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Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [12 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Net community production and carbon export during the late summer in the
           Ross Sea, Antarctica
    • Authors: Hans. B. DeJong; Robert B. Dunbar, David A. Koweek, David A. Mucciarone, Sarah K. Bercovici, Dennis A. Hansell
      Abstract: The phytoplankton bloom in the Ross Sea is the largest in spatial extent and one of the most productive in Antarctica, yet the fate of the summer bloom remains poorly understood. Here we present carbon system data from the first biogeochemical process cruise to be conducted in both the western and central Ross Sea during late summer (February-March 2013). Using one-dimensional carbon budgets, we found evidence for substantial positive net community production (425 ± 204 mmol C m-2 d-1) during the late summer in Terra Nova Bay (TNB) of the western Ross Sea, which was rapidly exported to below 200 m. In addition, seasonally integrated carbon export was higher in diatom-dominated TNB (7.3 ± 0.9 mol C m-2) compared to the Phaeocystis antarctica-dominated central Ross Sea (3.4 ± 0.8 mol C m-2). Substantial late summer productivity and export may be a widespread phenomenon in Antarctic coastal regions that is not accounted for in regional carbon models.
      PubDate: 2017-02-18T02:55:42.493681-05:
      DOI: 10.1002/2016GB005417
  • Correlations of surface ocean pCO2 to satellite chlorophyll on monthly to
           interannual timescales
    • Authors: Amanda R. Fay; Galen A. McKinley
      Abstract: On the mean, ocean carbon uptake is linked to biological productivity, but how biological variability impacts carbon uptake is poorly quantified. Our ability to diagnose past change, understand present variability and predict the future state of the global carbon cycle requires improving mechanistic understanding in this area. Here, we make use of co-located pCO2 and temperature data, a merged surface ocean color product, and physical fields from an ocean state estimate to assess relationships between surface ocean biology and the carbon cycle on seasonal, monthly anomaly, and interannual timescales over the period 1998-2014. Using a correlation analysis on spatial scales from local to basin-scale biomes, we identify the timescales on which ocean productivity could be directly modifying ocean carbon uptake. On seasonal timescales outside of the equatorial Pacific, biome-scale correlations are negative between chlorophyll and pCO2. Though this relationship is pervasive, the underlying mechanisms vary across timescales and biomes. Consistent with previous findings, biological activity is a significant driver of pCO2 seasonality only in the subpolar biomes. For monthly anomalies acting on top of the mean seasonality, productivity and pCO2 changes are significantly correlated in the subpolar North Pacific and Southern Ocean. Only in the Southern Ocean are correlations consistent with a dominant role for biology in the surface ocean carbon cycle on all timescales.
      PubDate: 2017-02-18T02:40:38.240114-05:
      DOI: 10.1002/2016GB005563
  • Global and Regional Fluxes of Carbon from Land Use and Land-Cover Change
    • Authors: R. A. Houghton; Alexander A. Nassikas
      Abstract: The net flux of carbon from land use and land-cover change (LULCC) is an important term in the global carbon balance. Here we report a new estimate of annual fluxes from 1850 to 2015, updating earlier analyses with new estimates of both historical and current rates of LULCC and including emissions from draining and burning of peatlands in Southeast Asia. For most of the 186 countries included we relied on data from FAO to document changes in the areas of croplands and pastures since 1960 and changes in the areas of forests and “other land” since 1990. For earlier years we used other sources of information. We used a bookkeeping model that prescribed changes in carbon density of vegetation and soils for 20 types of ecosystems and five land uses. The total net flux attributable to LULCC over the period 1850-2015 is calculated to have been 145 ± 16 PgC (one standard deviation). Most of the emissions were from the tropics (102 ± 5.8 PgC), generally increasing over time to a maximum of 2.10 PgC yr-1 in 1997. Outside the tropics emissions were roughly constant at 0.5 PgC yr-1 until 1940, declined to zero around 1970, and then became negative. For the most recent decade (2006-2015) global net emissions from LULCC averaged 1.11 (±0.35) PgC yr-1, consisting of a net source from the tropics (1.41 ± 0.17 PgC yr-1), a net sink in northern mid-latitudes (-0.28 ± 0.21 PgC yr-1), and carbon neutrality in southern mid-latitudes.
      PubDate: 2017-02-15T12:36:00.450841-05:
      DOI: 10.1002/2016GB005546
  • A Global Ocean Climatology of Preindustrial and Modern Ocean δ13C
    • Authors: M. Eide; A. Olsen, U. Ninnemann, T. Johannessen
      Abstract: We present a global ocean climatology of dissolved inorganic carbon δ13C (‰) corrected for the 13C-Suess effect, preindustrial δ13C. This was constructed by first using Olsen and Ninnemann's [2010] back-calculation method on data from 25 World Ocean Circulation Experiment cruises to reconstruct the preindustrial δ13C on sections spanning all major oceans. Next, we developed five multilinear regression equations, one for each major ocean basin, which were applied on the World Ocean Atlas data to construct the climatology. This reveals the natural δ13C distribution in the global ocean. Compared to the modern distribution, the preindustrial δ13C spans a larger range of values. The maxima, of up to 1.8‰, occurs in the subtropical gyres of all basins, in the upper and intermediate waters of the North Atlantic, as well as in mode waters with a Southern Ocean origin. Particularly strong gradients occur at intermediate depths, revealing a strong potential for using δ13C as a tracer for changes in water mass geometry at these levels. Further, we identify a much tighter relationship between δ13C and Apparent Oxygen Utilization (AOU) than between δ13C and phosphate. This arises because, in contrast to phosphate, AOU and δ13C are both partly reset when waters are ventilated in the Southern Ocean, and underscores that δ13C is a highly robust proxy for past changes in ocean oxygen content and ocean ventilation. Our global preindustrial δ13C climatology is openly accessible, and can be used for example for improved model evaluation and interpretation of sediment δ13C records.
      PubDate: 2017-02-13T00:25:35.504076-05:
      DOI: 10.1002/2016GB005473
  • A Global Estimate of the Full Oceanic 13C Suess Effect Since the
    • Authors: M. Eide; A. Olsen, U. Ninnemann, T. Eldevik
      Abstract: We present the first estimate of the full global ocean 13C Suess effect since preindustrial times, based on observations. This has been derived by first using the method of Olsen and Ninnemann [2010] to calculate 13C Suess effect estimates on sections spanning the world ocean, which were next mapped on a global 1°x1° grid. We find a strong 13C Suess effect in the upper 1000 m of all basins, with strongest decrease in the Subtropical Gyres of the Northern Hemisphere, where δ13C of dissolved inorganic carbon has decreased by more than 0.8‰ since the industrial revolution. At greater depths, a significant 13C Suess effect can only be detected in the northern parts of the North Atlantic Ocean. The relationship between the 13C Suess effect and the concentration of anthropogenic carbon varies strongly between water masses, reflecting the degree to which source waters are equilibrated with the atmospheric 13C Suess effect before sinking. Finally, we estimate a global ocean inventory of anthropogenic CO2 of 92 ± 46 Gt C. This provides an estimate that is almost independent of, and consistent, within the uncertainties, with previous estimates.
      PubDate: 2017-02-13T00:25:34.277328-05:
      DOI: 10.1002/2016GB005472
  • Annual Cyclicity in Export Efficiency in the Inner Southern California
    • Authors: William Z. Haskell; Maria G. Prokopenko, Douglas E. Hammond, Rachel H. R. Stanley, Zoe O. Sandwith
      Abstract: The balance of marine autotrophy and heterotrophy regulates the ocean's ability to serve as a CO2 sink, as organic material produced by autotrophs sinks into the ocean interior to drive the biological pump. Marine ecosystems over the continental margins, especially coastal upwelling regions, account for a disproportionate amount of carbon export, thus even small fluctuations in export in these regions can have a large impact on the global carbon cycle. In this study, we estimated the rate of gross oxygen production (GOP), stoichiometrically related to gross primary production, by combining measurements of the triple isotope composition of dissolved oxygen with estimates of vertical advection, eddy diffusion and air-sea gas exchange in a one-dimensional two-box non-steady state model of the euphotic zone. Net oxygen production (NOP) estimates based on O2/Ar were then combined with GOP to estimate the NOP/GOP ratio, or potential export efficiency, out of the euphotic zone at the San Pedro Ocean Time series (SPOT) during an 18-month period between January 2013 and June 2014. GOP estimates ranged from 161 ± 44 to 477 ± 155 mmol m−2 d−1 during this period, peaking in May each year, and NOP/GOP ratios ranged from 0.05 ± 0.10 to 0.65 ± 0.28. The highest export efficiency occurred in late February/early March, following the onset of spring upwelling, declining as the upwelling season continued. This study demonstrates that export efficiency changes through time in this temperate coastal upwelling region on a repeated annual cycle and the magnitude of export efficiency suggests efficient photosynthetic energy conversion by phytoplankton in spring.
      PubDate: 2017-02-02T23:59:38.815386-05:
      DOI: 10.1002/2016GB005561
  • The biogeochemisty of cobalt in the Mediterranean Sea
    • Authors: Gabriel Dulaquais; Hélène Planquette, Stéphane L'Helguen, Micha J. A. Rijkenberg, Marie Boye
      Abstract: The soluble (sCo 
      PubDate: 2017-02-02T17:30:24.006607-05:
      DOI: 10.1002/2016GB005478
  • Removal of Atmospheric Ethanol by Wet Deposition
    • Authors: J. David Felix; Joan D. Willey, Rachel K. Thomas, Katherine M. Mullaugh, G. Brooks Avery, Robert J. Kieber, Ralph N. Mead, John Helms, Fernanda F. Giubbina, M. Lucia A. M. Campos, John Cala
      Abstract: The global wet deposition flux of ethanol is estimated to be 2.4 ± 1.6 Tg/yr with a conservative range of 0.2 - 4.6 Tg/yr based upon analyses of 219 wet deposition samples collected at 12 locations globally. This estimate calculated using observed wet deposition ethanol concentrations is in agreement with previous models (1.4 - 5 Tg/yr) predicting the wet deposition sink using Henry's Law coefficients and atmospheric ethanol concentrations. Wet deposition is estimated to remove between 6 - 17 % of the total ethanol emitted to the atmosphere on an annual basis. The concentration of ethanol in marine rain (25 ± 6 nM) is an order of magnitude less than in the majority of terrestrial rains (345 ± 280 nM). Terrestrial rain samples collected in locations impacted by high local sources of biofuel usage and locations downwind from ethanol distilleries were an order of magnitude higher in ethanol concentration (3090 ± 448 nM) compared to rain collected in terrestrial locations not impacted by these sources. These results indicate wet deposition of ethanol is heavily influenced by local sources. Results of this study are important because they suggest that as biofuel production and usage increases, the concentration of ethanol in the atmosphere will increase as will the wet deposition flux. Additional research constraining the sources, sinks and atmospheric impacts of ethanol is necessary to better assist in the debate as whether or not to increase consumption of the alcohol as a biofuel.
      PubDate: 2017-02-02T08:15:37.012878-05:
      DOI: 10.1002/2016GB005516
  • Development of a global ocean mercury model with a methylation cycle:
           outstanding issues
    • Authors: Kirill Semeniuk; Ashu Dastoor
      Abstract: We present a newly developed global ocean mercury (Hg) transport and biogeochemistry model and use pre-anthropogenic equilibrium simulations to highlight physical and chemical processes which reveal significant knowledge gaps that need to be addressed. As with previous 3D ocean Hg model work we use a bulk chemistry scheme based on particulate organic carbon remineralization. We also include an explicit methylation cycle based on available reaction rates. The methylation to demethylation rate ratio based on various field studies is found to be inconsistent with the concentration ratios measured in the Southern Ocean around Antarctica and in the Arctic. There is also model-measurement disagreement in the old waters of the Tropical and North Pacific Ocean. The model produces an intermediate water maximum in total Hg in this region reflecting the higher age of water which is absent in observations. The model also underestimates total Hg concentrations in the deepest waters in this region. These disagreements in depth profile shape point to an inadequate representation of scavenging and sedimentation and possibly seabed emission or remobilization of Hg. In addition, the total Hg distribution differences compared to previous model work reflect sensitivity to ocean model transport characteristics and in particular the tracer diffusion. The residence time of Hg in the global ocean and the surface evasion flux of elemental Hg is sensitive to such model aspects. We find a global ocean Hg turnover time against sediment burial to be about 1100 years which is within the range of previous studies.
      PubDate: 2017-02-01T01:16:04.473614-05:
      DOI: 10.1002/2016GB005452
  • Multiple metabolisms constrain the anaerobic nitrite budget in the Eastern
           Tropical South Pacific
    • Authors: Andrew R. Babbin; Brian D. Peters, Calvin W. Mordy, Brittany Widner, Karen L. Casciotti, Bess B. Ward
      Abstract: The Eastern Tropical South Pacific is one of the three major oxygen deficient zones (ODZs) in the global ocean, and is responsible for approximately one-third of marine water column nitrogen loss. It is the best studied of the ODZs, and like the others, features a broad nitrite maximum across the low oxygen layer. How the microbial processes that produce and consume nitrite in anoxic waters interact to sustain this feature is unknown. Here, we used 15N-tracer experiments to disentangle five of the biologically-mediated processes that control the nitrite pool, including a high-resolution profile of nitrogen loss rates. Nitrate reduction to nitrite likely depended on organic matter fluxes, but the organic matter did not drive detectable rates of denitrification to N2. However, multiple lines of evidence show that denitrification is important in shaping the biogeochemistry of this ODZ. Significant rates of anaerobic nitrite oxidation at the ODZ boundaries were also measured. Iodate was a potential oxidant that could support part of this nitrite consumption pathway. We additionally observed N2 production from labeled cyanate and postulate that anammox bacteria have the ability to harness cyanate as another form of reduced nitrogen rather than relying solely on ammonification of complex organic matter. The balance of the five anaerobic rates measured – anammox, denitrification, nitrate reduction, nitrite oxidation, and dissimilatory nitrite reduction to ammonium – are sufficient to reproduce broadly the observed nitrite and nitrate profiles in a simple one-dimensional model, but require an additional cryptic source of reduced nitrogen to the deeper ODZ to avoid ammonium overconsumption.
      PubDate: 2017-01-26T01:50:42.622383-05:
      DOI: 10.1002/2016GB005407
  • Mobility and Transport of Mercury and Methylmercury in Peat as a Function
           of Changes in Water Table Regime and Plant Functional Groups
    • Authors: Kristine M. Haynes; Evan Kane, Lynette Potvin, Erik Lilleskov, Randall K. Kolka, Carl P. J. Mitchell
      Abstract: Climate change is likely to significantly affect the hydrology, ecology and ecosystem function of peatlands, with potentially important but unclear impacts on mercury mobility within and transport from peatlands. Using a full-factorial mesocosm approach we investigated the potential impacts on mercury mobility of water table regime changes (high and low) and vegetation community shifts (sedge-dominated, Ericaceae-dominated or unmanipulated control) in peat monoliths at the PEATcosm mesocosm facility in Houghton, Michigan. Lower and more variable water table regimes and the loss of Ericaceae shrubs act significantly and independently to increase both total Hg (THg) and methylmercury (MeHg) concentrations in peat pore water and in spring snowmelt runoff. These differences are related to enhanced peat decomposition and internal regeneration of electron acceptors which are more strongly related to water table regime than to plant community changes. Loss of Ericaceae shrubs and an increase in sedge cover may also affect Hg concentrations and mobility via oxygen shuttling and/or the provision of labile root exudates. Altered hydrological regimes and shifting vegetation communities, as a result of global climate change, are likely to enhance Hg transport from peatlands to downstream aquatic ecosystems.
      PubDate: 2017-01-24T07:45:32.945815-05:
      DOI: 10.1002/2016GB005471
  • A re-evaluation of the magnitude and impacts of anthropogenic atmospheric
           nitrogen inputs on the ocean
    • Authors: Jickells T. D; Buitenhuis E, Altieri K, Baker A. R, Capone D, Duce R. A, Dentener F, Fennel K, Kanakidou M, LaRoche J, Lee K, Liss P, Middelburg J. J, Moore J. K, Okin G, Oschlies A, Sarin M, Seitzinger S, Sharples J, Singh A, Suntharalingam P, Uematsu M, Zamora L. M.
      Abstract: We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition, and compare this to fluvial inputs and di-nitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological di-nitrogen fixation is the main external source of nitrogen to the open ocean, i.e. beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr-1 and less than the Duce et al., 2008 estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.
      PubDate: 2017-01-21T13:05:27.48803-05:0
      DOI: 10.1002/2016GB005586
  • Two Decades of Pacific Anthropogenic Carbon Storage and Ocean
           Acidification Along GO-SHIP Sections P16 and P02
    • Authors: Carter B. R; Feely R. A, Mecking S, Cross J. N, Macdonald A. M, Siedlecki S. A, Talley L. D, Sabine C. L, Millero F. J, Swift J. H, Dickson A. G, Rodgers K. B.
      Abstract: A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (Canth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30°N and P16 is a meridional section crossing the North and South Pacific at ~150°W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to ~1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s-2000s) and recent (2000s-2010s) decades along P02, at rates of 0.53 ± 0.11 and 0.46 ± 0.11 mol C m-2 a˗1, respectively. By contrast, Canth storage accelerated from 0.29 ± 0.10 to 0.45 ± 0.11 mol C m˗2 a˗1 along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60°S and 60°N of 6.1 ± 1.5 PgC decade˗1 in the earlier decade and 8.8 ± 2.2 PgC decade˗1 in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by ˗0.08 to ˗0.14 and aragonite saturation state by ˗0.57 to ˗0.82.
      PubDate: 2017-01-21T12:30:31.728041-05:
      DOI: 10.1002/2016GB005485
  • Mixed layer carbon cycling at the Kuroshio Extension Observatory
    • Authors: Andrea J. Fassbender; Christopher L. Sabine, Meghan F. Cronin, Adrienne J. Sutton
      Abstract: Seven years of data from the NOAA Kuroshio Extension Observatory (KEO) surface mooring, located in the North Pacific Ocean carbon sink region, were used to evaluate drivers of mixed layer carbon cycling. A time-dependent mass balance approach relying on two carbon tracers was used to diagnostically evaluate how surface ocean processes influence mixed layer carbon concentrations over the annual cycle. Results indicate that the annual physical carbon input is predominantly balanced by biological carbon uptake during the intense spring bloom. Net annual gas exchange that adds carbon to the mixed layer and the opposing influence of net precipitation that dilutes carbon concentrations, make up smaller contributions to the annual mixed layer carbon budget. Decomposing the biological term into annual net community production (aNCP) and calcium carbonate production (aCaCO3) yields 7 ± 3 mol C m-2 yr-1 aNCP and 0.5 ± 0.3 mol C m-2 yr-1 aCaCO3, giving an annually integrated particulate inorganic carbon to particulate organic carbon production ratio of 0.07 ± 0.05, as a lower limit. Although we find that vertical physical processes dominate carbon input to the mixed layer at KEO, it remains unclear how horizontal features, such as eddies, influence carbon production and export by altering nutrient supply as well as the depth of winter ventilation. Further research evaluating linkages between Kuroshio Extension jet instabilities, eddy activity, and nutrient supply mechanisms is needed to adequately characterize the drivers and sensitivities of carbon cycling near KEO.
      PubDate: 2017-01-13T22:10:24.855091-05:
      DOI: 10.1002/2016GB005547
  • Declining spatial efficiency of global cropland nitrogen allocation
    • Authors: Nathaniel D. Mueller; Luis Lassaletta, Bryan Runck, Gilles Billen, Josette Garnier, James S. Gerber
      Abstract: Efficiently allocating nitrogen (N) across space maximizes crop productivity for a given amount of N input and reduces N losses to the environment. Here we quantify changes in the global spatial efficiency of cropland N use by calculating historical tradeoff frontiers relating N inputs to possible N yield assuming efficient allocation. Time-series cropland N budgets from 1961 to 2009 characterize the evolution of N input-yield response functions across 12 regions and are the basis for constructing tradeoff frontiers. Improvements in agronomic technology have substantially increased cropping system yield potentials and expanded N-driven crop production possibilities. However, we find these gains are compromised by the declining spatial efficiency of N use across regions. Since the start of the Green Revolution, N inputs and yields have moved farther from the optimal frontier over time; in recent years (1994–2009), global N surplus has grown to a value that is 69% greater than what is possible with efficient N allocation between regions. To reflect regional pollution and agricultural development goals, we construct scenarios that restrict reallocation, finding that these changes only slightly decrease potential gains in nitrogen use efficiency. Our results are inherently conservative due to the regional unit of analysis, meaning a larger potential exists than is quantified here for cross-scale policies to promote spatially efficient N use.
      PubDate: 2017-01-12T20:30:29.905431-05:
      DOI: 10.1002/2016GB005515
  • Thorium distributions in high and low dust regions and the significance
           for iron supply
    • Authors: Christopher T. Hayes; Jeffrey Rosen, David McGee, Edward A. Boyle
      Abstract: Thorium and uranium isotopes (232Th, 230Th, 238U, and 234U) were investigated to refine their use for estimating mineral dust deposition and Fe delivery to the ocean. U concentrations and isotope ratios were consistent with conservative behavior and can safely be described using published U-salinity relationships and global average seawater isotopic composition. Near Barbados, waters affected by the Amazon outflow contained elevated 232Th. This signals one region where the thorium-dust method is inaccurate because of a confounding continental input. Dissolved 232Th fluxes in this region suggest that Amazonian Fe supply to the adjacent open ocean is much larger than local atmospheric deposition. The colloidal content of dissolved Th south of Bermuda was found to be quite small (2-6%), similar to that found north of Hawaii, despite the order of magnitude higher dust deposition in the Atlantic. This finding supports the assumption that dissolved 232Th and 230Th are scavenged at the same rate despite their different sources and also sheds light on the increase of dissolved 232Th fluxes with integrated depth. Outside the region influenced by Amazon River waters, dissolved 232Th fluxes are compared with Bermudan aerosol Fe deposition to estimate that fractional Th solubility is around 20% in this region. Finally, new dissolved and soluble Fe, Mn and Cr data from the subtropical North Pacific support the idea that Fe concentrations in the remote ocean are highly buffered, whereas 232Th has a larger dynamic range between high and low dust regions.
      PubDate: 2017-01-11T15:05:22.293763-05:
      DOI: 10.1002/2016GB005511
  • Upwelling and Primary Production during the U.S. GEOTRACES East Pacific
           Zonal Transect
    • Authors: David Kadko
      Abstract: The 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling region (PCU). In this work the flux of nitrate into the euphotic zone is derived for stations within the PCU using a previously developed method whereby dilution of the water column 7Be inventory by upwelled 7Be-free water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles are used to constrain vertical diffusivity within the upper thermocline. These transport terms are applied to nitrate profiles to estimate net community production between 79oW and 104oW along the EPZT, which includes the zone of active upwelling to the edge of the oligotrophic gyre. With a simple, one-dimensional model the calculated upwelling rates were inversely related to mixed layer temperature and ranged from 0-3.0 m/d. Results using a depth-dependent upwelling rate with a component of horizontal advection are also described. Vertical diffusivities near the base of the euphotic zone were in the range 1.7 - 4.5 x 10-4 m2/s. These values are compared to those generated by analysis of temperature profiles. Net community production averaged 15 mmol C/m2/d for stations between 84oW and 104oW, and was 134 mmol C/m2/d for the furthest inshore station at 79oW which displayed the lowest SST and greatest rate of upwelling.
      PubDate: 2017-01-11T14:50:48.01206-05:0
      DOI: 10.1002/2016GB005554
  • Issue Information
    • First page: 1
      Abstract: No abstract is available for this article.
      PubDate: 2017-02-10T17:43:57.288834-05:
      DOI: 10.1002/gbc.20449
  • Influence of biological carbon export on ocean carbon uptake over the
           annual cycle across the North Pacific Ocean
    • Authors: Hilary I. Palevsky; Paul D. Quay
      First page: 81
      Abstract: We evaluate the influences of biological carbon export, physical circulation, and temperature-driven solubility changes on air-sea CO2 flux across the North Pacific basin (35°N–50°N, 142°E–125°W) throughout the full annual cycle by constructing mixed layer budgets for dissolved inorganic carbon (DIC) and pCO2, determined on fifteen container ship transects between Hong Kong and Long Beach, CA from 2008 to 2012. Annual air-sea CO2 flux is greatest in the western North Pacific and decreases eastward across the basin (2.7 ± 0.9 mol C m-2 yr-1 west of 170°E, as compared to 2.1 ± 0.3 mol C m-2 yr-1 east of 160°W). East of 160°W, DIC removal by annual net community production (NCP) more than fully offsets the DIC increase due to air-sea CO2 flux. However, in the region west of 170°E influenced by deep winter mixing, annual NCP only offsets ~20% of the DIC increase due to air-sea CO2 flux, requiring significant DIC removal by geostrophic advection. Temperature-driven solubility changes have no net influence on pCO2 and account for
      PubDate: 2017-01-05T20:30:21.474642-05:
      DOI: 10.1002/2016GB005527
  • Particle flux in the oceans: Challenging the steady state assumption
    • Authors: Sarah L. C. Giering; Richard Sanders, Adrian P. Martin, Stephanie A. Henson, Jennifer S. Riley, Chris M. Marsay, David Johns
      First page: 159
      Abstract: Atmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption; that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (
      PubDate: 2017-01-02T19:45:45.559175-05:
      DOI: 10.1002/2016GB005424
  • Mechanisms responsible for high N2O emissions from sub-Arctic permafrost
           peatlands studied via stable isotope techniques
    • Authors: J. Gil; T. Perez, K. Boering, P. J. Martikainen, C. Biasi
      First page: 172
      Abstract: Recent field studies have shown that there are habitats in the sub-Arctic tundra emitting N2O at exceptionally high rates. In this study stable isotope techniques were applied to characterize the processes responsible for these high N2O emissions which have been found from bare peat surfaces in permafrost peatlands. The results include the first data on the nitrogen and oxygen isotopic composition of N2O emitted from Arctic tundra. The emission-weighted average δ15Nbulk value for N2O of -13.0 ± 2.0‰ (mean ± s.d; n = 8) from the bare peat surfaces falls within the range of the emission-weighted average values from other natural ecosystems, but is distinct from those for managed/agricultural ecosystems. This implies that if in the future, a smaller rate in the overall decreasing trend of δ15NbulkN2O tropospheric isotopic composition is found, it cannot only be attributed to agricultural N2O emission reductions from mitigation actions but also from soils in natural ecosystems that may be emitting more N2O to the atmosphere due to warmer conditions. The SP values from emitted N2O range from -30‰ to 58‰, indicating a temporal shift of microbial production and consumption of N2O during the sampling period. Soil emission SP data suggest that the N2O emission in sub-Arctic tundra are more likely to be produced by nitrifier denitrification in the relatively drier study year, but due to variable published SP values for N2O production processes in soils, this interpretation has to be taken with caution. According to SP values at depth, denitrification was the main N2O production pathway. To better address the usefulness of SP in partitioning microbial mechanisms in soils further studies in soils mesocosms are required.
      PubDate: 2017-01-05T21:30:22.165009-05:
      DOI: 10.1002/2015GB005370
  • Organic matter remineralization in marine sediments: a Pan-Arctic
    • Authors: Solveig Bourgeois; Philippe Archambault, Ursula Witte
      First page: 190
      Abstract: Climate change in the Arctic is ongoing and causes drastic modification on the ecosystem functioning. In soft-bottom environments, organic matter remineralization is considered an important ecosystem function. Here we provide a large scale assessment of the current knowledge on the benthic organic matter remineralization and its potential response to climate change. Sediment oxygen demand (SOD) values (n = 1154), measured throughout the Arctic, were gathered from 30 publications and 16 databases and nutrient flux values, available in a far lesser extent (n 
      PubDate: 2017-01-05T20:20:26.706277-05:
      DOI: 10.1002/2016GB005378
  • Avoidable impacts of ocean warming on marine primary production: Insights
           from the CESM ensembles
    • Authors: Kristen M. Krumhardt; Nicole S. Lovenduski, Matthew C. Long, Keith Lindsay
      Abstract: As anthropogenic emissions and warming continue to alter Earth's environment, it is essential to highlight future impacts that can be avoided through mitigation. Here, we use two ensembles of the Community Earth System Model (CESM) run under the business-as-usual scenario, RCP 8.5, and the mitigation scenario, RCP 4.5, to identify avoidable impacts of anthropogenic warming on marine net primary production (NPP). We emphasize the use of ensembles so as to distinguish long-term, anthropogenic trends in marine productivity from internal variability. 20th century globally-integrated marine NPP is 55.7 ± 1 Pg C, with much of the variability attributable to certain regions (e.g., the equatorial Pacific). CESM projections indicate that global marine NPP will drop by ∼4% by 2080 if we follow RCP 8.5, but only by 2% under RCP 4.5. The response to warming on a global scale includes compensating regional effects; NPP increases in polar and eastern equatorial Pacific waters, but decreases in the Atlantic, western Pacific, and Indian Ocean. The two main phytoplankton groups simulated in CESM show distinct responses: diatoms decrease their NPP, while small phytoplankton NPP increases over the mid-21st century. Trends in NPP from mid-21st century to 2080 are significantly different between the two emission scenarios mainly in the Atlantic Ocean basin and therefore impacts here are “avoidable” if we follow RCP 4.5, rather than RCP 8.5. In contrast, changes in NPP on a global scale and in most areas of the Pacific and Indian basins and the Southern Ocean are not distinguishable between forcing scenarios.
      PubDate: 2016-12-16T22:30:29.122296-05:
      DOI: 10.1002/2016GB005528
  • Interactions between land use change and carbon cycle feedbacks
    • Authors: Natalie M. Mahowald; James T. Randerson, Keith Lindsay, Ernesto Munoz, Scott C. Doney, Peter Lawrence, Sarah Schlunegger, Daniel S. Ward, David Lawrence, Forrest Hoffman
      Abstract: Using the Community Earth System Model, we explore the role of human land use and land cover change (LULCC) in modifying the terrestrial carbon budget in simulations forced by Representative Concentration Pathway 8.5, extended to year 2300. Overall, conversion of land (e.g. from forest to croplands via deforestation) results in a model-estimated, cumulative carbon loss of 490 PgC between 1850 and 2300, larger than the 230 PgC loss of carbon caused by climate change over this same interval. The LULCC carbon loss is a combination of a direct loss at the time of conversion and an indirect loss from the reduction of potential terrestrial carbon sinks. Approximately 40% of the carbon loss associated with LULCC in the simulations arises from direct human modification of the land surface; the remaining 60% is an indirect consequence of the loss of potential natural carbon sinks. Because of the multi-century carbon cycle legacy of current land use decisions, a globally averaged amplification factor of 2.6 must be applied to 2015 land use carbon losses to adjust for indirect effects. This estimate is 30% higher when considering the carbon cycle evolution after 2100. Most of the terrestrial uptake of anthropogenic carbon in the model occurs from the influence of rising atmospheric CO2 on photosynthesis in trees, and thus model-projected carbon feedbacks are especially sensitive to deforestation.
      PubDate: 2016-12-16T11:00:26.595185-05:
      DOI: 10.1002/2016GB005374
  • Anthropogenic CO2 Accumulation and Uptake Rates in the Pacific Ocean based
           on Changes in the 13C/12C of Dissolved Inorganic Carbon
    • Authors: P. Quay; R. Sonnerup, D. Munro, C. Sweeney
      Abstract: The anthropogenic CO2 accumulation rate for the Pacific Ocean was estimated from the decrease in δ13C of the dissolved inorganic carbon measured on six WOCE cruises during the 1990s and repeated during CLIVAR in the 2000s. A mean depth-integrated anthropogenic δ13C change of -83 ± 20 ‰ m decade-1 was estimated for the basin using the multiple linear regression approach. The largest anthropogenic δ13C decreases occurred between 40°S and 60°S, whereas the smallest decreases occurred in the Southern Ocean and subpolar North Pacific. A mean anthropogenic CO2 accumulation rate of 0.41 ± 0.13 mol C m-2 yr-1 (0.82 ± 0.26 Pg C yr-1) was determined based on observed δ13C changes and is in agreement with previous observation- and model-based estimates. The mean DIC13 inventory change of -178 ± 43 ‰ mol m-2 decade-1 was primarily the result of air-sea CO2 exchange acting on the measured air-sea δ13C disequilibrium of ~ -1.2 ± 0.1 ‰. Regional differences between the DIC13 inventory change and air-sea 13CO2 flux yielded net anthropogenic CO2 uptake rates (independent of ∆pCO2) that ranged from ~0 to 1 mol m-2 yr-1 and basin-wide mean of 1.2 ± 1.5 Pg C yr-1. High rates of surface ocean DIC increase and δ13C decrease observed in the Drake Passage (53°-60°S) support above average anthropogenic CO2 accumulation since 2005. Observed δ13C changes in the Pacific Ocean indicate that ocean transport significantly impacted the anthropogenic CO2 distribution and illustrate the utility of δ13C as a tracer to unravel the processes controlling the present and future accumulation of anthropogenic CO2 in the ocean.
      PubDate: 2016-12-14T12:35:30.984635-05:
      DOI: 10.1002/2016GB005460
  • What proportion of riverine nutrients reaches the open ocean?
    • Authors: Jonathan Sharples; Jack J. Middelburg, Katja Fennel, Timothy D. Jickells
      Abstract: Globally rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported, and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time.
      PubDate: 2016-12-09T06:25:41.431956-05:
      DOI: 10.1002/2016GB005483
  • Two Centuries of Nitrogen Dynamics: Legacy Sources and Sinks in the
           Mississippi and Susquehanna River Basins
    • Authors: K. J. Van Meter; N. B. Basu, P. Van Cappellen
      Abstract: Global flows of reactive nitrogen (N) have increased significantly over the last century in response to agricultural intensification and elevated levels of atmospheric deposition. Despite widespread implementation conservation measures, N concentrations in surface waters are often remaining steady or continuing to increase. Although such lack of response has been attributed to time lags associated with legacy N stores in subsurface reservoirs, it is unclear what the magnitudes of such stores are and how they are partitioned between shallow soil and deeper groundwater reservoirs. Here we have synthesized data to develop a 214-year (1800 – 2014) trajectory of N inputs to the land surface of the continental U.S. We have concurrently developed a parsimonious, process-based model, ELEMeNT that pairs this input trajectory with a travel time-based approach to simulate transport and retention along subsurface pathways. Using the model, we have reconstructed historic nitrate yields at the outlets of two major U.S. watersheds, the Mississippi River Basin (MRB) and Susquehanna River Basin (SRB). Our results show significant N loading above baseline levels in both watersheds before the widespread use of commercial N fertilizers, largely due to the conversion of forest and grassland to row-crop agriculture. Model results also allow us to quantify the magnitudes of legacy N in soil and groundwater pools, and to highlight the dominance of soil legacies in MRB and groundwater legacies in SRB. Approximately 55% and 18% of the current annual N loads in the MRB and SRB were found to be older than 10 years of age.
      PubDate: 2016-12-01T18:21:03.807771-05:
      DOI: 10.1002/2016GB005498
  • Fire and deforestation dynamics in Amazonia (1973-2014)
    • Authors: M. J. E. Marle; R. D. Field, G. R. Werf, I. A. Estrada Wagt, R. A. Houghton, L. V. Rizzo, P. Artaxo, K. Tsigaridis
      Abstract: Consistent long-term estimates of fire emissions are important to understand the changing role of fire in the global carbon cycle and to assess the relative importance of humans and climate in shaping fire regimes. However, there is limited information on fire emissions from before the satellite-era. We show that in the Amazon region, including the Arc of Deforestation and Bolivia, visibility observations derived from weather stations could explain 61% of the variability in satellite-based estimates of bottom-up fire emissions since 1997, and 42% of the variability in satellite-based estimates of total column carbon monoxide concentrations since 2001. This enabled us to reconstruct the fire history of this region since 1973 when visibility information became available. Our estimates indicate that until 1987 relatively few fires occurred in this region and that fire emissions increased rapidly over the 1990s. We found that this pattern agreed reasonably well with forest loss datasets, indicating that although natural fires may occur here deforestation and degradation were the main cause of fires. Compared to fire emissions estimates based on Food and Agricultural Organization's Global Forest and Resources Assessment (FAO-FRA) data, our results were substantially lower up to the 1990s, after which they were more in line. Our visibility-based fire emissions dataset can help constrain dynamic global vegetation models and atmospheric models with a better representation of the anthropogenic fire regime.AGU Index terms: 0305 Aerosols and particles, 0315 Biosphere/atmosphere interactions, 0428 Carbon cycling, 0480 Remote sensing, 1632 Land cover change
      PubDate: 2016-11-16T16:45:26.858339-05:
      DOI: 10.1002/2016GB005445
  • Increasing net ecosystem biomass production of Canada's boreal and
           temperate forests despite decline in dry climates
    • Authors: Robbie A. Hember; Werner A. Kurz, Nicholas C. Coops
      First page: 134
      Abstract: Repeated measurements of tree biomass at field plots describe recovery from disturbances, sampling artifacts, and potential effects of environmental change on forest ecosystems. Challenges in differentiating between intrinsic and extrinsic sources of variation, both in theory and in practice, continues to confound claims of an anthropogenic carbon sink in forest biomass. Here, we analyzed observations at 10,307 plots across southern ecozones of Canada to investigate temporal trends in stand-level biomass growth (G), biomass loss due to mortality (M), and net ecosystem biomass production (NEBP) of intact stands. Net extrinsic forcing (Fex) was expressed by the collective dependence of biomass fluxes on climate anomalies, nitrogen deposition (N), and atmospheric carbon dioxide concentration (C). Inferences drawn directly from linear mixed effects model coefficients only reflect the static behaviour of the model specifically at field plot locations. We, therefore, defined a dynamic landscape-scale net extrinsic forcing (Fex′), which additionally accounted for potential negative feedback responses to anthropogenic growth enhancement. Simulations were performed over 1501-2012 to estimate Fex′. Overall, Fex′ was positive, suggesting that environmental changes drove a 90 percent increase in NEBP. The increase in NEBP was confined to wet regions, while the biomass sink in dry regions decreased, suggesting that large expanses of northern forests, historically located near the boundary between wet and dry climates, may be at high risk of decline under continued increases in evaporative demand. These results have important implications for the greenhouse gas balance of Canada's forest sector.
      PubDate: 2016-12-26T01:50:23.531446-05:
      DOI: 10.1002/2016GB005459
  • Ocean productivity from space: Commentary
    • Authors: Mati Kahru
      First page: 214
      Abstract: Ocean color measurements from satellites have been used to estimate global oceanic productivity for about 30 years but the approach still has many problems. A combination of more sophisticated satellite products with improved models has the potential of higher accuracy but in reality the improvement in accuracy during last two decades has been minimal. Persistent cloud cover over the oceans and low sun elevation over polar areas severely limit the potential of operational satellite ocean color measurements. A combination of remote measurements from both satellites and sub-orbital platforms as well as from a large number of autonomous devices in the ocean can overcome these limitations in the future.
      PubDate: 2016-12-28T16:30:21.450377-05:
      DOI: 10.1002/2016GB005582
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