Thanks for your reply. I am looking to track the source of individual leads to our site - e.g., adwords, organic search, referrals, etc. The type of thing you'd get from GA but at the individual level.
Could you clarify what you mean by source tracking?
If you are referring to what page sent a user to this page you are analyzing, you can create a report to figure this out. If you go to the report tab in Marketo, you can look at the HTTP Referrer and Entry Page for each lead.