Biology Reference
In-Depth Information
semblages indicate cooling, drying, and the spread of savanna/grassland in
the eastern Mediterranean region through the Miocene (Strömberg et al.
2007). The shift can also be widely detected from expanding dry paleosols,
a change in the isotopic carbon content of mammalian tooth enamel, and
by increasing dust in ocean sediments from the middle Miocene onward
(Rea et al. 1985). The beginning of the middle Miocene represents another
pivotal point in ecosystem history because it was a time favorable to the
development, coalescence and radiation of drier, seasonal communities and
lineages.
Although the focus here is on the New World, it is widely known that
climate systems are teleconnected (Markgraf 2001), meaning that events
in one part of the world usually have global impact. This is now widely
recognized through the highly publicized effects of El Niños (Nash 2002).
Another example is the glacial expansion beginning in the late Miocene
that caused a marine regression of about 40-50 m. This isolated the Medi-
terranean Sea, resulting in an accumulation of salt (gypsum) in the basin
that reduced salinity in the ocean by 6 percent. Reduced salinity in turn
allowed Northern Hemisphere seawater to freeze at higher temperatures,
created more heat-refl ecting snow and ice, and served as a positive feedback
to global cooling and seasonality. Much of the life of the Mediterranean Sea
died during this event, giving rise to the Messinian Salinity Crisis, named
after the interval of the Miocene when it occurred, around 4.8 Ma (Rouchy
et al. 2001; Rouchy et al. 2007). Although it happened in the Mediterra-
nean Sea, the effects were transmitted throughout the teleconnected cli-
mate and biotic systems of the Earth. A repeated lesson from the past is
that the effects of regional environmental alterations, whether an El Niño,
a salinity crisis, or a large-scale change or removal of vegetation, will be
experienced well beyond the area of origin.
Cooling after the middle Miocene was not a straight-line trend. There
was the MMCO between 17 and 15 Ma. At 4-3 Ma in the middle Pliocene,
atmospheric CO
2
values are estimated at 360-440 ppmv, and temperatures
increased to around 3.5°C warmer than at present (Salzmann et al. 2008).
The causes were a brief increase in atmospheric CO
2
concentration in the
middle Miocene and greater oceanic transport of heat from the lower lati-
tudes by the Gulf Stream with closure of the Isthmus of Panama in the
middle Pliocene. In the middle Pliocene, sea level was about 35 m higher
than at present along the Atlantic coast; winter temperatures in the Arctic
were an estimated 20°C-22°C warmer; summer temperatures were 6°C-
8°C warmer (Zubakov and Borzenkova 1990); and MAP increased. After
this warm interval, temperatures resumed the trend toward cooling, led
