Nd and Sr isotope record of ODP Sites 121-756 and 122-762 sediments

We have analyzed the Nd isotopic composition of both ancient seawater and detrital material from long sequences of carbonated oozes of the South Indian Ocean which are ODP Site 756 (Ninety East Ridge (-30°S), 1518 m water depth) and ODP Site 762 (Northwest Australian margin, 1360 m water depth). The measurements indicate that the epsilon-Nd changes in Indian seawater over the last 35 Ma result from changes in the oceanic circulation, large volcanic and continental weathering Nd inputs. This highlights the diverse nature of those controls and their interconnections in a small area of the ocean. These new records combined with those previously obtained at the equatorial ODP Sites 757 and 707 in the Indian Ocean (Gourlan et al., 2008, doi:10.1016/j.epsl.2007.11.054) established that the distribution of intermediate seawater epsilon-Nd was uniform over most of the Indian Ocean from 35 Ma to 10 Ma within a geographical area extending from 40°S to the equator and from -60°E to 120°E. However, the epsilon-Nd value of Indian Ocean seawater which kept an almost constant value (at about -7 to -8) from 35 to 15 Ma rose by 3 epsilon-Nd units from 15 to 10 Ma. This sharp increase has been caused by a radiogenic Nd enrichment of the water mass originating from the Pacific flowing through the Indonesian Passage. Using a two end-members model we calculated that the Nd transported to the Indian Ocean through the Indonesian Pathway was 1.7 times larger at 10 Ma than at 15 Ma. The Nd isotopic composition of ancient seawater and that of the sediment detrital component appear to be strongly correlated for some specific events. A first evidence occurs between 20 and 15 Ma with two positive spikes recorded in both epsilon-Nd signals that are clearly induced by a volcanic crisis of, most likely, the St. Paul hot-spot. A second evidence is the very large epsilon-Nd decrease recorded at ODP Sites 756 and 762 during the past 10 Ma which has never been previously observed. The synchronism between the epsilon-Nd decrease in seawater from 10 to 5 Ma and evidences of desertification in the western part of the nearly Australian continent suggests enhanced weathering inputs in this ocean from this continent as a result of climatic changes.

A BRDF-BPDF database for the analysis of Earth targets reflectances

A database of representative BRDF and BPDF derived from the POLDER measurements. From the huge amount of data acquired by the spaceborne instrument over a period of 7 years, we selected a set of targets with high quality observations. The selection aimed at a large number of observations, free of cloud or aerosol contamination, acquired in diverse observation geometry with a focus on the backscatter direction that shows the specific Hot-Spot signature. The targets are sorted according to the 16-classes IGBP land cover classification system and the target selection aims at a spatial representativeness within the class. The database thus provides a set of high quality BRDF and BPDF samples that can be used to assess the typical variability of natural surface reflectances or to evaluate models.

Geochemistry in the mid Atlantic Ocean ridge basalts

The main objective of Leg 82 of the Glomar Challenger was to document mantle heterogeneity in the vicinity of, and away from, a so-called hot spot: the Azores Triple Junction. One of the geochemical tools that permits, at least in part, the recognition of mantle heterogeneities uses hygromagmaphile elements, those elements that have an affinity for the liquid. This tool is presented in terms of an extended Coryell-Masuda plot, which incorporates within the rare earth elements the hygromagmaphile transition elements Th, Ta, Zr, Hf, Ti, Y, and V. The extended Coryell-Masuda plot is used to summarize our knowledge of mantle heterogeneity along the ridge axis at zero-age. It is also used by choosing those hygromagmaphile elements that can be analyzed on board by X-ray fluorescence spectrometry to give preliminary information on the enriched or depleted character of recovered samples. Shore-based results, which include analyses of most of the hygromagmaphile elements measured either by X-ray spectrometry or neutron activation analysis, confirm the shipboard data. From the point of view of comparative geochemistry, the variety of basalts recovered during Leg 82 provides a good opportunity to test and verify the classification of the hygromagmaphile elements. Analyses from Leg 82 provide new data about the relationship between extended rare earth patterns (enriched or depleted) that can be estimated either by La/Sm ratio or Nb/Zr (or Ta/Hf) ratios: samples from Hole 556 are depleted (low Nb/Zr ratio) but have a high 206Pb/ 204Pb (19.5) ratio; in Hole 558 a moderately enriched basalt unit with a La/Sm (= Nb/Zr) ratio (chondrite normalized) of 2 has a high 206Pb/204Pb (20) ratio. One of the most interesting results of Leg 82 lies in the crossing patterns of extended Coryell-Masuda plots for basalts from the same hole. This result enhances the notion of local mantle heterogeneity versus regional mantle heterogeneity and is confirmed by isotope data; it also favors a model of short-lived, discrete magma chambers. The data tend to confirm the Hayes Fracture Zone as a southern limit for the influence of Azores-type mantle. Nevertheless, north of the Hayes Fracture Zone, the influence of a plumelike mantle source is not simple and probably requires an explanation more complex than a contribution from a single fixed hot spot.

Chemical composition of igneous rocks and origin of the sill and pillow-basalt complex at DSDP Site 61-462

The sill and pillow complex cored on Deep Sea Drilling Project Leg 61 (Site 462) is divided into two groups, A and B types, on the basis of chemical composition and volcanostratigraphy. The A-type basalt is characterized by a higher FeO*/MgO ratio and abundant TiO2, whereas the B-type basalt is characterized by a lower FeO*/MgO ratio and scarcity of TiO2. The A type is composed of sills interbedded with hyaloclastic sediments, and the B type consists of basalt sills and pillow basalt with minor amounts of sediment. However, the structure of pillow basalts in the B type is atypical; they might be eruptive. From paleontological study of the interbedded sediments and radiometric age determination of the basalt, the volcanic event of A type is assumed to be Cenomanian to Aptian, and that of B type somewhat older. The oceanic crust in the Nauru Basin was assumed to be Oxfordian, based on the Mesozoic magnetic anomaly. Consequently, two events of intraplate volcanism are recognized. It is thus assumed that the sill-pillow complex did not come from a normal oceanic ridge, and that normal oceanic basement could therefore underlie the complex. The Site 462 basalts are quartz-normative, and strongly hypersthene-normative, and have a higher FeO*/MgO ratio and lower TiO2 content. Olivine from the Nauru Basin basalts has a lower Mg/(Mg + Fe**2+) ratio (0.83-0.84) and coexists with spinel of lower Mg/(Mg + Fe**2+) ratio when compared to olivine-spinel pairs from mid-ocean ridge (MAR) basalt. The glass of spinel-bearing basalts has a higher FeO*/(FeO* + MgO) ratio (0.58-0.60) than that of MAR (<0.575). Therefore, the Nauru Basin basalts are chemically and mineralogically distinct from ocean-ridge tholeiite. That the Nauru Basin basalts are quartz-normative and strongly hypersthene-normative and have a lower TiO2 content suggests that the basaltic liquids of Site 462 were generated at shallower depths (<5 kbar) than ocean-ridge tholeiite: Site 462 basalts are similar to basalts from the Manihiki Plateau and the Ontong-Java Plateau, but different from Hawaiian tholeiite of hot-spot type, with lower K2O and TiO2 content. We propose a new type of basalt, ocean-plateau tholeiite, a product of intraplate volcanism.

Mega-epibenthos at Bouvet Island (South Atlantic)

Mega-epibenthic diversity was analysed using a seabed photography at four stations off Bouvet Island and one station at the Spiess Seamount in the South Atlantic. Surprisingly, the intermediate-scale diversity within the area of investigation was not lower compared to that on the Patagonian shelf and only moderately lower than that on the Antarctic continental shelf. This result is incompatible with Mac Arthur and Wilson's Island Biogeography Theory describing species richness as a function of immigration of new species into an area and its extension. The relatively high species number and the very small extension of the Bouvet shelf compared to the much larger continental shelves of the other two areas can be explained by long-range dispersal of marine benthic animals in the Antarctic Circumpolar Current and high habitat heterogeneity. The observed uncoupling of intermediate-scale from large-scale background species diversity on the Antarctic shelf raises the question whether in these benthic systems an upper capacity limit for diversity exists.

Conventional and 40Ar/39Ar K-Ar ages of volcanic rocks at DSDP Leg 55 Holes

Conventional K-Ar, 40Ar/39Ar total fusion, and 40Ar/39Ar incremental heating data on hawaiite and tholeiitic basalt samples from Ojin (Site 430), alkalic basalt samples from Nintoku (Site 432), and alkalic and tholeiitic basalt samples from Suiko (Site 433) seamounts in the Emperor Seamount chain give the following best ages for these volcanoes: Ojin = 55.2 ± 0.7 m.y., Nintoku = 56.2 ± 0.6 m.y., and Suiko = 64.7 ± 1.1 m.y. These new data bring to 27 the number of dated volcanoes in the Hawaiian-Emperor volcanic chain. The new dates prove that the age progression from Kilauea Volcano on Hawaii (0 m.y.) through the Hawaiian-Emperor bend (- 43 m.y.) to Koko Seamount (48.1 m.y.) in the southernmost Emperor Seamounts continues more than halfway up the Emperor chain to Suiko Seamount. The age versus distance data for the Hawaiian-Emperor chain are consistent with the kinematic hot-spot hypothesis, which predicts that the volcanoes are progressively older west and north away from the active volcanoes of Kilauea and Mauna Loa. The data are consistent with an average volcanic propagation velocity of either 8 cm/year from Suiko to Kilauea or of 6 cm/year from Suiko to Midway followed by a velocity of 9 cm/year from Midway to Kilauea, but it appears that the change in direction that formed the Hawaiian- Emperor bend probably was not accompanied by a major change in velocity.