Neogene Radiolarians from the Kerguelen Plateau

Abundant, generally well-preserved radiolarians from Sites 737, 744, 745, 746, 747, 748, and 751 were used in stratigraphic analysis of Neogene, and particularly middle Miocene to Holocene, Kerguelen Plateau sediments. The composite Kerguelen section is more complete than the Weddell Sea sections recovered by Leg 113, and the radiolarians are better preserved. Leg 113 radiolarian zonations of Weddell Sea sites were applicable with only slight modification, and three new zones - Siphonosphaera vesuvius, Acrosphaeral labrata, and Amphymenium challengerae - are proposed for the latest Miocene. Geologic age estimates are given for all radiolarian zones used. Major hiatuses affecting most sites were seen within the middle Miocene, in the latest Miocene, and latest Pliocene. Five new species are described: Acrosphaera? labrata, Acrosphaera? mercurius, Siphonosphaera vesuvius, Actinomma? magnifenestra, and Helotholus? haysi.

Upper Triassic nannofossils from Wombat Plateau, Northwest Australia

A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofossils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagenetic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated by Prinsiosphaera triassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, with Crucirhabdus primulus as the ancestor. This species gave rise to C. minutus and Archaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences of C. primulus and Eoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphometric changes in this time-interval which possess biostratigraphic utility: (1) P. triassica increases in diameter from an average of 6 µm to over 9 µm; (2) E. zlambachensis evolves from a stubby to an elongated shape; and (3) C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).

Quaternary oxygen isotope record of planktonic foraminifera from ODP Site 805 on the Ontong Java Plateau

The oxygen isotope records of G. sacculifer and Pulleniatina in the uppermost three cores at Ocean Drilling Program Hole 805C span the last 1.6 m.y., an estimate based on Fourier stratigraphy. The last 700,000 yr are dominated by both eccentricity and obliquity-related orbital fluctuations. The range of variation of delta18O values is about 1.5?, of which ca. 75% may be assigned to global ice-volume effect. The remainder of the range is shared by the effects of surface temperature variation, thermocline depth change (in the case of Pulleniatina, especially), and differential dissolution. Before 1 Ma, obliquity-related fluctuations dominate. The transition between obliquity- and eccentricity-dominated time occurs between ca. 1 and 0.7 Ma. It is marked by irregularities in phase relationships, the source of which is not clear. The age of the Brunhes/Matuyama boundary is determined as 794,000 yr by obliquity counting. However, an age of 830,000 yr also is compatible with the counts of both eccentricity and obliquity cycles. In the first case, Stage 19 (which contains the boundary) is coincident with the crest of the 19th obliquity cycle, setting the first crest downcore equal to zero, and counting backward (o19). In the second, Stage 19 coincides with o20. No evidence was found for fluctuations related to precession (23 and 19 k.y.) rising above the noise level, using plain Fourier expansion on the age model of the entire series. Detailed stratigraphic comparison with the Quaternary record of Hole 806B allows the recognition of major dissolution events (which increase the difference in delta18O values of G. sacculifer at the two sites). These occur at Stages 11-13, 16-17, and near 1.5 Ma (below o33).

High-resolution multi-proxy record of the last glacial period from Lake Van, eastern Anatolian high plateau, Turkey

Detailed analyses of the Lake Van pollen, Ca/K ratio and stable oxygen isotope record allow the identification of millennial-scale vegetation and environmental changes in eastern Anatolia throughout the last glacial (~75-15 ka BP). The climate within the last glacial was cold and dry, with low arboreal pollen (AP) levels. The driest and coldest period corresponds to Marine Isotope Stage (MIS) 2 (~28-14.5 ka BP) dominated by the highest values of xerophytic steppe vegetation. Our high-resolution multi proxy record shows rapid expansions and contractions of tree populations that reflects variability in temperature and moisture availability. This rapid vegetation and environmental changes can be linked to the stadial-interstadial pattern of the Dansgaard-Oeschger (DO) events as recorded in the Greenland ice cores. Periods of reduced moisture availability were characterized by enhanced xerophytic species and high terrigenous input from the Lake Van catchment area. Furthermore, comparison with the marine realm reveals that the complex atmosphere-ocean interaction can be explained by the strength and position of the westerlies, which is responsible for the supply of humidity in eastern Anatolia. Influenced by diverse topography of the Lake Van catchment, larger DO interstadials (e.g. DO 19, 17-16, 14, 12 and 8) show the highest expansion of temperate species within the last glacial. However, Heinrich events (HE), characterized by highest concentrations of ice-rafted debris (IRD) in marine sediments, are identified in eastern Anatolia by AP values not lower and high steppe components not more abundant than during DO stadials. In addition, this work is a first attempt to establish a continuous microscopic charcoal record over the last glacial in the Near East, which documents an initial immediate response to millennial-scale climate and environmental variability and enables us to shed light on the history of fire activity during the last glacial.

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.

Chemical composition of basalts from ODP holes at the Kerguelen Plateau

Petrographic and geochemical study of basalts in the Kerguelen Plateau basement revealed changes in composition and character of volcanism during development of this tectonovolcanic structure. The Kerguelen Plateau is one of the largest intraplate rises in the World Ocean. It started to form about 120 Ma ago. Age of basalts and overlying sediments shows that plateau formation was in the northwest direction. Basalts of the Kerguelen Plateau basement are products of tholeiitic melts in terms of geochemistry, but differ from mid-ocean ridge basalt (MORB). They are enriched in incompatible trace elements and rare earth elements (REE) relative to MORB, and degree of enrichment varies in basalts from different segments of the plateau. Composition of basalts does not directly depend on their age. Specific features of plateau magmatism are commonly explained in terms of a long-living deep magma plume, which variously interacted with a depleted upper mantle source at different stages of plateau formation. However, taking into account block morphology and deep structure of the plateau, one can suggest that plateau volcanism was initiated by a large fault. As the volcanism prograded to the northwest, depth of fault penetration into the mantle changed. Composition of basalts in the plateau basement was also governed by formation depth of primary melts.

Geochemistry of Kerguelen-Heard Plateau basalts

During Leg 120 basalts were recovered at four drill holes on the Kerguelen-Heard Plateau. This paper reports the trace element and Sr, Nd, Hf, and Pb isotopic characteristics of these basalts and compares these basalts with Indian Ocean basalts and Kerguelen and Heard island volcanics. Kerguelen-Heard Plateau basalts are extremely heterogeneous in character. Intersite variations are larger than intrasite variations. Part of the chemical variations of the plateau volcanics overlap with those characteristics of Kerguelen Island volcanics, which indicates tapping of the same mantle source during the two different periods of activity. The estimates of the degree of melting for the plateau basalts (smaller degree of melting than for mid-ocean ridge basalts) and the heterogeneous character of the plateau exclude an origin that requires large degrees of melting or more rigorous convection than at ocean ridges. However, all characteristics indicate an oceanic origin for the Kerguelen-Heard Plateau.