Radium quartet reveals no less than four main processes along the GEOTRACES North Atlantic Ocean section (30°N)

Science Highlights

23 January 2015

The four radium (Ra) isotopes (224Ra, 223Ra, 228Ra, 226Ra, “radium quartet”) are produced in situ via decay of their insoluble  thorium isotope parents in sediments from the continental margins and deep-sea and then released to the ocean. In the ocean, their distributions are controlled by particle removal (226Ra) and radioactive decay with four different half-lives. These properties make the “quartet” an invaluable tracer of coast-to-ocean processes and a water mass spreading chronometer.

Thanks to a dense and beautiful data set documenting the radium quartet along the 30°N GEOTRACES US section (GA03), Charette and co-authors (2015, see reference below) were able to identify:

  • a Mediterranean outflow spreading rate of 0.52-0.60 cm/s derived from 228Ra,
  • evidence of substantial sediment/water interaction in the benthic boundary layer along the oxygen minimum zones
  • decoupling between 223Ra and the other Ra isotope sources over the mid-Atlantic Ridge, and
  • significant continental inputs (e.g. submarine groundwater discharge) in the western Atlantic.

Last but not least, they conclude that the 228Ra inventories in the upper water column have remained constant over the past 40 years, which suggests that submarine groundwater discharge (the primary 228Ra source) is steady-state for the North Atlantic on decadal time scales.

15 Charette l
Figure. Box average 0-1000 m inventories (15° x 15°) of 228Ra (x 1010 atoms m-2) for samples collected on the GEOTRACES Atlantic section (GT) (2010-2011) and the Transient Tracers in the Ocean cruises (TTO) (1981-1986). Each solid black dot is a TTO station, each red x is a GT station. Click here to view the figure larger.

 

Reference:

Charette, M. A., Morris, P. J., Henderson, P. B., & Moore, W. S. (2015). Radium isotope distributions during the US GEOTRACES North Atlantic cruises. Marine Chemistry, In Press doi:10.1016/j.marchem.2015.01.001 Click here to access the paper.

 

Latest highlights

Mercury content and isotopes in bird’s guano: a window to reconstruct past climates

By analysing peat cores, Chuxian Li and her colleagues have shown how populations of nesting seabirds have fluctuated on a sub-Antarctic Island over the last 8,000 years.

Contraction of North Atlantic Deep Water during glacial times: a paradigm called into question

Blaser and co-authors propose a new distribution of deep-water masses in the Atlantic Ocean during the Last Glacial Maximum and the Heinrich Stadial..

Authigenic radiogenic neodymium isotope composition traces millennial-scale overturning circulation variability in the Arabian Sea

The neodymium isotopic signature of the sediment authigenic fraction is, a priori, a valuable proxy for reconstructing deep-water mass trajectories…

Magmatic activity at the Carlsberg Ridge in the Indian Ocean triggered by glacial sea-level variation

De and colleagues provide a high-resolution record of magmatic input and associated hydrothermal activity over the past 49 kyr from a core recovered from a magmatic segment of the slow-spreading Carlsberg Ridge in the Indian Ocean.

Rechercher