eqn. 6.11The difference between the Wvalues in equations 6.9 and 6.11 is mainly due to
the effects of ionic strength, which are properly compensated for using total activ-
ity coefficients. Overall, the data show that surface seawater is about six to seven
times supersaturated with respect to calcite. We might then reasonably expect
CaCO 3 to precipitate spontaneously in the surface waters of the oceans.
The evidence from field studies is somewhat contrary to the predictions based
on equilibrium chemistry. Abiological precipitation of CaCO 3 seems to be very
limited, restricted to geographically and geochemically unusual conditions. The
reasons why carbonate minerals are reluctant to precipitate from surface sea-
water are still poorly understood, but probably include inhibiting effects of other
dissolved ions and compounds. Even where abiological precipitation is sus-
pected—for example, the famous ooid shoals and whitings of the Bahamas (Box
6.5)—it is often difficult to discount the effects of microbial involvement in the
precipitation process.
Volumetrically, the biological removal of Ca^2 +and HCO 3 - ions, built into the
skeletons of organisms, is much more important. In the modern oceans, the large
continental shelf areas created by sealevel rise in the last 11 000 years probably
account for about 45–50% of global carbonate deposition. Moreover, about half
of this sink for Ca^2 +and HCO 3 - ions occurs in the massive coral reefs of tropical
and subtropical oceans (e.g. the Australian Great Barrier Reef). It is tempting to
assume that coral reefs have always represented a major removal process for Ca^2 +
and HCO 3 - ions. However, over the last 150 million years it can be shown that it
is carbonate sedimentation in the deep oceans which has been volumetrically
more important, accounting for between 65 and 80% of the global CaCO 3 inven-
tory. These deep-sea deposits, which average about 0.5 km in thickness, mantle
about half the area of the deep ocean floor (Fig. 6.9). The ultra fine-grained
calcium carbonate muds (often referred to as oozes) are composed of phyto-
plankton (coccolithophores) and zooplankton (foraminifera) skeletons (Fig. 6.10).
Although these pelagic organisms live in the ocean surface waters, after death
their skeletons sink through the water column, either directly or within the faecal
pellets of zooplankton.
The controls on the distribution of pelagic oozes are partly related to the avail-
ability of nutrients, which must be capable of sustaining significant populations
of phytoplankton (see Section 5.5). More important, however, is the dissolution
of CaCO 3 as particles sink into ocean deep waters. In the deep ocean, carbon
dioxide (CO 2 ) concentrations increase, particularly in the deep Pacific, as a result
of the decomposition of sedimenting organic matter. Decreased temperature and
increased pressure also promote dissolution of CaCO 3 , favouring the reverse
reaction in equation 6.4.
By mapping the depth at which carbonate sediments exist on the floors of the
oceans, it is possible to identify the level where the rate of supply of biogenic
CaCO 3 is balanced by the rate of solution. This depth, known as the calcite
compensation depth (CCD), is variable in the world’s oceans, depending on the
W=
¥
¥
=
248 10
33 10
751
8
9.
.
.
The Oceans 201