<P> In particular and very rare conditions such as encountered in the past in Lechuguilla Cave in New Mexico (and more recently in the Frasassi Caves in Italy), other mechanisms may also play a role . The oxidation of sulfides leading to the formation of sulfuric acid can also be one of the corrosion factors in karst formation . As oxygen (O) - rich surface waters seep into deep anoxic karst systems, they bring oxygen, which reacts with sulfide present in the system (pyrite or hydrogen sulfide) to form sulfuric acid (H SO). Sulfuric acid then reacts with calcium carbonate, causing increased erosion within the limestone formation . This chain of reactions is: </P> <Dl> <Dd> <Table> <Tr> <Td> H S </Td> <Td> + </Td> <Td> 2 O </Td> <Td> → </Td> <Td_colspan="5"> H SO </Td> <Td> (sulfide oxidation) </Td> </Tr> <Tr> <Td> H SO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td> SO 2 − </Td> <Td> + </Td> <Td_colspan="3"> 2 H O </Td> <Td> (sulfuric acid dissociation) </Td> </Tr> <Tr> <Td> CaCO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td> Ca </Td> <Td> + </Td> <Td> H CO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> (calcium carbonate dissolution) </Td> </Tr> <Tr> <Td> CaCO </Td> <Td> + </Td> <Td> H SO </Td> <Td> → </Td> <Td> CaSO </Td> <Td> + </Td> <Td_colspan="3"> H CO </Td> <Td> (global reaction leading to calcium sulfate) </Td> </Tr> <Tr> <Td> CaSO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td_colspan="5"> CaSO 2 H O </Td> <Td> (hydration and gypsum formation) </Td> </Tr> </Table> </Dd> </Dl> <Dd> <Table> <Tr> <Td> H S </Td> <Td> + </Td> <Td> 2 O </Td> <Td> → </Td> <Td_colspan="5"> H SO </Td> <Td> (sulfide oxidation) </Td> </Tr> <Tr> <Td> H SO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td> SO 2 − </Td> <Td> + </Td> <Td_colspan="3"> 2 H O </Td> <Td> (sulfuric acid dissociation) </Td> </Tr> <Tr> <Td> CaCO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td> Ca </Td> <Td> + </Td> <Td> H CO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> (calcium carbonate dissolution) </Td> </Tr> <Tr> <Td> CaCO </Td> <Td> + </Td> <Td> H SO </Td> <Td> → </Td> <Td> CaSO </Td> <Td> + </Td> <Td_colspan="3"> H CO </Td> <Td> (global reaction leading to calcium sulfate) </Td> </Tr> <Tr> <Td> CaSO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td_colspan="5"> CaSO 2 H O </Td> <Td> (hydration and gypsum formation) </Td> </Tr> </Table> </Dd> <Table> <Tr> <Td> H S </Td> <Td> + </Td> <Td> 2 O </Td> <Td> → </Td> <Td_colspan="5"> H SO </Td> <Td> (sulfide oxidation) </Td> </Tr> <Tr> <Td> H SO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td> SO 2 − </Td> <Td> + </Td> <Td_colspan="3"> 2 H O </Td> <Td> (sulfuric acid dissociation) </Td> </Tr> <Tr> <Td> CaCO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td> Ca </Td> <Td> + </Td> <Td> H CO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> (calcium carbonate dissolution) </Td> </Tr> <Tr> <Td> CaCO </Td> <Td> + </Td> <Td> H SO </Td> <Td> → </Td> <Td> CaSO </Td> <Td> + </Td> <Td_colspan="3"> H CO </Td> <Td> (global reaction leading to calcium sulfate) </Td> </Tr> <Tr> <Td> CaSO </Td> <Td> + </Td> <Td> 2 H O </Td> <Td> → </Td> <Td_colspan="5"> CaSO 2 H O </Td> <Td> (hydration and gypsum formation) </Td> </Tr> </Table>

Which of the following is required for karst topography