Page 12 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINES 3

Present deep circulation of the ocean
A) Open-ocean Coriolis B) wind-driven upwelling
effect upwelling
The principal driver of present deep-water circulation is
Meteorological the formation of dense cold water in the northern North At-
equator offshore lantic and on the shelves of Antarctica. This water sinks
Southeast Northeast wind and provides the deep and bottom water for all major ocean
trades trades
basins. The first-order pattern is that most deep water forms
in the Atlantic and flows through the Indian Ocean to the
North Pacific whence it returns to the source areas via up-
welling and surface circulation. (Fig. 1.5; Broecker and Peng,
1982; Open University, 1989b).
This flow pattern forces the three principal oceans to con-
stantly exchange surface water and deep water at a large
scale. The effects of this exchange on the three basins are
very different, though, and this leads to differences in wa-
ter chemistry that are relevant for carbonate sedimentation.
C) Coriolis effect transport D) obstruction upwelling
(shown for northern (plan view) The Atlantic donates deep water and receives surface water,
hemisphere) the Pacific receives deep water and donates surface water,
and the situation of the Indian Ocean is intermediate. As
nutrient concentrations are generally low in surface water
and high in deep water, the Atlantic is depleted in nutri-
ents compared to the Pacific. Fig. 1.6 depicts this principle
current of “basin-basin fractionation” of dissolved substances.
Another side effect of deep circulation concerns carbonate
dissolution at depth. The deep water in the Atlantic is rela-
tively young, thus still rich in oxygen (from the time spent at
the sea surface) and low in carbon dioxide because the wa-
ter has not had time to oxidize much organic matter. Con-
versely, Pacific deep water is old, low in oxygen and high in
carbon dioxide because of the long time spent oxidizing or-
ganic matter without an opportunity to take up oxygen from
the atmosphere. The low CO 2 content of the deep Atlantic
Fig. 1.2.— Most efficient processes causing upwelling. A) Ek- leads to high carbonate saturation and a low position of the
man transport along the equator drives the surface layers away in carbonate compensation depth (CCD) – the level where the
opposite directions. B) Wind blowing offshore drives the surface rate of carbonate sedimentation equals the rate of carbonate
layer away from the coastal zone and exposes deeper water lay-
ers. (Ekman transport starts at high angles to the wind but quickly dissolution. The situation is reversed in the Pacific. There,
turns into wind-parallel motion because of space constraints). C) the deep water is old, rich in CO 2 and thus largely undersat-
Ekman transport induced by coast-parallel winds or surface cur- urated with respect to calcium carbonate. This increases the
rents drives the surface layer offshore and exposes deeper layers rate of dissolution and raises the carbonate compensation
(situation shown is for the northern hemisphere). D) Local up- depth. The world map in Fig. 1.7 clearly shows the differ-
welling where shore-parallel current is impeded by a promontory ences between Atlantic and Pacific.
in the coastline. After Pipkin et al. (1987), modified.
Distribution of temperature, salinity, nutrients and light in the
the reversing monsoon winds may induce reversing surface surface waters
currents. Currently, the strongest monsoons develop be-
tween southern Asia and the Indian Ocean. Temperature, salinity and nutrients in the surface layer of
Fig. 1.4 shows that the present world ocean shows fea- the sea are important controls on carbonate production.
tures of Weyl’s laterally bounded “gyre ocean” as well as a Their distribution is mainly governed by the latitudinal dif-
globe-circling current. Paleoceanographic studies indicate ferences of solar radiation and by the patterns of surface cir-
that the changing positions of continents, archipelagos and culation. The first-order trend is a decrease in temperature
island arcs of the geologic past also changed the coeval cir- with increasing latitude and salinity maxima in the horse lat-
culation patterns. It seems, however, that with proper ad- itudes that correspond to the desert belts on land. From the
justments, the concepts of oceanic gyres and globe-circling horse latitudes, salinity decreases pole-ward and towards
currents are extremely useful models for the reconstruction the equator where high rainfall dilutes the surface water.
of the past, including the reconstruction of shoal-water car- The effects of gyre circulation are more varied as outlined
bonate deposition. below.