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Biogeochemistry of Late Paleozoic North American brachiopods and secular variation of seawater composition (1989)

Brand, Uwe

Springer

Biodegradation 7 (1989), S. 159-193

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ISSN: 1572-9729

Keywords: Brachiopod biogeochemistry ; diagenesis ; secular isotopic variations ; carbon isotopes ; oxygen isotopes ; Sr/Ca ; temperature ; Recent ; Late Paleozoic

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition

Notes: Abstract Sr/Ca ratios in modern brachiopod shells reflect variations in ambient seawater, whereas their Na contents show no relationship with water depth or habitat. Their Mn and Fe contents are controlled, in part, by leaching of these elements from oxide coatings or the low input/sedimentation rate of detrital material into depositional areas such as Quatsino Sound. For most Carboniferous brachiopods from North America, the Mn and Fe contents are similar to those recorded by their Recent counterparts. The high Mn and Fe contents in the brachiopods from shales suggest several possibilities for these levels. One possibility is the leaching of Mn and Fe from oxide coatings/matrix which was not completely removed in the cleaning process, or the high levels in part reflect unusual depositional conditions (some degree of anoxia) for the local shaly environments. The Sr/Ca ratio of brachiopods and, by inference, complementary seawater, did not vary significantly during the Carboniferous. The Sr/Ca minimum observed in brachiopods of Mississippian age coincides with a dip in the 87Sr/86Sr curve and correlates with the Hercynian orogeny. This is attributed to the cycling of seawater through mid-ocean ridge basalts, and postulated exchange reactions account for variation in the composition of seawater-Ca. The unidirectional trend of heavier δ13C values from the Devonian to the Permian is intricately coupled with the evolution of the terrestrial biomass. In addition to expansion of terrestrial plants, burial of reduced carbon in the form of coal (organic matter) contributed to the observed shift. The start of the Permo-Pennsylvanian glaciation is marked by a negative excursion of the secular carbon trend, which is linked to weathering of reduced carbon and its return to the oceanic reservoir with its oxidized carbon. The oxygen isotope values reflect the unidirectional trend towards higher values of the carbon data with decreasing geologic age. Negative excursions of the trend may be related to extensive weathering of terrestrial and submarine rocks, whereas positive excursions may be related to hydrothermal alteration of submarine rocks and dehydration of oceanic crust during times of active sea-floor spreading. Oxygen-calculated water temperatures of unaltered brachiopod material are unrealistically high for all of the Devonian, and the Chesterian-Meramecian, Desmoinesian-Missourian, and Artinskian Epochs. During these times maximum water temperatures of 42° to 56°C are well above the thermal threshold of protein denaturation. This process, which is lethal to most higher organisms, demands an adjustment in oxygen of -2.5%. for samples older than Missourian, and of -1.250%. for samples spanning the Missourian-Artinskian interval. With these adjustments and salinity considerations made prior to calculations, water temperatures become reasonable for the Late Paleozoic epeiric, tropical seas of North America.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00004216

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Azo Bridges from Azines, XXIIIPart XXII. Ref.. 1,5-Laticyclic Conjugation Between Parallel Azo and o-Phenylene Bridges. Structure Dependence of [6 + 2] PhotocycloadditionsDedicated to Prof. Dr. J. Sauer on the occasion of his 65th birthday. (1996)

Beck, Karin ; Brand, Uwe ; Hünig, Siefried ; [et al.]

Weinheim : Wiley-Blackwell

Liebigs Annalen 1996 (1996), S. 1881-1892

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ISSN: 0947-3440

Keywords: Bridges, parallel o-phenylene and azo ; Dyotropic hydrogen transfer ; 1,5-Laticyclic conjugation ; [6 + 2] Photocycloaddition ; Photoelectron spectra ; Chemistry ; Organic Chemistry

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Chemistry and Pharmacology

Notes: Examples were synthesized of the four systems 1, 3, 5, and 7, in which rigid parallelo o-phenylene and azo bridges are connected to five- and/or six-membered carbocyclic moities. The o-phenylene bridge was introduced by two routes: (A) starting from precursors already containing that bridge (24, 29) and assembling the azo bridge in consecutive steps (→ 3a, 3b, 5c, 5d, 5e, 5f, 5g); (B) starting from the systems with parallel C=C/N=N bridges (9a, 11a, 13a, 42) and completing the dihydro-o-phenylene ring by tetrachlorothiopene dioxide. Dyotropic hydrogen transfer of the azo bridge enhances the dehydrogenation of the intermediate dihydro-o-phenylene derivatives (22, 3cH2, 25). This mechanism was proved by the domino hydrogen transfer 44 → 45 → 5h. Via route B, systems 1a, 1b, 3c, 3d, 5a, 5b, 5h, and 43 were obtained. In sharp contrast to the smooth [2 + 2] photocycloaddition of systems 9, 11, 13, and 15 (C=C/N=N bridges), [6 + 2] photocycloaddition occurs only with systems 1 and (5C/5N) and 3 (6C/5N) but not with systems 5 (5C/6N) and 7 (6C/6N). These differences are not caused by slightly varying distances of the two bridges (X-ray data) but by the higher n_ ionization energy of the azo group incorporated into a 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) instead of a 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) moiety, the hypsochromicity of the corresponding DBH n-π* state and the higher ground-state energy of DBH compared to DBO.

Additional Material: 3 Ill.