Page 15 - Carbonate Sedimentology and Sequence Stratigraphy

6 WOLFGANG SCHLAGER

-1
25 - 40 40 - 90 90 - 125 125 - 500 gCm .yr -1
Fig. 1.8.— Organic productivity in the surface layer of the ocean, calculated from phosphate distribution, latitude and distance from
shore. Typical are minima (“submarine deserts” in the subtropical gyres and maxima in the coastal upwelling zones, particularly along the
west-facing coasts. High productivity also occurs in the globe-circling Antarctic current where water of all depths mixes easily because
density gradients are very low. Carbonate production is strongly influenced by nutrient concentration (see chapter 2). After Berger
(1989), modified.

high salinity because the gyre centers lie in the arid horse ESSENTIALS OF CARBONATE MINERALOGY AND
latitudes. The centers of the gyres represent warm, nutrient- CHEMISTRY
depleted deserts of the ocean.
The eastern peripheries of the subtropical gyres represent
the other extreme. The flow pattern is such that wind and The material for carbonate sedimentation is extracted
Coriolis force drive the surface water away from the conti- from the dissolved load of the sea; the volume of carbonate
nent. Upwelling of cool, nutrient-rich water is the result. At rocks derived from erosion of older rocks is very small. The
the western periphery of the gyres, upwelling is weaker as precipitation reactions can be summarized as
wind drift and Coriolis force oppose one another.
−
In the equatorial belt, the opposed direction of the Corio- Ca 2+ +2HCO 3 → CO 2 +H 2 O + CaCO 3
lis force in the two hemispheres leads to surface divergence
and gentle upwelling with moderately high nutrient levels. Aquatic precipitation proceeds along biotic and abiotic
Upwelling and high rainfall lower the salinities. pathways that are discussed further in chapter 2.
Light is the basis for photosynthesis, i.e. organic growth
Precipitation, preservation and alteration of the carbon-
that relies on dissolved nutrients in the water and energy
ate rocks are strongly influenced by their mineralogy. Three
from the Sun. Organic growth by photosynthesis is the be-
minerals appear in significant amounts: aragonite, calcite
gin of the food chain of the ocean. Moreover, much carbon-
and dolomite (Fig. 1.10). In practice, calcite is further subdi-
ate sediment is formed as a byproduct of photosynthesis. vided into rather pure calcite (also called “low-magnesium
The decrease of light with water depth follows a simple calcite”) and magnesian calcite (or “high-magnesium cal-
exponential function
cite”). Magnesian calcites are generally defined as calcites
I z = I 0 .e −kz with more than 4 mol% of CaCO 3 replaced by MgCO 3
(J.A.D. Dickson in Tucker and Wright, 1990). This bound-
where I 0 and I z are the irradiance at sea level and depth ary is justified by the observed changes in solubility (Fig.
z, respectively, and k is an attentuation coefficient that de- 1.11). Up to 4mol% MgCO 3 , magnesium does not seem to
pends on the turbidity of the water. For instance, k is large significantly influence calcite solubility. Therefore it is prac-
in areas of high suspended sediment load or high plank- tical to draw the boundary at the 4% level. One should be
ton productivity; k is small in clear, low-productivity waters aware, however, that there is a continuous range of magne-
such as at the centers of the subtropical gyres (Fig. 1.9). sium contents in calcite from 0 to over 30 mol% MgCO 3 .