(Table 1) Textural characteristics of sediments at DSDP Leg 71 Holes

In conjunction with a study of ice-rafted detritus (IRD), textural analyses were carried out on Miocene to Quaternary sediments at Sites 511, 512, 513, and 514. Grain-size statistics were computed for the <62.5 µm fraction in order to identify changes in current velocity; the sand fraction was omitted from these determinations because it consists primarily of ice-raft and biogenic components. The contributions by terrigenous sediment in the 62.5-250 µm and the >250 µm fractions were determined.

Geochemistry of metasedimentary units of the Cambro-Ordovician Ross Orogen in North Victoria Land and Oates Land, Antarctica

Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately mature shales. Highly mature lithotypes with Chemical Index of Weathering values of >=95 are typically absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very homogeneous series lacking significant compositional variations. Major variations are only found in chemical parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical sorting of the detritus. Geochemical data, 87Sr/ 86Sr t=490 Ma ratios of 0.7120 - 0.7174, epsilonNd, t=490 Ma values of -7.6 to -10.3 and single-stage Nd-model ages of 1.7 - 1.9 Ga are indicative of an origin from a chemically evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for significant sedimentary infill from primitive "ophiolitic" sources. Metasediments of the Middle Cambrian Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace element data and Sm-Nd isotope data (epsilonNd, t=500 Ma values of -14.3 to -1.2 and single-stage Nd-model ages of 1.7 - 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor rocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from more western inboard areas of the Wilson Terrane (epsilonNd, t=510 Ma -7.0 to -14.3) indicate a relatively high proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age. Metasedimentary series in an eastern, more outboard position (epsilonNd, t=510 Ma -5.4 to -10.0; single-stage Nd model ages 1.4 - 1.9) on the contrary document stronger influence of a more primitive source with younger formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope geochemistry for an island-arc component in these series.

Nd and Sr isotope compositions, REE concentrations and Al/Ca ratios of different phases of surface sediments in the South Pacific

The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe-Mn coatings, "unclean" foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core-tops) covering the entire midlatitude South Pacific. Comparison of detrital Nd isotope compositions to deep water values from the same locations suggests that "boundary exchange" has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of "unclean" planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and "nondecarbonated" leachates of bulk sediment Fe-Mn oxyhydroxide coatings also agree with "unclean" foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyr, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases toward the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.

Mineralogy of Hawaiian Arch sediments

The mineralogy of both bulk- and clay-sized (<2 µm) fractions of sediments from Holes 842A and 842B of Ocean Drilling Program Leg 136 was determined by X-ray diffraction. The sediments consist of a combination of terrigenous (quartz, plagioclase, smectite, illite, kaolinite, and chlorite), volcaniclastic (augite, plagioclase, and volcanic glass), and diagenetic minerals (smectite, phillipsite, clinoptilolite, and opal-CT). Although biogenic silica (radiolarians and diatoms) is common in near-seafloor (<10 mbsf) sediments, biogenic calcite is rare. Variations with depth in abundances of the terrigenous minerals reflect temporal changes in the flux of eolian material to the site. Volcanogenic material derived from the Hawaiian Islands is present in lithologic Unit 1 (0-19.9 meters below seafloor) both as discrete layers and as finely disseminated silt- and clay-sized material. Volcanic glass is present only in the upper 10 m of the sediment column. In Unit 2 (19.9-35.7 mbsf), increased smectite and zeolite abundances with depth as well as indurated, zeolite-rich layers are thought to be the alteration products of volcanogenic material. The source of this older (late Oligocene to middle Miocene) volcanogenic detritus may be continental volcanism. Microfabrics imaged using back-scattered electron imaging reflect the effects of compaction and diagenesis on sediment porosity and matrix structure. As porosity decreases during burial, the matrix changes from an open, floc-like fabric, to an interlocking network of clay mineral domains, and finally to a dense intergrowth of clay minerals and zeolites. Despite the substantial changes in sediment microfabric and mineralogy, correlations between physical and acoustic properties and mineralogy are weak or absent. The sediment has maintained high porosity (>70%), and water content appears to dominate the sediment's physical character and acoustic response.