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

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 28, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 121, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 15, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 130)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 29)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 54)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 50)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 111)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 47)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 125)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 39, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 36, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 17, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 15, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 81, SJR: 2.661, h-index: 144)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [15 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Oceanic nitrogen cycling and N2O flux perturbations in the Anthropocene
    • Authors: A. Landolfi; C. Somes, W. Koeve, L. M. Zamora, A. Oschlies
      Abstract: There is currently no consensus on how humans are affecting the marine nitrogen (N) cycle, which limits marine biological production and CO2 uptake. Anthropogenic changes in ocean warming, deoxygenation, and atmospheric N deposition can all individually affect the marine N cycle and the oceanic production of the greenhouse gas nitrous oxide (N2O). However, the combined effect of these perturbations on marine N cycling, ocean productivity, and marine N2O production is poorly understood. Here we use an Earth system model of intermediate complexity to investigate the combined effects of estimated 21st century CO2 atmospheric forcing and atmospheric N deposition. Our simulations suggest that anthropogenic perturbations cause only a small imbalance to the N cycle relative to preindustrial conditions (∼+5 Tg N y−1 in 2100). More N-loss from water-column denitrification in expanded oxygen minimum zones (OMZs) is counteracted by less benthic denitrification, due to the stratification-induced reduction in organic matter export. The larger atmospheric N load is offset by reduced N inputs by marine N2 fixation. Our model predicts a decline in oceanic N2O emissions by 2100. This is induced by the decrease in organic matter export and associated N2O production, and by the anthropogenically-driven changes in ocean circulation and atmospheric N2O concentrations. After comprehensively accounting for a series of complex physical-biogeochemical interactions, this study suggests that N flux imbalances are limited by biogeochemical feedbacks that help stabilize the marine N inventory against anthropogenic changes. These findings support the hypothesis that strong negative feedbacks regulate the marine N inventory on centennial timescales.
      PubDate: 2017-07-24T00:45:35.656801-05:
      DOI: 10.1002/2017GB005633
       
  • Physically driven Patchy O2 Changes in the North Atlantic Ocean simulated
           by the CMIP5 Earth System Models
    • Authors: F. Tagklis; A. Bracco, T. Ito
      Abstract: The subpolar North Atlantic is a key region for the oceanic uptake of heat, oxygen and carbon dioxide. Centennial oxygen (O2) changes are investigated in the upper 700 m of the North Atlantic Ocean using a subset of Earth System Models (ESMs) included in the Coupled Model Intercomparison Project Phase 5. The climatological distributions of dissolved O2 averaged for the recent past period (1975-2005) are generally well captured although the convective activity differs among the models in space and strength, and most models show a cold bias south of Greenland. By the end of the 21st century, all models predict an increase in depth-integrated temperature of 2-3oC, resultant solubility decrease, weakened vertical mass transport, decreased nutrient supply into the euphotic layer, and weakened export production. Despite an overall tendency of the North Atlantic to lose oxygen, patchy regions of O2 increase are observed due to the weakening of the North Atlantic Current (NAC) causing a regional solubility increase (the warming hole effect) and a decrease in the advection of subtropical, low-O2 waters into the subpolar regions (the nutrient stream effect). Additionally, a shift in the NAC position contributes to localized O2 changes near the boundaries of water masses. The net O2 change reflects the combination of multiple factors leading to highly heterogeneous and model-dependent patterns. Our results imply that changes in the strength and position of the NAC will likely play crucial roles in setting the pattern of O2 changes in future projections.
      PubDate: 2017-07-22T16:10:27.436181-05:
      DOI: 10.1002/2016GB005617
       
  • In situ measurements of atmospheric O2 and CO2 reveal an unexpected O2
           signal over the tropical Atlantic Ocean
    • Authors: Penelope A. Pickers; Andrew C. Manning, William T. Sturges, Corinne Le Quéré, Sara E. Mikaloff Fletcher, Philip A. Wilson, Alex J. Etchells
      Abstract: We present the first meridional transects of atmospheric O2 and CO2 over the Atlantic Ocean. We combine these measurements into the tracer Atmospheric Potential Oxygen (APO), which is a measure of the oceanic contribution to atmospheric O2 variations. Our new in situ measurement system, deployed on board a commercial container ship during 2015, performs as well as or better than existing similar measurements systems. The data show small short-term variability (hours to days), a step-change corresponding to the position of the Intertropical Convergence Zone (ITCZ), and a seasonal cycle symmetrical around the equator. In contrast to data from the Pacific Ocean and to previous modelling studies, our new Atlantic Ocean APO data show no significant bulge in the tropics. This difference cannot be accounted for by interannual variability in the position of the ITCZ or the Atlantic Meridional Mode Index and appears to be a permanent feature of the Atlantic Ocean system. Modelled APO using the TM3 atmospheric transport model does exhibit a significant bulge over the Atlantic and over-estimates the interhemispheric gradient in APO over the Atlantic Ocean. These results indicate that either there are inaccuracies in the oceanic flux data products in the equatorial Atlantic Ocean region, or that there are atmospheric transport inaccuracies in the model, or a combination of both. Our shipboard O2 and CO2 measurements are on-going, and will reveal the long-term nature of equatorial APO outgassing over the Atlantic as more data become available.
      PubDate: 2017-07-19T07:03:25.975775-05:
      DOI: 10.1002/2017GB005631
       
  • Global Evaluation of Particulate Organic Carbon Flux Parameterizations and
           Implications for Atmospheric pCO2
    • Authors: Lucas Gloege; Galen A. McKinley, Colleen B. Mouw, Audrey B. Ciochetto
      Abstract: The shunt of photosynthetically derived particulate organic carbon (POC) from the euphotic zone and deep remineralization comprises the basic mechanism of the “biological carbon pump.” POC raining through the “twilight zone” (euphotic depth to 1 km) and “midnight zone” (1 km to 4 km) is remineralized back to inorganic form through respiration. Accurately modeling POC flux is critical for understanding the “biological pump” and its impacts on air-sea CO2 exchange and, ultimately, long-term ocean carbon sequestration. Yet, commonly used parameterizations have not been tested quantitatively against global datasets using identical modeling frameworks. Here, we use a single one-dimensional physical-biogeochemical modeling framework to assess three common POC flux parameterizations in capturing POC flux observations from moored sediment traps and thorium-234 depletion. The exponential decay, Martin curve, and ballast model are compared to data from 11 biogeochemical provinces distributed across the globe. In each province, the model captures satellite-based estimates of surface primary production within uncertainties. Goodness-of-fit is measured by how well the simulation captures the observations, quantified by bias and the root-mean-squared-error and displayed using “target diagrams.” Comparisons are presented separately for the twilight zone and midnight zone. We find the ballast hypothesis shows no improvement over a globally or regionally parameterized Martin curve. For all provinces taken together, Martin's b that best fits the data is [0.70, 0.98]; this finding reduces by at least a factor of 3 previous estimates of potential impacts on atmospheric pCO2 of uncertainty in POC export to a more modest range [-16 ppm, +12 ppm].
      PubDate: 2017-07-19T06:45:56.461676-05:
      DOI: 10.1002/2016GB005535
       
  • Untangling biogeochemical processes from the impact of ocean circulation:
           First insight on the Mediterranean dissolved barium dynamics
    • Authors: L. Jullion; S. H. M. Jacquet, T. Tanhua
      Abstract: Based on an unprecedented dissolved barium (D_Ba) data set collected in the Mediterranean Sea during a zonal transect between the Lebanon coast and Gibraltar (M84/3 cruise, April 2011), we decompose the D_Ba distribution to isolate the contribution of biogeochemical processes from the impact of the oceanic circulation. We have built a simple parametric water mass analysis (Parametric Optimum Multi Parameter - POMP- analysis) to reconstruct the contribution of the different Mediterranean water masses to the thermohaline structure. These water mass fractions have then been used to successfully reconstruct the background vertical gradient of D_Ba reflecting the balance between the large-scale oceanic circulation and the biological activity over long time scales. Superimposed on the background field, several D_Ba anomalies have been identified. Positive anomalies are associated with topographic obstacles and may be explained by the dissolution of particulate biogenic barium (P_Ba barite) of material resuspended by the local currents. The derived dissolution rates range from 0.06 to 0.21 μmol m−2 d−1. Negative anomalies are present in the mesopelagic region of the western and eastern basins (except in the easternmost Levantine basin) as well as in the abyssal western basin. This represents the first quantification of the non-conservative component of the D_Ba signal. These mesopelagic anomalies could reflect the subtraction of D_Ba during P_Ba barite formation occurring during organic carbon remineralisation. The deep anomalies may potentially reflect the transport of material towards the deep sea during winter deep convection and the subsequent remineralisation. The D_Ba subtraction fluxes range from -0.07 to -1.28 μmol m−2 d−1. D_Ba derived fluxes of P_Ba barite (up to 0.21 μmol m−2 d−1) and organic carbon (13 to 29 mmol C m−2 d−1) are in good agreement with other independent measurements suggesting that D_Ba can help constraining remineralisation horizons. This study highlights the importance of quantifying the impact of the large-scale oceanic circulation in order to better understand the biogeochemical cycling of elements and to build reliable geochemical proxies.
      PubDate: 2017-07-19T02:43:36.245085-05:
      DOI: 10.1002/2016GB005489
       
  • Sea-air exchange patterns along the central and outer East Siberian Arctic
           Shelf as inferred from continuous CO2, stable isotope and bulk chemistry
           measurements
    • Authors: Christoph Humborg; Marc C. Geibel, Leif G. Anderson, Göran Björk, Carl-Magnus Mörth, Marcus Sundbom, Brett F. Thornton, Barbara Deutsch, Erik Gustafsson, Bo Gustafsson, Jörgen Ek, Igor Semiletov
      Abstract: This large-scale quasi-synoptic study gives a comprehensive picture of sea-air CO2 fluxes during the melt season in the central and outer Laptev Sea (LS) and East Siberian Sea (ESS). During a 7-week cruise we compiled a continuous record of both surface water and air CO2 concentrations, in total 76 892 measurements. Overall, the central and outer parts of the ESAS constituted a sink for CO2, and we estimate a median uptake of 9.4 g C m-2 y-1 or 6.6 Tg C y-1. Our results suggest that while the ESS and shelf break waters adjacent to the LS and ESS are net autotrophic systems, the LS is a net heterotrophic system. CO2 sea-air fluxes for the LS were 4.7 g C m-2 y-1 and for the ESS we estimate an uptake of 7.2 g C m-2 y-1. Isotopic composition of dissolved inorganic carbon (δ13CDIC and δ13CCO2) in the water column indicate that the LS is depleted in δ13CDIC compared to the Arctic Ocean (ArcO) and ESS with an offset of 0.5‰ which can be explained by mixing of δ13CDIC depleted riverine waters and 4.0 Tg y-1 respiration of OCter; only a minor part (0.72 Tg y-1) of this respired OCter is exchanged with the atmosphere. Property-mixing diagrams of total organic carbon (TOC) and isotope ratio (δ13CSPE-DOC) vs. DOC concentration diagram indicate conservative and non-conservative mixing in the LS and ESS, respectively. We suggest land derived particulate organic carbon (POC) from coastal erosion as an additional significant source for the depleted δ13CDIC.
      PubDate: 2017-07-12T22:40:30.768823-05:
      DOI: 10.1002/2017GB005656
       
  • Temperature dependence of plankton community metabolism in the subtropical
           and tropical ocean
    • Authors: Lara S. Garcia-Corral; Johnna M. Holding, Paloma Carrillo-de-Albornoz, Alexandra Steckbauer, María Pérez-Lorenzo, Nuria Navarro, Pablo Serret, Josep M. Gasol, Xosé Anxelu G. Morán, Marta Estrada, Eugenio Fraile-Nuez, Verónica Benítez-Barrios, Susana Agusti, Carlos M. Duarte
      Abstract: Here we assess the temperature dependence of the metabolic rates (gross primary production - GPP, community respiration - CR and the ratio GPP/CR) of oceanic plankton communities. We compile data from 133 stations of the Malaspina 2010 Expedition, distributed among the subtropical and tropical Atlantic, Pacific and Indian oceans. We used the in vitro technique to measured metabolic rates during 24 h incubations at three different sampled depths: surface, 20% and 1% of the photosynthetically active radiation measured at surface. We also measured the % of ultraviolet B radiation (UVB) penetrating at surface waters. GPP and CR rates increased with warming, albeit different responses were observed for each sampled depth. The overall GPP/CR ratio declined with warming. Higher activation energies (Ea´s) were derived for both processes (GPPChla = 0.97; CRChla = 1.26; CRHPA= 0.95 eV) compared to those previously reported. The Indian Ocean showed the highest Ea (GPPChla = 1.70; CRChla = 1.48; CRHPA= 0.57 eV), while the Atlantic Ocean showed the lowest (GPPChla = 0.86; CRChla = 0.77; CRHPA= -0.13 eV). We believe that the difference between previous assessments and the ones presented here can be explained by the overrepresentation of Atlantic communities in the previous data sets. We found that UVB radiation also affects the temperature dependence of surface GPP, which decreased rather than increased under high levels of UVB. Ocean warming, which causes stratification and oligotrophication of the subtropical and tropical oceans, may lead to reduced surface GPP as a result of increased penetration of UVB radiation.
      PubDate: 2017-06-22T08:35:22.281049-05:
      DOI: 10.1002/2017GB005629
       
  • Biological production, export efficiency, and phytoplankton communities
           across 8000 km of the South Atlantic
    • Authors: E. M. Howard; C. A. Durkin, G. M. M. Hennon, F. Ribalet, R. H. R. Stanley
      Abstract: In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March-May 2013. We used observations of pico-, nano-, and microphytoplankton to evaluate community structure and diversity, and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O2 m-2 d-1, gross primary production (GPP) between 40 and 100 mmol O2 m-2 d-1, and NCP/GPP, a measure of export efficiency, ranging from 0.1-0.2 (0.05-0.1 in carbon units). However, notable exceptions to this basin scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of DOC export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.
      PubDate: 2017-06-16T01:19:57.336504-05:
      DOI: 10.1002/2016GB005488
       
  • Assessing trends and uncertainties in satellite-era ocean chlorophyll
           using space-time modeling
    • Authors: Matthew L. Hammond; Claudie Beaulieu, Sujit K. Sahu, Stephanie A. Henson
      Abstract: The presence, magnitude, and even direction of long-term trends in phytoplankton abundance over the past few decades is still debated in the literature, primarily due to differences in the data sets and methodologies used. Recent work has suggested that the satellite chlorophyll record is not yet long enough to distinguish climate change trends from natural variability, despite the high density of coverage in both space and time. Previous work has typically focused on using linear models to determine the presence of trends, where each grid cell is considered independently from its neighbors. However, trends can be more thoroughly evaluated using a spatially resolved approach. Here a Bayesian hierarchical spatio-temporal model is fitted to quantify trends in ocean chlorophyll from September 1997 to December 2013. The approach used in this study explicitly accounts for the dependence between neighboring grid cells, which allows us to estimate trend by ‘borrowing strength’ from the spatial correlation. By way of comparison, a model without spatial correlation is also fitted. This results in a notable loss of accuracy in model fit. Additionally, we find an order of magnitude smaller global trend, and larger uncertainty, when using the spatio-temporal model: -0.023 ± 0.12 % yr-1 as opposed to -0.38 ± 0.045 % yr-1 when the spatial correlation is not taken into account. The improvement in accuracy of trend estimates, and the more complete account of their uncertainty emphasizes the solution that space-time modeling offers for studying global long-term change.
      PubDate: 2017-06-15T20:00:23.82708-05:0
      DOI: 10.1002/2016GB005600
       
  • Quantification of uncertainties in global grazing systems assessments
    • Authors: T. Fetzel; P. Havlik, M. Herrero, J. O. Kaplan, T. Kastner, C. Kroisleitner, S. Rolinski, T. Searchinger, P. M. Bodegom, S. Wirsenius, K.-H. Erb
      Abstract: Livestock systems play a key role in global sustainability challenges like food security and climate change, yet, many unknowns and large uncertainties prevail. We present a systematic, spatially explicit assessment of uncertainties related to grazing intensity (GI), a key metric for assessing ecological impacts of grazing, by combining existing datasets on a) grazing feed intake, b) the spatial distribution of livestock, c) the extent of grazing land, and d) its net primary productivity (NPP). An analysis of the resulting 96 maps implies that on average 15% of the grazing land NPP is consumed by livestock. GI is low in most of worlds grazing lands but hotspots of very high GI prevail in 1% of the total grazing area. The agreement between GI maps is good on one fifth of the world's grazing area, while on the remainder it is low to very low. Largest uncertainties are found in global drylands and where grazing land bears trees (e.g., the Amazon basin or the Taiga belt). In some regions like India or Western Europe massive uncertainties even result in GI> 100% estimates. Our sensitivity analysis indicates that the input-data for NPP, animal distribution and grazing area contribute about equally to the total variability in GI maps, while grazing feed intake is a less critical variable. We argue that a general improvement in quality of the available global level datasets is a precondition for improving the understanding of the role of livestock systems in the context of global environmental change or food security.
      PubDate: 2017-06-15T18:35:18.67075-05:0
      DOI: 10.1002/2016GB005601
       
  • Slow Sinking Particulate Organic Carbon in the Atlantic Ocean: magnitude,
           flux and potential controls
    • Authors: Chelsey A. Baker; Stephanie A. Henson, Emma L. Cavan, Sarah L. C. Giering, Andrew Yool, Marion Gehlen, Anna Belcher, Jennifer S. Riley, Helen E. K. Smith, Richard Sanders
      Abstract: The remineralization depth of particulate organic carbon (POC) fluxes exported from the surface ocean exert a major control over atmospheric CO₂ levels. According to a long held paradigm most of the POC exported to depth is associated with large particles. However, recent lines of evidence suggest that slow sinking POC (SSPOC) may be an important contributor to this flux. Here we assess the circumstances under which this occurs. Our study uses samples collected using the Marine Snow Catcher throughout the Atlantic Ocean, from high latitudes to mid latitudes. We find median SSPOC concentrations of 5.5 μg L-1, 13 times smaller than suspended POC concentrations and 75 times higher than median fast sinking POC (FSPOC) concentrations (0.07 μg L-1). Export fluxes of SSPOC generally exceed FSPOC flux, with the exception being during a spring bloom sampled in the Southern Ocean. In the Southern Ocean SSPOC fluxes often increase with depth relative to FSPOC flux, likely due to midwater fragmentation of FSPOC, a process which may contribute to shallow mineralization of POC and hence to reduced carbon storage. Biogeochemical models do not generally reproduce this behaviour, meaning that they likely overestimate long term ocean carbon storage.
      PubDate: 2017-06-15T18:30:19.648295-05:
      DOI: 10.1002/2017GB005638
       
  • Effects of parameter indeterminacy in pelagic biogeochemical modules of
           Earth System Models on projections into a warming future: the scale of the
           problem
    • Authors: U. Löptien; H. Dietze
      Abstract: Numerical Earth System Models are generic tools used to extrapolate present climate conditions into a warming future and to explore geo-engineering options. Most of the current-generation models feature a simple pelagic biogeochemical model component that is embedded into a three dimensional general circulation ocean model. The dynamics of these biogeochemical model components is essentially controlled by so-called model parameters most of which are poorly known. Here we explore the feasibility to estimate these parameters in a full-fledged three dimensional Earth System Model by minimizing the misfit to noisy observations. The focus is on parameter identifiability. Based on earlier studies, we illustrate problems in determining a unique estimate of those parameters, that prescribe the limiting effect of nutrient and light-depleted conditions on carbon assimilation by autotrophic phytoplankton. Our results showcase that for typical models and evaluation metrics no meaningful “best” unique parameter set exists. We find very different parameter sets which are, on the one hand, equally consistent with our (synthetic) historical observations while, on the other hand, they propose strikingly differing projections into a warming climate.
      PubDate: 2017-06-15T18:20:21.712728-05:
      DOI: 10.1002/2017GB005690
       
  • Controls on the distribution of fluorescent dissolved organic matter
           during an under-ice algal bloom in the western Arctic Ocean
    • Authors: Wilson G. Mendoza; Elliot L. Weiss, Brian Schieber, B. Greg Mitchell
      Abstract: In this study we used fluorescence excitation and emission spectroscopy (EEMs), hydrographic data and a self-organizing map (SOM) analysis to assess the spatial distribution of labile and refractory FDOM for the Chukchi and Beaufort Seas at the time of a massive under-ice phytoplankton bloom during early summer 2011. Biogeochemical properties were assessed through decomposition of water property classes and sample classification that employed a SOM neural network-based analysis which classified ten clusters from 269 samples and 17 variables. The terrestrial, humic-like component FDOM (ArC1, 4.98±1.54 QSU) and protein-like component FDOM (ArC3, 1.63±0.88 QSU) were found to have elevated fluorescence in the LPML (salinity ~29.56±0.76). In the LPML water mass, the observed contribution of meteoric water fraction was 17%, relative to a 12% contribution from the sea-ice melt fraction. The labile ArC3-protein-like component (2.01±1.92 QSU) was also observed to be elevated in the PWW mass, where the under-ice algal bloom was observed (~40-50 m). We interpreted these relationships to indicate that the accumulation and variable distribution of the protein-like component on the shelf could be influenced directly by sea-ice melt, transport and mixing processes; and indirectly by the in situ algal bloom and microbial activity. ArC5, corresponding to what is commonly considered marine humic FDOM indicated a bimodal distribution with high values both in the freshest and saltiest waters. The association of ArC5 with deep, dense salty water is consistent with this component as refractory humic-like FDOM whereas our evidence of a terrestrial origin challenges this classic paradigm for this component.
      PubDate: 2017-06-14T10:35:20.487245-05:
      DOI: 10.1002/2016GB005569
       
  • Temperature and oxygen dependence of the remineralization of organic
           matter
    • Authors: C. Laufkötter; Jasmin G. John, Charles A. Stock, John P. Dunne
      Abstract: Accurate representation of the remineralization of sinking organic matter is crucial for reliable projections of the marine carbon cycle. Both water temperature and oxygen concentration are thought to influence remineralization rates, but limited data constraints have caused disagreement concerning the degree of these influences. We analyse a compilation of POC flux measurements from 19 globally distributed sites. Our results indicate that the attenuation of the flux of particulate organic matter depends on temperature with a Q10 between 1.5 and 2.01, and on oxygen described by a half saturation constant between 4 and 12 μmol/L. We assess the impact of the temperature and oxygen dependence in the biogeochemistry model COBALT, coupled to GFDL's Earth System Model ESM2M. The new remineralization parameterization results in shallower remineralization in the low latitudes but deeper remineralization in the high latitudes, redistributing POC flux towards the poles. It also decreases the volume of the oxygen minimum zones, partly addressing a long-standing bias in global climate models. Extrapolating temperature-dependent remineralization rates to the surface (i.e., beyond the depth range of POC flux data) resulted in rapid recycling and excessive surface nutrients. Surface nutrients could be ameliorated by reducing near surface rates in a manner consistent with bacterial colonization, suggesting the need for improved remineralization constraints within the euphotic zone. The temperature and oxygen dependence cause an additional 10% decrease in global POC flux at 500m depth, but no significant change in global POC flux at 2000m under the RCP8.5 future projection.
      PubDate: 2017-06-06T18:00:21.600766-05:
      DOI: 10.1002/2017GB005643
       
  • Characteristics of the surface water DMS and pCO2 distributions and their
           relationships in the Southern Ocean, southeast Indian Ocean and northwest
           Pacific Ocean
    • Abstract: Oceanic dimethyl sulfide (DMS) is of interest due to its critical influence on atmospheric sulfur compounds in the marine atmosphere and its hypothesized significant role in global climate. High resolution shipboard underway measurements of surface seawater DMS and the partial pressure of carbon dioxide (pCO2) were conducted in the Atlantic Ocean and Indian Ocean sectors of the Southern Ocean (SO), the southeast Indian Ocean and the northwest Pacific Ocean from February – April 2014 during the 30th Chinese Antarctic Research Expedition. The SO, particularly in the region south of 58 °S, had the highest mean surface seawater DMS concentration of 4.1 ± 8.3 nM (ranged from 0.1 to 73.2 nM) and lowest mean seawater pCO2 level of 337 ± 50 μatm (ranged from 221 to 411 μatm) over the entire cruise. Significant variations of surface seawater DMS and pCO2 in the seasonal ice zone (SIZ) of SO were observed, which are mainly controlled by biological process and sea ice activity. We found a significant negative relationship between DMS and pCO2 in the SO SIZ using 0.1 degree resolution, [DMS] seawater = - 0.160 [pCO2] seawater + 61.8 (r2 = 0.594, n = 924, p < 0.001). We anticipate that the relationship may possibly be utilized to reconstruct the surface seawater DMS climatology in the SO SIZ. Further studies are necessary to improve the universality of this approach.
       
  • Complex Terrain Influences Ecosystem Carbon Responses to Temperature and
           Precipitation
    • Abstract: Terrestrial ecosystem responses to temperature and precipitation have major implications for the global carbon cycle. Case studies demonstrate that complex terrain, which accounts for more than 50% of Earth's land surface, can affect ecological processes associated with land-atmosphere carbon fluxes. However, no studies have addressed the role of complex terrain in mediating ecophysiological responses of land-atmosphere carbon fluxes to climate variables. We synthesized data from AmeriFlux towers and found that for sites in complex terrain, responses of ecosystem CO2 fluxes to temperature and precipitation are organized according to terrain slope and drainage area, variables associated with water and energy availability. Specifically, we found that for tower sites in complex terrain, mean topographic slope and drainage area surrounding the tower explained between 51% and 78% of site-to-site variation in the response of CO2 fluxes to temperature and precipitation depending on the time scale. We found no such organization among sites in flat terrain, even though their flux responses exhibited similar ranges. These results challenge prevailing conceptual framework in terrestrial ecosystem modeling that assumes CO2 fluxes derive from vertical soil-plant-climate interactions. We conclude that the terrain in which ecosystems are situated can also have important influences on CO2 responses to temperature and precipitation. This work has implications for about 14% of the total land area of the conterminous US. This area is considered topographically complex and contributes to approximately 15% of gross ecosystem carbon production in the conterminous US.
       
  • Issue Information
    • Abstract: No abstract is available for this article.
       
  • North Atlantic Deep Water Formation Inhibits High Arctic Contamination by
           Continental Perfluorooctane Sulfonate (PFOS) Discharges
    • Abstract: Perfluorooctane sulfonate (PFOS) is an aliphatic fluorinated compound with eight carbon atoms that is extremely persistent in the environment and can adversely affect human and ecological health. The stability, low reactivity, and high water solubility of PFOS combined with the North American phase-out in production around the year 2000, make it a potentially useful new tracer for ocean circulation. Here we characterize processes affecting the lifetime and accumulation of PFOS in the North Atlantic Ocean and transport to sensitive Arctic regions by developing a 3-D simulation within the MITgcm. The model captures variability in measurements across biogeographical provinces (R2 = 0.90, p=0.01). In 2015, the North Atlantic PFOS reservoir was equivalent to 60% of cumulative inputs from the North American and European continents (1400 Mg). Cumulative inputs to the Arctic accounted for 30% of continental discharges, while the remaining 10% was transported to the tropical Atlantic and other regions. PFOS concentrations declined rapidly after 2002 in the surface mixed-layer (half-life: 1-2 years) but are still increasing below 1000 m depth. During peak production years (1980-2000), plumes of PFOS enriched seawater were transported to the Subarctic in energetic surface ocean currents. However, Atlantic Meridional Overturning Circulation (AMOC) and deep ocean transport returned a substantial fraction of this northward transport (20%, 530 Mg) to southern latitudes and reduced cumulative inputs to the Arctic (730 Mg) by 70%. Weakened AMOC due to climate change is thus likely to increase the magnitude of persistent bioaccumulative pollutants entering the Arctic Ocean.
       
  • Biological and land use controls on the isotopic composition of aquatic
           carbon in the Upper Mississippi River Basin
    • Abstract: Riverine ecosystems receive organic matter (OM) from terrestrial sources, internally produce new OM, and biogeochemically cycle and modify organic and inorganic carbon. Major gaps remain in the understanding of the relationships between carbon sources and processing in river systems. Here we synthesize isotopic, elemental, and molecular properties of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the Upper Mississippi River (UMR) system above Wabasha, MN, including the main stem Mississippi River and its four major tributaries (Minnesota, upper Mississippi, St. Croix, and Chippewa Rivers). Our goal was to elucidate how biological processing modifies the chemical and isotopic composition of aquatic carbon pools during transport downstream in a large river system with natural and man-made impoundments. Relationships between land cover and DOC carbon-isotope composition, absorbance, and hydrophobic acid content indicate that DOC retains terrestrial carbon source information, while the terrestrial POC signal is largely replaced by autochthonous organic matter, and DIC integrates the influence of in-stream photosynthesis and respiration of organic matter. The UMR is slightly heterotrophic throughout the year, but pools formed by low-head navigation dams and natural impoundments promote a shift towards autotrophic conditions, altering aquatic ecosystem dynamics and POC and DIC composition. Such changes likely occur in all major river systems affected by low-head dams and need to be incorporated into our understanding of inland water carbon dynamics and processes controlling CO2 emissions from rivers, as new navigation and flood control systems are planned for future river and water resources management.
       
  • A Model for CH2D2 and 13CH3D as Complementary Tracers for the Budget of
           Atmospheric CH4
    • Abstract: We present a theoretical model to investigate the potential of 13CH3D and 12CH2D2, the doubly substituted mass-18 isotopologues of methane, as tools for tracking atmospheric methane sources and sinks. We use electronic structure methods to estimate kinetic isotope fractionations associated with the major sink reactions of methane in air (reactions with OH and Cl radicals), and combine literature data with reconnaissance measurements of the relative abundances of 13CH3D and 12CH2D2 to estimate the compositions of the largest atmospheric sources. This model atmospheric budget is investigated with a simplified box model in which we explore both steady state and dynamical (non-steady state) conditions triggered by changes in emission or sink fluxes. The steady-state model predicts that sink reactions will generate a marked (>100‰) clumped isotope excess in atmospheric Δ12CH2D2 relative to the net source composition. 12CH2D2 measurements may thus be useful for tracing both atmospheric source and sink fluxes. The effect of sinks on Δ13CH3D is much less pronounced, indicating that 13CH3D in air will give a more focused picture of the source composition.
       
 
 
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