Yes, they can be. 

A trademark by definition is any 'sign' that one trader uses to distinguish its goods/services from those offered by others. Under the Trade Marks Act 1995:

sign includes the following or any combination of the following, namely, any letter, word, name, signature, numeral, device, brand, heading, label, ticket, aspect of packaging, shape, colour, sound or scent.  

No matter the type of 'sign' you apply for it will be more difficult to register if it's not capable of distinguishing the associated products and services. In Australia there is only one 'smell' trademark to be successfully registered. It's the smell of in connection with golf tees! There are several new applications currently pending so it will be interesting to see how they go.