Chemical element profiles by instrumental neutron activation analysis : in, part 1, two species of wheat bunt spores, Tilletia caries (DC) Tul. and Tilletia controversa Kühn; part 2, representative sediment and basalt samples taken from a DSDP 678 m core, site 525A, Leg 74, Walvis Ridge Public Deposited

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  • Part 1 The concentration of seventeen elements in two species of fungus which cause wheat bunt disease, Tilletia caries (DC.)Tul. (TS) and Tilletia controversa Kiihn (DS), were determined by instrumental neutron activation analysis in 37 TS spore samples, and 31 DS spore samples. Aluminum was chosen as a soil contamination indicator to correct for soil contamination. The plot of the concentrations of the ith element [X[subscript i]] versus Al, yielded the biological concentrations of [X.[subscript i]]. The results show that the biological concentrations of Sc, V, La, and Sm are insignificantly small and that their contents in the spores are essentially all derived from soil dust contamination. For Na and Fe, considerable fractions, 0.15 and 0.60, respectively, of their total concentrations are derived from soil contamination. For other elements, the soil contamination contributions are relatively small compared to their biological concentrations. The "student" t-test was used for comparisons of the geometric means of the element concentrations between the TS and DS spore series. The differences between the mean values of Cl, K, Ca, Mn, Zn, and Br for the TS and CS series are not totally due to random errors within the 95% confidence level. The differences for K and Cl between the TS and DS series are large and outside the ±10- limits; therefore, the concentrations of these two elements can be used as reliable criteria for distinguishing these two species. Also, Br may be useful as a diagnostic trace element due to the significant difference between the Br geometric means of the TS and DS spores. Part 2 Forty sediment and four basement basalt samples, taken from a 678 m core drilled by the DSDP (Deep Sea Drilling Project) at Site 525A, Leg 74 (June 10-15, 1980), as well as sixteen selected basalt samples around the south Atlantic Ocean were subjected to instrumental neutron activation analysis. Thirty-two major, minor, and trace elements were determined. The core from the Wavlis Ridge site (2467 m) consisted of 574.6 m of sediment and 103.5 m of basalt. The downcore element concentration profiles and regression analyses show that the rare earth elements (REE) are present in significant amounts in both the carbonate and non-carbonate phases in sediments; Sr is concentrated in the carbonate phase; most of the other elements determined exist mainly in the non-carbonate (mostly clay) phase. The calculated partition coefficients of the REEs between the carbonate phase and the free REE ion concentrations in sea water were high and increased with decreasing REE ionic radii or increasing atomic number from 3.9x10⁶ for La to 15x10⁶ for Lu. Using the partition coefficients of the REEs in the carbonate and non-carbonate (clay) phases, the REE concentrations in Atlantic sea water were calculated, and the results indicate that the lanthanide concentrations have not been changed significantly in south Atlantic sea water over the past 70 m.y.. The Ce anomaly observed in >95% carbonate sediments is related to the Ce⁺³ concentration in sea water; therefore, the Ce anomaly is a redox (reducing-oxidizing) indicator of sea water. (Essentially, >99.99% of soluble Ce in sea water is present as Ce⁺³.) The REE patterns show no Ce depletion in mollusc shell segments from the late Campanian, and a slight Ce depletion in carbonate phases from the late Paleocene sediments. From early Eocene on, the REE patterns in the carbonate phase show a marked Ce depletion, the same as is observed in carbonates from the late Pleistocene to early Holocene (about 0.3 m.y. ago). The abrupt and striking change in the Ce depletion indicates that sea water was anoxic over the Walvis Ridge during the late Campanian. As the gap between northeast South America (northeast Brazil) and west central Africa (near the southwest corner of the upper half of Africa, e.g., near Liberia) widened and the Walvis Ridge subsided between the late Campanian and late Paleocene, sea water flow between the north Atlantic and south Atlantic oceans increased steadily, and gradually flushed out the anoxic water layer over the Walvis Ridge, thereby achieving oxidation conditions at about 54 m.y. ago, that are similar to present day sea water redox conditions: in the world oceans. The chemical compositions of the basement rocks of the core corresponds to alkalic basalts, not MA-Crean Ridge basalts (MORBs). Only a few basement rocks had been recovered from other Walvis Ridge sites. The results add more evidence which supports the hypothesis that the Wavlis Ridge was formed by a series of volcanoes moving over a "hot spot" near the Mid-Atlantic Ridge. These volcanoes migrated eastward as the South American and African continents drifted away. Most of the basalt samples taken from the southeast Brazilian continental margin also are not similar to MORE. From the bulk chemical composition and the REE pattern, one 112 m.y. old basalt has been identified as an early-stage MORB. To date, this is the oldest oceanic tholeiite recovered from the south Atlantic. This direct evidence indicates that the continental split between South America and Africa commenced ≥112 m.y. ago, and is consistent with the suggestion that the rift between the two continents began about 125 m.y. ago.
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