CaCO 3 (eqn. 6.6). This allows CaCO 3 to precipitate as nodules (concretions) in
the sediment. These are not quantified in the global budget as they are volu-
metrically small sinks of CaCO 3.6.4.7 Hydrothermal processes
The hydrothermal (hot water) cycling of seawater at mid-ocean ridges has a pro-
found effect on the chemistry and budgets of some major and trace elements in
seawater. To understand the chemical changes it is necessary to know a little about
geological processes at mid-ocean ridges.
Basaltic ocean crust is emplaced at mid-ocean ridges by crystallization from
magma that is sourced from a magma chamber at shallow depth (about 2 km)
below the ridge. The magma chamber and newly emplaced crust is a discrete
heat source, localized below the ridge (Fig. 6.13). Successive emplacement of
new ocean crust gradually pushes the older crust laterally away from the ridge
axis at rates of a few millimetres per year. This ageing crust cools and subsides
as it travels away from the ridge axis. The resulting thermal structure, i.e. a
localized heat source underlying the ridge with cooler flanking areas, encourages
seawater to convect through fractures and fissures in the crust. This convection
is vigorous close to the ridge axis but more passive on the off-axis flanks (Fig.
6.14).
The deep waters of the oceans are cool (around 2–4°C) and dense relative to
overlying seawater. This dense water percolates into fissures in the basaltic crust.
Slowly it penetrates deep into the crust, gradually heating up, particularly as it
approaches the heat source of the underlying magma chamber. This massive heat208 Chapter Six
+
+
+
+
+
++
+
+
+
+++
+Pillow basaltsDirection of
seafloor spreading+ +++ ++ ++Emplacement
of doleriteTop of magma
chamberHeatSeawater~ 2 kmFig. 6.13Simplified structure of a fast spreading mid-ocean ridge (e.g. East Pacific Rise).