OFR 151.pdf - CRC LEME

7.1. Global backdrop
SECTION 7 (TERTIARY CLIMATES)
During the Tertiary, the earth as a whole underwent progressive warming, then cooling/drying
episodes (Frakes 1999). Although no single factor is likely to be paramount in any region,
patterns of Tertiary climatic change are increasingly being linked to changes in deep ocean
circulation associated with continental drift (Zachos et al. 1994).
As in the Cretaceous, global trends were interrupted or sharpened by a number of short-term
climatic excursions whose impact was variable. For example, there is increasing evidence
that the earth suffered a number of major impacts from extraterrestrial bolides during the
Tertiary although the climatic consequences continue to be debated. Farley et al. (1998) cite
geochemical evidence for a comet shower extending over a 2.5 million year period in the Late
Eocene. Two impacts produced major craters – off the East Coast of America (Chesapeake
Bay Crater) and in Siberia (Popigai Crater). The more important of these geologically shortterm
climatic excursions are: the Cretaceous-Tertiary boundary event at about 65 Ma; the
Paleocene-Eocene Thermal Maximum at about 55 Ma; and four events linked to changes in
ocean circulation, being the Eocene-Oligocene transition linked to the opening of the
Tasmanian Gateway between Australia and Antarctica at about 33 Ma, the mid Miocene
climatic transition between 21-27 Ma, mid Pliocene warming at about 3 Ma and Plio-
Pleistocene bipolar glaciation after 2.5 Ma.
7.1.1 Cretaceous-Tertiary boundary (K/T) event [~65 Ma]
A number of events, which began during the late Late Cretaceous, culminated near to, but
before, the Cretaceous-Tertiary (K/T) boundary. These range from abnormally high levels of
volcanism (Sutherland 1994) and increased terrestrial erosion (Robert and Chamley 1990) to a
decline in dinosaur diversity (Sereno 1999) and major changes in climate and ocean chemistry
(Barrera 1994).
A common explanation is marine regression during the latest Maastrichtian, e.g. Barrera et al.
(1997) and Eaton et al. (1997). Nevertheless, unlike earlier geological boundaries, the
evidence is compelling that the Cenozoic era was initiated by the impact of a ~10 km
diameter bolide moving at ~20 km sec -1 at Chicxulub in the Gulf of Mexico (Hildebrand et al.
1995, Melosh 1995, Kerr 1997, Kroon et al. 1998). Shocked quartz has been recorded as far
north as the Gosau Basin in Austria (Preisinger et al. 1986). More immediate effects include
the deposition of impact breccias up to 900 m thick at a distance of 100 km from the crater,
the collapse of the shelf margin and emplacement of tsunami (megawave) deposits on the
continental shelves in Texas and northern Mexico (references in Keller et al. 1997, Bralower
et al. 1998).
Observed or predicted global-scale phenomena include abrupt mortality in pelagic organisms
(Paul and Mitchell 1994, Kaiho et al. 1999), collapse of deepwater benthic foraminiferal
communities (Coccioni and Galeotti 1994), massive reduction in marine productivity (Barrera
and Keller 1994), a world-encompassing dust cloud (Covey et al. 1994), cooling due to
sulphur released into the atmosphere (Ward 1995), acid-rain (D’Hondt et al. 1994), the
possible collapse of the hydrological cycle (cf. Covey et al. 1994, Vonhof and Smit 1997), the
geologically instantaneous combustion of ~18-24% of the terrestrial above-ground biomass
(Ivany and Salawitch 1993, Arinobu et al. 1999), lava outpouring in India (Geotimes 1995),
and the release and ignition of huge amounts of methane trapped in seafloor sediments (Day
1999).
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