OFR 151.pdf - CRC LEME

East Antarctica and strengthening of the meridional surface temperature gradients. This
‘strengthening’ almost certainly was a major factor behind aridification of mid-latitude
regions in Africa, Australia, and North and South America during the Middle-Late Miocene.
Again, a primary driver is likely to be changes in deep-ocean circulation. For example, polar
cooling during the early Middle Miocene is seen to have been curtailed by the production of
warm, saline deep water in the eastern Paratethys/northern Indian Ocean and accelerated
cooling in the Antarctic region is linked to a reduction in the flow of deep water to the
Southern Ocean at about 14.8 Ma.
The roles of marine regression and orogeny in reinforcing global cooling and drying during
the Middle-Late Miocene are more equivocal (Molnar and England 1990). For example,
GCM modelling (Ramstein et al. 1997) links increasingly continental climates in central Asia
and increasingly monsoonal climates across India and Indo-China to the retreat of the
(epicontinental) Paratethys Sea during the Late Oligocene and Miocene; Burbank et al. (1993)
and Filippelli (1997) propose that uplift of the Tibetan Plateau had intensified the Asian
monsoon by about the same time (7-8 Ma). The consequences of renewed cooling and drying
during the Late Miocene include a world-wide increase in the biomass of plants such as
grasses that utilise C4 photosynthesis. This in turn is linked to major faunal changes (Cerling
et al. 1997, Cerling and Harris 1998). Modelling by Dutton and Barron (1997) predicts that
this trend, to shrub- and herb-dominated vegetation types, is likely to have aggravated the
cooling trend in the Northern Hemisphere. Dickens and Owen (1999) report that primary
productivity increased simultaneously at upwelling zones in the Indian and Pacific Oceans
during the latest Miocene and Early Pliocene. Whilst the consequences (if any) to terrestrial
ecosystems remains elusive, the data are evidence for an important change in the global
nutrient cycle (an increase in supply of organic carbon) during the Late Miocene.
Hodell and Kennett (1986) conclude that the latest Miocene (6.5-5 Ma) was characterised by
pulses of ice sheet expansion and contraction, with the most intense events occurring during
the latest Miocene at 5.5-5.1 Ma. The effects of mid and high-latitude cooling included an
increase in the volume of ice forming the East Antarctic Ice Sheet, the formation of ice
shelves in the Ross and Wedell Seas, the deposition of ice-rafted debris on the Falkland
Plateau in the subantarctic Atlantic Ocean, and repeated extension of glaciers beyond the
mountain-front in southern South America (Clapperton 1979, 1986, Hodell and Kennett
1986). The combined glacial events are estimated to have lowered global sea levels by 30-60
m, contributing to the isolation (and subsequent desiccation) of the Mediterranean Sea from
the Atlantic Ocean between 6-5 Ma (Aharon et al. 1993). The magnitude of this event
(Messinian salinity crisis) can be judged from the estimate that ~6% of the global oceanic salt
budget was deposited as evaporites in the Mediterranean Basin (McKenzie 1999). The
catastrophic reconnection of the basin to the world ocean at about 5 Ma defines the Miocene-
Pliocene boundary.
7.1.6 ‘Mid’ Pliocene warming [~3 Ma]
Consensus exists that the Early Pliocene [~5-3 Ma] was the most recent interval of sustained
global warmth (King 1996, Billups et al. 1998, Kohler et al. 1998). For example, Raymo et
al. (1996) conclude that the global mean temperatures at ~3 Ma may have been as much as
3.5 0 C warmer than at present. High-resolution biostratigraphies from the Southern Ocean
point to peak warming occurring at ~4.3 Ma, with other brief warming events recorded at
~4.5 Ma, ~4.2 Ma and ~3.6 Ma (Bohaty and Harwood 1998). SSTs at high latitudes in the
North and South Atlantic are estimated to have been up to 8 ° C and 2-3 0 C respectively higher
than today although little or no warming is recorded in the tropics (Dowsett et al. 1996).
However, there is no agreement whether high latitude warming led to partial decay or growth
of the Antarctic ice sheet (cf. Hodell and Warnke 1991, Wilson 1995, Burkle et al. 1996,
Harwood et al. 2000). Significant cooling episodes are recorded at 4.5 Ma and between ~3.5-
3.2 Ma (Hodell and Warnke 1991, Burkle et al. 1996).
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