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Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [11 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • 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
  • Historical variations of mercury stable isotope ratios in arctic glacier
           firn and ice cores.
    • 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
  • Decadal variations and trends of the global ocean carbon sink
    • 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
    • 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
  • 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
      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
  • 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
      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
  • A multi‐year estimate of methane fluxes in Alaska from CARVE
           atmospheric observations
    • 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
    • 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
  • Redistribution of pyrogenic carbon from hillslopes to stream corridors
           following a large montane wildfire
    • 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
  • Century‐long increasing trend and variability of dissolved organic
           carbon export from the Mississippi River basin driven by natural and
           anthropogenic forcing
    • 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
  • 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
      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
  • Phosphorus transformations along a large‐scale climosequence in arid and
           semi‐arid grasslands of northern China
    • 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
  • 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
      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
  • 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
    • First page: 1123
      Abstract: No abstract is available for this article.
      PubDate: 2016-09-09T05:18:18.338577-05:
      DOI: 10.1002/gbc.20338
  • Factors regulating the Great Calcite Belt in the Southern Ocean and its
           biogeochemical significance
    • Authors: William M. Balch; Nicholas R. Bates, Phoebe J. Lam, Benjamin S. Twining, Sarah Z. Rosengard, Bruce C. Bowler, Dave T. Drapeau, Rebecca Garley, Laura C. Lubelczyk, Catherine Mitchell, Sara Rauschenberg
      First page: 1124
      Abstract: The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here, we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the sub‐tropical, Agulhas, and sub‐Antarctic frontal regions. Coccolithophore‐rich regions were typically associated with high‐velocity frontal regions with higher seawater partial pressures of CO2 (pCO2) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO2 source. There was no calcium carbonate (CaCO3) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high‐flux particulate inorganic carbon (PIC) regions. Contemporaneous observations are synthesized with results of trace‐metal incubation experiments, 234Th‐based flux estimates, and remotely‐sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.
      PubDate: 2016-08-10T13:15:57.320553-05:
      DOI: 10.1002/2016GB005414
  • Influence of plankton community structure on the sinking velocity of
           marine aggregates
    • Authors: L. T. Bach; T. Boxhammer, A. Larsen, N. Hildebrandt, K. G. Schulz, U. Riebesell
      First page: 1145
      Abstract: About 50 gigatons of carbon are fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25 m deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ø 80 – 400 µm) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity (Pestimated) decreased as the picoautotroph (0.2‐2 µm) fraction of the phytoplankton biomass increased. Thus, picoautotroph‐dominated communities may be indicative for food‐webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500 cells/mL accelerate sinking by about 35‐40% which we estimate (by one‐dimensional modelling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging, both of which are controlled by plankton community structure.
      PubDate: 2016-08-13T17:26:48.467073-05:
      DOI: 10.1002/2016GB005372
  • Oxygen utilization rate (OUR) underestimates ocean respiration – a
           model study
    • First page: 1166
      Abstract: We use a simple 1D model representing an isolated density surface in the ocean and 3D global ocean biogeochemical models to evaluate the concept of computing the subsurface oceanic oxygen utilization rate (OUR) from the changes of apparent oxygen utilization (AOU) and water age. The distribution of AOU in the ocean is not only the imprint of respiration in the ocean's interior, but is strongly influenced by transport processes and eventually loss at the ocean surface. Since AOU and water age are subject to advection and diffusive mixing, it is only when they are affected both in the same way that OUR represents the correct rate of oxygen consumption. This is the case only when advection prevails or with uniform respiration rates, when the proportions of AOU and age are not changed by transport. In experiments with the 1D‐tube model, OUR underestimates respiration when maximum respiration rates occur near the outcrops of isopycnals, and overestimates when maxima occur far from the outcrops. Given the distribution of respiration in the ocean, i.e. elevated rates near high latitude outcrops of isopycnals and low rates below the oligotrophic gyres, underestimates are the rule. Integrating these effects globally in three coupled ocean biogeochemical and circulation models we find that AOU‐over‐age based calculations underestimate true model respiration by a factor of three. Most of this difference is observed in the upper 1000 m of the ocean with the discrepancies increasing towards the surface where OUR underestimates respiration by as much as factor of four.
      PubDate: 2016-08-13T17:15:57.267959-05:
      DOI: 10.1002/2015GB005354
  • Nitrate uptake across biomes and the influence of elemental stoichiometry:
           A new look at LINX II
    • First page: 1183
      Abstract: Considering recent increases in anthropogenic N loading it is essential to identify the controls on N removal and retention in aquatic ecosystems because the fate of N has consequences for water quality in streams and downstream ecosystems. Biological uptake of nitrate (NO3‐) is a major pathway by which N is removed from these ecosystems. Here we used data from the second Lotic Intersite Nitrogen eXperiment (LINX II) in a multivariate analysis to identify the primary drivers of variation in NO3‐ uptake velocity among biomes. Across 69 study watersheds in North America, DOC:NO3‐ ratios and photosynthetically active radiation were identified as the two most important predictor variables in explaining NO3‐ uptake velocity. However, within a specific biome the predictor variables of NO3‐ uptake velocity varied, and included various physical, chemical and biological attributes. . Our analysis demonstrates the broad control of elemental stoichiometry on NO3‐ uptake velocity as well as the importance of biome‐specific predictors. Understanding this spatial variation has important implications for biome‐specific watershed management and the downstream export of NO3‐, as well as for development of spatially explicit global models that describe N dynamics in streams and rivers.
      PubDate: 2016-08-13T17:31:20.611741-05:
      DOI: 10.1002/2016GB005468
  • Partitioning N2O Emissions within the US Corn Belt using an Inverse
           Modeling Approach
    • Authors: Zichong Chen; Timothy J. Griffis, Dylan B. Millet, Jeffrey D. Wood, Xuhui Lee, John M. Baker, Ke Xiao, Peter A. Turner, Ming Chen, John Zobitz, Kelley C. Wells
      First page: 1192
      Abstract: Nitrous oxide (N2O) emissions within the US Corn Belt have been previously estimated to be 200‐900% larger than predictions from emission inventories, implying that one or more source categories in bottom‐up approaches are underestimated. Here we interpret hourly N2O concentrations measured during 2010 and 2011 at a tall tower using a time‐inverted transport model and a scale factor Bayesian inverse method to simultaneously constrain direct and indirect agricultural emissions. The optimization revealed that both agricultural source categories were underestimated by the Intergovernmental Panel on Climate Change (IPCC) inventory approach. However, the magnitude of the discrepancies differed substantially, ranging from 42–58% and 200–525% for direct and indirect components, respectively. Optimized agricultural N2O budgets for the Corn Belt were 319 ± 184 (total), 188 ± 66 (direct), and 131 ± 118 Gg‐N yr‐1 (indirect) in 2010, versus 471 ± 326, 198 ± 80, and 273 ± 246 Gg‐N yr‐1 in 2011. We attribute the inter‐annual differences to varying moisture conditions, with increased precipitation in 2011 amplifying emissions. We found that indirect emissions represented 41–58% of the total agricultural budget, a considerably larger portion than the 25–30% predicted in bottom‐up inventories, further highlighting the need for improved constraints on this source category. These findings further support the hypothesis that indirect emissions are presently underestimated in bottom‐up inventories. Based on our results, we suggest an indirect emission factor for runoff and leaching ranging from 0.014–0.035 for the Corn Belt, which represents an upward adjustment of 1.9–4.6 times relative to the IPCC and is in agreement with recent bottom‐up field studies.
      PubDate: 2016-08-24T18:55:29.376875-05:
      DOI: 10.1002/2015GB005313
  • Quantifying Mesoscale‐Driven Nitrate Supply: A Case Study
    • Authors: Rosalind E. M. Pidcock; Adrian P. Martin, Stuart. C. Painter, John T. Allen, Meric A. Srokosz, Alex Forryan, Mark Stinchcombe, David A. Smeed
      First page: 1206
      Abstract: The supply of nitrate to surface waters plays a crucial role in maintaining marine life. Physical processes at the mesoscale (~10‐100 km) and smaller have been advocated to provide a major fraction of the global supply. Whilst observational studies have focussed on well‐defined features, such as isolated eddies, the vertical circulation and nutrient supply in a typical 100‐200 km square of ocean will involve a turbulent spectrum of interacting, evolving and decaying features. A crucial step in closing the ocean nitrogen budget is to be able to rank the importance of mesoscale fluxes against other sources of nitrate for surface waters for a representative area of open ocean. While this has been done using models, the vital observational equivalent is still lacking.To illustrate the difficulties that prevent us from putting a global estimate on the significance of the mesoscale observationally, we use data from a cruise in the Iceland Basin where vertical velocity and nitrate observations were made simultaneously at the same high spatial resolution. Local mesoscale nitrate flux is found to be an order of magnitude greater than that due to small‐scale vertical mixing and exceeds coincident nitrate uptake rates and estimates of nitrate supply due to winter convection. However, a non‐zero net vertical velocity for the region introduces a significant bias in regional estimates of the mesoscale vertical nitrate transport. The need for synopticity means that a more accurate estimate can not be simply found by using a larger survey area. It is argued that time‐series, rather than spatial surveys, may be the best means to quantify the contribution of mesoscale processes to the nitrate budget of the surface ocean.
      PubDate: 2016-08-29T01:36:50.899318-05:
      DOI: 10.1002/2016GB005383
  • Sources of uncertainties in 21st century projections of potential ocean
           ecosystem stressors
    • First page: 1224
      Abstract: Future projections of potential ocean ecosystem stressors, such as acidification, warming, deoxygenation and changes in ocean productivity, are uncertain due to incomplete understanding of fundamental processes, internal climate variability, and divergent carbon emissions scenarios. This complicates climate change impact assessments. We evaluate the relative importance of these uncertainty sources in projections of potential stressors as a function of projection lead‐time and spatial scale. Internally generated climate variability is the dominant source of uncertainty in mid‐to‐low latitudes and in most coastal Large Marine Ecosystems over the next few decades, suggesting irreducible uncertainty inherent in these short projections. Uncertainty in projections of century‐scale global sea surface temperature (SST), global thermocline oxygen, and regional surface pH is dominated by scenario uncertainty, highlighting the critical importance of policy decisions on carbon emissions. In contrast, uncertainty in century‐scale projections of net primary productivity (NPP), low oxygen waters, and Southern Ocean SST is dominated by model uncertainty, underscoring the importance of overcoming deficiencies in scientific understanding and improved process representation in Earth system models are critical for making more robust projections of these potential stressors. We also show that changes in the combined potential stressors emerge from the noise in 39% (34 – 44%) of the ocean by 2016‐2035 relative to the 1986‐2005 reference period and in 54% (50 – 60%) of the ocean by 2076‐2095 following a high carbon emissions scenario. Projected large changes in surface pH and SST can be reduced substantially and rapidly with aggressive carbon emission mitigation, but only marginally for oxygen. The regional importance of model uncertainty and internal variability underscores the need for expanded and improved multi‐model and large initial condition ensemble projections with Earth system models for evaluating regional marine resource impacts.
      PubDate: 2016-08-31T17:51:14.364207-05:
      DOI: 10.1002/2015GB005338
  • Oceanic teleconnection for carbon dioxide
    • Authors: Takamitsu Ito
      First page: 1244
      Abstract: Biogeochemical teleconnection links seemingly unrelated chemical/biological anomalies that are geographically separated by large distances. Bronselaer et al propose a new mechanism for an interhemispheric teleconnection of air‐sea carbon dioxide fluxes in which the upwelling of the Southern Ocean triggers a series of perturbations leading to the alteration of the carbon uptake in the North Atlantic.The westerly wind over the Southern Ocean has a unique role in the climate system. It energizes the strongest ocean current, Antarctic Circumpolar Current, and it lifts up the carbon‐ and nutrient‐rich deep waters all the way to the surface. It is an end point of the ocean's deep overturning circulation and associated biological carbon storage, where the excess carbon from accumulated decomposition of organic material is finally released back into the atmosphere. It is well established that the Southern Ocean upwelling regionally modulates the de‐gassing of carbon dioxide there. However, its global‐scale implication is not yet fully understood. What happens to the carbon uptake in the other parts of the oceans'In this volume of Global Biogeochemical Cycles, Bronselaer et al describes the chain of events that link the increased Southern Ocean wind to the ocean carbon uptake in the northern high latitudes. The authors conducted a set of computational experiments, showing that the Southern Ocean is a starting point of the oceanic teleconnection, where the excess nutrient is transported equatorward through the shallow overturning circulation. The stream of macro‐nutrient then fertilizes the low‐latitude productivity that eventually shifts the carbonate chemistry of the high latitude surface waters. This is an intriguing case of oceanic teleconnection, linking seemingly unrelated biogeochemical anomalies that are geographically separated by large distances.The surprising conclusion is that a stronger Southern Ocean wind increases the de‐gassing of carbon dioxide in both northern and southern high latitudes. This happens because more carbon is upwelling into the northern high latitudes due to the increased low‐latitude biological pump, approximately doubling the de‐gassing intensity relative to the Southern Ocean response alone. There may be more surprises from the Southern Ocean.
      PubDate: 2016-08-08T05:21:22.418969-05:
      DOI: 10.1002/2016GB005461
  • Rising atmospheric methane: 2007‐14 growth and isotopic shift.
    • 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.
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