scholarly journals Impact of dust deposition on Fe biogeochemistry at the Tropical Eastern North Atlantic Time-series Observatory site

2009 ◽  
Vol 6 (2) ◽  
pp. 4305-4359 ◽  
Author(s):  
Y. Ye ◽  
C. Völker ◽  
D. A. Wolf-Gladrow
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Organic Ligands ◽  
Particle Aggregation ◽  
Dust Particles ◽  
Dust Deposition ◽  
Colloidal Iron ◽  
Inorganic Particles ◽  
Dissolved Iron ◽  
Eastern North Atlantic

Abstract. A one-dimensional model of iron speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-series Observatory (TENATSO) site. Aimed at investigating the role of organic complexation and dust particles in Fe speciation and bioavailability, the model is extended in this study by a more complex description of the origin and fate of organic ligands and of particle aggregation and sinking. Model results show that the profile of dissolved iron is strongly influenced by the abundance of organic ligands. Modelled processes controlling the source and fate of ligands can well explain the abundance of strong ligands. However, a restoring of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profile of weak ligands, indicating that decay time of weak ligands might be too long for a 1d-model. High dust deposition brings not only considerable input of iron into surface waters but also fine inorganic particles for particle aggregation and Fe scavenging. Simulated profiles of dissolved iron show high sensitivity to re-dissolution of colloidal and particulate iron. The colloidal to soluble iron ratio is underestimated assuming that colloidal iron is mainly composed of inorganic colloids. That strongly argues for introducing organic colloids into the model in future work.

scholarly journals A model of Fe speciation and biogeochemistry at the Tropical Eastern North Atlantic Time-Series Observatory site

2009 ◽  
Vol 6 (10) ◽  
pp. 2041-2061 ◽  
Author(s):  
Y. Ye ◽  
C. Völker ◽  
D. A. Wolf-Gladrow
Keyword(s):  
Time Series ◽  
Vertical Distribution ◽  
North Atlantic ◽  
Decay Time ◽  
Particle Aggregation ◽  
Dust Particles ◽  
Dimensional Model ◽  
One Dimensional ◽  
Fe Speciation ◽  
Eastern North Atlantic

Abstract. A one-dimensional model of Fe speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-Series Observatory (TENATSO) site. Among diverse processes affecting Fe speciation, this study is focusing on investigating the role of dust particles in removing dissolved iron (DFe) by a more complex description of particle aggregation and sinking, and explaining the abundance of organic Fe-binding ligands by modelling their origin and fate. The vertical distribution of different particle classes in the model shows high sensitivity to changing aggregation rates. Using the aggregation rates from the sensitivity study in this work, modelled particle fluxes are close to observations, with dust particles dominating near the surface and aggregates deeper in the water column. POC export at 1000 m is a little higher than regional sediment trap measurements, suggesting further improvement of modelling particle aggregation, sinking or remineralisation. Modelled strong ligands have a high abundance near the surface and decline rapidly below the deep chlorophyll maximum, showing qualitative similarity to observations. Without production of strong ligands, phytoplankton concentration falls to 0 within the first 2 years in the model integration, caused by strong Fe-limitation. A nudging of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profiles, assuming a decay time of 7 years for weak ligands. This indicates that weak ligands have a longer decay time and therefore cannot be modelled adequately in a one-dimensional model. The modelled DFe profile is strongly influenced by particle concentration and vertical distribution, because the most important removal of DFe in deeper waters is colloid formation and aggregation. Redissolution of particulate iron is required to reproduce an observed DFe profile at TENATSO site. Assuming colloidal iron is mainly composed of inorganic colloids, the modelled colloidal to soluble iron ratio is lower that observations, indicating the importance of organic colloids.

scholarly journals Dust deposition: iron source or sink? A case study

2011 ◽  
Vol 8 (8) ◽  
pp. 2107-2124 ◽  
Author(s):  
Y. Ye ◽  
T. Wagener ◽  
C. Völker ◽  
C. Guieu ◽  
D. A. Wolf-Gladrow
Keyword(s):  
Dissolution Kinetics ◽  
Ecosystem Model ◽  
Particle Aggregation ◽  
Dust Particles ◽  
Dust Deposition ◽  
Short Term ◽  
Colloidal Iron ◽  
Excess Iron ◽  
Particle Adsorption ◽  
Adsorption Rates

Abstract. A significant decrease of dissolved iron (DFe) concentration has been observed after dust addition into mesocosms during the DUst experiment in a low Nutrient low chlorophyll Ecosystem (DUNE), carried out in the summer of 2008. Due to low biological productivity at the experiment site, biological consumption of iron can not explain the magnitude of DFe decrease. To understand processes regulating the observed DFe variation, we simulated the experiment using a one-dimensional model of the Fe biogeochemical cycle, coupled with a simple ecosystem model. Different size classes of particles and particle aggregation are taken into account to describe the particle dynamics. DFe concentration is regulated in the model by dissolution from dust particles and adsorption onto particle surfaces, biological uptake, and photochemical mobilisation of particulate iron. The model reproduces the observed DFe decrease after dust addition well. This is essentially explained by particle adsorption and particle aggregation that produces a high export within the first 24 h. The estimated particle adsorption rates range between the measured adsorption rates of soluble iron and those of colloidal iron, indicating both processes controlling the DFe removal during the experiment. A dissolution timescale of 3 days is used in the model, instead of an instantaneous dissolution, underlining the importance of dissolution kinetics on the short-term impact of dust deposition on seawater DFe. Sensitivity studies reveal that initial DFe concentration before dust addition was crucial for the net impact of dust addition on DFe during the DUNE experiment. Based on the balance between abiotic sinks and sources of DFe, a critical DFe concentration has been defined, above which dust deposition acts as a net sink of DFe, rather than a source. Taking into account the role of excess iron binding ligands and biotic processes, the critical DFe concentration might be applied to explain the short-term variability of DFe after natural dust deposition in various different ocean regions.

scholarly journals Effects of dry and wet Saharan dust deposition in the tropical North Atlantic Ocean

2018 ◽  
Author(s):  
Laura F. Korte ◽  
Franziska Pausch ◽  
Scarlett Trimborn ◽  
Corina P. D. Brussaard ◽  
Geert-Jan A. Brummer ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Temporal Dynamics ◽  
North Atlantic Ocean ◽  
Nutrient Release ◽  
Saharan Dust ◽  
Dust Particles ◽  
Initial Increase ◽  
Dust Deposition ◽  
Tropical North Atlantic

Abstract. Incubation experiments comprising Saharan dust additions were conducted in the tropical North Atlantic Ocean along an east-west transect at 12° N to study the phytoplankton response to nutrient release in oligotrophic seawater conditions. Experiments were performed at three stations (M1, M3, M4), mimicking wet and dry deposition of low and high amounts of Saharan dust deposition from two different dust sources (paleo-lake and sand dune). Dust particle sizes were adjusted to resemble dust that is naturally deposited over the ocean at the experiment sites. For wet dust deposition, the dust was pre-leached in acidified ‘artificial rainwater’ (H2SO4) for 16 to 24 hours, mimicking acid cloud processing at different pH values. Experiments were run up to eight days. Daily nutrient measurements of phosphate (PO43−), silicate (SiO44−), nitrate (NO3−) and cell abundances were performed in addition to measurements of concentrations of total dissolved iron (DFe), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) at the start and at the end of the experiments. A significant initial increase and subsequent gradual decrease in PO43−, SiO44− and DFe concentrations were observed after wet dust deposition using high amounts of dust previously leached in low pH rain (H2SO4, pH = 2). Remarkably, the experiments showed no nutrient release (PO43−, SiO44− and DFe) from dry-dust addition and the NO3− concentrations remained unaffected in all (dry and wet) experiments. The prokaryotic cyanobacterium Synechococcus spp. was the most prominent picophytoplankton in all mixed layer experiments. After an initial increase in cell abundance, a subsequent decrease (at M1) or a slight increase (at M3) with similar temporal dynamics was observed for dry and wet dust deposition experiments. The POC concentrations increased in all experiments and showed similar high values after both dry and wet dust deposition treatments, even though wet dust deposition is considered to have a higher potential to introduce bioavailable nutrients (i.e. PO43−, SiO44− and DFe) into the otherwise nutrient-starved oligotrophic ocean. Our observations suggest that such nutrients may be more likely to favor the growth of the phytoplankton community when an additional N-source is also available. In addition to acting as a fertilizer, our results from both dry and wet dust deposition experiments suggest that Saharan dust particles might be incorporated into marine snow aggregates leading to similar high POC concentrations.

scholarly journals Deep dissolved iron profiles in the eastern North Atlantic in relation to water masses

2003 ◽  
Vol 30 (17) ◽  
pp. n/a-n/a ◽  
Author(s):  
A. Laës ◽  
S. Blain ◽  
P. Laan ◽  
E. P. Achterberg ◽  
G. Sarthou ◽  
...  
Keyword(s):  
North Atlantic ◽  
Water Masses ◽  
Dissolved Iron ◽  
Eastern North Atlantic

scholarly journals Seasonality of intermediate waters hydrography west of the Iberian Peninsula from an 8 yr semiannual time series of an oceanographic section

Ocean Science ◽  
2013 ◽  
Vol 9 (2) ◽  
pp. 411-429 ◽  
Author(s):  
E. Prieto ◽  
C. González-Pola ◽  
A. Lavín ◽  
R. F. Sánchez ◽  
M. Ruiz-Villarreal
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Large Scale ◽  
Seasonal Migration ◽  
Ekman Transport ◽  
Wind Stress Curl ◽  
Pressure System ◽  
Vertical Displacements ◽  
Eastern North Atlantic ◽  
Mediterranean Water

Abstract. Seasonality of hydrographical properties at depth in the western Iberian margin (eastern North Atlantic) is analysed from a 2003–2010 time series of a semiannual oceanographic section extending ∼200 nm off Cape Finisterre (43° N). All water masses down to the permanent thermocline (2000 dbar) show a consistent seasonal signature in their thermohaline properties and there is a notable asymmetry between the slope region and the outer ocean (in the surroundings of the Galicia Bank). There is overall cooling and freshening of eastern North Atlantic central waters in summertime, which is larger and deeper-reaching on the slope. In summertime, Mediterranean Water (MW) gets tightly attached against the slope and is uplifted, reinforcing its thermohaline signature and diminishing its presence at the outer ocean. In wintertime the situation reverses, MW seems to detach from the slope and spreads out to the open ocean, even being observed a secondary branch around the Galicia Bank. Thermohaline seasonality at depth shows values up to 0.4 °C and 0.08 in salinity at the lower MW, of the order of 20% of the overall interannual variability observed during the whole period. Decomposition of thermohaline changes at isobaric levels to changes along isoneutral surfaces and changes due to vertical displacements help analyse the physical processes behind the observed seasonality in terms of (1) the large-scale seasonality of the subtropical gyre in response to the seasonal migration of the subtropical high pressure system and subsequent anomalies in Ekman transport and wind stress curl, (2) the continental slope dynamics, characterized by summer upwelling, winter development of the Iberian Poleward Current and Mediterranean water spreading, and (3) the possible influence of seasonal changes of water mass properties at their formation sources.

scholarly journals Sedimentary and mineral dust sources of dissolved iron to the World Ocean

2007 ◽  
Vol 4 (2) ◽  
pp. 1279-1327 ◽  
Author(s):  
J. K. Moore ◽  
O. Braucher
Keyword(s):  
Mineral Dust ◽  
Global Scale ◽  
Dust Particles ◽  
Dust Deposition ◽  
Iron Source ◽  
Dissolved Iron ◽  
Sinking Particles ◽  
Sedimentary Source ◽  
The Impact ◽  
Iron Concentrations

Abstract. A worldwide database of dissolved iron observations is used to improve simulations of the marine iron cycle within a global-scale, Biogeochemical Elemental Cycling (BEC) ocean model. Modifications to the model include: 1) an improved particle scavenging parameterization based on the sinking mass flux of particulate organic material, biogenic silica, calcium carbonate, and mineral dust particles; 2) desorption of dissolved iron from sinking particles; and 3) an improved sedimentary source for dissolved iron. Most scavenged iron (90%) is put on sinking particles to remineralize deeper in the water column. The model-observation mismatches are greatly reduced both in surface waters and in the deeper ocean. Inclusion of desorption has little effect on surface water iron concentrations where adsorption/scavenging is strongly dominant, but significantly increases simulated iron concentrations in the deep ocean. Our results suggest that there must be substantial removal of dissolved iron from subsurface waters (where iron concentrations are <0.6 nM in most regions) to match observed distributions. Aggregation and removal on sinking particles of Fe bound to organic colloids is a likely mechanism. The improved BEC model is used to address the relative contributions of mineral dust and marine sediments in driving ocean productivity and observed dissolved iron distributions. The sedimentary iron source from the continental margins has a strong impact on open ocean iron concentrations, particularly in the North Pacific. Plumes of elevated dissolved iron concentrations develop at depth in the Southern Ocean, extending from source regions in the SW Atlantic and around New Zealand. The lower particle flux and weaker scavenging in this region allows the continental iron source to be advected far from source areas. Both the margin sediment and mineral dust Fe sources significantly impact global scale primary production, export production, and nitrogen fixation, with inputs from dust deposition having a modestly stronger impact. Ocean biogeochemical models need to include the sedimentary source for dissolved iron, or they will overestimate the impact of dust deposition variations on the marine carbon cycle.

scholarly journals Effects of dust deposition on iron cycle in the surface Mediterranean Sea: results from a mesocosm seeding experiment

2010 ◽  
Vol 7 (11) ◽  
pp. 3769-3781 ◽  
Author(s):  
T. Wagener ◽  
C. Guieu ◽  
N. Leblond
Keyword(s):  
Water Column ◽  
Sediment Traps ◽  
Dust Particles ◽  
Dust Deposition ◽  
Dissolved Iron ◽  
Surface Ocean ◽  
Batch Dissolution ◽  
Particulate Iron ◽  
Dust Events ◽  
Scavenging Ratio

Abstract. Soil dust deposition is recognized as a major source of iron to the open ocean at global and regional scales. However, the processes that control the speciation and cycle of iron in the surface ocean after dust deposition are poorly documented mainly due to the logistical difficulties to investigate in-situ, natural dust events. The development of clean mesocosms in the frame of the DUNE project (a DUst experiment in a low Nutrient low chlorophyll Ecosystem) was a unique opportunity to investigate these processes at the unexplored scale of one dust deposition event. During the DUNE-1-P mesocosm seeding experiment, iron stocks (dissolved and particulate concentrations in the water column) and fluxes (export of particulate iron in sediment traps) were followed during 8 days after an artificial dust seeding mimicking a wet deposition of 10 g m−2. The addition of dust at the surface of the mesocosms was immediately followed by a decrease of dissolved iron [dFe] concentration in the 0–10 m water column. This decrease was likely due to dFe scavenging on settling dust particles and mineral organic aggregates. The scavenging ratio of dissolved iron on dust particles averaged 0.37 ± 0.12 nmol mg−1. Batch dissolution experiments conducted in parallel to the mesocosm experiment showed a increase (up to 600%) in dust iron dissolution capacity in dust-fertilized waters compared to control conditions. This study gives evidences of complex and unexpected effects of dust deposition on surface ocean biogeochemistry: (1) large dust deposition events may be a sink for surface ocean dissolved iron and (2) successive dust deposition events may induce different biogeochemical responses in the surface ocean.

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