Integrated stratigraphy and palaeoecology of the Lower and Middle Miocene of the Porcupine Basin

A high-resolution palynological analysis and a detailed palaeomagnetic study of a marine sequence recovered during IODP Expedition Leg 307 in the Porcupine Basin southwest of Ireland provide new insights into the regional depositional history and palaeoenvironmental evolution during Early Neogene times. The Hole 1318B studied was drilled on the upper slope of the continental margin in a water depth of 409 m, upslope from a province of carbonate mounds (the Belgica mound province). The diverse and well-preserved dinoflagellate cyst associations consist typically of deep neritic and oceanic species, mixed with a neritic component transported from the shelf, reflecting the deep depositional setting at the continental margin. The palaeomagnetic record together with the ranges of key dinoflagellate cyst species constrain the age of the studied sequence between 16.7 Ma and 12.01 Ma, that is, between the late Burdigalian and late Serravallian. The distinct unconformity terminating the Miocene sequence correlates to the global sequence boundary Ser4/Tor1 dated at 10.5 Ma, and represents, according to previous extensive seismic studies, a basin-wide erosional event. The overlying sediments are of Middle Pleistocene or younger age. Downslope from IODP Site 1318, carbonate mounds root on the erosional surface. The dinoflagellate cyst associations from the Porcupine Basin distinctly mirror the global cooling phase following the Middle Miocene Climatic Optimum. Cooling phase Mi3, a short-lived glaciation, is particularly well expressed and here dated at 13.6 Ma. The palynomorph record furthermore indicates a reduction of the productivity and an increase of oceanic oligotrophic species after 14 Ma, suggesting a reduction or perhaps even a shutdown of the upwelling

Strontium isotopic ages from IODP Exp307

Sr isotope stratigraphy provides a new age model for the first complete section drilled through a deep-water coral mound. The 155-m-long section from Challenger Mound in the Porcupine Sea-bight, southwest of Ireland, is on Miocene siliciclastics and consists entirely of sediments bearing well-preserved cold-water coral Lophelia pertusa. The 87Sr/86Sr values of 28 coral specimens from the mound show an upward-increasing trend, correspond to ages from 2.6 to 0.5 Ma, and identify a significant hiatus from ca. 1.7 to 1.0 Ma at 23.6 m below seafloor. The age of the basal mound sediments coincides with the intensification of Northern Hemisphere glaciations that set up the modern stratification of the northeast Atlantic and enabled coral growth. Mound growth persisted throughout glacial-interglacial fluctuations, reached a maximum rate (24 cm/k.y.) ca. 2.0 Ma, and ceased at 1.7 Ma. Unlike other buried mounds in Porcupine Seabight, Challenger Mound was only partly covered during its growth interruption, and growth restarted ca. 1.0 Ma