Volcanic rocks of the Witu Islands,Papua New Guinea: The origin of magmas above the deepest part of the New Britain Benioff zone

The Witu Islands are Quaternary volcanoes that overlie the deepest (about 300–580 km) part of the New Britain Benioff zone. The islands are about 100 km south of the transcurrent-divergent plate boundary that crosses the Bismarck Sea, and they surmount the southeastern end of the Willaumez-Manus Rise. The rocks are olivine- and quartz-normative tholeiitic basalts, low- and high-SiO₂ andesites, dacites, and rhyolites. Alkaline rocks that overlie the deep (greater than 300 km) parts of other Benioff zones have not been found in the Witu Islands. Compared to the Witu Islands rocks, those with similar SiO₂ contents from New Britain volcanoes that overlie progressively shallower parts of the Benioff zone to the south, are, for example, generally poorer in Na+K, Ti, and P, and higher in Ca and Al. There are similar progressive changes in trace-element abundances, but Zr and Nb contents are distinctly richer in Witu Islands rocks.⁸⁷Sr/⁸⁶Sr values range between 0.70311 and 0.7038, which are typical for rocks from New Britain as a whole and from other island arcs in the southwest Pacific. Two¹⁴³Nd/¹⁴⁴Nd values of 0.512211 and 0.512271, taken together with the Sr isotopic results, define a source region equivalent to those for oceanic-island basalts; there is no evidence for sea-water contamination of the sources. Perhaps the most striking feature of the Witu Islands rocks is their compositional diversity. Basalts range from olivine tholeiites similar to marginal-basin basalts from other areas, to quartz tholeiites similar in most respects to those typical of island arcs, and to incompatible-element-enriched tholeiites that are close to silica-undersaturation. Andesites on Unea Island have a strong island-arc signature, but the andesites, dacites, and rhyolite of Garove Island have some features that may be more in common with the silica-oversaturated rocks of oceanic areas. The mineralogy of Witu Islands basalts is characterised by phenocrysts of olivine (Fo_89-54), plagioclase (An_90-55) and Ca-augite. Cr-Al-rich spinels and aluminous magnetites are present as inclusions in some olivine phenocrysts. Groundmass fayalite, alkali feldspar, and dacitic to rhyolitic glasses high in K/Na are found in a few samples. In contrast to basalts from volcanoes above the shallower parts of the New Britain Benioff zone, those of the Witu Islands are characterised by rarity of low-Ca pyroxene, as phenocrysts or in the groundmass. Rocks richer in SiO₂ are characterised by the presence of orthopyroxene phenocrysts and lesser amounts of olivine. Hydrous minerals appear to be absent. Groundmass Fe-Ti oxides define crystallisation temperatures (about 800–1050°C) and oxygen fugacities (fO₂) corresponding to those of the Ni-NiO buffer, but up to two fO₂ log units above it. The suite as a whole is phenocryst-poor compared to most New Britain volcanic rocks. A significant degree of mantle heterogeneity is inferred by the chemical variability of the Witu Islands rocks. However, there are no compelling reasons in support of the interpretation that source heterogeneity is due to the effect of a slab-derived component. The cause of the heterogeneity is unclear, but may be due to mantle differentiation processes related to an anomalous tectonic setting.     

The origin of Island mammoths and the quaternary land bridge history of the Northern Channel Islands,California

Pygmy proboscidean remains of Mammuthus exilis occur abundantly in late Quaternary deposits on the Northern Channel Islands, California. On the assumption that ancestral elephants could not have swum to the islands and must therefore have walked out, various land bridges have been hypothesized that link the northern islands to the mainland by a peninsula. Geological evidence for a land bridge, however, is lacking, and new evidence shows that elephants are excellent swimmers and skilled at crossing watergaps. The Santa Barbara Channel was narrowed to only 6 km during glacially lowered sea levels. Modern elephants swim much further, and at speeds ranging from 0.96–2.70 km/hr. Motives for California elephants to cross Pleistocene watergaps are inferred from motives that lead modern elephants in Asia and Africa to cross watergaps. These are the visual and olfactory sensing of islands and of insular food during times of drought or fire-induced food shortage. Diminutive size of M. exilis principally reflects lack of island predators, an adaption to periodic food stress in a finite forage area affected by periodic drought and fire, and an adaptation for keeping population numbers high to maintain genetic variability and to ensure survival despite accidents. A late Quaternary scenario describes the environmental setting of the Santa Barbara Channel and the conditions that led to proboscidean dispersal to the preexistent super-island Santarosae.     

Strontium isotope ratios of quaternary volcanic rocks from Papua New Guinea

Strontium isotope data for Quaternary volcanic rocks from six separate areas in Papua New Guinea suggest a subdivision into two broad groups. One group consists of island volcanoes that have lower Sr³⁷/Sr³⁸ ratios (0.7034–0.7043) and show isotopic homogeneity within each area sampled. Except for the St. Andrew Strait samples, it is considered that these rocks were produced from relatively homogeneous source regions in the upper mantle. Volcanoes of the second group (on the Papua New Guinea mainland) have generally a higher and wider range of Sr³⁷/Sr³⁶ ratios, compared to those of the first group. It is thought that the magmas of these mainland volcanoes were affected by different degrees of sialic crustal contamination, or were derived from heterogeneous sources in the upper mantle.     

Greater India's place in Gondwanaland and in Asia

Evidence from the eastern Indian Ocean implies a Greater India in Gondwanaland. Translated without distortion to Asia, the northeast part of Greater India is found to correspond in shape with the Tibetan Plateau. With geophysical evidence, this suggests that the northeast part of Greater India underlies, presumably by underthrusting, the Tibetan Plateau.     

Paleopedology ofRamapithecus-bearing sediments,North India

The utility of using fossil soils in addition to other sedimentologic evidence in reconstructing past environments is considered with a preliminary analysis of the character of pedogenic alteration of selected floodplain deposits which have yielded the hominoid primatesRamapithecus andDryopithecus in the vicinity of Haritalyangar, District Bilaspur, H. P., India. The alluvial paleosols studied reflect variance of properties which are related to original depositional fabric modified by secondary pedogenic processes. The Nagri aged paleosols of Haritalyangar are ferruginous tropical soils or low-grade oxisols developed on typical floodplain toposequences, all reflecting somewhat varied histories as a function of proximity to ancient active stream courses.

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Zusammenfassung Es wird untersucht, inwieweit es für die Rekonstruktion des früheren Bildungsmilieus sinnvoll ist, in Verbindung mit anderen sedimentologischen Erscheinungen fossile Böden mit heranzuziehen, und zwar nach einer Analyse der Merkmale pedogenetischer Veränderungen in bestimmten Ablagerungen der Alluvialebenen in der Umgebung von Haritalyangar, Bilaspur, H. P., Indien, in denen die hominoiden PrimatenRamapithecus undDryopithecus gefunden worden sind. Die untersuchten alluvialen Paleoböden zeigen, verglichen mit der ursprünglichen Ablagerungsstruktur, Abweichungen in ihren Eigenschaften, die durch sekundäre pedogenetische Prozesse bestimmt sind.Die Paleoböden der Nagri-Stufe von Haritalyangar sind eisenreiche tropische Böden oder schwächere Oxiböden, die in den typischen Toposequenzen der Alluvialebenen entstanden sind und die aufgrund ihrer verschiedenen Entfernung zu alten Flußläufen alle eine etwas unterschiedliche Vergangenheit widerspiegeln.

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Résumé On considère ici la possibilité d'utiliser des sols fossiles en plus d'autres évidences sedimentologiques pour reconstruire les environnements du passé. On présente aussi une analyse préliminaire des caractéristiques de modifications pédogéniques de certains sédiments de plaines alluviales, qui ont livré les primates hominoïdesRamapithecus etDryopithecus, au voisinage de Haritalyangar, district de Bilaspur, H. P., Inde. Les paléosols alluviaux étudiés révèlent diverses propriétés caractéristiques de dé pôts originaux, modifiés par des processus pédogéniques secondaires. Les vieux paléosols de Nagri, aux environs de Haritalyangar, sont des oxisols peu évolués, formés sur des séquence topographique typique de plaines alluviales. Tous reflètent quelque peu des stages divers selon leur proximité d'anciens cours fluviatiles actifs.

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Estimation of Tigris-Euphrates streamflow from regional paleoenvironmental proxy data

A six thousand year history of streamflow in Mesopotamia is derived from a synthesis of regional paleoenvironmental proxy data. The proxy data are interpreted with consideration to the climatic signals represented in the records and to the temporal resolution of the records. A consideration of modern synoptic climatology suggests the spatial patterns of streamflow-generating precipitation in relation to atmospheric circulation. These patterns provide a framework for the interpretation of streamflow from the regional proxies. Given the nature of the data at hand, only a low frequency signal is reconstructed. Assessment of the role of small scale climatic fluctuations as a forcing function of population dynamics must await the availability of finer resolution environmental data.