Christensen does these things, and it's hard to know what he's getting at.

In this case, he's saying the TES could not find surface carbonates at all but did find carbonate concentrations of 2% to 5% in the ubiquitous dust blanketing the planet... (well, the dust doesn't actually blanket the planet. Instead, it is found in concentrations indicative of strongly differential erosion, but that's another story...) Then Christensen refers to 2%-5% concentrations of carbonates as trace amounts. Why? It's a significant amount, not a trace amount, but Christensen seems to be promoting a Cold Mars model that requires no more than "trace amounts," so he simply defines 2% ro 5% concentrations as "traces."

This sort of thing happens all the time in Mars study. Just re-define what you find so that it fits your paradigm and you're good to go.

As for the apparent absence of carbonates on the surface, no one has yet been able to come up with a plausible explanation for all the "white" outcrops -- not just the many White-Rock formations mostly found in craters, but the "white" deposits found in the Ares Vallis Pathfinder landing site and suspected elsewhere on the surface. If they are not carbonate, what are they? In order to fit the phenomena to the paradigm, the "white" formations have been re-defined as "red" -- thus for the moment overcoming the apparent contradiction.

The fact is that no one knows what the "white" formations are, and despite all the armwaving to the contrary, no one can say for certain that they AREN'T carbonate.

There are no apparent massive regional carbonate deposits discoverable at the surface, and that finding supports the contention that there may have never been significant, long-lasting oceans on Mars. On the other hand, such deposits may exist, but cannot be "seen" by current instrumentation due to dust cover or other reasons.

After all, Christensen himself has invoked "dust cover" to account for the preservation of snow packs which cannot be observed in mid and equatorial latitudes -- in order to make some sense of the ubiquitous contemporary channels. If "dust" can mask unobservable snow packs necessary for his "trickle down" theory of channelling, then dust can bloody well mask carbonates. But we don't know.

We don't even know for certain the origin of the dust, and we especially don't know what there is such massively differential erosion on the surface, leaving some areas essentially free of dust and burying others in drifts kilometers deep.

Much more study needed...

Cheers, BRFelix