Biology Reference
In-Depth Information
by a decline in atmospheric CO
2
(Lunt et al. 2008); and at 3 Ma near the
end of the Pliocene, ice volume in the Arctic had reached two-thirds that
of the Pleistocene (Crowley and North 1991). Haug and colleagues (2005)
place the beginning of signifi cant Northern Hemisphere glaciations at
2.7 Ma, based on alkenone unsaturation ratios and diatom oxygen isotope
ratios from cores in the western subarctic Pacifi c Ocean. Kleiven and others
(2002) note an intensifi cation of glaciations in the North Atlantic region
between 3.5 and 2.4 Ma. The North American Laurentide ice sheet was
present by 2.6 Ma; there was permafrost in the boreal region by 2.4 Ma;
tillites (glacial rock debris) are found in Puget Sound by around 2.1 Ma,
and in Yellowstone National Park by 2 Ma; and ice briefl y extended down
the Mississippi River Valley as far as Iowa around 2.1 Ma. After a warm
period around 1.7 Ma, cold conditions returned, continuing the sequence
of eighteen to twenty Quaternary glaciations. These glaciations were under
the infl uence of Milankovitch cycles lasting roughly100,000 years, inter-
rupted by brief 10,000-year-long interglacials like that of the present, and
modulated by changes in ocean circulation and fl uctuations in CO
2
. We are
currently about 11,000 years into this latest interglacial, and past patterns
suggest that were it not for increasing warmth created by use of fossil fuels
and burning of forests, the Earth would be entering about the twenty-fi rst
glacial cycle of the past 2.6 million years. The concern is that by delay-
ing the event, its eventual effects may be more lasting, drastic, and chaotic
than the dramatic-enough changes of the last interglacial-glacial transition
as demonstrated by the isotope and fossil record. Some believe present cli-
mates may be “approaching a bifurcation point, that is, a point at which the
system will undergo a transition to a new stable climate state of permanent
midlatitude northern hemisphere glaciation” (Crowley and Hyde 2008).
NORTH AMERICA (NORTH OF MEXICO)
In the far northwest, sequences of insects, ostracodes, soils, and fossil fl oras
all reveal the fl uctuating but inexorable trend toward colder, winter-drier,
and increasingly seasonal conditions (Axelrod et al. 1991). The Alaskan
Seldovia Point fl ora is early to middle Miocene in age, 16-14 Ma, and it
preserves a deciduous forest
Larix
, with some evergreen boreal forest trees
such as
Abies
,
Picea
, and
Tsuga
(see fi g. 7.1). These species began coalesc-
ing inland in the mid to late Miocene around 10-6 Ma into a recognizable
version of the boreal coniferous forest association. In the middle Miocene
Ballast Brook Formation on Banks Island, Northwest Territories, Canada
(74°N), large trees of
Glyptostrobus
,
Picea
, and
Pinus
were present with
