What the ~1.4 Ga Xiamaling Formation can and cannot tell us about the mid-Proterozoic ocean.

Despite a surge of recent work, the evolution of mid-Proterozoic oceanic-atmospheric redox remains heavily debated. Constraining the dynamics of Proterozoic redox evolution is essential to determine the role, if any, that anoxia played in protracting the development of eukaryotic diversity. We present a multiproxy suite of high-resolution geochemical measurements from a drill core capturing the ~1.4 Ga Xiamaling Formation, North China Craton. Specifically, we analyzed major and trace element concentrations, sulfur and molybdenum isotopes, and iron speciation not only to better understand the local redox conditions but also to establish how relevant our data are to understanding the contemporaneous global ocean. Our results suggest that throughout deposition of the Xiamaling Formation, the basin experienced varying degrees of isolation from the global ocean. During deposition of the lower organic-rich shales (130-85 m depth), the basin was extremely restricted, and the reservoirs of sulfate and trace metals were drawn down almost completely. Above a depth of 85 m, shales were deposited in dominantly euxinic waters that more closely resembled a marine system and thus potentially bear signatures of coeval seawater. In the most highly enriched sample from this upper interval, the concentration of molybdenum is 51 ppm with a δ98 Mo value of +1.7‰. Concentrations of Mo and other redox-sensitive elements in our samples are consistent with a deep ocean that was largely anoxic on a global scale. Our maximum δ98 Mo value, in contrast, is high compared to published mid-Proterozoic data. This high value raises the possibility that the Earth's surface environments were transiently more oxygenated at ~1.4 Ga compared to preceding or postdating times. More broadly, this study demonstrates the importance of integrating all available data when attempting to reconstruct surface O2 dynamics based on rocks of any age.

Spinal headaches after myelograms: comparison of needle types.

OBJECTIVE: We compared traditional bevel-tip end-hole spinal needles and pencil-point-tip side-hole needles for the incidence, severity, and duration of spinal headaches in subjects who had myelograms. Age, sex, and myelographic findings were examined. SUBJECTS AND METHODS: We studied 138 subjects referred for myelograms. For 108 procedures, we randomly used 22-gauge Quinke bevel-tip end-hole needles or 22-gauge Sprotte pencil-point-tip needles. The 30 additional subjects were examined with Gertie Marx pencil-point-tip needles. All myelograms were performed by one of two neuroradiologists using recommended doses of iohexol. The myelograms were examined by an independent neuroradiologist for quality of image and presence of extraarachnoid contrast material. Five to 14 days after myelography, subjects were telephoned by an independent observer and asked about the presence, severity, duration, and positional quality of headache. Spinal headache is defined by positional quality and increases in severity when the subject moves from horizontal to sitting or standing. RESULTS: We found that four (8%) of 52 subjects who had myelograms with Sprotte needles had spinal headaches. Likewise, 14 (25%) of 56 subjects who had myelograms with Quinke needles had spinal headaches. We calculated a statistically significant difference in the incidence of spinal headaches using chi-square analysis (p = .02). The average grade and duration of the spinal headaches did not differ significantly, although they were less marked in the Sprotte group. Spinal headaches occurred more frequently in young and middle-aged subjects than in older subjects. We found one definite extraarachnoid injection in each group. For the Gertie Marx needles, two (7%) of 30 subjects had spinal headaches. The average grade of postmyelogram headache was 2.5, and the mean duration was 1 day. There were no mixed injections. CONCLUSION: We found a significant reduction in spinal headaches after myelograms when we used the pencil-point-tip side-hole needle. These results support the routine use of these needles for myelography in young and middle-aged patients.