The open ocean tracer release experiment is an accurate method to investigate the turbulent vertical mixing and horizontal dispersion in the ocean. The experiment starts with the release of a chemical compound, concentrations of which are detectable even when strongly diluted. With time, the tracer patch spreads horizontally and vertically, its spread monitored during several cruise campaigns.

Across the density surfaces, the tracer speads by many sporadic turbulent mixing events. The mean tracer patch thickness indicates the vertical mixing rate integrated over time and space and can be accurately estimated. In the mid-ocean common vertical mixing rates of 10^-5 m² s^-1 have been estimated with the accuracy of several percents.
 
 Along the density surfaces (isopycnals), the spread of the tracer patch responds to the ocean's mean circulation and turbulent stirring by eddies. Hence, the horizontal dispersion and advection parameters can be inferred from the horizontal extent and shift of the tracer patch. The achievable accuracy of horizontal mixing rates along the density surfaces is much lower than the accuracy of the vertical turbulent mixing rates. Common open ocean isopycnal mixing rates of 10² to 10³ m² s^-1, are estimated with the accuracy of several orders of magnitude.

GUTRE:

 In 2008-2010 a tracer release experiment in the open ocean was performed in the tropical northeast Atlantic with the purpose to quantify the upper ventilation pathway of the oxygen minimum zone. The Guinea Upwelling Tracer Release Experiment (GUTRE) started with the release of the tracer, CF₃SF₅, on an isopycnal surface σ_θ= 26.88 kg m^-3, which corresponds to about 350 m depth. Three surveys, performed 7, 20, and 30 months after the release, sampled the laterally and vertically expanding tracer patch. After 30 months the tracer patch expanded about 200 m in the vertical and more than 1000 km in the horizontal. The experiment resulted in a vertical mixing rate of (1.2 ± 0.1) ∙ 10^-5 m² s^-1.

CF₃SF₅ 

Trifluoromethyl sulfur pentafluoride (CF₃SF₅) is detectable in quantities as low as 10^-16 mol kg^-1 in a seawater sample by a gas chromatograph with an electron capture detector. For comparison, the common trace metals in the ocean, like iron or zink, can be measured in concentrations of 10^-9 mol kg^-1, oxygen in 10^-6 mol kg^-1. The compound CF₃SF₅ is virtually inert in the marine environment, i.e., it is chemically conserved and is harmless to marine organisms. The only known environmental impact of the substance is that it is a minor greenhouse gas. 

Ocean Tracer Injection System (OTIS)

OTIS is an underwater sled used to release the tracer very close to one chosen density surface. The neutrally buoyant sled is equipped with the container of compressed tracer gas and a CTD (conductivity-temperature density) instrument. 
The density information is used to automatically control the sled position in the depth. When the sled deviates from the aimed density surface, the pumps injecting the tracer switch off. 
During the operation, the sled is towed with a speed of 1-2 knots for several hours. The tracer is released as a spray of very small droplets, because they have to dissolve quickly to avoid sinking to the ground. The achieved density error of the tracer injection is less than 0.005 kg m^-3, equivalent to a depth error of about 2 m.
Such high accuracy of the tracer injection is required to accurately estimate the rate of the tracer patch spread across the density surfaces with time.