Physical properties and geochemistry at DSDP Leg 70 Holes

Ashes occurring both as distinct layers and mixed with pelagic sediments of the hydrothermal mounds lying south of the Galapagos Rift are mainly rhyolitic and basaltic. The ashes, of rhyolitic to intermediate composition, appear to belong to a calc-alkalic series and were probably derived from Plinian eruptions in Ecuador or Colombia. Basaltic ashes are made of nonvesicular sideromelane spalling shards and are of tholeiitic composition. They probably were derived locally from fault scarps. Most rhyolitic and basaltic glass shards studied are fresh except for hydration of the rhyolitic shards. Some shards are severely altered, however. Basaltic ash may be more common in pelagic sediments deposited near accretion zones and may be a source of silica and other elements released during diagenesis

Mineralogy of ODP Site 105-645

Cores from the upper 70 meters below seafloor (mbsf) (upper Pleistocene) at Ocean Drilling Program (ODP) Site 645 in Baffin Bay show dramatic meter-scale changes in color and mineralogy. Below this interval, mineralogical changes are more gradual to the top of the Miocene at about 550 mbsf. The Pliocene-Pleistocene section can be divided into five facies: Facies 1 - massive, poorly sorted, gravel-bearing muds; Facies 2 - gray silty clays and silty muds; Facies 3 - laminated detricarbonate silty muds; Facies 4 - silty sand and sandy silt; and Facies 5 - poorly sorted muddy sands and silty muds. Facies 4 and 5 are restricted to the Pliocene section below depths of about 275 mbsf. The mineralogical/color cycles in the upper 70 mbsf are the result of alternations between Facies 2 and three lithotypes of Facies 1: lithotype A - tan-colored, carbonate-rich, gravel-bearing mud; lithotype B - weak, red-colored, gravel-bearing mud rich in sedimentary rock fragments; and lithotype C - gray, gravel-bearing mud. A fourth lithotype, D, is restricted to depths of 168-275 mbsf and is dark gray, carbonate-poor, gravel-bearing mud. We believe that all lithotypes of Facies 1 and the sand and gravel fractions of Facies 2 and 3 were deposited by ice rafting. Depositional processes for Facies 4 and 5 probably include ice rafting and bottom- and turbidity-current transport. Data from petrographic analyses of light and heavy sand-sized grains and X-ray analyses of silt- and clay-size fractions suggest that tan-colored sediments (lithotype A of Facies 1; Facies 3) were derived mainly from Paleozoic carbonates of Ellesmere, Devon, and northern Baffin islands. Weak red sediments (lithotype B) contain significant red sedimentary clasts, reworked quartzarenite grains and clasts, and rounded colorless garnets, all derived from Proterozoic sequences of the Borden and Thule basins, and from minor Mesozoic red beds. Other sediments in the upper 335 mbsf at Site 645 contain detritus from a heterogeneous mixture of sources, including Precambrian shield terranes around Baffin Bay. Sediments from 335 to 550 mbsf (Facies 5) are rich in friable sedimentary clasts and detrital micas and contain glauconite and, in a few samples, reworked diatoms. These components suggest derivation from poorly consolidated Mesozoic-Tertiary sediments in coastal outcrops and beneath the modern shelves of northeastern Baffin Island and western Greenland. For the upper Pleistocene section (about 0-100 mbsf), marked mineralogical cyclicity is attributed to fluctuating glacial margins, calving rates, and iceberg melting rates, particularly around the northern end of Baffin Bay. Tan-colored, carbonate-rich units were derived at times of maximum advance of glaciers on Ellesmere and Devon islands, during relatively warm intervals induced by incursion of warm Atlantic surface water into the bay. At the beginning of these warmer episodes, most icebergs were contributed by glaciers near sea level around the Arctic channels, which resulted in deposition of weak red, ice-rafted units rich in Proterozoic sedimentary clasts.

Ages, lithologie descriptions and mineral analyses from different DSDP holes in the Indian Ocean

The present work is based on mineralogical studies of sand and silt layers from a number of Deep Sea Drilling Project sites in the Indian Ocean belonging to different physiographic provinces of different ages. The minerals can be grouped into two major associations: a hornblende-opaque association with varying amounts of pyroxene, garnet, epidote, zircon, etc. and a biotite-chlorite-muscovite assemblage. The dominance of unstable minerals indicates a first generation, though evidence of reworking is reflected in the zircon and tourmaline grains at some sites. A large variety of minerals at some sites indicates a complex source. The mineral composition is nearly homogeneous at different sites for the entire length of the core, indicating that they have been derived from the same source during the deposition of that interval. However, the provenance changed by tectonic activity, the effect of which has been reflected in the mineralogy of some sites. An attempt was made to describe the mineralogic characteristics and their tectonic interpretations in the Pliocene and Miocene periods in the Ganges and Indus fan sites and also in the Wharton and Mozambique basin sites. Similar attempts could not be made for other ages in other physiographic provinces as the numbers of samples were too few. Within the limited scope, some idea about the mineralogical character of different basins and different physiographic provinces can be obtained from the present study. Mineralogical evidence also suggests very long transport of sediments in the deep sea. Regional variation of mineralogy has resulted due to source, sea-floor configuration, selective removal, reworking by different agencies and the processes operating in the ocean. There is no relation between a particular age and a set mineral assemblage for the Cenozoic sediments of the Indian Ocean.

Sedimentation rates and foraminiferal assemblages from ODP Site 162-184

Sedimentological and faunal records from the transitional period marking the onset of widespread northern hemisphere glaciation have been investigated at Ocean Drilling Program Site 984. The late Pliocene interglacial sediments of the northeast Atlantic are carbonate rich and show evidence of vigorous bottom water circulation at intermediate water depths. Contrasting this, the late Pliocene glacial sediments are characterised by carbonate dissolution and slower bottom current velocities. Weak or "leaky" Norwegian Sea overflows, undersaturated with respect to carbonate, influenced this region during the late Pliocene glacials. The early Pleistocene pattern of intermediate water circulation appears to have changed radically in the northeast Atlantic. At this time, interglacial carbonate values and inferred bottom current velocities are low. This suggests slow-flowing, undersaturated Norwegian Sea water bathing the site. The overflow increased during the early Pleistocene interglacials as the exchange between the Atlantic and Norwegian-Greenland Seas improved. The most significant feature of the early Pleistocene glacials is the increase in inferred bottom current velocity. These changes reflect a switch in deep North Atlantic convection to shallower depths during glacial periods, possibly in a manner similar to the increasing contribution of glacial intermediate water to the North Atlantic during the late Pleistocene glacials. Our results suggest that the late Pleistocene climate variability of the North Atlantic is a pervasive feature of the late Pliocene-early Pleistocene record.