OFR 151.pdf - CRC LEME

Explanations for the PETM are centred upon a transient increase in greenhouse gases in the
atmosphere (Barron 1985, Rhea et al. 1990) although suggestions as to the trigger vary.
Recent proposals include (1) the exhumation and oxidation of organic carbon during the
collision of India and Asia (Beck et al. 1995) and (2) catastrophic dissociation of gas
(methane) hydrates in the deep ocean (Sloan et al. 1992, Dickens et al. 1995, Bralower et al.
1997, Kerr 1997, Kroon et al. 1998, Yulsman 1999a, 1999b, Blunier 2000). An analogous
but earlier (Early Cretaceous) 'methane hydrate' event is suggested to reflect oceanic turnover
arising from major Caribbean volcanism and opening of the North-East Atlantic (Norris and
Rohl 1999).
Whilst the marine signatures of the PETM coincide with the Paleocene-Eocene boundary,
warmer temperatures continue to be recorded for the next 7 Ma. Globally averaged
temperatures are estimated to have been 2-4 0 C higher than at present throughout the Early
Eocene. As the warmest interval of the Cenozoic, the climate system of the Early Eocene
continues to be studied intensively, including by modelling experiments (Sloan 1994, Sloan
and Merrill 1998). These experiments predict that the warming of very large lakes had a
significant effect on Early Eocene climates in the continental interior of North America
(Sloan 1994) and the same may be true of the larger lake basins in central Australia.
Biological responses to Early Eocene warmth include the migration of the tropical mangrove
palm Nypa southwards to a palaeolatitude of ~66 0 S on the West Coast of Tasmania (Pole and
Macphail 1996) and increased levels of insect damage on fossil leaves preserved in Wyoming
(Wilf and Labandiera 1999). Whether rainfall became more or less seasonal is unclear. For
example, Robert and Chamley (1990) propose that variation in clay mineral associations in
oceanic sediments imply global warmth was associated with alternating wet and dry seasons
in continents bordering the palaeo-Atlantic Ocean, and the same may be true for central
Australia.
7.1.3 Mid Eocene climatic transition [~48 Ma]
The Middle-Late Eocene boundary marks the latest transition between the two opposing
climatic modes experienced by the Earth during Phanerozoic time – the greenhouse (largely
non-glacial) and icehouse (largely glacial) states. For example, high latitude climates
remained warm and equable relative to the present (cf. Greenwood and Wing 1995, Jordan
1996, Wing and Greenwood 1996) and the fossil remains of a basking shark, now unknown in
Subantarctic or Antarctic waters, occurs in Middle Eocene sediments on Seymour Island on
the Antarctic Peninsula (Cione and Reguero 1998). Summer SSTs during the early Middle
Eocene were similar to the present-day United States Gulf Coast whilst winters were 7-8 0 C
warmer due to strong oceanic heat transport (Andreasson and Schmitz 2000). Nevertheless,
for the first time since the Palaeozoic, changes in global ice-volume provide a convincing
explanation for rapid eustatic events (Browning et al. 1996).
Oxygen isotope data show that during the Middle to Late Eocene, global climate cooled in at
least two stages, which were separated by significant warming (Frederiksen 1991, Mackensen
and Ehrmann 1992, Graham 1994). This long-term cooling trend is recorded by climatic
proxies as diverse as clay mineral associations in the Southern Ocean (Robert and Chamley
1990) and megafloral associations in southern California and the Yukon (Frederiksen 1991,
Ridgway et al. 1995). Current explanations invoke plate tectonic movements (with a
subsequent reorganisation of ocean currents) and decreasing concentrations of greenhouse
gases, especially carbon dioxide, due to diminished hydrothermal, tectonic and volcanic
activity. On present indications, atmospheric concentrations of carbon dioxide (ρCO2) were
similar to, or only slightly higher than, present values, based on a pH profile of the Middle
Eocene tropical Pacific Ocean (Pearson and Palmer 1999).
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