Biology Reference
In-Depth Information
causes, and their effects on the biota. One such point was reached in the
middle Miocene, when temperatures began an accelerated decline toward
a new climate state.
The paleotemperature curve for 16.3 to 2.6 Ma shows that the relatively
slow pace of changes in the Oligocene and early Miocene picked up in the
middle Miocene around 15 Ma (fi g. 3.4) and began altering the Earth's biota
at an ever-increasing rate. Some physical changes were gradual, such as the
erosion of the Appalachian Mountains down to about modern elevations
by the end of the Pliocene. Others were faster, such as uplift of the Rocky
Mountains that began a rapid rise between 12 and 4 Ma. The Sierra Nevada
attained two-thirds of their current height since 10 Ma. At 3 Ma they stood
at around 2100 m, rising about 950 m since that time. The California Coast
Ranges, Cascade Mountains, and Alaska Ranges all reached signifi cant al-
titudes within the past 6 million years, creating new temperate zones, and
increasing habitat diversity, the weathering of silicate rocks, continentality,
and the extent of land affected by rain shadow. Another subtle but impor-
tant change was greater albedo, or refl ection of heat, as the dense, ever-
green, heat-absorbing tropical vegetation, widespread in the Paleocene and
Eocene, was replaced over large areas by heat-refl ecting open or deciduous
vegetation and exposed soil surfaces in the Oligocene and early Miocene,
and especially after the middle Miocene.
The most rapid change, however, was in climate. At the beginning of
the middle Miocene, 17-15 Ma, temperatures increased to what is known
as the Middle Miocene Climatic Optimum, or MMCO (Riishuus et al.
2006). The MMCO was followed by a steep decline around 15 Ma that
can be characterized as a tipping point in global climates. The cause was
a continuing drop in atmospheric CO
2
concentration, with less compen-
sating input because of slowing plate movement and no sudden release of
methane as had been the case in the early Eocene. Polar glaciations were
just underway in the Eocene, but they were extensive on Antarctica by the
Oligocene, and likely present on Greenland as shown by seasonal sea ice at
the Eocene-Oligocene transition 38 Ma (Eldrett et al. 2009; “An Insight
into Amber,” 2008). Ice masses had appeared elsewhere in the Arctic by
the middle to late Miocene, expanded in the Mio-Pliocene, and were in full
glacial mode by the late Pliocene. With glaciation, oceans further cool from
meltwater, creating reduced evaporation and precipitation, and resulting in
greater seasonality. The effects on New World vegetation will be discussed
below, but the trend was global, including, for example, New Zealand (Lee
et al. 2001), as shown by widespread increases in seasonally dry forests,
grasslands, and in populations of grazing animals. In Turkey, phytolith as-
