DIAGENESIS OF A MODERN DEEP-WATER CARBONATE MOUND: CHALLENGER MOUND, PORCUPINE BASIN

Expedition 307 of the Integrated Ocean Drilling Program (IODP) drilled a large deep-water carbonate mound (Challenger Mound) in the Porcupine Seabight, southwest of Ireland. A suite of mixed siliciclastic and carbonate sediments were recovered spanning the Miocene to Pleistocene. The mound itself is dominated by coral floatstones through rudstones, whereas the off-mound sediments consist of bioturbated siliciclastic mudstones. The main carbonate producer is Lophelia pertusa, an azooxanthellate coral. The corals provide the mound with an increased diagenetic potential in comparison to the off mound sediments due to the presence of aragonite within the coral skeletons. These skeletal fragments, many of which show both dissolution features and calcite overgrowths, are found to be variably bioeroded and often contain abundant accumulations of pyrite. Analysis of pore water major element and carbon isotopic geochemistries suggests that microbial sulfate reduction is occurring, both within the mound and off mound sites. Throughout much of the cored interval a down core trend of decreasing Mg and Ca, and increasing Sr content in pore water suggests the mineral-controlled diagenetic processes of aragonite dissolution and calcite precipitation. In other intervals, both the Ca and Sr contents are observed to increase and Mg content decreases, indicating that dolomitization is occurring at depth within the mound. Within Challenger Mound the intervals containing the most highly altered coral fragments coincide with abrupt changes in pore water geochemistry, and are interpreted as representing depositional hiatuses.