Benthic foraminiferal carbon isotopic records and the development of abyssal circulation at DSDP Sites 9-77, 12-119, 30-289, 82-563 and 94-608

The North Atlantic at present is ventilated by overflow of the Denmark Strait, Iceland-Faeroe Ridge, Faeroe Bank Channel, and Wyville-Thompson Ridge. The evolution of Cenozoic abyssal circulation of this region was related to tectonic opening and subsidence of these sills. We used d13C records of the benthic foraminifer Cibicidoides to decipher the timing of tectonically controlled changes in bottom-water circulation in the eastern basins (Biscay and Iberian) of the northern North Atlantic. Records from Site 608 (Kings Trough, northeastern North Atlantic) show that from about 24 to 15 Ma (early to early middle Miocene), d13C values in the Kings Trough area were depleted relative to western North Atlantic values and were more similar to Pacific d13C values. This reflects less ventilation of the Kings Trough region as compared to the well-oxygenated western North Atlantic. Comparison of Oligocene d13C records from Site 119 (Bay of Biscay) with western North Atlantic records suggests that the eastern basin was also relatively isolated prior to 24 Ma. At about 15 Ma, d13C values at Site 608 attained values similar to the western North Atlantic, indicating increased eastern basin ventilation in the middle Miocene. This increased advection into the eastern basin predated a major d18O increase which occurred at about 14.6 Ma. Subsidence estimates of the Greenland-Scotland Ridge indicate that the deepening of the Iceland-Faeroe Ridge was coincident with the marked change in eastern basin deep-water ventilation.

Shape-invariant multibeam backscatter data of the eastern slope of the Porcupine Seabight (50 m grid size)

In this study multibeam angular backscatter data acquired in the eastern slope of the Porcupine Seabight are analysed. Processing of the angular backscatter data using the 'NRGCOR' software was made for 29 locations comprising different geological provinces like: carbonate mounds, buried mounds, seafloor channels, and inter-channel areas. A detailed methodology is developed to produce a map of angle-invariant (normalized) backscatter data by correcting the local angular backscatter values. The present paper involves detailed processing steps and related technical aspects of the normalization approach. The presented angle-invariant backscatter map possesses 12 dB dynamic range in terms of grey scale. A clear distinction is seen between the mound dominated northern area (Belgica province) and the Gollum channel seafloor at the southern end of the site. Qualitative analyses of the calculated mean backscatter values i.e., grey scale levels, utilizing angle-invariant backscatter data generally indicate backscatter values are highest (lighter grey scale) in the mound areas followed by buried mounds. The backscatter values are lowest in the inter-channel areas (lowest grey scale level). Moderate backscatter values (medium grey level) are observed from the Gollum and Kings channel data, and significant variability within the channel seafloor provinces. The segmentation of the channel seafloor provinces are made based on the computed grey scale levels for further analyses based on the angular backscatter strength. Three major parameters are utilized to classify four different seafloor provinces of the Porcupine Seabight by employing a semi-empirical method to analyse multibeam angular backscatter data. The predicted backscatter response which has been computed at 20° is the highest for the mound areas. The coefficient of variation (CV) of the mean backscatter response is also the highest for the mound areas. Interestingly, the slope value of the buried mound areas are found to be the highest. However, the channel seafloor of moderate backscatter response presents the lowest slope and CV values. A critical examination of the inter-channel areas indicates less variability within the estimated three parameters.

Mineral composition of 0.1-0.05 mm grain size fraction from the surface layer of bottom sediments in the West European province of the Atlantic Ocean

Rock material sampled from the Mir manned deep-sea submersibles and by dradges, grabs, and sediment cores over a vast area of the North Atlantic was analyzed to show that this material is of continental origin, unlike original rocks of the ocean floor. It is proved to be related to iceberg rafting during Quaternary glaciations. Independent data on distribution and composition of sandy and silty grains in sediment cores also support this relation to the recent glaciation. New criteria for identification of iceberg rock matter in pelagic sediments are presented on the base of analysis of all available data.