Diapycnal nutrient fluxes across the benthic boundary layers at the continental slopes off Peru and Mauritania based on 224Ra/223Ra-ratios

Marine sediments may act as sources and/or sinks of nutrients in oxygen minimum zones (OMZs) where solute fluxes between sediments and the above water column are believed to be an important component for the bioproductivity in the mixed layer. However, solute fluxes between sediments and the water column above are difficult to obtain. We determined the diapycnal nutrient fluxes across the benthic boundary layer at the OMZs off Peru and Mauritania using nutrient concentration profiles and vertical diffusion coefficients derived from ²²⁴Ra/²²³Ra isotope ratios.

The radium isotopes ²²³Ra (half-life: 11.4 days) and ²²⁴Ra (half-life: 3.66 days) are produced in sediments by radioactive decay of ²²⁷Th and ²²⁸Th ,respectively: Diffusion of radium out of the sediments results in gradients of ²²⁴Ra/²²³Ra above the sediments; and these gradients may be used to estimate vertical diffusion coefficients. We determined ²²⁴Ra/²²³Ra along two transects at the continental slopes off Peru (12° S) and off Mauritania (18° N) in water depths between 65m – 1000mand 53m – 1100m, respectively. For the ²²³Ra/²²⁴Ra sampling we used acrylic fibers/cartridges coated with MnO₂. The Mn-fibers were attached to benthic landers (moored for ~ 24 hours at the sea floor) and the Mn-cartridges were placed in series on several in-situ filtration pumps (Challenger Oceanic and McLane pumps). ²²³Ra and ²²⁴Ra were measured on-board using a delayed coincidence counting system (RaDeCC).

In our presentation we will present the ²²⁴Ra/²²³Ra distribution along these transects investigated and we will compare the diapycnal radium-derived nutrient fluxes with those determined using benthic chambers and microstructure measurements.