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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 22, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 50, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
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J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 23)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 22)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.527, h-index: 47)
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Tectonics     Full-text available via subscription   (Followers: 8, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 198, SJR: 1.769, h-index: 110)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 2.4]   [H-I: 109]   [7 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Atmospheric iron deposition in the Northwestern Pacific Ocean and its
           adjacent marginal seas: The importance of coal burning
    • Authors: Yi‐Chiu Lin; Jen‐Ping Chen, Tung‐Yuan Ho, I‐Chun Tsai
      Abstract: This study applied a regional air‐quality model, incorporated with an emission module, to quantitatively differentiate the atmospheric iron sources originating from lithogenic dusts or coal‐burning fly ashes deposited in the Northwest Pacific Ocean and its marginal seas. Particular attention was paid to the high iron content of fly ashes emitted from steel and iron plants burning coals. Using the year 2007 as an example, the modeling results exhibit large seasonal variations in iron deposition, with highest deposition fluxes occurred during spring and autumn, which are comparable to the seasonal fluctuation of chlorophyll a concentrations estimated by satellite images in the oceanic regions. Fly ash from coal burning accounted for 7.2% of the total iron deposited over the Northwest Pacific Ocean and 15% of that over the northern South China Sea. After considering the difference of iron solubility in the aerosols, anthropogenic aerosol associated with coal burning would be the major bioavailable iron source in the surface water of the oceanic regions.
      PubDate: 2014-12-19T08:59:32.577783-05:
      DOI: 10.1002/2013GB004795
  • Predicting the long‐term fate of buried organic carbon in colluvial
    • Authors: Zhengang Wang; Kristof Van Oost, Gerard Govers
      Abstract: A significant part of the soil organic carbon that is eroded in uplands is deposited and buried in colluvial settings. Understanding the fate of this deposited soil organic carbon (SOC) is of key importance for the understanding of the role of (accelerated) erosion in the global C cycle: the residence time of the deposited carbon will determine if, and for how long, accelerated erosion due to human disturbance will induce sequestration of SOC from the atmosphere to the soil. Experimental studies may provide useful information, but, given the time scale under consideration, the response of the colluvial SOC can only be simulated using numerical models which need careful calibration using field data. In this study, we present a depth explicit SOC model (ICBM‐DE) including soil profile evolution due to sedimentation to simulate the long‐term C dynamics in colluvial soils. The SOC profile predicted by our model is in good agreement with field observations. The C burial efficiency (the ratio of current C content of the buried sediments to the original C content at the time of sedimentation) of deposited sediments exponentially decreases with time and gradually reached an equilibrium value. This equilibrium C burial efficiency is positively correlated with the sedimentation rate. The sedimentation rate is crucial for the long‐term dynamics of the deposited SOC as it controls the time that buried sediments spend at a given soil depth, thereby determining its temporal evolution of C input and decomposition rate during the burial process: C input and decomposition rate vary with depth due to the vertical variation of root distribution and soil environmental factors such as (but not limited to) humidity, temperature and aeration. The model demonstrates that, for the profiles studied, it takes ca. 300 yr for the buried SOC to lose half of its C load. It would also take centuries for the SOC accumulated in colluvial soils over the past decades due to soil redistribution under mechanized agriculture to be released to the atmosphere after the application of soil conservation measures such as conservation tillage.
      PubDate: 2014-12-15T00:39:42.891303-05:
      DOI: 10.1002/2014GB004912
  • Variability in efficiency of particulate organic carbon export: A model
    • Authors: Stephanie A. Henson; Andrew Yool, Richard Sanders
      Abstract: The flux of organic carbon from the surface ocean to mesopelagic depths is a key component of the global carbon cycle and is ultimately derived from primary production (PP) by phytoplankton. Only a small fraction of organic carbon produced by PP is exported from the upper ocean, referred to as the export efficiency (herein e‐ratio). Limited observations of the e‐ratio are available and there is thus considerable interest in using remotely‐sensed parameters such as sea surface temperature to extrapolate local estimates to global annual export flux. Currently, there are large discrepancies between export estimates derived in this way; one possible explanation is spatial or temporal sampling bias in the observations. Here we examine global patterns in the spatial and seasonal variability in e‐ratio and the subsequent effect on export estimates using a high resolution global biogeochemical model. NEMO‐MEDUSA represents export as separate slow and fast sinking detrital material whose remineralisation is respectively temperature dependent and a function of ballasting minerals. We find that both temperature and the fraction of export carried by slow sinking particles are factors in determining e‐ratio, suggesting that current empirical algorithms for e‐ratio that only consider temperature are overly simple. We quantify the temporal lag between PP and export, which is greatest in regions of strong variability in PP where seasonal decoupling can result in large e‐ratio variability. Extrapolating global export estimates from instantaneous measurements of e‐ratio is strongly affected by seasonal variability, and can result in errors in estimated export of up to ±60%.
      PubDate: 2014-12-11T08:07:02.3004-05:00
      DOI: 10.1002/2014GB004965
  • Dimethyl Sulfide in the Amazon Rain Forest
    • Authors: K. Jardine; A.M. Yañez‐Serrano, J. Williams, N. Kunert, A. Jardine, T. Taylor, L. Abrell, P. Artaxo, A. Guenther, C.N. Hewitt, E. House, A. P. Florentino, A. Manzi, N. Higuchi, J. Kesselmeier, T. Behrendt, P. R. Veres, B. Derstroff, J. D. Fuentes, S. Martin, M. O. Andreae
      Abstract: Surface‐to‐atmosphere emissions of dimethyl sulfide (DMS) may impact global climate through the formation of gaseous sulfuric acid, which can yield secondary sulfate aerosols and contribute to new particle formation. While oceans are generally considered the dominant source of DMS, a shortage of ecosystem observations prevents an accurate analysis of terrestrial DMS sources. Using mass spectrometry, we quantified ambient DMS mixing ratios within and above a primary rainforest ecosystem in the central Amazon Basin in real‐time (2010–2011) and at high vertical resolution (2013–2014). Elevated but highly variable DMS mixing ratios were observed within the canopy, showing clear evidence of a net ecosystem source to the atmosphere during both day and night in both the dry and wet seasons. Periods of high DMS mixing ratios lasting up to 8 hours (up to 160 ppt) often occurred within the canopy and near the surface during many evenings and nights. Daytime gradients showed mixing ratios (up to 80 ppt) peaking near the top of the canopy as well as near the ground following a rain event. The spatial and temporal distribution of DMS suggests that ambient levels and their potential climatic impacts are dominated by local soil and plant emissions. A soil source was confirmed by measurements of DMS emission fluxes from Amazon soils as a function of temperature and soil moisture. Furthermore, light and temperature dependent DMS emissions were measured from seven tropical tree species. Our study has important implications for understanding terrestrial DMS sources and their role in coupled land‐atmosphere climate feedbacks.
      PubDate: 2014-12-08T14:46:16.566824-05:
      DOI: 10.1002/2014GB004969
  • Benchmarking the seasonal cycle of CO2 fluxes simulated by terrestrial
           ecosystem models
    • Authors: Shushi Peng; Philippe Ciais, Frédéric Chevallier, Philippe Peylin, Patricia Cadule, Stephen Sitch, Shilong Piao, Anders Ahlström, Chris Huntingford, Peter Levy, Xiran Li, Yongwen Liu, Mark Lomas, Benjamin Poulter, Nicolas Viovy, Tao Wang, Xuhui Wang, Sönke Zaehle, Ning Zeng, Fang Zhao, Hongfang Zhao
      Pages: n/a - n/a
      Abstract: We evaluated the seasonality of CO2 fluxes simulated by nine terrestrial ecosystem models of the TRENDY project against 1) the seasonal cycle of gross primary production (GPP) and net ecosystem exchange (NEE) measured at flux tower sites over different biomes, 2) gridded monthly Model Tree Ensembles‐estimated GPP (MTE‐GPP) and MTE‐NEE obtained by interpolating many flux towers measurements with a machine‐learning algorithm, 3) atmospheric CO2 mole fraction measurements at surface sites, and 4) CO2 total columns (XCO2) measurements from the Total Carbon Column Observing Network (TCCON). For comparison with atmospheric CO2 measurements, the LMDZ4 transport model was run with time‐varying CO2 fluxes of each model as surface boundary conditions. Seven out of the nine models overestimate the seasonal amplitude of GPP, and produce a too early start in spring at most flux sites. Despite their positive bias for GPP, the nine models underestimate NEE at most flux sites, and in the Northern Hemisphere compared with MTE‐GPP. Comparison with surface atmospheric CO2 measurements confirms that most models underestimate the seasonal amplitude of NEE in the Northern Hemisphere (except CLM4C and SDGVM). Comparison with TCCON data also shows that the seasonal amplitude of XCO2 is underestimated by more than 10% for seven out of the nine models (except for CLM4C and SDGVM) and that the MTE‐NEE product is closer to the TCCON data using LMDZ4. From CO2 columns measured routinely at 10 TCCON sites, the constrained amplitude of NEE over the Northern Hemisphere is of 1.6 ± 0.4 gC m‐2 day‐1, which translates into a net CO2 uptake during the carbon uptake period in the Northern Hemisphere of 7.9 ± 2.0 PgC yr‐1.
      PubDate: 2014-11-28T05:07:56.325727-05:
      DOI: 10.1002/2014GB004931
  • CO2 and CH4 isotope compositions and production pathways in a tropical
    • Authors: M.E. Holmes; J. Chanton, M. Tfaily, A. Ogram
      Pages: n/a - n/a
      Abstract: While it is widely recognized that peatlands are important in the global carbon cycle, there is limited information on belowground gas production in tropical peatlands. We measured porewater methane (CH4) and carbon dioxide (CO2) concentrations and δ13C isotopic composition and CH4 and CO2 production rates in peat incubations from the Changuinola wetland in Panama. Our most striking finding was that CH4 was depleted in 13C (‐94‰ in porewater and produced at ‐107‰ in incubated peat) relative to CH4 found in most temperate and northern wetlands, potentially impacting the accuracy of approaches that use carbon isotopes to constrain global mass balance estimates. Fractionation factors between CH4 and CO2 showed that hydrogenotrophic methanogenesis was the dominant CH4 production pathway, with up to 100% of the CH4 produced via this route. Far more CO2 than CH4 (7 to 100 X) was measured in porewater, due in part to loss of CH4 through ebullition or oxidation and to the production of CO2 from pathways other than methanogenesis. We analyzed data on 58 wetlands from the literature to determine the dominant factors influencing the relative proportions of CH4 produced by hydrogenotrophic and acetoclastic methanogenesis and found that a combination of environmental parameters including pH, vegetation type, nutrient status and latitude are correlated to the dominant methanogenic pathway. Methane production pathways in tropical peatlands do not correlate with these variables in the same way as their more northerly counterparts and thus may be differently affected by climate change.
      PubDate: 2014-11-28T00:23:18.927988-05:
      DOI: 10.1002/2014GB004951
  • Nitrogen and phosphorus fluxes from watersheds of the Northeast U.S. from
           1930–2000: Role of anthropogenic nutrient inputs, infrastructure,
           and runoff.
    • Abstract: An ongoing challenge for society is to harness the benefits of nutrients, nitrogen (N) and phosphorus (P), while minimizing their negative effects on ecosystems. While there is a good understanding of the mechanisms of nutrient delivery at small scales, it is unknown how nutrient transport and processing scale up to larger watersheds and whole regions over long time periods. We used a model that incorporates nutrient inputs to watersheds, hydrology, and infrastructure (sewers, waste‐water treatment plants, and reservoirs) to reconstruct historic nutrient yields for the northeastern U.S. from 1930 to 2002. Over the study period, yields of nutrients increased significantly from some watersheds and decreased in others. As a result, at the regional scale, the total yield of N and P from the region did not change significantly. Temporal variation in regional N and P yields was correlated with runoff coefficient, but not with nutrient inputs. Spatial patterns of N and P yields were best predicted by nutrient inputs, but the correlation between inputs and yields across watersheds decreased over the study period. The effect of infrastructure on yields was minimal relative to the importance of soils and rivers. However, infrastructure appeared to alter the relationships between inputs and yields. The role of infrastructure changed over time and was important in creating spatial and temporal heterogeneity in nutrient input‐yield relationships.
  • Issue Information
    • Abstract: Cover: Temporal variation of C storage in colluvial settings with different sedimentation rates from the onset of soil erosion. See Wang et al. [pp. 65–79; doi: 10.1002/2014GB004912].
  • Dust deposition in the eastern Indian Ocean: the ocean perspective from
           Antarctica to the Bay of Bengal
    • Abstract: Atmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry deposition) in remote regions. In this work, we discuss the upper‐ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges‐Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large‐scale gradients in mineral dust deposition and multiple source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66±60 mg m‐2 yr‐1) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m‐2 yr‐1 at 18°S) and in the southern Bay of Bengal (2,500±570 mg m‐2 yr‐1). Our total deposition fluxes, which have an uncertainty on the order of a factor 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.
  • Dissolved Fe and Al in the upper 1000m of the eastern Indian Ocean: a
           high‐resolution transect along 95°E from the Antarctic margin
           to the Bay of Bengal
    • Abstract: A high‐resolution section of dissolved iron (dFe) and aluminium (dAl) was obtained along ~95°E in the upper 1000m of the eastern Indian Ocean from the Antarctic margin (66°S) to the Bay of Bengal (18°N) during the US‐CLIVAR‐CO2 Repeat Hydrography I08S and I09N sections (February‐April 2007). In the Southern Ocean, low concentrations of dAl (
  • Carbon dynamics of the Weddell Gyre, Southern Ocean
    • Abstract: The accumulation of carbon within the Weddell Gyre, and its exchanges across the gyre boundaries are investigated with three recent full‐depth oceanographic sections enclosing this climatically‐important region. The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveal that deep water transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre's carbon budget, while a dual‐cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058 ± 0.010 Pg C yr‐1 derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air‐to‐sea CO2 flux of 0.002 ± 0.007 Pg C yr‐1 (mean 1998‐2011) to 0.012 ± 0.024 Pg C yr‐1 (mean 2008‐2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived from in situ biogeochemical measurements reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033 ± 0.021 Pg C yr‐1. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deep water formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long timescales.
  • The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere
           and ocean
    • Abstract: It is well known that the equilibration timescale for the isotopic ratios 13C/12C and 14C/12C in the ocean mixed layer is on the order of a decade, two orders of magnitude slower than for oxygen. Less widely‐appreciated is the factthat the equilibration timescale is quite sensitive to the speciation of Dissolved Inorganic Carbon (DIC) in the mixed layer, scaling linearly with the ratio DIC/CO 2, which varies inversely with atmospheric pCO 2. Although this effect is included in models that resolve the role of carbon speciation in air‐sea exchange, its role is often unrecognized, and it is not commonly considered in the interpretation of carbon isotope observations. Here, we use a global 3‐dimensional ocean model to estimate the redistribution of the carbon isotopic ratios between the atmosphere and ocean due solely to variations in atmospheric pCO 2. Under Last Glacial Maximum (LGM) pCO 2, atmospheric Δ14 C is increased by ≈ 30 due to the speciation change, all else being equal, raising the surface reservoir age by about 250 years throughout most of the ocean. For 13 C, enhanced surface disequilibrium under LGM pCO2 causes the upper ocean, atmosphere and North Atlantic Deep Water δ13C to become at least 0.2 higher relative to deep waters ventilated by the Southern Ocean. Conversely, under high pCO2, rapid equilibration greatly decreases isotopic disequilibrium. As a result, during geological periods of high pCO2, vertical δ13C gradients may have been greatly weakened as a direct chemical consequence of the high pCO2, masquerading as very well‐ventilated or biologically‐deadÔStrangeloveÕ oceans. The ongoing anthropogenic rise of pCO2 is accelerating the equilibration of the carbon isotopes in the ocean, lowering atmospheric Δ14C and weakening δ13C gradients within the ocean to a degree that is similar to the traditional fossil fuel ’Suess’ effect.
  • Preferential remineralization of dissolved organic phosphorus and
           non‐Redfield DOM dynamics in the global ocean: Impacts on marine
           productivity, nitrogen fixation, and carbon export
    • Abstract: Selective removal of nitrogen (N) and phosphorus (P) from the marine dissolved organic matter (DOM) pool has been reported in several regional studies. Because DOM is an important advective/mixing pathway of carbon (C) export from the ocean surface layer and its non‐Redfieldian stoichiometry would affect estimates of marine export production per unit N and P, we investigated the stoichiometry of marine DOM and its remineralization globally using a compiled DOM dataset. Marine DOM is enriched in C and N compared to Redfield stoichiometry, averaging 317:39:1 and 810:48:1 for C:N:P within the degradable and total bulk pools, respectively. Dissolved organic phosphorus (DOP) is found to be preferentially remineralized about twice as rapidly with respect to the enriched C:N stoichiometry of marine DOM. Biogeochemical simulations with the Biogeochemical Elemental Cycling model using Redfield and variable DOM stoichiometry corroborate the need for non‐Redfield dynamics to match the observed DOM stoichiometry. From our model simulations, preferential DOP remineralization is found to increase the strength of the biological pump by ~9% versus the case of Redfield DOM cycling. Global net primary productivity increases ~10% including an increase in marine nitrogen fixation of ~26% when preferential DOP remineralization and direct utilization of DOP by phytoplankton are included. The largest increases in marine nitrogen fixation, NPP, and carbon export are observed within the western subtropical gyres, suggesting the lateral transfer of P in the form of DOP from the productive eastern and poleward gyre margins may be important for sustaining these processes downstream in the subtropical gyres.
  • Phenological characteristics of global coccolithophore blooms
    • Abstract: Coccolithophores are recognised as having a significant influence on the global carbon cycle through the production and export of calcium carbonate (often referred to as particulate inorganic carbon or PIC). Using remotely sensed PIC and chlorophyll data we investigate the seasonal dynamics of coccolithophores relative to a mixed phytoplankton community. Seasonal variability in PIC, here considered to indicate changes in coccolithophore biomass, is identified across much of the global ocean. Blooms, which typically start in February‐March in the low latitude (~30°) northern hemisphere and last for ~6‐7 months, get progressively later (April–May) and shorter (3–4 months) moving polewards. A similar pattern is observed in the southern hemisphere, where blooms that generally begin around August‐September in the lower latitudes and which last for ~8 months, get later and shorter with increasing latitude. It has previously been considered that phytoplankton blooms consist of a sequential succession of blooms of individual phytoplankton types. Comparison of PIC and chlorophyll peak dates suggest instead that in many open ocean regions, blooms of coccolithophores and other phytoplankton can co‐occur, conflicting with the traditional view of species succession that is thought to take place in temperate regions such as the North Atlantic.
  • Importance of vegetation for manganese cycling in temperate forested
    • Abstract: Many surface soils are enriched in metals due to anthropogenic atmospheric inputs. To predict the persistence of these contaminants in soils, factors that impact rates of metal removal from soils into streams must be understood. Experiments at containerized seedling (“mesocosm”), pedon, and catchment scales were used to investigate the influence of vegetation on manganese (Mn) transport at the Susquehanna/Shale Hills Critical Zone Observatory (SSHCZO) in Pennsylvania, USA, where past atmospheric inputs from industrial sources have enriched Mn in surface soils. Large quantities of Mn that were leached from soil components into solution were taken up by vegetation; as a result, only relatively small quantities of Mn were removed from soil into effluent and streams. Manganese uptake into vegetation exceeded Mn losses in soil leachate by 20‐200x at all scales, and net Mn loss from soils decreased in the presence of vegetation due to uptake into plant tissues. The majority of Mn taken up by forest vegetation at SSHCZO each year was returned to the soil in leaf litter and consequently immobilized as Mn‐oxides that formed during litter decomposition. Thus, plant uptake of Mn combined with rapid oxidation of Mn during litter decomposition contribute to long‐term retention. Current release rates of soluble Mn from SSHCZO soils were similar to release rates from the larger Susquehanna River Basin (SRB), indicating that the processes observed at SSHCZO may be widespread across the region. Indeed, although atmospheric deposition of Mn has declined, surface soils at SSHCZO and throughout the eastern United States remain enriched in Mn. If recycling through vegetation can attenuate the removal of Mn from soils, as observed in this study, then Mn concentrations in soils and river waters will likely decrease slowly over time following watershed contamination. Understanding the role of vegetation in regulating metal transport is important for evaluating the long‐term effects of historical and ongoing metal loading to soils.
  • Regulation of Redfield ratios in the deep ocean
    • Abstract: Biotic regulation of the environment at global scales has been debated for several decades. An example is the similarity between deep‐ocean and phytoplankton mean N:P ratios. N and P cycles are heavily altered by human activities, mainly through an increase in nutrient supply to the upper ocean. As phytoplankton only access nutrients in the upper ocean, it is critical to understand (1) to what extent phytoplankton are able to regulate N and P concentrations as well as their ratio in the deep, inaccessible layer, and (2) what mechanisms control the value of the deep‐water N:P ratio and the efficiency of its biotic regulation. With a model of N and P cycles in the global ocean separated in two layers, we show that the value of the deep‐water N:P ratio is determined by non‐fixer's N:P ratio, recycling and denitrification. Our model predicts that, although phytoplankton cannot efficiently regulate deep nutrient pools, they can maintain nearly constant ratios between nutrients because compensatory dynamics between non‐fixers and nitrogen‐fixers allows a control of deep‐water chemistry through nutrient recycling. This mechanism could explain the near‐constancy of the deep‐water N:P ratio, in agreement with Redfield's [1934, 1958] classical hypothesis. Surprisingly, N:P ratio of phytoplankton does not affect their ability to regulate the deep‐water N:P ratio. Our model suggests that increased water column stratification as a result of global climate change may decrease the stability of the N:P ratio in the deep ocean over long temporal and spatial scales.
  • Controls on the silicon isotope distribution in the ocean: New diagnostics
           from a data‐constrained model
    • Abstract: The global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized, data‐constrained model of the oceanic silicon cycle that is embedded in a data‐assimilated steady circulation. Including fractionation during opal dissolution improves the model's ability to capture the approximately linear relation between isotope ratio, δ30Si, and inverse silicic‐acid concentration observed in the deep Atlantic. To quantify the importance of hydrographic control on the isotope distribution, δ30Si is partitioned into contributions from preformed and regenerated silicic acid, further partitioned according to euphotic‐zone origin. We find that the large‐scale features of the isotope distribution in the Atlantic basin are dominated by preformed silicic acid, with regenerated silicic acid being important for setting vertical gradients. In the Pacific and Indian Oceans, preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio, with gradients of the preformed and regenerated contributions tending to cancel each other in the deep Pacific. The Southern‐Ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to δ30Si. Nearly the entire preformed part of δ30Si is of Southern‐Ocean and North‐Atlantic origin. The regenerated part of lta30Si in the Atlantic basin also has a contribution of central‐Atlantic (∼ 40∘ S – 40∘N) origin that is comparable in magnitude to the North‐Atlantic contribution. In other basins, the central Pacific and Indian Ocean are the second largest contributors to the regenerated part of δ30Si.
  • Seasonality of biological and physical controls on surface ocean CO2 from
           hourly observations at the Southern Ocean Time Series site south of
    • Abstract: The Subantarctic Zone (SAZ), which covers the northern half of the Southern Ocean between the Subtropical and Subantarctic Fronts, is important for air‐sea CO2 exchange, ventilation of the lower thermocline, and nutrient supply for global ocean productivity. Here we present the first high‐resolution autonomous observations of mixed layer CO2 partial pressure (pCO2) and hydrographic properties covering a full annual cycle in the SAZ. The amplitude of the seasonal cycle in pCO2 (~60 μatm), from near atmospheric equilibrium in late winter to ~330 μatm in mid summer, results from opposing physical and biological drivers. Decomposing these contributions demonstrates that the biological control on pCO2 (up to 100 μatm), is four times larger than the thermal component, and driven by annual net community production of 2.45±1.47 mol C m–2 yr–1. After the summer biological pCO2 depletion, the return to near atmospheric equilibrium proceeds slowly, driven in part by autumn entrainment into a deepening mixed layer, and achieving full equilibration in late winter and early spring as respiration and advection complete the annual cycle. The shutdown of winter convection and associated mixed layer shoaling proceeds intermittently, appearing to frustrate the initiation of production. Horizontal processes, identified from salinity anomalies, are associated with biological pCO2 signatures, but with differing impacts in winter (when they reflect far‐field variations in dissolved inorganic carbon and/or biomass) and summer (when they suggest promotion of local production by the relief of silicic acid or iron limitation). These results provide clarity on SAZ seasonal carbon cycling and demonstrate that the magnitude of the seasonal pCO2 cycle is twice as large as that in the subarctic high‐nutrient, low‐chlorophyll waters, which can inform the selection of optimal global models in this region.
  • The influence of photosynthetic acclimation to rising CO2 and warmer
           temperatures on leaf and canopy photosynthesis models.
    • Abstract: There is an increasing necessity to understand how climate change factors, particularly increasing atmospheric concentrations of CO2 ([CO2]) and rising temperature, will influence photosynthetic carbon assimilation (A). Based on theory, an increased [CO2] concomitant with a rise in temperature will increase A in C3 plants beyond that of an increase in [CO2] alone. However, uncertainty surrounding the acclimation response of key photosynthetic parameters to these changes can influence this response. In this work, the acclimation responses of C3 photosynthesis for soybean measured at the SoyFACE Temperature by Free Air CO2 Enrichment (T‐FACE) experiment is incorporated in a leaf biochemical and canopy photosynthesis model. The two key parameters used as model inputs, the maximum velocity for carboxylation (Vc,max) and maximum rate of electron transport (Jmax), were measured in a full factorial [CO2] by temperature experiment over two growing seasons and applied in leaf‐ and canopy‐scale models to (1) reassess the theory of combined increases in [CO2] and temperature on A, (2) determine the role of photosynthetic acclimation to increased growth [CO2] and/or temperature in leaf and canopy predictions of A for these treatments, and (3) assess the diurnal and seasonal differences in leaf‐ and canopy‐scale A associated with the imposed treatments. The results demonstrate that the theory behind combined increases in [CO2] and temperature are sound, however, incorporating more recent parameterizations into the photosynthesis model predicts greater increases in A when [CO2] and temperature are increased together. Photosynthetic acclimation is shown to decrease leaf‐level A for all treatments, however, in elevated [CO2] the impact of acclimation does not result in any appreciable loss in photosynthetic potential at the canopy scale. In this analysis, neglecting photosynthetic acclimation in heated treatments, with or without concomitant rise in [CO2], leads to modeled over‐estimates of carbon gain for soybean under future predicted conditions.
  • Sensitivity of global terrestrial carbon cycle dynamics to variability in
           satellite‐observed burned area
    • Abstract: Fire plays an important role in terrestrial ecosystems by regulating biogeochemistry, biogeography and energy budgets, yet despite the importance of fire as an integral ecosystem process, significant advances remain to improve its prognostic representation in carbon cycle models. To recommend and to help prioritize model improvements, this study investigates the sensitivity of a coupled global biogeography and biogeochemistry model, LPJ, to observed burned area measured by three independent satellite‐derived products, GFED v3.1, L3JRC, and GlobCarbon. Model variables are compared with benchmarks that include pan‐tropical aboveground biomass, global tree cover, and CO2 and CO trace gas concentrations. Depending on prescribed burned area product, global aboveground carbon stocks varied by 300 Pg C, and woody cover ranged from 50‐73 Mkm2. Tree cover and biomass were both reduced linearly with increasing burned area, i.e., at regional scales, a 10% reduction in tree cover per 1000 km2, and 0.04‐to‐0.40 Mg C reduction per 1000 km2. In boreal regions, satellite burned area improved simulated tree‐cover and biomass distributions, but in savanna regions, model‐data correlations decreased. Global net biome production was relatively insensitive to burned area, and the long‐term land carbon sink was robust, ~2.5 Pg C yr‐1, suggesting that feedbacks from ecosystem respiration compensated for reductions in fuel consumption via fire. CO2 transport provided further evidence that heterotrophic respiration compensated any emission reductions in the absence of fire, with minor differences in modeled CO2 fluxes among burned area products. CO was a more sensitive indicator for evaluating fire emissions, with MODIS‐GFED burned area producing CO concentrations largely in agreement with independent observations in high‐latitudes. This study illustrates how ensembles of burned area datasets can be used to diagnose model structures and parameters for further improvement, and also highlights the importance in considering uncertainties and variability in observed burned area data products for model applications.
  • The age of river‐transported carbon: a global perspective
    • Abstract: The role played by river networks in regional and global carbon (C) budgets is receiving increasing attention. Despite the potential of radiocarbon measurements (∆14C) to elucidate sources and cycling of different riverine C pools, there remain large regions for which no data are available, and no comprehensive attempts to synthesize the available information and examine global patterns in the 14C content of different riverine C pools. Here, we present new 14C data on particulate and dissolved organic C (POC and DOC) from six river basins in tropical and subtropical Africa, and compiled >1400 literature ∆14C data and ancillary parameters from rivers globally. Our analysis reveals a consistent pattern whereby POC is progressively older in systems carrying higher sediment loads, coinciding with a lower organic carbon content. At the global scale, this pattern leads to a proposed global median ∆14C signature of −203‰, corresponding to an age of ~1800 yr BP. For DOC exported to the coastal zone, we predict a modern (decadal) age (∆14C = +22 to +46‰), and paired datasets confirm that riverine DOC is generally more recent in origin than POC – in contrast to the situation in ocean environments. Weathering regimes complicate the interpretation of 14C ages of dissolved inorganic carbon (DIC), but the available data favors the hypothesis that in most cases, more recent organic C is preferentially mineralized.
  • Sinking velocities and microbial respiration rates alter the attenuation
           of particulate carbon fluxes through the mesopelagic zone
    • Abstract: The attenuation of sinking particle fluxes through the mesopelagic zone is an important process that controls the sequestration of carbon and the distribution of other elements throughout the oceans. Case studies at two contrasting sites, the oligotrophic regime of the Bermuda Atlantic Time Series (BATS) and the mesotrophic waters of the western Antarctic Peninsula (WAP) sector of the Southern Ocean, revealed large differences in the rates of particle‐attached microbial respiration and the average sinking velocities of marine particles, two parameters that affect the transfer efficiency of particulate matter from the base of the euphotic zone into the deep ocean. Rapid average sinking velocities of 270 ± 150 m d−1 were observed along the WAP, whereas the average velocity was 49 ± 25 m d−1 at the BATS site. Respiration rates of particle‐attached microbes were measured using novel RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan, Boyd et al., unpublished manuscript) sediment traps that first intercepts sinking particles then incubates them in situ. RESIRE experiments yielded flux‐normalized respiration rates of 0.4 ± 0.1 d−1 at BATS when excluding an outlier of 1.52 d−1, while these rates were undetectable along the WAP (0.01 ± 0.02 d−1). At BATS, flux‐normalized respiration rates decreased exponentially with respect to depth below the euphotic zone with a 75% reduction between the 150 and 500 m depths. These findings provide quantitative and mechanistic insights into the processes that control the transfer efficiency of particle flux through the mesopelagic and its variability throughout the global oceans.
  • Net Community Production in the North Atlantic Ocean derived from
           Volunteer Observing Ship data
    • Abstract: The magnitude of marine plankton net community production (NCP) is indicative of both the biologically driven exchange of carbon dioxide between the atmosphere and the surface ocean, and the export of organic carbon from the surface ocean to the oceaninterior. In this study the seasonal variability in the NCP of five biogeochemical regions in the North Atlantic was determined from measurements of surface water dissolved oxygen and dissolved inorganic carbon (DIC) sampled from a Volunteer Observing Ship (VOS). The magnitude of NCP derived from dissolved oxygen measurements (NCPO2) was consistent with previous geochemical estimates of NCP in the North Atlantic, with an average annual NCPO2 of 9.5 ± 6.5 mmol O2 m−2 d−2. Annual NCPO2 did not vary significantly over 35 degrees of latitude, and was not significantly different from NCP derived from DIC measurements (NCPDIC). The relatively simple method described here is applicable to any VOS route on which surface water dissolved oxygen concentrations can be accurately measured, thus providing estimates of NCP at higher spatial and temporal resolution than currently achieved.
  • The stoichiometry of carbon and nutrients in peat formation
    • Abstract: Northern peatlands have stored large amounts (~500 Pg) of carbon (C) since the last glaciation. Combined with peat C are nutrients such as nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg) and potassium (K), each of which plays an important role in plant production, litter decomposition and the biogeochemical functioning of peatlands. Yet little attention has been given to the amounts of these nutrients stored in northern peatlands and their stoichiometry with C. Here, we use data on nutrient concentrations in over 400 peat profiles in Ontario, Canada, representing bogs, fens and swamps and their vegetation. We show that the C:N ratio is high (> 40:1) in vegetation and litter, but declines through the peat profiles to reach ratios between 22:1 and 29:1 in peat below 50 cm. In contrast, the C:P ratio rises from vegetation and litter (500:1 to 1300:1) to 1500:1 to 2000:1 in the lower part of the peat profile. Ratios of C to Ca, Mg and K vary with peatland type. Most of these stoichiometric changes occur in the early stages of organic matter decomposition, where the litter structure remains intact. We estimate that ~18 Pg of N has been stored in northern peatlands since deglaciation, reflecting high N accumulation rates (~0.8 g m−2 y−1), whereas P accumulation is small (~0.3 Pg, ~0.016 g m−2 y−1), indicating that P is recycled quickly in the surface layers.
  • Net ecosystem production and organic carbon balance of U.S. east coast
           estuaries: A synthesis approach
    • Abstract: Net ecosystem production (NEP) and the overall organic carbon budget for the estuaries along the east coast of the United States are estimated. We focus on the open estuarine waters, excluding the fringing wetlands. We developed empirical models relating NEP to loading ratios of dissolved inorganic nitrogen to total organic carbon, and carbon burial in the sediment to estuarine water residence time and total nitrogen input across the landward boundary. Output from a data‐constrained water quality model was used to estimate inputs of total nitrogen and organic carbon to the estuaries across the landward boundary, including fluvial and tidal‐wetland sources. Organic carbon export from the estuaries to the continental shelf was computed by difference, assuming steady state. Uncertainties in the budget were estimated by allowing uncertainties in the supporting model relations. Collectively, U.S. east coast estuaries are net heterotrophic, with the area‐integrated NEP of −1.5 (−2.8, −1.0) Tg C yr−1 (best estimate and 95% confidence interval) and area‐normalized NEP of −3.2 (−6.1, −2.3) mol C m−2 yr−1. East coast estuaries serve as a source of organic carbon to the shelf, exporting 3.4 (2.0, 4.3) Tg C yr−1 or 7.6 (4.4, 9.5) mol C m−2 yr−1. Organic carbon inputs from fluvial and tidal‐wetland sources for the region are estimated at 5.4 (4.6, 6.5) Tg C yr−1 or 12 (10, 14) mol C m−2 yr−1 and carbon burial in the open estuarine waters at 0.50 (0.33, 0.78) Tg C yr−1 or 1.1 (0.73, 1.7) mol C m−2 yr−1. Our results highlight the importance of estuarine systems in the overall coastal budget of organic carbon, suggesting that in the aggregate, U.S. east coast estuaries assimilate (via respiration and burial) ~40% of organic carbon inputs from fluvial and tidal‐wetland sources and allow ~60% to be exported to the shelf.
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