Page 10 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 1

Essentials of neighboring disciplines

INTRODUCTION winds and the Coriolis force such that thermocline water
and deep water well up. Deep water also rises to the surface
Unraveling Earth history is a core business of geology. in the Antarctic Current because of the current’s extremely
However, understanding cause and effect of past events re- low density gradient. More gentle upwelling occurs in the
quires input from other disciplines that can study processes equatorial Pacific where the opposing directions of the Cori-
directly and do not have to reconstruct them from incom- olis force of the northern and southern hemisphere drive the
plete historic records. This introductory chapter summa- surface layer away from the equator. See Fig. 1.2 for cartoons
rizes a very limited number of concepts from neighboring of upwelling mechanisms.
disciplines that are relevant in this respect. The list is woe- An important principle in all interactions between atmo-
fully incomplete. I did not try to simply cover the most im- sphere and ocean is Ekman transport: wind blowing over
portant concepts; rather, I selected those that are highly rele- the ocean induces a current in the surface layer approxi-
vant for the topic of this book yet not sufficiently important mately at right angles to the wind. This current moves to
for geology at large to be routinely covered by introductory the right of the wind in the northern hemisphere and to the
texts or courses in geology. left in the southern hemisphere.
When using the modern oceans as models for the geologic
SOME PRINCIPLES OF OCEANOGRAPHY past, we should keep in mind that currently temperature has
a greater effect on the density stratification than salinity. The
Layering of the oceanic water column main reason for the dominance of temperature are the ice
caps of the poles. At times in the past when little ice was
The density of ocean water varies as a function of temper- present, salinity seems to have been the dominant factor in
ature and salinity. Under the influence of the Earth’s grav- density stratification (e.g. Hay, 1988).
ity field denser water sinks and lighter water rises and this
creates a density-stratified, layered ocean (Weyl, 1970; Open Present surface circulation of the oceans
University, 1989a, 1989b). In first approximation, three lay-
ers may be distinguished: (Fig. 1.1) The surface circulation greatly influences the distribution
➤ A surface layer, where waves and currents are suffi- of temperature, salinity and nutrients in the surface layer
ciently strong to preclude the formation of permanent and those properties, in turn, are important controls on car-
density gradients; the surface layer is thus well mixed bonate precipitation and deposition. We shall examine the
and in equilibrium with the atmosphere with regard to present circulation to derive guidelines for interpreting the
oxygen and other chemical agents. past. For more detail, see Weyl (1970), Broecker and Peng
➤ The thermocline, where density increases steadily (1982), Open University (1989a, 1989b), Emiliani (1992, p.
downwards and vertical mixing is greatly inhibited. 254-309).
➤ The oceanic deep water, where density slowly increases The chain of cause and effect between atmosphere and
downward but the vertical gradient is so low that it is surface ocean starts with the Sun. It heats the equatorial
easily disturbed by horizontal flow. belt of the Earth much more than the polar regions. As a
Water in all layers circulates. Circulation in the surface consequence, the air over a stationary Earth would rise over
layer is driven by winds in the atmosphere. The deep the equator, flow poleward at high altitude, cool, sink and
layer presently forms one world-wide circulation system flow back towards the equator near the Earth’s surface. This
that moves slowly under the influence of minor density gra- simple circulation model immediately calls for two essential
dients. modifications:
Surface layer and deep layers are generally well separated ➤ the Coriolis force of the rotating Earth deflects the flow
but they communicate in certain, well defined areas. In the of air. Air starting to move from pole to equator will be
northern North Atlantic and around Antarctica, surface wa- deflected to move from east to west as the Coriolis effect
ter becomes so dense that it sinks and joins the deep-water turns it to the right in the northern and to the left in the
body. Conversely, along the west-facing coastlines of Africa southern hemisphere. Conversely, air starting to flow
and the Americas the surface layer is driven offshore by poleward from the equator will turn to flow eastward.

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