That's an interesting question. Fe and P are probably the most relevant nutrients that are abundant in suspended glacial rock flour. Since Fe3+ is pretty insoluble in seawater, the suspended (vs. dissolved) load might actually matter -- wind-blown dust would be the #1 competitor. The first relevant paper I found suggests the dust still dominates [1], but it was clearly enough of a consideration for them to write a paper about it. And sometimes the wind-blown dust is actually glacial rock flour [2], so the plot thickens!
