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Geophysical Research Letters     Full-text available via subscription   (Followers: 80, SJR: 3.493, h-index: 157)
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Journal Cover Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [11 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Origins, Seasonality and Fluxes of Organic Matter in the Congo River
    • Authors: Robert G. M. Spencer; Peter J. Hernes, Bienvenu Dinga, Jose N. Wabakanghanzi, Travis W. Drake, Johan Six
      Abstract: The Congo River in central Africa represents a major source of organic matter (OM) to the Atlantic Ocean. This study examined elemental (%OC, %N, C:N), stable isotopic (δ13C, δ15N) and biomarker composition (lignin phenols) of particulate OM (POM) and dissolved OM (DOM) across the seasonal hydrograph. Even though the Congo exhibits an extremely stable intraannual discharge regime, seasonal variability in OM composition was evident. DOM appears predominantly derived from vascular plant inputs with greater relative contribution during the rising limb and peak in discharge associated with the major November‐December discharge maximum. Generally, POM appears to be sourced from soil‐derived mineral‐associated OM (low C:N, low Λ8, higher (Ad:Al)v) but the relative proportion of fresh vascular plant material (higher C:N, higher Λ8, lower (Ad:Al)v) increases with higher discharge. During the study period (September 2009‐November 2010) the Congo exported 29.21 Tg yr‐1 of total suspended sediment (TSS), 1.96 Tg yr‐1 of particulate organic carbon (POC) and 12.48 Tg yr‐1 of dissolved organic carbon (DOC). The Congo exports an order of magnitude lower TSS load in comparison to other major riverine sources of TSS (e.g. Ganges, Brahmaputra), but due to its OM rich character actually exports a comparable amount of POC. The Congo is also 2.5 times more efficient at exporting dissolved lignin per unit volume compared to the Amazon. Including Congo dissolved lignin data in residence time calculations for lignin in the Atlantic Ocean results in an approximately 10% reduction from the existing estimate, suggesting this material is more reactive than previously thought.
      PubDate: 2016-07-05T08:10:41.770855-05:
      DOI: 10.1002/2016GB005427
  • Carbon fate in a large temperate human‐impacted river system: focus
           on benthic dynamics
    • Authors: Lauriane Vilmin; Nicolas Flipo, Nicolas Escoffier, Vincent Rocher, Alexis Groleau
      Abstract: Fluvial networks play an important role in regional and global carbon (C) budgets. The Seine River, from the Paris urban area to the entrance of its estuary (220 km), is studied here as an example of a large human‐impacted river system subject to temperate climatic conditions. We assess organic C (OC) budgets up‐ and downstream from one of the world's largest waste water treatment plants and for different hydrological conditions using a hydro‐biogeochemical model. The fine representation of sediment accumulation on the river bed allows for the quantification of pelagic and benthic effects on OC export towards the estuary and on river metabolism (i.e. net CO2 production). OC export is significantly affected by benthic dynamics during the driest periods, when 25 % of the inputs to the system is transformed or stored in the sediment layer. Benthic processes also substantially affect river metabolism under any hydrological condition. On average, benthic respiration accounts for one third of the total river respiration along the studied stretch (0.27 out of 0.86 gC·m−2·d−1). Even though the importance of benthic processes was already acknowledged by the scientific community for headwater streams, these results stress the major influence of benthic dynamics, and thus of physical processes such as sedimentation and re‐suspension, on C cycling in downstream river systems. It opens the door to new developments in the quantification of C emissions by global models, whereby biogeochemical processing and benthic dynamics should be taken into account.
      PubDate: 2016-06-28T21:40:25.35896-05:0
      DOI: 10.1002/2015GB005271
  • Enzyme‐level Interconversion of Nitrate and Nitrite in the Fall Mixed
           Layer of the Antarctic Ocean
    • Authors: P.C. Kemeny; M. A. Weigand, R. Zhang, B.R. Carter, K.L. Karsh, S.E. Fawcett, D.M. Sigman
      Abstract: In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO3‐) have been used to investigate NO3‐ assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the 15N/14N (δ15N) and 18O/16O (δ18O) of NO3‐ and nitrite (NO2‐) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ15N and δ18O of NO3‐ + NO2‐ increase roughly equally, as expected for NO3‐ assimilation; however, the δ15N of NO3‐‐only (measured after NO2‐ removal) increases more than NO3‐‐only δ18O. Differencing indicates that NO2‐ has an extremely low δ15N, often 
      PubDate: 2016-06-20T05:15:26.900053-05:
      DOI: 10.1002/2015GB005350
  • Changes in anthropogenic nitrogen and phosphorus inputs to the St.
           Lawrence sub‐basin over 110 years and impacts on riverine export
    • Abstract: Human activities have increased the flow of nitrogen (N) and phosphorus (P) over much of the Earth, leading to increased agricultural production, but also the degradation of air, soil, and water quality. Here, we quantify the sources of anthropogenic N and P inputs to 76 watersheds of the St. Lawrence Basin (SLB) throughout the 20th century using NANI/NAPI (net anthropogenic N/P input to watersheds), a mass balance modeling approach, and estimate the fraction of these inputs exported to adjacent rivers. Our results show that since 1901, NANI and NAPI increased 4.5‐ and 3.8‐fold respectively with a peak in 1991 mainly due to high atmospheric N deposition and P fertilizer application. However the relative increase over the course of the last century was much higher in certain watersheds, particularly those where there was greater urbanization. Ranges in NANI and NAPI vary greatly among watersheds (110 to 9,351 kg N km‐2 yr‐1 and 0.16 to 1,938 kg P km‐2 yr‐1, respectively in 2011) and are strongly related to riverine fluxes (R2 = 0.87 and 0.71 for N and P, respectively). Our results suggest that 22% of NANI (ranging from 11% to 68% across watersheds) and 17% of NAPI (ranging from 3% to 173%) are exported to rivers. Predominant sources of inputs vary spatially and through time largely due to changes in farming practices. By tracking the main sources of inputs to specific watersheds and through time, our work provides insights for N and P management. Reduction strategies will likely need to be watershed specific, although through time, our results clearly show the large‐scale impact of targeted legislation.
      PubDate: 2016-06-15T00:30:38.217089-05:
      DOI: 10.1002/2016GB005384
  • Variability in the sensitivity among model simulations of permafrost and
           carbon dynamics in the permafrost region between 1960 and 2009
    • Abstract: A significant portion of large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO2 and CH4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near‐surface permafrost (within 3 m) area over the region, but there are large differences in the magnitude of the simulated rates of loss among the models (0.2 to 58.8 x 103 km2 y‐1). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954 Tg C y‐1 between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982‐2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. To improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback, and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.
      PubDate: 2016-06-15T00:30:33.346489-05:
      DOI: 10.1002/2016GB005405
  • Quantifying the drivers of ocean‐atmosphere CO2 fluxes
    • Authors: Jonathan M. Lauderdale; Stephanie Dutkiewicz, Richard G. Williams, Michael J. Follows
      Abstract: A mechanistic framework for quantitatively mapping the regional drivers of air–sea CO2 fluxes at a global scale is developed. The framework evaluates the interplay between: (1) surface heat and freshwater fluxes that influence the potential saturated carbon concentration, which depends on changes in sea surface temperature, salinity and alkalinity, (2) a residual, disequilibrium flux influenced by upwelling and entrainment of remineralized carbon‐ and nutrient‐rich waters from the ocean interior, as well as rapid subduction of surface waters, (3) carbon uptake and export by biological activity as both soft tissue and carbonate, and (4) the effect on surface carbon concentrations due to freshwater precipitation or evaporation. In a steady state simulation of a coarse resolution ocean circulation and biogeochemistry model, the sum of the individually determined components is close to the known total flux of the simulation. The leading order balance, identified in different dynamical regimes, is between the CO2 fluxes driven by surface heat fluxes and a combination of biologically‐driven carbon uptake and disequilibrium‐driven carbon outgassing. The framework is still able to reconstruct simulated fluxes when evaluated using monthly‐averaged data and takes a form that can be applied consistently in models of different complexity and observations of the ocean. In this way, the framework may reveal differences in the balance of drivers acting across an ensemble of climate model simulations or be applied to an analysis and interpretation of the observed, real‐world air–sea flux of CO2.
      PubDate: 2016-06-11T00:50:31.538076-05:
      DOI: 10.1002/2016GB005400
  • Eroding permafrost coasts release low amounts of dissolved organic carbon
           (DOC) from ground ice into the nearshore zone of the Arctic Ocean
    • Authors: George Tanski; Nicole Couture, Hugues Lantuit, Antje Eulenburg, Michael Fritz
      Abstract: Ice‐rich permafrost coasts in the Arctic are highly sensitive to climate warming and erode at a pace that exceeds the global average. Permafrost coasts deliver vast amounts of organic carbon into the nearshore zone of the Arctic Ocean. Numbers on flux exist for particulate and total soil organic carbon (POC and TOC). However, they do not exist for dissolved organic carbon (DOC), which is known to be highly bioavailable. This study aims to estimate DOC stocks in coastal permafrost as well as the annual flux into the ocean. DOC concentrations in ground ice were analyzed along the ice‐rich Yukon coast (YC) in the western Canadian Arctic. The annual DOC flux was estimated using available numbers for coast length, cliff height, annual erosion rate, and volumetric ice content in different stratigraphic horizons. Our results showed that DOC concentrations in ground ice range between 0.3 and 347.0 mg L‐1 with an estimated stock of 13.6 ± 3.0 g m‐3 along the YC. An annual DOC flux of 54.9 ± 0.9 Mg yr‐1 was computed. These DOC fluxes are low compared to POC fluxes from coastal erosion or POC and DOC fluxes from Arctic rivers. We conclude that DOC fluxes from permafrost coasts play a secondary role in the Arctic carbon budget. However, this DOC is assumed to be highly bioavailable. We hypothesize that DOC from coastal erosion is important for ecosystems in the Arctic nearshore zones, particularly in summer when river discharge is low, and in areas where rivers are absent.
      PubDate: 2016-06-01T21:40:26.783341-05:
      DOI: 10.1002/2015GB005337
  • Benthic marine calcifiers coexist with CaCO3‐undersaturated seawater
    • Abstract: Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO3) minerals have raised concerns about the consequences to marine organisms that build CaCO3 structures. A large proportion of benthic marine calcifiers incorporate Mg2+ into their skeletons (Mg‐calcite), which in general, reduces mineral stability. The relative vulnerability of some marine calcifiers to ocean acidification appears linked to the relative solubility of their shell or skeletal mineralogy, although some organisms have sophisticated mechanisms for constructing and maintaining their CaCO3 structures causing deviation from this dependence. Nevertheless, few studies consider seawater saturation state with respect to the actual Mg‐calcite mineralogy (ΩMg‐x) of a species when evaluating the effect of ocean acidification on that species. Here, a global dataset of skeletal mole % MgCO3 of benthic calcifiers and in situ environmental conditions spanning a depth range of 0 m (subtidal/neritic) to 5600 m (abyssal) was assembled to calculate in situ ΩMg‐x. This analysis shows that 24% of the studied benthic calcifiers currently experience seawater mineral undersaturation (ΩMg‐x 
      PubDate: 2016-05-27T13:30:27.667182-05:
      DOI: 10.1002/2015GB005260
  • Amazon forest response to repeated droughts
    • Abstract: The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin‐wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin‐wide ground‐based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground‐based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha‐1, CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long‐term biomass sink during the baseline pre‐2010 drought period (1998 − pre‐2010) of 1.33 Mg ha‐1 yr‐1 (CI: 0.90, 1.74, p 
      PubDate: 2016-04-30T07:57:00.036414-05:
      DOI: 10.1002/2015GB005133
  • The anthropogenic perturbation of the marine nitrogen cycle by atmospheric
           deposition: Nitrogen cycle feedbacks and the 15N Haber‐Bosch effect
    • 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.
  • 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 
  • Linking temperature sensitivity of soil CO2 release to substrate,
           environmental and microbial properties across alpine ecosystems
    • 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.
  • A structural equation model analysis of phosphorus transformations in
           global unfertilized and uncultivated soils
    • 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.
  • Sources of uncertainties in 21st century projections of potential ocean
           ecosystem stressors
    • 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.
  • 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.
  • Partitioning uncertainty in ocean carbon uptake projections: Internal
           variability, emission scenario, and model structure
    • 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.
  • 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.
  • Methane Emissions from global rice fields: Magnitude, spatio‐temporal
           patterns and environmental controls
    • 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.
  • Partitioning N2O Emissions within the US Corn Belt using an Inverse
           Modeling Approach
    • 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.
  • Issue Information
    • Abstract: No abstract is available for this article.
  • Factors regulating the Great Calcite Belt in the Southern Ocean and its
           biogeochemical significance
    • 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.
  • Nitrate uptake across biomes and the influence of elemental stoichiometry:
           A new look at LINX II
    • 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.
  • Oceanic teleconnection for carbon dioxide
    • 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.
  • Oxygen utilization rate (OUR) underestimates ocean respiration – a
           model study
    • 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.
  • Quantifying Mesoscale‐Driven Nitrate Supply: A Case Study
    • 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.
  • Influence of plankton community structure on the sinking velocity of
           marine aggregates
    • 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.
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
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