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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 56, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 7, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 3, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 8)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 25)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 15)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 24)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 17)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 4, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 8, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 66, SJR: 2.189, h-index: 121)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [7 followers]  Follow
    
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Trace metal‐limited growth affects the post mortem dissolution of
           the marine diatom Pseudo‐nitzschia delicatissima
    • Abstract: We investigated the effects of iron (Fe) and copper (Cu) limitations on biogenic silica (bSiO2) dissolution kinetics of the marine diatom Pseudo‐nitzschia delicatissima during a three week batch dissolution experiment. The dissolution of this species was faster during the first week than thereafter. Modeling results from four dissolution models and Scanning Electron Microcopy images suggested the successive dissolution of two phases of bSiO2, with two different dissolution constants. Micronutrient limitation during growth affected the respective proportion of the two phases, and their dissolution constants. After three weeks of dissolution, frustules from micronutrient‐limited diatoms were better preserved than those of replete cells. Our results also confirm that micronutrient‐limited cells may export more Si relative to N than replete cells, and may increase the silicate pump: This may not only be due to a higher degree of silicification of the live cells, but also to a decoupling between the recycling of Si and N during dissolution. We suggest that a mechanistic understanding of the evolution of the dissolution constant during dissolution is needed. This would improve the parameterization of dissolution in ecosystem models, and ultimately their predictions on the amount of bSiO2 that dissolves in the photic zone, and the amount of bSiO2 that is exported to the seafloor.
      PubDate: 2015-12-22T16:47:35.98529-05:0
      DOI: 10.1002/2015GB005088
       
  • Towards More Realistic Projections of Soil Carbon Dynamics by Earth System
           Models
    • Abstract: Soil carbon (C) is a critical component of Earth system models (ESMs) and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the 3rd to 5th assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real‐world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. Firstly, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by 1st‐order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic SOC dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth‐dependent environmental controls, and other processes that strongly affect soil C dynamics. Secondly, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool‐ and flux‐based datasets through data assimilation is among the highest priorities for near‐term research to reduce biases among ESMs. Thirdly, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable datasets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
      PubDate: 2015-12-18T21:55:34.283143-05:
      DOI: 10.1002/2015GB005239
       
  • Contrasting correlation patterns between environmental factors and
           chlorophyll levels in the global ocean
    • Abstract: In this study we analyze large‐scale satellite‐derived data using generalized additive models to characterize the global correlation patterns between environmental forcing and marine phytoplankton biomass. We found systematic differences in the relationships between key environmental drivers (temperature, light and wind) and ocean chlorophyll in the subtropical/tropical and temperate oceans. For the subtropical/tropical and equatorial oceans, the chlorophyll generally declined with increasing temperature and light, while in temperate oceans, chlorophyll was best explained by bell‐shaped or positive functions of temperature and light. The relationship between chlorophyll and wind speed is generally positive in low latitude oceans and bell‐shaped in temperate oceans. Our analyses also demonstrated strong and geographically consistent positive autoregressive effects of chlorophyll from one month to the next and negative autoregressive effects for measurements 2 months apart. These findings imply possibly different regional phytoplankton responses to environmental forcing, suggesting that future environmental change could affect the tropical and temperate upper ocean chlorophyll levels differently.
      PubDate: 2015-12-06T20:19:32.851192-05:
      DOI: 10.1002/2015GB005216
       
  • Decadal trends of ocean and land carbon fluxes from a regional joint
           ocean‐atmosphere inversion
    • Authors: K. Steinkamp; N. Gruber
      Abstract: From 1980 until 2010, the combined CO2 sink strengths of ocean and land increased by nearly 50% (‐0.55 PgCyr−1decade−1), but the spatial distribution of this trend is not well known. We address this by performing a joint cyclostationary ocean‐atmosphere inversion for the three decades 1980‐89, 1990‐99 and 2000‐08, using only carbon data from the ocean and atmosphere as constraints, i.e., without applying any prior information about the land fluxes. We find that in the inversion, most of the 30 year sink trend stems from the ocean (‐0.44 PgCyr−1decade−1). The contribution of the terrestrial biosphere is commensurably smaller, but has more decadal variability. First the land sink strength intensified in the 1990s by 0.4 (±0.3) PgCyr−1 compared to the 1980s, but then weakened slightly by 0.2 (±0.4) PgCyr−1 in the 2000s. The different land regions contributed very variedly to these global trends. While the northern extra‐tropical land acted as an increasing carbon sink throughout the examined period primarily driven by boreal regions, the tropical land is estimated to have acted as an increasing source of CO2, with source magnitude and trend dominated by enhanced release in tropical America during the Amazon‐mean wet season. This pattern is largely unchanged if the oceanic inversion constraint, which is based on a stationary ocean circulation, is replaced by an estimate based on simulation results from an ocean biogeochemical general circulation model that includes year‐to‐year variability in the air‐sea CO2 fluxes and also has a trend (‐0.07 PgCyr−1decade−1) that is at the very low end of current estimates. However, the land/ocean partitioning of the trend contribution is adjusted accordingly. Oceanic carbon data has a major impact on carbon exchange for all tropical regions and southern Africa, but also for observationally better constrained regions in North America and temperate Asia. The European trend exhibits a strong sensitivity to the choice of the atmospheric CO2 network.
      PubDate: 2015-11-25T08:23:42.770457-05:
      DOI: 10.1002/2014GB004907
       
  • An impulse response function for the ‘long tail’ of excess
           atmospheric CO2 in an Earth system model
    • Authors: N. S. Lord; A. Ridgwell, M. C. Thorne, D. J. Lunt
      Abstract: The ultimate fate of (fossil fuel) CO2 emitted to the atmosphere is governed by a range of sedimentological and geological processes operating on timescales of up to the ca. hundred thousand year response of the silicate weathering feedback. However, how the various geological CO2 sinks might saturate and feedbacks weaken in response to increasing total emissions is poorly known. Here, we explore the relative importance and timescales of these processes using a 3D ocean‐based Earth system model. We first generate an ensemble of 1 Myr duration CO2 decay curves spanning cumulative emissions of up to 20,000 PgC. To aid characterization and understanding of the model response to increasing emission size, we then generate an impulse response function description for the long‐term fate of CO2 in the model. Our analysis is consistent with a progressively increasing fraction of total emissions that are removed from the atmosphere as emissions increase, via carbonate weathering and burial, due to the ocean carbon sink becoming saturated, together with a lengthening of the timescale of removal. However, we find that in our model the ultimate CO2 sink – silicate weathering feedback – is approximately invariant with respect to cumulative emissions, both in terms of its importance (it removes the remaining excess ~7% of total emissions from the atmosphere) and timescale (~270 kyr). Because a simple pulse‐response description leads to initially large predictive errors for a realistic time‐varying carbon release, we also develop a convolution‐based description of atmospheric CO2 decay which can be used as a simple and efficient means of making long‐term carbon cycle perturbation projections.
      PubDate: 2015-11-18T05:40:17.504919-05:
      DOI: 10.1002/2014GB005074
       
  • Nitrogen cycling in the secondary nitrite maximum of the Eastern Tropical
           North Pacific off Costa Rica
    • Authors: Carolyn Buchwald; Alyson E. Santoro, Rachel H. R. Stanley, Karen L. Casciotti
      Abstract: Nitrite is a central intermediate in the marine nitrogen cycle and represents a critical juncture where nitrogen can be reduced to the less bioavailable N2 gas or oxidized to nitrate and retained in a more bioavailable form. We present an analysis of rates of microbial nitrogen transformations in the oxygen deficient zone (ODZ) within the eastern tropical north Pacific ocean (ETNP). We determined rates using a novel one‐dimensional model using the distribution of nitrite and nitrate concentrations, along with their natural abundance nitrogen (N) and oxygen (O) isotope profiles. We predict rate profiles for nitrate reduction, nitrite reduction, and nitrite oxidation throughout the ODZ, as well as the contributions of anammox to nitrite reduction and nitrite oxidation. Nitrate reduction occurs at a maximum rate of 25 nM d−1 at the top of the ODZ, at the same depth as the maximum rate of nitrite reduction, 15 nM d−1. Nitrite oxidation occurs at maximum rates of 10 nM d−1 above the secondary nitrite maximum (SNM), but also in the secondary nitrite maximum, within the ODZ. Anammox contributes to nitrite oxidation within the ODZ, but cannot account for all of it. Nitrite oxidation within the ODZ that is not through anammox is also supported by microbial gene abundance profiles. Our results suggest the presence of nitrite oxidation within the ETNP ODZ, with implications for the distribution and physiology of marine nitrite‐oxidizing bacteria, and for total nitrogen loss in the largest marine ODZ.
      PubDate: 2015-11-17T02:17:04.312454-05:
      DOI: 10.1002/2015GB005187
       
  • Cadmium regeneration within the North Atlantic
    • Authors: Saeed Roshan; Jingfeng Wu
      Abstract: Cadmium (Cd) incorporated into the benthic microfossils has been widely used in reconstructing the past water circulation in the North Atlantic. This requires a major control by conservative mixing over regeneration process on the seawater cadmium distribution in the North Atlantic. Through coupling the recently‐reported Cd data at depths below 300 m with quantitative water mass analysis along the GA03 transect, we tested two models for cadmium cycling within the North Atlantic: conservative mixing alone and conservative mixing plus regeneration which is termed as regenerative mixing. The results show that the regenerative mixing model reproduces the observations (Slope=0.99 and R2=0.97) much better than the conservative mixing model (Slope=0.99 and R2=0.88). The regenerative mixing model was applied to estimate the amount of dissolved cadmium regenerated “within” the North Atlantic. This regionally‐regenerated cadmium contributes to ~10% and >50% of the total cadmium in the North Atlantic at depths >1000 m and 300–1000 m, respectively, indicating that the microfossil‐reconstructed seawater Cd can be attributed to the mixing from the South Atlantic to the North Atlantic with high accuracy for depths below 1000 m, whilst within 300–1000 m cadmium is more controlled by the regeneration process than the mixing process, complicating the attribution of the microfossil‐reconstructed seawater Cd to the changes in water mass geometry within this 300–1000 m depth range. A regionally and vertically constant Cd/PO43− regeneration ratio ~ 262 pM/µmol/kg was derived from the regenerative mixing model which is comparable with ratios estimated in the recent studies for the North Atlantic.
      PubDate: 2015-11-11T14:36:51.303273-05:
      DOI: 10.1002/2015GB005215
       
  • Marine Biological Production from In Situ Oxygen Measurements on a
           Profiling Float in the Subarctic Pacific Ocean
    • Authors: Seth M. Bushinsky; Steven Emerson
      Abstract: Evaluating the organic carbon flux from the surface ocean to the interior (the marine biological pump) is essential for predictions of ocean carbon cycle feedback to climate change. One approach for determining these fluxes is to measure the concentration of oxygen in the upper ocean over a seasonal cycle, calculate the net O2 flux using an upper ocean model, and then use a stoichiometric relationship between oxygen evolved and organic carbon produced. Applying this tracer in a variety of ocean areas over seasonal cycles requires accurate O2 measurements on autonomous vehicles. Here we demonstrate this approach using an O2 sensor on a profiling float that is periodically calibrated against atmospheric pO2. Using accurate data and a model that includes all physical and biological processes influencing oxygen, we determine an annual net community production (ANCP) of 0.7 ± 0.5 mol C m−2 yr−1 in the Northeast Pacific Ocean (50°N, 145°W) from June 2012 to June 2013. There is a strong seasonal cycle in net biological oxygen production with wintertime fluxes caused by bubble processes critical to determining the annual flux. Approximately 50% of net autotrophic production during summer months is consumed by net respiration during the winter. The result is a biological pump in the subarctic Pacific Ocean that is less than that determined by similar methods in the subtropics to the south. This estimate is significantly lower than that predicted by satellite remote sensing and global circulation models.
      PubDate: 2015-11-10T13:41:13.929491-05:
      DOI: 10.1002/2015GB005251
       
  • Ammonia and nitrite oxidation in the Eastern Tropical North Pacific
    • Abstract: Nitrification plays a key role in the marine nitrogen (N) cycle, including in oceanic oxygen minimum zones (OMZs), which are hot spots for denitrification and anaerobic ammonia oxidation (anammox). Recent evidence suggests that nitrification links the source (remineralized organic matter) and sink (denitrification and anammox) of fixed N directly in the steep oxycline in the OMZs. We performed shipboard incubations with 15N tracers to characterize the depth distribution of nitrification in the Eastern Tropical North Pacific (ETNP). Additional experiments were conducted to investigate photoinhibition. Allylthiourea (ATU) was used to distinguish the contribution of archaeal and bacterial ammonia oxidation. The abundance of archaeal and β‐proteobacterial ammonia monooxygenase gene subunit A (amoA) was determined by qPCR. The rates of ammonia and nitrite oxidation showed distinct subsurface maxima, with the latter slightly deeper than the former. The ammonia oxidation maximum coincided with the primary nitrite concentration maximum, archaeal amoA gene maximum, and the subsurface nitrous oxide maximum. Negligible rates of ammonia oxidation were found at anoxic depths, where high rates of nitrite oxidation were measured. Archaeal amoA gene abundance was generally one to two orders of magnitude higher than bacterial amoA gene abundance, and inhibition of ammonia‐oxidizing bacteria with 10 μM ATU did not affect ammonia oxidation rates, indicating the dominance of archaea in ammonia oxidation. These results depict highly dynamic activities of ammonia and nitrite oxidation in the oxycline of the ETNP OMZ.
      PubDate: 2015-11-07T04:14:14.168913-05:
      DOI: 10.1002/2015GB005278
       
  • Changing Amazon biomass and the role of atmospheric CO2 concentration,
           climate and land use
    • Authors: Andrea D. Almeida Castanho; David Galbraith, Ke Zhang, Michael T. Coe, Marcos H. Costa, Paul Moorcroft
      Abstract: The Amazonian tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure and function are sensitive to changes in climate, atmospheric composition and land use. In this study biomass and productivity simulated by three DGVMs (IBIS, ED2 and JULES) for the period 1970–2008 are compared with observations from forest plots (RAINFOR). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long‐term increase in simulated biomass in undisturbed Amazonian forests, while land use change dominates a reduction in AGB, of similar magnitude to the CO2 biomass gain, in the south and southeastern Amazonia. Climate extremes exert a strong effect on the biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: 1) spatially variable plant traits; 2) soil and nutrients mediated processes; 3) extreme event mortality; 4) sensitivity to climatic variability. Finally, continued long‐term observations and ecosystem‐scale experiments (e.g. FACE experiments) are essential for a better understanding of the changing dynamics of tropical forests.
      PubDate: 2015-11-06T06:40:57.29023-05:0
      DOI: 10.1002/2015GB005135
       
  • Tracing terrestrial DOC in the Baltic Sea ‐ a 3‐D model study
    • Abstract: The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi‐enclosed sea, by releasing it as a tracer in a 3‐D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land‐sea‐atmosphere cycling of carbon in coastal‐ and shelf seas that are heavily influenced by riverine DOC.
      PubDate: 2015-10-28T21:30:23.634165-05:
      DOI: 10.1002/2014GB005078
       
  • Low particulate carbon to nitrogen ratios in arctic surface waters
    • Abstract: During the Canada Three Oceans and Joint Ocean Ice Study projects in the summers of 2007 and 2008, we measured particulate organic carbon to nitrogen ratios (POC:PON) throughout the euphotic zone in subarctic and arctic waters. Depth‐integrated values averaged 2.65 (±0.19) in the Beaufort Sea and Canada Basin (BS‐CB domain), and were much lower than both the Redfield ratio (6.6) and the average ratios (3.9 to 5.6) measured across other arctic‐subarctic domains. Average uptake ratios of C and N (ρC:ρN) were also lower (0.87±0.14) in BS‐CB than in the other four domains (2.10 to 3.51). Decreasing POC:PON ratios were associated with low concentrations of phytoplankton C, reduced abundance of biogenic silica (bSiO2), a smaller relative contribution of the >5 µm fraction to total chlorophyll a and a larger relative contribution of small flagellates (
      PubDate: 2015-10-15T00:48:11.614376-05:
      DOI: 10.1002/2015GB005200
       
  • Multi‐decadal accumulation of anthropogenic and remineralized
           dissolved inorganic carbon along the Extended Ellett Line in the northeast
           Atlantic Ocean
    • Abstract: Marine carbonate chemistry measurements have been carried out annually since 2009 during UK research cruises along the Extended Ellett Line (EEL), a hydrographic transect in the northeast Atlantic Ocean. The EEL intersects several water masses that are key to the global thermohaline circulation, and therefore the cruises sample a region in which it is critical to monitor secular physical and biogeochemical changes. We have combined results from these EEL cruises with existing quality‐controlled observational data syntheses to produce a hydrographic time‐series for the EEL from 1981 to 2013. This reveals multi‐decadal increases in dissolved inorganic carbon (DIC) throughout the water column, with a near‐surface maximum rate of 1.80 ± 0.45 µmol kg‐1 yr‐1. Anthropogenic CO2 accumulation was assessed, using simultaneous changes in apparent oxygen utilization (AOU) and total alkalinity (TA) as proxies for the biogeochemical processes that influence DIC. The stable carbon isotope composition of DIC (δ13CDIC) was also determined, and used as an independent test of our method. We calculated a volume‐integrated anthropogenic CO2 accumulation rate of 2.8 ± 0.4 mg‐C m‐3 yr‐1 along the EEL, which is about double the global mean. The anthropogenic CO2 component accounts for only 31 ± 6 % of the total DIC increase. The remainder is derived from increased organic matter remineralization, which we attribute to the lateral redistribution of water masses that accompanies subpolar gyre contraction. Output from a general circulation‐ecosystem model demonstrates that spatiotemporal heterogeneity in the observations has not significantly biased our multi‐decadal rate‐of‐change calculations, and indicates that the EEL observations have been tracking distal changes in the surrounding North Atlantic and Nordic Seas.
       
  • Geochemical and tectonic uplift controls on rock nitrogen inputs across
           terrestrial ecosystems
    • Abstract: Rock contains > 99% of Earth's reactive nitrogen (N); but questions remain over the direct importance of rock N weathering inputs to terrestrial biochemical cycling. Here we investigate the factors that regulate rock N abundance and develop a new model for quantifying rock N mobilization fluxes across desert to temperate rainforest ecosystems in California, USA. We analyzed the N content of 968 rock samples from 531 locations, and compiled 178 cosmogenically derived denudation estimates from across the region to identify landscapes and ecosystems where rocks account for a significant fraction of terrestrial N inputs. Strong coherence between rock N content and geophysical factors, such as protolith (i.e. parent rock), grain‐size, and thermal history are observed. A spatial model that combines rock geochemistry with lithology and topography demonstrates that average rock N reservoirs range from 0.18 – 1.2 kg N m−3 (80 to 534 mg N kg−1) across the 9 geomorphic provinces of California, and estimates a rock N denudation flux of 20 – 92 Gg yr−1 across the entire study area (natural atmospheric inputs ~ 140 Gg yr−1). The model highlights regional differences in rock N mobilization, and points to the Coast Ranges, Transverse Ranges, and the Klamath Mountains as regions where rock N could contribute meaningfully to ecosystem N cycling. Contrasting these data to global compilations suggests that our findings are broadly applicable beyond California and that the N abundance and variability in rock are well constrained across most of the Earth system.
       
  • Towards a quantitative and empirical dissolved organic carbon budget for
           the Gulf of Maine, a semi‐enclosed shelf sea
    • Abstract: A time series of organic carbon export from Gulf of Maine (GoM) watersheds was compared to a time series of biological, chemical, bio‐optical and hydrographic properties, measured across the GoM between Yarmouth, NS, Canada and Portland, ME, USA. Optical proxies were used to quantify the dissolved organic carbon (DOC) and particulate organic carbon (POC) in the GoM. The LOADEST regression model applied to river discharge data demonstrated that riverine DOC export (and its decadal variance) has increased over the last 80 years. Several extraordinarily wet years (2006–2010) resulted in a massive pulse of chromophoric dissolved organic matter (CDOM; proxy for DOC) into the western GoM along with unidentified optically‐scattering material (
       
  • Global boron cycle in the anthropocene
    • Abstract: This paper presents a revised and updated synthesis of the biogeochemical cycle of boron at the Earth's surface, where the largest fluxes are associated with the injection of seasalt aerosols to the atmosphere (1.44 Tg B/yr), production and combustion of fossil fuels (1.2 Tg B/yr), atmospheric deposition (3.48 Tg B/yr), the mining of B ores (1.1 Tg B/yr) and the transport of dissolved and suspended matter in rivers (0.80 Tg B/yr). The new estimates show that anthropogenic mobilization of B from the continental crust is equivalent to and in some cases exceeding the naturally occurring processes, resulting in substantial fluxes to the ocean and the hydrosphere. The anthropogenic component contributes 81% of the flux in rivers. The mean residence time for B in seawater supports the use of δ11B in marine carbonates as an index of changes in the pH of seawater over time periods of >1 million years.
       
  • Issue Information
    • Abstract: No abstract is available for this article.
       
  • Multi‐decadal variations in Southern Hemisphere atmospheric 14C:
           Evidence against a Southern Ocean sink at the end of the Little Ice Age
           CO2 anomaly
    • Abstract: Northern Hemisphere‐wide cooling during the Little Ice Age (LIA; CE 1650‐1775) is associated with a ~5 ppmv decrease in atmospheric carbon dioxide. Changes in terrestrial and ocean carbon reservoirs have been postulated as possible drivers of this relatively large shift in atmospheric CO2, potentially providing insights into the mechanisms and sensitivity of the global carbon cycle. Here we report decadally‐resolved radiocarbon (14C) levels in a network of tree rings series spanning CE 1700‐1950 located along the northern boundary of, and within, the Southern Ocean. We observe regional dilutions in atmospheric radiocarbon (relative to the Northern Hemisphere) associated with upwelling of 14CO2–depleted abyssal waters. We find the inter‐hemispheric 14C offset approaches zero during increasing global atmospheric CO2 at the end of the LIA, with reduced ventilation in the Southern Ocean and a Northern Hemisphere source of old carbon (most probably originating from deep Arctic peat layers). The coincidence of the atmospheric CO2 increase and reduction in the inter‐hemispheric 14C offset imply a common climate control. Possible mechanisms of synchronous change in the high latitudes of both hemispheres are discussed.
       
  • Microzooplankton regulation of surface ocean POC:PON ratios
    • Abstract: The elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of the oceanâĂŹs store of carbon. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon‐to‐nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (Particulate Organic Carbon: Particulate Organic Nitrogen; POC:PON). Here we use an idealized ecosystem model to show how interactions between primary and secondary producers maintain the mean composition of surface particulates, and the difference between primary producers and bulk material. Idealized physiological models of phytoplankton and microzooplankton, constrained by laboratory data, reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: under nitrogen limitation, phytoplankton accumulate carbon in carbohydrates and lipids while microzooplankton deplete internal C reserves to fuel respiration. Global ecosystem simulations yield hypothetical global distributions of phytoplankton and microzooplankton C:N ratio predicting elevated phytoplankton C:N ratios in the high light, low nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio due to respiration of excess carbon in systems subject to top‐down control. The model qualitatively captures, and provides a simple interpretation for, a global compilation of surface ocean POC:PON data.
       
  • The Greenland Ice Sheet as a hotspot of phosphorus weathering and export
           in the Arctic
    • Abstract: The contribution of ice sheets to the global biogeochemical cycle of phosphorus is largely unknown, due to the lack of field data. Here, we present the first comprehensive study of phosphorus export from two Greenland Ice Sheet glaciers. Our results indicate that the ice sheet is a hot spot of phosphorus export in the Arctic. Soluble reactive phosphorus (SRP) concentrations, up to 0.35 μM, are similar to those observed in Arctic rivers. Yields of SRP are among the highest in the literature, with denudation rates of 17–27 kg P km−2 year−1. Particulate phases, as with non‐glaciated catchments, dominate phosphorus export (>97 % of total phosphorus flux). The labile particulate fraction differs between the two glaciers studied, with significantly higher yields found at the larger glacier (57.3 versus 8.3 kg P km−2 year−1). Total phosphorus yields are an order of magnitude higher than riverine values reported in the literature. We estimate that the ice sheet contributes ~15 % of total bioavailable phosphorus input to the Arctic oceans (~11 Gg year−1), and dominates total phosphorus input (408 Gg year−1), which is more than three times that estimated from Arctic rivers (126 Gg year−1). We predict that these fluxes will rise with increasing ice sheet freshwater discharge in the future.
       
  • Simulating the modern δ30Si distribution in the oceans and in marine
           sediments
    • Abstract: The δ30Si of biogenic silica (δ30SiBSi) in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past. The application of this proxy, however, requires an understanding of the modern δ30Si distributions and their controlling mechanisms. Here we present results from a modern climate simulation with a coupled ocean‐sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation. In agreement with previous studies, biological fractionation combined with physical transport and mixing determine the oceanic distribution of simulated δ30Si. A new finding is a distinct seasonal cycle of δ30Si in the surface ocean, which is inversely related to that of silicic acid concentration and mixed layer depth. We also provide the first simulation results of sedimentary δ30Si, which reveal that (1) the δ30SiBSi distribution in the surface sediment reflects the exported δ30SiBSi signal from the euphotic zone and (2) the dissolution of biogenic silica in the sediment acts as a source of relatively light δ30Si into the bottom waters of the polar oceans, while it is a source of heavier δ30Si to the subtropical South Atlantic and South Pacific.
       
  • Long‐term P weathering and recent N deposition control contemporary
           plant‐soil C, N and P
    • Abstract: Models are needed to understand how plant‐soil nutrient stores and fluxes have responded to the last two centuries of widespread anthropogenic nutrient pollution and predict future change. These models need to integrate across carbon, nitrogen and phosphorus (C, N, & P) cycles and simulate changes over suitable timescales using available driving data. It is also vital that they are constrainable against observed data to provide confidence in their outputs. To date, no models address all of these requirements. To meet this need, a new model, N14CP, is introduced, which is initially applied to Northern hemisphere temperate and boreal ecosystems over the Holocene. N14CP is parameterized and tested using 88 northern Europe plot‐scale studies, providing the most robust test of such a model to date. The model simulates long‐term P weathering, based on the assumption of a starting pool of weatherable P (Pweath0, g m−2), which is gradually transformed into organic and sorbed pools. Nitrogen fixation (and consequently primary production) is made dependent on available P. In the absence of knowledge about the spatial variability of Pweath0, N14CP produces good average soil and plant variables, but cannot simulate variations among sites. Allowing Pweath0 to vary between sites improves soil C, N and P results greatly, suggesting contemporary soil C, N and P are sensitive to long‐term P weathering. Most sites were found to be N limited. Anthropogenic N deposition since 1800 was calculated to have increased plant biomass substantially, in agreement with observations, and consequently increased soil carbon pools.
       
  • A century of human‐driven changes in the carbon dioxide
           concentration of lakes
    • Abstract: Now that evasion of carbon dioxide (CO2) from inland waters is accounted for in global carbon models, it is crucial to quantify how these fluxes have changed in the past and forecast how they may alter in the future in response to local and global change. Here, we developed a sediment proxy for the concentration of summer surface dissolved CO2 concentration and used it to reconstruct changes over the past 150 years for three large lakes that have been affected by climate warming, changes in nutrient load and detrital terrigenous supplies. Initially CO2‐neutral to the atmosphere, all three lakes subsequently fluctuated between near‐equilibrium and supersaturation. Although catchment inputs have supplied CO2 to the lakes, internal processes and re‐allocation have ultimately regulated decadal changes in lake surface CO2 concentration. Nutrient concentration has been the dominant driver of CO2 variability for a century although the reproducible, non‐monotonic relationship of CO2 to nutrient concentration suggests an interplay between metabolic and chemical processes. Yet, for two of these lakes, climatic control of CO2 concentrations has been important over the last 30 years, promoting higher surface CO2 concentrations, likely by decreasing hypolimnetic carbon storage. This new approach offers the unique opportunity to scale, a posteriori, the long‐term impact of human activities on lake CO2.
       
  • Rapid Anthropogenic Changes in CO2 and pH in the Atlantic Ocean:
           2003‐2014
    • Abstract: The extended multi‐linear regression (eMLR) method is used to determine the uptake and storage of anthropogenic carbon in the Atlantic Ocean based on repeat occupations of 4 cruises from 1989–2014 (A16, A20, A22, and A10), with an emphasis on the 2003–2014 period. The results show a significant increase in basin wide anthropogenic carbon storage in the North Atlantic, which absorbed 4.4 ± 0.9 Pg C decade‐1 from 2003–2014 compared to 1.9 ± 0.4 Pg C decade‐1 for the 1989–2003 period. This decadal variability is attributed to changing ventilation patterns associated with the North Atlantic Oscillation (NAO) and increasing release of anthropogenic carbon into the atmosphere. There are small changes in the uptake rate of CO2 in the South Atlantic for these time periods (3.7 ± 0.8 Pg C decade‐1 versus 3.2 ± 0.7 Pg C decade‐1). Several eddies are identified containing ~20% more anthropogenic carbon than the surrounding waters in the South Atlantic demonstrating the importance of eddies in transporting anthropogenic carbon. The uptake of carbon results in a decrease in pH of ~0.0021 ± 0.0007 yr‐1 for surface waters during the last 10 years, in line with the atmospheric increase in CO2.
       
  • Net community production and calcification from seven years of NOAA
           Station Papa Mooring measurements
    • Abstract: Seven years of near‐continuous observations from the Ocean Station Papa (OSP) surface mooring were used to evaluate drivers of marine carbon cycling in the eastern subarctic Pacific. Processes contributing to mixed layer carbon inventory changes throughout each deployment year were quantitatively assessed using a time‐dependent mass‐balance approach in which total alkalinity and dissolved inorganic carbon were used as tracers. By using two mixed layer carbon tracers it was possible to isolate the influences of net community production (NCP) and calcification. Our results indicate that the annual NCP at OSP is 2 ± 1 mol C m‐2 yr‐1 and the annual calcification is 0.3 ± 0.3 mol C m‐2 yr‐1. Piecing together evidence for potentially significant dissolved organic carbon cycling in this region, we estimate a particulate inorganic carbon to particulate organic carbon ratio between 0.15 and 0.25. This is at least double the global average, adding to the growing evidence that calcifying organisms play an important role in carbon export at this location. These results, coupled with significant seasonality in the NCP, suggest that carbon cycling near OSP may be more complex than previously thought, and highlight the importance of continuous observations for robust assessments of biogeochemical cycling.
       
  • Annual cycles of phytoplankton biomass in the Subarctic Atlantic and
           Pacific Ocean
    • Abstract: High latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here, we show that when evaluated using phytoplankton carbon biomass (Cphyto) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms observed in the North Atlantic. The annual increase in subarctic Pacific phytoplankton biomass is not readily observed in the Chl record because it is paralleled by light‐ and nutrient‐driven decreases in cellular pigment levels (Cphyto:Chl). Specifically, photoacclimation and iron stress effects on Cphyto:Chl oppose the biomass increase, leading to only modest changes in bulk Chl. The magnitude of the photoacclimation effect is quantified using descriptors of the near‐surface light environment and a photophysiological model. Iron‐stress effects are diagnosed from satellite chlorophyll fluorescence data. Last, we show that biomass accumulation in the Pacific is slower than the Atlantic, but is closely tied to similar levels of seasonal nutrient uptake in both basins. Annual cycles of satellite‐derived Chl and Cphyto are reproduced by in situ autonomous profiling floats. These results contradict the long‐standing paradigm that environmental conditions prevent phytoplankton accumulation in the subarctic Northeast Pacific and suggest a greater seasonal decoupling between phytoplankton growth and losses than traditionally implied. Further, our results highlight the role of physiological processes in shaping bulk properties, such as Chl, and their interpretation in studies of ocean ecosystem dynamics and climate change.
       
  • Mercury presence and speciation in the South Atlantic Ocean along the
           40°S transect
    • Abstract: Mercury (Hg) natural biogeochemical cycle is complex and a significant portion of biological and chemical transformation occurs in the marine environment. To better understand the presence and abundance of Hg species in the remote ocean regions, waters of South Atlantic Ocean along 40°S parallel were investigated during UK‐GEOTRACES cruise GA10. Total mercury (THg), methylated mercury (MeHg) and dissolved gaseous mercury (DGM) concentrations were determined. The concentrations were very low in the range of pg/L (femtomolar). All Hg species had higher concentration in western than in eastern basin. THg did not appear to be a useful geotracer. Elevated methylated Hg species were commonly associated with low‐oxygen water masses and occasionally with peaks of Chlorophyll a, both involved with carbon (re)cycling. The overall highest MeHg concentrations were observed in the mixed layer (500 m) and in the vicinity of the Gough Island.. Conversely, DGM concentrations showed distinct layering and differed between the water masses in a nutrient‐like manner. DGM was lowest at surface, indicating degassing to the atmosphere; and was highest in the Upper Circumpolar Deep Water, where the oxygen concentration was lowest. DGM increased also in Antarctic Bottom Water. At one station, dimethylmercury was determined and showed increase in region with lowest oxygen saturation. Altogether, our data indicate that the South Atlantic Ocean could be a source of Hg to the atmosphere and that its biogeochemical transformations depend primarily upon carbon cycling and are thereby additionally prone to global ocean change.
       
  • How well do global ocean biogeochemistry models simulate dissolved iron
           distributions'
    • Abstract: Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from thirteen global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES programme we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron‐fertilisation experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.
       
 
 
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