minimally-invasive-technique-for-injection-into-rat-optic-nerve

Minimally-invasive Technique for Injection into Rat Optic Nerve

The rat optic nerve is a useful model for stem cell regeneration research. Direct injection into the rat optic nerve allows delivery into the central ...

Johns Hopkins University

The most severe excursions in the Earth's climatic history are thought to be associated with Proterozoic glaciations. According to the 'Snowball Earth' hypothesis, the Marinoan glaciation, which ended about 635 million years ago, involved global or nearly global ice cover. At the termination of this glacial period, rapid melting of continental ice sheets must have caused a large rise in sea level. Here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples. These structures occur at homologous stratigraphic levels in Australia, Brazil, Canada, Namibia and Svalbard. Our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds, significantly longer than those typical for today's oceans. The reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch-unlimited ocean basins. We propose that these extraordinary wind and wave conditions were characteristic of the climatic transit, and provide observational targets for atmospheric circulation models.

Extreme winds and waves in the aftermath of a Neoproterozoic glaciation.

Fe speciation and S-isotope of pyrite data from the terminal Proterozoic Sheepbed Formation in Canada and Doushantuo Formation in China reveal that ocean deep waters were anoxic after the global glaciations (snowball Earth) ending 635 million years ago, but that marine sulfate concentrations and inferentially atmospheric oxygen levels were higher than before the glaciations. This supports a long-postulated link between oxygen levels and the emergence of eumetazoa. Subsequent ventilation of the deep ocean, inferred from shifts in Fe speciation in Newfoundland (previously published data) and western Canada (this report), paved the way for Ediacaran macrobiota to colonize the deep seafloors.

On the coevolution of Ediacaran oceans and animals.

The 'snowball Earth' hypothesis interprets geological evidence as indicating multi-million-year episodes of global glaciation near the beginning and end of the Proterozoic eon. On the basis of a coupled carbon cycle-climate model, Peltier et al. propose that temperature-dependent remineralization of organic carbon in a Neoproterozoic ocean with 100-1,000x more dissolved organic carbon than today could create a negative climate feedback, thereby preventing a snowball Earth. Their results are sensitive to initial conditions and model parameters; moreover, important geological observations and components of the carbon cycle are not considered-notably the absence of sources or sinks of carbon. Their model results fall short of explaining the geological evidence in the absence of global glaciation.

Article	Year
Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature\| PubMed ID: 15650730	2005
On the coevolution of Ediacaran oceans and animals. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 18469138	2008
Snowball prevention questioned. Nature\| PubMed ID: 19092866	2008

Paul F. Hoffman

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