Here are a couple of segements from:
http://ida.wr.usgs.gov/html/m13007/m1300766.html

to give an idea of the formations Nick's paper refers to.

We can see right away that the surface appearance here (which is similar to that found over much of Isidis -- except close to the margins of the basin) is quite a bit different than the surface in the previous image -- which is quite a bit farther north.

The mounds shown in the previous image are scattered, they are slick-sided, and they are light-colored in contrast to the dark surface they are found on.

Note another contrast: the Isidis surface is intensely pitted with overlapping circular formations that superficially resemble impact craters (but may not have any relation to impact). The more northerly sample, by contrast, shows mounds, buttes, and a very few craters -- some of which may be impact craters -- on a very smooth surface.

Well. Not entirely smooth.

At fine scale, the surface is rather rough, though not pitted, and note the thread-like ridges that extend from the mounds and connect the mounds, especially in the lower part of the image.

In the Isidis image, there are no such ridges, though there are lots of light colored dunes (or at least these formations are interpreted as dunes.) Note also that the cones in the Isidis image are quite rough, apparently heavily eroded, in contrast to the smooth, white, fresh appearance of the mounds further north.

Are the cones in Isidis and the mounds further north formed by the same processes? Are they just different ages, perhaps?

Nick argues that the cones in Isidis are produced by release of liquid CO2 from subsurface reservoirs. As he knows, CO2 is very difficult to maintain in a liquid state close to the surface of Mars, and it is probably impossible to maintain liquid CO2 at substantial depth.
At the surface, liquid CO2 would flash to vapor much more violently that liquid water would. Thus, if a suitable reservoir for liquid CO2 could form below ground, any breach of that reservoir's cap could indeed lead to the formation of vents that could resemble the Isidis cones. But for that to happen, the subsurface of Mars must be very cold. It also must be able to sustain very high pressures.

Nick argues that the subsurface is cold and can sustain high pressures. So far, he has not demonstrated the truth of his claims. Of course it is difficult to do so remotely. So far, especially given the findings of water signs essentially everywhere near the surface of Mars, in some areas signs of water (ice) in great abundance, the argument for CO2 as the sole or rather the chief volatile on Mars is not holding up well.

But is water or water ice the cause of the cones in Isidis or the mounds further north? Or is there some volcanic process involving lava in interaction with ice responsible? Or is it something else entirely?

Heck if I know. But what we can say is that the intensity of the pitting in Isidis is remarkable -- there are few areas as heavily pitted -- and I wonder: could the pits in combination with the cones suggest there was a period of rapid surface heating beneath an ice cover?

And could the more northerly mounds be largely ice?

We might revisit the question of ice at the surface of Mars later on.

As for the notion that "most" of the circular formations (apart from the mounds) I've referred to in the images here are modified impact craters, I would ask anyone who believes that to demonstrate how an impact crater is transformed into a ring formation. Try experiments. See if you can do it.

And do some compare and contrast between actual impact craters on Mars, Earth, and Venus, and the circular formations on Mars that more strongly resemble circular formations on Earth (and possibly Venus) that are caused by internal forces.

Cheers, BFelix