Carbon isotope and magnetic polarity evidence for nondepositional events within the Cambrian-Ordovician Boundary section near Dayangcha, Jilin Province, China

Carbon isotope and magnetic polarity stratigraphic results from the Cambrian-Ordovician Boundary section at Xiaoyangqiao, near Dayangcha, Jilin Province, China, in comparison to a contemporaneous section at Black Mountain, Australia, indicate strata equivalent to major portions of the Australian sequence are either absent or are restricted to highly condensed intervals. These intervals are correlative with regressive sea level events identified in Australia and western North America, suggesting regional or eustatic sea level changes strongly influenced deposition of the Xiaoyangqiao sequence. These results also suggest the Xiaoyangqiao section is unfavourable as the site of the Cambrian-Ordovician Boundary Global Stratotype Section and Point

A Case for Catastrophic True Polar Wander in the Cambrian

Of all the periods in the Phanerozoic time scale, the Cambrian system is the most perplexing. Several lines of evidence which together suggest that the Cambrian Earth may have experienced a large, rapid episode of true polar wander (TPW) include: (1) The Cambrian is short. Once thought to have a duration approaching 100 Ma, stratigraphic Rb/Sr and U/Pb geochronometry now suggests a time span of about 20 Ma. (2) Lithostratigraphic analyses suggest that Northern Africa went from high to low and back to high latitudes within this time interval. North America at least moved into the carbonate belt during the early Cambrian. (3) Improvements in biostratigraphy, magnetostratigraphy, and carbon isotopic stratigraphy around the Precambrian-Cambrian boundary now allow precise correlations to be made between strata in Siberia, China, Morocco, Australia, and with somewhat less precision to North America. Isochronous poles from Australia and Africa demonstrate that they were moving separately in the late Precambrian. (4) Paleomagnetic poles, derived from biostratigraphically-dated units, suggest nearly 90° of APW for many, if not all, of these continents during Cambrian time. We interpret these effects as an episode of TPW resulting from the collision of East and West Gondwana during the early Cambrian. This suturing event would stop subduction of a large equatorial plate dipping under Africa (responsible at least in part for the Pan-African metamorphic events). Thermal warming of the orphaned slab then removes its mass anomaly, causing the magnitudes of the minimum and intermediate eigenvectors of the total earth moment of inertia tensor to switch, leading to a 90° TPW reorientation of the crust. The Cambrian sea-level transgression may then be the result of moving a large ocean basin from the polar region onto the equatorial buldge. Similarly, the enormously accelerated rates of organic evolution observed during Cambrian time may be the result of paleoenvironmental dislocation driven by TPW

Magnetostratigraphic investigations of the lower paleozoic system boundaries, and associated paleogeographic implications

Continued refinement of a global Geologic Timescale solely through increased precision of biostratigraphic correlations philosophically suffers from the inherent lack of a universal reference frame. Geomagnetic polarity reversals, which occur relatively rapidly and simultaneously on a global scale, can provide the necessary universal reference frame, provided the polarity reversals are correlated within a well-defined biostratigraphic framework and occur with a fairly distinctive pattern. Magnetostratigraphic correlations across the Cambrian-Ordovician boundary interval indicate that normal polarity zones correlative to Late Cambrian conodont zones occur within sections from Texas, northern China, western Newfoundland, central Australia, and possibly Kazakhstan. These correlations strongly suggest that temporal differences may exist between sections in the absolute time value of key biostratigraphic horizons. There may also be very brief normal polarity zones correlative with Early Ordovician conodont and graptolite zonations, but those relationships have not yet been well-established. Magnetostratigraphic correlations allow polarity to be unambiguously determined for the relevant continental unit, even in the absence of previous paleomagnetic investigation. Extension of this to Late Cambrian and Early Ordovician paleogeographic problems indicate that North China, and probably also South China, underwent approximately 90° counterclockwise rotation during the Cambrian, and were most likely attached to or very near the present northern margin of Australia during that time. Paleomagnetic results from Upper Silurian through Middle Devonian carbonates of the Barrandian area, Czechoslovakia have at least three components of magnetization preserved within them. Two of the components appear to pass the fold test, indicating that they pre-date the deformation creating the basin, constrained to be not later than Late Carboniferous. Differences between the two components probably correspond to different times of acquisition, and may record rapid plate motion of the Bohemian Massif during the Middle Paleozoic. Paleomagnetic results from Upper Ordovician to Lower Silurian carbonates from Anticosti Island, Quebec are not reliable because of the extremely weak magnetization of these rocks. Sharp increases in intensity during thermal demagnetization experiments may provide insight into the chemical changes which occur within carbonate rocks during thermal demagnetization, but at the present time those phenomenon are not well understood.</p

Paleomagnetic results from the Cambrian-Ordovician boundary section at Black Mountain, Georgina Basin, western Queensland, Australia

Zones of alternating magnetic polarity have been identified throughout the Cambrian-Ordovician sequence at Black Mountain, and can be presumed to record geomagnetic field reversals during or immediately after deposition. A strong correlation can be made to polarity zones recognized at Dayangcha, northeastern China, a candidate site for establishment of the Cambrian-Ordovician boundary Global Stratotype Section and Point. Polarity zones associated with the Hispidodontus discretus and Hirsutodontus simplex Assemblage-Zones at Black Mountain are absent at Dayangcha, suggesting hiatuses at these levels in the Dayangcha sequence. Secondary components preserved in the Black Mountain section may provide temporal constraints on Middle Paleozoic diagenetic events in the Burke River Structural Belt

Very low doses of heavy oxygen ion radiation induce premature ovarian failure

Astronauts are exposed to charged particles during space travel, and charged particles are also used for cancer radiotherapy. Premature ovarian failure is a well-known side effect of conventional, low linear energy transfer (LET) cancer radiotherapy, but little is known about the effects of high LET charged particles on the ovary. We hypothesized that lower LET (16.5 keV/µm) oxygen particles would be less damaging to the ovary than we previously found for iron (LET = 179 keV/µm). Adult female mice were irradiated with 0, 5, 30 or 50 cGy oxygen ions or 50 cGy oxygen plus dietary supplementation with the antioxidant alpha lipoic acid (ALA). Six-hour after irradiation, percentages of ovarian follicles immunopositive for γH2AX, a marker of DNA double strand breaks, 4-HNE, a marker of oxidative lipid damage and BBC3 (PUMA), a proapoptotic BCL-2 family protein, were dose dependently increased in irradiated mice compared to controls. One week after irradiation, numbers of primordial, primary and secondary follicles per ovary were dose dependently decreased, with complete absence of follicles in the 50 cGy groups. The ED50 for primordial follicle destruction was 4.6 cGy for oxygen compared to 27.5 cGy for iron in our previous study. Serum FSH and LH concentrations were significantly elevated in 50 cGy groups at 8 week. Supplementation with ALA mitigated the early effects, but not the ultimate depletion of ovarian follicles. In conclusion, oxygen charged particles are even more potent inducers of ovarian follicle depletion than charged iron particles, raising concern for premature ovarian failure in astronauts exposed to both particles during space travel