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Geophysical Research Letters     Full-text available via subscription   (Followers: 102, SJR: 3.323, h-index: 185)
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Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [12 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Two Centuries of Nitrogen Dynamics: Legacy Sources and Sinks in the
           Mississippi and Susquehanna River Basins
    • Authors: K. J. Van Meter; N. B. Basu, P. Van Cappellen
      Abstract: Global flows of reactive nitrogen (N) have increased significantly over the last century in response to agricultural intensification and elevated levels of atmospheric deposition. Despite widespread implementation conservation measures, N concentrations in surface waters are often remaining steady or continuing to increase. Although such lack of response has been attributed to time lags associated with legacy N stores in subsurface reservoirs, it is unclear what the magnitudes of such stores are and how they are partitioned between shallow soil and deeper groundwater reservoirs. Here we have synthesized data to develop a 214-year (1800 – 2014) trajectory of N inputs to the land surface of the continental U.S. We have concurrently developed a parsimonious, process-based model, ELEMeNT that pairs this input trajectory with a travel time-based approach to simulate transport and retention along subsurface pathways. Using the model, we have reconstructed historic nitrate yields at the outlets of two major U.S. watersheds, the Mississippi River Basin (MRB) and Susquehanna River Basin (SRB). Our results show significant N loading above baseline levels in both watersheds before the widespread use of commercial N fertilizers, largely due to the conversion of forest and grassland to row-crop agriculture. Model results also allow us to quantify the magnitudes of legacy N in soil and groundwater pools, and to highlight the dominance of soil legacies in MRB and groundwater legacies in SRB. Approximately 55% and 18% of the current annual N loads in the MRB and SRB were found to be older than 10 years of age.
      PubDate: 2016-12-01T18:21:03.807771-05:
      DOI: 10.1002/2016GB005498
  • Dissolved organic matter composition of Arctic rivers: linking permafrost
           and parent material to riverine carbon
    • Authors: Jonathan A. O'Donnell; George R. Aiken, David K. Swanson, Santosh Panda, Kenna D. Butler, Andrew P. Baltensperger
      Abstract: Recent climate change in the Arctic is driving permafrost thaw, which has important implications for regional hydrology and global carbon dynamics. Permafrost is an important control on groundwater dynamics and the amount and chemical composition of dissolved organic matter (DOM) transported by high-latitude rivers. The consequences of permafrost thaw for riverine DOM dynamics will likely vary across space and time, due in part to spatial variation in ecosystem properties in Arctic watersheds. Here, we examined watershed controls on DOM composition in 69 rivers draining heterogeneous landscapes across a broad region of Arctic Alaska. We characterized DOM using bulk dissolved organic carbon (DOC) concentration, optical properties, and chemical fractionation, and classified watersheds based on permafrost characteristics (mapping of parent material and ground-ice content, modeling of thermal state) and ecotypes. Parent material and ground-ice content significantly affected the amount and composition of DOM. DOC concentrations were higher in watersheds underlain by fine-grained loess compared to watersheds underlain by coarse-grained sand or shallow bedrock. DOC concentration was also higher in rivers draining ice-rich landscapes compared to rivers draining ice-poor landscapes. Similarly, specific ultraviolet absorbance (SUVA254, an index of DOM aromaticity) values were highest in watersheds underlain by fine-grained deposits or ice-rich permafrost. We also observed differences in hydrophobic organic acids, hydrophilic compounds, and DOM fluorescence across watersheds. Both DOC concentration and SUVA254 were negatively correlated with watershed active layer thickness, as determined by high-resolution permafrost modeling. Together, these findings highlight how spatial variations in permafrost physical and thermal properties can influence riverine DOM.
      PubDate: 2016-11-24T13:50:23.023529-05:
      DOI: 10.1002/2016GB005482
  • Modelled Chl:C ratio and derived estimates of phytoplankton carbon biomass
           and its contribution to total particulate organic carbon in the global
           surface ocean
    • Authors: Lionel Arteaga; Markus Pahlow, Andreas Oschlies
      Abstract: Chlorophyll (Chl) is a distinctive component of autotrophic organisms, often used as an indicator of phytoplankton biomass in the ocean. However, assessment of phytoplankton biomass from Chl relies on the accurate estimation of the Chl:C ratio. Here we present global patterns of Chl:C ratios in the surface ocean obtained from a phytoplankton growth model that accounts for the optimal acclimation of phytoplankton to ambient nutrient, light, and temperature conditions. The model agrees largely with observed/expected global patterns of Chl:C. Combining our Chl:C estimates with satellite Chl and particulate organic carbon (POC), we infer phytoplankton C concentration in the surface ocean and its contribution to the total POC pool. Our results suggest that the portion of POC corresponding to living phytoplankton is higher in subtropical latitudes and less productive regions (∼ 30–70%), and decreases to ∼ 10–30% towards high latitudes and productive regions. An important caveat of our model is the lack of iron limiting effects on phytoplankton physiology. Comparison of our predicted phytoplankton biomass with an independent estimate of total POC reveals a positive correlation between nitrate concentrations and non-photosynthetic POC in the surface ocean. This correlation disappears when a constant Chl:C is applied. Our analysis is not constrained by assumptions of constant Chl:C or phytoplankton:POC ratio, providing a novel independent analysis of phytoplankton biomass in the surface ocean. These results highlight the importance of accounting for the variability in Chl:C, and its application in distinguishing the autotrophic and heterotrophic components in the assemblage of the marine plankton ecosystem.
      PubDate: 2016-11-18T01:15:31.056892-05:
      DOI: 10.1002/2016GB005458
  • Two black carbon pools transported by the Changjiang and Huanghe Rivers in
    • Authors: Xuchen Wang; Caili Xu, Ellen M. Druffel, Yuejun Xue, Yuanzhi Qi
      Abstract: Major rivers play important roles in transporting large amounts of terrestrial organic matter from land to the ocean each year, and the organic matter carried by rivers contains a significant fraction of black carbon (BC). A recent study estimated that 0.027 Gt of BC is transported in the dissolved phase by rivers each year, which accounts for ~ 10% of the global flux of dissolved organic carbon (DOC) [Jaffé et al., 2013]. The relative sources of this large amount of riverine dissolved black carbon (DBC) from biomass burning (young, modern 14C) and fossil fuel (old, 14C-free) combustion are not known. We present radiocarbon measurements of BC in both dissolved and particulate phases transported by the Changjiang and Huanghe Rivers, the two largest rivers in China, during 2015. We show that two, distinct BC pools (young and old) were carried by the rivers. The DBC pool was much younger than the particulate BC (PBC) pool. Mass balance calculations indicate that most (78 - 85%) of the DBC in the Changjiang and Huanghe Rivers was derived from biomass burning, and only 15 – 22% was from fossil fuel combustion. In contrast, PBC from biomass burning and fossil fuel combustion were approximately equal in these two rivers. Export of PBC and DBC by the rivers are decoupled, and fluxes of PBC were 4.1 and 6.7 times higher than DBC in the Changjiang and Huanghe Rivers, respectively. The 14C age differences of the two BC pools suggest that BC derived from biomass burning and fossil fuel combustion are mobilized in different phases and on different time scales in these rivers.
      PubDate: 2016-11-16T18:12:56.976585-05:
      DOI: 10.1002/2016GB005509
  • Fire and deforestation dynamics in Amazonia (1973-2014)
    • Authors: M. J. E. Marle; R. D. Field, G. R. Werf, I. A. Estrada Wagt, R. A. Houghton, L. V. Rizzo, P. Artaxo, K. Tsigaridis
      Abstract: Consistent long-term estimates of fire emissions are important to understand the changing role of fire in the global carbon cycle and to assess the relative importance of humans and climate in shaping fire regimes. However, there is limited information on fire emissions from before the satellite-era. We show that in the Amazon region, including the Arc of Deforestation and Bolivia, visibility observations derived from weather stations could explain 61% of the variability in satellite-based estimates of bottom-up fire emissions since 1997, and 42% of the variability in satellite-based estimates of total column carbon monoxide concentrations since 2001. This enabled us to reconstruct the fire history of this region since 1973 when visibility information became available. Our estimates indicate that until 1987 relatively few fires occurred in this region and that fire emissions increased rapidly over the 1990s. We found that this pattern agreed reasonably well with forest loss datasets, indicating that although natural fires may occur here deforestation and degradation were the main cause of fires. Compared to fire emissions estimates based on Food and Agricultural Organization's Global Forest and Resources Assessment (FAO-FRA) data, our results were substantially lower up to the 1990s, after which they were more in line. Our visibility-based fire emissions dataset can help constrain dynamic global vegetation models and atmospheric models with a better representation of the anthropogenic fire regime.AGU Index terms: 0305 Aerosols and particles, 0315 Biosphere/atmosphere interactions, 0428 Carbon cycling, 0480 Remote sensing, 1632 Land cover change
      PubDate: 2016-11-16T16:45:26.858339-05:
      DOI: 10.1002/2016GB005445
  • Trichodesmium latitudinal distribution on the shelf-break in the
           southwestern Atlantic Ocean during spring and autumn
    • Authors: Amália Maria Sacilotto Detoni; Áurea Maria Ciotti, Paulo H. R. Calil, Virginia Maria Tavano, João Sarkis Yunes
      Abstract: We report Trichodesmium slicks in the South Atlantic associated with fronts produced by the southward displacement of the Brazil Current. For the first time, a relatively high spatial sampling grid of trichomes density is shown over an undersampled region with regards to Trichodesmium. In this study, the distributions of Trichodesmium along the southeastern Brazilian continental shelf-break were characterized during four oceanographic cruises and related to seawater temperature, upper-layer nutrient concentrations and wind dynamics. Meridional changes in the densities of trichomes followed an expected trend with higher (lower) values found in warmer (colder) and more saline (less saline) water masses. Between 23° and 28°S, large accumulations of trichomes were associated with high temperature, increased phosphate concentrations (>0.08 μM), nitrogen below 2.4 μM and low wind speeds (
      PubDate: 2016-11-12T13:00:21.669547-05:
      DOI: 10.1002/2016GB005431
  • The CAFE Model: A Net Production Model for Global Ocean Phytoplankton
    • Authors: Greg M. Silsbe; Michael J. Beherenfeld, Kimberly H. Halsey, Allen J. Milligan, Toby K. Westberry
      Abstract: The Carbon, Absorption, and Fluorescence Euphotic-resolving (CAFE) net primary production model is an adaptable framework for advancing global ocean productivity assessments by exploiting state-of-the-art satellite ocean color analyses and addressing key physiological and ecological attributes of phytoplankton. Here, we present the first implementation of the CAFE model that incorporates inherent optical properties derived from ocean color measurements into a mechanistic and accurate model of phytoplankton growth rates (μ) and net phytoplankton production (NPP). The CAFE model calculates NPP as the product of energy absorption (QPAR), and the efficiency (φμ) by which absorbed energy is converted into carbon biomass (CPhyto), while μ is calculated as NPP normalized to CPhyto. The CAFE model performance is evaluated alongside 21 other NPP models against a spatially robust and globally representative set of direct NPP measurements. This analysis demonstrates that the CAFE model explains the greatest amount of variance and has the lowest model bias relative to other NPP models analyzed with this dataset. Global oceanic NPP from the CAFE model (52 Pg C m-2 yr-1) and mean division rates (0.34 d-1) are derived from climatological satellite data (2002-2014). This manuscript discusses and validates individual CAFE model parameters (e.g. QPAR, φμ), provides detailed sensitivity analyses, and compares the CAFE model results and parameterization to other widely cited models.
      PubDate: 2016-11-03T13:08:30.775007-05:
      DOI: 10.1002/2016GB005521
  • Large Saharan dust storms: Implications for chlorophyll dynamics in the
           Mediterranean Sea.
    • Authors: Rachele Gallisai; Gianluca Volpe, Francesc Peters
      Abstract: We investigate the large (LDE) and very large (VLDE) Saharan dust deposition events that occurred between 2000 and 2007 and their short-term impact on the dynamics of marine phytoplankton in the Mediterranean Sea. A total of 153 LDE were identified unevenly distributed over the years. Events were more frequent during winter, in the Eastern Mediterranean, and autumn, when they affected both the Western and the Central Mediterranean. Most of the 31 VLDE occurred during winter and autumn in the Central Mediterranean. The dynamics of chlorophyll after VLDE were studied as a proxy for phytoplankton response to atmospheric dust. A significant response of chlorophyll to dust addition was evident; this appeared to be especially true for the Western Mediterranean where a chlorophyll increases of up to 345% was recorded, whereas in the Central Mediterranean it was up to 146% and in the Eastern Mediterranean up to 121%. Chlorophyll response behavior was quite heterogeneous probably as a result of the uniqueness of each VLDE, the differences between Mediterranean areas, the community structure of phytoplankton and the interaction between bacteria and phytoplankton for new resources. An eastward decreasing trend in chlorophyll response was observed, which is in accordance with the relative importance of bacterial activity with respect to phytoplankton. The increase in mineral aerosols with increased aridity in the region together with the decrease in the depth of the mixed layer of the oceans should boost the importance of aerosols fueling marine production.
      PubDate: 2016-11-03T10:13:17.667149-05:
      DOI: 10.1002/2016GB005404
  • Relationships between the El Niño Southern Oscillation, precipitation,
           and nitrogen wet deposition rates in the contiguous United States
    • Authors: Tsengel Nergui; R. David Evans, Jennifer C. Adam, Serena H. Chung
      Abstract: Human activities have significantly increased reactive nitrogen (N) in the environment, leading to adverse effects on various ecosystems. We used 1979-2012 seasonal inorganic N wet deposition data from the National Atmospheric Deposition Program to evaluate the relationship between the El Niño Southern Oscillation (ENSO) and N wet deposition in the contiguous U.S. The correlations between precipitation and inorganic N wet deposition were the strongest and most spatially extensive during winter; up to 62% and 53% of the 2- to 6-year variations of precipitation and N wet deposition rates, respectively, in the Rocky Mountains, along the coast of the Gulf of Mexico, and near the Great Lakes can be explained by variation in the NINO3.4 climate index, which was used as a measure of ENSO activity. During El Niño winters, precipitation and N wet deposition rates were higher than normal in the southern U.S., while La Niña events brought higher precipitation and N wet deposition to the Rocky Mountains and Great Lakes regions. Wintertime N wet deposition correlations held through springtime in the Great Lakes and the Northeast; however, correlations between NINO3.4 and precipitation were not significant at most sites, suggesting factors besides precipitation amount contributed to the 2- to 6-year variation of N wet deposition in these regions. As the frequency, strength, and types of ENSO change in the future, inter-annual variability of N wet deposition will be affected, indirectly affecting spatial distribution of dry N deposition and potentially changing the overall spatial patterns of N deposition.
      PubDate: 2016-10-29T08:15:26.55969-05:0
      DOI: 10.1002/2016GB005439
  • Anthropogenic carbon in the ocean, surface to interior connections
    • Authors: Sjoerd Groeskamp; Andrew Lenton, Richard Matear, Bernadette M. Sloyan, Clothilde Langlais
      Abstract: Quantifying the surface to interior transport of Anthropogenic carbon (CA) is critical for projecting future carbon uptake and, for improved understanding of the role of the oceans in the global carbon cycle. Here we develop and apply a diagnostic tool that provides a volumetric streamfunction in (CA,σ0) coordinates to calculate the total diapycnal CA transport in the ocean. We combine this with air-sea fluxes of CA to infer the internal ocean mixing of CA to obtain a closed globally integrated budget analyses of the ocean's CA transport. This diagnostic separates the contribution from the mean flow, seasonal cycles, trend, surface fluxes and mixing in the distribution and the accumulation of CA in the ocean. We find that the redistribution of CA from the surface to the interior of the ocean is due to an interplay between circulation and mixing. The circulation component is dominated by the mean flow, however effects due to seasonal cycles are significant for the CA redistribution. The two most important pathways for CA subduction are through the transformation of Thermocline Water (TW) into Subantarctic Mode Water (SAMW) and, by transformation of Circumpolar Deep Water (CDW) into lighter Antarctic Intermediate Water (AAIW). The results suggest that an accurate representation of intermediate and mode water formation, deep water formation and spatial and temporal distribution of ocean mixing in ocean models is essential to simulate and project the oceanic uptake of CA.
      PubDate: 2016-10-20T08:15:20.939913-05:
      DOI: 10.1002/2016GB005476
  • Vertical modeling of the nitrogen cycle in the eastern tropical South
           Pacific oxygen deficient zone using high-resolution concentration and
           isotope measurements
    • Authors: Brian D. Peters; Andrew R. Babbin, Karsten A. Lettmann, Calvin W. Mordy, O. Ulloa, Bess B. Ward, Karen L. Casciotti
      Abstract: Marine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss processes have been debated, and traditional methods for measuring these rates are labor-intensive and may miss hot spots in spatially and temporally variable environments. Here, we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high-resolution concentration and natural abundance stable isotope measurements of nitrate (NO3-) and nitrite (NO2-). These measurements were used to estimate process rates using a two-step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO3- reduction and NO2- oxidation. Nevertheless, we addressed two questions surrounding the fates of NO2- in the ODZ: 1) Is NO2- being primarily reduced to N2 or oxidized to NO3- in the ODZ? and 2) what are the contributions of anammox and denitrification to NO2- removal? Depth-integrated rates from the model suggest that 72-88% of the NO2- produced in the ODZ was oxidized back to NO3-, while 12-28% of NO2- was reduced to N2. Furthermore, our model suggested that 36-74% of NO2- loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long-term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.
      PubDate: 2016-10-15T14:55:25.827321-05:
      DOI: 10.1002/2016GB005415
  • Multi-decadal Wind-Driven Shifts in Northwest Pacific Temperature,
           Salinity, O2 and PO4
    • Authors: Eun Young Kwon; Young Ho Kim, Young-Gyu Park, Young-Hyang Park, John Dunne, Kyung-Il Chang
      Abstract: The North Pacific gyre boundaries are characterized by stark contrasts in physical and biogeochemical properties. Meridional movement of gyre boundaries, influenced by climate change, can therefore exert a large influence on not only marine ecosystems but also on climate. We examine the evidence for wind-driven southward shifts in subsurface temperature, salinity, PO4, and O2 within the Northwest Pacific from the 1950s to the 2000s. Gyre boundary shifts can explain 30 ~ 60% of temperature and salinity trends zonally averaged in the Northwest Pacific, and observed PO4 and O2 trends along the 137°E and 144°E meridians. The close tie between the wind-driven shifts in gyre boundaries and the tracer distributions is further supported by results from an eddy-resolving (0.1° × 0.1°) GFDL climate model, suggesting that the physical and biogeochemical properties averaged within the Northwest Pacific gyre boundaries closely follow the latitude changes of the zero Sverdrup stream function with lags of zero to three years. The gyre shift effect on tracer distribution is poorly represented in a coarse resolution (1° × 1°) model due partly to poor representations of fronts and eddies. This study suggests that future changes in Northwest Pacific PO4 and O2 content may depend not only on ocean temperature and stratification, but also on the ocean gyre response to winds.
      PubDate: 2016-10-15T13:45:28.599388-05:
      DOI: 10.1002/2016GB005442
  • Interdecadal Trichodesmium variability in cold North Atlantic waters
    • Authors: Sara Rivero-Calle; Carlos E. Del Castillo, Anand Gnanadesikan, Amin Dezfuli, Benjamin Zaitchik, David G. Johns
      Abstract: Studies of the nitrogen cycle in the ocean generally assume that the distribution of the marine diazotroph, Trichodesmium, is restricted to warm, tropical and sub-tropical oligotrophic waters. Here we show evidence that Trichodesmium are widely distributed in the North Atlantic. We report an approximately five-fold increase during the 1980s and 1990s in Trichodesmium presence near the British Isles with respect to the average over the last 50 years. A potential explanation is an increase in the Saharan dust source starting in the 1980s, coupled with changes in North Atlantic winds that opened a pathway for dust transport. Results from a coarse-resolution model in which winds vary but iron deposition is climatologically fixed, suggest frequent nitrogen limitation in the region and reversals of the Portugal current, but it does not simulate the observed changes in Trichodesmium. In addition, modeling results suggest frequent nitrogen limitation in the region and possible reversals of the Portugal current. These factors, when coupled with iron fertilization, could benefit Trichodesmium. Our results suggest that Trichodesmium may be capable of growth at temperatures below 20 oC and challenge assumptions about their latitudinal distribution. Therefore, we need to reevaluate assumptions about the temperature limitations of Trichodesmium and the dinitrogen (N2) fixation capabilities of extra-tropical strains, which may have important implications for the global nitrogen budget.
      PubDate: 2016-10-15T13:20:27.610541-05:
      DOI: 10.1002/2015GB005361
  • Phytoplankton Size Impact on Export Flux in the Global Ocean
    • Authors: Colleen B. Mouw; Audrey Barnett, Galen A. McKinley, Lucas Gloege, Darren Pilcher
      Abstract: Efficiency of the biological pump of carbon to the deep ocean depends largely on biologically mediated export of carbon from the surface ocean and its remineralization with depth. Global satellite studies have primarily focused on chlorophyll concentration and net primary production (NPP) to understand the role of phytoplankton in these processes. Recent satellite retrievals of phytoplankton composition now allow for the size of phytoplankton cells to be considered. Here, we improve understanding of phytoplankton size structure impacts on particle export, remineralization and transfer. A global compilation of particulate organic carbon (POC) flux estimated from sediment traps and 234Th are utilized. Annual climatologies of NPP, percent microplankton, and POC flux at four time series locations and within biogeochemical provinces are constructed. Parameters that characterize POC flux vs. depth (export flux ratio, labile fraction, remineralization length scale) are fit for time series locations, biogeochemical provinces and times of the year dominated by small and large phytoplankton cells where phytoplankton cell size show enough dynamic range over the annual cycle. Considering all data together, our findings support the idea of high export flux but low transfer efficiency in productive regions and vice versa for oligotrophic regions. However, when parsing by dominant size class, we find periods dominated by small cells to have both greater export flux efficiency and lower transfer efficiency than periods when large cells comprise a greater proportion of the phytoplankton community.
      PubDate: 2016-10-14T17:10:55.974888-05:
      DOI: 10.1002/2015GB005355
  • Nitrogen deposition slows down soil sulfur cycling
    • Authors: Hao Chen; Liqiong Yang, Li Wen, Pan Luo, Lu Liu, Yi Yang, Kelin Wang, Dejun Li
      Abstract: Increased atmospheric nitrogen (N) deposition has been found to alter processes and functions of terrestrial ecosystems including the biogeochemical cycling of N and other elements, e.g., phosphorus (P), calcium (Ca) and potassium (K). Nevertheless, how N deposition changes sulfur (S) cycling is largely unknown. Based on a meta-analysis and a lab N addition experiment, here we show that N addition significantly suppresses the activity of soil arylsulfatase, which is a major enzyme involved in the mineralization of organic S. The evidence suggests that N-induced decrease in soil pH is responsible for the decrease of arylsulfatase activity. Soil buffering capacity plays a critical role in mediating the extent of arylsulfatase activity response to N inputs via its regulation on soil pH. Our results suggest that N deposition may slow down S cycling by suppressing soil organic S mineralization.
      PubDate: 2016-10-11T09:10:54.984972-05:
      DOI: 10.1002/2016GB005423
  • Climatic Drivers for Multi-Decadal Shifts in Solute Transport and Methane
           Production Zones within a Large Peat Basin
    • Authors: Paul H. Glaser; Donald I. Siegel, Jeffrey P. Chanton, Andrew S. Reeve, Donald O. Rosenberry, J. Elizabeth Corbett, Zeno Levy
      Abstract: Northern peatlands are an important source for greenhouse gases but their capacity to produce methane remains uncertain under changing climatic conditions. We therefore analyzed a 43-year time series of pore-water chemistry to determine if long-term shifts in precipitation altered the vertical transport of solutes within a large peat basin in northern Minnesota. These data suggest that rates of methane production can be finely tuned to multi-decadal shifts in precipitation that drive the vertical penetration of labile carbon substrates within the Glacial Lake Agassiz Peatlands.Tritium and cation profiles demonstrate that only the upper meter of these peat deposits was flushed by downwardly moving recharge from 1965 through 1983 during a Transitional Dry-to-Moist Period. However, a shift to a moister climate after 1984 drove surface waters much deeper, largely flushing the pore waters of all bogs and fens to depths of 2 m. Labile carbon compounds were transported downward from the rhizosphere to the basal peat at this time producing a substantial enrichment of methane in ∆14C with respect to the solid-phase peat from 1991 to 2008. These data indicate that labile carbon substrates can fuel deep production zones of methanogenesis that more than doubled in thickness across this large peat basin after 1984. Moreover, the entire peat profile apparently has the capacity to produce methane from labile carbon substrates depending on climate-driven modes of solute transport. Future changes in precipitation may therefore play a central role in determining the source strength of peatlands in the global methane cycle.
      PubDate: 2016-10-06T02:36:03.589529-05:
      DOI: 10.1002/2016GB005397
  • Aboveground biomass variability across intact and degraded forests in the
           Brazilian Amazon
    • Authors: Marcos Longo; Michael M. Keller, Maiza N. dos-Santos, Veronika Leitold, Ekena R. Pinagé, Alessandro Baccini, Sassan Saatchi, Euler M. Nogueira, Mateus Batistella, Douglas C. Morton
      Abstract: Deforestation rates have declined in the Brazilian Amazon since 2005, yet degradation from logging, fire, and fragmentation has continued in frontier forests. In this study we quantified the aboveground carbon density (ACD) in intact and degraded forests using the largest data set of integrated forest inventory plots (n = 359) and airborne lidar data (18,000ha) assembled to date for the Brazilian Amazon. We developed statistical models relating inventory ACD estimates to lidar metrics that explained 70% of the variance across forest types. Airborne lidar ACD estimates for intact forests ranged between 5.0 ± 2.5 and 31.9±10.8kgCm−2. Degradation carbon losses were large and persistent. Sites that burned multiple times within a decade lost up to 15.0±0.7kgCm−2(94%) of ACD. Forests that burned nearly 15 years ago had between 4.1 ± 0.5 and 6.8±0.3kgCm−2(22 − 40%) less ACD than intact forests. Even for low-impact logging disturbances, ACD was between 0.7 ± 0.3 to 4.4±0.4kgCm−2(4 − 21%) lower than unlogged forests. Comparing biomass estimates from airborne lidar to existing biomass maps, we found that regional and pan-tropical products consistently overestimated ACD in degraded forests, underestimated ACD in intact forests, and showed little sensitivity to fires and logging. Fine-scale heterogeneity in ACD across intact and degraded forests highlights the benefits of airborne lidar for carbon mapping. Differences between airborne lidar and regional biomass maps underscore the need to improve and update biomass estimates for dynamic land use frontiers, to better characterize deforestation and degradation carbon emissions for regional carbon budgets and REDD+.
      PubDate: 2016-10-03T03:36:25.695017-05:
      DOI: 10.1002/2016GB005465
  • The role of metabolism in modulating CO2 fluxes in boreal lakes
    • Authors: Matthew J. Bogard; Paul A. Giorgio
      Abstract: Lake CO2 emissions are increasingly recognized as an important component of the global CO2 cycle, yet the origin of these emissions is not clear, as specific contributions from metabolism and in-lake cycling, versus external inputs, are not well defined. To assess the coupling of lake metabolism with CO2 concentrations and fluxes, we estimated steady-state ratios of gross primary production to respiration (GPP:R) and rates of net ecosystem production (NEP = GPP-R) from surface water O2 dynamics (concentration and stable isotopes) in 187 boreal lakes spanning long environmental gradients. Our findings suggest internal metabolism plays a dominant role in regulating CO2 fluxes in most lakes, but this pattern only emerges when examined at a resolution that accounts for the vastly differing relationships between lake metabolism and CO2 fluxes: Fluxes of CO2 exceeded those from NEP in over half the lakes, but unexpectedly, these effects were most common and typically largest in a subset (~30% of total) of net autotrophic lakes that nevertheless emitted CO2. Equally surprising, we found no environmental characteristics that distinguished this category from the more common net heterotrophic, CO2 outgassing lakes. Excess CO2 fluxes relative to NEP were best predicted by catchment structure and hydrologic properties, and we infer from a combination of methods that both catchment inputs and internal anaerobic processes may have contributed this excess CO2. Together, our findings show the link between lake metabolism and CO2 fluxes is often strong, but can vary widely across the boreal biome, having important implications for catchment-wide C budgets.
      PubDate: 2016-09-28T00:55:51.303708-05:
      DOI: 10.1002/2016GB005463
  • Analysis of longitudinal variations in North Pacific alkalinity to improve
           predictive algorithms
    • Authors: Claudia H. Fry; Toby Tyrrell, Eric P. Achterberg
      Abstract: The causes of natural variation in alkalinity in the North Pacific surface ocean need to be investigated to understand the carbon cycle and to improve predictive algorithms. We used GLODAPv2 to test hypotheses on the causes of three longitudinal phenomena in Alk*, a tracer of calcium carbonate cycling. These phenomena are: (a) an increase from east to west between 45°N and 55°N, (b) an increase from west to east between 25°N and 40°N, and (c) a minor increase from west to east in the equatorial upwelling region. Between 45°N and 55°N, Alk* is higher on the western than on the eastern side and this is associated with denser isopycnals with higher Alk* lying at shallower depths. Between 25°N and 40°N, upwelling along the North American continental shelf causes higher Alk* in the east. Along the equator, a strong east-west trend was not observed, even though the upwelling on the eastern side of the basin is more intense, because the water brought to the surface is not high in Alk*. We created two algorithms to predict alkalinity, one for the entire Pacific Ocean north of 30°S and one for the eastern margin. The Pacific Ocean algorithm is more accurate than the commonly-used algorithm published by Lee et al. [2006], of similar accuracy to the best previously published algorithm by Sasse et al. [2013], and is less biased with longitude than other algorithms in the subpolar North Pacific. Our eastern margin algorithm is more accurate than previously published algorithms.
      PubDate: 2016-09-28T00:55:48.503362-05:
      DOI: 10.1002/2016GB005398
  • Tracing Dust Input to the Global Ocean using Thorium Isotopes in Marine
           Sediments: ThoroMap
    • Authors: S. S. Kienast; G. Winckler, J. Lippold, S. Albani, N. M. Mahowald
      Abstract: Continental dust input into the ocean-atmosphere system has significant ramifications for biogeochemical cycles and global climate, yet direct observations of dust deposition in the ocean remain scarce. The long-lived isotope thorium-232 (232Th) is greatly enriched in upper continental crust compared to oceanic crust and mid-ocean ridge basalt (MORB)-like volcanogenic material. In open ocean sediments, away from fluvial and ice-rafted sources of continental material, 232Th is often assumed to be of predominantly eolian origin. In conjunction with flux normalization based on the particle reactive radioisotope thorium-230 (230Th), 232Th measurements in marine sediments are a promising proxy for dust accumulation in the modern and past ocean. Here we present ThoroMap, a new global data compilation of 230Th-normalized fluxes of 232Th. After careful screening, we derive dust deposition estimates in the global ocean averaged for the late Holocene (0-4 ka) and the Last Glacial Maximum (LGM, 19-23 ka). ThoroMap is compared with dust deposition estimates derived from CCSM3 and CCSM4, two coupled atmosphere, land, ocean, and sea-ice models. Model-data correlation factors are 0.63 (CCSM3) and 0.59 (CCSM4) in the late Holocene and 0.82 (CCSM3) and 0.83 (CCSM4) in the LGM. ThoroMap is the first compilation that is built on a single, specific proxy for dust and that exclusively uses flux-normalization to derive dust deposition rates.
      PubDate: 2016-09-28T00:55:46.684301-05:
      DOI: 10.1002/2016GB005408
  • Mercury isotope compositions across North American forests
    • Authors: Wang Zheng; Daniel Obrist, Dominique Weis, Bridget A. Bergquist
      Abstract: Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg.The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0) and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere-forest-soil interface.
      PubDate: 2016-09-28T00:55:42.817945-05:
      DOI: 10.1002/2015GB005323
  • Quantifying uncertainty in future ocean carbon uptake
    • Authors: John P. Dunne
      Abstract: Attributing uncertainty in ocean carbon uptake between societal trajectory (scenarios), earth system model construction (structure), and inherent natural variation in climate (internal), is critical to make progress in identifying, understanding and reducing those uncertainties. In the present issue of Global Biogeochemical Cycles, Lovenduski et al. (2016) disentangle these drivers of uncertainty in ocean carbon uptake over time and space and assess the resulting implications for the emergence timescales of structural and scenario uncertainty over internal variability. Such efforts are critical for establishing realizable and efficient monitoring goals and prioritizing areas of continued model development. Under recently proposed climate stabilization targets, such efforts to partition uncertainty also become increasingly critical to societal decision‐making in the context of carbon stabilization.
      PubDate: 2016-09-22T10:35:20.447163-05:
      DOI: 10.1002/2016GB005525
  • Decadal variations and trends of the global ocean carbon sink
    • Authors: Peter Landschützer; Nicolas Gruber, Dorothee C. E. Bakker
      Abstract: We investigate the variations of the ocean CO2 sink during the past three decades using global surface ocean maps of the partial pressure of CO2 reconstructed from observations contained in the Surface Ocean CO2 Atlas Version 2. To create these maps, we used the neural network‐based data‐interpolation method of [Landschützer2014], but extended the work in time from 1998 through 2011 to the period from 1982 through 2011. Our results suggest strong decadal variations in the global ocean carbon sink around a long‐term increase that corresponds roughly to that expected from the rise in atmospheric CO2. The sink is estimated to have weakened during the 1990s toward a minimum uptake of only ‐0.8 ± 0.5 Pg C yr − 1 in 2000, and thereafter to have strengthened considerably to rates of more than ‐2.0 ± 0.5 Pg C yr − 1. These decadal variations originate mostly from the extratropical oceans while the tropical regions contribute primarily to interannual variations. Changes in sea‐surface temperature affecting the solubility of CO2 explain part of these variations, particularly at subtropical latitudes. But most of the higher latitude changes are attributed to modifications in the surface concentration of dissolved inorganic carbon and alkalinity, induced by decadal variations in atmospheric forcing, with patterns that are reminiscent of those of the Northern and Southern Annular Modes. These decadal variations lead to a substantially smaller cumulative anthropogenic CO2 uptake of the ocean over the 1982 through 2011 period (reduction of 7.5 ± 5.5 Pg C) relative to that derived by the Global Carbon Budget.
      PubDate: 2016-09-20T06:55:23.555706-05:
      DOI: 10.1002/2015GB005359
  • The age of iron and iron source attribution in the ocean
    • Authors: Mark Holzer; Marina Frants, Benoît Pasquier
      Abstract: We use tracers to partition dissolved iron (dFe) into the contributions from each source within a numerical model of the iron cycle without perturbing the system. These contributions are further partitioned according to the time since injection into the ocean, which defines their iron‐age spectrum and mean iron age. The utility of these diagnostics is illustrated for a family of inverse‐model estimates of the iron cycle, constrained by a data‐assimilated circulation and available dFe measurements. The source contributions are compared with source anomalies defined as the differences between solutions with and without the source in question. We find that in the Southern Ocean euphotic zone, the hydrothermal and sediment contributions range from 15% to 30% of the total each, which the anomalies underestimate by a factor of  ∼ 2 because of the nonlinearity of scavenging. The iron age is only reset by scavenging and attains a mean of several hundred years in the Southern Ocean euphotic zone, revealing that aeolian iron there is supplied primarily from depth as regenerated dFe. Tagging iron according to source region and pathways shows that 70–80% of the aeolian dFe in the euphotic zone near Antarctica is supplied from north of 46° S via paths that reach below 1 km depth. Hydrothermal iron has the oldest surface mean ages on the order of mid‐depth ventilation times. A measure of uncertainty is provided by the systematic variations of our diagnostics across the family of iron‐cycle estimates, each member of which has a different aeolian source strength.
      PubDate: 2016-09-20T06:41:42.039895-05:
      DOI: 10.1002/2016GB005418
  • Issue Information
    • Pages: 1371 - 1371
      Abstract: No abstract is available for this article.
      PubDate: 2016-11-08T15:19:09.845504-05:
      DOI: 10.1002/gbc.20340
  • Dissolved iron and iron isotopes in the Southeastern Pacific Ocean
    • Authors: Jessica N. Fitzsimmons; Tim M. Conway, Jong‐Mi Lee, Richard Kayser, Kristen M. Thyng, Seth G. John, Edward A. Boyle
      First page: 1372
      Abstract: The Southeast Pacific Ocean is a severely understudied yet dynamic region for trace metals such as iron, since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters. In this study, we report the dissolved iron (dFe) distribution from seven stations and Fe isotope ratios (δ56Fe) from three of these stations across a near‐zonal transect from 20‐27°S. We found elevated dFe concentrations associated with the oxygen deficient zone (ODZ), with light δ56Fe implicating reduced Fe porewater fluxes. However, temporal dFe variability and rapid δ56Fe shifts with depth suggest gradients in ODZ Fe source and/or redox processes vary over short depth/spatial scales. The dFe concentrations decreased rapidly offshore, and in the upper ocean dFe was controlled by biological processes, resulting in an Fe:C ratio of 4.2 µmol/mol. Calculated vertical diffusive Fe fluxes were greater than published dust inputs to surface waters, but both were orders of magnitude lower than horizontal diffusive fluxes, which dominate dFe delivery to the gyre. The δ56Fe data in the deep sea showed evidence for a ‐0.2‰ AAIW end‐member and a heavy δ56Fe of +0.55‰ for distally‐transported hydrothermal dissolved Fe from the East Pacific Rise. These heavy δ56Fe values were contrasted with the near‐crustal δ56Fe recorded in the hydrothermal plume reaching Station ALOHA in the North Pacific. The heavy hydrothermal δ56Fe precludes a nanopyrite composition of hydrothermal dFe and instead suggests the presence of oxides or, more likely, binding of hydrothermal dFe by organic ligands in the distal plume.
      PubDate: 2016-09-14T05:05:35.642126-05:
      DOI: 10.1002/2015GB005357
  • The anthropogenic perturbation of the marine nitrogen cycle by atmospheric
           deposition: Nitrogen cycle feedbacks and the 15N Haber‐Bosch effect
    • Authors: Simon Yang; Nicolas Gruber
      First page: 1418
      Abstract: Over the last 100 years, anthropogenic emissions have led to a strong increase of atmospheric nitrogen deposition over the ocean, yet the resulting impacts and feedbacks are neither well understood nor quantified. To this end, we run a suite of simulations with the ocean component of the Community Earth System Model v1.2 forced with five scenarios of nitrogen deposition over the period from 1850 through 2100, while keeping all other forcings unchanged. Even though global oceanic net primary production increases little in response to this fertilization, the higher export and the resulting expansion of the oxygen minimum zones cause an increase in pelagic and benthic denitrification and burial by about 5%. In addition, the enhanced availability of fixed nitrogen in the surface ocean reduces global ocean N2‐fixation by more than 10%. Despite the compensating effects through these negative feedbacks that eliminate by the year 2000 about 60% of the deposited nitrogen, the anthropogenic nitrogen input forced the upper ocean N‐budget into an imbalance of between 9 to 22 Tg N yr−1 depending on the deposition scenario. The excess nitrogen accumulates to highly detectable levels and causes in most areas a distinct negative trend in the δ15N of the oceanic fixed nitrogen pools ‐ a trend we refer to as the 15N Haber‐Bosch effect. Changes in surface nitrate utilization and the nitrogen feedbacks induce further changes in the δ15N of NO3, making it a good, but complex recorder of the overall impact of the changes in atmospheric deposition.
      PubDate: 2016-08-25T15:01:05.380467-05:
      DOI: 10.1002/2016GB005421
  • A multi‐year estimate of methane fluxes in Alaska from CARVE
           atmospheric observations
    • Authors: Scot M. Miller; Charles E. Miller, Roisin Commane, Rachel Y.‐W. Chang, Steven J. Dinardo, John M. Henderson, Anna Karion, Jakob Lindaas, Joe R. Melton, John B. Miller, Colm Sweeney, Steven C. Wofsy, Anna M. Michalak
      First page: 1441
      Abstract: Methane (CH4) fluxes from Alaska and other arctic regions may be sensitive to thawing permafrost and future climate change, but estimates of both current and future fluxes from the region are uncertain. This study estimates CH4 fluxes across Alaska for 2012 – 2014 using aircraft observations from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) and a geostatistical inverse model (GIM). We find that a simple flux model based on a daily soil temperature map and a static map of wetland extent reproduces the atmospheric CH4 observations at the state‐wide, multi‐year scale more effectively than global‐scale process‐based models. This result points to a simple and effective way of representing CH4 fluxes across Alaska. It further suggests that process‐based models can improve their representation of key processes, and that more complex processes included in these models cannot be evaluated given the information content of available atmospheric CH4 observations. In addition, we find that CH4 emissions from the North Slope of Alaska account for 24% of the total statewide flux of 1.74  ±  0.26 Tg CH4 (for May – Oct.). Global‐scale process models only attribute an average of 3% of the total flux to this region. This mismatch occurs for two reasons: process models likely underestimate wetland extent in regions without visible surface water, and these models prematurely shut down CH4 fluxes at soil temperatures near 0° C. Lastly, we find that the seasonality of CH4 fluxes varied during 2012 – 2014, but that total emissions did not differ significantly among years, despite substantial differences in soil temperature and precipitation.
      PubDate: 2016-09-15T07:41:25.616771-05:
      DOI: 10.1002/2016GB005419
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