Geoscience Reference
In-Depth Information
Figure 10.2.
The SPECMAP (Spectral Mapping Project) composite chronology for
a set of seven stacked (superimposed) δ
18
O records from different ocean basins of the
world. Dating involves tuning of the marine isotope records by orbital forcing (from
Martinson et al.,
1987
, by permission of Elsevier).
results from the El'gygytgyn lake core record is evidence for a series of “super
interglacials” in the Arctic through the Quaternary, warmer than the Eemian (Melles
et al.,
2012
)
Over the past 400,000 years, which is better known, there have been at least four
major global-scale ice advances. During the past 0.9 million years, there were eight
ice age cycles with durations of about 100,000 years and these fluctuations appear
to have increased in amplitude over the past 430,000 years. Prior to about 0.9 ma,
ice age cycles had a 41,000 year duration. The reason for this shift is not under-
stood. The Antarctic ice core from Dome C (EPICA Community Members,
2004
)
spans eight glacial cycles and clearly shows the increased amplitude of ice volume
over the past four cycles. Between 750 and 430 ka, warm intervals were longer (up
to 28,000 years duration) but these interglacials had less pronounced warmth than
later ones.
The Last Glacial Cycle, which we know most about, started about 124 ka at the
peak of the Eemian Interglacial, and extends to the start of the Holocene. The paleo-
climate community often refers to history in terms of marine “stages,” based on
18
O in benthic foraminifera. Odd-numbered stages correspond to warmer conditions
and/or less terrestrial ice, whereas even-numbered stages correspond to colder con-
ditions and/or more terrestrial ice. There are also substages. The Eemian interglacial
(Kolfschoten, Gibbard, and Knudsen,
2003
; Shackleton et al.,
2003
) is associated
with Marine Isotope Substage 5e (MIS 5e also known as MIS5.5).
Figure 10.2
is a
composite MIS chronology extending back to 300 ka (in normalized units). In this
figure, substages 5a-e correspond to 5.1-5.5. While 5a, 5c and 5e (5.1, 5.3, 5.5)
were periods of reduced ice volume and/or higher temperature within stage 5, 5b,
and 5d (5.2 and 5.4) were periods of cooler conditions and/or terrestrial ice growth,
but on a smaller scale than in stage 4 (Bradley,
1999
).
