Environments - Geochemistry: An Introduction - page 188

4.5

4.0

Bulk carbonate
(Walvis Ridge, S. Atlantic)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Eocene Paleocene Cretaceous

0.0

45 50 55 60 65 70

Age (Ma)

Figure 9.7 Carbon isotope compositions of the bulk carbonate in sediments from the Deep Sea Drilling

Project hole 237 cored on the Walvis Ridge, South Atlantic (Shackleton and Hall, 1984 ). The sharp
drop of the

13 C values at the Cretaceous-Paleocene boundary and the lack of a C isotope
difference between benthic and pelagic species (not shown) indicate that burial of organic
material with negative

δ

13 C suddenly came to an end, thus signaling the failure of organic
productivity. In contrast, the sharp decline of the

δ

13 C values at the end of the Paleocene is visible
in both benthic and pelagic species: it is believed that this event represents the sudden release of
methane from isotopically light methane hydrate trapped in sediments into the atmosphere.

δ

Other events have a different geochemical signature and the associated mass extinction
hence presumably must be due to different effects. The Paleocene-Eocene boundary at
55 Ma ( Fig. 9.7 ) is also characterized by a sharp

13 C decline, but this time both the ben-
thic and pelagic organisms are shifted by approximately the same amount. It is believed that
large amounts of isotopically light carbon were suddenly released into the ocean and the
atmosphere. A common interpretation is the destabilization, for reasons that still remain
to be established, of very large masses of methane hydrates present in sediments and
permafrost and the liberation of large amounts of lethal methane gas into the atmosphere.
A third type of biogeochemical catastrophes which, however, have rarely led to extinc-
tions of the same amplitude as meteoritic impacts, are the oceanic anoxic events (OAEs).
These are episodes of deposition of sulfide-rich anoxic sediments rich in heavy metals
known as black shales and resembling the organic-matter-rich, oxygen-starved laminites
deposited at the bottom of the Black Sea today. The best known of these short events took
place during the Toarcian (183 Ma), the early Aptian (120 Ma), and at the Cenomanian-
Turonian boundary (93 Ma). It is not well understood whether they represent oxygen
deficiency in the water column such as at the bottom of the Black Sea, or rather increased
burial of organic material.
The unstable character or the redox conditions of the oceanic system is best
demonstrated by the secular evolution of sulfur isotopes since the Cretaceous ( Fig. 9.8 ).

δ