Page 88 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 5: RHYTHMS AND EVENTS IN CARBONATE STRATIGRAPHY 79

LONG OSCILLATIONS IN THE Hardie (1996) showed that the greenhouse phases of the
OCEAN-ATMOSPHERE SYSTEM Fischer-Sandberg wave correlate with times of marine evap-
orites rich in KCl, the icehouse phases with evaporites rich
in MgSO 4 (Fig. 5.9c, d). He proposed that changes in sea-
Autocycles as well as orbital rhythms can be studied in
water composition rather than fluctuations in atmospheric
outcrop. In addition, the past few decades have produced
CO 2 levels had caused the alternation of aragonite seas and
convincing evidence for rhythms in the sediment record at
7
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time scales of 10 to 10 years. These rhythms are rarely calcite seas in the past 600 My. Hardie (1996) also pointed
out that hydrothermal alteration of young basalt at spread-
obvious at the outcrop scale; they were discovered by inten-
ing ridges leads to net transfer of Mg and SO 4 from seawa-
sive collaboration among paleoceanographers, paleoclima-
tologists, geochemists, sedimentologists and stratigraphers ter to rock, and of K (and Ca) from rock to seawater. Thus,
and tectonicists. For most of these rhythms it remains open high rates of sea-floor spreading reduce the concentration
if they are truly periodic. However, almost all of them can of Mg and SO 4 in the sea and increase the concentration
be shown to be true oscillations rather than undirectional of K. The hydrothermal pump, therefore, offers an expla-
trends with occasional incisions, such as the record of biotic nation for the abundance of KCl-evaporites at times of high
evolution. sea level and fast spreading, and the abundance of MgSO 4
Environmental variables showing long oscillations that evaporites at times of slow spreading. Consequently, Hardie
are particularly relevant for carbonate sedimentation and (1996), like Sandberg (1983), considered changes in the rate
of plate tectonics the ultimate driver of the aragonite-calcite
sequence stratigraphy are eustatic sea level, sea-surface
oscillations, albeit with a coupling mechanism in the form
temperatures, ice cover and the primary mineralogy of
of sea-water chemistry rather than atmospheric CO 2 .Stan-
carbonates and evaporites. The causes invoked for these
ley and Hardie (1998) expanded the concept of aragonite-
oscillations fall in three categories:
calcite oscillations in the sea by demonstrating that it is not
(1) oscillations of the rate of plate motion;
restricted to abiotic precipitates. Organisms with relatively
(2) celestial oscillations (solar system and beyond);
weak internal control on the calcification process or organ-
(3) autocycles in complex systems with numerous feed-
isms that calcify very rapidly (“hypercalcifiers”) also fol-
backs.
low the aragonite-calcite wave in the Phanerozoic. Dickson
(2004) added important data gleaned from echinoderm re-
The course of research on carbonate-relevant oscillations
mains.
in the past three decades shows first a convergence of opin-
Veizer et al. (2000) challenged the 300 My icehouse-
ions followed by renewed discrepancies and divergence, de-
greenhouse cycle with a large data set on temperature-
spite growing data and insight (Fig. 5.9).
sensitive oxygen-isotope ratios from pristine carbonate
Long-term eustasy was the first variable to be quantified shells (Fig. 5.9e). They discovered persistent oscillations
for the Phanerozoic by Hallam (1977) and Vail et al. (1977) with an approximate period of 135±9 My. The strong cor-
(Fig. 5.9a). Both curves showed a little less than two cycles relation of this signal with the distribution of ice-rafted de-
in the Phanerozoic with highs in the mid-Paleozoic and Cre- bris and other glacial indicators supports its climatic signif-
taceous. In a bold synthesis, Fischer (1982) combined this icance. Veizer et al. (2000) report a drastic mismatch with
eustatic curve with the new insights on the history of cli- the 300 My icehouse-greenhouse cycle as well as with di-
mate and ocean circulation obtained mainly by the Deep-Sea
rect estimates of atmospheric CO 2 levels; they reject CO 2
Drilling Program (Fig. 5.9b). He proposed that the Earth was variations as a possible driver of the oscillations. Shaviv
oscillating between a greenhouse state with high sea level, and Veizer (2003) argue for an astronomic cause of the 135
equable climate and high levels of atmospheric CO 2 ,and an My cycle (Fig. 5.9f). They point out that the flux of cos-
icehouse state of low sea level, large climatic gradients be- mic rays is modulated by the passage of the solar system
tween poles and equator, and low CO 2 levels. Fischer (1982) through the spiral arms of the milky way, creating an oscil-
estimated the ice-house-greenhouse cycle to be about 300 lation of 143±10 My, very similar to the 135±9 My oscilla-
My long and be driven by changes in the rate of mantle con- tion in oxygen-isotope ratios. High cosmic-ray flux leads to
vection and plate motion. Sandberg (1983) linked Fischer’s more low-level clouds, higher albedo and therefore cooler
icehouse-greenhouse cycles with changes in the mineralogy climate. Royer et al. (2004) reject the notion of strong celes-
of abiotic carbonate precipitates in the ocean and proposed
tial control on climate and maintain that the case for CO 2
changes in atmospheric CO 2 content as the causal connec- as the principal driver is robust. They correct Veizer et al.’s
tion (Fig. 5.9d). (2000) isotope data for likely pH effects. This modification
The contributions by Fischer (1982) and Sandberg (1983) produces a better match with the CO 2 record and climate
were very influential. “Icehouse world” and “greenhouse models but the 135 My oscillation remains.
world” became household terms and many aspects of the This brief tour through the science of long oscillations
sediment record were tied to this long oscillation. In in Phanerozoic climate, sea-level and carbonate chemistry
the past decade, the notion of long, CO 2 -driven icehouse- shows that, at present, the situation is heaven for a scientist
greenhouse cycles was both significantly expanded and fun- writing proposals for funding and hell for a book author try-
damentally challenged. ing to extract the eternal truth of the matter. This quandary