The use of anthropogenic tritium and helium-3 to study subtropical gyre ventilation and circulation

Abstract
With tritium and helium-3 ( 3 He) data from the Transient Tracers in the Ocean (tto) expedition and from two other contemporaneous cruises, a synoptic picture of the ventilation and circulation of the subtropical North Atlantic is built. We will see clear evidence of gyre circulation in the tritium- 3 He age distributions on the shallower isopycnals, permitting estimates of the rates of circulation averaged over timescales from months to decades. The entry points of fluid into the main thermocline and the pathways of exchange with the upper ocean on seasonal to decade timescales appear clearly. It is the time-averaged transport processes on those timescales that are important to the uptake of carbon dioxide by the ocean. The overall relation between tritium and 3He within the subtropics exhibits a systematic hook-like pattern that is consistent with ‘strong’ ventilation of the gyre thermocline; a fluid parcel entering the gyre thermocline making only about one circuit around the gyre before being ventilated. Finally, we present a time-series of 3 He measurements made over a period of two years near Bermuda. The mixed layer is demonstrably supersaturated in this isotope throughout a large part of the year, requiring a gas-exchange flux of this isotope to the atmosphere. Model results are presented that permit the calculation of the in situ solubility isotope ratio anomaly for helium (as affected by bubble injection and gas exchange), and that can be used to estimate the upward flux of this isotope. Because only a small fraction of this flux can be produced in the mixed layer, this helium must be ‘mined’ from the main thermocline. The computed flux is consistent with the long-term evolution of the inventories of tritium and 3 He within the main thermocline. This flux has implications regarding the vertical transport of material within and from the permanent thermocline. A single observation of what may be one of the processes responsible for this upward flux is discussed.

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