Page 11 - Carbonate Sedimentology and Sequence Stratigraphy

2 WOLFGANG SCHLAGER

Fig. 1.1.— Upper water column of the tropical and temperate parts of the ocean. A) Principal layering of the water column.
B) Temperature profile (solid line) indicates low gradients in the mixed layer, extremely high gradients in the uppermost thermocline,
high gradients in the lower thermocline and low gradients in the oceanic deep water. C) Characteristic profiles of the concentration
of dissolved oxgen and phosphate (a nutrient). These profiles reflect the interplay of the density structure of the water, gas exchange
with the atmosphere and organic growth and decay. Oxygen and nutrient levels are crucial for subdividing the carbonate producing
environments. See text for details.

➤ a single circulation cell per hemisphere cannot be sus- der the trades and returns to the east under the westerlies.
tained because of rapid cooling in the upper atmo- The smaller subpolar gyres circle counterclockwise under
sphere and the speed of the Earth’s rotation. The single the combined effect of westerlies in the temperate zone and
cell breaks up into three cells where air rises and sinks easterlies in the subpolar region. In the equatorial region,
as shown in Fig. 1.3A. The three cells induce three ma- our model ocean shows two narrow, counterclockwise gyres
jor windfields per hemisphere – easterly trade winds in as part of the water piled up against the western border of
the low latitudes, westerlies in the temperate belt and the ocean returns eastward in the equatorial calm zone, the
easterlies again in the subpolar regions. doldrums.
The three wind fields between pole and equator induce sur- Fig. 1.4 depicts the real surface circulation of the world
face currents in the ocean. If the Earth were entirely cov- ocean. It shows most features predicted Weyl’s (1970) model
ered by water, we would see globe-circling currents flow- ocean. The most significant departure is around Antarctica
ing from east to west near the poles, from west to east in where north-south land barriers are absent so that a globe-
the temperate zone of westerly winds and from east to west circling current, the Antarctic Current, could develop. Fur-
again in the subtropical belt of trade winds. Around the thermore, the narrow equatorial gyres and the counter cur-
equator where winds are weak and irregular, surface cur- rent separating them are weakly developed in the Atlantic
rents will also be weak. The present distribution of land where the equatorial system points WNW-ESE because of
masses severely obstructs the development of globe-circling the peculiar shape of the continental borders, and in the In-
currents. Pacific and Atlantic both extend from the Arctic to dian Ocean, where India deforms the northern subtropical
the Antarctic but are bounded in the west and east by con- gyre and monsoonal circulation disturbs the trade-wind sys-
tinents or archipelagos. The Indian Ocean extends over a tem.
little more than one hemisphere in the same fashion. Thus, Monsoons are winds that reverse direction in opposite
is behooves us to consider a model ocean stretching from seasons because of the different heating and cooling of the
pole to pole but bounded in the east and west by land (Fig. air over land and sea. Air over a large land mass at tem-
1.3B; Weyl, 1970). In this model, the latitudinal currents in- perate latitudes becomes cool and dense in winter; a high-
duced by the wind fields are deflected to form circular cur- pressure cell forms and wind streams out from it. Con-
rents called gyres. The largest ones, the subtropical gyres, versely, air in the summer is hot and light; a cell of low pres-
develop in both hemispheres as water flows westward un- sure develops and sucks in air from adjacent oceans where