OFR 151.pdf - CRC LEME

7.1.4 Eocene-Oligocene climatic transition [~33 Ma]
The Eocene-Oligocene transition has emerged as one of the more critical intervals in recent
geological history. Reasons include major changes in world ocean circulation, e.g. the
development of the Circumantarctic Current, continental-scale glaciation of Antarctica and
increased volcanism in the South Pacific. Opening of the second gateway allowing deep
water circulation around Antarctica – the Drake Passage Gateway between western Antarctica
and South America – is insecurely dated but circulation of surface and intermediate water was
occurring in the Late Eocene (Diester-Haass and Zahn 1996). Co-eval events include the reorganisation
of the Indo-Australian and Pacific Plate boundaries, significant stepwise floral
and faunal turnover in both marine and terrestrial realms spanning >10 Ma of the Eocene
(Zachos et al. 1993, Ridgway et al. 1995), and several closely spaced bolide impacts in the
Northern Hemisphere (Kennett 1977, Kennett et al. 1985, Stoffler and Claeys 1997, Vonhof
et al. 2000). Radiometric ages imply that two of the large impacts were contemporaneous
(35.7 ± 0.2 Ma) in central Siberia and off the East Coast of North America (Poag et al. 1992,
Clymer et al. 1996, Bottomley et al. 1997). Interestingly, this date coincides with the Eocene-
Oligocene boundary as defined by Harland et al. (1990) but is ~2 million years older than the
currently adopted date of 33.7 Ma in Australia (Young and Laurie 1996). Vonhof et al.
(2000) propose that these impacts accelerated global cooling during the Late Eocene whilst
Ivany et al. (2000) see cooler winters (cause unspecified) as a possible explanation for
observed mass extinctions at the Eocene-Oligocene boundary.
Recent δ 18 O and magnesium/calcium ratio data indicate that the first major accumulation of
ice on East Antarctica occurred very rapidly during the earliest Oligocene (cf. Zachos et al.
1992, Lear et al. 2000). Associated events include diminished equatorial circulation (Kennett
1977, Barron and Peterson 1991), an abrupt drop of >3 0 C in SSTs in the Southern Ocean (Wei
1991) and the earliest known Tertiary glacial event on any landmass outside of Antarctica –
the transient development of a valley glacier in northwestern Tasmania (Macphail et al. 1993,
Macphail and Hill 1994).
Abrupt cooling at about the Eocene-Oligocene boundary was temporarily reversed during the
Early Oligocene, and warming SSTs recorded in the Great Australian Bight (Kamp et al.
1990) and around Antarctica (Mackensen and Ehrmann 1992) during the Early Oligocene
match global trends elsewhere (Feary et al. 1991). Nevertheless, Shackleton (1986) has
inferred that intense glacial events occurred in East Antarctica at about 31 Mar (Event E) and
24 Mar (Event F). Event E precedes a dramatic (>140 m) lowering of global sea level during
the Early-mid Oligocene (cf. Haq et al. 1987, Pekar and Millar 1996), and cooling in the
equatorial Pacific (Keigwin and Keller 1984). Event F coincides with the first and largest
(Mi-1 Event of Zachos et al. 1997) of several brief glacial episodes marking the Oligocene-
Miocene transition (23.8 Ma).
Brief cooling episodes at high latitudes, which are linked to longer term (100 ka, 400 ka)
Milankovitch cycles, are recorded between ~27-21 Ma (Zachos et al. 1997). Otherwise the
Late Oligocene-Early Miocene is usually considered to have been a time of relative global
warmth in the sense that ocean temperatures were slowly increasing and continental ice
volumes were decreasing from mid-Late Oligocene levels (cf. Miller et al. 1991).
7.1.5 Middle Miocene climatic transition [~13-16 Ma]
On current indications, Oligo-Miocene warming climaxed in the late Early Miocene [~16 Ma]
and was followed by sustained global cooling. Flower and Kennett (1994) propose that the
early [~16-15 Ma] stage of this transition was marked by major short-term instability in
climate and relative sea-level, linked to changes in deep-sea circulation and the volume of ice
making up the East Antarctica Ice Sheet. The late [~15-13 Ma] stage was marked by large
fluctuations in relative sea level followed by a global sea level fall, major ice sheet growth in
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