Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
189
The Cenomanian–Turonian black shale episodes over the Mediterranean might have
been related to a massive input of plant material with terrestrial runoff from Africa in
much the same way as the mid-Pleistocene sapropel deposition reflect high flood periods
of the Nile (Rossignol-Strick et al., 1998). The coeval plant localities in shallow-water
marine facies, e.g., the Cenomanian of Nammoura, Lebanon (Krassilov & Bacchia,
2000) or the Turonian of Gerofit, Israel, are evidence of terrestrial injections, from an
enormously productive coastal vegetation in the latter case (Fig. 80).
Shallow-water black shale deposition is ascribed either to a high biotic productivity of
surface waters or to a density stratification providing for an enhanced preservation of
organic matter below the picnocline (Demaison & Moore, 1980; Wignall, 1984; Wignall
& Newton, 2001). Black shale facies occur either at the beginning or over the culmination
of transgressive sequences (Wignall & Newton, 2001). The origins of the basal
black shales (boundary clay) seem to have been related to ponding of estuaries by a
rising sea and a concomitant eutrophication by nutrients coming with continental runoff
(as in the Permian/Triassic Nedubrovo sequence: Krassilov et al., 1999a, 2000; IX-3).
At the maximal transgression phase, the decisive factor might have been a circulation of
water masses in the arising deep ocean – epeiric sea system, with an outflow of hypersaline
waters from epeiric seas imposing a thermal/density stratification in the ocean.
This mechanism explains a constant association of thick black shale sequences with
climate warming (Fig. 81): density stratification results in a lesser CO 2
uptake by the
oceans that counterbalance a sink of atmospheric CO 2
to massive organic deposition.
The lacustrine black shales owe their origin to high biotic production rates compensated
for by a rapid removal of organic matter from epilimnion to anoxic hypolimnion.
These conditions preferentially occur in taphrogenic lakes of intracontinental rift zones,
such as Tanganyika of the African rift system or the Cretaceous trans-Asiatic rift zone
of Transbaikalia, Mongolia and northern China. The climatic control is primarily related
to seasonality of precipitation – terrestrial runoff enhancing biotic production and stratification
of the water column.
Fig. 80. Gerofit locality, western flank of the Dead Sea rift valley, southern Israel: view of a corbonate/shale
sequence and accumulation of plant debris in the Turonian turbidite deposits, a possible source of eutrophication.