Mn, N02

Redox chemistry in oxygen minimum

5. Bottom-water enrichment

222Rn, 228Ra, Mn, Si

Flux out of the sediments

6. Deep-ocean depletion

210Pb, 230Th, Cu

Scavenging by settling particles

10.4.2 Composition of seawater Major ions

The salinity of seawater is defined as the grams of dissolved salt per kg of seawater. Using good technique, salinity can be reported to 0.001%o or 1 ppm(m). By tradition the major ions have been defined as those that make a significant contribution to the salinity. Thus, major ions are those with concentrations greater than 1 mg/kg or lppm(m). By this definition there are 11 major ions in seawater (Table 10-9).

The elements Na, K, CI, SO, Br, B, and F are the most conservative major elements. No significant variations in the ratios of these elements to chlorine have been demonstrated. Strontium has a small (< 0.5%) depletion in the euphotic zone (Brass and Turekian, 1974) possibly due to the plankton Acantharia, which makes its shell from SrS04 (celestite). Calcium has been known since the 19th century to be about 0.5% enriched in the deep sea relative to surface waters. Alkalinity (HC03 ) also shows a deep enrichment. These elements are controlled by the formation

Table 10-9 The major ions of seawater: concentration at 35%o, ratio to chlorinity, and molar concentration"

The major ions of seawater

g/kg g/kg Ion at S = 35%o (chlorinity %o) mol/L


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