Geoscience Reference
In-Depth Information
of the tree (Figure 9.16b) and then an approximately 6-mm
(0.2-in.) wide core is extracted that cuts across the tree rings,
thus showing all of them. The resulting hole is plugged with a
small stick, and it then seals shut, resulting in no harm to the
tree. Some trees live a very long time and thus produce long,
continuous tree ring records. An excellent example of such a
tree is the famous bristlecone pines in the White Mountains of
California (Figure 9.16c), which are the oldest living things
on Earth. Some of these trees are over 4000 years old and still
growing! Thus, by looking at the rings of one tree alone, it is
possible to reconstruct the growth cycles, and therefore the his-
tory of climate change, for the past 4000 years in the area where
the pine trees have lived.
Although the living bristlecone pines have yielded an
excellent record of climate change, paleoclimatologists are
constantly looking for ways to extend the record farther and
farther back in time. In the context of dendrochronology, this
goal is accomplished through a method called
cross dating
. The
premise of cross dating is that ring patterns from similar trees
that overlapped in their life spans can be matched to extend the
record of ring variability back in time.
To see how this method works, look at Figure 9.16d.
Imagine that a core is extracted from a living tree (A) that pro-
duces a distinctive ring record. Near this living tree is another
tree (B) that is dead but still standing. Upon coring (or, in the
case of a dead tree, cutting) the second tree, it is discovered
that the ring pattern in the latter half of its life was the same as
that during the early years of the first tree's (A) life. It is thus
logical to conclude that the trees overlapped in their life spans
and that they responded to the same climate conditions as
far as their growth was concerned. Imagine then that another
tree (C) is discovered that was used as a support structure in
an old house structure. Analysis of its ring pattern indicates
that in the latter part of its life it overlapped with the first part
of the second (B) tree's life. We now have a record of climate
change that extends back to a period before the life of the in-
termediate tree. Using this method, paleoclimatologists have
extended the history of climate change in the White Mountain
area to beyond 9000 years ago!
Ice Core Analysis
Yet another way that prehistoric climate records can be recon-
structed is by investigating oxygen isotopes contained in ice
cores recovered from glaciers in Greenland and Antarctica.
These glaciers contain annual layers of ice that accumulated
over thousands of years. Assuming again that the more deeply
buried ice layers are the oldest, it is possible to count layers to
determine when they formed. The premise of using oxygen iso-
topes is that oxygen contains two primary isotopes, O-16 and
O-18, that differ in their atomic weight. Although both isotopes
are found in liquid water, O-16 water is almost 500 times more
common and more easily evaporated than its heavier counter-
part, O-18 water. This variation depends somewhat on the water
temperature at the time it evaporates. When water is warmer, for
example, proportionately more O-18 water is evaporated than
when the water temperatures are colder. Thus, it is possible to
examine the ratio of O-16 to O-18 in layers of ice and yearly
accumulations of marine sediments to indirectly reconstruct
past climate cycles.
To see an example of how this method of climate recon-
struction works, examine Figure 9.17, which illustrates the way
O-16 deposited in interior of ice
O-18 falls in initial
precipitation phase
Water vapor is
richer in O-16
Continental glacier
Evaporation
Water is proportionately
richer in O-18
Benthic
foraminifera
Ocean
Figure 9.17 Using oxygen isotope ratios to reconstruct glacial and interglacial cycles.
Seawater contains both O-16 and O-18
isotopes, which are preferentially evaporated (O-16) and precipitated (O-18) relative to each other during glacial cycles. Thus, micro-
scopic marine organisms (benthic foraminifera) absorb relatively more of the heavier O-18 isotope during glacial cycles. At the same
time, moisture-laden air flows toward growing ice sheets, where O-18 is precipitated first because it is heavier than O-16. By the time
the air mass reaches the center of the ice sheet, it is enriched in O-16 relative to O-18.
