Page 20 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINES 11

Fig. 1.16.— Salinity effects. Diver-
relative number of species the two most stable environments – fresh
sity of aquatic fauna and flora plotted
against water salinity. Diversity peaks in

water and open-ocean sea water. Re-
stricted marine environments are char-
acterized by large variability of salinity.
This reduces species diversity.
Tem-
perature variations have similar effects
and tend to occur together with salinity
variations. After Remane and Schlieper
(1971), modified.
50 100 200 300
water salinity
is probably more than a coincidence that the diversity max- Ecologic concepts such as logistic growth and r-K life his-
ima correspond to two salinity ranges that are commonly tories were certainly valid in the past. The difficulty is
available at the Earth’s surface: fresh water is continuously to properly apply them to organisms that differ from their
supplied by condensation of water vapor from the atmo- modern counterparts because of evolutionary change. For
sphere and normal sea water is present in large quantities instance, the question of which invertebrate carbonate pro-
in the ocean. Sea-water composition changes through geo- ducers of the past had photosynthetic symbionts is a matter
chemical cycling but the rate of change is very slow com- of intense debate among paleontologists. The answers are
pared to the rates of biotic speciation and extinction (see highly relevant for modeling carbonate production of the
chapter 5). Regardless of the reason for the salinity-diversity past (see chapter 2).
relationship in Fig. 1.16, the pattern provides an excellent A similar situation holds for oceanography. We have no
tool for determining the degree of restriction of carbonate reason to doubt that general principles of oceanic layering
depositional environments, i.e. the extent to which the envi- and circulation, such as mixed layer, thermocline, Ekman
ronment was cut off from exchange with the open sea (see transport etc. were valid in the past. It is also virtually
chapter 2). inevitable that surface circulation was always largely gov-
Paleontologists have collected large data sets on the num- erned by wind shear and deep circulation by temperature-
ber of fossil species in sedimentary rocks and their change salinity related density differences. This notwithstanding,
through geologic time. After correction for preservation ef- the circulation patterns of past oceans differed considerably
fects, these data provide a basis for estimating biodiversity from the modern situation. One reason is that plate tecton-
of past environments. ics continually changed the shapes and positions of ocean
basins and continents. For instance, the globe-circling cur-
rent around Antarctica developed in the Oligocene when
From oceanography , chemistry and biology to geology tectonic movements opened critical seaways. On the other
hand, Cenozoic closure of the seaways between North and
South America, and Europe and Africa blocked the globe-
The oceanographic, chemical and biological concepts pre- circling equatorial current that had existed in the Late Juras-
sented above were derived from the study of the modern sic and Cretaceous. Another reason for major changes in
world, typically based on observations at time scales of sec- oceanic circulation may be the conditions at the poles. For
onds to hundreds of years. Study of the geologic record re- instance, Hay et al. (2004) questioned that the large gyres
quires expanding the scope to scales of thousands of years to in the surface ocean could form under reduced temperature
hundreds of millions of years. The principles of this chapter gradients such as in the Mid-Cretaceous. They suggested an
remain valid but new processes come into view that operate alternative model of surface circulation by ephemeral, mi-
at these expanded time scales. The most important ones are grating eddies.
biotic evolution, plate tectonics, and geochemical cycling, Finally, ocean chemistry as well as the mineralogy of
i.e. the chemical fractionation and recombination of materi- carbonate skeletons varied in the past, mainly because of
als as a result of mantle convection and plate tectonics. The changing rates of chemical cycling through the crust and the
kind of interaction between these long-term processes and mantle and evolutionary changes of biota at the Earth’s sur-
the principles just described may be illustrated by a few ex- face. Chapter 5 discusses examples of such oscillations that
amples. directly affected carbonate sedimentation.