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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]
  • Vertical modeling of the nitrogen cycle in the eastern tropical South
           Pacific oxygen deficient zone using high-resolution concentration and
           isotope measurements
    • Authors: Brian D. Peters; Andrew R. Babbin, Karsten A. Lettmann, Calvin W. Mordy, O. Ulloa, Bess B. Ward, Karen L. Casciotti
      Abstract: Marine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss processes have been debated, and traditional methods for measuring these rates are labor-intensive and may miss hot spots in spatially and temporally variable environments. Here, we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high-resolution concentration and natural abundance stable isotope measurements of nitrate (NO3-) and nitrite (NO2-). These measurements were used to estimate process rates using a two-step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO3- reduction and NO2- oxidation. Nevertheless, we addressed two questions surrounding the fates of NO2- in the ODZ: 1) Is NO2- being primarily reduced to N2 or oxidized to NO3- in the ODZ? and 2) what are the contributions of anammox and denitrification to NO2- removal? Depth-integrated rates from the model suggest that 72-88% of the NO2- produced in the ODZ was oxidized back to NO3-, while 12-28% of NO2- was reduced to N2. Furthermore, our model suggested that 36-74% of NO2- loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long-term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.
      PubDate: 2016-10-15T14:55:25.827321-05:
      DOI: 10.1002/2016GB005415
  • Multi-decadal Wind-Driven Shifts in Northwest Pacific Temperature,
           Salinity, O2 and PO4
    • Authors: Eun Young Kwon; Young Ho Kim, Young-Gyu Park, Young-Hyang Park, John Dunne, Kyung-Il Chang
      Abstract: The North Pacific gyre boundaries are characterized by stark contrasts in physical and biogeochemical properties. Meridional movement of gyre boundaries, influenced by climate change, can therefore exert a large influence on not only marine ecosystems but also on climate. We examine the evidence for wind-driven southward shifts in subsurface temperature, salinity, PO4, and O2 within the Northwest Pacific from the 1950s to the 2000s. Gyre boundary shifts can explain 30 ~ 60% of temperature and salinity trends zonally averaged in the Northwest Pacific, and observed PO4 and O2 trends along the 137°E and 144°E meridians. The close tie between the wind-driven shifts in gyre boundaries and the tracer distributions is further supported by results from an eddy-resolving (0.1° × 0.1°) GFDL climate model, suggesting that the physical and biogeochemical properties averaged within the Northwest Pacific gyre boundaries closely follow the latitude changes of the zero Sverdrup stream function with lags of zero to three years. The gyre shift effect on tracer distribution is poorly represented in a coarse resolution (1° × 1°) model due partly to poor representations of fronts and eddies. This study suggests that future changes in Northwest Pacific PO4 and O2 content may depend not only on ocean temperature and stratification, but also on the ocean gyre response to winds.
      PubDate: 2016-10-15T13:45:28.599388-05:
      DOI: 10.1002/2016GB005442
  • Interdecadal Trichodesmium variability in cold North Atlantic waters
    • Authors: Sara Rivero-Calle; Carlos E. Del Castillo, Anand Gnanadesikan, Amin Dezfuli, Benjamin Zaitchik, David G. Johns
      Abstract: Studies of the nitrogen cycle in the ocean generally assume that the distribution of the marine diazotroph, Trichodesmium, is restricted to warm, tropical and sub-tropical oligotrophic waters. Here we show evidence that Trichodesmium are widely distributed in the North Atlantic. We report an approximately five-fold increase during the 1980s and 1990s in Trichodesmium presence near the British Isles with respect to the average over the last 50 years. A potential explanation is an increase in the Saharan dust source starting in the 1980s, coupled with changes in North Atlantic winds that opened a pathway for dust transport. Results from a coarse-resolution model in which winds vary but iron deposition is climatologically fixed, suggest frequent nitrogen limitation in the region and reversals of the Portugal current, but it does not simulate the observed changes in Trichodesmium. In addition, modeling results suggest frequent nitrogen limitation in the region and possible reversals of the Portugal current. These factors, when coupled with iron fertilization, could benefit Trichodesmium. Our results suggest that Trichodesmium may be capable of growth at temperatures below 20 oC and challenge assumptions about their latitudinal distribution. Therefore, we need to reevaluate assumptions about the temperature limitations of Trichodesmium and the dinitrogen (N2) fixation capabilities of extra-tropical strains, which may have important implications for the global nitrogen budget.
      PubDate: 2016-10-15T13:20:27.610541-05:
      DOI: 10.1002/2015GB005361
  • Phytoplankton Size Impact on Export Flux in the Global Ocean
    • Authors: Colleen B. Mouw; Audrey Barnett, Galen A. McKinley, Lucas Gloege, Darren Pilcher
      Abstract: Efficiency of the biological pump of carbon to the deep ocean depends largely on biologically mediated export of carbon from the surface ocean and its remineralization with depth. Global satellite studies have primarily focused on chlorophyll concentration and net primary production (NPP) to understand the role of phytoplankton in these processes. Recent satellite retrievals of phytoplankton composition now allow for the size of phytoplankton cells to be considered. Here, we improve understanding of phytoplankton size structure impacts on particle export, remineralization and transfer. A global compilation of particulate organic carbon (POC) flux estimated from sediment traps and 234Th are utilized. Annual climatologies of NPP, percent microplankton, and POC flux at four time series locations and within biogeochemical provinces are constructed. Parameters that characterize POC flux vs. depth (export flux ratio, labile fraction, remineralization length scale) are fit for time series locations, biogeochemical provinces and times of the year dominated by small and large phytoplankton cells where phytoplankton cell size show enough dynamic range over the annual cycle. Considering all data together, our findings support the idea of high export flux but low transfer efficiency in productive regions and vice versa for oligotrophic regions. However, when parsing by dominant size class, we find periods dominated by small cells to have both greater export flux efficiency and lower transfer efficiency than periods when large cells comprise a greater proportion of the phytoplankton community.
      PubDate: 2016-10-14T17:10:55.974888-05:
      DOI: 10.1002/2015GB005355
  • Nitrogen deposition slows down soil sulfur cycling
    • Authors: Hao Chen; Liqiong Yang, Li Wen, Pan Luo, Lu Liu, Yi Yang, Kelin Wang, Dejun Li
      Abstract: Increased atmospheric nitrogen (N) deposition has been found to alter processes and functions of terrestrial ecosystems including the biogeochemical cycling of N and other elements, e.g., phosphorus (P), calcium (Ca) and potassium (K). Nevertheless, how N deposition changes sulfur (S) cycling is largely unknown. Based on a meta-analysis and a lab N addition experiment, here we show that N addition significantly suppresses the activity of soil arylsulfatase, which is a major enzyme involved in the mineralization of organic S. The evidence suggests that N-induced decrease in soil pH is responsible for the decrease of arylsulfatase activity. Soil buffering capacity plays a critical role in mediating the extent of arylsulfatase activity response to N inputs via its regulation on soil pH. Our results suggest that N deposition may slow down S cycling by suppressing soil organic S mineralization.
      PubDate: 2016-10-11T09:10:54.984972-05:
      DOI: 10.1002/2016GB005423
  • Climatic Drivers for Multi-Decadal Shifts in Solute Transport and Methane
           Production Zones within a Large Peat Basin
    • Authors: Paul H. Glaser; Donald I. Siegel, Jeffrey P. Chanton, Andrew S. Reeve, Donald O. Rosenberry, J. Elizabeth Corbett, Zeno Levy
      Abstract: Northern peatlands are an important source for greenhouse gases but their capacity to produce methane remains uncertain under changing climatic conditions. We therefore analyzed a 43-year time series of pore-water chemistry to determine if long-term shifts in precipitation altered the vertical transport of solutes within a large peat basin in northern Minnesota. These data suggest that rates of methane production can be finely tuned to multi-decadal shifts in precipitation that drive the vertical penetration of labile carbon substrates within the Glacial Lake Agassiz Peatlands.Tritium and cation profiles demonstrate that only the upper meter of these peat deposits was flushed by downwardly moving recharge from 1965 through 1983 during a Transitional Dry-to-Moist Period. However, a shift to a moister climate after 1984 drove surface waters much deeper, largely flushing the pore waters of all bogs and fens to depths of 2 m. Labile carbon compounds were transported downward from the rhizosphere to the basal peat at this time producing a substantial enrichment of methane in ∆14C with respect to the solid-phase peat from 1991 to 2008. These data indicate that labile carbon substrates can fuel deep production zones of methanogenesis that more than doubled in thickness across this large peat basin after 1984. Moreover, the entire peat profile apparently has the capacity to produce methane from labile carbon substrates depending on climate-driven modes of solute transport. Future changes in precipitation may therefore play a central role in determining the source strength of peatlands in the global methane cycle.
      PubDate: 2016-10-06T02:36:03.589529-05:
      DOI: 10.1002/2016GB005397
  • Aboveground biomass variability across intact and degraded forests in the
           Brazilian Amazon
    • Authors: Marcos Longo; Michael M. Keller, Maiza N. dos-Santos, Veronika Leitold, Ekena R. Pinagé, Alessandro Baccini, Sassan Saatchi, Euler M. Nogueira, Mateus Batistella, Douglas C. Morton
      Abstract: Deforestation rates have declined in the Brazilian Amazon since 2005, yet degradation from logging, fire, and fragmentation has continued in frontier forests. In this study we quantified the aboveground carbon density (ACD) in intact and degraded forests using the largest data set of integrated forest inventory plots (n = 359) and airborne lidar data (18,000ha) assembled to date for the Brazilian Amazon. We developed statistical models relating inventory ACD estimates to lidar metrics that explained 70% of the variance across forest types. Airborne lidar ACD estimates for intact forests ranged between 5.0 ± 2.5 and 31.9±10.8kgCm−2. Degradation carbon losses were large and persistent. Sites that burned multiple times within a decade lost up to 15.0±0.7kgCm−2(94%) of ACD. Forests that burned nearly 15 years ago had between 4.1 ± 0.5 and 6.8±0.3kgCm−2(22 − 40%) less ACD than intact forests. Even for low-impact logging disturbances, ACD was between 0.7 ± 0.3 to 4.4±0.4kgCm−2(4 − 21%) lower than unlogged forests. Comparing biomass estimates from airborne lidar to existing biomass maps, we found that regional and pan-tropical products consistently overestimated ACD in degraded forests, underestimated ACD in intact forests, and showed little sensitivity to fires and logging. Fine-scale heterogeneity in ACD across intact and degraded forests highlights the benefits of airborne lidar for carbon mapping. Differences between airborne lidar and regional biomass maps underscore the need to improve and update biomass estimates for dynamic land use frontiers, to better characterize deforestation and degradation carbon emissions for regional carbon budgets and REDD+.
      PubDate: 2016-10-03T03:36:25.695017-05:
      DOI: 10.1002/2016GB005465
  • The role of metabolism in modulating CO2 fluxes in boreal lakes
    • Authors: Matthew J. Bogard; Paul A. Giorgio
      Abstract: Lake CO2 emissions are increasingly recognized as an important component of the global CO2 cycle, yet the origin of these emissions is not clear, as specific contributions from metabolism and in-lake cycling, versus external inputs, are not well defined. To assess the coupling of lake metabolism with CO2 concentrations and fluxes, we estimated steady-state ratios of gross primary production to respiration (GPP:R) and rates of net ecosystem production (NEP = GPP-R) from surface water O2 dynamics (concentration and stable isotopes) in 187 boreal lakes spanning long environmental gradients. Our findings suggest internal metabolism plays a dominant role in regulating CO2 fluxes in most lakes, but this pattern only emerges when examined at a resolution that accounts for the vastly differing relationships between lake metabolism and CO2 fluxes: Fluxes of CO2 exceeded those from NEP in over half the lakes, but unexpectedly, these effects were most common and typically largest in a subset (~30% of total) of net autotrophic lakes that nevertheless emitted CO2. Equally surprising, we found no environmental characteristics that distinguished this category from the more common net heterotrophic, CO2 outgassing lakes. Excess CO2 fluxes relative to NEP were best predicted by catchment structure and hydrologic properties, and we infer from a combination of methods that both catchment inputs and internal anaerobic processes may have contributed this excess CO2. Together, our findings show the link between lake metabolism and CO2 fluxes is often strong, but can vary widely across the boreal biome, having important implications for catchment-wide C budgets.
      PubDate: 2016-09-28T00:55:51.303708-05:
      DOI: 10.1002/2016GB005463
  • Analysis of longitudinal variations in North Pacific alkalinity to improve
           predictive algorithms
    • Authors: Claudia H. Fry; Toby Tyrrell, Eric P. Achterberg
      Abstract: The causes of natural variation in alkalinity in the North Pacific surface ocean need to be investigated to understand the carbon cycle and to improve predictive algorithms. We used GLODAPv2 to test hypotheses on the causes of three longitudinal phenomena in Alk*, a tracer of calcium carbonate cycling. These phenomena are: (a) an increase from east to west between 45°N and 55°N, (b) an increase from west to east between 25°N and 40°N, and (c) a minor increase from west to east in the equatorial upwelling region. Between 45°N and 55°N, Alk* is higher on the western than on the eastern side and this is associated with denser isopycnals with higher Alk* lying at shallower depths. Between 25°N and 40°N, upwelling along the North American continental shelf causes higher Alk* in the east. Along the equator, a strong east-west trend was not observed, even though the upwelling on the eastern side of the basin is more intense, because the water brought to the surface is not high in Alk*. We created two algorithms to predict alkalinity, one for the entire Pacific Ocean north of 30°S and one for the eastern margin. The Pacific Ocean algorithm is more accurate than the commonly-used algorithm published by Lee et al. [2006], of similar accuracy to the best previously published algorithm by Sasse et al. [2013], and is less biased with longitude than other algorithms in the subpolar North Pacific. Our eastern margin algorithm is more accurate than previously published algorithms.
      PubDate: 2016-09-28T00:55:48.503362-05:
      DOI: 10.1002/2016GB005398
  • Tracing Dust Input to the Global Ocean using Thorium Isotopes in Marine
           Sediments: ThoroMap
    • Authors: S. S. Kienast; G. Winckler, J. Lippold, S. Albani, N. M. Mahowald
      Abstract: Continental dust input into the ocean-atmosphere system has significant ramifications for biogeochemical cycles and global climate, yet direct observations of dust deposition in the ocean remain scarce. The long-lived isotope thorium-232 (232Th) is greatly enriched in upper continental crust compared to oceanic crust and mid-ocean ridge basalt (MORB)-like volcanogenic material. In open ocean sediments, away from fluvial and ice-rafted sources of continental material, 232Th is often assumed to be of predominantly eolian origin. In conjunction with flux normalization based on the particle reactive radioisotope thorium-230 (230Th), 232Th measurements in marine sediments are a promising proxy for dust accumulation in the modern and past ocean. Here we present ThoroMap, a new global data compilation of 230Th-normalized fluxes of 232Th. After careful screening, we derive dust deposition estimates in the global ocean averaged for the late Holocene (0-4 ka) and the Last Glacial Maximum (LGM, 19-23 ka). ThoroMap is compared with dust deposition estimates derived from CCSM3 and CCSM4, two coupled atmosphere, land, ocean, and sea-ice models. Model-data correlation factors are 0.63 (CCSM3) and 0.59 (CCSM4) in the late Holocene and 0.82 (CCSM3) and 0.83 (CCSM4) in the LGM. ThoroMap is the first compilation that is built on a single, specific proxy for dust and that exclusively uses flux-normalization to derive dust deposition rates.
      PubDate: 2016-09-28T00:55:46.684301-05:
      DOI: 10.1002/2016GB005408
  • Mercury isotope compositions across North American forests
    • Authors: Wang Zheng; Daniel Obrist, Dominique Weis, Bridget A. Bergquist
      Abstract: Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg.The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0) and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere-forest-soil interface.
      PubDate: 2016-09-28T00:55:42.817945-05:
      DOI: 10.1002/2015GB005323
  • Quantifying uncertainty in future ocean carbon uptake
    • Authors: John P. Dunne
      Abstract: Attributing uncertainty in ocean carbon uptake between societal trajectory (scenarios), earth system model construction (structure), and inherent natural variation in climate (internal), is critical to make progress in identifying, understanding and reducing those uncertainties. In the present issue of Global Biogeochemical Cycles, Lovenduski et al. (2016) disentangle these drivers of uncertainty in ocean carbon uptake over time and space and assess the resulting implications for the emergence timescales of structural and scenario uncertainty over internal variability. Such efforts are critical for establishing realizable and efficient monitoring goals and prioritizing areas of continued model development. Under recently proposed climate stabilization targets, such efforts to partition uncertainty also become increasingly critical to societal decision‐making in the context of carbon stabilization.
      PubDate: 2016-09-22T10:35:20.447163-05:
      DOI: 10.1002/2016GB005525
  • Decadal variations and trends of the global ocean carbon sink
    • Authors: Peter Landschützer; Nicolas Gruber, Dorothee C. E. Bakker
      Abstract: We investigate the variations of the ocean CO2 sink during the past three decades using global surface ocean maps of the partial pressure of CO2 reconstructed from observations contained in the Surface Ocean CO2 Atlas Version 2. To create these maps, we used the neural network‐based data‐interpolation method of [Landschützer2014], but extended the work in time from 1998 through 2011 to the period from 1982 through 2011. Our results suggest strong decadal variations in the global ocean carbon sink around a long‐term increase that corresponds roughly to that expected from the rise in atmospheric CO2. The sink is estimated to have weakened during the 1990s toward a minimum uptake of only ‐0.8 ± 0.5 Pg C yr − 1 in 2000, and thereafter to have strengthened considerably to rates of more than ‐2.0 ± 0.5 Pg C yr − 1. These decadal variations originate mostly from the extratropical oceans while the tropical regions contribute primarily to interannual variations. Changes in sea‐surface temperature affecting the solubility of CO2 explain part of these variations, particularly at subtropical latitudes. But most of the higher latitude changes are attributed to modifications in the surface concentration of dissolved inorganic carbon and alkalinity, induced by decadal variations in atmospheric forcing, with patterns that are reminiscent of those of the Northern and Southern Annular Modes. These decadal variations lead to a substantially smaller cumulative anthropogenic CO2 uptake of the ocean over the 1982 through 2011 period (reduction of 7.5 ± 5.5 Pg C) relative to that derived by the Global Carbon Budget.
      PubDate: 2016-09-20T06:55:23.555706-05:
      DOI: 10.1002/2015GB005359
  • The age of iron and iron source attribution in the ocean
    • Authors: Mark Holzer; Marina Frants, Benoît Pasquier
      Abstract: We use tracers to partition dissolved iron (dFe) into the contributions from each source within a numerical model of the iron cycle without perturbing the system. These contributions are further partitioned according to the time since injection into the ocean, which defines their iron‐age spectrum and mean iron age. The utility of these diagnostics is illustrated for a family of inverse‐model estimates of the iron cycle, constrained by a data‐assimilated circulation and available dFe measurements. The source contributions are compared with source anomalies defined as the differences between solutions with and without the source in question. We find that in the Southern Ocean euphotic zone, the hydrothermal and sediment contributions range from 15% to 30% of the total each, which the anomalies underestimate by a factor of  ∼ 2 because of the nonlinearity of scavenging. The iron age is only reset by scavenging and attains a mean of several hundred years in the Southern Ocean euphotic zone, revealing that aeolian iron there is supplied primarily from depth as regenerated dFe. Tagging iron according to source region and pathways shows that 70–80% of the aeolian dFe in the euphotic zone near Antarctica is supplied from north of 46° S via paths that reach below 1 km depth. Hydrothermal iron has the oldest surface mean ages on the order of mid‐depth ventilation times. A measure of uncertainty is provided by the systematic variations of our diagnostics across the family of iron‐cycle estimates, each member of which has a different aeolian source strength.
      PubDate: 2016-09-20T06:41:42.039895-05:
      DOI: 10.1002/2016GB005418
  • A multi‐year estimate of methane fluxes in Alaska from CARVE
           atmospheric observations
    • Authors: Scot M. Miller; Charles E. Miller, Roisin Commane, Rachel Y.‐W. Chang, Steven J. Dinardo, John M. Henderson, Anna Karion, Jakob Lindaas, Joe R. Melton, John B. Miller, Colm Sweeney, Steven C. Wofsy, Anna M. Michalak
      Abstract: Methane (CH4) fluxes from Alaska and other arctic regions may be sensitive to thawing permafrost and future climate change, but estimates of both current and future fluxes from the region are uncertain. This study estimates CH4 fluxes across Alaska for 2012 – 2014 using aircraft observations from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) and a geostatistical inverse model (GIM). We find that a simple flux model based on a daily soil temperature map and a static map of wetland extent reproduces the atmospheric CH4 observations at the state‐wide, multi‐year scale more effectively than global‐scale process‐based models. This result points to a simple and effective way of representing CH4 fluxes across Alaska. It further suggests that process‐based models can improve their representation of key processes, and that more complex processes included in these models cannot be evaluated given the information content of available atmospheric CH4 observations. In addition, we find that CH4 emissions from the North Slope of Alaska account for 24% of the total statewide flux of 1.74  ±  0.26 Tg CH4 (for May – Oct.). Global‐scale process models only attribute an average of 3% of the total flux to this region. This mismatch occurs for two reasons: process models likely underestimate wetland extent in regions without visible surface water, and these models prematurely shut down CH4 fluxes at soil temperatures near 0° C. Lastly, we find that the seasonality of CH4 fluxes varied during 2012 – 2014, but that total emissions did not differ significantly among years, despite substantial differences in soil temperature and precipitation.
      PubDate: 2016-09-15T07:41:25.616771-05:
      DOI: 10.1002/2016GB005419
  • Dissolved iron and iron isotopes in the Southeastern Pacific Ocean
    • Authors: Jessica N. Fitzsimmons; Tim M. Conway, Jong‐Mi Lee, Richard Kayser, Kristen M. Thyng, Seth G. John, Edward A. Boyle
      Abstract: The Southeast Pacific Ocean is a severely understudied yet dynamic region for trace metals such as iron, since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters. In this study, we report the dissolved iron (dFe) distribution from seven stations and Fe isotope ratios (δ56Fe) from three of these stations across a near‐zonal transect from 20‐27°S. We found elevated dFe concentrations associated with the oxygen deficient zone (ODZ), with light δ56Fe implicating reduced Fe porewater fluxes. However, temporal dFe variability and rapid δ56Fe shifts with depth suggest gradients in ODZ Fe source and/or redox processes vary over short depth/spatial scales. The dFe concentrations decreased rapidly offshore, and in the upper ocean dFe was controlled by biological processes, resulting in an Fe:C ratio of 4.2 µmol/mol. Calculated vertical diffusive Fe fluxes were greater than published dust inputs to surface waters, but both were orders of magnitude lower than horizontal diffusive fluxes, which dominate dFe delivery to the gyre. The δ56Fe data in the deep sea showed evidence for a ‐0.2‰ AAIW end‐member and a heavy δ56Fe of +0.55‰ for distally‐transported hydrothermal dissolved Fe from the East Pacific Rise. These heavy δ56Fe values were contrasted with the near‐crustal δ56Fe recorded in the hydrothermal plume reaching Station ALOHA in the North Pacific. The heavy hydrothermal δ56Fe precludes a nanopyrite composition of hydrothermal dFe and instead suggests the presence of oxides or, more likely, binding of hydrothermal dFe by organic ligands in the distal plume.
      PubDate: 2016-09-14T05:05:35.642126-05:
      DOI: 10.1002/2015GB005357
  • The anthropogenic perturbation of the marine nitrogen cycle by atmospheric
           deposition: Nitrogen cycle feedbacks and the 15N Haber‐Bosch effect
    • Authors: Simon Yang; Nicolas Gruber
      Abstract: Over the last 100 years, anthropogenic emissions have led to a strong increase of atmospheric nitrogen deposition over the ocean, yet the resulting impacts and feedbacks are neither well understood nor quantified. To this end, we run a suite of simulations with the ocean component of the Community Earth System Model v1.2 forced with five scenarios of nitrogen deposition over the period from 1850 through 2100, while keeping all other forcings unchanged. Even though global oceanic net primary production increases little in response to this fertilization, the higher export and the resulting expansion of the oxygen minimum zones cause an increase in pelagic and benthic denitrification and burial by about 5%. In addition, the enhanced availability of fixed nitrogen in the surface ocean reduces global ocean N2‐fixation by more than 10%. Despite the compensating effects through these negative feedbacks that eliminate by the year 2000 about 60% of the deposited nitrogen, the anthropogenic nitrogen input forced the upper ocean N‐budget into an imbalance of between 9 to 22 Tg N yr−1 depending on the deposition scenario. The excess nitrogen accumulates to highly detectable levels and causes in most areas a distinct negative trend in the δ15N of the oceanic fixed nitrogen pools ‐ a trend we refer to as the 15N Haber‐Bosch effect. Changes in surface nitrate utilization and the nitrogen feedbacks induce further changes in the δ15N of NO3, making it a good, but complex recorder of the overall impact of the changes in atmospheric deposition.
      PubDate: 2016-08-25T15:01:05.380467-05:
      DOI: 10.1002/2016GB005421
  • Issue Information
    • Pages: 1245 - 1245
      Abstract: No abstract is available for this article.
      PubDate: 2016-10-12T17:12:43.539556-05:
      DOI: 10.1002/gbc.20339
  • Methane Emissions from global rice fields: Magnitude, spatio‐temporal
           patterns and environmental controls
    • Authors: Bowen Zhang; Hanqin Tian, Wei Ren, Bo Tao, Chaoqun Lu, Jia Yang, Kamaljit Banger, Shufen Pan
      First page: 1246
      Abstract: Given the importance of the potential positive feedback between methane (CH4) emissions and climate change, it is critical to accurately estimate the magnitude and spatio‐temporal patterns of CH4 emissions from global rice fields and better understand the underlying determinants governing the emissions. Here, we used a coupled biogeochemical model in combination with satellite‐derived contemporary inundation area to quantify the magnitude and spatio‐temporal variation of CH4 emissions from global rice fields and attribute the environmental controls of CH4 emissions during 1901‐2010. Our study estimated that CH4 emissions from global rice fields varied from 18.3 ± 0.1 Tg CH4/yr (Avg. ± 1 std. dev.) under intermittent irrigation to 38.8 ± 1.0 Tg CH4/yr under continuous flooding in the 2000s, indicating that the magnitude of CH4 emissions from global rice fields was largely dependent on different water schemes. Over the past 110 years, our simulated results showed that global CH4 emissions from rice cultivation increased 85%. The expansion of rice fields was the dominant factor for the increasing trends of CH4 emissions, followed by elevated CO2 concentration, and nitrogen fertilizer use. On the contrary, climate had the negative effect on the cumulative CH4 emissions for most of the years over the study period. Our results imply that CH4 emissions from global rice fields could be reduced through implementing optimized irrigation practices. Therefore, the future magnitude of CH4 emissions from rice fields will be determined by the human demand for rice production as well as the implementation of optimized water management practices.
      PubDate: 2016-09-01T11:05:27.783375-05:
      DOI: 10.1002/2016GB005381
  • Phosphorus transformations along a large‐scale climosequence in arid and
           semi‐arid grasslands of northern China
    • Authors: Jiao Feng; Benjamin L. Turner, Xiaotao Lü, Zhenhua Chen, Kai Wei, Jihui Tian, Chao Wang, Wentao Luo, Lijun Chen
      First page: 1264
      Abstract: The Walker and Syers model of phosphorus (P) transformations during long‐term soil development has been verified along many chronosequences, but has rarely been examined along climosequences, particularly in arid regions. We hypothesized that decreasing aridity would have similar effects on soil P transformations as time by increasing the rate of pedogenesis. To assess this, we examined P fractions in arid and semi‐arid grassland soils along a 3,700 km aridity gradient in northern China (aridity between 0.43 and 0.97, calculated as 1–[mean annual precipitation / potential evapotranspiration]). Primary mineral P declined as aridity decreased, although it still accounted for about 30% of the total P in the wettest sites. In contrast, the proportions of organic and occluded P increased as aridity decreased. These changes in soil P composition occurred in parallel with marked shifts in soil nutrient stoichiometry, with organic carbon:organic P and nitrogen:organic P ratios increasing with decreasing aridity. These results indicate increasing P demand relative to carbon or nitrogen along the climosequence. Overall, our results indicate a broad shift from abiotic to biotic control on P cycling at an aridity threshold of approximately 0.7 (corresponding to about 250 mm mean annual rainfall). We conclude that the Walker and Syers model can be extended to climosequences in arid and semi‐arid ecosystems, and that the apparent decoupling of nutrient cycles in arid soils is a consequence of their pedogenic immaturity.
      PubDate: 2016-09-01T11:05:35.56328-05:0
      DOI: 10.1002/2015GB005331
  • Partitioning uncertainty in ocean carbon uptake projections: Internal
           variability, emission scenario, and model structure
    • Authors: Nicole S. Lovenduski; Galen A. McKinley, Amanda R. Fay, Keith Lindsay, Matthew C. Long
      First page: 1276
      Abstract: We quantify and isolate the sources of projection uncertainty in annual‐mean sea‐air CO2 flux over the period 2006‐2080 on global and regional scales using output from two sets of ensembles with the Community Earth System Model (CESM) and models participating in the 5th Coupled Model Intercomparison Project (CMIP5). For annual‐mean, globally‐integrated sea‐air CO2 flux, uncertainty grows with prediction lead time and is primarily attributed to uncertainty in emission scenario. At the regional scale of the California Current System, we observe relatively high uncertainty that is nearly constant for all prediction lead times, and is dominated by internal climate variability and model structure, respectively in the CESM and CMIP5 model suites. Analysis of CO2 flux projections over 17 biogeographical biomes reveals a spatially heterogenous pattern of projection uncertainty. On the biome scale, uncertainty is driven by a combination of internal climate variability and model structure, with emission scenario emerging as the dominant source for long projection lead times in both modeling suites.
      PubDate: 2016-09-01T11:10:41.740407-05:
      DOI: 10.1002/2016GB005426
  • Century‐long increasing trend and variability of dissolved organic
           carbon export from the Mississippi River basin driven by natural and
           anthropogenic forcing
    • Authors: Wei Ren; Hanqin Tian, Wei‐Jun Cai, Steven E. Lohrenz, Charles S. Hopkinson, Wei‐Jen Huang, Jia Yang, Bo Tao, Shufen Pan, Ruoying He
      First page: 1288
      Abstract: There has been considerable debate as to how natural forcing and anthropogenic activities alter the timing and magnitude of the delivery of dissolved organic carbon (DOC) to the coastal ocean, which has ramifications for the ocean carbon budget, land‐ocean interactions, and coastal life. Here, we present an analysis of DOC export from the Mississippi River to the Gulf of Mexico during 1901‐2010 as influenced by changes in climate, land use and management practices, atmospheric CO2, and nitrogen deposition, through the integration of observational data with a coupled hydrologic‐biogeochemical land model. Model simulations show that DOC export in the 2000s increased more than 40% since the 1900s. For the recent three decades (1981‐2010), however, our simulated DOC export did not show a significant increasing trend, which is consistent with observations by USGS. Our factorial analyses suggest that land use and land cover change, including land management practices (LMPs: i.e., fertilization, irrigation, tillage, etc.), were the dominant contributors to the century‐scale trend of rising total riverine DOC export, followed by changes in atmospheric carbon dioxide, nitrogen deposition, and climate. Decadal and inter‐annual variations of DOC export were largely attributed to year‐to‐year climatic variability and extreme flooding events, which have been exacerbated by human activity. LMPs show incremental contributions to DOC increase since the 1960s, indicating the importance of sustainable agricultural practices in coping with future environmental changes such as extreme flooding events. Compared to the observational‐based estimate, the modeled DOC export was 20% higher, while DOC concentrations were slightly lower. Further refinements in model structure and input datasets should enable reductions in uncertainties in our prediction of century‐long trends in DOC.
      PubDate: 2016-09-05T17:45:46.004861-05:
      DOI: 10.1002/2016GB005395
  • A structural equation model analysis of phosphorus transformations in
           global unfertilized and uncultivated soils
    • Authors: Enqing Hou; Chengrong Chen, Yuanwen Kuang, Yuguang Zhang, Marijke Heenan, Dazhi Wen
      First page: 1300
      Abstract: Understanding the soil phosphorus (P) cycle is a prerequisite for predicting how environmental changes may influence the dynamics and availability of P in soil. We compiled a database of P fractions sequentially extracted by the Hedley procedure and its modification in 626 unfertilized and uncultivated soils worldwide. With this database, we applied structural equation modeling to test hypothetical soil P transformation models and to quantify the importance of different soil P pools and P transformation pathways in shaping soil P availability at a global scale. Our models revealed that soluble inorganic P (Pi, a readily available P pool) was positively and directly influenced by labile Pi, labile organic P (Po), and primary mineral P, and negatively and directly influenced by secondary mineral P; soluble Pi was not directly influenced by moderately labile Po or occluded P. The overall effect on soluble Pi was greatest for labile Pi followed by the organic P pools, occluded P, and then primary mineral P; the overall influence from secondary mineral P was small. Labile Pi was directly linked to all other soil P pools and was more strongly linked than soluble Pi to labile Po and primary mineral P. Our study highlights the important roles of labile Pi in mediating P transformations and in determining overall P availability in soils throughout the world.
      PubDate: 2016-09-17T17:36:14.767436-05:
      DOI: 10.1002/2016GB005371
  • Linking temperature sensitivity of soil CO2 release to substrate,
           environmental and microbial properties across alpine ecosystems
    • Authors: Jinzhi Ding; Leiyi Chen, Beibei Zhang, Li Liu, Guibiao Yang, Kai Fang, Yongliang Chen, Fei Li, Dan Kou, Chengjun Ji, Yiqi Luo, Yuanhe Yang
      First page: 1310
      Abstract: Our knowledge of fundamental drivers of the temperature sensitivity (Q10) of soil carbon dioxide (CO2) release is crucial for improving the predictability of soil carbon dynamics in Earth System Models. However, patterns and determinants of Q10 over a broad geographic scale are not fully understood, especially in alpine ecosystems. Here, we address this issue by incubating surface soils (0‐10 cm) obtained from 156 sites across Tibetan alpine grasslands. Q10 was estimated from the dynamics of the soil CO2 release rate under varying temperatures of 5‐25 oC. Structure equation modeling was performed to evaluate the relative importance of substrate, environmental and microbial properties in regulating the soil CO2 release rate and Q10. Our results indicated that steppe soils had significantly lower CO2 release rates but higher Q10 than meadow soils. The combination of substrate properties and environmental variables could predict 52% of the variation in soil CO2 release rate across all grassland sites, and explained 37% and 58% of the variation in Q10 across the steppe and meadow sites, respectively. Of these, precipitation was the best predictor of soil CO2 release rate. Basal microbial respiration rate (B) was the most important predictor of Q10 in steppe soils, whereas soil pH outweighed B as the major regulator in meadow soils. These results demonstrate that carbon quality and environmental variables co‐regulate Q10 across alpine ecosystems, implying that modelers can rely on the ‘carbon‐quality temperature’ hypothesis for estimating apparent temperature sensitivities, but relevant environmental factors, especially soil pH, should be considered in higher‐productivity alpine regions.
      PubDate: 2016-09-17T17:40:44.5297-05:00
      DOI: 10.1002/2015GB005333
  • Historical variations of mercury stable isotope ratios in arctic glacier
           firn and ice cores.
    • Authors: C. M. Zdanowicz; E. M. Krümmel, A. J. Poulain, E. Yumvihoze, J. Chen, M. Štrok, M. Scheer, H. Hintelmann
      First page: 1324
      Abstract: The concentration and isotopic composition of mercury (Hg) were determined in glacier core samples from Canadian Arctic ice caps dating from pre‐industrial to recent time (early 21st century). Mean Hg levels increased from ≤ 0.2 ng L‐1 in pre‐industrial time to ~0.8‐1.2 ng L‐1 in the modern industrial era (last ~200 years). Hg accumulated on Arctic ice caps has Δ199Hg and Δ201Hg that are higher (~‐1 to 2.9 ‰) than previously reported for Arctic snow (mostly 
      PubDate: 2016-09-20T16:35:35.832207-05:
      DOI: 10.1002/2016GB005411
  • Redistribution of pyrogenic carbon from hillslopes to stream corridors
           following a large montane wildfire
    • Authors: M. Francesca Cotrufo; Claudia M. Boot, Stephanie Kampf, Peter A. Nelson, Daniel J. Brogan, Tim Covino, Michelle L. Haddix, Lee H. MacDonald, Sarah Rathburn, Sandra Ryan‐Bukett, Sarah Schmeer, Ed Hall
      First page: 1348
      Abstract: Pyrogenic carbon (PyC) constitutes a significant fraction of organic carbon in most soils. However PyC soil stocks are generally smaller than what is expected from estimates of PyC produced from fire and decomposition losses, implying that other processes cause PyC loss from soils. Surface erosion has been previously suggested as one such process. To address this, following a large wildfire in the Rocky Mountains (CO, USA), we tracked PyC from the litter layer and soil, through eroded, suspended, and dissolved solids to alluvial deposits along river sides. We separated deposited sediment into high‐ and low‐density fractions to identify preferential forms of PyC transport, and quantified PyC in all samples and density fractions using benzene polycarboxylic acid markers. A few months after the fire, PyC had yet to move vertically into the mineral soil and remained in the organic layer or had been transported off site by rainfall driven overland flow. During major storm events PyC was associated with suspended sediments in river water, and later identified in low‐density riverbank deposits. Flows from an unusually long‐duration and high magnitude rain storm either removed or buried the riverbank sediments approximately one year after their deposition. We conclude that PyC redistributes after wildfire in patterns that are consistent with erosion and deposition of low‐density sediments. A more complete understanding of PyC dynamics requires attention to the interaction of post‐fire precipitation patterns and geomorphological features that control surface erosion and deposition throughout the watershed.Index Terms: Carbon Cycling, Soils, Biogeochemistry.
      PubDate: 2016-09-09T01:37:37.665065-05:
      DOI: 10.1002/2016GB005467
  • Rising atmospheric methane: 2007‐14 growth and isotopic shift.
    • Authors: E. G. Nisbet; E. J. Dlugokencky, M. R. Manning, D. Lowry, R. E. Fisher, J. L. France, S. E. Michel, J. B. Miller, J. W. C. White, B. Vaughn, P. Bousquet, J. A. Pyle, N. J. Warwick, M. Cain, R. Brownlow, G. Zazzeri, M. Lanoisellé, A. C. Manning, E. Gloor, D. E. J. Worthy, E.-G. Brunke, C. Labuschagne, E. W. Wolff, A. L. Ganesan
      First page: 1356
      Abstract: From 2007 to 2013, the globally‐averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr‐1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics: for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies, or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C‐depleted values together with its significant interannual variability, and the tropical and Southern Hemisphere loci of post‐2007 growth, both indicate fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.
      PubDate: 2016-09-27T17:41:46.943872-05:
      DOI: 10.1002/2016GB005406
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