Environments - Geochemistry: An Introduction - page 187

0.7100

Cenoz. Cretac. Jurass. Tri. Per. Carbonif. Dev. Sil. Ordovic. Cambr.

0.7095

0.7090

Continental runoff

0.7085

0.7080

0.7075

Interaction with the oceanic crust

0.7070

0.7065

0 100 200 300 400 500

Geological age (Ma)

Figure 9.6

Evolution of the 87 Sr

86 Sr ratio in marine carbonates over geological time. The very low Rb
content of these rocks ensures that the record of the ratio has not been significantly altered by
radioactive ingrowth since deposition. The rapid increase in this ratio since the end of the
Cretaceous is attributed to uplift of the high- 87 Sr

/

86 Sr Himalayas, which accelerated the input to
the ocean of radiogenic strontium from the ancient basement of the mountain range. This
indicates tectonic forcing of the rate of erosion with enormous climatic consequences.

/

9.1.3 Biogeochemical catastrophes in the Phanerozoic

The biogeochemical evolution of our planet is perturbed by occasional events perturbing
the long-term trends described in the previous section. At about 65 Ma, i.e. at the bound-
ary between the Cretaceous and the Paleocene, the Earth was impacted by a large asteroid
(recently identified as the impactor of the Chixculub crater in the Yucatan) which trig-
gered such huge atmospheric and oceanic effects that most living groups, and in particular
dinosaurs and ammonites, disappeared in the aftermath of the collision. The correla-
tion between the so-called K/T boundary mass extinction and a large meteoritic impact
was first identified by the presence all over the world of a thin layer of sediments rich
in iridium, a metal from the platinum family, that is orders of magnitude more abun-
dant in meteorites than crustal rocks. Other mass extinctions, such as that taking place
at the boundary between the Permian and the Triassic, have been ascribed to similar
planetary-wide catastrophes.
The geochemical consequences of the K/T boundary event are quite spectacular: the

13 C of the bulk carbonate deposited either at the bottom of the ocean, as depicted in
Fig. 9.7 for the South Atlantic (Shackleton and Hall, 1984 ) , or on land, decreases instan-
taneously by some 2-3 per mil. It has also been found that the

δ

13 C difference between

δ

13 C gradient in the water column
( Fig. 7.13 ) disappears, which indicates a nearly instantaneous collapse of global biological
productivity.

benthic and pelagic organisms which reflects the δ