My first thought when I first saw this story (the BBC covered it a couple of weeks ago) was 'how have these things not corroded to nothing through electrolytic action in the Millions of years since they were formed?'. I've no doubt that it's (probably) true - it's fairly easily provable one way or the other anyway. [Edit: I'm sure the mining companies will be falling over themselves to prove otherwise!]
Maybe these are the remnants and they started out as big as houses!
They seem to be layered growths, not fragments or concretions (aside from what seabed gets trapped in the bottom resting side).
The concentrations of Fe, Mn, etc. are extremely small in seawater. The (2+) ions are more soluble than others, but are easily oxidized; pure manganese deposits are usually present in Mn(4+) form (pyrolusite, etc.), and iron, mixed (2+, 3+) (magnetite) or (3+) (hematite). Evidently, these ions are at a low enough concentration not to spontaneously nucleate (that's happened long ago, at river deltas, deep sea smokers, etc.), and keep in mind, these things have been here
for millions of years, and likely will continue to (give or take areas where new spontaneous conditions form, or old fields get buried, subducted, etc.); these are
extremely slow equilibria we're talking about!
Note that these mixed oxides are moderately good conductors (they're used for battery and electrolysis electrodes), and given the layered composition (varying with global nutrient conditions, weather patterns, etc.), it could well be that cells are randomly created with enough potential to (endothermically) electrolyze water; or there are spontaneous or bacterial processes at work that liberate oxygen from the more oxidized ions (Mn(3+) is about as stable (bixbyite, though I gather it doesn't form at these temperatures, another more hydrated mineral would likely form?). In any case, there is some potential energy stored in these, despite the extremely slow deposition suggesting a low-energy process. I suppose one way to put it is: it might be low energy, but storing up that tiny margin over millions of years, can eventually count for something.
It could very well be that these nodules imply something about an overall balance of redox potential on the planet; they almost certainly wouldn't form without a surplus of oxygen, but so too their growth might not be straightforward, perhaps stalling or even dissolving as conditions change -- particularly over glacial cycles, perhaps, but also broader cycles or trends, such as mass volcanic eruptions, or astronomical changes. It might be that their existence is something of an accident, a balance that merely happens to trend upward at the present time (and in just those locations where they do).
They are present in the geologic record, so it's not like we live in unique times; but oxygen has been present for quite a long time, and, as long as the continents have been moving around, there have been vast swaths of otherwise stationary seabed, hardly accumulating insoluble matter, while the thermohaline cycle starts, stops and shifts paths -- bringing in trace elements, or stagnating in place.
Tim
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