We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition and compare this to fluvial inputs and dinitrogen fixation. We evaluate the ...scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate that about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological dinitrogen fixation is the main external source of nitrogen to the open ocean, i.e., beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land‐based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr−1 and less than the Duce et al. (2008) estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.
Key Points
A new estimate of total atmospheric fixed nitrogen inputs to the ocean (39 Tg N yr−1) and its spatial distribution is presented
The effects of atmospheric deposition on the oceans are estimated as an increase of productivity equivalent to 0.15 Pg C yr−1
Four key uncertainties in these estimates are identified
The environmental conditions of Earth, including the climate, are determined by physical, chemical, biological, and human interactions that transform and transport materials and energy. This is the ..."Earth system": a highly complex entity characterized by multiple nonlinear responses and thresholds, with linkages between disparate components. One important part of this system is the iron cycle, in which iron-containing soil dust is transported from land through the atmosphere to the oceans, affecting ocean biogeochemistry and hence having feedback effects on climate and dust production. Here we review the key components of this cycle, identifying critical uncertainties and priorities for future research.
Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the ocean's external (nonrecycled) nitrogen supply and up to ∼30% of ...the annual new marine biological production, ∼0.3 petagram of carbon per year. This input could account for the production of up to ∼1.6 teragrams of nitrous oxide ($\text{N}_{2}\text{O}$) per year. Although ∼10% of the ocean's drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in$\text{N}_{2}\text{O}$emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future.
Despite over a century of published reports of dissolved organic nitrogen (DON) in precipitation, its implications are still being appraised. The number of studies focusing on atmospheric organic ...nitrogen deposition has increased steadily in recent years, but comparatively little has been done to draw together this disparate knowledge. This is partly a consequence of valid concerns about the comparability of analysis and sampling methodologies. Given the current global trends in anthropogenic nitrogen fixation, an improved qualitative and quantitative understanding of the organic nitrogen component is needed to complement the well-established knowledge base pertaining to nitrate and ammonium deposition. This global review confirms the quantitative importance of bulk DON in precipitation. This cumulative data set also helps to resolve some of the uncertainty that arises from the generally locally and temporally limited scale of the individual studies. Because of analytical and procedural changes in recent decades, assessments are made of the comparability of the data sets; caution is needed in comparisons of individual studies, but the overall trends in the compiled set are more robust. Despite the large number of reports considered, evidence for long-term temporal changes in rainwater organic nitrogen concentrations is ambiguous. With regard to sources, it is likely that some of the organic material observed is not locally generated, but undergoes extensive or long-range atmospheric transport. The compiled data set shows a land-to-sea gradient in organic nitrogen concentration. Possible precursors, reported data on the most likely component groups, and potential source mechanisms are also outlined.
Since iron is an important micronutrient, deposition of iron in mineral aerosols can impact the carbon cycle and atmospheric CO2. This paper reviews our current understanding of the global dust cycle ...and identifies future research needs. The global distribution of desert dust is estimated from a combination of observations of dust from in situ concentration, optical depth, and deposition data; observations from satellite; and global atmospheric models. The anthropogenically influenced portion of atmospheric desert dust flux is thought to be smaller than the natural portion, but is difficult to quantify due to the poorly understood response of desert dust to changes in climate, land use, and water use. The iron content of aerosols is thought to vary by a factor of 2, while the uncertainty in dust deposition is at least a factor of 10 in some regions due to the high spatial and temporal variability and limited observations. Importantly, we have a limited understanding of the processes by which relatively insoluble soil iron (typically ∼0.5% is soluble) becomes more soluble (1–80%) during atmospheric transport, but these processes could be impacted by anthropogenic emissions of sulfur or organic acids. In order to understand how humans will impact future iron deposition to the oceans, we need to improve our understanding of: iron deposition to remote oceans, iron chemistry in aerosols, how desert dust sources will respond to climate change, and how humans will impact the transport of bioavailable fraction of iron to the oceans.
Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO
) and ammonium (NH
) to the surface waters of the open ocean, with potential impacts ...on marine productivity and the global carbon cycle. Global-scale understanding of the impacts of N deposition to the oceans is reliant on our ability to produce and validate models of nitrogen emission, atmospheric chemistry, transport and deposition. In this work, ~2900 observations of aerosol NO
and NH
concentrations, acquired from sampling aboard ships in the period 1995 - 2012, are used to assess the performance of modelled N concentration and deposition fields over the remote ocean. Three ocean regions (the eastern tropical North Atlantic, the northern Indian Ocean and northwest Pacific) were selected, in which the density and distribution of observational data were considered sufficient to provide effective comparison to model products. All of these study regions are affected by transport and deposition of mineral dust, which alters the deposition of N, due to uptake of nitrogen oxides (NO
) on mineral surfaces. Assessment of the impacts of atmospheric N deposition on the ocean requires atmospheric chemical transport models to report deposition fluxes, however these fluxes cannot be measured over the ocean. Modelling studies such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), which only report deposition flux are therefore very difficult to validate for dry deposition. Here the available observational data were averaged over a 5° × 5° grid and compared to ACCMIP dry deposition fluxes (ModDep) of oxidised N (NO
) and reduced N (NH
) and to the following parameters from the TM4-ECPL (TM4) model: ModDep for NO
, NH
and particulate NO
and NH
, and surface-level particulate NO
and NH
concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO
and NH
) that are more relevant to the observed parameters and which are not available in ACCMIP. Dry deposition fluxes (CalDep) were calculated from the observed concentrations using estimates of dry deposition velocities. Model - observation ratios, weighted by grid-cell area and numbers of observations, (R
) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO
concentrations (R
= 1.4 - 2.9) and under-estimates NH
concentrations (R
= 0.5 - 0.7), with spatial distributions in the tropical Atlantic and northern Indian Ocean not being reproduced by the model. In the case of NH
in the Indian Ocean, this discrepancy was probably due to seasonal biases in the sampling. Similar patterns were observed in the various comparisons of CalDep to ModDep (R
= 0.6 - 2.6 for NO
, 0.6 - 3.1 for NH
). Values of R
for NH
CalDep - ModDep comparisons were approximately double the corresponding values for NH
CalDep - ModDep comparisons due to the significant fraction of gas-phase NH
deposition incorporated in the TM4 and ACCMIP NH
model products. All of the comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities used to derive deposition fluxes from concentrations. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades. Recommendations are made for improvements in N deposition estimation through changes in observations, modelling and model - observation comparison procedures. Validation of modelled dry deposition requires effective comparisons to observable aerosol-phase species concentrations and this cannot be achieved if model products only report dry deposition flux over the ocean.
This work reports on the current status of the global modeling of iron (Fe)
deposition fluxes and atmospheric concentrations and the analyses of the
differences between models, as well as between ...models and observations. A
total of four global 3-D chemistry transport (CTMs) and general circulation
(GCMs) models participated in this intercomparison, in the framework of
the United Nations Joint Group of Experts on the Scientific Aspects of Marine
Environmental Protection (GESAMP) Working Group 38, “The Atmospheric Input
of Chemicals to the Ocean”. The global total Fe (TFe) emission strength in
the models is equal to ∼72 Tg Fe yr−1 (38–134 Tg Fe yr−1)
from mineral dust sources and around 2.1 Tg Fe yr−1 (1.8–2.7 Tg Fe yr−1)
from combustion processes (the sum of anthropogenic
combustion/biomass burning and wildfires). The mean global labile Fe (LFe)
source strength in the models, considering both the primary emissions and the
atmospheric processing, is calculated to be 0.7 (±0.3) Tg Fe yr−1,
accounting for both mineral dust and combustion aerosols. The
mean global deposition fluxes into the global ocean are estimated to be in the range
of 10–30 and 0.2–0.4 Tg Fe yr−1 for TFe and LFe, respectively,
which roughly corresponds to a respective 15 and 0.3 Tg Fe yr−1 for the multi-model ensemble model mean. The model intercomparison analysis indicates that the representation of the
atmospheric Fe cycle varies among models, in terms of both the magnitude of
natural and combustion Fe emissions as well as the complexity of atmospheric
processing parameterizations of Fe-containing aerosols. The model comparison
with aerosol Fe observations over oceanic regions indicates that most models
overestimate surface level TFe mass concentrations near dust source
regions and tend to underestimate the low concentrations observed in remote
ocean regions. All models are able to simulate the tendency of higher Fe
concentrations near and downwind from the dust source regions, with the mean
normalized bias for the Northern Hemisphere (∼14), larger
than that of the Southern Hemisphere (∼2.4) for the ensemble model
mean. This model intercomparison and model–observation comparison study
reveals two critical issues in LFe simulations that require further
exploration: (1) the Fe-containing aerosol size distribution and (2) the
relative contribution of dust and combustion sources of Fe to labile Fe in
atmospheric aerosols over the remote oceanic regions.
From March through early May of 2000, rain and bulk aerosol samples were collected at a coastal site on the eastern Mediterranean Sea at Erdemli, Turkey, and analyzed for nitrogen (N) species, ...including nitrate (NO3−), nitrite (NO2−), ammonium (NH4+), water‐soluble organic N, urea, and dissolved free amino acids. Other ions were also analyzed, including Ca2+, Mg2+, K+, Na+, Cl−, and SO42−. Water‐soluble organic N was found to contribute ∼17% and ∼26% of the total water‐soluble N in rain and aerosols, respectively. Organic N concentrations within rain and aerosols exhibited statistically significant linear relationships to Ca2+ ion (Rsqr ∼ 0.75, P < 0.05), suggesting a relationship to calcite (CaCO3) in atmospheric dust. Kinematic trajectory analyses indicated the origin of winds from arid regions, mainly in northern Africa, in 70% of the aerosols sampled. Earth Probe/Total Ozone Mapping Spectrometer aerosol index data also confirmed the influence of atmospheric dust in the region on days when Ca2+ concentrations were elevated, and trajectory analyses suggested northern Africa as a source region. The combined ion, trajectory, and aerosol index data suggest that organic N is associated with atmospheric dust in this region. Urea N and amino N represented a small percentage of the organic N fraction. In rain and aerosols, urea represented ∼11% and <1%, respectively, of the total organic N. While amino N contributed minimally to organic N totals (∼1% of total organic N in aerosols), the individual amino acids contributing ∼75% of amino N were indicative of biological organisms. Further research is needed to decipher the influence from biology and gas phase adsorption of anthropogenically derived water‐soluble organics on organic N totals.
As part of the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA), PM10 aerosol was collected during both the wet and dry (biomass burning) seasons of 1999 and analyzed for total ...water‐soluble organic nitrogen (WSON), urea, and 17 amino acids. In addition to total WSON the inorganic N species nitrate (NO3−), nitrite (NO2−), and ammonium (NH4+) were also analyzed. WSON was found to represent ∼45% (mean concentration ∼3.5 nmol N/m3) and ∼43% (mean concentration ∼61 nmol N/m3) of the total N in wet and dry season aerosol samples, respectively. Urea and amino N made up ∼19% of the total organic N in dry season aerosols and ∼2.5% of the total organic N in wet season aerosols; the majority of WSON, ∼80% in the dry season and ∼97% in the wet season, remained uncharacterized. The results suggest that biomass burning is a source of WSON, yet poorly understood (since this data set represents the first study of WSON in the context of biomass burning). Future studies aimed at determining the magnitude of WSON released from biomass burning globally, its species composition, and its biogeochemical significance are needed.
Palaeoclimatic periodicity recorded by Chinese loess-paleosol sequence has been investigated for a number of years. However, conclusions from previous investigations are still controversial, and ...interpretation of cycle evolution is quite equivocal. In this study, two typical loess-paleosol sequences (148 and 191m in thickness, respectively) in the central Chinese Loess Plateau are sampled (3872 samples total) and measured for grain size distribution and magnetic susceptibility in order to reconstruct the palaeoclimatic changes over the past three million years. On the basis of a new, sensitive proxy indicator of palaeoclimate and a newly developed independent time scale (not orbitally-tuned), two time series of Asian dust storm variations, which are highly related to the palaeoclimate system changes, are obtained. Wavelet and spectrum analyses indicate that there are approximately 400, 200, 100, 66, 57, 41, 31, 27 and 22kyr cycles in these typical loess-paleosol records. Some orbital-driven cycles are weak and are not well presented in the new time series while some non-orbital cycles are found. Since the eccentricity frequencies of the solar irradiance of approximately 400 and 100kyr are preserved in these palaeoclimatic sequences, the lack of relatively short-time orbital cycles of 41-kyr-obliquity and 22-kyr-precession cycles in part of the two time series may be explained by the relatively low time-resolution of the loess-paleosol deposits. Through an astronomical estimate, the obliquity and precession cycles should leave stronger footprints on records of palaeoclimatic variations at the middle latitudes of the northern hemisphere. The presence of non-orbital cycles may be explained by unstable dust deposition processes and pedogenic processes in the paleosol units, which could misrepresent or obliterate the imprint of the solar irradiance frequency. This conclusion may indicate that one should be cautious when investigating specific palaeoclimatic changes (e.g., at sub-orbital time scales) recorded in loess deposits, especially in the paleosol units.
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