OFR 151.pdf - CRC LEME

SECTION 6 (LATE CRETACEOUS CLIMATES)
6.1 Global backdrop
Equator to pole temperature gradients remained relatively low during the Cenomanian to
Maastrichtian but the poleward transport of heat via warm, saline water was far from uniform.
For example, differences in surface water temperature between low and high latitudes are
estimated to have been about 1-4 0 C during the Coniacian-Santonian and up to 14 0 C during the
Late Albian and Late Maastrichtian.
Most palaeotemperature estimates are based on stable isotope (δ 18 O, δ 13 C) analyses of Albian-
Maastrichtian foraminifera recovered from deep-sea drilling (DSDP/ODP) sites. An
exception is Horrell (1991) who has used a combination of geological and biological evidence
to reconstruct global climatic zones for the Maastrichtian. Leaf margin analysis of floras
preserved near the North Pole show mean annual air temperatures of 10 ± 3 0° C during the
Cenomanian (Herman and Spicer 1997) whilst temperatures during the Coniacian and
Campanian-Maastrichtian were 2-3 0 C and 2-8 0 C higher, respectively (Parrish and Spicer
1988). δ 18 O data from the Naturaliste Plateau (palaeolatitude 58 0 S) in the southern Indian
Ocean, the Falkland Plateau (palaeolatitude 58 0 -62 0 S) in the South-west Atlantic Ocean, and
Weddell Sea (palaeolatitude 65 0 S) indicate similar warming at middle to high palaeolatitudes
in the Southern Hemisphere during the Cenomanian. Clarke and Jenkyns (1999) propose that
global SSTs peaked sometime between the Cenomanian-Turonian boundary and the Middle
Turonian, a period when global relative sea levels were higher than at any other time during
the Mesozoic (Haq et al. 1987). The timing has been challenged by Kuypers et al. (1999),
based on δ 13 C evidence for a large drawdown in atmospheric CO2 at the
Cenomanian/Turonian boundary, which implies major climatic cooling during the Early
Turonian.
SSTs remained relatively warm into the early Early Campanian but began to cool during the
late Early Campanian (Huber et al. 1995). This cooling, which continued throughout the
Maastrichtian, has been linked to the regression of epicontinental seaways from North
America, Europe, Asia, South America and Africa (Frank and Arthur 1999). The same
authors propose that a major reorganisation of ocean circulation patterns occurred at the midlate
Maastrichtian boundary, resulting in the development of a thermohaline circulation
system similar to that of the modern oceans. Miller et al. (1999) interpret a very rapid drop of
30-40 m in relative sea level, which occurred at about the same time, as evidence for the
development of moderate-sized ice sheets on high latitude landmasses.
Evidence is mounting that Late Cretaceous cooling was terminated by a short-lived warming
event during the last 0.5 million years of Maastrichtian time although SSTs in the equatorial
Pacific were only as warm (~27-29 0 C) as now (Wilson and Opdyke 1996). Conversely, Li
and Keller (1998) report rapid cooling occurred within 100,000 years of the
Cretaceous/Tertiary (K/T) boundary at one mid latitude site in the South Atlantic. Johnson et
al. (1989) report increased megafloral turnover below the K/T boundary in North Dakota but
the change is smaller than turnover at the K/T boundary. Jeffery (1997) proposes climates
became increasingly seasonal and unstable over a broad latitudinal range across the K/T
boundary. The hypothesis that the K/T boundary reflects the impact of a large (~10 km)
bolide onto the Yucatan Peninsula, Mexico is almost universally accepted (Norris et al. 1999).
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