversy remains over their importance as drivers of seasonal dynamics of net primary productivity in tropical forests. We use ground data from nine lowland Amazonian forest plots collected over three years to quantify the monthly NPP of leaves, reproductive material, woody material, and fine roots over an annual cycle. We distinguish between forests that do not experience substantial seasonal moisture stress (“humid sites”) and forests that experience a stronger dry season (“dry sites”). We find that forests from both precipitation regimes maximise leaf NPP over the drier season, with a peak in production in August at both humid (mean 0.39 ± 0.03 Mg C ha‐1 mo‐1 in July, n = 4) and dry sites (mean 0.49 ± 0.03 Mg C ha‐1 mo‐1 in September, n = 8). We identify two distinct seasonal carbon allocation patterns (the allocation of NPP to a specific organ such as wood leaves or fine roots divided by total NPP). The forests monitored in the present study show evidence of either: (i) constant allocation to roots and a seasonal trade‐off between leaf and woody material; or (ii) constant allocation to wood and a seasonal trade‐off between roots and leaves. Finally, we find strong evidence of synchronised flowering at the end of the dry season in both precipitation regimes. Flower production reaches a maximum of 0.047 ± 0.013 and 0.031 ± 0.004 Mg C ha‐1 mo‐1 in November, in humid and dry sites respectively. Fruitfall production was staggered throughout the year, probably reflecting the high variation in varying times to development and loss of fruit amongst species.
       
  • Particulate Organic Carbon and Nitrogen Export from Major Arctic Rivers
    • Abstract: Northern rivers connect a land area of approximately 20.5 million km2 to the Arctic Ocean and surrounding seas. These rivers account for ~10% of global river discharge, and transport massive quantities of dissolved and particulate materials that reflect watershed sources and impact biogeochemical cycling in the ocean. In this paper, multi‐year datasets from a coordinated sampling program are used to characterize particulate organic carbon (POC) and nitrogen (PN) export from the six largest rivers within the pan‐Arctic watershed (Yenisey, Lena, Ob’, Mackenzie, Yukon, Kolyma). Together these rivers export an average of 3055 x 109 g of POC and 368 x 109 g of PN each year. Scaled up to the pan‐Arctic watershed as a whole, fluvial export estimates increase to 5767 x 109 g and 695 x 109 g of POC and PN per year respectively. POC export is substantially lower than dissolved organic carbon export by these rivers, whereas PN export is roughly equal to dissolved nitrogen export. Seasonal patterns in concentrations and source/composition indicators (C:N, δ13C, Δ14C, δ15N) are broadly similar among rivers, but distinct regional differences are also evident. For example, average radiocarbon ages of POC range from ~2000 (Ob’) to ~5500 (Mackenzie) years before present. Rapid changes within the Arctic system as a consequence of global warming make it challenging to establish a contemporary baseline of fluvial export, but the results presented in this paper capture variability and quantify average conditions for nearly a decade at the beginning of the 21st century.
       
  • Topographic variability and the influence of soil erosion on the carbon
           cycle
    • Abstract: Soil erosion, particularly that caused by agriculture, is closely linked to the global carbon (C) cycle. There is a wide range of contrasting global estimates of how erosion alters soil‐atmosphere C exchange. This can be partly attributed to limited understanding of how geomorphology, topography, and management practices affect erosion and oxidation of soil organic C (SOC). This work presents a physically‐based approach that stresses the heterogeneity at fine spatial scales of SOC erosion, SOC burial, and associated soil‐atmosphere C fluxes. The Holcombe's Branch watershed, part of the Calhoun Critical Zone Observatory in South Carolina, USA is the case study used. The site has experienced some of the most serious agricultural soil erosion in North America. We use SOC content measurements from constrastins soil profiles, and estimates of SOC oxidation rates at multiple soil depths. The methodology was implemented in the tRIBS‐ECO (Triangulated Irregular Network‐based Real‐time Integrated Basin Simulator‐Erosion and Carbon Oxidation), a spatially‐ and depth‐explicit model of SOC dynamics built within an existing coupled physically‐based hydro‐geomorphic model. According to observations from multiple soil profiles, about 32% of the original SOC content has been eroded in the study area. The results indicate that C erosion and its replacement exhibit significant topographic variation at relatively small scales (tens of meters). The episodic representation of SOC erosion reproduces the history of SOC erosion better than models that use an assumption of constant erosion in space and time. The net atmospheric C exchange at the study site is estimated to range range from a maximum source of 14.5 g m‐2 yr‐1 to a maximum sink of ‐18.2 g m‐2 yr‐1. The small‐scale complexity of C erosion and burial driven by topography exerts a strong control on the landscape's capacity to serve as a C source or a sink.
       
 
 
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