What causes the ice ages in the late Plioceneand Pleistocene
2016
For more than a century, two families of ice age theories have
been proposed: insolation based theories proposed by Adhemar,
and atmospheric CO 2 ones proposed by Tyndall. The major
technique advance of deep-sea sediment drilling, as well as new interpretations
of stable oxygen isotopic composition of the deep-sea fossil foraminifera,
which is regarded as a proxy index of continental ice volume and sea
water temperature, established the now well recognized glacial-interglacial
variations in climate over the late Pliocene and Pleistocene, e.g.
~3 Ma. A landmark progress came from Hays et al. (1976) who unambiguously
demonstrated that the change in insolation induced by the Earth’s
axial tilt (obliquity), the wobble of this tilt (precession), and
the degree of circularity of the Earth’s orbit around the sun
(eccentricity) brings on ice ages (glaciations) every hundred thousand
years or so during the Pleistocene. Many subsequent observations,
from the deep-sea to continental records, confirmed the co-variation
between insolation and the alternation of glacial ice age phases,
and warmer interglacial phases, at least in terms of frequency domains.
However, these findings cannot explain the so called ~100 ka conundrum,
where the size of 100 ka insolation forcing is relatively small yet
the apparent ice sheet response is large, as well as the bi-polar
symmetry in climate changes at precessional frequency of ~23 ka, despite
the fact that the precession cycle drives insolation changes in antiphase
between the two hemispheres. As such, other factors may be considered
in the driving of Northern Hemisphere Glaciation (NHG) global glacial-interglacial
variations. Several hypotheses have been put forward, such as: (1)
the gradual decreasing CO 2 drove global cooling through
to a threshold value where insolation changes may force the ice age
cycles; (2) the effect of local insolation that modulated the East
Antarctica Ice Sheet after the gradual CO 2 decline that
drove its growth to maximal extent; (3) the Southern Ocean “marine
biological pump” changes forced by the ocean water stratification
and current changes that modulated the atmospheric CO 2 thus
triggered ice ages; (4) the clearing of regolith under the North American
ice sheets which modulate ice sheet thickness and extent changes which
allow the ice sheets to grow further south and be influenced more
by precessional cycles, as well as strengthened the fresh bedrock
weathering and drawdown more atmospheric CO 2 ; and (5) because
ice mass balance depends on whether the temperature is above or below
the freezing point, a physically more relevant parameter to measure
insolation forcing should be the insolation integrated over a given
threshold that allows for ice melting, and thus triggering the glaciations.
Unfortunately, we cannot determine which one is right or several of
them acting together so far. The most distinct features of the ice
age climate are both the onset of the NHG at ~2.7 Ma and the transition
of ~41 ka based glacial-interglacial cycles to cycles of a ~100 ka
dominated frequency at ~1.0 Ma, namely the Early Middle Pleistocene
Transition. We suggest that, both insolation and CO 2 changes
together forced the climate of the ice ages: the stepwise cooling
that occurred at ~2.7 Ma and ~1.0 Ma may be linked with gradual atmospheric
CO 2 reduction, during which the atmospheric CO 2 levels fell to a threshold value that triggered high-latitude (both
poles) ice expansion. Under a new cooling state since ~2.7 Ma, the
dominant climatic cycle at ~41 ka was forced by obliquity (orbital
tilt), while the ~23 ka precession cycle was cancelled by opposing
effects at both poles. On the other hand, we propose that the ~100
ka cycles that became dominant following approximately 1 Ma were forced
by a CO 2 based “marine biological pump” change
in the Southern Ocean, or is related to stochastic behavior of the
ice sheets. The ~400 ka eccentricity cycle, which is a stronger absolute
influence on insolation is an outstanding rhythm in the late Cenozoic,
yet is not as strong in climate records as the precessional and obliquity
cycles during the late Pliocene and Pleistocene, and thus needs further
examination. In summary, we suggest ice ages are caused by the integrated
forcing of insolation, atmospheric CO 2 , ocean conveyer
and ice sheet feedback etc., in which the insolation forcing is primary,
a threshold value of the other facts was attained, the ice age happened.
However, our hypothesis is very tentative, more investigation is still
needed on what causes the ice ages.
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