Page 89 - Carbonate Sedimentology and Sequence Stratigraphy

80 WOLFGANG SCHLAGER

notwithstanding, some important messages clearly emerge. the rate of change in accommodation and the rate of sedi-
➤ There is strong evidence that ice-free greenhouse condi- ment production are very similar. So similar, in fact, that
tions were rare in the Phanerozoic. According to Frakes we could probably not distinguish Proterozoic rimmed plat-
et al. (1992) and Crowell (2000), ice-rafted debris occurs forms from Neogene ones in seismic data.
in about 60% of Phanerozoic time if one uses a 5 My Empty buckets may serve as a case in point. This char-
window. This implies that glacio-eustatic fluctuations acteristic geometry of raised platform rims and deeper la-
are more of a rule than an exception in the Phanero- goons is conspicuously developed on extant platforms with
zoic. Gibbs et al. 2000 conclude from modelling studies scleractinian reefs or oolite shoals forming the rim. Analo-
that significant polar ice caps may form at 10 - 14 times gous geometries have been reported from various parts of
present CO 2 levels given suitable paleogeography. the Phanerozoic as well as the Proterozoic (Fig. 3.23 3.24;
18
➤ Evidence for ice and temperatures estimated from δ O Meyer, 1989; Wendte et al. 1992; Playford et al. 1989; Van
oscillated with an estimated period of 135 My, i.e. less Buchem et al., 2000; Grotzinger and James, 2000; Adams et
than half the period of the icehouse-greenhouse cycle of al. 2004).
Fischer (1982), estimated as 300 My. Perhaps the most conspicuous effects of Phanerozoic bi-
➤ The 300 My cycle is in phase with chemical changes, otic evolution are relatively short disturbances caused by
i.e. the alternation of KCl and MgSO 4 evaporites, of extinction events. The detailed record of the Phanerozoic
aragonite and calcite seas, and certain proxies for atmo- shows that certain extinction events did affect the carbon-
spheric CO 2 levels. ate production systems (Fig. 5.10). Some events caused pro-
duction to shift from the T factory to the M factory, but
there were also events that caused a shift in the opposite di-
BIOTIC EVOLUTION
rection and still other events shifted production from one
metazoan group to another within the T factory. Hottinger
The topic of evolution takes us back to a point made in
(1989) argued that extinction of reef builders may take mil-
chapter 1: Biotic evolution is one of the most significant
lions of years to repair because these organisms typically are
causes of change in the geologic record, rivaling the effects
K strategists with very long life cycles and therefore slow
of plate tectonics and chemical cycling. Throughout the Late evolution. For instance, Hottinger estimates that it took 5
Proterozoic and Phanerozoic, the time interval with detailed – 6 My to rebuild vigorous reef communities after the end-
sediment record, species and higher taxonomic units came Cretaceous extinction.
and went. Particularly in the Phanerozoic, severe extinc- Data on the number of Phanerozoic reefs (Fig. 5.11) show
tions are followed by short intervals of rapid speciation and significant variations with some rapid declines related to ex-
longer periods of slow change, in agreement with the con-
tinctions. There is also some indication that the maximum
cept of punctuated equilibria (Eldredge and Gould 1972).
carbonate production, a crude measure of the growth poten-
The profound effect of evolution on the sediment record is
tial of reef communities, decreased in the wake of major ex-
beyond doubt and provided a solid basis for biostratigraphy
tinctions (Bosscher and Schlager, 1993; Flügel and Kiessling,
already 150 years ago.
2002, Fig.1).
The scope of this book, however, leads to a more spe-
Arguably the most drastic change in Phanerozoic carbon-
cific question: what are the effects of evolution on the ba-
ate sedimentation is the advent of calcareous plankton and
sic functioning of the carbonate system, on loci and rates of
nannoplankton. Estimates of carbonate sediment accumula-
carbonate production, facies zonation, construction of plat-
tion on continental margins and epeiric seas, on slopes and
forms. In the Phanerozoic carbonate world, most evolution-
rises, and the pelagic centers of the ocean basins reveal a
ary change falls in the category: the actors change but the
fascinating pattern: the overall rate of carbonate accumu-
play goes on, the play being the basic functioning of the
lation seems to have increased several-fold in the past 100
system of carbonate production, deposition and early dia-
My and the locus of accumulation has shifted from conti-
genesis. This statement may sound bold and off the mark
nents to oceans (Fig. 5.12’. The onset of significant pelagic
in view of the drastic evolutionary changes in carbonate-
carbonate deposition on ocean crust about 100 My ago is the
secreting biota during the Phanerozoic. I consider it too con-
likely cause of both trends (Hay, 1985; Veizer and Macken-
servative. The processes for the construction of flat-topped, zie, 2004). In the Late Cretaceous and Cenozoic, planktonic
rimmed platforms with a zonation analogous to Wilson’s foraminifers and coccolithophorids progressively replaced
standard facies belts (chapter 4) evolved in the Archaean shallow-water benthos in precipitating carbonate from the
and the early part of the Proterozoic. The platform had ocean. This explains the shift from continents to oceans in
a “modern” anatomy by Neoproterozoic times (Hoffman, carbonate accumulation. The increase in total carbonate ac-
1974; Grotzinger and James, 2000). This does not mean, of cumulation probably relates to the fact that the pelagic sedi-
course, that modern scleractinian reefs and Proterozoic stro- ment is mainly deposited on ocean crust and therefore more
matolite reefs are comparable ecosystems with similar food rapidly recycled than the carbonate rocks on the continents.
webs, carbonate precipitation mechanisms etc. At that level, The “carbonate mill” seems to be grinding faster since the
the differences are enormous but the gross anatomy of the
changeover to pelagic sedimentation.
respective accumulations and their fundamental control by