Geoscience Reference
In-Depth Information
peat bogs; loess deposits, tree rings (dendrochronology), speleothems (mineral for-
mations in limestone caves), and geomorphic features such as raised beaches and
marine terraces, moraines, and glacial erratics. The Arctic is a rich source of paleo-
climate information and some detailed records have come from this area. A remark-
able record of Arctic climate extending back 2.8 million years, hence containing the
entire Quaternary as well as the late Pliocene, is now available from a core drilled
through the bottom sediments of Lake El'gygytgyn in northeastern Russia (Melles
et al.,
2012
). Lake El'gygytgyn is located in a meteorite impact crater that formed
3.58 Ma.
Paleoclimatology is an immense ield, and it is beyond the scope of this topic to
describe in depth the wide variety of climate reconstruction techniques. R. Bradley
(
1999
) provides a comprehensive review. However, we still need to address several
issues, such as radiocarbon dating. It is also necessary to come to grips with ice and
ocean cores, which provide many of the longest paleoclimate records. Some of our
most intriguing insights (as well as questions) regarding past climates have come
from these records. It is not only the long time coverage from marine and ice cores
that make them so valuable, but also that they are generally continuous and, in the
case of ice cores, provide information at high temporal resolution.
10.2.2
14
C Versus Calendar Dates
Although dates in this chapter (as ka or Ma) are cited as estimated calendar years
before present (with the present taken as the year 1950), there is still modern lit-
erature that reports ages in uncalibrated radiocarbon years (
14
C years before pre-
sent). We performed conversions as necessary. Radiocarbon dating is useful back
to about 40 ka. Radiocarbon is produced in the upper atmosphere by neutron bom-
bardment of nitrogen atoms. The neutrons are part of the cosmic radiation entering
the upper atmosphere. Plants and animals assimilate
14
C into their tissues through
photosynthesis and respiration, with the
14
C content in equilibrium with that of the
atmosphere. This equilibrium occurs as there is a constant exchange of new
14
C as
old cells die and are replaced. When organisms die, exchange and replacement of
14
C ends, and the
14
C decays to nitrogen as a function of time (radiocarbon has a
half-life of 5.73x10
3
years), activating a radioactive “clock.” However, for many
reasons, including variations in the cosmic ray flux,
14
C levels have varied over time.
Although
14
C age is close to calendar age for the past 2,500 years, before then there
is a substantial difference, with
14
C underestimating true age (Bradley,
1999
). For
example, a
14
C age of 10 ka corresponds to a calendar age of 11,500 ka; whereas a
14
C age of 20 ka corresponds to a calendar age of about 23.9 ka.
10.2.3
Ice Cores
Snow that falls on an ice sheet is compressed by subsequent snowfall, becomes firm,
with a density of 550 kg m
−3
around 10 m depth, and then ice (a density of at least
840 kg m
−3
), where pore close-off occurs at depths of about 60 m in Greenland.
