The chemistry of natural waters - Geochemistry: An Introduction - page 157

The downward transfer of nutrients is therefore achieved by settling of the particles and
not by the sinking of surface waters depleted by biological activity. In contrast, upward
transfer of nutrients is most efficiently effected by deep-water upwelling. This mechanism
explains why deep-water upwellings off the coasts of Mauritania, Peru, Namibia, etc., are
associated with intense primary productivity.
The recycling of soft parts is accompanied by the redissolution of calcite from carbon-
ate tests. Alkalinity and

CO 2 increase while pH decreases with depth, typically from
values of 8.2 in surface water to 7.8 in deep water. As discussed earlier, the “surface-
water” and “deep-water” boxes may, by way of fractionation throughout the thermocline,
maintain a sharp contrast in spite of some nutrients having very long overall residence
times.
The deep ocean is cold and isolated from the atmosphere, contrary to surface water.
It is undersaturated in CaCO 3 whereas surface water is saturated, explaining why liv-
ing organisms have found an energy advantage in using this compound preferentially for
the fabrication of hard parts. The saturation limit, or lysocline, and the “carbonate com-
pensation depth” (CCD), which is the depth below which carbonates are not observed
in sediments, currently lie at a depth of about 4500 m. Above this level, carbonate sed-
iment is abundant, as on either side of the mid-ocean ridges, which may rise to just
2500 m below the surface; at greater depths, all calcite is dissolved and only residue
is deposited, very slowly, as red clay, the composition of which is much affected by
atmospheric dust.
In addition to vertical chemical variations, horizontal variations are important too. This
is partly because the largest rivers (e.g. the Amazon, Mississippi, and Congo) flow into
the Atlantic, which therefore receives a greater supply of nutrients than the Pacific, hence
its rich biota. The largest mass of deep water forms in the North Atlantic by downwelling
of the very salty water of the Gulf Stream, made even denser at high latitudes by the for-
mation of the ice cap ( Fig. 7.17 ). This is the North Atlantic Deep Water (NADW). Other
masses form near the Antarctic, such as the Antarctic Bottom Water (AABW) in the Wed-
del Sea. This very cold water, which is less salty than the NADW as it is less subjected
to evaporation, lies at the bottom of the main oceans. The NADW begins its long jour-
ney along the coast of the Americas to the Indian Ocean and the Pacific Ocean by way
of the circum-Antarctic current ( Fig. 7.18 ). There it rises to the surface and returns to the
Atlantic via Indonesia and Madagascar. The whole oceanic circulation system is driven by
temperature and salinity contrasts (thermohaline circulation) and is often referred to, for
obvious reasons, as the oceanic “conveyor belt.” ( Fig. 7.18 ) In Chapter 6 we calculated the
age for renewal of the deep water at 1600 years, which is approximately the duration of a
cycle. During this deep journey, the water that forms at the surface, and hence is poor in
nutrients but rich in oxygen, receives surface organic input. As the debris raining from the
surface is dissolved at depth, deep water progressively gains nutrients and alkalinity, and
loses oxygen. It also gains CO 2 through the effects of respiration, which is reflected by the
smaller carbonate sediment production in the Pacific than in the Atlantic. Generally, these
characteristics clearly distinguish the old water of the deep Pacific from the young Atlantic
waters.