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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)   [Sort by number of followers]

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 23, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 57, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 8, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 3, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 40)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 9)
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J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 21)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 28)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 17)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 5, SJR: 0.959, h-index: 51)
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Water Resources Research     Full-text available via subscription   (Followers: 68, SJR: 2.189, h-index: 121)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [8 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • When Can Ocean Acidification Impacts be Detected from Decadal Alkalinity
    • Abstract: We use a large initial condition suite of simulations (30 runs) with an Earth system model to assess the detectability of biogeochemical impacts of ocean acidification (OA) on the marine alkalinity distribution from decadally‐repeated hydrographic measurements such as those produced by the Global Ship‐Based Hydrographic Investigations Program (GO‐SHIP). Detection of these impacts is complicated by alkalinity changes from variability and long term trends in freshwater and organic matter cycling and ocean circulation. In our ensemble simulation, variability in freshwater cycling generates large changes in alkalinity that obscure the changes of interest and prevent the attribution of observed alkalinity redistribution to OA. These complications from freshwater cycling can be mostly avoided through salinity normalization of alkalinity. With the salinity‐normalized alkalinity, modeled OA impacts are broadly detectable in the surface of the subtropical gyres by 2030. Discrepancies between this finding and the finding of an earlier analysis suggest these estimates are strongly sensitive to the patterns of calcium carbonate export simulated by the model. OA impacts are detectable later in the subpolar and equatorial regions due to slower responses of alkalinity to OA in these regions and greater seasonal equatorial alkalinity variability. OA impacts are detectable later at depth despite lower variability due to smaller rates of change and consistent measurement uncertainty.
      PubDate: 2016-04-18T19:05:38.338592-05:
      DOI: 10.1002/2015GB005308
  • Potential use of the N2/Ar ratio as a constraint on the oceanic fixed
           nitrogen loss
    • Authors: M. Shigemitsu; N. Gruber, A. Oka, Y. Yamanaka
      Abstract: Using a global ocean biogeochemical model, we investigate the suitability of the N2/Ar supersaturation ratio (ΔN2/Ar) as a tracer of marine nitrogen fixation and denitrification, i.e., the main biological processes that add or remove fixed nitrogen from the ocean. In a series of factorial simulations, we demonstrate that in regions away from the oxygen minimum zones (OMZs) the ΔN2/Ar characteristics are mostly determined by benthic denitrification occurring in the deep ocean with minor contributions from benthic and water‐column denitrification at shallower depths. In the OMZs, the subsurface maxima of ΔN2/Ar are mainly determined by water‐column denitrification. In contrast, nitrogen fixation has little impact on ΔN2/Ar owing to the rapid loss of the N2 supersaturation signal through air‐sea exchange. We thus conclude that ΔN2/Ar can act as a powerful constraint on water column and benthic denitrification occurring in intermediate‐to‐deep waters, but it cannot be used to estimate nitrogen fixation. The comparison of the currently very limited observations of the ΔN2/Ar with our model results shows an acceptable level of agreement, suggesting that the model's prescribed rates and distribution of benthic and water‐column denitrification (i.e., 140 and 52 TgN yr‐1, respectively) are reasonable and confirms the results derived from other constraints.
      PubDate: 2016-04-07T20:42:21.238923-05:
      DOI: 10.1002/2015GB005297
  • Nitrogen trace gas fluxes from a semi‐arid subtropical savanna under
           woody legume encroachment
    • Authors: Fiona M Soper; Thomas W Boutton, Peter M Groffman, Jed P Sparks
      Abstract: Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and climate. These systems are experiencing widespread encroachment by woody plants, frequently associated with large increases in soil N, with no consensus on implications for trace gas emissions. We investigated the impact of encroachment by N‐fixing tree Prosopis glandulosa on total reactive N gas flux (Nt = NO + N2O + NOy + NH3) from south Texas savanna soils over two years. Contrary to expectations, upland Prosopis groves did not have greater Nt fluxes than adjacent unencroached grasslands. However, abiotic conditions (temperature, rainfall, and topography) were strong drivers. Emissions from moist, low‐lying Prosopis playas were up to three times higher than from Prosopis uplands. Though NO dominated emissions, NH3 and NOy (non‐NO oxidized N) comprised 12‐16% of the total summer N flux (up to 7.9 µg N m‐2 h‐1). Flux responses to soil wetting were temperature‐dependent for NO, NH3 and NOy: a 15 mm rainfall event increased flux 3‐22 fold after 24 hours in summer, but had no effect in winter. Repeated soil wetting reduced N flux responses, indicating substrate depletion as a likely control. Rapid (
      PubDate: 2016-03-30T18:55:39.513151-05:
      DOI: 10.1002/2015GB005298
  • Temperature influence on phytoplankton community growth rates
    • Authors: Elliot Sherman; J. Keith Moore, Francois Primeau, David Tanouye
      Abstract: A large database of field estimates of phytoplankton community growth rates in natural populations was compiled and analyzed to determine the apparent temperature effect on phytoplankton community growth rate. We conducted an ordinary least squares regression to optimize the parameters in two commonly used growth‐temperature relations (Arrhenius and Q10 models). Both equations fit the observational data equally with the optimized parameter values. The optimum apparent Q10 value was 1.47 ± 0.08 [95% C.I.]. Microzooplankton grazing rates closely matched the temperature trends for phytoplankton growth. This likely reflects a dynamic adjustment of biomass and grazing rates by the microzooplankton to match their available food source, illustrating tight coupling of phytoplankton growth and microzooplankton grazing rates. The field‐measured temperature effect and growth rates were compared with estimates from the satellite Carbon‐based Productivity Model (CbPM) and three Earth System Models (ESMs), with model output extracted at the same month and sampling locations as the observations. The optimized, apparent Q10 value calculated for the CbPM was 1.51, with overestimation of growth rates. The apparent Q10 value in the Community Earth System Model (CESM V1.0) was 1.65, with modest underestimation of growth rates. The GFDL‐ESM2M and GFDL‐ESM2G models produced apparent Q10 values of 1.52 and 1.39 respectively. Models with an apparent Q10 that is significantly greater than ~1.5 will overestimate the phytoplankton community growth response to the ongoing climate warming, and will have spatial biases in estimated growth rates for the current era.
      PubDate: 2016-03-18T01:06:47.331976-05:
      DOI: 10.1002/2015GB005272
  • A Mass Budget for Mercury and Methylmercury in the Arctic Ocean
    • Abstract: Elevated biological concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, are observed throughout the Arctic Ocean but major sources and degradation pathways in seawater are not well understood. We develop a mass budget for mercury species in the Arctic Ocean based on available data since 2004 and discuss implications and uncertainties. Our calculations show that high total Hg in Arctic seawater relative to other basins reflect large freshwater inputs and sea ice cover that inhibits losses through evasion. We find that most net MeHg production (20 Mg a‐1) occurs in the subsurface ocean (20‐200 m). There, it is converted to dimethylmercury (Me2Hg: 17 Mg a‐1), which diffuses to the polar mixed layer and evades to the atmosphere (14 Mg a‐1). Me2Hg has a short atmospheric lifetime and rapidly degrades back to MeHg. We postulate that most evaded Me2Hg is re‐deposited as MeHg and that atmospheric deposition is the largest net MeHg source (8 Mg a‐1) to the biologically productive surface ocean. MeHg concentrations in Arctic Ocean seawater are elevated compared to lower latitudes. Riverine MeHg inputs accounts for an approximately 15% of inputs to the surface ocean (2.5 Mg a‐1) but greater importance in the future is likely given increasing freshwater discharges and permafrost melt. This may offset potential declines driven by increasing evasion from ice‐free surface waters. Geochemical model simulations illustrate that for the most biologically relevant regions of the ocean, regulatory actions that decrease Hg inputs have the capacity to rapidly affect aquatic Hg concentrations.
      PubDate: 2016-02-03T17:18:26.409506-05:
      DOI: 10.1002/2015GB005280
  • On which timescales do gas transfer velocities control North Atlantic CO2
           flux variability'
    • Abstract: The North Atlantic is an important basin for the global ocean's uptake of anthropogenic and natural carbon dioxide (CO2), but the mechanisms controlling this carbon flux are not fully understood. The air‐sea flux of CO2, F, is the product of a gas transfer velocity, k, the air‐sea CO2 concentration gradient, ΔpCO2, and the temperature and salinity‐dependent solubility coefficient, α. k is difficult to constrain, representing the dominant uncertainty in F on short (instantaneous to interannual) timescales. Previous work shows that in the North Atlantic, ΔpCO2 and k both contribute significantly to interannual F variability, but that k is unimportant for multidecadal variability. On some timescale between interannual and multidecadal, gas transfer velocity variability and its associated uncertainty become negligible. Here, we quantify this critical timescale for the first time. Using an ocean model, we determine the importance of k, ΔpCO2 and α on a range of timescales. On interannual and shorter timescales, both ΔpCO2 and k are important controls on F. In contrast, pentadal to multidecadal North Atlantic flux variability is driven almost entirely by ΔpCO2; k contributes less than 25%. Finally, we explore how accurately one can estimate North Atlantic F without a knowledge of non‐seasonal k variability, finding it possible for interannual and longer timescales. These findings suggest that continued efforts to better constrain gas transfer velocities are necessary to quantify interannual variability in the North Atlantic carbon sink. However, uncertainty in k variability is unlikely to limit the accuracy of estimates of longer term flux variability.
  • Forest harvest contribution to Boreal freshwater methyl mercury load
    • Abstract: Clear‐cutting of Boreal coniferous forests enhances MeHg pool in organic topsoil 7 times Forest clear‐cutting enhances stream export of MeHg more than three times in undulating terrain Forest harvest increases MeHg export to Boreal headwaters by 12‐20% in Sweden and 2 % globally
  • The Influence of Southern Ocean Winds on the North Atlantic Carbon Sink
    • Abstract: Observed and predicted increases in Southern Ocean winds are thought to upwell deep ocean carbon and increase atmospheric CO2. However, Southern Ocean dynamics affect biogeochemistry and circulation pathways on a global scale. Using idealised MITgcm simulations, we demonstrate that an increase in Southern Ocean winds reduces the carbon sink in the North Atlantic sub‐polar gyre. The increase in atmospheric CO2 due to the reduction of the North Atlantic carbon sink is shown to be of the same magnitude as the increase in atmospheric CO2 due to Southern Ocean outgassing. The mechanism can be described as follows: The increase in Southern Ocean winds leads to an increase in upper ocean northward nutrient transport. Biological productivity is therefore enhanced in the tropics, which alters the chemistry of the sub‐thermocline waters that are ultimately upwelled in the subpolar gyre. The results demonstrate the influence of Southern Ocean winds on the North Atlantic carbon sink and show that the effect of Southern Ocean winds on atmospheric CO2 is likely twice as large as previously thought in past, present, and future climates.
  • Are mangroves drivers or buffers of coastal
           acidification' < mt > Insights from
           alkalinity and dissolved inorganic carbon export estimates across a
           latitudinal transect.
    • Abstract: Mangrove forests are hotspots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO2 [CO2*] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia, and calculated the exchange of DIC, alkalinity and [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m‐2 d‐1 across the six systems, ranging from import of 97 mmol m‐2 d‐1 to an export of 85 mmol m‐2 d‐1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr‐1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m‐2 day‐1 and an export of 117 mmol m‐2 day‐1 with an estimated global export of 4.2 ± 1.3 Tmol yr‐1. A net import of free CO2 was estimated (‐11.4 ± 14.8 mmol m‐2 d‐1), and was equivalent to approximately one third of the air water CO2 flux (33.1 ± 6.3 mmol m‐2 d‐1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.
  • Satellite estimates of net community production based on O2/Ar
           observations and comparison to other estimates.
    • Abstract: We present two statistical algorithms for predicting global oceanic net community production (NCP) from satellite observations. To calibrate these two algorithms, we compiled a large dataset of in situ O2/Ar‐NCP and remotely sensed observations, including sea surface temperature (SST), net primary production (NPP), phytoplankton size composition, and inherent optical properties. The first algorithm is based on genetic programming (GP) which simultaneously searches for the optimal form and coefficients of NCP equations. We find that several GP solutions are consistent with NPP and SST being strong predictors of NCP. The second algorithm uses support vector regression (SVR) to optimize a numerical relationship between O2/Ar‐NCP measurements and satellite observations. Both statistical algorithms can predict NCP relatively well, with a coefficient of determination (R2) of 0.68 for GP and 0.72 for SVR, which is comparable to other algorithms in the literature. However, our new algorithms predict more spatially uniform annual NCP distribution for the world's oceans and higher annual NCP values in the Southern Ocean and the five oligotrophic gyres.
  • Issue Information
    • Abstract: No abstract is available for this article.
  • Dynamics of carbonate chemistry, production and calcification of the
           Florida Reef Tract (2009‐2010): evidence for seasonal dissolution
    • Abstract: Ocean acidification is projected to lower the Ωar of reef waters by 0.3‐0.4 units by the end of century making it more difficult for calcifying organisms to secrete calcium carbonate while at the same time making the environment more favorable for abiotic and biotic dissolution of the reef framework. There is great interest in being able to project the point in time when coral reefs will cross the tipping point between being net depositional to net erosional in terms of their carbonate budgets. Periodic in situ assessments of the balance between carbonate production and dissolution that spans seasonal time scales may prove useful in monitoring and formulating projections of the impact of ocean acidification on reef carbonate production. This study represents the first broad scale geochemical survey of the rates of net community production (NCP) and net community calcification (NCC) across the Florida Reef Tract (FRT). Surveys were performed at approximately quarterly intervals in 2009‐10 across seven onshore‐offshore transects spanning the upper, middle and lower Florida Keys. Averaged across the FRT, the rates of NCP and NCC were positive during the spring/summer at 62 ± 7 and 17 ± 2 mmol m‐2 d‐1, respectively, and negative during the fall/winter at ‐33 ± 6 and ‐7 ± 2 mmol m‐2 d‐1. The most significant finding of the study was that the northern‐most reef is already net erosional (‐1.1 ± 0.4 kg CaCO3 m‐2 y‐1) and mid‐reefs to the south were net depositional on an annual basis (0.4 ± 0.1 kg CaCO3 m‐2 y‐1) but erosional during the fall and winter. Only the two southern‐most reefs were net depositional year‐round. These results indicate that parts of the FRT have already crossed the tipping point for carbonate production and other parts are getting close.
  • Global impact of tropical cyclones on primary production
    • Abstract: In this paper, we explore the global responses of surface temperature, chlorophyll and primary production to tropical cyclones (TCs). Those ocean responses are first characterized from the statistical analysis of satellite data under ~1000 TCs over the 1998‐2007 period. Besides the cold wake, the vast majority of TCs induce a weak chlorophyll response, with only ~10% of induced blooms exceeding 0.1 mg.m‐3. The largest chlorophyll responses mostly occur within coastal regions, in contrast to the strongest cold wakes that generally occur farther offshore. To understand this decoupling, we analyze a coupled dynamical‐biogeochemical oceanic simulation forced by realistic wind vortices applied along observed TC tracks. The simulation displays a realistic spatial structure of TC‐induced blooms and its observed decoupling with TC cold wakes. In regions of strong TC energy input, the strongest cold wakes occur in regions of shallow thermocline (
  • A Novel Molecular Approach for Tracing Terrigenous Dissolved Organic
           Matter into the Deep Ocean
    • Abstract: Marine dissolved organic matter (DOM) contains one of the largest exchangeable organic carbon pools on Earth. Riverine input represents an important source of DOM to the oceans, yet much remains to be learned about the fate of the DOM linking terrestrial to oceanic carbon cycles through rivers at the global scale. Here, we use ultrahigh resolution mass spectrometry to identify 184 molecular formulae that are indicators of riverine inputs (referred to as t‐Peaks) and to track their distribution in the deep North Atlantic and North Pacific Oceans. The t‐Peaks were found to be enriched in the Amazon River, to be highly correlated with known tracers of terrigenous input, and to be observed in all samples from four different rivers characterized by vastly different landscapes and vegetation coverage spanning equatorial (Amazon and Congo), subtropical (Altamaha) and Arctic (Kolyma) regions. Their distribution reveals that terrigenous organic matter is injected into the deep ocean by the global meridional overturning circulation, indicating that a fraction of the terrigenous DOM introduced by rivers contributes to the DOM pool observed in the deep ocean and to the storage of terrigenous organic carbon. This novel molecular approach can be used to further constrain the transfer of DOM from land to sea, especially considering that FT‐ICR MS analysis is becoming increasingly frequent in studies characterizing the molecular composition of DOM in lakes, rivers and the ocean.
  • Effects of African Dust Deposition on Phytoplankton in the Western
           Tropical Atlantic Ocean off Barbados
    • Abstract: Bioassay incubation experiments conducted with nutrients and local atmospheric aerosol amendments indicate that phosphorus (P) availability limited phytoplankton growth in the low nutrient low chlorophyll (LNLC) ocean off Barbados. Atmospheric deposition provides a relatively large influx of new nutrients and trace metals to the surface ocean in this region in comparison to other nutrient sources. However, the impact on native phytoplankton is muted due to the high ratio of nitrogen (N) to P (NO3:SRP > 40) and the low P solubility of these aerosols. Atmospheric deposition induces P limitation in this LNLC region by adding more N and iron (Fe) relative to P. This favors the growth of Prochlorococcus, a genus characterized by low P requirements and highly efficient P acquisition mechanisms. A global three‐dimensional marine ecosystem model that includes species‐specific phytoplankton elemental quotas/stoichiometry and the atmospheric deposition of N, P and Fe supports this conclusion. Future increases in aerosol N loading may therefore influence phytoplankton community structure in other LNLC areas, thereby affecting the biological pump and associated carbon sequestration.
  • 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 
  • Heavy silicon isotopic composition of silicic acid and biogenic silica in
           Arctic waters over the Beaufort shelf and the Canada Basin
    • Abstract: The silicon isotopic composition of silicic acid (δ30Si(OH)4) and biogenic silica (δ30Si‐bSiO2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ30Si(OH)4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30ε = ‐0.97 ± 0.17‰, agrees well with laboratory and global‐ocean estimates. However, the δ30Si dynamics of this region are better represented by closed system isotope models that yield lower values of 30ε, between ‐0.33‰ and ‐0.41‰, depending on how the contribution of sea ice diatoms is incorporated. In the upper 400 m, δ30Si‐bSiO2 values were among the heaviest ever measured in marine suspended bSiO2 (+2.03‰ to +3.51‰). A positive correlation between δ30Si‐bSiO2 and sea‐ice cover implies that heavy signals can result from isotopically‐heavy sea‐ice diatoms introduced to pelagic assemblages. Below the surface bSiO2 production zone, δ30Si(OH)4 distribution followed that of major water masses. Vertical δ30Si(OH)4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125‐200 m) composed of modified Pacific water, and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300‐500 m deep). In the Canada Basin Deep Water (below 2,000 m), δ30Si(OH)4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH)4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ30Si(OH)4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box‐model simulation of the global marine δ30Si(OH)4 distribution successfully reproduced the observed patterns, with the δ30Si(OH)4 of the simulated deep Arctic Ocean being the heaviest of all deep‐ocean basins.
  • Seasonal trends of Amazonian rainforest phenology, net primary
           productivity, and carbon allocation.
    • Abstract: The seasonality of solar irradiance and precipitation may regulate seasonal variations in tropical forests carbon cycling. Controversy remains over their importance as drivers of seasonal dynamics of net primary productivity in tropical forests. We use ground data from nine lowland Amazonian forest plots collected over three years to quantify the monthly NPP of leaves, reproductive material, woody material, and fine roots over an annual cycle. We distinguish between forests that do not experience substantial seasonal moisture stress (“humid sites”) and forests that experience a stronger dry season (“dry sites”). We find that forests from both precipitation regimes maximise leaf NPP over the drier season, with a peak in production in August at both humid (mean 0.39 ± 0.03 Mg C ha‐1 mo‐1 in July, n = 4) and dry sites (mean 0.49 ± 0.03 Mg C ha‐1 mo‐1 in September, n = 8). We identify two distinct seasonal carbon allocation patterns (the allocation of NPP to a specific organ such as wood leaves or fine roots divided by total NPP). The forests monitored in the present study show evidence of either: (i) constant allocation to roots and a seasonal trade‐off between leaf and woody material; or (ii) constant allocation to wood and a seasonal trade‐off between roots and leaves. Finally, we find strong evidence of synchronised flowering at the end of the dry season in both precipitation regimes. Flower production reaches a maximum of 0.047 ± 0.013 and 0.031 ± 0.004 Mg C ha‐1 mo‐1 in November, in humid and dry sites respectively. Fruitfall production was staggered throughout the year, probably reflecting the high variation in varying times to development and loss of fruit amongst species.
  • Particulate Organic Carbon and Nitrogen Export from Major Arctic Rivers
    • Abstract: Northern rivers connect a land area of approximately 20.5 million km2 to the Arctic Ocean and surrounding seas. These rivers account for ~10% of global river discharge, and transport massive quantities of dissolved and particulate materials that reflect watershed sources and impact biogeochemical cycling in the ocean. In this paper, multi‐year datasets from a coordinated sampling program are used to characterize particulate organic carbon (POC) and nitrogen (PN) export from the six largest rivers within the pan‐Arctic watershed (Yenisey, Lena, Ob’, Mackenzie, Yukon, Kolyma). Together these rivers export an average of 3055 x 109 g of POC and 368 x 109 g of PN each year. Scaled up to the pan‐Arctic watershed as a whole, fluvial export estimates increase to 5767 x 109 g and 695 x 109 g of POC and PN per year respectively. POC export is substantially lower than dissolved organic carbon export by these rivers, whereas PN export is roughly equal to dissolved nitrogen export. Seasonal patterns in concentrations and source/composition indicators (C:N, δ13C, Δ14C, δ15N) are broadly similar among rivers, but distinct regional differences are also evident. For example, average radiocarbon ages of POC range from ~2000 (Ob’) to ~5500 (Mackenzie) years before present. Rapid changes within the Arctic system as a consequence of global warming make it challenging to establish a contemporary baseline of fluvial export, but the results presented in this paper capture variability and quantify average conditions for nearly a decade at the beginning of the 21st century.
  • Topographic variability and the influence of soil erosion on the carbon
    • Abstract: Soil erosion, particularly that caused by agriculture, is closely linked to the global carbon (C) cycle. There is a wide range of contrasting global estimates of how erosion alters soil‐atmosphere C exchange. This can be partly attributed to limited understanding of how geomorphology, topography, and management practices affect erosion and oxidation of soil organic C (SOC). This work presents a physically‐based approach that stresses the heterogeneity at fine spatial scales of SOC erosion, SOC burial, and associated soil‐atmosphere C fluxes. The Holcombe's Branch watershed, part of the Calhoun Critical Zone Observatory in South Carolina, USA is the case study used. The site has experienced some of the most serious agricultural soil erosion in North America. We use SOC content measurements from constrastins soil profiles, and estimates of SOC oxidation rates at multiple soil depths. The methodology was implemented in the tRIBS‐ECO (Triangulated Irregular Network‐based Real‐time Integrated Basin Simulator‐Erosion and Carbon Oxidation), a spatially‐ and depth‐explicit model of SOC dynamics built within an existing coupled physically‐based hydro‐geomorphic model. According to observations from multiple soil profiles, about 32% of the original SOC content has been eroded in the study area. The results indicate that C erosion and its replacement exhibit significant topographic variation at relatively small scales (tens of meters). The episodic representation of SOC erosion reproduces the history of SOC erosion better than models that use an assumption of constant erosion in space and time. The net atmospheric C exchange at the study site is estimated to range range from a maximum source of 14.5 g m‐2 yr‐1 to a maximum sink of ‐18.2 g m‐2 yr‐1. The small‐scale complexity of C erosion and burial driven by topography exerts a strong control on the landscape's capacity to serve as a C source or a sink.
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