The field grew to 12 with the addition last week of Christiana Figueres of Costa Rica, formerly the executive Secretary of the UN Framework Convention on Climate Change. She will appear with Clark in the second debate, with Irina Bokova of Bulgaria, Igor Luksic of Montenegro and Danilo Turk of Slovenia.
Figueres is the second woman candidate from Latin America. Argentina's foreign minister Susana Malcorra is considered one of the strongest candidates but as Ban Ki Moon's chief of staff for many years, is also seen as the status quo candidate. She has also been subject to criticism within South America and accused of turning a blind eye to human rights abuses in Venezuela in exchange for its support.
Venezuela is on the Security Council, along with New Zealand.
The debates will be live-streamed on the UN webcast and on Al Jazeera. They will be chaired by Al Jazeera diplomatic editor James Bays.
Two other candidates are expected to be nominated, former Australian Prime Minister Kevin Rudd, and Bulgaria's EU Commissioner Kristalina Georgieva.
Rudd has been waiting for the new Australian government to be formed before seeking its support for his nomination. Although Rudd is a former Labor PM, the Coalition Government is expected to officially support his bid.