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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 24, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 52, 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: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
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: 4, 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: 201, SJR: 2.189, h-index: 121)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [7 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • A meta‐analysis of oceanic DMS and DMSP cycling processes:
           Disentangling the summer paradox
    • Authors: Martí Galí; Rafel Simó
      Pages: n/a - n/a
      Abstract: The biogenic volatile compound dimethylsulfide (DMS) is produced in the ocean mainly from the ubiquitous phytoplankton osmolyte dimethylsulfoniopropionate (DMSP). In the upper mixed layer, DMS concentration and the daily averaged solar irradiance are roughly proportional across latitudes and seasons. This translates into a seasonal mismatch between DMS and phytoplankton biomass at low latitudes, termed the “DMS summer paradox”, which remains difficult to reproduce with biogeochemical models. Here we report on a global meta‐analysis of DMSP and DMS cycling processes and their relationship to environmental factors. We show that DMS seasonality reflects progressive changes in a short‐term dynamic equilibrium, set by the quotient between gross DMS production rates and the sum of biotic and abiotic DMS consumption rate constants. Gross DMS production is the principal driver of DMS seasonality, due to the synergistic increases towards summer in two of its underlying factors: phytoplankton DMSP content (linked to species succession) and short‐term community DMSP‐to‐DMS conversion yields (linked to physiological stress). We also show that particulate DMSP transformations (linked to grazing‐induced phytoplankton mortality) generally contribute a larger share of gross DMS production than dissolved‐phase DMSP metabolism. The summer paradox is amplified by a decrease in microbial DMS consumption rate constants towards summer. However, this effect is partially compensated by a concomitant increase in abiotic DMS loss rate constants. Besides seasonality, we identify consistent covariation between key sulfur cycling variables and trophic status. These findings should improve the modeling projections of the main natural source of climatically active atmospheric sulfur.
      PubDate: 2015-03-12T17:22:41.039729-05:
      DOI: 10.1002/2014GB004940
  • Controls on the silicon isotope distribution in the ocean: New diagnostics
           from a data‐constrained model
    • Authors: Mark Holzer; Mark A. Brzezinski
      Abstract: The global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized, data‐constrained model of the oceanic silicon cycle that is embedded in a data‐assimilated steady circulation. Including fractionation during opal dissolution improves the model's ability to capture the approximately linear relation between isotope ratio, δ30Si, and inverse silicic‐acid concentration observed in the deep Atlantic. To quantify the importance of hydrographic control on the isotope distribution, δ30Si is partitioned into contributions from preformed and regenerated silicic acid, further partitioned according to euphotic‐zone origin. We find that the large‐scale features of the isotope distribution in the Atlantic basin are dominated by preformed silicic acid, with regenerated silicic acid being important for setting vertical gradients. In the Pacific and Indian Oceans, preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio, with gradients of the preformed and regenerated contributions tending to cancel each other in the deep Pacific. The Southern‐Ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to δ30Si. Nearly the entire preformed part of δ30Si is of Southern‐Ocean and North‐Atlantic origin. The regenerated part of lta30Si in the Atlantic basin also has a contribution of central‐Atlantic (∼ 40∘ S – 40∘N) origin that is comparable in magnitude to the North‐Atlantic contribution. In other basins, the central Pacific and Indian Ocean are the second largest contributors to the regenerated part of δ30Si.
      PubDate: 2015-03-12T16:56:27.931535-05:
      DOI: 10.1002/2014GB004967
  • Issue Information
    • Pages: i - iii
      Abstract: Cover: The Betsiboka River, Madagascar, during wet season discharge at the RN4 bridge in the vicinity of Maevatanana. The age‐old adage, regarding rivers as the ‘arteries of the Earth,’ could rarely be imbued more strongly than by the floodwaters of the second largest river in Madagascar. See Marwick et al. [pp. 122–137; doi: 10.1002/2014GB004911].
      PubDate: 2015-03-12T11:59:53.79612-05:0
      DOI: 10.1002/gbc.20193
  • Observing multi‐decadal trends in Southern Ocean CO2 uptake: What
           can we learn from an ocean model'
    • Authors: Nicole S. Lovenduski; Amanda R. Fay, Galen A. McKinley
      Abstract: We use output from a hindcast simulation (1958–2007) of an ocean biogeochemical and ecological model to inform an observational strategy for detection of a weakening Southern Ocean CO2 sink from surface ocean pCO2 data. Particular emphasis isplaced on resolving disparate conclusions about the Southern Ocean CO2 sink that have been drawn from surface ocean pCO2 observation studies in the past. We find that long‐term trends in ΔpCO2 (pCO2oc ‐ pCO2atm) can be used as a proxy for changes in the strength of the CO2 sink, but must be interpreted with caution, as they are calculated from small differences in the oceanic and atmospheric pCO2 trends. Large interannual, decadal, and multi‐decadal variability in ΔpCO2 persists throughout the simulation, suggesting that one must consider a range of start and end years for trend analysis before drawing conclusions about changes in the CO2 sink. Winter‐mean CO2 flux trends are statistically indistinguishable from annual‐mean trends, arguing for inclusion of all available pCO2oc data in future analyses of the CO2 sink. The weakening of the CO2 sink emerges during the observed period of our simulation (1981–2007) in the subpolar seasonally stratified biome (4∘C < average climatological temperature < 9∘C); the weakening is most evident during periods with positive trends in the Southern Annular Mode. With perfect temporal and spatial coverage, 13 years of pCO2oc data would be required to detect a weakening CO2 sink in this biome. Given available data, it is not yet possible to detect a weakening of the Southern Ocean CO2 sink with much certainty, due to imperfect data coverage and high variability in Southern Ocean surface pCO2.
      PubDate: 2015-03-11T16:44:53.674191-05:
      DOI: 10.1002/2014GB004933
  • Geographic variability in organic carbon stock and accumulation rate in
           sediments of East and Southeast Asian seagrass meadows
    • Authors: Toshihiro Miyajima; Masakazu Hori, Masami Hamaguchi, Hiromori Shimabukuro, Hiroshi Adachi, Hiroya Yamano, Masahiro Nakaoka
      Abstract: Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long‐term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780–1080 mmol g−1) than in sediments without seagrass cover (52–430 mmol g−1). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140–440 mmol g−1. Carbon isotope mass balancing suggested that the contribution of seagrass‐derived carbon to OC stored in sediments was often relatively minor (temperate: 10–40%; subtropical: 35–82%; tropical: 4–34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38–120 Mg ha−1. The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32–1.34 mm yr−1). The long‐term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24–101 kg ha−1 yr−1) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha−1 yr−1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.
      PubDate: 2015-03-08T18:11:13.059669-05:
      DOI: 10.1002/2014GB004979
  • The relation of mixed‐layer net community production to
           phytoplankton community composition in the Southern Ocean
    • Authors: Nicolas Cassar; Simon W. Wright, Paul G. Thomson, Thomas W. Trull, Karen J. Westwood, Miguel Salas, Andrew Davidson, Imojen Pearce, Diana M. Davies, Richard J. Matear
      Abstract: Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure,build biomass and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Subantarctic Zone (SAZ) and northern reaches of the Polar Frontal Zone (PFZ) in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O2/Ar‐net community oxygen production (NCP) and triple‐O2 isotopes‐gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed‐layer depth‐integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export.
      PubDate: 2015-03-08T18:11:11.548602-05:
      DOI: 10.1002/2014GB004936
  • Nitrogen and phosphorus fluxes from watersheds of the Northeast U.S. from
           1930–2000: Role of anthropogenic nutrient inputs, infrastructure,
           and runoff.
    • Authors: Rebecca L. Hale; Nancy B. Grimm, Charles J. Vörösmarty, Balazs Fekete
      Abstract: An ongoing challenge for society is to harness the benefits of nutrients, nitrogen (N) and phosphorus (P), while minimizing their negative effects on ecosystems. While there is a good understanding of the mechanisms of nutrient delivery at small scales, it is unknown how nutrient transport and processing scale up to larger watersheds and whole regions over long time periods. We used a model that incorporates nutrient inputs to watersheds, hydrology, and infrastructure (sewers, waste‐water treatment plants, and reservoirs) to reconstruct historic nutrient yields for the northeastern U.S. from 1930 to 2002. Over the study period, yields of nutrients increased significantly from some watersheds and decreased in others. As a result, at the regional scale, the total yield of N and P from the region did not change significantly. Temporal variation in regional N and P yields was correlated with runoff coefficient, but not with nutrient inputs. Spatial patterns of N and P yields were best predicted by nutrient inputs, but the correlation between inputs and yields across watersheds decreased over the study period. The effect of infrastructure on yields was minimal relative to the importance of soils and rivers. However, infrastructure appeared to alter the relationships between inputs and yields. The role of infrastructure changed over time and was important in creating spatial and temporal heterogeneity in nutrient input‐yield relationships.
      PubDate: 2015-02-25T07:58:55.328816-05:
      DOI: 10.1002/2014GB004909
  • Carbon dynamics of the Weddell Gyre, Southern Ocean
    • Authors: Peter J. Brown; Loïc Jullion, Peter Landschützer, Dorothee C. E. Bakker, Alberto C. Naveira Garabato, Michael P. Meredith, Sinhue Torres‐Valdés, Andrew Watson, Mario Hoppema, Brice Loose, Elizabeth M. Jones, Maciej Telszewski, Steve D. Jones, Rik Wanninkhof
      Abstract: The accumulation of carbon within the Weddell Gyre, and its exchanges across the gyre boundaries are investigated with three recent full‐depth oceanographic sections enclosing this climatically‐important region. The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveal that deep water transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre's carbon budget, while a dual‐cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058 ± 0.010 Pg C yr‐1 derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air‐to‐sea CO2 flux of 0.002 ± 0.007 Pg C yr‐1 (mean 1998‐2011) to 0.012 ± 0.024 Pg C yr‐1 (mean 2008‐2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived from in situ biogeochemical measurements reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033 ± 0.021 Pg C yr‐1. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deep water formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long timescales.
      PubDate: 2015-02-19T00:54:45.332305-05:
      DOI: 10.1002/2014GB005006
  • Dissolved Fe and Al in the upper 1000m of the eastern Indian Ocean: a
           high‐resolution transect along 95°E from the Antarctic margin
           to the Bay of Bengal
    • Authors: Maxime M. Grand; Christopher I. Measures, Mariko Hatta, William T. Hiscock, William M. Landing, Peter L. Morton, Clifton S. Buck, Pamela M. Barrett, Joseph A. Resing
      Abstract: A high‐resolution section of dissolved iron (dFe) and aluminium (dAl) was obtained along ~95°E in the upper 1000m of the eastern Indian Ocean from the Antarctic margin (66°S) to the Bay of Bengal (18°N) during the US‐CLIVAR‐CO2 Repeat Hydrography I08S and I09N sections (February‐April 2007). In the Southern Ocean, low concentrations of dAl (
      PubDate: 2015-02-17T21:16:28.327974-05:
      DOI: 10.1002/2014GB004920
  • Dust deposition in the eastern Indian Ocean: the ocean perspective from
           Antarctica to the Bay of Bengal
    • Authors: Maxime M. Grand; Christopher I. Measures, Mariko Hatta, William T. Hiscock, Clifton S. Buck, William M. Landing
      Abstract: Atmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry deposition) in remote regions. In this work, we discuss the upper‐ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges‐Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large‐scale gradients in mineral dust deposition and multiple source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66±60 mg m‐2 yr‐1) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m‐2 yr‐1 at 18°S) and in the southern Bay of Bengal (2,500±570 mg m‐2 yr‐1). Our total deposition fluxes, which have an uncertainty on the order of a factor 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.
      PubDate: 2015-02-16T16:34:08.73026-05:0
      DOI: 10.1002/2014GB004898
  • Preferential remineralization of dissolved organic phosphorus and
           non‐Redfield DOM dynamics in the global ocean: Impacts on marine
           productivity, nitrogen fixation, and carbon export
    • Authors: Robert T. Letscher; J. Keith Moore
      Abstract: Selective removal of nitrogen (N) and phosphorus (P) from the marine dissolved organic matter (DOM) pool has been reported in several regional studies. Because DOM is an important advective/mixing pathway of carbon (C) export from the ocean surface layer and its non‐Redfieldian stoichiometry would affect estimates of marine export production per unit N and P, we investigated the stoichiometry of marine DOM and its remineralization globally using a compiled DOM dataset. Marine DOM is enriched in C and N compared to Redfield stoichiometry, averaging 317:39:1 and 810:48:1 for C:N:P within the degradable and total bulk pools, respectively. Dissolved organic phosphorus (DOP) is found to be preferentially remineralized about twice as rapidly with respect to the enriched C:N stoichiometry of marine DOM. Biogeochemical simulations with the Biogeochemical Elemental Cycling model using Redfield and variable DOM stoichiometry corroborate the need for non‐Redfield dynamics to match the observed DOM stoichiometry. From our model simulations, preferential DOP remineralization is found to increase the strength of the biological pump by ~9% versus the case of Redfield DOM cycling. Global net primary productivity increases ~10% including an increase in marine nitrogen fixation of ~26% when preferential DOP remineralization and direct utilization of DOP by phytoplankton are included. The largest increases in marine nitrogen fixation, NPP, and carbon export are observed within the western subtropical gyres, suggesting the lateral transfer of P in the form of DOP from the productive eastern and poleward gyre margins may be important for sustaining these processes downstream in the subtropical gyres.
      PubDate: 2015-02-13T07:02:48.931038-05:
      DOI: 10.1002/2014GB004904
  • The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere
           and ocean
    • Authors: Eric D. Galbraith; Eun Young Kwon, Daniele Bianchi, Mathis P. Hain, Jorge L. Sarmiento
      Abstract: It is well known that the equilibration timescale for the isotopic ratios 13C/12C and 14C/12C in the ocean mixed layer is on the order of a decade, two orders of magnitude slower than for oxygen. Less widely‐appreciated is the factthat the equilibration timescale is quite sensitive to the speciation of Dissolved Inorganic Carbon (DIC) in the mixed layer, scaling linearly with the ratio DIC/CO 2, which varies inversely with atmospheric pCO 2. Although this effect is included in models that resolve the role of carbon speciation in air‐sea exchange, its role is often unrecognized, and it is not commonly considered in the interpretation of carbon isotope observations. Here, we use a global 3‐dimensional ocean model to estimate the redistribution of the carbon isotopic ratios between the atmosphere and ocean due solely to variations in atmospheric pCO 2. Under Last Glacial Maximum (LGM) pCO 2, atmospheric Δ14 C is increased by ≈ 30 due to the speciation change, all else being equal, raising the surface reservoir age by about 250 years throughout most of the ocean. For 13 C, enhanced surface disequilibrium under LGM pCO2 causes the upper ocean, atmosphere and North Atlantic Deep Water δ13C to become at least 0.2 higher relative to deep waters ventilated by the Southern Ocean. Conversely, under high pCO2, rapid equilibration greatly decreases isotopic disequilibrium. As a result, during geological periods of high pCO2, vertical δ13C gradients may have been greatly weakened as a direct chemical consequence of the high pCO2, masquerading as very well‐ventilated or biologically‐deadÔStrangeloveÕ oceans. The ongoing anthropogenic rise of pCO2 is accelerating the equilibration of the carbon isotopes in the ocean, lowering atmospheric Δ14C and weakening δ13C gradients within the ocean to a degree that is similar to the traditional fossil fuel ’Suess’ effect.
      PubDate: 2015-02-09T08:26:52.665271-05:
      DOI: 10.1002/2014GB004929
  • The stoichiometry of carbon and nutrients in peat formation
    • Authors: Meng Wang; Tim R. Moore, Julie Talbot, John L. Riley
      First page: 113
      Abstract: Northern peatlands have stored large amounts (~500 Pg) of carbon (C) since the last glaciation. Combined with peat C are nutrients such as nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg) and potassium (K), each of which plays an important role in plant production, litter decomposition and the biogeochemical functioning of peatlands. Yet little attention has been given to the amounts of these nutrients stored in northern peatlands and their stoichiometry with C. Here, we use data on nutrient concentrations in over 400 peat profiles in Ontario, Canada, representing bogs, fens and swamps and their vegetation. We show that the C:N ratio is high (> 40:1) in vegetation and litter, but declines through the peat profiles to reach ratios between 22:1 and 29:1 in peat below 50 cm. In contrast, the C:P ratio rises from vegetation and litter (500:1 to 1300:1) to 1500:1 to 2000:1 in the lower part of the peat profile. Ratios of C to Ca, Mg and K vary with peatland type. Most of these stoichiometric changes occur in the early stages of organic matter decomposition, where the litter structure remains intact. We estimate that ~18 Pg of N has been stored in northern peatlands since deglaciation, reflecting high N accumulation rates (~0.8 g m−2 y−1), whereas P accumulation is small (~0.3 Pg, ~0.016 g m−2 y−1), indicating that P is recycled quickly in the surface layers.
      PubDate: 2015-02-03T04:08:35.655911-05:
      DOI: 10.1002/2014GB005000
  • The age of river‐transported carbon: a global perspective
    • Authors: Trent R. Marwick; Fredrick Tamooh, Cristian R. Teodoru, Alberto V. Borges, François Darchambeau, Steven Bouillon
      First page: 122
      Abstract: The role played by river networks in regional and global carbon (C) budgets is receiving increasing attention. Despite the potential of radiocarbon measurements (∆14C) to elucidate sources and cycling of different riverine C pools, there remain large regions for which no data are available, and no comprehensive attempts to synthesize the available information and examine global patterns in the 14C content of different riverine C pools. Here, we present new 14C data on particulate and dissolved organic C (POC and DOC) from six river basins in tropical and subtropical Africa, and compiled >1400 literature ∆14C data and ancillary parameters from rivers globally. Our analysis reveals a consistent pattern whereby POC is progressively older in systems carrying higher sediment loads, coinciding with a lower organic carbon content. At the global scale, this pattern leads to a proposed global median ∆14C signature of −203‰, corresponding to an age of ~1800 yr BP. For DOC exported to the coastal zone, we predict a modern (decadal) age (∆14C = +22 to +46‰), and paired datasets confirm that riverine DOC is generally more recent in origin than POC – in contrast to the situation in ocean environments. Weathering regimes complicate the interpretation of 14C ages of dissolved inorganic carbon (DIC), but the available data favors the hypothesis that in most cases, more recent organic C is preferentially mineralized.
      PubDate: 2015-02-07T00:25:00.470953-05:
      DOI: 10.1002/2014GB004911
  • Importance of vegetation for manganese cycling in temperate forested
    • Authors: Elizabeth M. Herndon; Lixin Jin, Danielle M. Andrews, David M. Eissenstat, Susan L. Brantley
      First page: 160
      Abstract: Many surface soils are enriched in metals due to anthropogenic atmospheric inputs. To predict the persistence of these contaminants in soils, factors that impact rates of metal removal from soils into streams must be understood. Experiments at containerized seedling (“mesocosm”), pedon, and catchment scales were used to investigate the influence of vegetation on manganese (Mn) transport at the Susquehanna/Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA, where past atmospheric inputs from industrial sources have enriched Mn in surface soils. Large quantities of Mn that were leached from soil components into solution were taken up by vegetation; as a result, only relatively small quantities of Mn were removed from soil into effluent and streams. Manganese uptake into vegetation exceeded Mn losses in soil leachate by 20‐200x at all scales, and net Mn loss from soils decreased in the presence of vegetation due to uptake into plant tissues. The majority of Mn taken up by forest vegetation at SSHCZO each year was returned to the soil in leaf litter and consequently immobilized as Mn‐oxides that formed during litter decomposition. Thus, plant uptake of Mn combined with rapid oxidation of Mn during litter decomposition contribute to long‐term retention. Current release rates of soluble Mn from SSHCZO soils were similar to release rates from the larger Susquehanna River Basin (SRB), indicating that the processes observed at SSHCZO may be widespread across the region. Indeed, although atmospheric deposition of Mn has declined, surface soils at SSHCZO and throughout the eastern United States remain enriched in Mn. If recycling through vegetation can attenuate the removal of Mn from soils, as observed in this study, then Mn concentrations in soils and river waters will likely decrease slowly over time following watershed contamination. Understanding the role of vegetation in regulating metal transport is important for evaluating the long‐term effects of historical and ongoing metal loading to soils.
      PubDate: 2015-02-21T05:27:10.100473-05:
      DOI: 10.1002/2014GB004858
  • Sinking velocities and microbial respiration rates alter the attenuation
           of particulate carbon fluxes through the mesopelagic zone
    • Authors: A. M. P. McDonnell; P. W. Boyd, K. O. Buesseler
      First page: 175
      Abstract: The attenuation of sinking particle fluxes through the mesopelagic zone is an important process that controls the sequestration of carbon and the distribution of other elements throughout the oceans. Case studies at two contrasting sites, the oligotrophic regime of the Bermuda Atlantic Time Series (BATS) and the mesotrophic waters of the western Antarctic Peninsula (WAP) sector of the Southern Ocean, revealed large differences in the rates of particle‐attached microbial respiration and the average sinking velocities of marine particles, two parameters that affect the transfer efficiency of particulate matter from the base of the euphotic zone into the deep ocean. Rapid average sinking velocities of 270 ± 150 m d−1 were observed along the WAP, whereas the average velocity was 49 ± 25 m d−1 at the BATS site. Respiration rates of particle‐attached microbes were measured using novel RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan, Boyd et al., unpublished manuscript) sediment traps that first intercepts sinking particles then incubates them in situ. RESIRE experiments yielded flux‐normalized respiration rates of 0.4 ± 0.1 d−1 at BATS when excluding an outlier of 1.52 d−1, while these rates were undetectable along the WAP (0.01 ± 0.02 d−1). At BATS, flux‐normalized respiration rates decreased exponentially with respect to depth below the euphotic zone with a 75% reduction between the 150 and 500 m depths. These findings provide quantitative and mechanistic insights into the processes that control the transfer efficiency of particle flux through the mesopelagic and its variability throughout the global oceans.
      PubDate: 2015-02-25T07:08:44.73746-05:0
      DOI: 10.1002/2014GB004935
  • The influence of photosynthetic acclimation to rising CO2 and warmer
           temperatures on leaf and canopy photosynthesis models.
    • Authors: Justin Bagley; David M. Rosenthal, Ursula M. Ruiz‐Vera, Matthew H. Siebers, Praveen Kumar, Donald R. Ort, Carl J. Bernacchi
      First page: 194
      Abstract: There is an increasing necessity to understand how climate change factors, particularly increasing atmospheric concentrations of CO2 ([CO2]) and rising temperature, will influence photosynthetic carbon assimilation (A). Based on theory, an increased [CO2] concomitant with a rise in temperature will increase A in C3 plants beyond that of an increase in [CO2] alone. However, uncertainty surrounding the acclimation response of key photosynthetic parameters to these changes can influence this response. In this work, the acclimation responses of C3 photosynthesis for soybean measured at the SoyFACE Temperature by Free Air CO2 Enrichment (T‐FACE) experiment is incorporated in a leaf biochemical and canopy photosynthesis model. The two key parameters used as model inputs, the maximum velocity for carboxylation (Vc,max) and maximum rate of electron transport (Jmax), were measured in a full factorial [CO2] by temperature experiment over two growing seasons and applied in leaf‐ and canopy‐scale models to (1) reassess the theory of combined increases in [CO2] and temperature on A, (2) determine the role of photosynthetic acclimation to increased growth [CO2] and/or temperature in leaf and canopy predictions of A for these treatments, and (3) assess the diurnal and seasonal differences in leaf‐ and canopy‐scale A associated with the imposed treatments. The results demonstrate that the theory behind combined increases in [CO2] and temperature are sound, however, incorporating more recent parameterizations into the photosynthesis model predicts greater increases in A when [CO2] and temperature are increased together. Photosynthetic acclimation is shown to decrease leaf‐level A for all treatments, however, in elevated [CO2] the impact of acclimation does not result in any appreciable loss in photosynthetic potential at the canopy scale. In this analysis, neglecting photosynthetic acclimation in heated treatments, with or without concomitant rise in [CO2], leads to modeled over‐estimates of carbon gain for soybean under future predicted conditions.
      PubDate: 2015-02-25T08:03:22.614257-05:
      DOI: 10.1002/2014GB004848
  • Sensitivity of global terrestrial carbon cycle dynamics to variability in
           satellite‐observed burned area
    • Authors: Benjamin Poulter; Patricia Cadule, Audrey Cheiney, Philippe Ciais, Elke Hodson, Philippe Peylin, Stephen Plummer, Allan Spessa, Sassan Saatchi, Chao Yue, Niklaus E. Zimmermann
      First page: 207
      Abstract: Fire plays an important role in terrestrial ecosystems by regulating biogeochemistry, biogeography and energy budgets, yet despite the importance of fire as an integral ecosystem process, significant advances remain to improve its prognostic representation in carbon cycle models. To recommend and to help prioritize model improvements, this study investigates the sensitivity of a coupled global biogeography and biogeochemistry model, LPJ, to observed burned area measured by three independent satellite‐derived products, GFED v3.1, L3JRC, and GlobCarbon. Model variables are compared with benchmarks that include pan‐tropical aboveground biomass, global tree cover, and CO2 and CO trace gas concentrations. Depending on prescribed burned area product, global aboveground carbon stocks varied by 300 Pg C, and woody cover ranged from 50‐73 Mkm2. Tree cover and biomass were both reduced linearly with increasing burned area, i.e., at regional scales, a 10% reduction in tree cover per 1000 km2, and 0.04‐to‐0.40 Mg C reduction per 1000 km2. In boreal regions, satellite burned area improved simulated tree‐cover and biomass distributions, but in savanna regions, model‐data correlations decreased. Global net biome production was relatively insensitive to burned area, and the long‐term land carbon sink was robust, ~2.5 Pg C yr‐1, suggesting that feedbacks from ecosystem respiration compensated for reductions in fuel consumption via fire. CO2 transport provided further evidence that heterotrophic respiration compensated any emission reductions in the absence of fire, with minor differences in modeled CO2 fluxes among burned area products. CO was a more sensitive indicator for evaluating fire emissions, with MODIS‐GFED burned area producing CO concentrations largely in agreement with independent observations in high‐latitudes. This study illustrates how ensembles of burned area datasets can be used to diagnose model structures and parameters for further improvement, and also highlights the importance in considering uncertainties and variability in observed burned area data products for model applications.
      PubDate: 2015-02-25T06:53:24.557131-05:
      DOI: 10.1002/2013GB004655
  • Seasonality of biological and physical controls on surface ocean CO2 from
           hourly observations at the Southern Ocean Time Series site south of
    • Authors: E. H. Shadwick; T. W. Trull, B. Tilbrook, A. J. Sutton, E. Schulz, C. L. Sabine
      First page: 223
      Abstract: The Subantarctic Zone (SAZ), which covers the northern half of the Southern Ocean between the Subtropical and Subantarctic Fronts, is important for air‐sea CO2 exchange, ventilation of the lower thermocline, and nutrient supply for global ocean productivity. Here we present the first high‐resolution autonomous observations of mixed layer CO2 partial pressure (pCO2) and hydrographic properties covering a full annual cycle in the SAZ. The amplitude of the seasonal cycle in pCO2 (~60 μatm), from near atmospheric equilibrium in late winter to ~330 μatm in mid summer, results from opposing physical and biological drivers. Decomposing these contributions demonstrates that the biological control on pCO2 (up to 100 μatm), is four times larger than the thermal component, and driven by annual net community production of 2.45±1.47 mol C m–2 yr–1. After the summer biological pCO2 depletion, the return to near atmospheric equilibrium proceeds slowly, driven in part by autumn entrainment into a deepening mixed layer, and achieving full equilibration in late winter and early spring as respiration and advection complete the annual cycle. The shutdown of winter convection and associated mixed layer shoaling proceeds intermittently, appearing to frustrate the initiation of production. Horizontal processes, identified from salinity anomalies, are associated with biological pCO2 signatures, but with differing impacts in winter (when they reflect far‐field variations in dissolved inorganic carbon and/or biomass) and summer (when they suggest promotion of local production by the relief of silicic acid or iron limitation). These results provide clarity on SAZ seasonal carbon cycling and demonstrate that the magnitude of the seasonal pCO2 cycle is twice as large as that in the subarctic high‐nutrient, low‐chlorophyll waters, which can inform the selection of optimal global models in this region.
      PubDate: 2015-02-27T11:41:12.002794-05:
      DOI: 10.1002/2014GB004906
  • Phenological characteristics of global coccolithophore blooms
    • Authors: Jason Hopkins; Stephanie A. Henson, Stuart C. Painter, Toby Tyrrell, Alex J. Poulton
      First page: 239
      Abstract: Coccolithophores are recognised as having a significant influence on the global carbon cycle through the production and export of calcium carbonate (often referred to as particulate inorganic carbon or PIC). Using remotely sensed PIC and chlorophyll data we investigate the seasonal dynamics of coccolithophores relative to a mixed phytoplankton community. Seasonal variability in PIC, here considered to indicate changes in coccolithophore biomass, is identified across much of the global ocean. Blooms, which typically start in February‐March in the low latitude (~30°) northern hemisphere and last for ~6‐7 months, get progressively later (April–May) and shorter (3–4 months) moving polewards. A similar pattern is observed in the southern hemisphere, where blooms that generally begin around August‐September in the lower latitudes and which last for ~8 months, get later and shorter with increasing latitude. It has previously been considered that phytoplankton blooms consist of a sequential succession of blooms of individual phytoplankton types. Comparison of PIC and chlorophyll peak dates suggest instead that in many open ocean regions, blooms of coccolithophores and other phytoplankton can co‐occur, conflicting with the traditional view of species succession that is thought to take place in temperate regions such as the North Atlantic.
      PubDate: 2015-02-27T11:01:04.328185-05:
      DOI: 10.1002/2014GB004919
  • Regulation of Redfield ratios in the deep ocean
    • Authors: Anne‐Sophie Auguères; Michel Loreau
      First page: 254
      Abstract: Biotic regulation of the environment at global scales has been debated for several decades. An example is the similarity between deep‐ocean and phytoplankton mean N:P ratios. N and P cycles are heavily altered by human activities, mainly through an increase in nutrient supply to the upper ocean. As phytoplankton only access nutrients in the upper ocean, it is critical to understand (1) to what extent phytoplankton are able to regulate N and P concentrations as well as their ratio in the deep, inaccessible layer, and (2) what mechanisms control the value of the deep‐water N:P ratio and the efficiency of its biotic regulation. With a model of N and P cycles in the global ocean separated in two layers, we show that the value of the deep‐water N:P ratio is determined by non‐fixer's N:P ratio, recycling and denitrification. Our model predicts that, although phytoplankton cannot efficiently regulate deep nutrient pools, they can maintain nearly constant ratios between nutrients because compensatory dynamics between non‐fixers and nitrogen‐fixers allows a control of deep‐water chemistry through nutrient recycling. This mechanism could explain the near‐constancy of the deep‐water N:P ratio, in agreement with Redfield's [1934, 1958] classical hypothesis. Surprisingly, N:P ratio of phytoplankton does not affect their ability to regulate the deep‐water N:P ratio. Our model suggests that increased water column stratification as a result of global climate change may decrease the stability of the N:P ratio in the deep ocean over long temporal and spatial scales.
      PubDate: 2015-02-27T10:52:35.27283-05:0
      DOI: 10.1002/2014GB005066
  • Atmospheric iron deposition in the Northwestern Pacific Ocean and its
           adjacent marginal seas: The importance of coal burning
    • Authors: Yi‐Chiu Lin; Jen‐Ping Chen, Tung‐Yuan Ho, I‐Chun Tsai
      First page: 138
      Abstract: This study applied a regional air‐quality model, incorporated with an emission module, to quantitatively differentiate the atmospheric iron sources originating from lithogenic dusts or coal‐burning fly ashes deposited in the Northwest Pacific Ocean and its marginal seas. Particular attention was paid to the high iron content of fly ashes emitted from steel and iron plants burning coals. Using the year 2007 as an example, the modeling results exhibit large seasonal variations in iron deposition, with highest deposition fluxes occurred during spring and autumn, which are comparable to the seasonal fluctuation of chlorophyll a concentrations estimated by satellite images in the oceanic regions. Fly ash from coal burning accounted for 7.2% of the total iron deposited over the Northwest Pacific Ocean and 15% of that over the northern South China Sea. After considering the difference of iron solubility in the aerosols, anthropogenic aerosol associated with coal burning would be the major bioavailable iron source in the surface water of the oceanic regions.
      PubDate: 2014-12-19T08:59:32.577783-05:
      DOI: 10.1002/2013GB004795
  • Detecting the progression of ocean acidification from the saturation state
           of CaCO3 in the subtropical South Pacific
    • Abstract: Progression of ocean acidification in the subtropical South Pacific was investigated by using high‐quality data from trans‐Pacific zonal section at 17°S (World Ocean Circulation Experiment section P21) collected in 1994 and 2009. During this 15‐year period, the CaCO3 saturation state of seawater with respect to calcite (Ωcal) and aragonite (Ωarg) in the upper water column (
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