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the middle of the paleomagnetic chron 29r (Keller et al., 2009, fig. 5). Oxygen isotope
analyses (Li & Keller, 1998) and the response of microorganisms to water temperature (e.g.,
Abramovich & Keller, 2002; Abramovich et al., 2010) reflect fluctuations in surface and
intermediate depth ocean water temperature during the Late Maastrichtian, especially in
the latest 0.5 Myr. Fluctuating cool temperatures (average degrees of 9.9
0
C intermediate
and 15.4
0
C surface water) during 66.85 and 65.52 Myr were followed by a short-term
warming between 65.45 and 65.11 Myr which increased intermediate water temperatures
by 2-3
0
C, and decreased the vertical thermal gradient to an average of 2.7
0
C (Li & Keller,
1998). A previous study by Stüben et al. (2003) on hemipelagic sediments of Tunisia
differentiated three cool periods (65.50-65.55, 65.26-65.33, 65.04-65.12 Myr) and three
warm periods (65.33-65.38, 65.12-65.26, 65.00-65.04 Myr). Tantawy et al. (2009, p. 85) point
out that “
the biotic effects of volcanism have long been the unknown factors in creating
biotic stress. The contribution of the Deccan volcanism to the K-T mass extinction
remained largely unknown, although recent investigations revealed that the main
phase of Deccan volcanism coincided with the K-T mass extinction
”. Keller et al. (2009,
p. 723-4) refer to “
the dust clouds obscuring sunlight and causing short-term global
cooling”
as the result of the volcanic eruptions, but
“how Deccan volcanism affected the
environment and how it may have led to the mass extinction of dinosaurs and other
organisms in India and globally is still speculative
”. The direct cause for seawater
temperature fluctuations during the Maastrichtian last half million years and the dwarfed
microfossils in this time interval (Keller, 2008) are herein related to sunlight screening by
volcaniclastic dust from the Deccan volcanism, suggesting that its main activity extended
over the same period.
Most Late Maastrichtian planktonic microorganisms reduced their size (dwarfing) at about
65.4 Myr (Keller, 2008) reaching sexual maturity at smaller dimensions and probably more
rapid than their normally-sized ancestors. This assemblage became dominated by low-
oxygen-tolerant small heterohelicids (Keller & Abramovich, 2009). All these globally
detected abnormal morphological and ecological aspects of the latest Cretaceous marine
microfossils attest to the deterioration of the ecological conditions, as the result of sunlight
screening and darkening of the Earth to various extents and periods. Global darkening of
the atmosphere by fine volcaniclasts decreased photosynthetic activity of the symbiotic
zooxanthellae in extreme cases these useless symbionts were digested by their host. The
dwarfing of the latest Maastrichtian microfossils was artificially demonstrated by the
elimination of these symbiotic dinoflagellates from within the planktonic foraminifer
Globigerinoides sacculifer
(Bé et al., 1982). The loss of symbionts resulted in early
gametogenesis (at small size), short life span of the foraminifer and its smaller shell size at
sexual maturity (dwarfing), exactly as described from the latest Maastrichtian planktonic
foraminifera and calcareous nannoplankton. When the tested live foraminifers were
reinfected by zooxanthellae they resumed normal shell growth and size as before the
removal of the symbiotic zooxanthellae (Bé et al., 1982). The lack of planktonic microfossils
of normal size in the latest Maastrichtian 0.5 Myr indicates that solar radiation was, during
this period, too low to resume symbiotic relationships between these photosynthesizing
dinoflagellates and the microorganisms. The drastically reduced photosynthetic activity
lowered the oxygen content in the upper water column as attested to by the increased
abundance of low-oxygen-tolerant small heterohelicids and the blooming of the disaster
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