204 Chapter Six
(a) (b)Fig. 6.10(a) Planktonic coccolithophore Emiliania huxleyi, a very common species in the modern oceans. This
specimen has a diameter of 8mm. The skeleton is clearly composed of circular shields packed around a single
algal cell. After death the coccosphere breaks down, releasing the shields to form the microscopic particles of
deep-sea oozes and chalks (photograph courtesy of D. Harbour). (b) Skeleton of modern planktonic foraminifer
Globigerinoides sacculifer, common in tropical oceans. Scale bar= 50 mm (photograph courtesy of B. Funnell).
degree of CaCO 3 undersaturation in the deep waters (Fig. 6.11). In the Atlantic
Ocean the CCD is at about 4.5 km depth; above the CCD, at about 4 km depth
in the Atlantic, there is a critical depth, known as the lysocline (Fig. 6.11). Here
the rate of calcite dissolution increases markedly and all but the most robust
particles dissolve rapidly. It is estimated that about 80% of the CaCO 3 settling
into deep waters is dissolved, either during transit through the water column or
on the seabed. As a consequence, pelagic carbonate deposits are most common
on the shallower parts of the deep ocean floors (Fig. 6.11) or on topographic
highs that project above the CCD.
Planktonic coccolithophores and foraminifera did not evolve until the mid-
Mesozoic (about 150 million years ago), whereas shallow-water shelly organisms
are known to have existed throughout Phanerozoic time (570 million years to the
present day). This means that the locus of carbonate deposition has shifted to the
deep oceans only in the last quarter of Phanerozoic time.
The removal of Ca^2 +by CaCO 3 precipitation can be estimated directly from
the abundance of CaCO 3 -rich ocean sediments and their sedimentation rates
(Table 6.2). From equation 6.4, we see that two moles of HCO 3 are removed with
each mole of Ca^2 +, a process that releases dissolved CO 2 into seawater, ultimately
to be returned to the atmosphere. Calcium carbonate also incorporates a small
but significant amount of Mg^2 +by isomorphous substitution for Ca^2 +(see Box
4.6) and this is used to derive the Mg^2 +removal in Table 6.2.