41
dock. The Earth is not a solid sphere, but instead has a structure of layers with different chemical compositions and physical characteristics. We live on a solid layer
called the lithosphere or crust, but beneath that is the mantle, which flows and
deforms if force is exerted upon it. We can draw an analogy with a waterbed; the
crust is the surface that we can lie on and the mantle is the water that fills the bed.
When someone gets on one side of the waterbed, the surface sinks, but on the other
side a compensatory bulge emerges as the water is displaced. Similarly, when the
Laurentide Ice Sheet grew over North America it exerted a force on the crust which
displaced material in the mantle beneath and uplifted a bulge around the periphery
of the ice sheet. When the person gets off the waterbed, the pattern is reversed by the
rush of water back to the disturbed side. Likewise, when the Laurentide Ice Sheet
was diminished by melting, the peripheral bulge collapsed (subsided) and retreated
back toward the former center of the ice sheet near modern-day Hudson Bay, while
the area formerly buried by ice uplifted as mantle material returned. A key difference between the waterbed analogy and real-life Earth is how quickly these
responses take to occur. The bed responds almost instantly because the medium is
water. In contrast, Earth takes thousands of years to adjust because the medium is
mantle material with a very different viscosity (think of decadently filling your
waterbed with cold honey instead of water for example!). The key point to recognize is that even thousands of years after the ice is gone, the Earth is still seeking a
new equilibrium and GIA is ongoing.
The U.S. Atlantic coast lies on the peripheral bulge of the former Laurentide Ice
Sheet, which today causes the land to subside and our dock to experience an additional source of relative sea-level rise. In the tide-gauge measurements made at
Oregon Inlet (Fig. 3.3), GIA contributes about 1 mm/yr. of the measured relative
sea-level rise. Since the process occurs slowly, this rate is likely to have persisted
unchanged over the past ~2000 years and will continue at the same rate into the
future on human timescales of decades to a couple of centuries. If we examine longer timescales (say 5000 or 10,000 years), GIA does not make a linear contribution
to relative sea level, but instead slows down from the past toward the present.
Reconstructing Relative Sea-Level
The instrumental record of direct relative sea-level measurements from North
Carolina and elsewhere illustrates that profound changes are underway in our
oceans, but these are short records. The very longest observational time series of
relative sea level began in the 1700s in northwestern Europe (Amsterdam), but most
records began much more recently. If we want to provide a context for modern
changes and understand why they happened, then we need to know how relative
sea-level varied naturally in geologically recent times. Generating such a paleo perspective can enhance our ability to peer into the future and to answer fundamental
questions such as what fraction of relative sea-level rise since 1900 AD can be
assigned to human activities? And can we anticipate the magnitude and
3 Time and Tide Wait for No Man
dock. The Earth is not a solid sphere, but instead has a structure of layers with different chemical compositions and physical characteristics. We live on a solid layer
called the lithosphere or crust, but beneath that is the mantle, which flows and
deforms if force is exerted upon it. We can draw an analogy with a waterbed; the
crust is the surface that we can lie on and the mantle is the water that fills the bed.
When someone gets on one side of the waterbed, the surface sinks, but on the other
side a compensatory bulge emerges as the water is displaced. Similarly, when the
Laurentide Ice Sheet grew over North America it exerted a force on the crust which
displaced material in the mantle beneath and uplifted a bulge around the periphery
of the ice sheet. When the person gets off the waterbed, the pattern is reversed by the
rush of water back to the disturbed side. Likewise, when the Laurentide Ice Sheet
was diminished by melting, the peripheral bulge collapsed (subsided) and retreated
back toward the former center of the ice sheet near modern-day Hudson Bay, while
the area formerly buried by ice uplifted as mantle material returned. A key difference between the waterbed analogy and real-life Earth is how quickly these
responses take to occur. The bed responds almost instantly because the medium is
water. In contrast, Earth takes thousands of years to adjust because the medium is
mantle material with a very different viscosity (think of decadently filling your
waterbed with cold honey instead of water for example!). The key point to recognize is that even thousands of years after the ice is gone, the Earth is still seeking a
new equilibrium and GIA is ongoing.
The U.S. Atlantic coast lies on the peripheral bulge of the former Laurentide Ice
Sheet, which today causes the land to subside and our dock to experience an additional source of relative sea-level rise. In the tide-gauge measurements made at
Oregon Inlet (Fig. 3.3), GIA contributes about 1 mm/yr. of the measured relative
sea-level rise. Since the process occurs slowly, this rate is likely to have persisted
unchanged over the past ~2000 years and will continue at the same rate into the
future on human timescales of decades to a couple of centuries. If we examine longer timescales (say 5000 or 10,000 years), GIA does not make a linear contribution
to relative sea level, but instead slows down from the past toward the present.
Reconstructing Relative Sea-Level
The instrumental record of direct relative sea-level measurements from North
Carolina and elsewhere illustrates that profound changes are underway in our
oceans, but these are short records. The very longest observational time series of
relative sea level began in the 1700s in northwestern Europe (Amsterdam), but most
records began much more recently. If we want to provide a context for modern
changes and understand why they happened, then we need to know how relative
sea-level varied naturally in geologically recent times. Generating such a paleo perspective can enhance our ability to peer into the future and to answer fundamental
questions such as what fraction of relative sea-level rise since 1900 AD can be
assigned to human activities? And can we anticipate the magnitude and
3 Time and Tide Wait for No Man