What has caused the secular increase in solar nitrogen-15?

While the N-15/N-14 ratio variations noted in lunar surface samples appear to be due to a secular increase in this ratio in the solar wind, over a period of several billion years, whose ultimate cause may be found in the solar convective zone, the change resists explanation within the currently accepted framework of solar processes. A 'mixing model' approach is presently invoked which implies that the isotopic composition of each component has remained constant over time, thereby obviating the postulation of the large change in composition of so large an isotopic reservoir as the solar convective zone. An evaluation is made of the drawbacks of this approach.