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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 41, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 18)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 5)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 12)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 21)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 13)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 17, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 123, SJR: 1.769, h-index: 110)
Journal Cover Global Biogeochemical Cycles
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     ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.4]   [H-I: 109]
  • Experimental assessment of diazotroph responses to elevated seawater pCO2
           in the North Pacific Subtropical Gyre
    • Abstract: We examined short‐term (24‐72 hours) responses of naturally occurring marine N2 fixing microorganisms (termed diazotrophs) to abrupt increases in the partial pressure of carbon dioxide (pCO2) in seawater during 9 incubation experiments conducted between May 2010 and September 2012 at Station ALOHA (22°45’N, 158˚W) in the North Pacific Subtropical Gyre (NPSG). Rates of N2 fixation, nitrogenase (nifH) gene abundances and transcripts of six major groups of cyanobacterial diazotrophs (including both unicellular and filamentous phylotypes), and rates of primary productivity (as measured by 14C‐bicarbonate assimilation into plankton biomass) were determined under contemporary (~390 ppm) and elevated pCO2 conditions (~1100 ppm). Quantitative polymerase chain reaction (QPCR) amplification of planktonic nifH genes revealed that unicellular cyanobacteria phylotypes dominated gene abundances during these experiments. In the majority of experiments (7 out of 9), elevated pCO2 did not significantly influence rates of dinitrogen (N2) fixation or primary productivity (two‐way ANOVA, P > 0.05). During two experiments, rates of N2 fixation rates and primary productivity were significantly lower (by 79 to 82% and 52 to 72%, respectively) in the elevated pCO2 treatments relative to the ambient controls (two‐way ANOVA, P 
       
  • Reconciling modeled and observed atmospheric deposition of soluble organic
           nitrogen at coastal locations
    • Abstract: Atmospheric deposition of reactive nitrogen (N) species from air pollutants is a significant source of exogenous nitrogen in marine ecosystems. Here we use an atmospheric chemical transport model to investigate the supply of soluble organic nitrogen (ON) from anthropogenic sources to the ocean. Comparisons of modeled deposition with observations at coastal and marine locations show good overall agreement for inorganic nitrogen and total soluble nitrogen. However, previous modeling approaches result in significant underestimates of the soluble ON deposition if the model only includes the primary soluble ON and the secondary oxidized ON in gases and aerosols. Our model results suggest that including the secondary reduced ON in aerosols as a source of soluble ON contributes to an improved prediction of the deposition rates (g N m−2 yr−1). The model results show a clear distinction in the vertical distribution of soluble ON in aerosols between different processes from the primary sources and the secondary formation. The model results (excluding the biomass burning and natural emission changes) suggest an increase in soluble ON outflow from atmospheric pollution, in particular from East Asia, to the oceans in the twentieth century. These results highlight the necessity of improving the process‐based quantitative understanding of the chemical reactions of inorganic nitrogen species with organics in aerosol and cloud water.
       
  • Issue Information
    • Abstract: No abstract is available for this article.
       
  • Impact of Large‐Scale Climate Extremes on Biospheric Carbon Fluxes:
           An Intercomparison Based on MsTMIP Data
    • Abstract: Understanding the role of climate extremes and their impact on the carbon (C) cycle is increasingly a focus of Earth system science. Climate extremes such as droughts, heat waves, or heavy precipitation events can cause substantial changes in terrestrial C fluxes. On the other hand, extreme changes in C fluxes are often, but not always, driven by extreme climate conditions. Here we present an analysis of how extremes in temperature and precipitation, and extreme changes in terrestrial C fluxes are related to each other in 10 state‐of‐the‐art terrestrial carbon models, all driven by the same climate forcing. We use model outputs from the North American Carbon Program (NACP) Multi‐scale synthesis and Terrestrial Model Intercomparison Project (MsTMIP). A global scale analysis shows that both droughts and heatwaves translate into anomalous net releases of CO2 from the land surface via different mechanisms: Droughts largely decrease gross primary production (GPP) and to a lower extent total respiration (TR), while heat waves slightly decrease GPP but increase TR. Cold and wet periods have a smaller opposite effect. Analyzing extremes in C fluxes reveals that extreme changes in GPP and TR are often caused by strong shifts in water availability, but for extremes in TR shifts in temperature are also important. Extremes in net CO2 exchange are equally strongly driven by deviations in temperature and precipitation. Models mostly agree on the sign of the C flux response to climate extremes, but model spread is large. In tropical forests, C cycle extremes are driven by water availability, whereas in boreal forests temperature plays a more important role. Models are particularly uncertain about the C flux response to extreme heat in boreal forests.
       
  • Satellite‐detected fluorescence: decoupling non‐photochemical
           quenching from iron stress signals in the South Atlantic and Southern
           Ocean
    • Abstract: Satellite detected sunlight induced chlorophyll fluorescence could offer valuable information about the physiological status of phytoplankton on a global scale. Realization of this potential is confounded by the considerable uncertainty that exists in deconvolving the multiple ecophysiological processes that can influence the satellite signal. A dominant source of current uncertainty arises from the extent of reductions in chlorophyll fluorescence caused by the high light intensities phytoplankton are typically exposed to when satellite images are captured. In this study, results from over two hundred non‐photochemical quenching (NPQ) experiments conducted on cruises spanning from subtropical gyre to Southern Ocean waters have confirmed that satellite fluorescence quantum yields have the potential to reveal broad regions of iron (Fe) stress. However, our results suggest significant variability in phytoplankton NPQ behaviour between oceanic regimes. Dynamic NPQ must therefore be considered to achieve a reliable interpretation of satellite fluorescence in terms of Fe stress. Specifically, significantly lower NPQ was found in stratified subtropical gyre‐type waters than in well‐mixed Southern Ocean waters. Such variability is suggested to result from differences in incident irradiance fluctuation experienced by phytoplankton, with highly variable irradiance conditions likely driving phytoplankton to acclimate or adapt towards a higher dynamic NPQ capacity. Sea surface temperature empirically demonstrated the strongest correlation with NPQ parameters and is presented as a means of correcting the chlorophyll fluorescence signature for the region studied. With these corrections, a decadal composite of satellite austral summer observations is presented for the Southern Ocean, potentially reflecting spatial variability in the distribution and extent of Fe stress.
       
  • Variability in under‐ice export fluxes of biogenic matter in the
           Arctic Ocean
    • Abstract: A critical question regarding the organic carbon cycle in the Arctic Ocean is whether the decline in ice extent and thickness and the associated increase in solar irradiance in the upper ocean will result in increased primary production and particulate organic carbon (POC) export. To assess spatial and temporal variability in POC export, under‐ice export fluxes were measured with short‐term sediment traps in the northern Laptev Sea in July‐August‐September 1995, north of the Fram Strait in July 1997 and in the Central Arctic in August‐September 2012. Sediment traps were deployed at 2‐5 m and 20‐25 m under ice for periods ranging from 8.5 to 71 hours. In addition to POC fluxes, total particulate matter, chlorophyll a, biogenic particulate silica, phytoplankton, and zooplankton fecal pellet fluxes were measured to evaluate the amount and composition of the material exported in the upper Arctic Ocean. Whereas elevated export fluxes observed on and near the Laptev Sea shelf were likely the combined result of high primary production, resuspension, and release of particulate matter from melting ice, low export fluxes above the central basins despite increased light availability during the record minimum ice extent of 2012 suggest that POC export was limited by nutrient supply during summer. These results suggest that the ongoing decline in ice cover affects export fluxes differently on Arctic shelves and over the deep Arctic Ocean, and that POC export is likely to remain low above the central basins unless additional nutrients are supplied to surface waters.
       
  • Natural biogeochemical cycle of mercury in a global
           three‐dimensional ocean tracer model
    • Abstract: We implement mercury (Hg) biogeochemistry in the OFFline global 3‐D ocean TRACer model (OFFTRAC) to investigate the natural Hg cycle, prior to any anthropogenic input. The simulation includes three Hg tracers: dissolved elemental (Hg0aq), dissolved divalent (HgIIaq) and particle‐bound mercury (HgPaq). Our Hg parameterization takes into account redox chemistry in ocean waters, air‐sea exchange of Hg0, scavenging of HgIIaq onto sinking particles and resupply of HgIIaq at depth by remineralization of sinking particles. Atmospheric boundary conditions are provided by a global simulation of the natural atmospheric Hg cycle in the GEOS‐Chem model. In the surface ocean, the OFFTRAC model predicts global mean concentrations of 0.16 pM for total Hg, partitioned as 80% HgIIaq, 14% Hg0aq and 6% HgPaq. Total Hg concentrations increase to 0.38 pM in the thermocline/intermediate waters (between the mixed layer and 1,000 m depth) and 0.82 pM in deep waters (below 1,000 m), reflecting removal of Hg from the surface to the subsurface ocean by particle sinking followed by remineralization at depth. Our model predicts that Hg concentrations in the deep N. Pacific Ocean (>2,000 m) are a factor of 2‐3 higher than in the deep N. Atlantic Ocean. This is the result of cumulative input of Hg from particle remineralization as deep waters transit from the N. Atlantic to the N. Pacific on their ~2,000‐year journey. The model is able to reproduce the relatively uniform concentrations of total Hg observed in the old deep waters of the N. Pacific Ocean (observations: 1.2 ± 0.4 pM; model: 1.1 ± 0.04 pM) and Southern Ocean (observations: 1.1 ± 0.2 pM; model: 0.8 ± 0.02 pM). However, the modeled concentrations are factors of 5‐6 too low compared to observed concentrations in the surface ocean and in the young water masses of the deep N. Atlantic Ocean. This large underestimate for these regions implies a factor of 5‐6 anthropogenic enhancement in Hg concentrations.
       
  • Influences of Glacier Melt and Permafrost Thaw on the Age of Dissolved
           Organic Carbon in the Yukon River Basin
    • Abstract: Responses of near‐surface permafrost and glacial ice to climate change are of particular significance for understanding long term effects on global carbon cycling and carbon export by high‐latitude northern rivers. Here we report Δ14C ‐dissolved organic carbon (DOC) values and dissolved organic matter optical data for the Yukon River, 15 tributaries of the Yukon River, glacial meltwater, and groundwater and soil water endmember sources draining to the Yukon River, with the goal of assessing mobilization of aged DOC within the watershed. Ancient DOC was associated with glacial meltwater and groundwater sources. In contrast, DOC from watersheds dominated by peat soils and underlain by permafrost was typically enriched in Δ14C indicating that degradation of ancient carbon stores is currently not occurring at large enough scales to quantitatively influence bulk DOC exports from those landscapes. On an annual basis, DOC exported was predominantly modern during the spring period throughout the Yukon River basin, and became older through summer‐fall and winter periods, suggesting that contributions of older DOC from soils, glacial meltwaters and groundwater are significant during these months. Our data indicate that rapidly receding glaciers and increasing groundwater inputs will likely result in greater contributions of older DOC in the Yukon River and its tributaries in coming decades.
       
  • The triple oxygen isotope tracer of primary productivity in a dynamic
           ocean model
    • Abstract: The triple oxygen isotopic composition of dissolved oxygen (17Δdis) was added to the ocean ecosystem and biogeochemistry component of the Community Earth System Model, version 1.1.1. Model simulations were used to investigate the biological and physical dynamics of 17Δdis and assess its application as a tracer of gross photosynthetic production (GOP) of O2 in the ocean mixed layer. The model reproduced large‐scale patterns of 17Δdis found in observational data across diverse biogeographical provinces. Mixed layer model performance was best in the Pacific, had a negative bias in the North Atlantic and a positive bias in the Southern Ocean. Based on model results, the steady‐state equation commonly used to calculate GOP from tracer values overestimated the globally averaged model GOP by 29%. Vertical entrainment/mixing and the time rate of change of 17Δdis were the two largest sources of bias when applying the steady‐state method to calculate GOP. Entrainment/mixing resulted in the largest overestimation in mid latitudes and during summer and fall and almost never caused an underestimation of GOP. The tracer time‐rate of change bias resulted both in underestimation of GOP (e.g., during spring blooms at high latitudes) and overestimation (e.g., during the summer following a bloom). Seasonally, bias was highest in the fall (Sept‐Oct‐Nov in the Northern Hemisphere, Mar‐Apr‐May in the Southern), overestimating GOP by 62%, globally averaged. Overall, the steady‐state method was most accurate in equatorial and low latitude regions where it estimated GOP to within ±10%. Field applicable correction terms are derived for entrainment and mixing that capture 86% of model vertical bias and require only mixed layer depth history and triple oxygen isotope measurements from two depths.
       
  • Linking variability in soil solution dissolved organic carbon to climate,
           soil type and vegetation type
    • Abstract: Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux i.e. the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, firstly, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil and vegetation types at global scale and, secondly, to identify potential determinants of the site‐to‐site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R2 = 0.67‐0.99) in DOC concentrations in mineral European forest soils correlates with NH4+, C/N, Al and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.
       
  • Intra‐annual variability of organic carbon concentrations in running
           waters: drivers along a climatic gradient
    • Abstract: Trends in surface water dissolved organic carbon (DOC) concentrations have received considerable scientific interest during recent decades. However, intra‐annual DOC variability is often orders of magnitude larger than long‐term trends. Unravelling the controls on intra‐annual DOC dynamics holds the key to a better understanding of long‐term changes and their ecological significance. We quantified and characterized intra‐annual DOC variability and compared it with long‐term DOC trends in 136 streams and rivers, varying in size and geographical characteristics, across a 1400 km latitudinal gradient during 2000–2010. Discharge, temperature and month of the year were the most significant predictors of intra‐annual DOC variability in a majority of the running waters. Relationships between DOC, discharge, and temperature were however different along a mean annual temperature (MAT) gradient. Running waters with low MAT generally displayed positive DOC‐discharge correlations whereas the relationships in sites with higher MAT were more variable. This reflected contrasting relationships between temperature and discharge with discharge positively correlated with temperature in cold areas, while it was negatively correlated with temperature in catchments with higher MAT. Sites where flow, temperature and month were poorly related to intra‐annual DOC dynamics were large catchments or sites with extensive upstream lake cover. DOC trends were generally much smaller than intra‐annual DOC variability and did not show any North–south gradient. Our findings suggest that DOC in running waters could respond to a changing climate in ways not predictable, or even discernible, from extrapolation of recent inter‐annual trends.
       
  • Natural and anthropogenic variations in atmospheric mercury deposition
           during the Holocene near Quelccaya Ice Cap, Peru
    • Abstract: Mercury (Hg) is a toxic metal that is transported globally through the atmosphere. The emission of Hg from mineral reservoirs and subsequent recycling in surface reservoirs (i.e., soil/biomass, ocean, and atmosphere) are fundamental to the modern global Hg cycle, yet past emissions from anthropogenic and natural sources are not fully constrained. We use a sediment core from Yanacocha, a headwater lake in southeastern Peru, to study the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. From 12.3 to 3.5 ka, Hg fluxes in the record are relatively constant (mean ± 1σ: 1.4 ± 0.6 µg m‐2 a‐1, n = 189). Past Hg deposition does not correlate with changes in regional temperature and precipitation, inferred from nearby paleoclimate records, or with most large volcanic events that occurred regionally, in the Andean Central Volcanic Zone (~300‐400 km from Yanacocha), and globally. In B.C. 1450 (3.4 ka), Hg fluxes abruptly increased and reached the Holocene‐maximum flux (6.7 µg m‐2 a‐1) in B.C. 1200, concurrent with a ~100‐year peak in Fe and chalcophile metals (As, Ag, Tl) and the presence of framboidal pyrite. Continuously elevated Hg fluxes from B.C. 1200‐500 suggest a protracted mining‐dust source near Yanacocha that is identical in timing to documented pre‐Incan cinnabar mining in central Peru. During Incan and Colonial time (A.D. 1450‐1650), Hg deposition remains elevated relative to background levels but lower relative to other Hg records from sediment cores in central Peru, indicating a limited spatial extent of preindustrial Hg emissions. Hg fluxes from A.D. 1980 to 2011 (4.0 ± 1.0 µg m‐2 a‐1, n = 5) are 3.0 ± 1.5 times greater than pre‐anthropogenic fluxes and are similar to modern fluxes documented in remote lakes around the world.
       
  • Representative regional sampling of carbon dioxide and methane
           concentrations in hemiboreal headwater streams reveal underestimates in
           less systematic approaches
    • Abstract: Boreal headwater streams have been identified as hotspots for water‐air exchange of greenhouse gases (GHG´s). Despite these findings GHG concentrations and fluxes in headwaters are to a great extent unexplored at large (regional/national) scales. This study was the first to systematically determine the concentrations of CO2 and CH4 in hemiboreal (southern boreal and boreonemoral) headwater streams. The use of a headspace sampling method focusing on GHG´s in combination with a statistically representative selection of more than 200 streams across two regions in Sweden was the basis for defining the base flow supersaturation level of CO2 and CH4. All streams were supersaturated relative to the atmosphere in CO2 (median concentration, 1.9 (±1.1) mg C L‐1) and the majority in CH4 (median concentration, 7.1 (±54.0) µg C L‐1 for the 82% of streams in which CH4 was detected). The spatial variability in both CO2 and CH4 was high but positively related to total organic carbon, mean annual temperature and proportion of peatland in the catchment. There were however regional differences in the spatial controls, which is something that predictive models need to consider. The large and representative data set allowed for comparison between a headspace and an alkalinity‐based method for determining CO2 in these headwaters. More than 50% of the streams contained no alkalinity which made the alkalinity based determination of CO2 impossible. In addition, half of the streams with alkalinity alkalinities low enough (
       
  • APPLICATION OF REMOTE SENSING TO UNDERSTANDING FIRE REGIMES AND BIOMASS
           BURNING EMISSIONS OF THE TROPICAL ANDES
    • Abstract: In the tropical Andes, fires play an important cultural and ecological role. However, there have been very few systematic studies aimed at understanding the biomass burning dynamics in the area. This paper seeks to advance on our understanding of burning regimes in this region, with the first detailed and comprehensive assessment of fire occurrence and the derived gross biomass burning emissions of an area of the Peruvian tropical Andes. We selected for analysis an area of 2.8 million hectares at altitudes over 2000 m. We analyzed fire occurrence over a 12‐year period with three types of satellite data: active fire pixels from the MODerate Resolution Imaging Spectroradiometer (MODIS) MCD14ML product, burned area scars from the MODIS MCD45 product, and higher resolution Landsat 5 TM imagery. Fire dynamics showed a large intra‐ and inter‐annual variability, with most fires occurring May‐October (the period coinciding with the dry season), and year 2005 having the largest number of fires and burned area. Total area burned decreased with increasing rainfall until a given rainfall threshold beyond which no relationship was found. The estimated fire return interval (FRI) for the area is 37 years for grasslands, which is within the range reported for grasslands and 65 years for forests, which is remarkably shorter than other reported FRI in tropical moist forests. The greatest contribution (60‐70%, depending of the data source) to biomass burning emissions came from burned montane cloud forests (4.5 million Mg CO2 over the study period), despite accounting for only 7.4‐10% of the total burned area. Gross aboveground biomass emissions (7.55 ± 2.14 Tg CO2; 0.43 ± 0.04 Tg CO; 24,012 ± 2,685 Mg CH4 for the study area) were larger than previously reported for the Tropical Andes.
       
  • The contribution of aeolian sand and dust to iron fertilization of
           phytoplankton blooms in southwestern Ross Sea, Antarctica
    • Abstract: Iron (Fe) limitation during the austral summer is a persistent characteristic of primary production in the Ross Sea, Antarctica. Recent observations reveal low dissolved Fe (dFe) concentrations in the Ross Sea polynya after the dFe winter reserve has been consumed in association with high rates of primary production, suggesting significant new sources of dFe are required to sustain the phytoplankton bloom during this period. The accumulation of locally‐derived aeolian sand and dust (ASD) on sea ice is a potential source. To constrain aeolian Fe inputs from melting sea ice, we determined ASD mass accumulation rates as well as the total and soluble Fe content on first year sea ice in McMurdo Sound (Latitude 77.5ºS; Longitude 165ºE), southwestern (SW) Ross Sea. The mean ASD mass accumulation rate was ~1.5 g m‐2 yr‐1, total Fe content of this ASD was 4 ± 1 wt % and the percentage of soluble Fe was 11 ± 1 %. Assuming our results are representative of the 7400 km2 McMurdo Sound region, we use our mean estimate of the bulk aeolian dFe flux of 122.1 µmol m‐2 yr‐1 to calculate that aeolian Fe potentially supports between 9.0 x 109 and 4.1 x 1011 mol C yr‐1 (0.1‐4.9 Tg C yr‐1) of new primary production in McMurdo Sound. This equates to ~15 % of new primary production in the SW Ross Sea, suggesting that locally‐derived aeolian dFe is a minor component of seasonal Fe supply. Consequently, this study highlights the need to quantify other sources of dFe to the Ross Sea region, which can potentially sustain phytoplankton blooms during the austral summer. In comparison to other estimates in the Antarctic region, McMurdo Sound has very high ASD and represents an upper limit of dFe that can be contributed to the ocean from melting sea ice in the Ross Sea.
       
  • Ecological processes dominate the 13C land disequilibrium in a Rocky
           Mountain subalpine forest
    • Abstract: Fossil fuel combustion has increased atmospheric CO2 by ≈ 115 µmol mol‐1 since 1750, and decreased its carbon isotope composition (δ13C) by 1.7‐2 ‰ (the 13C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ13C of CO2 released by terrestrial ecosystems is expected to differ from the δ13C of CO2 assimilated by land plants during photosynthesis. This isotopic difference between land‐atmosphere respiration (δR) and photosynthetic assimilation (δA) fluxes gives rise to the 13C land disequilibrium (D). Contemporary understanding suggests that over annual and longer time scales, D is determined primarily by the Suess effect, and thus D is generally positive (δR > δA). A seven‐year record of biosphere‐atmosphere carbon exchange was used to evaluate the seasonality of δA and δR, and the 13C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ13C of net land‐atmosphere exchange during day and night, and combined with tower‐based flux observations to assess δA and δR. The disequilibrium varied seasonally, and when flux‐weighted was opposite in sign than expected from the Suess effect (D = ‐0.75 ± 0.21 ‰ or ‐0.88 ± 0.10 ‰ depending on method). Seasonality in D appeared to be driven by photosynthetic discrimination (Δcanopy) responding to environmental factors. Possible explanations for negative D include: 1) changes in Δcanopy over decades as CO2 and temperature have risen, and/or 2) post‐photosynthetic fractionation processes leading to sequestration of isotopically‐enriched carbon in long‐lived pools like wood and soil.
       
  • Drivers of pCO2 variability in two contrasting coral reef lagoons: The
           influence of submarine groundwater discharge
    • Abstract: The impact of groundwater on pCO2 variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of pCO2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high pCO2 (5,501 µatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and pCO2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO2 (549 µatm) than Heron Island (471 µatm), however, pCO2 exhibited a greater diel range in Heron Island (778 µatm) than in Rarotonga (507 µatm). SGD flux rates were quantified using a radon (222Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free‐CO2 m−2 d−1 from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free‐CO2 m−2 d−1. Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO2 m−2 d−1 in Rarotonga and Heron Island, respectively), with SGD‐derived free‐CO2 most likely contributing to the outgassing of CO2. Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO3) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.
       
  • Air‐sea CO2 fluxes in the California Current: Impacts of model
           resolution and coastal topography
    • Abstract: The present study uses a suite of coupled physical‐biogeochemical model simulations at 1/3°, 1/10°, and 1/30° to assess the impact of horizontal resolution on air‐sea CO2 fluxes in the California Current System (CCS), a relevant issue for downscaling between coarser resolution global climate models and higher resolution regional models. The results demonstrate that horizontal resolution is important to reproduce the sharp transition between near‐shore outgassing and offshore absorption, as well as to resolve the regions of enhanced near‐shore outgassing in the lee of capes. The width of the outgassing region is overestimated when horizontal resolution is not eddy‐resolving (i.e., 1/3°), but becomes more dependent on shelf topography for eddy‐resolving simulations (i.e., 1/10° and 1/30°). Enhanced near‐shore outgassing is associated with local increases in wind‐driven upwelling in the lee of capes (i.e., expansion fans), meaning that sufficient horizontal resolution is needed both in the ocean circulation model and in the wind field forcing the model. From a global carbon budget perspective, the model indicates that biological production generates sufficient absorption within a few hundred kilometers of the coast to offset near‐shore outgassing, which is consistent with the notion that mid‐latitude eastern boundary current upwelling systems act both as a sink and source for atmospheric CO2. Based on the 1/30° solution, the CCS between 35‐45N and out to 600 km offshore is as a net carbon sink of ca. 6 TgC yr−1, with the 1/10° solution underestimating this value by less than 10% and the 1/3° solution by a factor of three.
       
  • Calcium carbonate dissolution in the upper 1000 m of the eastern
           North Atlantic
    • Abstract: Recent analyses suggest that considerable CaCO3 dissolution may occur in the upper water column of the ocean (< 1500 m). This study uses the distribution of particulate calcium from high‐resolution suspended matter sampling along the CLIVAR/CO2 Repeat Hydrography A16N transect in 2003 to estimate CaCO3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400–1000 m) in the eastern tropical North Atlantic. The CaCO3 dissolution rate in this region is estimated to be 0.9 mmol CaCO3 m‐2 d‐1, indicating this region is a hotspot for upper water column CaCO3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO3 m‐2 d‐1) and are comparable to previous estimates of CaCO3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface‐ocean CaCO3 standing stocks suggests that biologically‐mediated CaCO3 dissolution may be occurring in the top 1000 m of the Atlantic.
       
  • The viscosity effect on marine particle flux – a climate relevant
           feedback mechanism
    • Abstract: Oceanic uptake and long‐term storage of atmospheric carbon dioxide (CO2) are strongly driven by the marine ‘biological pump’, i.e. sinking of biotically fixed inorganic carbon and nutrients from the surface into the deep ocean [Sarmiento and Bender, 1994; Volk and Hoffert, 1985]. Sinking velocity of marine particles depends on seawater viscosity, which is strongly controlled by temperature [Sharqawy et al., 2010]. Consequently, marine particle flux is accelerated as ocean temperatures increase under global warming [Bach et al., 2012]. Here we show that this previously overlooked 'viscosity effect' could have profound impacts on marine biogeochemical cycling and carbon uptake over the next centuries to millennia. In our global‐warming simulation, the viscosity effect accelerates particle sinking by up to 25%, thereby effectively reducing the portion of organic matter that is respired in the surface ocean. Accordingly, the biological carbon pump's efficiency increases, enhancing the sequestration of atmospheric CO2 into the ocean. This effect becomes particularly important on longer timescales when warming reaches the ocean interior. At the end of our simulation (4000 AD) oceanic carbon uptake is 17% higher, atmospheric CO2 concentration is 180 ppm lower, and the increase in global average surface temperature is 8% weaker when considering the viscosity effect. Consequently, the viscosity effect could act as a long‐term negative feedback mechanism in the global climate system.
       
  • A Growing Oceanic Carbon Uptake: Results from an inversion study of
           surface pCO2 data
    • Abstract: Concerted community efforts have been devoted to producing an authoritative climatology of air‐sea CO 2 fluxes [Takahashi et al., 2009], but identifying decadal trends in CO 2 fluxes has proven to be more challenging. The available surface pCO 2 estimates are too sparse to separate long‐term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo [MCMC] sampling as a novel technique for estimating the historical pCO 2 at the ocean surface. The result is a plausible history of surface pCO 2 based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO 2 data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO 2, with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr − 1 of anthropogenic carbon and an increase in the global annual uptake over the 30‐year time period of 0.4 ± 0.1 PgC yr − 1 decade − 1. The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling.
       
  • Climate Warming Shifts Carbon Allocation from Stemwood to Roots in
           Calcium‐Depleted Spruce Forests
    • Abstract: Increased greening of northern forests, measured by the Normalized Difference Vegetation Index (NDVI), has been presented as evidence that a warmer climate has increased both net primary productivity (NPP) and the carbon sink in boreal forests. However, higher production and greener canopies may accompany changes in carbon allocation that favor foliage or fine roots over less decomposable woody biomass. Furthermore, tree core data throughout mid‐ and northern latitudes have revealed a divergence problem (DP), a weakening in tree ring responses to warming over the past half century that is receiving increasing attention, but remains poorly understood. Often, the same sites exhibit trend inconsistency phenomenon (TIP), namely positive, or no trends in growing season NDVI where negative trends in tree ring indexes are observed. Here we studied growth of two Norway spruce (Picea abies) stands in western Russia that exhibited both the DP and TIP but were subject to soil acidification and calcium depletion of differing timing and severity. Our results link the decline in radial growth starting in 1980 to a shift in carbon allocation from wood to roots driven by a combination of two factors: (a) soil acidification that depleted calcium and impaired root function and (b) earlier onset of the growing season that further taxed the root system. The latter change in phenology appears to act as a trigger at both sites to push trees into nutrient limitation as the demand for Ca increased with the longer growing season, thereby causing the shift in carbon allocation. © 2013 American Geophysical Union. All rights reserved.
       
 
 
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