Tracing nutrients released from anoxic sediments with a Tracer Release Experiment - POSTRE

Toste S Tanhua, Martin Visbeck and Madeleine Freund, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Anoxic sediments tend be a source of a significant flux of nutrients such as phosphate, ammonia and iron into the water column. These nutrients has the potential to fuel primary production if the nutrient reach the photic layer, and can thus create a positive feedback loop. In the Peruvian Oxygen minimum zone System Tracer Release Experiment (POSTRE) about 70 kg of trifluoromethyl sulfur pentafluoride (SF5CF3) was injected close to the bottom in the bottom boundary layer of the upper Peruvian continental slope at 250m depth in October 2015 to simulate such nutrient flux. This is the first large-scale TRE where the tracer is injected this way rather than on an isopycnal. A tracer survey cruise in early 2017 mapped the tracer distribution between 30 and 12°S along the shelf and in offshore waters. A southward transport of the tracer along the shelf is observed, together with a westward propagation of the tracer (representing nutrient from the sediment) mainly between 20 and 15°S. A significant fraction of the offshore transport seems to be connected to transport by eddies. The transport along isopycnals move the tracer towards the surface by isopycnal heave at higher latitudes.

Simultaneous observations of transient tracers show interesting patterns of ventilation in the Oxygen Minimum Zone off Peru and Chile. Here we demonstrate the utility of a tracer dispersion experiment together with observations of transient tracers and a range of biogeochemical variables to elucidate horizontal transport and ventilation in the Oxygen Minumum Zone in the Southeast tropical Pacific Ocean.