© 2013. American Geophysical Union. All Rights Reserved.

Eos, Vol. 94, No. 30, 23 July 2013

PAGE 268

Rock varnish points to Younger Dryaswet 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 Sandywas 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 compositionof “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.

Taeh

youn

g Le

e

Samples of cloud water affected by biomass

burning. The samples were collected at Mount

Tai, China.