Argentinian pesos are used in Argentina, Chilean pesos are used in Chile. US dollars may be exchanged into local currency at either banks or Money Exchange kiosks, at the open market exchange rate of the day. Some shops might accept US cash, though this is not a common practice. You can also withdraw cash from ATM's using your credit card and PIN code.
When visiting the Falkland Islands, local pounds (FIP) are used as well as pound sterling. When landing in Antarctica, some bases do accept USD cash only. In Port Lockroy, the following currencies are accepted: GBP, USD, Euro, and major credit cards (for purchases above 100 USD).