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Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [15 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Biological production, export efficiency, and phytoplankton communities
           across 8000 km of the South Atlantic
    • Authors: E. M. Howard; C. A. Durkin, G. M. M. Hennon, F. Ribalet, R. H. R. Stanley
      Abstract: In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March-May 2013. We used observations of pico-, nano-, and microphytoplankton to evaluate community structure and diversity, and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O2 m-2 d-1, gross primary production (GPP) between 40 and 100 mmol O2 m-2 d-1, and NCP/GPP, a measure of export efficiency, ranging from 0.1-0.2 (0.05-0.1 in carbon units). However, notable exceptions to this basin scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of DOC export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.
      PubDate: 2017-06-16T01:19:57.336504-05:
      DOI: 10.1002/2016GB005488
  • Assessing trends and uncertainties in satellite-era ocean chlorophyll
           using space-time modeling
    • Authors: Matthew L. Hammond; Claudie Beaulieu, Sujit K. Sahu, Stephanie A. Henson
      Abstract: The presence, magnitude, and even direction of long-term trends in phytoplankton abundance over the past few decades is still debated in the literature, primarily due to differences in the data sets and methodologies used. Recent work has suggested that the satellite chlorophyll record is not yet long enough to distinguish climate change trends from natural variability, despite the high density of coverage in both space and time. Previous work has typically focused on using linear models to determine the presence of trends, where each grid cell is considered independently from its neighbors. However, trends can be more thoroughly evaluated using a spatially resolved approach. Here a Bayesian hierarchical spatio-temporal model is fitted to quantify trends in ocean chlorophyll from September 1997 to December 2013. The approach used in this study explicitly accounts for the dependence between neighboring grid cells, which allows us to estimate trend by ‘borrowing strength’ from the spatial correlation. By way of comparison, a model without spatial correlation is also fitted. This results in a notable loss of accuracy in model fit. Additionally, we find an order of magnitude smaller global trend, and larger uncertainty, when using the spatio-temporal model: -0.023 ± 0.12 % yr-1 as opposed to -0.38 ± 0.045 % yr-1 when the spatial correlation is not taken into account. The improvement in accuracy of trend estimates, and the more complete account of their uncertainty emphasizes the solution that space-time modeling offers for studying global long-term change.
      PubDate: 2017-06-15T20:00:23.82708-05:0
      DOI: 10.1002/2016GB005600
  • Quantification of uncertainties in global grazing systems assessments
    • Authors: T. Fetzel; P. Havlik, M. Herrero, J. O. Kaplan, T. Kastner, C. Kroisleitner, S. Rolinski, T. Searchinger, P. M. Bodegom, S. Wirsenius, K.-H. Erb
      Abstract: Livestock systems play a key role in global sustainability challenges like food security and climate change, yet, many unknowns and large uncertainties prevail. We present a systematic, spatially explicit assessment of uncertainties related to grazing intensity (GI), a key metric for assessing ecological impacts of grazing, by combining existing datasets on a) grazing feed intake, b) the spatial distribution of livestock, c) the extent of grazing land, and d) its net primary productivity (NPP). An analysis of the resulting 96 maps implies that on average 15% of the grazing land NPP is consumed by livestock. GI is low in most of worlds grazing lands but hotspots of very high GI prevail in 1% of the total grazing area. The agreement between GI maps is good on one fifth of the world's grazing area, while on the remainder it is low to very low. Largest uncertainties are found in global drylands and where grazing land bears trees (e.g., the Amazon basin or the Taiga belt). In some regions like India or Western Europe massive uncertainties even result in GI> 100% estimates. Our sensitivity analysis indicates that the input-data for NPP, animal distribution and grazing area contribute about equally to the total variability in GI maps, while grazing feed intake is a less critical variable. We argue that a general improvement in quality of the available global level datasets is a precondition for improving the understanding of the role of livestock systems in the context of global environmental change or food security.
      PubDate: 2017-06-15T18:35:18.67075-05:0
      DOI: 10.1002/2016GB005601
  • Slow Sinking Particulate Organic Carbon in the Atlantic Ocean: magnitude,
           flux and potential controls
    • Authors: Chelsey A. Baker; Stephanie A. Henson, Emma L. Cavan, Sarah L. C. Giering, Andrew Yool, Marion Gehlen, Anna Belcher, Jennifer S. Riley, Helen E. K. Smith, Richard Sanders
      Abstract: The remineralization depth of particulate organic carbon (POC) fluxes exported from the surface ocean exert a major control over atmospheric CO₂ levels. According to a long held paradigm most of the POC exported to depth is associated with large particles. However, recent lines of evidence suggest that slow sinking POC (SSPOC) may be an important contributor to this flux. Here we assess the circumstances under which this occurs. Our study uses samples collected using the Marine Snow Catcher throughout the Atlantic Ocean, from high latitudes to mid latitudes. We find median SSPOC concentrations of 5.5 μg L-1, 13 times smaller than suspended POC concentrations and 75 times higher than median fast sinking POC (FSPOC) concentrations (0.07 μg L-1). Export fluxes of SSPOC generally exceed FSPOC flux, with the exception being during a spring bloom sampled in the Southern Ocean. In the Southern Ocean SSPOC fluxes often increase with depth relative to FSPOC flux, likely due to midwater fragmentation of FSPOC, a process which may contribute to shallow mineralization of POC and hence to reduced carbon storage. Biogeochemical models do not generally reproduce this behaviour, meaning that they likely overestimate long term ocean carbon storage.
      PubDate: 2017-06-15T18:30:19.648295-05:
      DOI: 10.1002/2017GB005638
  • Effects of parameter indeterminacy in pelagic biogeochemical modules of
           Earth System Models on projections into a warming future: the scale of the
    • Authors: U. Löptien; H. Dietze
      Abstract: Numerical Earth System Models are generic tools used to extrapolate present climate conditions into a warming future and to explore geo-engineering options. Most of the current-generation models feature a simple pelagic biogeochemical model component that is embedded into a three dimensional general circulation ocean model. The dynamics of these biogeochemical model components is essentially controlled by so-called model parameters most of which are poorly known. Here we explore the feasibility to estimate these parameters in a full-fledged three dimensional Earth System Model by minimizing the misfit to noisy observations. The focus is on parameter identifiability. Based on earlier studies, we illustrate problems in determining a unique estimate of those parameters, that prescribe the limiting effect of nutrient and light-depleted conditions on carbon assimilation by autotrophic phytoplankton. Our results showcase that for typical models and evaluation metrics no meaningful “best” unique parameter set exists. We find very different parameter sets which are, on the one hand, equally consistent with our (synthetic) historical observations while, on the other hand, they propose strikingly differing projections into a warming climate.
      PubDate: 2017-06-15T18:20:21.712728-05:
      DOI: 10.1002/2017GB005690
  • Controls on the distribution of fluorescent dissolved organic matter
           during an under-ice algal bloom in the western Arctic Ocean
    • Authors: Wilson G. Mendoza; Elliot L. Weiss, Brian Schieber, B. Greg Mitchell
      Abstract: In this study we used fluorescence excitation and emission spectroscopy (EEMs), hydrographic data and a self-organizing map (SOM) analysis to assess the spatial distribution of labile and refractory FDOM for the Chukchi and Beaufort Seas at the time of a massive under-ice phytoplankton bloom during early summer 2011. Biogeochemical properties were assessed through decomposition of water property classes and sample classification that employed a SOM neural network-based analysis which classified ten clusters from 269 samples and 17 variables. The terrestrial, humic-like component FDOM (ArC1, 4.98±1.54 QSU) and protein-like component FDOM (ArC3, 1.63±0.88 QSU) were found to have elevated fluorescence in the LPML (salinity ~29.56±0.76). In the LPML water mass, the observed contribution of meteoric water fraction was 17%, relative to a 12% contribution from the sea-ice melt fraction. The labile ArC3-protein-like component (2.01±1.92 QSU) was also observed to be elevated in the PWW mass, where the under-ice algal bloom was observed (~40-50 m). We interpreted these relationships to indicate that the accumulation and variable distribution of the protein-like component on the shelf could be influenced directly by sea-ice melt, transport and mixing processes; and indirectly by the in situ algal bloom and microbial activity. ArC5, corresponding to what is commonly considered marine humic FDOM indicated a bimodal distribution with high values both in the freshest and saltiest waters. The association of ArC5 with deep, dense salty water is consistent with this component as refractory humic-like FDOM whereas our evidence of a terrestrial origin challenges this classic paradigm for this component.
      PubDate: 2017-06-14T10:35:20.487245-05:
      DOI: 10.1002/2016GB005569
  • Temperature and oxygen dependence of the remineralization of organic
    • Authors: C. Laufkötter; Jasmin G. John, Charles A. Stock, John P. Dunne
      Abstract: Accurate representation of the remineralization of sinking organic matter is crucial for reliable projections of the marine carbon cycle. Both water temperature and oxygen concentration are thought to influence remineralization rates, but limited data constraints have caused disagreement concerning the degree of these influences. We analyse a compilation of POC flux measurements from 19 globally distributed sites. Our results indicate that the attenuation of the flux of particulate organic matter depends on temperature with a Q10 between 1.5 and 2.01, and on oxygen described by a half saturation constant between 4 and 12 μmol/L. We assess the impact of the temperature and oxygen dependence in the biogeochemistry model COBALT, coupled to GFDL's Earth System Model ESM2M. The new remineralization parameterization results in shallower remineralization in the low latitudes but deeper remineralization in the high latitudes, redistributing POC flux towards the poles. It also decreases the volume of the oxygen minimum zones, partly addressing a long-standing bias in global climate models. Extrapolating temperature-dependent remineralization rates to the surface (i.e., beyond the depth range of POC flux data) resulted in rapid recycling and excessive surface nutrients. Surface nutrients could be ameliorated by reducing near surface rates in a manner consistent with bacterial colonization, suggesting the need for improved remineralization constraints within the euphotic zone. The temperature and oxygen dependence cause an additional 10% decrease in global POC flux at 500m depth, but no significant change in global POC flux at 2000m under the RCP8.5 future projection.
      PubDate: 2017-06-06T18:00:21.600766-05:
      DOI: 10.1002/2017GB005643
  • No long-term trends in pCO2 despite increasing organic carbon
           concentrations in boreal lakes, streams and rivers
    • Authors: Anna C. Nydahl; Marcus B. Wallin, Gesa A. Weyhenmeyer
      Abstract: Concentrations of dissolved organic carbon (DOC) from terrestrial sources have been increasing in freshwaters across large parts of the boreal region. According to results from large scale field and detailed laboratory studies such a DOC increase could potentially stimulate carbon dioxide (CO2) production, subsequently increasing the partial pressure of CO2 (pCO2) in freshwaters. However, the response of pCO2 to the presently observed long-term increase in DOC in freshwaters is still unknown. Here we tested whether the commonly found spatial DOC-pCO2 relationship is also valid on a temporal scale. Analysing time series of water chemical data from 71 lakes, 30 streams and four river mouths distributed across all of Sweden over a 17 year period, we observed significant DOC concentration increases in 39 lakes, 15 streams and four river mouths. Significant pCO2 increases were however only observed in six of these 58 waters, indicating that long-term DOC increases in Swedish waters are disconnected from temporal pCO2 trends. We suggest that the uncoupling of trends in DOC concentration and pCO2 are a result of increased surface water runoff. When surface water runoff increases, there is likely less CO2 relative to DOC imported from soils into waters due to a changed balance between surface and groundwater flow. Additionally, increased surface water runoff causes faster water flushing through the landscape giving less time for in situ CO2 production in freshwaters. We conclude that pCO2 is presently not following DOC concentration trends, which has important implications for modelling future CO2 emissions from boreal waters.
      PubDate: 2017-06-06T12:20:19.990682-05:
      DOI: 10.1002/2016GB005539
  • Basin scale variability of active diazotrophs and nitrogen fixation in the
           North Pacific, from the tropics to the subarctic Bering Sea
    • Authors: Takuhei Shiozaki; Deniz Bombar, Lasse Riemann, Fuminori Hashihama, Shigenobu Takeda, Tamaha Yamaguchi, Makoto Ehama, Koji Hamasaki, Ken Furuya
      Abstract: Nitrogen-fixing micro-organisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later non-cyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here, we examined diazotroph community structure and activity along a 7,500-km south-north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ-24774A11 and highly active cyanobacterial phylotypes (>50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN-A2, which was abundant near the surface (1.2 × 105 nifH gene copies L-1). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.
      PubDate: 2017-06-06T11:30:19.977521-05:
      DOI: 10.1002/2017GB005681
  • Autonomous observing platform CO2 data shed new light on the Southern
           Ocean carbon cycle.
    • Authors: Are Olsen
      Abstract: While the number of surface ocean CO2 partial pressure (pCO2) measurements has soared the recent decades, the Southern Ocean remains undersampled. Williams et al. [2017] now present pCO2 estimates based on data from pH-sensor equipped Bio-Argo floats, which have been measuring in the Southern Ocean since 2014. The authors demonstrate the utility of these data for understanding the carbon cycle in this region, which has a large influence on the distribution of CO2 between the ocean and atmosphere. Biogeochemical sensors deployed on autonomous platforms hold the potential to shape our view of the ocean carbon cycle in the coming decades.
      PubDate: 2017-06-06T10:25:21.555552-05:
      DOI: 10.1002/2017GB005676
  • Respiration of new and old carbon in the surface ocean: implications for
           estimates of global oceanic gross primary productivity
    • Authors: Matheus C. Carvalho; Kai G. Schulz, Bradley D. Eyre
      Abstract: New respiration (Rnew, of freshly fixated carbon) and old respiration (Rold, of storage carbon) were estimated for different regions of the global surface ocean using published data on simultaneous measurements of: 1) primary productivity using 14C (14PP); 2) gross primary productivity (GPP) based on 18O or O2; and 3) net community productivity (NCP) using O2. The ratio Rnew / GPP in 24 h incubations was typically between 0.1 and 0.3 regardless of depth and geographical area, demonstrating that values were almost constant regardless of large variations in temperature (0 to 27 °C), irradiance (surface to ~100 m deep), nutrients (nutrient rich and poor waters), and community composition (diatoms, flagellates, etc). As such, between 10 and 30% of primary production in the surface ocean is respired in less than 24 h, and most respiration (between 55 and 75%) was of older carbon. Rnew was most likely associated with autotrophs, with minor contribution from heterotrophic bacteria. Patterns were less clear for Rold. Short 14C incubations are less affected by respiratory losses. Global oceanic GPP is estimated to be between 70 and 145 Gt C y-1.
      PubDate: 2017-05-31T19:20:27.986092-05:
      DOI: 10.1002/2016GB005583
  • Understanding the unique biogeochemistry of the Mediterranean Sea:
           Insights from a coupled phosphorus and nitrogen model
    • Authors: Helen R. Powley; Michael D. Krom, Philippe Van Cappellen
      Abstract: The Mediterranean Sea (MS) is an oligotrophic basin whose offshore water column exhibits low dissolved inorganic phosphorus (P) and nitrogen (N) concentrations, unusually high nitrate (NO3) to phosphate (PO4) ratios, and distinct biogeochemical differences between the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). A new mass balance model of P and N cycling in the WMS is coupled to a pre-existing EMS model to understand these biogeochemical features. Estimated land-derived inputs of reactive P and N to the WMS and EMS are similar per unit surface area, but marine inputs are four to five times greater for the WMS, which helps explain the approximately three times higher primary productivity of the WMS. The lateral inputs of marine sourced inorganic and organic P support significant fractions of new production in the WMS and EMS, similar to subtropical gyres. The mass balance calculations imply that the MS is net heterotrophic: dissolved organic P and N entering the WMS and EMS, primarily via the straits of Gibraltar and Sicily, are mineralized to PO4 and NO3 and subsequently exported out of the basin by the prevailing anti-estuarine circulation. The high deep water (DW) molar NO3:PO4 ratios reflect the high reactive N:P ratio of inputs to the WMS and EMS, combined with low denitrification rates. The lower DW NO3:PO4 ratio of the WMS (21) compared to the EMS (28) reflects lower reactive N:P ratios of inputs to the WMS, including the relatively low N:P ratio of Atlantic surface water flowing into the WMS.
      PubDate: 2017-05-20T01:40:22.520293-05:
      DOI: 10.1002/2017GB005648
  • Variability in the mechanisms controlling Southern Ocean phytoplankton
           bloom phenology in an ocean model and satellite observations
    • Authors: Tyler Rohr; Matthew C. Long, Maria T. Kavanaugh, Keith Lindsay, Scott C. Doney
      Abstract: A coupled global numerical simulation (conducted with the Community Earth System Model) is used in conjunction with satellite remote sensing observations to examine the role of top-down (grazing pressure) and bottom-up (light, nutrients) controls on marine phytoplankton bloom dynamics in the Southern Ocean. Phytoplankton seasonal phenology is evaluated in the context of the recently proposed ‘disturbance-recovery’ hypothesis relative to more traditional, exclusively ‘bottom-up’ frameworks. All blooms occur when phytoplankton division rates exceed loss rates to permit sustained net population growth, however the nature of this decoupling period varies regionally in CESM. Regional case studies illustrate how unique pathways allow blooms to emerge despite very poor division rates or very strong grazing rates. In the Subantarctic, southeast Pacific small spring blooms initiate early co-occurring with deep mixing and low division rates, consistent with the ‘disturbance-recovery’ hypothesis. Similar systematics are present in the Subantarctic, southwest Atlantic during the spring, but are eclipsed by a subsequent, larger summer bloom that is coincident with shallow mixing and the annual maximum in division rates, consistent with a ‘bottom-up’, light limited framework. In the model simulation, increased iron stress prevents a similar summer bloom in the southeast Pacific. In the simulated Antarctic zone (70°S - 65°S) seasonal sea ice acts as a dominant phytoplankton-zooplankton decoupling agent, triggering a delayed but substantial bloom as ice recedes. Satellite ocean color remote sensing and ocean physical reanalysis products do not precisely match model predicted phenology, but observed patterns do indicate regional variability in mechanism across the Atlantic and Pacific.
      PubDate: 2017-05-11T02:05:33.790705-05:
      DOI: 10.1002/2016GB005615
  • Impacts of ENSO on air-sea oxygen exchange: observations and mechanisms
    • Authors: Yassir A. Eddebbar; Matthew C. Long, Laure Resplandy, Christian Rödenbeck, Keith B. Rodgers, Manfredi Manizza, Ralph F. Keeling
      Abstract: Models and observations of Atmospheric Potential Oxygen (APO ≃ O2 + 1.1*CO2) are used to investigate the influence of El Niño Southern Oscillation (ENSO) on air-sea O2 exchange. An atmospheric transport inversion of APO data from the Scripps flask network shows significant interannual variability in tropical APO fluxes that is positively correlated with the Niño3.4 index, indicating anomalous ocean outgassing of APO during El Niño. Hindcast simulations of the Community Earth System Model (CESM) and the Institut Pierre-Simon Laplace (IPSL) model show similar APO sensitivity to ENSO, differing from the Geophysical Fluid Dynamic Laboratory (GFDL) model, which shows an opposite APO response. In all models, O2 accounts for most APO flux variations. Detailed analysis in CESM shows the O2 response is driven primarily by ENSO-modulation of the source and rate of equatorial upwelling, which moderate the intensity of O2 uptake due to vertical transport of low-O2 waters. These upwelling changes dominate over counteracting effects of biological productivity and thermally-driven O2 exchange. During El Niño, shallower and weaker upwelling leads to anomalous O2 outgassing, whereas deeper and intensified upwelling during La Niña drives enhanced O2 uptake. This response is strongly localized along the central and eastern equatorial Pacific, leading to an equatorial zonal dipole in atmospheric anomalies of APO. This dipole is further intensified by ENSO-related changes in winds, reconciling apparently conflicting APO observations in the tropical Pacific. These findings suggest a substantial and complex response of the oceanic O2 cycle to climate variability that is significantly (>50%) underestimated in magnitude by ocean models.
      PubDate: 2017-05-11T02:05:32.125588-05:
      DOI: 10.1002/2017GB005630
  • The impact of changing wind speeds on gas transfer and its effect on
           global air-sea CO2 fluxes
    • Authors: R. Wanninkhof; J. Trinanes
      Abstract: An increase in global wind speeds over time is affecting the global uptake of CO2 by the ocean. We determine the impact of changing winds on gas transfer and CO2 uptake by using the recently updated, global high-resolution, cross-calibrated multi-platform wind product (CCMP-V2) and a fixed monthly pCO2 climatology. In particular, we assess global changes in the context of regional wind speed changes that are attributed to large-scale climate reorganizations. The impact of wind on global CO2 gas fluxes as determined by the bulk formula is dependent on several factors, including the functionality of the gas exchange-wind speed relationship and the regional and seasonal differences in the air-water partial pressure of CO2 gradient (∆pCO2). The latter also controls the direction of the flux. Fluxes out of the ocean are influenced more by changes in the low-to-intermediate wind speed range, while ingassing is impacted more by changes in higher winds because of the regional correlations between wind and ∆pCO2. Gas exchange-wind speed parameterizations with a quadratic and third-order polynomial dependency on wind, each of which meets global constraints, are compared. The changes in air-sea CO2 fluxes resulting from wind speed trends are greatest in the equatorial Pacific and cause a 0.03-0.04 Pg C dec-1 increase in outgassing over the 27 year time span. This leads to a small overall decrease of 0.00 to 0.02 Pg C dec-1 in global net CO2 uptake, contrary to expectations that increasing winds increase net CO2 uptake.
      PubDate: 2017-05-09T12:15:25.135771-05:
      DOI: 10.1002/2016GB005592
  • Seasonal and spatial variability in northern Gulf of Alaska surface-water
           iron concentrations driven by shelf sediment resuspension, glacial
           meltwater, a Yakutat eddy, and dust
    • Authors: John Crusius; Andrew W. Schroth, Joseph A. Resing, Jay Cullen, Robert W. Campbell
      Abstract: Phytoplankton growth in the Gulf of Alaska (GoA) is limited by iron (Fe), yet Fe sources are poorly constrained. We examine the temporal and spatial distributions of Fe, and its sources in the GoA, based on data from three cruises carried out in 2010 from the Copper River (AK) mouth to beyond the shelf break. April data are the first to describe late winter Fe behavior before surface-water nitrate depletion began. Sediment resuspension during winter and spring storms generated high “total dissolvable Fe” (TDFe) concentrations of ~1000 nmol kg-1 along the entire continental shelf, which decreased beyond the shelf break. In July, high TDFe concentrations were similar on the shelf, but more spatially variable, and driven by low-salinity glacial meltwater. Conversely, dissolved Fe (DFe) concentrations in surface waters were far lower and more seasonally consistent, ranging from ~4 nmol kg-1 in nearshore waters to ~0.6-1.5 nmol kg-1 seaward of the shelf break during April and July, despite dramatic depletion of nitrate over that period. The reasonably constant DFe concentrations are likely maintained during the year across the shelf by complexation by strong organic ligands, coupled with ample supply of labile particulate Fe. The April DFe data can be simulated using a simple numerical model that assumes a DFe flux from shelf sediments, horizontal transport by eddy diffusion, and removal by scavenging. Given how global change is altering many processes impacting the Fe cycle, additional studies are needed to examine controls on DFe in the Gulf of Alaska.
      PubDate: 2017-05-06T01:20:35.673674-05:
      DOI: 10.1002/2016GB005493
  • Contributions of wildland fire to terrestrial ecosystem carbon dynamics in
           North America from 1990 - 2012
    • Authors: Guangsheng Chen; Daniel J. Hayes, A. David McGuire
      Abstract: Burn area and the frequency of extreme fire events have been increasing during recent decades in North America, and this trend is expected to continue over the 21st century. While many aspects of the North American carbon budget have been intensively studied, the net contribution of fire disturbance to the overall net carbon flux at the continental scale remains uncertain. Based on national scale, spatially-explicit and long-term fire data, along with the improved model parameterization in a process-based ecosystem model, we simulated the impact of fire disturbance on both direct carbon emissions and net terrestrial ecosystem carbon balance in North America. Fire-caused direct carbon emissions were 106.55 ± 15.98 Tg C/yr during 1990-2012; however, the net ecosystem carbon balance associated with fire was -26.09 ± 5.22 Tg C/yr, indicating that most of the emitted carbon was re-sequestered by the terrestrial ecosystem. Direct carbon emissions showed an increase in Alaska and Canada during 1990-2012 as compared to prior periods due to more extreme fire events, resulting in a large carbon source from these two regions. Among biomes, the largest carbon source was found to be from the boreal forest, primarily due to large reductions in soil organic matter during, and with slower recovery after, fire events. The interactions between fire and environmental factors reduced the fire-caused ecosystem carbon source. Fire disturbance only caused a weak carbon source as compared to the best-estimate terrestrial carbon sink in North America owing to the long-term legacy effects of historical burn area coupled with fast ecosystem recovery during 1990-2012.
      PubDate: 2017-05-05T22:47:24.810957-05:
      DOI: 10.1002/2016GB005548
  • Energetic Costs of Calcification Under Ocean Acidification
    • Authors: Christopher Spalding; Seth Finnegan, Woodward W. Fischer
      Abstract: Anthropogenic ocean acidification threatens to negatively impact marine organisms that precipitate calcium carbonate skeletons. Past geological events, such as the Permian-Triassic Mass Extinction, together with modern experiments generally support these concerns. However, the physiological costs of producing a calcium carbonate skeleton under different acidification scenarios remain poorly understood. Here, we present an idealized mathematical model to quantify whole-skeleton costs, concluding that they rise only modestly (up to ∼10%) under acidification expected for 2100. The modest magnitude of this effect reflects in part the low energetic cost of inorganic, calcium carbonate relative to the proteinaceous organic matrix component of skeletons. Our analysis does, however, point to an important kinetic constraint that depends on seawater carbonate chemistry, and we hypothesize that the impact of acidification is more likely to cause extinctions within groups where the timescale of larval development is tightly constrained. The cheapness of carbonate skeletons compared to organic materials also helps explain the widespread evolutionary convergence upon calcification within the metazoa.
      PubDate: 2017-05-05T20:54:14.575023-05:
      DOI: 10.1002/2016GB005597
  • Productivity patterns in the Equatorial Pacific over the last
           30,000 years
    • Authors: Kassandra M. Costa; Allison W. Jacobel, Jerry F. McManus, Robert F. Anderson, Gisela Winckler, Nivedita Thiagarajan
      Abstract: The Equatorial Pacific traverses a number of productivity regimes, from the highly productive coastal upwelling along Peru to the near gyre-like productivity lows along the international dateline, making it an ideal target for investigating how biogeochemical systems respond to changing oceanographic conditions over time. However, conflicting reconstructions of productivity during periods of rapid climate change, like the last deglaciation, render the spatio-temporal response of Equatorial Pacific productivity ambiguous. In this study, surface productivity since the last glacial period (30,000 years ago) is reconstructed from seven cores near the Line Islands, Central Equatorial Pacific, and integrated with productivity records from across the Equatorial Pacific. Three coherent deglacial patterns in productivity are identified: 1) a monotonic glacial-Holocene increase in productivity, primarily along the Equator, associated with increasing nutrient concentrations over time, 2) a deglacial peak in productivity ~15,000 years ago due to transient entrainment of nutrient rich southern-sourced deep waters and 3) possible precessional cycles in productivity in the Eastern Equatorial Pacific that may be related to ITCZ migration and potential interactions with ENSO dynamics. These findings suggest that productivity was generally lower during the glacial period, a trend observed zonally across the Equatorial Pacific, while deglacial peaks in productivity may be prominent only in the East.
      PubDate: 2017-05-04T05:05:45.12659-05:0
      DOI: 10.1002/2016GB005579
  • Nitrogen dynamic in Eurasian coastal Arctic ecosystem: Insight from
           nitrogen isotope
    • Authors: Benoit Thibodeau; Dorothea Bauch, Maren Voss
      Abstract: Primary productivity is limited by the availability of nitrogen (N) in most of the coastal Arctic, as a large portion of N is released by the spring freshet and completely consumed during the following summer. Thus, understanding the fate of riverine nitrogen is critical to identify the link between dissolved nitrogen dynamic and coastal primary productivity to foresee upcoming changes in the Arctic seas, such as increase riverine discharge and permafrost thaw. Here, we provide a field-based study of nitrogen dynamic over the Laptev Sea shelf based on isotope geochemistry. We demonstrate that while most of the nitrate found under the surface fresh water layer is of remineralized origin, some of the nitrate originates from atmospheric input and was probably transported at depth by the mixing of brine-enriched denser water during sea-ice formation. Moreover, our results suggest that riverine dissolved organic nitrogen (DON) represents up to 6 times the total riverine release of nitrate and that about 62 to 76% of the DON is removed within the shelf waters. This is a crucial information regarding the near-future impact of climate change on primary productivity in the Eurasian coastal Arctic.
      PubDate: 2017-04-24T00:26:14.70952-05:0
      DOI: 10.1002/2016GB005593
  • Global patterns of woody residence time and its influence on model
           simulation of aboveground biomass
    • Authors: Bao-Lin Xue; Qinghua Guo, Tianyu Hu, Jingfeng Xiao, Yuanhe Yang, Guoqiang Wang, Shengli Tao, Yanjun Su, Jin Liu, Xiaoqian Zhao
      Abstract: Woody residence time (τw) is an important parameter that expresses the balance between mature forest recruitment/growth and mortality. Using field data collected from the literature, this study explored the global forest τw and investigated its influence on model simulations of aboveground biomass (AGB) at a global scale. Specifically, τw was found to be related to forest age, annual temperature and precipitation at a global scale, but its determinants were different among various plant function types. The estimated global forest τw based on the filed data showed large spatial heterogeneity, which plays an important role in model simulation of AGB by a Dynamic Global Vegetation Model (DGVM). τw could change the resulting AGB in 10-fold based on a site-level test using the Monte Carlo method. At the global level, different parameterization schemes of the Integrated Biosphere Simulator using the estimated τw resulted in a two-fold change in the AGB simulation for 2100. Our results highlight the influences of various biotic and abiotic variables on forest τw. The estimation of τw in our study may help improve the model simulations and reduce the parameter's uncertainty over the projection of future AGB in the current DGVM or Earth System Models. A clearer understanding of the responses of τw to climate change and the corresponding sophisticated description of forest growth/mortality in model structure is also needed for the improvement of carbon stock prediction in future studies.
      PubDate: 2017-04-23T22:52:26.852908-05:
      DOI: 10.1002/2016GB005557
  • Modeling radiocarbon constraints on the dilution of dissolved organic
           carbon in the deep ocean.
    • Authors: Jamie D. Wilson; Sandra Arndt
      Abstract: The recalcitrance of dissolved organic carbon (DOC) that leads to its accumulation in the deep ocean is typically considered a function of its reactivity. Yet, recent experimental evidence has shown that DOC from the deep ocean, if concentrated, can support significant microbial growth. This supports an alternative hypothesis that [DOC] may become too dilute to support microbial growth. The radiocarbon signature of DOC is a key constraint on the DOC cycling that allows testing of the plausibility of this hypothesis. Here, we use a box model of diluted DOC in the deep ocean and its radiocarbon signature that is constrained on the basis of the new experimental evidence, as well as current knowledge of deep ocean DOC cycling to quantitatively test the dilution hypothesis. We explore the uncertainty in model results across a range of plausible dilution thresholds, additional processes, and fluxes of DOC to the deep ocean. Results show that the model is able to predict the observed radiocarbon signature for a dilution threshold close to the observed deep ocean [DOC] and for fluxes close to published estimates. Sensitivity analysis shows that this result is highly sensitive to variations in the dilution threshold, and the assumption that diluted DOC is able to survive ocean overturning. The experimental findings can be alternatively reconciled over a large range of different conditions assuming a small pool of diluted DOC with a modern radiocarbon signature, consistent with recent observations, and offering a parsimonious interpretation of dilution with existing hypotheses on DOC recalcitrance.
      PubDate: 2017-04-19T06:22:19.587587-05:
      DOI: 10.1002/2016GB005520
  • Strong biotic influences on regional patterns of climate regulation
    • Authors: H. M. Serna-Chavez; N. G. Swenson, M. D. Weiser, E. E. Loon, W. Bouten, M. D. Davidson, P. M. Bodegom
      Abstract: Climate regulation services from forests are an important leverage in global-change mitigation treaties. Like most ecosystem services, climate regulation is the product of various ecological phenomena with unique spatial features. Elucidating which abiotic and biotic factors relate to spatial patterns of climate regulation services advances our understanding of what underlies climate-mitigation potential, and its variation within and across ecosystems. Here we quantify and contrast the statistical relations between climate regulation services (albedo and evapotranspiration, primary productivity and soil carbon) and abiotic and biotic factors. We focus on 16,955 forest plots in a regional extent across the eastern United States. We find the statistical effects of forest litter and understory carbon on climate regulation services to be as strong as those of temperature-precipitation interactions. These biotic factors likely influence climate regulation through changes in vegetation and canopy density, radiance scattering, and decomposition rates. We also find a moderate relation between leaf nitrogen traits and primary productivity at this regional scale. The statistical relation between climate regulation and temperature-precipitation ranges, seasonality, and climatic thresholds highlights a strong feedback with global climate change. Our assessment suggests the expression of strong biotic influences on climate regulation services at a regional, temperate extent. Biotic homogenization and management practices manipulating forests structure and succession will likely strongly impact climate-mitigation potential. The identity, strength, and direction of primary influences differed for each process involved in climate regulation. Hence, different abiotic and biotic factors are needed to monitor and quantify the full climate-mitigation potential of temperate forest ecosystems.
      PubDate: 2017-04-18T07:16:10.659312-05:
      DOI: 10.1002/2017GB005627
  • One possible uncertainty in CMIP5 projections of low-oxygen water volume
           in the Eastern Tropical Pacific
    • Authors: M. Shigemitsu; A. Yamamoto, A. Oka, Y. Yamanaka
      Abstract: Using the results from nine Earth system models submitted to the Coupled Model Intercomparison Project Phase 5 (CMIP5), we identify the Eastern Tropical Pacific (ETP) as the region with the greatest uncertainty of future changes in oxygen-deficient (
      PubDate: 2017-04-17T08:50:24.915209-05:
      DOI: 10.1002/2016GB005447
  • Picoplankton contribution to biogenic silica stocks and production rates
           in the Sargasso Sea
    • Authors: Jeffrey W. Krause; Mark A. Brzezinski, Stephen B. Baines, Jackie L. Collier, Benjamin S. Twining, Daniel C. Ohnemus
      Abstract: Picocyanobacteria in the Sargasso Sea accumulate significant amounts of Si, a finding which questions how we interpret previous regional measurements of biogenic silica (bSi) production and the role of diatoms in the open ocean. The picoplankton (
      PubDate: 2017-04-13T10:45:14.871372-05:
      DOI: 10.1002/2017GB005619
  • Issue Information
    • Pages: 761 - 761
      Abstract: No abstract is available for this article.
      PubDate: 2017-06-14T00:23:21.785449-05:
      DOI: 10.1002/gbc.20453
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