Jquery Ajax Posting JSON to webservice

I am trying to post a JSON object to a asp.net webservice.

My json looks like this:

var markers = { "markers": [
  { "position": "128.3657142857143", "markerPosition": "7" },
  { "position": "235.1944023323615", "markerPosition": "19" },
  { "position": "42.5978231292517", "markerPosition": "-3" }
]};

I am using the json2.js to stringyfy my JSON object.

and I am using jquery to post it to my webservice.

  $.ajax({
        type: "POST",
        url: "/webservices/PodcastService.asmx/CreateMarkers",
        data: markers,
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function(data){alert(data);},
        failure: function(errMsg) {
            alert(errMsg);
        }
  });

I am getting the following error:

Invalid JSON primitive

I have found a bunch of posts relating to this and it seems to be a really common problem but nothing i try fixes the issue.

When firebug what is being posted to the server it looks like this:

markers%5B0%5D%5Bposition%5D=128.3657142857143&markers%5B0%5D%5BmarkerPosition%5D=7&markers%5B1%5D%5Bposition%5D=235.1944023323615&markers%5B1%5D%5BmarkerPosition%5D=19&markers%5B2%5D%5Bposition%5D=42.5978231292517&markers%5B2%5D%5BmarkerPosition%5D=-3

My webservice function that is being called is:

[WebMethod]
public string CreateMarkers(string markerArray)
{
    return "received markers";
}