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

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.

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.