Geoscience Reference
In-Depth Information
Pacii c Ocean for evidence of the past behavior of the ocean-atmosphere phe-
nomenon known as the El Niño-Southern Oscillation (ENSO). During an
El Niño event, the trade winds fail, causing the warm waters in the western
Pacii c Ocean to shit eastward, warming surface temperatures and shallow
coral reefs across the tropical Pacii c.
Palmyra Atoll, one of the most spectacular and diverse coral reefs in the
world, is located just north of the equator in the central Pacii c Ocean, making
it ideal for studying past changes in the ENSO. Geochemist Kim Cobb of the
Georgia Institute of Technology has been studying coral reefs at Palmyra Atoll
for over a decade. She uses scuba gear to dive down to and collect cores from
the massive heads of the coral genus
Porites,
which grow up to several meters
(6-10 feet) high. Cobb also collects fossilized corals on the island in order to
extend the record to older time periods. Once all of these coral cores are taken
to her laboratory, they are X-rayed to delineate density changes in their calcium
carbonate skeleton. Denser layers are formed during periods of slower growth
in the cooler winter months; less dense layers are formed during the warmer
summer months, when the coral grows more rapidly. h ese annual layers, like
tree rings, provide a chronology of the coral. Each sample is analyzed for geo-
chemical signals of climate events that took place when the coral was growing.
Cobb and her team have found that the Palmyra corals have faithfully
recorded the El Niño events of the twentieth century as rel ected in the ratio
of oxygen-18 to oxygen-16 in the coral skeletons. Armed with this knowledge,
Cobb has explored the frequency and intensity of El Niño over past periods
of climatic change. She found that, during El Niño events, the waters sur-
rounding Palmyra become warmer and rainfall increases—both leading to
a reduced amount of the heavier isotope (oxygen-18) relative to the lighter
isotope (oxygen-16) in the coral skeleton. Conversely, La Niña brings cooler
waters and reduced rainfall to the site, leading to a greater proportion of
oxygen-18 relative to oxygen-16.
Cobb was able to extend the record back 1,100 years by splicing, drilling,
and dating dozens of exposed fossilized coral heads on the beaches of Palmyra.
h ese studies reveal that, during the Medieval Climate Anomaly between the
tenth and fourteenth centuries, sea surface temperatures were cooler in the cen-
tral Pacii c than today—conditions associated with La Niña. We know from
studying modern climate patterns that cooler waters in the central and eastern
Pacii c are associated with drier conditions over the American West and north-
ern Mexico. Cobb's results suggest that the Medieval droughts (described in
chapter 9), which coincided with the collapse and migration of the Ancestral
