The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 119

allied species) and notoungulates (an extinct order of South American mammals). The
advantages of looking at teeth, as opposed to soils, are that animals' selective feeding
enhances the isotope signal, and tooth identification and dating is comparatively easy,
whereas soil contains a mix of 13 C and 12 C originating from both C 3 and C 4 remains.
The results of Cerling's team show that large mammals before 8 mya from around
the world all had diets either dominated by C 3 plants or which consisted purely of
C 3 plants. In contrast, by 6 mya equids and some other large mammals from many
regions in latitudes below 37 ◦ (the subtropics and tropics) had C 4 diets.
During this time the Drake and Tasmanian Passages widened further, so increasing
Antarctica's thermal isolation and facilitating growth of its ice sheets. By 6 mya
ice-sheet growth in west Antarctica had become significant. This ice sheet and the
one already existing in east Antarctica were subject to positive albedo/temperature-
feedback effects and so ice slowly grew to fill the Antarctic continent.
4.3 ThePliocene(5.3-2.6mya)
The Pliocene was the last epoch in the Tertiary period. It was a time of global
transition from warm conditions with global surface temperatures about 3 ◦ Cwarmer
than today, smaller ice sheets and higher sea levels compared with current cooler
conditions. In 2004 a team led by Ana Ravelo from the University of California and
Boise State University published an analysis of several palaeoclimate proxy records.
These included an 18 O isotope record from benthic Foraminifera, an alkenone record, a
biogenic carbonate record, an organic carbon record and a biogenic isotopic carbon-
sediment record. Together these records and those of other researchers provide a
reasonably clear picture of how the climate changed, although of course the detail is
limited.
Relative to today (the dawn of the 21st century) the approximately 3 ◦ Cwarmer
early Pliocene Earth had a sea level some 10-20 m higher than now and an enhanced
thermohaline circulation (to which we will return later). Atmospheric and ocean-
circulation patterns were in places significantly different from today. It was thought
that during the Pliocene the Pacific Ocean was subject to permanent El Ni no-like
conditions and different ocean- and atmosphere-circulation patterns (Wara et al.,
2005). (Today the El Nino Southern Oscillation [ENSO] is a semi-regular event
the effects of which have repercussions a hemisphere away; see Chapter 5 and the
Holocene summary as well Chapter 6.) However, more recently evidence has been
gathering that El Ni no events were probably taking place in past warm times much as
they do today. For example, in 2011 Japanese researchers led by Tsuyoshi Watanabe
reported that the idea of a permanent El Ni no mode during the Pliocene warm period
was not supported by coral evidence. Indeed, in even warmer times such as the
Eocene maximum the ENSO still seems to be present in the same year. In the same
year, 2011, Linda Ivany at Syracuse University in New York and her team analysed
Antarctic fossils from the Eocene, 56-34 mya, when the average global temperature
exceeded today's by at least 10 ◦ C. They measured the width of growth bands in the
fossils of wood and bivalves (a class of mollusc) as indicators of annual growth rate.