Ivar O Murdmaa

Local variations in distribution and composition of ferromanganese nodules in the Clarion-Clipperton Nodule province

Abstract Local variations in nodule distribution and composition are discussed based on investigations in seven areas surveyed during five Soviet expeditions beteween 1968 and 1988 to the Clarion-Clipperton Nodule Province. Variations in nodule abundance are related to seafloor topography and to local changes in sedimentation rates caused both by bottom current activity and by carbonate dissolution below the lysocline and CCD. Most variations in nodule composition can be attributed to the two principal mechanisms of nodule growth, hydrogenetic and diagenetic, which create three genetic types of nodules differing in morphology, mineralogy, inner structure and chemistry. Hydrogenetic nodules, relatively enriched in Fe, Co and Pb, poor in Mn, Cu, Ni and Mo, and with low Mn/Fe ratios, were found both on abyssal hills, where they are associated with siliceous marly ooze, and on the abyssal basin floor, where they are associated with radiolarian ooze. Rugged bottom topography favours their occurrence and high abundance. Hydrogenetic nodules are though to be formed under distinct stable bottom current conditions. Diagenetic nodules enriched in Mn, Cu, Ni and Mo, and with a high Mn/Fe ratio, occur on clayey radiolarian ooze on the abyssal basin floor below the CCD. These nodules are deposited in high densities and are found predominantly on gentle slopes, where upper Quaternary clayey radiolarian ooze is only a few tens of centimetres thick and is unconformably underlain by pre-Quaternary pelagic clay. We assume that the diagenetic growth of hydroxides is favoured by long-term oscillations of bottom currents, which lead to periodical redeposition of the soft, unstable diagenetically active surface layer of radiolarian ooze. The latter serves as a direct source of metals for nodule growth.

Late Weichselian to Holocene paleoenvironments in the Barents Sea

Paleoceanographic changes since the Late Weichselian have been studied in three sediment cores raised from shelf depressions along a north–south transect across the central Barents Sea. AMS radiocarbon dating offers a resolution of several hundred years for the Holocene. The results of lithological and micropaleontological study reveal the response of the Barents Sea to global climatic changes and Atlantic water inflow. Four evolutionary stages were distinguished. The older sediments are moraine deposits. The destruction of the Barents Sea ice sheet during the beginning of the deglaciation in response to climate warming and sea level rise resulted in proximal glaciomarine sedimentation. Then, the retreat of the glacier front to archipelagoes during the main phase of deglaciation caused meltwater discharge and restricted iceberg calving. Fine-grained distal glaciomarine sediments were deposited from periodic near-bottom nepheloid flows and the area was almost permanently covered with sea ice. The dramatic change in paleoenvironment occurred near the Pleistocene/Holocene boundary when normal marine conditions ultimately established resulting in a sharp increase of biological productivity. This event was diachronous and started prior to 10 14C ka BP in the southern and about 9.2 14C ka in the northern Barents Sea. Variations in sediment supply, paleoproductivity, sea-ice conditions, and Atlantic water inflow controlled paleoenvironmental changes during the Holocene.

Afanasy Nikitin Seamount within the intraplate deformation zone, Indian Ocean

Abstract Geological and geophysical investigations of the Afanasy Nikitin Seamount during Cruise 20 of the R/V Academic Mstislav Keldysh , including multichannel seismic profiling, side-scan sonar survey and observations with manned mir submersibles, revealed the nature and geological history of the volcano within the North Indian Ocean intraplate deformation zone. The seamount is composed of Late Cretaceous effusive rocks comprising a bimodal series of olivine picrite-basalt, and of trachybasalt to trachyte erupted near a spreading center. Conglomerates from the upper slope testify that the volcano emerged above sea level in the Paleocene, and subsided in the Eocene. The Neogene to Recent intraplate stresses led to the youngest deformations (faulting and fracturing) of the volcanic edifice.

Radiocarbon dates of plant detritus from sedimentary deposits of drill hole Bav-480, Pechora Sea

Data obtained during cruises 7, 11, 13, and 14 of R/V Akademik Sergei Vavilov (1990, 1997, and 1998) contributed a lot to reconstruction of paleogeographic environments that existed in the Barents Sea during the last glaciation maximum. This was possible owing to extensive acoustic profiling with Parasound equipment and use of a new approach to study of the upper part of the sedimentary cover. It is established that during the last glaciation maximum the Barents Sea was substantially smaller. It was almost completely surrounded by ice caps that descended to the shelf from glaciation centers on the land. The sea was connected with the World Ocean only by a narrow strait corresponding to the Bear Trough. Sea areas free of bottom glaciers were covered with floating ice: either by old pack ice or by plates of widespread shelf glaciers. Particular glaciomarine sediments accumulated under the floating ice. Destruction of the shelf glaciers during deglaciation resulted in formation of anomalously thick bottom sediment layers. The Pechora Sea shelf appeared to be above the sea level and was eroded by rivers. During entire Late Quaternary and, probably, the earlier epoch, the South Novaya Zemlya Trough represented an area of continuous marine and glaciomarine sedimentation.

Recent and Last Glacial deep-sea facies: Response to global climatic oscillations

Abstract The geographic zonation of the Recent World Ocean, as expressed by distribution of the surface water masses is reflected by latitudinal lithofacial belts and by planktonic foraminiferal assemblages. The lithofacies are controlled mainly by biological productivity and the microfossil assemblages are sensitive to sea-surface temperatures (SST). Migration of the latitudinal lithofacial belts and their boundaries during the Last Glaciation was inferred from displacement of the SST zones.