Over the past/last decade or during the past/last decade

Which is the correct usage: over the past/last decade or during the past/last decade?

Though often used interchangeably, there are subtle differences in meaning between "over" and "during." You might also consider using "throughout."

Something that happened at a specific time will generally be described with "during." E.g.: "During the last decade, scientists discovered life on Mars." Both "during" and "over" can be used for something that may have spanned the entire period. E.g.: The earth's population increased 5% over/during the last decade.

A good discussion of the usage of over, during and throughout is in S. Lindstromberg's English Prepositions Explained. ([Link to Google Books][1])

(Edited to delete the following discussion of an issue you didn't ask about.)

In most contexts, "past decade" and "last decade" have the same meaning. The exception is when you are using "last" as a synonym for "final". For example: "The last decade of the 20th Century."

Perhaps to avoid ambiguity, the "style guides" of some publications sometimes require the use of "past" except when being used synonymously with "final."