geoNear with Pagination and sorting on created_at - mongodb

Question

I´m having an App with the capability of listing articles within a user specified location + radius surrounding. See ruby code with mongo query below.
For Web App, it uses pagination based on "last item" (min_distance) and "excluded_ids" in combination with "limit", what I believe to be the most performant and also better than "limit" and "skip". This solution approach supports "previous page" and "next page". For "jump to page" it makes use of the combination of "limit" and "skip".
geoNear by nature sorts by "distance".
New requirement is to have, additionally, sort by "created_at" (user toggle).
My question:
For sort by "created_at" to work, will it be sufficient to add sort into the query? I´m afraid this won´t work for the current implemented pagination approach and I will need to somehow rewrite to have "last item" based on "created_at", rather than on "min_distance".
Can someone tell for sure how Mongo handles internally the sort and if simply adding sort for created_at will work with this type of pagination?
Or else, how this would look for replacing mind_distance for created_at.
Also, for supporting both sort types, what index/es is/are needed?
One for both, or one for each?
def self.articles_with_distance(params, excluded_ids=[])
if params[:min_distance].present?
Article.collection.aggregate([
{ :"$geoNear" => {
:near => {
:type => "Point",
:coordinates => [params[:lon].to_f, params[:lat].to_f],
},
:query => {_id: { :"$nin" => excluded_ids }}, # to exclude items already shown if min distance between two pages are same
:distanceField => "distance_from",
:maxDistance => params[:radius].to_i,
:minDistance => params[:min_distance].to_i, # only need to reduce the result set by exclude items had been shown previous pages
:spherical => true,
:limit => (params[:per_page].to_i || 10)
}}
]);
else
Article.collection.aggregate([
{ :"$geoNear" => {
:near => {
:type => "Point",
:coordinates => [params[:lon].to_f, params[:lat].to_f],
},
:distanceField => "distance_from",
:maxDistance => params[:radius].to_i,
:spherical => true,
#
}},
{
:"$skip" => (params[:per_page].to_i * (params[:page].to_i-1))
},
{
:"$limit" => (params[:per_page].to_i || 10)
}
]);
end
end