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Journal Cover Global Biogeochemical Cycles
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   Published by AGU Homepage  [17 journals]
  • Basin‐wide N2 fixation in the deep waters of the Mediterranean Sea
    • Abstract: Recent findings indicate that N2 fixation is significant in aphotic waters, presumably due to heterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationship between organic matter and heterotrophic N2 fixation remains unknown. Here we explore N2 fixation in the deep chlorophyll maximum (DCM) and underneath deep waters across the whole Mediterranean Sea and relate it to organic matter composition, characterized by optical and molecular methods. Our N2 fixation rates were in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to 0.43 nmol N L‐1 d‐1), and were significantly correlated to the presence of relatively labile organic matter with fluorescence and molecular formula properties representative for peptides and unsaturated aliphatics, and associated with the presence of more oxygenated ventilated water masses. Finally, and despite aphotic N2 fixation contributes largely to total water column diazotrophic activity (>50%), its contribution to overall nitrogen inputs to the basin is negligible (
      PubDate: 2016-06-20T05:45:24.218765-05:
      DOI: 10.1002/2015GB005326
  • Enzyme‐level Interconversion of Nitrate and Nitrite in the Fall
           Mixed Layer of the Antarctic Ocean
    • Authors: P.C. Kemeny; M. A. Weigand, R. Zhang, B.R. Carter, K.L. Karsh, S.E. Fawcett, D.M. Sigman
      Abstract: In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO3‐) have been used to investigate NO3‐ assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the 15N/14N (δ15N) and 18O/16O (δ18O) of NO3‐ and nitrite (NO2‐) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ15N and δ18O of NO3‐ + NO2‐ increase roughly equally, as expected for NO3‐ assimilation; however, the δ15N of NO3‐‐only (measured after NO2‐ removal) increases more than NO3‐‐only δ18O. Differencing indicates that NO2‐ has an extremely low δ15N, often 
      PubDate: 2016-06-20T05:15:26.900053-05:
      DOI: 10.1002/2015GB005350
  • Changes in anthropogenic nitrogen and phosphorus inputs to the St.
           Lawrence sub‐basin over 110 years and impacts on riverine
    • Abstract: Human activities have increased the flow of nitrogen (N) and phosphorus (P) over much of the Earth, leading to increased agricultural production, but also the degradation of air, soil, and water quality. Here, we quantify the sources of anthropogenic N and P inputs to 76 watersheds of the St. Lawrence Basin (SLB) throughout the 20th century using NANI/NAPI (net anthropogenic N/P input to watersheds), a mass balance modeling approach, and estimate the fraction of these inputs exported to adjacent rivers. Our results show that since 1901, NANI and NAPI increased 4.5‐ and 3.8‐fold respectively with a peak in 1991 mainly due to high atmospheric N deposition and P fertilizer application. However the relative increase over the course of the last century was much higher in certain watersheds, particularly those where there was greater urbanization. Ranges in NANI and NAPI vary greatly among watersheds (110 to 9,351 kg N km‐2 yr‐1 and 0.16 to 1,938 kg P km‐2 yr‐1, respectively in 2011) and are strongly related to riverine fluxes (R2 = 0.87 and 0.71 for N and P, respectively). Our results suggest that 22% of NANI (ranging from 11% to 68% across watersheds) and 17% of NAPI (ranging from 3% to 173%) are exported to rivers. Predominant sources of inputs vary spatially and through time largely due to changes in farming practices. By tracking the main sources of inputs to specific watersheds and through time, our work provides insights for N and P management. Reduction strategies will likely need to be watershed specific, although through time, our results clearly show the large‐scale impact of targeted legislation.
      PubDate: 2016-06-15T00:30:38.217089-05:
      DOI: 10.1002/2016GB005384
  • Variability in the sensitivity among model simulations of permafrost and
           carbon dynamics in the permafrost region between 1960 and 2009
    • Abstract: A significant portion of large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO2 and CH4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near‐surface permafrost (within 3 m) area over the region, but there are large differences in the magnitude of the simulated rates of loss among the models (0.2 to 58.8 x 103 km2 y‐1). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954 Tg C y‐1 between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982‐2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. To improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback, and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.
      PubDate: 2016-06-15T00:30:33.346489-05:
      DOI: 10.1002/2016GB005405
  • Quantifying the drivers of ocean‐atmosphere CO2 fluxes
    • Authors: Jonathan M. Lauderdale; Stephanie Dutkiewicz, Richard G. Williams, Michael J. Follows
      Abstract: A mechanistic framework for quantitatively mapping the regional drivers of air–sea CO2 fluxes at a global scale is developed. The framework evaluates the interplay between: (1) surface heat and freshwater fluxes that influence the potential saturated carbon concentration, which depends on changes in sea surface temperature, salinity and alkalinity, (2) a residual, disequilibrium flux influenced by upwelling and entrainment of remineralized carbon‐ and nutrient‐rich waters from the ocean interior, as well as rapid subduction of surface waters, (3) carbon uptake and export by biological activity as both soft tissue and carbonate, and (4) the effect on surface carbon concentrations due to freshwater precipitation or evaporation. In a steady state simulation of a coarse resolution ocean circulation and biogeochemistry model, the sum of the individually determined components is close to the known total flux of the simulation. The leading order balance, identified in different dynamical regimes, is between the CO2 fluxes driven by surface heat fluxes and a combination of biologically‐driven carbon uptake and disequilibrium‐driven carbon outgassing. The framework is still able to reconstruct simulated fluxes when evaluated using monthly‐averaged data and takes a form that can be applied consistently in models of different complexity and observations of the ocean. In this way, the framework may reveal differences in the balance of drivers acting across an ensemble of climate model simulations or be applied to an analysis and interpretation of the observed, real‐world air–sea flux of CO2.
      PubDate: 2016-06-11T00:50:31.538076-05:
      DOI: 10.1002/2016GB005400
  • The Influence of Southern Ocean Winds on the North Atlantic Carbon Sink
    • Authors: Ben Bronselaer; Laure Zanna, David R. Munday, Jason Lowe
      Abstract: Observed and predicted increases in Southern Ocean winds are thought to upwell deep ocean carbon and increase atmospheric CO2. However, Southern Ocean dynamics affect biogeochemistry and circulation pathways on a global scale. Using idealised MITgcm simulations, we demonstrate that an increase in Southern Ocean winds reduces the carbon sink in the North Atlantic sub‐polar gyre. The increase in atmospheric CO2 due to the reduction of the North Atlantic carbon sink is shown to be of the same magnitude as the increase in atmospheric CO2 due to Southern Ocean outgassing. The mechanism can be described as follows: The increase in Southern Ocean winds leads to an increase in upper ocean northward nutrient transport. Biological productivity is therefore enhanced in the tropics, which alters the chemistry of the sub‐thermocline waters that are ultimately upwelled in the subpolar gyre. The results demonstrate the influence of Southern Ocean winds on the North Atlantic carbon sink and show that the effect of Southern Ocean winds on atmospheric CO2 is likely twice as large as previously thought in past, present, and future climates.
      PubDate: 2016-06-08T21:55:41.021524-05:
      DOI: 10.1002/2015GB005364
  • Profiling float‐based observations of net respiration beneath the
           mixed layer
    • Authors: Tyler D. Hennon; Stephen C. Riser, Sabine Mecking
      Abstract: We employ profiling floats with dissolved oxygen sensors to observe in situ temporal oxygen evolution below the mixed layer, allowing us to characterize net respiration of organic carbon in eight distinct regions over the globe. Export and export efficiency are generally high in locations with strong seasonal variability, and low in locations of weak seasonality. Vertically integrated respiration is weakly, yet significantly, correlated with remote observations of chlorophyll, net primary production, and planktonic community size structure. These correlations suggest that regimes of high net primary production and large phytoplankton fuel elevated respiration at depth. Several regions of float‐based observations intersect with sites of other detailed observations (e.g. Hawaii and Sargasso Sea), which allows us to compare our results to independent studies. We find that there is good agreement among export production estimates at highly seasonal locations, and that float‐based observations may be biased low at weakly seasonal locations. We posit that the reason for the low‐latitude discrepancy is the relative steady‐state of oxygen concentration caused by weak seasonality and shallow wintertime mixed layer depths.
      PubDate: 2016-06-08T02:20:58.985053-05:
      DOI: 10.1002/2016GB005380
  • Convergent approaches to determine an ecosystem's transpiration fraction
    • Authors: M. Berkelhammer; D. Noone, T.E. Wong, S.P. Burns, J.F. Knowles, A. Kaushik, P.D. Blanken, M.W. Williams
      Abstract: The transpiration (T) fraction of total terrestrial evapotranspiration (ET), T/ET, can vary across ecosystems between 20‐95% with a global average of ∼60%. The wide range may either reflect true heterogeneity between ecosystems and/or uncertainties in the techniques used to derive this property. Here we compared independent approaches to estimate T/ET at two needle‐leaf forested sites with a factor of three difference in leaf area index (LAI). The first method utilized water vapor isotope profiles and the second derived transpiration through its functional relationship with gross primary production (GPP). We found strong agreement between T/ET values from these two independent approaches although we noted a discrepancy at low vapor pressure deficits (VPD). We hypothesize that this divergence arises because stomatal conductance is independent of humidity at low VPD. Overall, we document significant synoptic‐scale T/ET variability but minimal growing season‐scale variability. This result indicates a high sensitivity of T/ET to passing weather but convergence towards a stable mean state, which is set by LAI. While changes in T/ET could emerge from a myriad of processes, including above‐ (LAI) or below‐ (rooting depth) ground changes, there was only minimal interannual variability and no secular trend in our analysis of T/ET from the 15‐year eddy covariance timeseries at Niwot Ridge. If the lack of trend observed here is apparent elsewhere, it suggests that the processes controlling the T and E fluxes are coupled in a way to maintain a stable ratio.
      PubDate: 2016-06-06T16:36:13.554688-05:
      DOI: 10.1002/2016GB005392
  • Forest harvest contribution to Boreal freshwater methyl mercury load
    • Abstract: Clear‐cutting of Boreal coniferous forests enhances MeHg pool in organic topsoil 7 times Forest clear‐cutting enhances stream export of MeHg more than three times in undulating terrain Forest harvest increases MeHg export to Boreal headwaters by 12‐20% in Sweden and 2 % globally
      PubDate: 2016-06-06T00:50:32.601745-05:
      DOI: 10.1002/2015GB005316
  • Dynamic biogeochemical controls on river pCO2 and recent changes under
           aggravating river impoundment: an example of the subtropical Yangtze River
    • Authors: Shaoda Liu; Xi Xi Lu, Xinghui Xia, Shurong Zhang, Lishan Ran, Xiankun Yang, Ting Liu
      Abstract: This paper highlights two aspects of the dynamic biogeochemical controls of riverine pCO2 in an increasingly impounded large subtropical river (the Yangtze): the terrestrial dominance through internal respiration of land‐derived organic carbon and the influence of increased autotrophic activity in impounded areas on river pCO2. River pCO2 and total organic carbon (TOC) increase downstream on the mainstem (pCO2: 528–1703 µatm; TOC: 137–263 µmol/L) and vary significantly among tributaries (464–3300 µatm; TOC: 109–340 µmol/L). pCO2 displays larger spatial variability than temporal variability and is spatially correlated with river organic carbon across the river (p 
      PubDate: 2016-06-05T21:40:27.553861-05:
      DOI: 10.1002/2016GB005388
  • Heavy silicon isotopic composition of silicic acid and biogenic silica in
           Arctic waters over the Beaufort shelf and the Canada Basin
    • Authors: D. E. Varela; M. A. Brzezinski, C. P. Beucher, J. L. Jones, K. E. Giesbrecht, B. Lansard, A. Mucci
      Abstract: The silicon isotopic composition of silicic acid (δ30Si(OH)4) and biogenic silica (δ30Si‐bSiO2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ30Si(OH)4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30ε = ‐0.97 ± 0.17‰, agrees well with laboratory and global‐ocean estimates. However, the δ30Si dynamics of this region are better represented by closed system isotope models that yield lower values of 30ε, between ‐0.33‰ and ‐0.41‰, depending on how the contribution of sea ice diatoms is incorporated. In the upper 400 m, δ30Si‐bSiO2 values were among the heaviest ever measured in marine suspended bSiO2 (+2.03‰ to +3.51‰). A positive correlation between δ30Si‐bSiO2 and sea‐ice cover implies that heavy signals can result from isotopically‐heavy sea‐ice diatoms introduced to pelagic assemblages. Below the surface bSiO2 production zone, δ30Si(OH)4 distribution followed that of major water masses. Vertical δ30Si(OH)4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125‐200 m) composed of modified Pacific water, and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300‐500 m deep). In the Canada Basin Deep Water (below 2,000 m), δ30Si(OH)4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH)4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ30Si(OH)4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box‐model simulation of the global marine δ30Si(OH)4 distribution successfully reproduced the observed patterns, with the δ30Si(OH)4 of the simulated deep Arctic Ocean being the heaviest of all deep‐ocean basins.
      PubDate: 2016-06-04T16:00:36.778142-05:
      DOI: 10.1002/2015GB005277
  • Different sources and degradation state of dissolved, particulate and
           sedimentary organic matter along the Eurasian Arctic coastal margin
    • Abstract: Thawing Arctic permafrost causes massive fluvial and erosional releases of dissolved and particulate organic carbon (DOC and POC) to coastal waters. Here we investigate how different sources and degradation of remobilized terrestrial carbon may affect large‐scale carbon cycling, by comparing molecular and dual‐isotope composition of waterborne high‐molecular weight DOC (>1kD, a.k.a. colloidal OC), POC and sedimentary OC (SOC) across the East Siberian Arctic Shelves. Lignin phenol fingerprints demonstrate a longitudinal trend in relative contribution of terrestrial sources to coastal OC. Wax lipids and cutins were not detected in COC, in contrast to POC and SOC, suggesting that different terrestrial carbon pools partition into different aquatic carrier phases. The Δ14C signal suggests overwhelmingly contemporary sources for COC, while POC and SOC are dominated by old C from Ice Complex Deposit (ICD) permafrost. Monte Carlo source apportionment (δ13C, Δ14C) constrained that COC was dominated by terrestrial OC from Topsoil permafrost (65%) and marine plankton (25%) with smaller contribution ICD and other older permafrost stocks (9%). This distribution is likely a result of inherent compositional matrix differences, possibly driven by organomineral associations. Modern OC found suspended in the surface water may be more exposed to degradation, in contrast to older OC that preferentially settles to the seafloor where it may be degraded on a longer timescale. The different sources which partition into DOC, POC and SOC appear to have vastly different fates along the Eurasian Arctic coastal margin, and may possibly respond on different timescales to climate change.
      PubDate: 2016-06-02T14:25:25.477014-05:
      DOI: 10.1002/2015GB005307
  • Eroding permafrost coasts release low amounts of dissolved organic carbon
           (DOC) from ground ice into the nearshore zone of the Arctic Ocean
    • Authors: George Tanski; Nicole Couture, Hugues Lantuit, Antje Eulenburg, Michael Fritz
      Abstract: Ice‐rich permafrost coasts in the Arctic are highly sensitive to climate warming and erode at a pace that exceeds the global average. Permafrost coasts deliver vast amounts of organic carbon into the nearshore zone of the Arctic Ocean. Numbers on flux exist for particulate and total soil organic carbon (POC and TOC). However, they do not exist for dissolved organic carbon (DOC), which is known to be highly bioavailable. This study aims to estimate DOC stocks in coastal permafrost as well as the annual flux into the ocean. DOC concentrations in ground ice were analyzed along the ice‐rich Yukon coast (YC) in the western Canadian Arctic. The annual DOC flux was estimated using available numbers for coast length, cliff height, annual erosion rate, and volumetric ice content in different stratigraphic horizons. Our results showed that DOC concentrations in ground ice range between 0.3 and 347.0 mg L‐1 with an estimated stock of 13.6 ± 3.0 g m‐3 along the YC. An annual DOC flux of 54.9 ± 0.9 Mg yr‐1 was computed. These DOC fluxes are low compared to POC fluxes from coastal erosion or POC and DOC fluxes from Arctic rivers. We conclude that DOC fluxes from permafrost coasts play a secondary role in the Arctic carbon budget. However, this DOC is assumed to be highly bioavailable. We hypothesize that DOC from coastal erosion is important for ecosystems in the Arctic nearshore zones, particularly in summer when river discharge is low, and in areas where rivers are absent.
      PubDate: 2016-06-01T21:40:26.783341-05:
      DOI: 10.1002/2015GB005337
  • Net community production at Ocean Station Papa observed with nitrate and
           oxygen sensors on profiling floats
    • Authors: Joshua N. Plant; Kenneth S. Johnson, Carole M. Sakamoto, Hans W. Jannasch, Luke J. Coletti, Stephen C. Riser, Dana D. Swift
      Abstract: Six profiling floats equipped with nitrate and oxygen sensors were deployed at Ocean Station P in the Gulf of Alaska. The resulting six calendar years and ten float years of nitrate and oxygen data were used to determine an average annual cycle for net community production (NCP) in the top 35 meters of the water column. NCP became positive in February as soon as the mixing activity in the surface layer began to weaken, but nearly three months before the traditionally defined mixed layer began to shoal from its winter time maximum. NCP displayed two maxima, one towards the end of May and another in August with a summertime minimum in June corresponding to the historical peak in mesozooplankton biomass. The average annual NCP was determined to be 1.5 ± 0.6 mol C m‐2 yr‐1 using nitrate and 1.5 ± 0.7 mol C m‐2 yr‐1 using oxygen. The results from oxygen data proved to be quite sensitive to the gas exchange model used as well as the accuracy of the oxygen measurement. Gas exchange models optimized for carbon dioxide flux generally ignore transport due to gas exchange through the injection of bubbles and these models yield NCP values that are two to three time higher than the nitrate based estimates. If nitrate and oxygen NCP rates are assumed to be related by the Redfield model, we show that the oxygen gas exchange model can be optimized by tuning the exchange terms to reproduce the nitrate NCP annual cycle.
      PubDate: 2016-06-01T20:55:32.422847-05:
      DOI: 10.1002/2015GB005349
  • Benthic marine calcifiers coexist with CaCO3‐undersaturated seawater
    • Abstract: Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms that build CaCO3 structures. A large proportion of benthic marine calcifiers incorporate Mg2+ into their skeletons (Mg‐calcite), which in general, reduces mineral stability. The relative vulnerability of some marine calcifiers to ocean acidification appears linked to the relative solubility of their shell or skeletal mineralogy, although some organisms have sophisticated mechanisms for constructing and maintaining their CaCO3 structures causing deviation from this dependence. Nevertheless, few studies consider seawater saturation state with respect to the actual Mg‐calcite mineralogy (ΩMg‐x) of a species when evaluating the effect of ocean acidification on that species. Here, a global dataset of skeletal mole % MgCO3 of benthic calcifiers and in situ environmental conditions spanning a depth range of 0 m (subtidal/neritic) to 5600 m (abyssal) was assembled to calculate in situ ΩMg‐x. This analysis shows that 24% of the studied benthic calcifiers currently experience seawater mineral undersaturation (ΩMg‐x 
      PubDate: 2016-05-27T13:30:27.667182-05:
      DOI: 10.1002/2015GB005260
  • Amazon forest response to repeated droughts
    • Abstract: The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin‐wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin‐wide ground‐based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground‐based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha‐1, CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long‐term biomass sink during the baseline pre‐2010 drought period (1998 − pre‐2010) of 1.33 Mg ha‐1 yr‐1 (CI: 0.90, 1.74, p 
      PubDate: 2016-04-30T07:57:00.036414-05:
      DOI: 10.1002/2015GB005133
  • 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
  • Issue Information
    • First page: 613
      Abstract: No abstract is available for this article.
      PubDate: 2016-06-11T03:57:49.692945-05:
      DOI: 10.1002/gbc.20335
  • Particulate Organic Carbon and Nitrogen Export from Major Arctic Rivers
    • Authors: J. W. McClelland; R. M. Holmes, B. J. Peterson, P. A. Raymond, R. G. Striegl, A. V. Zhulidov, S. A. Zimov, N. Zimov, S. E. Tank, R. G. M. Spencer, R. Staples, T. Y. Gurtovaya, C. G. Griffin
      First page: 629
      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.
      PubDate: 2016-05-11T19:00:29.998164-05:
      DOI: 10.1002/2015GB005351
  • Topographic variability and the influence of soil erosion on the carbon
    • Authors: Yannis G. Dialynas; Satish Bastola, Rafael L. Bras, Sharon A. Billings, Daniel Markewitz, Daniel deB. Richter
      First page: 644
      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.
      PubDate: 2016-05-11T19:11:03.925071-05:
      DOI: 10.1002/2015GB005302
  • Dynamics of carbonate chemistry, production and calcification of the
           Florida Reef Tract (2009‐2010): evidence for seasonal dissolution
    • Authors: Nancy Muehllehner; Chris Langdon, Alyson Venti, David Kadko
      First page: 661
      Abstract: Ocean acidification is projected to lower the Ωar of reef waters by 0.3‐0.4 units by the end of century making it more difficult for calcifying organisms to secrete calcium carbonate while at the same time making the environment more favorable for abiotic and biotic dissolution of the reef framework. There is great interest in being able to project the point in time when coral reefs will cross the tipping point between being net depositional to net erosional in terms of their carbonate budgets. Periodic in situ assessments of the balance between carbonate production and dissolution that spans seasonal time scales may prove useful in monitoring and formulating projections of the impact of ocean acidification on reef carbonate production. This study represents the first broad scale geochemical survey of the rates of net community production (NCP) and net community calcification (NCC) across the Florida Reef Tract (FRT). Surveys were performed at approximately quarterly intervals in 2009‐10 across seven onshore‐offshore transects spanning the upper, middle and lower Florida Keys. Averaged across the FRT, the rates of NCP and NCC were positive during the spring/summer at 62 ± 7 and 17 ± 2 mmol m‐2 d‐1, respectively, and negative during the fall/winter at ‐33 ± 6 and ‐7 ± 2 mmol m‐2 d‐1. The most significant finding of the study was that the northern‐most reef is already net erosional (‐1.1 ± 0.4 kg CaCO3 m‐2 y‐1) and mid‐reefs to the south were net depositional on an annual basis (0.4 ± 0.1 kg CaCO3 m‐2 y‐1) but erosional during the fall and winter. Only the two southern‐most reefs were net depositional year‐round. These results indicate that parts of the FRT have already crossed the tipping point for carbonate production and other parts are getting close.
      PubDate: 2016-05-14T19:25:42.153584-05:
      DOI: 10.1002/2015GB005327
  • A Novel Molecular Approach for Tracing Terrigenous Dissolved Organic
           Matter into the Deep Ocean
    • Authors: Patricia M. Medeiros; Michael Seidel, Jutta Niggemann, Robert G. M. Spencer, Peter J. Hernes, Patricia L. Yager, William L. Miller, Thorsten Dittmar, Dennis A. Hansell
      First page: 689
      Abstract: Marine dissolved organic matter (DOM) contains one of the largest exchangeable organic carbon pools on Earth. Riverine input represents an important source of DOM to the oceans, yet much remains to be learned about the fate of the DOM linking terrestrial to oceanic carbon cycles through rivers at the global scale. Here, we use ultrahigh resolution mass spectrometry to identify 184 molecular formulae that are indicators of riverine inputs (referred to as t‐Peaks) and to track their distribution in the deep North Atlantic and North Pacific Oceans. The t‐Peaks were found to be enriched in the Amazon River, to be highly correlated with known tracers of terrigenous input, and to be observed in all samples from four different rivers characterized by vastly different landscapes and vegetation coverage spanning equatorial (Amazon and Congo), subtropical (Altamaha) and Arctic (Kolyma) regions. Their distribution reveals that terrigenous organic matter is injected into the deep ocean by the global meridional overturning circulation, indicating that a fraction of the terrigenous DOM introduced by rivers contributes to the DOM pool observed in the deep ocean and to the storage of terrigenous organic carbon. This novel molecular approach can be used to further constrain the transfer of DOM from land to sea, especially considering that FT‐ICR MS analysis is becoming increasingly frequent in studies characterizing the molecular composition of DOM in lakes, rivers and the ocean.
      PubDate: 2016-05-14T19:30:28.468126-05:
      DOI: 10.1002/2015GB005320
  • Seasonal trends of Amazonian rainforest phenology, net primary
           productivity, and carbon allocation.
    • First page: 700
      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.
      PubDate: 2016-05-14T19:20:49.018887-05:
      DOI: 10.1002/2015GB005270
  • Effects of African Dust Deposition on Phytoplankton in the Western
           Tropical Atlantic Ocean off Barbados
    • First page: 716
      Abstract: Bioassay incubation experiments conducted with nutrients and local atmospheric aerosol amendments indicate that phosphorus (P) availability limited phytoplankton growth in the low nutrient low chlorophyll (LNLC) ocean off Barbados. Atmospheric deposition provides a relatively large influx of new nutrients and trace metals to the surface ocean in this region in comparison to other nutrient sources. However, the impact on native phytoplankton is muted due to the high ratio of nitrogen (N) to P (NO3:SRP > 40) and the low P solubility of these aerosols. Atmospheric deposition induces P limitation in this LNLC region by adding more N and iron (Fe) relative to P. This favors the growth of Prochlorococcus, a genus characterized by low P requirements and highly efficient P acquisition mechanisms. A global three‐dimensional marine ecosystem model that includes species‐specific phytoplankton elemental quotas/stoichiometry and the atmospheric deposition of N, P and Fe supports this conclusion. Future increases in aerosol N loading may therefore influence phytoplankton community structure in other LNLC areas, thereby affecting the biological pump and associated carbon sequestration.
      PubDate: 2016-05-21T04:50:35.235916-05:
      DOI: 10.1002/2015GB005334
  • Satellite estimates of net community production based on O2/Ar
           observations and comparison to other estimates.
    • Authors: Zuchuan Li; Nicolas Cassar
      First page: 735
      Abstract: We present two statistical algorithms for predicting global oceanic net community production (NCP) from satellite observations. To calibrate these two algorithms, we compiled a large dataset of in situ O2/Ar‐NCP and remotely sensed observations, including sea surface temperature (SST), net primary production (NPP), phytoplankton size composition, and inherent optical properties. The first algorithm is based on genetic programming (GP) which simultaneously searches for the optimal form and coefficients of NCP equations. We find that several GP solutions are consistent with NPP and SST being strong predictors of NCP. The second algorithm uses support vector regression (SVR) to optimize a numerical relationship between O2/Ar‐NCP measurements and satellite observations. Both statistical algorithms can predict NCP relatively well, with a coefficient of determination (R2) of 0.68 for GP and 0.72 for SVR, which is comparable to other algorithms in the literature. However, our new algorithms predict more spatially uniform annual NCP distribution for the world's oceans and higher annual NCP values in the Southern Ocean and the five oligotrophic gyres.
      PubDate: 2016-05-21T05:05:42.819892-05:
      DOI: 10.1002/2015GB005314
  • Are mangroves drivers or buffers of coastal
           acidification' < mt > Insights from
           alkalinity and dissolved inorganic carbon export estimates across a
           latitudinal transect.
    • Authors: James Z. Sippo; Damien T. Maher, Douglas R. Tait, Ceylena Holloway, Isaac R. Santos
      First page: 753
      Abstract: Mangrove forests are hotspots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO2 [CO2*] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia, and calculated the exchange of DIC, alkalinity and [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m‐2 d‐1 across the six systems, ranging from import of 97 mmol m‐2 d‐1 to an export of 85 mmol m‐2 d‐1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr‐1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m‐2 day‐1 and an export of 117 mmol m‐2 day‐1 with an estimated global export of 4.2 ± 1.3 Tmol yr‐1. A net import of free CO2 was estimated (‐11.4 ± 14.8 mmol m‐2 d‐1), and was equivalent to approximately one third of the air water CO2 flux (33.1 ± 6.3 mmol m‐2 d‐1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.
      PubDate: 2016-05-24T22:30:34.756468-05:
      DOI: 10.1002/2015GB005324
  • Global impact of tropical cyclones on primary production
    • First page: 767
      Abstract: In this paper, we explore the global responses of surface temperature, chlorophyll and primary production to tropical cyclones (TCs). Those ocean responses are first characterized from the statistical analysis of satellite data under ~1000 TCs over the 1998‐2007 period. Besides the cold wake, the vast majority of TCs induce a weak chlorophyll response, with only ~10% of induced blooms exceeding 0.1 mg.m‐3. The largest chlorophyll responses mostly occur within coastal regions, in contrast to the strongest cold wakes that generally occur farther offshore. To understand this decoupling, we analyze a coupled dynamical‐biogeochemical oceanic simulation forced by realistic wind vortices applied along observed TC tracks. The simulation displays a realistic spatial structure of TC‐induced blooms and its observed decoupling with TC cold wakes. In regions of strong TC energy input, the strongest cold wakes occur in regions of shallow thermocline (
      PubDate: 2016-05-27T02:12:40.894692-05:
      DOI: 10.1002/2015GB005214
  • On which timescales do gas transfer velocities control North Atlantic CO2
           flux variability'
    • Authors: Matthew P. Couldrey; Kevin I. C. Oliver, Andrew Yool, Paul R. Halloran, Eric P. Achterberg
      First page: 787
      Abstract: The North Atlantic is an important basin for the global ocean's uptake of anthropogenic and natural carbon dioxide (CO2), but the mechanisms controlling this carbon flux are not fully understood. The air‐sea flux of CO2, F, is the product of a gas transfer velocity, k, the air‐sea CO2 concentration gradient, ΔpCO2, and the temperature and salinity‐dependent solubility coefficient, α. k is difficult to constrain, representing the dominant uncertainty in F on short (instantaneous to interannual) timescales. Previous work shows that in the North Atlantic, ΔpCO2 and k both contribute significantly to interannual F variability, but that k is unimportant for multidecadal variability. On some timescale between interannual and multidecadal, gas transfer velocity variability and its associated uncertainty become negligible. Here, we quantify this critical timescale for the first time. Using an ocean model, we determine the importance of k, ΔpCO2 and α on a range of timescales. On interannual and shorter timescales, both ΔpCO2 and k are important controls on F. In contrast, pentadal to multidecadal North Atlantic flux variability is driven almost entirely by ΔpCO2; k contributes less than 25%. Finally, we explore how accurately one can estimate North Atlantic F without a knowledge of non‐seasonal k variability, finding it possible for interannual and longer timescales. These findings suggest that continued efforts to better constrain gas transfer velocities are necessary to quantify interannual variability in the North Atlantic carbon sink. However, uncertainty in k variability is unlikely to limit the accuracy of estimates of longer term flux variability.
      PubDate: 2016-05-31T13:11:04.419169-05:
      DOI: 10.1002/2015GB005267
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