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© 2013. American Geophysical Union. All Rights Reserved. Eos, Vol. 94, No. 30, 23 July 2013 PAGE 268 Rock varnish points to Younger Dryas wet period in Dead Sea Basin During the Younger Dryas period, from about 12,900 to 11,600 years ago, the North Atlantic region cooled rapidly. This cooling, one of several rapid periods of climate change in the Earth’s history, had wide effects across the globe. However, in desert regions evidence for the Younger Dryas climate change has been hard to find. Now Liu et al. report new evidence of a cold, wet Younger Dryas period in the Dead Sea Basin from rock varnish. Rock varnish is a slowly accreting coating on exposed rock in deserts; this varnish tends to form in layers that can provide records of past climate change. Previous studies of rock varnish in the western United States have provided evidence of wet conditions there during the Younger Dryas. Studying rock varnish from the late glacial Lake Lisan shorelines, the authors found a layering pattern of thick orange/yellow sur- face layers with low levels of manganese and barium as well as thin dark basal layers rich in manganese and barium. The scientists sug- gest that the dark layers indicate a wet period coinciding with the Younger Dryas, during which the lake level rose at least 100 meters from its Bølling-Allerød lowstand. The authors also note that the layered patterns representing the Younger Dryas wet event are similar to those previously observed in western U.S. dry lands. The study provides new evidence of Younger Dryas climate changes in the Dead Sea region and demon- strates the usefulness of rock varnish in recon- structing the past wetness history of the world’s deserts. ( Geophysical Research Letters, doi:10.1002/grl.50492, 2013) —EB Tropical storm Sandy was a 1-in-700-year event On 29 October 2012 tropical storm Sandy slammed into the New Jersey shoreline, bringing wind and water that killed more than 100 people and caused tens of billions of dollars in damage. Though its wind speeds were only equivalent to those of a low-level hurricane, Sandy caused record-breaking flooding in New Jersey, New York, and elsewhere. In lower Manhattan, water levels hit 4.28 meters above the mean low water level—the highest flood waters in the region since sensors were installed in 1920. One of the drivers behind Sandy’s extreme storm surge was the unusual angle Sandy took as it hit the New Jersey coast. Most tropical cyclones in the North Atlantic sweep up the coast on a northward or northeast- ward track. Sandy, on the other hand, drove into New Jersey traveling toward the northwest—the only tropical cyclone in the historical record to do so. With this near- perpendicular approach, Sandy’s onshore winds had more time to drive a wall of water onto one coastal region, rather than moving along a swath of coastline. Using information of tropical cyclone tracks for the whole North Atlantic from 1950 to 2010, Hall and Sobel calculated the odds that a similar storm—a category 1 or higher hurricane with an approach angle to New Jersey at least as close to perpendicular as Sandy—could happen again. According to the authors’ statistical model, the occurrence rate of a Sandy-style storm is 0.0014 per year, meaning that if future hurricane activity matches the recent past, a storm like Sandy could be expected on average about once every 700 years. The fact that Sandy happened, the authors say, means either that New York and New Jersey were very unlucky or that climate change has increased the probability of a Sandy-like storm beyond what they found with their steady-climate statistical model. ( Geophysical Research Letters, doi:10.1002/ grl.50395, 2013) —CS Identifying the chemical composition of “brown carbon” in the atmosphere Aerosol particles in the atmosphere can either absorb or scatter incoming solar radiation, thus either heating or cooling the atmosphere. One of the most studied types of aerosols that absorb radiation is black carbon (also called soot), which comes from incomplete combustion of fossil fuels, biofuel, and biomass. Black carbon has been identified as a significant factor contributing to global warming. Somewhat less well studied is brown carbon, which also absorbs solar radiation but does so slightly differently than black carbon: Brown carbon absorbs light most strongly in ultraviolet and short visible wavelengths, giving it a yellowish or brownish appearance. Biomass burning is a major source of brown carbon. To learn more about the chemical compo- sition of brown carbon in the atmosphere, Desyaterik et al. analyzed cloud water sam- ples from regions of China where high levels of agricultural biomass burning take place. They found a significant amount of brown carbon in the clouds—it was easy to spot by the yellowish-brownish color of the cloud water. Using liquid chromatography, the authors identified 16 chemical compounds in the brown carbon and found that the most important classes of light-absorbing com- pounds were nitrophenols and aromatic carbonyls. The study is one of the first quantitative studies to identify the chemical species that constitute brown carbon in the atmosphere. (Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50561, 2013) —EB —ERNIE BALCERAK, Staff Writer, and COLIN SCHULTZ, Writer Microstratigraphy in rock varnish from the Dead Sea Basin showing the Younger Dryas wet period, represented by the manganese- and barium-rich dark basal layer.The overlying manganese- and barium-poor orange/yellow layers are diagnostic of the Holocene relatively dry climate in the region.The maximum varnish thickness in the image is about 200 micrometers. Taehyoung Lee Samples of cloud water affected by biomass burning.The samples were collected at Mount Tai, China.
