A simple photometer for precise determination of dissolved oxygen concentration by the Winkler method with recommendations for improving respiration rate measurements in aquatic organisms

A simple inexpensive photometer designed for Winkler titration end-point detection is described. The precision of replicate dissolved oxygen measurements using this instrument was 0.06–0.22%. This high precision is needed to measure the small changes in dissolved oxygen concentration for determining the respiration rate of small aquatic organisms. Other recommendations for improving precision are also presented.     

Paleomagnetism of the triassic chugwater redbeds revisited (Wyoming, U.S.A.)

Paleomagnetic results from 107 samples of the Chugwater Group near Lander, Wyoming, show a regular progression in pole positions from bottom to top of the sequence. This pole position trend of about 25° matches very well the North American apparent polar wander path between Early Permian and Early Triassic. It could be argued that this “agreement” results in a conflict between the apparent magnetic age (Permian) and the Early to Late Triassic age generally assigned to the Chugwater Group. However, similar progressions of paleomagnetic pole positions have been reported for the Early Triassic Moenkopi Formation in Colorado; thus it appears that long-term variations and swings characterized the geomagnetic field at that time. With detailed paleomagnetic sampling, these features can be utilized for stratigraphic correlation in addition to magnetic-reversal stratigraphy. This will eliminate, to some degree, part of the non-uniqueness inherently present in correlations based on reversal stratigraphy only.     

A shale buchite xenolith with Al-armalcolite and native iron in a lava from Asuk,Disko, central West Greenland

A native iron bearing buchite xenolith enclosed in basalt in a native iron bearing lava from Disko is similar in chemistry, apart from volatile components, to unheated Mesozoic to early Tertiary carbonaceous shale sediments from West Greenland, believed to have been the major contaminant and reducing agent causing the formation of the native iron in the volcanic rocks on Disko. The shale buchite has preserved original sedimentary lamination and experienced only a comparatively short-lived heating which allowed only short-range equilibration. The xenolith contains native iron and graphite, while the immediately enclosing basalt does not, and demonstrates the indigenous reducing properties of the shales when heated to high (1,150–1,200 ° C) temperatures at low pressures. Sedimentary clastic iron titanium oxide grains have reacted with the immediately surrounding matrix and completely recrystallized to aggregates of rutile and Al-armalcolite with up to 24 mol% Al₂TiO₅. The Al-armalcolites contain 5 to 11 mol% of the anosovite (Ti₃O₅) component. They show a considerable variation in iron-magnesium ratio due to small-scale variations in bulk chemistry to varying f _O2 and to the effects of progressive melting of the sediment. Glassy veins in the buchite, formed along original fractures in the sediment, contain an assemblage of cordierite, low Ca-pyroxene, plagioclase, ilmenite, ferropseudobrookite-rich armalcolite, iron metal, troilite and acid glass, and crystallized under higher oxygen fugacities than the buchite interior, probably due to influx from the less reduced enclosing basalt magma.     

Inversion of magnetic anomalies and sea-floor spreading in the Cayman Trough

For some time, sea-floor spreading has been hypothesized for the Mid-Cayman Rise based on inferences from seismicity, heat flow, topography and plate geometry. Here we present magnetic anomaly inversions from which a reasonable record of sea-floor spreading emerges. We obtain total opening rates of 20 ± 2 mm/yr for 0–2.4 m.y. B.P. and 40 ± 2 mm/yr for 2.4–6.0 m.y. B.P. Data on the west flank extend the half-opening rate of 20 mm/yr back to 8.3 m.y. B.P. Spreading has been very nearly symmetric. These new observations place important constraints on plate tectonic reconstructions by defining the relative motion between the North American and Caribbean plates. They also shed some light on sea-floor spreading processes in which the spreading center is a secondary feature in the sense that it is over an order of magnitude shorter than the adjoining transform faults.     

Evolution of rifted ocean ridges

A major question in seafloor tectonics has been, how does the 2-km-deep rift valley characteristic of slow-spreading ridges evolve into the relatively horizontal undulating relief of the rift mountains? Deep-tow studies of the Mid-Atlantic Ridge suggest that the primary mechanism for transformation of the rift valley topography is normal faulting along fault planes which dip away from the valley axis. The faulting occurs in a narrow zone just beyond the outer walls of the rift valley. This model allows for a steady-state evolution of the rift valley into the rift mountains in which the state of stress in the oceanic lithosphere continues to be in horizontal deviatoric tension throughout the entire process. Alternate mechanisms involving reverse faulting or regional tilt may be active but are found to be of less importance. Implications for various dynamic models of the rift valley are discussed.