As opposed to natural C-14/C-12 which enters the ocean through the air-sea interface, waters rich in primordial helium (high He-3/He-4 ratio) derive from mid-ocean ridges at the ocean floor. This unique boundary condition means that mantle helium can provide independant information concerning patterns of deep-ocean circulation. OCMIP modelers have already made simulations for natural C-14/C-12. Here we descibe protocols to make OCMIP simulations for mantle helium.
Primordial He, which also invades the deep sea from the earth's interior via mid-ocean ridges, has a distinctly different He-3/He-4 ratio than does atmospheric helium. Vent waters, along with their He-3/He-4 signature, enter the ocean at several hundred degrees centigrade and typically rise a few hundred meters in the water column, before coming to equilibrium and being transported along surfaces of constant density (isopycnals). When it was found that oceanographic circulation patterns deduced from He-3/He-4 data (Stommel, 1982) contradicted the simple, yet classic model of deep-ocean circulation (Stommel and Arons, 1960), it became obvious to many that the unique signature of He-3/He-4 could help shed light on the mysteries of deep-ocean circulation. Since then, many He-3/He-4 samples have been collected throughout the world ocean, largely during the WOCE sampling campaign. One ocean modeling study has been published that has focused on simulating He-3/He-4 in the LSG model (Farley et al., 1995). They assumed that the boundary condition for He-3/He-4 is proportional to the spreading rate of the mid-ocean ridges. Other groups have also begun making preliminary simulations for He-3/He-4.
For this comparison, all 3-D models will make one standard He-3/He-4 simulation, to near steady state. Following Farley et al. (1995), primordial He-3 and He-4 will be added to the ocean at mid-ocean ridges along the sea floor as a function of ridge positions and spreading rates. He-3 and He-4 will be carried as individual tracers. At the air-sea interface, all ocean models will exchange He-3 and He-4 with the atmosphere using sea-air flux boundary conditions that are analogous to those developed for CO2 during the first phase of OCMIP. Intercomparison of model results will focus on the He-3/He-4 distribution in the sea and to a lesser extent on helium fluxes from the ocean to atmosphere. Such comparison should provide unique insight, into the performance of each model's deep-ocean circulation, particularly in regard to evaluating the performance of OCMIP purposeful, deep-ocean CO2 injection simulations.