Terrestrial Palaeoecology and Global Change

Chapter 9. Crises
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a
b
c
d
Fig. 138. Iridium spike at the Cretaceous–Tertiary boundary at Sussex, Powder River Basin, Wyoming: (a)
view of the outcrop, the transboundary sequence dug up, (b) the boundary of the Maastrichtian marine shales
and the Palaeocene coal-bearing strata; red pen-knife points to the “boundary clay” beneath the lower coal
seam, (c) light grey “boundary clay” on eroded surface of the dark grey Maastrichtian shale; the palynological
boundary occurs at the base of the tonstein, iridium is concentrated in the pedogenic ochric layer on top of it,
(d) surface of the ochric layer with plant debris.
Since ferns often spread over fresh ashbeds (Kornaú, 1978), the association of fern
spores with tonsteins has an ecological explanation (in addition to the taphonomic one,
above). In a thick trans-KTB tuffaceous sequence of Augustovka River, Sakhalin (Krassilov,
1979; VII.6), a fern bed, the greenish-grey pelitic tuffite with fronds of Cladophlebis
(Osmunda), Dicksonia and Onoclea on the bedding plane, overlays a coaly fossil
soil with horsetail rhizomes, indicating waterlogging in association with the ashfall.
Fern spikes have also been recorded at the equivalent stratigraphic levels in other
parts of the world allegedly indicating a global deforestation event (Vajda et al., 2001).
However, if representing a pioneer phase of temperate rainforests, like the presentday
tree-fern groves in the Nothofagus belt of southern Australia (Fig. 139), they
might have actually signalled afforestation at the expense of the Cretaceous open
shrublands (IX.9.1).
The iridium enrichment at the KTB has inspired a search for similar phenomena
elsewhere over the Phanerozoic sequences. And the anomalies have been actually found
at several critical stratigraphic levels from the Precambrian to Eocene, including the