The Earth Story

Did a Gamma Ray Burst Irradiate Earth in the 8th Century?

In 2012 scientist Fusa Miyake announced the detection of high levels of the isotopes Carbon-14 and Beryllium-10 in tree rings formed in 775 CE, suggesting that a burst of radiation struck the Earth in the year 774 or 775. These isotopes form when radiation from space collides with nitrogen atoms, which then decay into these heavier forms of carbon and beryllium.

A nearby supernova was ruled out as the cause of the burst, as no observations were recorded at this time, aside from a possible sighting in 776 CE, which was too late to account for the C-14 data. Miyake also ruled out solar flares because they are not powerful enough to account for the observed excess of C-14.

Astronomers Valeri Hambaryan and Ralph Neuhӓuser of the Astrophysics Institute of the University of Jena in Germany may have a better answer: a nearby short duration gamma burst. They suggest that two compact stellar remnants (black holes, white dwarfs or neutron stars) collided together, releasing a gamma ray burst. In these mergers the burst is short and intense, lasting less than two seconds. This explanation would be consistent with both the C-14 measurements and the absence of any recorded events in the sky. The merging stars could not have been closer than 3,000 light years away, of the burst would have led to the extinction of some terrestrial life. Hambaryan and Neuhӓuser believe the gamma ray burst originated 3,000 - 12,000 light years from the Sun.

-JF

Image: artist's impression of the merger of two neutron stars (Credit: NASA/Dana Berry) Larger image: http://www.ras.org.uk/images/stories/press/114914main_neutcollidecloseup_strip.jpg Sources: http://www.sciencedaily.com/releases/2013/01/130121083255.htm http://www.ras.org.uk/news-and-press/224-news-2013/2215-did-an-8th-century-gamma-ray-burst-irradiate-the-earth

Dating ice gets a boost Scientists trying to understand how our climate works, whether in order to understand climate change, the vagaries of ice ages or reconstruct ancient environments, have to rely on a variety of proxies from places as dispersed as cave stalagtites and bristlecone pines in the high Sierras of the American west. One major set of proxies used in studying more recent climates are ice cores, that have been drilled over the years from locations as diverse as Greenland, the Patagonian ice cap and Mt Kilimanjaro. Researchers studying the whopper of them all, the Antarctic ice cap, have come up with an innovative new way of dating ancient ice that should prove applicable worldwide. Ice cores tell us about past climate in a variety of ways. They contain precious samples of atmosphere preserved as bubbles, chemical traces of volcanic eruptions (including some that can be radiometrically dated, giving us golden nails to estimate the time in between) and climate proxies such as oxygen isotopes (used to track the growth and collapse of ice sheets during the ice ages, and also taken from other sources such as plankton skeletons to infer climates back in deep time). The new discovery may allow researchers to find and accurately date Antactica's oldest ice, potentially pushing back these rich records much further in time than those we have now. The new technique involves the noble gas krypton, which doesn't interact much chemically with other elements in a manner similar to noble metals like gold, because their outer electron shells are full, making bonding difficult. Like carbon 14, there exists a radioactive isotope of krypton that decays at known rates and has a much longer half life of 230,000 years. Created by cosmic ray bombardment in the atmosphere like C14, it settled in the bubbles in the ice ( C14 enters organic matter through breathing and eating instead) . Once shielded from further radiation, began to decay giving us a long lived atomic clock to date the ice with. Carbon 14 is limited in usefulness due to its short half life, and while chunks of wood extracted from glaciers have been dated, the method stops being of use when the detection limit of the instruments is reached, making it hard to date thing past 50,000 years ago. Krypton is rare, and while the potential of this method was long understood, the breakthrough came in 2011 with a massive improvement in detector technology. They still need to melt fairly large samples of ice, and extract the air from the bubbles for analysis. The oldest ice dated so far is 800,000 years old, and, having successfully dated ice of known ages to prove the method works, the team are hoping to drill new cores potentially going back even further in time. Finding this ice will prove tricky, since much of the oldest ice will have flowed into the ocean. Hopefully some patches remain at the base of the ice sheet or at its edges. This will help understand ice ages better, by allowing us to date much older ice accurately, since the C14 limit barely takes us back into the last interglacial, though the ice sheets have waxed and waned around 50 times over the last two million years. The period they are hoping to reach, around 1.5 million years back, was a crucial one, during which the frequency of ice ages changed to being influenced of the 100,000 year orbital cycle rather than the 40,000 year one (or maybe 4-5 precessional cycles, for an deeper explanation of these cycles see our piece on astronomical rocks at http://tinyurl.com/kaq6nuf). The reason for this transition is poorly understood, and the highly detailed records old ice can provide might prove the crucial piece to move the debate forward. Loz Image credit: NASA/Christy Hansen http://www.nsf.gov/news/news_summ.jsp?cntn_id=131136&WT.mc_id=USNSF_51&WT.mc_ev=click http://oregonstate.edu/ua/ncs/archives/2014/apr/scientists-successfully-use-krypton-accurately-date-ancient-antarctic-ice http://www.sci-news.com/geology/science-noble-gas-krypton-antarctic-ice-01872.html