Geology Reference
In-Depth Information
careful layer-by-layer measurements of oxygen isotope ratios in coastal sediments can re-
veal how Earth's surface ice (or lack thereof) has varied through time.
Such exacting work is the domain of geologist Ken Miller and his colleagues at Rutgers
University, who have spent decades scrutinizing the thick accumulations of marine sedi-
ments that blanket coastal New Jersey. These sediments, with a record extending back one
hundred million years, contain a wealth of microscopic fossil shells called foraminifera, or
forams for short. Each tiny foram preserves the oxygen isotope content of the ocean at the
time when it grew. Layer-by-layer measurements of oxygen isotopes in New Jersey sedi-
ments thus provide a simple and accurate estimate of the ice volume through time.
In the recent geological past, the ice cover appears to have ebbed and flowed constantly,
with correspondingly large changes in sea level on a timescale of a few thousand years.
At the height of recent ice ages, more than 5 percent of Earth's water has been trapped
in ice, lowering sea levels by perhaps three hundred feet from their present mark. About
twenty thousand years ago, such a period of low ocean level is thought to have created a
land bridge between Asia and North America across what is now the Bering Strait—the
original passageway for humans and other mammals to the New World. During the same
icy interval, there was no English Channel, as a dry valley connected the British Isles to
France. By contrast, at times of maximum warming, when glaciers largely disappear and
ice caps recede, sea levels have repeatedly risen by as much as three hundred feet higher
than today, submerging hundreds of thousands of square miles of coastal areas around the
globe.
Miller and his colleagues identified more than a hundred cycles of glacial advance and
retreat in the past nine million years, with at least a dozen such events in just the past
million years alone—changes that point to wild swings of as much as six hundred feet in
ocean level. While details may vary from cycle to cycle, these events are clearly period-
ic and are related to so-called Milankovitch cycles, named for the Serbian astrophysicist
Milutin Milankovitch, who discovered them about a century ago. He realized that well-
knownvariationsinEarth'sorbitaroundtheSun,includingourplanet'stilt,itsellipticalor-
bit,andaslightwobbleinitsrotationaxis,imposeperiodsofclimate changewithintervals
of roughly twenty, forty-one, and one hundred thousand years. These variations all affect
the amount of sunlight hitting Earth and thus exert a profound effect on global climate.
So what of the next fifty thousand years? We can be confident that sea levels will con-
tinue to vary dramatically, with many more rises and falls. At times, very possibly over the
nexttwentythousandyears,icecapswillgrow,glacierswilladvance,andsealevelwillde-
crease by two hundred feet or more—a level that has occurred at least eight times over the
past million years. Such a change will have powerful effects on the world's coastlines. The
East Coast of the United States will shift many miles to the east, as the shallow continental
slope is exposed. All of the great East Coast harbors, from Boston to Miami, will become
