An Introduction to Environmental Chemistry

iron- and manganese-rich. These sediments derive from FeS particles that pre-

cipitate in the ‘black smoke’ plume as the hot and acidic hydrothermal water is

rapidly cooled and mixed with alkali seawater. The particles fall out both close

to the vent and some kilometres from it. It is currently difficult to quantify the

global metal fluxes from these processes, although estimates have been made. In

the case of manganese, mid-ocean ridge hydrothermal sources can be up to

hundreds of mmol kg-^1 , 10^6 times higher than ambient seawater, and thus may be

significant to the global oceanic budget (Table 6.7). Modern atmospheric fluxes

of some metals are larger than river inputs (Table 6.8), caused by various com-

bustion processes—coal burning, metal smelting and automobile engines. The

shift towards a larger industrial atmospheric source for some metals may increase

their concentrations in open-ocean waters, since riverine metal inputs are often

removed in estuaries (Section 6.2).

The chemistry of dissolved metals in seawater can be grouped into

three classes, which describe the behaviour of the metal during chemical

cycling. These classes—conservative, nutrient-like and scavenged—have been

recognized by the shapes of concentration profiles when plotted against depth in

the oceans.

The Oceans 217

10–100 μg l–1 103 –10^6 μg l–1 Rivers

Resuspension

common

Resuspension

rare

Resuspension

(^10) common
(^3) –10 (^6) μg l–1
100’S
m
100 m
1–2 μg l–1 Rain of
biological
particles
Biological
particles
Euphotic Zone Estuary
Hydrothermal plume
MOR
Fig. 6.18Sketch to show variability in the concentration of particles in seawater. MOR, mid-ocean ridge.
Table 6.7Source of dissolved manganese to the oceans (10^9 mol yr-^1 ). After Chester (2000).
Rivers Atmosphere Hydrothermal (MOR)*
Source 5 0.5 11–34

• An unknown but potentially significant amount of manganese emitted from mid-ocean ridge (MOR)
  hydrothermal sources is precipitated near black smokers (i.e. immediately removed), so this estimate is not
  directly comparable to the other inputs.