Sunset in the Atacama desert

I don't shoot a lot of landscapes. Highest point in my county is the landfill! But sometimes, I can't avoid it.

This was taken looking east at sunset from a small oasis in the Atacama Desert in Chile. Only adjustment is a slight curves to lighten the foreground and darken the sky and then USM. I don't like the vignetting at all and am curious if anyone has any thoughts either about what caused it (it's there even before the curves adjustment) or how to remove it. Only thing I can think of for a cause is the haze filter pretty much permanently attached to the lens, but it's not something that shows up regularly as I would have expected with that cause.

I should add that the light stuff in the foreground is trash, which is ubiquitous in the Atacama. Leave it and it will be there 100 years later. Was tempted to crop it out, but it wouldn't be the Atacama without it.

1Ds, 100-400 at 340, 1/60 at f8, ISO 250.

Rich