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Science Highlights

Astonishing protactinium and thorium profiles in the Mediterranean Sea

In the framework of the GEOTRACES cruise along the GA04 section in the Meditterranean Sea, Gdaniec et al. (2018, see reference below) measured thorium (Th) and protactinium (Pa) isotope distributions on 8 profiles across the Mediterranean Sea. Contrasting with what is observed in the open ocean:

  • Depth profiles of these tracers are non linear, indicating that these profiles are overprinted by deep water circulation. These bended shapes allow identifying convection processes in the NW basin and occurrence of depleted Aegean and enriched Adriatic waters.
  • 99% of the 230Th and 94% of the (although more soluble) 231Pa in situ produced are scavenged and deposited within the Mediterranean Sea.
  • The fractionation factors between Th and Pa (FTh/Pa) are low, reflecting the important removal of the 231Pa compare to the open ocean.
  • The 232Th distribution mainly reflects the input of lithogenic material from rivers and/or sediment resuspension.

18 Gdniec
Figure:
 Map of (a) Pa-Th sampling sites and and section plots of (b) 231Pa, (c) 230Th, and (d) 232Th measured in unfiltered seawater sampled along the GEOTRACES GA04 section in the Mediterranean Sea. SABB: Southern Algero-Balearic Basin, CABB: Central Algero-Balearic Basin and NABB: Northern Algero-Balearic Basin.

Reference:

Gdaniec, S., Roy-Barman, M., Foliot, L., Thil, F., Dapoigny, A., Burckel, P., Garcia-Orellana, J., Masqué, P., Mörth, C-M., Andersson, P. S. (2018). Thorium and protactinium isotopes as tracers of marine particle fluxes and deep water circulation in the Mediterranean Sea. Marine Chemistry, 199, 12–23. http://doi.org/10.1016/J.MARCHEM.2017.12.002

Surface South Pacific ecosystems reflect the availability of the nutrients iron, nitrate and phosphate

Thanks to a high resolution section across the South Pacific (150°E-150°W, GEOTRACES GP13 cruise), Ellwood and co-workers (2018, see reference below) identify that the gradient of sources and fates of the 3 nutrients iron (Fe), nitrogen (N) and phosphorus (P) is explaining the observed ecosystem west-east gradient. In the west, phytoplankton able to fix atmospheric nitrogen (diazotroph species) is abundant while it is the opposite in the eastern end of the section. As shown in the figure, such drop of the diazotroph species is due to the low abundance of Fe in the most remote part of the section.

18 Ellwood figureFigure: Cartoon showing the input fluxes for iron (Fe), nitrogen (N) and phosphorus (P) into surface ocean across the GP13 zonal section. In the west, diazotrophs are abundant while it is the opposite in the eastern end of the section due to the low abundance of Fe, in the most remote part of the section. Click here to view the figure larger.

Reference:

Ellwood, M. J., Bowie, A. R., Baker, A., Gault-Ringold, M., Hassler, C., Law, C. S., Maher, W. A., Marriner, A., Nodder, S., Sander, S., Stevens, C., Townsend, A., van der Merwe, P., Woodward, E. M. S., Wuttig, K., Boyd, P. W. (2018). Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5,300 km South Pacific Zonal Section (153°E-150°W). Global Biogeochemical Cycles, 32(2), 187–207. http://doi.org/10.1002/2017GB005736

Particle distribution in repeated ocean sections and sediment resuspension

This is the first compilation of an expansive data base of transmissometer data on a decadal period of time. More than 7376 stations have been analyzed for “cp” value, a proxy for particle concentrations, by Gardner and co-workers (2018, see reference below). Full water-column sections confirm that particle concentrations are higher in surface waters, decrease rapidly below 200 m, most often down to the seafloor. However, cloudy near-bottom waters, known as “benthic nepheloid layers”, are generated by sediment erosion and resuspension at specific geographic areas. These locations are directly linked to energetic surface dynamics that produce regions of high Eddy Kinetic Energy (EKE). More fascinating, however, is the decadal persistence of this close surface-to-deep connection. We invite you to read this very interesting article!

18 Gardner2Figures: (A) Map of log of surface eddy kinetic energy (modified from Dixon et al., 2011) with cp transects indicated. (B) Section of cp (proxy for particle concentration) along 53° W in spring, 2012 in the Western North Atlantic. Black contours are dissolved oxygen (µmol kg-1). Click here to view the figure larger.

Reference:

Gardner, W. D., Mishonov, A. V., & Richardson, M. J. (2018). Decadal Comparisons of Particulate Matter in Repeat Transects in the Atlantic, Pacific, and Indian Ocean Basins. Geophysical Research Letters. http://doi.org/10.1002/2017GL076571

Why did the concentration of atmospheric carbon dioxide rise so much and so quickly during the last deglaciation? 

During the Last Glacial Maximum, the deep southern Pacific waters were stratified, efficiently accumulating old, CO2 rich waters. Basak and co-authors (2018, see reference below) measured neodymium isotopes in sediment cores that clearly show that when these deep waters became less stratified as the climate warmed they released their carbon which could escape to the atmosphere...what a tempting prospect and beautiful teaser for the forthcoming PAGES-GEOTRACES workshop of December 2018!

 18 Pahnke l
Figure: View from RV Polarstern while collecting sediment samples used in the study by Basak et al.
Read more at: https://phys.org/news/2018-02-scientists-theory-role-south-pacific.html#jCp
Credit: Dr. Katharina Pahnke


Reference:

Basak, C., Fröllje, H., Lamy, F., Gersonde, R., Benz, V., Anderson, R. F., Molina-Kescher, M., Pahnke, K. (2018). Breakup of last glacial deep stratification in the South Pacific. Science, 359(6378), 900–904. http://doi.org/10.1126/science.aao2473

Read more also at: https://phys.org/news/2018-02-scientists-theory-role-south-pacific.html#jCp

 

Where, how and which trace elements are released from dust at the sea surface?

Alex Baker and Tim Jickells (2017, see reference below) propose to answer to this question thanks to analysis of aerosols collected in the framework of the Atlantic Meridional Transect (AMT). They established the soluble concentrations of a range of trace metals (iron, aluminium, manganese, titanium, zinc, vanadium, nickel and copper) and major ions. They reveal much higher inputs to the North Atlantic Ocean compared to the South Atlantic Ocean, reflecting stronger land based emission sources in the Northern Hemisphere. Comparison of these inputs with the surface water contents of the same trace metals compiled in the GEOTRACES intermediate data product show surprising features that you will discover if you read this paper…

18 Baker lowFigures: (A) Approximate tracks of the AMT cruises (dots and triangles) and general flow directions of the seven major atmospheric transport routes encountered during the cruises (arrows). Abbreviations for the air transport regimes are: continental Europe (EUR), North Africa including the Sahara and Sahel: (SAH), Southern Africa impacted by biomass burning emissions (SAB), Southern Africa not impacted by biomass burning (SAF), South America (SAM), remote North or South Atlantic i.e. not crossing land for at least 5 days prior to collection (RNA and RSA respectively). (B) Box and whisker plots showing the variations in the concentrations of iron, aluminium, manganese, titanium, zinc, vanadium, nickel and copper with air transport/source type for the AMT transect. They reveal much higher inputs to the North Atlantic Ocean, reflecting stronger land based emission sources in the Northern Hemisphere. Please click here to view the figure larger. (Figures modified from Progress in Oceanography)

Reference:

Baker, A. R., & Jickells, T. D. (2017). Atmospheric deposition of soluble trace elements along the Atlantic Meridional Transect (AMT). Progress in Oceanography, 158, 41–51. http://doi.org/10.1016/j.pocean.2016.10.002

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