Interstitial-water geochemistry of Kerguelen Plateau sediments

This report synthesizes all of the interstitial-water chemistry studies associated with the Kerguelen Plateau phase of ODP Leg 119. Sediments were cored at six sites (49°24'S to 59°36'S) in water depths ranging from 564 to 4082 m. A total of 77 interstitial-water samples was recovered as part of the routine sampling protocol. In addition, a novel, highresolution pore-water sampling program was tested during Leg 119 that enabled us to pinpoint reaction zones and extend our data base to deeper, drier levels that were heretofore inaccessible. Data collected include interstitial-water sodium, potassium, calcium, magnesium, pH, alkalinity, sulfate, ammonia, phosphate, aqueous silica, salinity, chloride, oxidation-reduction potentials, and sediment chemistry. The northern sector (Sites 736 and 737) is characterized by the highest sedimentation rates (up to 140 m/m.y.) and thermal gradients (70°-98°C/km) encountered on the Kerguelen Plateau during Leg 119. Site 737 represents the most reactive sediment column cored on the Kerguelen Plateau. Major cation fluxes at Site 737 are the strongest measured during Leg 119. High dissolved calcium concentrations (141.5 mM) were encountered near the bottom of Hole 737B. Elevated temperatures promote silica diagenesis and the alteration of volcanic material below 300 mbsf, and a diagenetic front was discovered near 370 mbsf at Site 737. The southern portion of the Kerguelen Plateau (Sites 738 and 744) records the lowest sedimentation rates (less than 5 m/m.y.) and thermal gradients (43°C/km) of the three study areas. Major cation fluxes at the southern sites are the lowest that we measured on the Kerguelen Plateau. High-resolution sampling provided evidence for significant silica release to the pore waters during the weathering of basement basalt. The relatively low thermal gradient does not appear to be sufficient for the formation of the opal-CT and quartz chert beds and nodules that were encountered below 120 mbsf at Site 738. Sediment-accumulation rates on the Eastern Kerguelen Sediment Ridge (Sites 745 and 746) are intermediate to those of the northern and southern sites. Deposition below the regional CCD accounts for the nearly carbonate-free, siliceous sediments. Despite their low organic carbon contents (mean = 0.15%), sediments on the Eastern Kerguelen Sediment Ridge exhibit the highest pore-water alkalinity (6.77 mM), ammonium (0.50 mM), and phosphate (23 µM) concentrations measured on the Kerguelen Plateau. Major cation fluxes are intermediate to those calculated for the northern and southern sites. The Eastern Kerguelen Sediment Ridge interstitial waters are unusual, however, in that the downward flux of magnesium is greater than the upward flux of calcium.

Geochemistry, clay mineralogy and grain size distribution in various sediment cores drilled during ODP Leg 119 on the Antarctic continental shelf off Prydz Bay

Drilling was undertaken at five sites (739-743) on ODP Leg 119 on a transect across the continental shelf of Prydz Bay, East Antarctica, to elucidate the long-term glacial history of the area and to examine the importance of the area with respect to the development of the East Antarctic ice sheet as a whole. In addition to providing a record of glaciation spanning 36 m.y. or more, Leg 119 has provided information concerning the development of a continental margin under the prolonged influence of a major ice sheet. This has allowed the development of a sedimentary model that may be applicable not only to other parts of the Antarctic continental margin, but also to northern high-latitude continental shelves. The cored glacial sedimentary record in Prydz Bay consists of three major sequences, dominated by diamictite: 1. An upper flat-lying sequence that ranges in thickness from a few meters in inner and western Prydz Bay to nearly 250 m in the outer or eastern parts of the bay. The uppermost few meters consist of Holocene diatom ooze and diatomaceous mud with a minor ice-rafted component overlying diamicton and diamictite of late Miocene to Quaternary age. The diamictite is mainly massive, but stratified varieties and minor mudstone and diatomite also occur. 2. An upper prograding sequence cored at Sites 739 and 743, unconformly below the flat-lying sequence. This consists of a relatively steep (4° inclination) prograding wedge with a number of discrete sedimentary packages. At Sites 739 and 743 the sequence is dominated by massive and stratified diamictite, some of which shows evidence of slumping and minor debris flowage. 3. A lower, more gently inclined, prograding sequence lies unconformably below the flat-lying sequence at Site 742 and the upper prograding sequence at Site 739. This extends to the base of both sites, to 316 and 487 mbsf, respectively. It is dominated by massive, relatively clast-poor diamictite which is kaolinite-rich, light in color, and contains sporadic carbonate-cemented layers. The lower part of Site 742 includes well-stratified diamictites and very poorly sorted mudstones. The base of this site has indications of large-scale soft-sediment deformation and probably represents proximity to the base of the glacial sequence. Facies analysis of the Prydz Bay glacial sequence indicates a range of depositional environments. Massive diamictite is interpreted largely as waterlain till, deposited close to the grounding line of a floating glacier margin, although basal till and debris flow facies are also present. Weakly stratified diamictite is interpreted as having formed close to or under the floating ice margin and influenced by the input of marine diatomaceous sediment (proximal glaciomarine setting). Well-stratified diamictite has a stronger marine input, being more diatom-rich, and probably represents a proximal-distal glaciomarine sediment with the glaciogenic component being supplied by icebergs. Other facies include a variety of mudstones and diatom-rich sediments of marine origin, in which an ice-rafted component is still significant. None of the recovered sediments are devoid of a glacial influence. The overall depositional setting of the prograding sequence is one in which the grounded ice margin is situated close to the shelf edge. Progradation was achieved primarily by deposition of waterlain till. The flat-lying sequence illustrates a complex sequence of advances and retreats across the outer part of the shelf, with intermittent phases of ice loading and erosion. The glacial chronology is based largely on diatom stratigraphy, which has limited resolution. It appears that ice reached the paleoshelf break by earliest Oligocene, suggesting full-scale development of the East Antarctic ice sheet by that time. The ice sheet probably dominated the continental margin for much of Oligocene to middle Miocene time. Retreat, but not total withdrawal of the ice sheet, took place in late Miocene to mid-Pliocene time. The late Pliocene to Pleistocene was characterized by further advances across, and progradation of, the continental shelf. Holocene time has been characterized by reduced glacial conditions and a limited influence of glacial processes on sedimentation.

Mineralogical and grain size analysis of ODP Hole 119-745B from the Southern Ocean

The upper Miocene to Pleistocene sediments recovered at ODP Sites 745 and 746 in the Australian-Antarctic Basin are characterized by cyclic facies changes. Sedimentological investigations of a detailed Quaternary section reveal that facies A is dominated by a high content of siliceous microfossils, a relatively low terrigenous sediment content, an ice-rafted component, low concentrations of fine sediment particles, and a relatively high smectite content. This facies corresponds to interglacial sedimentary conditions. Facies B, in contrast, is characteristic of glacial conditions and is dominated by a large amount of terrigenous material and a smaller opaline component. There is also a prominent ice-rafted component. The microfossils commonly are reworked and broken. The clay mineral assemblages show higher proportions of glacially derived illite and chlorite. A combination of four different processes, attributed to glacial-interglacial cycles, was responsible for the cyclic facies changes during Quaternary time: transport by gravity, ice, and current and changes in primary productivity. Of great importance was the movement of the grounding line of the ice shelves, which directly influenced the intensity of ice rafting and of gravitational sediment transport to the deep sea. The extension of the ice shelves was also responsible for the generation of cold and erosive Antarctic Bottom Water, which controlled the grain-size distribution, particularly of the fine fraction, in the investigated area.

Andreas Markantonatos (Ed.) Brill's Companion to Sophocles, Brill, Leiden, Boston, 2012, xxii + 737 pp

Fil: Saravia de Grossi, María Inés. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación. Instituto de Investigaciones en Humanidades y Ciencias Sociales (UNLP-CONICET); Argentina.