CONSTRAINTS ON MANTLE HE-3 FLUXES AND DEEP-SEA CIRCULATION FROM AN OCEANIC GENERAL-CIRCULATION MODEL

We have simulated the steady-state distribution of helium in the deep sea to investigate the magnitude and spatial and temporal variability of mantle degassing and to characterize deep-sea circulation and venti lation. The simulation was produced by linking a simple source functio n for helium injected at mid-ocean ridges with an oceanic general circ ulation model (GCM). By assuming that the flux of mantle helium is lin early proportional to the seafloor spreading rate and by using previou s estimates for the total flux of mantle helium into the oceans, the G CM yields an oceanic He-3 distribution which is in qualitative agreeme nt with observations both in overall magnitude and in general distribu tion. This provides new evidence that the flux of mantle He-3 into the oceans is about 1000 mol/yr and that mid-ocean ridges are the dominan t source of mantle helium. Although the match with observations is goo d in the Pacific and Indian Oceans, the simulated He-3 anomalies throu ghout the Atlantic Ocean are much higher than has been measured. Becau se the GCM is thought to reproduce Atlantic circulation reasonably wel l, this discrepancy suggests an error in the helium source function. E ither helium injection is not a linear function of seafloor emplacemen t rate, or eruption and concomitant degassing are highly episodic at t he slow spreading rates characteristic of the Mid-Atlantic Ridge (MAR) . The latter explanation would imply minimal volcanic activity along t he entire length of the MAR over the last few centuries. In addition t o constraints on the degassing nux, our work provides information on t he transport and ventilation of deep ocean waters and constrains the d egree to which current GCMs can reproduce deep-water circulation patte rns. While the results generally support the GCM's abyssal circulation , our simulation reveals regions of overly-intense lateral diffusion a nd upwelling in the model, particularly in the equatorial Pacific. Sim ilarly, there appears to be insufficient production of He-ventilated b ottom waters in the model Antarctic. These observations suggest that f urther refinement of the GCM abyssal circulation is required.