It is a trade block and minke whale defence pact signed between Río de la Plata (A better world TL), Patagonia (A better world TL) and Chile (A better world TL) in 2008. Brazil (A better world TL) joined in 2009.
While Territory of Shag Rocks, South Georgia and the South Sandwich Islands (A better world TL) is subject to all UK treaties and obligations, it is de facto part of Mercasur due to its remote location in relation to the UK.