They're there probably debris covered.
I'll go out on a limb and predict that there also probably debris or ice covered lakes or seas currently in the northern plains of Mars. The ground penetrating radar on Mars Express or Mars Reconnaissance Orbiter may detect them.
I also don't agree with the phrasing of the discovery of the carbonates in the dust all over Mars. The researchers rather imply it stems from small amounts of liquid water suspended in the atmosphere. But as Gil Levin has continually argued and has been confirmed in experiment liquid water can exist in thin films on the surface of Mars. Then the carbonate containing dust probably stems from these thin liquid water films that can form diurnally and seasonally.
This also would mean microbial life could probably survive there.
Bob Clark
Space/Science » in reply to Uh...Robert...
Origins of the carbonate dust on Mars.