58 research outputs found
Revisiting the distribution of oceanic N<sub>2</sub> fixation and estimating diazotrophic contribution to marine production
Marine N2 fixation supports a significant portion of oceanic primary production by making N2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles
Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic
The remineralisation of sinking particles by prokaryotic heterotrophic activity is important for controlling oceanic carbon sequestration. Here, we report mesopelagic particulate organic carbon (POC) remineralisation fluxes in the North Atlantic along the GEOTRACES-GA01 section (GEOVIDE cruise; May-June 2014) using the particulate biogenic barium (excess barium; Baxs/ proxy. Important mesopelagic (100-1000 m) Baxs differences were observed along the transect depending on the intensity of past blooms, the phytoplankton community structure, and the physical forcing, including downwelling. The subpolar province was characterized by the highest mesopelagic Baxs content (up to 727 pmol L-1/, which was attributed to an intense bloom averaging 6 mg chl a m-3 between January and June 2014 and by an intense 1500m deep convection in the central Labrador Sea during the winter preceding the sampling. This downwelling could have promoted a deepening of the prokaryotic heterotrophic activity, increasing the Baxs content. In comparison, the temperate province, characterized by the lowest Baxs content (391 pmol L-1/, was sampled during the bloom period and phytoplankton appear to be dominated by small and calcifying species, such as coccolithophorids. The Baxs content, related to oxygen consumption, was converted into a remineralisation flux using an updated relationship, proposed for the first time in the North Atlantic. The estimated fluxes were of the same order of magnitude as other fluxes obtained using independent methods (moored sediment traps, incubations) in the North Atlantic. Interestingly, in the subpolar and subtropical provinces, mesopelagic POC remineralisation fluxes (up to 13 and 4.6 mmol Cm-2 d-1, respectively) were equalling and occasionally even exceeding upper-ocean POC export fluxes, deduced using the 234Th method. These results highlight the important impact of the mesopelagic remineralisation on the biological carbon pump of the studied area with a near-zero, deep (> 1000 m) carbon sequestration efficiency in spring 2014
Revisiting the distribution of oceanic N 2 fixation and estimating diazotrophic contribution to marine production
Marine N 2 fixation supports a significant portion of oceanic primary production by making N 2 bioavailable to planktonic communities, in the process influencing atmosphere-ocean carbon fluxes and our global climate. However, the geographical distribution and controlling factors of marine N 2 fixation remain elusive largely due to sparse observations. Here we present unprecedented high-resolution underway N 2 fixation estimates across over 6000 kilometers of the western North Atlantic. Unexpectedly, we find increasing N 2 fixation rates from the oligotrophic Sargasso Sea to North America coastal waters, driven primarily by cyanobacterial diazotrophs. N 2 fixation is best correlated to phosphorus availability and chlorophyll-a concentration. Globally, intense N 2 fixation activity in the coastal oceans is validated by a meta-analysis of published observations and we estimate the annual coastal N 2 fixation flux to be 16.7 Tg N. This study broadens the biogeography of N 2 fixation, highlights the interplay of regulating factors, and reveals thriving diazotrophic communities in coastal waters with potential significance to the global nitrogen and carbon cycles
Atmospheric aerosols at the Pierre Auger Observatory and environmental implications
The Pierre Auger Observatory detects the highest energy cosmic rays.
Calorimetric measurements of extensive air showers induced by cosmic rays are
performed with a fluorescence detector. Thus, one of the main challenges is the
atmospheric monitoring, especially for aerosols in suspension in the
atmosphere. Several methods are described which have been developed to measure
the aerosol optical depth profile and aerosol phase function, using lasers and
other light sources as recorded by the fluorescence detector. The origin of
atmospheric aerosols traveling through the Auger site is also presented,
highlighting the effect of surrounding areas to atmospheric properties. In the
aim to extend the Pierre Auger Observatory to an atmospheric research platform,
a discussion about a collaborative project is presented.Comment: Regular Article, 16 pages, 12 figure
Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect
We report here the results of total mercury (HgT) determinations along the 2014 Geotraces Geovide cruise (GA01 transect) in the North Atlantic Ocean (NA) from Lisbon (Portugal) to the coast of Labrador (Canada). HgT concentrations in unfiltered samples (HgTUNF) were log-normally distributed and ranged between 0.16 and 1.54âŻpmolâŻLâ1, with a geometric mean of 0.51âŻpmolâŻLâ1 for the 535 samples analysed. The dissolved fraction (<âŻ0.45âŻÂ”m) of HgT (HgTF), determined on 141 samples, averaged 78âŻ% of the HgTUNF for the entire data set, 84âŻ% for open seawaters (below 100âŻm) and 91âŻ% if the Labrador Sea data are excluded, where the primary production was high (with a winter convection down to 1400âŻm). HgTUNF concentrations increased eastwards and with depth from Greenland to Europe and from subsurface to bottom waters. The HgTUNF concentrations were similarly low in the subpolar gyre waters (ââŒââŻ0.45âŻpmolâŻLâ1), whereas they exceeded 0.60âŻpmolâŻLâ1 in the subtropical gyre waters. The HgTUNF distribution mirrored that of dissolved oxygen concentration, with highest concentration levels associated with oxygen-depleted zones. The relationship between HgTF and the apparent oxygen utilization confirms the nutrient-like behaviour of Hg in the NA. An extended optimum multiparameter analysis allowed us to characterize HgTUNF concentrations in the different source water types (SWTs) present along the transect. The distribution pattern of HgTUNF, modelled by the mixing of SWTs, show Hg enrichment in Mediterranean waters and North East Atlantic Deep Water and low concentrations in young waters formed in the subpolar gyre and Nordic seas. The change in anthropogenic Hg concentrations in the Labrador Sea Water during its eastward journey suggests a continuous decrease in Hg content in this water mass over the last decades. Calculation of the water transport driven by the Atlantic Meridional Overturning Circulation across the PortugalâGreenland transect indicates northward Hg transport within the upper limb and southward Hg transport within the lower limb, with resulting net northward transport of about 97.2âŻkmolâŻyrâ1
Climatically sensitive transfer of iron to maritime Antarctic ecosystems by surface runoff
Iron supplied by glacial weathering results in pronounced hotspots of biological production in an otherwise iron-limited Southern Ocean Ecosystem. However, glacial iron inputs are thought to be dominated by icebergs. Here we show that surface runoff from three island groups of the maritime Antarctic exports more filterable (<0.45âÎŒm) iron (6â81âkgâkmâ2âaâ1) than icebergs (0.0â1.2âkgâkmâ2âaâ1). Glacier-fed streams also export more acid-soluble iron (27.0â18,500âkgâkmâ2âaâ1) associated with suspended sediment than icebergs (0â241âkgâkmâ2âaâ1). Significant fluxes of filterable and sediment-derived iron (1â10âGgâaâ1 and 100â1,000âGgâaâ1, respectively) are therefore likely to be delivered by runoff from the Antarctic continent. Although estuarine removal processes will greatly reduce their availability to coastal ecosystems, our results clearly indicate that riverine iron fluxes need to be accounted for as the volume of Antarctic melt increases in response to 21st century climate change
Atmospheric deposition fluxes over the Atlantic Ocean: a GEOTRACES case study
Atmospheric deposition is an important source of micronutrients to the
ocean, but atmospheric deposition fluxes remain poorly constrained in most
ocean regions due to the limited number of field observations of wet and dry
atmospheric inputs. Here we present the distribution of dissolved aluminium
(dAl), as a tracer of atmospheric inputs, in surface waters of the Atlantic
Ocean along GEOTRACES sections GA01, GA06, GA08, and GA10. We used the
surface mixed-layer concentrations of dAl to calculate atmospheric
deposition fluxes using a simple steady state model. We have optimized the
Al fractional aerosol solubility, the dAl residence time within the surface
mixed layer and the depth of the surface mixed layer for each separate cruise to
calculate the atmospheric deposition fluxes. We calculated the lowest
deposition fluxes of 0.15±0.1 and 0.27±0.13 g mâ2 yrâ1
for the South and North Atlantic Ocean (>40â S and >40â N) respectively, and the highest fluxes of 1.8
and 3.09 g mâ2 yrâ1 for the south-east Atlantic and tropical
Atlantic Ocean, respectively. Overall, our estimations are comparable to
atmospheric dust deposition model estimates and reported field-based
atmospheric deposition estimates. We note that our estimates diverge from
atmospheric dust deposition model flux estimates in regions influenced by
riverine Al inputs and in upwelling regions. As dAl is a key trace element
in the GEOTRACES programme, the approach presented in this study allows
calculations of atmospheric deposition fluxes at high spatial resolution for
remote ocean regions.</p
Dissolved Pb and Pb isotopes in the North Atlantic from the GEOVIDE transect (GEOTRACES GA-01) and their decadal evolution
During the 2014 GEOVIDE transect, seawater samples were collected for
dissolved Pb and Pb isotope analysis. These samples provide a high-resolution
snapshot of the source regions for the present Pb distribution in the
North Atlantic Ocean. Some of these stations were previously occupied for Pb
from as early as 1981, and we compare the 2014 data with these older data,
some of which are reported here for the first time. Lead concentrations were
highest in subsurface Mediterranean Water (MW) near the coast of Portugal,
which agrees well with other recent observations by the US GEOTRACES program
(Noble et al., 2015). The recently formed Labrador Sea Water (LSW) between
Greenland and Nova Scotia is much lower in Pb concentration than the older
LSW found in the West European Basin due to decreases in Pb
emissions into the atmosphere during the past 20Â years. Comparison of North
Atlantic data from 1989 to 2014 shows decreasing Pb concentrations consistent
with decreased anthropogenic inputs, active scavenging, and
advection/convection. Although the isotopic composition of northern North
Atlantic seawater appears more homogenous compared to previous decades, a
clear spatiotemporal trend in isotope ratios is evident over the past
15Â years and implies that small changes to atmospheric Pb emissions continue.
Emissions data indicate that the relative proportions of US and European Pb
sources to the ocean have been relatively uniform during the past 2 decades,
while aerosol data may suggest a greater relative proportion of natural
mineral Pb. Using our measurements in conjunction with emissions inventories,
we support the findings of previous atmospheric analyses that a significant
portion of the Pb deposited to the ocean in 2014 was natural, although it is
obscured by the much greater solubility of anthropogenic aerosols over
natural ones.</p
Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise
The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special_issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue
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