bluuebunny/crossref_metadata_2025 huggingface dataset:
Retrieval of aerosol optical depth from surface solar radiation measurements using machine learning algorithms, nonlinear regression and a radiative transfer based look-up table
Authors: Huttunen J. , Kokkola H. , Mielonen T. , Mononen M. , Lipponen A. , Reunanen J. , Lindfors A. V. , Mikkonen S. , Lehtinen K. E. J. , Kouremeti N. , Bais A. , Niska H. , Arola A.
Published: January 2016

Abstract. In order to have a good estimate of the current forcing by anthropogenic aerosols knowledge on past aerosol levels is needed. Aerosol optical depth (AOD) is a good measure for aerosol loading. However, dedicated measurements of AOD are only available from 1990’s onward. One option to lengthen the AOD time series beyond 1990’s is to retrieve AOD from surface solar radiation (SSR) measurements done with pyranometers. In this work, we have evaluated several inversion methods designed for this task. We compared a look-up table method based on radiative transfer modelling, a nonlinear regression method and four machine learning methods (Gaussian Process, Neural Network, Random Forest and Support Vector Machine) with AOD observations done with a sun photometer at an Aerosol Robotic Network (AERONET) site in Thessaloniki, Greece. Our results show that most of the machine learning methods produce AOD estimates comparable to the look-up table and nonlinear regression methods. All of the applied methods produced AOD values that corresponded well to the AERONET observations with the lowest correlation coefficient value being 0.87 for the Random Forest method. While many of the methods tended to slightly overestimate low AODs and underestimate high AODs, Neural network and support vector machine showed overall better correspondence for the whole AOD range. The differences in producing both ends of the AOD range seem to be caused by differences in the aerosol composition. High AODs were in most cases those with high water vapour content which might affect the aerosol single scattering albedo (SSA) through uptake of water into aerosols. Our study indicates that machine learning methods benefit from the fact that they do not constrain the aerosol SSA in the retrieval where as the LUT method assumes a constant value for it. This would also mean that machine learning methods could have potential in reproducing AOD from SSR even though SSA would have changed during the observation period.
Water Vapor Inhibits Hydrogen Sulfide Detection in Pulsed Fluorescence Sulfur Monitors
Authors: Bluhme Anders B. , Ingemar Jonas L. , Meusinger Carl , Johnson Matthew S.
Published: January 2016

Abstract. The Thermo Scientiﬁc 450 Hydrogen Sulﬁde - Sulfur Dioxide Analyzer measures both H2S and SO2. SO2 is measured by pulsed ﬂuorescence, while H2S is converted to SO2 with a molybdenum catalyst prior to detection. The 450 is widely used to measure ambient concentrations, e.g. for emissions monitoring and pollution control. An air stream with a constant H2S concentration was generated and the output of the analyzer recorded as a function of relative humidity. The analyzer under-reported H2S as soon as the relative humidity was increased. The fraction of undetected H2S increased from 8.3 % at 5.3 % RH (294 K) to over 34 % at RH > 80 %. H2S mole fractions of 573, 1142, and 5145 ppb were tested. The ﬁndings indicate that previous results obtained with instruments using similar catalysts should be re-evaluated to correct for interference from water vapor. It is suspected that water decreases the efﬁciency of the converter unit and thereby reduces the measured H2S concentration.
Direct assimilation of Chinese FY-3C Microwave Temperature Sounder-2 radiances in the global GRAPES system
Authors: Li J. L. , Liu G. L.
Published: January 2016

Abstract. FengYun-3C (FY-3C) is an operational polar-orbiting satellite carrying the new-generation microwave sounding instruments in China. This paper describes the assimilation of the FY-3C Microwave Temperature Sounder-2 (MWTS-2) radiances in the Global and Regional Assimilation and PrEdiction System (GRAPES) of China Meteoroligcal Administration. A quality control (QC) procedure for the assimilation of MWTS-2 radiance is proposed. Extensive monitoring before assimilation shows that MWTS-2 observations exhibit a clear striping pattern. A technique combining principal component analysis (PCA) and Ensemble Empirical Mode Decomposition (EEMD) is applied to the observations to remove the striping noise. Cloudy field-of-views (FOVs) are identified by applying the Visible and InfrarRed Radiometer (VIRR) cloud fraction threshold of 76%. Other QC steps include the following: (i) eight outmost FOVs, (ii) channel 6 if the terrain altitude is greater than 500 m, (iii) channel 5 over sea ice and land, (iv) coastal FOVs, and (v) outliers with large differences between observations and model simulations. Approximately 83%, 75%, 40% and 40% of the observations are removed by the proposed QC for channels 5-8, respectively. After QC, the global biases and standard deviations are reduced significantly. The assimilation of the MWTS-2 radiances shows a positive impact when the control experiment assimilates only conventional observations. The experiments also show that the analysis and forecast errors are slightly reduced when the striping noise is removed from the observations. The quality control scheme of extracting the striping noise may contribute to the analysis and forecast. The impact of MWTS-2 is neutral when the conventional data and other satellite data are all assimilated.
Comparison of XCO abundances from the Total Carbon Column Observing Network and the Network for the Detection of Atmospheric Composition Change measured in Karlsruhe
Authors: Kiel M. , Hase F. , Blumenstock T. , Kirner O.
Published: January 2016

Abstract. We present a comparison of Karlsruhe XCO records (April 2010–December 2014) from the Total Carbon Column Observing Network (TCCON) and retrieved from the spectral region covered by the Network for the Detection of Atmospheric Composition Change (NDACC). The Karlsruhe TCCON Fourier Transform Infrared (FTIR) spectrometer allows to record spectra in the mid-infrared (MIR) and near-infrared (NIR) spectral region simultaneously which makes Karlsruhe a favorable FTIR site to directly compare measurements from both spectral regions. We compare XCO retrieved from the fundamental absorption band at 4.5 µm (as used by NDACC) and ﬁrst overtone absorption band at 2.3 µm (TCCON-style measurements). We observe a bias of (4.47 ± 0.17) ppb between both data sets with a standard deviation of 2.39 ppb in seasonal variation. This corresponds to a relative bias of (4.76 ± 0.18) % and a standard deviation of 2.28 %. We identify different sources which contribute to the observed bias (airmass-independent correction factor, airmass-dependent correction factor, isotopic identities, differing a priori volume mixing ratio proﬁles) and quantify their contributions. We show that the seasonality in the residual of NDACC and TCCON XCO can be estimated by the smoothing effect caused by differing averaging kernel sensitivities between the MIR and NIR spectral region. This study aims to improve the comparability of NDACC and TCCON XCO validation data sets as desired for potential future satellite missions and model studies.
Shape-temperature relationships of pristine ice crystals derived from polarimetric cloud radar observations during the ACCEPT campaign
Authors: Myagkov A. , Seifert P. , Wandinger U. , Bühl J. , Engelmann R.
Published: January 2016

Abstract. This paper presents first quantitative estimations of ice particle shape at the top of liquid-topped clouds. Analyzed ice particles were formed in the presence of supercooled water and in the temperature range from −20 to −3 °C. The estimation is based on polarizability ratios of ice particles measured by a Ka-band cloud radar MIRA-35 with hybrid polarimetric configuration, manufactured by METEK GmbH. For this study, 22 cases observed during the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign were used. Polarizability ratios retrieved for cloud layers with the cloud top temperatures of ~−5, ~−8, ~−15, and ~−20 °C were 1.6, 0.9, 0.6, and 0.9, respectively. Such values correspond to prolate, quasi-isotropic, oblate, and quasi-isotropic particles, respectively. Data from a free-fall chamber were used for the comparison. A good agreement of detected shapes with well-known shape-temperature dependencies observed in laboratories was found. Polarizability ratios used for the analysis were estimated for areas located close to the cloud top where aggregation and riming processes do not strongly affect ice particles. We concluded, that in microwave scattering models ice particles detected in these areas can be assumed to have pristine shapes. It was also found that even slight variations of ambient conditions at the cloud top with temperatures warmer than ~−5 °C can lead to rapid changes of ice crystal shape.
High Spatial Resolution mapping of Precipitable Water Vapor using SAR interferograms, GPS observations and ERA-Interim reanalysis
Authors: Tang W. , Liao M. S. , Zhang L. , Li W. , Yu W. M.
Published: January 2016

Abstract. A high spatial resolution of the Precipitable Water Vapor (PW V) in the atmosphere is a key requirement for the short-scale weather forecasting and climate research. The aim of this work is to derive temporally-differenced maps of the spatial distribution of PWV by analyzing the atmospheric delay "noise" in Interferometric Synthetic Aperture Radar (InSA R). A time series maps of differential PW V were obtained by processing a set of ENVISAT ASAR images cover the area of Southern California, USA from 06 October 2007 to 29 November 2008. To get a more accurate PW V, the component of dry delay is calculated by using ERA-Interim reanalysis products. In addition, the ERA-Interim was used to compute the conversion factors required to convert the zenith wet delay to water vapor. The InSA R-derived differential PW V maps were calibrated by means of the GPS PW V measurements over the study area. We validated our results against the measurements of PW V derived from the MEdium Resolution Imaging Spectrometer (MERIS) which is located together with ASAR sensor onboard the ENVISAT satellite. Our comparative results show strong spatial correlations between the two data sets. The difference maps have Gaussian distributions with mean values close to zero and standard deviations below 2 mm. The advantages of the InSAR technique is that it provides water vapor distribution with a spatial resolution as fine as 20 m and an accuracy of ~2 mm. Such a high spatial resolution maps of PW V could lead to much greater accuracy in meteorological understanding and quantitative precipitation forecasts.
Uncertainties in cloud phase and optical thickness retrievals from the Earth Polychromatic Imaging Camera (EPIC)
Authors: Meyer Kerry , Yang Yuekui , Platnick Steven
Published: January 2016

Abstract. This paper presents an investigation of the expected uncertainties of a single channel cloud optical thickness (COT) retrieval technique, as well as a simple cloud temperature threshold based thermodynamic phase approach, in support of the Deep Space Climate Observatory (DSCOVR) mission. DSCOVR cloud products will be derived from Earth Polychromatic Imaging Camera (EPIC) observations in the ultraviolet and visible spectra. Since EPIC is not equipped with a spectral channel in the shortwave or mid-wave infrared that is sensitive to cloud effective radius (CER), COT will be inferred from a single visible channel with the assumption of appropriate CER values for liquid and ice phase clouds. One month of Aqua MODIS daytime granules from April 2005 is selected for investigating cloud phase sensitivity, and a subset of these granules that has similar EPIC sun-view geometry is selected for investigating COT uncertainties. EPIC COT retrievals are simulated with the same algorithm as the operational MODIS cloud products (MOD06), except using fixed phase-dependent CER values. Uncertainty estimates are derived by comparing the single channel COT retrievals with the baseline bi-spectral MODIS retrievals. Results show that a single channel COT retrieval is feasible for EPIC. For ice clouds, single channel retrieval errors are minimal (< 2 %) due to the particle size insensitivity of the assumed ice crystal (i.e., severely roughened aggregate of hexagonal columns) scattering properties at visible wavelengths, while for liquid clouds the error is mostly limited to within 10 %, although for thin clouds (COT < 2) the error can be higher. Potential uncertainties in EPIC cloud masking and cloud temperature retrievals are not considered in this study.
Characterization and source apportionment of organic aerosol using offline aerosol mass spectrometry
Authors: Daellenbach K. R. , Bozzetti C. , Křepelová A. , Canonaco F. , Wolf R. , Zotter P. , Fermo P. , Crippa M. , Slowik J. G. , Sosedova Y. , Zhang Y. , Huang R.-J. , Poulain L. , Szidat S. , Baltensperger U. , El Haddad I. , Prévôt A. S. H.
Published: January 2016

Abstract. Field deployments of the Aerodyne Aerosol Mass Spectrometer (AMS) have significantly advanced real-time measurements and source apportionment of non-refractory particulate matter. However, the cost and complex maintenance requirements of the AMS make its deployment at sufficient sites to determine regional characteristics impractical. Furthermore, the negligible transmission efficiency of the AMS inlet for supermicron particles significantly limits the characterization of their chemical nature and contributing sources. In this study, we utilize the AMS to characterize the water-soluble organic fingerprint of ambient particles collected onto conventional quartz filters, which are routinely sampled at many air quality sites. The method was applied to 256 particulate matter (PM) filter samples (PM1, PM2.5, and PM10, i.e., PM with aerodynamic diameters smaller than 1, 2.5, and 10 µm, respectively), collected at 16 urban and rural sites during summer and winter. We show that the results obtained by the present technique compare well with those from co-located online measurements, e.g., AMS or Aerosol Chemical Speciation Monitor (ACSM). The bulk recoveries of organic aerosol (60–91 %) achieved using this technique, together with low detection limits (0.8 µg of organic aerosol on the analyzed filter fraction) allow its application to environmental samples. We will discuss the recovery variability of individual hydrocarbon ions, ions containing oxygen, and other ions. The performance of such data in source apportionment is assessed in comparison to ACSM data. Recoveries of organic components related to different sources as traffic, wood burning, and secondary organic aerosol are presented. This technique, while subjected to the limitations inherent to filter-based measurements (e.g., filter artifacts and limited time resolution) may be used to enhance the AMS capabilities in measuring size-fractionated, spatially resolved long-term data sets.
The microwave properties of simulated melting precipitation particles: sensitivity to initial melting
Authors: Johnson B. T. , Olson W. S. , Skofronick-Jackson G.
Published: January 2016

Abstract. A simplified approach is presented for assessing the microwave response to the initial melting of realistically shaped ice particles. This paper is divided into two parts: (1) a description of the Single Particle Melting Model (SPMM), a heuristic melting simulation for ice-phase precipitation particles of any shape or size (SPMM is applied to two simulated aggregate snow particles, simulating melting up to 0.15 melt fraction by mass), and (2) the computation of the single-particle microwave scattering and extinction properties of these hydrometeors, using the discrete dipole approximation (via DDSCAT), at the following selected frequencies: 13.4, 35.6, and 94.0 GHz for radar applications and 89, 165.0, and 183.31 GHz for radiometer applications. These selected frequencies are consistent with current microwave remote-sensing platforms, such as CloudSat and the Global Precipitation Measurement (GPM) mission. Comparisons with calculations using variable-density spheres indicate significant deviations in scattering and extinction properties throughout the initial range of melting (liquid volume fractions less than 0.15). Integration of the single-particle properties over an exponential particle size distribution provides additional insight into idealized radar reflectivity and passive microwave brightness temperature sensitivity to variations in size/mass, shape, melt fraction, and particle orientation.
Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-Gerasimenko
Authors: Volwerk M. , Richter I. , Tsurutani B. , Götz C. , Altwegg K. , Broiles T. , Burch J. , Carr C. , Cupido E. , Delva M. , Dósa M. , Edberg N. J. T. , Eriksson A. , Henri P. , Koenders C. , Lebreton J.-P. , Mandt K. E. , Nilsson H. , Opitz A. , Rubin M. , Schwingenschuh K. , Stenberg Wieser G. , Szegö K. , Vallat C. , Vallieres X. , Glassmeier K.-H.
Published: January 2016

Abstract. The data from all Rosetta plasma consortium instruments and from the ROSINA COPS instrument are used to study the interaction of the solar wind with the outgassing cometary nucleus of 67P/Churyumov-Gerasimenko. During 6 and 7 June 2015, the interaction was first dominated by an increase in the solar wind dynamic pressure, caused by a higher solar wind ion density. This pressure compressed the draped magnetic field around the comet, and the increase in solar wind electrons enhanced the ionization of the outflow gas through collisional ionization. The new ions are picked up by the solar wind magnetic field, and create a ring/ring-beam distribution, which, in a high-β plasma, is unstable for mirror mode wave generation. Two different kinds of mirror modes are observed: one of small size generated by locally ionized water and one of large size generated by ionization and pick-up farther away from the comet.
Mapping of steady-state electric fields and convective drifts in geomagnetic fields – Part 2: The IGRF
Authors: Walker A. D. M.
Published: January 2016

Abstract. A method of mapping electric fields along geomagnetic field lines is applied to the IGRF (International Geomagnetic Reference Field) model. The method involves integrating additional sets of first order differential equations simultaneously with those for tracing a magnetic field line. These provide a measure of the rate of change of the separation of two magnetic field lines separated by an infinitesimal amount. From the results of the integration Faraday's law is used to compute the electric field as a function of position along the field line. Examples of computations from a software package developed to implement the method are presented. This is expected to be of use in conjugate studies of magnetospheric phenomena such as SuperDARN (Super Dual Auroral Radar) observations of convection in conjugate hemispheres, or comparison of satellite electric field observations with fields measured in the ionosphere.
Ion Bernstein waves in the magnetic reconnection region
Authors: Narita Y. , Nakamura R. , Baumjohann W. , Glassmeier K.-H. , Motschmann U. , Comişel H.
Published: January 2016

Abstract. Four-dimensional energy spectra and a diagram for dispersion relations are determined for the first time in a magnetic reconnection region in the magnetotail using data from four-spacecraft measurements by the Cluster mission on a spatial scale of 200 km, about 0.1 ion inertial lengths. The energy spectra are anisotropic with an extension in the perpendicular direction and axially asymmetric with respect to the mean magnetic field. The dispersion diagram in the plasma rest frame is in reasonably good agreement with the ion Bernstein waves at the second and higher harmonics of the proton gyrofrequency. Perpendicular-propagating ion Bernstein waves likely exist in an outflow region of magnetic reconnection, which may contribute to bifurcation of the current sheet in the outflow region.
Millennial changes in North Atlantic oxygen concentrations
Authors: Hoogakker B. A. A. , Thornalley D. J. R. , Barker S.
Published: January 2016

Abstract. Glacial–interglacial changes in bottom water oxygen concentrations [O2] in the deep northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O2] changes in relation to millennial climate oscillations in the North Atlantic over the last glacial cycle, using bottom water [O2] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015) and (2) ODP (Ocean Drilling Program) Site 1055 from the intermediate northwest Atlantic. The deep northeast Atlantic core MD95-2042 shows decreased bottom water [O2] during millennial-scale cool events, with lowest bottom water [O2] of 170, 144, and 166 ± 17 µmol kg−1 during Heinrich ice rafting events H6, H4, and H1. Importantly, at intermediate depth core ODP Site 1055, bottom water [O2] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with the lowest values of 179 and 194 µmol kg−1 recorded during millennial cool event C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O2] across the North Atlantic below  ∼ 1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O2] during North Atlantic Heinrich events and earlier cold events at the two sites can be linked to water mass changes in relation to ocean circulation changes and possibly productivity changes. At the intermediate depth site a possible strong North Atlantic Intermediate Water cell would preclude water mass changes as a cause for decreased bottom water [O2]. Instead, we propose that the lower bottom [O2] there can be linked to productivity changes through increased export of organic material from the surface ocean and its subsequent remineralization in the water column and the sediment.
Precipitation legacy effects on dryland ecosystem carbon fluxes: direction, magnitude and biogeochemical carryovers
Authors: Shen W. , Jenerette G. D. , Hui D. , Scott R. L.
Published: January 2016

Abstract. The precipitation legacy effect, defined as the impact of historical precipitation (PPT) on extant ecosystem dynamics, has been recognized as an important driver in shaping the temporal variability of dryland aboveground net primary production (ANPP) and soil respiration. How the PPT legacy influences whole ecosystem-level carbon (C) fluxes has rarely been quantitatively assessed, particularly at longer temporal scales. We parameterized a process-based ecosystem model to a semiarid savanna ecosystem in the southwestern USA, calibrated and evaluated the model performance based on 7 years of eddy-covariance measurements, and conducted two sets of simulation experiments to assess interdecadal and interannual PPT legacy effects over a 30-year simulation period. The results showed that decreasing the previous period/year PPT (dry legacy) always increased subsequent net ecosystem production (NEP) whereas increasing the previous period/year PPT (wet legacy) decreased NEP. The simulated dry-legacy impacts mostly increased subsequent gross ecosystem production (GEP) and reduced ecosystem respiration (Re), but the wet legacy mostly reduced GEP and increased Re. Although the direction and magnitude of GEP and Re responses to the simulated dry and wet legacies were influenced by both the previous and current PPT conditions, the NEP responses were predominantly determined by the previous PPT characteristics including rainfall amount, seasonality and event size distribution. Larger PPT difference between periods/years resulted in larger legacy impacts, with dry legacies fostering more C sequestration and wet legacies more C release. The carryover of soil N between periods/years was mainly responsible for the GEP responses, while the carryovers of plant biomass, litter and soil organic matter were mainly responsible for the Re responses. These simulation results suggest that previous PPT conditions can exert substantial legacy impacts on current ecosystem C balance, which should be taken into account while assessing the response of dryland ecosystem C dynamics to future PPT regime changes.
Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea
Authors: Li Q. P. , Dong Y. , Wang Y.
Published: January 2016

Abstract. A field survey from the coastal ocean zones to the offshore pelagic zones of the northeastern South China Sea (nSCS) was conducted during the inter-monsoon period of May 2014 when the region was characterized by prevailing low-nutrient conditions. Comprehensive field measurements were made for not only hydrographic and biogeochemical properties but also phytoplankton growth and microzooplankton grazing rates. We also performed estimations of the vertical turbulent diffusivity and diffusive nutrient fluxes using a Thorpe-scale method and the upwelling nutrient fluxes by Ekman pumping using satellite-derived wind stress curl. Our results indicated a positive correlation between the integrated phytoplankton chlorophyll a and vertical nutrient fluxes in the offshore region of the nSCS during the study period. We generally found an increasing role of turbulent diffusion but a decreasing role of curl-driven upwelling in vertical transport of nutrients from the coastal ocean zones to the offshore pelagic zones. Elevated nutrient fluxes near Dongsha Islands supported high new production leading to net growth of the phytoplankton community, whereas the low fluxes near the southwest of Taiwan had resulted in a negative net community growth leading to decline of a surface phytoplankton bloom. Overall, phytoplankton dynamics in the large part of the nSCS could be largely driven by vertical nutrient fluxes including turbulent diffusion and curl-driven upwelling during the spring inter-monsoon period.
Spatial and seasonal contrasts of sedimentary organic matter in floodplain lakes of the central Amazon basin
Authors: Sobrinho R. L. , Bernardes M. C. , Abril G. , Kim J.-H. , Zell C. I , Mortillaro J.-M. , Meziane T. , Moreira-Turcq P. , Sinninghe Damsté J. S.
Published: January 2016

Abstract. In this study, we investigated the seasonal and spatial pattern of sedimentary organic matter (SOM) in five floodplain lakes of the central Amazon basin (Cabaliana, Janauaca, Canaçari, Mirituba and Curuai) which have different morphologies, hydrodynamics and vegetation coverages. Surface sediments were collected in four hydrological seasons: low water (LW), rising water (RW), high water (HW) and falling water (FW) in 2009 and 2010. We investigated commonly used bulk geochemical tracers such as the C : N ratio and the stable isotopic composition of organic carbon (δ13Corg). These results were compared with lignin phenol parameters as an indicator of vascular plant detritus and branched glycerol dialkyl glycerol tetraethers (brGDGTs) to trace the input of soil organic matter (OM) from land to the aquatic settings. We also applied the crenarchaeol as an indicator of aquatic (rivers and lakes) OM. Our data showed that during the RW and FW seasons, the surface sediments were enriched in lignin and brGDGTs in comparison to other seasons. Our study also indicated that floodplain lake sediments primarily consisted of allochthonous, C3 plant-derived OM. However, a downstream increase in C4 macrophyte-derived OM contribution was observed along the gradient of increasing open waters – i.e., from upstream to downstream. Accordingly, we attribute the temporal and spatial difference in SOM composition to the hydrological dynamics between the floodplain lakes and the surrounding flooded forests.
Dissolved organic carbon lability and stable isotope shifts during microbial decomposition in a tropical river system
Authors: Geeraert N. , Omengo F. O. , Govers G. , Bouillon S.
Published: January 2016

Abstract. A significant amount of carbon is transported to the ocean as dissolved organic carbon (DOC) in rivers. During transport, it can be transformed through microbial consumption and photochemical oxidation. In dark incubation experiments with water from the Tana River, Kenya, we examined the consumption of DOC through microbial decomposition and the associated change in its carbon stable isotope composition (δ13C). In 15 of the 18 incubations, DOC concentrations decreased significantly by 10 to 60 %, with most of the decomposition taking place within the first 24–48 h. After 8 days, the remaining DOC was up to 3 ‰ more depleted in 13C compared with the initial pool, and the change in δ13C correlated strongly with the fraction of DOC remaining. We hypothesize that the shift in δ13C is consistent with greater microbial lability of DOC originating from herbaceous C4 vegetation than DOC derived from woody C3 vegetation in the semi-arid lower Tana. The results complement earlier findings that the stable isotope concentration of riverine DOC does not necessarily reflect the proportion of C3 and C4-derived DOC in the catchment: besides spatial distribution patterns of different vegetation types, processing within the river can further influence the δ13C of riverine OC.
Iron-Bound Organic Carbon in Forest Soils: Quantification and Characterization
Authors: Zhao Qian , Poulson Simon R. , Obrist Daniel , Sumaila Samira , Dynes James J. , McBeth Joyce M. , Yang Yu
Published: January 2016

Abstract. Iron oxide minerals play an important role in stabilizing organic carbon (OC) and regulating the biogeochemical cycles of OC on the earth surface. To predict the fate of OC, it is essential to completely understand the amount, spatial variability and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concentrations and characteristics of Fe-bound OC in soils collected from 14 forests in the United States, and determined the impact of ecogeographical variables and soil physicochemical properties o n the association of OC and Fe minerals. We found that Fe-bound OC contributed up to 57.8 % of total OC (TOC) in forest soils. Atomic ratios of OC:Fe ranged from 0.56 to 17.7 with values of 1–10 for most samples, and these ratios indicate an importance of both sorptive and incorporativ e interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concentration of reactive Fe, which suggests that the importance of association with Fe in OC accumulation was not governed by the concentration of reactive Fe. Concentrations of Fe-bound OC and fFe-OC increased with the latitude and reached peak values at a site with a mean annual temperature of 6.6 °C Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliphatic than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C/N ratios did not differ substantially. In summary, 13C-enriched OC with less aliphatic carbon and more carboxylic carbon was associated with Fe minerals in the soils, with values of fFe-OC being controlled by both sorptive and incorporative associations between Fe and OC. Overall, this study demonstrates that Fe oxides play an important role in regulating the biogeochemical cycles of C in forest soils, and uncovers the governing factors for the spa tial variability and characteristics of Fe-bound OC.
Temperature affects the morphology and calcification of Emiliania huxleyi strains
Authors: Rosas-Navarro Anaid , Langer Gerald , Ziveri Patrizia
Published: January 2016

Abstract. Three strains of E. huxleyi type A were grown under non-limiting nutrient and light conditions, at 10, 15, 20 and 25 °C. All three strains displayed similar growth rate versus temperature relationships, with an optimum at 20-25 °C. Elemental production (particulate inorganic carbon (PIC), particulate organic carbon (POC), total particulate nitrogen (TPN)), coccolith mass, coccolith size, and width of the tube elements cycle were positively correlated with temperature over the sub-optimum to optimum temperature range. The correlation between PIC production and coccolith mass/size supports the notion that coccolith mass can be used as a proxy for PIC production in sediment samples. Increasing PIC production was positively correlated with the percentage of incomplete coccoliths in one strain only. Generally, coccoliths were heavier when PIC production was higher. This shows that incompleteness of coccoliths is not due to time shortage at high PIC production. Sub-optimal growth temperatures lead to an increase in the percentage of malformed coccoliths in a strain-speciﬁc fashion. Since in total only six strains have been tested thus far, it is presently difﬁcult to say whether sub-optimal temperature is an important factor causing malformations in the ﬁeld. The most important parameter in biogeochemical terms, the PIC:POC, shows a minimum at optimum growth temperature in all investigated strains. This clariﬁes the ambiguous picture featuring in the literature.
Introduction to the project VAHINE: VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific
Authors: Bonnet S. , Moutin T. , Rodier M. , Grisoni J.M. , Louis F. , Folcher E. , Bourgeois B. , Boré J.M. , Renaud A.
Published: January 2016

Abstract. At the global scale, N2 fixations provides the major external source of reactive nitrogen to the surface ocean, before atmospheric and riverine inputs, and sustains ~50 % of new primary production in oligotrophic environments. The main goal of the VAHINE project was to study the fate of nitrogen newly fixed by diazotrophs (or diazotroph-derived nitrogen) in oceanic food webs, how it impact heterotrophic bacteria, phytoplankton and zooplankton dynamics, stocks and fluxes of biogenic elements and particle export. Three large-volume (~50 m3) mesocosms were deployed in a tropical oligotrophic ecosystem (the New Caledonia lagoon, south-eastern Pacific) and intentionally fertilized with ~0.8 μM of dissolved inorganic phosphorus (DIP) to stimulate diazotrophy and follow subsequent ecosystem and fluxes changes. VAHINE was a multidisciplinary project involving close collaborations between biogeochemists, molecular ecologist, chemists, marine opticians and modelers. This introductory paper describes in detail the scientific objectives of the project as well as the implementation plan: the mesocosm description and deployment, the selection of the study site (New Caledonian lagoon) and the logistical and sampling strategy. The description of the main hydrological and biogeochemical conditions of the study site before the mesocosms deployment and during the experiment itself is then detailed, and a general overview of the papers published in this special issue is presented.
Spring Blooms in the Baltic Sea have weakened but lengthened from 2000 to 2014
Authors: Groetsch P. M. M. , Simis S. G. H. , Eleveld M. A. , Peters S. W. M.
Published: January 2016

Abstract. Phytoplankton spring bloom phenology was derived from a 15-year time-series (2000-2014) of ship-of-opportunity chlorophyll-a fluorescence observations in the Baltic Sea. Decadal trends were analysed against inter-annual variability in bloom timing and intensity, and environmental drivers (nutrient concentration, temperature, radiation level, wind speed). Spring blooms developed along a south to north gradient with the first blooms peaking mid-March in the Bay of Mecklenburg and the latest peaks occurring mid-April in the Gulf of Finland. Bloom duration was similar between sea areas (43 ± 2 d), except for shorter bloom duration in the Bay of Mecklenburg (36 ± 11 d). Variability in bloom timing increased towards the south. Bloom peak concentrations were highest (and most variable) in the Gulf of Finland (20.2 ± 5.7 mg m−3) and the Bay of Mecklenburg (12.3 ± 5.2 mg m−3). Bloom peak concentration showed a negative trend of −0.31 ± 0.10 mg m−3 yr−1. Trend-agnostic distribution-based (Weibull-type) bloom metrics showed a positive trend in bloom duration of 1.04 ± 0.20 d yr−1, which was not found for any of the threshold-based metrics. The Weibull bloom metric results were considered representative in presence of bloom intensity trends. Bloom intensity was mainly determined by winter nutrient concentration, while bloom timing and duration co-varied with meteorological conditions. Longer blooms corresponded to higher water temperature, more intense solar radiation, and lower wind speed. It is concluded that nutrient reduction efforts led to decreasing bloom intensity, while changes in Baltic Sea environmental conditions associated with global change correspond to a lengthening spring bloom period.
Mercury Methylation in paddy soil: Source and distribution of mercury species at a Hg mining area, Guizhou Province, China
Authors: Zhao L. , Anderson C. W. N. , Qiu G. L. , Meng B. , Wang D. Y. , Feng X. B.
Published: January 2016

Abstract. Rice paddy plantation for human consumption is dominant land uses throughout Asia. Rice paddy fields have been identified as important sites for methylmerucry (MeHg) production in the terrestrial ecosystem, and a primary pathway of MeHg exposure to human in mercury (Hg) mining areas. We compared the source and distribution of Hg species in different compartments of the rice paddy during a complete rice-growing season at two different typical Hg-contaminated mining sites: an abandoned site with high Hg concentration in soil but low concentration in atmosphere, and a current-day artisanal site with low concentration in soil but high concentration in atmosphere. The contribution of new Hg to the ecosystem from irrigation and atmospheric deposition was insignificant relative to the pool of old Hg; the dominant source of MeHg to paddy soil is in situ methylation of inorganic Hg. Elevated MeHg concentrations jointly with the high proportion of Hg as MeHg in paddy water and the surface soil layer at the artisanal s ite demonstrated active Hg methylation at this site only. We propose that the in situ production of MeHg is dependent on elevated IHg in the atmosphere, and the deposition of new Hg into a low pH anoxic geochemical system. In contrast, the absence of depth-dependent variability in the MeHg concentration in soil cores collected from abandoned Hg mining site, consistent with the low concentration of Hg in atmospheric deposition and high pH of the paddy water/irrigation water, suggested that the net production of MeHg was limited. We also propose that the concentration of Hg in ambient air is an indicator for the risk of MeHg accumulation in paddy rice.
Are flood-driven turbidity currents hot-spots for priming effect in lakes?
Authors: Bouffard D. , Perga M.-E.
Published: January 2016

Abstract. In deep stratified lakes, such as Lake Geneva, flood-driven turbidity currents are thought to contribute to the replenishment of deep oxygen by significant transport of river waters supersaturated with oxygen into the hypolimnion. The overarching aim of this study was to test directly this long-standing hypothesis. It combines direct observational data collected during an extreme flooding event that occurred in May 2015 with dark bioassays designed to evaluate the consequences of riverborne inputs on the hypolimnetic respiration. The exceptional precipitations of May 2015 caused floods with annual return time for the Rhône River, the dominant tributary of Lake Geneva, and with 50-year return time for the Dranse River, the second most important tributary. Sediment loaded river flows generated turbidity currents plunging into the lake hypolimnion. The observed river intrusions contributed to the redistribution of dissolved oxygen, with no net gain, when occurring in the lowermost hypolimnetic layer. In the uppermost hypolimnion above the last deep mixing event the intrusions coincided with a net oxygen deficit. Consistent with field observations, dark bioassays showed that 1% to 50% substitution of riverine organic matter to deep (<200 m) hypolimnetic water did not affect microbial respiration, while addition of 1 to 10% of riverine water to the uppermost hypolimnetic waters resulted in a respiration overyielding, i.e. excess respiration of both riverborne and lacustrine organic matter. The results of our study conflict the hypothesis that flood-driven turbidity currents necessarily increase hypolimnetic oxygen stocks in Lake Geneva. In contrast, they show that flood-driven turbidity currents can be potential hot-spots for priming effect in lakes.
Effect of light on photosynthetic efficiency of sequestered chloroplasts in intertidal benthic foraminifera (Haynesina germanica and Ammonia tepida)
Authors: Jauffrais Thierry , Jesus Bruno , Metzger Edouard , Mouget Jean-Luc , Jorissen Frans , Geslin Emmanuelle
Published: January 2016

Abstract. Some benthic foraminifera have the ability to incorporate functional chloroplasts from diatoms (kleptoplasty). Our objective was to investigate chloroplast functionality of two benthic foraminifera (Haynesina germanica and Ammonia tepida) exposed to different irradiance levels (0, 25, 70 μmol photon m-2 s-1) using spectral reflectance, epifluorescence observations, oxygen evolution and pulse amplitude modulated (PAM) fluorometry. Our results clearly showed that H. germanica was capable of using its kleptoplasts for more than one week while A. tepida showed very limited kleptoplastic ability with maximum photosystem II quantum efficiency (Fv/Fm = 0.4), much lower than H. germanica and decreasing to zero in only one day. Only H. germanica showed net oxygen production with a compensation point at 24 μmol photon m-2 s-1 and a production up to 1000 pmol O2 cell-1 day-1 at 300 μmol photon m-2 s-1. Haynesina germanica Fv/Fm slowly decreased from 0.65 to 0.55 in 7 days when kept in darkness; however, it quickly decreased to 0.2 under high light. Kleptoplast functional time was thus estimated between 11 and 21 days in darkness and between 7 and 8 days at high light. These results emphasize that studies about foraminifera kleptoplasty must take into account light history. Additionally, this study showed that the kleptoplasts are unlikely to be completely functional, thus requiring continuous chloroplast resupply from foraminifera food source. The advantages of keeping functional chloroplasts are discussed but more information is needed to better understand foraminifera feeding strategies.
Surfactant control of gas transfer velocity along an offshore coastal transect: results from a laboratory gas exchange tank
Authors: Pereira R. , Schneider-Zapp K. , Upstill-Goddard R. C.
Published: January 2016

Abstract. We measured total surfactant activity (SA; Triton T-X-100 equivalent) and CDOM absorbance (cm-1) in the surface microlayer (SML) and subsurface water (SSW) seasonally (2012-2013) along a 20 km coastal transect (North East UK), and evaluated corresponding values of the gas transfer velocity (kw; cm hr-1) using a custom designed air-sea gas exchange tank. Spatial SA variability exceeded its temporal variability. Overall, SA varied five-fold between all samples (0.08 - 0.38 mg L-1 T-X-100), being highest in the SML during summer. SML SA enrichment factors relative to SSW were ~ 1.0 - 1.9, except for two values (0.75; 0.89: February 2013). CDOM absorbance (250 - 450 nm), the CDOM spectral slope ratio (SR = S275–295 / S350–400) and the 250:365 nm CDOM absorbance ratio (E2 : E3) demonstrate the potential for terrestrially-derived CDOM to be biogeochemically processed in North Sea coastal waters. The range in corresponding k660 (kw for CO2; freshwater; 20 °C) was 6.8 - 22.0 cm hr-1. The film factor R660 (the ratio of the scaled transfer velocity k660 to the one of clean water) strongly correlates with SA (r ≥0.70, p ≤0.002, each n = 16) with high SML SA correlated to k660 suppression ~14 - 51 % relative to clean laboratory water, highlighting strong spatio-temporal gradients in gas exchange due to varying surfactant in these coastal waters. Such variability should be taken account of when evaluating marine trace gas sources and sinks.
Terrestrial biosphere changes over the last 120 kyr
Authors: Hoogakker B. A. A. , Smith R. S. , Singarayer J. S. , Marchant R. , Prentice I. C. , Allen J. R. M. , Anderson R. S. , Bhagwat S. A. , Behling H. , Borisova O. , Bush M. , Correa-Metrio A. , de Vernal A. , Finch J. M. , Fréchette B. , Lozano-Garcia S. , Gosling W. D. , Granoszewski W. , Grimm E. C. , Grüger E. , Hanselman J. , Harrison S. P. , Hill T. R. , Huntley B. , Jiménez-Moreno G. , Kershaw P. , Ledru M.-P. , Magri D. , McKenzie M. , Müller U. , Nakagawa T. , Novenko E. , Penny D. , Sadori L. , Scott L. , Stevenson J. , Valdes P. J. , Vandergoes M. , Velichko A. , Whitlock C. , Tzedakis C.
Published: January 2016

Abstract. A new global synthesis and biomization of long (> 40 kyr) pollen-data records is presented and used with simulations from the HadCM3 and FAMOUS climate models and the BIOME4 vegetation model to analyse the dynamics of the global terrestrial biosphere and carbon storage over the last glacial–interglacial cycle. Simulated biome distributions using BIOME4 driven by HadCM3 and FAMOUS at the global scale over time generally agree well with those inferred from pollen data. Global average areas of grassland and dry shrubland, desert, and tundra biomes show large-scale increases during the Last Glacial Maximum, between ca. 64 and 74 ka BP and cool substages of Marine Isotope Stage 5, at the expense of the tropical forest, warm-temperate forest, and temperate forest biomes. These changes are reflected in BIOME4 simulations of global net primary productivity, showing good agreement between the two models. Such changes are likely to affect terrestrial carbon storage, which in turn influences the stable carbon isotopic composition of seawater as terrestrial carbon is depleted in 13C.